sshd.0   [plain text]


SSHD(8)                     System Manager's Manual                    SSHD(8)

NAME
     sshd - OpenSSH SSH daemon

SYNOPSIS
     sshd [-deiqtD46] [-b bits] [-f config_file] [-g login_grace_time]
          [-h host_key_file] [-k key_gen_time] [-p port] [-u len]

DESCRIPTION
     sshd (SSH Daemon) is the daemon program for ssh(1).  Together these proM--
     grams replace rlogin and rsh, and provide secure encrypted communications
     between two untrusted hosts over an insecure network.  The programs are
     intended to be as easy to install and use as possible.

     sshd is the daemon that listens for connections from clients.  It is norM--
     mally started at boot from /etc/rc.  It forks a new daemon for each
     incoming connection.  The forked daemons handle key exchange, encryption,
     authentication, command execution, and data exchange.  This implementaM--
     tion of sshd supports both SSH protocol version 1 and 2 simultaneously.
     sshd works as follows.

   SSH protocol version 1

     Each host has a host-specific RSA key (normally 1024 bits) used to idenM--
     tify the host.  Additionally, when the daemon starts, it generates a
     server RSA key (normally 768 bits).  This key is normally regenerated
     every hour if it has been used, and is never stored on disk.

     Whenever a client connects the daemon responds with its public host and
     server keys.  The client compares the RSA host key against its own
     database to verify that it has not changed.  The client then generates a
     256 bit random number.  It encrypts this random number using both the
     host key and the server key, and sends the encrypted number to the
     server.  Both sides then use this random number as a session key which is
     used to encrypt all further communications in the session.  The rest of
     the session is encrypted using a conventional cipher, currently Blowfish
     or 3DES, with 3DES being used by default.  The client selects the encrypM--
     tion algorithm to use from those offered by the server.

     Next, the server and the client enter an authentication dialog.  The
     client tries to authenticate itself using .rhosts authentication, .rhosts
     authentication combined with RSA host authentication, RSA challenge-
     response authentication, or password based authentication.

     Rhosts authentication is normally disabled because it is fundamentally
     insecure, but can be enabled in the server configuration file if desired.
     System security is not improved unless rshd(8), rlogind(8), and rexecd(8)
     are disabled (thus completely disabling rlogin(1) and rsh(1) into the
     machine).

   SSH protocol version 2

     Version 2 works similarly: Each host has a host-specific key (RSA or DSA)
     used to identify the host.  However, when the daemon starts, it does not
     generate a server key.  Forward security is provided through a Diffie-
     Hellman key agreement.  This key agreement results in a shared session
     key.

     The rest of the session is encrypted using a symmetric cipher, currently
     128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit
     AES.  The client selects the encryption algorithm to use from those
     offered by the server.  Additionally, session integrity is provided
     through a cryptographic message authentication code (hmac-sha1 or hmac-
     md5).

     Protocol version 2 provides a public key based user (PubkeyAuthenticaM--
     tion) or client host (HostbasedAuthentication) authentication method,
     conventional password authentication and challenge response based methM--
     ods.

   Command execution and data forwarding

     If the client successfully authenticates itself, a dialog for preparing
     the session is entered.  At this time the client may request things like
     allocating a pseudo-tty, forwarding X11 connections, forwarding TCP/IP
     connections, or forwarding the authentication agent connection over the
     secure channel.

     Finally, the client either requests a shell or execution of a command.
     The sides then enter session mode.  In this mode, either side may send
     data at any time, and such data is forwarded to/from the shell or command
     on the server side, and the user terminal in the client side.

     When the user program terminates and all forwarded X11 and other connecM--
     tions have been closed, the server sends command exit status to the
     client, and both sides exit.

     sshd can be configured using command-line options or a configuration
     file.  Command-line options override values specified in the configuraM--
     tion file.

     sshd rereads its configuration file when it receives a hangup signal,
     SIGHUP, by executing itself with the name it was started as, i.e.,
     /usr/sbin/sshd.

     The options are as follows:

     -b bits
             Specifies the number of bits in the ephemeral protocol version 1
             server key (default 768).

     -d      Debug mode.  The server sends verbose debug output to the system
             log, and does not put itself in the background.  The server also
             will not fork and will only process one connection.  This option
             is only intended for debugging for the server.  Multiple -d
             options increase the debugging level.  Maximum is 3.

     -e      When this option is specified, sshd will send the output to the
             standard error instead of the system log.

     -f configuration_file
             Specifies the name of the configuration file.  The default is
             /etc/sshd_config.  sshd refuses to start if there is no configuM--
             ration file.

     -g login_grace_time
             Gives the grace time for clients to authenticate themselves
             (default 600 seconds).  If the client fails to authenticate the
             user within this many seconds, the server disconnects and exits.
             A value of zero indicates no limit.

     -h host_key_file
             Specifies the file from which the host key is read (default
             /etc/ssh_host_key).  This option must be given if sshd is not run
             as root (as the normal host file is normally not readable by anyM--
             one but root).  It is possible to have multiple host key files
             for the different protocol versions and host key algorithms.

     -i      Specifies that sshd is being run from inetd.  sshd is normally
             not run from inetd because it needs to generate the server key
             before it can respond to the client, and this may take tens of
             seconds.  Clients would have to wait too long if the key was
             regenerated every time.  However, with small key sizes (e.g.,
             512) using sshd from inetd may be feasible.

     -k key_gen_time
             Specifies how often the ephemeral protocol version 1 server key
             is regenerated (default 3600 seconds, or one hour).  The motivaM--
             tion for regenerating the key fairly often is that the key is not
             stored anywhere, and after about an hour, it becomes impossible
             to recover the key for decrypting intercepted communications even
             if the machine is cracked into or physically seized.  A value of
             zero indicates that the key will never be regenerated.

     -p port
             Specifies the port on which the server listens for connections
             (default 22).

     -q      Quiet mode.  Nothing is sent to the system log.  Normally the
             beginning, authentication, and termination of each connection is
             logged.

     -t      Test mode.  Only check the validity of the configuration file and
             sanity of the keys.  This is useful for updating sshd reliably as
             configuration options may change.

     -u len  This option is used to specify the size of the field in the utmp
             structure that holds the remote host name.  If the resolved host
             name is longer than len, the dotted decimal value will be used
             instead.  This allows hosts with very long host names that overM--
             flow this field to still be uniquely identified.  Specifying -u0
             indicates that only dotted decimal addresses should be put into
             the utmp file.  -u0 is also be used to prevent sshd from making
             DNS requests unless the authentication mechanism or configuration
             requires it.  Authentication mechanisms that may require DNS
             include RhostsAuthentication, RhostsRSAAuthentication,
             HostbasedAuthentication and using a from="pattern-list" option in
             a key file.

     -D      When this option is specified sshd will not detach and does not
             become a daemon.  This allows easy monitoring of sshd.

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

CONFIGURATION FILE
     sshd reads configuration data from /etc/sshd_config (or the file speciM--
     fied with -f on the command line).  The file contains keyword-argument
     pairs, one per line.  Lines starting with `#' and empty lines are interM--
     preted as comments.

     The possible keywords and their meanings are as follows (note that keyM--
     words are case-insensitive and arguments are case-sensitive):

     AFSTokenPassing
             Specifies whether an AFS token may be forwarded to the server.
             Default is ``yes''.

     AllowGroups
             This keyword can be followed by a list of group names, separated
             by spaces.  If specified, login is allowed only for users whose
             primary group or supplementary group list matches one of the patM--
             terns.  `*' and `'?  can be used as wildcards in the patterns.
             Only group names are valid; a numerical group ID is not recogM--
             nized.  By default login is allowed regardless of the group list.

     AllowTcpForwarding
             Specifies whether TCP forwarding is permitted.  The default is
             ``yes''.  Note that disabling TCP forwarding does not improve
             security unless users are also denied shell access, as they can
             always install their own forwarders.

     AllowUsers
             This keyword can be followed by a list of user names, separated
             by spaces.  If specified, login is allowed only for users names
             that match one of the patterns.  `*' and `'?  can be used as
             wildcards in the patterns.  Only user names are valid; a numeriM--
             cal user ID is not recognized.  By default login is allowed
             regardless of the user name.  If the pattern takes the form
             USER@HOST then USER and HOST are separately checked, restricting
             logins to particular users from particular hosts.

     AuthorizedKeysFile
             Specifies the file that contains the public keys that can be used
             for user authentication.  AuthorizedKeysFile may contain tokens
             of the form %T which are substituted during connection set-up.
             The following tokens are defined: %% is replaced by a literal
             '%', %h is replaced by the home directory of the user being
             authenticated and %u is replaced by the username of that user.
             After expansion, AuthorizedKeysFile is taken to be an absolute
             path or one relative to the user's home directory.  The default
             is ``.ssh/authorized_keys''

     Banner  In some jurisdictions, sending a warning message before authentiM--
             cation may be relevant for getting legal protection.  The conM--
             tents of the specified file are sent to the remote user before
             authentication is allowed.  This option is only available for
             protocol version 2.

     ChallengeResponseAuthentication
             Specifies whether challenge response authentication is allowed.
             All authentication styles from login.conf(5) are supported.  The
             default is ``yes''.

     Ciphers
             Specifies the ciphers allowed for protocol version 2.  Multiple
             ciphers must be comma-separated.  The default is
             ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.''

     ClientAliveInterval
             Sets a timeout interval in seconds after which if no data has
             been received from the client, sshd will send a message through
             the encrypted channel to request a response from the client.  The
             default is 0, indicating that these messages will not be sent to
             the client.  This option applies to protocol version 2 only.

     ClientAliveCountMax
             Sets the number of client alive messages (see above) which may be
             sent without sshd receiving any messages back from the client. If
             this threshold is reached while client alive messages are being
             sent, sshd will disconnect the client, terminating the session.
             It is important to note that the use of client alive messages is
             very different from KeepAlive (below). The client alive messages
             are sent through the encrypted channel and therefore will not be
             spoofable. The TCP keepalive option enabled by KeepAlive is
             spoofable. The client alive mechanism is valuable when the client
             or server depend on knowing when a connection has become inacM--
             tive.

             The default value is 3. If ClientAliveInterval (above) is set to
             15, and ClientAliveCountMax is left at the default, unresponsive
             ssh clients will be disconnected after approximately 45 seconds.

     DenyGroups
             This keyword can be followed by a number of group names, sepaM--
             rated by spaces.  Users whose primary group or supplementary
             group list matches one of the patterns aren't allowed to log in.
             `*' and `'?  can be used as wildcards in the patterns.  Only
             group names are valid; a numerical group ID is not recognized.
             By default login is allowed regardless of the group list.

     DenyUsers
             This keyword can be followed by a number of user names, separated
             by spaces.  Login is disallowed for user names that match one of
             the patterns.  `*' and `'?  can be used as wildcards in the patM--
             terns.  Only user names are valid; a numerical user ID is not
             recognized.  By default login is allowed regardless of the user
             name.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports
             forwarded for the client.  By default, sshd binds remote port
             forwardings to the loopback addresss.  This prevents other remote
             hosts from connecting to forwarded ports.  GatewayPorts can be
             used to specify that sshd should bind remote 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''.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication
             together with successful public key client host authentication is
             allowed (hostbased authentication).  This option is similar to
             RhostsRSAAuthentication and applies to protocol version 2 only.
             The default is ``no''.

     HostKey
             Specifies the file containing the private host keys (default
             /etc/ssh_host_key) used by SSH protocol versions 1 and 2.  Note
             that sshd will refuse to use a file if it is group/world-accessiM--
             ble.  It is possible to have multiple host key files.  ``rsa1''
             keys are used for version 1 and ``dsa'' or ``rsa'' are used for
             version 2 of the SSH protocol.

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in
             RhostsAuthentication, RhostsRSAAuthentication or
             HostbasedAuthentication.

             /etc/hosts.equiv and /etc/shosts.equiv are still used.  The
             default is ``yes''.

     IgnoreUserKnownHosts
             Specifies whether sshd should ignore the user's
             $HOME/.ssh/known_hosts during RhostsRSAAuthentication or
             HostbasedAuthentication.  The default is ``no''.

     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.  On the other hand,
             if keepalives are not sent, sessions may hang indefinitely on the
             server, leaving ``ghost'' users and consuming server resources.

             The default is ``yes'' (to send keepalives), and the server will
             notice if the network goes down or the client host reboots.  This
             avoids infinitely hanging sessions.

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

     KerberosAuthentication
             Specifies whether Kerberos authentication is allowed.  This can
             be in the form of a Kerberos ticket, or if PasswordAuthentication
             is yes, the password provided by the user will be validated
             through the Kerberos KDC.  To use this option, the server needs a
             Kerberos servtab which allows the verification of the KDC's idenM--
             tity.  Default is ``yes''.

     KerberosOrLocalPasswd
             If set then if password authentication through Kerberos fails
             then the password will be validated via any additional local
             mechanism such as /etc/passwd.  Default is ``yes''.

     KerberosTgtPassing
             Specifies whether a Kerberos TGT may be forwarded to the server.
             Default is ``no'', as this only works when the Kerberos KDC is
             actually an AFS kaserver.

     KerberosTicketCleanup
             Specifies whether to automatically destroy the user's ticket
             cache file on logout.  Default is ``yes''.

     KeyRegenerationInterval
             In protocol version 1, the ephemeral server key is automatically
             regenerated after this many seconds (if it has been used).  The
             purpose of regeneration is to prevent decrypting captured sesM--
             sions by later breaking into the machine and stealing the keys.
             The key is never stored anywhere.  If the value is 0, the key is
             never regenerated.  The default is 3600 (seconds).

     ListenAddress
             Specifies the local addresses sshd should listen on.  The followM--
             ing forms may be used:

                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port

             If port is not specified, sshd will listen on the address and all
             prior Port options specified. The default is to listen on all
             local addresses.  Multiple ListenAddress options are permitted.
             Additionally, any Port options must precede this option for non
             port qualified addresses.

     LoginGraceTime
             The server disconnects after this time if the user has not sucM--
             cessfully logged in.  If the value is 0, there is no time limit.
             The default is 600 (seconds).

     LogLevel
             Gives the verbosity level that is used when logging messages from
             sshd.  The possible values are: QUIET, FATAL, ERROR, INFO, VERM--
             BOSE and DEBUG.  The default is INFO.  Logging with level DEBUG
             violates the privacy of users and is not recommended.

     MACs    Specifies the available MAC (message authentication code) algoM--
             rithms.  The MAC algorithm is used in protocol version 2 for data
             integrity protection.  Multiple algorithms must be comma-sepaM--
             rated.  The default is
             ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated conM--
             nections to the sshd daemon.  Additional connections will be
             dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10.

             Alternatively, random early drop can be enabled by specifying the
             three colon separated values ``start:rate:full'' (e.g.,
             "10:30:60").  sshd will refuse connection attempts with a probaM--
             bility of ``rate/100'' (30%) if there are currently ``start''
             (10) unauthenticated connections.  The probability increases linM--
             early and all connection attempts are refused if the number of
             unauthenticated connections reaches ``full'' (60).

     PAMAuthenticationViaKbdInt
             Specifies whether PAM challenge response authentication is
             allowed. This allows the use of most PAM challenge response
             authentication modules, but it will allow password authentication
             regardless of whether PasswordAuthentication is disabled.  The
             default is ``no''.

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The
             default is ``yes''.

     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the
             server allows login to accounts with empty password strings.  The
             default is ``no''.

     PermitRootLogin
             Specifies whether root can login using ssh(1).  The argument must
             be ``yes'', ``without-password'', ``forced-commands-only'' or
             ``no''.  The default is ``yes''.

             If this option is set to ``without-password'' password authentiM--
             cation is disabled for root.

             If this option is set to ``forced-commands-only'' root login with
             public key authentication will be allowed, but only if the
             command option has been specified (which may be useful for taking
             remote backups even if root login is normally not allowed). All
             other authentication methods are disabled for root.

             If this option is set to ``no'' root is not allowed to login.

     PidFile
             Specifies the file that contains the process identifier of the
             sshd daemon.  The default is /var/run/sshd.pid.

     Port    Specifies the port number that sshd listens on.  The default is
             22.  Multiple options of this type are permitted.  See also
             ListenAddress.

     PrintLastLog
             Specifies whether sshd should print the date and time when the
             user last logged in.  The default is ``yes''.

     PrintMotd
             Specifies whether sshd should print /etc/motd when a user logs in
             interactively.  (On some systems it is also printed by the shell,
             /etc/profile, or equivalent.)  The default is ``yes''.

     Protocol
             Specifies the protocol versions sshd should support.  The possiM--
             ble values are ``1'' and ``2''.  Multiple versions must be comma-
             separated.  The default is ``2,1''.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The
             default is ``yes''.  Note that this option applies to protocol
             version 2 only.

     ReverseMappingCheck
             Specifies whether sshd should try to verify the remote host name
             and check that the resolved host name for the remote IP address
             maps back to the very same IP address.  The default is ``no''.

     RhostsAuthentication
             Specifies whether authentication using rhosts or /etc/hosts.equiv
             files is sufficient.  Normally, this method should not be permitM--
             ted because it is insecure.  RhostsRSAAuthentication should be
             used instead, because it performs RSA-based host authentication
             in addition to normal rhosts or /etc/hosts.equiv authentication.
             The default is ``no''.  This option applies to protocol version 1
             only.

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication
             together with successful RSA host authentication is allowed.  The
             default is ``no''.  This option applies to protocol version 1
             only.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The
             default is ``yes''.  This option applies to protocol version 1
             only.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1
             server key.  The minimum value is 512, and the default is 768.

     StrictModes
             Specifies whether sshd should check file modes and ownership of
             the user's files and home directory before accepting login.  This
             is normally desirable because novices sometimes accidentally
             leave their directory or files world-writable.  The default is
             ``yes''.

     Subsystem
             Configures an external subsystem (e.g., file transfer daemon).
             Arguments should be a subsystem name and a command to execute
             upon subsystem request.  The command sftp-server(8) implements
             the ``sftp'' file transfer subsystem.  By default no subsystems
             are defined.  Note that this option applies to protocol version 2
             only.

     SyslogFacility
             Gives the facility code that is used when logging messages from
             sshd.  The possible values are: DAEMON, USER, AUTH, LOCAL0,
             LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The
             default is AUTH.

     UseLogin
             Specifies whether login(1) is used for interactive login sesM--
             sions.  The default is ``no''.  Note that login(1) is never used
             for remote command execution.  Note also, that if this is
             enabled, X11Forwarding will be disabled because login(1) does not
             know how to handle xauth(1) cookies.

     X11DisplayOffset
             Specifies the first display number available for sshd's X11 forM--
             warding.  This prevents sshd from interfering with real X11
             servers.  The default is 10.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The default is
             ``no''.  Note that disabling X11 forwarding does not improve
             security in any way, as users can always install their own forM--
             warders.  X11 forwarding is automatically disabled if UseLogin is
             enabled.

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

   Time Formats

     sshd command-line arguments and configuration file options that specify
     time may be expressed using a sequence of the form: time[qualifier],
     where time is a positive integer value and qualifier is one of the folM--
     lowing:

           <none>  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

     Each member of the sequence is added together to calculate the total time
     value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

LOGIN PROCESS
     When a user successfully logs in, sshd does the following:

           1.   If the login is on a tty, and no command has been specified,
                prints last login time and /etc/motd (unless prevented in the
                configuration file or by $HOME/.hushlogin; see the FILES secM--
                tion).

           2.   If the login is on a tty, records login time.

           3.   Checks /etc/nologin; if it exists, prints contents and quits
                (unless root).

           4.   Changes to run with normal user privileges.

           5.   Sets up basic environment.

           6.   Reads $HOME/.ssh/environment if it exists.

           7.   Changes to user's home directory.

           8.   If $HOME/.ssh/rc exists, runs it; else if /etc/sshrc exists,
                runs it; otherwise runs xauth.  The ``rc'' files are given the
                X11 authentication protocol and cookie in standard input.

           9.   Runs user's shell or command.

AUTHORIZED_KEYS FILE FORMAT
     $HOME/.ssh/authorized_keys is the default file that lists the public keys
     that are permitted for RSA authentication in protocol version 1 and for
     public key authentication (PubkeyAuthentication) in protocol version 2.
     AuthorizedKeysFile may be used to specify an alternative file.

     Each line of the file contains one key (empty lines and lines starting
     with a `#' are ignored as comments).  Each RSA public key consists of the
     following fields, separated by spaces: options, bits, exponent, modulus,
     comment.  Each protocol version 2 public key consists of: options, keyM--
     type, base64 encoded key, comment.  The options fields are optional; its
     presence is determined by whether the line starts with a number or not
     (the option field never starts with a number).  The bits, exponent, moduM--
     lus and comment fields give the RSA key for protocol version 1; the comM--
     ment field is not used for anything (but may be convenient for the user
     to identify the key).  For protocol version 2 the keytype is ``ssh-dss''
     or ``ssh-rsa''.

     Note that lines in this file are usually several hundred bytes long
     (because of the size of the RSA key modulus).  You don't want to type
     them in; instead, copy the identity.pub, id_dsa.pub or the id_rsa.pub
     file and edit it.

     The options (if present) consist of comma-separated option specificaM--
     tions.  No spaces are permitted, except within double quotes.  The folM--
     lowing option specifications are supported (note that option keywords are
     case-insensitive):

     from="pattern-list"
             Specifies that in addition to RSA authentication, the canonical
             name of the remote host must be present in the comma-separated
             list of patterns (`*' and `'?  serve as wildcards).  The list may
             also contain patterns negated by prefixing them with `'!; if the
             canonical host name matches a negated pattern, the key is not
             accepted.  The purpose of this option is to optionally increase
             security: RSA authentication by itself does not trust the network
             or name servers or anything (but the key); however, if somebody
             somehow steals the key, the key permits an intruder to log in
             from anywhere in the world.  This additional option makes using a
             stolen key more difficult (name servers and/or routers would have
             to be compromised in addition to just the key).

     command="command"
             Specifies that the command is executed whenever this key is used
             for authentication.  The command supplied by the user (if any) is
             ignored.  The command is run on a pty if the client requests a
             pty; otherwise it is run without a tty.  If a 8-bit clean channel
             is required, one must not request a pty or should specify no-pty.
             A quote may be included in the command by quoting it with a backM--
             slash.  This option might be useful to restrict certain RSA keys
             to perform just a specific operation.  An example might be a key
             that permits remote backups but nothing else.  Note that the
             client may specify TCP/IP and/or X11 forwarding unless they are
             explicitly prohibited.  Note that this option applies to shell,
             command or subsystem execution.

     environment="NAME=value"
             Specifies that the string is to be added to the environment when
             logging in using this key.  Environment variables set this way
             override other default environment values.  Multiple options of
             this type are permitted.  This option is automatically disabled
             if UseLogin is enabled.

     no-port-forwarding
             Forbids TCP/IP forwarding when this key is used for authenticaM--
             tion.  Any port forward requests by the client will return an
             error.  This might be used, e.g., in connection with the command
             option.

     no-X11-forwarding
             Forbids X11 forwarding when this key is used for authentication.
             Any X11 forward requests by the client will return an error.

     no-agent-forwarding
             Forbids authentication agent forwarding when this key is used for
             authentication.

     no-pty  Prevents tty allocation (a request to allocate a pty will fail).

     permitopen="host:port"
             Limit local ``ssh -L'' port forwarding such that it may only conM--
             nect to the specified host and port.  IPv6 addresses can be specM--
             ified with an alternative syntax: host/port.  Multiple permitopen
             options may be applied separated by commas. No pattern matching
             is performed on the specified hostnames, they must be literal
             domains or addresses.

   Examples
     1024 33 12121...312314325 ylo@foo.bar

     from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula

     command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323
     backup.hut.fi

     permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323

SSH_KNOWN_HOSTS FILE FORMAT
     The /etc/ssh_known_hosts, and $HOME/.ssh/known_hosts files contain host
     public keys for all known hosts.  The global file should be prepared by
     the administrator (optional), and the per-user file is maintained autoM--
     matically: whenever the user connects from an unknown host its key is
     added to the per-user file.

     Each line in these files contains the following fields: hostnames, bits,
     exponent, modulus, comment.  The fields are separated by spaces.

     Hostnames is a comma-separated list of patterns ('*' and '?' act as wildM--
     cards); each pattern in turn is matched against the canonical host name
     (when authenticating a client) or against the user-supplied name (when
     authenticating a server).  A pattern may also be preceded by `'!  to
     indicate negation: if the host name matches a negated pattern, it is not
     accepted (by that line) even if it matched another pattern on the line.

     Bits, exponent, and modulus are taken directly from the RSA host key;
     they can be obtained, e.g., from /etc/ssh_host_key.pub.  The optional
     comment field continues to the end of the line, and is not used.

     Lines starting with `#' and empty lines are ignored as comments.

     When performing host authentication, authentication is accepted if any
     matching line has the proper key.  It is thus permissible (but not recomM--
     mended) to have several lines or different host keys for the same names.
     This will inevitably happen when short forms of host names from different
     domains are put in the file.  It is possible that the files contain conM--
     flicting information; authentication is accepted if valid information can
     be found from either file.

     Note that the lines in these files are typically hundreds of characters
     long, and you definitely don't want to type in the host keys by hand.
     Rather, generate them by a script or by taking /etc/ssh_host_key.pub and
     adding the host names at the front.

   Examples

     closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
     cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=

FILES
     /etc/sshd_config
             Contains configuration data for sshd.  This file should be
             writable by root only, but it is recommended (though not necesM--
             sary) that it be world-readable.

     /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
             These three files contain the private parts of the host keys.
             These files should only be owned by root, readable only by root,
             and not accessible to others.  Note that sshd does not start if
             this file is group/world-accessible.

     /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub,
             /etc/ssh_host_rsa_key.pub
             These three files contain the public parts of the host keys.
             These files should be world-readable but writable only by root.
             Their contents should match the respective private parts.  These
             files are not really used for anything; they are provided for the
             convenience of the user so their contents can be copied to known
             hosts files.  These files are created using ssh-keygen(1).

     /etc/moduli
             Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
             Exchange".

     /var/run/sshd.pid
             Contains the process ID of the sshd listening for connections (if
             there are several daemons running concurrently for different
             ports, this contains the pid of the one started last).  The conM--
             tent of this file is not sensitive; it can be world-readable.

     $HOME/.ssh/authorized_keys
             Lists the public keys (RSA or DSA) that can be used to log into
             the user's account.  This file must be readable by root (which
             may on some machines imply it being world-readable if the user's
             home directory resides on an NFS volume).  It is recommended that
             it not be accessible by others.  The format of this file is
             described above.  Users will place the contents of their
             identity.pub, id_dsa.pub and/or id_rsa.pub files into this file,
             as described in ssh-keygen(1).

     /etc/ssh_known_hosts and $HOME/.ssh/known_hosts
             These files are consulted when using rhosts with RSA host authenM--
             tication or protocol version 2 hostbased authentication to check
             the public key of the host.  The key must be listed in one of
             these files to be accepted.  The client uses the same files to
             verify that it is connecting to the correct remote host.  These
             files should be writable only by root/the owner.
             /etc/ssh_known_hosts should be world-readable, and
             $HOME/.ssh/known_hosts can but need not be world-readable.

     /etc/nologin
             If this file exists, sshd refuses to let anyone except root log
             in.  The contents of the file are displayed to anyone trying to
             log in, and non-root connections are refused.  The file should be
             world-readable.

     /etc/hosts.allow, /etc/hosts.deny
             Access controls that should be enforced by tcp-wrappers are
             defined here.  Further details are described in hosts_access(5).

     $HOME/.rhosts
             This file contains host-username pairs, separated by a space, one
             per line.  The given user on the corresponding host is permitted
             to log in without password.  The same file is used by rlogind and
             rshd.  The file must be writable only by the user; it is recomM--
             mended that it not be accessible by others.

             If is also possible to use netgroups in the file.  Either host or
             user name may be of the form +@groupname to specify all hosts or
             all users in the group.

     $HOME/.shosts
             For ssh, this file is exactly the same as for .rhosts.  However,
             this file is not used by rlogin and rshd, so using this permits
             access using SSH only.

     /etc/hosts.equiv
             This file is used during .rhosts authentication.  In the simplest
             form, this file contains host names, one per line.  Users on
             those hosts are permitted to log in without a password, provided
             they have the same user name on both machines.  The host name may
             also be followed by a user name; such users are permitted to log
             in as any user on this machine (except root).  Additionally, the
             syntax ``+@group'' can be used to specify netgroups.  Negated
             entries start with `-'.

             If the client host/user is successfully matched in this file,
             login is automatically permitted provided the client and server
             user names are the same.  Additionally, successful RSA host
             authentication is normally required.  This file must be writable
             only by root; it is recommended that it be world-readable.

             Warning: It is almost never a good idea to use user names in
             hosts.equiv.  Beware that it really means that the named user(s)
             can log in as anybody, which includes bin, daemon, adm, and other
             accounts that own critical binaries and directories.  Using a
             user name practically grants the user root access.  The only
             valid use for user names that I can think of is in negative
             entries.

             Note that this warning also applies to rsh/rlogin.

     /etc/shosts.equiv
             This is processed exactly as /etc/hosts.equiv.  However, this
             file may be useful in environments that want to run both
             rsh/rlogin and ssh.

     $HOME/.ssh/environment
             This file is read into the environment at login (if it exists).
             It can only contain empty lines, comment lines (that start with
             `#'), and assignment lines of the form name=value.  The file
             should be writable only by the user; it need not be readable by
             anyone else.

     $HOME/.ssh/rc
             If this file exists, it is run with /bin/sh after reading the
             environment files but before starting the user's shell or comM--
             mand.  If X11 spoofing is in use, this will receive the "proto
             cookie" pair in standard input (and DISPLAY in environment).
             This must call xauth(1) in that case.

             The primary purpose of this file is to run any initialization
             routines which may be needed before the user's home directory
             becomes accessible; AFS is a particular example of such an enviM--
             ronment.

             This file will probably contain some initialization code followed
             by something similar to:

                     if read proto cookie; then
                             echo add $DISPLAY $proto $cookie | xauth -q -
                     fi

             If this file does not exist, /etc/sshrc is run, and if that does
             not exist either, xauth is used to store the cookie.

             This file should be writable only by the user, and need not be
             readable by anyone else.

     /etc/sshrc
             Like $HOME/.ssh/rc.  This can be used to specify machine-specific
             login-time initializations globally.  This file should be
             writable only by root, and should be world-readable.

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
     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     login.conf(5), moduli(5), sftp-server(8)

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
     Protocol Architecture, draft-ietf-secsh-architecture-09.txt, July 2001,
     work in progress material.

     M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange
     for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
     exchange-01.txt, April 2001, work in progress material.

BSD                           September 25, 1999                           BSD