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

NAME
     ssh -- OpenSSH SSH client (remote login program)

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

DESCRIPTION
     ssh (SSH client) is a program for logging into a remote machine and for
     executing commands on a remote machine.  It is intended to provide secure
     encrypted communications between two untrusted hosts over an insecure
     network.  X11 connections, arbitrary TCP ports and UNIX-domain sockets
     can also be forwarded over the secure channel.

     ssh connects and logs into the specified hostname (with optional user
     name).  The user must prove his/her identity to the remote machine using
     one of several methods (see below).

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

     The options are as follows:

     -1	     Forces ssh to try protocol version 1 only.

     -2	     Forces ssh to try protocol version 2 only.

     -4	     Forces ssh to use IPv4 addresses only.

     -6	     Forces ssh to use IPv6 addresses only.

     -A	     Enables forwarding of the authentication agent connection.	 This
	     can also be specified on a per-host basis in a 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	     Disables forwarding of the authentication agent connection.

     -b bind_address
	     Use bind_address on the local machine as the source address of
	     the connection.  Only useful on systems with more than one
	     address.

     -C	     Requests compression of all data (including stdin, stdout,
	     stderr, and data for forwarded X11, TCP and UNIX-domain connec-
	     tions).  The compression algorithm is the same used by gzip(1),
	     and the ``level'' can be controlled by the CompressionLevel
	     option for protocol version 1.  Compression is desirable on modem
	     lines and other slow connections, but will only slow down things
	     on fast networks.	The default value can be set on a host-by-host
	     basis in the configuration files; see the Compression option.

     -c cipher_spec
	     Selects the cipher specification for encrypting the session.

	     Protocol version 1 allows specification of a single cipher.  The
	     supported values are ``3des'', ``blowfish'', and ``des''.	For
	     protocol version 2, cipher_spec is a comma-separated list of
	     ciphers listed in order of preference.  See the Ciphers keyword
	     in ssh_config(5) for more information.

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

     -f	     Requests ssh to go to background just before command execution.
	     This is useful if ssh is going to ask for passwords or
	     passphrases, but the user wants it in the background.  This
	     implies -n.  The recommended way to start X11 programs at a
	     remote site is with something like ssh -f host xterm.

	     If the ExitOnForwardFailure configuration option is set to
	     ``yes'', then a client started with -f will wait for all remote
	     port forwards to be successfully established before placing
	     itself in the background.

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

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

     -I pkcs11
	     Specify the PKCS#11 shared library ssh should use to communicate
	     with a PKCS#11 token providing the user's private RSA key.

     -i identity_file
	     Selects a file from which the identity (private key) for public
	     key authentication is read.  The default is ~/.ssh/identity for
	     protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
	     ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
	     Identity files may also be specified on a per-host basis in the
	     configuration file.  It is possible to have multiple -i options
	     (and multiple identities specified in configuration files).  If
	     no certificates have been explicitly specified by the
	     CertificateFile directive, ssh will also try to load certificate
	     information from the filename obtained by appending -cert.pub to
	     identity filenames.

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

     -K	     Enables GSSAPI-based authentication and forwarding (delegation)
	     of GSSAPI credentials to the server.

     -k	     Disables forwarding (delegation) of GSSAPI credentials to the
	     server.

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

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

	     By default, the local port is bound in accordance with the
	     GatewayPorts setting.  However, an explicit bind_address may be
	     used to bind the connection to a specific address.	 The
	     bind_address of ``localhost'' indicates that the listening port
	     be bound for local use only, while an empty address or `*' indi-
	     cates 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 shar-
	     ing.  Multiple -M options places ssh into ``master'' mode with
	     confirmation required before slave connections are accepted.
	     Refer to the description of ControlMaster in ssh_config(5) for
	     details.

     -m mac_spec
	     A comma-separated list of MAC (message authentication code) 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
	     machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
	     start an emacs on shadows.cs.hut.fi, and the X11 connection will
	     be automatically forwarded over an encrypted channel.  The ssh
	     program will be put in the background.  (This does not work if
	     ssh needs to ask for a password or passphrase; see also the -f
	     option.)

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

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

		   AddKeysToAgent
		   AddressFamily
		   BatchMode
		   BindAddress
		   CanonicalDomains
		   CanonicalizeFallbackLocal
		   CanonicalizeHostname
		   CanonicalizeMaxDots
		   CanonicalizePermittedCNAMEs
		   CertificateFile
		   ChallengeResponseAuthentication
		   CheckHostIP
		   Cipher
		   Ciphers
		   ClearAllForwardings
		   Compression
		   CompressionLevel
		   ConnectionAttempts
		   ConnectTimeout
		   ControlMaster
		   ControlPath
		   ControlPersist
		   DynamicForward
		   EscapeChar
		   ExitOnForwardFailure
		   FingerprintHash
		   ForwardAgent
		   ForwardX11
		   ForwardX11Timeout
		   ForwardX11Trusted
		   GatewayPorts
		   GlobalKnownHostsFile
		   GSSAPIAuthentication
		   GSSAPIKeyExchange
		   GSSAPIClientIdentity
		   GSSAPIDelegateCredentials
		   GSSAPIRenewalForcesRekey
		   GSSAPITrustDns
		   GSSAPIKexAlgorithms
		   HashKnownHosts
		   Host
		   HostbasedAuthentication
		   HostbasedKeyTypes
		   HostKeyAlgorithms
		   HostKeyAlias
		   HostName
		   IdentitiesOnly
		   IdentityAgent
		   IdentityFile
		   Include
		   IPQoS
		   KbdInteractiveAuthentication
		   KbdInteractiveDevices
		   KexAlgorithms
		   LocalCommand
		   LocalForward
		   LogLevel
		   MACs
		   Match
		   NoHostAuthenticationForLocalhost
		   NumberOfPasswordPrompts
		   PasswordAuthentication
		   PermitLocalCommand
		   PKCS11Provider
		   Port
		   PreferredAuthentications
		   Protocol
		   ProxyCommand
		   ProxyJump
		   ProxyUseFdpass
		   PubkeyAcceptedKeyTypes
		   PubkeyAuthentication
		   RekeyLimit
		   RemoteForward
		   RequestTTY
		   RhostsRSAAuthentication
		   RSAAuthentication
		   SendEnv
		   ServerAliveInterval
		   ServerAliveCountMax
		   StreamLocalBindMask
		   StreamLocalBindUnlink
		   StrictHostKeyChecking
		   TCPKeepAlive
		   Tunnel
		   TunnelDevice
		   UpdateHostKeys
		   UsePrivilegedPort
		   User
		   UserKnownHostsFile
		   VerifyHostKeyDNS
		   VisualHostKey
		   XAuthLocation

     -p port
	     Port to connect to on the remote host.  This can be specified on
	     a per-host basis in the configuration file.

     -Q query_option
	     Queries ssh for the algorithms supported for the specified ver-
	     sion 2.  The available features are: cipher (supported symmetric
	     ciphers), cipher-auth (supported symmetric ciphers that support
	     authenticated encryption), mac (supported message integrity
	     codes), kex (key exchange algorithms), key (key types), key-cert
	     (certificate key types), key-plain (non-certificate key types),
	     and protocol-version (supported SSH protocol versions).

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

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

	     Port forwardings can also be specified in the configuration file.
	     Privileged ports can be forwarded only when logging in as root on
	     the remote machine.  IPv6 addresses can be specified by enclosing
	     the address in square brackets.

	     By default, TCP listening sockets on the server will be bound to
	     the loopback interface only.  This may be overridden by 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
	     options.

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

	     The devices may be specified by numerical ID or the keyword
	     ``any'', which uses the next available tunnel device.  If
	     remote_tun is not specified, it defaults to ``any''.  See also
	     the Tunnel and TunnelDevice directives in ssh_config(5).  If the
	     Tunnel directive is unset, it is set to the default tunnel mode,
	     which is ``point-to-point''.

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

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

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

     -x	     Disables X11 forwarding.

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

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

     ssh may additionally obtain configuration data from a per-user 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 protocols 1 and 2.  The default is to
     use protocol 2 only, though this can be changed via the Protocol option
     in ssh_config(5) or the -1 and -2 options (see above).  Protocol 1 should
     not be used and is only offered to support legacy devices.	 It suffers
     from a number of cryptographic weaknesses and doesn't support many of the
     advanced features available for protocol 2.

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

     Host-based authentication works as follows: If the machine the user logs
     in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
     remote machine, and the user names are the same on both sides, or if the
     files ~/.rhosts or ~/.shosts exist in the user's home directory on the
     remote machine and contain a line containing the name of the client
     machine and the name of the user on that machine, the user is considered
     for login.	 Additionally, the server must be able to verify the client's
     host key (see the description of /etc/ssh/ssh_known_hosts and
     ~/.ssh/known_hosts, below) for login to be permitted.  This authentica-
     tion method closes security holes due to IP spoofing, DNS spoofing, and
     routing spoofing.	[Note to the administrator: /etc/hosts.equiv,
     ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse-
     cure and should be disabled if security is desired.]

     Public key authentication works as follows: The scheme is based on pub-
     lic-key cryptography, using cryptosystems where encryption and 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) contains
     a brief discussion of the DSA and RSA algorithms.

     The file ~/.ssh/authorized_keys lists the public keys that are permitted
     for logging in.  When the user logs in, the ssh program tells the server
     which key pair it would like to use for authentication.  The client
     proves that it has access to the private key and the server checks that
     the corresponding public key is authorized to accept the account.

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

     A variation on public key authentication is available in the form of 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
     authentication to prevent server spoofing or man-in-the-middle attacks,
     which could otherwise be used to circumvent the encryption.  The
     StrictHostKeyChecking option can be used to control logins to machines
     whose host key is not known or has changed.

     When the user's identity has been accepted by the server, the server
     either executes the given command in a non-interactive session or, if no
     command has been specified, logs into the machine and gives the user a
     normal shell as an interactive session.  All communication with the
     remote command or shell will be automatically encrypted.

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

     If a pseudo-terminal has been allocated the user may use the escape 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
	     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 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 the secure channel can be
     specified either on the command line or in a configuration file.  One
     possible application of TCP forwarding is a secure connection to a mail
     server; another is going through firewalls.

     In the example below, we look at encrypting communication between an IRC
     client and server, even though the IRC server does not directly support
     encrypted communications.	This works as follows: the user connects to
     the remote host using ssh, specifying a port to be used to forward con-
     nections to the remote server.  After that it is possible to start the
     service which is to be encrypted on the client machine, connecting to the
     same local port, and ssh will encrypt and forward the connection.

     The following example tunnels an IRC session from client machine
     ``127.0.0.1'' (localhost) to remote server ``server.example.com'':

	 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
	 $ irc -c '#users' -p 1234 pinky 127.0.0.1

     This tunnels a connection to IRC server ``server.example.com'', joining
     channel ``#users'', nickname ``pinky'', using port 1234.  It doesn't mat-
     ter which port is used, as long as it's greater than 1023 (remember, only
     root can open sockets on privileged ports) and doesn't conflict with any
     ports already in use.  The connection is forwarded to port 6667 on the
     remote server, since that's the standard port for IRC services.

     The -f option backgrounds ssh and the remote command ``sleep 10'' is
     specified to allow an amount of time (10 seconds, in the example) to
     start the service which is to be tunnelled.  If no connections are made
     within the time specified, ssh will exit.

X11 FORWARDING
     If the ForwardX11 variable is set to ``yes'' (or see the description of
     the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
     environment variable is set), the connection to the X11 display is auto-
     matically forwarded to the remote side in such a way that any X11 pro-
     grams started from the shell (or command) will go through the encrypted
     channel, and the connection to the real X server will be made from the
     local machine.  The user should not manually set DISPLAY.	Forwarding of
     X11 connections can be configured on the command line or in configuration
     files.

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

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

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

VERIFYING HOST KEYS
     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
     securely.	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
     remote network 10.0.99.0/24 using a point-to-point connection from
     10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
     to the remote network, at 192.168.1.15, allows it.

     On the client:

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

     On the server:

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

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

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

     Since an SSH-based setup entails a fair amount of overhead, it may be
     more suited to temporary setups, such as for wireless VPNs.  More 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_USER_AUTH	   This variable contains, for SSH2 only, a comma-sep-
			   arated list of authentication methods that were
			   successfuly used to authenticate. When possible,
			   these methods are extended with detailed informa-
			   tion on the credential used.

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

     TZ			   This variable is set to indicate the present time
			   zone if it was set when the daemon was started
			   (i.e. the daemon passes the value on to new 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
     allowed to change their environment.  For more information, see the
     PermitUserEnvironment option in sshd_config(5).

ENVIRONMENT
     SSH_USE_STRONG_RNG
	     The reseeding of the OpenSSL random generator is usually done
	     from /dev/urandom.	 If the SSH_USE_STRONG_RNG environment vari-
	     able is set to value other than 0 the OpenSSL random generator is
	     reseeded from /dev/random.	 The number of bytes read is defined
	     by the SSH_USE_STRONG_RNG value.  Minimum is 14 bytes.  This set-
	     ting is not recommended on the computers without the hardware
	     random generator because insufficient entropy causes the connec-
	     tion to be blocked until enough entropy is available.

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
	     described in the sshd(8) manual page.  This file is not highly
	     sensitive, but the recommended permissions are read/write for the
	     user, and not accessible by others.

     ~/.ssh/config
	     This is the per-user configuration file.  The file format and
	     configuration options are described in ssh_config(5).  Because of
	     the potential for abuse, this file must have strict permissions:
	     read/write for the user, and not writable by others.

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

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.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 3DES.

     ~/.ssh/identity.pub
     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_rsa.pub
	     Contains the public key for authentication.  These files are not
	     sensitive and can (but need not) be readable by anyone.

     ~/.ssh/known_hosts
	     Contains a list of host keys for all hosts the user has logged
	     into that are not already in the systemwide list of known host
	     keys.  See sshd(8) for further details of the format of this
	     file.

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

     /etc/hosts.equiv
	     This file is for host-based authentication (see above).  It
	     should only be writable by root.

     /etc/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 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.

IPV6
     IPv6 address can be used everywhere where IPv4 address. In all entries
     must be the IPv6 address enclosed in square brackets. Note: The square
     brackets are metacharacters for the shell and must be escaped in shell.

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

STANDARDS
     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			       December 3, 2024				   BSD