openvpn

openvpn(8)                  System Manager's Manual                 openvpn(8)



NAME
       openvpn - secure IP tunnel daemon.

SYNOPSIS
       openvpn [ options ... ]

INTRODUCTION
       OpenVPN is an open source VPN daemon by James Yonan.  Because OpenVPN
       tries to be a universal VPN tool offering a great deal of flexibility,
       there are a lot of options on this manual page.  If you're new to
       OpenVPN, you might want to skip ahead to the examples section where you
       will see how to construct simple VPNs on the command line without even
       needing a configuration file.

       Also note that there's more documentation and examples on the OpenVPN
       web site: http://openvpn.net/

       And if you would like to see a shorter version of this manual, see the
       openvpn usage message which can be obtained by running openvpn without
       any parameters.

DESCRIPTION
       OpenVPN is a robust and highly flexible VPN daemon.  OpenVPN supports
       SSL/TLS security, ethernet bridging, TCP or UDP tunnel transport
       through proxies or NAT, support for dynamic IP addresses and DHCP,
       scalability to hundreds or thousands of users, and portability to most
       major OS platforms.

       OpenVPN is tightly bound to the OpenSSL library, and derives much of
       its crypto capabilities from it.

       OpenVPN supports conventional encryption using a pre-shared secret key
       (Static Key mode) or public key security (SSL/TLS mode) using client &
       server certificates.  OpenVPN also supports non-encrypted TCP/UDP
       tunnels.

       OpenVPN is designed to work with the TUN/TAP virtual networking
       interface that exists on most platforms.

       Overall, OpenVPN aims to offer many of the key features of IPSec but
       with a relatively lightweight footprint.

OPTIONS
       OpenVPN allows any option to be placed either on the command line or in
       a configuration file.  Though all command line options are preceded by
       a double-leading-dash ("--"), this prefix can be removed when an option
       is placed in a configuration file.

       --help Show options.

       --config file
              Load additional config options from file where each line
              corresponds to one command line option, but with the leading
              '--' removed.

              If --config file is the only option to the openvpn command, the
              --config can be removed, and the command can be given as openvpn
              file

              Note that configuration files can be nested to a reasonable
              depth.

              Double quotation or single quotation characters ("", '') can be
              used to enclose single parameters containing whitespace, and "#"
              or ";" characters in the first column can be used to denote
              comments.

              Note that OpenVPN 2.0 and higher performs backslash-based shell
              escaping for characters not in single quotations, so the
              following mappings should be observed:

                  \\       Maps to a single backslash character (\).
                  \"       Pass a literal doublequote character ("), don't
                           interpret it as enclosing a parameter.
                  \[SPACE] Pass a literal space or tab character, don't
                           interpret it as a parameter delimiter.

              For example on Windows, use double backslashes to represent
              pathnames:

                  secret "c:\\OpenVPN\\secret.key"

              For examples of configuration files, see
              http://openvpn.net/examples.html

              Here is an example configuration file:

                  #
                  # Sample OpenVPN configuration file for
                  # using a pre-shared static key.
                  #
                  # '#' or ';' may be used to delimit comments.

                  # Use a dynamic tun device.
                  dev tun

                  # Our remote peer
                  remote mypeer.mydomain

                  # 10.1.0.1 is our local VPN endpoint
                  # 10.1.0.2 is our remote VPN endpoint
                  ifconfig 10.1.0.1 10.1.0.2

                  # Our pre-shared static key
                  secret static.key

   Tunnel Options:
       --mode m
              Set OpenVPN major mode.  By default, OpenVPN runs in
              point-to-point mode ("p2p").  OpenVPN 2.0 introduces a new mode
              ("server") which implements a multi-client server capability.

       --local host
              Local host name or IP address for bind.  If specified, OpenVPN
              will bind to this address only.  If unspecified, OpenVPN will
              bind to all interfaces.

       --remote host [port] [proto]
              Remote host name or IP address.  On the client, multiple
              --remote options may be specified for redundancy, each referring
              to a different OpenVPN server.  Specifying multiple --remote
              options for this purpose is a special case of the more general
              connection-profile feature.  See the <connection> documentation
              below.

              The OpenVPN client will try to connect to a server at host:port
              in the order specified by the list of --remote options.

              proto indicates the protocol to use when connecting with the
              remote, and may be "tcp" or "udp".

              For forcing IPv4 or IPv6 connection suffix tcp or udp with 4/6
              like udp4/udp6/tcp4/tcp6.

              The client will move on to the next host in the list, in the
              event of connection failure.  Note that at any given time, the
              OpenVPN client will at most be connected to one server.

              Note that since UDP is connectionless, connection failure is
              defined by the --ping and --ping-restart options.

              Note the following corner case:  If you use multiple --remote
              options, AND you are dropping root privileges on the client with
              --user and/or --group, AND the client is running a non-Windows
              OS, if the client needs to switch to a different server, and
              that server pushes back different TUN/TAP or route settings, the
              client may lack the necessary privileges to close and reopen the
              TUN/TAP interface.  This could cause the client to exit with a
              fatal error.

              If --remote is unspecified, OpenVPN will listen for packets from
              any IP address, but will not act on those packets unless they
              pass all authentication tests.  This requirement for
              authentication is binding on all potential peers, even those
              from known and supposedly trusted IP addresses (it is very easy
              to forge a source IP address on a UDP packet).

              When used in TCP mode, --remote will act as a filter, rejecting
              connections from any host which does not match host.

              If host is a DNS name which resolves to multiple IP addresses,
              OpenVPN will try them in the order that the system getaddrinfo()
              presents them, so priorization and DNS randomization is done by
              the system library.  Unless an IP version is forced by the
              protocol specification (4/6 suffix), OpenVPN will try both IPv4
              and IPv6 addresses, in the order getaddrinfo() returns them.

       --remote-random-hostname
              Prepend a random string (6 bytes, 12 hex characters) to hostname
              to prevent DNS caching.  For example, "foo.bar.gov" would be
              modified to "<random-chars>.foo.bar.gov".

       <connection>
              Define a client connection profile.  Client connection profiles
              are groups of OpenVPN options that describe how to connect to a
              given OpenVPN server.  Client connection profiles are specified
              within an OpenVPN configuration file, and each profile is
              bracketed by <connection> and </connection>.

              An OpenVPN client will try each connection profile sequentially
              until it achieves a successful connection.

              --remote-random can be used to initially "scramble" the
              connection list.

              Here is an example of connection profile usage:

                  client
                  dev tun

                  <connection>
                  remote 198.19.34.56 1194 udp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  http-proxy 192.168.0.8 8080
                  </connection>

                  <connection>
                  remote 198.19.36.99 443 tcp
                  http-proxy 192.168.0.8 8080
                  </connection>

                  persist-key
                  persist-tun
                  pkcs12 client.p12
                  remote-cert-tls server
                  verb 3

              First we try to connect to a server at 198.19.34.56:1194 using
              UDP.  If that fails, we then try to connect to 198.19.34.56:443
              using TCP.  If that also fails, then try connecting through an
              HTTP proxy at 192.168.0.8:8080 to 198.19.34.56:443 using TCP.
              Finally, try to connect through the same proxy to a server at
              198.19.36.99:443 using TCP.

              The following OpenVPN options may be used inside of a
              <connection> block:

              bind, connect-retry, connect-retry-max, connect-timeout,
              explicit-exit-notify, float, fragment, http-proxy,
              http-proxy-option, key-direction, link-mtu, local, lport,
              mssfix, mtu-disc, nobind, port, proto, remote, rport,
              socks-proxy, tls-auth, tls-crypt, tun-mtu and tun-mtu-extra.

              A defaulting mechanism exists for specifying options to apply to
              all <connection> profiles.  If any of the above options (with
              the exception of remote ) appear outside of a <connection>
              block, but in a configuration file which has one or more
              <connection> blocks, the option setting will be used as a
              default for <connection> blocks which follow it in the
              configuration file.

              For example, suppose the nobind option were placed in the sample
              configuration file above, near the top of the file, before the
              first <connection> block.  The effect would be as if nobind were
              declared in all <connection> blocks below it.

       --proto-force p
              When iterating through connection profiles, only consider
              profiles using protocol p ('tcp'|'udp').

       --remote-random
              When multiple --remote address/ports are specified, or if
              connection profiles are being used, initially randomize the
              order of the list as a kind of basic load-balancing measure.

       --proto p
              Use protocol p for communicating with remote host.  p can be
              udp, tcp-client, or tcp-server.

              The default protocol is udp when --proto is not specified.

              For UDP operation, --proto udp should be specified on both
              peers.

              For TCP operation, one peer must use --proto tcp-server and the
              other must use --proto tcp-client.  A peer started with
              tcp-server will wait indefinitely for an incoming connection.  A
              peer started with tcp-client will attempt to connect, and if
              that fails, will sleep for 5 seconds (adjustable via the
              --connect-retry option) and try again infinite or up to N
              retries (adjustable via the --connect-retry-max option).  Both
              TCP client and server will simulate a SIGUSR1 restart signal if
              either side resets the connection.

              OpenVPN is designed to operate optimally over UDP, but TCP
              capability is provided for situations where UDP cannot be used.
              In comparison with UDP, TCP will usually be somewhat less
              efficient and less robust when used over unreliable or congested
              networks.

              This article outlines some of problems with tunneling IP over
              TCP:

              http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

              There are certain cases, however, where using TCP may be
              advantageous from a security and robustness perspective, such as
              tunneling non-IP or application-level UDP protocols, or
              tunneling protocols which don't possess a built-in reliability
              layer.

       --connect-retry n [max]
              Wait n seconds  between connection attempts (default=5).
              Repeated reconnection attempts are slowed down after 5 retries
              per remote by doubling the wait time after each unsuccessful
              attempt. The optional argument max specifies the maximum value
              of wait time in seconds at which it gets capped (default=300).

       --connect-retry-max n
              n specifies the number of times each --remote or <connection>
              entry is tried. Specifying n as one would try each entry exactly
              once. A successful connection resets the counter.
              (default=unlimited).

       --show-proxy-settings
              Show sensed HTTP or SOCKS proxy settings. Currently, only
              Windows clients support this option.

       --http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
              Connect to remote host through an HTTP proxy at address server
              and port port.  If HTTP Proxy-Authenticate is required, authfile
              is a file containing a username and password on 2 lines, or
              "stdin" to prompt from console. Its content can also be
              specified in the config file with the --http-proxy-user-pass
              option. (See section on inline files)

              auth-method should be one of "none", "basic", or "ntlm".

              HTTP Digest authentication is supported as well, but only via
              the auto or auto-nct flags (below).

              The auto flag causes OpenVPN to automatically determine the
              auth-method and query stdin or the management interface for
              username/password credentials, if required.  This flag exists on
              OpenVPN 2.1 or higher.

              The auto-nct flag (no clear-text auth) instructs OpenVPN to
              automatically determine the authentication method, but to reject
              weak authentication protocols such as HTTP Basic Authentication.

       --http-proxy-option type [parm]
              Set extended HTTP proxy options.  Repeat to set multiple
              options.

              VERSION version -- Set HTTP version number to version
              (default=1.0).

              AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.

              CUSTOM-HEADER name content -- Adds the custom Header with name
              as name and content as the content of the custom HTTP header.

       --socks-proxy server [port] [authfile]
              Connect to remote host through a Socks5 proxy at address server
              and port port (default=1080).  authfile (optional) is a file
              containing a username and password on 2 lines, or "stdin" to
              prompt from console.

       --resolv-retry n
              If hostname resolve fails for --remote, retry resolve for n
              seconds before failing.

              Set n to "infinite" to retry indefinitely.

              By default, --resolv-retry infinite is enabled.  You can disable
              by setting n=0.

       --float
              Allow remote peer to change its IP address and/or port number,
              such as due to DHCP (this is the default if --remote is not
              used).  --float when specified with --remote allows an OpenVPN
              session to initially connect to a peer at a known address,
              however if packets arrive from a new address and pass all
              authentication tests, the new address will take control of the
              session.  This is useful when you are connecting to a peer which
              holds a dynamic address such as a dial-in user or DHCP client.

              Essentially, --float tells OpenVPN to accept authenticated
              packets from any address, not only the address which was
              specified in the --remote option.

       --ipchange cmd
              Run command cmd when our remote ip-address is initially
              authenticated or changes.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              When cmd is executed two arguments are appended after any
              arguments specified in cmd , as follows:

              cmd ip_address port_number

              Don't use --ipchange in --mode server mode.  Use a
              --client-connect script instead.

              See the "Environmental Variables" section below for additional
              parameters passed as environmental variables.

              If you are running in a dynamic IP address environment where the
              IP addresses of either peer could change without notice, you can
              use this script, for example, to edit the /etc/hosts file with
              the current address of the peer.  The script will be run every
              time the remote peer changes its IP address.

              Similarly if our IP address changes due to DHCP, we should
              configure our IP address change script (see man page for
              dhcpcd(8) ) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN.
              OpenVPN will then reestablish a connection with its most
              recently authenticated peer on its new IP address.

       --port port
              TCP/UDP port number or port name for both local and remote (sets
              both --lport and --rport options to given port).  The current
              default of 1194 represents the official IANA port number
              assignment for OpenVPN and has been used since version
              2.0-beta17.  Previous versions used port 5000 as the default.

       --lport port
              Set local TCP/UDP port number or name.  Cannot be used together
              with --nobind option.

       --rport port
              Set TCP/UDP port number or name used by the --remote option. The
              port can also be set directly using the --remote option.

       --bind [ipv6only]
              Bind to local address and port. This is the default unless any
              of --proto tcp-client , --http-proxy or --socks-proxy are used.

              If the ipv6only keyword is present OpenVPN will bind only to
              IPv6 (as opposed to IPv6 and IPv4) when a IPv6 socket is opened.


       --nobind
              Do not bind to local address and port.  The IP stack will
              allocate a dynamic port for returning packets.  Since the value
              of the dynamic port could not be known in advance by a peer,
              this option is only suitable for peers which will be initiating
              connections by using the --remote option.

       --dev tunX | tapX | null
              TUN/TAP virtual network device ( X can be omitted for a dynamic
              device.)

              See examples section below for an example on setting up a TUN
              device.

              You must use either tun devices on both ends of the connection
              or tap devices on both ends.  You cannot mix them, as they
              represent different underlying network layers.

              tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap
              devices encapsulate Ethernet 802.3 (OSI Layer 2).

       --dev-type device-type
              Which device type are we using?  device-type should be tun (OSI
              Layer 3) or tap (OSI Layer 2).  Use this option only if the
              TUN/TAP device used with --dev does not begin with tun or tap.

       --topology mode
              Configure virtual addressing topology when running in --dev tun
              mode.  This directive has no meaning in --dev tap mode, which
              always uses a subnet topology.

              If you set this directive on the server, the --server and
              --server-bridge directives will automatically push your chosen
              topology setting to clients as well.  This directive can also be
              manually pushed to clients.  Like the --dev directive, this
              directive must always be compatible between client and server.

              mode can be one of:

              net30 -- Use a point-to-point topology, by allocating one /30
              subnet per client.  This is designed to allow point-to-point
              semantics when some or all of the connecting clients might be
              Windows systems.  This is the default on OpenVPN 2.0.

              p2p -- Use a point-to-point topology where the remote endpoint
              of the client's tun interface always points to the local
              endpoint of the server's tun interface.  This mode allocates a
              single IP address per connecting client.  Only use when none of
              the connecting clients are Windows systems.  This mode is
              functionally equivalent to the --ifconfig-pool-linear directive
              which is available in OpenVPN 2.0, is deprecated and will be
              removed in OpenVPN 2.5

              subnet -- Use a subnet rather than a point-to-point topology by
              configuring the tun interface with a local IP address and subnet
              mask, similar to the topology used in --dev tap and ethernet
              bridging mode.  This mode allocates a single IP address per
              connecting client and works on Windows as well.  Only available
              when server and clients are OpenVPN 2.1 or higher, or OpenVPN
              2.0.x which has been manually patched with the --topology
              directive code.  When used on Windows, requires version 8.2 or
              higher of the TAP-Win32 driver.  When used on *nix, requires
              that the tun driver supports an ifconfig(8) command which sets a
              subnet instead of a remote endpoint IP address.

              This option exists in OpenVPN 2.1 or higher.

              Note: Using --topology subnet changes the interpretation of the
              arguments of --ifconfig to mean "address netmask", no longer
              "local remote".

       --dev-node node
              Explicitly set the device node rather than using /dev/net/tun,
              /dev/tun, /dev/tap, etc.  If OpenVPN cannot figure out whether
              node is a TUN or TAP device based on the name, you should also
              specify --dev-type tun or --dev-type tap.

              Under Mac OS X this option can be used to specify the default
              tun implementation. Using --dev-node utun forces usage of the
              native Darwin tun kernel support. Use --dev-node utunN to select
              a specific utun instance. To force using the tun.kext
              (/dev/tunX) use --dev-node tun.  When not specifying a
              --dev-node option openvpn will first try to open utun, and fall
              back to tun.kext.

              On Windows systems, select the TAP-Win32 adapter which is named
              node in the Network Connections Control Panel or the raw GUID of
              the adapter enclosed by braces.  The --show-adapters option
              under Windows can also be used to enumerate all available
              TAP-Win32 adapters and will show both the network connections
              control panel name and the GUID for each TAP-Win32 adapter.

       --lladdr address
              Specify the link layer address, more commonly known as the MAC
              address.  Only applied to TAP devices.

       --iproute cmd
              Set alternate command to execute instead of default iproute2
              command.  May be used in order to execute OpenVPN in
              unprivileged environment.

       --ifconfig l rn
              Set TUN/TAP adapter parameters.  l is the IP address of the
              local VPN endpoint.  For TUN devices in point-to-point mode, rn
              is the IP address of the remote VPN endpoint.  For TAP devices,
              or TUN devices used with --topology subnet, rn is the subnet
              mask of the virtual network segment which is being created or
              connected to.

              For TUN devices, which facilitate virtual point-to-point IP
              connections (when used in --topology net30 or p2p mode), the
              proper usage of --ifconfig is to use two private IP addresses
              which are not a member of any existing subnet which is in use.
              The IP addresses may be consecutive and should have their order
              reversed on the remote peer.  After the VPN is established, by
              pinging rn, you will be pinging across the VPN.

              For TAP devices, which provide the ability to create virtual
              ethernet segments, or TUN devices in --topology subnet mode
              (which create virtual "multipoint networks"), --ifconfig is used
              to set an IP address and subnet mask just as a physical ethernet
              adapter would be similarly configured.  If you are attempting to
              connect to a remote ethernet bridge, the IP address and subnet
              should be set to values which would be valid on the the bridged
              ethernet segment (note also that DHCP can be used for the same
              purpose).

              This option, while primarily a proxy for the ifconfig(8)
              command, is designed to simplify TUN/TAP tunnel configuration by
              providing a standard interface to the different ifconfig
              implementations on different platforms.

              --ifconfig parameters which are IP addresses can also be
              specified as a DNS or /etc/hosts file resolvable name.

              For TAP devices, --ifconfig should not be used if the TAP
              interface will be getting an IP address lease from a DHCP
              server.

       --ifconfig-noexec
              Don't actually execute ifconfig/netsh commands, instead pass
              --ifconfig parameters to scripts using environmental variables.

       --ifconfig-nowarn
              Don't output an options consistency check warning if the
              --ifconfig option on this side of the connection doesn't match
              the remote side.  This is useful when you want to retain the
              overall benefits of the options consistency check (also see
              --disable-occ option) while only disabling the ifconfig
              component of the check.

              For example, if you have a configuration where the local host
              uses --ifconfig but the remote host does not, use
              --ifconfig-nowarn on the local host.

              This option will also silence warnings about potential address
              conflicts which occasionally annoy more experienced users by
              triggering "false positive" warnings.

       --route network/IP [netmask] [gateway] [metric]
              Add route to routing table after connection is established.
              Multiple routes can be specified.  Routes will be automatically
              torn down in reverse order prior to TUN/TAP device close.

              This option is intended as a convenience proxy for the route(8)
              shell command, while at the same time providing portable
              semantics across OpenVPN's platform space.

              netmask default -- 255.255.255.255

              gateway default -- taken from --route-gateway or the second
              parameter to --ifconfig when --dev tun is specified.

              metric default -- taken from --route-metric otherwise 0.

              The default can be specified by leaving an option blank or
              setting it to "default".

              The network and gateway parameters can also be specified as a
              DNS or /etc/hosts file resolvable name, or as one of three
              special keywords:

              vpn_gateway -- The remote VPN endpoint address (derived either
              from --route-gateway or the second parameter to --ifconfig when
              --dev tun is specified).

              net_gateway -- The pre-existing IP default gateway, read from
              the routing table (not supported on all OSes).

              remote_host -- The --remote address if OpenVPN is being run in
              client mode, and is undefined in server mode.

       --route-gateway gw|'dhcp'
              Specify a default gateway gw for use with --route.

              If dhcp is specified as the parameter, the gateway address will
              be extracted from a DHCP negotiation with the OpenVPN
              server-side LAN.

       --route-metric m
              Specify a default metric m for use with --route.

       --route-delay [n] [w]
              Delay n seconds (default=0) after connection establishment,
              before adding routes. If n is 0, routes will be added
              immediately upon connection establishment.  If --route-delay is
              omitted, routes will be added immediately after TUN/TAP device
              open and --up script execution, before any --user or --group
              privilege downgrade (or --chroot execution.)

              This option is designed to be useful in scenarios where DHCP is
              used to set tap adapter addresses.  The delay will give the DHCP
              handshake time to complete before routes are added.

              On Windows, --route-delay tries to be more intelligent by
              waiting w seconds (w=30 by default) for the TAP-Win32 adapter to
              come up before adding routes.

       --route-up cmd
              Run command cmd after routes are added, subject to
              --route-delay.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              See the "Environmental Variables" section below for additional
              parameters passed as environmental variables.

       --route-pre-down cmd
              Run command cmd before routes are removed upon disconnection.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              See the "Environmental Variables" section below for additional
              parameters passed as environmental variables.

       --route-noexec
              Don't add or remove routes automatically.  Instead pass routes
              to --route-up script using environmental variables.

       --route-nopull
              When used with --client or --pull, accept options pushed by
              server EXCEPT for routes, block-outside-dns and dhcp options
              like DNS servers.

              When used on the client, this option effectively bars the server
              from adding routes to the client's routing table, however note
              that this option still allows the server to set the TCP/IP
              properties of the client's TUN/TAP interface.

       --allow-pull-fqdn
              Allow client to pull DNS names from server (rather than being
              limited to IP address) for --ifconfig, --route, and
              --route-gateway.

       --client-nat snat|dnat network netmask alias
              This pushable client option sets up a stateless one-to-one NAT
              rule on packet addresses (not ports), and is useful in cases
              where routes or ifconfig settings pushed to the client would
              create an IP numbering conflict.

              network/netmask (for example 192.168.0.0/255.255.0.0) defines
              the local view of a resource from the client perspective, while
              alias/netmask (for example 10.64.0.0/255.255.0.0) defines the
              remote view from the server perspective.

              Use snat (source NAT) for resources owned by the client and dnat
              (destination NAT) for remote resources.

              Set --verb 6 for debugging info showing the transformation of
              src/dest addresses in packets.

       --redirect-gateway flags...
              Automatically execute routing commands to cause all outgoing IP
              traffic to be redirected over the VPN.  This is a client-side
              option.

              This option performs three steps:

              (1) Create a static route for the --remote address which
              forwards to the pre-existing default gateway.  This is done so
              that (3) will not create a routing loop.

              (2) Delete the default gateway route.

              (3) Set the new default gateway to be the VPN endpoint address
              (derived either from --route-gateway or the second parameter to
              --ifconfig when --dev tun is specified).

              When the tunnel is torn down, all of the above steps are
              reversed so that the original default route is restored.

              Option flags:

              local -- Add the local flag if both OpenVPN peers are directly
              connected via a common subnet, such as with wireless.  The local
              flag will cause step 1 above to be omitted.

              autolocal -- Try to automatically determine whether to enable
              local flag above.

              def1 -- Use this flag to override the default gateway by using
              0.0.0.0/1 and 128.0.0.0/1 rather than 0.0.0.0/0.  This has the
              benefit of overriding but not wiping out the original default
              gateway.

              bypass-dhcp -- Add a direct route to the DHCP server (if it is
              non-local) which bypasses the tunnel (Available on Windows
              clients, may not be available on non-Windows clients).

              bypass-dns -- Add a direct route to the DNS server(s) (if they
              are non-local) which bypasses the tunnel (Available on Windows
              clients, may not be available on non-Windows clients).

              block-local -- Block access to local LAN when the tunnel is
              active, except for the LAN gateway itself.  This is accomplished
              by routing the local LAN (except for the LAN gateway address)
              into the tunnel.

              ipv6 -- Redirect IPv6 routing into the tunnel.  This works
              similar to the def1 flag, that is, more specific IPv6 routes are
              added (2000::/4, 3000::/4), covering the whole IPv6 unicast
              space.

              !ipv4 -- Do not redirect IPv4 traffic - typically used in the
              flag pair ipv6 !ipv4 to redirect IPv6-only.

       --link-mtu n
              Sets an upper bound on the size of UDP packets which are sent
              between OpenVPN peers.  It's best not to set this parameter
              unless you know what you're doing.

       --redirect-private [flags]
              Like --redirect-gateway, but omit actually changing the default
              gateway.  Useful when pushing private subnets.

       --block-ipv6
              On the client, instead of sending IPv6 packets over the VPN
              tunnel, all IPv6 packets are answered with an ICMPv6 no route
              host message. On the server, all IPv6 packets from clients are
              answered with an ICMPv6 no route to host message. This options
              is intended for cases when IPv6 should be blocked and other
              options are not available.  --block-ipv6 will use the remote
              IPv6 as source address of the ICMPv6 packets if set, otherwise
              will use fe80::7 as source address.

              For this option to make sense you actually have to route traffic
              to the tun interface. The following example config block would
              send all IPv6 traffic to OpenVPN and answer all requests with no
              route to host, effectively blocking IPv6.

              # client config
              --ifconfig-ipv6 fd15:53b6:dead::2/64  fd15:53b6:dead::1
              --redirect-gateway ipv6
              --block-ipv6

              # Server config, push a "valid" ipv6 config to the client and
              block # on the server
              --push "ifconfig-ipv6 fd15:53b6:dead::2/64  fd15:53b6:dead::1"
              --push "redirect-gateway ipv6"
              --block-ipv6

       --tun-mtu n
              Take the TUN device MTU to be n and derive the link MTU from it
              (default=1500).  In most cases, you will probably want to leave
              this parameter set to its default value.

              The MTU (Maximum Transmission Units) is the maximum datagram
              size in bytes that can be sent unfragmented over a particular
              network path.  OpenVPN requires that packets on the control or
              data channels be sent unfragmented.

              MTU problems often manifest themselves as connections which hang
              during periods of active usage.

              It's best to use the --fragment and/or --mssfix options to deal
              with MTU sizing issues.

       --tun-mtu-extra n
              Assume that the TUN/TAP device might return as many as n bytes
              more than the --tun-mtu size on read.  This parameter defaults
              to 0, which is sufficient for most TUN devices.  TAP devices may
              introduce additional overhead in excess of the MTU size, and a
              setting of 32 is the default when TAP devices are used.  This
              parameter only controls internal OpenVPN buffer sizing, so there
              is no transmission overhead associated with using a larger
              value.

       --mtu-disc type
              Should we do Path MTU discovery on TCP/UDP channel?  Only
              supported on OSes such as Linux that supports the necessary
              system call to set.

              'no' -- Never send DF (Don't Fragment) frames
              'maybe' -- Use per-route hints
              'yes' -- Always DF (Don't Fragment)

       --mtu-test
              To empirically measure MTU on connection startup, add the
              --mtu-test option to your configuration.  OpenVPN will send ping
              packets of various sizes to the remote peer and measure the
              largest packets which were successfully received.  The
              --mtu-test process normally takes about 3 minutes to complete.

       --fragment max
              Enable internal datagram fragmentation so that no UDP datagrams
              are sent which are larger than max bytes.

              The max parameter is interpreted in the same way as the
              --link-mtu parameter, i.e. the UDP packet size after
              encapsulation overhead has been added in, but not including the
              UDP header itself.

              The --fragment option only makes sense when you are using the
              UDP protocol ( --proto udp ).

              --fragment adds 4 bytes of overhead per datagram.

              See the --mssfix option below for an important related option to
              --fragment.

              It should also be noted that this option is not meant to replace
              UDP fragmentation at the IP stack level.  It is only meant as a
              last resort when path MTU discovery is broken.  Using this
              option is less efficient than fixing path MTU discovery for your
              IP link and using native IP fragmentation instead.

              Having said that, there are circumstances where using OpenVPN's
              internal fragmentation capability may be your only option, such
              as tunneling a UDP multicast stream which requires
              fragmentation.

       --mssfix max
              Announce to TCP sessions running over the tunnel that they
              should limit their send packet sizes such that after OpenVPN has
              encapsulated them, the resulting UDP packet size that OpenVPN
              sends to its peer will not exceed max bytes. The default value
              is 1450.

              The max parameter is interpreted in the same way as the
              --link-mtu parameter, i.e. the UDP packet size after
              encapsulation overhead has been added in, but not including the
              UDP header itself. Resulting packet would be at most 28 bytes
              larger for IPv4 and 48 bytes for IPv6 (20/40 bytes for IP header
              and 8 bytes for UDP header). Default value of 1450 allows IPv4
              packets to be transmitted over a link with MTU 1473 or higher
              without IP level fragmentation.

              The --mssfix option only makes sense when you are using the UDP
              protocol for OpenVPN peer-to-peer communication, i.e.  --proto
              udp.

              --mssfix and --fragment can be ideally used together, where
              --mssfix will try to keep TCP from needing packet fragmentation
              in the first place, and if big packets come through anyhow (from
              protocols other than TCP), --fragment will internally fragment
              them.

              Both --fragment and --mssfix are designed to work around cases
              where Path MTU discovery is broken on the network path between
              OpenVPN peers.

              The usual symptom of such a breakdown is an OpenVPN connection
              which successfully starts, but then stalls during active usage.

              If --fragment and --mssfix are used together, --mssfix will take
              its default max parameter from the --fragment max option.

              Therefore, one could lower the maximum UDP packet size to 1300
              (a good first try for solving MTU-related connection problems)
              with the following options:

              --tun-mtu 1500 --fragment 1300 --mssfix

       --sndbuf size
              Set the TCP/UDP socket send buffer size.  Defaults to operation
              system default.

       --rcvbuf size
              Set the TCP/UDP socket receive buffer size.  Defaults to
              operation system default.

       --mark value
              Mark encrypted packets being sent with value. The mark value can
              be matched in policy routing and packetfilter rules. This option
              is only supported in Linux and does nothing on other operating
              systems.

       --socket-flags flags...
              Apply the given flags to the OpenVPN transport socket.
              Currently, only TCP_NODELAY is supported.

              The TCP_NODELAY socket flag is useful in TCP mode, and causes
              the kernel to send tunnel packets immediately over the TCP
              connection without trying to group several smaller packets into
              a larger packet.  This can result in a considerably improvement
              in latency.

              This option is pushable from server to client, and should be
              used on both client and server for maximum effect.

       --txqueuelen n
              (Linux only) Set the TX queue length on the TUN/TAP interface.
              Currently defaults to 100.

       --shaper n
              Limit bandwidth of outgoing tunnel data to n bytes per second on
              the TCP/UDP port.  Note that this will only work if mode is set
              to p2p.  If you want to limit the bandwidth in both directions,
              use this option on both peers.

              OpenVPN uses the following algorithm to implement traffic
              shaping: Given a shaper rate of n bytes per second, after a
              datagram write of b bytes is queued on the TCP/UDP port, wait a
              minimum of (b / n) seconds before queuing the next write.

              It should be noted that OpenVPN supports multiple tunnels
              between the same two peers, allowing you to construct full-speed
              and reduced bandwidth tunnels at the same time, routing
              low-priority data such as off-site backups over the reduced
              bandwidth tunnel, and other data over the full-speed tunnel.

              Also note that for low bandwidth tunnels (under 1000 bytes per
              second), you should probably use lower MTU values as well (see
              above), otherwise the packet latency will grow so large as to
              trigger timeouts in the TLS layer and TCP connections running
              over the tunnel.

              OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.

       --inactive n [bytes]
              Causes OpenVPN to exit after n seconds of inactivity on the
              TUN/TAP device. The time length of inactivity is measured since
              the last incoming or outgoing tunnel packet.  The default value
              is 0 seconds, which disables this feature.

              If the optional bytes parameter is included, exit if less than
              bytes of combined in/out traffic are produced on the tun/tap
              device in n seconds.

              In any case, OpenVPN's internal ping packets (which are just
              keepalives) and TLS control packets are not considered
              "activity", nor are they counted as traffic, as they are used
              internally by OpenVPN and are not an indication of actual user
              activity.

       --ping n
              Ping remote over the TCP/UDP control channel if no packets have
              been sent for at least n seconds (specify --ping on both peers
              to cause ping packets to be sent in both directions since
              OpenVPN ping packets are not echoed like IP ping packets).  When
              used in one of OpenVPN's secure modes (where --secret,
              --tls-server, or --tls-client is specified), the ping packet
              will be cryptographically secure.

              This option has two intended uses:

              (1) Compatibility with stateful firewalls.  The periodic ping
              will ensure that a stateful firewall rule which allows OpenVPN
              UDP packets to pass will not time out.

              (2) To provide a basis for the remote to test the existence of
              its peer using the --ping-exit option.

       --ping-exit n
              Causes OpenVPN to exit after n seconds pass without reception of
              a ping or other packet from remote.  This option can be combined
              with --inactive, --ping, and --ping-exit to create a two-tiered
              inactivity disconnect.

              For example,

              openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

              when used on both peers will cause OpenVPN to exit within 60
              seconds if its peer disconnects, but will exit after one hour if
              no actual tunnel data is exchanged.

       --ping-restart n
              Similar to --ping-exit, but trigger a SIGUSR1 restart after n
              seconds pass without reception of a ping or other packet from
              remote.

              This option is useful in cases where the remote peer has a
              dynamic IP address and a low-TTL DNS name is used to track the
              IP address using a service such as http://dyndns.org/ + a
              dynamic DNS client such as ddclient.

              If the peer cannot be reached, a restart will be triggered,
              causing the hostname used with --remote to be re-resolved (if
              --resolv-retry is also specified).

              In server mode, --ping-restart, --inactive, or any other type of
              internally generated signal will always be applied to individual
              client instance objects, never to whole server itself.  Note
              also in server mode that any internally generated signal which
              would normally cause a restart, will cause the deletion of the
              client instance object instead.

              In client mode, the --ping-restart parameter is set to 120
              seconds by default.  This default will hold until the client
              pulls a replacement value from the server, based on the
              --keepalive setting in the server configuration.  To disable the
              120 second default, set --ping-restart 0 on the client.

              See the signals section below for more information on SIGUSR1.

              Note that the behavior of SIGUSR1 can be modified by the
              --persist-tun, --persist-key, --persist-local-ip, and
              --persist-remote-ip options.

              Also note that --ping-exit and --ping-restart are mutually
              exclusive and cannot be used together.

       --keepalive interval timeout
              A helper directive designed to simplify the expression of --ping
              and --ping-restart.

              This option can be used on both client and server side, but it
              is enough to add this on the server side as it will push
              appropriate --ping and --ping-restart options to the client.  If
              used on both server and client, the values pushed from server
              will override the client local values.

              The timeout argument will be twice as long on the server side.
              This ensures that a timeout is detected on client side before
              the server side drops the connection.

              For example, --keepalive 10 60 expands as follows:

                   if mode server:
                     ping 10                    # Argument: interval
                     ping-restart 120           # Argument: timeout*2
                     push "ping 10"             # Argument: interval
                     push "ping-restart 60"     # Argument: timeout
                   else
                     ping 10                    # Argument: interval
                     ping-restart 60            # Argument: timeout

       --ping-timer-rem
              Run the --ping-exit / --ping-restart timer only if we have a
              remote address.  Use this option if you are starting the daemon
              in listen mode (i.e. without an explicit --remote peer), and you
              don't want to start clocking timeouts until a remote peer
              connects.

       --persist-tun
              Don't close and reopen TUN/TAP device or run up/down scripts
              across SIGUSR1 or --ping-restart restarts.

              SIGUSR1 is a restart signal similar to SIGHUP, but which offers
              finer-grained control over reset options.

       --persist-key
              Don't re-read key files across SIGUSR1 or --ping-restart.

              This option can be combined with --user nobody to allow restarts
              triggered by the SIGUSR1 signal.  Normally if you drop root
              privileges in OpenVPN, the daemon cannot be restarted since it
              will now be unable to re-read protected key files.

              This option solves the problem by persisting keys across SIGUSR1
              resets, so they don't need to be re-read.

       --persist-local-ip
              Preserve initially resolved local IP address and port number
              across SIGUSR1 or --ping-restart restarts.

       --persist-remote-ip
              Preserve most recently authenticated remote IP address and port
              number across SIGUSR1 or --ping-restart restarts.

       --mlock
              Disable paging by calling the POSIX mlockall function.  Requires
              that OpenVPN be initially run as root (though OpenVPN can
              subsequently downgrade its UID using the --user option).

              Using this option ensures that key material and tunnel data are
              never written to disk due to virtual memory paging operations
              which occur under most modern operating systems.  It ensures
              that even if an attacker was able to crack the box running
              OpenVPN, he would not be able to scan the system swap file to
              recover previously used ephemeral keys, which are used for a
              period of time governed by the --reneg options (see below), then
              are discarded.

              The downside of using --mlock is that it will reduce the amount
              of physical memory available to other applications.

       --up cmd
              Run command cmd after successful TUN/TAP device open (pre --user
              UID change).

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              The up command is useful for specifying route commands which
              route IP traffic destined for private subnets which exist at the
              other end of the VPN connection into the tunnel.

              For --dev tun execute as:

              cmd tun_dev tun_mtu link_mtu ifconfig_local_ip
              ifconfig_remote_ip [ init | restart ]

              For --dev tap execute as:

              cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask
              [ init | restart ]

              See the "Environmental Variables" section below for additional
              parameters passed as environmental variables.

              Note that if cmd includes arguments, all OpenVPN-generated
              arguments will be appended to them to build an argument list
              with which the executable will be called.

              Typically, cmd will run a script to add routes to the tunnel.

              Normally the up script is called after the TUN/TAP device is
              opened.  In this context, the last command line parameter passed
              to the script will be init.  If the --up-restart option is also
              used, the up script will be called for restarts as well.  A
              restart is considered to be a partial reinitialization of
              OpenVPN where the TUN/TAP instance is preserved (the
              --persist-tun option will enable such preservation).  A restart
              can be generated by a SIGUSR1 signal, a --ping-restart timeout,
              or a connection reset when the TCP protocol is enabled with the
              --proto option.  If a restart occurs, and --up-restart has been
              specified, the up script will be called with restart as the last
              parameter.

              NOTE: on restart, OpenVPN will not pass the full set of
              environment variables to the script.  Namely, everything related
              to routing and gateways will not be passed, as nothing needs to
              be done anyway - all the routing setup is already in place.
              Additionally, the up-restart script will run with the downgraded
              UID/GID settings (if configured).

              The following standalone example shows how the --up script can
              be called in both an initialization and restart context.  (NOTE:
              for security reasons, don't run the following example unless UDP
              port 9999 is blocked by your firewall.  Also, the example will
              run indefinitely, so you should abort with control-c).

              openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up
              'echo up' --down 'echo down' --persist-tun --up-restart

              Note that OpenVPN also provides the --ifconfig option to
              automatically ifconfig the TUN device, eliminating the need to
              define an --up script, unless you also want to configure routes
              in the --up script.

              If --ifconfig is also specified, OpenVPN will pass the ifconfig
              local and remote endpoints on the command line to the --up
              script so that they can be used to configure routes such as:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw $5

       --up-delay
              Delay TUN/TAP open and possible --up script execution until
              after TCP/UDP connection establishment with peer.

              In --proto udp mode, this option normally requires the use of
              --ping to allow connection initiation to be sensed in the
              absence of tunnel data, since UDP is a "connectionless"
              protocol.

              On Windows, this option will delay the TAP-Win32 media state
              transitioning to "connected" until connection establishment,
              i.e. the receipt of the first authenticated packet from the
              peer.

       --down cmd
              Run command cmd after TUN/TAP device close (post --user UID
              change and/or --chroot ).  cmd consists of a path to script (or
              executable program), optionally followed by arguments. The path
              and arguments may be single- or double-quoted and/or escaped
              using a backslash, and should be separated by one or more
              spaces.

              Called with the same parameters and environmental variables as
              the --up option above.

              Note that if you reduce privileges by using --user and/or
              --group, your --down script will also run at reduced privilege.

       --down-pre
              Call --down cmd/script before, rather than after, TUN/TAP close.

       --up-restart
              Enable the --up and --down scripts to be called for restarts as
              well as initial program start.  This option is described more
              fully above in the --up option documentation.

       --setenv name value
              Set a custom environmental variable name=value to pass to
              script.

       --setenv FORWARD_COMPATIBLE 1
              Relax config file syntax checking so that unknown directives
              will trigger a warning but not a fatal error, on the assumption
              that a given unknown directive might be valid in future OpenVPN
              versions.

              This option should be used with caution, as there are good
              security reasons for having OpenVPN fail if it detects problems
              in a config file.  Having said that, there are valid reasons for
              wanting new software features to gracefully degrade when
              encountered by older software versions.

              It is also possible to tag a single directive so as not to
              trigger a fatal error if the directive isn't recognized.  To do
              this, prepend the following before the directive: setenv opt

              Versions prior to OpenVPN 2.3.3 will always ignore options set
              with the setenv opt directive.

              See also --ignore-unknown-option

       --setenv-safe name value
              Set a custom environmental variable OPENVPN_name=value to pass
              to script.

              This directive is designed to be pushed by the server to
              clients, and the prepending of "OPENVPN_" to the environmental
              variable is a safety precaution to prevent a LD_PRELOAD style
              attack from a malicious or compromised server.

       --ignore-unknown-option opt1 opt2 opt3 ... optN
              When one of options opt1 ... optN is encountered in the
              configuration file the configuration file parsing does not fail
              if this OpenVPN version does not support the option. Multiple
              --ignore-unknown-option options can be given to support a larger
              number of options to ignore.

              This option should be used with caution, as there are good
              security reasons for having OpenVPN fail if it detects problems
              in a config file. Having said that, there are valid reasons for
              wanting new software features to gracefully degrade when
              encountered by older software versions.

              --ignore-unknown-option is available since OpenVPN 2.3.3.

       --script-security level
              This directive offers policy-level control over OpenVPN's usage
              of external programs and scripts.  Lower level values are more
              restrictive, higher values are more permissive.  Settings for
              level:

              0 -- Strictly no calling of external programs.
              1 -- (Default) Only call built-in executables such as ifconfig,
              ip, route, or netsh.
              2 -- Allow calling of built-in executables and user-defined
              scripts.
              3 -- Allow passwords to be passed to scripts via environmental
              variables (potentially unsafe).

              OpenVPN releases before v2.3 also supported a method flag which
              indicated how OpenVPN should call external commands and scripts.
              This could be either execve or system.  As of OpenVPN 2.3, this
              flag is no longer accepted.  In most *nix environments the
              execve() approach has been used without any issues.

              Some directives such as --up allow options to be passed to the
              external script. In these cases make sure the script name does
              not contain any spaces or the configuration parser will choke
              because it can't determine where the script name ends and script
              options start.

              To run scripts in Windows in earlier OpenVPN versions you needed
              to either add a full path to the script interpreter which can
              parse the script or use the system flag to run these scripts.
              As of OpenVPN 2.3 it is now a strict requirement to have full
              path to the script interpreter when running non-executables
              files.  This is not needed for executable files, such as .exe,
              .com, .bat or .cmd files.  For example, if you have a Visual
              Basic script, you must use this syntax now:

                  --up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'

              Please note the single quote marks and the escaping of the
              backslashes (\) and the space character.

              The reason the support for the system flag was removed is due to
              the security implications with shell expansions when executing
              scripts via the system() call.

       --disable-occ
              Don't output a warning message if option inconsistencies are
              detected between peers.  An example of an option inconsistency
              would be where one peer uses --dev tun while the other peer uses
              --dev tap.

              Use of this option is discouraged, but is provided as a
              temporary fix in situations where a recent version of OpenVPN
              must connect to an old version.

       --user user
              Change the user ID of the OpenVPN process to user after
              initialization, dropping privileges in the process.  This option
              is useful to protect the system in the event that some hostile
              party was able to gain control of an OpenVPN session.  Though
              OpenVPN's security features make this unlikely, it is provided
              as a second line of defense.

              By setting user to nobody or somebody similarly unprivileged,
              the hostile party would be limited in what damage they could
              cause.  Of course once you take away privileges, you cannot
              return them to an OpenVPN session.  This means, for example,
              that if you want to reset an OpenVPN daemon with a SIGUSR1
              signal (for example in response to a DHCP reset), you should
              make use of one or more of the --persist options to ensure that
              OpenVPN doesn't need to execute any privileged operations in
              order to restart (such as re-reading key files or running
              ifconfig on the TUN device).

       --group group
              Similar to the --user option, this option changes the group ID
              of the OpenVPN process to group after initialization.

       --cd dir
              Change directory to dir prior to reading any files such as
              configuration files, key files, scripts, etc.  dir should be an
              absolute path, with a leading "/", and without any references to
              the current directory such as "." or "..".

              This option is useful when you are running OpenVPN in --daemon
              mode, and you want to consolidate all of your OpenVPN control
              files in one location.

       --chroot dir
              Chroot to dir after initialization.  --chroot essentially
              redefines dir as being the top level directory tree (/).
              OpenVPN will therefore be unable to access any files outside
              this tree.  This can be desirable from a security standpoint.

              Since the chroot operation is delayed until after
              initialization, most OpenVPN options that reference files will
              operate in a pre-chroot context.

              In many cases, the dir parameter can point to an empty
              directory, however complications can result when scripts or
              restarts are executed after the chroot operation.

              Note: The SSL library will probably need /dev/urandom to be
              available inside the chroot directory dir.  This is because SSL
              libraries occasionally need to collect fresh random.  Newer
              linux kernels and some BSDs implement a getrandom() or
              getentropy() syscall that removes the need for /dev/urandom to
              be available.

       --setcon context
              Apply SELinux context after initialization. This essentially
              provides the ability to restrict OpenVPN's rights to only
              network I/O operations, thanks to SELinux. This goes further
              than --user and --chroot in that those two, while being great
              security features, unfortunately do not protect against
              privilege escalation by exploitation of a vulnerable system
              call. You can of course combine all three, but please note that
              since setcon requires access to /proc you will have to provide
              it inside the chroot directory (e.g. with mount --bind).

              Since the setcon operation is delayed until after
              initialization, OpenVPN can be restricted to just
              network-related system calls, whereas by applying the context
              before startup (such as the OpenVPN one provided in the SELinux
              Reference Policies) you will have to allow many things required
              only during initialization.

              Like with chroot, complications can result when scripts or
              restarts are executed after the setcon operation, which is why
              you should really consider using the --persist-key and
              --persist-tun options.

       --daemon [progname]
              Become a daemon after all initialization functions are
              completed.  This option will cause all message and error output
              to be sent to the syslog file (such as /var/log/messages),
              except for the output of scripts and ifconfig commands, which
              will go to /dev/null unless otherwise redirected.  The syslog
              redirection occurs immediately at the point that --daemon is
              parsed on the command line even though the daemonization point
              occurs later.  If one of the --log options is present, it will
              supersede syslog redirection.

              The optional progname parameter will cause OpenVPN to report its
              program name to the system logger as progname.  This can be
              useful in linking OpenVPN messages in the syslog file with
              specific tunnels.  When unspecified, progname defaults to
              "openvpn".

              When OpenVPN is run with the --daemon option, it will try to
              delay daemonization until the majority of initialization
              functions which are capable of generating fatal errors are
              complete.  This means that initialization scripts can test the
              return status of the openvpn command for a fairly reliable
              indication of whether the command has correctly initialized and
              entered the packet forwarding event loop.

              In OpenVPN, the vast majority of errors which occur after
              initialization are non-fatal.

              Note: as soon as OpenVPN has daemonized, it can not ask for
              usernames, passwords, or key pass phrases anymore.  This has
              certain consequences, namely that using a password-protected
              private key will fail unless the --askpass option is used to
              tell OpenVPN to ask for the pass phrase (this requirement is new
              in v2.3.7, and is a consequence of calling daemon() before
              initializing the crypto layer).

              Further, using --daemon together with --auth-user-pass (entered
              on console) and --auth-nocache will fail as soon as key
              renegotiation (and reauthentication) occurs.

       --syslog [progname]
              Direct log output to system logger, but do not become a daemon.
              See --daemon directive above for description of progname
              parameter.

       --errors-to-stderr
              Output errors to stderr instead of stdout unless log output is
              redirected by one of the --log options.

       --passtos
              Set the TOS field of the tunnel packet to what the payload's TOS
              is.

       --inetd [wait|nowait] [progname]
              Use this option when OpenVPN is being run from the inetd or
              xinetd(8) server.

              The wait/nowait option must match what is specified in the
              inetd/xinetd config file.  The nowait mode can only be used with
              --proto tcp-server.  The default is wait.  The nowait mode can
              be used to instantiate the OpenVPN daemon as a classic TCP
              server, where client connection requests are serviced on a
              single port number.  For additional information on this kind of
              configuration, see the OpenVPN FAQ:
              http://openvpn.net/faq.html#oneport

              This option precludes the use of --daemon, --local, or --remote.
              Note that this option causes message and error output to be
              handled in the same way as the --daemon option.  The optional
              progname parameter is also handled exactly as in --daemon.

              Also note that in wait mode, each OpenVPN tunnel requires a
              separate TCP/UDP port and a separate inetd or xinetd entry.  See
              the OpenVPN 1.x HOWTO for an example on using OpenVPN with
              xinetd: http://openvpn.net/1xhowto.html

       --log file
              Output logging messages to file, including output to
              stdout/stderr which is generated by called scripts.  If file
              already exists it will be truncated.  This option takes effect
              immediately when it is parsed in the command line and will
              supersede syslog output if --daemon or --inetd is also
              specified.  This option is persistent over the entire course of
              an OpenVPN instantiation and will not be reset by SIGHUP,
              SIGUSR1, or --ping-restart.

              Note that on Windows, when OpenVPN is started as a service,
              logging occurs by default without the need to specify this
              option.

       --log-append file
              Append logging messages to file.  If file does not exist, it
              will be created.  This option behaves exactly like --log except
              that it appends to rather than truncating the log file.

       --suppress-timestamps
              Avoid writing timestamps to log messages, even when they
              otherwise would be prepended. In particular, this applies to log
              messages sent to stdout.

       --machine-readable-output
              Always write timestamps and message flags to log messages, even
              when they otherwise would not be prefixed. In particular, this
              applies to log messages sent to stdout.

       --writepid file
              Write OpenVPN's main process ID to file.

       --nice n
              Change process priority after initialization ( n greater than 0
              is lower priority, n less than zero is higher priority).

       --fast-io
              (Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a
              call to poll/epoll/select prior to the write operation.  The
              purpose of such a call would normally be to block until the
              device or socket is ready to accept the write.  Such blocking is
              unnecessary on some platforms which don't support write blocking
              on UDP sockets or TUN/TAP devices.  In such cases, one can
              optimize the event loop by avoiding the poll/epoll/select call,
              improving CPU efficiency by 5% to 10%.

              This option can only be used on non-Windows systems, when
              --proto udp is specified, and when --shaper is NOT specified.

       --multihome
              Configure a multi-homed UDP server.  This option needs to be
              used when a server has more than one IP address (e.g. multiple
              interfaces, or secondary IP addresses), and is not using --local
              to force binding to one specific address only.  This option will
              add some extra lookups to the packet path to ensure that the UDP
              reply packets are always sent from the address that the client
              is talking to. This is not supported on all platforms, and it
              adds more processing, so it's not enabled by default.

              Note: this option is only relevant for UDP servers.

              Note 2: if you do an IPv6+IPv4 dual-stack bind on a Linux
              machine with multiple IPv4 address, connections to IPv4
              addresses will not work right on kernels before 3.15, due to
              missing kernel support for the IPv4-mapped case (some
              distributions have ported this to earlier kernel versions,
              though).

       --echo [parms...]
              Echo parms to log output.

              Designed to be used to send messages to a controlling
              application which is receiving the OpenVPN log output.

       --remap-usr1 signal
              Control whether internally or externally generated SIGUSR1
              signals are remapped to SIGHUP (restart without persisting
              state) or SIGTERM (exit).

              signal can be set to "SIGHUP" or "SIGTERM".  By default, no
              remapping occurs.

       --verb n
              Set output verbosity to n (default=1).  Each level shows all
              info from the previous levels.  Level 3 is recommended if you
              want a good summary of what's happening without being swamped by
              output.

              0 -- No output except fatal errors.
              1 to 4 -- Normal usage range.
              5 -- Output R and W characters to the console for each packet
              read and write, uppercase is used for TCP/UDP packets and
              lowercase is used for TUN/TAP packets.
              6 to 11 -- Debug info range (see errlevel.h for additional
              information on debug levels).

       --status file [n]
              Write operational status to file every n seconds.

              Status can also be written to the syslog by sending a SIGUSR2
              signal.

              With multi-client capability enabled on a server, the status
              file includes a list of clients and a routing table. The output
              format can be controlled by the --status-version option in that
              case.

              For clients or instances running in point-to-point mode, it will
              contain the traffic statistics.

       --status-version [n]
              Set the status file format version number to n.

              This only affects the status file on servers with multi-client
              capability enabled.

              1 -- traditional format (default). The client list contains the
              following fields comma-separated: Common Name, Real Address,
              Bytes Received, Bytes Sent, Connected Since.
              2 -- a more reliable format for external processing. Compared to
              version 1, the client list contains some additional fields:
              Virtual Address, Virtual IPv6 Address, Username, Client ID, Peer
              ID, Data Channel Cipher.  Future versions may extend the number
              of fields.
              3 -- identical to 2, but fields are tab-separated.


       --mute n
              Log at most n consecutive messages in the same category.  This
              is useful to limit repetitive logging of similar message types.

       --compress [algorithm]
              Enable a compression algorithm.

              The algorithm parameter may be "lzo", "lz4", or empty.  LZO and
              LZ4 are different compression algorithms, with LZ4 generally
              offering the best performance with least CPU usage.  For
              backwards compatibility with OpenVPN versions before v2.4, use
              "lzo" (which is identical to the older option "--comp-lzo yes").

              If the algorithm parameter is empty, compression will be turned
              off, but the packet framing for compression will still be
              enabled, allowing a different setting to be pushed later.

              Security Considerations

              Compression and encryption is a tricky combination.  If an
              attacker knows or is able to control (parts of) the plaintext of
              packets that contain secrets, the attacker might be able to
              extract the secret if compression is enabled.  See e.g. the
              CRIME and BREACH attacks on TLS which also leverage compression
              to break encryption.  If you are not entirely sure that the
              above does not apply to your traffic, you are advised to *not*
              enable compression.


       --comp-lzo [mode]
              DEPRECATED This option will be removed in a future OpenVPN
              release.  Use the newer --compress instead.

              Use LZO compression -- may add up to 1 byte per packet for
              incompressible data.  mode may be "yes", "no", or "adaptive"
              (default).

              In a server mode setup, it is possible to selectively turn
              compression on or off for individual clients.

              First, make sure the client-side config file enables selective
              compression by having at least one --comp-lzo directive, such as
              --comp-lzo no.  This will turn off compression by default, but
              allow a future directive push from the server to dynamically
              change the on/off/adaptive setting.

              Next in a --client-config-dir file, specify the compression
              setting for the client, for example:

                  comp-lzo yes
                  push "comp-lzo yes"

              The first line sets the comp-lzo setting for the server side of
              the link, the second sets the client side.

       --comp-noadapt
              When used in conjunction with --comp-lzo, this option will
              disable OpenVPN's adaptive compression algorithm.  Normally,
              adaptive compression is enabled with --comp-lzo.

              Adaptive compression tries to optimize the case where you have
              compression enabled, but you are sending predominantly
              incompressible (or pre-compressed) packets over the tunnel, such
              as an FTP or rsync transfer of a large, compressed file.  With
              adaptive compression, OpenVPN will periodically sample the
              compression process to measure its efficiency.  If the data
              being sent over the tunnel is already compressed, the
              compression efficiency will be very low, triggering openvpn to
              disable compression for a period of time until the next
              re-sample test.

       --management socket-name unix [pw-file]      (recommended)
       --management IP port [pw-file]
              Enable a management server on a socket-name Unix socket on those
              platforms supporting it, or on a designated TCP port.

              pw-file , if specified, is a password file where the password
              must be on first line.  Instead of a filename it can use the
              keyword stdin which will prompt the user for a password to use
              when OpenVPN is starting.

              For unix sockets, the  default  behaviour  is to create a unix
              domain socket that may be connected to by any process.  Use the
              --management-client-user and --management-client-group
              directives to restrict access.

              The management interface provides a special mode where the TCP
              management link can operate over the tunnel itself.  To enable
              this mode, set IP to tunnel.  Tunnel mode will cause the
              management interface to listen for a TCP connection on the local
              VPN address of the TUN/TAP interface.

              BEWARE of enabling the management interface over TCP.  In  these
              cases you should ALWAYS make use of pw-file to password protect
              the management interface.  Any user who can connect to this TCP
              IP:port will be able to manage and control (and interfere with)
              the OpenVPN process.  It is also strongly recommended to set IP
              to 127.0.0.1 (localhost) to restrict accessibility of the
              management server to local clients.

              While the management port is designed for  programmatic control
              of OpenVPN by other applications, it is possible to telnet to
              the port, using a telnet client in "raw" mode.  Once  connected,
              type "help" for a list of commands.

              For detailed documentation on the management interface, see the
              management-notes.txt file in the management folder of the
              OpenVPN source distribution.


       --management-client
              Management interface will connect as a TCP/unix domain client to
              IP:port specified by --management rather than listen as a TCP
              server or on a unix domain socket.

              If the client connection fails to connect or is disconnected, a
              SIGTERM signal will be generated causing OpenVPN to quit.

       --management-query-passwords
              Query management channel for private key password and
              --auth-user-pass username/password.  Only query the management
              channel for inputs which ordinarily would have been queried from
              the console.

       --management-query-proxy
              Query management channel for proxy server information for a
              specific --remote (client-only).

       --management-query-remote
              Allow management interface to override --remote directives
              (client-only).

       --management-external-key
              Allows usage for external private key file instead of --key
              option (client-only).

       --management-external-cert certificate-hint
              Allows usage for external certificate instead of --cert option
              (client-only).  certificate-hint is an arbitrary string which is
              passed to a management interface client as an argument of
              NEED-CERTIFICATE notification.  Requires
              --management-external-key.

       --management-forget-disconnect
              Make OpenVPN forget passwords when management session
              disconnects.

              This directive does not affect the --http-proxy
              username/password.  It is always cached.

       --management-hold
              Start OpenVPN in a hibernating state, until a client of the
              management interface explicitly starts it with the hold release
              command.

       --management-signal
              Send SIGUSR1 signal to OpenVPN if management session
              disconnects.  This is useful when you wish to disconnect an
              OpenVPN session on user logoff. For --management-client this
              option is not needed since a disconnect will always generate a
              SIGTERM.

       --management-log-cache n
              Cache the most recent n lines of log file history for usage by
              the management channel.

       --management-up-down
              Report tunnel up/down events to management interface.

       --management-client-auth
              Gives management interface client the responsibility to
              authenticate clients after their client certificate has been
              verified.  See management-notes.txt in OpenVPN distribution for
              detailed notes.

       --management-client-pf
              Management interface clients must specify a packet filter file
              for each connecting client.  See management-notes.txt in OpenVPN
              distribution for detailed notes.

       --management-client-user u
              When the management interface is listening on a unix domain
              socket, only allow connections from user u.

       --management-client-group g
              When the management interface is listening on a unix domain
              socket, only allow connections from group g.

       --plugin module-pathname [init-string]
              Load plug-in module from the file module-pathname, passing
              init-string as an argument to the module initialization
              function.  Multiple plugin modules may be loaded into one
              OpenVPN process.

              The module-pathname argument can be just a filename or a
              filename with a relative or absolute path.  The format of the
              filename and path defines if the plug-in will be loaded from a
              default plug-in directory or outside this directory.

                  --plugin path        Effective directory used
                  ====================================================
                   myplug.so            DEFAULT_DIR/myplug.so
                   subdir/myplug.so     DEFAULT_DIR/subdir/myplug.so
                   ./subdir/myplug.so   CWD/subdir/myplug.so
                   /usr/lib/my/plug.so  /usr/lib/my/plug.so

              DEFAULT_DIR is replaced by the default plug-in directory, which
              is configured at the build time of OpenVPN.  CWD is the current
              directory where OpenVPN was started or the directory OpenVPN
              have switched into via the --cd option before the --plugin
              option.

              For more information and examples on how to build OpenVPN
              plug-in modules, see the README file in the plugin folder of the
              OpenVPN source distribution.

              If you are using an RPM install of OpenVPN, see
              /usr/share/openvpn/plugin.  The documentation is in doc and the
              actual plugin modules are in lib.

              Multiple plugin modules can be cascaded, and modules can be used
              in tandem with scripts.  The modules will be called by OpenVPN
              in the order that they are declared in the config file.  If both
              a plugin and script are configured for the same callback, the
              script will be called last.  If the return code of the
              module/script controls an authentication function (such as
              tls-verify, auth-user-pass-verify, or client-connect), then
              every module and script must return success (0) in order for the
              connection to be authenticated.

       --keying-material-exporter label len
              Save Exported Keying Material [RFC5705] of len bytes (must be
              between 16 and 4095 bytes) using label in environment
              (exported_keying_material) for use by plugins in
              OPENVPN_PLUGIN_TLS_FINAL callback.

              Note that exporter labels have the potential to collide with
              existing PRF labels. In order to prevent this, labels MUST begin
              with "EXPORTER".


   Server Mode
       Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is
       supported, and can be enabled with the --mode server option.  In server
       mode, OpenVPN will listen on a single port for incoming client
       connections.  All client connections will be routed through a single
       tun or tap interface.  This mode is designed for scalability and should
       be able to support hundreds or even thousands of clients on
       sufficiently fast hardware.  SSL/TLS authentication must be used in
       this mode.

       --server network netmask ['nopool']
              A helper directive designed to simplify the configuration of
              OpenVPN's server mode.  This directive will set up an OpenVPN
              server which will allocate addresses to clients out of the given
              network/netmask.  The server itself will take the ".1" address
              of the given network for use as the server-side endpoint of the
              local TUN/TAP interface.

              For example, --server 10.8.0.0 255.255.255.0 expands as follows:

                   mode server
                   tls-server
                   push "topology [topology]"

                   if dev tun AND (topology == net30 OR topology == p2p):
                     ifconfig 10.8.0.1 10.8.0.2
                     if !nopool:
                       ifconfig-pool 10.8.0.4 10.8.0.251
                     route 10.8.0.0 255.255.255.0
                     if client-to-client:
                       push "route 10.8.0.0 255.255.255.0"
                     else if topology == net30:
                       push "route 10.8.0.1"

                   if dev tap OR (dev tun AND topology == subnet):
                     ifconfig 10.8.0.1 255.255.255.0
                     if !nopool:
                       ifconfig-pool 10.8.0.2 10.8.0.253 255.255.255.0
                     push "route-gateway 10.8.0.1"
                     if route-gateway unset:
                       route-gateway 10.8.0.2


              Don't use --server if you are ethernet bridging.  Use
              --server-bridge instead.

       --server-bridge gateway netmask pool-start-IP pool-end-IP

       --server-bridge ['nogw']

              A helper directive similar to --server which is designed to
              simplify the configuration of OpenVPN's server mode in ethernet
              bridging configurations.

              If --server-bridge is used without any parameters, it will
              enable a DHCP-proxy mode, where connecting OpenVPN clients will
              receive an IP address for their TAP adapter from the DHCP server
              running on the OpenVPN server-side LAN.  Note that only clients
              that support the binding of a DHCP client with the TAP adapter
              (such as Windows) can support this mode.  The optional nogw flag
              (advanced) indicates that gateway information should not be
              pushed to the client.

              To configure ethernet bridging, you must first use your OS's
              bridging capability to bridge the TAP interface with the
              ethernet NIC interface.  For example, on Linux this is done with
              the brctl tool, and with Windows XP it is done in the Network
              Connections Panel by selecting the ethernet and TAP adapters and
              right-clicking on "Bridge Connections".

              Next you you must manually set the IP/netmask on the bridge
              interface.  The gateway and netmask parameters to
              --server-bridge can be set to either the IP/netmask of the
              bridge interface, or the IP/netmask of the default
              gateway/router on the bridged subnet.

              Finally, set aside a IP range in the bridged subnet, denoted by
              pool-start-IP and pool-end-IP, for OpenVPN to allocate to
              connecting clients.

              For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128
              10.8.0.254 expands as follows:

                  mode server
                  tls-server

                  ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
                  push "route-gateway 10.8.0.4"

              In another example, --server-bridge (without parameters) expands
              as follows:

                  mode server
                  tls-server

                  push "route-gateway dhcp"

              Or --server-bridge nogw expands as follows:

                  mode server
                  tls-server

       --push option
              Push a config file option back to the client for remote
              execution.  Note that option must be enclosed in double quotes
              ("").  The client must specify --pull in its config file.  The
              set of options which can be pushed is limited by both
              feasibility and security.  Some options such as those which
              would execute scripts are banned, since they would effectively
              allow a compromised server to execute arbitrary code on the
              client.  Other options such as TLS or MTU parameters cannot be
              pushed because the client needs to know them before the
              connection to the server can be initiated.

              This is a partial list of options which can currently be pushed:
              --route, --route-gateway, --route-delay, --redirect-gateway,
              --ip-win32, --dhcp-option, --inactive, --ping, --ping-exit,
              --ping-restart, --setenv, --auth-token, --persist-key,
              --persist-tun, --echo, --comp-lzo, --socket-flags, --sndbuf,
              --rcvbuf

       --push-reset
              Don't inherit the global push list for a specific client
              instance.  Specify this option in a client-specific context such
              as with a --client-config-dir configuration file.  This option
              will ignore --push options at the global config file level.

       --push-remove opt
              selectively remove all --push options matching "opt" from the
              option list for a client.  "opt" is matched as a substring
              against the whole option string to-be-pushed to the client, so
              --push-remove route would remove all --push route ...  and
              --push route-ipv6 ...  statements, while --push-remove
              'route-ipv6 2001:' would only remove IPv6 routes for 2001:...
              networks.

              --push-remove can only be used in a client-specific context,
              like in a --client-config-dir file, or --client-connect script
              or plugin -- similar to --push-reset, just more selective.

              NOTE: to change an option, --push-remove can be used to first
              remove the old value, and then add a new --push option with the
              new value.

              NOTE2: due to implementation details, 'ifconfig' and 'ifconfig-
              ipv6' can only be removed with an exact match on the option
              ("push-remove ifconfig"), no substring matching and no matching
              on the IPv4/IPv6 address argument is possible.

       --push-peer-info
              Push additional information about the client to server.  The
              following data is always pushed to the server:

              IV_VER=<version> -- the client OpenVPN version

              IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the
              client OS platform

              IV_LZO_STUB=1 -- if client was built with LZO stub capability

              IV_LZ4=1 -- if the client supports LZ4 compressions.

              IV_PROTO=2 -- if the client supports peer-id floating mechanism

              IV_NCP=2 -- negotiable ciphers, client supports --cipher pushed
              by the server, a value of 2 or greater indicates client supports
              AES-GCM-128 and AES-GCM-256.

              IV_GUI_VER=<gui_id> <version> -- the UI version of a UI if one
              is running, for example "de.blinkt.openvpn 0.5.47" for the
              Android app.

              When --push-peer-info is enabled the additional information
              consists of the following data:

              IV_HWADDR=<mac address> -- the MAC address of clients default
              gateway

              IV_SSL=<version string> -- the ssl version used by the client,
              e.g. "OpenSSL 1.0.2f 28 Jan 2016".

              IV_PLAT_VER=x.y - the version of the operating system, e.g. 6.1
              for Windows 7.

              UV_<name>=<value> -- client environment variables whose names
              start with "UV_"

       --disable
              Disable a particular client (based on the common name) from
              connecting.  Don't use this option to disable a client due to
              key or password compromise.  Use a CRL (certificate revocation
              list) instead (see the --crl-verify option).

              This option must be associated with a specific client instance,
              which means that it must be specified either in a client
              instance config file using --client-config-dir or dynamically
              generated using a --client-connect script.

       --ifconfig-pool start-IP end-IP [netmask]
              Set aside a pool of subnets to be dynamically allocated to
              connecting clients, similar to a DHCP server.  For tun-style
              tunnels, each client will be given a /30 subnet (for
              interoperability with Windows clients).  For tap-style tunnels,
              individual addresses will be allocated, and the optional netmask
              parameter will also be pushed to clients.


       --ifconfig-pool-persist file [seconds]
              Persist/unpersist ifconfig-pool data to file, at seconds
              intervals (default=600), as well as on program startup and
              shutdown.

              The goal of this option is to provide a long-term association
              between clients (denoted by their common name) and the virtual
              IP address assigned to them from the ifconfig-pool.  Maintaining
              a long-term association is good for clients because it allows
              them to effectively use the --persist-tun option.

              file is a comma-delimited ASCII file, formatted as
              <Common-Name>,<IP-address>.

              If seconds = 0, file will be treated as read-only.  This is
              useful if you would like to treat file as a configuration file.

              Note that the entries in this file are treated by OpenVPN as
              suggestions only, based on past associations between a common
              name and IP address.  They do not guarantee that the given
              common name will always receive the given IP address.  If you
              want guaranteed assignment, use --ifconfig-push


       --ifconfig-pool-linear
              DEPRECATED This option will be removed in OpenVPN 2.5

              Modifies the --ifconfig-pool directive to allocate individual
              TUN interface addresses for clients rather than /30 subnets.
              NOTE:  This option is incompatible with Windows clients.

              This option is deprecated, and should be replaced with
              --topology p2p which is functionally equivalent.

       --ifconfig-push local remote-netmask [alias]
              Push virtual IP endpoints for client tunnel, overriding the
              --ifconfig-pool dynamic allocation.

              The parameters local and remote-netmask are set according to the
              --ifconfig directive which you want to execute on the client
              machine to configure the remote end of the tunnel.  Note that
              the parameters local and remote-netmask are from the perspective
              of the client, not the server.  They may be DNS names rather
              than IP addresses, in which case they will be resolved on the
              server at the time of client connection.

              The optional alias parameter may be used in cases where NAT
              causes the client view of its local endpoint to differ from the
              server view.  In this case local/remote-netmask will refer to
              the server view while alias/remote-netmask will refer to the
              client view.

              This option must be associated with a specific client instance,
              which means that it must be specified either in a client
              instance config file using --client-config-dir or dynamically
              generated using a --client-connect script.

              Remember also to include a --route directive in the main OpenVPN
              config file which encloses local, so that the kernel will know
              to route it to the server's TUN/TAP interface.

              OpenVPN's internal client IP address selection algorithm works
              as follows:

              1 -- Use --client-connect script generated file for static IP
              (first choice).
              2 -- Use --client-config-dir file for static IP (next choice).
              3 -- Use --ifconfig-pool allocation for dynamic IP (last
              choice).

       --iroute network [netmask]
              Generate an internal route to a specific client. The netmask
              parameter, if omitted, defaults to 255.255.255.255.

              This directive can be used to route a fixed subnet from the
              server to a particular client, regardless of where the client is
              connecting from.  Remember that you must also add the route to
              the system routing table as well (such as by using the --route
              directive).  The reason why two routes are needed is that the
              --route directive routes the packet from the kernel to OpenVPN.
              Once in OpenVPN, the --iroute directive routes to the specific
              client.

              This option must be specified either in a client instance config
              file using --client-config-dir or dynamically generated using a
              --client-connect script.

              The --iroute directive also has an important interaction with
              --push "route ...".  --iroute essentially defines a subnet which
              is owned by a particular client (we will call this client A).
              If you would like other clients to be able to reach A's subnet,
              you can use --push "route ..."  together with --client-to-client
              to effect this.  In order for all clients to see A's subnet,
              OpenVPN must push this route to all clients EXCEPT for A, since
              the subnet is already owned by A.  OpenVPN accomplishes this by
              not not pushing a route to a client if it matches one of the
              client's iroutes.

       --client-to-client
              Because the OpenVPN server mode handles multiple clients through
              a single tun or tap interface, it is effectively a router.  The
              --client-to-client flag tells OpenVPN to internally route
              client-to-client traffic rather than pushing all
              client-originating traffic to the TUN/TAP interface.

              When this option is used, each client will "see" the other
              clients which are currently connected.  Otherwise, each client
              will only see the server.  Don't use this option if you want to
              firewall tunnel traffic using custom, per-client rules.

       --duplicate-cn
              Allow multiple clients with the same common name to concurrently
              connect.  In the absence of this option, OpenVPN will disconnect
              a client instance upon connection of a new client having the
              same common name.

       --client-connect cmd
              Run command cmd on client connection.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              The command is passed the common name and IP address of the
              just-authenticated client as environmental variables (see
              environmental variable section below).  The command is also
              passed the pathname of a freshly created temporary file as the
              last argument (after any arguments specified in cmd ), to be
              used by the command to pass dynamically generated config file
              directives back to OpenVPN.

              If the script wants to generate a dynamic config file to be
              applied on the server when the client connects, it should write
              it to the file named by the last argument.

              See the --client-config-dir option below for options which can
              be legally used in a dynamically generated config file.

              Note that the return value of script is significant.  If script
              returns a non-zero error status, it will cause the client to be
              disconnected.

       --client-disconnect cmd
              Like --client-connect but called on client instance shutdown.
              Will not be called unless the --client-connect script and
              plugins (if defined) were previously called on this instance
              with successful (0) status returns.

              The exception to this rule is if the --client-disconnect command
              or plugins are cascaded, and at least one client-connect
              function succeeded, then ALL of the client-disconnect functions
              for scripts and plugins will be called on client instance object
              deletion, even in cases where some of the related client-connect
              functions returned an error status.

              The --client-disconnect command is passed the same pathname as
              the corresponding --client-connect command as its last argument.
              (after any arguments specified in cmd ).

       --client-config-dir dir
              Specify a directory dir for custom client config files.  After a
              connecting client has been authenticated, OpenVPN will look in
              this directory for a file having the same name as the client's
              X509 common name.  If a matching file exists, it will be opened
              and parsed for client-specific configuration options.  If no
              matching file is found, OpenVPN will instead try to open and
              parse a default file called "DEFAULT", which may be provided but
              is not required. Note that the configuration files must be
              readable by the OpenVPN process after it has dropped it's root
              privileges.

              This file can specify a fixed IP address for a given client
              using --ifconfig-push, as well as fixed subnets owned by the
              client using --iroute.

              One of the useful properties of this option is that it allows
              client configuration files to be conveniently created, edited,
              or removed while the server is live, without needing to restart
              the server.

              The following options are legal in a client-specific context:
              --push, --push-reset, --push-remove, --iroute, --ifconfig-push,
              and --config.

       --ccd-exclusive
              Require, as a condition of authentication, that a connecting
              client has a --client-config-dir file.

       --tmp-dir dir
              Specify a directory dir for temporary files.  This directory
              will be used by openvpn processes and script to communicate
              temporary data with openvpn main process. Note that the
              directory must be writable by the OpenVPN process after it has
              dropped it's root privileges.

              This directory will be used by in the following cases:

              * --client-connect scripts to dynamically generate
              client-specific configuration files.

              * OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return
              success/failure via auth_control_file when using deferred auth
              method

              * OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules
              via pf_file

       --hash-size r v
              Set the size of the real address hash table to r and the virtual
              address table to v.  By default, both tables are sized at 256
              buckets.

       --bcast-buffers n
              Allocate n buffers for broadcast datagrams (default=256).

       --tcp-queue-limit n
              Maximum number of output packets queued before TCP (default=64).

              When OpenVPN is tunneling data from a TUN/TAP device to a remote
              client over a TCP connection, it is possible that the TUN/TAP
              device might produce data at a faster rate than the TCP
              connection can support.  When the number of output packets
              queued before sending to the TCP socket reaches this limit for a
              given client connection, OpenVPN will start to drop outgoing
              packets directed at this client.

       --tcp-nodelay
              This macro sets the TCP_NODELAY socket flag on the server as
              well as pushes it to connecting clients.  The TCP_NODELAY flag
              disables the Nagle algorithm on TCP sockets causing packets to
              be transmitted immediately with low latency, rather than waiting
              a short period of time in order to aggregate several packets
              into a larger containing packet.  In VPN applications over TCP,
              TCP_NODELAY is generally a good latency optimization.

              The macro expands as follows:

                   if mode server:
                     socket-flags TCP_NODELAY
                     push "socket-flags TCP_NODELAY"

       --max-clients n
              Limit server to a maximum of n concurrent clients.

       --max-routes-per-client n
              Allow a maximum of n internal routes per client (default=256).
              This is designed to help contain DoS attacks where an
              authenticated client floods the server with packets appearing to
              come from many unique MAC addresses, forcing the server to
              deplete virtual memory as its internal routing table expands.
              This directive can be used in a --client-config-dir file or
              auto-generated by a --client-connect script to override the
              global value for a particular client.

              Note that this directive affects OpenVPN's internal routing
              table, not the kernel routing table.

       --stale-routes-check n [t]
              Remove routes haven't had activity for n seconds (i.e. the
              ageing time).

              This check is ran every t seconds (i.e. check interval).

              If t is not present it defaults to n

              This option helps to keep the dynamic routing table small.  See
              also --max-routes-per-client

       --connect-freq n sec
              Allow a maximum of n new connections per sec seconds from
              clients.  This is designed to contain DoS attacks which flood
              the server with connection requests using certificates which
              will ultimately fail to authenticate.

              This is an imperfect solution however, because in a real DoS
              scenario, legitimate connections might also be refused.

              For the best protection against DoS attacks in server mode, use
              --proto udp and either --tls-auth or --tls-crypt.

       --learn-address cmd
              Run command cmd to validate client virtual addresses or routes.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              Three arguments will be appended to any arguments in cmd as
              follows:

              [1] operation -- "add", "update", or "delete" based on whether
              or not the address is being added to, modified, or deleted from
              OpenVPN's internal routing table.
              [2] address -- The address being learned or unlearned.  This can
              be an IPv4 address such as "198.162.10.14", an IPv4 subnet such
              as "198.162.10.0/24", or an ethernet MAC address (when --dev tap
              is being used) such as "00:FF:01:02:03:04".
              [3] common name -- The common name on the certificate associated
              with the client linked to this address.  Only present for "add"
              or "update" operations, not "delete".

              On "add" or "update" methods, if the script returns a failure
              code (non-zero), OpenVPN will reject the address and will not
              modify its internal routing table.

              Normally, the cmd script will use the information provided above
              to set appropriate firewall entries on the VPN TUN/TAP
              interface.  Since OpenVPN provides the association between
              virtual IP or MAC address and the client's authenticated common
              name, it allows a user-defined script to configure firewall
              access policies with regard to the client's high-level common
              name, rather than the low level client virtual addresses.

       --auth-user-pass-verify cmd method
              Require the client to provide a username/password (possibly in
              addition to a client certificate) for authentication.

              OpenVPN will run command cmd to validate the username/password
              provided by the client.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              If method is set to "via-env", OpenVPN will call script with the
              environmental variables username and password set to the
              username/password strings provided by the client.  Be aware that
              this method is insecure on some platforms which make the
              environment of a process publicly visible to other unprivileged
              processes.

              If method is set to "via-file", OpenVPN will write the username
              and password to the first two lines of a temporary file.  The
              filename will be passed as an argument to script, and the file
              will be automatically deleted by OpenVPN after the script
              returns.  The location of the temporary file is controlled by
              the --tmp-dir option, and will default to the current directory
              if unspecified.  For security, consider setting --tmp-dir to a
              volatile storage medium such as /dev/shm (if available) to
              prevent the username/password file from touching the hard drive.

              The script should examine the username and password, returning a
              success exit code (0) if the client's authentication request is
              to be accepted, or a failure code (1) to reject the client.

              This directive is designed to enable a plugin-style interface
              for extending OpenVPN's authentication capabilities.

              To protect against a client passing a maliciously formed
              username or password string, the username string must consist
              only of these characters: alphanumeric, underbar ('_'), dash
              ('-'), dot ('.'), or at ('@').  The password string can consist
              of any printable characters except for CR or LF.  Any illegal
              characters in either the username or password string will be
              converted to underbar ('_').

              Care must be taken by any user-defined scripts to avoid creating
              a security vulnerability in the way that these strings are
              handled.  Never use these strings in such a way that they might
              be escaped or evaluated by a shell interpreter.

              For a sample script that performs PAM authentication, see
              sample-scripts/auth-pam.pl in the OpenVPN source distribution.

       --auth-gen-token [lifetime]
              After successful user/password authentication, the OpenVPN
              server will with this option generate a temporary authentication
              token and push that to client.  On the following renegotiations,
              the OpenVPN client will pass this token instead of the users
              password.  On the server side the server will do the token
              authentication internally and it will NOT do any additional
              authentications against configured external user/password
              authentication mechanisms.

              The lifetime argument defines how long the generated token is
              valid.  The lifetime is defined in seconds.  If lifetime is not
              set or it is set to 0, the token will never expire.

              This feature is useful for environments which is configured to
              use One Time Passwords (OTP) as part of the user/password
              authentications and that authentication mechanism does not
              implement any auth-token support.

       --opt-verify
              Clients that connect with options that are incompatible with
              those of the server will be disconnected.

              Options that will be compared for compatibility include
              dev-type, link-mtu, tun-mtu, proto, ifconfig, comp-lzo,
              fragment, keydir, cipher, auth, keysize, secret, no-replay,
              no-iv, tls-auth, key-method, tls-server, and tls-client.

              This option requires that --disable-occ NOT be used.

       --auth-user-pass-optional
              Allow connections by clients that do not specify a
              username/password.  Normally, when --auth-user-pass-verify or
              --management-client-auth is specified (or an authentication
              plugin module), the OpenVPN server daemon will require
              connecting clients to specify a username and password.  This
              option makes the submission of a username/password by clients
              optional, passing the responsibility to the user-defined
              authentication module/script to accept or deny the client based
              on other factors (such as the setting of X509 certificate
              fields).  When this option is used, and a connecting client does
              not submit a username/password, the user-defined authentication
              module/script will see the username and password as being set to
              empty strings ("").  The authentication module/script MUST have
              logic to detect this condition and respond accordingly.

       --client-cert-not-required
              DEPRECATED This option will be removed in OpenVPN 2.5

              Don't require client certificate, client will authenticate using
              username/password only.  Be aware that using this directive is
              less secure than requiring certificates from all clients.

              Please note: This is replaced by --verify-client-cert which
              allows for more flexibility. The option --verify-client-cert
              none is functionally equivalent to --client-cert-not-required


       --verify-client-cert none|optional|require
              Specify whether the client is required to supply a valid
              certificate.

              Possible options are

              none : a client certificate is not required. the client need to
              authenticate using username/password only.  Be aware that using
              this directive is less secure than requiring certificates from
              all clients.

              If you use this directive, the entire responsibility of
              authentication will rest on your --auth-user-pass-verify script,
              so keep in mind that bugs in your script could potentially
              compromise the security of your VPN.

              --verify-client-cert none is functionally equivalent to
              --client-cert-not-required.

              optional : a client may present a certificate but it is not
              required to do so.  When using this directive, you should also
              use a --auth-user-pass-verify script to ensure that clients are
              authenticated using a certificate, a username and password, or
              possibly even both.

              Again, the entire responsibility of authentication will rest on
              your --auth-user-pass-verify script, so keep in mind that bugs
              in your script could potentially compromise the security of your
              VPN.

              require : this is the default option. A client is required to
              present a certificate, otherwise VPN access is refused.

              If you don't use this directive (or use --verify-client-cert
              require ) but you also specify an --auth-user-pass-verify
              script, then OpenVPN will perform double authentication.  The
              client certificate verification AND the --auth-user-pass-verify
              script will need to succeed in order for a client to be
              authenticated and accepted onto the VPN.

       --username-as-common-name
              For --auth-user-pass-verify authentication, use the
              authenticated username as the common name, rather than the
              common name from the client cert.

       --port-share host port [dir]
              When run in TCP server mode, share the OpenVPN port with another
              application, such as an HTTPS server.  If OpenVPN senses a
              connection to its port which is using a non-OpenVPN protocol, it
              will proxy the connection to the server at host:port.  Currently
              only designed to work with HTTP/HTTPS, though it would be
              theoretically possible to extend to other protocols such as ssh.

              dir specifies an optional directory where a temporary file with
              name N containing content C will be dynamically generated for
              each proxy connection, where N is the source IP:port of the
              client connection and C is the source IP:port of the connection
              to the proxy receiver.  This directory can be used as a
              dictionary by the proxy receiver to determine the origin of the
              connection.  Each generated file will be automatically deleted
              when the proxied connection is torn down.

              Not implemented on Windows.

   Client Mode
       Use client mode when connecting to an OpenVPN server which has
       --server, --server-bridge, or --mode server in it's configuration.

       --client
              A helper directive designed to simplify the configuration of
              OpenVPN's client mode.  This directive is equivalent to:

                   pull
                   tls-client

       --pull This option must be used on a client which is connecting to a
              multi-client server.  It indicates to OpenVPN that it should
              accept options pushed by the server, provided they are part of
              the legal set of pushable options (note that the --pull option
              is implied by --client ).

              In particular, --pull allows the server to push routes to the
              client, so you should not use --pull or --client in situations
              where you don't trust the server to have control over the
              client's routing table.

       --pull-filter accept|ignore|reject text
              Filter options received from the server if the option starts
              with text.  Runs on client. The action flag accept allows the
              option, ignore removes it and reject flags an error and triggers
              a SIGUSR1 restart.  The filters may be specified multiple times,
              and each filter is applied in the order it is specified. The
              filtering of each option stops as soon as a match is found.
              Unmatched options are accepted by default.

              Prefix comparison is used to match text against the received
              option so that

                  --pull-filter ignore "route"

              would remove all pushed options starting with route which would
              include, for example, route-gateway.  Enclose text in quotes to
              embed spaces.

                  --pull-filter accept "route 192.168.1."
                  --pull-filter ignore "route "

              would remove all routes that do not start with 192.168.1.

              This option may be used only on clients.  Note that reject may
              result in a repeated cycle of failure and reconnect, unless
              multiple remotes are specified and connection to the next remote
              succeeds. To silently ignore an option pushed by the server, use
              ignore.

       --auth-user-pass [up]
              Authenticate with server using username/password.  up is a file
              containing username/password on 2 lines. If the password line is
              missing, OpenVPN will prompt for one.

              If up is omitted, username/password will be prompted from the
              console.

              The server configuration must specify an --auth-user-pass-verify
              script to verify the username/password provided by the client.

       --auth-retry type
              Controls how OpenVPN responds to username/password verification
              errors such as the client-side response to an AUTH_FAILED
              message from the server or verification failure of the private
              key password.

              Normally used to prevent auth errors from being fatal on the
              client side, and to permit username/password requeries in case
              of error.

              An AUTH_FAILED message is generated by the server if the client
              fails --auth-user-pass authentication, or if the server-side
              --client-connect script returns an error status when the client
              tries to connect.

              type can be one of:

              none -- Client will exit with a fatal error (this is the
              default).
              nointeract -- Client will retry the connection without
              requerying for an --auth-user-pass username/password.  Use this
              option for unattended clients.
              interact -- Client will requery for an --auth-user-pass
              username/password and/or private key password before attempting
              a reconnection.

              Note that while this option cannot be pushed, it can be
              controlled from the management interface.

       --static-challenge t e
              Enable static challenge/response protocol using challenge text
              t, with echo flag given by e (0|1).

              The echo flag indicates whether or not the user's response to
              the challenge should be echoed.

              See management-notes.txt in the OpenVPN distribution for a
              description of the OpenVPN challenge/response protocol.

       --server-poll-timeout n, --connect-timeout n
              When connecting to a remote server do not wait for more than n
              seconds waiting for a response before trying the next server.
              The default value is 120s. This timeout includes proxy and TCP
              connect timeouts.

       --explicit-exit-notify [n]
              In UDP client mode or point-to-point mode, send server/peer an
              exit notification if tunnel is restarted or OpenVPN process is
              exited.  In client mode, on exit/restart, this option will tell
              the server to immediately close its client instance object
              rather than waiting for a timeout.  The n parameter (default=1)
              controls the maximum number of attempts that the client will try
              to resend the exit notification message.

              In UDP server mode, send RESTART control channel command to
              connected clients. The n parameter (default=1) controls client
              behavior. With n = 1 client will attempt to reconnect to the
              same server, with n = 2 client will advance to the next server.

              OpenVPN will not send any exit notifications unless this option
              is enabled.

       --allow-recursive-routing
              When this option is set, OpenVPN will not drop incoming tun
              packets with same destination as host.

   Data Channel Encryption Options:
       These options are meaningful for both Static & TLS-negotiated key modes
       (must be compatible between peers).

       --secret file [direction]
              Enable Static Key encryption mode (non-TLS).  Use pre-shared
              secret file which was generated with --genkey.

              The optional direction parameter enables the use of 4 distinct
              keys (HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt),
              so that each data flow direction has a different set of HMAC and
              cipher keys.  This has a number of desirable security properties
              including eliminating certain kinds of DoS and message replay
              attacks.

              When the direction parameter is omitted, 2 keys are used
              bidirectionally, one for HMAC and the other for
              encryption/decryption.

              The direction parameter should always be complementary on either
              side of the connection, i.e. one side should use "0" and the
              other should use "1", or both sides should omit it altogether.

              The direction parameter requires that file contains a 2048 bit
              key.  While pre-1.5 versions of OpenVPN generate 1024 bit key
              files, any version of OpenVPN which supports the direction
              parameter, will also support 2048 bit key file generation using
              the --genkey option.

              Static key encryption mode has certain advantages, the primary
              being ease of configuration.

              There are no certificates or certificate authorities or
              complicated negotiation handshakes and protocols.  The only
              requirement is that you have a pre-existing secure channel with
              your peer (such as ssh ) to initially copy the key.  This
              requirement, along with the fact that your key never changes
              unless you manually generate a new one, makes it somewhat less
              secure than TLS mode (see below).  If an attacker manages to
              steal your key, everything that was ever encrypted with it is
              compromised.  Contrast that to the perfect forward secrecy
              features of TLS mode (using Diffie Hellman key exchange), where
              even if an attacker was able to steal your private key, he would
              gain no information to help him decrypt past sessions.

              Another advantageous aspect of Static Key encryption mode is
              that it is a handshake-free protocol without any distinguishing
              signature or feature (such as a header or protocol handshake
              sequence) that would mark the ciphertext packets as being
              generated by OpenVPN.  Anyone eavesdropping on the wire would
              see nothing but random-looking data.

       --key-direction
              Alternative way of specifying the optional direction parameter
              for the --tls-auth and --secret options. Useful when using
              inline files (See section on inline files).

       --auth alg
              Authenticate data channel packets and (if enabled) tls-auth
              control channel packets with HMAC using message digest algorithm
              alg.  (The default is SHA1 ).  HMAC is a commonly used message
              authentication algorithm (MAC) that uses a data string, a secure
              hash algorithm, and a key, to produce a digital signature.

              The OpenVPN data channel protocol uses encrypt-then-mac (i.e.
              first encrypt a packet, then HMAC the resulting ciphertext),
              which prevents padding oracle attacks.

              If an AEAD cipher mode (e.g. GCM) is chosen, the specified
              --auth algorithm is ignored for the data channel, and the
              authentication method of the AEAD cipher is used instead.  Note
              that alg still specifies the digest used for tls-auth.

              In static-key encryption mode, the HMAC key is included in the
              key file generated by --genkey.  In TLS mode, the HMAC key is
              dynamically generated and shared between peers via the TLS
              control channel.  If OpenVPN receives a packet with a bad HMAC
              it will drop the packet.  HMAC usually adds 16 or 20 bytes per
              packet.  Set alg=none to disable authentication.

              For more information on HMAC see
              http://www.cs.ucsd.edu/users/mihir/papers/hmac.html

       --cipher alg
              Encrypt data channel packets with cipher algorithm alg.

              The default is BF-CBC, an abbreviation for Blowfish in Cipher
              Block Chaining mode.  When cipher negotiation (NCP) is allowed,
              OpenVPN 2.4 and newer on both client and server side will
              automatically upgrade to AES-256-GCM.  See --ncp-ciphers and
              --ncp-disable for more details on NCP.

              Using BF-CBC is no longer recommended, because of its 64-bit
              block size.  This small block size allows attacks based on
              collisions, as demonstrated by SWEET32.  See
              https://community.openvpn.net/openvpn/wiki/SWEET32 for details.
              Due to this, support for BF-CBC, DES, CAST5, IDEA and RC2
              ciphers will be removed in OpenVPN 2.6.

              To see other ciphers that are available with OpenVPN, use the
              --show-ciphers option.

              Set alg=none to disable encryption.


       --ncp-ciphers cipher_list
              Restrict the allowed ciphers to be negotiated to the ciphers in
              cipher_list.  cipher_list is a colon-separated list of ciphers,
              and defaults to "AES-256-GCM:AES-128-GCM".

              For servers, the first cipher from cipher_list will be pushed to
              clients that support cipher negotiation.

              Cipher negotiation is enabled in client-server mode only.  I.e.
              if --mode is set to 'server' (server-side, implied by setting
              --server ), or if --pull is specified (client-side, implied by
              setting --client).

              If both peers support and do not disable NCP, the negotiated
              cipher will override the cipher specified by --cipher.

              Additionally, to allow for more smooth transition, if NCP is
              enabled, OpenVPN will inherit the cipher of the peer if that
              cipher is different from the local --cipher setting, but the
              peer cipher is one of the ciphers specified in --ncp-ciphers.
              E.g. a non-NCP client (<=v2.3, or with --ncp-disabled set)
              connecting to a NCP server (v2.4+) with "--cipher BF-CBC" and
              "--ncp-ciphers AES-256-GCM:AES-256-CBC" set can either specify
              "--cipher BF-CBC" or "--cipher AES-256-CBC" and both will work.


       --ncp-disable
              Disable "negotiable crypto parameters".  This completely
              disables cipher negotiation.

       --keysize n
              DEPRECATED This option will be removed in OpenVPN 2.6.

              Size of cipher key in bits (optional).  If unspecified, defaults
              to cipher-specific default.  The --show-ciphers option (see
              below) shows all available OpenSSL ciphers, their default key
              sizes, and whether the key size can be changed.  Use care in
              changing a cipher's default key size.  Many ciphers have not
              been extensively cryptanalyzed with non-standard key lengths,
              and a larger key may offer no real guarantee of greater
              security, or may even reduce security.

       --prng alg [nsl]
              (Advanced) For PRNG (Pseudo-random number generator), use digest
              algorithm alg (default=sha1), and set nsl (default=16) to the
              size in bytes of the nonce secret length (between 16 and 64).

              Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes
              function instead for all of OpenVPN's pseudo-random number
              needs.

       --engine [engine-name]
              Enable OpenSSL hardware-based crypto engine functionality.

              If engine-name is specified, use a specific crypto engine.  Use
              the --show-engines standalone option to list the crypto engines
              which are supported by OpenSSL.

       --no-replay
              DEPRECATED This option will be removed in OpenVPN 2.5.

              (Advanced) Disable OpenVPN's protection against replay attacks.
              Don't use this option unless you are prepared to make a tradeoff
              of greater efficiency in exchange for less security.

              OpenVPN provides datagram replay protection by default.

              Replay protection is accomplished by tagging each outgoing
              datagram with an identifier that is guaranteed to be unique for
              the key being used.  The peer that receives the datagram will
              check for the uniqueness of the identifier.  If the identifier
              was already received in a previous datagram, OpenVPN will drop
              the packet.  Replay protection is important to defeat attacks
              such as a SYN flood attack, where the attacker listens in the
              wire, intercepts a TCP SYN packet (identifying it by the context
              in which it occurs in relation to other packets), then floods
              the receiving peer with copies of this packet.

              OpenVPN's replay protection is implemented in slightly different
              ways, depending on the key management mode you have selected.

              In Static Key mode or when using an CFB or OFB mode cipher,
              OpenVPN uses a 64 bit unique identifier that combines a time
              stamp with an incrementing sequence number.

              When using TLS mode for key exchange and a CBC cipher mode,
              OpenVPN uses only a 32 bit sequence number without a time stamp,
              since OpenVPN can guarantee the uniqueness of this value for
              each key.  As in IPSec, if the sequence number is close to
              wrapping back to zero, OpenVPN will trigger a new key exchange.

              To check for replays, OpenVPN uses the sliding window algorithm
              used by IPSec.

       --replay-window n [t]
              Use a replay protection sliding-window of size n and a time
              window of t seconds.

              By default n is 64 (the IPSec default) and t is 15 seconds.

              This option is only relevant in UDP mode, i.e.  when either
              --proto udp is specified, or no --proto option is specified.

              When OpenVPN tunnels IP packets over UDP, there is the
              possibility that packets might be dropped or delivered out of
              order.  Because OpenVPN, like IPSec, is emulating the physical
              network layer, it will accept an out-of-order packet sequence,
              and will deliver such packets in the same order they were
              received to the TCP/IP protocol stack, provided they satisfy
              several constraints.

              (a) The packet cannot be a replay (unless --no-replay is
              specified, which disables replay protection altogether).

              (b) If a packet arrives out of order, it will only be accepted
              if the difference between its sequence number and the highest
              sequence number received so far is less than n.

              (c) If a packet arrives out of order, it will only be accepted
              if it arrives no later than t seconds after any packet
              containing a higher sequence number.

              If you are using a network link with a large pipeline (meaning
              that the product of bandwidth and latency is high), you may want
              to use a larger value for n.  Satellite links in particular
              often require this.

              If you run OpenVPN at --verb 4, you will see the message
              "Replay-window backtrack occurred [x]" every time the maximum
              sequence number backtrack seen thus far increases.  This can be
              used to calibrate n.

              There is some controversy on the appropriate method of handling
              packet reordering at the security layer.

              Namely, to what extent should the security layer protect the
              encapsulated protocol from attacks which masquerade as the kinds
              of normal packet loss and reordering that occur over IP
              networks?

              The IPSec and OpenVPN approach is to allow packet reordering
              within a certain fixed sequence number window.

              OpenVPN adds to the IPSec model by limiting the window size in
              time as well as sequence space.

              OpenVPN also adds TCP transport as an option (not offered by
              IPSec) in which case OpenVPN can adopt a very strict attitude
              towards message deletion and reordering:  Don't allow it.  Since
              TCP guarantees reliability, any packet loss or reordering event
              can be assumed to be an attack.

              In this sense, it could be argued that TCP tunnel transport is
              preferred when tunneling non-IP or UDP application protocols
              which might be vulnerable to a message deletion or reordering
              attack which falls within the normal operational parameters of
              IP networks.

              So I would make the statement that one should never tunnel a
              non-IP protocol or UDP application protocol over UDP, if the
              protocol might be vulnerable to a message deletion or reordering
              attack that falls within the normal operating parameters of what
              is to be expected from the physical IP layer.  The problem is
              easily fixed by simply using TCP as the VPN transport layer.

       --mute-replay-warnings
              Silence the output of replay warnings, which are a common false
              alarm on WiFi networks.  This option preserves the security of
              the replay protection code without the verbosity associated with
              warnings about duplicate packets.

       --replay-persist file
              Persist replay-protection state across sessions using file to
              save and reload the state.

              This option will strengthen protection against replay attacks,
              especially when you are using OpenVPN in a dynamic context (such
              as with --inetd) when OpenVPN sessions are frequently started
              and stopped.

              This option will keep a disk copy of the current replay
              protection state (i.e. the most recent packet timestamp and
              sequence number received from the remote peer), so that if an
              OpenVPN session is stopped and restarted, it will reject any
              replays of packets which were already received by the prior
              session.

              This option only makes sense when replay protection is enabled
              (the default) and you are using either --secret (shared-secret
              key mode) or TLS mode with --tls-auth.

       --no-iv
              DEPRECATED This option will be removed in OpenVPN 2.5.

              (Advanced) Disable OpenVPN's use of IV (cipher initialization
              vector).  Don't use this option unless you are prepared to make
              a tradeoff of greater efficiency in exchange for less security.

              OpenVPN uses an IV by default, and requires it for CFB and OFB
              cipher modes (which are totally insecure without it).  Using an
              IV is important for security when multiple messages are being
              encrypted/decrypted with the same key.

              IV is implemented differently depending on the cipher mode used.

              In CBC mode, OpenVPN uses a pseudo-random IV for each packet.

              In CFB/OFB mode, OpenVPN uses a unique sequence number and time
              stamp as the IV.  In fact, in CFB/OFB mode, OpenVPN uses a
              datagram space-saving optimization that uses the unique
              identifier for datagram replay protection as the IV.

       --use-prediction-resistance
              Enable prediction resistance on mbed TLS's RNG.

              Enabling prediction resistance causes the RNG to reseed in each
              call for random. Reseeding this often can quickly deplete the
              kernel entropy pool.

              If you need this option, please consider running a daemon that
              adds entropy to the kernel pool.


       --test-crypto
              Do a self-test of OpenVPN's crypto options by encrypting and
              decrypting test packets using the data channel encryption
              options specified above.  This option does not require a peer to
              function, and therefore can be specified without --dev or
              --remote.

              The typical usage of --test-crypto would be something like this:

              openvpn --test-crypto --secret key

              or

              openvpn --test-crypto --secret key --verb 9

              This option is very useful to test OpenVPN after it has been
              ported to a new platform, or to isolate problems in the
              compiler, OpenSSL crypto library, or OpenVPN's crypto code.
              Since it is a self-test mode, problems with encryption and
              authentication can be debugged independently of network and
              tunnel issues.

   TLS Mode Options:
       TLS mode is the most powerful crypto mode of OpenVPN in both security
       and flexibility.  TLS mode works by establishing control and data
       channels which are multiplexed over a single TCP/UDP port.  OpenVPN
       initiates a TLS session over the control channel and uses it to
       exchange cipher and HMAC keys to protect the data channel.  TLS mode
       uses a robust reliability layer over the UDP connection for all control
       channel communication, while the data channel, over which encrypted
       tunnel data passes, is forwarded without any mediation.  The result is
       the best of both worlds: a fast data channel that forwards over UDP
       with only the overhead of encrypt, decrypt, and HMAC functions, and a
       control channel that provides all of the security features of TLS,
       including certificate-based authentication and Diffie Hellman forward
       secrecy.

       To use TLS mode, each peer that runs OpenVPN should have its own local
       certificate/key pair ( --cert and --key ), signed by the root
       certificate which is specified in --ca.

       When two OpenVPN peers connect, each presents its local certificate to
       the other.  Each peer will then check that its partner peer presented a
       certificate which was signed by the master root certificate as
       specified in --ca.

       If that check on both peers succeeds, then the TLS negotiation will
       succeed, both OpenVPN peers will exchange temporary session keys, and
       the tunnel will begin passing data.

       The OpenVPN project provides a set of scripts for managing RSA
       certificates & keys: https://github.com/OpenVPN/easy-rsa

       --tls-server
              Enable TLS and assume server role during TLS handshake.  Note
              that OpenVPN is designed as a peer-to-peer application.  The
              designation of client or server is only for the purpose of
              negotiating the TLS control channel.

       --tls-client
              Enable TLS and assume client role during TLS handshake.

       --ca file
              Certificate authority (CA) file in .pem format, also referred to
              as the root certificate.  This file can have multiple
              certificates in .pem format, concatenated together.  You can
              construct your own certificate authority certificate and private
              key by using a command such as:

              openssl req -nodes -new -x509 -keyout ca.key -out ca.crt

              Then edit your openssl.cnf file and edit the certificate
              variable to point to your new root certificate ca.crt.

              For testing purposes only, the OpenVPN distribution includes a
              sample CA certificate (ca.crt).  Of course you should never use
              the test certificates and test keys distributed with OpenVPN in
              a production environment, since by virtue of the fact that they
              are distributed with OpenVPN, they are totally insecure.

       --capath dir
              Directory containing trusted certificates (CAs and CRLs).  Not
              available with mbed TLS.

              When using the --capath option, you are required to supply valid
              CRLs for the CAs too.  CAs in the capath directory are expected
              to be named <hash>.<n>.  CRLs are expected to be named
              <hash>.r<n>.  See the -CApath option of openssl verify , and the
              -hash option of openssl x509 and openssl crl for more
              information.

       --dh file
              File containing Diffie Hellman parameters in .pem format
              (required for --tls-server only).

              Set file=none to disable Diffie Hellman key exchange (and use
              ECDH only). Note that this requires peers to be using an SSL
              library that supports ECDH TLS cipher suites (e.g. OpenSSL
              1.0.1+, or mbed TLS 2.0+).

              Use openssl dhparam -out dh2048.pem 2048 to generate 2048-bit DH
              parameters. Diffie Hellman parameters may be considered public.

       --ecdh-curve name
              Specify the curve to use for elliptic curve Diffie Hellman.
              Available curves can be listed with --show-curves.  The
              specified curve will only be used for ECDH TLS-ciphers.

              This option is not supported in mbed TLS builds of OpenVPN.

       --cert file
              Local peer's signed certificate in .pem format -- must be signed
              by a certificate authority whose certificate is in --ca file.
              Each peer in an OpenVPN link running in TLS mode should have its
              own certificate and private key file.  In addition, each
              certificate should have been signed by the key of a certificate
              authority whose public key resides in the --ca certificate
              authority file.  You can easily make your own certificate
              authority (see above) or pay money to use a commercial service
              such as thawte.com (in which case you will be helping to finance
              the world's second space tourist :).  To generate a certificate,
              you can use a command such as:

              openssl req -nodes -new -keyout mycert.key -out mycert.csr

              If your certificate authority private key lives on another
              machine, copy the certificate signing request (mycert.csr) to
              this other machine (this can be done over an insecure channel
              such as email).  Now sign the certificate with a command such
              as:

              openssl ca -out mycert.crt -in mycert.csr

              Now copy the certificate (mycert.crt) back to the peer which
              initially generated the .csr file (this can be over a public
              medium).  Note that the openssl ca command reads the location of
              the certificate authority key from its configuration file such
              as /usr/share/ssl/openssl.cnf -- note also that for certificate
              authority functions, you must set up the files index.txt (may be
              empty) and serial (initialize to 01 ).

       --extra-certs file
              Specify a file containing one or more PEM certs (concatenated
              together) that complete the local certificate chain.

              This option is useful for "split" CAs, where the CA for server
              certs is different than the CA for client certs.  Putting certs
              in this file allows them to be used to complete the local
              certificate chain without trusting them to verify the
              peer-submitted certificate, as would be the case if the certs
              were placed in the ca file.

       --key file
              Local peer's private key in .pem format.  Use the private key
              which was generated when you built your peer's certificate (see
              --cert file above).

       --tls-version-min version ['or-highest']
              Sets the minimum TLS version we will accept from the peer
              (default is "1.0").  Examples for version include "1.0", "1.1",
              or "1.2".  If 'or-highest' is specified and version is not
              recognized, we will only accept the highest TLS version
              supported by the local SSL implementation.

       --tls-version-max version
              Set the maximum TLS version we will use (default is the highest
              version supported).  Examples for version include "1.0", "1.1",
              or "1.2".

       --pkcs12 file
              Specify a PKCS #12 file containing local private key, local
              certificate, and root CA certificate.  This option can be used
              instead of --ca, --cert, and --key.  Not available with mbed
              TLS.

       --verify-hash hash [algo]
              Specify SHA1 or SHA256 fingerprint for level-1 cert.  The
              level-1 cert is the CA (or intermediate cert) that signs the
              leaf certificate, and is one removed from the leaf certificate
              in the direction of the root.  When accepting a connection from
              a peer, the level-1 cert fingerprint must match hash or
              certificate verification will fail.  Hash is specified as
              XX:XX:... For example:

                  AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16

              The algo flag can be either SHA1 or SHA256.  If not provided, it
              defaults to SHA1.

       --pkcs11-cert-private [0|1]...
              Set if access to certificate object should be performed after
              login.  Every provider has its own setting.

       --pkcs11-id name
              Specify the serialized certificate id to be used. The id can be
              gotten by the standalone --show-pkcs11-ids option.

       --pkcs11-id-management
              Acquire PKCS#11 id from management interface. In this case a
              NEED-STR 'pkcs11-id-request' real-time message will be
              triggered, application may use pkcs11-id-count command to
              retrieve available number of certificates, and pkcs11-id-get
              command to retrieve certificate id and certificate body.

       --pkcs11-pin-cache seconds
              Specify how many seconds the PIN can be cached, the default is
              until the token is removed.

       --pkcs11-protected-authentication [0|1]...
              Use PKCS#11 protected authentication path, useful for biometric
              and external keypad devices.  Every provider has its own
              setting.

       --pkcs11-providers provider...
              Specify a RSA Security Inc. PKCS #11 Cryptographic Token
              Interface (Cryptoki) providers to load.  This option can be used
              instead of --cert, --key, and --pkcs12.

              If p11-kit is present on the system, its p11-kit-proxy.so module
              will be loaded by default if either the --pkcs11-id or
              --pkcs11-id-management options are specified without
              --pkcs11-provider being given.

       --pkcs11-private-mode mode...
              Specify which method to use in order to perform private key
              operations.  A different mode can be specified for each
              provider.  Mode is encoded as hex number, and can be a mask one
              of the following:

              0 (default) -- Try to determine automatically.
              1 -- Use sign.
              2 -- Use sign recover.
              4 -- Use decrypt.
              8 -- Use unwrap.

       --cryptoapicert select-string
              Load the certificate and private key from the Windows
              Certificate System Store (Windows/OpenSSL Only).

              Use this option instead of --cert and --key.

              This makes it possible to use any smart card, supported by
              Windows, but also any kind of certificate, residing in the Cert
              Store, where you have access to the private key.  This option
              has been tested with a couple of different smart cards (GemSAFE,
              Cryptoflex, and Swedish Post Office eID) on the client side, and
              also an imported PKCS12 software certificate on the server side.

              To select a certificate, based on a substring search in the
              certificate's subject:

              cryptoapicert "SUBJ:Peter Runestig"

              To select a certificate, based on certificate's thumbprint:

              cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."

              The thumbprint hex string can easily be copy-and-pasted from the
              Windows Certificate Store GUI.


       --key-method m
              DEPRECATED This option will be removed in OpenVPN 2.5

              Use data channel key negotiation method m.  The key method must
              match on both sides of the connection.

              After OpenVPN negotiates a TLS session, a new set of keys for
              protecting the tunnel data channel is generated and exchanged
              over the TLS session.

              In method 1 (the default for OpenVPN 1.x), both sides generate
              random encrypt and HMAC-send keys which are forwarded to the
              other host over the TLS channel. Method 1 is deprecated in
              OpenVPN 2.4 , and will be removed in OpenVPN 2.5.

              In method 2, (the default for OpenVPN 2.0) the client generates
              a random key.  Both client and server also generate some random
              seed material.  All key source material is exchanged over the
              TLS channel. The actual keys are generated using the TLS PRF
              function, taking source entropy from both client and server.
              Method 2 is designed to closely parallel the key generation
              process used by TLS 1.0.

              Note that in TLS mode, two separate levels of keying occur:

              (1) The TLS connection is initially negotiated, with both sides
              of the connection producing certificates and verifying the
              certificate (or other authentication info provided) of the other
              side.  The --key-method parameter has no effect on this process.

              (2) After the TLS connection is established, the tunnel session
              keys are separately negotiated over the existing secure TLS
              channel.  Here, --key-method determines the derivation of the
              tunnel session keys.

       --tls-cipher l
       --tls-ciphersuites l
              A list l of allowable TLS ciphers delimited by a colon (":").

              These setting can be used to ensure that certain cipher suites
              are used (or not used) for the TLS connection.  OpenVPN uses TLS
              to secure the control channel, over which the keys that are used
              to protect the actual VPN traffic are exchanged.

              The supplied list of ciphers is (after potential OpenSSL/IANA
              name translation) simply supplied to the crypto library.  Please
              see the OpenSSL and/or mbed TLS documentation for details on the
              cipher list interpretation.

              For OpenSSL, the --tls-cipher is used for TLS 1.2 and below. For
              TLS 1.3 and up, the --tls-ciphersuites setting is used. mbed TLS
              has no TLS 1.3 support yet and only the --tls-cipher setting is
              used.

              Use --show-tls to see a list of TLS ciphers supported by your
              crypto library.

              Warning!  --tls-cipher and --tls-ciphersuites are expert
              features, which - if used correctly - can improve the security
              of your VPN connection.  But it is also easy to unwittingly use
              them to carefully align a gun with your foot, or just break your
              connection.  Use with care!

              The default for --tls-cipher is to use mbed TLS's default cipher
              list when using mbed TLS or
              "DEFAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA"
              when using OpenSSL.

              The default for --tls-ciphersuites is to use the crypto
              library's default.

       --tls-cert-profile profile
              Set the allowed cryptographic algorithms for certificates
              according to profile.

              The following profiles are supported:

              legacy (default): SHA1 and newer, RSA 2048-bit+, any elliptic
              curve.

              preferred : SHA2 and newer, RSA 2048-bit+, any elliptic curve.

              suiteb : SHA256/SHA384, ECDSA with P-256 or P-384.

              This option is only fully supported for mbed TLS builds.
              OpenSSL builds use the following approximation:

              legacy (default): sets "security level 1"

              preferred : sets "security level 2"

              suiteb : sets "security level 3" and --tls-cipher "SUITEB128".

              OpenVPN will migrate to 'preferred' as default in the future.
              Please ensure that your keys already comply.

       --tls-timeout n
              Packet retransmit timeout on TLS control channel if no
              acknowledgment from remote within n seconds (default=2).  When
              OpenVPN sends a control packet to its peer, it will expect to
              receive an acknowledgement within n seconds or it will
              retransmit the packet, subject to a TCP-like exponential backoff
              algorithm.  This parameter only applies to control channel
              packets.  Data channel packets (which carry encrypted tunnel
              data) are never acknowledged, sequenced, or retransmitted by
              OpenVPN because the higher level network protocols running on
              top of the tunnel such as TCP expect this role to be left to
              them.

       --reneg-bytes n
              Renegotiate data channel key after n bytes sent or received
              (disabled by default with an exception, see below).  OpenVPN
              allows the lifetime of a key to be expressed as a number of
              bytes encrypted/decrypted, a number of packets, or a number of
              seconds.  A key renegotiation will be forced if any of these
              three criteria are met by either peer.

              If using ciphers with cipher block sizes less than 128-bits,
              --reneg-bytes is set to 64MB by default, unless it is explicitly
              disabled by setting the value to 0, but this is HIGHLY
              DISCOURAGED as this is designed to add some protection against
              the SWEET32 attack vector.  For more information see the
              --cipher option.

       --reneg-pkts n
              Renegotiate data channel key after n packets sent and received
              (disabled by default).

       --reneg-sec max [min]
              Renegotiate data channel key after at most max seconds
              (default=3600) and at least min seconds (default is 90% of max
              for servers, and equal to max for clients).

              The effective reneg-sec value used is per session pseudo-
              uniform-randomized between min and max.

              With the default value of 3600 this results in an effective per
              session value in the range of 3240..3600 seconds for servers, or
              just 3600 for clients.

              When using dual-factor authentication, note that this default
              value may cause the end user to be challenged to reauthorize
              once per hour.

              Also, keep in mind that this option can be used on both the
              client and server, and whichever uses the lower value will be
              the one to trigger the renegotiation.  A common mistake is to
              set --reneg-sec to a higher value on either the client or
              server, while the other side of the connection is still using
              the default value of 3600 seconds, meaning that the
              renegotiation will still occur once per 3600 seconds.  The
              solution is to increase --reneg-sec on both the client and
              server, or set it to 0 on one side of the connection (to
              disable), and to your chosen value on the other side.

       --hand-window n
              Handshake Window -- the TLS-based key exchange must finalize
              within n seconds of handshake initiation by any peer (default =
              60 seconds).  If the handshake fails we will attempt to reset
              our connection with our peer and try again.  Even in the event
              of handshake failure we will still use our expiring key for up
              to --tran-window seconds to maintain continuity of transmission
              of tunnel data.

       --tran-window n
              Transition window -- our old key can live this many seconds
              after a new a key renegotiation begins (default = 3600 seconds).
              This feature allows for a graceful transition from old to new
              key, and removes the key renegotiation sequence from the
              critical path of tunnel data forwarding.

       --single-session
              After initially connecting to a remote peer, disallow any new
              connections.  Using this option means that a remote peer cannot
              connect, disconnect, and then reconnect.

              If the daemon is reset by a signal or --ping-restart, it will
              allow one new connection.

              --single-session can be used with --ping-exit or --inactive to
              create a single dynamic session that will exit when finished.

       --tls-exit
              Exit on TLS negotiation failure.

       --tls-auth file [direction]
              Add an additional layer of HMAC authentication on top of the TLS
              control channel to mitigate DoS attacks and attacks on the TLS
              stack.

              In a nutshell, --tls-auth enables a kind of "HMAC firewall" on
              OpenVPN's TCP/UDP port, where TLS control channel packets
              bearing an incorrect HMAC signature can be dropped immediately
              without response.

              file (required) is a file in OpenVPN static key format which can
              be generated by --genkey

              Older versions (up to OpenVPN 2.3) supported a freeform
              passphrase file.  This is no longer supported in newer versions
              (v2.4+).

              See the --secret option for more information on the optional
              direction parameter.

              --tls-auth is recommended when you are running OpenVPN in a mode
              where it is listening for packets from any IP address, such as
              when --remote is not specified, or --remote is specified with
              --float.

              The rationale for this feature is as follows.  TLS requires a
              multi-packet exchange before it is able to authenticate a peer.
              During this time before authentication, OpenVPN is allocating
              resources (memory and CPU) to this potential peer.  The
              potential peer is also exposing many parts of OpenVPN and the
              OpenSSL library to the packets it is sending.  Most successful
              network attacks today seek to either exploit bugs in programs
              (such as buffer overflow attacks) or force a program to consume
              so many resources that it becomes unusable.  Of course the first
              line of defense is always to produce clean, well-audited code.
              OpenVPN has been written with buffer overflow attack prevention
              as a top priority.  But as history has shown, many of the most
              widely used network applications have, from time to time, fallen
              to buffer overflow attacks.

              So as a second line of defense, OpenVPN offers this special
              layer of authentication on top of the TLS control channel so
              that every packet on the control channel is authenticated by an
              HMAC signature and a unique ID for replay protection.  This
              signature will also help protect against DoS (Denial of Service)
              attacks.  An important rule of thumb in reducing vulnerability
              to DoS attacks is to minimize the amount of resources a
              potential, but as yet unauthenticated, client is able to
              consume.

              --tls-auth does this by signing every TLS control channel packet
              with an HMAC signature, including packets which are sent before
              the TLS level has had a chance to authenticate the peer.  The
              result is that packets without the correct signature can be
              dropped immediately upon reception, before they have a chance to
              consume additional system resources such as by initiating a TLS
              handshake.  --tls-auth can be strengthened by adding the
              --replay-persist option which will keep OpenVPN's replay
              protection state in a file so that it is not lost across
              restarts.

              It should be emphasized that this feature is optional and that
              the key file used with --tls-auth gives a peer nothing more than
              the power to initiate a TLS handshake.  It is not used to
              encrypt or authenticate any tunnel data.

              Use --tls-crypt instead if you want to use the key file to not
              only authenticate, but also encrypt the TLS control channel.

       --tls-crypt keyfile

              Encrypt and authenticate all control channel packets with the
              key from keyfile.  (See --tls-auth for more background.)

              Encrypting (and authenticating) control channel packets:

              · provides more privacy by hiding the certificate used for the
                TLS connection,

              · makes it harder to identify OpenVPN traffic as such,

              · provides "poor-man's" post-quantum security, against attackers
                who will never know the pre-shared key (i.e. no forward
                secrecy).


              In contrast to --tls-auth, --tls-crypt does *not* require the
              user to set --key-direction.

              Security Considerations

              All peers use the same --tls-crypt pre-shared group key to
              authenticate and encrypt control channel messages.  To ensure
              that IV collisions remain unlikely, this key should not be used
              to encrypt more than 2^48 client-to-server or 2^48
              server-to-client control channel messages.  A typical initial
              negotiation is about 10 packets in each direction.  Assuming
              both initial negotiation and renegotiations are at most 2^16
              (65536) packets (to be conservative), and (re)negotiations
              happen each minute for each user (24/7), this limits the
              tls-crypt key lifetime to 8171 years divided by the number of
              users.  So a setup with 1000 users should rotate the key at
              least once each eight years.  (And a setup with 8000 users each
              year.)

              If IV collisions were to occur, this could result in the
              security of --tls-crypt degrading to the same security as using
              --tls-auth.  That is, the control channel still benefits from
              the extra protection against active man-in-the-middle-attacks
              and DoS attacks, but may no longer offer extra privacy and
              post-quantum security on top of what TLS itself offers.

              For large setups or setups where clients are not trusted,
              consider using --tls-crypt-v2 instead.  That uses per-client
              unique keys, and thereby improves the bounds to 'rotate a client
              key at least once per 8000 years'.

       --tls-crypt-v2 keyfile

              Use client-specific tls-crypt keys.

              For clients, keyfile is a client-specific tls-crypt key.  Such a
              key can be generated using the --tls-crypt-v2-genkey option.

              For servers, keyfile is used to unwrap client-specific keys
              supplied by the client during connection setup.  This key must
              be the same as the key used to generate the client-specific key
              (see --tls-crypt-v2-genkey).

              On servers, this option can be used together with the --tls-auth
              or --tls-crypt option.  In that case, the server will detect
              whether the client is using client-specific keys, and
              automatically select the right mode.

       --tls-crypt-v2-genkey client|server keyfile [metadata]

              If the first parameter equals "server", generate a
              --tls-crypt-v2 server key and store the key in keyfile.


              If the first parameter equals "client", generate a
              --tls-crypt-v2 client key, and store the key in keyfile.

              If supplied, include the supplied metadata in the wrapped client
              key.  This metadata must be supplied in base64-encoded form.
              The metadata must be at most 735 bytes long (980 bytes in
              base64).

              If no metadata is supplied, OpenVPN will use a 64-bit unix
              timestamp representing the current time in UTC, encoded in
              network order, as metadata for the generated key.

              A tls-crypt-v2 client key is wrapped using a server key.  To
              generate a client key, the user must therefore supply the server
              key using the --tls-crypt-v2 option.

              Servers can use --tls-crypt-v2-verify to specify a metadata
              verification command.

       --tls-crypt-v2-verify cmd

              Run command cmd to verify the metadata of the client-specific
              tls-crypt-v2 key of a connecting client.  This allows server
              administrators to reject client connections, before exposing the
              TLS stack (including the notoriously dangerous X.509 and ASN.1
              stacks) to the connecting client.

              OpenVPN supplies the following env vars to the command:

              · script_type is set to "tls-crypt-v2-verify"

              · metadata_type is set to "0" if the metadata was user supplied,
                or "1" if it's a 64-bit unix timestamp representing the key
                creation time.

              · metadata_file contains the filename of a temporary file that
                contains the client metadata.


              The command can reject the connection by exiting with a non-zero
              exit code.

       --askpass [file]
              Get certificate password from console or file before we
              daemonize.

              For the extremely security conscious, it is possible to protect
              your private key with a password.  Of course this means that
              every time the OpenVPN daemon is started you must be there to
              type the password.  The --askpass option allows you to start
              OpenVPN from the command line.  It will query you for a password
              before it daemonizes.  To protect a private key with a password
              you should omit the -nodes option when you use the openssl
              command line tool to manage certificates and private keys.

              If file is specified, read the password from the first line of
              file.  Keep in mind that storing your password in a file to a
              certain extent invalidates the extra security provided by using
              an encrypted key.

       --auth-nocache
              Don't cache --askpass or --auth-user-pass username/passwords in
              virtual memory.

              If specified, this directive will cause OpenVPN to immediately
              forget username/password inputs after they are used.  As a
              result, when OpenVPN needs a username/password, it will prompt
              for input from stdin, which may be multiple times during the
              duration of an OpenVPN session.

              When using --auth-nocache in combination with a user/password
              file and --chroot or --daemon, make sure to use an absolute
              path.

              This directive does not affect the --http-proxy
              username/password.  It is always cached.

       --auth-token token
              This is not an option to be used directly in any configuration
              files, but rather push this option from a --client-connect
              script or a --plugin which hooks into the
              OPENVPN_PLUGIN_CLIENT_CONNECT or
              OPENVPN_PLUGIN_CLIENT_CONNECT_V2 calls.  This option provides a
              possibility to replace the clients password with an
              authentication token during the lifetime of the OpenVPN client.

              Whenever the connection is renegotiated and the
              --auth-user-pass-verify script or --plugin making use of the
              OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY hook is triggered, it will
              pass over this token as the password instead of the password the
              user provided.  The authentication token can only be reset by a
              full reconnect where the server can push new options to the
              client.  The password the user entered is never preserved once
              an authentication token have been set.  If the OpenVPN server
              side rejects the authentication token, the client will receive
              an AUTH_FAIL and disconnect.

              The purpose of this is to enable two factor authentication
              methods, such as HOTP or TOTP, to be used without needing to
              retrieve a new OTP code each time the connection is
              renegotiated.  Another use case is to cache authentication data
              on the client without needing to have the users password cached
              in memory during the life time of the session.

              To make use of this feature, the --client-connect script or
              --plugin needs to put

                  push "auth-token UNIQUE_TOKEN_VALUE"

              into the file/buffer for dynamic configuration data.  This will
              then make the OpenVPN server to push this value to the client,
              which replaces the local password with the UNIQUE_TOKEN_VALUE.

              Newer clients (2.4.7+) will fall back to the original password
              method after a failed auth. Older clients will keep using the
              token value and react according to --auth-retry

       --tls-verify cmd
              Run command cmd to verify the X509 name of a pending TLS
              connection that has otherwise passed all other tests of
              certification (except for revocation via --crl-verify directive;
              the revocation test occurs after the --tls-verify test).

              cmd should return 0 to allow the TLS handshake to proceed, or 1
              to fail.

              cmd consists of a path to script (or executable program),
              optionally followed by arguments. The path and arguments may be
              single- or double-quoted and/or escaped using a backslash, and
              should be separated by one or more spaces.

              When cmd is executed two arguments are appended after any
              arguments specified in cmd , as follows:

              cmd certificate_depth subject

              These arguments are, respectively, the current certificate depth
              and the X509 common name (cn) of the peer.

              This feature is useful if the peer you want to trust has a
              certificate which was signed by a certificate authority who also
              signed many other certificates, where you don't necessarily want
              to trust all of them, but rather be selective about which peer
              certificate you will accept.  This feature allows you to write a
              script which will test the X509 name on a certificate and decide
              whether or not it should be accepted.  For a simple perl script
              which will test the common name field on the certificate, see
              the file verify-cn in the OpenVPN distribution.

              See the "Environmental Variables" section below for additional
              parameters passed as environmental variables.

       --tls-export-cert directory
              Store the certificates the clients uses upon connection to this
              directory. This will be done before --tls-verify is called.  The
              certificates will use a temporary name and will be deleted when
              the tls-verify script returns.  The file name used for the
              certificate is available via the peer_cert environment variable.

       --x509-username-field [ext:]fieldname
              Field in the X.509 certificate subject to be used as the
              username (default=CN).  Typically, this option is specified with
              fieldname as either of the following:

              --x509-username-field emailAddress
              --x509-username-field ext:subjectAltName

              The first example uses the value of the "emailAddress" attribute
              in the certificate's Subject field as the username.  The second
              example uses the ext: prefix to signify that the X.509 extension
              fieldname "subjectAltName" be searched for an rfc822Name (email)
              field to be used as the username.  In cases where there are
              multiple email addresses in ext:fieldname, the last occurrence
              is chosen.

              When this option is used, the --verify-x509-name option will
              match against the chosen fieldname instead of the Common Name.

              Only the subjectAltName and issuerAltName X.509 extensions are
              supported.

              Please note: This option has a feature which will convert an
              all-lowercase fieldname to uppercase characters, e.g., ou -> OU.
              A mixed-case fieldname or one having the ext: prefix will be
              left as-is.  This automatic upcasing feature is deprecated and
              will be removed in a future release.

       --verify-x509-name name type
              Accept connections only if a host's X.509 name is equal to name.
              The remote host must also pass all other tests of verification.

              Which X.509 name is compared to name depends on the setting of
              type.  type can be "subject" to match the complete subject DN
              (default), "name" to match a subject RDN or "name-prefix" to
              match a subject RDN prefix.  Which RDN is verified as name
              depends on the --x509-username-field option. But it defaults to
              the common name (CN), e.g. a certificate with a subject DN
              "C=KG, ST=NA, L=Bishkek, CN=Server-1" would be matched by:

              --verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1' and
              --verify-x509-name Server-1 name or you could use
              --verify-x509-name Server- name-prefix if you want a client to
              only accept connections to "Server-1", "Server-2", etc.

              --verify-x509-name is a useful replacement for the --tls-verify
              option to verify the remote host, because --verify-x509-name
              works in a --chroot environment without any dependencies.

              Using a name prefix is a useful alternative to managing a CRL
              (Certificate Revocation List) on the client, since it allows the
              client to refuse all certificates except for those associated
              with designated servers.

              NOTE: Test against a name prefix only when you are using OpenVPN
              with a custom CA certificate that is under your control.  Never
              use this option with type "name-prefix" when your client
              certificates are signed by a third party, such as a commercial
              web CA.

       --x509-track attribute
              Save peer X509 attribute value in environment for use by plugins
              and management interface.  Prepend a '+' to attribute to save
              values from full cert chain.  Values will be encoded as
              X509_<depth>_<attribute>=<value>.  Multiple --x509-track options
              can be defined to track multiple attributes.

       --ns-cert-type client|server
              DEPRECATED This option will be removed in OpenVPN 2.5.  Use the
              more modern equivalent --remote-cert-tls instead.  This option
              will be removed in OpenVPN 2.5.

              Require that peer certificate was signed with an explicit
              nsCertType designation of "client" or "server".

              This is a useful security option for clients, to ensure that the
              host they connect with is a designated server.

              See the easy-rsa/build-key-server script for an example of how
              to generate a certificate with the nsCertType field set to
              "server".

              If the server certificate's nsCertType field is set to "server",
              then the clients can verify this with --ns-cert-type server.

              This is an important security precaution to protect against a
              man-in-the-middle attack where an authorized client attempts to
              connect to another client by impersonating the server.  The
              attack is easily prevented by having clients verify the server
              certificate using any one of --ns-cert-type, --verify-x509-name,
              or --tls-verify.

       --remote-cert-ku [v...]
              Require that peer certificate was signed with an explicit key
              usage.

              If present in the certificate, the keyUsage value is validated
              by the TLS library during the TLS handshake.  Specifying this
              option without arguments requires this extension to be present
              (so the TLS library will verify it).

              If the list v...  is also supplied, the keyUsage field must have
              at least the same bits set as the bits in one of the values
              supplied in the list v...

              The key usage values in the list must be encoded in hex, e.g.
              "--remote-cert-ku a0"

       --remote-cert-eku oid
              Require that peer certificate was signed with an explicit
              extended key usage.

              This is a useful security option for clients, to ensure that the
              host they connect to is a designated server.

              The extended key usage should be encoded in oid notation, or
              OpenSSL symbolic representation.

       --remote-cert-tls client|server
              Require that peer certificate was signed with an explicit key
              usage and extended key usage based on RFC3280 TLS rules.

              This is a useful security option for clients, to ensure that the
              host they connect to is a designated server.  Or the other way
              around; for a server to verify that only hosts with a client
              certificate can connect.

              The --remote-cert-tls client option is equivalent to
              --remote-cert-ku --remote-cert-eku "TLS Web Client
              Authentication"

              The --remote-cert-tls server option is equivalent to
              --remote-cert-ku --remote-cert-eku "TLS Web Server
              Authentication"

              This is an important security precaution to protect against a
              man-in-the-middle attack where an authorized client attempts to
              connect to another client by impersonating the server.  The
              attack is easily prevented by having clients verify the server
              certificate using any one of --remote-cert-tls,
              --verify-x509-name, or --tls-verify.

       --crl-verify crl ['dir']
              Check peer certificate against the file crl in PEM format.

              A CRL (certificate revocation list) is used when a particular
              key is compromised but when the overall PKI is still intact.

              Suppose you had a PKI consisting of a CA, root certificate, and
              a number of client certificates.  Suppose a laptop computer
              containing a client key and certificate was stolen.  By adding
              the stolen certificate to the CRL file, you could reject any
              connection which attempts to use it, while preserving the
              overall integrity of the PKI.

              The only time when it would be necessary to rebuild the entire
              PKI from scratch would be if the root certificate key itself was
              compromised.

              If the optional dir flag is specified, enable a different mode
              where crl is a directory containing files named as revoked
              serial numbers (the files may be empty, the contents are never
              read).  If a client requests a connection, where the client
              certificate serial number (decimal string) is the name of a file
              present in the directory, it will be rejected.

              Note: As the crl file (or directory) is read every time a peer
              connects, if you are dropping root privileges with --user, make
              sure that this user has sufficient privileges to read the file.

   SSL Library information:
       --show-ciphers
              (Standalone) Show all cipher algorithms to use with the --cipher
              option.

       --show-digests
              (Standalone) Show all message digest algorithms to use with the
              --auth option.

       --show-tls
              (Standalone) Show all TLS ciphers supported by the crypto
              library.  OpenVPN uses TLS to secure the control channel, over
              which the keys that are used to protect the actual VPN traffic
              are exchanged.  The TLS ciphers will be sorted from highest
              preference (most secure) to lowest.

              Be aware that whether a cipher suite in this list can actually
              work depends on the specific setup of both peers (e.g. both
              peers must support the cipher, and an ECDSA cipher suite will
              not work if you are using an RSA certificate, etc.).

       --show-engines
              (Standalone) Show currently available hardware-based crypto
              acceleration engines supported by the OpenSSL library.

       --show-curves
              (Standalone) Show all available elliptic curves to use with the
              --ecdh-curve option.

   Generate a random key:
       Used only for non-TLS static key encryption mode.

       --genkey file
              (Standalone) Generate a random key to be used as a shared
              secret, for use with the --secret , --tls-auth or --tls-crypt
              options.  Stores the key in file.

              If using this for --secret , this file must be shared with the
              peer over a pre-existing secure channel such as scp(1).

   TUN/TAP persistent tunnel config mode:
       Available with Linux 2.4.7+.  These options comprise a standalone mode
       of OpenVPN which can be used to create and delete persistent tunnels.

       --mktun
              (Standalone) Create a persistent tunnel on platforms which
              support them such as Linux.  Normally TUN/TAP tunnels exist only
              for the period of time that an application has them open.  This
              option takes advantage of the TUN/TAP driver's ability to build
              persistent tunnels that live through multiple instantiations of
              OpenVPN and die only when they are deleted or the machine is
              rebooted.

              One of the advantages of persistent tunnels is that they
              eliminate the need for separate --up and --down scripts to run
              the appropriate ifconfig(8) and route(8) commands.  These
              commands can be placed in the the same shell script which starts
              or terminates an OpenVPN session.

              Another advantage is that open connections through the
              TUN/TAP-based tunnel will not be reset if the OpenVPN peer
              restarts.  This can be useful to provide uninterrupted
              connectivity through the tunnel in the event of a DHCP reset of
              the peer's public IP address (see the --ipchange option above).

              One disadvantage of persistent tunnels is that it is harder to
              automatically configure their MTU value (see --link-mtu and
              --tun-mtu above).

              On some platforms such as Windows, TAP-Win32 tunnels are
              persistent by default.

       --rmtun
              (Standalone) Remove a persistent tunnel.

       --dev tunX | tapX
              TUN/TAP device

       --user user
              Optional user to be owner of this tunnel.

       --group group
              Optional group to be owner of this tunnel.

   Windows-Specific Options:
       --win-sys path
              Set the Windows system directory pathname to use when looking
              for system executables such as route.exe and netsh.exe.  By
              default, if this directive is not specified, OpenVPN will use
              the SystemRoot environment variable.

              This option have changed behaviour in OpenVPN 2.3.  Earlier you
              had to define --win-sys env to use the SystemRoot environment
              variable, otherwise it defaulted to C:\WINDOWS.  It is not
              needed to use the env keyword any more, and it will just be
              ignored. A warning is logged when this is found in the
              configuration file.

       --ip-win32 method
              When using --ifconfig on Windows, set the TAP-Win32 adapter IP
              address and netmask using method.  Don't use this option unless
              you are also using --ifconfig.

              manual -- Don't set the IP address or netmask automatically.
              Instead output a message to the console telling the user to
              configure the adapter manually and indicating the IP/netmask
              which OpenVPN expects the adapter to be set to.

              dynamic [offset] [lease-time] -- Automatically set the IP
              address and netmask by replying to DHCP query messages generated
              by the kernel.  This mode is probably the "cleanest" solution
              for setting the TCP/IP properties since it uses the well-known
              DHCP protocol.  There are, however, two prerequisites for using
              this mode: (1) The TCP/IP properties for the TAP-Win32 adapter
              must be set to "Obtain an IP address automatically," and (2)
              OpenVPN needs to claim an IP address in the subnet for use as
              the virtual DHCP server address.  By default in --dev tap mode,
              OpenVPN will take the normally unused first address in the
              subnet.  For example, if your subnet is 192.168.4.0 netmask
              255.255.255.0, then OpenVPN will take the IP address 192.168.4.0
              to use as the virtual DHCP server address.  In --dev tun mode,
              OpenVPN will cause the DHCP server to masquerade as if it were
              coming from the remote endpoint.  The optional offset parameter
              is an integer which is > -256 and < 256 and which defaults to
              -1.  If offset is positive, the DHCP server will masquerade as
              the IP address at network address + offset.  If offset is
              negative, the DHCP server will masquerade as the IP address at
              broadcast address + offset.  The Windows ipconfig /all command
              can be used to show what Windows thinks the DHCP server address
              is.  OpenVPN will "claim" this address, so make sure to use a
              free address.  Having said that, different OpenVPN
              instantiations, including different ends of the same connection,
              can share the same virtual DHCP server address.  The lease-time
              parameter controls the lease time of the DHCP assignment given
              to the TAP-Win32 adapter, and is denoted in seconds.  Normally a
              very long lease time is preferred because it prevents routes
              involving the TAP-Win32 adapter from being lost when the system
              goes to sleep.  The default lease time is one year.

              netsh -- Automatically set the IP address and netmask using the
              Windows command-line "netsh" command.  This method appears to
              work correctly on Windows XP but not Windows 2000.

              ipapi -- Automatically set the IP address and netmask using the
              Windows IP Helper API.  This approach does not have ideal
              semantics, though testing has indicated that it works okay in
              practice.  If you use this option, it is best to leave the
              TCP/IP properties for the TAP-Win32 adapter in their default
              state, i.e. "Obtain an IP address automatically."

              adaptive -- (Default) Try dynamic method initially and fail over
              to netsh if the DHCP negotiation with the TAP-Win32 adapter does
              not succeed in 20 seconds.  Such failures have been known to
              occur when certain third-party firewall packages installed on
              the client machine block the DHCP negotiation used by the
              TAP-Win32 adapter.  Note that if the netsh failover occurs, the
              TAP-Win32 adapter TCP/IP properties will be reset from DHCP to
              static, and this will cause future OpenVPN startups using the
              adaptive mode to use netsh immediately, rather than trying
              dynamic first.  To "unstick" the adaptive mode from using netsh,
              run OpenVPN at least once using the dynamic mode to restore the
              TAP-Win32 adapter TCP/IP properties to a DHCP configuration.

       --route-method m
              Which method m to use for adding routes on Windows?

              adaptive (default) -- Try IP helper API first.  If that fails,
              fall back to the route.exe shell command.
              ipapi -- Use IP helper API.
              exe -- Call the route.exe shell command.

       --dhcp-option type [parm]
              Set extended TAP-Win32 TCP/IP properties, must be used with
              --ip-win32 dynamic or --ip-win32 adaptive.  This option can be
              used to set additional TCP/IP properties on the TAP-Win32
              adapter, and is particularly useful for configuring an OpenVPN
              client to access a Samba server across the VPN.

              DOMAIN name -- Set Connection-specific DNS Suffix.

              DNS addr -- Set primary domain name server IPv4 or IPv6 address.
              Repeat this option to set secondary DNS server addresses.

              Note: DNS IPv6 servers are currently set using netsh (the
              existing DHCP code can only do IPv4 DHCP, and that protocol only
              permits IPv4 addresses anywhere).  The option will be put into
              the environment, so an --up script could act upon it if needed.

              WINS addr -- Set primary WINS server address (NetBIOS over
              TCP/IP Name Server).  Repeat this option to set secondary WINS
              server addresses.

              NBDD addr -- Set primary NBDD server address (NetBIOS over
              TCP/IP Datagram Distribution Server) Repeat this option to set
              secondary NBDD server addresses.

              NTP addr -- Set primary NTP server address (Network Time
              Protocol).  Repeat this option to set secondary NTP server
              addresses.

              NBT type -- Set NetBIOS over TCP/IP Node type.  Possible
              options: 1 = b-node (broadcasts), 2 = p-node (point-to-point
              name queries to a WINS server), 4 = m-node (broadcast then query
              name server), and 8 = h-node (query name server, then
              broadcast).

              NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope
              ID provides an extended naming service for the NetBIOS over
              TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS
              scope ID is to isolate NetBIOS traffic on a single network to
              only those nodes with the same NetBIOS scope ID.  The NetBIOS
              scope ID is a character string that is appended to the NetBIOS
              name. The NetBIOS scope ID on two hosts must match, or the two
              hosts will not be able to communicate. The NetBIOS Scope ID also
              allows computers to use the same computer name, as they have
              different scope IDs. The Scope ID becomes a part of the NetBIOS
              name, making the name unique.  (This description of NetBIOS
              scopes courtesy of NeonSurge@abyss.com)

              DISABLE-NBT -- Disable Netbios-over-TCP/IP.

              Note that if --dhcp-option is pushed via --push to a non-windows
              client, the option will be saved in the client's environment
              before the up script is called, under the name
              "foreign_option_{n}".

       --tap-sleep n
              Cause OpenVPN to sleep for n seconds immediately after the
              TAP-Win32 adapter state is set to "connected".

              This option is intended to be used to troubleshoot problems with
              the --ifconfig and --ip-win32 options, and is used to give the
              TAP-Win32 adapter time to come up before Windows IP Helper API
              operations are applied to it.

       --show-net-up
              Output OpenVPN's view of the system routing table and network
              adapter list to the syslog or log file after the TUN/TAP adapter
              has been brought up and any routes have been added.

       --block-outside-dns
              Block DNS servers on other network adapters to prevent DNS
              leaks. This option prevents any application from accessing TCP
              or UDP port 53 except one inside the tunnel. It uses Windows
              Filtering Platform (WFP) and works on Windows Vista or later.

              This option is considered unknown on non-Windows platforms and
              unsupported on Windows XP, resulting in fatal error.  You may
              want to use --setenv opt or --ignore-unknown-option (not
              suitable for Windows XP) to ignore said error.  Note that
              pushing unknown options from server does not trigger fatal
              errors.

       --dhcp-renew
              Ask Windows to renew the TAP adapter lease on startup.  This
              option is normally unnecessary, as Windows automatically
              triggers a DHCP renegotiation on the TAP adapter when it comes
              up, however if you set the TAP-Win32 adapter Media Status
              property to "Always Connected", you may need this flag.

       --dhcp-release
              Ask Windows to release the TAP adapter lease on shutdown.  This
              option has no effect now, as it is enabled by default starting
              with OpenVPN 2.4.1.

       --register-dns
              Run ipconfig /flushdns and ipconfig /registerdns on connection
              initiation.  This is known to kick Windows into recognizing
              pushed DNS servers.

       --pause-exit
              Put up a "press any key to continue" message on the console
              prior to OpenVPN program exit.  This option is automatically
              used by the Windows explorer when OpenVPN is run on a
              configuration file using the right-click explorer menu.

       --service exit-event [0|1]
              Should be used when OpenVPN is being automatically executed by
              another program in such a context that no interaction with the
              user via display or keyboard is possible.  In general, end-users
              should never need to explicitly use this option, as it is
              automatically added by the OpenVPN service wrapper when a given
              OpenVPN configuration is being run as a service.

              exit-event is the name of a Windows global event object, and
              OpenVPN will continuously monitor the state of this event object
              and exit when it becomes signaled.

              The second parameter indicates the initial state of exit-event
              and normally defaults to 0.

              Multiple OpenVPN processes can be simultaneously executed with
              the same exit-event parameter.  In any case, the controlling
              process can signal exit-event, causing all such OpenVPN
              processes to exit.

              When executing an OpenVPN process using the --service directive,
              OpenVPN will probably not have a console window to output
              status/error messages, therefore it is useful to use --log or
              --log-append to write these messages to a file.

       --show-adapters
              (Standalone) Show available TAP-Win32 adapters which can be
              selected using the --dev-node option.  On non-Windows systems,
              the ifconfig(8) command provides similar functionality.

       --allow-nonadmin [TAP-adapter]
              (Standalone) Set TAP-adapter to allow access from
              non-administrative accounts.  If TAP-adapter is omitted, all TAP
              adapters on the system will be configured to allow non-admin
              access.  The non-admin access setting will only persist for the
              length of time that the TAP-Win32 device object and driver
              remain loaded, and will need to be re-enabled after a reboot, or
              if the driver is unloaded and reloaded.  This directive can only
              be used by an administrator.

       --show-valid-subnets
              (Standalone) Show valid subnets for --dev tun emulation.  Since
              the TAP-Win32 driver exports an ethernet interface to Windows,
              and since TUN devices are point-to-point in nature, it is
              necessary for the TAP-Win32 driver to impose certain constraints
              on TUN endpoint address selection.

              Namely, the point-to-point endpoints used in TUN device
              emulation must be the middle two addresses of a /30 subnet
              (netmask 255.255.255.252).

       --show-net
              (Standalone) Show OpenVPN's view of the system routing table and
              network adapter list.

   PKCS#11 Standalone Options:
       --show-pkcs11-ids [provider] [cert_private]
              (Standalone) Show PKCS#11 token object list. Specify
              cert_private as 1 if certificates are stored as private objects.

              If p11-kit is present on the system, the provider argument is
              optional; if omitted the default p11-kit-proxy.so module will be
              queried.

              --verb option can be used BEFORE this option to produce
              debugging information.

   Standalone Debug Options:
       --show-gateway [v6target]
              (Standalone) Show current IPv4 and IPv6 default gateway and
              interface towards the gateway (if the protocol in question is
              enabled).  If an IPv6 address is passed as argument, the IPv6
              route for this host is reported.

   IPv6 Related Options
       The following options exist to support IPv6 tunneling in peer-to-peer
       and client-server mode.  All options are modeled after their IPv4
       counterparts, so more detailed explanations given there apply here as
       well (except for --topology , which has no effect on IPv6).

       --ifconfig-ipv6 ipv6addr/bits ipv6remote
              configure IPv6 address ipv6addr/bits on the ``tun'' device.  The
              second parameter is used as route target for --route-ipv6 if no
              gateway is specified.

       --route-ipv6 ipv6addr/bits [gateway] [metric]
              setup IPv6 routing in the system to send the specified IPv6
              network into OpenVPN's ``tun''.  The gateway parameter is only
              used for IPv6 routes across ``tap'' devices, and if missing, the
              ``ipv6remote'' field from --ifconfig-ipv6 or
              --route-ipv6-gateway is used.

       --route-ipv6-gateway gw
              Specify a default gateway gw for use with --route-ipv6.

       --server-ipv6 ipv6addr/bits
              convenience-function to enable a number of IPv6 related options
              at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool and --push
              tun-ipv6 Is only accepted if ``--mode server'' or ``--server''
              is set. Pushing of the --tun-ipv6 directive is done for older
              clients which require an explicit ``--tun-ipv6'' in their
              configuration.

       --ifconfig-ipv6-pool ipv6addr/bits
              Specify an IPv6 address pool for dynamic assignment to clients.
              The pool starts at ipv6addr and matches the offset determined
              from the start of the IPv4 pool.

       --ifconfig-ipv6-push ipv6addr/bits ipv6remote
              for ccd/ per-client static IPv6 interface configuration, see
              --client-config-dir and --ifconfig-push for more details.

       --iroute-ipv6 ipv6addr/bits
              for ccd/ per-client static IPv6 route configuration, see
              --iroute for more details how to setup and use this, and how
              --iroute and --route interact.


SCRIPTING AND ENVIRONMENTAL VARIABLES
       OpenVPN exports a series of environmental variables for use by
       user-defined scripts.

   Script Order of Execution
       --up   Executed after TCP/UDP socket bind and TUN/TAP open.

       --tls-verify
              Executed when we have a still untrusted remote peer.

       --ipchange
              Executed after connection authentication, or remote IP address
              change.

       --client-connect
              Executed in --mode server mode immediately after client
              authentication.

       --route-up
              Executed after connection authentication, either immediately
              after, or some number of seconds after as defined by the
              --route-delay option.

       --route-pre-down
              Executed right before the routes are removed.

       --client-disconnect
              Executed in --mode server mode on client instance shutdown.

       --down Executed after TCP/UDP and TUN/TAP close.

       --learn-address
              Executed in --mode server mode whenever an IPv4 address/route or
              MAC address is added to OpenVPN's internal routing table.

       --auth-user-pass-verify
              Executed in --mode server mode on new client connections, when
              the client is still untrusted.

   String Types and Remapping
       In certain cases, OpenVPN will perform remapping of characters in
       strings.  Essentially, any characters outside the set of permitted
       characters for each string type will be converted to underbar ('_').

       Q: Why is string remapping necessary?

       A: It's an important security feature to prevent the malicious coding
       of strings from untrusted sources to be passed as parameters to
       scripts, saved in the environment, used as a common name, translated to
       a filename, etc.

       Q: Can string remapping be disabled?

       A: Yes, by using the --no-name-remapping option, however this should be
       considered an advanced option.

       Here is a brief rundown of OpenVPN's current string types and the
       permitted character class for each string:

       X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at
       ('@'), colon (':'), slash ('/'), and equal ('=').  Alphanumeric is
       defined as a character which will cause the C library isalnum()
       function to return true.

       Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and
       at ('@').

       --auth-user-pass username: Same as Common Name, with one exception:
       starting with OpenVPN 2.0.1, the username is passed to the
       OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without
       string remapping.

       --auth-user-pass password: Any "printable" character except CR or LF.
       Printable is defined to be a character which will cause the C library
       isprint() function to return true.

       --client-config-dir filename as derived from common name or username:
       Alphanumeric, underbar ('_'), dash ('-'), and dot ('.') except for "."
       or ".." as standalone strings.  As of v2.0.1-rc6, the at ('@')
       character has been added as well for compatibility with the common name
       character class.

       Environmental variable names: Alphanumeric or underbar ('_').

       Environmental variable values: Any printable character.

       For all cases, characters in a string which are not members of the
       legal character class for that string type will be remapped to underbar
       ('_').

   Environmental Variables
       Once set, a variable is persisted indefinitely until it is reset by a
       new value or a restart,

       As of OpenVPN 2.0-beta12, in server mode, environmental variables set
       by OpenVPN are scoped according to the client objects they are
       associated with, so there should not be any issues with scripts having
       access to stale, previously set variables which refer to different
       client instances.

       bytes_received
              Total number of bytes received from client during VPN session.
              Set prior to execution of the --client-disconnect script.

       bytes_sent
              Total number of bytes sent to client during VPN session.  Set
              prior to execution of the --client-disconnect script.

       common_name
              The X509 common name of an authenticated client.  Set prior to
              execution of --client-connect, --client-disconnect, and
              --auth-user-pass-verify scripts.

       config Name of first --config file.  Set on program initiation and
              reset on SIGHUP.

       daemon Set to "1" if the --daemon directive is specified, or "0"
              otherwise.  Set on program initiation and reset on SIGHUP.

       daemon_log_redirect
              Set to "1" if the --log or --log-append directives are
              specified, or "0" otherwise.  Set on program initiation and
              reset on SIGHUP.

       dev    The actual name of the TUN/TAP device, including a unit number
              if it exists.  Set prior to --up or --down script execution.

       dev_idx
              On Windows, the device index of the TUN/TAP adapter (to be used
              in netsh.exe calls which sometimes just do not work right with
              interface names).  Set prior to --up or --down script execution.

       foreign_option_{n}
              An option pushed via --push to a client which does not natively
              support it, such as --dhcp-option on a non-Windows system, will
              be recorded to this environmental variable sequence prior to
              --up script execution.

       ifconfig_broadcast
              The broadcast address for the virtual ethernet segment which is
              derived from the --ifconfig option when --dev tap is used.  Set
              prior to OpenVPN calling the ifconfig or netsh (windows version
              of ifconfig) commands which normally occurs prior to --up script
              execution.

       ifconfig_ipv6_local
              The local VPN endpoint IPv6 address specified in the
              --ifconfig-ipv6 option (first parameter).  Set prior to OpenVPN
              calling the ifconfig or netsh (windows version of ifconfig)
              commands which normally occurs prior to --up script execution.

       ifconfig_ipv6_netbits
              The prefix length of the IPv6 network on the VPN interface.
              Derived from the /nnn parameter of the IPv6 address in the
              --ifconfig-ipv6 option (first parameter).  Set prior to OpenVPN
              calling the ifconfig or netsh (windows version of ifconfig)
              commands which normally occurs prior to --up script execution.

       ifconfig_ipv6_remote
              The remote VPN endpoint IPv6 address specified in the
              --ifconfig-ipv6 option (second parameter).  Set prior to OpenVPN
              calling the ifconfig or netsh (windows version of ifconfig)
              commands which normally occurs prior to --up script execution.

       ifconfig_local
              The local VPN endpoint IP address specified in the --ifconfig
              option (first parameter).  Set prior to OpenVPN calling the
              ifconfig or netsh (windows version of ifconfig) commands which
              normally occurs prior to --up script execution.

       ifconfig_remote
              The remote VPN endpoint IP address specified in the --ifconfig
              option (second parameter) when --dev tun is used.  Set prior to
              OpenVPN calling the ifconfig or netsh (windows version of
              ifconfig) commands which normally occurs prior to --up script
              execution.

       ifconfig_netmask
              The subnet mask of the virtual ethernet segment that is
              specified as the second parameter to --ifconfig when --dev tap
              is being used.  Set prior to OpenVPN calling the ifconfig or
              netsh (windows version of ifconfig) commands which normally
              occurs prior to --up script execution.

       ifconfig_pool_local_ip
              The local virtual IP address for the TUN/TAP tunnel taken from
              an --ifconfig-push directive if specified, or otherwise from the
              ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  Only set for --dev tun tunnels.  This option is set
              on the server prior to execution of the --client-connect and
              --client-disconnect scripts.

       ifconfig_pool_netmask
              The virtual IP netmask for the TUN/TAP tunnel taken from an
              --ifconfig-push directive if specified, or otherwise from the
              ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  Only set for --dev tap tunnels.  This option is set
              on the server prior to execution of the --client-connect and
              --client-disconnect scripts.

       ifconfig_pool_remote_ip
              The remote virtual IP address for the TUN/TAP tunnel taken from
              an --ifconfig-push directive if specified, or otherwise from the
              ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  This option is set on the server prior to execution
              of the --client-connect and --client-disconnect scripts.

       link_mtu
              The maximum packet size (not including the IP header) of tunnel
              data in UDP tunnel transport mode.  Set prior to --up or --down
              script execution.

       local  The --local parameter.  Set on program initiation and reset on
              SIGHUP.

       local_port
              The local port number or name, specified by --port or --lport.
              Set on program initiation and reset on SIGHUP.

       password
              The password provided by a connecting client.  Set prior to
              --auth-user-pass-verify script execution only when the via-env
              modifier is specified, and deleted from the environment after
              the script returns.

       proto  The --proto parameter.  Set on program initiation and reset on
              SIGHUP.

       remote_{n}
              The --remote parameter.  Set on program initiation and reset on
              SIGHUP.

       remote_port_{n}
              The remote port number, specified by --port or --rport.  Set on
              program initiation and reset on SIGHUP.

       route_net_gateway
              The pre-existing default IP gateway in the system routing table.
              Set prior to --up script execution.

       route_vpn_gateway
              The default gateway used by --route options, as specified in
              either the --route-gateway option or the second parameter to
              --ifconfig when --dev tun is specified.  Set prior to --up
              script execution.

       route_{parm}_{n}
              A set of variables which define each route to be added, and are
              set prior to --up script execution.

              parm will be one of "network", "netmask", "gateway", or
              "metric".

              n is the OpenVPN route number, starting from 1.

              If the network or gateway are resolvable DNS names, their IP
              address translations will be recorded rather than their names as
              denoted on the command line or configuration file.

       route_ipv6_{parm}_{n}
              A set of variables which define each IPv6 route to be added, and
              are set prior to --up script execution.

              parm will be one of "network" or "gateway" ("netmask" is
              contained as "/nnn" in the route_ipv6_network_{n}, unlike IPv4
              where it is passed in a separate environment variable).

              n is the OpenVPN route number, starting from 1.

              If the network or gateway are resolvable DNS names, their IP
              address translations will be recorded rather than their names as
              denoted on the command line or configuration file.

       peer_cert
              Temporary file name containing the client certificate upon
              connection.  Useful in conjunction with --tls-verify

       script_context
              Set to "init" or "restart" prior to up/down script execution.
              For more information, see documentation for --up.

       script_type
              Prior to execution of any script, this variable is set to the
              type of script being run.  It can be one of the following: up,
              down, ipchange, route-up, tls-verify, auth-user-pass-verify,
              client-connect, client-disconnect, or learn-address.  Set prior
              to execution of any script.

       signal The reason for exit or restart.  Can be one of sigusr1, sighup,
              sigterm, sigint, inactive (controlled by --inactive option),
              ping-exit (controlled by --ping-exit option), ping-restart
              (controlled by --ping-restart option), connection-reset
              (triggered on TCP connection reset), error, or unknown (unknown
              signal).  This variable is set just prior to down script
              execution.

       time_ascii
              Client connection timestamp, formatted as a human-readable time
              string.  Set prior to execution of the --client-connect script.

       time_duration
              The duration (in seconds) of the client session which is now
              disconnecting.  Set prior to execution of the
              --client-disconnect script.

       time_unix
              Client connection timestamp, formatted as a unix integer
              date/time value.  Set prior to execution of the --client-connect
              script.

       tls_digest_{n} / tls_digest_sha256_{n}
              Contains the certificate SHA1 / SHA256 fingerprint, where n is
              the verification level.  Only set for TLS connections.  Set
              prior to execution of --tls-verify script.

       tls_id_{n}
              A series of certificate fields from the remote peer, where n is
              the verification level.  Only set for TLS connections.  Set
              prior to execution of --tls-verify script.

       tls_serial_{n}
              The serial number of the certificate from the remote peer, where
              n is the verification level.  Only set for TLS connections.  Set
              prior to execution of --tls-verify script. This is in the form
              of a decimal string like "933971680", which is suitable for
              doing serial-based OCSP queries (with OpenSSL, do not prepend
              "0x" to the string) If something goes wrong while reading the
              value from the certificate it will be an empty string, so your
              code should check that.  See the
              contrib/OCSP_check/OCSP_check.sh script for an example.

       tls_serial_hex_{n}
              Like tls_serial_{n}, but in hex form (e.g. "12:34:56:78:9A").

       tun_mtu
              The MTU of the TUN/TAP device.  Set prior to --up or --down
              script execution.

       trusted_ip (or trusted_ip6)
              Actual IP address of connecting client or peer which has been
              authenticated.  Set prior to execution of --ipchange,
              --client-connect, and --client-disconnect scripts.  If using
              ipv6 endpoints (udp6, tcp6), trusted_ip6 will be set instead.

       trusted_port
              Actual port number of connecting client or peer which has been
              authenticated.  Set prior to execution of --ipchange,
              --client-connect, and --client-disconnect scripts.

       untrusted_ip (or untrusted_ip6)
              Actual IP address of connecting client or peer which has not
              been authenticated yet.  Sometimes used to nmap the connecting
              host in a --tls-verify script to ensure it is firewalled
              properly.  Set prior to execution of --tls-verify and
              --auth-user-pass-verify scripts.  If using ipv6 endpoints (udp6,
              tcp6), untrusted_ip6 will be set instead.

       untrusted_port
              Actual port number of connecting client or peer which has not
              been authenticated yet.  Set prior to execution of --tls-verify
              and --auth-user-pass-verify scripts.

       username
              The username provided by a connecting client.  Set prior to
              --auth-user-pass-verify script execution only when the via-env
              modifier is specified.

       X509_{n}_{subject_field}
              An X509 subject field from the remote peer certificate, where n
              is the verification level.  Only set for TLS connections.  Set
              prior to execution of --tls-verify script.  This variable is
              similar to tls_id_{n} except the component X509 subject fields
              are broken out, and no string remapping occurs on these field
              values (except for remapping of control characters to "_").  For
              example, the following variables would be set on the OpenVPN
              server using the sample client certificate in sample-keys
              (client.crt).  Note that the verification level is 0 for the
              client certificate and 1 for the CA certificate.

                  X509_0_emailAddress=me@myhost.mydomain
                  X509_0_CN=Test-Client
                  X509_0_O=OpenVPN-TEST
                  X509_0_ST=NA
                  X509_0_C=KG
                  X509_1_emailAddress=me@myhost.mydomain
                  X509_1_O=OpenVPN-TEST
                  X509_1_L=BISHKEK
                  X509_1_ST=NA
                  X509_1_C=KG

INLINE FILE SUPPORT
       OpenVPN allows including files in the main configuration for the --ca,
       --cert, --dh, --extra-certs, --key, --pkcs12, --secret, --crl-verify,
       --http-proxy-user-pass, --tls-auth, --tls-crypt, and --tls-crypt-v2
       options.

       Each inline file started by the line <option> and ended by the line
       </option>

       Here is an example of an inline file usage

           <cert>
           -----BEGIN CERTIFICATE-----
           [...]
           -----END CERTIFICATE-----
           </cert>

       When using the inline file feature with --pkcs12 the inline file has to
       be base64 encoded. Encoding of a .p12 file into base64 can be done for
       example with OpenSSL by running openssl base64 -in input.p12


SIGNALS
       SIGHUP Cause OpenVPN to close all TUN/TAP and network connections,
              restart, re-read the configuration file (if any), and reopen
              TUN/TAP and network connections.

       SIGUSR1
              Like SIGHUP, except don't re-read configuration file, and
              possibly don't close and reopen TUN/TAP device, re-read key
              files, preserve local IP address/port, or preserve most recently
              authenticated remote IP address/port based on --persist-tun,
              --persist-key, --persist-local-ip, and --persist-remote-ip
              options respectively (see above).

              This signal may also be internally generated by a timeout
              condition, governed by the --ping-restart option.

              This signal, when combined with --persist-remote-ip, may be sent
              when the underlying parameters of the host's network interface
              change such as when the host is a DHCP client and is assigned a
              new IP address.  See --ipchange above for more information.

       SIGUSR2
              Causes OpenVPN to display its current statistics (to the syslog
              file if --daemon is used, or stdout otherwise).

       SIGINT, SIGTERM
              Causes OpenVPN to exit gracefully.

TUN/TAP DRIVER SETUP
       If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP
       driver already installed.  If so, there are still a few things you need
       to do:

       Make device: mknod /dev/net/tun c 10 200

       Load driver: modprobe tun

EXAMPLES
       Prior to running these examples, you should have OpenVPN installed on
       two machines with network connectivity between them.  If you have not
       yet installed OpenVPN, consult the INSTALL file included in the OpenVPN
       distribution.

   TUN/TAP Setup:
       If you are using Linux 2.4 or higher, make the tun device node and load
       the tun module:

              mknod /dev/net/tun c 10 200

              modprobe tun

       If you installed from RPM, the mknod step may be omitted, because the
       RPM install does that for you.

       Only Linux 2.4 and newer are supported.

       For other platforms, consult the INSTALL file at
       http://openvpn.net/install.html for more information.

   Firewall Setup:
       If firewalls exist between the two machines, they should be set to
       forward UDP port 1194 in both directions.  If you do not have control
       over the firewalls between the two machines, you may still be able to
       use OpenVPN by adding --ping 15 to each of the openvpn commands used
       below in the examples (this will cause each peer to send out a UDP ping
       to its remote peer once every 15 seconds which will cause many stateful
       firewalls to forward packets in both directions without an explicit
       firewall rule).

       If you are using a Linux iptables-based firewall, you may need to enter
       the following command to allow incoming packets on the TUN device:

              iptables -A INPUT -i tun+ -j ACCEPT

       See the firewalls section below for more information on configuring
       firewalls for use with OpenVPN.

   VPN Address Setup:
       For purposes of our example, our two machines will be called
       bob.example.com and alice.example.com.  If you are constructing a VPN
       over the internet, then replace bob.example.com and alice.example.com
       with the internet hostname or IP address that each machine will use to
       contact the other over the internet.

       Now we will choose the tunnel endpoints.  Tunnel endpoints are private
       IP addresses that only have meaning in the context of the VPN.  Each
       machine will use the tunnel endpoint of the other machine to access it
       over the VPN.  In our example, the tunnel endpoint for bob.example.com
       will be 10.4.0.1 and for alice.example.com, 10.4.0.2.

       Once the VPN is established, you have essentially created a secure
       alternate path between the two hosts which is addressed by using the
       tunnel endpoints.  You can control which network traffic passes between
       the hosts (a) over the VPN or (b) independently of the VPN, by choosing
       whether to use (a) the VPN endpoint address or (b) the public internet
       address, to access the remote host. For example if you are on
       bob.example.com and you wish to connect to alice.example.com via ssh
       without using the VPN (since ssh has its own built-in security) you
       would use the command ssh alice.example.com.  However in the same
       scenario, you could also use the command telnet 10.4.0.2 to create a
       telnet session with alice.example.com over the VPN, that would use the
       VPN to secure the session rather than ssh.

       You can use any address you wish for the tunnel endpoints but make sure
       that they are private addresses (such as those that begin with 10 or
       192.168) and that they are not part of any existing subnet on the
       networks of either peer, unless you are bridging.  If you use an
       address that is part of your local subnet for either of the tunnel
       endpoints, you will get a weird feedback loop.

   Example 1: A simple tunnel without security
       On bob:

              openvpn --remote alice.example.com --dev tun1 --ifconfig
              10.4.0.1 10.4.0.2 --verb 9

       On alice:

              openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
              10.4.0.1 --verb 9

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

       The --verb 9 option will produce verbose output, similar to the
       tcpdump(8) program.  Omit the --verb 9 option to have OpenVPN run
       quietly.

   Example 2: A tunnel with static-key security (i.e. using a pre-shared
       secret)
       First build a static key on bob.

              openvpn --genkey --secret key

       This command will build a random key file called key (in ascii format).
       Now copy key to alice over a secure medium such as by using the scp(1)
       program.

       On bob:

              openvpn --remote alice.example.com --dev tun1 --ifconfig
              10.4.0.1 10.4.0.2 --verb 5 --secret key

       On alice:

              openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
              10.4.0.1 --verb 5 --secret key

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

   Example 3: A tunnel with full TLS-based security
       For this test, we will designate bob as the TLS client and alice as the
       TLS server.  Note that client or server designation only has meaning
       for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer,
       UDP-based communication model.

       First, build a separate certificate/key pair for both bob and alice
       (see above where --cert is discussed for more info).  Then construct
       Diffie Hellman parameters (see above where --dh is discussed for more
       info).  You can also use the included test files client.crt,
       client.key, server.crt, server.key and ca.crt.  The .crt files are
       certificates/public-keys, the .key files are private keys, and ca.crt
       is a certification authority who has signed both client.crt and
       server.crt.  For Diffie Hellman parameters you can use the included
       file dh1024.pem.  Note that all client, server, and certificate
       authority certificates and keys included in the OpenVPN distribution
       are totally insecure and should be used for testing only.

       On bob:

              openvpn --remote alice.example.com --dev tun1 --ifconfig
              10.4.0.1 10.4.0.2 --tls-client --ca ca.crt --cert client.crt
              --key client.key --reneg-sec 60 --verb 5

       On alice:

              openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
              10.4.0.1 --tls-server --dh dh1024.pem --ca ca.crt --cert
              server.crt --key server.key --reneg-sec 60 --verb 5

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

       Notice the --reneg-sec 60 option we used above.  That tells OpenVPN to
       renegotiate the data channel keys every minute.  Since we used --verb 5
       above, you will see status information on each new key negotiation.

       For production operations, a key renegotiation interval of 60 seconds
       is probably too frequent.  Omit the --reneg-sec 60 option to use
       OpenVPN's default key renegotiation interval of one hour.

   Routing:
       Assuming you can ping across the tunnel, the next step is to route a
       real subnet over the secure tunnel.  Suppose that bob and alice have
       two network interfaces each, one connected to the internet, and the
       other to a private network.  Our goal is to securely connect both
       private networks.  We will assume that bob's private subnet is
       10.0.0.0/24 and alice's is 10.0.1.0/24.

       First, ensure that IP forwarding is enabled on both peers.  On Linux,
       enable routing:

              echo 1 > /proc/sys/net/ipv4/ip_forward

       and enable TUN packet forwarding through the firewall:

              iptables -A FORWARD -i tun+ -j ACCEPT

       On bob:

              route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2

       On alice:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1

       Now any machine on the 10.0.0.0/24 subnet can access any machine on the
       10.0.1.0/24 subnet over the secure tunnel (or vice versa).

       In a production environment, you could put the route command(s) in a
       script and execute with the --up option.

FIREWALLS
       OpenVPN's usage of a single UDP port makes it fairly firewall-friendly.
       You should add an entry to your firewall rules to allow incoming
       OpenVPN packets.  On Linux 2.4+:

              iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT

       This will allow incoming packets on UDP port 1194 (OpenVPN's default
       UDP port) from an OpenVPN peer at 1.2.3.4.

       If you are using HMAC-based packet authentication (the default in any
       of OpenVPN's secure modes), having the firewall filter on source
       address can be considered optional, since HMAC packet authentication is
       a much more secure method of verifying the authenticity of a packet
       source.  In that case:

              iptables -A INPUT -p udp --dport 1194 -j ACCEPT

       would be adequate and would not render the host inflexible with respect
       to its peer having a dynamic IP address.

       OpenVPN also works well on stateful firewalls.  In some cases, you may
       not need to add any static rules to the firewall list if you are using
       a stateful firewall that knows how to track UDP connections.  If you
       specify --ping n, OpenVPN will be guaranteed to send a packet to its
       peer at least once every n seconds.  If n is less than the stateful
       firewall connection timeout, you can maintain an OpenVPN connection
       indefinitely without explicit firewall rules.

       You should also add firewall rules to allow incoming IP traffic on TUN
       or TAP devices such as:

              iptables -A INPUT -i tun+ -j ACCEPT

       to allow input packets from tun devices,

              iptables -A FORWARD -i tun+ -j ACCEPT

       to allow input packets from tun devices to be forwarded to other hosts
       on the local network,

              iptables -A INPUT -i tap+ -j ACCEPT

       to allow input packets from tap devices, and

              iptables -A FORWARD -i tap+ -j ACCEPT

       to allow input packets from tap devices to be forwarded to other hosts
       on the local network.

       These rules are secure if you use packet authentication, since no
       incoming packets will arrive on a TUN or TAP virtual device unless they
       first pass an HMAC authentication test.

FAQ
       http://openvpn.net/faq.html

HOWTO
       For a more comprehensive guide to setting up OpenVPN in a production
       setting, see the OpenVPN HOWTO at http://openvpn.net/howto.html

PROTOCOL
       For a description of OpenVPN's underlying protocol, see
       http://openvpn.net/security.html

WEB
       OpenVPN's web site is at http://openvpn.net/

       Go here to download the latest version of OpenVPN, subscribe to the
       mailing lists, read the mailing list archives, or browse the SVN
       repository.

BUGS
       Report all bugs to the OpenVPN team <info@openvpn.net>.

SEE ALSO
       dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)

NOTES
       This product includes software developed by the OpenSSL Project (
       http://www.openssl.org/ )

       For more information on the TLS protocol, see
       http://www.ietf.org/rfc/rfc2246.txt

       For more information on the LZO real-time compression library see
       http://www.oberhumer.com/opensource/lzo/

COPYRIGHT
       Copyright (C) 2002-2018 OpenVPN Inc This program is free software; you
       can redistribute it and/or modify it under the terms of the GNU General
       Public License version 2 as published by the Free Software Foundation.

AUTHORS
       James Yonan <jim@yonan.net>



                               28 February 2018                     openvpn(8)