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 1.35 2004/06/26 09:14:40 jmc 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 This keyword can be followed by a list of group name patterns, separated
90 If specified, login is allowed only for users whose primary
91 group or supplementary group list matches one of the patterns.
96 wildcards in the patterns.
97 Only group names are valid; a numerical group ID is not recognized.
98 By default, login is allowed for all groups.
99 .It Cm AllowTcpForwarding
100 Specifies whether TCP forwarding is permitted.
103 Note that disabling TCP forwarding does not improve security unless
104 users are also denied shell access, as they can always install their
107 This keyword can be followed by a list of user name patterns, separated
109 If specified, login is allowed only for user names that
110 match one of the patterns.
115 wildcards in the patterns.
116 Only user names are valid; a numerical user ID is not recognized.
117 By default, login is allowed for all users.
118 If the pattern takes the form USER@HOST then USER and HOST
119 are separately checked, restricting logins to particular
120 users from particular hosts.
121 .It Cm AuthorizedKeysFile
122 Specifies the file that contains the public keys that can be used
123 for user authentication.
124 .Cm AuthorizedKeysFile
125 may contain tokens of the form %T which are substituted during connection
127 The following tokens are defined: %% is replaced by a literal '%',
128 %h is replaced by the home directory of the user being authenticated and
129 %u is replaced by the username of that user.
131 .Cm AuthorizedKeysFile
132 is taken to be an absolute path or one relative to the user's home
135 .Dq .ssh/authorized_keys .
137 In some jurisdictions, sending a warning message before authentication
138 may be relevant for getting legal protection.
139 The contents of the specified file are sent to the remote user before
140 authentication is allowed.
141 This option is only available for protocol version 2.
142 By default, no banner is displayed.
143 .It Cm ChallengeResponseAuthentication
144 Specifies whether challenge-response authentication is allowed.
147 this controls the use of PAM (see
150 Note that this affects the effectiveness of the
151 .Cm PasswordAuthentication
158 Specifies the ciphers allowed for protocol version 2.
159 Multiple ciphers must be comma-separated.
160 The supported ciphers are
174 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
175 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
177 .It Cm ClientAliveInterval
178 Sets a timeout interval in seconds after which if no data has been received
181 will send a message through the encrypted
182 channel to request a response from the client.
184 is 0, indicating that these messages will not be sent to the client.
185 This option applies to protocol version 2 only.
186 .It Cm ClientAliveCountMax
187 Sets the number of client alive messages (see above) which may be
190 receiving any messages back from the client.
191 If this threshold is reached while client alive messages are being sent,
193 will disconnect the client, terminating the session.
194 It is important to note that the use of client alive messages is very
198 The client alive messages are sent through the encrypted channel
199 and therefore will not be spoofable.
200 The TCP keepalive option enabled by
203 The client alive mechanism is valuable when the client or
204 server depend on knowing when a connection has become inactive.
206 The default value is 3.
208 .Cm ClientAliveInterval
209 (above) is set to 15, and
210 .Cm ClientAliveCountMax
211 is left at the default, unresponsive ssh clients
212 will be disconnected after approximately 45 seconds.
214 Specifies whether compression is allowed.
222 This keyword can be followed by a list of group name patterns, separated
224 Login is disallowed for users whose primary group or supplementary
225 group list matches one of the patterns.
230 wildcards in the patterns.
231 Only group names are valid; a numerical group ID is not recognized.
232 By default, login is allowed for all groups.
234 This keyword can be followed by a list of user name patterns, separated
236 Login is disallowed for user names that match one of the patterns.
240 can be used as wildcards in the patterns.
241 Only user names are valid; a numerical user ID is not recognized.
242 By default, login is allowed for all users.
243 If the pattern takes the form USER@HOST then USER and HOST
244 are separately checked, restricting logins to particular
245 users from particular hosts.
247 Specifies whether remote hosts are allowed to connect to ports
248 forwarded for the client.
251 binds remote port forwardings to the loopback address.
252 This prevents other remote hosts from connecting to forwarded ports.
254 can be used to specify that
256 should bind remote port forwardings to the wildcard address,
257 thus allowing remote hosts to connect to forwarded ports.
264 .It Cm GSSAPIAuthentication
265 Specifies whether user authentication based on GSSAPI is allowed.
268 Note that this option applies to protocol version 2 only.
269 .It Cm GSSAPICleanupCredentials
270 Specifies whether to automatically destroy the user's credentials cache
274 Note that this option applies to protocol version 2 only.
275 .It Cm HostbasedAuthentication
276 Specifies whether rhosts or /etc/hosts.equiv authentication together
277 with successful public key client host authentication is allowed
278 (hostbased authentication).
279 This option is similar to
280 .Cm RhostsRSAAuthentication
281 and applies to protocol version 2 only.
285 Specifies a file containing a private host key
288 .Pa /etc/ssh/ssh_host_key
289 for protocol version 1, and
290 .Pa /etc/ssh/ssh_host_dsa_key
291 for protocol version 2.
294 will refuse to use a file if it is group/world-accessible.
295 It is possible to have multiple host key files.
297 keys are used for version 1 and
301 are used for version 2 of the SSH protocol.
307 files will not be used in
308 .Cm RhostsRSAAuthentication
310 .Cm HostbasedAuthentication .
314 .Pa /etc/ssh/shosts.equiv
318 .It Cm IgnoreUserKnownHosts
321 should ignore the user's
322 .Pa $HOME/.ssh/known_hosts
324 .Cm RhostsRSAAuthentication
326 .Cm HostbasedAuthentication .
329 .It Cm KerberosAuthentication
330 Specifies whether the password provided by the user for
331 .Cm PasswordAuthentication
332 will be validated through the Kerberos KDC.
333 To use this option, the server needs a
334 Kerberos servtab which allows the verification of the KDC's identity.
337 .It Cm KerberosGetAFSToken
338 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
339 an AFS token before accessing the user's home directory.
342 .It Cm KerberosOrLocalPasswd
343 If set then if password authentication through Kerberos fails then
344 the password will be validated via any additional local mechanism
349 .It Cm KerberosTicketCleanup
350 Specifies whether to automatically destroy the user's ticket cache
354 .It Cm KeyRegenerationInterval
355 In protocol version 1, the ephemeral server key is automatically regenerated
356 after this many seconds (if it has been used).
357 The purpose of regeneration is to prevent
358 decrypting captured sessions by later breaking into the machine and
360 The key is never stored anywhere.
361 If the value is 0, the key is never regenerated.
362 The default is 3600 (seconds).
364 Specifies the local addresses
367 The following forms may be used:
369 .Bl -item -offset indent -compact
373 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
378 .Ar host No | Ar IPv4_addr No : Ar port
384 .Ar host No | Ar IPv6_addr Oc : Ar port
392 will listen on the address and all prior
395 The default is to listen on all local addresses.
398 options are permitted.
401 options must precede this option for non port qualified addresses.
402 .It Cm LoginGraceTime
403 The server disconnects after this time if the user has not
404 successfully logged in.
405 If the value is 0, there is no time limit.
406 The default is 120 seconds.
408 Gives the verbosity level that is used when logging messages from
410 The possible values are:
411 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
413 DEBUG and DEBUG1 are equivalent.
414 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
415 Logging with a DEBUG level violates the privacy of users and is not recommended.
417 Specifies the available MAC (message authentication code) algorithms.
418 The MAC algorithm is used in protocol version 2
419 for data integrity protection.
420 Multiple algorithms must be comma-separated.
422 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
424 Specifies the maximum number of authentication attempts permitted per
426 Once the number of failures reaches half this value,
427 additional failures are logged.
430 Specifies the maximum number of concurrent unauthenticated connections to the
433 Additional connections will be dropped until authentication succeeds or the
435 expires for a connection.
438 Alternatively, random early drop can be enabled by specifying
439 the three colon separated values
443 will refuse connection attempts with a probability of
446 if there are currently
449 unauthenticated connections.
450 The probability increases linearly and all connection attempts
451 are refused if the number of unauthenticated connections reaches
454 .It Cm PasswordAuthentication
455 Specifies whether password authentication is allowed.
460 was built without PAM support, in which case the default is
463 .Cm ChallengeResponseAuthentication
466 and the PAM authentication policy for
470 password authentication will be allowed through the challenge-response
471 mechanism regardless of the value of
472 .Cm PasswordAuthentication .
473 .It Cm PermitEmptyPasswords
474 When password authentication is allowed, it specifies whether the
475 server allows login to accounts with empty password strings.
478 .It Cm PermitRootLogin
479 Specifies whether root can login using
483 .Dq without-password ,
484 .Dq forced-commands-only
490 .Cm ChallengeResponseAuthentication
493 the root user may be allowed in with its password even if
494 .Cm PermitRootLogin is set to
495 .Dq without-password .
497 If this option is set to
499 password authentication is disabled for root. Note that other authentication
500 methods (e.g., keyboard-interactive/PAM) may still allow root to login using
503 If this option is set to
504 .Dq forced-commands-only
505 root login with public key authentication will be allowed,
508 option has been specified
509 (which may be useful for taking remote backups even if root login is
510 normally not allowed).
511 All other authentication methods are disabled for root.
513 If this option is set to
515 root is not allowed to login.
516 .It Cm PermitUserEnvironment
518 .Pa ~/.ssh/environment
522 .Pa ~/.ssh/authorized_keys
527 Enabling environment processing may enable users to bypass access
528 restrictions in some configurations using mechanisms such as
531 Specifies the file that contains the process ID of the
535 .Pa /var/run/sshd.pid .
537 Specifies the port number that
541 Multiple options of this type are permitted.
547 should print the date and time when the user last logged in.
555 when a user logs in interactively.
556 (On some systems it is also printed by the shell,
562 Specifies the protocol versions
565 The possible values are
569 Multiple versions must be comma-separated.
572 Note that the order of the protocol list does not indicate preference,
573 because the client selects among multiple protocol versions offered
579 .It Cm PubkeyAuthentication
580 Specifies whether public key authentication is allowed.
583 Note that this option applies to protocol version 2 only.
584 .It Cm RhostsRSAAuthentication
585 Specifies whether rhosts or
587 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 lookup 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
680 and PAM account and session module processing for all authentication types.
682 Because PAM challenge-response authentication usually serves an equivalent
683 role to password authentication, you should disable either
684 .Cm PasswordAuthentication
686 .Cm ChallengeResponseAuthentication.
690 is enabled, you will not be able to run
695 .It Cm UsePrivilegeSeparation
698 separates privileges by creating an unprivileged child process
699 to deal with incoming network traffic.
700 After successful authentication, another process will be created that has
701 the privilege of the authenticated user.
702 The goal of privilege separation is to prevent privilege
703 escalation by containing any corruption within the unprivileged processes.
706 .It Cm VersionAddendum
707 Specifies a string to append to the regular version string to identify
708 OS- or site-specific modifications.
710 .Dq FreeBSD-20041028 .
711 .It Cm X11DisplayOffset
712 Specifies the first display number available for
717 from interfering with real X11 servers.
720 Specifies whether X11 forwarding is permitted.
728 When X11 forwarding is enabled, there may be additional exposure to
729 the server and to client displays if the
731 proxy display is configured to listen on the wildcard address (see
733 below), however this is not the default.
734 Additionally, the authentication spoofing and authentication data
735 verification and substitution occur on the client side.
736 The security risk of using X11 forwarding is that the client's X11
737 display server may be exposed to attack when the ssh client requests
738 forwarding (see the warnings for
742 A system administrator may have a stance in which they want to
743 protect clients that may expose themselves to attack by unwittingly
744 requesting X11 forwarding, which can warrant a
748 Note that disabling X11 forwarding does not prevent users from
749 forwarding X11 traffic, as users can always install their own forwarders.
750 X11 forwarding is automatically disabled if
753 .It Cm X11UseLocalhost
756 should bind the X11 forwarding server to the loopback address or to
757 the wildcard address.
760 binds the forwarding server to the loopback address and sets the
763 environment variable to
765 This prevents remote hosts from connecting to the proxy display.
766 However, some older X11 clients may not function with this
771 to specify that the forwarding server should be bound to the wildcard
780 Specifies the full pathname of the
784 .Pa /usr/X11R6/bin/xauth .
788 command-line arguments and configuration file options that specify time
789 may be expressed using a sequence of the form:
791 .Ar time Op Ar qualifier ,
795 is a positive integer value and
797 is one of the following:
799 .Bl -tag -width Ds -compact -offset indent
814 Each member of the sequence is added together to calculate
815 the total time value.
817 Time format examples:
819 .Bl -tag -width Ds -compact -offset indent
821 600 seconds (10 minutes)
825 1 hour 30 minutes (90 minutes)
829 .It Pa /etc/ssh/sshd_config
830 Contains configuration data for
832 This file should be writable by root only, but it is recommended
833 (though not necessary) that it be world-readable.
838 OpenSSH is a derivative of the original and free
839 ssh 1.2.12 release by Tatu Ylonen.
840 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
841 Theo de Raadt and Dug Song
842 removed many bugs, re-added newer features and
844 Markus Friedl contributed the support for SSH
845 protocol versions 1.5 and 2.0.
846 Niels Provos and Markus Friedl contributed support
847 for privilege separation.