ssh

SSH(1)                     BSD General Commands Manual                    SSH(1)

NAME
     ssh — OpenSSH remote login client

SYNOPSIS
     ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
         [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
         [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
         [-J destination] [-L address] [-l login_name] [-m mac_spec]
         [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
         [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
         [command]

DESCRIPTION
     ssh (SSH client) is a program for logging into a remote machine and for
     executing commands on a remote machine.  It is intended to provide secure
     encrypted communications between two untrusted hosts over an insecure
     network.  X11 connections, arbitrary TCP ports and UNIX-domain sockets can
     also be forwarded over the secure channel.

     ssh connects and logs into the specified destination, which may be
     specified as either [user@]hostname or a URI of the form
     ssh://[user@]hostname[:port].  The user must prove his/her identity to the
     remote machine using one of several methods (see below).

     If a command is specified, it is executed on the remote host instead of a
     login shell.

     The options are as follows:

     -4      Forces ssh to use IPv4 addresses only.

     -6      Forces ssh to use IPv6 addresses only.

     -A      Enables forwarding of connections from an authentication agent such
             as ssh-agent(1).  This can also be specified on a per-host basis in
             a configuration file.

             Agent forwarding should be enabled with caution.  Users with the
             ability to bypass file permissions on the remote host (for the
             agent's UNIX-domain socket) can access the local agent through the
             forwarded connection.  An attacker cannot obtain key material from
             the agent, however they can perform operations on the keys that
             enable them to authenticate using the identities loaded into the
             agent.  A safer alternative may be to use a jump host (see -J).

     -a      Disables forwarding of the authentication agent connection.

     -B bind_interface
             Bind to the address of bind_interface before attempting to connect
             to the destination host.  This is only useful on systems with more
             than one address.

     -b bind_address
             Use bind_address on the local machine as the source address of the
             connection.  Only useful on systems with more than one address.

     -C      Requests compression of all data (including stdin, stdout, stderr,
             and data for forwarded X11, TCP and UNIX-domain connections).  The
             compression algorithm is the same used by gzip(1).  Compression is
             desirable on modem lines and other slow connections, but will only
             slow down things on fast networks.  The default value can be set on
             a host-by-host basis in the configuration files; see the
             Compression option.

     -c cipher_spec
             Selects the cipher specification for encrypting the session.
             cipher_spec is a comma-separated list of ciphers listed in order of
             preference.  See the Ciphers keyword in ssh_config(5) for more
             information.

     -D [bind_address:]port
             Specifies a local “dynamic” application-level port forwarding.
             This works by allocating a socket to listen to port on the local
             side, optionally bound to the specified bind_address.  Whenever a
             connection is made to this port, the connection is forwarded over
             the secure channel, and the application protocol is then used to
             determine where to connect to from the remote machine.  Currently
             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as
             a SOCKS server.  Only root can forward privileged ports.  Dynamic
             port forwardings can also be specified in the configuration file.

             IPv6 addresses can be specified by enclosing the address in square
             brackets.  Only the superuser can forward privileged ports.  By
             default, the local port is bound in accordance with the
             GatewayPorts setting.  However, an explicit bind_address may be
             used to bind the connection to a specific address.  The
             bind_address of “localhost” indicates that the listening port be
             bound for local use only, while an empty address or ‘*’ indicates
             that the port should be available from all interfaces.

     -E log_file
             Append debug logs to log_file instead of standard error.

     -e escape_char
             Sets the escape character for sessions with a pty (default: ‘~’).
             The escape character is only recognized at the beginning of a line.
             The escape character followed by a dot (‘.’) closes the connection;
             followed by control-Z suspends the connection; and followed by
             itself sends the escape character once.  Setting the character to
             “none” disables any escapes and makes the session fully
             transparent.

     -F configfile
             Specifies an alternative per-user configuration file.  If a
             configuration file is given on the command line, the system-wide
             configuration file (/etc/ssh/ssh_config) will be ignored.  The
             default for the per-user configuration file is ~/.ssh/config.  If
             set to “none”, no configuration files will be read.

     -f      Requests ssh to go to background just before command execution.
             This is useful if ssh is going to ask for passwords or passphrases,
             but the user wants it in the background.  This implies -n.  The
             recommended way to start X11 programs at a remote site is with
             something like ssh -f host xterm.

             If the ExitOnForwardFailure configuration option is set to “yes”,
             then a client started with -f will wait for all remote port
             forwards to be successfully established before placing itself in
             the background.

     -G      Causes ssh to print its configuration after evaluating Host and
             Match blocks and exit.

     -g      Allows remote hosts to connect to local forwarded ports.  If used
             on a multiplexed connection, then this option must be specified on
             the master process.

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to communicate
             with a PKCS#11 token providing keys for user authentication.

     -i identity_file
             Selects a file from which the identity (private key) for public key
             authentication is read.  The default is ~/.ssh/id_dsa,
             ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
             ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa.  Identity files may also be
             specified on a per-host basis in the configuration file.  It is
             possible to have multiple -i options (and multiple identities
             specified in configuration files).  If no certificates have been
             explicitly specified by the CertificateFile directive, ssh will
             also try to load certificate information from the filename obtained
             by appending -cert.pub to identity filenames.

     -J destination
             Connect to the target host by first making a ssh connection to the
             jump host described by destination and then establishing a TCP
             forwarding to the ultimate destination from there.  Multiple jump
             hops may be specified separated by comma characters.  This is a
             shortcut to specify a ProxyJump configuration directive.  Note that
             configuration directives supplied on the command-line generally
             apply to the destination host and not any specified jump hosts.
             Use ~/.ssh/config to specify configuration for jump hosts.

     -K      Enables GSSAPI-based authentication and forwarding (delegation) of
             GSSAPI credentials to the server.

     -k      Disables forwarding (delegation) of GSSAPI credentials to the
             server.

     -L [bind_address:]port:host:hostport
     -L [bind_address:]port:remote_socket
     -L local_socket:host:hostport
     -L local_socket:remote_socket
             Specifies that connections to the given TCP port or Unix socket on
             the local (client) host are to be forwarded to the given host and
             port, or Unix socket, on the remote side.  This works by allocating
             a socket to listen to either a TCP port on the local side,
             optionally bound to the specified bind_address, or to a Unix
             socket.  Whenever a connection is made to the local port or socket,
             the connection is forwarded over the secure channel, and a
             connection is made to either host port hostport, or the Unix socket
             remote_socket, from the remote machine.

             Port forwardings can also be specified in the configuration file.
             Only the superuser can forward privileged ports.  IPv6 addresses
             can be specified by enclosing the address in square brackets.

             By default, the local port is bound in accordance with the
             GatewayPorts setting.  However, an explicit bind_address may be
             used to bind the connection to a specific address.  The
             bind_address of “localhost” indicates that the listening port be
             bound for local use only, while an empty address or ‘*’ indicates
             that the port should be available from all interfaces.

     -l login_name
             Specifies the user to log in as on the remote machine.  This also
             may be specified on a per-host basis in the configuration file.

     -M      Places the ssh client into “master” mode for connection sharing.
             Multiple -M options places ssh into “master” mode but with
             confirmation required using ssh-askpass(1) before each operation
             that changes the multiplexing state (e.g. opening a new session).
             Refer to the description of ControlMaster in ssh_config(5) for
             details.

     -m mac_spec
             A comma-separated list of MAC (message authentication code)
             algorithms, specified in order of preference.  See the MACs keyword
             for more information.

     -N      Do not execute a remote command.  This is useful for just
             forwarding ports.

     -n      Redirects stdin from /dev/null (actually, prevents reading from
             stdin).  This must be used when ssh is run in the background.  A
             common trick is to use this to run X11 programs on a remote
             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
             an emacs on shadows.cs.hut.fi, and the X11 connection will be
             automatically forwarded over an encrypted channel.  The ssh program
             will be put in the background.  (This does not work if ssh needs to
             ask for a password or passphrase; see also the -f option.)

     -O ctl_cmd
             Control an active connection multiplexing master process.  When the
             -O option is specified, the ctl_cmd argument is interpreted and
             passed to the master process.  Valid commands are: “check” (check
             that the master process is running), “forward” (request forwardings
             without command execution), “cancel” (cancel forwardings), “exit”
             (request the master to exit), and “stop” (request the master to
             stop accepting further multiplexing requests).

     -o option
             Can be used to give options in the format used in the configuration
             file.  This is useful for specifying options for which there is no
             separate command-line flag.  For full details of the options listed
             below, and their possible values, see ssh_config(5).

                   AddKeysToAgent
                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   CASignatureAlgorithms
                   CertificateFile
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Ciphers
                   ClearAllForwardings
                   Compression
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   FingerprintHash
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAcceptedAlgorithms
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   Hostname
                   IdentitiesOnly
                   IdentityAgent
                   IdentityFile
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   KnownHostsCommand
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PermitRemoteOpen
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   ProxyCommand
                   ProxyJump
                   ProxyUseFdpass
                   PubkeyAcceptedAlgorithms
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteCommand
                   RemoteForward
                   RequestTTY
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   SetEnv
                   StreamLocalBindMask
                   StreamLocalBindUnlink
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UpdateHostKeys
                   User
                   UserKnownHostsFile
                   VerifyHostKeyDNS
                   VisualHostKey
                   XAuthLocation

     -p port
             Port to connect to on the remote host.  This can be specified on a
             per-host basis in the configuration file.

     -Q query_option
             Queries ssh for the algorithms supported for the specified version
             2.  The available features are: cipher (supported symmetric
             ciphers), cipher-auth (supported symmetric ciphers that support
             authenticated encryption), help (supported query terms for use with
             the -Q flag), mac (supported message integrity codes), kex (key
             exchange algorithms), key (key types), key-cert (certificate key
             types), key-plain (non-certificate key types), key-sig (all key
             types and signature algorithms), protocol-version (supported SSH
             protocol versions), and sig (supported signature algorithms).
             Alternatively, any keyword from ssh_config(5) or sshd_config(5)
             that takes an algorithm list may be used as an alias for the
             corresponding query_option.

     -q      Quiet mode.  Causes most warning and diagnostic messages to be
             suppressed.

     -R [bind_address:]port:host:hostport
     -R [bind_address:]port:local_socket
     -R remote_socket:host:hostport
     -R remote_socket:local_socket
     -R [bind_address:]port
             Specifies that connections to the given TCP port or Unix socket on
             the remote (server) host are to be forwarded to the local side.

             This works by allocating a socket to listen to either a TCP port or
             to a Unix socket on the remote side.  Whenever a connection is made
             to this port or Unix socket, the connection is forwarded over the
             secure channel, and a connection is made from the local machine to
             either an explicit destination specified by host port hostport, or
             local_socket, or, if no explicit destination was specified, ssh
             will act as a SOCKS 4/5 proxy and forward connections to the
             destinations requested by the remote SOCKS client.

             Port forwardings can also be specified in the configuration file.
             Privileged ports can be forwarded only when logging in as root on
             the remote machine.  IPv6 addresses can be specified by enclosing
             the address in square brackets.

             By default, TCP listening sockets on the server will be bound to
             the loopback interface only.  This may be overridden by specifying
             a bind_address.  An empty bind_address, or the address ‘*’,
             indicates that the remote socket should listen on all interfaces.
             Specifying a remote bind_address will only succeed if the server's
             GatewayPorts option is enabled (see sshd_config(5)).

             If the port argument is ‘0’, the listen port will be dynamically
             allocated on the server and reported to the client at run time.
             When used together with -O forward the allocated port will be
             printed to the standard output.

     -S ctl_path
             Specifies the location of a control socket for connection sharing,
             or the string “none” to disable connection sharing.  Refer to the
             description of ControlPath and ControlMaster in ssh_config(5) for
             details.

     -s      May be used to request invocation of a subsystem on the remote
             system.  Subsystems facilitate the use of SSH as a secure transport
             for other applications (e.g. sftp(1)).  The subsystem is specified
             as the remote command.

     -T      Disable pseudo-terminal allocation.

     -t      Force pseudo-terminal allocation.  This can be used to execute
             arbitrary screen-based programs on a remote machine, which can be
             very useful, e.g. when implementing menu services.  Multiple -t
             options force tty allocation, even if ssh has no local tty.

     -V      Display the version number and exit.

     -v      Verbose mode.  Causes ssh to print debugging messages about its
             progress.  This is helpful in debugging connection, authentication,
             and configuration problems.  Multiple -v options increase the
             verbosity.  The maximum is 3.

     -W host:port
             Requests that standard input and output on the client be forwarded
             to host on port over the secure channel.  Implies -N, -T,
             ExitOnForwardFailure and ClearAllForwardings, though these can be
             overridden in the configuration file or using -o command line
             options.

     -w local_tun[:remote_tun]
             Requests tunnel device forwarding with the specified tun(4) devices
             between the client (local_tun) and the server (remote_tun).

             The devices may be specified by numerical ID or the keyword “any”,
             which uses the next available tunnel device.  If remote_tun is not
             specified, it defaults to “any”.  See also the Tunnel and
             TunnelDevice directives in ssh_config(5).

             If the Tunnel directive is unset, it will be set to the default
             tunnel mode, which is “point-to-point”.  If a different Tunnel
             forwarding mode it desired, then it should be specified before -w.

     -X      Enables X11 forwarding.  This can also be specified on a per-host
             basis in a configuration file.

             X11 forwarding should be enabled with caution.  Users with the
             ability to bypass file permissions on the remote host (for the
             user's X authorization database) can access the local X11 display
             through the forwarded connection.  An attacker may then be able to
             perform activities such as keystroke monitoring.

             For this reason, X11 forwarding is subjected to X11 SECURITY
             extension restrictions by default.  Please refer to the ssh -Y
             option and the ForwardX11Trusted directive in ssh_config(5) for
             more information.

     -x      Disables X11 forwarding.

     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
             subjected to the X11 SECURITY extension controls.

     -y      Send log information using the syslog(3) system module.  By default
             this information is sent to stderr.

     ssh may additionally obtain configuration data from a per-user
     configuration file and a system-wide configuration file.  The file format
     and configuration options are described in ssh_config(5).

AUTHENTICATION
     The OpenSSH SSH client supports SSH protocol 2.

     The methods available for authentication are: GSSAPI-based authentication,
     host-based authentication, public key authentication, challenge-response
     authentication, and password authentication.  Authentication methods are
     tried in the order specified above, though PreferredAuthentications can be
     used to change the default order.

     Host-based authentication works as follows: If the machine the user logs in
     from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote
     machine, the user is non-root and the user names are the same on both
     sides, or if the files ~/.rhosts or ~/.shosts exist in the user's home
     directory on the remote machine and contain a line containing the name of
     the client machine and the name of the user on that machine, the user is
     considered for login.  Additionally, the server must be able to verify the
     client's host key (see the description of /etc/ssh/ssh_known_hosts and
     ~/.ssh/known_hosts, below) for login to be permitted.  This authentication
     method closes security holes due to IP spoofing, DNS spoofing, and routing
     spoofing.  [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
     rlogin/rsh protocol in general, are inherently insecure and should be
     disabled if security is desired.]

     Public key authentication works as follows: The scheme is based on public-
     key cryptography, using cryptosystems where encryption and decryption are
     done using separate keys, and it is unfeasible to derive the decryption key
     from the encryption key.  The idea is that each user creates a
     public/private key pair for authentication purposes.  The server knows the
     public key, and only the user knows the private key.  ssh implements public
     key authentication protocol automatically, using one of the DSA, ECDSA,
     Ed25519 or RSA algorithms.  The HISTORY section of ssl(8) contains a brief
     discussion of the DSA and RSA algorithms.

     The file ~/.ssh/authorized_keys lists the public keys that are permitted
     for logging in.  When the user logs in, the ssh program tells the server
     which key pair it would like to use for authentication.  The client proves
     that it has access to the private key and the server checks that the
     corresponding public key is authorized to accept the account.

     The server may inform the client of errors that prevented public key
     authentication from succeeding after authentication completes using a
     different method.  These may be viewed by increasing the LogLevel to DEBUG
     or higher (e.g. by using the -v flag).

     The user creates his/her key pair by running ssh-keygen(1).  This stores
     the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
     ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
     (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
     ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
     ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
     ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub
     (authenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
     home directory.  The user should then copy the public key to
     ~/.ssh/authorized_keys in his/her home directory on the remote machine.
     The authorized_keys file corresponds to the conventional ~/.rhosts file,
     and has one key per line, though the lines can be very long.  After this,
     the user can log in without giving the password.

     A variation on public key authentication is available in the form of
     certificate authentication: instead of a set of public/private keys, signed
     certificates are used.  This has the advantage that a single trusted
     certification authority can be used in place of many public/private keys.
     See the CERTIFICATES section of ssh-keygen(1) for more information.

     The most convenient way to use public key or certificate authentication may
     be with an authentication agent.  See ssh-agent(1) and (optionally) the
     AddKeysToAgent directive in ssh_config(5) for more information.

     Challenge-response authentication works as follows: The server sends an
     arbitrary "challenge" text, and prompts for a response.  Examples of
     challenge-response authentication include BSD Authentication (see
     login.conf(5)) and PAM (some non-OpenBSD systems).

     Finally, if other authentication methods fail, ssh prompts the user for a
     password.  The password is sent to the remote host for checking; however,
     since all communications are encrypted, the password cannot be seen by
     someone listening on the network.

     ssh automatically maintains and checks a database containing identification
     for all hosts it has ever been used with.  Host keys are stored in
     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file
     /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any new
     hosts are automatically added to the user's file.  If a host's
     identification ever changes, ssh warns about this and disables password
     authentication to prevent server spoofing or man-in-the-middle attacks,
     which could otherwise be used to circumvent the encryption.  The
     StrictHostKeyChecking option can be used to control logins to machines
     whose host key is not known or has changed.

     When the user's identity has been accepted by the server, the server either
     executes the given command in a non-interactive session or, if no command
     has been specified, logs into the machine and gives the user a normal shell
     as an interactive session.  All communication with the remote command or
     shell will be automatically encrypted.

     If an interactive session is requested ssh by default will only request a
     pseudo-terminal (pty) for interactive sessions when the client has one.
     The flags -T and -t can be used to override this behaviour.

     If a pseudo-terminal has been allocated the user may use the escape
     characters noted below.

     If no pseudo-terminal has been allocated, the session is transparent and
     can be used to reliably transfer binary data.  On most systems, setting the
     escape character to “none” will also make the session transparent even if a
     tty is used.

     The session terminates when the command or shell on the remote machine
     exits and all X11 and TCP connections have been closed.

ESCAPE CHARACTERS
     When a pseudo-terminal has been requested, ssh supports a number of
     functions through the use of an escape character.

     A single tilde character can be sent as ~~ or by following the tilde by a
     character other than those described below.  The escape character must
     always follow a newline to be interpreted as special.  The escape character
     can be changed in configuration files using the EscapeChar configuration
     directive or on the command line by the -e option.

     The supported escapes (assuming the default ‘~’) are:

     ~.      Disconnect.

     ~^Z     Background ssh.

     ~#      List forwarded connections.

     ~&      Background ssh at logout when waiting for forwarded connection /
             X11 sessions to terminate.

     ~?      Display a list of escape characters.

     ~B      Send a BREAK to the remote system (only useful if the peer supports
             it).

     ~C      Open command line.  Currently this allows the addition of port
             forwardings using the -L, -R and -D options (see above).  It also
             allows the cancellation of existing port-forwardings with
             -KL[bind_address:]port for local, -KR[bind_address:]port for remote
             and -KD[bind_address:]port for dynamic port-forwardings.  !command
             allows the user to execute a local command if the
             PermitLocalCommand option is enabled in ssh_config(5).  Basic help
             is available, using the -h option.

     ~R      Request rekeying of the connection (only useful if the peer
             supports it).

     ~V      Decrease the verbosity (LogLevel) when errors are being written to
             stderr.

     ~v      Increase the verbosity (LogLevel) when errors are being written to
             stderr.

TCP FORWARDING
     Forwarding of arbitrary TCP connections over a secure channel can be
     specified either on the command line or in a configuration file.  One
     possible application of TCP forwarding is a secure connection to a mail
     server; another is going through firewalls.

     In the example below, we look at encrypting communication for an IRC
     client, even though the IRC server it connects to does not directly support
     encrypted communication.  This works as follows: the user connects to the
     remote host using ssh, specifying the ports to be used to forward the
     connection.  After that it is possible to start the program locally, and
     ssh will encrypt and forward the connection to the remote server.

     The following example tunnels an IRC session from the client to an IRC
     server at “server.example.com”, joining channel “#users”, nickname “pinky”,
     using the standard IRC port, 6667:

         $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
         $ irc -c '#users' pinky IRC/127.0.0.1

     The -f option backgrounds ssh and the remote command “sleep 10” is
     specified to allow an amount of time (10 seconds, in the example) to start
     the program which is going to use the tunnel.  If no connections are made
     within the time specified, ssh will exit.

X11 FORWARDING
     If the ForwardX11 variable is set to “yes” (or see the description of the
     -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
     environment variable is set), the connection to the X11 display is
     automatically forwarded to the remote side in such a way that any X11
     programs started from the shell (or command) will go through the encrypted
     channel, and the connection to the real X server will be made from the
     local machine.  The user should not manually set DISPLAY.  Forwarding of
     X11 connections can be configured on the command line or in configuration
     files.

     The DISPLAY value set by ssh will point to the server machine, but with a
     display number greater than zero.  This is normal, and happens because ssh
     creates a “proxy” X server on the server machine for forwarding the
     connections over the encrypted channel.

     ssh will also automatically set up Xauthority data on the server machine.
     For this purpose, it will generate a random authorization cookie, store it
     in Xauthority on the server, and verify that any forwarded connections
     carry this cookie and replace it by the real cookie when the connection is
     opened.  The real authentication cookie is never sent to the server machine
     (and no cookies are sent in the plain).

     If the ForwardAgent variable is set to “yes” (or see the description of the
     -A and -a options above) and the user is using an authentication agent, the
     connection to the agent is automatically forwarded to the remote side.

VERIFYING HOST KEYS
     When connecting to a server for the first time, a fingerprint of the
     server's public key is presented to the user (unless the option
     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
     using ssh-keygen(1):

           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

     If the fingerprint is already known, it can be matched and the key can be
     accepted or rejected.  If only legacy (MD5) fingerprints for the server are
     available, the ssh-keygen(1) -E option may be used to downgrade the
     fingerprint algorithm to match.

     Because of the difficulty of comparing host keys just by looking at
     fingerprint strings, there is also support to compare host keys visually,
     using random art.  By setting the VisualHostKey option to “yes”, a small
     ASCII graphic gets displayed on every login to a server, no matter if the
     session itself is interactive or not.  By learning the pattern a known
     server produces, a user can easily find out that the host key has changed
     when a completely different pattern is displayed.  Because these patterns
     are not unambiguous however, a pattern that looks similar to the pattern
     remembered only gives a good probability that the host key is the same, not
     guaranteed proof.

     To get a listing of the fingerprints along with their random art for all
     known hosts, the following command line can be used:

           $ ssh-keygen -lv -f ~/.ssh/known_hosts

     If the fingerprint is unknown, an alternative method of verification is
     available: SSH fingerprints verified by DNS.  An additional resource record
     (RR), SSHFP, is added to a zonefile and the connecting client is able to
     match the fingerprint with that of the key presented.

     In this example, we are connecting a client to a server,
     “host.example.com”.  The SSHFP resource records should first be added to
     the zonefile for host.example.com:

           $ ssh-keygen -r host.example.com.

     The output lines will have to be added to the zonefile.  To check that the
     zone is answering fingerprint queries:

           $ dig -t SSHFP host.example.com

     Finally the client connects:

           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
           [...]
           Matching host key fingerprint found in DNS.
           Are you sure you want to continue connecting (yes/no)?

     See the VerifyHostKeyDNS option in ssh_config(5) for more information.

SSH-BASED VIRTUAL PRIVATE NETWORKS
     ssh contains support for Virtual Private Network (VPN) tunnelling using the
     tun(4) network pseudo-device, allowing two networks to be joined securely.
     The sshd_config(5) configuration option PermitTunnel controls whether the
     server supports this, and at what level (layer 2 or 3 traffic).

     The following example would connect client network 10.0.50.0/24 with remote
     network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to
     10.1.1.2, provided that the SSH server running on the gateway to the remote
     network, at 192.168.1.15, allows it.

     On the client:

           # ssh -f -w 0:1 192.168.1.15 true
           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
           # route add 10.0.99.0/24 10.1.1.2

     On the server:

           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
           # route add 10.0.50.0/24 10.1.1.1

     Client access may be more finely tuned via the /root/.ssh/authorized_keys
     file (see below) and the PermitRootLogin server option.  The following
     entry would permit connections on tun(4) device 1 from user “jane” and on
     tun device 2 from user “john”, if PermitRootLogin is set to
     “forced-commands-only”:

       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

     Since an SSH-based setup entails a fair amount of overhead, it may be more
     suited to temporary setups, such as for wireless VPNs.  More permanent VPNs
     are better provided by tools such as ipsecctl(8) and isakmpd(8).

ENVIRONMENT
     ssh will normally set the following environment variables:

     DISPLAY               The DISPLAY variable indicates the location of the
                           X11 server.  It is automatically set by ssh to point
                           to a value of the form “hostname:n”, where “hostname”
                           indicates the host where the shell runs, and ‘n’ is
                           an integer ≥ 1.  ssh uses this special value to
                           forward X11 connections over the secure channel.  The
                           user should normally not set DISPLAY explicitly, as
                           that will render the X11 connection insecure (and
                           will require the user to manually copy any required
                           authorization cookies).

     HOME                  Set to the path of the user's home directory.

     LOGNAME               Synonym for USER; set for compatibility with systems
                           that use this variable.

     MAIL                  Set to the path of the user's mailbox.

     PATH                  Set to the default PATH, as specified when compiling
                           ssh.

     SSH_ASKPASS           If ssh needs a passphrase, it will read the
                           passphrase from the current terminal if it was run
                           from a terminal.  If ssh does not have a terminal
                           associated with it but DISPLAY and SSH_ASKPASS are
                           set, it will execute the program specified by
                           SSH_ASKPASS and open an X11 window to read the
                           passphrase.  This is particularly useful when calling
                           ssh from a .xsession or related script.  (Note that
                           on some machines it may be necessary to redirect the
                           input from /dev/null to make this work.)

     SSH_ASKPASS_REQUIRE   Allows further control over the use of an askpass
                           program.  If this variable is set to “never” then ssh
                           will never attempt to use one.  If it is set to
                           “prefer”, then ssh will prefer to use the askpass
                           program instead of the TTY when requesting passwords.
                           Finally, if the variable is set to “force”, then the
                           askpass program will be used for all passphrase input
                           regardless of whether DISPLAY is set.

     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to
                           communicate with the agent.

     SSH_CONNECTION        Identifies the client and server ends of the
                           connection.  The variable contains four space-
                           separated values: client IP address, client port
                           number, server IP address, and server port number.

     SSH_ORIGINAL_COMMAND  This variable contains the original command line if a
                           forced command is executed.  It can be used to
                           extract the original arguments.

     SSH_TTY               This is set to the name of the tty (path to the
                           device) associated with the current shell or command.
                           If the current session has no tty, this variable is
                           not set.

     SSH_TUNNEL            Optionally set by sshd(8) to contain the interface
                           names assigned if tunnel forwarding was requested by
                           the client.

     SSH_USER_AUTH         Optionally set by sshd(8), this variable may contain
                           a pathname to a file that lists the authentication
                           methods successfully used when the session was
                           established, including any public keys that were
                           used.

     TZ                    This variable is set to indicate the present time
                           zone if it was set when the daemon was started (i.e.
                           the daemon passes the value on to new connections).

     USER                  Set to the name of the user logging in.

     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
     “VARNAME=value” to the environment if the file exists and users are allowed
     to change their environment.  For more information, see the
     PermitUserEnvironment option in sshd_config(5).

FILES
     ~/.rhosts
             This file is used for host-based authentication (see above).  On
             some machines this file may need to be world-readable if the user's
             home directory is on an NFS partition, because sshd(8) reads it as
             root.  Additionally, this file must be owned by the user, and must
             not have write permissions for anyone else.  The recommended
             permission for most machines is read/write for the user, and not
             accessible by others.

     ~/.shosts
             This file is used in exactly the same way as .rhosts, but allows
             host-based authentication without permitting login with rlogin/rsh.

     ~/.ssh/
             This directory is the default location for all user-specific
             configuration and authentication information.  There is no general
             requirement to keep the entire contents of this directory secret,
             but the recommended permissions are read/write/execute for the
             user, and not accessible by others.

     ~/.ssh/authorized_keys
             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
             for logging in as this user.  The format of this file is described
             in the sshd(8) manual page.  This file is not highly sensitive, but
             the recommended permissions are read/write for the user, and not
             accessible by others.

     ~/.ssh/config
             This is the per-user configuration file.  The file format and
             configuration options are described in ssh_config(5).  Because of
             the potential for abuse, this file must have strict permissions:
             read/write for the user, and not writable by others.

     ~/.ssh/environment
             Contains additional definitions for environment variables; see
             ENVIRONMENT, above.

     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ecdsa_sk
     ~/.ssh/id_ed25519
     ~/.ssh/id_ed25519_sk
     ~/.ssh/id_rsa
             Contains the private key for authentication.  These files contain
             sensitive data and should be readable by the user but not
             accessible by others (read/write/execute).  ssh will simply ignore
             a private key file if it is accessible by others.  It is possible
             to specify a passphrase when generating the key which will be used
             to encrypt the sensitive part of this file using AES-128.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ecdsa_sk.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_ed25519_sk.pub
     ~/.ssh/id_rsa.pub
             Contains the public key for authentication.  These files are not
             sensitive and can (but need not) be readable by anyone.

     ~/.ssh/known_hosts
             Contains a list of host keys for all hosts the user has logged into
             that are not already in the systemwide list of known host keys.
             See sshd(8) for further details of the format of this file.

     ~/.ssh/rc
             Commands in this file are executed by ssh when the user logs in,
             just before the user's shell (or command) is started.  See the
             sshd(8) manual page for more information.

     /etc/hosts.equiv
             This file is for host-based authentication (see above).  It should
             only be writable by root.

     /etc/ssh/shosts.equiv
             This file is used in exactly the same way as hosts.equiv, but
             allows host-based authentication without permitting login with
             rlogin/rsh.

     /etc/ssh/ssh_config
             Systemwide configuration file.  The file format and configuration
             options are described in ssh_config(5).

     /etc/ssh/ssh_host_key
     /etc/ssh/ssh_host_dsa_key
     /etc/ssh/ssh_host_ecdsa_key
     /etc/ssh/ssh_host_ed25519_key
     /etc/ssh/ssh_host_rsa_key
             These files contain the private parts of the host keys and are used
             for host-based authentication.

     /etc/ssh/ssh_known_hosts
             Systemwide list of known host keys.  This file should be prepared
             by the system administrator to contain the public host keys of all
             machines in the organization.  It should be world-readable.  See
             sshd(8) for further details of the format of this file.

     /etc/ssh/sshrc
             Commands in this file are executed by ssh when the user logs in,
             just before the user's shell (or command) is started.  See the
             sshd(8) manual page for more information.

EXIT STATUS
     ssh exits with the exit status of the remote command or with 255 if an
     error occurred.

SEE ALSO
     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

STANDARDS
     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
     Numbers, RFC 4250, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC
     4251, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
     RFC 4252, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol,
     RFC 4253, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
     4254, January 2006.

     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
     (SSH) Key Fingerprints, RFC 4255, January 2006.

     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
     Secure Shell Protocol (SSH), RFC 4256, January 2006.

     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
     Extension, RFC 4335, January 2006.

     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
     Layer Encryption Modes, RFC 4344, January 2006.

     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
     Layer Protocol, RFC 4345, January 2006.

     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the
     Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.

     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format,
     RFC 4716, November 2006.

     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure
     Shell Transport Layer, RFC 5656, December 2009.

     A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-
     World Security, 1999, International Workshop on Cryptographic Techniques
     and E-Commerce (CrypTEC '99).

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu
     Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de
     Raadt and Dug Song removed many bugs, re-added newer features and created
     OpenSSH.  Markus Friedl contributed the support for SSH protocol versions
     1.5 and 2.0.

BSD                             February 15, 2021                            BSD