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 1.70 2006/08/21 08:14:01 dtucker 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.
60 Arguments may optionally be enclosed in double quotes
62 in order to represent arguments containing spaces.
65 keywords and their meanings are as follows (note that
66 keywords are case-insensitive and arguments are case-sensitive):
69 Specifies what environment variables sent by the client will be copied into
76 for how to configure the client.
77 Note that environment passing is only supported for protocol 2.
78 Variables are specified by name, which may contain the wildcard characters
82 Multiple environment variables may be separated by whitespace or spread
86 Be warned that some environment variables could be used to bypass restricted
88 For this reason, care should be taken in the use of this directive.
89 The default is not to accept any environment variables.
91 Specifies which address family should be used by
102 This keyword can be followed by a list of group name patterns, separated
104 If specified, login is allowed only for users whose primary
105 group or supplementary group list matches one of the patterns.
106 Only group names are valid; a numerical group ID is not recognized.
107 By default, login is allowed for all groups.
108 The allow/deny directives are processed in the following order:
119 for more information on patterns.
120 .It Cm AllowTcpForwarding
121 Specifies whether TCP forwarding is permitted.
124 Note that disabling TCP forwarding does not improve security unless
125 users are also denied shell access, as they can always install their
128 This keyword can be followed by a list of user name patterns, separated
130 If specified, login is allowed only for user names that
131 match one of the patterns.
132 Only user names are valid; a numerical user ID is not recognized.
133 By default, login is allowed for all users.
134 If the pattern takes the form USER@HOST then USER and HOST
135 are separately checked, restricting logins to particular
136 users from particular hosts.
137 The allow/deny directives are processed in the following order:
148 for more information on patterns.
149 .It Cm AuthorizedKeysFile
150 Specifies the file that contains the public keys that can be used
151 for user authentication.
152 .Cm AuthorizedKeysFile
153 may contain tokens of the form %T which are substituted during connection
155 The following tokens are defined: %% is replaced by a literal '%',
156 %h is replaced by the home directory of the user being authenticated, and
157 %u is replaced by the username of that user.
159 .Cm AuthorizedKeysFile
160 is taken to be an absolute path or one relative to the user's home
163 .Dq .ssh/authorized_keys .
165 In some jurisdictions, sending a warning message before authentication
166 may be relevant for getting legal protection.
167 The contents of the specified file are sent to the remote user before
168 authentication is allowed.
169 This option is only available for protocol version 2.
170 By default, no banner is displayed.
171 .It Cm ChallengeResponseAuthentication
172 Specifies whether challenge-response authentication is allowed.
175 this controls the use of PAM (see
178 Note that this affects the effectiveness of the
179 .Cm PasswordAuthentication
186 Specifies the ciphers allowed for protocol version 2.
187 Multiple ciphers must be comma-separated.
188 The supported ciphers are
203 .Bd -literal -offset 3n
204 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
205 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
206 aes192-ctr,aes256-ctr
208 .It Cm ClientAliveCountMax
209 Sets the number of client alive messages (see below) which may be
212 receiving any messages back from the client.
213 If this threshold is reached while client alive messages are being sent,
214 sshd will disconnect the client, terminating the session.
215 It is important to note that the use of client alive messages is very
219 The client alive messages are sent through the encrypted channel
220 and therefore will not be spoofable.
221 The TCP keepalive option enabled by
224 The client alive mechanism is valuable when the client or
225 server depend on knowing when a connection has become inactive.
227 The default value is 3.
229 .Cm ClientAliveInterval
230 (see below) is set to 15, and
231 .Cm ClientAliveCountMax
232 is left at the default, unresponsive SSH clients
233 will be disconnected after approximately 45 seconds.
234 This option applies to protocol version 2 only.
235 .It Cm ClientAliveInterval
236 Sets a timeout interval in seconds after which if no data has been received
239 will send a message through the encrypted
240 channel to request a response from the client.
242 is 0, indicating that these messages will not be sent to the client.
243 This option applies to protocol version 2 only.
245 Specifies whether compression is allowed, or delayed until
246 the user has authenticated successfully.
255 This keyword can be followed by a list of group name patterns, separated
257 Login is disallowed for users whose primary group or supplementary
258 group list matches one of the patterns.
259 Only group names are valid; a numerical group ID is not recognized.
260 By default, login is allowed for all groups.
261 The allow/deny directives are processed in the following order:
272 for more information on patterns.
274 This keyword can be followed by a list of user name patterns, separated
276 Login is disallowed for user names that match one of the patterns.
277 Only user names are valid; a numerical user ID is not recognized.
278 By default, login is allowed for all users.
279 If the pattern takes the form USER@HOST then USER and HOST
280 are separately checked, restricting logins to particular
281 users from particular hosts.
282 The allow/deny directives are processed in the following order:
293 for more information on patterns.
295 Forces the execution of the command specified by
297 ignoring any command supplied by the client.
298 The command is invoked by using the user's login shell with the -c option.
299 This applies to shell, command, or subsystem execution.
300 It is most useful inside a
303 The command originally supplied by the client is available in the
304 .Ev SSH_ORIGINAL_COMMAND
305 environment variable.
307 Specifies whether remote hosts are allowed to connect to ports
308 forwarded for the client.
311 binds remote port forwardings to the loopback address.
312 This prevents other remote hosts from connecting to forwarded ports.
314 can be used to specify that sshd
315 should allow remote port forwardings to bind to non-loopback addresses, thus
316 allowing other hosts to connect.
319 to force remote port forwardings to be available to the local host only,
321 to force remote port forwardings to bind to the wildcard address, or
323 to allow the client to select the address to which the forwarding is bound.
326 .It Cm GSSAPIAuthentication
327 Specifies whether user authentication based on GSSAPI is allowed.
330 Note that this option applies to protocol version 2 only.
331 .It Cm GSSAPICleanupCredentials
332 Specifies whether to automatically destroy the user's credentials cache
336 Note that this option applies to protocol version 2 only.
337 .It Cm HostbasedAuthentication
338 Specifies whether rhosts or /etc/hosts.equiv authentication together
339 with successful public key client host authentication is allowed
340 (host-based authentication).
341 This option is similar to
342 .Cm RhostsRSAAuthentication
343 and applies to protocol version 2 only.
346 .It Cm HostbasedUsesNameFromPacketOnly
347 Specifies whether or not the server will attempt to perform a reverse
348 name lookup when matching the name in the
354 .Cm HostbasedAuthentication .
359 uses the name supplied by the client rather than
360 attempting to resolve the name from the TCP connection itself.
364 Specifies a file containing a private host key
367 .Pa /etc/ssh/ssh_host_key
368 for protocol version 1, and
369 .Pa /etc/ssh/ssh_host_dsa_key
370 for protocol version 2.
373 will refuse to use a file if it is group/world-accessible.
374 It is possible to have multiple host key files.
376 keys are used for version 1 and
380 are used for version 2 of the SSH protocol.
386 files will not be used in
387 .Cm RhostsRSAAuthentication
389 .Cm HostbasedAuthentication .
393 .Pa /etc/ssh/shosts.equiv
397 .It Cm IgnoreUserKnownHosts
400 should ignore the user's
401 .Pa ~/.ssh/known_hosts
403 .Cm RhostsRSAAuthentication
405 .Cm HostbasedAuthentication .
408 .It Cm KerberosAuthentication
409 Specifies whether the password provided by the user for
410 .Cm PasswordAuthentication
411 will be validated through the Kerberos KDC.
412 To use this option, the server needs a
413 Kerberos servtab which allows the verification of the KDC's identity.
416 .It Cm KerberosGetAFSToken
417 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
418 an AFS token before accessing the user's home directory.
421 .It Cm KerberosOrLocalPasswd
422 If password authentication through Kerberos fails then
423 the password will be validated via any additional local mechanism
428 .It Cm KerberosTicketCleanup
429 Specifies whether to automatically destroy the user's ticket cache
433 .It Cm KeyRegenerationInterval
434 In protocol version 1, the ephemeral server key is automatically regenerated
435 after this many seconds (if it has been used).
436 The purpose of regeneration is to prevent
437 decrypting captured sessions by later breaking into the machine and
439 The key is never stored anywhere.
440 If the value is 0, the key is never regenerated.
441 The default is 3600 (seconds).
443 Specifies the local addresses
446 The following forms may be used:
448 .Bl -item -offset indent -compact
452 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
457 .Ar host No | Ar IPv4_addr No : Ar port
463 .Ar host No | Ar IPv6_addr Oc : Ar port
470 sshd will listen on the address and all prior
473 The default is to listen on all local addresses.
476 options are permitted.
479 options must precede this option for non-port qualified addresses.
480 .It Cm LoginGraceTime
481 The server disconnects after this time if the user has not
482 successfully logged in.
483 If the value is 0, there is no time limit.
484 The default is 120 seconds.
486 Gives the verbosity level that is used when logging messages from
488 The possible values are:
489 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
491 DEBUG and DEBUG1 are equivalent.
492 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
493 Logging with a DEBUG level violates the privacy of users and is not recommended.
495 Specifies the available MAC (message authentication code) algorithms.
496 The MAC algorithm is used in protocol version 2
497 for data integrity protection.
498 Multiple algorithms must be comma-separated.
500 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
502 Introduces a conditional block.
503 If all of the criteria on the
505 line are satisfied, the keywords on the following lines override those
506 set in the global section of the config file, until either another
508 line or the end of the file.
511 are one or more criteria-pattern pairs.
512 The available criteria are
518 Only a subset of keywords may be used on the lines following a
521 Available keywords are
522 .Cm AllowTcpForwarding ,
526 .Cm X11DisplayOffset ,
529 .Cm X11UseLocalHost .
531 Specifies the maximum number of authentication attempts permitted per
533 Once the number of failures reaches half this value,
534 additional failures are logged.
537 Specifies the maximum number of concurrent unauthenticated connections to the
539 Additional connections will be dropped until authentication succeeds or the
541 expires for a connection.
544 Alternatively, random early drop can be enabled by specifying
545 the three colon separated values
549 will refuse connection attempts with a probability of
552 if there are currently
555 unauthenticated connections.
556 The probability increases linearly and all connection attempts
557 are refused if the number of unauthenticated connections reaches
560 .It Cm PasswordAuthentication
561 Specifies whether password authentication is allowed.
566 was built without PAM support, in which case the default is
569 .Cm ChallengeResponseAuthentication
572 and the PAM authentication policy for
576 password authentication will be allowed through the challenge-response
577 mechanism regardless of the value of
578 .Cm PasswordAuthentication .
579 .It Cm PermitEmptyPasswords
580 When password authentication is allowed, it specifies whether the
581 server allows login to accounts with empty password strings.
585 Specifies the destinations to which TCP port forwarding is permitted.
586 The forwarding specification must be one of the following forms:
588 .Bl -item -offset indent -compact
602 .Ar \&[ IPv6_addr \&] : port
606 Multiple forwards may be specified by separating them with whitespace.
609 can be used to remove all restrictions and permit any forwarding requests.
610 By default all port forwarding requests are permitted.
611 .It Cm PermitRootLogin
612 Specifies whether root can log in using
616 .Dq without-password ,
617 .Dq forced-commands-only ,
623 .Cm ChallengeResponseAuthentication
626 the root user may be allowed in with its password even if
627 .Cm PermitRootLogin is set to
628 .Dq without-password .
630 If this option is set to
631 .Dq without-password ,
632 password authentication is disabled for root.
634 If this option is set to
635 .Dq forced-commands-only ,
636 root login with public key authentication will be allowed,
639 option has been specified
640 (which may be useful for taking remote backups even if root login is
641 normally not allowed).
642 All other authentication methods are disabled for root.
644 If this option is set to
646 root is not allowed to log in.
650 device forwarding is allowed.
666 .It Cm PermitUserEnvironment
668 .Pa ~/.ssh/environment
672 .Pa ~/.ssh/authorized_keys
677 Enabling environment processing may enable users to bypass access
678 restrictions in some configurations using mechanisms such as
681 Specifies the file that contains the process ID of the
684 .Pa /var/run/sshd.pid .
686 Specifies the port number that
690 Multiple options of this type are permitted.
696 should print the date and time of the last user login when a user logs
705 when a user logs in interactively.
706 (On some systems it is also printed by the shell,
712 Specifies the protocol versions
715 The possible values are
719 Multiple versions must be comma-separated.
722 Note that the order of the protocol list does not indicate preference,
723 because the client selects among multiple protocol versions offered
729 .It Cm PubkeyAuthentication
730 Specifies whether public key authentication is allowed.
733 Note that this option applies to protocol version 2 only.
734 .It Cm RhostsRSAAuthentication
735 Specifies whether rhosts or
737 authentication together
738 with successful RSA host authentication is allowed.
741 This option applies to protocol version 1 only.
742 .It Cm RSAAuthentication
743 Specifies whether pure RSA authentication is allowed.
746 This option applies to protocol version 1 only.
748 Defines the number of bits in the ephemeral protocol version 1 server key.
749 The minimum value is 512, and the default is 768.
753 should check file modes and ownership of the
754 user's files and home directory before accepting login.
755 This is normally desirable because novices sometimes accidentally leave their
756 directory or files world-writable.
760 Configures an external subsystem (e.g. file transfer daemon).
761 Arguments should be a subsystem name and a command (with optional arguments)
762 to execute upon subsystem request.
767 file transfer subsystem.
768 By default no subsystems are defined.
769 Note that this option applies to protocol version 2 only.
770 .It Cm SyslogFacility
771 Gives the facility code that is used when logging messages from
773 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
774 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
777 Specifies whether the system should send TCP keepalive messages to the
779 If they are sent, death of the connection or crash of one
780 of the machines will be properly noticed.
781 However, this means that
782 connections will die if the route is down temporarily, and some people
784 On the other hand, if TCP keepalives are not sent,
785 sessions may hang indefinitely on the server, leaving
787 users and consuming server resources.
791 (to send TCP keepalive messages), and the server will notice
792 if the network goes down or the client host crashes.
793 This avoids infinitely hanging sessions.
795 To disable TCP keepalive messages, the value should be set to
800 should look up the remote host name and check that
801 the resolved host name for the remote IP address maps back to the
802 very same IP address.
808 is used for interactive login sessions.
813 is never used for remote command execution.
814 Note also, that if this is enabled,
816 will be disabled because
818 does not know how to handle
822 .Cm UsePrivilegeSeparation
823 is specified, it will be disabled after authentication.
825 Enables the Pluggable Authentication Module interface.
828 this will enable PAM authentication using
829 .Cm ChallengeResponseAuthentication
831 .Cm PasswordAuthentication
832 in addition to PAM account and session module processing for all
833 authentication types.
835 Because PAM challenge-response authentication usually serves an equivalent
836 role to password authentication, you should disable either
837 .Cm PasswordAuthentication
839 .Cm ChallengeResponseAuthentication.
843 is enabled, you will not be able to run
848 .It Cm UsePrivilegeSeparation
851 separates privileges by creating an unprivileged child process
852 to deal with incoming network traffic.
853 After successful authentication, another process will be created that has
854 the privilege of the authenticated user.
855 The goal of privilege separation is to prevent privilege
856 escalation by containing any corruption within the unprivileged processes.
859 .It Cm VersionAddendum
860 Specifies a string to append to the regular version string to identify
861 OS- or site-specific modifications.
863 .Dq FreeBSD-20061110 .
864 .It Cm X11DisplayOffset
865 Specifies the first display number available for
868 This prevents sshd from interfering with real X11 servers.
871 Specifies whether X11 forwarding is permitted.
879 When X11 forwarding is enabled, there may be additional exposure to
880 the server and to client displays if the
882 proxy display is configured to listen on the wildcard address (see
884 below), though this is not the default.
885 Additionally, the authentication spoofing and authentication data
886 verification and substitution occur on the client side.
887 The security risk of using X11 forwarding is that the client's X11
888 display server may be exposed to attack when the SSH client requests
889 forwarding (see the warnings for
893 A system administrator may have a stance in which they want to
894 protect clients that may expose themselves to attack by unwittingly
895 requesting X11 forwarding, which can warrant a
899 Note that disabling X11 forwarding does not prevent users from
900 forwarding X11 traffic, as users can always install their own forwarders.
901 X11 forwarding is automatically disabled if
904 .It Cm X11UseLocalhost
907 should bind the X11 forwarding server to the loopback address or to
908 the wildcard address.
910 sshd binds the forwarding server to the loopback address and sets the
913 environment variable to
915 This prevents remote hosts from connecting to the proxy display.
916 However, some older X11 clients may not function with this
921 to specify that the forwarding server should be bound to the wildcard
930 Specifies the full pathname of the
934 .Pa /usr/local/bin/xauth .
938 command-line arguments and configuration file options that specify time
939 may be expressed using a sequence of the form:
941 .Ar time Op Ar qualifier ,
945 is a positive integer value and
947 is one of the following:
949 .Bl -tag -width Ds -compact -offset indent
964 Each member of the sequence is added together to calculate
965 the total time value.
967 Time format examples:
969 .Bl -tag -width Ds -compact -offset indent
971 600 seconds (10 minutes)
975 1 hour 30 minutes (90 minutes)
979 .It Pa /etc/ssh/sshd_config
980 Contains configuration data for
982 This file should be writable by root only, but it is recommended
983 (though not necessary) that it be world-readable.
988 OpenSSH is a derivative of the original and free
989 ssh 1.2.12 release by Tatu Ylonen.
990 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
991 Theo de Raadt and Dug Song
992 removed many bugs, re-added newer features and
994 Markus Friedl contributed the support for SSH
995 protocol versions 1.5 and 2.0.
996 Niels Provos and Markus Friedl contributed support
997 for privilege separation.