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 1.25 2003/09/01 09:50:04 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):
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 GSSAPICleanupCreds
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 .
308 Specifies whether the system should send TCP keepalive messages to the
310 If they are sent, death of the connection or crash of one
311 of the machines will be properly noticed.
312 However, this means that
313 connections will die if the route is down temporarily, and some people
315 On the other hand, if keepalives are not sent,
316 sessions may hang indefinitely on the server, leaving
318 users and consuming server resources.
322 (to send keepalives), and the server will notice
323 if the network goes down or the client host crashes.
324 This avoids infinitely hanging sessions.
326 To disable keepalives, the value should be set to
328 .It Cm KerberosAuthentication
329 Specifies whether the password provided by the user for
330 .Cm PasswordAuthentication
331 will be validated through the Kerberos KDC.
332 To use this option, the server needs a
333 Kerberos servtab which allows the verification of the KDC's identity.
336 .It Cm KerberosOrLocalPasswd
337 If set then if password authentication through Kerberos fails then
338 the password will be validated via any additional local mechanism
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
389 The default is to listen on all local addresses.
392 options are permitted.
395 options must precede this option for non port qualified addresses.
396 .It Cm LoginGraceTime
397 The server disconnects after this time if the user has not
398 successfully logged in.
399 If the value is 0, there is no time limit.
400 The default is 120 seconds.
402 Gives the verbosity level that is used when logging messages from
404 The possible values are:
405 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
407 DEBUG and DEBUG1 are equivalent.
408 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
409 Logging with a DEBUG level violates the privacy of users and is not recommended.
411 Specifies the available MAC (message authentication code) algorithms.
412 The MAC algorithm is used in protocol version 2
413 for data integrity protection.
414 Multiple algorithms must be comma-separated.
416 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
418 Specifies the maximum number of concurrent unauthenticated connections to the
421 Additional connections will be dropped until authentication succeeds or the
423 expires for a connection.
426 Alternatively, random early drop can be enabled by specifying
427 the three colon separated values
431 will refuse connection attempts with a probability of
434 if there are currently
437 unauthenticated connections.
438 The probability increases linearly and all connection attempts
439 are refused if the number of unauthenticated connections reaches
442 .It Cm PasswordAuthentication
443 Specifies whether password authentication is allowed.
446 .It Cm PermitEmptyPasswords
447 When password authentication is allowed, it specifies whether the
448 server allows login to accounts with empty password strings.
451 .It Cm PermitRootLogin
452 Specifies whether root can login using
456 .Dq without-password ,
457 .Dq forced-commands-only
463 If this option is set to
465 password authentication is disabled for root.
467 If this option is set to
468 .Dq forced-commands-only
469 root login with public key authentication will be allowed,
472 option has been specified
473 (which may be useful for taking remote backups even if root login is
474 normally not allowed).
475 All other authentication methods are disabled for root.
477 If this option is set to
479 root is not allowed to login.
480 .It Cm PermitUserEnvironment
482 .Pa ~/.ssh/environment
486 .Pa ~/.ssh/authorized_keys
491 Enabling environment processing may enable users to bypass access
492 restrictions in some configurations using mechanisms such as
495 Specifies the file that contains the process ID of the
499 .Pa /var/run/sshd.pid .
501 Specifies the port number that
505 Multiple options of this type are permitted.
511 should print the date and time when the user last logged in.
519 when a user logs in interactively.
520 (On some systems it is also printed by the shell,
526 Specifies the protocol versions
529 The possible values are
533 Multiple versions must be comma-separated.
536 Note that the order of the protocol list does not indicate preference,
537 because the client selects among multiple protocol versions offered
543 .It Cm PubkeyAuthentication
544 Specifies whether public key authentication is allowed.
547 Note that this option applies to protocol version 2 only.
548 .Cm RhostsRSAAuthentication
550 instead, because it performs RSA-based host authentication in addition
551 to normal rhosts or /etc/hosts.equiv authentication.
554 This option applies to protocol version 1 only.
555 .It Cm RhostsRSAAuthentication
556 Specifies whether rhosts or /etc/hosts.equiv authentication together
557 with successful RSA host authentication is allowed.
560 This option applies to protocol version 1 only.
561 .It Cm RSAAuthentication
562 Specifies whether pure RSA authentication is allowed.
565 This option applies to protocol version 1 only.
567 Defines the number of bits in the ephemeral protocol version 1 server key.
568 The minimum value is 512, and the default is 768.
572 should check file modes and ownership of the
573 user's files and home directory before accepting login.
574 This is normally desirable because novices sometimes accidentally leave their
575 directory or files world-writable.
579 Configures an external subsystem (e.g., file transfer daemon).
580 Arguments should be a subsystem name and a command to execute upon subsystem
586 file transfer subsystem.
587 By default no subsystems are defined.
588 Note that this option applies to protocol version 2 only.
589 .It Cm SyslogFacility
590 Gives the facility code that is used when logging messages from
592 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
593 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
598 should lookup the remote host name and check that
599 the resolved host name for the remote IP address maps back to the
600 very same IP address.
606 is used for interactive login sessions.
611 is never used for remote command execution.
612 Note also, that if this is enabled,
614 will be disabled because
616 does not know how to handle
620 .Cm UsePrivilegeSeparation
621 is specified, it will be disabled after authentication.
623 Enables PAM authentication (via challenge-response) and session set up.
624 If you enable this, you should probably disable
625 .Cm PasswordAuthentication .
628 then you will not be able to run sshd as a non-root user.
629 .It Cm UsePrivilegeSeparation
632 separates privileges by creating an unprivileged child process
633 to deal with incoming network traffic.
634 After successful authentication, another process will be created that has
635 the privilege of the authenticated user.
636 The goal of privilege separation is to prevent privilege
637 escalation by containing any corruption within the unprivileged processes.
640 .It Cm X11DisplayOffset
641 Specifies the first display number available for
646 from interfering with real X11 servers.
649 Specifies whether X11 forwarding is permitted.
657 When X11 forwarding is enabled, there may be additional exposure to
658 the server and to client displays if the
660 proxy display is configured to listen on the wildcard address (see
662 below), however this is not the default.
663 Additionally, the authentication spoofing and authentication data
664 verification and substitution occur on the client side.
665 The security risk of using X11 forwarding is that the client's X11
666 display server may be exposed to attack when the ssh client requests
667 forwarding (see the warnings for
671 A system administrator may have a stance in which they want to
672 protect clients that may expose themselves to attack by unwittingly
673 requesting X11 forwarding, which can warrant a
677 Note that disabling X11 forwarding does not prevent users from
678 forwarding X11 traffic, as users can always install their own forwarders.
679 X11 forwarding is automatically disabled if
682 .It Cm X11UseLocalhost
685 should bind the X11 forwarding server to the loopback address or to
686 the wildcard address.
689 binds the forwarding server to the loopback address and sets the
692 environment variable to
694 This prevents remote hosts from connecting to the proxy display.
695 However, some older X11 clients may not function with this
700 to specify that the forwarding server should be bound to the wildcard
709 Specifies the full pathname of the
713 .Pa /usr/X11R6/bin/xauth .
717 command-line arguments and configuration file options that specify time
718 may be expressed using a sequence of the form:
720 .Ar time Op Ar qualifier ,
724 is a positive integer value and
726 is one of the following:
728 .Bl -tag -width Ds -compact -offset indent
743 Each member of the sequence is added together to calculate
744 the total time value.
746 Time format examples:
748 .Bl -tag -width Ds -compact -offset indent
750 600 seconds (10 minutes)
754 1 hour 30 minutes (90 minutes)
758 .It Pa /etc/ssh/sshd_config
759 Contains configuration data for
761 This file should be writable by root only, but it is recommended
762 (though not necessary) that it be world-readable.
767 OpenSSH is a derivative of the original and free
768 ssh 1.2.12 release by Tatu Ylonen.
769 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
770 Theo de Raadt and Dug Song
771 removed many bugs, re-added newer features and
773 Markus Friedl contributed the support for SSH
774 protocol versions 1.5 and 2.0.
775 Niels Provos and Markus Friedl contributed support
776 for privilege separation.