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 1.71 2007/01/02 09:57:25 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon configuration file
45 .Nm /etc/ssh/sshd_config
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
53 The file contains keyword-argument pairs, one per line.
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
59 in order to represent arguments containing spaces.
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
66 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
79 Multiple environment variables may be separated by whitespace or spread
83 Be warned that some environment variables could be used to bypass restricted
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
88 Specifies which address family should be used by
99 This keyword can be followed by a list of group name patterns, separated
101 If specified, login is allowed only for users whose primary
102 group or supplementary group list matches one of the patterns.
103 Only group names are valid; a numerical group ID is not recognized.
104 By default, login is allowed for all groups.
105 The allow/deny directives are processed in the following order:
116 for more information on patterns.
117 .It Cm AllowTcpForwarding
118 Specifies whether TCP forwarding is permitted.
121 Note that disabling TCP forwarding does not improve security unless
122 users are also denied shell access, as they can always install their
125 This keyword can be followed by a list of user name patterns, separated
127 If specified, login is allowed only for user names that
128 match one of the patterns.
129 Only user names are valid; a numerical user ID is not recognized.
130 By default, login is allowed for all users.
131 If the pattern takes the form USER@HOST then USER and HOST
132 are separately checked, restricting logins to particular
133 users from particular hosts.
134 The allow/deny directives are processed in the following order:
145 for more information on patterns.
146 .It Cm AuthorizedKeysFile
147 Specifies the file that contains the public keys that can be used
148 for user authentication.
149 .Cm AuthorizedKeysFile
150 may contain tokens of the form %T which are substituted during connection
152 The following tokens are defined: %% is replaced by a literal '%',
153 %h is replaced by the home directory of the user being authenticated, and
154 %u is replaced by the username of that user.
156 .Cm AuthorizedKeysFile
157 is taken to be an absolute path or one relative to the user's home
160 .Dq .ssh/authorized_keys .
162 In some jurisdictions, sending a warning message before authentication
163 may be relevant for getting legal protection.
164 The contents of the specified file are sent to the remote user before
165 authentication is allowed.
166 This option is only available for protocol version 2.
167 By default, no banner is displayed.
168 .It Cm ChallengeResponseAuthentication
169 Specifies whether challenge-response authentication is allowed.
170 All authentication styles from
176 Specifies the ciphers allowed for protocol version 2.
177 Multiple ciphers must be comma-separated.
178 The supported ciphers are
193 .Bd -literal -offset 3n
194 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
195 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
196 aes192-ctr,aes256-ctr
198 .It Cm ClientAliveCountMax
199 Sets the number of client alive messages (see below) which may be
202 receiving any messages back from the client.
203 If this threshold is reached while client alive messages are being sent,
204 sshd will disconnect the client, terminating the session.
205 It is important to note that the use of client alive messages is very
209 The client alive messages are sent through the encrypted channel
210 and therefore will not be spoofable.
211 The TCP keepalive option enabled by
214 The client alive mechanism is valuable when the client or
215 server depend on knowing when a connection has become inactive.
217 The default value is 3.
219 .Cm ClientAliveInterval
220 (see below) is set to 15, and
221 .Cm ClientAliveCountMax
222 is left at the default, unresponsive SSH clients
223 will be disconnected after approximately 45 seconds.
224 This option applies to protocol version 2 only.
225 .It Cm ClientAliveInterval
226 Sets a timeout interval in seconds after which if no data has been received
229 will send a message through the encrypted
230 channel to request a response from the client.
232 is 0, indicating that these messages will not be sent to the client.
233 This option applies to protocol version 2 only.
235 Specifies whether compression is allowed, or delayed until
236 the user has authenticated successfully.
245 This keyword can be followed by a list of group name patterns, separated
247 Login is disallowed for users whose primary group or supplementary
248 group list matches one of the patterns.
249 Only group names are valid; a numerical group ID is not recognized.
250 By default, login is allowed for all groups.
251 The allow/deny directives are processed in the following order:
262 for more information on patterns.
264 This keyword can be followed by a list of user name patterns, separated
266 Login is disallowed for user names that match one of the patterns.
267 Only user names are valid; a numerical user ID is not recognized.
268 By default, login is allowed for all users.
269 If the pattern takes the form USER@HOST then USER and HOST
270 are separately checked, restricting logins to particular
271 users from particular hosts.
272 The allow/deny directives are processed in the following order:
283 for more information on patterns.
285 Forces the execution of the command specified by
287 ignoring any command supplied by the client.
288 The command is invoked by using the user's login shell with the -c option.
289 This applies to shell, command, or subsystem execution.
290 It is most useful inside a
293 The command originally supplied by the client is available in the
294 .Ev SSH_ORIGINAL_COMMAND
295 environment variable.
297 Specifies whether remote hosts are allowed to connect to ports
298 forwarded for the client.
301 binds remote port forwardings to the loopback address.
302 This prevents other remote hosts from connecting to forwarded ports.
304 can be used to specify that sshd
305 should allow remote port forwardings to bind to non-loopback addresses, thus
306 allowing other hosts to connect.
309 to force remote port forwardings to be available to the local host only,
311 to force remote port forwardings to bind to the wildcard address, or
313 to allow the client to select the address to which the forwarding is bound.
316 .It Cm GSSAPIAuthentication
317 Specifies whether user authentication based on GSSAPI is allowed.
320 Note that this option applies to protocol version 2 only.
321 .It Cm GSSAPICleanupCredentials
322 Specifies whether to automatically destroy the user's credentials cache
326 Note that this option applies to protocol version 2 only.
327 .It Cm HostbasedAuthentication
328 Specifies whether rhosts or /etc/hosts.equiv authentication together
329 with successful public key client host authentication is allowed
330 (host-based authentication).
331 This option is similar to
332 .Cm RhostsRSAAuthentication
333 and applies to protocol version 2 only.
336 .It Cm HostbasedUsesNameFromPacketOnly
337 Specifies whether or not the server will attempt to perform a reverse
338 name lookup when matching the name in the
344 .Cm HostbasedAuthentication .
349 uses the name supplied by the client rather than
350 attempting to resolve the name from the TCP connection itself.
354 Specifies a file containing a private host key
357 .Pa /etc/ssh/ssh_host_key
358 for protocol version 1, and
359 .Pa /etc/ssh/ssh_host_rsa_key
361 .Pa /etc/ssh/ssh_host_dsa_key
362 for protocol version 2.
365 will refuse to use a file if it is group/world-accessible.
366 It is possible to have multiple host key files.
368 keys are used for version 1 and
372 are used for version 2 of the SSH protocol.
378 files will not be used in
379 .Cm RhostsRSAAuthentication
381 .Cm HostbasedAuthentication .
385 .Pa /etc/shosts.equiv
389 .It Cm IgnoreUserKnownHosts
392 should ignore the user's
393 .Pa ~/.ssh/known_hosts
395 .Cm RhostsRSAAuthentication
397 .Cm HostbasedAuthentication .
400 .It Cm KerberosAuthentication
401 Specifies whether the password provided by the user for
402 .Cm PasswordAuthentication
403 will be validated through the Kerberos KDC.
404 To use this option, the server needs a
405 Kerberos servtab which allows the verification of the KDC's identity.
408 .It Cm KerberosGetAFSToken
409 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
410 an AFS token before accessing the user's home directory.
413 .It Cm KerberosOrLocalPasswd
414 If password authentication through Kerberos fails then
415 the password will be validated via any additional local mechanism
420 .It Cm KerberosTicketCleanup
421 Specifies whether to automatically destroy the user's ticket cache
425 .It Cm KeyRegenerationInterval
426 In protocol version 1, the ephemeral server key is automatically regenerated
427 after this many seconds (if it has been used).
428 The purpose of regeneration is to prevent
429 decrypting captured sessions by later breaking into the machine and
431 The key is never stored anywhere.
432 If the value is 0, the key is never regenerated.
433 The default is 3600 (seconds).
435 Specifies the local addresses
438 The following forms may be used:
440 .Bl -item -offset indent -compact
444 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
449 .Ar host No | Ar IPv4_addr No : Ar port
455 .Ar host No | Ar IPv6_addr Oc : Ar port
462 sshd will listen on the address and all prior
465 The default is to listen on all local addresses.
468 options are permitted.
471 options must precede this option for non-port qualified addresses.
472 .It Cm LoginGraceTime
473 The server disconnects after this time if the user has not
474 successfully logged in.
475 If the value is 0, there is no time limit.
476 The default is 120 seconds.
478 Gives the verbosity level that is used when logging messages from
480 The possible values are:
481 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
483 DEBUG and DEBUG1 are equivalent.
484 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
485 Logging with a DEBUG level violates the privacy of users and is not recommended.
487 Specifies the available MAC (message authentication code) algorithms.
488 The MAC algorithm is used in protocol version 2
489 for data integrity protection.
490 Multiple algorithms must be comma-separated.
492 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
494 Introduces a conditional block.
495 If all of the criteria on the
497 line are satisfied, the keywords on the following lines override those
498 set in the global section of the config file, until either another
500 line or the end of the file.
503 are one or more criteria-pattern pairs.
504 The available criteria are
510 Only a subset of keywords may be used on the lines following a
513 Available keywords are
514 .Cm AllowTcpForwarding ,
518 .Cm X11DisplayOffset ,
521 .Cm X11UseLocalHost .
523 Specifies the maximum number of authentication attempts permitted per
525 Once the number of failures reaches half this value,
526 additional failures are logged.
529 Specifies the maximum number of concurrent unauthenticated connections to the
531 Additional connections will be dropped until authentication succeeds or the
533 expires for a connection.
536 Alternatively, random early drop can be enabled by specifying
537 the three colon separated values
541 will refuse connection attempts with a probability of
544 if there are currently
547 unauthenticated connections.
548 The probability increases linearly and all connection attempts
549 are refused if the number of unauthenticated connections reaches
552 .It Cm PasswordAuthentication
553 Specifies whether password authentication is allowed.
556 .It Cm PermitEmptyPasswords
557 When password authentication is allowed, it specifies whether the
558 server allows login to accounts with empty password strings.
562 Specifies the destinations to which TCP port forwarding is permitted.
563 The forwarding specification must be one of the following forms:
565 .Bl -item -offset indent -compact
579 .Ar \&[ IPv6_addr \&] : port
583 Multiple forwards may be specified by separating them with whitespace.
586 can be used to remove all restrictions and permit any forwarding requests.
587 By default all port forwarding requests are permitted.
588 .It Cm PermitRootLogin
589 Specifies whether root can log in using
593 .Dq without-password ,
594 .Dq forced-commands-only ,
600 If this option is set to
601 .Dq without-password ,
602 password authentication is disabled for root.
604 If this option is set to
605 .Dq forced-commands-only ,
606 root login with public key authentication will be allowed,
609 option has been specified
610 (which may be useful for taking remote backups even if root login is
611 normally not allowed).
612 All other authentication methods are disabled for root.
614 If this option is set to
616 root is not allowed to log in.
620 device forwarding is allowed.
636 .It Cm PermitUserEnvironment
638 .Pa ~/.ssh/environment
642 .Pa ~/.ssh/authorized_keys
647 Enabling environment processing may enable users to bypass access
648 restrictions in some configurations using mechanisms such as
651 Specifies the file that contains the process ID of the
654 .Pa /var/run/sshd.pid .
656 Specifies the port number that
660 Multiple options of this type are permitted.
666 should print the date and time of the last user login when a user logs
675 when a user logs in interactively.
676 (On some systems it is also printed by the shell,
682 Specifies the protocol versions
685 The possible values are
689 Multiple versions must be comma-separated.
692 Note that the order of the protocol list does not indicate preference,
693 because the client selects among multiple protocol versions offered
699 .It Cm PubkeyAuthentication
700 Specifies whether public key authentication is allowed.
703 Note that this option applies to protocol version 2 only.
704 .It Cm RhostsRSAAuthentication
705 Specifies whether rhosts or /etc/hosts.equiv authentication together
706 with successful RSA host authentication is allowed.
709 This option applies to protocol version 1 only.
710 .It Cm RSAAuthentication
711 Specifies whether pure RSA authentication is allowed.
714 This option applies to protocol version 1 only.
716 Defines the number of bits in the ephemeral protocol version 1 server key.
717 The minimum value is 512, and the default is 768.
721 should check file modes and ownership of the
722 user's files and home directory before accepting login.
723 This is normally desirable because novices sometimes accidentally leave their
724 directory or files world-writable.
728 Configures an external subsystem (e.g. file transfer daemon).
729 Arguments should be a subsystem name and a command (with optional arguments)
730 to execute upon subsystem request.
735 file transfer subsystem.
736 By default no subsystems are defined.
737 Note that this option applies to protocol version 2 only.
738 .It Cm SyslogFacility
739 Gives the facility code that is used when logging messages from
741 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
742 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
745 Specifies whether the system should send TCP keepalive messages to the
747 If they are sent, death of the connection or crash of one
748 of the machines will be properly noticed.
749 However, this means that
750 connections will die if the route is down temporarily, and some people
752 On the other hand, if TCP keepalives are not sent,
753 sessions may hang indefinitely on the server, leaving
755 users and consuming server resources.
759 (to send TCP keepalive messages), and the server will notice
760 if the network goes down or the client host crashes.
761 This avoids infinitely hanging sessions.
763 To disable TCP keepalive messages, the value should be set to
768 should look up the remote host name and check that
769 the resolved host name for the remote IP address maps back to the
770 very same IP address.
776 is used for interactive login sessions.
781 is never used for remote command execution.
782 Note also, that if this is enabled,
784 will be disabled because
786 does not know how to handle
790 .Cm UsePrivilegeSeparation
791 is specified, it will be disabled after authentication.
793 Enables the Pluggable Authentication Module interface.
796 this will enable PAM authentication using
797 .Cm ChallengeResponseAuthentication
799 .Cm PasswordAuthentication
800 in addition to PAM account and session module processing for all
801 authentication types.
803 Because PAM challenge-response authentication usually serves an equivalent
804 role to password authentication, you should disable either
805 .Cm PasswordAuthentication
807 .Cm ChallengeResponseAuthentication.
811 is enabled, you will not be able to run
816 .It Cm UsePrivilegeSeparation
819 separates privileges by creating an unprivileged child process
820 to deal with incoming network traffic.
821 After successful authentication, another process will be created that has
822 the privilege of the authenticated user.
823 The goal of privilege separation is to prevent privilege
824 escalation by containing any corruption within the unprivileged processes.
827 .It Cm X11DisplayOffset
828 Specifies the first display number available for
831 This prevents sshd from interfering with real X11 servers.
834 Specifies whether X11 forwarding is permitted.
842 When X11 forwarding is enabled, there may be additional exposure to
843 the server and to client displays if the
845 proxy display is configured to listen on the wildcard address (see
847 below), though this is not the default.
848 Additionally, the authentication spoofing and authentication data
849 verification and substitution occur on the client side.
850 The security risk of using X11 forwarding is that the client's X11
851 display server may be exposed to attack when the SSH client requests
852 forwarding (see the warnings for
856 A system administrator may have a stance in which they want to
857 protect clients that may expose themselves to attack by unwittingly
858 requesting X11 forwarding, which can warrant a
862 Note that disabling X11 forwarding does not prevent users from
863 forwarding X11 traffic, as users can always install their own forwarders.
864 X11 forwarding is automatically disabled if
867 .It Cm X11UseLocalhost
870 should bind the X11 forwarding server to the loopback address or to
871 the wildcard address.
873 sshd binds the forwarding server to the loopback address and sets the
876 environment variable to
878 This prevents remote hosts from connecting to the proxy display.
879 However, some older X11 clients may not function with this
884 to specify that the forwarding server should be bound to the wildcard
893 Specifies the full pathname of the
897 .Pa /usr/X11R6/bin/xauth .
901 command-line arguments and configuration file options that specify time
902 may be expressed using a sequence of the form:
904 .Ar time Op Ar qualifier ,
908 is a positive integer value and
910 is one of the following:
912 .Bl -tag -width Ds -compact -offset indent
927 Each member of the sequence is added together to calculate
928 the total time value.
930 Time format examples:
932 .Bl -tag -width Ds -compact -offset indent
934 600 seconds (10 minutes)
938 1 hour 30 minutes (90 minutes)
942 .It Pa /etc/ssh/sshd_config
943 Contains configuration data for
945 This file should be writable by root only, but it is recommended
946 (though not necessary) that it be world-readable.
951 OpenSSH is a derivative of the original and free
952 ssh 1.2.12 release by Tatu Ylonen.
953 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
954 Theo de Raadt and Dug Song
955 removed many bugs, re-added newer features and
957 Markus Friedl contributed the support for SSH
958 protocol versions 1.5 and 2.0.
959 Niels Provos and Markus Friedl contributed support
960 for privilege separation.