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 1.53 2006/02/24 23:51:17 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.
102 Only group names are valid; a numerical group ID is not recognized.
103 By default, login is allowed for all groups.
109 for more information on patterns.
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.
122 Only user names are valid; a numerical user ID is not recognized.
123 By default, login is allowed for all users.
124 If the pattern takes the form USER@HOST then USER and HOST
125 are separately checked, restricting logins to particular
126 users from particular hosts.
132 for more information on patterns.
133 .It Cm AuthorizedKeysFile
134 Specifies the file that contains the public keys that can be used
135 for user authentication.
136 .Cm AuthorizedKeysFile
137 may contain tokens of the form %T which are substituted during connection
139 The following tokens are defined: %% is replaced by a literal '%',
140 %h is replaced by the home directory of the user being authenticated, and
141 %u is replaced by the username of that user.
143 .Cm AuthorizedKeysFile
144 is taken to be an absolute path or one relative to the user's home
147 .Dq .ssh/authorized_keys .
149 In some jurisdictions, sending a warning message before authentication
150 may be relevant for getting legal protection.
151 The contents of the specified file are sent to the remote user before
152 authentication is allowed.
153 This option is only available for protocol version 2.
154 By default, no banner is displayed.
155 .It Cm ChallengeResponseAuthentication
156 Specifies whether challenge-response authentication is allowed.
157 All authentication styles from
163 Specifies the ciphers allowed for protocol version 2.
164 Multiple ciphers must be comma-separated.
165 The supported ciphers are
180 .Bd -literal -offset 3n
181 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
182 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
183 aes192-ctr,aes256-ctr
185 .It Cm ClientAliveCountMax
186 Sets the number of client alive messages (see below) which may be
189 receiving any messages back from the client.
190 If this threshold is reached while client alive messages are being sent,
191 sshd will disconnect the client, terminating the session.
192 It is important to note that the use of client alive messages is very
196 The client alive messages are sent through the encrypted channel
197 and therefore will not be spoofable.
198 The TCP keepalive option enabled by
201 The client alive mechanism is valuable when the client or
202 server depend on knowing when a connection has become inactive.
204 The default value is 3.
206 .Cm ClientAliveInterval
207 (see below) is set to 15, and
208 .Cm ClientAliveCountMax
209 is left at the default, unresponsive SSH clients
210 will be disconnected after approximately 45 seconds.
211 .It Cm ClientAliveInterval
212 Sets a timeout interval in seconds after which if no data has been received
215 will send a message through the encrypted
216 channel to request a response from the client.
218 is 0, indicating that these messages will not be sent to the client.
219 This option applies to protocol version 2 only.
221 Specifies whether compression is allowed, or delayed until
222 the user has authenticated successfully.
231 This keyword can be followed by a list of group name patterns, separated
233 Login is disallowed for users whose primary group or supplementary
234 group list matches one of the patterns.
235 Only group names are valid; a numerical group ID is not recognized.
236 By default, login is allowed for all groups.
242 for more information on patterns.
244 This keyword can be followed by a list of user name patterns, separated
246 Login is disallowed for user names that match one of the patterns.
247 Only user names are valid; a numerical user ID is not recognized.
248 By default, login is allowed for all users.
249 If the pattern takes the form USER@HOST then USER and HOST
250 are separately checked, restricting logins to particular
251 users from particular hosts.
257 for more information on patterns.
259 Specifies whether remote hosts are allowed to connect to ports
260 forwarded for the client.
263 binds remote port forwardings to the loopback address.
264 This prevents other remote hosts from connecting to forwarded ports.
266 can be used to specify that sshd
267 should allow remote port forwardings to bind to non-loopback addresses, thus
268 allowing other hosts to connect.
271 to force remote port forwardings to be available to the local host only,
273 to force remote port forwardings to bind to the wildcard address, or
275 to allow the client to select the address to which the forwarding is bound.
278 .It Cm GSSAPIAuthentication
279 Specifies whether user authentication based on GSSAPI is allowed.
282 Note that this option applies to protocol version 2 only.
283 .It Cm GSSAPICleanupCredentials
284 Specifies whether to automatically destroy the user's credentials cache
288 Note that this option applies to protocol version 2 only.
289 .It Cm HostbasedAuthentication
290 Specifies whether rhosts or /etc/hosts.equiv authentication together
291 with successful public key client host authentication is allowed
292 (host-based authentication).
293 This option is similar to
294 .Cm RhostsRSAAuthentication
295 and applies to protocol version 2 only.
299 Specifies a file containing a private host key
302 .Pa /etc/ssh/ssh_host_key
303 for protocol version 1, and
304 .Pa /etc/ssh/ssh_host_rsa_key
306 .Pa /etc/ssh/ssh_host_dsa_key
307 for protocol version 2.
310 will refuse to use a file if it is group/world-accessible.
311 It is possible to have multiple host key files.
313 keys are used for version 1 and
317 are used for version 2 of the SSH protocol.
323 files will not be used in
324 .Cm RhostsRSAAuthentication
326 .Cm HostbasedAuthentication .
330 .Pa /etc/shosts.equiv
334 .It Cm IgnoreUserKnownHosts
337 should ignore the user's
338 .Pa ~/.ssh/known_hosts
340 .Cm RhostsRSAAuthentication
342 .Cm HostbasedAuthentication .
345 .It Cm KerberosAuthentication
346 Specifies whether the password provided by the user for
347 .Cm PasswordAuthentication
348 will be validated through the Kerberos KDC.
349 To use this option, the server needs a
350 Kerberos servtab which allows the verification of the KDC's identity.
353 .It Cm KerberosGetAFSToken
354 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
355 an AFS token before accessing the user's home directory.
358 .It Cm KerberosOrLocalPasswd
359 If password authentication through Kerberos fails then
360 the password will be validated via any additional local mechanism
365 .It Cm KerberosTicketCleanup
366 Specifies whether to automatically destroy the user's ticket cache
370 .It Cm KeyRegenerationInterval
371 In protocol version 1, the ephemeral server key is automatically regenerated
372 after this many seconds (if it has been used).
373 The purpose of regeneration is to prevent
374 decrypting captured sessions by later breaking into the machine and
376 The key is never stored anywhere.
377 If the value is 0, the key is never regenerated.
378 The default is 3600 (seconds).
380 Specifies the local addresses
383 The following forms may be used:
385 .Bl -item -offset indent -compact
389 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
394 .Ar host No | Ar IPv4_addr No : Ar port
400 .Ar host No | Ar IPv6_addr Oc : Ar port
407 sshd will listen on the address and all prior
410 The default is to listen on all local addresses.
413 options are permitted.
416 options must precede this option for non-port qualified addresses.
417 .It Cm LoginGraceTime
418 The server disconnects after this time if the user has not
419 successfully logged in.
420 If the value is 0, there is no time limit.
421 The default is 120 seconds.
423 Gives the verbosity level that is used when logging messages from
425 The possible values are:
426 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
428 DEBUG and DEBUG1 are equivalent.
429 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
430 Logging with a DEBUG level violates the privacy of users and is not recommended.
432 Specifies the available MAC (message authentication code) algorithms.
433 The MAC algorithm is used in protocol version 2
434 for data integrity protection.
435 Multiple algorithms must be comma-separated.
437 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
439 Specifies the maximum number of authentication attempts permitted per
441 Once the number of failures reaches half this value,
442 additional failures are logged.
445 Specifies the maximum number of concurrent unauthenticated connections to the
447 Additional connections will be dropped until authentication succeeds or the
449 expires for a connection.
452 Alternatively, random early drop can be enabled by specifying
453 the three colon separated values
457 will refuse connection attempts with a probability of
460 if there are currently
463 unauthenticated connections.
464 The probability increases linearly and all connection attempts
465 are refused if the number of unauthenticated connections reaches
468 .It Cm PasswordAuthentication
469 Specifies whether password authentication is allowed.
472 .It Cm PermitEmptyPasswords
473 When password authentication is allowed, it specifies whether the
474 server allows login to accounts with empty password strings.
477 .It Cm PermitRootLogin
478 Specifies whether root can log in using
482 .Dq without-password ,
483 .Dq forced-commands-only ,
489 If this option is set to
490 .Dq without-password ,
491 password authentication is disabled for root.
493 If this option is set to
494 .Dq forced-commands-only ,
495 root login with public key authentication will be allowed,
498 option has been specified
499 (which may be useful for taking remote backups even if root login is
500 normally not allowed).
501 All other authentication methods are disabled for root.
503 If this option is set to
505 root is not allowed to log in.
509 device forwarding is allowed.
518 .It Cm PermitUserEnvironment
520 .Pa ~/.ssh/environment
524 .Pa ~/.ssh/authorized_keys
529 Enabling environment processing may enable users to bypass access
530 restrictions in some configurations using mechanisms such as
533 Specifies the file that contains the process ID of the
536 .Pa /var/run/sshd.pid .
538 Specifies the port number that
542 Multiple options of this type are permitted.
548 should print the date and time of the last user login when a user logs
557 when a user logs in interactively.
558 (On some systems it is also printed by the shell,
564 Specifies the protocol versions
567 The possible values are
571 Multiple versions must be comma-separated.
574 Note that the order of the protocol list does not indicate preference,
575 because the client selects among multiple protocol versions offered
581 .It Cm PubkeyAuthentication
582 Specifies whether public key authentication is allowed.
585 Note that this option applies to protocol version 2 only.
586 .It Cm RhostsRSAAuthentication
587 Specifies whether rhosts or /etc/hosts.equiv authentication together
588 with successful RSA host authentication is allowed.
591 This option applies to protocol version 1 only.
592 .It Cm RSAAuthentication
593 Specifies whether pure RSA authentication is allowed.
596 This option applies to protocol version 1 only.
598 Defines the number of bits in the ephemeral protocol version 1 server key.
599 The minimum value is 512, and the default is 768.
603 should check file modes and ownership of the
604 user's files and home directory before accepting login.
605 This is normally desirable because novices sometimes accidentally leave their
606 directory or files world-writable.
610 Configures an external subsystem (e.g. file transfer daemon).
611 Arguments should be a subsystem name and a command to execute upon subsystem
617 file transfer subsystem.
618 By default no subsystems are defined.
619 Note that this option applies to protocol version 2 only.
620 .It Cm SyslogFacility
621 Gives the facility code that is used when logging messages from
623 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
624 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
627 Specifies whether the system should send TCP keepalive messages to the
629 If they are sent, death of the connection or crash of one
630 of the machines will be properly noticed.
631 However, this means that
632 connections will die if the route is down temporarily, and some people
634 On the other hand, if TCP keepalives are not sent,
635 sessions may hang indefinitely on the server, leaving
637 users and consuming server resources.
641 (to send TCP keepalive messages), and the server will notice
642 if the network goes down or the client host crashes.
643 This avoids infinitely hanging sessions.
645 To disable TCP keepalive messages, the value should be set to
650 should look up the remote host name and check that
651 the resolved host name for the remote IP address maps back to the
652 very same IP address.
658 is used for interactive login sessions.
663 is never used for remote command execution.
664 Note also, that if this is enabled,
666 will be disabled because
668 does not know how to handle
672 .Cm UsePrivilegeSeparation
673 is specified, it will be disabled after authentication.
675 Enables the Pluggable Authentication Module interface.
678 this will enable PAM authentication using
679 .Cm ChallengeResponseAuthentication
681 .Cm PasswordAuthentication
682 in addition to PAM account and session module processing for all
683 authentication types.
685 Because PAM challenge-response authentication usually serves an equivalent
686 role to password authentication, you should disable either
687 .Cm PasswordAuthentication
689 .Cm ChallengeResponseAuthentication.
693 is enabled, you will not be able to run
698 .It Cm UsePrivilegeSeparation
701 separates privileges by creating an unprivileged child process
702 to deal with incoming network traffic.
703 After successful authentication, another process will be created that has
704 the privilege of the authenticated user.
705 The goal of privilege separation is to prevent privilege
706 escalation by containing any corruption within the unprivileged processes.
709 .It Cm X11DisplayOffset
710 Specifies the first display number available for
713 This prevents sshd from interfering with real X11 servers.
716 Specifies whether X11 forwarding is permitted.
724 When X11 forwarding is enabled, there may be additional exposure to
725 the server and to client displays if the
727 proxy display is configured to listen on the wildcard address (see
729 below), though this is not the default.
730 Additionally, the authentication spoofing and authentication data
731 verification and substitution occur on the client side.
732 The security risk of using X11 forwarding is that the client's X11
733 display server may be exposed to attack when the SSH client requests
734 forwarding (see the warnings for
738 A system administrator may have a stance in which they want to
739 protect clients that may expose themselves to attack by unwittingly
740 requesting X11 forwarding, which can warrant a
744 Note that disabling X11 forwarding does not prevent users from
745 forwarding X11 traffic, as users can always install their own forwarders.
746 X11 forwarding is automatically disabled if
749 .It Cm X11UseLocalhost
752 should bind the X11 forwarding server to the loopback address or to
753 the wildcard address.
755 sshd binds the forwarding server to the loopback address and sets the
758 environment variable to
760 This prevents remote hosts from connecting to the proxy display.
761 However, some older X11 clients may not function with this
766 to specify that the forwarding server should be bound to the wildcard
775 Specifies the full pathname of the
779 .Pa /usr/X11R6/bin/xauth .
783 command-line arguments and configuration file options that specify time
784 may be expressed using a sequence of the form:
786 .Ar time Op Ar qualifier ,
790 is a positive integer value and
792 is one of the following:
794 .Bl -tag -width Ds -compact -offset indent
809 Each member of the sequence is added together to calculate
810 the total time value.
812 Time format examples:
814 .Bl -tag -width Ds -compact -offset indent
816 600 seconds (10 minutes)
820 1 hour 30 minutes (90 minutes)
824 .It Pa /etc/ssh/sshd_config
825 Contains configuration data for
827 This file should be writable by root only, but it is recommended
828 (though not necessary) that it be world-readable.
833 OpenSSH is a derivative of the original and free
834 ssh 1.2.12 release by Tatu Ylonen.
835 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
836 Theo de Raadt and Dug Song
837 removed many bugs, re-added newer features and
839 Markus Friedl contributed the support for SSH
840 protocol versions 1.5 and 2.0.
841 Niels Provos and Markus Friedl contributed support
842 for privilege separation.