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ssh(1)

NAME

     ssh -- OpenSSH SSH client (remote login program)

SYNOPSIS

     ssh [-1246AaCfgkNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D port]
	 [-e escape_char] [-F configfile] [-i identity_file]
	 [-L port:host:hostport] [-l login_name] [-m mac_spec] [-o option]
	 [-p port] [-R port:host:hostport] [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/IP 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.

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

   SSH protocol version 1
     First, 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, the user is immediately permitted to log in.
     Second, if .rhosts or .shosts exists in the user's home directory on the
     remote machine and contains a line containing the name of the client
     machine and the name of the user on that machine, the user is permitted
     to log in.  This form of authentication alone is normally not allowed by
     the server because it is not secure.

     The second authentication method is the rhosts or hosts.equiv method com-
     bined with RSA-based host authentication.	It means that if the login
     would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or
     /etc/ssh/shosts.equiv, and if additionally the server can verify the
     client's host key (see /etc/ssh/ssh_known_hosts and
     $HOME/.ssh/known_hosts in the FILES section), only then is login permit-
     ted.  This authentication method closes security holes due to IP spoof-
     ing, DNS spoofing and routing spoofing.  [Note to the administrator:
     /etc/hosts.equiv, $HOME/.rhosts, and the rlogin/rsh protocol in general,
     are inherently insecure and should be disabled if security is desired.]

     As a third authentication method, ssh supports RSA based authentication.
     The scheme is based on public-key cryptography: there are cryptosystems
     where encryption and decryption are done using separate keys, and it is
     not possible to derive the decryption key from the encryption key.  RSA
     is one such system.  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.

     The file $HOME/.ssh/authorized_keys lists the public keys that are per-
     mitted 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
     server checks if this key is permitted, and if so, sends the user (actu-
     ally the ssh program running on behalf of the user) a challenge, a random
     $HOME/.ssh/identity.pub in the user's home directory.  The user should
     then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
     directory on the remote machine (the authorized_keys file corresponds to
     the conventional $HOME/.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.  RSA authentication is much more secure than rhosts authen-
     tication.

     The most convenient way to use RSA authentication may be with an authen-
     tication agent.  See ssh-agent(1) for more information.

     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 protocol version 2
     When a user connects using protocol version 2, similar authentication
     methods are available.  Using the default values for
     PreferredAuthentications, the client will try to authenticate first using
     the hostbased method; if this method fails, public key authentication is
     attempted, and finally if this method fails, keyboard-interactive and
     password authentication are tried.

     The public key method is similar to RSA authentication described in the
     previous section and allows the RSA or DSA algorithm to be used: The
     client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to
     sign the session identifier and sends the result to the server.  The
     server checks whether the matching public key is listed in
     $HOME/.ssh/authorized_keys and grants access if both the key is found and
     the signature is correct.	The session identifier is derived from a
     shared Diffie-Hellman value and is only known to the client and the
     server.

     If public key authentication fails or is not available, a password can be
     sent encrypted to the remote host to prove the user's identity.

     Additionally, ssh supports hostbased or challenge response authentica-
     tion.

     Protocol 2 provides additional mechanisms for confidentiality (the traf-
     fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
     (hmac-md5, hmac-sha1).  Note that protocol 1 lacks a strong mechanism for
     ensuring the integrity of the connection.

   Login session and remote execution
     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.

     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 for SSH protocol
	     version 2 and if the peer supports it).

     ~C      Open command line (only useful for adding port forwardings using
	     the -L and -R options).

     ~R      Request rekeying of the connection (only useful for SSH protocol
	     version 2 and if the peer supports it).

   X11 and TCP forwarding
     If the ForwardX11 variable is set to ``yes'' (or see the description of
     the -X and -x options described later) 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 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.  For-
     warding of X11 connections can be configured on the command line or in
     configuration files.  Take note that X11 forwarding can represent a secu-
     rity hazard.

     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 described later) and the user is using an authenti-
     cation agent, the connection to the agent is automatically forwarded to
     the remote side.

     $HOME/.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 a trojan horse from getting the user's pass-
     word.  Another purpose of this mechanism is to prevent man-in-the-middle
     attacks which could otherwise be used to circumvent the encryption.  The
     StrictHostKeyChecking option can be used to prevent logins to machines
     whose host key is not known or has changed.

     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
	     Specify the interface to transmit from on machines with multiple
	     interfaces or aliased addresses.

     -C      Requests compression of all data (including stdin, stdout,
	     stderr, and data for forwarded X11 and TCP/IP connections).  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 blowfish | 3des | des
	     Selects the cipher to use for encrypting the session.  3des is
	     used by default.  It is believed to be secure.  3des (triple-des)
	     is an encrypt-decrypt-encrypt triple with three different keys.
	     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 implementations that
	     do not support the 3des cipher.  Its use is strongly discouraged
	     due to cryptographic weaknesses.

     -c cipher_spec
	     tion 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.

     -e ch | ^ch | none
	     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 $HOME/.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.

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

     -I smartcard_device
	     Specifies which smartcard device to use.  The argument is the
	     device ssh should use to communicate with a smartcard used for
	     storing the user's private RSA key.

     -i identity_file
	     Selects a file from which the identity (private key) for RSA or
	     DSA authentication is read.  The default is $HOME/.ssh/identity
	     for protocol version 1, and $HOME/.ssh/id_rsa and
	     $HOME/.ssh/id_dsa 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 identi-
	     ties specified in configuration files).

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

     -L 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,
	     and whenever a connection is made to this port, the connection is
	     forwarded over the secure channel, and a connection is made to
	     host port hostport from the remote machine.  Port forwardings can
	     also be specified in the configuration file.  Only root can for-
	     ward privileged ports.  IPv6 addresses can be specified with an
	     alternative syntax: port/host/hostport.


     -N      Do not execute a remote command.  This is useful for just for-
	     warding 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 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).

		   AddressFamily
		   BatchMode
		   BindAddress
		   ChallengeResponseAuthentication
		   CheckHostIP
		   Cipher
		   Ciphers
		   ClearAllForwardings
		   Compression
		   CompressionLevel
		   ConnectionAttempts
		   ConnectionTimeout
		   DynamicForward
		   EscapeChar
		   ForwardAgent
		   ForwardX11
		   ForwardX11Trusted
		   GatewayPorts
		   GlobalKnownHostsFile
		   GSSAPIAuthentication
		   GSSAPIDelegateCredentials
		   Host
		   HostbasedAuthentication
		   HostKeyAlgorithms
		   HostKeyAlias
		   HostName
		   IdentityFile
		   IdentitiesOnly
		   LocalForward
		   LogLevel
		   MACs
		   NoHostAuthenticationForLocalhost
		   NumberOfPasswordPrompts
		   PasswordAuthentication
		   Port
		   PreferredAuthentications
		   Protocol
		   StrictHostKeyChecking
		   TCPKeepAlive
		   UsePrivilegedPort
		   User
		   UserKnownHostsFile
		   VerifyHostKeyDNS
		   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 all warning and diagnostic messages to be
	     suppressed.

     -R 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 connec-
	     tion is forwarded over the secure channel, and a connection is
	     made to host port hostport from the local machine.  Port forward-
	     ings 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 with an alternative
	     syntax: port/host/hostport.

     -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 appli-
	     cations (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 arbi-
	     trary 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.

     -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.

     -x      Disables X11 forwarding.

     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 manu-
	      ally 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 particu-
	      larly useful when calling ssh from a .Xsession or related
	      script.  (Note that on some machines it may be necessary to re-
	      direct 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
	      The variable contains the original command line if a forced com-
	      mand is executed.  It can be used to extract the original argu-
	      ments.

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

     TZ       The timezone variable is set to indicate the present timezone 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 $HOME/.ssh/environment, and adds lines of the
     format ``VARNAME=value'' to the environment if the file exists and if
     users are allowed to change their environment.  For more information, see
	     Contains the authentication identity of the user.	They are for
	     protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
	     These files contain sensitive data and should be readable by the
	     user but not accessible by others (read/write/execute).  Note
	     that ssh ignores a private key file if it is accessible by oth-
	     ers.  It is possible to specify a passphrase when generating the
	     key; the passphrase will be used to encrypt the sensitive part of
	     this file using 3DES.

     $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
	     Contains the public key for authentication (public part of the
	     identity file in human-readable form).  The contents of the
	     $HOME/.ssh/identity.pub file should be added to the file
	     $HOME/.ssh/authorized_keys on all machines where the user wishes
	     to log in using protocol version 1 RSA authentication.  The con-
	     tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
	     should be added to $HOME/.ssh/authorized_keys on all machines
	     where the user wishes to log in using protocol version 2 DSA/RSA
	     authentication.  These files are not sensitive and can (but need
	     not) be readable by anyone.  These files are never used automati-
	     cally and are not necessary; they are only provided for the con-
	     venience of the user.

     $HOME/.ssh/config
	     This is the per-user configuration file.  The file format and
	     configuration options are described in ssh_config(5).

     $HOME/.ssh/authorized_keys
	     Lists the public keys (RSA/DSA) that can be used for logging in
	     as this user.  The format of this file is described in the
	     sshd(8) manual page.  In the simplest form the format is the same
	     as the .pub identity files.  This file is not highly sensitive,
	     but the recommended permissions are read/write for the user, and
	     not accessible by others.

     /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.	This file should be world-
	     readable.	This file contains public keys, one per line, in the
	     following format (fields separated by spaces): system name, pub-
	     lic key and optional comment field.  When different names are
	     used for the same machine, all such names should be listed, sepa-
	     rated by commas.  The format is described in the sshd(8) manual
	     page.

	     The canonical system name (as returned by name servers) is used
	     by sshd(8) to verify the client host when logging in; other names
	     are needed because ssh does not convert the user-supplied name to
	     a canonical name before checking the key, because someone with
	     access to the name servers would then be able to fool host
	     authentication.

     /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,
	     method is used.  By default ssh is not setuid root.

     $HOME/.rhosts
	     This file is used in rhosts authentication to list the host/user
	     pairs that are permitted to log in.  (Note that this file is also
	     used by rlogin and rsh, which makes using this file insecure.)
	     Each line of the file contains a host name (in the canonical form
	     returned by name servers), and then a user name on that host,
	     separated by a space.  On some machines this file may need to be
	     world-readable if the user's home directory is on a 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 permission for most machines is
	     read/write for the user, and not accessible by others.

	     Note that by default sshd(8) will be installed so that it
	     requires successful RSA host authentication before permitting
	     rhosts authentication.  If the server machine does not have the
	     client's host key in /etc/ssh/ssh_known_hosts, it can be stored
	     in $HOME/.ssh/known_hosts.  The easiest way to do this is to con-
	     nect back to the client from the server machine using ssh; this
	     will automatically add the host key to $HOME/.ssh/known_hosts.

     $HOME/.shosts
	     This file is used exactly the same way as .rhosts.  The purpose
	     for having this file is to be able to use rhosts authentication
	     with ssh without permitting login with rlogin or rsh(1).

     /etc/hosts.equiv
	     This file is used during rhosts authentication.  It contains
	     canonical hosts names, one per line (the full format is described
	     in the sshd(8) manual page).  If the client host is found in this
	     file, login is automatically permitted provided client and server
	     user names are the same.  Additionally, successful RSA host
	     authentication is normally required.  This file should only be
	     writable by root.

     /etc/ssh/shosts.equiv
	     This file is processed exactly as /etc/hosts.equiv.  This file
	     may be useful to permit logins using ssh but not using
	     rsh/rlogin.

     /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.

     $HOME/.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.

     $HOME/.ssh/environment
	     Contains additional definitions for environment variables, see
	     section ENVIRONMENT above.

DIAGNOSTICS

     ssh exits with the exit status of the remote command or with 255 if an
     Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
     2002, work in progress material.

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.

FreeBSD 5.4		      September 25, 1999		   FreeBSD 5.4

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