NAME | SYNOPSIS | DESCRIPTION | AUTHENTICATION | ESCAPE CHARACTERS | TCP FORWARDING | X11 FORWARDING | VERIFYING HOST KEYS | SSH-BASED VIRTUAL PRIVATE NETWORKS | ENVIRONMENT | FILES | EXIT STATUS | SEE ALSO | STANDARDS | AUTHORS | COLOPHON

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

NAME         top

     ssh — OpenSSH SSH client (remote login program)

SYNOPSIS         top

     ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
         [-D [bind_address:]port] [-E log_file] [-e escape_char]
         [-F configfile] [-I pkcs11] [-i identity_file]
         [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec]
         [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
         [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
         [user@]hostname [command]

DESCRIPTION         top

     ssh (SSH client) is a program for logging into a remote machine and for
     executing commands on a remote machine.  It is intended to provide
     secure encrypted communications between two untrusted hosts over an
     insecure network.  X11 connections, arbitrary TCP ports and UNIX-domain
     sockets 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 (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 configura‐
             tion 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 opera‐
             tions 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, TCP and UNIX-domain connec‐
             tions).  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”.  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 sup‐
             ported, and ssh will act as a SOCKS server.  Only root can for‐
             ward 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 inter‐
             faces.

     -E log_file
             Append debug logs to log_file instead of standard error.

     -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.  Set‐
             ting 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      Causes ssh to print its configuration after evaluating Host and
             Match blocks and exit.

     -g      Allows remote hosts to connect to local forwarded ports.  If
             used on a multiplexed connection, then this option must be
             specified on the master process.

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to communi‐
             cate 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,
             ~/.ssh/id_ed25519 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).  If
             no certificates have been explicitly specified by the
             CertificateFile directive, ssh will also try to load certifi‐
             cate information from the filename obtained by appending
             -cert.pub to identity filenames.

     -J [user@]host[:port]
             Connect to the target host by first making a ssh connection to
             the jump host and then establishing a TCP forwarding to the
             ultimate destination from there.  Multiple jump hops may be
             specified separated by comma characters.  This is a shortcut to
             specify a ProxyJump configuration directive.

     -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
     -L [bind_address:]port:remote_socket
     -L local_socket:host:hostport
     -L local_socket:remote_socket
             Specifies that connections to the given TCP port or Unix socket
             on the local (client) host are to be forwarded to the given
             host and port, or Unix socket, on the remote side.  This works
             by allocating a socket to listen to either a TCP port on the
             local side, optionally bound to the specified bind_address, or
             to a Unix socket.  Whenever a connection is made to the local
             port or socket, the connection is forwarded over the secure
             channel, and a connection is made to either host port hostport,
             or the Unix socket remote_socket, from the remote machine.

             Port forwardings can also be specified in the configuration
             file.  Only the superuser can forward privileged ports.  IPv6
             addresses can be specified by enclosing the address in square
             brackets.

             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 inter‐
             faces.

     -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.
             Refer to the description of ControlMaster in ssh_config(5) for
             details.

     -m mac_spec
             A comma-separated list of MAC (message authentication code)
             algorithms, 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.

     -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), “cancel” (cancel for‐
             wardings), “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 configu‐
             ration 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).

                   AddKeysToAgent
                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   CertificateFile
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   FingerprintHash
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostbasedKeyTypes
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentitiesOnly
                   IdentityAgent
                   IdentityFile
                   Include
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   ProxyJump
                   ProxyUseFdpass
                   PubkeyAcceptedKeyTypes
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   RequestTTY
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   StreamLocalBindMask
                   StreamLocalBindUnlink
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UpdateHostKeys
                   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 query_option
             Queries ssh for the algorithms supported for the specified ver‐
             sion 2.  The available features are: cipher (supported symmet‐
             ric ciphers), cipher-auth (supported symmetric ciphers that
             support authenticated encryption), mac (supported message
             integrity codes), kex (key exchange algorithms), key (key
             types), key-cert (certificate key types), key-plain (non-cer‐
             tificate key types), and protocol-version (supported SSH proto‐
             col versions).

     -q      Quiet mode.  Causes most warning and diagnostic messages to be
             suppressed.

     -R [bind_address:]port:host:hostport
     -R [bind_address:]port:local_socket
     -R remote_socket:host:hostport
     -R remote_socket:local_socket
             Specifies that connections to the given TCP port or Unix socket
             on the remote (server) host are to be forwarded to the given
             host and port, or Unix socket, on the local side.  This works
             by allocating a socket to listen to either a TCP port or to a
             Unix socket on the remote side.  Whenever a connection is made
             to this port or Unix socket, the connection is forwarded over
             the secure channel, and a connection is made to either host
             port hostport, or local_socket, 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 brackets.

             By default, TCP listening sockets 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 dynami‐
             cally 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 shar‐
             ing, 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 facilitate the use of SSH as a secure
             transport for other applications (e.g. sftp(1)).  The subsystem
             is specified as the remote command.

     -T      Disable pseudo-terminal allocation.

     -t      Force pseudo-terminal 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, authentica‐
             tion, 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 for‐
             warded to host on port over the secure channel.  Implies -N,
             -T, ExitOnForwardFailure and ClearAllForwardings, though these
             can be overridden in the configuration file or using -o command
             line options.

     -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 dis‐
             play 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 configu‐
     ration file and a system-wide configuration file.  The file format and
     configuration options are described in ssh_config(5).

AUTHENTICATION         top

     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).  Protocol
     1 should not be used and is only offered to support legacy devices.  It
     suffers from a number of cryptographic weaknesses and doesn't support
     many of the advanced features available for protocol 2.

     The methods available for authentication are: GSSAPI-based authentica‐
     tion, host-based authentication, public key authentication, challenge-
     response authentication, and password authentication.  Authentication
     methods are tried in the order specified above, though
     PreferredAuthentications can be used to change the default order.

     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 consid‐
     ered 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 authentica‐
     tion 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 inse‐
     cure 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 decryp‐
     tion 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, Ed25519 or RSA algorithms.  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 permit‐
     ted 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
     (DSA), ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519 (Ed25519), or
     ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/identity.pub
     (protocol 1), ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
     ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (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 pub‐
     lic/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) and (optionally)
     the AddKeysToAgent directive in ssh_config(5) for more information.

     Challenge-response authentication works as follows: The server sends an
     arbitrary "challenge" text, and prompts for a 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; how‐
     ever, since all communications are encrypted, the password cannot be
     seen by someone listening on the network.

     ssh automatically maintains and checks a database containing identifi‐
     cation 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 in a non-interactive session or, if
     no command has been specified, logs into the machine and gives the user
     a normal shell as an interactive session.  All communication with the
     remote command or shell will be automatically encrypted.

     If an interactive session is requested ssh by default will only request
     a pseudo-terminal (pty) for interactive sessions when the client has
     one.  The flags -T and -t can be used to override this behaviour.

     If a pseudo-terminal has been allocated the user may use the escape
     characters noted below.

     If no pseudo-terminal 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         top

     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 if the peer sup‐
             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 if the peer
             supports 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         top

     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, con‐
     necting 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         top

     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 for‐
     warding 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 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         top

     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 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.  If only legacy (MD5) fingerprints for the
     server are available, the ssh-keygen(1) -E option may be used to down‐
     grade the fingerprint algorithm to match.

     Because of the difficulty of comparing host keys just by looking at
     fingerprint strings, there is also support to compare host keys visual‐
     ly, using random art.  By setting the VisualHostKey option to “yes”, a
     small ASCII graphic gets displayed on every login to a server, no mat‐
     ter if the session itself is interactive or not.  By learning the pat‐
     tern 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         top

     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 con‐
     trols 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 gate‐
     way 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 per‐
     manent VPNs are better provided by tools such as ipsecctl(8) and
     isakmpd(8).

ENVIRONMENT         top

     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 nor‐
                           mally 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 com‐
                           piling 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 ter‐
                           minal 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 con‐
                           nection.  The variable contains four space-sepa‐
                           rated values: client IP address, client port num‐
                           ber, 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 com‐
                           mand.  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 con‐
                           nections).

     USER                  Set to the name of the user logging in.

     Additionally, ssh reads ~/.ssh/environment, and adds lines of the for‐
     mat “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         top

     ~/.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 gen‐
             eral 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, Ed25519, 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 permis‐
             sions: read/write for the user, and not writable by others.

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

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.ssh/id_rsa
             Contains the private key for authentication.  These files con‐
             tain 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_ed25519.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 configura‐
             tion 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_ed25519_key
     /etc/ssh/ssh_host_rsa_key
             These files contain the private parts of the host keys and are
             used for host-based authentication.

     /etc/ssh/ssh_known_hosts
             Systemwide list of known host keys.  This file should be pre‐
             pared 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         top

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

SEE ALSO         top

     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

STANDARDS         top

     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         top

     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.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.  Informa‐
     tion about the project can be found at
     http://www.openssh.com/portable.html.  If you have a bug report for
     this manual page, see http://www.openssh.com/report.html.  This page
     was obtained from the tarball openssh-7.3p1.tar.gz fetched from
     http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2016-10-04.
     If you discover any rendering problems in this HTML version of the
     page, or you believe there is a better or more up-to-date source for
     the page, or you have corrections or improvements to the information in
     this COLOPHON (which is not part of the original manual page), send a
     mail to man-pages@man7.org

BSD                             July 16, 2016                            BSD