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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.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 simul-
     taneously.  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
     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 data-
     base 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 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-
     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 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).
     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 (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 it was started as, i.e.,
     /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
	     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.

	     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 ver-
	     sions 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 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
	     zero 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
	     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 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
	     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 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
	     include RhostsRSAAuthentication, HostbasedAuthentication and

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 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 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(1).  The ``rc'' files
		are given the X11 authentication protocol and cookie (if
		applicable) 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
     (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 specifica-
	     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
	     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 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 cer-
	     tain public keys to perform just a specific operation.  An exam-
	     ple 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
	     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 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
	     domains or addresses.

   Examples

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 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
     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 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
	     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 con-
	     venience of the user so their contents can be copied to known
	     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
	     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, $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
	     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 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

	     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/ssh/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
	     environment files but before starting the user's shell or com-
	     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 envi-
	     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
			     echo add unix:`echo $DISPLAY |
				 cut -c11-` $proto $cookie
		     else
			     # X11UseLocalhost=no
			     echo add $DISPLAY $proto $cookie
		     fi | xauth -q -
	     fi

	     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 cre-
     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.

FreeBSD 5.4		      September 25, 1999		   FreeBSD 5.4

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