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 1.34 2004/06/13 14:01:42 dtucker 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 This keyword can be followed by a list of group name patterns, separated
89 If specified, login is allowed only for users whose primary
90 group or supplementary group list matches one of the patterns.
95 wildcards in the patterns.
96 Only group names are valid; a numerical group ID is not recognized.
97 By default, login is allowed for all groups.
98 .It Cm AllowTcpForwarding
99 Specifies whether TCP forwarding is permitted.
102 Note that disabling TCP forwarding does not improve security unless
103 users are also denied shell access, as they can always install their
106 This keyword can be followed by a list of user name patterns, separated
108 If specified, login is allowed only for user names that
109 match one of the patterns.
114 wildcards in the patterns.
115 Only user names are valid; a numerical user ID is not recognized.
116 By default, login is allowed for all users.
117 If the pattern takes the form USER@HOST then USER and HOST
118 are separately checked, restricting logins to particular
119 users from particular hosts.
120 .It Cm AuthorizedKeysFile
121 Specifies the file that contains the public keys that can be used
122 for user authentication.
123 .Cm AuthorizedKeysFile
124 may contain tokens of the form %T which are substituted during connection
126 The following tokens are defined: %% is replaced by a literal '%',
127 %h is replaced by the home directory of the user being authenticated and
128 %u is replaced by the username of that user.
130 .Cm AuthorizedKeysFile
131 is taken to be an absolute path or one relative to the user's home
134 .Dq .ssh/authorized_keys .
136 In some jurisdictions, sending a warning message before authentication
137 may be relevant for getting legal protection.
138 The contents of the specified file are sent to the remote user before
139 authentication is allowed.
140 This option is only available for protocol version 2.
141 By default, no banner is displayed.
142 .It Cm ChallengeResponseAuthentication
143 Specifies whether challenge response authentication is allowed.
144 All authentication styles from
150 Specifies the ciphers allowed for protocol version 2.
151 Multiple ciphers must be comma-separated.
152 The supported ciphers are
166 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
167 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
169 .It Cm ClientAliveInterval
170 Sets a timeout interval in seconds after which if no data has been received
173 will send a message through the encrypted
174 channel to request a response from the client.
176 is 0, indicating that these messages will not be sent to the client.
177 This option applies to protocol version 2 only.
178 .It Cm ClientAliveCountMax
179 Sets the number of client alive messages (see above) which may be
182 receiving any messages back from the client.
183 If this threshold is reached while client alive messages are being sent,
185 will disconnect the client, terminating the session.
186 It is important to note that the use of client alive messages is very
190 The client alive messages are sent through the encrypted channel
191 and therefore will not be spoofable.
192 The TCP keepalive option enabled by
195 The client alive mechanism is valuable when the client or
196 server depend on knowing when a connection has become inactive.
198 The default value is 3.
200 .Cm ClientAliveInterval
201 (above) is set to 15, and
202 .Cm ClientAliveCountMax
203 is left at the default, unresponsive ssh clients
204 will be disconnected after approximately 45 seconds.
206 Specifies whether compression is allowed.
214 This keyword can be followed by a list of group name patterns, separated
216 Login is disallowed for users whose primary group or supplementary
217 group list matches one of the patterns.
222 wildcards in the patterns.
223 Only group names are valid; a numerical group ID is not recognized.
224 By default, login is allowed for all groups.
226 This keyword can be followed by a list of user name patterns, separated
228 Login is disallowed for user names that match one of the patterns.
232 can be used as wildcards in the patterns.
233 Only user names are valid; a numerical user ID is not recognized.
234 By default, login is allowed for all users.
235 If the pattern takes the form USER@HOST then USER and HOST
236 are separately checked, restricting logins to particular
237 users from particular hosts.
239 Specifies whether remote hosts are allowed to connect to ports
240 forwarded for the client.
243 binds remote port forwardings to the loopback address.
244 This prevents other remote hosts from connecting to forwarded ports.
246 can be used to specify that
248 should bind remote port forwardings to the wildcard address,
249 thus allowing remote hosts to connect to forwarded ports.
256 .It Cm GSSAPIAuthentication
257 Specifies whether user authentication based on GSSAPI is allowed.
260 Note that this option applies to protocol version 2 only.
261 .It Cm GSSAPICleanupCredentials
262 Specifies whether to automatically destroy the user's credentials cache
266 Note that this option applies to protocol version 2 only.
267 .It Cm HostbasedAuthentication
268 Specifies whether rhosts or /etc/hosts.equiv authentication together
269 with successful public key client host authentication is allowed
270 (hostbased authentication).
271 This option is similar to
272 .Cm RhostsRSAAuthentication
273 and applies to protocol version 2 only.
277 Specifies a file containing a private host key
280 .Pa /etc/ssh/ssh_host_key
281 for protocol version 1, and
282 .Pa /etc/ssh/ssh_host_rsa_key
284 .Pa /etc/ssh/ssh_host_dsa_key
285 for protocol version 2.
288 will refuse to use a file if it is group/world-accessible.
289 It is possible to have multiple host key files.
291 keys are used for version 1 and
295 are used for version 2 of the SSH protocol.
301 files will not be used in
302 .Cm RhostsRSAAuthentication
304 .Cm HostbasedAuthentication .
308 .Pa /etc/shosts.equiv
312 .It Cm IgnoreUserKnownHosts
315 should ignore the user's
316 .Pa $HOME/.ssh/known_hosts
318 .Cm RhostsRSAAuthentication
320 .Cm HostbasedAuthentication .
323 .It Cm KerberosAuthentication
324 Specifies whether the password provided by the user for
325 .Cm PasswordAuthentication
326 will be validated through the Kerberos KDC.
327 To use this option, the server needs a
328 Kerberos servtab which allows the verification of the KDC's identity.
331 .It Cm KerberosGetAFSToken
332 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
333 an AFS token before accessing the user's home directory.
336 .It Cm KerberosOrLocalPasswd
337 If set then if password authentication through Kerberos fails then
338 the password will be validated via any additional local mechanism
343 .It Cm KerberosTicketCleanup
344 Specifies whether to automatically destroy the user's ticket cache
348 .It Cm KeyRegenerationInterval
349 In protocol version 1, the ephemeral server key is automatically regenerated
350 after this many seconds (if it has been used).
351 The purpose of regeneration is to prevent
352 decrypting captured sessions by later breaking into the machine and
354 The key is never stored anywhere.
355 If the value is 0, the key is never regenerated.
356 The default is 3600 (seconds).
358 Specifies the local addresses
361 The following forms may be used:
363 .Bl -item -offset indent -compact
367 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
372 .Ar host No | Ar IPv4_addr No : Ar port
378 .Ar host No | Ar IPv6_addr Oc : Ar port
386 will listen on the address and all prior
389 The default is to listen on all local addresses.
392 options are permitted.
395 options must precede this option for non port qualified addresses.
396 .It Cm LoginGraceTime
397 The server disconnects after this time if the user has not
398 successfully logged in.
399 If the value is 0, there is no time limit.
400 The default is 120 seconds.
402 Gives the verbosity level that is used when logging messages from
404 The possible values are:
405 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
407 DEBUG and DEBUG1 are equivalent.
408 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
409 Logging with a DEBUG level violates the privacy of users and is not recommended.
411 Specifies the available MAC (message authentication code) algorithms.
412 The MAC algorithm is used in protocol version 2
413 for data integrity protection.
414 Multiple algorithms must be comma-separated.
416 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
418 Specifies the maximum number of authentication attempts permitted per
419 connection. Once the number of failures reaches half this value, additional
420 failures are logged. The default is 6.
422 Specifies the maximum number of concurrent unauthenticated connections to the
425 Additional connections will be dropped until authentication succeeds or the
427 expires for a connection.
430 Alternatively, random early drop can be enabled by specifying
431 the three colon separated values
435 will refuse connection attempts with a probability of
438 if there are currently
441 unauthenticated connections.
442 The probability increases linearly and all connection attempts
443 are refused if the number of unauthenticated connections reaches
446 .It Cm PasswordAuthentication
447 Specifies whether password authentication is allowed.
450 .It Cm PermitEmptyPasswords
451 When password authentication is allowed, it specifies whether the
452 server allows login to accounts with empty password strings.
455 .It Cm PermitRootLogin
456 Specifies whether root can login using
460 .Dq without-password ,
461 .Dq forced-commands-only
467 If this option is set to
469 password authentication is disabled for root. Note that other authentication
470 methods (e.g., keyboard-interactive/PAM) may still allow root to login using
473 If this option is set to
474 .Dq forced-commands-only
475 root login with public key authentication will be allowed,
478 option has been specified
479 (which may be useful for taking remote backups even if root login is
480 normally not allowed).
481 All other authentication methods are disabled for root.
483 If this option is set to
485 root is not allowed to login.
486 .It Cm PermitUserEnvironment
488 .Pa ~/.ssh/environment
492 .Pa ~/.ssh/authorized_keys
497 Enabling environment processing may enable users to bypass access
498 restrictions in some configurations using mechanisms such as
501 Specifies the file that contains the process ID of the
505 .Pa /var/run/sshd.pid .
507 Specifies the port number that
511 Multiple options of this type are permitted.
517 should print the date and time when the user last logged in.
525 when a user logs in interactively.
526 (On some systems it is also printed by the shell,
532 Specifies the protocol versions
535 The possible values are
539 Multiple versions must be comma-separated.
542 Note that the order of the protocol list does not indicate preference,
543 because the client selects among multiple protocol versions offered
549 .It Cm PubkeyAuthentication
550 Specifies whether public key authentication is allowed.
553 Note that this option applies to protocol version 2 only.
554 .It Cm RhostsRSAAuthentication
555 Specifies whether rhosts or /etc/hosts.equiv authentication together
556 with successful RSA host authentication is allowed.
559 This option applies to protocol version 1 only.
560 .It Cm RSAAuthentication
561 Specifies whether pure RSA authentication is allowed.
564 This option applies to protocol version 1 only.
566 Defines the number of bits in the ephemeral protocol version 1 server key.
567 The minimum value is 512, and the default is 768.
571 should check file modes and ownership of the
572 user's files and home directory before accepting login.
573 This is normally desirable because novices sometimes accidentally leave their
574 directory or files world-writable.
578 Configures an external subsystem (e.g., file transfer daemon).
579 Arguments should be a subsystem name and a command to execute upon subsystem
585 file transfer subsystem.
586 By default no subsystems are defined.
587 Note that this option applies to protocol version 2 only.
588 .It Cm SyslogFacility
589 Gives the facility code that is used when logging messages from
591 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
592 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
595 Specifies whether the system should send TCP keepalive messages to the
597 If they are sent, death of the connection or crash of one
598 of the machines will be properly noticed.
599 However, this means that
600 connections will die if the route is down temporarily, and some people
602 On the other hand, if TCP keepalives are not sent,
603 sessions may hang indefinitely on the server, leaving
605 users and consuming server resources.
609 (to send TCP keepalive messages), and the server will notice
610 if the network goes down or the client host crashes.
611 This avoids infinitely hanging sessions.
613 To disable TCP keepalive messages, the value should be set to
618 should lookup the remote host name and check that
619 the resolved host name for the remote IP address maps back to the
620 very same IP address.
626 is used for interactive login sessions.
631 is never used for remote command execution.
632 Note also, that if this is enabled,
634 will be disabled because
636 does not know how to handle
640 .Cm UsePrivilegeSeparation
641 is specified, it will be disabled after authentication.
643 Enables the Pluggable Authentication Module interface.
646 this will enable PAM authentication using
647 .Cm ChallengeResponseAuthentication
648 and PAM account and session module processing for all authentication types.
650 Because PAM challenge-response authentication usually serves an equivalent
651 role to password authentication, you should disable either
652 .Cm PasswordAuthentication
654 .Cm ChallengeResponseAuthentication.
658 is enabled, you will not be able to run
663 .It Cm UsePrivilegeSeparation
666 separates privileges by creating an unprivileged child process
667 to deal with incoming network traffic.
668 After successful authentication, another process will be created that has
669 the privilege of the authenticated user.
670 The goal of privilege separation is to prevent privilege
671 escalation by containing any corruption within the unprivileged processes.
674 .It Cm X11DisplayOffset
675 Specifies the first display number available for
680 from interfering with real X11 servers.
683 Specifies whether X11 forwarding is permitted.
691 When X11 forwarding is enabled, there may be additional exposure to
692 the server and to client displays if the
694 proxy display is configured to listen on the wildcard address (see
696 below), however this is not the default.
697 Additionally, the authentication spoofing and authentication data
698 verification and substitution occur on the client side.
699 The security risk of using X11 forwarding is that the client's X11
700 display server may be exposed to attack when the ssh client requests
701 forwarding (see the warnings for
705 A system administrator may have a stance in which they want to
706 protect clients that may expose themselves to attack by unwittingly
707 requesting X11 forwarding, which can warrant a
711 Note that disabling X11 forwarding does not prevent users from
712 forwarding X11 traffic, as users can always install their own forwarders.
713 X11 forwarding is automatically disabled if
716 .It Cm X11UseLocalhost
719 should bind the X11 forwarding server to the loopback address or to
720 the wildcard address.
723 binds the forwarding server to the loopback address and sets the
726 environment variable to
728 This prevents remote hosts from connecting to the proxy display.
729 However, some older X11 clients may not function with this
734 to specify that the forwarding server should be bound to the wildcard
743 Specifies the full pathname of the
747 .Pa /usr/X11R6/bin/xauth .
751 command-line arguments and configuration file options that specify time
752 may be expressed using a sequence of the form:
754 .Ar time Op Ar qualifier ,
758 is a positive integer value and
760 is one of the following:
762 .Bl -tag -width Ds -compact -offset indent
777 Each member of the sequence is added together to calculate
778 the total time value.
780 Time format examples:
782 .Bl -tag -width Ds -compact -offset indent
784 600 seconds (10 minutes)
788 1 hour 30 minutes (90 minutes)
792 .It Pa /etc/ssh/sshd_config
793 Contains configuration data for
795 This file should be writable by root only, but it is recommended
796 (though not necessary) that it be world-readable.
801 OpenSSH is a derivative of the original and free
802 ssh 1.2.12 release by Tatu Ylonen.
803 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
804 Theo de Raadt and Dug Song
805 removed many bugs, re-added newer features and
807 Markus Friedl contributed the support for SSH
808 protocol versions 1.5 and 2.0.
809 Niels Provos and Markus Friedl contributed support
810 for privilege separation.