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SSH(1)                     OpenBSD Reference Manual                     SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

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

     ssh [-afgknqstvxACNPTX1246] [-b bind_address] [-c cipher_spec] [-e
         escape_char] [-i identity_file] [-l login_name] [-m mac_spec] [-o
         option] [-p port] [-F configfile] [-L port:host:hostport] [-R
         port:host:hostport] [-D port] 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 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 /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 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/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 login is permitted.  This authentication method
     closes security holes due to IP spoofing, DNS spoofing and routing spoof-
     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 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 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 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.

     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 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
     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 sta-
     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 func-
     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 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

     ~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 encrypt-
     ed 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 the com-
     mand line or in a configuration file.

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

   Server authentication

     ssh automatically maintains and checks a database containing identifica-
     tions for all hosts it has ever been used with.  Host keys are stored in
     $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 (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.

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

     -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
             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 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 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 al-
             so 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).

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

     -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 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 useful for just for-
             warding ports (protocol version 2 only).

     -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.

     -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 a 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.

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

     -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.

     -T      Disable pseudo-tty allocation.

     -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 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 con-
             figuration files; see the Compression option below.

     -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.

     -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.  IPv6 addresses can be specified with an alternative
             syntax: port/host/hostport

     -D port
             Specifies a local ``dynamic'' application-level port forwarding.
             This works by allocating a socket to listen to port on the local
             side, and whenever a connection is made to this port, the connec-
             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 protocol is supported, and
             ssh will act as a SOCKS4 server.  Only root can forward privi-
             leged ports.  Dynamic port forwardings can also be specified in
             the configuration file.

     -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 the follow-
     ing order: command line options, user's configuration file
     ($HOME/.ssh/config), and system-wide configuration file
     (/etc/ssh/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''. Configuration
     options may be separated by whitespace or optional whitespace and exactly
     one `='; the latter format is useful to avoid the need to quote whites-
     pace when specifying configuration options using the ssh, scp and sftp -o
     option.

     The possible keywords and their meanings are as follows (note that key-
     words are case-insensitive and arguments 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''. This option ap-
             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 no
             user is present to supply the password.  The argument must be
             ``yes'' or ``no''. The default is ``no''.

     BindAddress
             Specify the interface to transmit from on machines with multiple
             interfaces or aliased addresses.  Note that this option does not
             work if UsePrivilegedPort is set to ``yes''.

     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.  The default is
             ``yes''.

     Cipher  Specifies the cipher to use for encrypting the session in proto-
             col version 1.  Currently, ``blowfish'', ``3des'', and ``des''
             are supported.  des is only supported in the ssh client for in-
             teroperability with legacy protocol 1 implementations that do not
             support the 3des cipher.  Its use is strongly discouraged due to
             cryptographic weaknesses.  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

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

     ClearAllForwardings
             Specifies that all local, remote and dynamic port forwardings
             specified in the configuration files or on the command line be
             cleared.  This option is primarily useful when used from the ssh
             command line to clear port forwardings set in configuration
             files, and is automatically set by scp(1) and sftp(1). The argu-
             ment must be ``yes'' or ``no''. The default is ``no''.

     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 1.

     DynamicForward
             Specifies that a TCP/IP port on the local machine be forwarded
             over the secure channel, and the application protocol is then
             used to determine where to connect to from the remote machine.
             The argument must be a port number.  Currently the SOCKS4 proto-
             col is supported, and ssh will act as a SOCKS4 server.  Multiple
             forwardings may be specified, and additional forwardings can be
             given on the command line.  Only the superuser can forward privi-
             leged ports.

     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''. 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 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.  By default, ssh binds local port forwardings to
             the loopback addresss.  This prevents other remote hosts from
             connecting to forwarded ports.  GatewayPorts can be used to spec-
             ify that ssh should bind local port forwardings to the wildcard
             address, thus allowing remote hosts to connect to forwarded
             ports.  The argument must be ``yes'' or ``no''. The default is
             ``no''.

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

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

     HostKeyAlgorithms
             Specifies 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 connec-
             tions or for 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 specifica-
             tions).

     IdentityFile
             Specifies a file from which the user's RSA or DSA authentication
             identity is read. The default is $HOME/.ssh/identity for protocol
             version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for proto-
             col version 2.  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 directo-
             ry.  It is possible to have multiple identity files specified in
             configuration files; all these identities will be tried in se-
             quence.

     KeepAlive
             Specifies whether the system should send TCP 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''.

     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 the specified host and port from the
             remote machine.  The first argument must be a port number, and
             the second must be host:port. IPv6 addresses can be specified
             with an alternative syntax: host/port. Multiple forwardings may
             be specified, and additional forwardings can be given on the com-
             mand 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,
             DEBUG, DEBUG1, DEBUG2 and DEBUG3.  The default is INFO.  DEBUG
             and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify higher
             levels of verbose output.

     MACs    Specifies the MAC (message authentication code) algorithms in or-
             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-sha1-96,hmac-md5-96''.

     NoHostAuthenticationForLocalhost
             This option can be used if the home directory is shared across
             machines.  In this case localhost will refer to a different ma-
             chine on each of the machines and the user will get many warnings
             about changed host keys.  However, this option disables host au-
             thentication for localhost.  The argument to this keyword must be
             ``yes'' or ``no''. The default is to check the host key for lo-
             calhost.

     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 au-
             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: ``hostbased,publickey,keyboard-
             interactive,password''.

     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 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.

     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 the specified host and port from the
             local machine.  The first argument must be a port number, and the
             second must be host:port. IPv6 addresses can be specified with an
             alternative syntax: host/port. Multiple forwardings may be speci-
             fied, 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''. The de-
             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 on-
             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 authentica-
             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.  The
             argument to this keyword must be ``yes'' or ``no''. The default
             is ``yes''.

     SmartcardDevice
             Specifies which smartcard device to use. The argument to this
             keyword is the device ssh should use to communicate with a smart-
             card used for storing the user's private RSA key. By default, no
             device is specified and smartcard support is not activated.

     StrictHostKeyChecking
             If this flag is set to ``yes'', ssh will never automatically add
             host keys to the $HOME/.ssh/known_hosts file, and refuses to con-
             nect to hosts whose host key has changed.  This provides maximum
             protection against trojan horse attacks, however, can be annoying
             when the /etc/ssh/ssh_known_hosts file is poorly maintained, or
             connections to new hosts are frequently made.  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 connec-
             tions.  The argument must be ``yes'' or ``no''. The default is
             ``no''. Note that this option must be set to ``yes'' if
             RhostsAuthentication and RhostsRSAAuthentication authentications
             are needed with older servers.

     User    Specifies the user to log in as.  This can be useful when a dif-
             ferent user name is used 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 for the user host key database 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 the path of the user's mailbox.

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

     SSH_ASKPASS
             If ssh needs a passphrase, it will read the passphrase from the
             current terminal if it was run from a terminal.  If ssh does not
             have a terminal associated with it but DISPLAY and SSH_ASKPASS
             are set, it will execute the program specified by SSH_ASKPASS and
             open an X11 window to read the passphrase.  This is particularly
             useful when calling ssh from a .Xsession or related script.
             (Note that on some machines it may be necessary to redirect the
             input from /dev/null to make this work.)

     SSH_AUTH_SOCK
             Identifies the path of a unix-domain socket used to 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_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) 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 (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
             Records host keys for all hosts the user has logged into that are
             not in /etc/ssh/ssh_known_hosts. 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 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
             $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 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 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 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.

     /etc/ssh/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.

     /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
             /etc/ssh/ssh_host_rsa_key
             These three files contain the private parts of the host keys and
             are used for RhostsRSAAuthentication and HostbasedAuthentication.
             Since they are readable only by root ssh must be setuid root if
             these authentication methods are desired.

     $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 re-
             quires 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(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.

     /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/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
     error occurred.

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.

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-12.txt, January
     2002, work in progress material.

OpenBSD 3.1                   September 25, 1999                            15

NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FILES | ENVIRONMENT | FILES | DIAGNOSTICS | AUTHORS | SEE ALSO

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