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 HostbasedAuthentication
240 Specifies whether rhosts or /etc/hosts.equiv authentication together
241 with successful public key client host authentication is allowed
242 (hostbased authentication).
243 This option is similar to
244 .Cm RhostsRSAAuthentication
245 and applies to protocol version 2 only.
249 Specifies a file containing a private host key
252 .Pa /etc/ssh/ssh_host_key
253 for protocol version 1, and
254 .Pa /etc/ssh/ssh_host_rsa_key
256 .Pa /etc/ssh/ssh_host_dsa_key
257 for protocol version 2.
260 will refuse to use a file if it is group/world-accessible.
261 It is possible to have multiple host key files.
263 keys are used for version 1 and
267 are used for version 2 of the SSH protocol.
273 files will not be used in
274 .Cm RhostsRSAAuthentication
276 .Cm HostbasedAuthentication .
280 .Pa /etc/shosts.equiv
284 .It Cm IgnoreUserKnownHosts
287 should ignore the user's
288 .Pa $HOME/.ssh/known_hosts
290 .Cm RhostsRSAAuthentication
292 .Cm HostbasedAuthentication .
295 .It Cm KerberosAuthentication
296 Specifies whether the password provided by the user for
297 .Cm PasswordAuthentication
298 will be validated through the Kerberos KDC.
299 To use this option, the server needs a
300 Kerberos servtab which allows the verification of the KDC's identity.
303 .It Cm KerberosOrLocalPasswd
304 If set then if password authentication through Kerberos fails then
305 the password will be validated via any additional local mechanism
310 .It Cm KerberosTicketCleanup
311 Specifies whether to automatically destroy the user's ticket cache
315 .It Cm KeyRegenerationInterval
316 In protocol version 1, the ephemeral server key is automatically regenerated
317 after this many seconds (if it has been used).
318 The purpose of regeneration is to prevent
319 decrypting captured sessions by later breaking into the machine and
321 The key is never stored anywhere.
322 If the value is 0, the key is never regenerated.
323 The default is 3600 (seconds).
325 Specifies the local addresses
328 The following forms may be used:
330 .Bl -item -offset indent -compact
334 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
339 .Ar host No | Ar IPv4_addr No : Ar port
345 .Ar host No | Ar IPv6_addr Oc : Ar port
353 will listen on the address and all prior
356 The default is to listen on all local addresses.
359 options are permitted.
362 options must precede this option for non port qualified addresses.
363 .It Cm LoginGraceTime
364 The server disconnects after this time if the user has not
365 successfully logged in.
366 If the value is 0, there is no time limit.
367 The default is 120 seconds.
369 Gives the verbosity level that is used when logging messages from
371 The possible values are:
372 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
374 DEBUG and DEBUG1 are equivalent.
375 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
376 Logging with a DEBUG level violates the privacy of users and is not recommended.
378 Specifies the available MAC (message authentication code) algorithms.
379 The MAC algorithm is used in protocol version 2
380 for data integrity protection.
381 Multiple algorithms must be comma-separated.
383 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
385 Specifies the maximum number of concurrent unauthenticated connections to the
388 Additional connections will be dropped until authentication succeeds or the
390 expires for a connection.
393 Alternatively, random early drop can be enabled by specifying
394 the three colon separated values
398 will refuse connection attempts with a probability of
401 if there are currently
404 unauthenticated connections.
405 The probability increases linearly and all connection attempts
406 are refused if the number of unauthenticated connections reaches
409 .It Cm PasswordAuthentication
410 Specifies whether password authentication is allowed.
413 .It Cm PermitEmptyPasswords
414 When password authentication is allowed, it specifies whether the
415 server allows login to accounts with empty password strings.
418 .It Cm PermitRootLogin
419 Specifies whether root can login using
423 .Dq without-password ,
424 .Dq forced-commands-only
430 If this option is set to
432 password authentication is disabled for root.
434 If this option is set to
435 .Dq forced-commands-only
436 root login with public key authentication will be allowed,
439 option has been specified
440 (which may be useful for taking remote backups even if root login is
441 normally not allowed).
442 All other authentication methods are disabled for root.
444 If this option is set to
446 root is not allowed to login.
447 .It Cm PermitUserEnvironment
449 .Pa ~/.ssh/environment
453 .Pa ~/.ssh/authorized_keys
458 Enabling environment processing may enable users to bypass access
459 restrictions in some configurations using mechanisms such as
462 Specifies the file that contains the process ID of the
466 .Pa /var/run/sshd.pid .
468 Specifies the port number that
472 Multiple options of this type are permitted.
478 should print the date and time when the user last logged in.
486 when a user logs in interactively.
487 (On some systems it is also printed by the shell,
493 Specifies the protocol versions
496 The possible values are
500 Multiple versions must be comma-separated.
503 Note that the order of the protocol list does not indicate preference,
504 because the client selects among multiple protocol versions offered
510 .It Cm PubkeyAuthentication
511 Specifies whether public key authentication is allowed.
514 Note that this option applies to protocol version 2 only.
515 .It Cm RhostsRSAAuthentication
516 Specifies whether rhosts or /etc/hosts.equiv authentication together
517 with successful RSA host authentication is allowed.
520 This option applies to protocol version 1 only.
521 .It Cm RSAAuthentication
522 Specifies whether pure RSA authentication is allowed.
525 This option applies to protocol version 1 only.
527 Defines the number of bits in the ephemeral protocol version 1 server key.
528 The minimum value is 512, and the default is 768.
532 should check file modes and ownership of the
533 user's files and home directory before accepting login.
534 This is normally desirable because novices sometimes accidentally leave their
535 directory or files world-writable.
539 Configures an external subsystem (e.g., file transfer daemon).
540 Arguments should be a subsystem name and a command to execute upon subsystem
546 file transfer subsystem.
547 By default no subsystems are defined.
548 Note that this option applies to protocol version 2 only.
549 .It Cm SyslogFacility
550 Gives the facility code that is used when logging messages from
552 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
553 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
556 Specifies whether the system should send TCP keepalive messages to the
558 If they are sent, death of the connection or crash of one
559 of the machines will be properly noticed.
560 However, this means that
561 connections will die if the route is down temporarily, and some people
563 On the other hand, if TCP keepalives are not sent,
564 sessions may hang indefinitely on the server, leaving
566 users and consuming server resources.
570 (to send TCP keepalive messages), and the server will notice
571 if the network goes down or the client host crashes.
572 This avoids infinitely hanging sessions.
574 To disable TCP keepalive messages, the value should be set to
579 should lookup the remote host name and check that
580 the resolved host name for the remote IP address maps back to the
581 very same IP address.
587 is used for interactive login sessions.
592 is never used for remote command execution.
593 Note also, that if this is enabled,
595 will be disabled because
597 does not know how to handle
601 .Cm UsePrivilegeSeparation
602 is specified, it will be disabled after authentication.
604 Enables PAM authentication (via challenge-response) and session set up.
605 If you enable this, you should probably disable
606 .Cm PasswordAuthentication .
609 then you will not be able to run sshd as a non-root user. The default is
611 .It Cm UsePrivilegeSeparation
614 separates privileges by creating an unprivileged child process
615 to deal with incoming network traffic.
616 After successful authentication, another process will be created that has
617 the privilege of the authenticated user.
618 The goal of privilege separation is to prevent privilege
619 escalation by containing any corruption within the unprivileged processes.
622 .It Cm X11DisplayOffset
623 Specifies the first display number available for
628 from interfering with real X11 servers.
631 Specifies whether X11 forwarding is permitted.
639 When X11 forwarding is enabled, there may be additional exposure to
640 the server and to client displays if the
642 proxy display is configured to listen on the wildcard address (see
644 below), however this is not the default.
645 Additionally, the authentication spoofing and authentication data
646 verification and substitution occur on the client side.
647 The security risk of using X11 forwarding is that the client's X11
648 display server may be exposed to attack when the ssh client requests
649 forwarding (see the warnings for
653 A system administrator may have a stance in which they want to
654 protect clients that may expose themselves to attack by unwittingly
655 requesting X11 forwarding, which can warrant a
659 Note that disabling X11 forwarding does not prevent users from
660 forwarding X11 traffic, as users can always install their own forwarders.
661 X11 forwarding is automatically disabled if
664 .It Cm X11UseLocalhost
667 should bind the X11 forwarding server to the loopback address or to
668 the wildcard address.
671 binds the forwarding server to the loopback address and sets the
674 environment variable to
676 This prevents remote hosts from connecting to the proxy display.
677 However, some older X11 clients may not function with this
682 to specify that the forwarding server should be bound to the wildcard
691 Specifies the full pathname of the
695 .Pa /usr/X11R6/bin/xauth .
699 command-line arguments and configuration file options that specify time
700 may be expressed using a sequence of the form:
702 .Ar time Op Ar qualifier ,
706 is a positive integer value and
708 is one of the following:
710 .Bl -tag -width Ds -compact -offset indent
725 Each member of the sequence is added together to calculate
726 the total time value.
728 Time format examples:
730 .Bl -tag -width Ds -compact -offset indent
732 600 seconds (10 minutes)
736 1 hour 30 minutes (90 minutes)
740 .It Pa /etc/ssh/sshd_config
741 Contains configuration data for
743 This file should be writable by root only, but it is recommended
744 (though not necessary) that it be world-readable.
749 OpenSSH is a derivative of the original and free
750 ssh 1.2.12 release by Tatu Ylonen.
751 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
752 Theo de Raadt and Dug Song
753 removed many bugs, re-added newer features and
755 Markus Friedl contributed the support for SSH
756 protocol versions 1.5 and 2.0.
757 Niels Provos and Markus Friedl contributed support
758 for privilege separation.