path: root/sshd.0

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

NAME
     sshd - OpenSSH SSH daemon

SYNOPSIS
     sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]
          [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]

DESCRIPTION
     sshd (SSH Daemon) is the daemon program for ssh(1).  Together these pro-
     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 nor-
     mally started at boot from /etc/rc.  It forks a new daemon for each in-
     coming connection.  The forked daemons handle key exchange, encryption,
     authentication, command execution, and data exchange.  This implementa-
     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 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.

     Regardless of the authentication type, the account is checked to ensure
     that it is accessible.  An account is not accessible if it is locked,
     listed in DenyUsers or its group is listed in DenyGroups .  The defini-
     tion of a locked account is system dependant. Some platforms have their
     own account database (eg AIX) and some modify the passwd field ( `*LK*'
     on Solaris, `*' on HP-UX, containing `Nologin' on Tru64 and a leading
     `!!' on Linux).  If there is a requirement to disable password authenti-
     cation for the account while allowing still public-key, then the passwd
     field should be set to something other than these values (eg `NP' or
     `*NP*' ).

     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, rlogind, and rexecd are dis-
     abled (thus completely disabling rlogin and rsh 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 of-
     fered 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 (PubkeyAuthentica-
     tion) or client host (HostbasedAuthentication) authentication method,
     conventional password authentication and challenge response based meth-
     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 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
     (by default sshd_config(5)).  Command-line options override values speci-
     fied in the configuration file.

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

     The options are as follows:

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

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

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

     -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 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/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 120 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 a file from which a host key is read.  This option must
             be given if sshd is not run as root (as the normal host key files
             are normally not readable by anyone but root).  The default is
             /etc/ssh/ssh_host_key for protocol version 1, and
             /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
             tocol version 2.  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(8).  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 ephemeral protocol version 1 server key
             is regenerated (default 3600 seconds, or one hour).  The motiva-
             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 ze-
             ro indicates that the key will never be regenerated.

     -o option
             Can be used to give options in the format used in the configura-
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.  For full details of the op-
             tions, and their values, see sshd_config(5).

     -p port
             Specifies the port on which the server listens for connections
             (default 22).  Multiple port options are permitted.  Ports speci-
             fied in the configuration file are ignored when a command-line
             port is specified.

     -q      Quiet mode.  Nothing is sent to the system log.  Normally the be-
             ginning, 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 over-
             flow this field to still be uniquely identified.  Specifying -u0
             indicates that only dotted decimal addresses should be put into
             the utmp file.  -u0 may also be used to prevent sshd from making
             DNS requests unless the authentication mechanism or configuration
             requires it.  Authentication mechanisms that may require DNS in-
             clude RhostsRSAAuthentication, HostbasedAuthentication and using
             a from="pattern-list" option in a key file.  Configuration op-
             tions that require DNS include using a USER@HOST pattern in
             AllowUsers or DenyUsers.

CONFIGURATION FILE
     sshd reads configuration data from /etc/ssh/sshd_config (or the file
     specified with -f on the command line).  The file format and configura-
     tion options are described in sshd_config(5).

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 the file $HOME/.ssh/environment, if it exists, and users
                are allowed to change their environment.  See the
                PermitUserEnvironment option in sshd_config(5).

           7.   Changes to user's home directory.

           8.   If $HOME/.ssh/rc exists, runs it; else if /etc/ssh/sshrc ex-
                ists, 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, key-
     type, base64 encoded key, comment.  The options field is optional; its
     presence is determined by whether the line starts with a number or not
     (the options field never starts with a number).  The bits, exponent, mod-
     ulus and comment fields give the RSA key for protocol version 1; the com-
     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 (be-
     cause of the size of the public key encoding).  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.

     sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
     2 keys of 768 bits.

     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 (note that option keywords are
     case-insensitive):

     from="pattern-list"
             Specifies that in addition to public key 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: public key authentication by itself does not
             trust the network or name servers or anything (but the key); how-
             ever, if somebody somehow steals the key, the key permits an in-
             truder to log in from anywhere in the world.  This additional op-
             tion 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 an 8-bit clean chan-
             nel 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
             backslash.  This option might be useful to restrict certain pub-
             lic 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.  Environment processing is disabled by
             default and is controlled via the PermitUserEnvironment option.
             This option is automatically disabled if UseLogin is enabled.

     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).

     permitopen="host:port"
             Limit local ``ssh -L'' port forwarding such that it may only con-
             nect to the specified host and port.  IPv6 addresses can be spec-
             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 do-
             mains 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 back-
     up.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/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 au-
     tomatically: 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,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
     cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd.  The file format and con-
             figuration options are described in sshd_config(5).

     /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
             /etc/ssh/ssh_host_rsa_key
             These three files contain the private parts of the host keys.
             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/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub,
             /etc/ssh/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".  The file format is described in moduli(5).

     /var/empty
             chroot(2) directory used by sshd during privilege separation in
             the pre-authentication phase.  The directory should not contain
             any files and must be owned by root and not group or world-
             writable.

     /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 process ID of the one started last).
             The content of this file is not sensitive; it can be world-read-
             able.

     $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 de-
             scribed 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/ssh_known_hosts, $HOME/.ssh/known_hosts
             These files are consulted when using rhosts with RSA host authen-
             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/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 de-
             fined 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 a password.  The same file is used by rlogind
             and rshd.  The file must be writable only by the user; it is rec-
             ommended that it not be accessible by others.

             It 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/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.  Environment processing is disabled by default and
             is controlled via the PermitUserEnvironment option.

     $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.
             It must not produce any output on stdout; stderr must be used in-
             stead.  If X11 forwarding is in use, it will receive the "proto
             cookie" pair in its standard input (and DISPLAY in its environ-
             ment).  The script must call xauth(1) because sshd will not run
             xauth automatically to add X11 cookies.

             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 && [ -n "$DISPLAY" ]; then
                     if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
                             # X11UseLocalhost=yes
                             echo add unix:`echo $DISPLAY |
                                 cut -c11-` $proto $cookie
                     else
                             # X11UseLocalhost=no
                             echo add $DISPLAY $proto $cookie
                     fi | xauth -q -
             fi

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

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

     /etc/ssh/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.

SEE ALSO
     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     chroot(2), hosts_access(5), login.conf(5), moduli(5), sshd_config(5),
     inetd(8), sftp-server(8)

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
     Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
     2002, work in progress material.

     M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange
     for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
     exchange-02.txt, January 2002, work in progress material.

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
     created OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
     for privilege separation.

OpenBSD 3.6                   September 25, 1999                             9