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+SSHD(8)                     System Manager's Manual                    SSHD(8)
+
+NAME
+     sshd - OpenSSH SSH daemon
+
+SYNOPSIS
+     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
+     sshd (SSH Daemon) is the daemon program for ssh(1).  Together these proM--
+     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 norM--
+     mally 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 implementaM--
+     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 idenM--
+     tify 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 encrypM--
+     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-
+     response 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 disM--
+     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
+     offered 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 (PubkeyAuthenticaM--
+     tion) or client host (HostbasedAuthentication) authentication method,
+     conventional password authentication and challenge response based methM--
+     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 connecM--
+     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 configuraM--
+     tion file.
+
+     sshd rereads its configuration file when it receives a hangup signal,
+     SIGHUP, by executing itself with the name it was started as, i.e.,
+     /usr/sbin/sshd.
+
+     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 conM--
+             figuration file.
+
+     -g login_grace_time
+             Gives the grace time for clients to authenticate themselves
+             (default 600 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 proM--
+             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.  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 motivaM--
+             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
+             zero indicates that the key will never be regenerated.
+
+     -o option
+             Can be used to give options in the format used in the configuraM--
+             tion 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 speciM--
+             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
+             beginning, 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 overM--
+             flow this field to still be uniquely identified.  Specifying -u0
+             indicates that only dotted decimal addresses should be put into
+             the utmp file.  -u0 is 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.
+
+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 configuraM--
+     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 secM--
+                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/ssh/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
+     $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, keyM--
+     type, base64 encoded key, comment.  The options fields are 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, moduM--
+     lus and comment fields give the RSA key for protocol version 1; the comM--
+     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
+     (because of the size of the RSA key modulus).  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 specificaM--
+     tions.  No spaces are permitted, except within double quotes.  The folM--
+     lowing option specifications are supported (note that option keywords are
+     case-insensitive):
+
+     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
+             accepted.  The purpose of this option is to optionally increase
+             security: 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 client requests a
+             pty; otherwise it is run without a tty.  If a 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 backM--
+             slash.  This option might be useful to restrict certain RSA 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.  This option is automatically disabled
+             if UseLogin is enabled.
+
+     no-port-forwarding
+             Forbids TCP/IP forwarding when this key is used for authenticaM--
+             tion.  Any port forward requests by the client will return an
+             error.  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 conM--
+             nect to the specified host and port.  IPv6 addresses can be specM--
+             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
+             domains 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
+     backup.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
+     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 wildM--
+     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
+     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 recomM--
+     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 conM--
+     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 conM--
+             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".
+
+     /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-readM--
+             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
+             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 authenM--
+             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
+             defined 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 password.  The same file is used by rlogind and
+             rshd.  The file must be writable only by the user; it is recomM--
+             mended that it not be accessible by others.
+
+             If is also possible to use netgroups in the file.  Either host or
+             user name may be of the form +@groupname to specify all hosts or
+             all users in the group.
+
+     $HOME/.shosts
+             For ssh, this file is exactly the same as for .rhosts.  However,
+             this file is not used by rlogin and rshd, so using this permits
+             access using SSH only.
+
+     /etc/hosts.equiv
+             This file is used during .rhosts authentication.  In the simplest
+             form, this file contains host names, one per line.  Users on
+             those hosts are permitted to log in without a password, provided
+             they have the same user name on both machines.  The host name may
+             also be followed by a user name; such users are permitted to log
+             in as any user on this machine (except root).  Additionally, the
+             syntax ``+@group'' can be used to specify netgroups.  Negated
+             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
+             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.
+
+     $HOME/.ssh/rc
+             If this file exists, it is run with /bin/sh after reading the
+             environment files but before starting the user's shell or comM--
+             mand.  It must not produce any output on stdout; stderr must be
+             used instead.  If X11 forwarding is in use, it will receive the
+             "proto cookie" pair in its standard input (and DISPLAY in its
+             environment).  The script must call xauth(1) because sshd will
+             not run xauth automatically to add X11 cookies.
+
+             The primary purpose of this file is to run any initialization
+             routines which may be needed before the user's home directory
+             becomes accessible; AFS is a particular example of such an enviM--
+             ronment.
+
+             This file will probably contain some initialization code followed
+             by something similar to:
+
+             if read proto cookie && [ -n "$DISPLAY" ]; then
+                     if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
+                             # X11UseLocalhost=yes
+                             xauth add unix:`echo $DISPLAY |
+                                 cut -c11-` $proto $cookie
+                     else
+                             # X11UseLocalhost=no
+                             xauth add $DISPLAY $proto $cookie
+                     fi
+             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.
+
+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 creM--
+     ated 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
+     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-group-
+     exchange-02.txt, January 2002, work in progress material.
+
+BSD                           September 25, 1999                           BSD