SSH(1)			    System Reference Manual			SSH(1)

NAME
     ssh - OpenSSH secure shell client (remote login program)

SYNOPSIS
     ssh [-l login_name] [hostname | user@hostname] [command]

     ssh [-afgknqtvxACNPTX246] [-c cipher_spec] [-e escape_char] [-i
	 identity_file] [-l login_name] [-o option] [-p port] [-L
	 port:host:hostport] [-R port:host:hostport] [hostname |
	 user@hostname] [command]

DESCRIPTION
     ssh (Secure Shell) 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 untrust-
     ed 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. The user must prove
     his/her identity to the remote machine using one of several methods de-
     pending on the protocol version used:

   SSH protocol version 1

     First, if the machine the user logs in from is listed in /etc/hosts.equiv
     or /usr/local/etc/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 ma-
     chine 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 (and primary) authentication method is the rhosts or
     hosts.equiv method combined with RSA-based host authentication.  It means
     that if the login would be permitted by $HOME/.rhosts, $HOME/.shosts,
     /etc/hosts.equiv, or /usr/local/etc/shosts.equiv, and if additionally the
     server can verify the client's host key (see /etc/ssh_known_hosts and
     $HOME/.ssh/known_hosts in the FILES section), only then login is 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 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 server
     checks if this key is permitted, and if so, sends the user (actually the
     ssh program running on behalf of the user) a challenge, a random number,
     encrypted by the user's public key.  The challenge can only be decrypted
     using the proper private key.  The user's client then decrypts the chal-
     lenge using the private key, proving that he/she knows the private key
     but without disclosing it to the server.


     ssh implements the RSA authentication protocol automatically.  The user
     creates his/her RSA key pair by running ssh-keygen(1).  This stores the
     private key in $HOME/.ssh/identity and the public key in
     $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 the protocol version 2 different authentica-
     tion methods are available: At first, the client attempts to authenticate
     using the public key method.  If this method fails password authentica-
     tion is tried.

     The public key method is similar to RSA authentication described in the
     previous section except that the DSA algorithm is used instead of the
     patented RSA algorithm.  The client uses his private DSA key
     $HOME/.ssh/id_dsa 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_keys2 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 serv-
     er.

     If public key authentication fails or is not available a password can be
     sent encrypted to the remote host for proving the user's identity.	 This
     protocol 2 implementation does not yet support Kerberos or S/Key authen-
     tication.

     Protocol 2 provides additional mechanisms for confidentiality (the traf-
     fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
     (hmac-sha1, hmac-md5).  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 ei-
     ther 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
     can disconnect with ~., and suspend ssh with ~^Z. All forwarded connec-
     tions can be listed with ~# and if the session blocks waiting for for-
     warded X11 or TCP/IP connections to terminate, it can be backgrounded
     with ~& (this should not be used while the user shell is active, as it
     can cause the shell to hang).  All available escapes can be listed with
     ~?.

     A single tilde character can be sent as ~~ (or by following the tilde by
     a character other than those described above).  The escape character must
     always follow a newline to be interpreted as special.  The escape charac-
     ter can be changed in configuration files or on the command line.


     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 in on the remote machine
     exists and all X11 and TCP/IP connections have been closed.  The exit
     status of the remote program is returned as the exit status of ssh.

   X11 and TCP forwarding

     If 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 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 user is using an authentication agent, the connection to the agent
     is automatically forwarded to the remote side unless disabled on command
     line or in a configuration file.

     Forwarding of arbitrary TCP/IP connections over the secure channel can be
     specified either on command line or in a configuration file.  One possi-
     ble application of TCP/IP forwarding is a secure connection to an elec-
     tronic purse; another is going trough firewalls.

   Server authentication

     ssh automatically maintains and checks a database containing identifica-
     tions for all hosts it has ever been used with.  RSA host keys are stored
     in $HOME/.ssh/known_hosts and DSA host keys are stored in
     $HOME/.ssh/known_hosts2 in the user's home directory.  Additionally, the
     files /etc/ssh_known_hosts and /etc/ssh_known_hosts2 are 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 password.  Another purpose of this mechanism is to
     prevent man-in-the-middle attacks which could otherwise be used to cir-
     cumvent the encryption.  The StrictHostKeyChecking option (see below) can
     be used to prevent logins to machines whose host key is not known or has
     changed.

OPTIONS
     -a	     Disables forwarding of the authentication agent connection.

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

     -c blowfish|3des
	     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.
	     It is presumably more secure than the des cipher which is no
	     longer supported in ssh. blowfish is a fast block cipher, it ap-
	     pears very secure and is much faster than 3des.

     -c 3des-cbc,blowfish-cbc,arcfour,cast128-cbc
	     Additionally, for protocol version 2 a comma-separated list of
	     ciphers can be specified in order of preference. Protocol version
	     2 supports 3DES, Blowfish and CAST128 in CBC mode and Arcfour.

     -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	     Requests ssh to go to background just before command execution.
	     This is useful if ssh is going to ask for passwords or passphras-
	     es, 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 identity_file
	     Selects the file from which the identity (private key) for RSA
	     authentication is read.  Default is $HOME/.ssh/identity in the
	     user's home directory.  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 config-
	     uration files).

     -k	     Disables forwarding of Kerberos tickets and AFS tokens.  This may
	     also be specified on a per-host basis in the configuration file.

     -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.

     -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 ma-
	     chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
	     an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
	     tomatically 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.)

     -N	     Do not execute a remote command.  This is usefull if you just
	     want to forward ports (protocol version 2 only).

     -o option
	     Can be used to give options in the format used in the config
	     file.  This is useful for specifying options for which there is
	     no separate command-line flag.  The option has the same format as
	     a line in the configuration file.

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

     -P	     Use a non-privileged port for outgoing connections.  This can be
	     used if your firewall does not permit connections from privileged
	     ports.  Note that this option turns off RhostsAuthentication and
	     RhostsRSAAuthentication.

     -q	     Quiet mode.  Causes all warning and diagnostic messages to be
	     suppressed.  Only fatal errors are displayed.

     -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.

     -T	     Disable pseudo-tty allocation (protocol version 2 only).

     -v	     Verbose mode.  Causes ssh to print debugging messages about its
	     progress.	This is helpful in debugging connection, authentica-
	     tion, and configuration problems.	The verbose mode is also used
	     to display skey(1) challenges, if the user entered "s/key" as
	     password.

     -x	     Disables X11 forwarding.

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

     -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 (see
	     below).  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 con-
	     figuration files; see the Compress option below.

     -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

     -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.

     -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.

CONFIGURATION FILES
     ssh obtains configuration data from the following sources (in this or-
     der): command line options, user's configuration file
     ($HOME/.ssh/config), and system-wide configuration file
     (/usr/local/etc/ssh_config). For each parameter, the first obtained value
     will be used.  The configuration files contain sections bracketed by
     ``Host'' specifications, and that section is only applied for hosts that
     match one of the patterns given in the specification.  The matched host
     name is the one given on the command line.

     Since the first obtained value for each parameter is used, more host-spe-
     cific declarations should be given near the beginning of the file, and
     general defaults at the end.

     The configuration file has the following format:

     Empty lines and lines starting with `#' are comments.

     Otherwise a line is of the format ``keyword arguments''. The possible
     keywords and their meanings are as follows (note that the configuration
     files are case-sensitive):

     Host    Restricts the following declarations (up to the next Host key-
	     word) to be only for those hosts that match one of the patterns
	     given after the keyword.  `*' and `?' can be used as wildcards in
	     the patterns.  A single `*' as a pattern can be used to provide
	     global defaults for all hosts.  The host is the hostname argument
	     given on the command line (i.e., the name is not converted to a
	     canonicalized host name before matching).

     AFSTokenPassing
	     Specifies whether to pass AFS tokens to remote host.  The argu-
	     ment to this keyword must be ``yes'' or ``no''.

     BatchMode
	     If set to ``yes'', passphrase/password querying will be disabled.
	     This option is useful in scripts and other batch jobs where you
	     have no user to supply the password.  The argument must be
	     ``yes'' or ``no''.

     CheckHostIP
	     If this flag is set to ``yes'', ssh will additionally check the
	     host ip address in the known_hosts file.  This allows ssh to de-
	     tect if a host key changed due to DNS spoofing.  If the option is
	     set to ``no'', the check will not be executed.

     Cipher  Specifies the cipher to use for encrypting the session.  Current-
	     ly, ``blowfish'', and ``3des'' are supported.  The default is
	     ``3des''.

     Ciphers
	     Specifies the ciphers allowed for protocol version 2 in order of
	     preference.  Multiple ciphers must be comma-separated.  The de-
	     fault is ``3des-cbc,blowfish-cbc,arcfour,cast128-cbc''.

     Compression
	     Specifies whether to use compression.  The argument must be
	     ``yes'' or ``no''.

     CompressionLevel
	     Specifies the compression level to use if compression is enable.
	     The argument must be an integer from 1 (fast) to 9 (slow, best).
	     The default level is 6, which is good for most applications.  The
	     meaning of the values is the same as in gzip(1).

     ConnectionAttempts
	     Specifies the number of tries (one per second) to make before
	     falling back to rsh or exiting.  The argument must be an integer.
	     This may be useful in scripts if the connection sometimes fails.

     DSAAuthentication
	     Specifies whether to try DSA authentication.  The argument to
	     this keyword must be ``yes'' or ``no''. DSA authentication will
	     only be attempted if a DSA identity file exists.  Note that this
	     option applies to protocol version 2 only.

     EscapeChar
	     Sets the escape character (default: `~'). The escape character
	     can also be set on the command line.  The argument should be a
	     single character, `^' followed by a letter, or ``none'' to dis-
	     able the escape character entirely (making the connection trans-
	     parent for binary data).

     FallBackToRsh
	     Specifies that if connecting via ssh fails due to a connection
	     refused error (there is no sshd(8) listening on the remote host),
	     rsh(1) should automatically be used instead (after a suitable
	     warning about the session being unencrypted).  The argument must
	     be ``yes'' or ``no''.

     ForwardAgent
	     Specifies whether the connection to the authentication agent (if
	     any) will be forwarded to the remote machine.  The argument must
	     be ``yes'' or ``no''. The default is ``no''.

     ForwardX11
	     Specifies whether X11 connections will be automatically redirect-
	     ed over the secure channel and DISPLAY set.  The argument must be
	     ``yes'' or ``no''. The default is ``no''.

     GatewayPorts
	     Specifies whether remote hosts are allowed to connect to local
	     forwarded ports.  The argument must be ``yes'' or ``no''. The de-
	     fault is ``no''.

     GlobalKnownHostsFile
	     Specifies a file to use instead of /etc/ssh_known_hosts.

     HostName
	     Specifies the real host name to log into.	This can be used to
	     specify nicknames or abbreviations for hosts.  Default is the
	     name given on the command line.  Numeric IP addresses are also
	     permitted (both on the command line and in HostName specifica-
	     tions).

     IdentityFile
	     Specifies the file from which the user's RSA authentication iden-
	     tity is read (default $HOME/.ssh/identity in the user's home di-
	     rectory).	Additionally, any identities represented by the au-
	     thentication agent will be used for authentication.  The file
	     name may use the tilde syntax to refer to a user's home directo-
	     ry.  It is possible to have multiple identity files specified in
	     configuration files; all these identities will be tried in se-
	     quence.

     IdentityFile2
	     Specifies the file from which the user's DSA authentication iden-
	     tity is read (default $HOME/.ssh/id_dsa in the user's home direc-
	     tory).  The file name may use the tilde syntax to refer to a us-
	     er's home directory.  It is possible to have multiple identity
	     files specified in configuration files; all these identities will
	     be tried in sequence.

     KeepAlive
	     Specifies whether the system should send keepalive messages to
	     the other side.  If they are sent, death of the connection or
	     crash of one of the machines will be properly noticed.  However,
	     this means that connections will die if the route is down tem-
	     porarily, and some people find it annoying.

	     The default is ``yes'' (to send keepalives), and the client will
	     notice if the network goes down or the remote host dies.  This is
	     important in scripts, and many users want it too.

	     To disable keepalives, the value should be set to ``no'' in both
	     the server and the client configuration files.

     KerberosAuthentication
	     Specifies whether Kerberos authentication will be used.  The ar-
	     gument to this keyword must be ``yes'' or ``no''.

     KerberosTgtPassing
	     Specifies whether a Kerberos TGT will be forwarded to the server.
	     This will only work if the Kerberos server is actually an AFS
	     kaserver.	The argument to this keyword must be ``yes'' or
	     ``no''.

     LocalForward
	     Specifies that a TCP/IP port on the local machine be forwarded
	     over the secure channel to given host:port from the remote ma-
	     chine.  The first argument must be a port number, and the second
	     must be host:port.	 Multiple forwardings may be specified, and
	     additional forwardings can be given on the command line.  Only
	     the superuser can forward privileged ports.

     LogLevel
	     Gives the verbosity level that is used when logging messages from
	     ssh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE
	     and DEBUG.	 The default is INFO.

     NumberOfPasswordPrompts
	     Specifies the number of password prompts before giving up.	 The
	     argument to this keyword must be an integer.  Default is 3.

     PasswordAuthentication
	     Specifies whether to use password authentication.	The argument
	     to this keyword must be ``yes'' or ``no''. Note that this option
	     applies to both protocol version 1 and 2.

     Port    Specifies the port number to connect on the remote host.  Default
	     is 22.

     Protocol
	     Specifies the protocol versions ssh should support in order of
	     preference.  The possible values are ``1'' and ``2''. Multiple
	     versions must be comma-separated.	The default is ``1,2''. This
	     means that ssh tries version 1 and falls back to version 2 if
	     version 1 is not available.

     ProxyCommand
	     Specifies the command to use to connect to the server.  The com-
	     mand string extends to the end of the line, and is executed with
	     /bin/sh. In the command string, `%h' will be substituted by the
	     host name to connect and `%p' by the port.	 The command can be
	     basically anything, and should read from its standard input and
	     write to its standard output.  It should eventually connect an
	     sshd(8) server running on some machine, or execute sshd -i some-
	     where.  Host key management will be done using the HostName of
	     the host being connected (defaulting to the name typed by the us-
	     er).  Note that CheckHostIP is not available for connects with a
	     proxy command.




     RemoteForward
	     Specifies that a TCP/IP port on the remote machine be forwarded
	     over the secure channel to given host:port from the local ma-
	     chine.  The first argument must be a port number, and the second
	     must be host:port.	 Multiple forwardings may be specified, and
	     additional forwardings can be given on the command line.  Only
	     the superuser can forward privileged ports.

     RhostsAuthentication
	     Specifies whether to try rhosts based authentication.  Note that
	     this declaration only affects the client side and has no effect
	     whatsoever on security.  Disabling rhosts authentication may re-
	     duce authentication time on slow connections when rhosts authen-
	     tication is not used.  Most servers do not permit RhostsAuthenti-
	     cation because it is not secure (see RhostsRSAAuthentication).
	     The argument to this keyword must be ``yes'' or ``no''.

     RhostsRSAAuthentication
	     Specifies whether to try rhosts based authentication with RSA
	     host authentication.  This is the primary authentication method
	     for most sites.  The argument must be ``yes'' or ``no''.

     RSAAuthentication
	     Specifies whether to try RSA authentication.  The argument to
	     this keyword must be ``yes'' or ``no''. RSA authentication will
	     only be attempted if the identity file exists, or an authentica-
	     tion agent is running.  Note that this option applies to protocol
	     version 1 only.

     SkeyAuthentication
	     Specifies whether to use skey(1) authentication.  The argument to
	     this keyword must be ``yes'' or ``no''. The default is ``no''.

     StrictHostKeyChecking
	     If this flag is set to ``yes'', ssh ssh will never automatically
	     add host keys to the $HOME/.ssh/known_hosts and
	     $HOME/.ssh/known_hosts2 files, and refuses to connect hosts whose
	     host key has changed.  This provides maximum protection against
	     trojan horse attacks.  However, it can be somewhat annoying if
	     you don't have good /etc/ssh_known_hosts and
	     /etc/ssh_known_hosts2 files installed and frequently connect new
	     hosts.  Basically this option forces the user to manually add any
	     new hosts.	 Normally this option is disabled, and new hosts will
	     automatically be added to the known host files.  The host keys of
	     known hosts will be verified automatically in either case.	 The
	     argument must be ``yes'' or ``no''.

     UsePrivilegedPort
	     Specifies whether to use a privileged port for outgoing connec-
	     tions.  The argument must be ``yes'' or ``no''. The default is
	     ``yes''. Note that setting this option to ``no'' turns off
	     RhostsAuthentication and RhostsRSAAuthentication.

     User    Specifies the user to log in as.  This can be useful if you have
	     a different user name on different machines.  This saves the
	     trouble of having to remember to give the user name on the com-
	     mand line.

     UserKnownHostsFile
	     Specifies a file to use instead of $HOME/.ssh/known_hosts.

     UseRsh  Specifies that rlogin/rsh should be used for this host.  It is
	     possible that the host does not at all support the ssh protocol.
	     This causes ssh to immediately execute rsh(1).  All other options
	     (except HostName) are ignored if this has been specified.	The
	     argument must be ``yes'' or ``no''.

     XAuthLocation
	     Specifies the location of the xauth(1) program.  The default is
	     /usr/X11R6/bin/xauth.

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 point the user's mailbox.

     PATH    Set to the default PATH, as specified when compiling ssh.

     SSH_AUTH_SOCK
	     indicates the path of a unix-domain socket used to communicate
	     with the agent.

     SSH_CLIENT
	     Identifies the client end of the connection.  The variable con-
	     tains three space-separated values: client ip-address, client
	     port number, and server port number.

     SSH_TTY
	     This is set to the name of the tty (path to the device) associat-
	     ed 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 (e.i., 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.

FILES
     $HOME/.ssh/known_hosts
	     Records host keys for all hosts the user has logged into (that
	     are not in /etc/ssh_known_hosts). See sshd(8).

     $HOME/.ssh/identity, $HOME/.ssh/id_dsa
	     Contains the RSA and the DSA authentication identity of the user.
	     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
	     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
	     $HOME/.ssh/authorized_keys on all machines where you wish to log
	     in using RSA authentication.  The contents of the
	     $HOME/.ssh/id_dsa.pub file should be added to
	     $HOME/.ssh/authorized_keys2 on all machines where you wish to log
	     in using DSA authentication.  These files are not sensitive and
	     can (but need not) be readable by anyone.	These files are never
	     used automatically and are not necessary; they is only provided
	     for the convenience of the user.

     $HOME/.ssh/config
	     This is the per-user configuration file.  The format of this file
	     is described above.  This file is used by the ssh client.	This
	     file does not usually contain any sensitive information, but the
	     recommended permissions are read/write for the user, and not ac-
	     cessible by others.

     $HOME/.ssh/authorized_keys
	     Lists the RSA keys 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 (that is, each line contains the number of bits in modulus,
	     public exponent, modulus, and comment fields, separated by
	     spaces).  This file is not highly sensitive, but the recommended
	     permissions are read/write for the user, and not accessible by
	     others.

     $HOME/.ssh/authorized_keys2
	     Lists the DSA keys that can be used for logging in as this user.
	     This file is not highly sensitive, but the recommended permis-
	     sions are read/write for the user, and not accessible by others.

     /etc/ssh_known_hosts, /etc/ssh_known_hosts2
	     Systemwide list of known host keys.  /etc/ssh_known_hosts con-
	     tains RSA and /etc/ssh_known_hosts2 contains DSA keys.  These
	     files 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, number of bits in modulus, public exponent,
	     modulus, 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 on 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 au-
	     thentication.

     /usr/local/etc/ssh_config
	     Systemwide configuration file.  This file provides defaults for
	     those values that are not specified in the user's configuration
	     file, and for those users who do not have a configuration file.
	     This file must be world-readable.

     $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.  One 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 re-
	     quires successful RSA host authentication before permitting
	     .rhosts authentication.  If your server machine does not have the
	     client's host key in /etc/ssh_known_hosts, you can store it in
	     $HOME/.ssh/known_hosts. The easiest way to do this is to connect
	     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(1) 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
	     on 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 au-
	     thentication is normally required.	 This file should only be
	     writable by root.

     /usr/local/etc/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/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.

     libcrypto.so.X.1
	     A version of this library which includes support for the RSA al-
	     gorithm is required for proper operation.

AUTHOR
     OpenSSH is a derivative of the original (free) ssh 1.2.12 release by Tatu
     Ylonen, but with bugs removed and newer features re-added.	 Rapidly after
     the 1.2.12 release, newer versions of the original ssh bore successively
     more restrictive licenses, and thus demand for a free version was born.

     This version of OpenSSH

     o	 has all components of a restrictive nature (i.e., patents) directly
	 removed from the source code; any licensed or patented components are


	 chosen from external libraries.

     o	 has been updated to support SSH protocol 1.5 and 2, making it compat-
	 ible with all other SSH clients and servers.

     o	 contains added support for kerberos(8) authentication and ticket
	 passing.

     o	 supports one-time password authentication with skey(1).

     OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
     Niels Provos, Theo de Raadt, and Dug Song.

     The support for SSH protocol 2 was written by Markus Friedl.

SEE ALSO
     rlogin(1),	 rsh(1),  scp(1),  ssh-add(1),	ssh-agent(1),  ssh-keygen(1),
     telnet(1),	 sshd(8),

BSD Experimental	      September 25, 1999			    13
