ssh

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

NAME
     ssh — OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D
         [bind_address:]port] [-e escape_char] [-F configfile]
         [-i identity_file] [-L  [bind_address:]port:host:hostport]
         [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R
         [bind_address:]port:host:hostport] [-S ctl_path] [-w tunnel:tunnel]
         [user@]hostname [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 replace rlogin
     and rsh, and provide secure encrypted communications between two untrusted
     hosts over an insecure network.  X11 connections and arbitrary TCP ports
     can also be forwarded over the secure channel.

     ssh connects and logs into the specified hostname (with optional user
     name).  The user must prove his/her identity to the remote machine using
     one of several methods depending on the protocol version used (see below).

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

     The options are as follows:

     -1      Forces ssh to try protocol version 1 only.

     -2      Forces ssh to try protocol version 2 only.

     -4      Forces ssh to use IPv4 addresses only.

     -6      Forces ssh to use IPv6 addresses only.

     -A      Enables forwarding of the authentication agent connection.  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      Disables forwarding of the authentication agent connection.

     -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 and TCP connections).  The compression
             algorithm is the same used by gzip(1), and the “level” can be
             controlled by the CompressionLevel option for protocol version 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.

             Protocol version 1 allows specification of a single cipher.  The
             supported values are “3des”, “blowfish”, and “des”.  3des (triple-
             des) is an encrypt-decrypt-encrypt triple with three different
             keys.  It is believed to be secure.  blowfish is a fast block
             cipher; it appears very secure and is much faster than 3des.  des
             is only supported in the ssh client for interoperability with
             legacy protocol 1 implementations that do not support the 3des
             cipher.  Its use is strongly discouraged due to cryptographic
             weaknesses.  The default is “3des”.

             For protocol version 2, cipher_spec is a comma-separated list of
             ciphers listed in order of preference.  The supported ciphers are:
             3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,
             aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blowfish-
             cbc, and cast128-cbc.  The default is:

                   aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
                   arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
                   aes192-ctr,aes256-ctr

     -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 with an alternative syntax:
             [bind_address/]port or 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 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.

     -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.

     -g      Allows remote hosts to connect to local forwarded ports.

     -I smartcard_device
             Specify the device ssh should use to communicate with a smartcard
             used for storing the user's private RSA key.  This option is only
             available if support for smartcard devices is compiled in (default
             is no support).

     -i identity_file
             Selects a file from which the identity (private key) for RSA or DSA
             authentication is read.  The default is ~/.ssh/identity for
             protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for
             protocol version 2.  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).

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

     -L [bind_address:]port:host:hostport
             Specifies that the given port on the local (client) host is to be
             forwarded to the given host and port on the remote side.  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 a connection is made to host port hostport
             from the remote machine.  Port forwardings can also be specified in
             the configuration file.  IPv6 addresses can be specified with an
             alternative syntax: [bind_address/]port/host/hostport or 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.

     -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 with confirmation
             required before slave connections are accepted.  Refer to the
             description of ControlMaster in ssh_config(5) for details.

     -m mac_spec
             Additionally, for protocol version 2 a comma-separated list of MAC
             (message authentication code) algorithms can be 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 (protocol version 2 only).

     -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) and “exit” (request the master
             to exit).

     -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).

                   AddressFamily
                   BatchMode
                   BindAddress
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   DynamicForward
                   EscapeChar
                   ForwardAgent
                   ForwardX11
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   KbdInteractiveDevices
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   SmartcardDevice
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UsePrivilegedPort
                   User
                   UserKnownHostsFile
                   VerifyHostKeyDNS
                   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      Quiet mode.  Causes all warning and diagnostic messages to be
             suppressed.

     -R [bind_address:]port:host:hostport
             Specifies that the given port on the remote (server) host is to be
             forwarded to the given host and port on the local side.  This works
             by allocating a socket to listen to port on the remote side, and
             whenever a connection is made to this port, the connection is
             forwarded over the secure channel, and a connection is made to host
             port hostport from the local machine.

             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 braces or using an alternative syntax:
             [bind_address/]host/port/hostport.

             By default, the listening socket on the server will be bound to the
             loopback interface only.  This may be overriden 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)).

     -S ctl_path
             Specifies the location of a control socket for 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 are a feature of the SSH2 protocol which
             facilitate the use of SSH as a secure transport for other
             applications (eg. sftp(1)).  The subsystem is specified as the
             remote command.

     -T      Disable pseudo-tty allocation.

     -t      Force pseudo-tty 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 tunnel:tunnel
             Requests a tun(4) device on the client (first tunnel arg) and
             server (second tunnel arg).  The devices may be specified by
             numerical ID or the keyword “any”, which uses the next available
             tunnel device.  See also the Tunnel directive in ssh_config(5).

     -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.

     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).

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

AUTHENTICATION
     The OpenSSH SSH client supports SSH protocols 1 and 2.  Protocol 2 is the
     default, with ssh falling back to protocol 1 if it detects protocol 2 is
     unsupported.  These settings may be altered using the Protocol option in
     ssh_config(5), or enforced using the -1 and -2 options (see above).  Both
     protocols support similar authentication methods, but protocol 2 is
     preferred since it provides additional mechanisms for confidentiality (the
     traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
     integrity (hmac-md5, hmac-sha1, hmac-ripemd160).  Protocol 1 lacks a strong
     mechanism for ensuring the integrity of the connection.

     The methods available for authentication are: host-based authentication,
     public key authentication, challenge-response authentication, and password
     authentication.  Authentication methods are tried in the order specified
     above, though protocol 2 has a configuration option to change the default
     order: PreferredAuthentications.

     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, 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 either the RSA or DSA
     algorithms.  Protocol 1 is restricted to using only RSA keys, but protocol
     2 may use either.  The HISTORY section of ssl(8) contains a brief
     discussion of the two 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 user creates his/her key pair by running ssh-keygen(1).  This stores
     the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2
     DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
     ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
     ~/.ssh/id_rsa.pub (protocol 2 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.

     The most convenient way to use public key authentication may be with an
     authentication agent.  See ssh-agent(1) for more information.

     Challenge-response authentication works as follows: The server sends an
     arbitrary "challenge" text, and prompts for a response.  Protocol 2 allows
     multiple challenges and responses; protocol 1 is restricted to just one
     challenge/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, or logs into the machine and gives the user a
     normal shell on the remote machine.  All communication with the remote
     command or shell will be automatically encrypted.

     If a pseudo-terminal has been allocated (normal login session), the user
     may use the escape characters noted below.

     If no pseudo-tty 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 for SSH protocol
             version 2 and if the peer supports it).

     ~C      Open command line.  Currently this allows the addition of port
             forwardings using the -L and -R options (see above).  It also
             allows the cancellation of existing remote port-forwardings using
             -KR hostport.  !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 for SSH protocol
             version 2 and if the peer supports it).

TCP FORWARDING
     Forwarding of arbitrary TCP connections over the 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 between an IRC
     client and server, even though the IRC server does not directly support
     encrypted communications.  This works as follows: the user connects to the
     remote host using ssh, specifying a port to be used to forward connections
     to the remote server.  After that it is possible to start the service which
     is to be encrypted on the client machine, connecting to the same local
     port, and ssh will encrypt and forward the connection.

     The following example tunnels an IRC session from client machine
     “127.0.0.1” (localhost) to remote server “server.example.com”:

         $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
         $ irc -c '#users' -p 1234 pinky 127.0.0.1

     This tunnels a connection to IRC server “server.example.com”, joining
     channel “#users”, nickname “pinky”, using port 1234.  It doesn't matter
     which port is used, as long as it's greater than 1023 (remember, only root
     can open sockets on privileged ports) and doesn't conflict with any ports
     already in use.  The connection is forwarded to port 6667 on the remote
     server, since that's the standard port for IRC services.

     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 service which is to be tunnelled.  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 verified, and
     the key can be accepted.  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 -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com.
           $ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -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, provided that the SSH server running on the gateway
     to the remote network, at 192.168.1.15, allows it:

           # ssh -f -w 0:1 192.168.1.15 true
           # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252

     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 the first tun(4) device from user “jane”
     and on the second device 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 tun1" ssh-rsa ... john

     Since a 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_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.

     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).

ENVIRONMENT
     SSH_USE_STRONG_RNG
             The reseeding of the OpenSSL random generator is usually done from
             /dev/urandom.  If the SSH_USE_STRONG_RNG environment variable is
             set to value other than 0 the OpenSSL random generator is reseeded
             from /dev/random.  The number of bytes read is defined by the
             SSH_USE_STRONG_RNG value.  Minimum is 6 bytes.  This setting is not
             recommended on the computers without the hardware random generator
             because insufficient entropy causes the connection to be blocked
             until enough entropy is available.

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/authorized_keys
             Lists the public keys (RSA/DSA) 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 accessible by others.

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

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.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 3DES.

     ~/.ssh/identity.pub
     ~/.ssh/id_dsa.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_rsa_key
             These three files contain the private parts of the host keys and
             are used for host-based authentication.  If protocol version 1 is
             used, ssh must be setuid root, since the host key is readable only
             by root.  For protocol version 2, ssh uses ssh-keysign(8) to access
             the host keys, eliminating the requirement that ssh be setuid root
             when host-based authentication is used.  By default ssh is not
             setuid root.

     /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.

IPV6
     IPv6 address can be used everywhere where IPv4 address. In all entries must
     be the IPv6 address enclosed in square brackets. Note: The square brackets
     are metacharacters for the shell and must be escaped in shell.

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

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH Protocol
     Architecture, draft-ietf-secsh-architecture-12.txt, January 2002, work in
     progress material.

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                            September 25, 1999                            BSD