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 1.77 2007/06/08 07:48:09 jmc Exp $
38 .Dd $Mdocdate: June 11 2007 $
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 GSSAPIKeyExchange
322 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
323 doesn't rely on ssh keys to verify host identity.
326 Note that this option applies to protocol version 2 only.
327 .It Cm GSSAPICleanupCredentials
328 Specifies whether to automatically destroy the user's credentials cache
332 Note that this option applies to protocol version 2 only.
333 .It Cm GSSAPIStrictAcceptorCheck
334 Determines whether to be strict about the identity of the GSSAPI acceptor
335 a client authenticates against. If
337 then the client must authenticate against the
339 service on the current hostname. If
341 then the client may authenticate against any service key stored in the
342 machine's default store. This facility is provided to assist with operation
343 on multi homed machines.
346 Note that this option applies only to protocol version 2 GSSAPI connections,
349 may only work with recent Kerberos GSSAPI libraries.
350 .It Cm GSSAPICredentialsPath
351 If specified, the delegated GSSAPI credential is stored in the
352 given path, overwriting any existing credentials.
353 Paths can be specified with syntax similar to the AuthorizedKeysFile
354 option (i.e., accepting %h and %u tokens).
355 When using this option,
356 setting 'GssapiCleanupCredentials no' is recommended,
357 so logging out of one session
358 doesn't remove the credentials in use by another session of
360 Currently only implemented for the GSI mechanism.
361 .It Cm GSIAllowLimitedProxy
362 Specifies whether to accept limited proxy credentials for
366 .It Cm HostbasedAuthentication
367 Specifies whether rhosts or /etc/hosts.equiv authentication together
368 with successful public key client host authentication is allowed
369 (host-based authentication).
370 This option is similar to
371 .Cm RhostsRSAAuthentication
372 and applies to protocol version 2 only.
375 .It Cm HostbasedUsesNameFromPacketOnly
376 Specifies whether or not the server will attempt to perform a reverse
377 name lookup when matching the name in the
383 .Cm HostbasedAuthentication .
388 uses the name supplied by the client rather than
389 attempting to resolve the name from the TCP connection itself.
393 Specifies a file containing a private host key
396 .Pa /etc/ssh/ssh_host_key
397 for protocol version 1, and
398 .Pa /etc/ssh/ssh_host_rsa_key
400 .Pa /etc/ssh/ssh_host_dsa_key
401 for protocol version 2.
404 will refuse to use a file if it is group/world-accessible.
405 It is possible to have multiple host key files.
407 keys are used for version 1 and
411 are used for version 2 of the SSH protocol.
417 files will not be used in
418 .Cm RhostsRSAAuthentication
420 .Cm HostbasedAuthentication .
424 .Pa /etc/shosts.equiv
428 .It Cm IgnoreUserKnownHosts
431 should ignore the user's
432 .Pa ~/.ssh/known_hosts
434 .Cm RhostsRSAAuthentication
436 .Cm HostbasedAuthentication .
439 .It Cm KerberosAuthentication
440 Specifies whether the password provided by the user for
441 .Cm PasswordAuthentication
442 will be validated through the Kerberos KDC.
443 To use this option, the server needs a
444 Kerberos servtab which allows the verification of the KDC's identity.
447 .It Cm KerberosGetAFSToken
448 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
449 an AFS token before accessing the user's home directory.
452 .It Cm KerberosOrLocalPasswd
453 If password authentication through Kerberos fails then
454 the password will be validated via any additional local mechanism
459 .It Cm KerberosTicketCleanup
460 Specifies whether to automatically destroy the user's ticket cache
464 .It Cm KeyRegenerationInterval
465 In protocol version 1, the ephemeral server key is automatically regenerated
466 after this many seconds (if it has been used).
467 The purpose of regeneration is to prevent
468 decrypting captured sessions by later breaking into the machine and
470 The key is never stored anywhere.
471 If the value is 0, the key is never regenerated.
472 The default is 3600 (seconds).
474 Specifies the local addresses
477 The following forms may be used:
479 .Bl -item -offset indent -compact
483 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
488 .Ar host No | Ar IPv4_addr No : Ar port
494 .Ar host No | Ar IPv6_addr Oc : Ar port
501 sshd will listen on the address and all prior
504 The default is to listen on all local addresses.
507 options are permitted.
510 options must precede this option for non-port qualified addresses.
511 .It Cm LoginGraceTime
512 The server disconnects after this time if the user has not
513 successfully logged in.
514 If the value is 0, there is no time limit.
515 The default is 120 seconds.
517 Gives the verbosity level that is used when logging messages from
519 The possible values are:
520 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
522 DEBUG and DEBUG1 are equivalent.
523 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
524 Logging with a DEBUG level violates the privacy of users and is not recommended.
526 Specifies the available MAC (message authentication code) algorithms.
527 The MAC algorithm is used in protocol version 2
528 for data integrity protection.
529 Multiple algorithms must be comma-separated.
531 .Bd -literal -offset indent
532 hmac-md5,hmac-sha1,umac-64@openssh.com,
533 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
536 Introduces a conditional block.
537 If all of the criteria on the
539 line are satisfied, the keywords on the following lines override those
540 set in the global section of the config file, until either another
542 line or the end of the file.
545 are one or more criteria-pattern pairs.
546 The available criteria are
552 Only a subset of keywords may be used on the lines following a
555 Available keywords are
556 .Cm AllowTcpForwarding ,
560 .Cm GSSApiAuthentication ,
561 .Cm KbdInteractiveAuthentication ,
562 .Cm KerberosAuthentication ,
563 .Cm PasswordAuthentication ,
565 .Cm RhostsRSAAuthentication ,
566 .Cm RSAAuthentication ,
567 .Cm X11DisplayOffset ,
570 .Cm X11UseLocalHost .
572 Specifies the maximum number of authentication attempts permitted per
574 Once the number of failures reaches half this value,
575 additional failures are logged.
578 Specifies the maximum number of concurrent unauthenticated connections to the
580 Additional connections will be dropped until authentication succeeds or the
582 expires for a connection.
585 Alternatively, random early drop can be enabled by specifying
586 the three colon separated values
590 will refuse connection attempts with a probability of
593 if there are currently
596 unauthenticated connections.
597 The probability increases linearly and all connection attempts
598 are refused if the number of unauthenticated connections reaches
601 .It Cm PasswordAuthentication
602 Specifies whether password authentication is allowed.
605 .It Cm PermitEmptyPasswords
606 When password authentication is allowed, it specifies whether the
607 server allows login to accounts with empty password strings.
611 Specifies the destinations to which TCP port forwarding is permitted.
612 The forwarding specification must be one of the following forms:
614 .Bl -item -offset indent -compact
628 .Ar \&[ IPv6_addr \&] : port
632 Multiple forwards may be specified by separating them with whitespace.
635 can be used to remove all restrictions and permit any forwarding requests.
636 By default all port forwarding requests are permitted.
637 .It Cm PermitRootLogin
638 Specifies whether root can log in using
642 .Dq without-password ,
643 .Dq forced-commands-only ,
649 If this option is set to
650 .Dq without-password ,
651 password authentication is disabled for root.
653 If this option is set to
654 .Dq forced-commands-only ,
655 root login with public key authentication will be allowed,
658 option has been specified
659 (which may be useful for taking remote backups even if root login is
660 normally not allowed).
661 All other authentication methods are disabled for root.
663 If this option is set to
665 root is not allowed to log in.
669 device forwarding is allowed.
685 .It Cm PermitUserEnvironment
687 .Pa ~/.ssh/environment
691 .Pa ~/.ssh/authorized_keys
696 Enabling environment processing may enable users to bypass access
697 restrictions in some configurations using mechanisms such as
700 Specifies the file that contains the process ID of the
703 .Pa /var/run/sshd.pid .
705 Specifies the port number that
709 Multiple options of this type are permitted.
715 should print the date and time of the last user login when a user logs
724 when a user logs in interactively.
725 (On some systems it is also printed by the shell,
731 Specifies the protocol versions
734 The possible values are
738 Multiple versions must be comma-separated.
741 Note that the order of the protocol list does not indicate preference,
742 because the client selects among multiple protocol versions offered
748 .It Cm PubkeyAuthentication
749 Specifies whether public key authentication is allowed.
752 Note that this option applies to protocol version 2 only.
753 .It Cm RhostsRSAAuthentication
754 Specifies whether rhosts or /etc/hosts.equiv authentication together
755 with successful RSA host authentication is allowed.
758 This option applies to protocol version 1 only.
759 .It Cm RSAAuthentication
760 Specifies whether pure RSA authentication is allowed.
763 This option applies to protocol version 1 only.
765 Defines the number of bits in the ephemeral protocol version 1 server key.
766 The minimum value is 512, and the default is 768.
770 should check file modes and ownership of the
771 user's files and home directory before accepting login.
772 This is normally desirable because novices sometimes accidentally leave their
773 directory or files world-writable.
777 Configures an external subsystem (e.g. file transfer daemon).
778 Arguments should be a subsystem name and a command (with optional arguments)
779 to execute upon subsystem request.
784 file transfer subsystem.
785 By default no subsystems are defined.
786 Note that this option applies to protocol version 2 only.
787 .It Cm SyslogFacility
788 Gives the facility code that is used when logging messages from
790 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
791 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
794 Specifies whether the system should send TCP keepalive messages to the
796 If they are sent, death of the connection or crash of one
797 of the machines will be properly noticed.
798 However, this means that
799 connections will die if the route is down temporarily, and some people
801 On the other hand, if TCP keepalives are not sent,
802 sessions may hang indefinitely on the server, leaving
804 users and consuming server resources.
808 (to send TCP keepalive messages), and the server will notice
809 if the network goes down or the client host crashes.
810 This avoids infinitely hanging sessions.
812 To disable TCP keepalive messages, the value should be set to
817 should look up the remote host name and check that
818 the resolved host name for the remote IP address maps back to the
819 very same IP address.
825 is used for interactive login sessions.
830 is never used for remote command execution.
831 Note also, that if this is enabled,
833 will be disabled because
835 does not know how to handle
839 .Cm UsePrivilegeSeparation
840 is specified, it will be disabled after authentication.
842 Enables the Pluggable Authentication Module interface.
845 this will enable PAM authentication using
846 .Cm ChallengeResponseAuthentication
848 .Cm PasswordAuthentication
849 in addition to PAM account and session module processing for all
850 authentication types.
852 Because PAM challenge-response authentication usually serves an equivalent
853 role to password authentication, you should disable either
854 .Cm PasswordAuthentication
856 .Cm ChallengeResponseAuthentication.
860 is enabled, you will not be able to run
865 .It Cm UsePrivilegeSeparation
868 separates privileges by creating an unprivileged child process
869 to deal with incoming network traffic.
870 After successful authentication, another process will be created that has
871 the privilege of the authenticated user.
872 The goal of privilege separation is to prevent privilege
873 escalation by containing any corruption within the unprivileged processes.
876 .It Cm X11DisplayOffset
877 Specifies the first display number available for
880 This prevents sshd from interfering with real X11 servers.
883 Specifies whether X11 forwarding is permitted.
891 When X11 forwarding is enabled, there may be additional exposure to
892 the server and to client displays if the
894 proxy display is configured to listen on the wildcard address (see
896 below), though this is not the default.
897 Additionally, the authentication spoofing and authentication data
898 verification and substitution occur on the client side.
899 The security risk of using X11 forwarding is that the client's X11
900 display server may be exposed to attack when the SSH client requests
901 forwarding (see the warnings for
905 A system administrator may have a stance in which they want to
906 protect clients that may expose themselves to attack by unwittingly
907 requesting X11 forwarding, which can warrant a
911 Note that disabling X11 forwarding does not prevent users from
912 forwarding X11 traffic, as users can always install their own forwarders.
913 X11 forwarding is automatically disabled if
916 .It Cm X11UseLocalhost
919 should bind the X11 forwarding server to the loopback address or to
920 the wildcard address.
922 sshd binds the forwarding server to the loopback address and sets the
925 environment variable to
927 This prevents remote hosts from connecting to the proxy display.
928 However, some older X11 clients may not function with this
933 to specify that the forwarding server should be bound to the wildcard
942 Specifies the full pathname of the
946 .Pa /usr/X11R6/bin/xauth .
950 command-line arguments and configuration file options that specify time
951 may be expressed using a sequence of the form:
953 .Ar time Op Ar qualifier ,
957 is a positive integer value and
959 is one of the following:
961 .Bl -tag -width Ds -compact -offset indent
976 Each member of the sequence is added together to calculate
977 the total time value.
979 Time format examples:
981 .Bl -tag -width Ds -compact -offset indent
983 600 seconds (10 minutes)
987 1 hour 30 minutes (90 minutes)
991 .It Pa /etc/ssh/sshd_config
992 Contains configuration data for
994 This file should be writable by root only, but it is recommended
995 (though not necessary) that it be world-readable.
1000 OpenSSH is a derivative of the original and free
1001 ssh 1.2.12 release by Tatu Ylonen.
1002 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1003 Theo de Raadt and Dug Song
1004 removed many bugs, re-added newer features and
1006 Markus Friedl contributed the support for SSH
1007 protocol versions 1.5 and 2.0.
1008 Niels Provos and Markus Friedl contributed support
1009 for privilege separation.
andersk / gssapi-openssh.git / blob