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 1.48 2006/01/02 17:09:49 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon configuration file
45 .Bl -tag -width Ds -compact
46 .It Pa /etc/ssh/sshd_config
50 reads configuration data from
51 .Pa /etc/ssh/sshd_config
52 (or the file specified with
55 The file contains keyword-argument pairs, one per line.
58 and empty lines are interpreted as comments.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
65 Specifies what environment variables sent by the client will be copied into
72 for how to configure the client.
73 Note that environment passing is only supported for protocol 2.
74 Variables are specified by name, which may contain the wildcard characters
78 Multiple environment variables may be separated by whitespace or spread
82 Be warned that some environment variables could be used to bypass restricted
84 For this reason, care should be taken in the use of this directive.
85 The default is not to accept any environment variables.
87 Specifies which address family should be used by
98 This keyword can be followed by a list of group name patterns, separated
100 If specified, login is allowed only for users whose primary
101 group or supplementary group list matches one of the patterns.
106 wildcards in the patterns.
107 Only group names are valid; a numerical group ID is not recognized.
108 By default, login is allowed for all groups.
109 .It Cm AllowTcpForwarding
110 Specifies whether TCP forwarding is permitted.
113 Note that disabling TCP forwarding does not improve security unless
114 users are also denied shell access, as they can always install their
117 This keyword can be followed by a list of user name patterns, separated
119 If specified, login is allowed only for user names that
120 match one of the patterns.
125 wildcards in the patterns.
126 Only user names are valid; a numerical user ID is not recognized.
127 By default, login is allowed for all users.
128 If the pattern takes the form USER@HOST then USER and HOST
129 are separately checked, restricting logins to particular
130 users from particular hosts.
131 .It Cm AuthorizedKeysFile
132 Specifies the file that contains the public keys that can be used
133 for user authentication.
134 .Cm AuthorizedKeysFile
135 may contain tokens of the form %T which are substituted during connection
137 The following tokens are defined: %% is replaced by a literal '%',
138 %h is replaced by the home directory of the user being authenticated and
139 %u is replaced by the username of that user.
141 .Cm AuthorizedKeysFile
142 is taken to be an absolute path or one relative to the user's home
145 .Dq .ssh/authorized_keys .
147 In some jurisdictions, sending a warning message before authentication
148 may be relevant for getting legal protection.
149 The contents of the specified file are sent to the remote user before
150 authentication is allowed.
151 This option is only available for protocol version 2.
152 By default, no banner is displayed.
153 .It Cm ChallengeResponseAuthentication
154 Specifies whether challenge response authentication is allowed.
155 All authentication styles from
161 Specifies the ciphers allowed for protocol version 2.
162 Multiple ciphers must be comma-separated.
163 The supported ciphers are
179 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
180 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
181 aes192-ctr,aes256-ctr''
183 .It Cm ClientAliveCountMax
184 Sets the number of client alive messages (see below) which may be
187 receiving any messages back from the client.
188 If this threshold is reached while client alive messages are being sent,
190 will disconnect the client, terminating the session.
191 It is important to note that the use of client alive messages is very
195 The client alive messages are sent through the encrypted channel
196 and therefore will not be spoofable.
197 The TCP keepalive option enabled by
200 The client alive mechanism is valuable when the client or
201 server depend on knowing when a connection has become inactive.
203 The default value is 3.
205 .Cm ClientAliveInterval
206 (see below) is set to 15, and
207 .Cm ClientAliveCountMax
208 is left at the default, unresponsive ssh clients
209 will be disconnected after approximately 45 seconds.
210 .It Cm ClientAliveInterval
211 Sets a timeout interval in seconds after which if no data has been received
214 will send a message through the encrypted
215 channel to request a response from the client.
217 is 0, indicating that these messages will not be sent to the client.
218 This option applies to protocol version 2 only.
220 Specifies whether compression is allowed, or delayed until
221 the user has authenticated successfully.
230 This keyword can be followed by a list of group name patterns, separated
232 Login is disallowed for users whose primary group or supplementary
233 group list matches one of the patterns.
238 wildcards in the patterns.
239 Only group names are valid; a numerical group ID is not recognized.
240 By default, login is allowed for all groups.
242 This keyword can be followed by a list of user name patterns, separated
244 Login is disallowed for user names that match one of the patterns.
248 can be used as wildcards in the patterns.
249 Only user names are valid; a numerical user ID is not recognized.
250 By default, login is allowed for all users.
251 If the pattern takes the form USER@HOST then USER and HOST
252 are separately checked, restricting logins to particular
253 users from particular hosts.
255 Specifies whether remote hosts are allowed to connect to ports
256 forwarded for the client.
259 binds remote port forwardings to the loopback address.
260 This prevents other remote hosts from connecting to forwarded ports.
262 can be used to specify that
264 should allow remote port forwardings to bind to non-loopback addresses, thus
265 allowing other hosts to connect.
268 to force remote port forwardings to be available to the local host only,
270 to force remote port forwardings to bind to the wildcard address, or
272 to allow the client to select the address to which the forwarding is bound.
275 .It Cm GSSAPIAuthentication
276 Specifies whether user authentication based on GSSAPI is allowed.
279 Note that this option applies to protocol version 2 only.
280 .It Cm GSSAPIKeyExchange
281 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
282 doesn't rely on ssh keys to verify host identity.
285 Note that this option applies to protocol version 2 only.
286 .It Cm GSSAPICleanupCredentials
287 Specifies whether to automatically destroy the user's credentials cache
291 Note that this option applies to protocol version 2 only.
292 .It Cm HostbasedAuthentication
293 Specifies whether rhosts or /etc/hosts.equiv authentication together
294 with successful public key client host authentication is allowed
295 (hostbased authentication).
296 This option is similar to
297 .Cm RhostsRSAAuthentication
298 and applies to protocol version 2 only.
302 Specifies a file containing a private host key
305 .Pa /etc/ssh/ssh_host_key
306 for protocol version 1, and
307 .Pa /etc/ssh/ssh_host_rsa_key
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/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 acquire
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.
477 .It Cm PermitEmptyPasswords
478 When password authentication is allowed, it specifies whether the
479 server allows login to accounts with empty password strings.
482 .It Cm PermitRootLogin
483 Specifies whether root can log in using
487 .Dq without-password ,
488 .Dq forced-commands-only
494 If this option is set to
496 password authentication is disabled for root.
498 If this option is set to
499 .Dq forced-commands-only
500 root login with public key authentication will be allowed,
503 option has been specified
504 (which may be useful for taking remote backups even if root login is
505 normally not allowed).
506 All other authentication methods are disabled for root.
508 If this option is set to
510 root is not allowed to log in.
514 device forwarding is allowed.
523 .It Cm PermitUserEnvironment
525 .Pa ~/.ssh/environment
529 .Pa ~/.ssh/authorized_keys
534 Enabling environment processing may enable users to bypass access
535 restrictions in some configurations using mechanisms such as
538 Specifies the file that contains the process ID of the
542 .Pa /var/run/sshd.pid .
544 Specifies the port number that
548 Multiple options of this type are permitted.
554 should print the date and time of the last user login when a user logs
563 when a user logs in interactively.
564 (On some systems it is also printed by the shell,
570 Specifies the protocol versions
573 The possible values are
577 Multiple versions must be comma-separated.
580 Note that the order of the protocol list does not indicate preference,
581 because the client selects among multiple protocol versions offered
587 .It Cm PubkeyAuthentication
588 Specifies whether public key authentication is allowed.
591 Note that this option applies to protocol version 2 only.
592 .It Cm RhostsRSAAuthentication
593 Specifies whether rhosts or /etc/hosts.equiv authentication together
594 with successful RSA host authentication is allowed.
597 This option applies to protocol version 1 only.
598 .It Cm RSAAuthentication
599 Specifies whether pure RSA authentication is allowed.
602 This option applies to protocol version 1 only.
604 Defines the number of bits in the ephemeral protocol version 1 server key.
605 The minimum value is 512, and the default is 768.
609 should check file modes and ownership of the
610 user's files and home directory before accepting login.
611 This is normally desirable because novices sometimes accidentally leave their
612 directory or files world-writable.
616 Configures an external subsystem (e.g., file transfer daemon).
617 Arguments should be a subsystem name and a command to execute upon subsystem
623 file transfer subsystem.
624 By default no subsystems are defined.
625 Note that this option applies to protocol version 2 only.
626 .It Cm SyslogFacility
627 Gives the facility code that is used when logging messages from
629 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
630 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
633 Specifies whether the system should send TCP keepalive messages to the
635 If they are sent, death of the connection or crash of one
636 of the machines will be properly noticed.
637 However, this means that
638 connections will die if the route is down temporarily, and some people
640 On the other hand, if TCP keepalives are not sent,
641 sessions may hang indefinitely on the server, leaving
643 users and consuming server resources.
647 (to send TCP keepalive messages), and the server will notice
648 if the network goes down or the client host crashes.
649 This avoids infinitely hanging sessions.
651 To disable TCP keepalive messages, the value should be set to
656 should look up the remote host name and check that
657 the resolved host name for the remote IP address maps back to the
658 very same IP address.
664 is used for interactive login sessions.
669 is never used for remote command execution.
670 Note also, that if this is enabled,
672 will be disabled because
674 does not know how to handle
678 .Cm UsePrivilegeSeparation
679 is specified, it will be disabled after authentication.
681 Enables the Pluggable Authentication Module interface.
684 this will enable PAM authentication using
685 .Cm ChallengeResponseAuthentication
686 and PAM account and session module processing for all authentication types.
688 Because PAM challenge-response authentication usually serves an equivalent
689 role to password authentication, you should disable either
690 .Cm PasswordAuthentication
692 .Cm ChallengeResponseAuthentication.
696 is enabled, you will not be able to run
701 .It Cm UsePrivilegeSeparation
704 separates privileges by creating an unprivileged child process
705 to deal with incoming network traffic.
706 After successful authentication, another process will be created that has
707 the privilege of the authenticated user.
708 The goal of privilege separation is to prevent privilege
709 escalation by containing any corruption within the unprivileged processes.
712 .It Cm X11DisplayOffset
713 Specifies the first display number available for
718 from interfering with real X11 servers.
721 Specifies whether X11 forwarding is permitted.
729 When X11 forwarding is enabled, there may be additional exposure to
730 the server and to client displays if the
732 proxy display is configured to listen on the wildcard address (see
734 below), however this is not the default.
735 Additionally, the authentication spoofing and authentication data
736 verification and substitution occur on the client side.
737 The security risk of using X11 forwarding is that the client's X11
738 display server may be exposed to attack when the ssh client requests
739 forwarding (see the warnings for
743 A system administrator may have a stance in which they want to
744 protect clients that may expose themselves to attack by unwittingly
745 requesting X11 forwarding, which can warrant a
749 Note that disabling X11 forwarding does not prevent users from
750 forwarding X11 traffic, as users can always install their own forwarders.
751 X11 forwarding is automatically disabled if
754 .It Cm X11UseLocalhost
757 should bind the X11 forwarding server to the loopback address or to
758 the wildcard address.
761 binds the forwarding server to the loopback address and sets the
764 environment variable to
766 This prevents remote hosts from connecting to the proxy display.
767 However, some older X11 clients may not function with this
772 to specify that the forwarding server should be bound to the wildcard
781 Specifies the full pathname of the
785 .Pa /usr/X11R6/bin/xauth .
789 command-line arguments and configuration file options that specify time
790 may be expressed using a sequence of the form:
792 .Ar time Op Ar qualifier ,
796 is a positive integer value and
798 is one of the following:
800 .Bl -tag -width Ds -compact -offset indent
815 Each member of the sequence is added together to calculate
816 the total time value.
818 Time format examples:
820 .Bl -tag -width Ds -compact -offset indent
822 600 seconds (10 minutes)
826 1 hour 30 minutes (90 minutes)
830 .It Pa /etc/ssh/sshd_config
831 Contains configuration data for
833 This file should be writable by root only, but it is recommended
834 (though not necessary) that it be world-readable.
839 OpenSSH is a derivative of the original and free
840 ssh 1.2.12 release by Tatu Ylonen.
841 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
842 Theo de Raadt and Dug Song
843 removed many bugs, re-added newer features and
845 Markus Friedl contributed the support for SSH
846 protocol versions 1.5 and 2.0.
847 Niels Provos and Markus Friedl contributed support
848 for privilege separation.