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

NAME
     sshd - secure shell daemon

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

DESCRIPTION
     sshd (Secure Shell Daemon) is the daemon program for ssh(1).  Together
     these programs replace rlogin and rsh programs, and provide secure en-
     crypted communications between two untrusted hosts over an insecure net-
     work.  The programs are intended to be as easy to install and use as pos-
     sible.

     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.

     sshd works as follows.  Each host has a host-specific RSA key (normally
     1024 bits) used to identify 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, the daemon sends its host and
     server public keys to the client.  The client compares the 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 num-
     ber to the server.  Both sides then start to 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 ci-
     pher, currently Blowfish and 3DES, with 3DES being is used by default.
     The client selects the encryption 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 that machine).

     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.

     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.

     -f configuration_file
             Specifies the name of the configuration file.  The default is
             /etc/sshd_config. sshd refuses to start if there is no configura-
             tion 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 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 any-
             one 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.

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

CONFIGURATION FILE
     sshd reads configuration data from /etc/sshd_config (or the file speci-
     fied 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.

     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.

     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 allowed disallowed for user names that match
             one of the patterns.  `*' and `?' can be used as wildcards 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.

     FascistLogging
             Specifies whether to use verbose logging.  Verbose logging vio-
             lates the privacy of users and is not recommended.  The argument
             must be ``yes'' or ``no''. The default is ``no''.

     HostKey
             Specifies the file containing the private host key (default
             /etc/ssh_host_key). Note that sshd does not start 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/shosts.equiv are still
             used.  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 tem-
             porarily, and some people find it annoying.  On the other hand,
             if keepalives are not send, 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. 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 or SecurID. 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.

     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).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The de-
             fault is ``yes''.

     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the
             server allows login to accounts with empty password strings.  The
             default is ``yes''.

     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).

     Port    Specifies the port number that sshd listens on.  The default is
             22.

     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''.

     QuietMode
             Specifies whether the system runs in quiet mode.  In quiet mode,
             nothing is logged in the system log, except fatal errors.  The
             default is ``no''.

     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 ``yes''.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The de-
             fault is ``yes''.

     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''.

     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 DAEMON.

     UseLogin
             Specifies whether login(1) is used. The default is ``no''.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The default is
             ``yes''. Note that disabling X11 forwarding does not improve se-
             curity in any way, as users can always install their own for-
             warders.

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

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.  Each line of the file contains 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, ex-
     ponent, 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 com-
     ment 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) consists 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.  Notice that the client may speci-
             fy TCP/IP and/or X11 forwardings unless they are explicitly pro-
             hibited.

     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_known_hosts and $HOME/.ssh/known_hosts files contain host
     public keys for all known hosts.  The global file should be prepared by
     the admistrator (optional), and the per-user file is maintained automati-
     cally: whenever the user connects 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 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 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_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/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_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_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-
             tents of this file are 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.

     /etc/ssh_known_hosts
             This file is consulted when using rhosts with RSA host authenti-
             cation to check the public key of the host.  The key must be
             listed in this file to be accepted.

     $HOME/.ssh/known_hosts
             The client uses this file and /etc/ssh_known_hosts to verify that
             the remote host is the one we intended to connect. 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
             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 ma-
             chine (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, 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/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
     Tatu Ylonen <ylo@cs.hut.fi>

     Information about new releases, mailing lists, and other related issues
     can be found from the SSH WWW home page:

           http://www.cs.hut.fi/ssh.

     OpenSSH is a derivative of the original (free) ssh 1.2.12 release, but
     with bugs removed and newer features re-added.   Rapidly after the 1.2.12
     release, newer versions bore successively more restrictive licenses.
     This version of OpenSSH

     o   has all components of a restrictive nature (ie. patents, see ssl(8))
         directly removed from the source code; any licensed or patented com-
         ponents are chosen from external libraries.

     o   has been updated to support ssh protocol 1.5.

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

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

     The libraries described in ssl(8) are required for proper operation.

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

OpenBSD 2.6                   September 25, 1999                            10
