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 1.40 2005/03/18 17:05:00 jmc 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.
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.
107 wildcards in the patterns.
108 Only group names are valid; a numerical group ID is not recognized.
109 By default, login is allowed for all groups.
110 .It Cm AllowTcpForwarding
111 Specifies whether TCP forwarding is permitted.
114 Note that disabling TCP forwarding does not improve security unless
115 users are also denied shell access, as they can always install their
118 This keyword can be followed by a list of user name patterns, separated
120 If specified, login is allowed only for user names that
121 match one of the patterns.
126 wildcards in the patterns.
127 Only user names are valid; a numerical user ID is not recognized.
128 By default, login is allowed for all users.
129 If the pattern takes the form USER@HOST then USER and HOST
130 are separately checked, restricting logins to particular
131 users from particular hosts.
132 .It Cm AuthorizedKeysFile
133 Specifies the file that contains the public keys that can be used
134 for user authentication.
135 .Cm AuthorizedKeysFile
136 may contain tokens of the form %T which are substituted during connection
138 The following tokens are defined: %% is replaced by a literal '%',
139 %h is replaced by the home directory of the user being authenticated and
140 %u is replaced by the username of that user.
142 .Cm AuthorizedKeysFile
143 is taken to be an absolute path or one relative to the user's home
146 .Dq .ssh/authorized_keys .
148 In some jurisdictions, sending a warning message before authentication
149 may be relevant for getting legal protection.
150 The contents of the specified file are sent to the remote user before
151 authentication is allowed.
152 This option is only available for protocol version 2.
153 By default, no banner is displayed.
154 .It Cm ChallengeResponseAuthentication
155 Specifies whether challenge-response authentication is allowed.
158 this controls the use of PAM (see
161 Note that this affects the effectiveness of the
162 .Cm PasswordAuthentication
169 Specifies the ciphers allowed for protocol version 2.
170 Multiple ciphers must be comma-separated.
171 The supported ciphers are
185 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
186 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
188 .It Cm ClientAliveInterval
189 Sets a timeout interval in seconds after which if no data has been received
192 will send a message through the encrypted
193 channel to request a response from the client.
195 is 0, indicating that these messages will not be sent to the client.
196 This option applies to protocol version 2 only.
197 .It Cm ClientAliveCountMax
198 Sets the number of client alive messages (see above) which may be
201 receiving any messages back from the client.
202 If this threshold is reached while client alive messages are being sent,
204 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 (above) 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.
225 Specifies whether compression is allowed.
233 This keyword can be followed by a list of group name patterns, separated
235 Login is disallowed for users whose primary group or supplementary
236 group list matches one of the patterns.
241 wildcards in the patterns.
242 Only group names are valid; a numerical group ID is not recognized.
243 By default, login is allowed for all groups.
245 This keyword can be followed by a list of user name patterns, separated
247 Login is disallowed for user names that match one of the patterns.
251 can be used as wildcards in the patterns.
252 Only user names are valid; a numerical user ID is not recognized.
253 By default, login is allowed for all users.
254 If the pattern takes the form USER@HOST then USER and HOST
255 are separately checked, restricting logins to particular
256 users from particular hosts.
258 Specifies whether remote hosts are allowed to connect to ports
259 forwarded for the client.
262 binds remote port forwardings to the loopback address.
263 This prevents other remote hosts from connecting to forwarded ports.
265 can be used to specify that
267 should allow remote port forwardings to bind to non-loopback addresses, thus
268 allowing other hosts to connect.
271 to force remote port forwardings to be available to the local host only,
273 to force remote port forwardings to bind to the wildcard address, or
275 to allow the client to select the address to which the forwarding is bound.
278 .It Cm GSSAPIAuthentication
279 Specifies whether user authentication based on GSSAPI is allowed.
282 Note that this option applies to protocol version 2 only.
283 .It Cm GSSAPICleanupCredentials
284 Specifies whether to automatically destroy the user's credentials cache
288 Note that this option applies to protocol version 2 only.
289 .It Cm HostbasedAuthentication
290 Specifies whether rhosts or /etc/hosts.equiv authentication together
291 with successful public key client host authentication is allowed
292 (hostbased authentication).
293 This option is similar to
294 .Cm RhostsRSAAuthentication
295 and applies to protocol version 2 only.
299 Specifies a file containing a private host key
302 .Pa /etc/ssh/ssh_host_key
303 for protocol version 1, and
304 .Pa /etc/ssh/ssh_host_dsa_key
305 for protocol version 2.
308 will refuse to use a file if it is group/world-accessible.
309 It is possible to have multiple host key files.
311 keys are used for version 1 and
315 are used for version 2 of the SSH protocol.
321 files will not be used in
322 .Cm RhostsRSAAuthentication
324 .Cm HostbasedAuthentication .
328 .Pa /etc/ssh/shosts.equiv
332 .It Cm IgnoreUserKnownHosts
335 should ignore the user's
336 .Pa $HOME/.ssh/known_hosts
338 .Cm RhostsRSAAuthentication
340 .Cm HostbasedAuthentication .
343 .It Cm KerberosAuthentication
344 Specifies whether the password provided by the user for
345 .Cm PasswordAuthentication
346 will be validated through the Kerberos KDC.
347 To use this option, the server needs a
348 Kerberos servtab which allows the verification of the KDC's identity.
351 .It Cm KerberosGetAFSToken
352 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
353 an AFS token before accessing the user's home directory.
356 .It Cm KerberosOrLocalPasswd
357 If set then if password authentication through Kerberos fails then
358 the password will be validated via any additional local mechanism
363 .It Cm KerberosTicketCleanup
364 Specifies whether to automatically destroy the user's ticket cache
368 .It Cm KeyRegenerationInterval
369 In protocol version 1, the ephemeral server key is automatically regenerated
370 after this many seconds (if it has been used).
371 The purpose of regeneration is to prevent
372 decrypting captured sessions by later breaking into the machine and
374 The key is never stored anywhere.
375 If the value is 0, the key is never regenerated.
376 The default is 3600 (seconds).
378 Specifies the local addresses
381 The following forms may be used:
383 .Bl -item -offset indent -compact
387 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
392 .Ar host No | Ar IPv4_addr No : Ar port
398 .Ar host No | Ar IPv6_addr Oc : Ar port
406 will listen on the address and all prior
409 The default is to listen on all local addresses.
412 options are permitted.
415 options must precede this option for non port qualified addresses.
416 .It Cm LoginGraceTime
417 The server disconnects after this time if the user has not
418 successfully logged in.
419 If the value is 0, there is no time limit.
420 The default is 120 seconds.
422 Gives the verbosity level that is used when logging messages from
424 The possible values are:
425 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
427 DEBUG and DEBUG1 are equivalent.
428 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
429 Logging with a DEBUG level violates the privacy of users and is not recommended.
431 Specifies the available MAC (message authentication code) algorithms.
432 The MAC algorithm is used in protocol version 2
433 for data integrity protection.
434 Multiple algorithms must be comma-separated.
436 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
438 Specifies the maximum number of authentication attempts permitted per
440 Once the number of failures reaches half this value,
441 additional failures are logged.
444 Specifies the maximum number of concurrent unauthenticated connections to the
447 Additional connections will be dropped until authentication succeeds or the
449 expires for a connection.
452 Alternatively, random early drop can be enabled by specifying
453 the three colon separated values
457 will refuse connection attempts with a probability of
460 if there are currently
463 unauthenticated connections.
464 The probability increases linearly and all connection attempts
465 are refused if the number of unauthenticated connections reaches
468 .It Cm PasswordAuthentication
469 Specifies whether password authentication is allowed.
474 was built without PAM support, in which case the default is
477 .Cm ChallengeResponseAuthentication
480 and the PAM authentication policy for
484 password authentication will be allowed through the challenge-response
485 mechanism regardless of the value of
486 .Cm PasswordAuthentication .
487 .It Cm PermitEmptyPasswords
488 When password authentication is allowed, it specifies whether the
489 server allows login to accounts with empty password strings.
492 .It Cm PermitRootLogin
493 Specifies whether root can log in using
497 .Dq without-password ,
498 .Dq forced-commands-only
504 .Cm ChallengeResponseAuthentication
507 the root user may be allowed in with its password even if
508 .Cm PermitRootLogin is set to
509 .Dq without-password .
511 If this option is set to
513 password authentication is disabled for root.
515 If this option is set to
516 .Dq forced-commands-only
517 root login with public key authentication will be allowed,
520 option has been specified
521 (which may be useful for taking remote backups even if root login is
522 normally not allowed).
523 All other authentication methods are disabled for root.
525 If this option is set to
527 root is not allowed to log in.
528 .It Cm PermitUserEnvironment
530 .Pa ~/.ssh/environment
534 .Pa ~/.ssh/authorized_keys
539 Enabling environment processing may enable users to bypass access
540 restrictions in some configurations using mechanisms such as
543 Specifies the file that contains the process ID of the
547 .Pa /var/run/sshd.pid .
549 Specifies the port number that
553 Multiple options of this type are permitted.
559 should print the date and time of the last user login when a user logs
568 when a user logs in interactively.
569 (On some systems it is also printed by the shell,
575 Specifies the protocol versions
578 The possible values are
582 Multiple versions must be comma-separated.
585 Note that the order of the protocol list does not indicate preference,
586 because the client selects among multiple protocol versions offered
592 .It Cm PubkeyAuthentication
593 Specifies whether public key authentication is allowed.
596 Note that this option applies to protocol version 2 only.
597 .It Cm RhostsRSAAuthentication
598 Specifies whether rhosts or
600 authentication together
601 with successful RSA host authentication is allowed.
604 This option applies to protocol version 1 only.
605 .It Cm RSAAuthentication
606 Specifies whether pure RSA authentication is allowed.
609 This option applies to protocol version 1 only.
611 Defines the number of bits in the ephemeral protocol version 1 server key.
612 The minimum value is 512, and the default is 768.
616 should check file modes and ownership of the
617 user's files and home directory before accepting login.
618 This is normally desirable because novices sometimes accidentally leave their
619 directory or files world-writable.
623 Configures an external subsystem (e.g., file transfer daemon).
624 Arguments should be a subsystem name and a command to execute upon subsystem
630 file transfer subsystem.
631 By default no subsystems are defined.
632 Note that this option applies to protocol version 2 only.
633 .It Cm SyslogFacility
634 Gives the facility code that is used when logging messages from
636 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
637 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
640 Specifies whether the system should send TCP keepalive messages to the
642 If they are sent, death of the connection or crash of one
643 of the machines will be properly noticed.
644 However, this means that
645 connections will die if the route is down temporarily, and some people
647 On the other hand, if TCP keepalives are not sent,
648 sessions may hang indefinitely on the server, leaving
650 users and consuming server resources.
654 (to send TCP keepalive messages), and the server will notice
655 if the network goes down or the client host crashes.
656 This avoids infinitely hanging sessions.
658 To disable TCP keepalive messages, the value should be set to
663 should look up the remote host name and check that
664 the resolved host name for the remote IP address maps back to the
665 very same IP address.
671 is used for interactive login sessions.
676 is never used for remote command execution.
677 Note also, that if this is enabled,
679 will be disabled because
681 does not know how to handle
685 .Cm UsePrivilegeSeparation
686 is specified, it will be disabled after authentication.
688 Enables the Pluggable Authentication Module interface.
691 this will enable PAM authentication using
692 .Cm ChallengeResponseAuthentication
693 and PAM account and session module processing for all authentication types.
695 Because PAM challenge-response authentication usually serves an equivalent
696 role to password authentication, you should disable either
697 .Cm PasswordAuthentication
699 .Cm ChallengeResponseAuthentication.
703 is enabled, you will not be able to run
708 .It Cm UsePrivilegeSeparation
711 separates privileges by creating an unprivileged child process
712 to deal with incoming network traffic.
713 After successful authentication, another process will be created that has
714 the privilege of the authenticated user.
715 The goal of privilege separation is to prevent privilege
716 escalation by containing any corruption within the unprivileged processes.
719 .It Cm VersionAddendum
720 Specifies a string to append to the regular version string to identify
721 OS- or site-specific modifications.
723 .Dq FreeBSD-20050605 .
724 .It Cm X11DisplayOffset
725 Specifies the first display number available for
730 from interfering with real X11 servers.
733 Specifies whether X11 forwarding is permitted.
741 When X11 forwarding is enabled, there may be additional exposure to
742 the server and to client displays if the
744 proxy display is configured to listen on the wildcard address (see
746 below), however this is not the default.
747 Additionally, the authentication spoofing and authentication data
748 verification and substitution occur on the client side.
749 The security risk of using X11 forwarding is that the client's X11
750 display server may be exposed to attack when the ssh client requests
751 forwarding (see the warnings for
755 A system administrator may have a stance in which they want to
756 protect clients that may expose themselves to attack by unwittingly
757 requesting X11 forwarding, which can warrant a
761 Note that disabling X11 forwarding does not prevent users from
762 forwarding X11 traffic, as users can always install their own forwarders.
763 X11 forwarding is automatically disabled if
766 .It Cm X11UseLocalhost
769 should bind the X11 forwarding server to the loopback address or to
770 the wildcard address.
773 binds the forwarding server to the loopback address and sets the
776 environment variable to
778 This prevents remote hosts from connecting to the proxy display.
779 However, some older X11 clients may not function with this
784 to specify that the forwarding server should be bound to the wildcard
793 Specifies the full pathname of the
797 .Pa /usr/X11R6/bin/xauth .
801 command-line arguments and configuration file options that specify time
802 may be expressed using a sequence of the form:
804 .Ar time Op Ar qualifier ,
808 is a positive integer value and
810 is one of the following:
812 .Bl -tag -width Ds -compact -offset indent
827 Each member of the sequence is added together to calculate
828 the total time value.
830 Time format examples:
832 .Bl -tag -width Ds -compact -offset indent
834 600 seconds (10 minutes)
838 1 hour 30 minutes (90 minutes)
842 .It Pa /etc/ssh/sshd_config
843 Contains configuration data for
845 This file should be writable by root only, but it is recommended
846 (though not necessary) that it be world-readable.
851 OpenSSH is a derivative of the original and free
852 ssh 1.2.12 release by Tatu Ylonen.
853 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
854 Theo de Raadt and Dug Song
855 removed many bugs, re-added newer features and
857 Markus Friedl contributed the support for SSH
858 protocol versions 1.5 and 2.0.
859 Niels Provos and Markus Friedl contributed support
860 for privilege separation.