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Linux 2.6 - man page for sshd (linux section 8)

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

     sshd -- OpenSSH SSH daemon

     sshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-c host_certificate_file] [-f config_file]
	  [-g login_grace_time] [-h host_key_file] [-k key_gen_time] [-o option] [-p port]
	  [-u len]

     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these programs replace
     rlogin(1) and rsh(1), and provide secure encrypted communications between two untrusted
     hosts over an insecure network.

     sshd listens for connections from clients.  It is normally started at boot from
     /etc/init/ssh.conf.  It forks a new daemon for each incoming connection.  The forked daemons
     handle key exchange, encryption, authentication, command execution, and data exchange.

     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 and options it was started with, e.g. /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

     -C connection_spec
	     Specify the connection parameters to use for the -T extended test mode.  If pro-
	     vided, any Match directives in the configuration file that would apply to the speci-
	     fied user, host, and address will be set before the configuration is written to
	     standard output.  The connection parameters are supplied as keyword=value pairs.
	     The keywords are ``user'', ``host'', and ``addr''.  All are required and may be sup-
	     plied in any order, either with multiple -C options or as a comma-separated list.

     -c host_certificate_file
	     Specifies a path to a certificate file to identify sshd during key exchange.  The
	     certificate file must match a host key file specified using the -h option or the
	     HostKey configuration directive.

     -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 standard error, 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 config_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_dsa_key, /etc/ssh/ssh_host_ecdsa_key and /etc/ssh/ssh_host_rsa_key
	     for protocol version 2.  It is possible to have multiple host key files for the dif-
	     ferent protocol versions and host key algorithms.

     -i      Specifies that sshd is being run from inetd(8).  sshd is normally not run from inetd
	     because it needs to generate the server key before it can respond to the client, and
	     this may take tens of seconds.  Clients would have to wait too long if the key was
	     regenerated every time.  However, with small key sizes (e.g. 512) using sshd from
	     inetd may be feasible.

     -k key_gen_time
	     Specifies how often the ephemeral protocol version 1 server key is regenerated
	     (default 3600 seconds, or one hour).  The motivation for regenerating the key fairly
	     often is that the key is not stored anywhere, and after about an hour it becomes
	     impossible to recover the key for decrypting intercepted communications even if the
	     machine is cracked into or physically seized.  A value of zero indicates that the
	     key will never be regenerated.

     -o option
	     Can be used to give options in the format used in the configuration file.	This is
	     useful for specifying options for which there is no separate command-line flag.  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).  Multi-
	     ple port options are permitted.  Ports specified in the configuration file with the
	     Port option are ignored when a command-line port is specified.  Ports specified
	     using the ListenAddress option override command-line ports.

     -q      Quiet mode.  Nothing is sent to the system log.  Normally the beginning, authentica-
	     tion, and termination of each connection is logged.

     -T      Extended test mode.  Check the validity of the configuration file, output the effec-
	     tive configuration to stdout and then exit.  Optionally, Match rules may be applied
	     by specifying the connection parameters using one or more -C options.

     -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 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.

     The OpenSSH SSH daemon supports SSH protocols 1 and 2.  The default is to use protocol 2
     only, though this can be changed via the Protocol option in sshd_config(5).  Protocol 2 sup-
     ports DSA, ECDSA and RSA keys; protocol 1 only supports RSA keys.	For both protocols, each
     host has a host-specific key, normally 2048 bits, used to identify the host.

     Forward security for protocol 1 is provided through an additional server key, normally 768
     bits, generated when the server starts.  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 num-
     ber.  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

     For protocol 2, 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 authen-
     tication code (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).

     Finally, the server and the client enter an authentication dialog.  The client tries to
     authenticate itself using host-based authentication, public key authentication, challenge-
     response authentication, or password authentication.

     Regardless of the authentication type, the account is checked to ensure that it is accessi-
     ble.  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 plat-
     forms have their own account database (eg AIX) and some modify the passwd field ( '*LK*' on
     Solaris and UnixWare, '*' on HP-UX, containing 'Nologin' on Tru64, a leading '*LOCKED*' on
     FreeBSD and a leading '!' on most Linuxes).  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*' ).

     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 connections, or forwarding the authentication agent con-
     nection over the secure channel.

     After this, 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.

     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
		~/.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 the file ~/.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 ~/.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.  See SSHRC, below.

	   9.	Runs user's shell or command.

     If the file ~/.ssh/rc exists, sh(1) runs it 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.

     AuthorizedKeysFile specifies the file containing public keys for public key authentication;
     if none is specified, the default is ~/.ssh/authorized_keys.  Each line of the file contains
     one key (empty lines and lines starting with a '#' are ignored as comments).  Protocol 1
     public keys consist of the following space-separated fields: options, bits, exponent, modu-
     lus, comment.  Protocol 2 public key consist of: options, keytype, base64-encoded key, com-
     ment.  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 proto-
     col version 2 the keytype is ``ecdsa-sha2-nistp256'', ``ecdsa-sha2-nistp384'',
     ``ecdsa-sha2-nistp521'', ``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) up to a limit of 8 kilobytes, which permits DSA keys up to 8 kilo-
     bits and RSA keys up to 16 kilobits.  You don't want to type them in; instead, copy the
     identity.pub, id_dsa.pub, id_ecdsa.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 the listed key is a certification authority (CA) that is trusted to
	     validate signed certificates for user authentication.

	     Certificates may encode access restrictions similar to these key options.	If both
	     certificate restrictions and key options are present, the most restrictive union of
	     the two is applied.

	     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 and/or X11 forwarding unless they are explicitly prohibited.
	     The command originally supplied by the client is available in the
	     SSH_ORIGINAL_COMMAND environment variable.  Note that this option applies to shell,
	     command or subsystem execution.  Also note that this command may be superseded by
	     either a sshd_config(5) ForceCommand directive or a command embedded in a certifi-

	     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.

	     Specifies that in addition to public key authentication, either the canonical name
	     of the remote host or its IP address must be present in the comma-separated list of
	     patterns.	See PATTERNS in ssh_config(5) for more information on patterns.

	     In addition to the wildcard matching that may be applied to hostnames or addresses,
	     a from stanza may match IP addresses using CIDR address/masklen notation.

	     The purpose of this option is to optionally increase security: public key authenti-
	     cation 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).

	     Forbids authentication agent forwarding when this key is used for authentication.

	     Forbids TCP 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.

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

	     Disables execution of ~/.ssh/rc.

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

	     Limit local ``ssh -L'' port forwarding such that it may only connect to the speci-
	     fied host and port.  IPv6 addresses can be specified by enclosing the address in
	     square brackets.  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.

	     On a cert-authority line, specifies allowed principals for certificate authentica-
	     tion as a comma-separated list.  At least one name from the list must appear in the
	     certificate's list of principals for the certificate to be accepted.  This option is
	     ignored for keys that are not marked as trusted certificate signers using the
	     cert-authority option.

	     Force a tun(4) device on the server.  Without this option, the next available device
	     will be used if the client requests a tunnel.

     An example authorized_keys file:

	# Comments allowed at start of line
	ssh-rsa AAAAB3Nza...LiPk== user@example.net
	from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
	AAAAB2...19Q== john@example.net
	command="dump /home",no-pty,no-port-forwarding ssh-dss
	AAAAC3...51R== example.net
	permitopen="",permitopen="" ssh-dss
	tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==

     The /etc/ssh/ssh_known_hosts and ~/.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: markers (optional), hostnames, bits,
     exponent, modulus, comment.  The fields are separated by spaces.

     The marker is optional, but if it is present then it must be one of ``@cert-authority'', to
     indicate that the line contains a certification authority (CA) key, or ``@revoked'', to
     indicate that the key contained on the line is revoked and must not ever be accepted.  Only
     one marker should be used on a key line.

     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.  A hostname or address may
     optionally be enclosed within '[' and ']' brackets then followed by ':' and a non-standard
     port number.

     Alternately, hostnames may be stored in a hashed form which hides host names and addresses
     should the file's contents be disclosed.  Hashed hostnames start with a '|' character.  Only
     one hashed hostname may appear on a single line and none of the above negation or wildcard
     operators may be applied.

     Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained,
     for example, 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; either one that matches exactly or, if the server has presented a certificate
     for authentication, the key of the certification authority that signed the certificate.  For
     a key to be trusted as a certification authority, it must use the ``@cert-authority'' marker
     described above.

     The known hosts file also provides a facility to mark keys as revoked, for example when it
     is known that the associated private key has been stolen.	Revoked keys are specified by
     including the ``@revoked'' marker at the beginning of the key line, and are never accepted
     for authentication or as certification authorities, but instead will produce a warning from
     ssh(1) when they are encountered.

     It is 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 defi-
     nitely don't want to type in the host keys by hand.  Rather, generate them by a script,
     ssh-keyscan(1) or by taking /etc/ssh/ssh_host_key.pub and adding the host names at the
     front.  ssh-keygen(1) also offers some basic automated editing for ~/.ssh/known_hosts
     including removing hosts matching a host name and converting all host names to their hashed

     An example ssh_known_hosts file:

	# Comments allowed at start of line
	closenet,..., 1024 37 159...93 closenet.example.net
	cvs.example.net, ssh-rsa AAAA1234.....=
	# A hashed hostname
	|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
	# A revoked key
	@revoked * ssh-rsa AAAAB5W...
	# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
	@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...

	     This file is used to suppress printing the last login time and /etc/motd, if
	     PrintLastLog and PrintMotd, respectively, are enabled.  It does not suppress print-
	     ing of the banner specified by Banner.

	     This file is used for host-based authentication (see ssh(1) for more information).
	     On some machines this file may need to be world-readable if the user's home direc-
	     tory is on an NFS partition, because sshd reads it as root.  Additionally, this file
	     must be owned by the user, and must not have write permissions for anyone else.  The
	     recommended permission for most machines is read/write for the user, and not acces-
	     sible by others.

	     This file is used in exactly the same way as .rhosts, but allows host-based authen-
	     tication without permitting login with rlogin/rsh.

	     This directory is the default location for all user-specific configuration and
	     authentication information.  There is no general requirement to keep the entire con-
	     tents of this directory secret, but the recommended permissions are read/write/exe-
	     cute for the user, and not accessible by others.

	     Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as this user.
	     The format of this file is described above.  The content of the file is not highly
	     sensitive, but the recommended permissions are read/write for the user, and not
	     accessible by others.

	     If this file, the ~/.ssh directory, or the user's home directory are writable by
	     other users, then the file could be modified or replaced by unauthorized users.  In
	     this case, sshd will not allow it to be used unless the StrictModes option has been
	     set to ``no''.

	     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.

	     Contains a list of host keys for all hosts the user has logged into that are not
	     already in the systemwide list of known host keys.  The format of this file is
	     described above.  This file should be writable only by root/the owner and can, but
	     need not be, world-readable.

	     Contains initialization routines to be run before the user's home directory becomes
	     accessible.  This file should be writable only by the user, and need not be readable
	     by anyone else.

	     Access controls that should be enforced by tcp-wrappers are defined here.	Further
	     details are described in hosts_access(5).

	     This file is for host-based authentication (see ssh(1)).  It should only be writable
	     by root.

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

	     See motd(5).

	     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.

	     This file is used in exactly the same way as hosts.equiv, but allows host-based
	     authentication without permitting login with rlogin/rsh.

	     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 these files are group/world-accessible.

	     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).

	     Systemwide list of known host keys.  This file should be prepared by the system
	     administrator to contain the public host keys of all machines in the organization.
	     The format of this file is described above.  This file should be writable only by
	     root/the owner and should be world-readable.

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

	     Similar to ~/.ssh/rc, it can be used to specify machine-specific login-time initial-
	     izations globally.  This file should be writable only by root, and should be world-

	     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-

     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
     ssh-vulnkey(1), chroot(2), hosts_access(5), moduli(5), sshd_config(5), inetd(8),

     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.

     System security is not improved unless rshd, rlogind, and rexecd are disabled (thus com-
     pletely disabling rlogin and rsh into the machine).

BSD					 October 28, 2010				      BSD

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