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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 for-warded forwarded
     warded 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 permis-sions permissions
             sions 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, how-ever however
             ever they can perform operations on the keys that enable them to authenticate using the identi-ties identities
             ties 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 desir-able desirable
             able on modem lines and other slow connections, but will only slow down things on fast net-works. networks.
             works.  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 interoper-ability interoperability
             ability 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 chan-nel, channel,
             nel, 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 speci-fied specified
             fied in the configuration file.

             IPv6 addresses can be specified by enclosing the address in square brackets.  Only the supe-ruser superuser
             ruser 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 configu-ration configuration
             ration 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 accor-dance accordance
             dance 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 listen-ing listening
             ing 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 perhost
             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 infor-mation. information.
             mation.

     -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 shad-ows.cs.hut.fi, shadows.cs.hut.fi,
             ows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted chan-nel. channel.
             nel.  The ssh program will be put in the background.  (This does not work if ssh needs to ask
             for a password or passphrase; see also the -f option.)

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

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

                   AddressFamily
                   BatchMode
                   BindAddress
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   IPQoS
                   KbdInteractiveAuthentication
                   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 con-figuration configuration
             figuration 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 for-warded forwarded
             warded only when logging in as root on the remote machine.  IPv6 addresses can be specified by
             enclosing the address in square brackets.

             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 fea-ture feature
             ture 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.  Works with Pro-tocol Protocol
             tocol 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 avail-able available
             able 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 permis-sions permissions
             sions 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, umac-128, hmac-ripemd160). hmacripemd160).
     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.  Authentica-tion Authentication
     tion 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 cryp-tosystems cryptosystems
     tosystems 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 (pro-tocol (protocol
     tocol 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 cor-responds corresponds
     responds 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 Authentica-tion Authentication
     tion (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.  Addition-ally, Additionally,
     ally, 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 con-nections connections
     nections 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 sup-ports supports
             ports it).

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

     ~R      Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer sup-ports supports
             ports it).

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

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

TCP FORWARDING
     Forwarding of arbitrary TCP connections over 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 con-nections connections
     nections 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 for-warded forwarded
     warded 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 deter-mined determined
     mined 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 prob-ability probability
     ability 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 finger-print fingerprint
     print 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, pseudodevice,
     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 automati-cally automatically
                           cally 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 cur-rent current
                           rent 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 envi-ronment environment
     ronment 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 files contain the private parts of the host keys and are used for host-based authentica-tion. authentication.
             tion.  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, elimi-nating eliminating
             nating 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-read-able. world-readable.
             able.  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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

BSD                            October 11, 2013                            BSD

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