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 1.3 2002/06/20 23:37:12 markus 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.
237 Specifies a file containing a private host key
240 .Pa /etc/ssh/ssh_host_key
241 for protocol version 1, and
242 .Pa /etc/ssh/ssh_host_rsa_key
244 .Pa /etc/ssh/ssh_host_dsa_key
245 for protocol version 2.
248 will refuse to use a file if it is group/world-accessible.
249 It is possible to have multiple host key files.
251 keys are used for version 1 and
255 are used for version 2 of the SSH protocol.
261 files will not be used in
262 .Cm RhostsAuthentication ,
263 .Cm RhostsRSAAuthentication
265 .Cm HostbasedAuthentication .
269 .Pa /etc/shosts.equiv
273 .It Cm IgnoreUserKnownHosts
276 should ignore the user's
277 .Pa $HOME/.ssh/known_hosts
279 .Cm RhostsRSAAuthentication
281 .Cm HostbasedAuthentication .
285 Specifies whether the system should send TCP keepalive messages to the
287 If they are sent, death of the connection or crash of one
288 of the machines will be properly noticed.
289 However, this means that
290 connections will die if the route is down temporarily, and some people
292 On the other hand, if keepalives are not sent,
293 sessions may hang indefinitely on the server, leaving
295 users and consuming server resources.
299 (to send keepalives), and the server will notice
300 if the network goes down or the client host crashes.
301 This avoids infinitely hanging sessions.
303 To disable keepalives, the value should be set to
305 .It Cm KerberosAuthentication
306 Specifies whether Kerberos authentication is allowed.
307 This can be in the form of a Kerberos ticket, or if
308 .It Cm PAMAuthenticationViaKbdInt
309 Specifies whether PAM challenge response authentication is allowed. This
310 allows the use of most PAM challenge response authentication modules, but
311 it will allow password authentication regardless of whether
312 .Cm PasswordAuthentication
313 is yes, the password provided by the user will be validated through
315 To use this option, the server needs a
316 Kerberos servtab which allows the verification of the KDC's identity.
319 .It Cm KerberosOrLocalPasswd
320 If set then if password authentication through Kerberos fails then
321 the password will be validated via any additional local mechanism
326 .It Cm KerberosTgtPassing
327 Specifies whether a Kerberos TGT may be forwarded to the server.
330 as this only works when the Kerberos KDC is actually an AFS kaserver.
331 .It Cm KerberosTicketCleanup
332 Specifies whether to automatically destroy the user's ticket cache
336 .It Cm KeyRegenerationInterval
337 In protocol version 1, the ephemeral server key is automatically regenerated
338 after this many seconds (if it has been used).
339 The purpose of regeneration is to prevent
340 decrypting captured sessions by later breaking into the machine and
342 The key is never stored anywhere.
343 If the value is 0, the key is never regenerated.
344 The default is 3600 (seconds).
346 Specifies the local addresses
349 The following forms may be used:
351 .Bl -item -offset indent -compact
355 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
360 .Ar host No | Ar IPv4_addr No : Ar port
366 .Ar host No | Ar IPv6_addr Oc : Ar port
374 will listen on the address and all prior
376 options specified. The default is to listen on all local
379 options are permitted. Additionally, any
381 options must precede this option for non port qualified addresses.
382 .It Cm LoginGraceTime
383 The server disconnects after this time if the user has not
384 successfully logged in.
385 If the value is 0, there is no time limit.
386 The default is 600 (seconds).
388 Gives the verbosity level that is used when logging messages from
390 The possible values are:
391 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
392 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
393 and DEBUG3 each specify higher levels of debugging output.
394 Logging with a DEBUG level violates the privacy of users
395 and is not recommended.
397 Specifies the available MAC (message authentication code) algorithms.
398 The MAC algorithm is used in protocol version 2
399 for data integrity protection.
400 Multiple algorithms must be comma-separated.
402 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
404 Specifies the maximum number of concurrent unauthenticated connections to the
407 Additional connections will be dropped until authentication succeeds or the
409 expires for a connection.
412 Alternatively, random early drop can be enabled by specifying
413 the three colon separated values
417 will refuse connection attempts with a probability of
420 if there are currently
423 unauthenticated connections.
424 The probability increases linearly and all connection attempts
425 are refused if the number of unauthenticated connections reaches
428 .It Cm PasswordAuthentication
429 Specifies whether password authentication is allowed.
432 .It Cm PermitEmptyPasswords
433 When password authentication is allowed, it specifies whether the
434 server allows login to accounts with empty password strings.
437 .It Cm PermitRootLogin
438 Specifies whether root can login using
442 .Dq without-password ,
443 .Dq forced-commands-only
449 If this option is set to
451 password authentication is disabled for root.
453 If this option is set to
454 .Dq forced-commands-only
455 root login with public key authentication will be allowed,
458 option has been specified
459 (which may be useful for taking remote backups even if root login is
460 normally not allowed). All other authentication methods are disabled
463 If this option is set to
465 root is not allowed to login.
467 Specifies the file that contains the process identifier of the
471 .Pa /var/run/sshd.pid .
473 Specifies the port number that
477 Multiple options of this type are permitted.
483 should print the date and time when the user last logged in.
491 when a user logs in interactively.
492 (On some systems it is also printed by the shell,
498 Specifies the protocol versions
501 The possible values are
505 Multiple versions must be comma-separated.
508 .It Cm PubkeyAuthentication
509 Specifies whether public key authentication is allowed.
512 Note that this option applies to protocol version 2 only.
513 .It Cm RhostsAuthentication
514 Specifies whether authentication using rhosts or /etc/hosts.equiv
516 Normally, this method should not be permitted because it is insecure.
517 .Cm RhostsRSAAuthentication
519 instead, because it performs RSA-based host authentication in addition
520 to normal rhosts or /etc/hosts.equiv authentication.
523 This option applies to protocol version 1 only.
524 .It Cm RhostsRSAAuthentication
525 Specifies whether rhosts or /etc/hosts.equiv authentication together
526 with successful RSA host authentication is allowed.
529 This option applies to protocol version 1 only.
530 .It Cm RSAAuthentication
531 Specifies whether pure RSA authentication is allowed.
534 This option applies to protocol version 1 only.
536 Defines the number of bits in the ephemeral protocol version 1 server key.
537 The minimum value is 512, and the default is 768.
541 should check file modes and ownership of the
542 user's files and home directory before accepting login.
543 This is normally desirable because novices sometimes accidentally leave their
544 directory or files world-writable.
548 Configures an external subsystem (e.g., file transfer daemon).
549 Arguments should be a subsystem name and a command to execute upon subsystem
555 file transfer subsystem.
556 By default no subsystems are defined.
557 Note that this option applies to protocol version 2 only.
558 .It Cm SyslogFacility
559 Gives the facility code that is used when logging messages from
561 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
562 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
567 is used for interactive login sessions.
572 is never used for remote command execution.
573 Note also, that if this is enabled,
575 will be disabled because
577 does not know how to handle
580 .Cm UsePrivilegeSeparation
581 is specified, it will be disabled after authentication.
582 .It Cm UsePrivilegeSeparation
585 separates privileges by creating an unprivileged child process
586 to deal with incoming network traffic. After successful authentication,
587 another process will be created that has the privilege of the authenticated
588 user. The goal of privilege separation is to prevent privilege
589 escalation by containing any corruption within the unprivileged processes.
592 .It Cm VerifyReverseMapping
595 should try to verify the remote host name and check that
596 the resolved host name for the remote IP address maps back to the
597 very same IP address.
600 .It Cm X11DisplayOffset
601 Specifies the first display number available for
606 from interfering with real X11 servers.
609 Specifies whether X11 forwarding is permitted.
612 Note that disabling X11 forwarding does not improve security in any
613 way, as users can always install their own forwarders.
614 X11 forwarding is automatically disabled if
617 .It Cm X11UseLocalhost
620 should bind the X11 forwarding server to the loopback address or to
621 the wildcard address. By default,
623 binds the forwarding server to the loopback address and sets the
626 environment variable to
628 This prevents remote hosts from connecting to the fake display.
629 However, some older X11 clients may not function with this
634 to specify that the forwarding server should be bound to the wildcard
643 Specifies the location of the
647 .Pa /usr/X11R6/bin/xauth .
652 command-line arguments and configuration file options that specify time
653 may be expressed using a sequence of the form:
655 .Ar time Oo Ar qualifier Oc ,
659 is a positive integer value and
661 is one of the following:
663 .Bl -tag -width Ds -compact -offset indent
678 Each member of the sequence is added together to calculate
679 the total time value.
681 Time format examples:
683 .Bl -tag -width Ds -compact -offset indent
685 600 seconds (10 minutes)
689 1 hour 30 minutes (90 minutes)
693 .It Pa /etc/ssh/sshd_config
694 Contains configuration data for
696 This file should be writable by root only, but it is recommended
697 (though not necessary) that it be world-readable.
700 OpenSSH is a derivative of the original and free
701 ssh 1.2.12 release by Tatu Ylonen.
702 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
703 Theo de Raadt and Dug Song
704 removed many bugs, re-added newer features and
706 Markus Friedl contributed the support for SSH
707 protocol versions 1.5 and 2.0.
708 Niels Provos and Markus Friedl contributed support
709 for privilege separation.