ssh(1) — OpenSSH SSH client (remote login program)



  • 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 commu‐
         nications 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 meth‐
         ods (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 con‐
    	     nection.  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 con‐
    	     nection.  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 compres‐
    	     sion algorithm is the same used by gzip(1), and the “level” can be con‐
    	     trolled by the CompressionLevel option for protocol version 1.  Compres‐
    	     sion 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 sup‐
    	     ported values are “3des”, “blowfish”, and “des”.  For protocol version 2,
    	     cipher_spec is a comma-separated list of ciphers listed in order of pref‐
    	     erence.  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, option‐
    	     ally 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 brack‐
    	     ets.  Only the superuser can forward privileged ports.  By default, the
    	     local port is bound in accordance with the GatewayPorts setting.  How‐
    	     ever, an explicit bind_address may be used to bind the connection to a
    	     specific address.	The bind_address of “localhost” indicates that the
    	     listening port be bound for local use only, while an empty address or ‘*’
    	     indicates that the port should be available from all interfaces.
    
         -E log_file
    	     Append debug logs to log_file instead of standard error.
    
         -e escape_char
    	     Sets the escape character for sessions with a pty (default: ‘~’).	The
    	     escape character is only recognized at the beginning of a line.  The
    	     escape character followed by a dot (‘.’) closes the connection; followed
    	     by control-Z suspends the connection; and followed by itself sends the
    	     escape character once.  Setting the character to “none” disables any
    	     escapes and makes the session fully transparent.
    
         -F configfile
    	     Specifies an alternative per-user configuration file.  If a configuration
    	     file is given on the command line, the system-wide configuration file
    	     (/etc/ssh/ssh_config) will be ignored.  The default for the per-user con‐
    	     figuration 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 suc‐
    	     cessfully 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 ver‐
    	     sion 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and
    	     ~/.ssh/id_rsa for protocol version 2.  Identity files may also be speci‐
    	     fied on a per-host basis in the configuration file.  It is possible to
    	     have multiple -i options (and multiple identities specified in configura‐
    	     tion files).  If no certificates have been explicitly specified by the
    	     CertificateFile directive, ssh will also try to load certificate informa‐
    	     tion from the filename obtained by appending -cert.pub to identity file‐
    	     names.
    
         -J [user@]host[:port]
    	     Connect to the target host by first making a ssh connection to the jump
    	     host and then establishing a TCP forwarding to the ultimate destination
    	     from there.  Multiple jump hops may be specified separated by comma char‐
    	     acters.  This is a shortcut to specify a ProxyJump configuration direc‐
    	     tive.
    
         -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 speci‐
    	     fied 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 con‐
    	     nection to a specific address.  The bind_address of “localhost” indicates
    	     that the listening port be bound for local use only, while an empty
    	     address or ‘*’ indicates that the port should be available from all
    	     interfaces.
    
         -l login_name
    	     Specifies the user to log in as on the remote machine.  This also may be
    	     specified on a per-host basis in the configuration file.
    
         -M	     Places the ssh client into “master” mode for connection sharing.  Multi‐
    	     ple -M options places ssh into “master” mode with confirmation required
    	     before slave connections are accepted.  Refer to the description of
    	     ControlMaster in ssh_config(5) for details.
    
         -m mac_spec
    	     A comma-separated list of MAC (message authentication code) algorithms,
    	     specified in order of preference.	See the MACs keyword for more informa‐
    	     tion.
    
         -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 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 exe‐
    	     cution), “cancel” (cancel forwardings), “exit” (request the master to
    	     exit), and “stop” (request the master to stop accepting further multi‐
    	     plexing requests).
    
         -o option
    	     Can be used to give options in the format used in the configuration file.
    	     This is useful for specifying options for which there is no separate com‐
    	     mand-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 version 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 sup‐
    	     pressed.
    
         -R [bind_address:]port:host:hostport
         -R [bind_address:]port:local_socket
         -R remote_socket:host:hostport
         -R remote_socket:local_socket
    	     Specifies that connections to the given TCP port or Unix socket on the
    	     remote (server) host are to be forwarded to the given host and port, or
    	     Unix socket, on the local side.  This works by allocating a socket to
    	     listen to either a TCP port or to a Unix socket on the remote side.
    	     Whenever a connection is made to this port or Unix socket, the connection
    	     is forwarded over the secure channel, and a connection is made to either
    	     host port hostport, or local_socket, from the local machine.
    
    	     Port forwardings can also be specified in the configuration file.	Privi‐
    	     leged ports can be forwarded only when logging in as root on the remote
    	     machine.  IPv6 addresses can be specified by enclosing the address in
    	     square brackets.
    
    	     By default, TCP listening sockets on the server will be bound to the
    	     loopback interface only.  This may be overridden by specifying a
    	     bind_address.  An empty bind_address, or the address ‘*’, indicates that
    	     the remote socket should listen on all interfaces.	 Specifying a remote
    	     bind_address will only succeed if the server's GatewayPorts option is
    	     enabled (see sshd_config(5)).
    
    	     If the port argument is ‘0’, the listen port will be dynamically allo‐
    	     cated 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 stan‐
    	     dard output.
    
         -S ctl_path
    	     Specifies the location of a control socket for connection sharing, or the
    	     string “none” to disable connection sharing.  Refer to the description of
    	     ControlPath and ControlMaster in ssh_config(5) for details.
    
         -s	     May be used to request invocation of a subsystem on the remote system.
    	     Subsystems facilitate the use of SSH as a secure transport for other
    	     applications (e.g. sftp(1)).  The subsystem is specified as the remote
    	     command.
    
         -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 alloca‐
    	     tion, 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 configura‐
    	     tion 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 over‐
    	     ridden 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 authoriza‐
    	     tion 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 sub‐
    	     jected to the X11 SECURITY extension controls.
    
         -y	     Send log information using the syslog(3) system module.  By default this
    	     information is sent to stderr.
    
         ssh may additionally obtain configuration data from a per-user configuration file
         and a system-wide configuration file.  The file format and configuration options
         are described in ssh_config(5).
    
    AUTHENTICATION
         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 avail‐
         able for protocol 2.
    
         The methods available for authentication are: GSSAPI-based authentication, host-
         based authentication, public key authentication, challenge-response authentica‐
         tion, 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 con‐
         taining 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 ver‐
         ify the client's host key (see the description of /etc/ssh/ssh_known_hosts and
         ~/.ssh/known_hosts, below) for login to be permitted.  This authentication method
         closes security holes due to IP spoofing, DNS spoofing, and routing spoofing.
         [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh pro‐
         tocol 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 encryp‐
         tion 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 auto‐
         matically, 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 log‐
         ging 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 autho‐
         rized to accept the account.
    
         The user creates his/her key pair by running ssh-keygen(1).  This stores the pri‐
         vate 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 pub‐
         lic key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (DSA),
         ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), or
         ~/.ssh/id_rsa.pub (RSA) in the user's home directory.  The user should then copy
         the public key to ~/.ssh/authorized_keys in his/her home directory on the remote
         machine.  The authorized_keys file corresponds to the conventional ~/.rhosts
         file, and has one key per line, though the lines can be very long.	 After this,
         the user can log in without giving the password.
    
         A variation on public key authentication is available in the form of certificate
         authentication: instead of a set of public/private keys, signed certificates are
         used.  This has the advantage that a single trusted certification authority can
         be used in place of many 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 pass‐
         word.  The password is sent to the remote host for checking; however, since all
         communications are encrypted, the password cannot be seen by someone listening on
         the network.
    
         ssh automatically maintains and checks a database containing identification for
         all hosts it has ever been used with.  Host keys are stored in ~/.ssh/known_hosts
         in the user's home directory.  Additionally, the file /etc/ssh/ssh_known_hosts is
         automatically checked for known hosts.  Any new hosts are automatically added to
         the user's file.  If a host's identification ever changes, ssh warns about this
         and disables password authentication to prevent server spoofing or man-in-the-
         middle attacks, which could otherwise be used to circumvent the encryption.  The
         StrictHostKeyChecking option can be used to control logins to machines whose host
         key is not known or has changed.
    
         When the user's identity has been accepted by the server, the server either exe‐
         cutes 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 interac‐
         tive session.  All communication with the remote command or shell will be auto‐
         matically 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 char‐
         acter 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 functions
         through the use of an escape character.
    
         A single tilde character can be sent as ~~ or by following the tilde by a charac‐
         ter other than those described below.  The escape character must always follow a
         newline to be interpreted as special.  The escape character can be changed in
         configuration files using the EscapeChar configuration directive or on the com‐
         mand 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 ses‐
    	     sions 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 forward‐
    	     ings 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 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 commu‐
         nications.	 This works as follows: the user connects to the remote host using
         ssh, specifying a port to be used to forward connections to the remote server.
         After that it is possible to start the service which is to be encrypted on the
         client machine, connecting to the same local port, and ssh will encrypt and for‐
         ward the connection.
    
         The following example tunnels an IRC session from client machine “127.0.0.1”
         (localhost) to remote server “server.example.com”:
    
    	 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
    	 $ irc -c '#users' -p 1234 pinky 127.0.0.1
    
         This tunnels a connection to IRC server “server.example.com”, joining channel
         “#users”, nickname “pinky”, using port 1234.  It doesn't matter which port is
         used, as long as it's greater than 1023 (remember, only root can open sockets on
         privileged ports) and doesn't conflict with any ports already in use.  The con‐
         nection 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 automatically forwarded to the
         remote side in such a way that any X11 programs started from the shell (or com‐
         mand) 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 Xau‐
         thority 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 connec‐
         tion 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 “yes”, a small ASCII graphic gets dis‐
         played 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 dis‐
         played.  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 avail‐
         able: 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 fin‐
         gerprint 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.exam‐
         ple.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 sup‐
         ports this, and at what level (layer 2 or 3 traffic).
    
         The following example would connect client network 10.0.50.0/24 with remote net‐
         work 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2,
         provided that the SSH server running on the gateway to the remote network, at
         192.168.1.15, allows it.
    
         On the client:
    
    	   # ssh -f -w 0:1 192.168.1.15 true
    	   # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
    	   # route add 10.0.99.0/24 10.1.1.2
    
         On the server:
    
    	   # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
    	   # route add 10.0.50.0/24 10.1.1.1
    
         Client access may be more finely tuned via the /root/.ssh/authorized_keys file
         (see below) and the PermitRootLogin server option.	 The following entry would
         permit connections on tun(4) device 1 from user “jane” and on tun device 2 from
         user “john”, if PermitRootLogin is set to “forced-commands-only”:
    
           tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
           tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
    
         Since an SSH-based setup entails a fair amount of overhead, it may be more suited
         to temporary setups, such as for wireless VPNs.  More permanent VPNs are better
         provided by tools such as ipsecctl(8) and isakmpd(8).
    
    ENVIRONMENT
         ssh will normally set the following environment variables:
    
         DISPLAY		   The DISPLAY variable indicates the location of the X11
    			   server.  It is automatically set by ssh to point to a value
    			   of the form “hostname:n”, where “hostname” indicates the
    			   host where the shell runs, and ‘n’ is an integer ≥ 1.  ssh
    			   uses this special value to forward X11 connections over the
    			   secure channel.  The user should normally not set DISPLAY
    			   explicitly, as that will render the X11 connection insecure
    			   (and will require the user to manually copy any required
    			   authorization cookies).
    
         HOME		   Set to the path of the user's home directory.
    
         LOGNAME		   Synonym for USER; set for compatibility with systems that
    			   use this variable.
    
         MAIL		   Set to the path of the user's mailbox.
    
         PATH		   Set to the default PATH, as specified when compiling ssh.
    
         SSH_ASKPASS	   If ssh needs a passphrase, it will read the passphrase from
    			   the current terminal if it was run from a terminal.	If ssh
    			   does not have a terminal associated with it but DISPLAY and
    			   SSH_ASKPASS are set, it will execute the program specified
    			   by SSH_ASKPASS and open an X11 window to read the
    			   passphrase.	This is particularly useful when calling ssh
    			   from a .xsession or related script.	(Note that on some
    			   machines it may be necessary to redirect the input from
    			   /dev/null to make this work.)
    
         SSH_AUTH_SOCK	   Identifies the path of a UNIX-domain socket used to commu‐
    			   nicate with the agent.
    
         SSH_CONNECTION	   Identifies the client and server ends of the connection.
    			   The variable contains four space-separated values: client
    			   IP address, client port number, server IP address, and
    			   server port number.
    
         SSH_ORIGINAL_COMMAND  This variable contains the original command line if a
    			   forced command is executed.	It can be used to extract the
    			   original arguments.
    
         SSH_USER_AUTH	   This variable contains, for SSH2 only, a comma-separated
    			   list of authentication methods that were successfuly used
    			   to authenticate. When possible, these methods are extended
    			   with detailed information 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 command.  If the cur‐
    			   rent 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 connections).
    
         USER		   Set to the name of the user logging in.
    
         Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
         “VARNAME=value” to the environment if the file exists and users are allowed to
         change their environment.	For more information, see the PermitUserEnvironment
         option in sshd_config(5).
    
    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 variable 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 setting is not rec‐
    	     ommended on the computers without the hardware random generator because
    	     insufficient entropy causes the connection 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 configura‐
    	     tion 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 recom‐
    	     mended 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 configura‐
    	     tion 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 sensi‐
    	     tive 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 gener‐
    	     ating 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 sensi‐
    	     tive 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 Ylo‐
         nen.  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				    March 16, 2019				   BSD
    

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