ssh(1) — Linux manual page

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 remote login client

SYNOPSIS         top

     ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface]
         [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
         [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
         [-i identity_file] [-J destination] [-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]] destination
         [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 destination, which may be
     specified as either [user@]hostname or a URI of the form
     ssh://[user@]hostname[:port].  The user must prove their identity
     to the remote machine using one of several methods (see below).

     If a command is specified, it is executed on the remote host
     instead of a login shell.

     The options are as follows:

     -4      Forces ssh to use IPv4 addresses only.

     -6      Forces ssh to use IPv6 addresses only.

     -A      Enables forwarding of connections from an authentication
             agent such as ssh-agent(1).  This can also be specified on
             a per-host basis in a configuration file.

             Agent forwarding should be enabled with caution.  Users
             with the ability to bypass file permissions on the remote
             host (for the agent's UNIX-domain socket) can access the
             local agent through the forwarded connection.  An attacker
             cannot obtain key material from the agent, however they can
             perform operations on the keys that enable them to
             authenticate using the identities loaded into the agent.  A
             safer alternative may be to use a jump host (see -J).

     -a      Disables forwarding of the authentication agent connection.

     -B bind_interface
             Bind to the address of bind_interface before attempting to
             connect to the destination host.  This is only useful on
             systems with more than one address.

     -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
             connections).  The compression algorithm is the same used
             by gzip(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.  cipher_spec is a comma-separated list of ciphers
             listed in order of preference.  See the Ciphers keyword in
             ssh_config(5) for more information.

     -D [bind_address:]port
             Specifies a local “dynamic” application-level port
             forwarding.  This works by allocating a socket to listen to
             port on the local side, optionally bound to the specified
             bind_address.  Whenever a connection is made to this port,
             the connection is forwarded over the secure channel, and
             the application protocol is then used to determine where to
             connect to from the remote machine.  Currently the SOCKS4
             and SOCKS5 protocols are supported, and ssh will act as a
             SOCKS server.  Only root can forward privileged ports.
             Dynamic port forwardings can also be specified in the
             configuration file.

             IPv6 addresses can be specified by enclosing the address in
             square brackets.  Only the superuser can forward privileged
             ports.  By default, the local port is bound in accordance
             with the GatewayPorts setting.  However, an explicit
             bind_address may be used to bind the connection to a
             specific address.  The bind_address of “localhost”
             indicates that the listening port be bound for local use
             only, while an empty address or ‘*’ indicates that the port
             should be available from all interfaces.

     -E 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.  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.  If set to “none”, no configuration
             files will be read.

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

     -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
             communicate with a PKCS#11 token providing keys for user
             authentication.

     -i identity_file
             Selects a file from which the identity (private key) for
             public key authentication is read.  The default is
             ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk,
             ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa.
             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
             certificate information from the filename obtained by
             appending -cert.pub to identity filenames.

     -J destination
             Connect to the target host by first making a ssh connection
             to the jump host described by destination 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.  Note that configuration
             directives supplied on the command-line generally apply to
             the destination host and not any specified jump hosts.  Use
             ~/.ssh/config to specify configuration for jump hosts.

     -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 interfaces.

     -l login_name
             Specifies the user to log in as on the remote machine.
             This also may be specified on a per-host basis in the
             configuration file.

     -M      Places the ssh client into “master” mode for connection
             sharing.  Multiple -M options places ssh into “master” mode
             but with confirmation required using ssh-askpass(1) before
             each operation that changes the multiplexing state (e.g.
             opening a new session).  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
             forwarding ports.  Refer to the description of SessionType
             in ssh_config(5) for details.

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

     -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).

                   AddKeysToAgent
                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   CASignatureAlgorithms
                   CertificateFile
                   CheckHostIP
                   Ciphers
                   ClearAllForwardings
                   Compression
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   FingerprintHash
                   ForkAfterAuthentication
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAcceptedAlgorithms
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   Hostname
                   IdentitiesOnly
                   IdentityAgent
                   IdentityFile
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   KnownHostsCommand
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PermitRemoteOpen
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   ProxyCommand
                   ProxyJump
                   ProxyUseFdpass
                   PubkeyAcceptedAlgorithms
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteCommand
                   RemoteForward
                   RequestTTY
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   SessionType
                   SetEnv
                   StdinNull
                   StreamLocalBindMask
                   StreamLocalBindUnlink
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UpdateHostKeys
                   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 for the algorithms supported by one of the
             following features: cipher (supported symmetric ciphers),
             cipher-auth (supported symmetric ciphers that support
             authenticated encryption), help (supported query terms for
             use with the -Q flag), mac (supported message integrity
             codes), kex (key exchange algorithms), key (key types),
             key-cert (certificate key types), key-plain (non-
             certificate key types), key-sig (all key types and
             signature algorithms), protocol-version (supported SSH
             protocol versions), and sig (supported signature
             algorithms).  Alternatively, any keyword from ssh_config(5)
             or sshd_config(5) that takes an algorithm list may be used
             as an alias for the corresponding query_option.

     -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
     -R [bind_address:]port
             Specifies that connections to the given TCP port or Unix
             socket on the remote (server) host are to be forwarded to
             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 from the local machine to either an
             explicit destination specified by host port hostport, or
             local_socket, or, if no explicit destination was specified,
             ssh will act as a SOCKS 4/5 proxy and forward connections
             to the destinations requested by the remote SOCKS client.

             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
             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 facilitate the use of SSH as a
             secure transport for other applications (e.g. sftp(1)).
             The subsystem is specified as the remote command.  Refer to
             the description of SessionType in ssh_config(5) for
             details.

     -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,
             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,
             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 will be set to the
             default tunnel mode, which is “point-to-point”.  If a
             different Tunnel forwarding mode it desired, then it should
             be specified before -w.

     -X      Enables X11 forwarding.  This can also be specified on a
             per-host basis in a configuration file.

             X11 forwarding should be enabled with caution.  Users with
             the ability to bypass file permissions on the remote host
             (for the user's X authorization database) can access the
             local X11 display through the forwarded connection.  An
             attacker may then be able to perform activities such as
             keystroke monitoring.

             For this reason, X11 forwarding is subjected to X11
             SECURITY extension restrictions by default.  Please refer
             to the ssh -Y option and the ForwardX11Trusted directive in
             ssh_config(5) for more information.

     -x      Disables X11 forwarding.

     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings
             are not subjected to the X11 SECURITY extension controls.

     -y      Send log information using the syslog(3) system module.  By
             default this information is sent to stderr.

     ssh may additionally obtain configuration data from a per-user
     configuration file and a system-wide configuration file.  The file
     format and configuration options are described in ssh_config(5).

AUTHENTICATION         top

     The OpenSSH SSH client supports SSH protocol 2.

     The methods available for authentication are: GSSAPI-based
     authentication, host-based authentication, public key
     authentication, keyboard-interactive 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, the user is non-root and the user names are the
     same on both sides, or if the files ~/.rhosts or ~/.shosts exist in
     the user's home directory on the remote machine and contain a line
     containing the name of the client machine and the name of the user
     on that machine, the user is considered for login.  Additionally,
     the server must be able to verify the client's host key (see the
     description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
     below) for login to be permitted.  This authentication method
     closes security holes due to IP spoofing, DNS spoofing, and routing
     spoofing.  [Note to the administrator: /etc/hosts.equiv, ~/.rhosts,
     and the rlogin/rsh protocol in general, are inherently insecure and
     should be disabled if security is desired.]

     Public key authentication works as follows: The scheme is based on
     public-key cryptography, using cryptosystems where encryption and
     decryption are done using separate keys, and it is unfeasible to
     derive the decryption key from the encryption key.  The idea is
     that each user creates a public/private key pair for authentication
     purposes.  The server knows the public key, and only the user knows
     the private key.  ssh implements public key authentication protocol
     automatically, using one of the DSA, ECDSA, 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
     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 server may inform the client of errors that prevented public
     key authentication from succeeding after authentication completes
     using a different method.  These may be viewed by increasing the
     LogLevel to DEBUG or higher (e.g. by using the -v flag).

     The user creates their key pair by running ssh-keygen(1).  This
     stores the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa
     (ECDSA), ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA),
     ~/.ssh/id_ed25519 (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-
     hosted Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public key
     in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
     ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA),
     ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub
     (authenticator-hosted 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 their home directory on the remote
     machine.  The authorized_keys file corresponds to the conventional
     ~/.rhosts file, and has one key per line, though the lines can be
     very long.  After this, the user can log in without giving the
     password.

     A variation on public key authentication is available in the form
     of certificate authentication: instead of a set of public/private
     keys, signed certificates are used.  This has the advantage that a
     single trusted certification authority can be used in place of many
     public/private keys.  See the CERTIFICATES section of ssh-keygen(1)
     for more information.

     The most convenient way to use public key or certificate
     authentication may be with an authentication agent.  See
     ssh-agent(1) and (optionally) the AddKeysToAgent directive in
     ssh_config(5) for more information.

     Keyboard-interactive authentication works as follows: The server
     sends an arbitrary "challenge" text and prompts for a response,
     possibly multiple times.  Examples of keyboard-interactive
     authentication include BSD Authentication (see login.conf(5)) and
     PAM (some non-OpenBSD systems).

     Finally, if other authentication methods fail, ssh prompts the user
     for a password.  The password is sent to the remote host for
     checking; however, since all communications are encrypted, the
     password cannot be seen by someone listening on the network.

     ssh automatically maintains and checks a database containing
     identification for all hosts it has ever been used with.  Host keys
     are stored in ~/.ssh/known_hosts in the user's home directory.
     Additionally, the file /etc/ssh/ssh_known_hosts is automatically
     checked for known hosts.  Any new hosts are automatically added to
     the user's file.  If a host's identification ever changes, ssh
     warns about this and disables password authentication to prevent
     server spoofing or man-in-the-middle attacks, which could otherwise
     be used to circumvent the encryption.  The StrictHostKeyChecking
     option can be used to control logins to machines whose host key is
     not known or has changed.

     When the user's identity has been accepted by the server, the
     server either executes the given command 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
             supports it).

     ~C      Open command line.  Currently this allows the addition of
             port forwardings using the -L, -R and -D options (see
             above).  It also allows the cancellation of existing 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 a 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 for an
     IRC client, even though the IRC server it connects to does not
     directly support encrypted communication.  This works as follows:
     the user connects to the remote host using ssh, specifying the
     ports to be used to forward the connection.  After that it is
     possible to start the program locally, and ssh will encrypt and
     forward the connection to the remote server.

     The following example tunnels an IRC session from the client to an
     IRC server at “server.example.com”, joining channel “#users”,
     nickname “pinky”, using the standard IRC port, 6667:

         $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
         $ irc -c '#users' pinky IRC/127.0.0.1

     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 program which is going to use the tunnel.  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 forwarding the connections over the encrypted channel.

     ssh will also automatically set up Xauthority data on the server
     machine.  For this purpose, it will generate a random authorization
     cookie, store it in Xauthority on the server, and verify that any
     forwarded connections carry this cookie and replace it by the real
     cookie when the connection is opened.  The real authentication
     cookie is never sent to the server machine (and no cookies are sent
     in the plain).

     If the ForwardAgent variable is set to “yes” (or see the
     description of the -A and -a options above) and the user is using
     an authentication agent, the connection to the agent is
     automatically forwarded to the remote side.

VERIFYING HOST KEYS         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
     determined using ssh-keygen(1):

           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

     If the fingerprint is already known, it can be matched and the key
     can be accepted or rejected.  If only legacy (MD5) fingerprints for
     the server are available, the ssh-keygen(1) -E option may be used
     to downgrade 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
     visually, using random art.  By setting the VisualHostKey option to
     “yes”, a small ASCII graphic gets displayed on every login to a
     server, no matter if the session itself is interactive or not.  By
     learning the pattern a known server produces, a user can easily
     find out that the host key has changed when a completely different
     pattern is displayed.  Because these patterns are not unambiguous
     however, a pattern that looks similar to the pattern remembered
     only gives a good probability that the host key is the same, not
     guaranteed proof.

     To get a listing of the fingerprints along with their random art
     for all known hosts, the following command line can be used:

           $ ssh-keygen -lv -f ~/.ssh/known_hosts

     If the fingerprint is unknown, an alternative method of
     verification is available: SSH fingerprints verified by DNS.  An
     additional resource record (RR), SSHFP, is added to a zonefile and
     the connecting client is able to match the fingerprint with that of
     the key presented.

     In this example, we are connecting a client to a server,
     “host.example.com”.  The SSHFP resource records should first be
     added to the zonefile for host.example.com:

           $ ssh-keygen -r host.example.com.

     The output lines will have to be added to the zonefile.  To check
     that the zone is answering fingerprint queries:

           $ dig -t SSHFP host.example.com

     Finally the client connects:

           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
           [...]
           Matching host key fingerprint found in DNS.
           Are you sure you want to continue connecting (yes/no)?

     See the VerifyHostKeyDNS option in ssh_config(5) for more
     information.

SSH-BASED VIRTUAL PRIVATE NETWORKS         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 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         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 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_ASKPASS_REQUIRE   Allows further control over the use of an
                           askpass program.  If this variable is set to
                           “never” then ssh will never attempt to use
                           one.  If it is set to “prefer”, then ssh will
                           prefer to use the askpass program instead of
                           the TTY when requesting passwords.  Finally,
                           if the variable is set to “force”, then the
                           askpass program will be used for all
                           passphrase input regardless of whether
                           DISPLAY is set.

     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket
                           used to communicate with the agent.

     SSH_CONNECTION        Identifies the client and server ends of the
                           connection.  The variable contains four
                           space-separated values: client IP address,
                           client port number, server IP address, and
                           server port number.

     SSH_ORIGINAL_COMMAND  This variable contains the original command
                           line if a forced command is executed.  It can
                           be used to extract the original arguments.

     SSH_TTY               This is set to the name of the tty (path to
                           the device) associated with the current shell
                           or command.  If the current session has no
                           tty, this variable is not set.

     SSH_TUNNEL            Optionally set by sshd(8) to contain the
                           interface names assigned if tunnel forwarding
                           was requested by the client.

     SSH_USER_AUTH         Optionally set by sshd(8), this variable may
                           contain a pathname to a file that lists the
                           authentication methods successfully used when
                           the session was established, including any
                           public keys that were used.

     TZ                    This variable is set to indicate the present
                           time zone if it was set when the daemon was
                           started (i.e. the daemon passes the value on
                           to new connections).

     USER                  Set to the name of the user logging in.

     Additionally, ssh reads ~/.ssh/environment, and adds lines of the
     format “VARNAME=value” to the environment if the file exists and
     users are allowed to change their environment.  For more
     information, see the PermitUserEnvironment option in
     sshd_config(5).

FILES         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 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, 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 permissions: read/write for the user, and not
             writable by others.

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

     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ecdsa_sk
     ~/.ssh/id_ed25519
     ~/.ssh/id_ed25519_sk
     ~/.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 AES-128.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ecdsa_sk.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_ed25519_sk.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_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
             prepared by the system administrator to contain the public
             host keys of all machines in the organization.  It should
             be world-readable.  See sshd(8) for further details of the
             format of this file.

     /etc/ssh/sshrc
             Commands in this file are executed by ssh when the user
             logs in, just before the user's shell (or command) is
             started.  See the sshd(8) manual page for more information.

EXIT STATUS         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.
     Information 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-8.7p1.tar.gz fetched
     from ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
     2021-08-27.  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 28, 2021                          BSD

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