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manpage ssh

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

NAME

ssh âАФ 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 specified 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 alâАР
             ternative 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 comâАР
             pression 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 preferâАР
             ence.  See the Ciphers keyword in ssh_config(5) for more information.

     -D [bind_address:]port
             Specifies a local âАЬdynamicâАЭ application-level port forwarding.  This works by allocating a socket to listen to port on the local side, opâАР
             tionally 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 âАЬlocalhostâАЭ indicates that the listening port be bound for local use only, while an empty adâАР
             dress or âА~*âАTM 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: âА~~âАTM).  The escape character is only recognized at the beginning of a line.  The
             escape character followed by a dot (âА~.âАTM) closes the connection; followed by control-Z suspends the connection; and followed by itself sends
             the escape character once.  Setting the character to âАЬnoneâАЭ 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 âАЬnoneâАЭ, no conâАР
             figuration 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 âАЬyesâАЭ, then a client started with -f will wait for all remote port forwards to
             be successfully established before placing itself in the background.

     -G      Causes ssh to print its configuration after evaluating Host and Match blocks and exit.

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

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to 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 cerâАР
             tificates have been explicitly specified by the CertificateFile directive, ssh will also try to load certificate information from the fileâАР
             name 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 forwardâАР
             ing 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 destiâАР
             nation 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 âАЬlocalhostâАЭ indicates that the listening port be bound for local use only, while an
             empty address or âА~*âАTM 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 âАЬmasterâАЭ mode for connection sharing.  Multiple -M options places ssh into âАЬmasterâАЭ 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 deâАР
             scription 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.

     -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 shadâАР
             ows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel.  The ssh program will be put in the backâАР
             ground.  (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: âАЬcheckâАЭ (check that the master process is running), âАЬforwardâАЭ (request forwardings without comâАР
             mand execution), âАЬcancelâАЭ (cancel forwardings), âАЬexitâАЭ (request the master to exit), and âАЬstopâАЭ (request the master to stop accepting furâАР
             ther 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 sepâАР
             arate 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
                   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 version 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 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 (supâАР
             ported 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 âА~*âАTM, 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 âА~0âАTM, 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 âАЬnoneâАЭ to disable connection sharing.  Refer to the deâАР
             scription of ControlPath and ControlMaster in ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote system.  Subsystems facilitate the use of SSH as a secure transport for
             other applications (e.g. sftp(1)).  The subsystem is specified as the remote command.

     -T      Disable pseudo-terminal allocation.

     -t      Force pseudo-terminal allocation.  This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useâАР
             ful, 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 âАЬanyâАЭ, which uses the next available tunnel device.  If remote_tun is not specâАР
             ified, it defaults to âАЬanyâАЭ.  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 âАЬpoint-to-pointâАЭ.  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 auâАР
             thorization database) can access the local X11 display through the forwarded connection.  An attacker may then be able to perform activiâАР
             ties 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.

             (Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension restrictions by default, because too many programs currently
             crash in this mode.  Set the ForwardX11Trusted option to âАЬnoâАЭ to restore the upstream behaviour.  This may change in future 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 âАЬyesâАЭ as described
             above.  Set the ForwardX11Trusted option to âАЬnoâАЭ to restore the upstream behaviour.  This may change in future depending on client-side imâАР
             provements.)

     -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 conâАР
     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 authentication, host-based authentication, public key authentication, challenge-response
     authentication, and password authentication.  Authentication methods are tried in the order specified above, though PreferredAuthentications can be
     used to change the default order.

     Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the reâАР
     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 routâАР
     ing 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 pubâАР
     lic/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) 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 differâАР
     ent 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 (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 his/her home directory on the remote machine.  The authorized_keys file corâАР
     responds to the conventional ~/.rhosts file, and has one key per line, though the lines can be very long.  After this, the user can log in without
     giving the password.

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

     The most convenient way to use public key or certificate authentication may be with an authentication agent.  See ssh-agent(1) 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 chalâАР
     lenge-response authentication include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD systems).

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

     ssh automatically maintains and checks a database containing identification for all hosts it has ever been used with.  Host keys are stored in
     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any
     new hosts are automatically added to the user's file.  If a host's identification ever changes, ssh warns about this and disables password authenâАР
     tication 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 comâАР
     mand has been specified, logs into the machine and gives the user a normal shell as an interactive session.  All communication with the remote comâАР
     mand 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 âАЬnoneâАЭ will also make the session transparent even if a tty is used.

     The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.

ESCAPE CHARACTERS

When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.

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

     The supported escapes (assuming the default âА~~âАTM) 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

     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 âАЬserver.example.comâАЭ, joining channel âАЬ#usersâАЭ, nickname âАЬpinkyâАЭ,
     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 âАЬsleep 10âАЭ 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

     If the ForwardX11 variable is set to âАЬyesâАЭ (or see the description of the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY enâАР
     vironment 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 maâАР
     chine.  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 âАЬproxyâАЭ 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 âАЬyesâАЭ (or see the description of the -A and -a options above) and the user is using an authentication agent,
     the connection to the agent is automatically forwarded to the remote side.

VERIFYING HOST KEYS

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

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

     If the fingerprint is already known, it can be matched and the key can be accepted or rejected.  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 âАЬyesâАЭ, a small ASCII graphic gets displayed on every login to a server, no matter if the sesâАР
     sion 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 reâАР
     membered 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, âАЬhost.example.comâАЭ.  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 seâАР
     curely.  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 âАЬjaneâАЭ and on tun device 2 from user âАЬjohnâАЭ, if PermitRootLogin is set to
     âАЬforced-commands-onlyâАЭ:

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

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

ENVIRONMENT

ssh will normally set the following environment variables:

     DISPLAY               The DISPLAY variable indicates the location of the X11 server.  It is automatically set by ssh to point to a value of the
                           form âАЬhostname:nâАЭ, where âАЬhostnameâАЭ indicates the host where the shell runs, and âА~nâАTM is an integer âЙ¥ 1.  ssh uses this speâАР
                           cial 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 âАЬneverâАЭ then ssh will never attempt to
                           use one.  If it is set to âАЬpreferâАЭ, then ssh will prefer to use the askpass program instead of the TTY when requesting passâАР
                           words.  Finally, if the variable is set to âАЬforceâАЭ, 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 adâАР
                           dress, 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 arâАР
                           guments.

     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 âАЬVARNAME=valueâАЭ to the environment if the file exists and users are alâАР
     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 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 accessiâАР
             ble 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 potenâАР
             tial for abuse, this file must have strict permissions: read/write for the user, and not writable by others.  It may be group-writable proâАР
             vided 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 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/ssh/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 maâАР
             chines 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.

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

AUTHORS

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

BSD                                                                   July 15, 2020                                                                  BSD

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