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RedHat 9 (Linux i386) - man page for sshd (redhat section 8)

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

     sshd -- OpenSSH SSH daemon

     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]

     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.  Addi-
     tionally, 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 com-
     munications 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 authen-
     ticate 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 disabled (thus completely disabling rlogin and rsh into the

   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, Blow-
     fish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.  The client selects the encryp-
     tion 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 (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, forward-
     ing 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 for-
     warded to/from the shell or command on the server side, and the user terminal in the client

     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 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

     -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.  Multi-
	     ple -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 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 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).  Multi-
	     ple 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, authentica-
	     tion, 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

     -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 dot-
	     ted 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 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.

     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

     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; 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 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; other-
		wise runs xauth.  The ``rc'' files are given the X11 authentication protocol and
		cookie 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 (PubkeyAu-
     thentication) in protocol version 2.  AuthorizedKeysFile may be used to specify an alterna-
     tive 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 con-
     sists 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 conve-
     nient 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 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 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 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 prohib-
	     ited.  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 for-
	     ward requests by the client will return an error.	This might be used, e.g., in con-
	     nection 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 authentication.

     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 speci-
	     fied host and port.  IPv6 addresses can be specified with an alternative syntax:
	     host/port.  Multiple permitopen options may be applied separated by commas. No pat-
	     tern matching is performed on the specified hostnames, they must be literal domains
	     or addresses.

     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="",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 conflict-
     ing information; authentication is accepted if valid information can be found from either

     Note that the lines in these files are typically hundreds of characters long, and you defi-
     nitely 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.

     closenet,..., 1024 37 159...93 closenet.hut.fi
     cvs.openbsd.org, 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, /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 respec-
	     tive 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-authentica-
	     tion phase.  The directory should not contain any files and must be owned by root
	     and not group or world-writable.

	     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 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 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).

	     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 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 entries.

	     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 std-
	     out; 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

	     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 simi-
	     lar 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

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

     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.

     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

BSD					September 25, 1999				      BSD

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