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 1.28 2004/02/17 19:35:21 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.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
65 This keyword can be followed by a list of group name patterns, separated
67 If specified, login is allowed only for users whose primary
68 group or supplementary group list matches one of the patterns.
73 wildcards in the patterns.
74 Only group names are valid; a numerical group ID is not recognized.
75 By default, login is allowed for all groups.
77 .It Cm AllowTcpForwarding
78 Specifies whether TCP forwarding is permitted.
81 Note that disabling TCP forwarding does not improve security unless
82 users are also denied shell access, as they can always install their
86 This keyword can be followed by a list of user name patterns, separated
88 If specified, login is allowed only for user names that
89 match one of the patterns.
94 wildcards in the patterns.
95 Only user names are valid; a numerical user ID is not recognized.
96 By default, login is allowed for all users.
97 If the pattern takes the form USER@HOST then USER and HOST
98 are separately checked, restricting logins to particular
99 users from particular hosts.
101 .It Cm AuthorizedKeysFile
102 Specifies the file that contains the public keys that can be used
103 for user authentication.
104 .Cm AuthorizedKeysFile
105 may contain tokens of the form %T which are substituted during connection
107 The following tokens are defined: %% is replaced by a literal '%',
108 %h is replaced by the home directory of the user being authenticated and
109 %u is replaced by the username of that user.
111 .Cm AuthorizedKeysFile
112 is taken to be an absolute path or one relative to the user's home
115 .Dq .ssh/authorized_keys .
117 In some jurisdictions, sending a warning message before authentication
118 may be relevant for getting legal protection.
119 The contents of the specified file are sent to the remote user before
120 authentication is allowed.
121 This option is only available for protocol version 2.
122 By default, no banner is displayed.
124 .It Cm ChallengeResponseAuthentication
125 Specifies whether challenge response authentication is allowed.
126 All authentication styles from
132 Specifies the ciphers allowed for protocol version 2.
133 Multiple ciphers must be comma-separated.
137 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
138 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
140 .It Cm ClientAliveInterval
141 Sets a timeout interval in seconds after which if no data has been received
144 will send a message through the encrypted
145 channel to request a response from the client.
147 is 0, indicating that these messages will not be sent to the client.
148 This option applies to protocol version 2 only.
149 .It Cm ClientAliveCountMax
150 Sets the number of client alive messages (see above) which may be
153 receiving any messages back from the client.
154 If this threshold is reached while client alive messages are being sent,
156 will disconnect the client, terminating the session.
157 It is important to note that the use of client alive messages is very
161 The client alive messages are sent through the encrypted channel
162 and therefore will not be spoofable.
163 The TCP keepalive option enabled by
166 The client alive mechanism is valuable when the client or
167 server depend on knowing when a connection has become inactive.
169 The default value is 3.
171 .Cm ClientAliveInterval
172 (above) is set to 15, and
173 .Cm ClientAliveCountMax
174 is left at the default, unresponsive ssh clients
175 will be disconnected after approximately 45 seconds.
177 Specifies whether compression is allowed.
185 This keyword can be followed by a list of group name patterns, separated
187 Login is disallowed for users whose primary group or supplementary
188 group list matches one of the patterns.
193 wildcards in the patterns.
194 Only group names are valid; a numerical group ID is not recognized.
195 By default, login is allowed for all groups.
198 This keyword can be followed by a list of user name patterns, separated
200 Login is disallowed for user names that match one of the patterns.
204 can be used as wildcards in the patterns.
205 Only user names are valid; a numerical user ID is not recognized.
206 By default, login is allowed for all users.
207 If the pattern takes the form USER@HOST then USER and HOST
208 are separately checked, restricting logins to particular
209 users from particular hosts.
211 Specifies whether remote hosts are allowed to connect to ports
212 forwarded for the client.
215 binds remote port forwardings to the loopback address.
216 This prevents other remote hosts from connecting to forwarded ports.
218 can be used to specify that
220 should bind remote port forwardings to the wildcard address,
221 thus allowing remote hosts to connect to forwarded ports.
228 .It Cm GSSAPIAuthentication
229 Specifies whether user authentication based on GSSAPI is allowed.
232 Note that this option applies to protocol version 2 only.
233 .It Cm GSSAPICleanupCredentials
234 Specifies whether to automatically destroy the user's credentials cache
238 Note that this option applies to protocol version 2 only.
239 .It Cm GSSAPIKeyExchange
240 Specifies whether key exchange based on GSSAPI may be used. When using
241 GSSAPI key exchange the server need not have a host key.
244 Note that this option applies to protocol version 2 only.
245 .It Cm GSSAPIUseSessionCredCache
246 Specifies whether a unique credentials cache name should be generated per
247 session for storing delegated credentials.
250 Note that this option applies to protocol version 2 only.
251 .It Cm HostbasedAuthentication
252 Specifies whether rhosts or /etc/hosts.equiv authentication together
253 with successful public key client host authentication is allowed
254 (hostbased authentication).
255 This option is similar to
256 .Cm RhostsRSAAuthentication
257 and applies to protocol version 2 only.
261 Specifies a file containing a private host key
264 .Pa /etc/ssh/ssh_host_key
265 for protocol version 1, and
266 .Pa /etc/ssh/ssh_host_rsa_key
268 .Pa /etc/ssh/ssh_host_dsa_key
269 for protocol version 2.
272 will refuse to use a file if it is group/world-accessible.
273 It is possible to have multiple host key files.
275 keys are used for version 1 and
279 are used for version 2 of the SSH protocol.
285 files will not be used in
286 .Cm RhostsRSAAuthentication
288 .Cm HostbasedAuthentication .
292 .Pa /etc/shosts.equiv
296 .It Cm IgnoreUserKnownHosts
299 should ignore the user's
300 .Pa $HOME/.ssh/known_hosts
302 .Cm RhostsRSAAuthentication
304 .Cm HostbasedAuthentication .
307 .It Cm KerberosAuthentication
308 Specifies whether the password provided by the user for
309 .Cm PasswordAuthentication
310 will be validated through the Kerberos KDC.
311 To use this option, the server needs a
312 Kerberos servtab which allows the verification of the KDC's identity.
315 .It Cm KerberosOrLocalPasswd
316 If set then if password authentication through Kerberos fails then
317 the password will be validated via any additional local mechanism
322 .It Cm KerberosTicketCleanup
323 Specifies whether to automatically destroy the user's ticket cache
327 .It Cm KeyRegenerationInterval
328 In protocol version 1, the ephemeral server key is automatically regenerated
329 after this many seconds (if it has been used).
330 The purpose of regeneration is to prevent
331 decrypting captured sessions by later breaking into the machine and
333 The key is never stored anywhere.
334 If the value is 0, the key is never regenerated.
335 The default is 3600 (seconds).
337 Specifies the local addresses
340 The following forms may be used:
342 .Bl -item -offset indent -compact
346 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
351 .Ar host No | Ar IPv4_addr No : Ar port
357 .Ar host No | Ar IPv6_addr Oc : Ar port
365 will listen on the address and all prior
368 The default is to listen on all local addresses.
371 options are permitted.
374 options must precede this option for non port qualified addresses.
375 .It Cm LoginGraceTime
376 The server disconnects after this time if the user has not
377 successfully logged in.
378 If the value is 0, there is no time limit.
379 The default is 120 seconds.
381 Gives the verbosity level that is used when logging messages from
383 The possible values are:
384 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
386 DEBUG and DEBUG1 are equivalent.
387 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
388 Logging with a DEBUG level violates the privacy of users and is not recommended.
390 Specifies the available MAC (message authentication code) algorithms.
391 The MAC algorithm is used in protocol version 2
392 for data integrity protection.
393 Multiple algorithms must be comma-separated.
395 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
397 Specifies the maximum number of concurrent unauthenticated connections to the
400 Additional connections will be dropped until authentication succeeds or the
402 expires for a connection.
405 Alternatively, random early drop can be enabled by specifying
406 the three colon separated values
410 will refuse connection attempts with a probability of
413 if there are currently
416 unauthenticated connections.
417 The probability increases linearly and all connection attempts
418 are refused if the number of unauthenticated connections reaches
421 .It Cm PasswordAuthentication
422 Specifies whether password authentication is allowed.
425 .It Cm PermitEmptyPasswords
426 When password authentication is allowed, it specifies whether the
427 server allows login to accounts with empty password strings.
430 .It Cm PermitRootLogin
431 Specifies whether root can login using
435 .Dq without-password ,
436 .Dq forced-commands-only
442 If this option is set to
444 password authentication is disabled for root.
446 If this option is set to
447 .Dq forced-commands-only
448 root login with public key authentication will be allowed,
451 option has been specified
452 (which may be useful for taking remote backups even if root login is
453 normally not allowed).
454 All other authentication methods are disabled for root.
456 If this option is set to
458 root is not allowed to login.
459 .It Cm PermitUserEnvironment
461 .Pa ~/.ssh/environment
465 .Pa ~/.ssh/authorized_keys
470 Enabling environment processing may enable users to bypass access
471 restrictions in some configurations using mechanisms such as
474 Specifies the file that contains the process ID of the
478 .Pa /var/run/sshd.pid .
480 Specifies the port number that
484 Multiple options of this type are permitted.
490 should print the date and time when the user last logged in.
498 when a user logs in interactively.
499 (On some systems it is also printed by the shell,
505 Specifies the protocol versions
508 The possible values are
512 Multiple versions must be comma-separated.
515 Note that the order of the protocol list does not indicate preference,
516 because the client selects among multiple protocol versions offered
522 .It Cm PubkeyAuthentication
523 Specifies whether public key authentication is allowed.
526 Note that this option applies to protocol version 2 only.
527 .It Cm RhostsRSAAuthentication
528 Specifies whether rhosts or /etc/hosts.equiv authentication together
529 with successful RSA host authentication is allowed.
532 This option applies to protocol version 1 only.
533 .It Cm RSAAuthentication
534 Specifies whether pure RSA authentication is allowed.
537 This option applies to protocol version 1 only.
539 Defines the number of bits in the ephemeral protocol version 1 server key.
540 The minimum value is 512, and the default is 768.
544 should check file modes and ownership of the
545 user's files and home directory before accepting login.
546 This is normally desirable because novices sometimes accidentally leave their
547 directory or files world-writable.
551 Configures an external subsystem (e.g., file transfer daemon).
552 Arguments should be a subsystem name and a command to execute upon subsystem
558 file transfer subsystem.
559 By default no subsystems are defined.
560 Note that this option applies to protocol version 2 only.
561 .It Cm SyslogFacility
562 Gives the facility code that is used when logging messages from
564 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
565 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
568 Specifies whether the system should send TCP keepalive messages to the
570 If they are sent, death of the connection or crash of one
571 of the machines will be properly noticed.
572 However, this means that
573 connections will die if the route is down temporarily, and some people
575 On the other hand, if TCP keepalives are not sent,
576 sessions may hang indefinitely on the server, leaving
578 users and consuming server resources.
582 (to send TCP keepalive messages), and the server will notice
583 if the network goes down or the client host crashes.
584 This avoids infinitely hanging sessions.
586 To disable TCP keepalive messages, the value should be set to
591 should lookup the remote host name and check that
592 the resolved host name for the remote IP address maps back to the
593 very same IP address.
599 is used for interactive login sessions.
604 is never used for remote command execution.
605 Note also, that if this is enabled,
607 will be disabled because
609 does not know how to handle
613 .Cm UsePrivilegeSeparation
614 is specified, it will be disabled after authentication.
616 Enables PAM authentication (via challenge-response) and session set up.
617 If you enable this, you should probably disable
618 .Cm PasswordAuthentication .
621 then you will not be able to run sshd as a non-root user. The default is
623 .It Cm UsePrivilegeSeparation
626 separates privileges by creating an unprivileged child process
627 to deal with incoming network traffic.
628 After successful authentication, another process will be created that has
629 the privilege of the authenticated user.
630 The goal of privilege separation is to prevent privilege
631 escalation by containing any corruption within the unprivileged processes.
634 .It Cm X11DisplayOffset
635 Specifies the first display number available for
640 from interfering with real X11 servers.
643 Specifies whether X11 forwarding is permitted.
651 When X11 forwarding is enabled, there may be additional exposure to
652 the server and to client displays if the
654 proxy display is configured to listen on the wildcard address (see
656 below), however this is not the default.
657 Additionally, the authentication spoofing and authentication data
658 verification and substitution occur on the client side.
659 The security risk of using X11 forwarding is that the client's X11
660 display server may be exposed to attack when the ssh client requests
661 forwarding (see the warnings for
665 A system administrator may have a stance in which they want to
666 protect clients that may expose themselves to attack by unwittingly
667 requesting X11 forwarding, which can warrant a
671 Note that disabling X11 forwarding does not prevent users from
672 forwarding X11 traffic, as users can always install their own forwarders.
673 X11 forwarding is automatically disabled if
676 .It Cm X11UseLocalhost
679 should bind the X11 forwarding server to the loopback address or to
680 the wildcard address.
683 binds the forwarding server to the loopback address and sets the
686 environment variable to
688 This prevents remote hosts from connecting to the proxy display.
689 However, some older X11 clients may not function with this
694 to specify that the forwarding server should be bound to the wildcard
703 Specifies the full pathname of the
707 .Pa /usr/X11R6/bin/xauth .
711 command-line arguments and configuration file options that specify time
712 may be expressed using a sequence of the form:
714 .Ar time Op Ar qualifier ,
718 is a positive integer value and
720 is one of the following:
722 .Bl -tag -width Ds -compact -offset indent
737 Each member of the sequence is added together to calculate
738 the total time value.
740 Time format examples:
742 .Bl -tag -width Ds -compact -offset indent
744 600 seconds (10 minutes)
748 1 hour 30 minutes (90 minutes)
752 .It Pa /etc/ssh/sshd_config
753 Contains configuration data for
755 This file should be writable by root only, but it is recommended
756 (though not necessary) that it be world-readable.
761 OpenSSH is a derivative of the original and free
762 ssh 1.2.12 release by Tatu Ylonen.
763 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
764 Theo de Raadt and Dug Song
765 removed many bugs, re-added newer features and
767 Markus Friedl contributed the support for SSH
768 protocol versions 1.5 and 2.0.
769 Niels Provos and Markus Friedl contributed support
770 for privilege separation.