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

NAME

     ssh — OpenSSH SSH client (remote login program)

SYNOPSIS

     ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11]
         [-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 host:port] [-w local_tun[:remote_tun]] [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.  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 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.

             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      Allows remote hosts to connect to local forwarded ports.

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user's private RSA key.

     -i identity_file
             Selects a file from which the identity (private key) for public key authentication is read.  The default is ~/.ssh/identity for
             protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and ~/.ssh/id_rsa 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).  ssh will also try to load certificate information from the filename obtained by appending
             -cert.pub to identity filenames.

     -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
             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 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), “forward” (request
             forwardings without command execution), “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).

                   AddressFamily
                   BatchMode
                   BindAddress
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   ForwardAgent
                   ForwardX11
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   IPQoS
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   RequestTTY
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UsePrivilegedPort
                   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      Quiet mode.  Causes most 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.

             By default, the listening socket 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 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 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 and works with Protocol version 2 only.

     -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 is set to the default tunnel mode, which is “point-to-point”.

     -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 protocols 1 and 2.  The default is to use protocol 2 only, though this can be changed via the
     Protocol option in ssh_config(5) or the -1 and -2 options (see above).  Both protocols support similar authentication methods, but
     protocol 2 is the default 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-sha2-256, hmac-sha2-512, umac-64, hmac-ripemd160).  Protocol 1
     lacks a strong mechanism for ensuring the integrity of the connection.

     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
     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 one of the DSA, ECDSA or RSA algorithms.  Protocol 1
     is restricted to using only RSA keys, but protocol 2 may use any.  The HISTORY section of ssl(8) (on non-OpenBSD systems, see
     http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) 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 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), ~/.ssh/id_ecdsa (protocol 2 ECDSA), 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), ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), 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.

     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) 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, -R and -D options (see above).  It also
             allows the cancellation of existing remote port-forwardings using -KR[bind_address:]port.  !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 the key can be accepted or rejected.  Because of the difficulty of comparing
     host keys just by looking at hex 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_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).

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/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 accessible by others.  It may
             be group-writable provided that the group in question contains only the user.

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

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.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_ecdsa.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_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.

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-argv0(1), ssh-keygen(1), ssh-keyscan(1), ssh-vulnkey(1), tun(4), hosts.equiv(5),
     ssh_config(5), ssh-keysign(8), sshd(8)

     The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.

     The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.

     The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.

     The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.

     The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.

     Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.

     Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, 2006.

     The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.

     The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.

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

     Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, 2006.

     The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

     Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, 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.
 

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