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 1.39 2005/03/01 10:09:52 djm 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
177 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
178 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
180 .It Cm ClientAliveInterval
181 Sets a timeout interval in seconds after which if no data has been received
184 will send a message through the encrypted
185 channel to request a response from the client.
187 is 0, indicating that these messages will not be sent to the client.
188 This option applies to protocol version 2 only.
189 .It Cm ClientAliveCountMax
190 Sets the number of client alive messages (see above) which may be
193 receiving any messages back from the client.
194 If this threshold is reached while client alive messages are being sent,
196 will disconnect the client, terminating the session.
197 It is important to note that the use of client alive messages is very
201 The client alive messages are sent through the encrypted channel
202 and therefore will not be spoofable.
203 The TCP keepalive option enabled by
206 The client alive mechanism is valuable when the client or
207 server depend on knowing when a connection has become inactive.
209 The default value is 3.
211 .Cm ClientAliveInterval
212 (above) is set to 15, and
213 .Cm ClientAliveCountMax
214 is left at the default, unresponsive ssh clients
215 will be disconnected after approximately 45 seconds.
217 Specifies whether compression is allowed.
225 This keyword can be followed by a list of group name patterns, separated
227 Login is disallowed for users whose primary group or supplementary
228 group list matches one of the patterns.
233 wildcards in the patterns.
234 Only group names are valid; a numerical group ID is not recognized.
235 By default, login is allowed for all groups.
237 This keyword can be followed by a list of user name patterns, separated
239 Login is disallowed for user names that match one of the patterns.
243 can be used as wildcards in the patterns.
244 Only user names are valid; a numerical user ID is not recognized.
245 By default, login is allowed for all users.
246 If the pattern takes the form USER@HOST then USER and HOST
247 are separately checked, restricting logins to particular
248 users from particular hosts.
250 Specifies whether remote hosts are allowed to connect to ports
251 forwarded for the client.
254 binds remote port forwardings to the loopback address.
255 This prevents other remote hosts from connecting to forwarded ports.
257 can be used to specify that
259 should allow remote port forwardings to bind to non-loopback addresses, thus
260 allowing other hosts to connect.
263 to force remote port forwardings to be available to the local host only,
265 to force remote port forwardings to bind to the wildcard address, or
267 to allow the client to select the address to which the forwarding is bound.
270 .It Cm GSSAPIAuthentication
271 Specifies whether user authentication based on GSSAPI is allowed.
274 Note that this option applies to protocol version 2 only.
275 .It Cm GSSAPICleanupCredentials
276 Specifies whether to automatically destroy the user's credentials cache
280 Note that this option applies to protocol version 2 only.
281 .It Cm GSSAPIKeyExchange
282 Specifies whether key exchange based on GSSAPI may be used. When using
283 GSSAPI key exchange the server need not have a host key.
286 Note that this option applies to protocol version 2 only.
287 .It Cm GSSAPIUseSessionCredCache
288 Specifies whether a unique credentials cache name should be generated per
289 session for storing delegated credentials.
292 Note that this option applies to protocol version 2 only.
293 .It Cm HostbasedAuthentication
294 Specifies whether rhosts or /etc/hosts.equiv authentication together
295 with successful public key client host authentication is allowed
296 (hostbased authentication).
297 This option is similar to
298 .Cm RhostsRSAAuthentication
299 and applies to protocol version 2 only.
303 Specifies a file containing a private host key
306 .Pa /etc/ssh/ssh_host_key
307 for protocol version 1, and
308 .Pa /etc/ssh/ssh_host_rsa_key
310 .Pa /etc/ssh/ssh_host_dsa_key
311 for protocol version 2.
314 will refuse to use a file if it is group/world-accessible.
315 It is possible to have multiple host key files.
317 keys are used for version 1 and
321 are used for version 2 of the SSH protocol.
327 files will not be used in
328 .Cm RhostsRSAAuthentication
330 .Cm HostbasedAuthentication .
334 .Pa /etc/shosts.equiv
338 .It Cm IgnoreUserKnownHosts
341 should ignore the user's
342 .Pa $HOME/.ssh/known_hosts
344 .Cm RhostsRSAAuthentication
346 .Cm HostbasedAuthentication .
349 .It Cm KerberosAuthentication
350 Specifies whether the password provided by the user for
351 .Cm PasswordAuthentication
352 will be validated through the Kerberos KDC.
353 To use this option, the server needs a
354 Kerberos servtab which allows the verification of the KDC's identity.
357 .It Cm KerberosGetAFSToken
358 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
359 an AFS token before accessing the user's home directory.
362 .It Cm KerberosOrLocalPasswd
363 If set then if password authentication through Kerberos fails then
364 the password will be validated via any additional local mechanism
369 .It Cm KerberosTicketCleanup
370 Specifies whether to automatically destroy the user's ticket cache
374 .It Cm KeyRegenerationInterval
375 In protocol version 1, the ephemeral server key is automatically regenerated
376 after this many seconds (if it has been used).
377 The purpose of regeneration is to prevent
378 decrypting captured sessions by later breaking into the machine and
380 The key is never stored anywhere.
381 If the value is 0, the key is never regenerated.
382 The default is 3600 (seconds).
384 Specifies the local addresses
387 The following forms may be used:
389 .Bl -item -offset indent -compact
393 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
398 .Ar host No | Ar IPv4_addr No : Ar port
404 .Ar host No | Ar IPv6_addr Oc : Ar port
412 will listen on the address and all prior
415 The default is to listen on all local addresses.
418 options are permitted.
421 options must precede this option for non port qualified addresses.
422 .It Cm LoginGraceTime
423 The server disconnects after this time if the user has not
424 successfully logged in.
425 If the value is 0, there is no time limit.
426 The default is 120 seconds.
428 Gives the verbosity level that is used when logging messages from
430 The possible values are:
431 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
433 DEBUG and DEBUG1 are equivalent.
434 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
435 Logging with a DEBUG level violates the privacy of users and is not recommended.
437 Specifies the available MAC (message authentication code) algorithms.
438 The MAC algorithm is used in protocol version 2
439 for data integrity protection.
440 Multiple algorithms must be comma-separated.
442 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
444 Specifies the maximum number of authentication attempts permitted per
446 Once the number of failures reaches half this value,
447 additional failures are logged.
450 Specifies the maximum number of concurrent unauthenticated connections to the
453 Additional connections will be dropped until authentication succeeds or the
455 expires for a connection.
458 Alternatively, random early drop can be enabled by specifying
459 the three colon separated values
463 will refuse connection attempts with a probability of
466 if there are currently
469 unauthenticated connections.
470 The probability increases linearly and all connection attempts
471 are refused if the number of unauthenticated connections reaches
474 .It Cm PasswordAuthentication
475 Specifies whether password authentication is allowed.
478 .It Cm PermitEmptyPasswords
479 When password authentication is allowed, it specifies whether the
480 server allows login to accounts with empty password strings.
483 .It Cm PermitRootLogin
484 Specifies whether root can log in using
488 .Dq without-password ,
489 .Dq forced-commands-only
495 If this option is set to
497 password authentication is disabled for root.
499 If this option is set to
500 .Dq forced-commands-only
501 root login with public key authentication will be allowed,
504 option has been specified
505 (which may be useful for taking remote backups even if root login is
506 normally not allowed).
507 All other authentication methods are disabled for root.
509 If this option is set to
511 root is not allowed to log in.
512 .It Cm PermitUserEnvironment
514 .Pa ~/.ssh/environment
518 .Pa ~/.ssh/authorized_keys
523 Enabling environment processing may enable users to bypass access
524 restrictions in some configurations using mechanisms such as
527 Specifies the file that contains the process ID of the
531 .Pa /var/run/sshd.pid .
533 Specifies the port number that
537 Multiple options of this type are permitted.
543 should print the date and time of the last user login when a user logs
552 when a user logs in interactively.
553 (On some systems it is also printed by the shell,
559 Specifies the protocol versions
562 The possible values are
566 Multiple versions must be comma-separated.
569 Note that the order of the protocol list does not indicate preference,
570 because the client selects among multiple protocol versions offered
576 .It Cm PubkeyAuthentication
577 Specifies whether public key authentication is allowed.
580 Note that this option applies to protocol version 2 only.
581 .It Cm RhostsRSAAuthentication
582 Specifies whether rhosts or /etc/hosts.equiv authentication together
583 with successful RSA host authentication is allowed.
586 This option applies to protocol version 1 only.
587 .It Cm RSAAuthentication
588 Specifies whether pure RSA authentication is allowed.
591 This option applies to protocol version 1 only.
593 Defines the number of bits in the ephemeral protocol version 1 server key.
594 The minimum value is 512, and the default is 768.
598 should check file modes and ownership of the
599 user's files and home directory before accepting login.
600 This is normally desirable because novices sometimes accidentally leave their
601 directory or files world-writable.
605 Configures an external subsystem (e.g., file transfer daemon).
606 Arguments should be a subsystem name and a command to execute upon subsystem
612 file transfer subsystem.
613 By default no subsystems are defined.
614 Note that this option applies to protocol version 2 only.
615 .It Cm SyslogFacility
616 Gives the facility code that is used when logging messages from
618 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
619 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
622 Specifies whether the system should send TCP keepalive messages to the
624 If they are sent, death of the connection or crash of one
625 of the machines will be properly noticed.
626 However, this means that
627 connections will die if the route is down temporarily, and some people
629 On the other hand, if TCP keepalives are not sent,
630 sessions may hang indefinitely on the server, leaving
632 users and consuming server resources.
636 (to send TCP keepalive messages), and the server will notice
637 if the network goes down or the client host crashes.
638 This avoids infinitely hanging sessions.
640 To disable TCP keepalive messages, the value should be set to
645 should lookup the remote host name and check that
646 the resolved host name for the remote IP address maps back to the
647 very same IP address.
653 is used for interactive login sessions.
658 is never used for remote command execution.
659 Note also, that if this is enabled,
661 will be disabled because
663 does not know how to handle
667 .Cm UsePrivilegeSeparation
668 is specified, it will be disabled after authentication.
670 Enables the Pluggable Authentication Module interface.
673 this will enable PAM authentication using
674 .Cm ChallengeResponseAuthentication
675 and PAM account and session module processing for all authentication types.
677 Because PAM challenge-response authentication usually serves an equivalent
678 role to password authentication, you should disable either
679 .Cm PasswordAuthentication
681 .Cm ChallengeResponseAuthentication.
685 is enabled, you will not be able to run
690 .It Cm UsePrivilegeSeparation
693 separates privileges by creating an unprivileged child process
694 to deal with incoming network traffic.
695 After successful authentication, another process will be created that has
696 the privilege of the authenticated user.
697 The goal of privilege separation is to prevent privilege
698 escalation by containing any corruption within the unprivileged processes.
701 .It Cm X11DisplayOffset
702 Specifies the first display number available for
707 from interfering with real X11 servers.
710 Specifies whether X11 forwarding is permitted.
718 When X11 forwarding is enabled, there may be additional exposure to
719 the server and to client displays if the
721 proxy display is configured to listen on the wildcard address (see
723 below), however this is not the default.
724 Additionally, the authentication spoofing and authentication data
725 verification and substitution occur on the client side.
726 The security risk of using X11 forwarding is that the client's X11
727 display server may be exposed to attack when the ssh client requests
728 forwarding (see the warnings for
732 A system administrator may have a stance in which they want to
733 protect clients that may expose themselves to attack by unwittingly
734 requesting X11 forwarding, which can warrant a
738 Note that disabling X11 forwarding does not prevent users from
739 forwarding X11 traffic, as users can always install their own forwarders.
740 X11 forwarding is automatically disabled if
743 .It Cm X11UseLocalhost
746 should bind the X11 forwarding server to the loopback address or to
747 the wildcard address.
750 binds the forwarding server to the loopback address and sets the
753 environment variable to
755 This prevents remote hosts from connecting to the proxy display.
756 However, some older X11 clients may not function with this
761 to specify that the forwarding server should be bound to the wildcard
770 Specifies the full pathname of the
774 .Pa /usr/X11R6/bin/xauth .
778 command-line arguments and configuration file options that specify time
779 may be expressed using a sequence of the form:
781 .Ar time Op Ar qualifier ,
785 is a positive integer value and
787 is one of the following:
789 .Bl -tag -width Ds -compact -offset indent
804 Each member of the sequence is added together to calculate
805 the total time value.
807 Time format examples:
809 .Bl -tag -width Ds -compact -offset indent
811 600 seconds (10 minutes)
815 1 hour 30 minutes (90 minutes)
819 .It Pa /etc/ssh/sshd_config
820 Contains configuration data for
822 This file should be writable by root only, but it is recommended
823 (though not necessary) that it be world-readable.
828 OpenSSH is a derivative of the original and free
829 ssh 1.2.12 release by Tatu Ylonen.
830 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
831 Theo de Raadt and Dug Song
832 removed many bugs, re-added newer features and
834 Markus Friedl contributed the support for SSH
835 protocol versions 1.5 and 2.0.
836 Niels Provos and Markus Friedl contributed support
837 for privilege separation.