Provided by: ssh-krb5_3.8.1p1-10_i386 bug


     sshd - OpenSSH SSH daemon


     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]


     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.  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
     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 challenge-
     response 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
     definition 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
     authentication 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
     disabled (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

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

     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
     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
     (by default sshd_config(5)).  Command-line options override values
     specified in the configuration file.

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

     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
             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 file.

     -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_rsa_key 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

     -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

     -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.  For full details
             of the options, 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
             specified in the configuration file are ignored when a command-
             line port is specified.

     -q      Quiet mode.  Only fatal errors are sent to the system log.
             Normally the beginning, authentication, and termination of each
             connection is logged.  If a second -q is given then nothing is
             sent to the system log.

     -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 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.


     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).


     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

           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
                exists, runs it; otherwise runs xauth.  The “rc” files are
                given the X11 authentication protocol and cookie in standard

           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, or the
     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 case-insensitive):

             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);
             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 option.

             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.

     1024 33 12121...312314325

     from="*,!" 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/  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/
     and adding the host names at the front.

     closenet,..., 1024 37 159...93, ssh-rsa AAAA1234.....=


             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 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/, /etc/ssh/,
             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).

             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 world-

             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-

             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
   , and/or 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
             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.

             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.

             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

             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.


     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.


     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.