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 1.37 2004/12/23 23:11:00 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 bind remote port forwardings to the wildcard address,
260 thus allowing remote hosts to connect to forwarded ports.
267 .It Cm GSSAPIAuthentication
268 Specifies whether user authentication based on GSSAPI is allowed.
271 Note that this option applies to protocol version 2 only.
272 .It Cm GSSAPICleanupCredentials
273 Specifies whether to automatically destroy the user's credentials cache
277 Note that this option applies to protocol version 2 only.
278 .It Cm HostbasedAuthentication
279 Specifies whether rhosts or /etc/hosts.equiv authentication together
280 with successful public key client host authentication is allowed
281 (hostbased authentication).
282 This option is similar to
283 .Cm RhostsRSAAuthentication
284 and applies to protocol version 2 only.
288 Specifies a file containing a private host key
291 .Pa /etc/ssh/ssh_host_key
292 for protocol version 1, and
293 .Pa /etc/ssh/ssh_host_rsa_key
295 .Pa /etc/ssh/ssh_host_dsa_key
296 for protocol version 2.
299 will refuse to use a file if it is group/world-accessible.
300 It is possible to have multiple host key files.
302 keys are used for version 1 and
306 are used for version 2 of the SSH protocol.
312 files will not be used in
313 .Cm RhostsRSAAuthentication
315 .Cm HostbasedAuthentication .
319 .Pa /etc/shosts.equiv
323 .It Cm IgnoreUserKnownHosts
326 should ignore the user's
327 .Pa $HOME/.ssh/known_hosts
329 .Cm RhostsRSAAuthentication
331 .Cm HostbasedAuthentication .
334 .It Cm KerberosAuthentication
335 Specifies whether the password provided by the user for
336 .Cm PasswordAuthentication
337 will be validated through the Kerberos KDC.
338 To use this option, the server needs a
339 Kerberos servtab which allows the verification of the KDC's identity.
342 .It Cm KerberosGetAFSToken
343 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
344 an AFS token before accessing the user's home directory.
347 .It Cm KerberosOrLocalPasswd
348 If set then if password authentication through Kerberos fails then
349 the password will be validated via any additional local mechanism
354 .It Cm KerberosTicketCleanup
355 Specifies whether to automatically destroy the user's ticket cache
359 .It Cm KeyRegenerationInterval
360 In protocol version 1, the ephemeral server key is automatically regenerated
361 after this many seconds (if it has been used).
362 The purpose of regeneration is to prevent
363 decrypting captured sessions by later breaking into the machine and
365 The key is never stored anywhere.
366 If the value is 0, the key is never regenerated.
367 The default is 3600 (seconds).
369 Specifies the local addresses
372 The following forms may be used:
374 .Bl -item -offset indent -compact
378 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
383 .Ar host No | Ar IPv4_addr No : Ar port
389 .Ar host No | Ar IPv6_addr Oc : Ar port
397 will listen on the address and all prior
400 The default is to listen on all local addresses.
403 options are permitted.
406 options must precede this option for non port qualified addresses.
407 .It Cm LoginGraceTime
408 The server disconnects after this time if the user has not
409 successfully logged in.
410 If the value is 0, there is no time limit.
411 The default is 120 seconds.
413 Gives the verbosity level that is used when logging messages from
415 The possible values are:
416 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
418 DEBUG and DEBUG1 are equivalent.
419 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
420 Logging with a DEBUG level violates the privacy of users and is not recommended.
422 Specifies the available MAC (message authentication code) algorithms.
423 The MAC algorithm is used in protocol version 2
424 for data integrity protection.
425 Multiple algorithms must be comma-separated.
427 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
429 Specifies the maximum number of authentication attempts permitted per
431 Once the number of failures reaches half this value,
432 additional failures are logged.
435 Specifies the maximum number of concurrent unauthenticated connections to the
438 Additional connections will be dropped until authentication succeeds or the
440 expires for a connection.
443 Alternatively, random early drop can be enabled by specifying
444 the three colon separated values
448 will refuse connection attempts with a probability of
451 if there are currently
454 unauthenticated connections.
455 The probability increases linearly and all connection attempts
456 are refused if the number of unauthenticated connections reaches
459 .It Cm PasswordAuthentication
460 Specifies whether password authentication is allowed.
463 .It Cm PermitEmptyPasswords
464 When password authentication is allowed, it specifies whether the
465 server allows login to accounts with empty password strings.
468 .It Cm PermitRootLogin
469 Specifies whether root can login using
473 .Dq without-password ,
474 .Dq forced-commands-only
480 If this option is set to
482 password authentication is disabled for root. Note that other authentication
483 methods (e.g., keyboard-interactive/PAM) may still allow root to login using
486 If this option is set to
487 .Dq forced-commands-only
488 root login with public key authentication will be allowed,
491 option has been specified
492 (which may be useful for taking remote backups even if root login is
493 normally not allowed).
494 All other authentication methods are disabled for root.
496 If this option is set to
498 root is not allowed to login.
499 .It Cm PermitUserEnvironment
501 .Pa ~/.ssh/environment
505 .Pa ~/.ssh/authorized_keys
510 Enabling environment processing may enable users to bypass access
511 restrictions in some configurations using mechanisms such as
514 Specifies the file that contains the process ID of the
518 .Pa /var/run/sshd.pid .
520 Specifies the port number that
524 Multiple options of this type are permitted.
530 should print the date and time of the last user login when a user logs
539 when a user logs in interactively.
540 (On some systems it is also printed by the shell,
546 Specifies the protocol versions
549 The possible values are
553 Multiple versions must be comma-separated.
556 Note that the order of the protocol list does not indicate preference,
557 because the client selects among multiple protocol versions offered
563 .It Cm PubkeyAuthentication
564 Specifies whether public key authentication is allowed.
567 Note that this option applies to protocol version 2 only.
568 .It Cm RhostsRSAAuthentication
569 Specifies whether rhosts or /etc/hosts.equiv authentication together
570 with successful RSA host authentication is allowed.
573 This option applies to protocol version 1 only.
574 .It Cm RSAAuthentication
575 Specifies whether pure RSA authentication is allowed.
578 This option applies to protocol version 1 only.
580 Defines the number of bits in the ephemeral protocol version 1 server key.
581 The minimum value is 512, and the default is 768.
585 should check file modes and ownership of the
586 user's files and home directory before accepting login.
587 This is normally desirable because novices sometimes accidentally leave their
588 directory or files world-writable.
592 Configures an external subsystem (e.g., file transfer daemon).
593 Arguments should be a subsystem name and a command to execute upon subsystem
599 file transfer subsystem.
600 By default no subsystems are defined.
601 Note that this option applies to protocol version 2 only.
602 .It Cm SyslogFacility
603 Gives the facility code that is used when logging messages from
605 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
606 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
609 Specifies whether the system should send TCP keepalive messages to the
611 If they are sent, death of the connection or crash of one
612 of the machines will be properly noticed.
613 However, this means that
614 connections will die if the route is down temporarily, and some people
616 On the other hand, if TCP keepalives are not sent,
617 sessions may hang indefinitely on the server, leaving
619 users and consuming server resources.
623 (to send TCP keepalive messages), and the server will notice
624 if the network goes down or the client host crashes.
625 This avoids infinitely hanging sessions.
627 To disable TCP keepalive messages, the value should be set to
632 should lookup the remote host name and check that
633 the resolved host name for the remote IP address maps back to the
634 very same IP address.
640 is used for interactive login sessions.
645 is never used for remote command execution.
646 Note also, that if this is enabled,
648 will be disabled because
650 does not know how to handle
654 .Cm UsePrivilegeSeparation
655 is specified, it will be disabled after authentication.
657 Enables the Pluggable Authentication Module interface.
660 this will enable PAM authentication using
661 .Cm ChallengeResponseAuthentication
662 and PAM account and session module processing for all authentication types.
664 Because PAM challenge-response authentication usually serves an equivalent
665 role to password authentication, you should disable either
666 .Cm PasswordAuthentication
668 .Cm ChallengeResponseAuthentication.
672 is enabled, you will not be able to run
677 .It Cm UsePrivilegeSeparation
680 separates privileges by creating an unprivileged child process
681 to deal with incoming network traffic.
682 After successful authentication, another process will be created that has
683 the privilege of the authenticated user.
684 The goal of privilege separation is to prevent privilege
685 escalation by containing any corruption within the unprivileged processes.
688 .It Cm X11DisplayOffset
689 Specifies the first display number available for
694 from interfering with real X11 servers.
697 Specifies whether X11 forwarding is permitted.
705 When X11 forwarding is enabled, there may be additional exposure to
706 the server and to client displays if the
708 proxy display is configured to listen on the wildcard address (see
710 below), however this is not the default.
711 Additionally, the authentication spoofing and authentication data
712 verification and substitution occur on the client side.
713 The security risk of using X11 forwarding is that the client's X11
714 display server may be exposed to attack when the ssh client requests
715 forwarding (see the warnings for
719 A system administrator may have a stance in which they want to
720 protect clients that may expose themselves to attack by unwittingly
721 requesting X11 forwarding, which can warrant a
725 Note that disabling X11 forwarding does not prevent users from
726 forwarding X11 traffic, as users can always install their own forwarders.
727 X11 forwarding is automatically disabled if
730 .It Cm X11UseLocalhost
733 should bind the X11 forwarding server to the loopback address or to
734 the wildcard address.
737 binds the forwarding server to the loopback address and sets the
740 environment variable to
742 This prevents remote hosts from connecting to the proxy display.
743 However, some older X11 clients may not function with this
748 to specify that the forwarding server should be bound to the wildcard
757 Specifies the full pathname of the
761 .Pa /usr/X11R6/bin/xauth .
765 command-line arguments and configuration file options that specify time
766 may be expressed using a sequence of the form:
768 .Ar time Op Ar qualifier ,
772 is a positive integer value and
774 is one of the following:
776 .Bl -tag -width Ds -compact -offset indent
791 Each member of the sequence is added together to calculate
792 the total time value.
794 Time format examples:
796 .Bl -tag -width Ds -compact -offset indent
798 600 seconds (10 minutes)
802 1 hour 30 minutes (90 minutes)
806 .It Pa /etc/ssh/sshd_config
807 Contains configuration data for
809 This file should be writable by root only, but it is recommended
810 (though not necessary) that it be world-readable.
815 OpenSSH is a derivative of the original and free
816 ssh 1.2.12 release by Tatu Ylonen.
817 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
818 Theo de Raadt and Dug Song
819 removed many bugs, re-added newer features and
821 Markus Friedl contributed the support for SSH
822 protocol versions 1.5 and 2.0.
823 Niels Provos and Markus Friedl contributed support
824 for privilege separation.