sshd_config.0

OpenSSH 是 SSH （Secure SHell） 协议的免费开源实现。它用安全、加密的网络连接工具代替了 telnet、ftp、 rlogin、rsh 和 rcp 工具。OpenSSH 支持

             Specifies the maximum number of authentication attempts permitted
             per connection.  Once the number of failures reaches half this
             value, additional failures are logged.  The default is 6.

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

             Alternatively, random early drop can be enabled by specifying the
             three colon separated values ``start:rate:full'' (e.g.,
             "10:30:60").  sshd will refuse connection attempts with a proba-
             bility of ``rate/100'' (30%) if there are currently ``start''
             (10) unauthenticated connections.  The probability increases lin-
             early and all connection attempts are refused if the number of
             unauthenticated connections reaches ``full'' (60).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The de-
             fault is ``yes''.

     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the
             server allows login to accounts with empty password strings.  The
             default is ``no''.

     PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument
             must be ``yes'', ``without-password'', ``forced-commands-only''
             or ``no''.  The default is ``yes''.

             If this option is set to ``without-password'' password authenti-
             cation is disabled for root.

             If this option is set to ``forced-commands-only'' root login with
             public key authentication will be allowed, but only if the
             command option has been specified (which may be useful for taking
             remote backups even if root login is normally not allowed).  All
             other authentication methods are disabled for root.

             If this option is set to ``no'' root is not allowed to log in.

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

     PidFile
             Specifies the file that contains the process ID of the sshd dae-
             mon.  The default is /var/run/sshd.pid.

     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 print the date and time of the last
             user login when a user logs in interactively.  The default is
             ``yes''.

     PrintMotd
             Specifies whether sshd should print /etc/motd when a user logs in
             interactively.  (On some systems it is also printed by the shell,
             /etc/profile, or equivalent.)  The default is ``yes''.

     Protocol
             Specifies the protocol versions sshd supports.  The possible val-
             ues are ``1'' and ``2''.  Multiple versions must be comma-sepa-
             rated.  The default is ``2,1''.  Note that the order of the pro-
             tocol list does not indicate preference, because the client se-
             lects among multiple protocol versions offered by the server.
             Specifying ``2,1'' is identical to ``1,2''.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The de-
             fault is ``yes''.  Note that this option applies to protocol ver-
             sion 2 only.

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication to-
             gether with successful RSA host authentication is allowed.  The
             default is ``no''.  This option applies to protocol version 1 on-
             ly.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The de-
             fault is ``yes''.  This option applies to protocol version 1 on-
             ly.

     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 (e.g., file transfer daemon).
             Arguments should be a subsystem name and a command to execute up-
             on subsystem request.  The command sftp-server(8) implements the
             ``sftp'' file transfer subsystem.  By default no subsystems are
             defined.  Note that this option applies to protocol version 2 on-
             ly.

     SyslogFacility
             Gives the facility code that is used when logging messages from
             sshd.  The possible values are: DAEMON, USER, AUTH, LOCAL0, LO-
             CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The de-
             fault is AUTH.

     TCPKeepAlive
             Specifies whether the system should send TCP 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 tem-
             porarily, and some people find it annoying.  On the other hand,
             if TCP keepalives are not sent, sessions may hang indefinitely on
             the server, leaving ``ghost'' users and consuming server re-
             sources.

             The default is ``yes'' (to send TCP keepalive messages), and the
             server will notice if the network goes down or the client host
             crashes.  This avoids infinitely hanging sessions.

             To disable TCP keepalive messages, the value should be set to
             ``no''.

     UseDNS  Specifies whether sshd should lookup the remote host name and
             check that the resolved host name for the remote IP address maps
             back to the very same IP address.  The default is ``yes''.

     UseLogin
             Specifies whether login(1) is used for interactive login ses-
             sions.  The default is ``no''.  Note that login(1) is never used
             for remote command execution.  Note also, that if this is en-
             abled, X11Forwarding will be disabled because login(1) does not
             know how to handle xauth(1) cookies.  If UsePrivilegeSeparation
             is specified, it will be disabled after authentication.

     UsePAM  Enables the Pluggable Authentication Module interface.  If set to
             ``yes'' this will enable PAM authentication using
             ChallengeResponseAuthentication and PAM account and session mod-
             ule processing for all authentication types.

             Because PAM challenge-response authentication usually serves an
             equivalent role to password authentication, you should disable
             either PasswordAuthentication or ChallengeResponseAuthentication.

             If UsePAM is enabled, you will not be able to run sshd(8) as a
             non-root user.  The default is ``no''.

     UsePrivilegeSeparation
             Specifies whether sshd separates privileges by creating an un-
             privileged child process to deal with incoming network traffic.
             After successful authentication, another process will be created
             that has the privilege of the authenticated user.  The goal of
             privilege separation is to prevent privilege escalation by con-
             taining any corruption within the unprivileged processes.  The
             default is ``yes''.

     X11DisplayOffset
             Specifies the first display number available for sshd's X11 for-
             warding.  This prevents sshd from interfering with real X11
             servers.  The default is 10.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The argument must
             be ``yes'' or ``no''.  The default is ``no''.

             When X11 forwarding is enabled, there may be additional exposure
             to the server and to client displays if the sshd proxy display is
             configured to listen on the wildcard address (see X11UseLocalhost
             below), however this is not the default.  Additionally, the au-
             thentication spoofing and authentication data verification and
             substitution occur on the client side.  The security risk of us-
             ing X11 forwarding is that the client's X11 display server may be
             exposed to attack when the ssh client requests forwarding (see
             the warnings for ForwardX11 in ssh_config(5)).  A system adminis-
             trator may have a stance in which they want to protect clients
             that may expose themselves to attack by unwittingly requesting
             X11 forwarding, which can warrant a ``no'' setting.

             Note that disabling X11 forwarding does not prevent users from
             forwarding X11 traffic, as users can always install their own
             forwarders.  X11 forwarding is automatically disabled if UseLogin
             is enabled.

     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 may not function
             with this configuration.  X11UseLocalhost may be set to ``no'' to
             specify that the forwarding server should be bound to the wild-
             card address.  The argument must be ``yes'' or ``no''.  The de-
             fault is ``yes''.

     XAuthLocation
             Specifies the full pathname of the xauth(1) program.  The default
             is /usr/X11R6/bin/xauth.

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

           <none>  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

     Each member of the sequence is added together to calculate the total time
     value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd.  This file should be
             writable by root only, but it is recommended (though not neces-
             sary) that it be world-readable.

SEE ALSO
     sshd(8)

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.

OpenBSD 3.6                   September 25, 1999                             8