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 1.44 2005/07/25 11:59:40 markus Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH daemon configuration file
46 .Bl -tag -width Ds -compact
47 .It Pa /etc/ssh/sshd_config
51 reads configuration data from
52 .Pa /etc/ssh/sshd_config
53 (or the file specified with
56 The file contains keyword-argument pairs, one per line.
59 and empty lines are interpreted as comments.
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.
107 wildcards in the patterns.
108 Only group names are valid; a numerical group ID is not recognized.
109 By default, login is allowed for all groups.
110 .It Cm AllowTcpForwarding
111 Specifies whether TCP forwarding is permitted.
114 Note that disabling TCP forwarding does not improve security unless
115 users are also denied shell access, as they can always install their
118 This keyword can be followed by a list of user name patterns, separated
120 If specified, login is allowed only for user names that
121 match one of the patterns.
126 wildcards in the patterns.
127 Only user names are valid; a numerical user ID is not recognized.
128 By default, login is allowed for all users.
129 If the pattern takes the form USER@HOST then USER and HOST
130 are separately checked, restricting logins to particular
131 users from particular hosts.
132 .It Cm AuthorizedKeysFile
133 Specifies the file that contains the public keys that can be used
134 for user authentication.
135 .Cm AuthorizedKeysFile
136 may contain tokens of the form %T which are substituted during connection
138 The following tokens are defined: %% is replaced by a literal '%',
139 %h is replaced by the home directory of the user being authenticated and
140 %u is replaced by the username of that user.
142 .Cm AuthorizedKeysFile
143 is taken to be an absolute path or one relative to the user's home
146 .Dq .ssh/authorized_keys .
148 In some jurisdictions, sending a warning message before authentication
149 may be relevant for getting legal protection.
150 The contents of the specified file are sent to the remote user before
151 authentication is allowed.
152 This option is only available for protocol version 2.
153 By default, no banner is displayed.
154 .It Cm ChallengeResponseAuthentication
155 Specifies whether challenge-response authentication is allowed.
158 this controls the use of PAM (see
161 Note that this affects the effectiveness of the
162 .Cm PasswordAuthentication
169 Specifies the ciphers allowed for protocol version 2.
170 Multiple ciphers must be comma-separated.
171 The supported ciphers are
187 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
188 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
189 aes192-ctr,aes256-ctr''
191 .It Cm ClientAliveCountMax
192 Sets the number of client alive messages (see above) which may be
195 receiving any messages back from the client.
196 If this threshold is reached while client alive messages are being sent,
198 will disconnect the client, terminating the session.
199 It is important to note that the use of client alive messages is very
203 The client alive messages are sent through the encrypted channel
204 and therefore will not be spoofable.
205 The TCP keepalive option enabled by
208 The client alive mechanism is valuable when the client or
209 server depend on knowing when a connection has become inactive.
211 The default value is 3.
213 .Cm ClientAliveInterval
214 (above) is set to 15, and
215 .Cm ClientAliveCountMax
216 is left at the default, unresponsive ssh clients
217 will be disconnected after approximately 45 seconds.
218 .It Cm ClientAliveInterval
219 Sets a timeout interval in seconds after which if no data has been received
222 will send a message through the encrypted
223 channel to request a response from the client.
225 is 0, indicating that these messages will not be sent to the client.
226 This option applies to protocol version 2 only.
228 Specifies whether compression is allowed, or delayed until
229 the user has authenticated successfully.
238 This keyword can be followed by a list of group name patterns, separated
240 Login is disallowed for users whose primary group or supplementary
241 group list matches one of the patterns.
246 wildcards in the patterns.
247 Only group names are valid; a numerical group ID is not recognized.
248 By default, login is allowed for all groups.
250 This keyword can be followed by a list of user name patterns, separated
252 Login is disallowed for user names that match one of the patterns.
256 can be used as wildcards in the patterns.
257 Only user names are valid; a numerical user ID is not recognized.
258 By default, login is allowed for all users.
259 If the pattern takes the form USER@HOST then USER and HOST
260 are separately checked, restricting logins to particular
261 users from particular hosts.
263 Specifies whether remote hosts are allowed to connect to ports
264 forwarded for the client.
267 binds remote port forwardings to the loopback address.
268 This prevents other remote hosts from connecting to forwarded ports.
270 can be used to specify that
272 should allow remote port forwardings to bind to non-loopback addresses, thus
273 allowing other hosts to connect.
276 to force remote port forwardings to be available to the local host only,
278 to force remote port forwardings to bind to the wildcard address, or
280 to allow the client to select the address to which the forwarding is bound.
283 .It Cm GSSAPIAuthentication
284 Specifies whether user authentication based on GSSAPI is allowed.
287 Note that this option applies to protocol version 2 only.
288 .It Cm GSSAPICleanupCredentials
289 Specifies whether to automatically destroy the user's credentials cache
293 Note that this option applies to protocol version 2 only.
294 .It Cm HostbasedAuthentication
295 Specifies whether rhosts or /etc/hosts.equiv authentication together
296 with successful public key client host authentication is allowed
297 (hostbased authentication).
298 This option is similar to
299 .Cm RhostsRSAAuthentication
300 and applies to protocol version 2 only.
304 Specifies a file containing a private host key
307 .Pa /etc/ssh/ssh_host_key
308 for protocol version 1, and
309 .Pa /etc/ssh/ssh_host_dsa_key
310 for protocol version 2.
313 will refuse to use a file if it is group/world-accessible.
314 It is possible to have multiple host key files.
316 keys are used for version 1 and
320 are used for version 2 of the SSH protocol.
326 files will not be used in
327 .Cm RhostsRSAAuthentication
329 .Cm HostbasedAuthentication .
333 .Pa /etc/ssh/shosts.equiv
337 .It Cm IgnoreUserKnownHosts
340 should ignore the user's
341 .Pa ~/.ssh/known_hosts
343 .Cm RhostsRSAAuthentication
345 .Cm HostbasedAuthentication .
348 .It Cm KerberosAuthentication
349 Specifies whether the password provided by the user for
350 .Cm PasswordAuthentication
351 will be validated through the Kerberos KDC.
352 To use this option, the server needs a
353 Kerberos servtab which allows the verification of the KDC's identity.
356 .It Cm KerberosGetAFSToken
357 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
358 an AFS token before accessing the user's home directory.
361 .It Cm KerberosOrLocalPasswd
362 If set then if password authentication through Kerberos fails then
363 the password will be validated via any additional local mechanism
368 .It Cm KerberosTicketCleanup
369 Specifies whether to automatically destroy the user's ticket cache
373 .It Cm KeyRegenerationInterval
374 In protocol version 1, the ephemeral server key is automatically regenerated
375 after this many seconds (if it has been used).
376 The purpose of regeneration is to prevent
377 decrypting captured sessions by later breaking into the machine and
379 The key is never stored anywhere.
380 If the value is 0, the key is never regenerated.
381 The default is 3600 (seconds).
383 Specifies the local addresses
386 The following forms may be used:
388 .Bl -item -offset indent -compact
392 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
397 .Ar host No | Ar IPv4_addr No : Ar port
403 .Ar host No | Ar IPv6_addr Oc : Ar port
411 will listen on the address and all prior
414 The default is to listen on all local addresses.
417 options are permitted.
420 options must precede this option for non port qualified addresses.
421 .It Cm LoginGraceTime
422 The server disconnects after this time if the user has not
423 successfully logged in.
424 If the value is 0, there is no time limit.
425 The default is 120 seconds.
427 Gives the verbosity level that is used when logging messages from
429 The possible values are:
430 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
432 DEBUG and DEBUG1 are equivalent.
433 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
434 Logging with a DEBUG level violates the privacy of users and is not recommended.
436 Specifies the available MAC (message authentication code) algorithms.
437 The MAC algorithm is used in protocol version 2
438 for data integrity protection.
439 Multiple algorithms must be comma-separated.
441 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
443 Specifies the maximum number of authentication attempts permitted per
445 Once the number of failures reaches half this value,
446 additional failures are logged.
449 Specifies the maximum number of concurrent unauthenticated connections to the
452 Additional connections will be dropped until authentication succeeds or the
454 expires for a connection.
457 Alternatively, random early drop can be enabled by specifying
458 the three colon separated values
462 will refuse connection attempts with a probability of
465 if there are currently
468 unauthenticated connections.
469 The probability increases linearly and all connection attempts
470 are refused if the number of unauthenticated connections reaches
473 .It Cm PasswordAuthentication
474 Specifies whether password authentication is allowed.
479 was built without PAM support, in which case the default is
482 .Cm ChallengeResponseAuthentication
485 and the PAM authentication policy for
489 password authentication will be allowed through the challenge-response
490 mechanism regardless of the value of
491 .Cm PasswordAuthentication .
492 .It Cm PermitEmptyPasswords
493 When password authentication is allowed, it specifies whether the
494 server allows login to accounts with empty password strings.
497 .It Cm PermitRootLogin
498 Specifies whether root can log in using
502 .Dq without-password ,
503 .Dq forced-commands-only
509 .Cm ChallengeResponseAuthentication
512 the root user may be allowed in with its password even if
513 .Cm PermitRootLogin is set to
514 .Dq without-password .
516 If this option is set to
518 password authentication is disabled for root.
520 If this option is set to
521 .Dq forced-commands-only
522 root login with public key authentication will be allowed,
525 option has been specified
526 (which may be useful for taking remote backups even if root login is
527 normally not allowed).
528 All other authentication methods are disabled for root.
530 If this option is set to
532 root is not allowed to log in.
533 .It Cm PermitUserEnvironment
535 .Pa ~/.ssh/environment
539 .Pa ~/.ssh/authorized_keys
544 Enabling environment processing may enable users to bypass access
545 restrictions in some configurations using mechanisms such as
548 Specifies the file that contains the process ID of the
552 .Pa /var/run/sshd.pid .
554 Specifies the port number that
558 Multiple options of this type are permitted.
564 should print the date and time of the last user login when a user logs
573 when a user logs in interactively.
574 (On some systems it is also printed by the shell,
580 Specifies the protocol versions
583 The possible values are
587 Multiple versions must be comma-separated.
590 Note that the order of the protocol list does not indicate preference,
591 because the client selects among multiple protocol versions offered
597 .It Cm PubkeyAuthentication
598 Specifies whether public key authentication is allowed.
601 Note that this option applies to protocol version 2 only.
602 .It Cm RhostsRSAAuthentication
603 Specifies whether rhosts or
605 authentication together
606 with successful RSA host authentication is allowed.
609 This option applies to protocol version 1 only.
610 .It Cm RSAAuthentication
611 Specifies whether pure RSA authentication is allowed.
614 This option applies to protocol version 1 only.
616 Defines the number of bits in the ephemeral protocol version 1 server key.
617 The minimum value is 512, and the default is 768.
621 should check file modes and ownership of the
622 user's files and home directory before accepting login.
623 This is normally desirable because novices sometimes accidentally leave their
624 directory or files world-writable.
628 Configures an external subsystem (e.g., file transfer daemon).
629 Arguments should be a subsystem name and a command to execute upon subsystem
635 file transfer subsystem.
636 By default no subsystems are defined.
637 Note that this option applies to protocol version 2 only.
638 .It Cm SyslogFacility
639 Gives the facility code that is used when logging messages from
641 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
642 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
645 Specifies whether the system should send TCP keepalive messages to the
647 If they are sent, death of the connection or crash of one
648 of the machines will be properly noticed.
649 However, this means that
650 connections will die if the route is down temporarily, and some people
652 On the other hand, if TCP keepalives are not sent,
653 sessions may hang indefinitely on the server, leaving
655 users and consuming server resources.
659 (to send TCP keepalive messages), and the server will notice
660 if the network goes down or the client host crashes.
661 This avoids infinitely hanging sessions.
663 To disable TCP keepalive messages, the value should be set to
668 should look up the remote host name and check that
669 the resolved host name for the remote IP address maps back to the
670 very same IP address.
676 is used for interactive login sessions.
681 is never used for remote command execution.
682 Note also, that if this is enabled,
684 will be disabled because
686 does not know how to handle
690 .Cm UsePrivilegeSeparation
691 is specified, it will be disabled after authentication.
693 Enables the Pluggable Authentication Module interface.
696 this will enable PAM authentication using
697 .Cm ChallengeResponseAuthentication
698 and PAM account and session module processing for all authentication types.
700 Because PAM challenge-response authentication usually serves an equivalent
701 role to password authentication, you should disable either
702 .Cm PasswordAuthentication
704 .Cm ChallengeResponseAuthentication.
708 is enabled, you will not be able to run
713 .It Cm UsePrivilegeSeparation
716 separates privileges by creating an unprivileged child process
717 to deal with incoming network traffic.
718 After successful authentication, another process will be created that has
719 the privilege of the authenticated user.
720 The goal of privilege separation is to prevent privilege
721 escalation by containing any corruption within the unprivileged processes.
724 .It Cm VersionAddendum
725 Specifies a string to append to the regular version string to identify
726 OS- or site-specific modifications.
728 .Dq FreeBSD-20050903 .
729 .It Cm X11DisplayOffset
730 Specifies the first display number available for
735 from interfering with real X11 servers.
738 Specifies whether X11 forwarding is permitted.
746 When X11 forwarding is enabled, there may be additional exposure to
747 the server and to client displays if the
749 proxy display is configured to listen on the wildcard address (see
751 below), however this is not the default.
752 Additionally, the authentication spoofing and authentication data
753 verification and substitution occur on the client side.
754 The security risk of using X11 forwarding is that the client's X11
755 display server may be exposed to attack when the ssh client requests
756 forwarding (see the warnings for
760 A system administrator may have a stance in which they want to
761 protect clients that may expose themselves to attack by unwittingly
762 requesting X11 forwarding, which can warrant a
766 Note that disabling X11 forwarding does not prevent users from
767 forwarding X11 traffic, as users can always install their own forwarders.
768 X11 forwarding is automatically disabled if
771 .It Cm X11UseLocalhost
774 should bind the X11 forwarding server to the loopback address or to
775 the wildcard address.
778 binds the forwarding server to the loopback address and sets the
781 environment variable to
783 This prevents remote hosts from connecting to the proxy display.
784 However, some older X11 clients may not function with this
789 to specify that the forwarding server should be bound to the wildcard
798 Specifies the full pathname of the
802 .Pa /usr/X11R6/bin/xauth .
806 command-line arguments and configuration file options that specify time
807 may be expressed using a sequence of the form:
809 .Ar time Op Ar qualifier ,
813 is a positive integer value and
815 is one of the following:
817 .Bl -tag -width Ds -compact -offset indent
832 Each member of the sequence is added together to calculate
833 the total time value.
835 Time format examples:
837 .Bl -tag -width Ds -compact -offset indent
839 600 seconds (10 minutes)
843 1 hour 30 minutes (90 minutes)
847 .It Pa /etc/ssh/sshd_config
848 Contains configuration data for
850 This file should be writable by root only, but it is recommended
851 (though not necessary) that it be world-readable.
856 OpenSSH is a derivative of the original and free
857 ssh 1.2.12 release by Tatu Ylonen.
858 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
859 Theo de Raadt and Dug Song
860 removed many bugs, re-added newer features and
862 Markus Friedl contributed the support for SSH
863 protocol versions 1.5 and 2.0.
864 Niels Provos and Markus Friedl contributed support
865 for privilege separation.