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+SSH(1)                     OpenBSD Reference 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) 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
+                   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
+                   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, 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/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) 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.
+
+     ~/.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/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-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.
+
+     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.
+
+OpenBSD 4.9                    November 18, 2010                   OpenBSD 4.9