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 1.15 2003/03/28 10:11:43 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):
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 user 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.
215 This 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 .Cm PasswordAuthentication
309 is yes, the password provided by the user will be validated through
311 To use this option, the server needs a
312 Kerberos servtab which allows the verification of the KDC's identity.
315 .It Cm KerberosOrLocalPasswd
316 If set then if password authentication through Kerberos fails then
317 the password will be validated via any additional local mechanism
322 .It Cm KerberosTgtPassing
323 Specifies whether a Kerberos TGT may be forwarded to the server.
326 as this only works when the Kerberos KDC is actually an AFS kaserver.
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
372 options specified. The default is to listen on all local
376 options are permitted. Additionally, any
378 options must precede this option for non port qualified addresses.
379 .It Cm LoginGraceTime
380 The server disconnects after this time if the user has not
381 successfully logged in.
382 If the value is 0, there is no time limit.
383 The default is 120 seconds.
385 Gives the verbosity level that is used when logging messages from
387 The possible values are:
388 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
390 DEBUG and DEBUG1 are equivalent.
391 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
392 Logging with a DEBUG level violates the privacy of users and is not recommended.
394 Specifies the available MAC (message authentication code) algorithms.
395 The MAC algorithm is used in protocol version 2
396 for data integrity protection.
397 Multiple algorithms must be comma-separated.
399 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
401 Specifies the maximum number of concurrent unauthenticated connections to the
404 Additional connections will be dropped until authentication succeeds or the
406 expires for a connection.
409 Alternatively, random early drop can be enabled by specifying
410 the three colon separated values
414 will refuse connection attempts with a probability of
417 if there are currently
420 unauthenticated connections.
421 The probability increases linearly and all connection attempts
422 are refused if the number of unauthenticated connections reaches
425 .It Cm PAMAuthenticationViaKbdInt
426 Specifies whether PAM challenge response authentication is allowed. This
427 allows the use of most PAM challenge response authentication modules, but
428 it will allow password authentication regardless of whether
429 .Cm PasswordAuthentication
431 .It Cm PasswordAuthentication
432 Specifies whether password authentication is allowed.
435 .It Cm PermitEmptyPasswords
436 When password authentication is allowed, it specifies whether the
437 server allows login to accounts with empty password strings.
440 .It Cm PermitRootLogin
441 Specifies whether root can login using
445 .Dq without-password ,
446 .Dq forced-commands-only
452 If this option is set to
454 password authentication is disabled for root.
456 If this option is set to
457 .Dq forced-commands-only
458 root login with public key authentication will be allowed,
461 option has been specified
462 (which may be useful for taking remote backups even if root login is
463 normally not allowed). All other authentication methods are disabled
466 If this option is set to
468 root is not allowed to login.
469 .It Cm PermitUserEnvironment
471 .Pa ~/.ssh/environment
475 .Pa ~/.ssh/authorized_keys
480 Enabling environment processing may enable users to bypass access
481 restrictions in some configurations using mechanisms such as
484 Specifies the file that contains the process ID of the
488 .Pa /var/run/sshd.pid .
490 Specifies the port number that
494 Multiple options of this type are permitted.
500 should print the date and time when the user last logged in.
508 when a user logs in interactively.
509 (On some systems it is also printed by the shell,
515 Specifies the protocol versions
518 The possible values are
522 Multiple versions must be comma-separated.
525 Note that the order of the protocol list does not indicate preference,
526 because the client selects among multiple protocol versions offered
532 .It Cm PubkeyAuthentication
533 Specifies whether public key authentication is allowed.
536 Note that this option applies to protocol version 2 only.
537 .It Cm RhostsAuthentication
538 Specifies whether authentication using rhosts or /etc/hosts.equiv
540 Normally, this method should not be permitted because it is insecure.
541 .Cm RhostsRSAAuthentication
543 instead, because it performs RSA-based host authentication in addition
544 to normal rhosts or /etc/hosts.equiv authentication.
547 This option applies to protocol version 1 only.
548 .It Cm RhostsRSAAuthentication
549 Specifies whether rhosts or /etc/hosts.equiv authentication together
550 with successful RSA host authentication is allowed.
553 This option applies to protocol version 1 only.
554 .It Cm RSAAuthentication
555 Specifies whether pure RSA authentication is allowed.
558 This option applies to protocol version 1 only.
560 Defines the number of bits in the ephemeral protocol version 1 server key.
561 The minimum value is 512, and the default is 768.
565 should check file modes and ownership of the
566 user's files and home directory before accepting login.
567 This is normally desirable because novices sometimes accidentally leave their
568 directory or files world-writable.
572 Configures an external subsystem (e.g., file transfer daemon).
573 Arguments should be a subsystem name and a command to execute upon subsystem
579 file transfer subsystem.
580 By default no subsystems are defined.
581 Note that this option applies to protocol version 2 only.
582 .It Cm SyslogFacility
583 Gives the facility code that is used when logging messages from
585 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
586 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
591 is used for interactive login sessions.
596 is never used for remote command execution.
597 Note also, that if this is enabled,
599 will be disabled because
601 does not know how to handle
605 .Cm UsePrivilegeSeparation
606 is specified, it will be disabled after authentication.
607 .It Cm UsePrivilegeSeparation
610 separates privileges by creating an unprivileged child process
611 to deal with incoming network traffic.
612 After successful authentication, another process will be created that has
613 the privilege of the authenticated user.
614 The goal of privilege separation is to prevent privilege
615 escalation by containing any corruption within the unprivileged processes.
618 .It Cm VerifyReverseMapping
621 should try to verify the remote host name and check that
622 the resolved host name for the remote IP address maps back to the
623 very same IP address.
626 .It Cm X11DisplayOffset
627 Specifies the first display number available for
632 from interfering with real X11 servers.
635 Specifies whether X11 forwarding is permitted.
643 When X11 forwarding is enabled, there may be additional exposure to
644 the server and to client displays if the
646 proxy display is configured to listen on the wildcard address (see
648 below), however this is not the default.
649 Additionally, the authentication spoofing and authentication data
650 verification and substitution occur on the client side.
651 The security risk of using X11 forwarding is that the client's X11
652 display server may be exposed to attack when the ssh client requests
653 forwarding (see the warnings for
657 A system administrator may have a stance in which they want to
658 protect clients that may expose themselves to attack by unwittingly
659 requesting X11 forwarding, which can warrant a
663 Note that disabling X11 forwarding does not prevent users from
664 forwarding X11 traffic, as users can always install their own forwarders.
665 X11 forwarding is automatically disabled if
668 .It Cm X11UseLocalhost
671 should bind the X11 forwarding server to the loopback address or to
672 the wildcard address.
675 binds the forwarding server to the loopback address and sets the
678 environment variable to
680 This prevents remote hosts from connecting to the proxy display.
681 However, some older X11 clients may not function with this
686 to specify that the forwarding server should be bound to the wildcard
695 Specifies the full pathname of the
699 .Pa /usr/X11R6/bin/xauth .
704 command-line arguments and configuration file options that specify time
705 may be expressed using a sequence of the form:
707 .Ar time Op Ar qualifier ,
711 is a positive integer value and
713 is one of the following:
715 .Bl -tag -width Ds -compact -offset indent
730 Each member of the sequence is added together to calculate
731 the total time value.
733 Time format examples:
735 .Bl -tag -width Ds -compact -offset indent
737 600 seconds (10 minutes)
741 1 hour 30 minutes (90 minutes)
745 .It Pa /etc/ssh/sshd_config
746 Contains configuration data for
748 This file should be writable by root only, but it is recommended
749 (though not necessary) that it be world-readable.
752 OpenSSH is a derivative of the original and free
753 ssh 1.2.12 release by Tatu Ylonen.
754 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
755 Theo de Raadt and Dug Song
756 removed many bugs, re-added newer features and
758 Markus Friedl contributed the support for SSH
759 protocol versions 1.5 and 2.0.
760 Niels Provos and Markus Friedl contributed support
761 for privilege separation.