Unix/Linux Go Back    


Linux 2.6 - man page for ssh (linux section 1)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


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

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

SYNOPSIS
     ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
	 [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
	 [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec] [-O ctl_cmd]
	 [-o option] [-p port] [-R [bind_address:]port:host:hostport] [-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 replace rlogin and rsh, and provide secure encrypted
     communications between two untrusted hosts over an insecure network.  X11 connections and
     arbitrary TCP ports 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 depending on the
     protocol version used (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 speci-
	     fied 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 for-
	     warded X11 and TCP connections).  The compression algorithm is the same used by
	     gzip(1), and the ``level'' can be controlled by the CompressionLevel option for pro-
	     tocol version 1.  Compression is desirable on modem lines and other slow connec-
	     tions, 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''.  3des (triple-des) is an encrypt-decrypt-
	     encrypt triple with three different keys.	It is believed to be secure.  blowfish is
	     a fast block cipher; it appears very secure and is much faster than 3des.	des is
	     only supported in the ssh client for interoperability with legacy protocol 1 imple-
	     mentations that do not support the 3des cipher.  Its use is strongly discouraged due
	     to cryptographic weaknesses.  The default is ``3des''.

	     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
	     configuration file.

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

     -e escape_char
	     Sets the escape character for sessions with a pty (default: '~').	The escape char-
	     acter is only recognized at the beginning of a line.  The escape character followed
	     by a dot ('.') closes the connection; followed by control-Z suspends the connection;
	     and followed by itself sends the escape character once.  Setting the character to
	     ``none'' disables any escapes and makes the session fully transparent.

     -F configfile
	     Specifies an alternative per-user configuration file.  If a configuration file is
	     given on the command line, the system-wide configuration file (/etc/ssh/ssh_config)
	     will be ignored.  The default for the per-user configuration file is ~/.ssh/config.

     -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 back-
	     ground.  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 estab-
	     lished before placing itself in the background.

     -g      Allows remote hosts to connect to local forwarded ports.

     -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 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).  ssh
	     will also try to load certificate information from the filename obtained by append-
	     ing -cert.pub to identity filenames.

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

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

     -L [bind_address:]port:host:hostport
	     Specifies that the given port on the local (client) host is to be forwarded to the
	     given host and port on the remote side.  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 chan-
	     nel, and a connection is made to host port hostport from the remote machine.  Port
	     forwardings can also be specified in the configuration file.  IPv6 addresses can be
	     specified by enclosing the address in square brackets.  Only the superuser can for-
	     ward 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.

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

     -M      Places the ssh client into ``master'' mode for connection sharing.  Multiple -M
	     options places ssh into ``master'' mode with confirmation required before slave con-
	     nections are accepted.  Refer to the description of ControlMaster in ssh_config(5)
	     for details.

     -m mac_spec
	     Additionally, for protocol version 2 a comma-separated list of MAC (message authen-
	     tication code) algorithms can be specified in order of preference.  See the MACs
	     keyword for more information.

     -N      Do not execute a remote command.  This is useful for just forwarding ports (protocol
	     version 2 only).

     -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) and ``exit'' (request
	     the master to exit).

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

		   AddressFamily
		   BatchMode
		   BindAddress
		   ChallengeResponseAuthentication
		   CheckHostIP
		   Cipher
		   Ciphers
		   ClearAllForwardings
		   Compression
		   CompressionLevel
		   ConnectionAttempts
		   ConnectTimeout
		   ControlMaster
		   ControlPath
		   DynamicForward
		   EscapeChar
		   ExitOnForwardFailure
		   ForwardAgent
		   ForwardX11
		   ForwardX11Trusted
		   GatewayPorts
		   GlobalKnownHostsFile
		   GSSAPIAuthentication
		   GSSAPIDelegateCredentials
		   HashKnownHosts
		   Host
		   HostbasedAuthentication
		   HostKeyAlgorithms
		   HostKeyAlias
		   HostName
		   IdentityFile
		   IdentitiesOnly
		   IPQoS
		   KbdInteractiveDevices
		   KexAlgorithms
		   LocalCommand
		   LocalForward
		   LogLevel
		   MACs
		   NoHostAuthenticationForLocalhost
		   NumberOfPasswordPrompts
		   PasswordAuthentication
		   PermitLocalCommand
		   PKCS11Provider
		   Port
		   PreferredAuthentications
		   Protocol
		   ProxyCommand
		   PubkeyAuthentication
		   RekeyLimit
		   RemoteForward
		   RhostsRSAAuthentication
		   RSAAuthentication
		   SendEnv
		   ServerAliveInterval
		   ServerAliveCountMax
		   StrictHostKeyChecking
		   TCPKeepAlive
		   Tunnel
		   TunnelDevice
		   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      Quiet mode.  Causes most warning and diagnostic messages to be suppressed.

     -R [bind_address:]port:host:hostport
	     Specifies that the given port on the remote (server) host is to be forwarded to the
	     given host and port on the local side.  This works by allocating a socket to listen
	     to port on the remote side, and whenever a connection is made to this port, the con-
	     nection is forwarded over the secure channel, and a connection is made to host port
	     hostport 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 braces.

	     By default, the listening socket on the server will be bound to the loopback inter-
	     face only.  This may be overridden by specifying a bind_address.  An empty
	     bind_address, or the address '*', indicates that the remote socket should listen on
	     all interfaces.  Specifying a remote bind_address will only succeed if the server's
	     GatewayPorts option is enabled (see sshd_config(5)).

	     If the port argument is '0', the listen port will be dynamically allocated on the
	     server and reported to the client at run time.  When used together with -O forward
	     the allocated port will be printed to the standard output.

     -S ctl_path
	     Specifies the location of a control socket for connection sharing, or the string
	     ``none'' to disable connection sharing.  Refer to the description of ControlPath and
	     ControlMaster in ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote system.  Subsystems
	     are a feature of the SSH2 protocol which facilitate the use of SSH as a secure
	     transport for other applications (eg. sftp(1)).  The subsystem is specified as the
	     remote command.

     -T      Disable pseudo-tty allocation.

     -t      Force pseudo-tty allocation.  This can be used to execute arbitrary screen-based
	     programs on a remote machine, which can be very useful, e.g. when implementing menu
	     services.	Multiple -t options force tty allocation, even if ssh has no local tty.

     -V      Display the version number and exit.

     -v      Verbose mode.  Causes ssh to print debugging messages about its progress.	This is
	     helpful in debugging connection, authentication, and configuration problems.  Multi-
	     ple -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 and works with Protocol version 2 only.

     -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 config-
	     uration file.

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

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

     -x      Disables X11 forwarding.

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

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

     ssh may additionally obtain configuration data from a per-user configuration file and a sys-
     tem-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).  Both protocols support similar authentication methods, but protocol 2
     is the default since it provides additional mechanisms for confidentiality (the traffic is
     encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and integrity (hmac-md5, hmac-
     sha1, umac-64, hmac-ripemd160).  Protocol 1 lacks a strong mechanism for ensuring the
     integrity of the connection.

     The methods available for authentication are: GSSAPI-based authentication, host-based
     authentication, public key authentication, challenge-response authentication, and password
     authentication.  Authentication methods are tried in the order specified above, though pro-
     tocol 2 has a configuration option to change the default order: PreferredAuthentications.

     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
     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 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 pro-
     tocol automatically, using one of the DSA, ECDSA or RSA algorithms.  Protocol 1 is
     restricted to using only RSA keys, but protocol 2 may use any.  The HISTORY section of
     ssl(8) (on non-OpenBSD systems, see
     http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) contains a brief discus-
     sion 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 (protocol 2 DSA), ~/.ssh/id_ecdsa (protocol 2
     ECDSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub
     (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), or
     ~/.ssh/id_rsa.pub (protocol 2 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 authentica-
     tion: 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 pub-
     lic/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) for more information.

     Challenge-response authentication works as follows: The server sends an arbitrary
     "challenge" text, and prompts for a response.  Protocol 2 allows multiple challenges and
     responses; protocol 1 is restricted to just one challenge/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 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 identi-
     fication 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, or logs into the machine and gives the user a normal shell on the remote
     machine.  All communication with the remote command or shell will be automatically
     encrypted.

     If a pseudo-terminal has been allocated (normal login session), the user may use the escape
     characters noted below.

     If no pseudo-tty 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 functions through the
     use of an escape character.

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

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

     ~.      Disconnect.

     ~^Z     Background ssh.

     ~#      List forwarded connections.

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

     ~?      Display a list of escape characters.

     ~B      Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the
	     peer supports it).

     ~C      Open command line.  Currently this allows the addition of port forwardings using the
	     -L, -R and -D options (see above).  It also allows the cancellation of existing
	     remote port-forwardings using -KR[bind_address:]port.  !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 for SSH protocol version 2 and if
	     the peer supports it).

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 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 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 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 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 con-
     nection to the X11 display is automatically forwarded to the remote side in such a way that
     any X11 programs started from the shell (or command) will go through the encrypted channel,
     and the connection to the real X server will be made from the local machine.  The user
     should not manually set DISPLAY.  Forwarding of X11 connections can be configured on the
     command line or in configuration files.

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

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

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

VERIFYING HOST KEYS
     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).  Finger-
     prints 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.	Because of the difficulty of comparing host keys just by looking at hex 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 com-
     pletely 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 fin-
     gerprints verified by DNS.  An additional resource record (RR), SSHFP, is added to a zone-
     file and the connecting client is able to match the fingerprint with that of the key pre-
     sented.

     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) configura-
     tion option PermitTunnel controls whether the server supports this, and at what level (layer
     2 or 3 traffic).

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

     On the client:

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

     On the server:

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

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

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

     Since an SSH-based setup entails a fair amount of overhead, it may be more suited to tempo-
     rary 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 for-
			   ward X11 connections over the secure channel.  The user should nor-
			   mally not set DISPLAY explicitly, as that will render the X11 connec-
			   tion 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 cur-
			   rent 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 win-
			   dow 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 communicate with
			   the agent.

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

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

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

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

     USER		   Set to the name of the user logging in.

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

FILES
     ~/.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 parti-
	     tion, 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 per-
	     mission 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 authen-
	     tication without permitting login with rlogin/rsh.

     ~/.ssh/
	     This directory is the default location for all user-specific configuration and
	     authentication information.  There is no general requirement to keep the entire con-
	     tents of this directory secret, but the recommended permissions are read/write/exe-
	     cute for the user, and not accessible by others.

     ~/.ssh/authorized_keys
	     Lists the public keys (DSA/ECDSA/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 accessible by others.  It
	     may be group-writable provided that the group in question contains only the user.

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

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_rsa
	     Contains the private key for authentication.  These files contain sensitive data and
	     should be readable by the user but not accessible by others (read/write/execute).
	     ssh will simply ignore a private key file if it is accessible by others.  It is pos-
	     sible 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_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 informa-
	     tion.

     /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_rsa_key
	     These three files contain the private parts of the host keys and are used for host-
	     based authentication.  If protocol version 1 is used, ssh must be setuid root, since
	     the host key is readable only by root.  For protocol version 2, ssh uses
	     ssh-keysign(8) to access the host keys, eliminating the requirement that ssh be
	     setuid root when host-based authentication is used.  By default ssh is not setuid
	     root.

     /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 informa-
	     tion.

EXIT STATUS
     ssh exits with the exit status of the remote command or with 255 if an error occurred.

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

     The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.

     The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.

     The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.

     The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.

     The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.

     Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.

     Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, 2006.

     The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.

     The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.

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

     Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419,
     2006.

     The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

     Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, 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 created OpenSSH.  Markus Friedl contributed the support
     for SSH protocol versions 1.5 and 2.0.

BSD					  March 24, 2017				      BSD
Unix & Linux Commands & Man Pages : ©2000 - 2017 Unix and Linux Forums


All times are GMT -4. The time now is 08:17 AM.