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 1.22 2003/08/13 08:46:31 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 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 .It Cm KerberosTicketCleanup
327 Specifies whether to automatically destroy the user's ticket cache
331 .It Cm KeyRegenerationInterval
332 In protocol version 1, the ephemeral server key is automatically regenerated
333 after this many seconds (if it has been used).
334 The purpose of regeneration is to prevent
335 decrypting captured sessions by later breaking into the machine and
337 The key is never stored anywhere.
338 If the value is 0, the key is never regenerated.
339 The default is 3600 (seconds).
341 Specifies the local addresses
344 The following forms may be used:
346 .Bl -item -offset indent -compact
350 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
355 .Ar host No | Ar IPv4_addr No : Ar port
361 .Ar host No | Ar IPv6_addr Oc : Ar port
369 will listen on the address and all prior
372 The default is to listen on all local addresses.
375 options are permitted.
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 PasswordAuthentication
426 Specifies whether password authentication is allowed.
429 .It Cm PermitEmptyPasswords
430 When password authentication is allowed, it specifies whether the
431 server allows login to accounts with empty password strings.
434 .It Cm PermitRootLogin
435 Specifies whether root can login using
439 .Dq without-password ,
440 .Dq forced-commands-only
446 If this option is set to
448 password authentication is disabled for root.
450 If this option is set to
451 .Dq forced-commands-only
452 root login with public key authentication will be allowed,
455 option has been specified
456 (which may be useful for taking remote backups even if root login is
457 normally not allowed).
458 All other authentication methods are disabled for root.
460 If this option is set to
462 root is not allowed to login.
463 .It Cm PermitUserEnvironment
465 .Pa ~/.ssh/environment
469 .Pa ~/.ssh/authorized_keys
474 Enabling environment processing may enable users to bypass access
475 restrictions in some configurations using mechanisms such as
478 Specifies the file that contains the process ID of the
482 .Pa /var/run/sshd.pid .
484 Specifies the port number that
488 Multiple options of this type are permitted.
494 should print the date and time when the user last logged in.
502 when a user logs in interactively.
503 (On some systems it is also printed by the shell,
509 Specifies the protocol versions
512 The possible values are
516 Multiple versions must be comma-separated.
519 Note that the order of the protocol list does not indicate preference,
520 because the client selects among multiple protocol versions offered
526 .It Cm PubkeyAuthentication
527 Specifies whether public key authentication is allowed.
530 Note that this option applies to protocol version 2 only.
531 .Cm RhostsRSAAuthentication
533 instead, because it performs RSA-based host authentication in addition
534 to normal rhosts or /etc/hosts.equiv authentication.
537 This option applies to protocol version 1 only.
538 .It Cm RhostsRSAAuthentication
539 Specifies whether rhosts or /etc/hosts.equiv authentication together
540 with successful RSA host authentication is allowed.
543 This option applies to protocol version 1 only.
544 .It Cm RSAAuthentication
545 Specifies whether pure RSA authentication is allowed.
548 This option applies to protocol version 1 only.
550 Defines the number of bits in the ephemeral protocol version 1 server key.
551 The minimum value is 512, and the default is 768.
555 should check file modes and ownership of the
556 user's files and home directory before accepting login.
557 This is normally desirable because novices sometimes accidentally leave their
558 directory or files world-writable.
562 Configures an external subsystem (e.g., file transfer daemon).
563 Arguments should be a subsystem name and a command to execute upon subsystem
569 file transfer subsystem.
570 By default no subsystems are defined.
571 Note that this option applies to protocol version 2 only.
572 .It Cm SyslogFacility
573 Gives the facility code that is used when logging messages from
575 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
576 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
581 should lookup the remote host name and check that
582 the resolved host name for the remote IP address maps back to the
583 very same IP address.
589 is used for interactive login sessions.
594 is never used for remote command execution.
595 Note also, that if this is enabled,
597 will be disabled because
599 does not know how to handle
603 .Cm UsePrivilegeSeparation
604 is specified, it will be disabled after authentication.
606 Enables PAM authentication (via challenge-response) and session set up.
607 If you enable this, you should probably disable
608 .Cm PasswordAuthentication .
611 then you will not be able to run sshd as a non-root user.
612 .It Cm UsePrivilegeSeparation
615 separates privileges by creating an unprivileged child process
616 to deal with incoming network traffic.
617 After successful authentication, another process will be created that has
618 the privilege of the authenticated user.
619 The goal of privilege separation is to prevent privilege
620 escalation by containing any corruption within the unprivileged processes.
623 .It Cm X11DisplayOffset
624 Specifies the first display number available for
629 from interfering with real X11 servers.
632 Specifies whether X11 forwarding is permitted.
640 When X11 forwarding is enabled, there may be additional exposure to
641 the server and to client displays if the
643 proxy display is configured to listen on the wildcard address (see
645 below), however this is not the default.
646 Additionally, the authentication spoofing and authentication data
647 verification and substitution occur on the client side.
648 The security risk of using X11 forwarding is that the client's X11
649 display server may be exposed to attack when the ssh client requests
650 forwarding (see the warnings for
654 A system administrator may have a stance in which they want to
655 protect clients that may expose themselves to attack by unwittingly
656 requesting X11 forwarding, which can warrant a
660 Note that disabling X11 forwarding does not prevent users from
661 forwarding X11 traffic, as users can always install their own forwarders.
662 X11 forwarding is automatically disabled if
665 .It Cm X11UseLocalhost
668 should bind the X11 forwarding server to the loopback address or to
669 the wildcard address.
672 binds the forwarding server to the loopback address and sets the
675 environment variable to
677 This prevents remote hosts from connecting to the proxy display.
678 However, some older X11 clients may not function with this
683 to specify that the forwarding server should be bound to the wildcard
692 Specifies the full pathname of the
696 .Pa /usr/X11R6/bin/xauth .
700 command-line arguments and configuration file options that specify time
701 may be expressed using a sequence of the form:
703 .Ar time Op Ar qualifier ,
707 is a positive integer value and
709 is one of the following:
711 .Bl -tag -width Ds -compact -offset indent
726 Each member of the sequence is added together to calculate
727 the total time value.
729 Time format examples:
731 .Bl -tag -width Ds -compact -offset indent
733 600 seconds (10 minutes)
737 1 hour 30 minutes (90 minutes)
741 .It Pa /etc/ssh/sshd_config
742 Contains configuration data for
744 This file should be writable by root only, but it is recommended
745 (though not necessary) that it be world-readable.
750 OpenSSH is a derivative of the original and free
751 ssh 1.2.12 release by Tatu Ylonen.
752 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
753 Theo de Raadt and Dug Song
754 removed many bugs, re-added newer features and
756 Markus Friedl contributed the support for SSH
757 protocol versions 1.5 and 2.0.
758 Niels Provos and Markus Friedl contributed support
759 for privilege separation.