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 1.63 2006/07/18 07:50:40 jmc 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 Keywords on lines following a
470 block are only applied if all of the criteria on the
475 block are one or more criteria-pattern pairs.
476 The available criteria are
481 Only a subset of keywords may be used on the lines following a
484 Available keywords are
485 .Cm AllowTcpForwarding ,
490 Specifies the maximum number of authentication attempts permitted per
492 Once the number of failures reaches half this value,
493 additional failures are logged.
496 Specifies the maximum number of concurrent unauthenticated connections to the
498 Additional connections will be dropped until authentication succeeds or the
500 expires for a connection.
503 Alternatively, random early drop can be enabled by specifying
504 the three colon separated values
508 will refuse connection attempts with a probability of
511 if there are currently
514 unauthenticated connections.
515 The probability increases linearly and all connection attempts
516 are refused if the number of unauthenticated connections reaches
519 .It Cm PasswordAuthentication
520 Specifies whether password authentication is allowed.
523 .It Cm PermitEmptyPasswords
524 When password authentication is allowed, it specifies whether the
525 server allows login to accounts with empty password strings.
529 Specifies the destinations to which TCP port forwarding is permitted.
530 The forwarding specification must be one of the following forms:
532 .Bl -item -offset indent -compact
546 .Ar \&[ IPv6_addr \&] : port
550 Multiple instances of
555 can be used to remove all restrictions and permit any forwarding requests.
556 By default all port forwarding requests are permitted.
557 .It Cm PermitRootLogin
558 Specifies whether root can log in using
562 .Dq without-password ,
563 .Dq forced-commands-only ,
569 If this option is set to
570 .Dq without-password ,
571 password authentication is disabled for root.
573 If this option is set to
574 .Dq forced-commands-only ,
575 root login with public key authentication will be allowed,
578 option has been specified
579 (which may be useful for taking remote backups even if root login is
580 normally not allowed).
581 All other authentication methods are disabled for root.
583 If this option is set to
585 root is not allowed to log in.
589 device forwarding is allowed.
605 .It Cm PermitUserEnvironment
607 .Pa ~/.ssh/environment
611 .Pa ~/.ssh/authorized_keys
616 Enabling environment processing may enable users to bypass access
617 restrictions in some configurations using mechanisms such as
620 Specifies the file that contains the process ID of the
623 .Pa /var/run/sshd.pid .
625 Specifies the port number that
629 Multiple options of this type are permitted.
635 should print the date and time of the last user login when a user logs
644 when a user logs in interactively.
645 (On some systems it is also printed by the shell,
651 Specifies the protocol versions
654 The possible values are
658 Multiple versions must be comma-separated.
661 Note that the order of the protocol list does not indicate preference,
662 because the client selects among multiple protocol versions offered
668 .It Cm PubkeyAuthentication
669 Specifies whether public key authentication is allowed.
672 Note that this option applies to protocol version 2 only.
673 .It Cm RhostsRSAAuthentication
674 Specifies whether rhosts or /etc/hosts.equiv authentication together
675 with successful RSA host authentication is allowed.
678 This option applies to protocol version 1 only.
679 .It Cm RSAAuthentication
680 Specifies whether pure RSA authentication is allowed.
683 This option applies to protocol version 1 only.
685 Defines the number of bits in the ephemeral protocol version 1 server key.
686 The minimum value is 512, and the default is 768.
690 should check file modes and ownership of the
691 user's files and home directory before accepting login.
692 This is normally desirable because novices sometimes accidentally leave their
693 directory or files world-writable.
697 Configures an external subsystem (e.g. file transfer daemon).
698 Arguments should be a subsystem name and a command (with optional arguments)
699 to execute upon subsystem request.
704 file transfer subsystem.
705 By default no subsystems are defined.
706 Note that this option applies to protocol version 2 only.
707 .It Cm SyslogFacility
708 Gives the facility code that is used when logging messages from
710 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
711 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
714 Specifies whether the system should send TCP keepalive messages to the
716 If they are sent, death of the connection or crash of one
717 of the machines will be properly noticed.
718 However, this means that
719 connections will die if the route is down temporarily, and some people
721 On the other hand, if TCP keepalives are not sent,
722 sessions may hang indefinitely on the server, leaving
724 users and consuming server resources.
728 (to send TCP keepalive messages), and the server will notice
729 if the network goes down or the client host crashes.
730 This avoids infinitely hanging sessions.
732 To disable TCP keepalive messages, the value should be set to
737 should look up the remote host name and check that
738 the resolved host name for the remote IP address maps back to the
739 very same IP address.
745 is used for interactive login sessions.
750 is never used for remote command execution.
751 Note also, that if this is enabled,
753 will be disabled because
755 does not know how to handle
759 .Cm UsePrivilegeSeparation
760 is specified, it will be disabled after authentication.
762 Enables the Pluggable Authentication Module interface.
765 this will enable PAM authentication using
766 .Cm ChallengeResponseAuthentication
768 .Cm PasswordAuthentication
769 in addition to PAM account and session module processing for all
770 authentication types.
772 Because PAM challenge-response authentication usually serves an equivalent
773 role to password authentication, you should disable either
774 .Cm PasswordAuthentication
776 .Cm ChallengeResponseAuthentication.
780 is enabled, you will not be able to run
785 .It Cm UsePrivilegeSeparation
788 separates privileges by creating an unprivileged child process
789 to deal with incoming network traffic.
790 After successful authentication, another process will be created that has
791 the privilege of the authenticated user.
792 The goal of privilege separation is to prevent privilege
793 escalation by containing any corruption within the unprivileged processes.
796 .It Cm X11DisplayOffset
797 Specifies the first display number available for
800 This prevents sshd from interfering with real X11 servers.
803 Specifies whether X11 forwarding is permitted.
811 When X11 forwarding is enabled, there may be additional exposure to
812 the server and to client displays if the
814 proxy display is configured to listen on the wildcard address (see
816 below), though this is not the default.
817 Additionally, the authentication spoofing and authentication data
818 verification and substitution occur on the client side.
819 The security risk of using X11 forwarding is that the client's X11
820 display server may be exposed to attack when the SSH client requests
821 forwarding (see the warnings for
825 A system administrator may have a stance in which they want to
826 protect clients that may expose themselves to attack by unwittingly
827 requesting X11 forwarding, which can warrant a
831 Note that disabling X11 forwarding does not prevent users from
832 forwarding X11 traffic, as users can always install their own forwarders.
833 X11 forwarding is automatically disabled if
836 .It Cm X11UseLocalhost
839 should bind the X11 forwarding server to the loopback address or to
840 the wildcard address.
842 sshd binds the forwarding server to the loopback address and sets the
845 environment variable to
847 This prevents remote hosts from connecting to the proxy display.
848 However, some older X11 clients may not function with this
853 to specify that the forwarding server should be bound to the wildcard
862 Specifies the full pathname of the
866 .Pa /usr/X11R6/bin/xauth .
870 command-line arguments and configuration file options that specify time
871 may be expressed using a sequence of the form:
873 .Ar time Op Ar qualifier ,
877 is a positive integer value and
879 is one of the following:
881 .Bl -tag -width Ds -compact -offset indent
896 Each member of the sequence is added together to calculate
897 the total time value.
899 Time format examples:
901 .Bl -tag -width Ds -compact -offset indent
903 600 seconds (10 minutes)
907 1 hour 30 minutes (90 minutes)
911 .It Pa /etc/ssh/sshd_config
912 Contains configuration data for
914 This file should be writable by root only, but it is recommended
915 (though not necessary) that it be world-readable.
920 OpenSSH is a derivative of the original and free
921 ssh 1.2.12 release by Tatu Ylonen.
922 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
923 Theo de Raadt and Dug Song
924 removed many bugs, re-added newer features and
926 Markus Friedl contributed the support for SSH
927 protocol versions 1.5 and 2.0.
928 Niels Provos and Markus Friedl contributed support
929 for privilege separation.
andersk / openssh.git / blob