sshd_config(4)															    sshd_config(4)

NAME

       sshd_config - sshd configuration file

SYNOPSIS

       /etc/ssh/sshd_config

       The  sshd(1M)  daemon reads configuration data from /etc/ssh/sshd_config (or the file specified with sshd -f on the command line). The file
       contains keyword-value pairs, one per line. A line starting with a hash mark (#) and empty lines are interpreted as comments.

       The sshd_config file supports the keywords listed below. Unless otherwise noted, keywords and their arguments are case-insensitive.

       AllowGroups

	   This keyword can be followed by a number of group names, separated by spaces. If specified, login is allowed only for users whose  pri-
	   mary  group	matches one of the patterns. Asterisk (*) and question mark (?) can be used as wildcards in the patterns. Only group names
	   are valid; a numerical group ID is not recognized. By default, login is allowed regardless of the primary group.

       AllowTcpForwarding

	   Specifies whether TCP forwarding is permitted. The default is yes. Note that disabling TCP forwarding does not improve security  unless
	   users are also denied shell access, as they can always install their own forwarders.

       AllowUsers

	   This  keyword  can  be followed by a number of user names, separated by spaces. If specified, login is allowed only for user names that
	   match one of the patterns. Asterisk (*) and question mark (?)  can be used as wildcards in the patterns. Only user names are  valid;  a
	   numerical user ID is not recognized. By default login is allowed regardless of the user name.

	   If  a specified pattern takes the form user@host then user and host are checked separately, restricting logins to particular users from
	   particular hosts.

       AuthorizedKeysFile

	   Specifies the file that contains the public keys that can be used for user authentication. AuthorizedKeysFile can contain tokens of the
	   form  %T,  which  are  substituted  during  connection  set-up.  The following tokens are defined: %% is replaced by a literal %, %h is
	   replaced by the home directory of the user being authenticated and %u is replaced by the username of that user. After expansion, Autho-
	   rizedKeysFile is taken to be an absolute path or one relative to the user's home directory. The default is .ssh/authorized_keys.

       Banner

	   In  some  jurisdictions,  sending a warning message before authentication can be relevant for getting legal protection. The contents of
	   the specified file are sent to the remote user before authentication is allowed. This option is only available for protocol version	2.
	   By default, no banner is displayed.

       Ciphers

	   Specifies   the   ciphers   allowed	 for   protocol   version   2.	 Multiple   ciphers   must  be	comma-separated.  The  default	is
	   aes128-ctr,aes128-cbc,arcfour,3des-cbc,blowfish-cbc.

       ClientAliveInterval

	   Sets a timeout interval in seconds after which, if no data has been received  from  the  client,  sshd  sends  a  message  through  the
	   encrypted  channel  to  request  a  response  from the client. The default is 0, indicating that these messages will not be sent to the
	   client. This option applies only to protocol version 2.

       ClientAliveCountMax

	   Sets the number of client alive messages (see ClientAliveInterval, above) that can be sent without sshd  receiving  any  messages  back
	   from  the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating
	   the session. It is important to note that the use of client alive messages is very different from KeepAlive	(see  below).  The  client
	   alive  messages  are  sent  through	the  encrypted	channel  and  therefore will not be spoofable. The TCP keepalive option enabled by
	   KeepAlive is spoofable. The client alive mechanism is valuable when a client or server depend on knowing when a connection  has  become
	   inactive.

	   The	default  value is 3. If ClientAliveInterval (above) is set to 15, and ClientAliveCountMax is left at the default, unresponsive ssh
	   clients will be disconnected after approximately 45 seconds.

       Compression

	   Controls whether the server allows the client to negotiate the use of compression. The default is yes.

       DenyGroups

	   Can be followed by a number of group names, separated by spaces. Users whose primary group matches one of the patterns are not  allowed
	   to  log  in. Asterisk (*) and question mark (?) can be used as wildcards in the patterns. Only group names are valid; a numerical group
	   ID is not recognized. By default, login is allowed regardless of the primary group.

       DenyUsers

	   Can be followed by a number of user names, separated by spaces. Login is disallowed for user names that  match  one	of  the  patterns.
	   Asterisk (*) and question mark (?) can be used as wildcards in the patterns. Only user names are valid; a numerical user ID is not rec-
	   ognized. By default, login is allowed regardless of the user name.

	   If a specified pattern takes the form user@host then user and host are checked separately, disallowing logins to particular users  from
	   particular hosts.

       GatewayPorts

	   Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd binds remote port forwardings
	   to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify  that
	   sshd  should  bind remote port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argu-
	   ment must be yes or no. The default is no.

       GSSAPIAuthentication

	   Enables/disables GSS-API user authentication. The default is yes.

	   Currently sshd authorizes client user principals to user accounts as follows: if the principal name matches the requested user account,
	   then the principal is authorized. Otherwise, GSS-API authentication fails.

       GSSAPIKeyExchange

	   Enables/disables GSS-API-authenticated key exchanges. The default is yes.

	   This  option  also  enables	the  use  of  the GSS-API to authenticate the user to server after the key exchange. Note that GSS-API key
	   exchange can succeed but the subsequent authentication using the GSS-API fail if the server does not authorize the user's GSS principal
	   name to the target user account.

	   Currently sshd authorizes client user principals to user accounts as follows: if the principal name matches the requested user account,
	   then the principal is authorized. Otherwise, GSS-API authentication fails.

       GSSAPIStoreDelegatedCredentials

	   Enables/disables the use of delegated GSS-API credentials on the server-side. The default is yes.

	   Specifically, this option, when enabled, causes the server to store delegated GSS-API credentials in the user's default GSS-API creden-
	   tial store (which for the Kerberos V mechanism means /tmp/krb5cc_<uid>).

	   Note -  sshd  does  not take any steps to explicitly destroy stored delegated GSS-API credentials upon logout. It is the responsibility
		   of PAM modules to destroy credentials associated with a session.

       HostbasedAuthentication

	   Specifies whether to try rhosts-based authentication with public key authentication. The argument must be yes or no. The default is no.
	   This  option  applies  to  protocol version 2 only and is similar to RhostsRSAAuthentication. See sshd(1M) for guidelines on setting up
	   host-based authentication.

       HostbasedUsesNameFromPacketOnly

	   Controls which hostname is searched for in the files ~/.shosts, /etc/shosts.equiv, and /etc/hosts.equiv. If this parameter  is  set	to
	   yes, the server uses the name the client claimed for itself and signed with that host's key. If set to no, the default, the server uses
	   the name to which the client's IP address resolves.

	   Setting this parameter to no disables host-based authentication when using NAT or when the client gets to the server indirectly through
	   a port-forwarding firewall.

       HostKey

	   Specifies  the  file  containing  the  private  host  key used by SSH. 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. Note that sshd will refuse to  use  a  file	if  it	is
	   group/world-accessible.  It	is  possible to have multiple host key files. rsa1 keys are used for version 1 and dsa or rsa are used for
	   version 2 of the SSH protocol.

       IgnoreRhosts

	   Specifies that .rhosts and .shosts files will not be used in authentication. /etc/hosts.equiv and /etc/shosts.equiv are still used. The
	   default is yes. This parameter applies to both protocol versions 1 and 2.

       IgnoreUserKnownHosts

	   Specifies  whether sshd should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default is no. This parame-
	   ter applies to both protocol versions 1 and 2.

       KbdInteractiveAuthentication

	   Specifies whether authentication by means of the "keyboard-interactive" authentication method (and PAM) is allowed.	Defaults  to  yes.
	   (Deprecated: this parameter can only be set to yes.)

       KeepAlive

	   Specifies  whether  the  system should send keepalive messages to the other side. If they are sent, death of the connection or crash of
	   one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, which can
	   be  an  annoyance. On the other hand, if keepalives are not sent, sessions can hang indefinitely on the server, leaving ``ghost'' users
	   and consuming server resources.

	   The default is yes (to send keepalives), and the server will notice if the network goes down or the client host  reboots.  This  avoids
	   infinitely hanging sessions.

	   To disable keepalives, the value should be set to no in both the server and the client configuration files.

       KeyRegenerationInterval

	   In protocol version 1, the ephemeral server key is automatically regenerated after this many seconds (if it has been used). The purpose
	   of regeneration is to prevent decrypting captured sessions by later breaking into the machine and stealing the keys. The key  is  never
	   stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds).

       ListenAddress

	   Specifies what local address sshd should listen on. The following forms can be used:

	   ListenAddress host|IPv4_addr|IPv6_addr
	   ListenAddress host|IPv4_addr:port
	   ListenAddress [host|IPv6_addr]:port

	   If  port  is not specified, sshd will listen on the address and all prior Port options specified. The default is to listen on all local
	   addresses. Multiple ListenAddress options are permitted. Additionally, any Port options must precede this option for non-port qualified
	   addresses.

	   The default is to listen on all local addresses. Multiple options of this type are permitted. Additionally, the Ports options must pre-
	   cede this option.

       LoginGraceTime

	   The server disconnects after this time (in seconds) if the user has not successfully logged in. If the value is 0,  there  is  no  time
	   limit. The default is 120 (seconds).

       LogLevel

	   Gives  the  verbosity  level that is used when logging messages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE,
	   DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging  with
	   level DEBUG violates the privacy of users and is not recommended.

       LookupClientHostnames

	   Specifies whether or not to lookup the names of client's addresses. Defaults to yes.

       MACs

	   Specifies  the  available  MAC  (message  authentication  code)  algorithms.  The  MAC algorithm is used in protocol version 2 for data
	   integrity protection. Multiple algorithms must be comma-separated. The default is hmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96.

       MaxStartups

	   Specifies the maximum number of concurrent unauthenticated connections to the sshd daemon. Additional connections will be dropped until
	   authentication succeeds or the LoginGraceTime expires for a connection. The default is 10.

	   Alternatively, random early drop can be enabled by specifying the three colon-separated values start:rate:full (for example, 10:30:60).
	   Referring to this example, sshd will refuse connection attempts with a probability of rate/100 (30% in our example) if there  are  cur-
	   rently  10  (from  the  start  field)  unauthenticated connections. The probabillity increases linearly and all connection attempts are
	   refused if the number of unauthenticated connections reaches full (60 in our example).

       PasswordAuthentication

	   Specifies whether password authentication is allowed. The default is yes. Note that this option applies to both protocol versions 1 and
	   2.

       PermitEmptyPasswords

	   When  password  authentication  is  allowed,  it  specifies whether the server allows login to accounts with empty password strings. In
	   /etc/default/login, if PASSREQ is not set, or PASSREQ=YES, then the default is no; if PASSREQ=NO, then the default is yes.

       PermitRootLogin

	   Specifies whether the root can log in using ssh(1). The argument must  be  yes,  without-password,  forced-commands-only,  or  no.  The
	   default  is	yes.  without-password means that root cannot be authenticated using the "password" or "keyboard-interactive" methods (see
	   description of KbdInteractiveAuthentication above). forced-commands-only means that authentication is allowed only for "publickey" (for
	   SSHv2, or "RSA", for SSHv1) and only if the matching authorized_keys entry for root has a "command=<cmd>" option.

	   The	without-password and forced-commands-only settings are useful for, for example, performing remote administration and backups using
	   trusted public keys for authentication of the remote client, without allowing access to the root account using passwords.

       PermitUserEnvironment

	   Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed  by  sshd.  The  default  is  no.
	   Enabling  environment  processing  can  enable  users  to  bypass  access  restrictions in some configurations using mechanisms such as
	   LD_PRELOAD.

       PidFile

	   Allows you to specify an alternative to /var/run/sshd.pid, the default file for storing the PID of the sshd listening for  connections.
	   See sshd(1M).

       Port

	   Specifies the port number that sshd listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress.

       PrintLastLog

	   Specifies whether sshd should display the date and time when the user last logged in. The default is yes.

       PrintMotd

	   Specifies  whether  sshd  should  display the contents of /etc/motd when a user logs in interactively. (On some systems it is also dis-
	   played by the shell or a shell startup file, such as /etc/profile.) The default is yes.

       Protocol

	   Specifies the protocol versions sshd should support in order of preference. The possible values are 1 and 2. Multiple versions must	be
	   comma-separated. The default is 2,1. This means that ssh tries version 2 and falls back to version 1 if version 2 is not available.

       PubkeyAuthentication

	   Specifies whether public key authentication is allowed. The default is yes. Note that this option applies to protocol version 2 only.

       RhostsAuthentication

	   Specifies  whether  authentication  using rhosts or /etc/hosts.equiv files is sufficient. Normally, this method should not be permitted
	   because it is insecure. RhostsRSAAuthentication should be used instead, because it performs RSA-based host authentication  in  addition
	   to normal rhosts or /etc/hosts.equiv authentication. The default is no. Note that this parameter applies only to protocol version 1.

       RhostsRSAAuthentication

	   Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is
	   no. Note that this parameter applies only to protocol version 1.

       RSAAuthentication

	   Specifies whether pure RSA authentication is allowed. The default is yes. Note that this option applies to protocol version 1 only.

       ServerKeyBits

	   Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 768.

       StrictModes

	   Specifies whether sshd should check file modes and ownership of the user's files and home directory before  accepting  login.  This	is
	   normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is yes.

       Subsystem

	   Configures  an  external subsystem (for example, a file transfer daemon). Arguments should be a subsystem name and a command to execute
	   upon subsystem request. The command sftp-server(1M) implements the sftp file transfer subsystem. By default, no subsystems are defined.
	   Note that this option applies to protocol version 2 only.

       SyslogFacility

	   Gives  the  facility  code  that  is used when logging messages from sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1,
	   LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, and LOCAL7. The default is AUTH.

       VerifyReverseMapping

	   Specifies whether sshd should try to verify the remote host name and check that the resolved host name for the remote IP  address  maps
	   back to the very same IP address. (A yes setting means "verify".) Setting this parameter to no can be useful where DNS servers might be
	   down and thus cause sshd to spend much time trying to resolve the client's IP address to a name. This feature is useful  for  Internet-
	   facing servers. The default is no.

       X11DisplayOffset

	   Specifies  the first display number available for sshd's X11 forwarding. This prevents sshd from interfering with real X11 servers. The
	   default is 10.

       X11Forwarding

	   Specifies whether X11 forwarding is permitted. The default is yes. Note that disabling X11 forwarding does not improve security in  any
	   way, as users can always install their own forwarders.

	   When X11 forwarding is enabled, there can be additional exposure to the server and to client displays if the sshd proxy display is con-
	   figured to listen on the wildcard address (see X11UseLocalhost below). However, this is not the default. Additionally, the  authentica-
	   tion spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding
	   is that the client's X11 display server can be exposed to attack when the ssh client requests forwarding (see  the  warnings  for  For-
	   wardX11 in ssh_config(4)). A system administrator who wants to protect clients that expose themselves to attack by unwittingly request-
	   ing X11 forwarding, should specify a ``no'' setting.

	   Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users  can  always  install  their  own  for-
	   warders.

       X11UseLocalhost

	   Specifies whether sshd should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, sshd binds
	   the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment	variable  to  ``localhost''.  This
	   prevents remote hosts from connecting to the proxy display. However, some older X11 clients might not function with this configuration.
	   X11UseLocalhost can be set to no to specify that the forwarding server should be bound to the wildcard address. The	argument  must	be
	   yes or no. The default is yes.

       XAuthLocation

	   Specifies the location of the xauth(1) program. The default is /usr/X/bin/xauth.

   Time Formats
       sshd  command-line  arguments  and  configuration file options that specify time can be expressed using a sequence of the form: time[quali-
       fier,] where time is a positive integer value and qualifier is one of the following:

       <none>

	   seconds

       s | S

	   seconds

       m | M

	   minutes

       h | H

	   hours

       d | D

	   days

       w |

	   weeks

       Each element of the sequence is added together to calculate the total time value. For example:

       600

	   600 seconds (10 minutes)

       10m

	   10 minutes

       1h30m

	   1 hour, 30 minutes (90 minutes)

       /etc/ssh/sshd_config

	   Contains configuration data for sshd. This file should be writable by root only, but it is recommended (though not necessary)  that	it
	   be world-readable.

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWsshu			   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Evolving			   |
       +-----------------------------+-----------------------------+

       login(1), sshd(1M), ssh_config(4), SEAM(5), attributes(5)

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 recent 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.

								    6 Oct 2005							    sshd_config(4)