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 1.21 2003/07/22 13:35:22 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 HostbasedAuthentication
229 Specifies whether rhosts or /etc/hosts.equiv authentication together
230 with successful public key client host authentication is allowed
231 (hostbased authentication).
232 This option is similar to
233 .Cm RhostsRSAAuthentication
234 and applies to protocol version 2 only.
238 Specifies a file containing a private host key
241 .Pa /etc/ssh/ssh_host_key
242 for protocol version 1, and
243 .Pa /etc/ssh/ssh_host_rsa_key
245 .Pa /etc/ssh/ssh_host_dsa_key
246 for protocol version 2.
249 will refuse to use a file if it is group/world-accessible.
250 It is possible to have multiple host key files.
252 keys are used for version 1 and
256 are used for version 2 of the SSH protocol.
262 files will not be used in
263 .Cm RhostsAuthentication ,
264 .Cm RhostsRSAAuthentication
266 .Cm HostbasedAuthentication .
270 .Pa /etc/shosts.equiv
274 .It Cm IgnoreUserKnownHosts
277 should ignore the user's
278 .Pa $HOME/.ssh/known_hosts
280 .Cm RhostsRSAAuthentication
282 .Cm HostbasedAuthentication .
286 Specifies whether the system should send TCP keepalive messages to the
288 If they are sent, death of the connection or crash of one
289 of the machines will be properly noticed.
290 However, this means that
291 connections will die if the route is down temporarily, and some people
293 On the other hand, if keepalives are not sent,
294 sessions may hang indefinitely on the server, leaving
296 users and consuming server resources.
300 (to send keepalives), and the server will notice
301 if the network goes down or the client host crashes.
302 This avoids infinitely hanging sessions.
304 To disable keepalives, the value should be set to
306 .It Cm KerberosAuthentication
307 Specifies whether Kerberos authentication is allowed.
308 This can be in the form of a Kerberos ticket, or if
309 .Cm PasswordAuthentication
310 is yes, the password provided by the user will be validated through
312 To use this option, the server needs a
313 Kerberos servtab which allows the verification of the KDC's identity.
316 .It Cm KerberosOrLocalPasswd
317 If set then if password authentication through Kerberos fails then
318 the password will be validated via any additional local mechanism
323 .It Cm KerberosTgtPassing
324 Specifies whether a Kerberos TGT may be forwarded to the server.
327 .It Cm KerberosTicketCleanup
328 Specifies whether to automatically destroy the user's ticket cache
332 .It Cm KeyRegenerationInterval
333 In protocol version 1, the ephemeral server key is automatically regenerated
334 after this many seconds (if it has been used).
335 The purpose of regeneration is to prevent
336 decrypting captured sessions by later breaking into the machine and
338 The key is never stored anywhere.
339 If the value is 0, the key is never regenerated.
340 The default is 3600 (seconds).
342 Specifies the local addresses
345 The following forms may be used:
347 .Bl -item -offset indent -compact
351 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
356 .Ar host No | Ar IPv4_addr No : Ar port
362 .Ar host No | Ar IPv6_addr Oc : Ar port
370 will listen on the address and all prior
373 The default is to listen on all local addresses.
376 options are permitted.
379 options must precede this option for non port qualified addresses.
380 .It Cm LoginGraceTime
381 The server disconnects after this time if the user has not
382 successfully logged in.
383 If the value is 0, there is no time limit.
384 The default is 120 seconds.
386 Gives the verbosity level that is used when logging messages from
388 The possible values are:
389 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
391 DEBUG and DEBUG1 are equivalent.
392 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
393 Logging with a DEBUG level violates the privacy of users and is not recommended.
395 Specifies the available MAC (message authentication code) algorithms.
396 The MAC algorithm is used in protocol version 2
397 for data integrity protection.
398 Multiple algorithms must be comma-separated.
400 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
402 Specifies the maximum number of concurrent unauthenticated connections to the
405 Additional connections will be dropped until authentication succeeds or the
407 expires for a connection.
410 Alternatively, random early drop can be enabled by specifying
411 the three colon separated values
415 will refuse connection attempts with a probability of
418 if there are currently
421 unauthenticated connections.
422 The probability increases linearly and all connection attempts
423 are refused if the number of unauthenticated connections reaches
426 .It Cm PasswordAuthentication
427 Specifies whether password authentication is allowed.
430 .It Cm PermitEmptyPasswords
431 When password authentication is allowed, it specifies whether the
432 server allows login to accounts with empty password strings.
435 .It Cm PermitRootLogin
436 Specifies whether root can login using
440 .Dq without-password ,
441 .Dq forced-commands-only
447 If this option is set to
449 password authentication is disabled for root.
451 If this option is set to
452 .Dq forced-commands-only
453 root login with public key authentication will be allowed,
456 option has been specified
457 (which may be useful for taking remote backups even if root login is
458 normally not allowed).
459 All other authentication methods are disabled for root.
461 If this option is set to
463 root is not allowed to login.
464 .It Cm PermitUserEnvironment
466 .Pa ~/.ssh/environment
470 .Pa ~/.ssh/authorized_keys
475 Enabling environment processing may enable users to bypass access
476 restrictions in some configurations using mechanisms such as
479 Specifies the file that contains the process ID of the
483 .Pa /var/run/sshd.pid .
485 Specifies the port number that
489 Multiple options of this type are permitted.
495 should print the date and time when the user last logged in.
503 when a user logs in interactively.
504 (On some systems it is also printed by the shell,
510 Specifies the protocol versions
513 The possible values are
517 Multiple versions must be comma-separated.
520 Note that the order of the protocol list does not indicate preference,
521 because the client selects among multiple protocol versions offered
527 .It Cm PubkeyAuthentication
528 Specifies whether public key authentication is allowed.
531 Note that this option applies to protocol version 2 only.
532 .It Cm RhostsAuthentication
533 Specifies whether authentication using rhosts or /etc/hosts.equiv
535 Normally, this method should not be permitted because it is insecure.
536 .Cm RhostsRSAAuthentication
538 instead, because it performs RSA-based host authentication in addition
539 to normal rhosts or /etc/hosts.equiv authentication.
542 This option applies to protocol version 1 only.
543 .It Cm RhostsRSAAuthentication
544 Specifies whether rhosts or /etc/hosts.equiv authentication together
545 with successful RSA host authentication is allowed.
548 This option applies to protocol version 1 only.
549 .It Cm RSAAuthentication
550 Specifies whether pure RSA authentication is allowed.
553 This option applies to protocol version 1 only.
555 Defines the number of bits in the ephemeral protocol version 1 server key.
556 The minimum value is 512, and the default is 768.
560 should check file modes and ownership of the
561 user's files and home directory before accepting login.
562 This is normally desirable because novices sometimes accidentally leave their
563 directory or files world-writable.
567 Configures an external subsystem (e.g., file transfer daemon).
568 Arguments should be a subsystem name and a command to execute upon subsystem
574 file transfer subsystem.
575 By default no subsystems are defined.
576 Note that this option applies to protocol version 2 only.
577 .It Cm SyslogFacility
578 Gives the facility code that is used when logging messages from
580 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
581 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
586 should lookup the remote host name and check that
587 the resolved host name for the remote IP address maps back to the
588 very same IP address.
594 is used for interactive login sessions.
599 is never used for remote command execution.
600 Note also, that if this is enabled,
602 will be disabled because
604 does not know how to handle
608 .Cm UsePrivilegeSeparation
609 is specified, it will be disabled after authentication.
611 Enables PAM authentication (via challenge-response) and session set up.
612 If you enable this, you should probably disable
613 .Cm PasswordAuthentication .
616 then you will not be able to run sshd as a non-root user.
617 .It Cm UsePrivilegeSeparation
620 separates privileges by creating an unprivileged child process
621 to deal with incoming network traffic.
622 After successful authentication, another process will be created that has
623 the privilege of the authenticated user.
624 The goal of privilege separation is to prevent privilege
625 escalation by containing any corruption within the unprivileged processes.
628 .It Cm X11DisplayOffset
629 Specifies the first display number available for
634 from interfering with real X11 servers.
637 Specifies whether X11 forwarding is permitted.
645 When X11 forwarding is enabled, there may be additional exposure to
646 the server and to client displays if the
648 proxy display is configured to listen on the wildcard address (see
650 below), however this is not the default.
651 Additionally, the authentication spoofing and authentication data
652 verification and substitution occur on the client side.
653 The security risk of using X11 forwarding is that the client's X11
654 display server may be exposed to attack when the ssh client requests
655 forwarding (see the warnings for
659 A system administrator may have a stance in which they want to
660 protect clients that may expose themselves to attack by unwittingly
661 requesting X11 forwarding, which can warrant a
665 Note that disabling X11 forwarding does not prevent users from
666 forwarding X11 traffic, as users can always install their own forwarders.
667 X11 forwarding is automatically disabled if
670 .It Cm X11UseLocalhost
673 should bind the X11 forwarding server to the loopback address or to
674 the wildcard address.
677 binds the forwarding server to the loopback address and sets the
680 environment variable to
682 This prevents remote hosts from connecting to the proxy display.
683 However, some older X11 clients may not function with this
688 to specify that the forwarding server should be bound to the wildcard
697 Specifies the full pathname of the
701 .Pa /usr/X11R6/bin/xauth .
705 command-line arguments and configuration file options that specify time
706 may be expressed using a sequence of the form:
708 .Ar time Op Ar qualifier ,
712 is a positive integer value and
714 is one of the following:
716 .Bl -tag -width Ds -compact -offset indent
731 Each member of the sequence is added together to calculate
732 the total time value.
734 Time format examples:
736 .Bl -tag -width Ds -compact -offset indent
738 600 seconds (10 minutes)
742 1 hour 30 minutes (90 minutes)
746 .It Pa /etc/ssh/sshd_config
747 Contains configuration data for
749 This file should be writable by root only, but it is recommended
750 (though not necessary) that it be world-readable.
755 OpenSSH is a derivative of the original and free
756 ssh 1.2.12 release by Tatu Ylonen.
757 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
758 Theo de Raadt and Dug Song
759 removed many bugs, re-added newer features and
761 Markus Friedl contributed the support for SSH
762 protocol versions 1.5 and 2.0.
763 Niels Provos and Markus Friedl contributed support
764 for privilege separation.