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NAME    [Toc]    [Back]

     sshd -- OpenSSH SSH daemon

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     sshd (SSH Daemon) is the daemon program for ssh(1).  Together these programs
 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 normally
 started at boot from /etc/rc.d/sshd.  It forks a new daemon for
     each incoming connection.  The forked daemons handle key exchange,
     encryption, authentication, 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    [Toc]    [Back]
     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 responds with its public host and
     server keys.  The client compares the RSA host key against its own database
 to verify that it has not changed.  The client then generates a 256
     bit random number.  It encrypts this random number using both the host
     key and the server key, and sends the encrypted number to the server.
     Both sides then use this random number as a session key which is used to
     encrypt all further communications in the session.  The rest of the session
 is encrypted using a conventional cipher, currently Blowfish or
     3DES, with 3DES being used by default.  The client selects the 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 challengeresponse
 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, rlogind, and rexecd are disabled
 (thus completely disabling rlogin and rsh into the machine).

   SSH protocol version 2    [Toc]    [Back]
     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 DiffieHellman
 key agreement.  This key agreement results in a shared session

     The rest of the session is encrypted using a symmetric cipher, currently
     128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit
     AES.  The client selects the encryption algorithm to use from those
     offered by the server.  Additionally, session integrity is provided
     through a cryptographic message authentication code (hmac-sha1 or hmacmd5).

     Protocol version 2 provides a public key based user (PubkeyAuthentication)
 or client host (HostbasedAuthentication) authentication method,
     conventional password authentication and challenge response based methods.

   Command execution and data forwarding    [Toc]    [Back]
     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 connections
 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 configuration

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

     The options are as follows:

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

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

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

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

     -g login_grace_time
             Gives the grace time for clients to authenticate themselves
             (default 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_dsa_key for protocol 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
             regenerated every time.  However, with small key sizes (e.g.,
             512) using sshd from inetd may be feasible.

     -k key_gen_time
             Specifies how often the ephemeral protocol version 1 server key
             is regenerated (default 3600 seconds, or one hour).  The 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 decrypting intercepted communications even
             if the machine is cracked into or physically seized.  A value of
             zero indicates that the key will never be regenerated.

     -o option
             Can be used to give options in the format used in the configuration
 file.  This is useful for specifying options for which there
             is no separate command-line flag.

     -p port
             Specifies the port on which the server listens for connections
             (default 22).  Multiple port options are permitted.  Ports specified
 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
             beginning, authentication, and termination of each connection is

     -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 overflow
 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
             include RhostsAuthentication, RhostsRSAAuthentication,
             HostbasedAuthentication and using a from="pattern-list" option in
             a key file.  Configuration options that require DNS include using
             a USER@HOST pattern in AllowUsers or DenyUsers.

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

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.


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

LOGIN PROCESS    [Toc]    [Back]

     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 section).

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

           3.   Checks /etc/nologin and /var/run/nologin; if one exists, it
                prints the 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 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
                exists, runs it; otherwise runs xauth(1).  The ``rc'' files
                are given the X11 authentication protocol and cookie (if
                applicable) in standard input.

           9.   Runs user's shell or command.


     $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, keytype,
 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, modulus
 and comment fields give the RSA key for protocol version 1; the comment
 field is not used for anything (but may be convenient for the user
     to identify the key).  For protocol version 2 the keytype is ``ssh-dss''
     or ``ssh-rsa''.

     Note that lines in this file are usually several hundred bytes long
     (because of the size of the 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 specifications.
  No spaces are permitted, except within double quotes.  The following
 option specifications are supported (note that option keywords are

             Specifies that in addition to public key authentication, the
             canonical name of the remote host must be present in the commaseparated
 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); 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).

             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 channel
 is required, one must not request a pty or should specify
             no-pty.  A quote may be included in the command by quoting it
             with a backslash.  This option might be useful to restrict certain
 public 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.

             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.

             Forbids TCP/IP forwarding when this key is used for authentication.
  Any port forward requests by the client will return an
             error.  This might be used, e.g., in connection with the command

             Forbids X11 forwarding when this key is used for authentication.
             Any X11 forward requests by the client will return an error.

             Forbids authentication agent forwarding when this key is used for

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

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

   Examples    [Toc]    [Back]
     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

     permitopen="",permitopen="" 1024 33 23...2323


     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
     automatically: 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 wildcards);
 each pattern in turn is matched against the canonical host name
     (when authenticating a client) or against the user-supplied name (when
     authenticating a server).  A pattern may also be preceded by `'!  to
     indicate negation: if the host name matches a negated pattern, it is not
     accepted (by that line) even if it matched another pattern on the line.

     Bits, exponent, and modulus are taken directly from the RSA host key;
     they can be obtained, e.g., from /etc/ssh/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 recommended)
 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 conflicting
 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    [Toc]    [Back]
     closenet,..., 1024 37 159...93 closenet.hut.fi
     cvs.openbsd.org, ssh-rsa AAAA1234.....=

FILES    [Toc]    [Back]

             Contains configuration data for sshd.  The file format and configuration
 options are described in sshd_config(5).

     /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key
             These two 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
             These two 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).

             Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
             Exchange".  The file format is described in moduli(5).

             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 worldwritable.

             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-readable.

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

     /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts
             These files are consulted when using rhosts with RSA host authentication
 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.

             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

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

             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 recommended
 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.

             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.

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

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

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

             Note that this warning also applies to rsh/rlogin.

             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.

             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.

             If this file exists, it is run with /bin/sh after reading the
             environment files but before starting the user's shell or command.
  It must not produce any output on stdout; stderr must be
             used instead.  If X11 forwarding is in use, it will receive the
             "proto cookie" pair in its standard input (and DISPLAY in its
             environment).  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
             becomes accessible; AFS is a particular example of such an environment.

             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
                             # X11UseLocalhost=no
                             echo add $DISPLAY $proto $cookie
                     fi | xauth -q -

             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.

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

AUTHORS    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     login.conf(5), moduli(5), sshd_config(5), 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-groupexchange-02.txt,
 January 2002, work in progress material.

FreeBSD 5.2.1                 September 25, 1999                 FreeBSD 5.2.1
[ Back ]
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