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 1.33 2004/05/23 23:59:53 dtucker 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 This keyword can be followed by a list of group name patterns, separated
89 If specified, login is allowed only for users whose primary
90 group or supplementary group list matches one of the patterns.
95 wildcards in the patterns.
96 Only group names are valid; a numerical group ID is not recognized.
97 By default, login is allowed for all groups.
98 .It Cm AllowTcpForwarding
99 Specifies whether TCP forwarding is permitted.
102 Note that disabling TCP forwarding does not improve security unless
103 users are also denied shell access, as they can always install their
106 This keyword can be followed by a list of user name patterns, separated
108 If specified, login is allowed only for user names that
109 match one of the patterns.
114 wildcards in the patterns.
115 Only user names are valid; a numerical user ID is not recognized.
116 By default, login is allowed for all users.
117 If the pattern takes the form USER@HOST then USER and HOST
118 are separately checked, restricting logins to particular
119 users from particular hosts.
120 .It Cm AuthorizedKeysFile
121 Specifies the file that contains the public keys that can be used
122 for user authentication.
123 .Cm AuthorizedKeysFile
124 may contain tokens of the form %T which are substituted during connection
126 The following tokens are defined: %% is replaced by a literal '%',
127 %h is replaced by the home directory of the user being authenticated and
128 %u is replaced by the username of that user.
130 .Cm AuthorizedKeysFile
131 is taken to be an absolute path or one relative to the user's home
134 .Dq .ssh/authorized_keys .
136 In some jurisdictions, sending a warning message before authentication
137 may be relevant for getting legal protection.
138 The contents of the specified file are sent to the remote user before
139 authentication is allowed.
140 This option is only available for protocol version 2.
141 By default, no banner is displayed.
142 .It Cm ChallengeResponseAuthentication
143 Specifies whether challenge response authentication is allowed.
144 All authentication styles from
150 Specifies the ciphers allowed for protocol version 2.
151 Multiple ciphers must be comma-separated.
154 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
155 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
157 .It Cm ClientAliveInterval
158 Sets a timeout interval in seconds after which if no data has been received
161 will send a message through the encrypted
162 channel to request a response from the client.
164 is 0, indicating that these messages will not be sent to the client.
165 This option applies to protocol version 2 only.
166 .It Cm ClientAliveCountMax
167 Sets the number of client alive messages (see above) which may be
170 receiving any messages back from the client.
171 If this threshold is reached while client alive messages are being sent,
173 will disconnect the client, terminating the session.
174 It is important to note that the use of client alive messages is very
178 The client alive messages are sent through the encrypted channel
179 and therefore will not be spoofable.
180 The TCP keepalive option enabled by
183 The client alive mechanism is valuable when the client or
184 server depend on knowing when a connection has become inactive.
186 The default value is 3.
188 .Cm ClientAliveInterval
189 (above) is set to 15, and
190 .Cm ClientAliveCountMax
191 is left at the default, unresponsive ssh clients
192 will be disconnected after approximately 45 seconds.
194 Specifies whether compression is allowed.
202 This keyword can be followed by a list of group name patterns, separated
204 Login is disallowed for users whose primary group or supplementary
205 group list matches one of the patterns.
210 wildcards in the patterns.
211 Only group names are valid; a numerical group ID is not recognized.
212 By default, login is allowed for all groups.
214 This keyword can be followed by a list of user name patterns, separated
216 Login is disallowed for user names that match one of the patterns.
220 can be used as wildcards in the patterns.
221 Only user names are valid; a numerical user ID is not recognized.
222 By default, login is allowed for all users.
223 If the pattern takes the form USER@HOST then USER and HOST
224 are separately checked, restricting logins to particular
225 users from particular hosts.
227 Specifies whether remote hosts are allowed to connect to ports
228 forwarded for the client.
231 binds remote port forwardings to the loopback address.
232 This prevents other remote hosts from connecting to forwarded ports.
234 can be used to specify that
236 should bind remote port forwardings to the wildcard address,
237 thus allowing remote hosts to connect to forwarded ports.
244 .It Cm GSSAPIAuthentication
245 Specifies whether user authentication based on GSSAPI is allowed.
248 Note that this option applies to protocol version 2 only.
249 .It Cm GSSAPICleanupCredentials
250 Specifies whether to automatically destroy the user's credentials cache
254 Note that this option applies to protocol version 2 only.
255 .It Cm HostbasedAuthentication
256 Specifies whether rhosts or /etc/hosts.equiv authentication together
257 with successful public key client host authentication is allowed
258 (hostbased authentication).
259 This option is similar to
260 .Cm RhostsRSAAuthentication
261 and applies to protocol version 2 only.
265 Specifies a file containing a private host key
268 .Pa /etc/ssh/ssh_host_key
269 for protocol version 1, and
270 .Pa /etc/ssh/ssh_host_rsa_key
272 .Pa /etc/ssh/ssh_host_dsa_key
273 for protocol version 2.
276 will refuse to use a file if it is group/world-accessible.
277 It is possible to have multiple host key files.
279 keys are used for version 1 and
283 are used for version 2 of the SSH protocol.
289 files will not be used in
290 .Cm RhostsRSAAuthentication
292 .Cm HostbasedAuthentication .
296 .Pa /etc/shosts.equiv
300 .It Cm IgnoreUserKnownHosts
303 should ignore the user's
304 .Pa $HOME/.ssh/known_hosts
306 .Cm RhostsRSAAuthentication
308 .Cm HostbasedAuthentication .
311 .It Cm KerberosAuthentication
312 Specifies whether the password provided by the user for
313 .Cm PasswordAuthentication
314 will be validated through the Kerberos KDC.
315 To use this option, the server needs a
316 Kerberos servtab which allows the verification of the KDC's identity.
319 .It Cm KerberosGetAFSToken
320 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
321 an AFS token before accessing the user's home directory.
324 .It Cm KerberosOrLocalPasswd
325 If set then if password authentication through Kerberos fails then
326 the password will be validated via any additional local mechanism
331 .It Cm KerberosTicketCleanup
332 Specifies whether to automatically destroy the user's ticket cache
336 .It Cm KeyRegenerationInterval
337 In protocol version 1, the ephemeral server key is automatically regenerated
338 after this many seconds (if it has been used).
339 The purpose of regeneration is to prevent
340 decrypting captured sessions by later breaking into the machine and
342 The key is never stored anywhere.
343 If the value is 0, the key is never regenerated.
344 The default is 3600 (seconds).
346 Specifies the local addresses
349 The following forms may be used:
351 .Bl -item -offset indent -compact
355 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
360 .Ar host No | Ar IPv4_addr No : Ar port
366 .Ar host No | Ar IPv6_addr Oc : Ar port
374 will listen on the address and all prior
377 The default is to listen on all local addresses.
380 options are permitted.
383 options must precede this option for non port qualified addresses.
384 .It Cm LoginGraceTime
385 The server disconnects after this time if the user has not
386 successfully logged in.
387 If the value is 0, there is no time limit.
388 The default is 120 seconds.
390 Gives the verbosity level that is used when logging messages from
392 The possible values are:
393 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
395 DEBUG and DEBUG1 are equivalent.
396 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
397 Logging with a DEBUG level violates the privacy of users and is not recommended.
399 Specifies the available MAC (message authentication code) algorithms.
400 The MAC algorithm is used in protocol version 2
401 for data integrity protection.
402 Multiple algorithms must be comma-separated.
404 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
406 Specifies the maximum number of authentication attempts permitted per
407 connection. Once the number of failures reaches half this value, additional
408 failures are logged. The default is 6.
410 Specifies the maximum number of concurrent unauthenticated connections to the
413 Additional connections will be dropped until authentication succeeds or the
415 expires for a connection.
418 Alternatively, random early drop can be enabled by specifying
419 the three colon separated values
423 will refuse connection attempts with a probability of
426 if there are currently
429 unauthenticated connections.
430 The probability increases linearly and all connection attempts
431 are refused if the number of unauthenticated connections reaches
434 .It Cm PasswordAuthentication
435 Specifies whether password authentication is allowed.
438 .It Cm PermitEmptyPasswords
439 When password authentication is allowed, it specifies whether the
440 server allows login to accounts with empty password strings.
443 .It Cm PermitRootLogin
444 Specifies whether root can login using
448 .Dq without-password ,
449 .Dq forced-commands-only
455 If this option is set to
457 password authentication is disabled for root. Note that other authentication
458 methods (e.g., keyboard-interactive/PAM) may still allow root to login using
461 If this option is set to
462 .Dq forced-commands-only
463 root login with public key authentication will be allowed,
466 option has been specified
467 (which may be useful for taking remote backups even if root login is
468 normally not allowed).
469 All other authentication methods are disabled for root.
471 If this option is set to
473 root is not allowed to login.
474 .It Cm PermitUserEnvironment
476 .Pa ~/.ssh/environment
480 .Pa ~/.ssh/authorized_keys
485 Enabling environment processing may enable users to bypass access
486 restrictions in some configurations using mechanisms such as
489 Specifies the file that contains the process ID of the
493 .Pa /var/run/sshd.pid .
495 Specifies the port number that
499 Multiple options of this type are permitted.
505 should print the date and time when the user last logged in.
513 when a user logs in interactively.
514 (On some systems it is also printed by the shell,
520 Specifies the protocol versions
523 The possible values are
527 Multiple versions must be comma-separated.
530 Note that the order of the protocol list does not indicate preference,
531 because the client selects among multiple protocol versions offered
537 .It Cm PubkeyAuthentication
538 Specifies whether public key authentication is allowed.
541 Note that this option applies to protocol version 2 only.
542 .It Cm RhostsRSAAuthentication
543 Specifies whether rhosts or /etc/hosts.equiv authentication together
544 with successful RSA host authentication is allowed.
547 This option applies to protocol version 1 only.
548 .It Cm RSAAuthentication
549 Specifies whether pure RSA authentication is allowed.
552 This option applies to protocol version 1 only.
554 Defines the number of bits in the ephemeral protocol version 1 server key.
555 The minimum value is 512, and the default is 768.
559 should check file modes and ownership of the
560 user's files and home directory before accepting login.
561 This is normally desirable because novices sometimes accidentally leave their
562 directory or files world-writable.
566 Configures an external subsystem (e.g., file transfer daemon).
567 Arguments should be a subsystem name and a command to execute upon subsystem
573 file transfer subsystem.
574 By default no subsystems are defined.
575 Note that this option applies to protocol version 2 only.
576 .It Cm SyslogFacility
577 Gives the facility code that is used when logging messages from
579 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
580 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
583 Specifies whether the system should send TCP keepalive messages to the
585 If they are sent, death of the connection or crash of one
586 of the machines will be properly noticed.
587 However, this means that
588 connections will die if the route is down temporarily, and some people
590 On the other hand, if TCP keepalives are not sent,
591 sessions may hang indefinitely on the server, leaving
593 users and consuming server resources.
597 (to send TCP keepalive messages), and the server will notice
598 if the network goes down or the client host crashes.
599 This avoids infinitely hanging sessions.
601 To disable TCP keepalive messages, the value should be set to
606 should lookup the remote host name and check that
607 the resolved host name for the remote IP address maps back to the
608 very same IP address.
614 is used for interactive login sessions.
619 is never used for remote command execution.
620 Note also, that if this is enabled,
622 will be disabled because
624 does not know how to handle
628 .Cm UsePrivilegeSeparation
629 is specified, it will be disabled after authentication.
631 Enables the Pluggable Authentication Module interface.
634 this will enable PAM authentication using
635 .Cm ChallengeResponseAuthentication
636 and PAM account and session module processing for all authentication types.
638 Because PAM challenge-response authentication usually serves an equivalent
639 role to password authentication, you should disable either
640 .Cm PasswordAuthentication
642 .Cm ChallengeResponseAuthentication.
646 is enabled, you will not be able to run
651 .It Cm UsePrivilegeSeparation
654 separates privileges by creating an unprivileged child process
655 to deal with incoming network traffic.
656 After successful authentication, another process will be created that has
657 the privilege of the authenticated user.
658 The goal of privilege separation is to prevent privilege
659 escalation by containing any corruption within the unprivileged processes.
662 .It Cm X11DisplayOffset
663 Specifies the first display number available for
668 from interfering with real X11 servers.
671 Specifies whether X11 forwarding is permitted.
679 When X11 forwarding is enabled, there may be additional exposure to
680 the server and to client displays if the
682 proxy display is configured to listen on the wildcard address (see
684 below), however this is not the default.
685 Additionally, the authentication spoofing and authentication data
686 verification and substitution occur on the client side.
687 The security risk of using X11 forwarding is that the client's X11
688 display server may be exposed to attack when the ssh client requests
689 forwarding (see the warnings for
693 A system administrator may have a stance in which they want to
694 protect clients that may expose themselves to attack by unwittingly
695 requesting X11 forwarding, which can warrant a
699 Note that disabling X11 forwarding does not prevent users from
700 forwarding X11 traffic, as users can always install their own forwarders.
701 X11 forwarding is automatically disabled if
704 .It Cm X11UseLocalhost
707 should bind the X11 forwarding server to the loopback address or to
708 the wildcard address.
711 binds the forwarding server to the loopback address and sets the
714 environment variable to
716 This prevents remote hosts from connecting to the proxy display.
717 However, some older X11 clients may not function with this
722 to specify that the forwarding server should be bound to the wildcard
731 Specifies the full pathname of the
735 .Pa /usr/X11R6/bin/xauth .
739 command-line arguments and configuration file options that specify time
740 may be expressed using a sequence of the form:
742 .Ar time Op Ar qualifier ,
746 is a positive integer value and
748 is one of the following:
750 .Bl -tag -width Ds -compact -offset indent
765 Each member of the sequence is added together to calculate
766 the total time value.
768 Time format examples:
770 .Bl -tag -width Ds -compact -offset indent
772 600 seconds (10 minutes)
776 1 hour 30 minutes (90 minutes)
780 .It Pa /etc/ssh/sshd_config
781 Contains configuration data for
783 This file should be writable by root only, but it is recommended
784 (though not necessary) that it be world-readable.
789 OpenSSH is a derivative of the original and free
790 ssh 1.2.12 release by Tatu Ylonen.
791 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
792 Theo de Raadt and Dug Song
793 removed many bugs, re-added newer features and
795 Markus Friedl contributed the support for SSH
796 protocol versions 1.5 and 2.0.
797 Niels Provos and Markus Friedl contributed support
798 for privilege separation.