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 1.48 2006/01/02 17:09:49 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.
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
179 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
180 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
181 aes192-ctr,aes256-ctr''
183 .It Cm ClientAliveCountMax
184 Sets the number of client alive messages (see below) which may be
187 receiving any messages back from the client.
188 If this threshold is reached while client alive messages are being sent,
190 will disconnect the client, terminating the session.
191 It is important to note that the use of client alive messages is very
195 The client alive messages are sent through the encrypted channel
196 and therefore will not be spoofable.
197 The TCP keepalive option enabled by
200 The client alive mechanism is valuable when the client or
201 server depend on knowing when a connection has become inactive.
203 The default value is 3.
205 .Cm ClientAliveInterval
206 (see below) is set to 15, and
207 .Cm ClientAliveCountMax
208 is left at the default, unresponsive ssh clients
209 will be disconnected after approximately 45 seconds.
210 .It Cm ClientAliveInterval
211 Sets a timeout interval in seconds after which if no data has been received
214 will send a message through the encrypted
215 channel to request a response from the client.
217 is 0, indicating that these messages will not be sent to the client.
218 This option applies to protocol version 2 only.
220 Specifies whether compression is allowed, or delayed until
221 the user has authenticated successfully.
230 This keyword can be followed by a list of group name patterns, separated
232 Login is disallowed for users whose primary group or supplementary
233 group list matches one of the patterns.
238 wildcards in the patterns.
239 Only group names are valid; a numerical group ID is not recognized.
240 By default, login is allowed for all groups.
242 This keyword can be followed by a list of user name patterns, separated
244 Login is disallowed for user names that match one of the patterns.
248 can be used as wildcards in the patterns.
249 Only user names are valid; a numerical user ID is not recognized.
250 By default, login is allowed for all users.
251 If the pattern takes the form USER@HOST then USER and HOST
252 are separately checked, restricting logins to particular
253 users from particular hosts.
255 Specifies whether remote hosts are allowed to connect to ports
256 forwarded for the client.
259 binds remote port forwardings to the loopback address.
260 This prevents other remote hosts from connecting to forwarded ports.
262 can be used to specify that
264 should allow remote port forwardings to bind to non-loopback addresses, thus
265 allowing other hosts to connect.
268 to force remote port forwardings to be available to the local host only,
270 to force remote port forwardings to bind to the wildcard address, or
272 to allow the client to select the address to which the forwarding is bound.
275 .It Cm GSSAPIAuthentication
276 Specifies whether user authentication based on GSSAPI is allowed.
279 Note that this option applies to protocol version 2 only.
280 .It Cm GSSAPIKeyExchange
281 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
282 doesn't rely on ssh keys to verify host identity.
285 Note that this option applies to protocol version 2 only.
286 .It Cm GSSAPICleanupCredentials
287 Specifies whether to automatically destroy the user's credentials cache
291 Note that this option applies to protocol version 2 only.
292 .It Cm GSIAllowLimitedProxy
293 Specifies whether to accept limited proxy credentials for
297 .It Cm HostbasedAuthentication
298 Specifies whether rhosts or /etc/hosts.equiv authentication together
299 with successful public key client host authentication is allowed
300 (hostbased authentication).
301 This option is similar to
302 .Cm RhostsRSAAuthentication
303 and applies to protocol version 2 only.
307 Specifies a file containing a private host key
310 .Pa /etc/ssh/ssh_host_key
311 for protocol version 1, and
312 .Pa /etc/ssh/ssh_host_rsa_key
314 .Pa /etc/ssh/ssh_host_dsa_key
315 for protocol version 2.
318 will refuse to use a file if it is group/world-accessible.
319 It is possible to have multiple host key files.
321 keys are used for version 1 and
325 are used for version 2 of the SSH protocol.
331 files will not be used in
332 .Cm RhostsRSAAuthentication
334 .Cm HostbasedAuthentication .
338 .Pa /etc/shosts.equiv
342 .It Cm IgnoreUserKnownHosts
345 should ignore the user's
346 .Pa ~/.ssh/known_hosts
348 .Cm RhostsRSAAuthentication
350 .Cm HostbasedAuthentication .
353 .It Cm KerberosAuthentication
354 Specifies whether the password provided by the user for
355 .Cm PasswordAuthentication
356 will be validated through the Kerberos KDC.
357 To use this option, the server needs a
358 Kerberos servtab which allows the verification of the KDC's identity.
361 .It Cm KerberosGetAFSToken
362 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
363 an AFS token before accessing the user's home directory.
366 .It Cm KerberosOrLocalPasswd
367 If set then if password authentication through Kerberos fails then
368 the password will be validated via any additional local mechanism
373 .It Cm KerberosTicketCleanup
374 Specifies whether to automatically destroy the user's ticket cache
378 .It Cm KeyRegenerationInterval
379 In protocol version 1, the ephemeral server key is automatically regenerated
380 after this many seconds (if it has been used).
381 The purpose of regeneration is to prevent
382 decrypting captured sessions by later breaking into the machine and
384 The key is never stored anywhere.
385 If the value is 0, the key is never regenerated.
386 The default is 3600 (seconds).
388 Specifies the local addresses
391 The following forms may be used:
393 .Bl -item -offset indent -compact
397 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
402 .Ar host No | Ar IPv4_addr No : Ar port
408 .Ar host No | Ar IPv6_addr Oc : Ar port
416 will listen on the address and all prior
419 The default is to listen on all local addresses.
422 options are permitted.
425 options must precede this option for non port qualified addresses.
426 .It Cm LoginGraceTime
427 The server disconnects after this time if the user has not
428 successfully logged in.
429 If the value is 0, there is no time limit.
430 The default is 120 seconds.
432 Gives the verbosity level that is used when logging messages from
434 The possible values are:
435 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
437 DEBUG and DEBUG1 are equivalent.
438 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
439 Logging with a DEBUG level violates the privacy of users and is not recommended.
441 Specifies the available MAC (message authentication code) algorithms.
442 The MAC algorithm is used in protocol version 2
443 for data integrity protection.
444 Multiple algorithms must be comma-separated.
446 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
448 Specifies the maximum number of authentication attempts permitted per
450 Once the number of failures reaches half this value,
451 additional failures are logged.
454 Specifies the maximum number of concurrent unauthenticated connections to the
457 Additional connections will be dropped until authentication succeeds or the
459 expires for a connection.
462 Alternatively, random early drop can be enabled by specifying
463 the three colon separated values
467 will refuse connection attempts with a probability of
470 if there are currently
473 unauthenticated connections.
474 The probability increases linearly and all connection attempts
475 are refused if the number of unauthenticated connections reaches
478 .It Cm PasswordAuthentication
479 Specifies whether password authentication is allowed.
482 .It Cm PermitEmptyPasswords
483 When password authentication is allowed, it specifies whether the
484 server allows login to accounts with empty password strings.
487 .It Cm PermitRootLogin
488 Specifies whether root can log in using
492 .Dq without-password ,
493 .Dq forced-commands-only
499 If this option is set to
501 password authentication is disabled for root.
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 log in.
519 device forwarding is allowed.
528 .It Cm PermitUserEnvironment
530 .Pa ~/.ssh/environment
534 .Pa ~/.ssh/authorized_keys
539 Enabling environment processing may enable users to bypass access
540 restrictions in some configurations using mechanisms such as
543 Specifies the file that contains the process ID of the
547 .Pa /var/run/sshd.pid .
549 Specifies the port number that
553 Multiple options of this type are permitted.
559 should print the date and time of the last user login when a user logs
568 when a user logs in interactively.
569 (On some systems it is also printed by the shell,
575 Specifies the protocol versions
578 The possible values are
582 Multiple versions must be comma-separated.
585 Note that the order of the protocol list does not indicate preference,
586 because the client selects among multiple protocol versions offered
592 .It Cm PubkeyAuthentication
593 Specifies whether public key authentication is allowed.
596 Note that this option applies to protocol version 2 only.
597 .It Cm RhostsRSAAuthentication
598 Specifies whether rhosts or /etc/hosts.equiv authentication together
599 with successful RSA host authentication is allowed.
602 This option applies to protocol version 1 only.
603 .It Cm RSAAuthentication
604 Specifies whether pure RSA authentication is allowed.
607 This option applies to protocol version 1 only.
609 Defines the number of bits in the ephemeral protocol version 1 server key.
610 The minimum value is 512, and the default is 768.
614 should check file modes and ownership of the
615 user's files and home directory before accepting login.
616 This is normally desirable because novices sometimes accidentally leave their
617 directory or files world-writable.
621 Configures an external subsystem (e.g., file transfer daemon).
622 Arguments should be a subsystem name and a command to execute upon subsystem
628 file transfer subsystem.
629 By default no subsystems are defined.
630 Note that this option applies to protocol version 2 only.
631 .It Cm SyslogFacility
632 Gives the facility code that is used when logging messages from
634 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
635 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
638 Specifies whether the system should send TCP keepalive messages to the
640 If they are sent, death of the connection or crash of one
641 of the machines will be properly noticed.
642 However, this means that
643 connections will die if the route is down temporarily, and some people
645 On the other hand, if TCP keepalives are not sent,
646 sessions may hang indefinitely on the server, leaving
648 users and consuming server resources.
652 (to send TCP keepalive messages), and the server will notice
653 if the network goes down or the client host crashes.
654 This avoids infinitely hanging sessions.
656 To disable TCP keepalive messages, the value should be set to
661 should look up the remote host name and check that
662 the resolved host name for the remote IP address maps back to the
663 very same IP address.
669 is used for interactive login sessions.
674 is never used for remote command execution.
675 Note also, that if this is enabled,
677 will be disabled because
679 does not know how to handle
683 .Cm UsePrivilegeSeparation
684 is specified, it will be disabled after authentication.
686 Enables the Pluggable Authentication Module interface.
689 this will enable PAM authentication using
690 .Cm ChallengeResponseAuthentication
691 and PAM account and session module processing for all authentication types.
693 Because PAM challenge-response authentication usually serves an equivalent
694 role to password authentication, you should disable either
695 .Cm PasswordAuthentication
697 .Cm ChallengeResponseAuthentication.
701 is enabled, you will not be able to run
706 .It Cm UsePrivilegeSeparation
709 separates privileges by creating an unprivileged child process
710 to deal with incoming network traffic.
711 After successful authentication, another process will be created that has
712 the privilege of the authenticated user.
713 The goal of privilege separation is to prevent privilege
714 escalation by containing any corruption within the unprivileged processes.
717 .It Cm X11DisplayOffset
718 Specifies the first display number available for
723 from interfering with real X11 servers.
726 Specifies whether X11 forwarding is permitted.
734 When X11 forwarding is enabled, there may be additional exposure to
735 the server and to client displays if the
737 proxy display is configured to listen on the wildcard address (see
739 below), however this is not the default.
740 Additionally, the authentication spoofing and authentication data
741 verification and substitution occur on the client side.
742 The security risk of using X11 forwarding is that the client's X11
743 display server may be exposed to attack when the ssh client requests
744 forwarding (see the warnings for
748 A system administrator may have a stance in which they want to
749 protect clients that may expose themselves to attack by unwittingly
750 requesting X11 forwarding, which can warrant a
754 Note that disabling X11 forwarding does not prevent users from
755 forwarding X11 traffic, as users can always install their own forwarders.
756 X11 forwarding is automatically disabled if
759 .It Cm X11UseLocalhost
762 should bind the X11 forwarding server to the loopback address or to
763 the wildcard address.
766 binds the forwarding server to the loopback address and sets the
769 environment variable to
771 This prevents remote hosts from connecting to the proxy display.
772 However, some older X11 clients may not function with this
777 to specify that the forwarding server should be bound to the wildcard
786 Specifies the full pathname of the
790 .Pa /usr/X11R6/bin/xauth .
794 command-line arguments and configuration file options that specify time
795 may be expressed using a sequence of the form:
797 .Ar time Op Ar qualifier ,
801 is a positive integer value and
803 is one of the following:
805 .Bl -tag -width Ds -compact -offset indent
820 Each member of the sequence is added together to calculate
821 the total time value.
823 Time format examples:
825 .Bl -tag -width Ds -compact -offset indent
827 600 seconds (10 minutes)
831 1 hour 30 minutes (90 minutes)
835 .It Pa /etc/ssh/sshd_config
836 Contains configuration data for
838 This file should be writable by root only, but it is recommended
839 (though not necessary) that it be world-readable.
844 OpenSSH is a derivative of the original and free
845 ssh 1.2.12 release by Tatu Ylonen.
846 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
847 Theo de Raadt and Dug Song
848 removed many bugs, re-added newer features and
850 Markus Friedl contributed the support for SSH
851 protocol versions 1.5 and 2.0.
852 Niels Provos and Markus Friedl contributed support
853 for privilege separation.