3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: sshd_config.5,v 1.84 2008/03/25 11:58:02 djm Exp $
38 .Dd $Mdocdate: March 27 2008 $
43 .Nd OpenSSH SSH daemon configuration file
45 .Nm /etc/ssh/sshd_config
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
53 The file contains keyword-argument pairs, one per line.
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
59 in order to represent arguments containing spaces.
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
66 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
79 Multiple environment variables may be separated by whitespace or spread
83 Be warned that some environment variables could be used to bypass restricted
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
88 Specifies which address family should be used by
99 This keyword can be followed by a list of group name patterns, separated
101 If specified, login is allowed only for users whose primary
102 group or supplementary group list matches one of the patterns.
103 Only group names are valid; a numerical group ID is not recognized.
104 By default, login is allowed for all groups.
105 The allow/deny directives are processed in the following order:
116 for more information on patterns.
117 .It Cm AllowTcpForwarding
118 Specifies whether TCP forwarding is permitted.
121 Note that disabling TCP forwarding does not improve security unless
122 users are also denied shell access, as they can always install their
125 This keyword can be followed by a list of user name patterns, separated
127 If specified, login is allowed only for user names that
128 match one of the patterns.
129 Only user names are valid; a numerical user ID is not recognized.
130 By default, login is allowed for all users.
131 If the pattern takes the form USER@HOST then USER and HOST
132 are separately checked, restricting logins to particular
133 users from particular hosts.
134 The allow/deny directives are processed in the following order:
145 for more information on patterns.
146 .It Cm AuthorizedKeysFile
147 Specifies the file that contains the public keys that can be used
148 for user authentication.
149 .Cm AuthorizedKeysFile
150 may contain tokens of the form %T which are substituted during connection
152 The following tokens are defined: %% is replaced by a literal '%',
153 %h is replaced by the home directory of the user being authenticated, and
154 %u is replaced by the username of that user.
156 .Cm AuthorizedKeysFile
157 is taken to be an absolute path or one relative to the user's home
160 .Dq .ssh/authorized_keys .
162 The contents of the specified file are sent to the remote user before
163 authentication is allowed.
166 then no banner is displayed.
167 This option is only available for protocol version 2.
168 By default, no banner is displayed.
169 .It Cm ChallengeResponseAuthentication
170 Specifies whether challenge-response authentication is allowed.
173 this controls the use of PAM (see
176 Note that this affects the effectiveness of the
177 .Cm PasswordAuthentication
183 .It Cm ChrootDirectory
186 to after authentication.
187 This path, and all its components, must be root-owned directories that are
188 not writable by any other user or group.
190 The path may contain the following tokens that are expanded at runtime once
191 the connecting user has been authenticated: %% is replaced by a literal '%',
192 %h is replaced by the home directory of the user being authenticated, and
193 %u is replaced by the username of that user.
197 must contain the necessary files and directories to support the
199 For an interactive session this requires at least a shell, typically
213 For file transfer sessions using
215 no additional configuration of the environment is necessary if the
216 in-process sftp server is used (see
220 The default is not to
223 Specifies the ciphers allowed for protocol version 2.
224 Multiple ciphers must be comma-separated.
225 The supported ciphers are
240 .Bd -literal -offset 3n
241 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
242 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
243 aes192-ctr,aes256-ctr
245 .It Cm ClientAliveCountMax
246 Sets the number of client alive messages (see below) which may be
249 receiving any messages back from the client.
250 If this threshold is reached while client alive messages are being sent,
251 sshd will disconnect the client, terminating the session.
252 It is important to note that the use of client alive messages is very
256 The client alive messages are sent through the encrypted channel
257 and therefore will not be spoofable.
258 The TCP keepalive option enabled by
261 The client alive mechanism is valuable when the client or
262 server depend on knowing when a connection has become inactive.
264 The default value is 3.
266 .Cm ClientAliveInterval
267 (see below) is set to 15, and
268 .Cm ClientAliveCountMax
269 is left at the default, unresponsive SSH clients
270 will be disconnected after approximately 45 seconds.
271 This option applies to protocol version 2 only.
272 .It Cm ClientAliveInterval
273 Sets a timeout interval in seconds after which if no data has been received
276 will send a message through the encrypted
277 channel to request a response from the client.
279 is 0, indicating that these messages will not be sent to the client.
280 This option applies to protocol version 2 only.
282 Specifies whether compression is allowed, or delayed until
283 the user has authenticated successfully.
292 This keyword can be followed by a list of group name patterns, separated
294 Login is disallowed for users whose primary group or supplementary
295 group list matches one of the patterns.
296 Only group names are valid; a numerical group ID is not recognized.
297 By default, login is allowed for all groups.
298 The allow/deny directives are processed in the following order:
309 for more information on patterns.
311 This keyword can be followed by a list of user name patterns, separated
313 Login is disallowed for user names that match one of the patterns.
314 Only user names are valid; a numerical user ID is not recognized.
315 By default, login is allowed for all users.
316 If the pattern takes the form USER@HOST then USER and HOST
317 are separately checked, restricting logins to particular
318 users from particular hosts.
319 The allow/deny directives are processed in the following order:
330 for more information on patterns.
332 Forces the execution of the command specified by
334 ignoring any command supplied by the client and
337 The command is invoked by using the user's login shell with the -c option.
338 This applies to shell, command, or subsystem execution.
339 It is most useful inside a
342 The command originally supplied by the client is available in the
343 .Ev SSH_ORIGINAL_COMMAND
344 environment variable.
345 Specifying a command of
347 will force the use of an in-process sftp server that requires no support
349 .Cm ChrootDirectory .
351 Specifies whether remote hosts are allowed to connect to ports
352 forwarded for the client.
355 binds remote port forwardings to the loopback address.
356 This prevents other remote hosts from connecting to forwarded ports.
358 can be used to specify that sshd
359 should allow remote port forwardings to bind to non-loopback addresses, thus
360 allowing other hosts to connect.
363 to force remote port forwardings to be available to the local host only,
365 to force remote port forwardings to bind to the wildcard address, or
367 to allow the client to select the address to which the forwarding is bound.
370 .It Cm GSSAPIAuthentication
371 Specifies whether user authentication based on GSSAPI is allowed.
374 Note that this option applies to protocol version 2 only.
375 .It Cm GSSAPICleanupCredentials
376 Specifies whether to automatically destroy the user's credentials cache
380 Note that this option applies to protocol version 2 only.
381 .It Cm HostbasedAuthentication
382 Specifies whether rhosts or /etc/hosts.equiv authentication together
383 with successful public key client host authentication is allowed
384 (host-based authentication).
385 This option is similar to
386 .Cm RhostsRSAAuthentication
387 and applies to protocol version 2 only.
390 .It Cm HostbasedUsesNameFromPacketOnly
391 Specifies whether or not the server will attempt to perform a reverse
392 name lookup when matching the name in the
398 .Cm HostbasedAuthentication .
403 uses the name supplied by the client rather than
404 attempting to resolve the name from the TCP connection itself.
408 Specifies a file containing a private host key
411 .Pa /etc/ssh/ssh_host_key
412 for protocol version 1, and
413 .Pa /etc/ssh/ssh_host_dsa_key
414 for protocol version 2.
417 will refuse to use a file if it is group/world-accessible.
418 It is possible to have multiple host key files.
420 keys are used for version 1 and
424 are used for version 2 of the SSH protocol.
430 files will not be used in
431 .Cm RhostsRSAAuthentication
433 .Cm HostbasedAuthentication .
437 .Pa /etc/ssh/shosts.equiv
441 .It Cm IgnoreUserKnownHosts
444 should ignore the user's
445 .Pa ~/.ssh/known_hosts
447 .Cm RhostsRSAAuthentication
449 .Cm HostbasedAuthentication .
452 .It Cm KerberosAuthentication
453 Specifies whether the password provided by the user for
454 .Cm PasswordAuthentication
455 will be validated through the Kerberos KDC.
456 To use this option, the server needs a
457 Kerberos servtab which allows the verification of the KDC's identity.
460 .It Cm KerberosGetAFSToken
461 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
462 an AFS token before accessing the user's home directory.
465 .It Cm KerberosOrLocalPasswd
466 If password authentication through Kerberos fails then
467 the password will be validated via any additional local mechanism
472 .It Cm KerberosTicketCleanup
473 Specifies whether to automatically destroy the user's ticket cache
477 .It Cm KeyRegenerationInterval
478 In protocol version 1, the ephemeral server key is automatically regenerated
479 after this many seconds (if it has been used).
480 The purpose of regeneration is to prevent
481 decrypting captured sessions by later breaking into the machine and
483 The key is never stored anywhere.
484 If the value is 0, the key is never regenerated.
485 The default is 3600 (seconds).
487 Specifies the local addresses
490 The following forms may be used:
492 .Bl -item -offset indent -compact
496 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
501 .Ar host No | Ar IPv4_addr No : Ar port
507 .Ar host No | Ar IPv6_addr Oc : Ar port
514 sshd will listen on the address and all prior
517 The default is to listen on all local addresses.
520 options are permitted.
523 options must precede this option for non-port qualified addresses.
524 .It Cm LoginGraceTime
525 The server disconnects after this time if the user has not
526 successfully logged in.
527 If the value is 0, there is no time limit.
528 The default is 120 seconds.
530 Gives the verbosity level that is used when logging messages from
532 The possible values are:
533 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
535 DEBUG and DEBUG1 are equivalent.
536 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
537 Logging with a DEBUG level violates the privacy of users and is not recommended.
539 Specifies the available MAC (message authentication code) algorithms.
540 The MAC algorithm is used in protocol version 2
541 for data integrity protection.
542 Multiple algorithms must be comma-separated.
544 .Bd -literal -offset indent
545 hmac-md5,hmac-sha1,umac-64@openssh.com,
546 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
549 Introduces a conditional block.
550 If all of the criteria on the
552 line are satisfied, the keywords on the following lines override those
553 set in the global section of the config file, until either another
555 line or the end of the file.
558 are one or more criteria-pattern pairs.
559 The available criteria are
565 Only a subset of keywords may be used on the lines following a
568 Available keywords are
569 .Cm AllowTcpForwarding ,
573 .Cm GSSApiAuthentication ,
574 .Cm KbdInteractiveAuthentication ,
575 .Cm KerberosAuthentication ,
576 .Cm PasswordAuthentication ,
578 .Cm PermitRootLogin ,
579 .Cm RhostsRSAAuthentication ,
580 .Cm RSAAuthentication ,
581 .Cm X11DisplayOffset ,
584 .Cm X11UseLocalHost .
586 Specifies the maximum number of authentication attempts permitted per
588 Once the number of failures reaches half this value,
589 additional failures are logged.
592 Specifies the maximum number of concurrent unauthenticated connections to the
594 Additional connections will be dropped until authentication succeeds or the
596 expires for a connection.
599 Alternatively, random early drop can be enabled by specifying
600 the three colon separated values
604 will refuse connection attempts with a probability of
607 if there are currently
610 unauthenticated connections.
611 The probability increases linearly and all connection attempts
612 are refused if the number of unauthenticated connections reaches
615 .It Cm PasswordAuthentication
616 Specifies whether password authentication is allowed.
620 .Cm ChallengeResponseAuthentication
626 and the PAM authentication policy for
630 password authentication will be allowed through the challenge-response
631 mechanism regardless of the value of
632 .Cm PasswordAuthentication .
633 .It Cm PermitEmptyPasswords
634 When password authentication is allowed, it specifies whether the
635 server allows login to accounts with empty password strings.
639 Specifies the destinations to which TCP port forwarding is permitted.
640 The forwarding specification must be one of the following forms:
642 .Bl -item -offset indent -compact
656 .Ar \&[ IPv6_addr \&] : port
660 Multiple forwards may be specified by separating them with whitespace.
663 can be used to remove all restrictions and permit any forwarding requests.
664 By default all port forwarding requests are permitted.
665 .It Cm PermitRootLogin
666 Specifies whether root can log in using
670 .Dq without-password ,
671 .Dq forced-commands-only ,
677 .Cm ChallengeResponseAuthentication
680 the root user may be allowed in with its password even if
681 .Cm PermitRootLogin is set to
682 .Dq without-password .
684 If this option is set to
685 .Dq without-password ,
686 password authentication is disabled for root.
688 If this option is set to
689 .Dq forced-commands-only ,
690 root login with public key authentication will be allowed,
693 option has been specified
694 (which may be useful for taking remote backups even if root login is
695 normally not allowed).
696 All other authentication methods are disabled for root.
698 If this option is set to
700 root is not allowed to log in.
704 device forwarding is allowed.
720 .It Cm PermitUserEnvironment
722 .Pa ~/.ssh/environment
726 .Pa ~/.ssh/authorized_keys
731 Enabling environment processing may enable users to bypass access
732 restrictions in some configurations using mechanisms such as
735 Specifies the file that contains the process ID of the
738 .Pa /var/run/sshd.pid .
740 Specifies the port number that
744 Multiple options of this type are permitted.
750 should print the date and time of the last user login when a user logs
759 when a user logs in interactively.
760 (On some systems it is also printed by the shell,
766 Specifies the protocol versions
769 The possible values are
773 Multiple versions must be comma-separated.
776 Note that the order of the protocol list does not indicate preference,
777 because the client selects among multiple protocol versions offered
783 .It Cm PubkeyAuthentication
784 Specifies whether public key authentication is allowed.
787 Note that this option applies to protocol version 2 only.
788 .It Cm RhostsRSAAuthentication
789 Specifies whether rhosts or
791 authentication together
792 with successful RSA host authentication is allowed.
795 This option applies to protocol version 1 only.
796 .It Cm RSAAuthentication
797 Specifies whether pure RSA authentication is allowed.
800 This option applies to protocol version 1 only.
802 Defines the number of bits in the ephemeral protocol version 1 server key.
803 The minimum value is 512, and the default is 768.
807 should check file modes and ownership of the
808 user's files and home directory before accepting login.
809 This is normally desirable because novices sometimes accidentally leave their
810 directory or files world-writable.
814 Configures an external subsystem (e.g. file transfer daemon).
815 Arguments should be a subsystem name and a command (with optional arguments)
816 to execute upon subsystem request.
822 file transfer subsystem.
826 implements an in-process
829 This may simplify configurations using
831 to force a different filesystem root on clients.
833 By default no subsystems are defined.
834 Note that this option applies to protocol version 2 only.
835 .It Cm SyslogFacility
836 Gives the facility code that is used when logging messages from
838 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
839 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
842 Specifies whether the system should send TCP keepalive messages to the
844 If they are sent, death of the connection or crash of one
845 of the machines will be properly noticed.
846 However, this means that
847 connections will die if the route is down temporarily, and some people
849 On the other hand, if TCP keepalives are not sent,
850 sessions may hang indefinitely on the server, leaving
852 users and consuming server resources.
856 (to send TCP keepalive messages), and the server will notice
857 if the network goes down or the client host crashes.
858 This avoids infinitely hanging sessions.
860 To disable TCP keepalive messages, the value should be set to
865 should look up the remote host name and check that
866 the resolved host name for the remote IP address maps back to the
867 very same IP address.
873 is used for interactive login sessions.
878 is never used for remote command execution.
879 Note also, that if this is enabled,
881 will be disabled because
883 does not know how to handle
887 .Cm UsePrivilegeSeparation
888 is specified, it will be disabled after authentication.
890 Enables the Pluggable Authentication Module interface.
893 this will enable PAM authentication using
894 .Cm ChallengeResponseAuthentication
896 .Cm PasswordAuthentication
897 in addition to PAM account and session module processing for all
898 authentication types.
900 Because PAM challenge-response authentication usually serves an equivalent
901 role to password authentication, you should disable either
902 .Cm PasswordAuthentication
904 .Cm ChallengeResponseAuthentication.
908 is enabled, you will not be able to run
913 .It Cm UsePrivilegeSeparation
916 separates privileges by creating an unprivileged child process
917 to deal with incoming network traffic.
918 After successful authentication, another process will be created that has
919 the privilege of the authenticated user.
920 The goal of privilege separation is to prevent privilege
921 escalation by containing any corruption within the unprivileged processes.
924 .It Cm VersionAddendum
925 Specifies a string to append to the regular version string to identify
926 OS- or site-specific modifications.
928 .Dq DragonFly-20080406 .
929 .It Cm X11DisplayOffset
930 Specifies the first display number available for
933 This prevents sshd from interfering with real X11 servers.
936 Specifies whether X11 forwarding is permitted.
944 When X11 forwarding is enabled, there may be additional exposure to
945 the server and to client displays if the
947 proxy display is configured to listen on the wildcard address (see
949 below), though this is not the default.
950 Additionally, the authentication spoofing and authentication data
951 verification and substitution occur on the client side.
952 The security risk of using X11 forwarding is that the client's X11
953 display server may be exposed to attack when the SSH client requests
954 forwarding (see the warnings for
958 A system administrator may have a stance in which they want to
959 protect clients that may expose themselves to attack by unwittingly
960 requesting X11 forwarding, which can warrant a
964 Note that disabling X11 forwarding does not prevent users from
965 forwarding X11 traffic, as users can always install their own forwarders.
966 X11 forwarding is automatically disabled if
969 .It Cm X11UseLocalhost
972 should bind the X11 forwarding server to the loopback address or to
973 the wildcard address.
975 sshd binds the forwarding server to the loopback address and sets the
978 environment variable to
980 This prevents remote hosts from connecting to the proxy display.
981 However, some older X11 clients may not function with this
986 to specify that the forwarding server should be bound to the wildcard
995 Specifies the full pathname of the
999 .Pa /usr/X11R6/bin/xauth .
1003 command-line arguments and configuration file options that specify time
1004 may be expressed using a sequence of the form:
1006 .Ar time Op Ar qualifier ,
1010 is a positive integer value and
1012 is one of the following:
1014 .Bl -tag -width Ds -compact -offset indent
1029 Each member of the sequence is added together to calculate
1030 the total time value.
1032 Time format examples:
1034 .Bl -tag -width Ds -compact -offset indent
1036 600 seconds (10 minutes)
1040 1 hour 30 minutes (90 minutes)
1044 .It Pa /etc/ssh/sshd_config
1045 Contains configuration data for
1047 This file should be writable by root only, but it is recommended
1048 (though not necessary) that it be world-readable.
1053 OpenSSH is a derivative of the original and free
1054 ssh 1.2.12 release by Tatu Ylonen.
1055 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1056 Theo de Raadt and Dug Song
1057 removed many bugs, re-added newer features and
1059 Markus Friedl contributed the support for SSH
1060 protocol versions 1.5 and 2.0.
1061 Niels Provos and Markus Friedl contributed support
1062 for privilege separation.
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