ssh.0

SSH(1)                     OpenBSD Reference Manual                     SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-1246AaCfgKkMNnqsTtVvXxY] [-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 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 untrust-
     ed 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 be-
     low).

     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 ad-
             dress.

     -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 dif-
             ferent 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 interoperabil-
             ity with legacy protocol 1 implementations that do not support
             the 3des cipher.  Its use is strongly discouraged due to crypto-
             graphic 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, blow-
             fish-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.  Dy-
             namic 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 brack-
             ets.  Only the superuser can forward privileged ports.  By de-
             fault, 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 `*' indi-
             cates 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 con-
             figuration 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 passphras-
             es, 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 it-
             self in the background.

     -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 (de-
             fault 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 pro-
             tocol 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 config-
             uration files).

     -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 con-
             nection 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 spec-
             ified with an alternative syntax: [bind_address/]port/host/host-
             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.  How-
             ever, an explicit bind_address may be used to bind the connection
             to a specific address.  The bind_address of ``localhost'' indi-
             cates that the listening port be bound for local use only, while
             an empty address or `*' indicates that the port should be avail-
             able 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 shar-
             ing.  Multiple -M options places ssh into ``master'' mode with
             confirmation required before slave connections are accepted.  Re-
             fer to the description of ControlMaster in ssh_config(5) for de-
             tails.

     -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 for-
             warding 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 ma-
             chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
             an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
             tomatically 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 configura-
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.  For full details of the op-
             tions 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
                   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
                   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 connec-
             tion 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 `*', indi-
             cates that the remote socket should listen on all interfaces.
             Specifying a remote bind_address will only succeed if the serv-
             er's GatewayPorts option is enabled (see sshd_config(5)).

     -S ctl_path
             Specifies the location of a control socket for connection shar-
             ing.  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 fa-
             cilitate the use of SSH as a secure transport for other applica-
             tions (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 arbi-
             trary 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, authentica-
             tion, and configuration problems.  Multiple -v options increase
             the verbosity.  The maximum is 3.

     -w local_tun[:remote_tun]
             Requests tunnel device forwarding with the specified tun(4) de-
             vices 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 ex-
             tension restrictions by default.  Please refer to the ssh -Y op-
             tion 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 configura-
     tion file and a system-wide configuration file.  The file format and con-
     figuration 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 pre-
     ferred since it provides additional mechanisms for confidentiality (the
     traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
     integrity (hmac-md5, hmac-sha1, 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 authentica-
     tion, host-based authentication, public key authentication, challenge-re-
     sponse authentication, and password authentication.  Authentication meth-
     ods are tried in the order specified above, though protocol 2 has a con-
     figuration 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/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 se-
     curity 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 pub-
     lic-key cryptography, using cryptosystems where encryption and decryption
     are done using separate keys, and it is unfeasible to derive the decryp-
     tion 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 us-
     er should then copy the public key to ~/.ssh/authorized_keys in his/her
     home directory on the remote machine.  The authorized_keys file corre-
     sponds 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 with-
     out 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 al-
     lows 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 identifica-
     tion 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 iden-
     tification ever changes, ssh warns about this and disables password au-
     thentication 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 ei-
     ther 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 func-
     tions 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 charac-
     ter can be changed in configuration files using the EscapeChar configura-
     tion 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 al-
             lows 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 con-
     nections 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 mat-
     ter 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 auto-
     matically forwarded to the remote side in such a way that any X11 pro-
     grams 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 re-
     mote 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.  Be-
     cause 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 se-
     curely.  The sshd_config(5) configuration option PermitTunnel controls
     whether the server supports this, and at what level (layer 2 or 3 traf-
     fic).

     The following example would connect client network 10.0.50.0/24 with re-
     mote 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 perma-
     nent 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 sys-
                           tems that use this variable.

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

     PATH                  Set to the default PATH, as specified when compil-
                           ing 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 call-
                           ing ssh from a .xsession or related script.  (Note
                           that on some machines it may be necessary to redi-
                           rect 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 con-
                           nection.  The variable contains four space-separat-
                           ed 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 de-
                           vice) 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 connec-
                           tions).

     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 al-
     lowed 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 us-
             er'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 recom-
             mended 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 con-
             figuration 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 (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 ac-
             cessible 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 acces-
             sible 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/shosts.equiv
             This file is used in exactly the same way as hosts.equiv, but al-
             lows 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 on-
             ly by root.  For protocol version 2, ssh uses ssh-keysign(8) to
             access the host keys, eliminating the requirement that ssh be se-
             tuid 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.

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)

     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.

     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.

OpenBSD 4.4                      July 2, 2008                               14