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 1.4 2002/06/22 16:45:29 stevesk 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):
64 .It Cm AFSTokenPassing
65 Specifies whether an AFS token may be forwarded to the server.
69 This keyword can be followed by a list of group name patterns, separated
71 If specified, login is allowed only for users whose primary
72 group or supplementary group list matches one of the patterns.
77 wildcards in the patterns.
78 Only group names are valid; a numerical group ID is not recognized.
79 By default, login is allowed for all groups.
81 .It Cm AllowTcpForwarding
82 Specifies whether TCP forwarding is permitted.
85 Note that disabling TCP forwarding does not improve security unless
86 users are also denied shell access, as they can always install their
90 This keyword can be followed by a list of user name patterns, separated
92 If specified, login is allowed only for users names that
93 match one of the patterns.
98 wildcards in the patterns.
99 Only user names are valid; a numerical user ID is not recognized.
100 By default, login is allowed for all users.
101 If the pattern takes the form USER@HOST then USER and HOST
102 are separately checked, restricting logins to particular
103 users from particular hosts.
105 .It Cm AuthorizedKeysFile
106 Specifies the file that contains the public keys that can be used
107 for user authentication.
108 .Cm AuthorizedKeysFile
109 may contain tokens of the form %T which are substituted during connection
110 set-up. The following tokens are defined: %% is replaced by a literal '%',
111 %h is replaced by the home directory of the user being authenticated and
112 %u is replaced by the username of that user.
114 .Cm AuthorizedKeysFile
115 is taken to be an absolute path or one relative to the user's home
118 .Dq .ssh/authorized_keys .
120 In some jurisdictions, sending a warning message before authentication
121 may be relevant for getting legal protection.
122 The contents of the specified file are sent to the remote user before
123 authentication is allowed.
124 This option is only available for protocol version 2.
125 By default, no banner is displayed.
127 .It Cm ChallengeResponseAuthentication
128 Specifies whether challenge response authentication is allowed.
129 All authentication styles from
135 Specifies the ciphers allowed for protocol version 2.
136 Multiple ciphers must be comma-separated.
140 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
141 aes192-cbc,aes256-cbc''
143 .It Cm ClientAliveInterval
144 Sets a timeout interval in seconds after which if no data has been received
147 will send a message through the encrypted
148 channel to request a response from the client.
150 is 0, indicating that these messages will not be sent to the client.
151 This option applies to protocol version 2 only.
152 .It Cm ClientAliveCountMax
153 Sets the number of client alive messages (see above) which may be
156 receiving any messages back from the client. If this threshold is
157 reached while client alive messages are being sent,
159 will disconnect the client, terminating the session. It is important
160 to note that the use of client alive messages is very different from
162 (below). The client alive messages are sent through the
163 encrypted channel and therefore will not be spoofable. The TCP keepalive
166 is spoofable. 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. If
170 .Cm ClientAliveInterval
171 (above) is set to 15, and
172 .Cm ClientAliveCountMax
173 is left at the default, unresponsive ssh clients
174 will be disconnected after approximately 45 seconds.
176 Specifies whether compression is allowed.
184 This keyword can be followed by a list of group name patterns, separated
186 Login is disallowed for users whose primary group or supplementary
187 group list matches one of the patterns.
192 wildcards in the patterns.
193 Only group names are valid; a numerical group ID is not recognized.
194 By default, login is allowed for all groups.
197 This keyword can be followed by a list of user name patterns, separated
199 Login is disallowed for user names that match one of the patterns.
203 can be used as wildcards in the patterns.
204 Only user names are valid; a numerical user ID is not recognized.
205 By default, login is allowed for all users.
206 If the pattern takes the form USER@HOST then USER and HOST
207 are separately checked, restricting logins to particular
208 users from particular hosts.
210 Specifies whether remote hosts are allowed to connect to ports
211 forwarded for the client.
214 binds remote port forwardings to the loopback address. This
215 prevents other remote hosts from connecting to forwarded ports.
217 can be used to specify that
219 should bind remote port forwardings to the wildcard address,
220 thus allowing remote hosts to connect to forwarded ports.
227 .It Cm HostbasedAuthentication
228 Specifies whether rhosts or /etc/hosts.equiv authentication together
229 with successful public key client host authentication is allowed
230 (hostbased authentication).
231 This option is similar to
232 .Cm RhostsRSAAuthentication
233 and applies to protocol version 2 only.
236 .It Cm GssapiAuthentication
237 Specifies whether authentication based on GSSAPI may be used, either using
238 the result of a successful key exchange, or using GSSAPI user
242 .It Cm GssapiKeyExchange
243 Specifies whether key exchange based on GSSAPI may be used. When using
244 GSSAPI key exchange the server need not have a host key.
247 .It Cm GssapiUseSessionCredCache
248 Specifies whether a unique credentials cache name should be generated per
249 session for storing delegated credentials.
253 Specifies a file containing a private host key
256 .Pa /etc/ssh/ssh_host_key
257 for protocol version 1, and
258 .Pa /etc/ssh/ssh_host_rsa_key
260 .Pa /etc/ssh/ssh_host_dsa_key
261 for protocol version 2.
264 will refuse to use a file if it is group/world-accessible.
265 It is possible to have multiple host key files.
267 keys are used for version 1 and
271 are used for version 2 of the SSH protocol.
277 files will not be used in
278 .Cm RhostsAuthentication ,
279 .Cm RhostsRSAAuthentication
281 .Cm HostbasedAuthentication .
285 .Pa /etc/shosts.equiv
289 .It Cm IgnoreUserKnownHosts
292 should ignore the user's
293 .Pa $HOME/.ssh/known_hosts
295 .Cm RhostsRSAAuthentication
297 .Cm HostbasedAuthentication .
301 Specifies whether the system should send TCP keepalive messages to the
303 If they are sent, death of the connection or crash of one
304 of the machines will be properly noticed.
305 However, this means that
306 connections will die if the route is down temporarily, and some people
308 On the other hand, if keepalives are not sent,
309 sessions may hang indefinitely on the server, leaving
311 users and consuming server resources.
315 (to send keepalives), and the server will notice
316 if the network goes down or the client host crashes.
317 This avoids infinitely hanging sessions.
319 To disable keepalives, the value should be set to
321 .It Cm KerberosAuthentication
322 Specifies whether Kerberos authentication is allowed.
323 This can be in the form of a Kerberos ticket, or if
324 .Cm PasswordAuthentication
325 is yes, the password provided by the user will be validated through
327 To use this option, the server needs a
328 Kerberos servtab which allows the verification of the KDC's identity.
331 .It Cm KerberosOrLocalPasswd
332 If set then if password authentication through Kerberos fails then
333 the password will be validated via any additional local mechanism
338 .It Cm KerberosTgtPassing
339 Specifies whether a Kerberos TGT may be forwarded to the server.
342 as this only works when the Kerberos KDC is actually an AFS kaserver.
343 .It Cm KerberosTicketCleanup
344 Specifies whether to automatically destroy the user's ticket cache
348 .It Cm KeyRegenerationInterval
349 In protocol version 1, the ephemeral server key is automatically regenerated
350 after this many seconds (if it has been used).
351 The purpose of regeneration is to prevent
352 decrypting captured sessions by later breaking into the machine and
354 The key is never stored anywhere.
355 If the value is 0, the key is never regenerated.
356 The default is 3600 (seconds).
358 Specifies the local addresses
361 The following forms may be used:
363 .Bl -item -offset indent -compact
367 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
372 .Ar host No | Ar IPv4_addr No : Ar port
378 .Ar host No | Ar IPv6_addr Oc : Ar port
386 will listen on the address and all prior
388 options specified. The default is to listen on all local
391 options are permitted. Additionally, any
393 options must precede this option for non port qualified addresses.
394 .It Cm LoginGraceTime
395 The server disconnects after this time if the user has not
396 successfully logged in.
397 If the value is 0, there is no time limit.
398 The default is 600 (seconds).
400 Gives the verbosity level that is used when logging messages from
402 The possible values are:
403 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
404 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
405 and DEBUG3 each specify higher levels of debugging output.
406 Logging with a DEBUG level violates the privacy of users
407 and is not recommended.
409 Specifies the available MAC (message authentication code) algorithms.
410 The MAC algorithm is used in protocol version 2
411 for data integrity protection.
412 Multiple algorithms must be comma-separated.
414 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
416 Specifies the maximum number of concurrent unauthenticated connections to the
419 Additional connections will be dropped until authentication succeeds or the
421 expires for a connection.
424 Alternatively, random early drop can be enabled by specifying
425 the three colon separated values
429 will refuse connection attempts with a probability of
432 if there are currently
435 unauthenticated connections.
436 The probability increases linearly and all connection attempts
437 are refused if the number of unauthenticated connections reaches
440 .It Cm PAMAuthenticationViaKbdInt
441 Specifies whether PAM challenge response authentication is allowed. This
442 allows the use of most PAM challenge response authentication modules, but
443 it will allow password authentication regardless of whether
444 .Cm PasswordAuthentication
446 .It Cm PasswordAuthentication
447 Specifies whether password authentication is allowed.
450 .It Cm PermitEmptyPasswords
451 When password authentication is allowed, it specifies whether the
452 server allows login to accounts with empty password strings.
455 .It Cm PermitRootLogin
456 Specifies whether root can login using
460 .Dq without-password ,
461 .Dq forced-commands-only
467 If this option is set to
469 password authentication is disabled for root.
471 If this option is set to
472 .Dq forced-commands-only
473 root login with public key authentication will be allowed,
476 option has been specified
477 (which may be useful for taking remote backups even if root login is
478 normally not allowed). All other authentication methods are disabled
481 If this option is set to
483 root is not allowed to login.
485 Specifies the file that contains the process ID of the
489 .Pa /var/run/sshd.pid .
491 Specifies the port number that
495 Multiple options of this type are permitted.
501 should print the date and time when the user last logged in.
509 when a user logs in interactively.
510 (On some systems it is also printed by the shell,
516 Specifies the protocol versions
519 The possible values are
523 Multiple versions must be comma-separated.
526 .It Cm PubkeyAuthentication
527 Specifies whether public key authentication is allowed.
530 Note that this option applies to protocol version 2 only.
531 .It Cm RhostsAuthentication
532 Specifies whether authentication using rhosts or /etc/hosts.equiv
534 Normally, this method should not be permitted because it is insecure.
535 .Cm RhostsRSAAuthentication
537 instead, because it performs RSA-based host authentication in addition
538 to normal rhosts or /etc/hosts.equiv authentication.
541 This option applies to protocol version 1 only.
542 .It Cm RhostsRSAAuthentication
543 Specifies whether rhosts or /etc/hosts.equiv authentication together
544 with successful RSA host authentication is allowed.
547 This option applies to protocol version 1 only.
548 .It Cm RSAAuthentication
549 Specifies whether pure RSA authentication is allowed.
552 This option applies to protocol version 1 only.
554 Defines the number of bits in the ephemeral protocol version 1 server key.
555 The minimum value is 512, and the default is 768.
559 should check file modes and ownership of the
560 user's files and home directory before accepting login.
561 This is normally desirable because novices sometimes accidentally leave their
562 directory or files world-writable.
566 Configures an external subsystem (e.g., file transfer daemon).
567 Arguments should be a subsystem name and a command to execute upon subsystem
573 file transfer subsystem.
574 By default no subsystems are defined.
575 Note that this option applies to protocol version 2 only.
576 .It Cm SyslogFacility
577 Gives the facility code that is used when logging messages from
579 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
580 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
585 is used for interactive login sessions.
590 is never used for remote command execution.
591 Note also, that if this is enabled,
593 will be disabled because
595 does not know how to handle
598 .Cm UsePrivilegeSeparation
599 is specified, it will be disabled after authentication.
600 .It Cm UsePrivilegeSeparation
603 separates privileges by creating an unprivileged child process
604 to deal with incoming network traffic. After successful authentication,
605 another process will be created that has the privilege of the authenticated
606 user. The goal of privilege separation is to prevent privilege
607 escalation by containing any corruption within the unprivileged processes.
610 .It Cm VerifyReverseMapping
613 should try to verify the remote host name and check that
614 the resolved host name for the remote IP address maps back to the
615 very same IP address.
618 .It Cm X11DisplayOffset
619 Specifies the first display number available for
624 from interfering with real X11 servers.
627 Specifies whether X11 forwarding is permitted.
630 Note that disabling X11 forwarding does not improve security in any
631 way, as users can always install their own forwarders.
632 X11 forwarding is automatically disabled if
635 .It Cm X11UseLocalhost
638 should bind the X11 forwarding server to the loopback address or to
639 the wildcard address. By default,
641 binds the forwarding server to the loopback address and sets the
644 environment variable to
646 This prevents remote hosts from connecting to the fake display.
647 However, some older X11 clients may not function with this
652 to specify that the forwarding server should be bound to the wildcard
661 Specifies the location of the
665 .Pa /usr/X11R6/bin/xauth .
670 command-line arguments and configuration file options that specify time
671 may be expressed using a sequence of the form:
673 .Ar time Oo Ar qualifier Oc ,
677 is a positive integer value and
679 is one of the following:
681 .Bl -tag -width Ds -compact -offset indent
696 Each member of the sequence is added together to calculate
697 the total time value.
699 Time format examples:
701 .Bl -tag -width Ds -compact -offset indent
703 600 seconds (10 minutes)
707 1 hour 30 minutes (90 minutes)
711 .It Pa /etc/ssh/sshd_config
712 Contains configuration data for
714 This file should be writable by root only, but it is recommended
715 (though not necessary) that it be world-readable.
718 OpenSSH is a derivative of the original and free
719 ssh 1.2.12 release by Tatu Ylonen.
720 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
721 Theo de Raadt and Dug Song
722 removed many bugs, re-added newer features and
724 Markus Friedl contributed the support for SSH
725 protocol versions 1.5 and 2.0.
726 Niels Provos and Markus Friedl contributed support
727 for privilege separation.