path: root/other/ssharp/ssh.0

SSH(1)                      System Reference Manual                     SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

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

     ssh [-afgknqstvxACNPTX1246] [-c cipher_spec] [-e escape_char] [-i
         identity_file] [-l login_name] [-m mac_spec] [-o option] [-p port]
         [-L port:host:hostport] [-R port:host:hostport] [hostname |
         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 untrustM--
     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 deM--
     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 /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 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 comM--
     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/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 permitted.  This authentication method
     closes security holes due to IP spoofing, DNS spoofing and routing spoofM--
     ing.  [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 chalM--
     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 authenM--
     tication.

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

     If other authentication methods fail, ssh prompts the user for a passM--
     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 authenticaM--
     tion methods are available.  Using the default values for
     PreferredAuthentications, the client will try to authenticate first using
     the public key method; if this method fails password authentication is
     attempted, and finally if this method fails keyboard-interactive authenM--
     tication is attempted.  If this method fails password authentication is
     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_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 servM--
     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.

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

     Protocol 2 provides additional mechanisms for confidentiality (the trafM--
     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 eiM--
     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
     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/IP connections have been closed.  The exit staM--
     tus of the remote program is returned as the exit status of ssh.

   Escape Characters

     When a pseudo terminal has been requested, ssh supports a number of funcM--
     tions through the use of an escape character.

     A single tilde character can be sent as ~~ (or by following the tilde by
     a character other than those described above).  The escape character must
     always follow a newline to be interpreted as special.  The escape characM--
     ter can be changed in configuration files using the EscapeChar configuraM--
     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 (protocol version 1 only)

     ~?      Display a list of escape characters

     ~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 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 comM--
     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 possiM--
     ble application of TCP/IP forwarding is a secure connection to an elecM--
     tronic purse; another is going through firewalls.

   Server authentication

     ssh automatically maintains and checks a database containing identificaM--
     tions for all hosts it has ever been used with.  RSA host keys are stored
     in $HOME/.ssh/known_hosts and host keys used in the protocol version 2
     are stored in $HOME/.ssh/known_hosts2 in the user's home directory.  AdM--
     ditionally, 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 mechaM--
     nism is to prevent man-in-the-middle attacks which could otherwise be
     used to circumvent the encryption.  The StrictHostKeyChecking option (see
     below) can be used to prevent logins to machines whose host key is not
     known or has changed.

     The options are as follows:

     -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 fully supported in ssh. blowfish is a fast block cipher,
             it appears very secure and is much faster than 3des.

     -c cipher_spec
             Additionally, for protocol version 2 a comma-separated list of
             ciphers can be specified in order of preference.  See Ciphers for
             more information.

     -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 passphrasM--
             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 or
             DSA 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
             configuration 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.

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

     -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 maM--
             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 auM--
             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 useful 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 for older servers.

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

     -s      May be used to request invocation of a subsystem on the remote
             system. Subsystems are a feature of the SSH2 protocol which faM--
             cilitate the use of SSH as a secure transport for other applicaM--
             tion (eg. sftp). The subsystem is specified as the remote comM--
             mand.

     -t      Force pseudo-tty allocation.  This can be used to execute arbiM--
             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.

     -T      Disable pseudo-tty allocation.

     -v      Verbose mode.  Causes ssh to print debugging messages about its
             progress.  This is helpful in debugging connection, authenticaM--
             tion, and configuration problems.  Multiple -v options increases
             the verbosity.  Maximum is 3.

     -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 conM--


             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 forM--
             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 connecM--
             tion is forwarded over the secure channel, and a connection is
             made to host port hostport from the local machine.  Port forwardM--
             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

     -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.

CONFIGURATION FILES
     ssh obtains configuration data from the following sources (in this orM--
     der): command line options, user's configuration file
     ($HOME/.ssh/config), and system-wide configuration file
     (/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-speM--
     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 keyM--
             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 arguM--
             ment to this keyword must be ``yes'' or ``no''. This option apM--

             plies to protocol version 1 only.

     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''. The default is ``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 deM--
             tect if a host key changed due to DNS spoofing.  If the option is
             set to ``no'', the check will not be executed.  The default is
             ``yes''.

     Cipher  Specifies the cipher to use for encrypting the session in protoM--
             col version 1.  Currently, ``blowfish'' and ``3des'' are supportM--
             ed.  The default is ``3des''.

     Ciphers
             Specifies the ciphers allowed for protocol version 2 in order of
             preference.  Multiple ciphers must be comma-separated.  The deM--
             fault is

               ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
                 aes192-cbc,aes256-cbc''

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

     CompressionLevel
             Specifies the compression level to use if compression is enabled.
             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).  Note that this
             option applies to protocol version 1 only.

     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.
             The default is 4.

     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 disM--
             able the escape character entirely (making the connection transM--
             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''. The default is ``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 redirectM--
             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 deM--
             fault is ``no''.

     GlobalKnownHostsFile
             Specifies a file to use for the protocol version 1 global host
             key database instead of /etc/ssh_known_hosts.

     GlobalKnownHostsFile2
             Specifies a file to use for the protocol version 2 global host
             key database instead of /etc/ssh_known_hosts2.

     HostbasedAuthentication
             Specifies whether to try rhosts based authentication with public
             key authentication.  The argument must be ``yes'' or ``no''. The
             default is ``yes''. This option applies to protocol version 2 onM--
             ly and is similar to RhostsRSAAuthentication.

     HostKeyAlgorithms
             Specfies the protocol version 2 host key algorithms that the
             client wants to use in order of preference.  The default for this
             option is: ``ssh-rsa,ssh-dss''

     HostKeyAlias
             Specifies an alias that should be used instead of the real host
             name when looking up or saving the host key in the host key
             database files.  This option is useful for tunneling ssh connecM--
             tions or if you have multiple servers running on a single host.

     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 specificaM--
             tions).

     IdentityFile
             Specifies the file from which the user's RSA or DSA authenticaM--
             tion identity is read (default $HOME/.ssh/identity in the user's
             home directory).  Additionally, any identities represented by the
             authentication agent will be used for authentication.  The file
             name may use the tilde syntax to refer to a user's home directoM--
             ry.  It is possible to have multiple identity files specified in
             configuration files; all these identities will be tried in seM--
             quence.

     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 temM--
             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 arM--

             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 maM--
             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.

     MACs    Specifies the MAC (message authentication code) algorithms in orM--
             der of preference.  The MAC algorithm is used in protocol version
             2 for data integrity protection.  Multiple algorithms must be
             comma-separated.  The default is

               ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
                 hmac-sha1-96,hmac-md5-96''

     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''. The default is
             ``yes''.

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

     PreferredAuthentications
             Specifies the order in which the client should try protocol 2 auM--
             thentication methods. This allows a client to prefer one method
             (e.g.  keyboard-interactive) over another method (e.g.  password)
             The default for this option is: ``publickey, password, keyboard-
             interactive''

     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 ``2,1''. This
             means that ssh tries version 2 and falls back to version 1 if
             version 2 is not available.

     ProxyCommand
             Specifies the command to use to connect to the server.  The comM--
             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 someM--
             where.  Host key management will be done using the HostName of
             the host being connected (defaulting to the name typed by the usM--
             er).  Note that CheckHostIP is not available for connects with a
             proxy command.

     PubkeyAuthentication
             Specifies whether to try public key authentication.  The argument
             to this keyword must be ``yes'' or ``no''. The default is
             ``yes''. This option applies to protocol version 2 only.

     RemoteForward
             Specifies that a TCP/IP port on the remote machine be forwarded
             over the secure channel to given host:port from the local maM--
             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 reM--
             duce authentication time on slow connections when rhosts authenM--
             tication is not used.  Most servers do not permit RhostsAuthentiM--
             cation because it is not secure (see RhostsRSAAuthentication ).
             The argument to this keyword must be ``yes'' or ``no''. The deM--
             fault is ``yes''. This option applies to protocol version 1 only.

     RhostsRSAAuthentication
             Specifies whether to try rhosts based authentication with RSA
             host authentication.  The argument must be ``yes'' or ``no''. The
             default is ``yes''. This option applies to protocol version 1 onM--
             ly.

     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 authenticaM--
             tion agent is running.  The default is ``yes''. Note that this
             option applies to protocol version 1 only.

     ChallengeResponseAuthentication
             Specifies whether to use challenge response authentication.  CurM--
             rently there is only support for skey(1) authentication.  The arM--
             gument to this keyword must be ``yes'' or ``no''. The default is
             ``no''.

     StrictHostKeyChecking
             If this flag is set to ``yes'', ssh will never automatically add
             host keys to the $HOME/.ssh/known_hosts and
             $HOME/.ssh/known_hosts2 files, and refuses to connect to hosts
             whose host key has changed.  This provides maximum protection
             against trojan horse attacks.  However, it can be somewhat annoyM--
             ing if you don't have good /etc/ssh_known_hosts and
             /etc/ssh_known_hosts2 files installed and frequently connect to
             new hosts.  This option forces the user to manually add all new
             hosts.  If this flag is set to ``no'', ssh will automatically add
             new host keys to the user known hosts files.  If this flag is set
             to ``ask'', new host keys will be added to the user known host
             files only after the user has confirmed that is what they really
             want to do, and ssh will refuse to connect to hosts whose host
             key has changed.  The host keys of known hosts will be verified
             automatically in all cases.  The argument must be ``yes'', ``no''
             or ``ask''. The default is ``ask''.

     UsePrivilegedPort
             Specifies whether to use a privileged port for outgoing connecM--
             tions.  The argument must be ``yes'' or ``no''. The default is
             ``no''. Note that you need to set this option to ``yes'' if you
             want to use RhostsAuthentication and RhostsRSAAuthentication with
             older servers.

     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 comM--
             mand line.

     UserKnownHostsFile
             Specifies a file to use for the protocol version 1 user host key
             database instead of $HOME/.ssh/known_hosts.

     UserKnownHostsFile2
             Specifies a file to use for the protocol version 2 user host key
             database instead of $HOME/.ssh/known_hosts2.

     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 conM--
             tains three space-separated values: client ip-address, client
             port number, and server port number.

     SSH_ORIGINAL_COMMAND
             The variable contains the original command line if a forced comM--
             mand is executed.  It can be used to extract the original arguM--
             ments.

     SSH_TTY
             This is set to the name of the tty (path to the device) associatM--
             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 (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.

FILES
     $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
             Records host keys for all hosts the user has logged into (that
             are not in /etc/ssh_known_hosts for protocol version 1 or
             /etc/ssh_known_hosts2 for protocol version 2).  See sshd(8).

     $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
             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 othM--
             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
             $HOME/.ssh/authorized_keys on all machines where you wish to log
             in using protocol version 1 RSA authentication.  The contents of
             the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file should
             be added to $HOME/.ssh/authorized_keys2 on all machines where you
             wish 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 automatically and are not
             necessary; they are only provided for the convenience of the usM--
             er.

     $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 acM--
             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 public keys (RSA/DSA) that can be used for logging in
             as this user.  This file is not highly sensitive, but the recomM--
             mended permissions are read/write for the user, and not accessiM--
             ble by others.

     /etc/ssh_known_hosts, /etc/ssh_known_hosts2
             Systemwide list of known host keys.  /etc/ssh_known_hosts conM--
             tains RSA and /etc/ssh_known_hosts2 contains RSA or DSA keys for
             protocol version 2.  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 modM--
             ulus, public exponent, modulus, and optional comment field.  When
             different names are used for the same machine, all such names
             should be listed, separated 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 auM--
             thentication.

     /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.  On some machines this file may need to be
             world-readable if the user's home directory is on a NFS partiM--
             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 reM--
             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 auM--
             thentication is normally required.  This file should only be
             writable by root.

     /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.

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 creM--
     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

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

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
     Protocol Architecture, draft-ietf-secsh-architecture-07.txt, January
     2001, work in progress material.

BSD Experimental              September 25, 1999                            14