sshd

     

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

NAME
     sshd - secure shell daemon

SYNOPSIS
     sshd [-diqQ46] 
          [-b bits] 
          [-f config_file] 
          [-g login_grace_time] 
          [-h host_key_file] 
          [-k key_gen_time] 
          [-p port] 
          [-u len] 
          [-V client_protocol_id]

DESCRIPTION
     sshd (Secure Shell Daemon) is the daemon program for ssh(1).  Together
     these programs replace rlogin and rsh, and provide secure encrypted com
     munications 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 nor
     mally started at boot from /etc/rc. It forks a new daemon for each incom
     ing connection.  The forked daemons handle key exchange, encryption, au
     thentication, command execution, and data exchange.  This implementation
     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 iden
     tify the host.  Additionally, when the daemon starts, it generates a
     server RSA key (normally 768 bits).  This key is normally regenerated ev
     ery 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 serv
     er.  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 encryp
     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-re
     sponse 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),  rexecd(8),
     and rexd(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 DSA key 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 Blowfish,
     3DES or CAST128 in CBC mode or Arcfour.  The client selects the encryp
     tion algorithm to use from those offered by the server.  Additionally,
     session integrity is provided through a cryptographic message authentica
     tion code (hmac-sha1 or hmac-md5).


     Protocol version 2 provides a public key based user authentication method
     (DSAAuthentication) and conventional password authentication.

   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 connec
     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 configura
     tion file.

     sshd rereads its configuration file when it receives a hangup signal,
     SIGHUP.

     The options are as follows:

     -b bits
             Specifies the number of bits in the 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 op
             tions increases the debugging level.  Maximum is 3.

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

     -g login_grace_time
             Gives the grace time for clients to authenticate themselves (de
             fault 300 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 RSA host key is read (default
             /etc/ssh/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
             anyone but root).

     -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 re
             generated 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 server key is regenerated (default 3600
             seconds, or one hour).  The motivation 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 de
             crypting 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 be
             ginning, authentication, and termination of each connection is
             logged.

     -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 over
             flow this field to still be uniquely identified.  Specifying -u0
             indicates that only dotted decimal addresses should be put into
             the utmp file.

     -Q      Do not print an error message if RSA support is missing.

     -V client_protocol_id
             SSH-2 compatibility mode.  When this option is specified sshd as
             sumes the client has sent the supplied version string and skips
             the Protocol Version Identification Exchange.  This option is not
             intended to be called directly.

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

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

     The following keywords are possible.

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

     AllowGroups
             This keyword can be followed by a number of group names, separat
             ed by spaces.  If specified, login is allowed only for users
             whose primary group matches one of the patterns.  `*' and `?' can
             be used as wildcards in the patterns.  Only group names are
             valid; a numerical group ID isn't recognized.  By default login
             is allowed regardless of the primary group.

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

     AllowUsers
             This keyword can be followed by a number 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 wild
             cards in the patterns.  Only user names are valid; a numerical
             user ID isn't recognized.  By default login is allowed regardless
             of the user name.

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

     CheckMail
             Specifies whether sshd should check for new mail for interactive
             logins.  The default is ``no''.

     DenyGroups
             This keyword can be followed by a number of group names, separat
             ed by spaces.  Users whose primary group matches one of the pat
             terns aren't allowed to log in.  `*' and `?' can be used as wild
             cards in the patterns.  Only group names are valid; a numerical
             group ID isn't recognized.  By default login is allowed regard
             less of the primary group.

     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 pat
             terns.  Only user names are valid; a numerical user ID isn't rec
             ognized.  By default login is allowed regardless of the user
             name.

     DSAAuthentication
             Specifies whether DSA authentication is allowed.  The default is
             ``yes''. Note that this option applies to protocol version 2 on
             ly.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports
             forwarded for the client.  The argument must be ``yes'' or
             ``no''. The default is ``no''.

     HostDSAKey
             Specifies the file containing the private DSA host key (default
             /etc/ssh_host_dsa_key) used by SSH protocol 2.0.  Note that sshd
             disables protocol 2.0 if this file is group/world-accessible.

     HostKey
             Specifies the file containing the private RSA host key (default
             /etc/ssh/ssh_host_key) used by SSH protocols 1.3 and 1.5.  Note
             that sshd disables protocols 1.3 and 1.5 if this file is
             group/world-accessible.

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in au
             thentication.  /etc/hosts.equiv and /etc/ssh/shosts.equiv are
             still used.  The default is ``yes''.

     IgnoreUserKnownHosts
             Specifies whether sshd should ignore the user's
             $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The de
             fault 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 tem
             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 iden
             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
             The server key is automatically regenerated after this many sec
             onds (if it has been used).  The purpose of regeneration is to
             prevent decrypting captured sessions 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 what local address sshd should listen on.  The default
             is to listen to all local addresses.  Multiple options of this
             type are permitted.  Additionally, the Ports options must precede
             this option.

     LoginGraceTime
             The server disconnects after this time if the user has not suc
             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, VERBOSE
             and DEBUG.  The default is INFO.  Logging with level DEBUG vio
             lates the privacy of users and is not recommended.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated con
             nections to the sshd daemon.  Additional connections will be
             dropped until authentication succeeds or the LoginGraceTime ex
             pires 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 proba
             billity of ``rate/100'' (30%) if there are currently ``start''
             (10) unauthenticated connections.  The probabillity increases
             linearly and all connection attempts are refused if the number of
             unauthenticated connections reaches ``full'' (60).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The de
             fault is ``yes''. Note that this option applies to both protocol
             versions 1 and 2.

     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 the root can log in using ssh(1).  The argument
             must be ``yes'', ``without-password'' or ``no''. The default is
             ``yes''. If this options is set to ``without-password'' only
             password authentication is disabled for root.

             Root login with RSA authentication when the command option has
             been specified will be allowed regardless of the value of this
             setting (which may be useful for taking remote backups even if
             root login is normally not allowed).

     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.

     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 possi
             ble values are ``1'' and ``2''. Multiple versions must be comma-
             separated.  The default is ``1''.

     RandomSeed
             Obsolete.  Random number generation uses other techniques.

     RhostsAuthentication
             Specifies whether authentication using rhosts or /etc/hosts.equiv"
             files is sufficient.  Normally, this method should not be permit
             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''.

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication to
             gether with successful RSA host authentication is allowed.  The
             default is ``no''.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The de
             fault is ``yes''. Note that this option applies to protocol ver
             sion 1 only.

     ServerKeyBits
             Defines the number of bits in the server key.  The minimum value


             is 512, and the default is 768.

     SkeyAuthentication
             Specifies whether skey(1) authentication is allowed.  The default
             is ``yes''. Note that s/key authentication is enabled only if
             PasswordAuthentication is allowed, too.

     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 up
             on 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 on
             ly.

     SyslogFacility
             Gives the facility code that is used when logging messages from
             sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LO
             CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The de
             fault is AUTH.

     UseLogin
             Specifies whether login(1) is used for interactive login ses
             sions.  Note that login(1) is never used for remote command exe
             cution.  The default is ``no''.

     X11DisplayOffset
             Specifies the first display number available for sshd's X11 for
             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 secu
             rity in any way, as users can always install their own for
             warders.

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

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 sec
                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
     The $HOME/.ssh/authorized_keys file lists the RSA keys that are permitted
     for RSA authentication in SSH protocols 1.3 and 1.5 Similarly, the
     $HOME/.ssh/authorized_keys2 file lists the DSA keys that are permitted
     for DSA authentication in SSH protocol 2.0.  Each line of the file con
     tains one key (empty lines and lines starting with a `#' are ignored as
     comments).  Each line consists of the following fields, separated by
     spaces: options, bits, exponent, modulus, comment.  The options field is
     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, modulus and comment fields give the RSA key; the comment field
     is not used for anything (but may be convenient for the user to identify
     the key).

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

     The options (if present) consist of comma-separated option specifica
     tions.  No spaces are permitted, except within double quotes.  The fol
     lowing option specifications are supported:

     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 ac
             cepted.  The purpose of this option is to optionally increase se
             curity: 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 connection requests
             a pty; otherwise it is run without a tty.  A quote may be includ
             ed in the command by quoting it with a backslash.  This option
             might be useful to restrict certain RSA keys to perform just a
             specific operation.  An example might be a key that permits re
             mote backups but nothing else.  Note that the client may specify
             TCP/IP and/or X11 forwarding unless they are explicitly prohibit
             ed.

     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.

     no-port-forwarding
             Forbids TCP/IP forwarding when this key is used for authentica
             tion.  Any port forward requests by the client will return an er
             ror.  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).

   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 back
     up.hut.fi

SSH_KNOWN_HOSTS FILE FORMAT
     The /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2,
     $HOME/.ssh/known_hosts, and $HOME/.ssh/known_hosts2 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 auto
     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 wild
     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 indi
     cate negation: if the host name matches a negated pattern, it is not ac
     cepted (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/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 recom
     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 con
     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/ssh_host_key.pub
     and adding the host names at the front.

   Examples
     closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93
     closenet.hut.fi

FILES



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

     /etc/ssh/ssh_host_key
             Contains the private part of the host key.  This file 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/ssh_host_key.pub
             Contains the public part of the host key.  This file should be
             world-readable but writable only by root.  Its contents should
             match the private part.  This file is not really used for any
             thing; it is only provided for the convenience of the user so its
             contents can be copied to known hosts files.  These two files are
             created using ssh-keygen(1).

     /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 con
             tent of this file is not sensitive; it can be world-readable.

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

     $HOME/.ssh/authorized_keys2
             Lists the DSA keys that can be used to log into the user's ac
             count.  This file must be readable by root (which may on some ma
             chines imply it being world-readable if the user's home directory
             resides on an NFS volume).  It is recommended that it not be ac
             cessible by others.  The format of this file is described above.
             Users will place the contents of their id_dsa.pub files into this
             file, as described in ssh-keygen(1).

     /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts
             These files are consulted when using rhosts with RSA host authen
             tication 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 the remote host is the one it intended
             to connect.  These files should be writable only by root/the own
             er.  /etc/ssh/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
             If compiled with LIBWRAP support, tcp-wrappers access controls
             may be defined here as 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 recom
             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 en
             tries start with `-'.

             If the client host/user is successfully matched in this file, lo
             gin is automatically permitted provided the client and server us
             er names are the same.  Additionally, successful RSA host authen
             tication 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 us
             er 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/ssh/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 en
             vironment files but before starting the user's shell or command.
             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 be
             comes accessible; AFS is a particular example of such an environ
             ment.

             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.

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

     This version of OpenSSH

        has all components of a restrictive nature (i.e., patents, see
         crypto(3))  directly removed from the source code; any licensed or
         patented components are chosen from external libraries.

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

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

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

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

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

SEE ALSO
     scp(1),  sftp-server(8),  ssh(1),  ssh-add(1),  ssh-agent(1),  ssh-
     keygen(1),  crypto(3),  rlogin(1),  rsh(1)

BSD Experimental              September 25, 1999                            12