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 1.78 2007/08/23 03:22:16 djm Exp $
43 .Nd OpenSSH SSH daemon configuration file
45 .Nm /etc/ssh/sshd_config
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
53 The file contains keyword-argument pairs, one per line.
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
59 in order to represent arguments containing spaces.
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
66 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
79 Multiple environment variables may be separated by whitespace or spread
83 Be warned that some environment variables could be used to bypass restricted
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
88 Specifies which address family should be used by
99 This keyword can be followed by a list of group name patterns, separated
101 If specified, login is allowed only for users whose primary
102 group or supplementary group list matches one of the patterns.
103 Only group names are valid; a numerical group ID is not recognized.
104 By default, login is allowed for all groups.
105 The allow/deny directives are processed in the following order:
116 for more information on patterns.
117 .It Cm AllowTcpForwarding
118 Specifies whether TCP forwarding is permitted.
121 Note that disabling TCP forwarding does not improve security unless
122 users are also denied shell access, as they can always install their
125 This keyword can be followed by a list of user name patterns, separated
127 If specified, login is allowed only for user names that
128 match one of the patterns.
129 Only user names are valid; a numerical user ID is not recognized.
130 By default, login is allowed for all users.
131 If the pattern takes the form USER@HOST then USER and HOST
132 are separately checked, restricting logins to particular
133 users from particular hosts.
134 The allow/deny directives are processed in the following order:
145 for more information on patterns.
146 .It Cm AuthorizedKeysFile
147 Specifies the file that contains the public keys that can be used
148 for user authentication.
149 .Cm AuthorizedKeysFile
150 may contain tokens of the form %T which are substituted during connection
152 The following tokens are defined: %% is replaced by a literal '%',
153 %h is replaced by the home directory of the user being authenticated, and
154 %u is replaced by the username of that user.
156 .Cm AuthorizedKeysFile
157 is taken to be an absolute path or one relative to the user's home
160 .Dq .ssh/authorized_keys .
162 The contents of the specified file are sent to the remote user before
163 authentication is allowed.
166 then no banner is displayed.
167 This option is only available for protocol version 2.
168 By default, no banner is displayed.
169 .It Cm ChallengeResponseAuthentication
170 Specifies whether challenge-response authentication is allowed.
171 All authentication styles from
177 Specifies the ciphers allowed for protocol version 2.
178 Multiple ciphers must be comma-separated.
179 The supported ciphers are
194 .Bd -literal -offset 3n
195 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
196 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
197 aes192-ctr,aes256-ctr
199 .It Cm ClientAliveCountMax
200 Sets the number of client alive messages (see below) which may be
203 receiving any messages back from the client.
204 If this threshold is reached while client alive messages are being sent,
205 sshd will disconnect the client, terminating the session.
206 It is important to note that the use of client alive messages is very
210 The client alive messages are sent through the encrypted channel
211 and therefore will not be spoofable.
212 The TCP keepalive option enabled by
215 The client alive mechanism is valuable when the client or
216 server depend on knowing when a connection has become inactive.
218 The default value is 3.
220 .Cm ClientAliveInterval
221 (see below) is set to 15, and
222 .Cm ClientAliveCountMax
223 is left at the default, unresponsive SSH clients
224 will be disconnected after approximately 45 seconds.
225 This option applies to protocol version 2 only.
226 .It Cm ClientAliveInterval
227 Sets a timeout interval in seconds after which if no data has been received
230 will send a message through the encrypted
231 channel to request a response from the client.
233 is 0, indicating that these messages will not be sent to the client.
234 This option applies to protocol version 2 only.
236 Specifies whether compression is allowed, or delayed until
237 the user has authenticated successfully.
246 This keyword can be followed by a list of group name patterns, separated
248 Login is disallowed for users whose primary group or supplementary
249 group list matches one of the patterns.
250 Only group names are valid; a numerical group ID is not recognized.
251 By default, login is allowed for all groups.
252 The allow/deny directives are processed in the following order:
263 for more information on patterns.
265 This keyword can be followed by a list of user name patterns, separated
267 Login is disallowed for user names that match one of the patterns.
268 Only user names are valid; a numerical user ID is not recognized.
269 By default, login is allowed for all users.
270 If the pattern takes the form USER@HOST then USER and HOST
271 are separately checked, restricting logins to particular
272 users from particular hosts.
273 The allow/deny directives are processed in the following order:
284 for more information on patterns.
286 Forces the execution of the command specified by
288 ignoring any command supplied by the client.
289 The command is invoked by using the user's login shell with the -c option.
290 This applies to shell, command, or subsystem execution.
291 It is most useful inside a
294 The command originally supplied by the client is available in the
295 .Ev SSH_ORIGINAL_COMMAND
296 environment variable.
298 Specifies whether remote hosts are allowed to connect to ports
299 forwarded for the client.
302 binds remote port forwardings to the loopback address.
303 This prevents other remote hosts from connecting to forwarded ports.
305 can be used to specify that sshd
306 should allow remote port forwardings to bind to non-loopback addresses, thus
307 allowing other hosts to connect.
310 to force remote port forwardings to be available to the local host only,
312 to force remote port forwardings to bind to the wildcard address, or
314 to allow the client to select the address to which the forwarding is bound.
317 .It Cm GSSAPIAuthentication
318 Specifies whether user authentication based on GSSAPI is allowed.
321 Note that this option applies to protocol version 2 only.
322 .It Cm GSSAPICleanupCredentials
323 Specifies whether to automatically destroy the user's credentials cache
327 Note that this option applies to protocol version 2 only.
328 .It Cm HostbasedAuthentication
329 Specifies whether rhosts or /etc/hosts.equiv authentication together
330 with successful public key client host authentication is allowed
331 (host-based authentication).
332 This option is similar to
333 .Cm RhostsRSAAuthentication
334 and applies to protocol version 2 only.
337 .It Cm HostbasedUsesNameFromPacketOnly
338 Specifies whether or not the server will attempt to perform a reverse
339 name lookup when matching the name in the
345 .Cm HostbasedAuthentication .
350 uses the name supplied by the client rather than
351 attempting to resolve the name from the TCP connection itself.
355 Specifies a file containing a private host key
358 .Pa /etc/ssh/ssh_host_key
359 for protocol version 1, and
360 .Pa /etc/ssh/ssh_host_rsa_key
362 .Pa /etc/ssh/ssh_host_dsa_key
363 for protocol version 2.
366 will refuse to use a file if it is group/world-accessible.
367 It is possible to have multiple host key files.
369 keys are used for version 1 and
373 are used for version 2 of the SSH protocol.
379 files will not be used in
380 .Cm RhostsRSAAuthentication
382 .Cm HostbasedAuthentication .
386 .Pa /etc/shosts.equiv
390 .It Cm IgnoreUserKnownHosts
393 should ignore the user's
394 .Pa ~/.ssh/known_hosts
396 .Cm RhostsRSAAuthentication
398 .Cm HostbasedAuthentication .
401 .It Cm KerberosAuthentication
402 Specifies whether the password provided by the user for
403 .Cm PasswordAuthentication
404 will be validated through the Kerberos KDC.
405 To use this option, the server needs a
406 Kerberos servtab which allows the verification of the KDC's identity.
409 .It Cm KerberosGetAFSToken
410 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
411 an AFS token before accessing the user's home directory.
414 .It Cm KerberosOrLocalPasswd
415 If password authentication through Kerberos fails then
416 the password will be validated via any additional local mechanism
421 .It Cm KerberosTicketCleanup
422 Specifies whether to automatically destroy the user's ticket cache
426 .It Cm KeyRegenerationInterval
427 In protocol version 1, the ephemeral server key is automatically regenerated
428 after this many seconds (if it has been used).
429 The purpose of regeneration is to prevent
430 decrypting captured sessions by later breaking into the machine and
432 The key is never stored anywhere.
433 If the value is 0, the key is never regenerated.
434 The default is 3600 (seconds).
436 Specifies the local addresses
439 The following forms may be used:
441 .Bl -item -offset indent -compact
445 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
450 .Ar host No | Ar IPv4_addr No : Ar port
456 .Ar host No | Ar IPv6_addr Oc : Ar port
463 sshd will listen on the address and all prior
466 The default is to listen on all local addresses.
469 options are permitted.
472 options must precede this option for non-port qualified addresses.
473 .It Cm LoginGraceTime
474 The server disconnects after this time if the user has not
475 successfully logged in.
476 If the value is 0, there is no time limit.
477 The default is 120 seconds.
479 Gives the verbosity level that is used when logging messages from
481 The possible values are:
482 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
484 DEBUG and DEBUG1 are equivalent.
485 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
486 Logging with a DEBUG level violates the privacy of users and is not recommended.
488 Specifies the available MAC (message authentication code) algorithms.
489 The MAC algorithm is used in protocol version 2
490 for data integrity protection.
491 Multiple algorithms must be comma-separated.
493 .Bd -literal -offset indent
494 hmac-md5,hmac-sha1,umac-64@openssh.com,
495 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
498 Introduces a conditional block.
499 If all of the criteria on the
501 line are satisfied, the keywords on the following lines override those
502 set in the global section of the config file, until either another
504 line or the end of the file.
507 are one or more criteria-pattern pairs.
508 The available criteria are
514 Only a subset of keywords may be used on the lines following a
517 Available keywords are
518 .Cm AllowTcpForwarding ,
522 .Cm GSSApiAuthentication ,
523 .Cm KbdInteractiveAuthentication ,
524 .Cm KerberosAuthentication ,
525 .Cm PasswordAuthentication ,
527 .Cm RhostsRSAAuthentication ,
528 .Cm RSAAuthentication ,
529 .Cm X11DisplayOffset ,
532 .Cm X11UseLocalHost .
534 Specifies the maximum number of authentication attempts permitted per
536 Once the number of failures reaches half this value,
537 additional failures are logged.
540 Specifies the maximum number of concurrent unauthenticated connections to the
542 Additional connections will be dropped until authentication succeeds or the
544 expires for a connection.
547 Alternatively, random early drop can be enabled by specifying
548 the three colon separated values
552 will refuse connection attempts with a probability of
555 if there are currently
558 unauthenticated connections.
559 The probability increases linearly and all connection attempts
560 are refused if the number of unauthenticated connections reaches
563 .It Cm PasswordAuthentication
564 Specifies whether password authentication is allowed.
567 .It Cm PermitEmptyPasswords
568 When password authentication is allowed, it specifies whether the
569 server allows login to accounts with empty password strings.
573 Specifies the destinations to which TCP port forwarding is permitted.
574 The forwarding specification must be one of the following forms:
576 .Bl -item -offset indent -compact
590 .Ar \&[ IPv6_addr \&] : port
594 Multiple forwards may be specified by separating them with whitespace.
597 can be used to remove all restrictions and permit any forwarding requests.
598 By default all port forwarding requests are permitted.
599 .It Cm PermitRootLogin
600 Specifies whether root can log in using
604 .Dq without-password ,
605 .Dq forced-commands-only ,
611 If this option is set to
612 .Dq without-password ,
613 password authentication is disabled for root.
615 If this option is set to
616 .Dq forced-commands-only ,
617 root login with public key authentication will be allowed,
620 option has been specified
621 (which may be useful for taking remote backups even if root login is
622 normally not allowed).
623 All other authentication methods are disabled for root.
625 If this option is set to
627 root is not allowed to log in.
631 device forwarding is allowed.
647 .It Cm PermitUserEnvironment
649 .Pa ~/.ssh/environment
653 .Pa ~/.ssh/authorized_keys
658 Enabling environment processing may enable users to bypass access
659 restrictions in some configurations using mechanisms such as
662 Specifies the file that contains the process ID of the
665 .Pa /var/run/sshd.pid .
667 Specifies the port number that
671 Multiple options of this type are permitted.
677 should print the date and time of the last user login when a user logs
686 when a user logs in interactively.
687 (On some systems it is also printed by the shell,
693 Specifies the protocol versions
696 The possible values are
700 Multiple versions must be comma-separated.
703 Note that the order of the protocol list does not indicate preference,
704 because the client selects among multiple protocol versions offered
710 .It Cm PubkeyAuthentication
711 Specifies whether public key authentication is allowed.
714 Note that this option applies to protocol version 2 only.
715 .It Cm RhostsRSAAuthentication
716 Specifies whether rhosts or /etc/hosts.equiv authentication together
717 with successful RSA host authentication is allowed.
720 This option applies to protocol version 1 only.
721 .It Cm RSAAuthentication
722 Specifies whether pure RSA authentication is allowed.
725 This option applies to protocol version 1 only.
727 Defines the number of bits in the ephemeral protocol version 1 server key.
728 The minimum value is 512, and the default is 768.
732 should check file modes and ownership of the
733 user's files and home directory before accepting login.
734 This is normally desirable because novices sometimes accidentally leave their
735 directory or files world-writable.
739 Configures an external subsystem (e.g. file transfer daemon).
740 Arguments should be a subsystem name and a command (with optional arguments)
741 to execute upon subsystem request.
746 file transfer subsystem.
747 By default no subsystems are defined.
748 Note that this option applies to protocol version 2 only.
749 .It Cm SyslogFacility
750 Gives the facility code that is used when logging messages from
752 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
753 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
756 Specifies whether the system should send TCP keepalive messages to the
758 If they are sent, death of the connection or crash of one
759 of the machines will be properly noticed.
760 However, this means that
761 connections will die if the route is down temporarily, and some people
763 On the other hand, if TCP keepalives are not sent,
764 sessions may hang indefinitely on the server, leaving
766 users and consuming server resources.
770 (to send TCP keepalive messages), and the server will notice
771 if the network goes down or the client host crashes.
772 This avoids infinitely hanging sessions.
774 To disable TCP keepalive messages, the value should be set to
779 should look up the remote host name and check that
780 the resolved host name for the remote IP address maps back to the
781 very same IP address.
787 is used for interactive login sessions.
792 is never used for remote command execution.
793 Note also, that if this is enabled,
795 will be disabled because
797 does not know how to handle
801 .Cm UsePrivilegeSeparation
802 is specified, it will be disabled after authentication.
804 Enables the Pluggable Authentication Module interface.
807 this will enable PAM authentication using
808 .Cm ChallengeResponseAuthentication
810 .Cm PasswordAuthentication
811 in addition to PAM account and session module processing for all
812 authentication types.
814 Because PAM challenge-response authentication usually serves an equivalent
815 role to password authentication, you should disable either
816 .Cm PasswordAuthentication
818 .Cm ChallengeResponseAuthentication.
822 is enabled, you will not be able to run
827 .It Cm UsePrivilegeSeparation
830 separates privileges by creating an unprivileged child process
831 to deal with incoming network traffic.
832 After successful authentication, another process will be created that has
833 the privilege of the authenticated user.
834 The goal of privilege separation is to prevent privilege
835 escalation by containing any corruption within the unprivileged processes.
838 .It Cm X11DisplayOffset
839 Specifies the first display number available for
842 This prevents sshd from interfering with real X11 servers.
845 Specifies whether X11 forwarding is permitted.
853 When X11 forwarding is enabled, there may be additional exposure to
854 the server and to client displays if the
856 proxy display is configured to listen on the wildcard address (see
858 below), though this is not the default.
859 Additionally, the authentication spoofing and authentication data
860 verification and substitution occur on the client side.
861 The security risk of using X11 forwarding is that the client's X11
862 display server may be exposed to attack when the SSH client requests
863 forwarding (see the warnings for
867 A system administrator may have a stance in which they want to
868 protect clients that may expose themselves to attack by unwittingly
869 requesting X11 forwarding, which can warrant a
873 Note that disabling X11 forwarding does not prevent users from
874 forwarding X11 traffic, as users can always install their own forwarders.
875 X11 forwarding is automatically disabled if
878 .It Cm X11UseLocalhost
881 should bind the X11 forwarding server to the loopback address or to
882 the wildcard address.
884 sshd binds the forwarding server to the loopback address and sets the
887 environment variable to
889 This prevents remote hosts from connecting to the proxy display.
890 However, some older X11 clients may not function with this
895 to specify that the forwarding server should be bound to the wildcard
904 Specifies the full pathname of the
908 .Pa /usr/X11R6/bin/xauth .
912 command-line arguments and configuration file options that specify time
913 may be expressed using a sequence of the form:
915 .Ar time Op Ar qualifier ,
919 is a positive integer value and
921 is one of the following:
923 .Bl -tag -width Ds -compact -offset indent
938 Each member of the sequence is added together to calculate
939 the total time value.
941 Time format examples:
943 .Bl -tag -width Ds -compact -offset indent
945 600 seconds (10 minutes)
949 1 hour 30 minutes (90 minutes)
953 .It Pa /etc/ssh/sshd_config
954 Contains configuration data for
956 This file should be writable by root only, but it is recommended
957 (though not necessary) that it be world-readable.
962 OpenSSH is a derivative of the original and free
963 ssh 1.2.12 release by Tatu Ylonen.
964 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
965 Theo de Raadt and Dug Song
966 removed many bugs, re-added newer features and
968 Markus Friedl contributed the support for SSH
969 protocol versions 1.5 and 2.0.
970 Niels Provos and Markus Friedl contributed support
971 for privilege separation.