SSH(1)



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

NAME
     ssh -- OpenSSH remote login client

SYNOPSIS
     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
     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 speci-
     fied as either [user@]hostname or a URI of the form
     ssh://[user@]hostname[:port].  The user must prove his/her 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 con-
             nect 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 ad-
             dress.

     -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).
             Compression is desirable on modem lines and other slow connec-
             tions, but will only slow down things on fast networks.  The de-
             fault value can be set on a host-by-host basis in the configura-
             tion 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.  Dy-
             namic 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 con-
             figuration file is given on the command line, the system-wide
             configuration file (/etc/ssh/ssh_config) will be ignored.  The
             default for the per-user configuration file is ~/.ssh/config.  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 im-
             plies -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 for-
             wards 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 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 ob-
             tained 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 al-
             locating 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 con-
             firmation 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) algo-
             rithms, 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 ma-
             chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
             an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
             tomatically forwarded over an encrypted channel.  The ssh program
             will be put in the background.  (This does not work if ssh needs
             to ask for a password or passphrase; see also the -f option.)

     -O ctl_cmd
             Control an active connection multiplexing master process.  When
             the -O option is specified, the ctl_cmd argument is interpreted
             and passed to the master process.  Valid commands are: "check"
             (check that the master process is running), "forward" (request
             forwardings without command execution), "cancel" (cancel forward-
             ings), "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 configura-
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.  For full details of the op-
             tions listed below, and their possible values, see ssh_config(5).

                   AddKeysToAgent
                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   CASignatureAlgorithms
                   CertificateFile
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Ciphers
                   ClearAllForwardings
                   Compression
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   FingerprintHash
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIKeyExchange
                   GSSAPIClientIdentity
                   GSSAPIDelegateCredentials
                   GSSAPIKexAlgorithms
                   GSSAPIRenewalForcesRekey
                   GSSAPIServerIdentity
                   GSSAPITrustDns
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostbasedKeyTypes
                   HostKeyAlgorithms
                   HostKeyAlias
                   Hostname
                   IdentitiesOnly
                   IdentityAgent
                   IdentityFile
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   ProxyCommand
                   ProxyJump
                   ProxyUseFdpass
                   PubkeyAcceptedKeyTypes
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteCommand
                   RemoteForward
                   RequestTTY
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   SetEnv
                   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 ssh for the algorithms supported for the specified ver-
             sion 2.  The available features are: 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), kex-gss (GSSAPI key exchange algo-
             rithms), 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).  Alterna-
             tively, any keyword from ssh_config(5) or sshd_config(5) that
             takes an algorithm list may be used as an alias for the corre-
             sponding 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 connec-
             tions 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 specify-
             ing 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 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 trans-
             port 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 op-
             tions.

     -w local_tun[:remote_tun]
             Requests tunnel device forwarding with the specified tun(4) de-
             vices between the client (local_tun) and the server (remote_tun).

             The devices may be specified by numerical ID or the keyword
             "any", which uses the next available tunnel device.  If
             remote_tun is not specified, it defaults to "any".  See also the
             Tunnel and TunnelDevice directives in ssh_config(5).

             If the Tunnel directive is unset, it 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 ex-
             tension restrictions by default.  Please refer to the ssh -Y op-
             tion and the ForwardX11Trusted directive in ssh_config(5) for
             more information.

             (Debian-specific: X11 forwarding is not subjected to X11 SECURITY
             extension restrictions by default, because too many programs cur-
             rently crash in this mode.  Set the ForwardX11Trusted option to
             "no" to restore the upstream behaviour.  This may change in fu-
             ture depending on client-side improvements.)

     -x      Disables X11 forwarding.

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

             (Debian-specific: In the default configuration, this option is
             equivalent to -X, since ForwardX11Trusted defaults to "yes" as
             described above.  Set the ForwardX11Trusted option to "no" to re-
             store the upstream behaviour.  This may change in future depend-
             ing on client-side improvements.)

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

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

AUTHENTICATION
     The OpenSSH SSH client supports SSH protocol 2.

     The methods available for authentication are: GSSAPI-based authentica-
     tion, host-based authentication, public key authentication, challenge-re-
     sponse authentication, and password authentication.  Authentication meth-
     ods are tried in the order specified above, though
     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/ssh/shosts.equiv on the re-
     mote 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 administra-
     tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
     are inherently insecure and should be disabled if security is desired.]

     Public key authentication works as follows: The scheme is based on pub-
     lic-key cryptography, using cryptosystems where encryption and decryption
     are done using separate keys, and it is unfeasible to derive the decryp-
     tion key from the encryption key.  The idea is that each user creates a
     public/private key pair for authentication purposes.  The server knows
     the public key, and only the user knows the private key.  ssh implements
     public key authentication protocol automatically, using one of the DSA,
     ECDSA, Ed25519 or RSA algorithms.  The HISTORY section of ssl(8) (on non-
     OpenBSD systems, see
     http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) con-
     tains 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 au-
     thentication from succeeding after authentication completes using a dif-
     ferent method.  These may be viewed by increasing the LogLevel to DEBUG
     or higher (e.g. by using the -v flag).

     The user creates his/her 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 (au-
     thenticator-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 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 cer-
     tificate authentication: instead of a set of public/private keys, signed
     certificates are used.  This has the advantage that a single trusted cer-
     tification 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.

     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; however,
     since all communications are encrypted, the password cannot be seen by
     someone listening on the network.

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

     When the user's identity has been accepted by the server, the server ei-
     ther executes the given command 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 re-
     mote 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 char-
     acters 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
     When a pseudo-terminal has been requested, ssh supports a number of func-
     tions through the use of an escape character.

     A single tilde character can be sent as ~~ or by following the tilde by a
     character other than those described below.  The escape character must
     always follow a newline to be interpreted as special.  The escape charac-
     ter can be changed in configuration files using the EscapeChar configura-
     tion directive or on the command line by the -e option.

     The supported escapes (assuming the default '~') are:

     ~.      Disconnect.

     ~^Z     Background ssh.

     ~#      List forwarded connections.

     ~&      Background ssh at logout when waiting for forwarded connection /
             X11 sessions to terminate.

     ~?      Display a list of escape characters.

     ~B      Send a BREAK to the remote system (only useful 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 re-
             mote 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 sup-
             ports it).

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

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

TCP FORWARDING
     Forwarding of arbitrary TCP connections over 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 sup-
     port 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 speci-
     fied 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
     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 en-
     vironment variable is set), the connection to the X11 display is automat-
     ically forwarded to the remote side in such a way that any X11 programs
     started from the shell (or command) will go through the encrypted chan-
     nel, and the connection to the real X server will be made from the local
     machine.  The user should not manually set DISPLAY.  Forwarding of X11
     connections can be configured on the command line or in configuration
     files.

     The DISPLAY value set by ssh will point to the server machine, but with a
     display number greater than zero.  This is normal, and happens because
     ssh creates a "proxy" X server on the server machine for forwarding the
     connections over the encrypted channel.

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

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

VERIFYING HOST KEYS
     When connecting to a server for the first time, a fingerprint of the
     server's public key is presented to the user (unless the option
     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
     using ssh-keygen(1):

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

     If the fingerprint is already known, it can be matched and the key can be
     accepted or rejected.  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 fin-
     gerprint strings, there is also support to compare host keys visually,
     using random art.  By setting the VisualHostKey option to "yes", a small
     ASCII graphic gets displayed on every login to a server, no matter if the
     session itself is interactive or not.  By learning the pattern a known
     server produces, a user can easily find out that the host key has changed
     when a completely different pattern is displayed.  Because these patterns
     are not unambiguous however, a pattern that looks similar to the pattern
     remembered only gives a good probability that the host key is the same,
     not guaranteed proof.

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

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

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

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

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

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

           $ dig -t SSHFP host.example.com

     Finally the client connects:

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

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

SSH-BASED VIRTUAL PRIVATE NETWORKS
     ssh contains support for Virtual Private Network (VPN) tunnelling using
     the tun(4) network pseudo-device, allowing two networks to be joined se-
     curely.  The sshd_config(5) configuration option PermitTunnel controls
     whether the server supports this, and at what level (layer 2 or 3 traf-
     fic).

     The following example would connect client network 10.0.50.0/24 with re-
     mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
     to 10.1.1.2, provided that the SSH server running on the gateway to the
     remote network, at 192.168.1.15, allows it.

     On the client:

           # ssh -f -w 0:1 192.168.1.15 true
           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
           # route add 10.0.99.0/24 10.1.1.2

     On the server:

           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
           # route add 10.0.50.0/24 10.1.1.1

     Client access may be more finely tuned via the /root/.ssh/authorized_keys
     file (see below) and the PermitRootLogin server option.  The following
     entry would permit connections on tun(4) device 1 from user "jane" and on
     tun device 2 from user "john", if PermitRootLogin is set to
     "forced-commands-only":

       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

     Since an SSH-based setup entails a fair amount of overhead, it may be
     more suited to temporary setups, such as for wireless VPNs.  More perma-
     nent VPNs are better provided by tools such as ipsecctl(8) and
     isakmpd(8).

ENVIRONMENT
     ssh will normally set the following environment variables:

     DISPLAY               The DISPLAY variable indicates the location of the
                           X11 server.  It is automatically set by ssh to
                           point to a value of the form "hostname:n", where
                           "hostname" indicates the host where the shell runs,
                           and 'n' is an integer >= 1.  ssh uses this special
                           value to forward X11 connections over the secure
                           channel.  The user should normally not set DISPLAY
                           explicitly, as that will render the X11 connection
                           insecure (and will require the user to manually
                           copy any required authorization cookies).

     HOME                  Set to the path of the user's home directory.

     LOGNAME               Synonym for USER; set for compatibility with sys-
                           tems that use this variable.

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

     PATH                  Set to the default PATH, as specified when compil-
                           ing ssh.

     SSH_ASKPASS           If ssh needs a passphrase, it will read the
                           passphrase from the current terminal if it was run
                           from a terminal.  If ssh does not have a terminal
                           associated with it but DISPLAY and SSH_ASKPASS are
                           set, it will execute the program specified by
                           SSH_ASKPASS and open an X11 window to read the
                           passphrase.  This is particularly useful when call-
                           ing ssh from a .xsession or related script.  (Note
                           that on some machines it may be necessary to redi-
                           rect the input from /dev/null to make this work.)

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

     SSH_CONNECTION        Identifies the client and server ends of the con-
                           nection.  The variable contains four space-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 de-
                           vice) 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 con-
                           tain a pathname to a file that lists the authenti-
                           cation 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 connec-
                           tions).

     USER                  Set to the name of the user logging in.

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

FILES
     ~/.rhosts
             This file is used for host-based authentication (see above).  On
             some machines this file may need to be world-readable if the
             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 con-
             figuration and authentication information.  There is no general
             requirement to keep the entire contents of this directory secret,
             but the recommended permissions are read/write/execute for the
             user, and not accessible by others.

     ~/.ssh/authorized_keys
             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
             for logging in as this user.  The format of this file is de-
             scribed in the sshd(8) manual page.  This file is not highly sen-
             sitive, 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.  It may be
             group-writable provided that the group in question contains only
             the user.

     ~/.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 acces-
             sible by others (read/write/execute).  ssh will simply ignore a
             private key file if it is accessible by others.  It is possible
             to specify a passphrase when generating the key which will be
             used to encrypt the sensitive part of this file using 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/ssh/shosts.equiv
             This file is used in exactly the same way as hosts.equiv, but al-
             lows host-based authentication without permitting login with
             rlogin/rsh.

     /etc/ssh/ssh_config
             Systemwide configuration file.  The file format and configuration
             options are described in ssh_config(5).

     /etc/ssh/ssh_host_key
     /etc/ssh/ssh_host_dsa_key
     /etc/ssh/ssh_host_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
     ssh exits with the exit status of the remote command or with 255 if an
     error occurred.

SEE ALSO
     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-argv0(1), ssh-keygen(1),
     ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

BSD                             April 17, 2020                             BSD

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