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 1.65 2006/07/18 08:22:23 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.
59 Arguments may optionally be enclosed in double quotes
61 in order to represent arguments containing spaces.
64 keywords and their meanings are as follows (note that
65 keywords are case-insensitive and arguments are case-sensitive):
68 Specifies what environment variables sent by the client will be copied into
75 for how to configure the client.
76 Note that environment passing is only supported for protocol 2.
77 Variables are specified by name, which may contain the wildcard characters
81 Multiple environment variables may be separated by whitespace or spread
85 Be warned that some environment variables could be used to bypass restricted
87 For this reason, care should be taken in the use of this directive.
88 The default is not to accept any environment variables.
90 Specifies which address family should be used by
101 This keyword can be followed by a list of group name patterns, separated
103 If specified, login is allowed only for users whose primary
104 group or supplementary group list matches one of the patterns.
105 Only group names are valid; a numerical group ID is not recognized.
106 By default, login is allowed for all groups.
107 The allow/deny directives are processed in the following order:
118 for more information on patterns.
119 .It Cm AllowTcpForwarding
120 Specifies whether TCP forwarding is permitted.
123 Note that disabling TCP forwarding does not improve security unless
124 users are also denied shell access, as they can always install their
127 This keyword can be followed by a list of user name patterns, separated
129 If specified, login is allowed only for user names that
130 match one of the patterns.
131 Only user names are valid; a numerical user ID is not recognized.
132 By default, login is allowed for all users.
133 If the pattern takes the form USER@HOST then USER and HOST
134 are separately checked, restricting logins to particular
135 users from particular hosts.
136 The allow/deny directives are processed in the following order:
147 for more information on patterns.
148 .It Cm AuthorizedKeysFile
149 Specifies the file that contains the public keys that can be used
150 for user authentication.
151 .Cm AuthorizedKeysFile
152 may contain tokens of the form %T which are substituted during connection
154 The following tokens are defined: %% is replaced by a literal '%',
155 %h is replaced by the home directory of the user being authenticated, and
156 %u is replaced by the username of that user.
158 .Cm AuthorizedKeysFile
159 is taken to be an absolute path or one relative to the user's home
162 .Dq .ssh/authorized_keys .
164 In some jurisdictions, sending a warning message before authentication
165 may be relevant for getting legal protection.
166 The contents of the specified file are sent to the remote user before
167 authentication is allowed.
168 This option is only available for protocol version 2.
169 By default, no banner is displayed.
170 .It Cm ChallengeResponseAuthentication
171 Specifies whether challenge-response authentication is allowed.
172 All authentication styles from
178 Specifies the ciphers allowed for protocol version 2.
179 Multiple ciphers must be comma-separated.
180 The supported ciphers are
195 .Bd -literal -offset 3n
196 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
197 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
198 aes192-ctr,aes256-ctr
200 .It Cm ClientAliveCountMax
201 Sets the number of client alive messages (see below) which may be
204 receiving any messages back from the client.
205 If this threshold is reached while client alive messages are being sent,
206 sshd will disconnect the client, terminating the session.
207 It is important to note that the use of client alive messages is very
211 The client alive messages are sent through the encrypted channel
212 and therefore will not be spoofable.
213 The TCP keepalive option enabled by
216 The client alive mechanism is valuable when the client or
217 server depend on knowing when a connection has become inactive.
219 The default value is 3.
221 .Cm ClientAliveInterval
222 (see below) is set to 15, and
223 .Cm ClientAliveCountMax
224 is left at the default, unresponsive SSH clients
225 will be disconnected after approximately 45 seconds.
226 This option applies to protocol version 2 only.
227 .It Cm ClientAliveInterval
228 Sets a timeout interval in seconds after which if no data has been received
231 will send a message through the encrypted
232 channel to request a response from the client.
234 is 0, indicating that these messages will not be sent to the client.
235 This option applies to protocol version 2 only.
237 Specifies whether compression is allowed, or delayed until
238 the user has authenticated successfully.
247 This keyword can be followed by a list of group name patterns, separated
249 Login is disallowed for users whose primary group or supplementary
250 group list matches one of the patterns.
251 Only group names are valid; a numerical group ID is not recognized.
252 By default, login is allowed for all groups.
253 The allow/deny directives are processed in the following order:
264 for more information on patterns.
266 This keyword can be followed by a list of user name patterns, separated
268 Login is disallowed for user names that match one of the patterns.
269 Only user names are valid; a numerical user ID is not recognized.
270 By default, login is allowed for all users.
271 If the pattern takes the form USER@HOST then USER and HOST
272 are separately checked, restricting logins to particular
273 users from particular hosts.
274 The allow/deny directives are processed in the following order:
285 for more information on patterns.
287 Specifies whether remote hosts are allowed to connect to ports
288 forwarded for the client.
291 binds remote port forwardings to the loopback address.
292 This prevents other remote hosts from connecting to forwarded ports.
294 can be used to specify that sshd
295 should allow remote port forwardings to bind to non-loopback addresses, thus
296 allowing other hosts to connect.
299 to force remote port forwardings to be available to the local host only,
301 to force remote port forwardings to bind to the wildcard address, or
303 to allow the client to select the address to which the forwarding is bound.
306 .It Cm GSSAPIAuthentication
307 Specifies whether user authentication based on GSSAPI is allowed.
310 Note that this option applies to protocol version 2 only.
311 .It Cm GSSAPICleanupCredentials
312 Specifies whether to automatically destroy the user's credentials cache
316 Note that this option applies to protocol version 2 only.
317 .It Cm HostbasedAuthentication
318 Specifies whether rhosts or /etc/hosts.equiv authentication together
319 with successful public key client host authentication is allowed
320 (host-based authentication).
321 This option is similar to
322 .Cm RhostsRSAAuthentication
323 and applies to protocol version 2 only.
327 Specifies a file containing a private host key
330 .Pa /etc/ssh/ssh_host_key
331 for protocol version 1, and
332 .Pa /etc/ssh/ssh_host_rsa_key
334 .Pa /etc/ssh/ssh_host_dsa_key
335 for protocol version 2.
338 will refuse to use a file if it is group/world-accessible.
339 It is possible to have multiple host key files.
341 keys are used for version 1 and
345 are used for version 2 of the SSH protocol.
351 files will not be used in
352 .Cm RhostsRSAAuthentication
354 .Cm HostbasedAuthentication .
358 .Pa /etc/shosts.equiv
362 .It Cm IgnoreUserKnownHosts
365 should ignore the user's
366 .Pa ~/.ssh/known_hosts
368 .Cm RhostsRSAAuthentication
370 .Cm HostbasedAuthentication .
373 .It Cm KerberosAuthentication
374 Specifies whether the password provided by the user for
375 .Cm PasswordAuthentication
376 will be validated through the Kerberos KDC.
377 To use this option, the server needs a
378 Kerberos servtab which allows the verification of the KDC's identity.
381 .It Cm KerberosGetAFSToken
382 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
383 an AFS token before accessing the user's home directory.
386 .It Cm KerberosOrLocalPasswd
387 If password authentication through Kerberos fails then
388 the password will be validated via any additional local mechanism
393 .It Cm KerberosTicketCleanup
394 Specifies whether to automatically destroy the user's ticket cache
398 .It Cm KeyRegenerationInterval
399 In protocol version 1, the ephemeral server key is automatically regenerated
400 after this many seconds (if it has been used).
401 The purpose of regeneration is to prevent
402 decrypting captured sessions by later breaking into the machine and
404 The key is never stored anywhere.
405 If the value is 0, the key is never regenerated.
406 The default is 3600 (seconds).
408 Specifies the local addresses
411 The following forms may be used:
413 .Bl -item -offset indent -compact
417 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
422 .Ar host No | Ar IPv4_addr No : Ar port
428 .Ar host No | Ar IPv6_addr Oc : Ar port
435 sshd will listen on the address and all prior
438 The default is to listen on all local addresses.
441 options are permitted.
444 options must precede this option for non-port qualified addresses.
445 .It Cm LoginGraceTime
446 The server disconnects after this time if the user has not
447 successfully logged in.
448 If the value is 0, there is no time limit.
449 The default is 120 seconds.
451 Gives the verbosity level that is used when logging messages from
453 The possible values are:
454 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
456 DEBUG and DEBUG1 are equivalent.
457 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
458 Logging with a DEBUG level violates the privacy of users and is not recommended.
460 Specifies the available MAC (message authentication code) algorithms.
461 The MAC algorithm is used in protocol version 2
462 for data integrity protection.
463 Multiple algorithms must be comma-separated.
465 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
467 Introduces a conditional block.
468 If all of the criteria on the
470 line are satisfied, the keywords on the following lines override those
471 set in the global section of the config file, until either another
473 line or the end of the file.
476 are one or more criteria-pattern pairs.
477 The available criteria are
482 Only a subset of keywords may be used on the lines following a
485 Available keywords are
486 .Cm AllowTcpForwarding ,
491 Specifies the maximum number of authentication attempts permitted per
493 Once the number of failures reaches half this value,
494 additional failures are logged.
497 Specifies the maximum number of concurrent unauthenticated connections to the
499 Additional connections will be dropped until authentication succeeds or the
501 expires for a connection.
504 Alternatively, random early drop can be enabled by specifying
505 the three colon separated values
509 will refuse connection attempts with a probability of
512 if there are currently
515 unauthenticated connections.
516 The probability increases linearly and all connection attempts
517 are refused if the number of unauthenticated connections reaches
520 .It Cm PasswordAuthentication
521 Specifies whether password authentication is allowed.
524 .It Cm PermitEmptyPasswords
525 When password authentication is allowed, it specifies whether the
526 server allows login to accounts with empty password strings.
530 Specifies the destinations to which TCP port forwarding is permitted.
531 The forwarding specification must be one of the following forms:
533 .Bl -item -offset indent -compact
547 .Ar \&[ IPv6_addr \&] : port
551 Multiple instances of
556 can be used to remove all restrictions and permit any forwarding requests.
557 By default all port forwarding requests are permitted.
558 .It Cm PermitRootLogin
559 Specifies whether root can log in using
563 .Dq without-password ,
564 .Dq forced-commands-only ,
570 If this option is set to
571 .Dq without-password ,
572 password authentication is disabled for root.
574 If this option is set to
575 .Dq forced-commands-only ,
576 root login with public key authentication will be allowed,
579 option has been specified
580 (which may be useful for taking remote backups even if root login is
581 normally not allowed).
582 All other authentication methods are disabled for root.
584 If this option is set to
586 root is not allowed to log in.
590 device forwarding is allowed.
606 .It Cm PermitUserEnvironment
608 .Pa ~/.ssh/environment
612 .Pa ~/.ssh/authorized_keys
617 Enabling environment processing may enable users to bypass access
618 restrictions in some configurations using mechanisms such as
621 Specifies the file that contains the process ID of the
624 .Pa /var/run/sshd.pid .
626 Specifies the port number that
630 Multiple options of this type are permitted.
636 should print the date and time of the last user login when a user logs
645 when a user logs in interactively.
646 (On some systems it is also printed by the shell,
652 Specifies the protocol versions
655 The possible values are
659 Multiple versions must be comma-separated.
662 Note that the order of the protocol list does not indicate preference,
663 because the client selects among multiple protocol versions offered
669 .It Cm PubkeyAuthentication
670 Specifies whether public key authentication is allowed.
673 Note that this option applies to protocol version 2 only.
674 .It Cm RhostsRSAAuthentication
675 Specifies whether rhosts or /etc/hosts.equiv authentication together
676 with successful RSA host authentication is allowed.
679 This option applies to protocol version 1 only.
680 .It Cm RSAAuthentication
681 Specifies whether pure RSA authentication is allowed.
684 This option applies to protocol version 1 only.
686 Defines the number of bits in the ephemeral protocol version 1 server key.
687 The minimum value is 512, and the default is 768.
691 should check file modes and ownership of the
692 user's files and home directory before accepting login.
693 This is normally desirable because novices sometimes accidentally leave their
694 directory or files world-writable.
698 Configures an external subsystem (e.g. file transfer daemon).
699 Arguments should be a subsystem name and a command (with optional arguments)
700 to execute upon subsystem request.
705 file transfer subsystem.
706 By default no subsystems are defined.
707 Note that this option applies to protocol version 2 only.
708 .It Cm SyslogFacility
709 Gives the facility code that is used when logging messages from
711 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
712 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
715 Specifies whether the system should send TCP keepalive messages to the
717 If they are sent, death of the connection or crash of one
718 of the machines will be properly noticed.
719 However, this means that
720 connections will die if the route is down temporarily, and some people
722 On the other hand, if TCP keepalives are not sent,
723 sessions may hang indefinitely on the server, leaving
725 users and consuming server resources.
729 (to send TCP keepalive messages), and the server will notice
730 if the network goes down or the client host crashes.
731 This avoids infinitely hanging sessions.
733 To disable TCP keepalive messages, the value should be set to
738 should look up the remote host name and check that
739 the resolved host name for the remote IP address maps back to the
740 very same IP address.
746 is used for interactive login sessions.
751 is never used for remote command execution.
752 Note also, that if this is enabled,
754 will be disabled because
756 does not know how to handle
760 .Cm UsePrivilegeSeparation
761 is specified, it will be disabled after authentication.
763 Enables the Pluggable Authentication Module interface.
766 this will enable PAM authentication using
767 .Cm ChallengeResponseAuthentication
769 .Cm PasswordAuthentication
770 in addition to PAM account and session module processing for all
771 authentication types.
773 Because PAM challenge-response authentication usually serves an equivalent
774 role to password authentication, you should disable either
775 .Cm PasswordAuthentication
777 .Cm ChallengeResponseAuthentication.
781 is enabled, you will not be able to run
786 .It Cm UsePrivilegeSeparation
789 separates privileges by creating an unprivileged child process
790 to deal with incoming network traffic.
791 After successful authentication, another process will be created that has
792 the privilege of the authenticated user.
793 The goal of privilege separation is to prevent privilege
794 escalation by containing any corruption within the unprivileged processes.
797 .It Cm X11DisplayOffset
798 Specifies the first display number available for
801 This prevents sshd from interfering with real X11 servers.
804 Specifies whether X11 forwarding is permitted.
812 When X11 forwarding is enabled, there may be additional exposure to
813 the server and to client displays if the
815 proxy display is configured to listen on the wildcard address (see
817 below), though this is not the default.
818 Additionally, the authentication spoofing and authentication data
819 verification and substitution occur on the client side.
820 The security risk of using X11 forwarding is that the client's X11
821 display server may be exposed to attack when the SSH client requests
822 forwarding (see the warnings for
826 A system administrator may have a stance in which they want to
827 protect clients that may expose themselves to attack by unwittingly
828 requesting X11 forwarding, which can warrant a
832 Note that disabling X11 forwarding does not prevent users from
833 forwarding X11 traffic, as users can always install their own forwarders.
834 X11 forwarding is automatically disabled if
837 .It Cm X11UseLocalhost
840 should bind the X11 forwarding server to the loopback address or to
841 the wildcard address.
843 sshd binds the forwarding server to the loopback address and sets the
846 environment variable to
848 This prevents remote hosts from connecting to the proxy display.
849 However, some older X11 clients may not function with this
854 to specify that the forwarding server should be bound to the wildcard
863 Specifies the full pathname of the
867 .Pa /usr/X11R6/bin/xauth .
871 command-line arguments and configuration file options that specify time
872 may be expressed using a sequence of the form:
874 .Ar time Op Ar qualifier ,
878 is a positive integer value and
880 is one of the following:
882 .Bl -tag -width Ds -compact -offset indent
897 Each member of the sequence is added together to calculate
898 the total time value.
900 Time format examples:
902 .Bl -tag -width Ds -compact -offset indent
904 600 seconds (10 minutes)
908 1 hour 30 minutes (90 minutes)
912 .It Pa /etc/ssh/sshd_config
913 Contains configuration data for
915 This file should be writable by root only, but it is recommended
916 (though not necessary) that it be world-readable.
921 OpenSSH is a derivative of the original and free
922 ssh 1.2.12 release by Tatu Ylonen.
923 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
924 Theo de Raadt and Dug Song
925 removed many bugs, re-added newer features and
927 Markus Friedl contributed the support for SSH
928 protocol versions 1.5 and 2.0.
929 Niels Provos and Markus Friedl contributed support
930 for privilege separation.