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 1.96 2008/07/02 02:24:18 djm Exp $
38 .Dd $Mdocdate: July 2 2008 $
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
98 .It Cm AllowAgentForwarding
101 forwarding is permitted.
104 Note that disabling agent forwarding does not improve security
105 unless users are also denied shell access, as they can always install
106 their own forwarders.
108 This keyword can be followed by a list of group name patterns, separated
110 If specified, login is allowed only for users whose primary
111 group or supplementary group list matches one of the patterns.
112 Only group names are valid; a numerical group ID is not recognized.
113 By default, login is allowed for all groups.
114 The allow/deny directives are processed in the following order:
125 for more information on patterns.
126 .It Cm AllowTcpForwarding
127 Specifies whether TCP forwarding is permitted.
130 Note that disabling TCP forwarding does not improve security unless
131 users are also denied shell access, as they can always install their
134 This keyword can be followed by a list of user name patterns, separated
136 If specified, login is allowed only for user names that
137 match one of the patterns.
138 Only user names are valid; a numerical user ID is not recognized.
139 By default, login is allowed for all users.
140 If the pattern takes the form USER@HOST then USER and HOST
141 are separately checked, restricting logins to particular
142 users from particular hosts.
143 The allow/deny directives are processed in the following order:
154 for more information on patterns.
155 .It Cm AuthorizedKeysFile
156 Specifies the file that contains the public keys that can be used
157 for user authentication.
158 .Cm AuthorizedKeysFile
159 may contain tokens of the form %T which are substituted during connection
161 The following tokens are defined: %% is replaced by a literal '%',
162 %h is replaced by the home directory of the user being authenticated, and
163 %u is replaced by the username of that user.
165 .Cm AuthorizedKeysFile
166 is taken to be an absolute path or one relative to the user's home
169 .Dq .ssh/authorized_keys .
171 The contents of the specified file are sent to the remote user before
172 authentication is allowed.
175 then no banner is displayed.
176 This option is only available for protocol version 2.
177 By default, no banner is displayed.
178 .It Cm ChallengeResponseAuthentication
179 Specifies whether challenge-response authentication is allowed.
180 All authentication styles from
185 .It Cm ChrootDirectory
188 to after authentication.
189 This path, and all its components, must be root-owned directories that are
190 not writable by any other user or group.
192 The path may contain the following tokens that are expanded at runtime once
193 the connecting user has been authenticated: %% is replaced by a literal '%',
194 %h is replaced by the home directory of the user being authenticated, and
195 %u is replaced by the username of that user.
199 must contain the necessary files and directories to support the
201 For an interactive session this requires at least a shell, typically
215 For file transfer sessions using
217 no additional configuration of the environment is necessary if the
218 in-process sftp server is used (see
222 The default is not to
225 Specifies the ciphers allowed for protocol version 2.
226 Multiple ciphers must be comma-separated.
227 The supported ciphers are
242 .Bd -literal -offset 3n
243 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
244 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
245 aes192-ctr,aes256-ctr
247 .It Cm ClientAliveCountMax
248 Sets the number of client alive messages (see below) which may be
251 receiving any messages back from the client.
252 If this threshold is reached while client alive messages are being sent,
253 sshd will disconnect the client, terminating the session.
254 It is important to note that the use of client alive messages is very
258 The client alive messages are sent through the encrypted channel
259 and therefore will not be spoofable.
260 The TCP keepalive option enabled by
263 The client alive mechanism is valuable when the client or
264 server depend on knowing when a connection has become inactive.
266 The default value is 3.
268 .Cm ClientAliveInterval
269 (see below) is set to 15, and
270 .Cm ClientAliveCountMax
271 is left at the default, unresponsive SSH clients
272 will be disconnected after approximately 45 seconds.
273 This option applies to protocol version 2 only.
274 .It Cm ClientAliveInterval
275 Sets a timeout interval in seconds after which if no data has been received
278 will send a message through the encrypted
279 channel to request a response from the client.
281 is 0, indicating that these messages will not be sent to the client.
282 This option applies to protocol version 2 only.
284 Specifies whether compression is allowed, or delayed until
285 the user has authenticated successfully.
294 This keyword can be followed by a list of group name patterns, separated
296 Login is disallowed for users whose primary group or supplementary
297 group list matches one of the patterns.
298 Only group names are valid; a numerical group ID is not recognized.
299 By default, login is allowed for all groups.
300 The allow/deny directives are processed in the following order:
311 for more information on patterns.
313 This keyword can be followed by a list of user name patterns, separated
315 Login is disallowed for user names that match one of the patterns.
316 Only user names are valid; a numerical user ID is not recognized.
317 By default, login is allowed for all users.
318 If the pattern takes the form USER@HOST then USER and HOST
319 are separately checked, restricting logins to particular
320 users from particular hosts.
321 The allow/deny directives are processed in the following order:
332 for more information on patterns.
334 Forces the execution of the command specified by
336 ignoring any command supplied by the client and
339 The command is invoked by using the user's login shell with the -c option.
340 This applies to shell, command, or subsystem execution.
341 It is most useful inside a
344 The command originally supplied by the client is available in the
345 .Ev SSH_ORIGINAL_COMMAND
346 environment variable.
347 Specifying a command of
349 will force the use of an in-process sftp server that requires no support
351 .Cm ChrootDirectory .
353 Specifies whether remote hosts are allowed to connect to ports
354 forwarded for the client.
357 binds remote port forwardings to the loopback address.
358 This prevents other remote hosts from connecting to forwarded ports.
360 can be used to specify that sshd
361 should allow remote port forwardings to bind to non-loopback addresses, thus
362 allowing other hosts to connect.
365 to force remote port forwardings to be available to the local host only,
367 to force remote port forwardings to bind to the wildcard address, or
369 to allow the client to select the address to which the forwarding is bound.
372 .It Cm GSSAPIAuthentication
373 Specifies whether user authentication based on GSSAPI is allowed.
376 Note that this option applies to protocol version 2 only.
377 .It Cm GSSAPIDelegateCredentials
378 Specifies whether delegated credentials are stored in the user's environment.
381 .It Cm GSSAPIKeyExchange
382 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
383 doesn't rely on ssh keys to verify host identity.
386 Note that this option applies to protocol version 2 only.
387 .It Cm GSSAPICleanupCredentials
388 Specifies whether to automatically destroy the user's credentials cache
392 Note that this option applies to protocol version 2 only.
393 .It Cm GSSAPIStrictAcceptorCheck
394 Determines whether to be strict about the identity of the GSSAPI acceptor
395 a client authenticates against. If
397 then the client must authenticate against the
399 service on the current hostname. If
401 then the client may authenticate against any service key stored in the
402 machine's default store. This facility is provided to assist with operation
403 on multi homed machines.
406 Note that this option applies only to protocol version 2 GSSAPI connections,
409 may only work with recent Kerberos GSSAPI libraries.
410 .It Cm GSSAPICredentialsPath
411 If specified, the delegated GSSAPI credential is stored in the
412 given path, overwriting any existing credentials.
413 Paths can be specified with syntax similar to the AuthorizedKeysFile
414 option (i.e., accepting %h and %u tokens).
415 When using this option,
416 setting 'GssapiCleanupCredentials no' is recommended,
417 so logging out of one session
418 doesn't remove the credentials in use by another session of
420 Currently only implemented for the GSI mechanism.
421 .It Cm GSIAllowLimitedProxy
422 Specifies whether to accept limited proxy credentials for
426 .It Cm HostbasedAuthentication
427 Specifies whether rhosts or /etc/hosts.equiv authentication together
428 with successful public key client host authentication is allowed
429 (host-based authentication).
430 This option is similar to
431 .Cm RhostsRSAAuthentication
432 and applies to protocol version 2 only.
435 .It Cm HostbasedUsesNameFromPacketOnly
436 Specifies whether or not the server will attempt to perform a reverse
437 name lookup when matching the name in the
443 .Cm HostbasedAuthentication .
448 uses the name supplied by the client rather than
449 attempting to resolve the name from the TCP connection itself.
453 Specifies a file containing a private host key
456 .Pa /etc/ssh/ssh_host_key
457 for protocol version 1, and
458 .Pa /etc/ssh/ssh_host_rsa_key
460 .Pa /etc/ssh/ssh_host_dsa_key
461 for protocol version 2.
464 will refuse to use a file if it is group/world-accessible.
465 It is possible to have multiple host key files.
467 keys are used for version 1 and
471 are used for version 2 of the SSH protocol.
477 files will not be used in
478 .Cm RhostsRSAAuthentication
480 .Cm HostbasedAuthentication .
484 .Pa /etc/shosts.equiv
488 .It Cm IgnoreUserKnownHosts
491 should ignore the user's
492 .Pa ~/.ssh/known_hosts
494 .Cm RhostsRSAAuthentication
496 .Cm HostbasedAuthentication .
499 .It Cm KerberosAuthentication
500 Specifies whether the password provided by the user for
501 .Cm PasswordAuthentication
502 will be validated through the Kerberos KDC.
503 To use this option, the server needs a
504 Kerberos servtab which allows the verification of the KDC's identity.
507 .It Cm KerberosGetAFSToken
508 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
509 an AFS token before accessing the user's home directory.
512 .It Cm KerberosOrLocalPasswd
513 If password authentication through Kerberos fails then
514 the password will be validated via any additional local mechanism
519 .It Cm KerberosTicketCleanup
520 Specifies whether to automatically destroy the user's ticket cache
524 .It Cm KeyRegenerationInterval
525 In protocol version 1, the ephemeral server key is automatically regenerated
526 after this many seconds (if it has been used).
527 The purpose of regeneration is to prevent
528 decrypting captured sessions by later breaking into the machine and
530 The key is never stored anywhere.
531 If the value is 0, the key is never regenerated.
532 The default is 3600 (seconds).
534 Specifies the local addresses
537 The following forms may be used:
539 .Bl -item -offset indent -compact
543 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
548 .Ar host No | Ar IPv4_addr No : Ar port
554 .Ar host No | Ar IPv6_addr Oc : Ar port
561 sshd will listen on the address and all prior
564 The default is to listen on all local addresses.
567 options are permitted.
570 options must precede this option for non-port qualified addresses.
571 .It Cm LoginGraceTime
572 The server disconnects after this time if the user has not
573 successfully logged in.
574 If the value is 0, there is no time limit.
575 The default is 120 seconds.
577 Gives the verbosity level that is used when logging messages from
579 The possible values are:
580 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
582 DEBUG and DEBUG1 are equivalent.
583 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
584 Logging with a DEBUG level violates the privacy of users and is not recommended.
586 Specifies the available MAC (message authentication code) algorithms.
587 The MAC algorithm is used in protocol version 2
588 for data integrity protection.
589 Multiple algorithms must be comma-separated.
591 .Bd -literal -offset indent
592 hmac-md5,hmac-sha1,umac-64@openssh.com,
593 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
596 Introduces a conditional block.
597 If all of the criteria on the
599 line are satisfied, the keywords on the following lines override those
600 set in the global section of the config file, until either another
602 line or the end of the file.
606 are one or more criteria-pattern pairs.
607 The available criteria are
613 The match patterns may consist of single entries or comma-separated
614 lists and may use the wildcard and negation operators described in the
621 criteria may additionally contain addresses to match in CIDR
622 address/masklen format, e.g.\&
626 Note that the mask length provided must be consistent with the address -
627 it is an error to specify a mask length that is too long for the address
628 or one with bits set in this host portion of the address.
635 Only a subset of keywords may be used on the lines following a
638 Available keywords are
639 .Cm AllowTcpForwarding ,
641 .Cm ChrootDirectory ,
644 .Cm GSSAPIAuthentication ,
645 .Cm HostbasedAuthentication ,
646 .Cm KbdInteractiveAuthentication ,
647 .Cm KerberosAuthentication ,
650 .Cm PasswordAuthentication ,
652 .Cm PermitRootLogin ,
653 .Cm RhostsRSAAuthentication ,
654 .Cm RSAAuthentication ,
655 .Cm X11DisplayOffset ,
658 .Cm X11UseLocalHost .
660 Specifies the maximum number of authentication attempts permitted per
662 Once the number of failures reaches half this value,
663 additional failures are logged.
666 Specifies the maximum number of open sessions permitted per network connection.
669 Specifies the maximum number of concurrent unauthenticated connections to the
671 Additional connections will be dropped until authentication succeeds or the
673 expires for a connection.
676 Alternatively, random early drop can be enabled by specifying
677 the three colon separated values
681 will refuse connection attempts with a probability of
684 if there are currently
687 unauthenticated connections.
688 The probability increases linearly and all connection attempts
689 are refused if the number of unauthenticated connections reaches
692 .It Cm PasswordAuthentication
693 Specifies whether password authentication is allowed.
696 .It Cm PermitEmptyPasswords
697 When password authentication is allowed, it specifies whether the
698 server allows login to accounts with empty password strings.
702 Specifies the destinations to which TCP port forwarding is permitted.
703 The forwarding specification must be one of the following forms:
705 .Bl -item -offset indent -compact
719 .Ar \&[ IPv6_addr \&] : port
723 Multiple forwards may be specified by separating them with whitespace.
726 can be used to remove all restrictions and permit any forwarding requests.
727 By default all port forwarding requests are permitted.
728 .It Cm PermitRootLogin
729 Specifies whether root can log in using
733 .Dq without-password ,
734 .Dq forced-commands-only ,
740 If this option is set to
741 .Dq without-password ,
742 password authentication is disabled for root.
744 If this option is set to
745 .Dq forced-commands-only ,
746 root login with public key authentication will be allowed,
749 option has been specified
750 (which may be useful for taking remote backups even if root login is
751 normally not allowed).
752 All other authentication methods are disabled for root.
754 If this option is set to
756 root is not allowed to log in.
760 device forwarding is allowed.
776 .It Cm PermitUserEnvironment
778 .Pa ~/.ssh/environment
782 .Pa ~/.ssh/authorized_keys
787 Enabling environment processing may enable users to bypass access
788 restrictions in some configurations using mechanisms such as
791 Specifies the file that contains the process ID of the
794 .Pa /var/run/sshd.pid .
796 Specifies the port number that
800 Multiple options of this type are permitted.
806 should print the date and time of the last user login when a user logs
815 when a user logs in interactively.
816 (On some systems it is also printed by the shell,
822 Specifies the protocol versions
825 The possible values are
829 Multiple versions must be comma-separated.
832 Note that the order of the protocol list does not indicate preference,
833 because the client selects among multiple protocol versions offered
839 .It Cm PubkeyAuthentication
840 Specifies whether public key authentication is allowed.
843 Note that this option applies to protocol version 2 only.
844 .It Cm RhostsRSAAuthentication
845 Specifies whether rhosts or /etc/hosts.equiv authentication together
846 with successful RSA host authentication is allowed.
849 This option applies to protocol version 1 only.
850 .It Cm RSAAuthentication
851 Specifies whether pure RSA authentication is allowed.
854 This option applies to protocol version 1 only.
856 Defines the number of bits in the ephemeral protocol version 1 server key.
857 The minimum value is 512, and the default is 1024.
861 should check file modes and ownership of the
862 user's files and home directory before accepting login.
863 This is normally desirable because novices sometimes accidentally leave their
864 directory or files world-writable.
868 Configures an external subsystem (e.g. file transfer daemon).
869 Arguments should be a subsystem name and a command (with optional arguments)
870 to execute upon subsystem request.
876 file transfer subsystem.
880 implements an in-process
883 This may simplify configurations using
885 to force a different filesystem root on clients.
887 By default no subsystems are defined.
888 Note that this option applies to protocol version 2 only.
889 .It Cm SyslogFacility
890 Gives the facility code that is used when logging messages from
892 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
893 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
896 Specifies whether the system should send TCP keepalive messages to the
898 If they are sent, death of the connection or crash of one
899 of the machines will be properly noticed.
900 However, this means that
901 connections will die if the route is down temporarily, and some people
903 On the other hand, if TCP keepalives are not sent,
904 sessions may hang indefinitely on the server, leaving
906 users and consuming server resources.
910 (to send TCP keepalive messages), and the server will notice
911 if the network goes down or the client host crashes.
912 This avoids infinitely hanging sessions.
914 To disable TCP keepalive messages, the value should be set to
919 should look up the remote host name and check that
920 the resolved host name for the remote IP address maps back to the
921 very same IP address.
927 is used for interactive login sessions.
932 is never used for remote command execution.
933 Note also, that if this is enabled,
935 will be disabled because
937 does not know how to handle
941 .Cm UsePrivilegeSeparation
942 is specified, it will be disabled after authentication.
944 Enables the Pluggable Authentication Module interface.
947 this will enable PAM authentication using
948 .Cm ChallengeResponseAuthentication
950 .Cm PasswordAuthentication
951 in addition to PAM account and session module processing for all
952 authentication types.
954 Because PAM challenge-response authentication usually serves an equivalent
955 role to password authentication, you should disable either
956 .Cm PasswordAuthentication
958 .Cm ChallengeResponseAuthentication.
962 is enabled, you will not be able to run
967 .It Cm UsePrivilegeSeparation
970 separates privileges by creating an unprivileged child process
971 to deal with incoming network traffic.
972 After successful authentication, another process will be created that has
973 the privilege of the authenticated user.
974 The goal of privilege separation is to prevent privilege
975 escalation by containing any corruption within the unprivileged processes.
978 .It Cm X11DisplayOffset
979 Specifies the first display number available for
982 This prevents sshd from interfering with real X11 servers.
985 Specifies whether X11 forwarding is permitted.
993 When X11 forwarding is enabled, there may be additional exposure to
994 the server and to client displays if the
996 proxy display is configured to listen on the wildcard address (see
998 below), though this is not the default.
999 Additionally, the authentication spoofing and authentication data
1000 verification and substitution occur on the client side.
1001 The security risk of using X11 forwarding is that the client's X11
1002 display server may be exposed to attack when the SSH client requests
1003 forwarding (see the warnings for
1006 .Xr ssh_config 5 ) .
1007 A system administrator may have a stance in which they want to
1008 protect clients that may expose themselves to attack by unwittingly
1009 requesting X11 forwarding, which can warrant a
1013 Note that disabling X11 forwarding does not prevent users from
1014 forwarding X11 traffic, as users can always install their own forwarders.
1015 X11 forwarding is automatically disabled if
1018 .It Cm X11UseLocalhost
1021 should bind the X11 forwarding server to the loopback address or to
1022 the wildcard address.
1024 sshd binds the forwarding server to the loopback address and sets the
1025 hostname part of the
1027 environment variable to
1029 This prevents remote hosts from connecting to the proxy display.
1030 However, some older X11 clients may not function with this
1035 to specify that the forwarding server should be bound to the wildcard
1037 The argument must be
1043 .It Cm XAuthLocation
1044 Specifies the full pathname of the
1048 .Pa /usr/X11R6/bin/xauth .
1052 command-line arguments and configuration file options that specify time
1053 may be expressed using a sequence of the form:
1055 .Ar time Op Ar qualifier ,
1059 is a positive integer value and
1061 is one of the following:
1063 .Bl -tag -width Ds -compact -offset indent
1078 Each member of the sequence is added together to calculate
1079 the total time value.
1081 Time format examples:
1083 .Bl -tag -width Ds -compact -offset indent
1085 600 seconds (10 minutes)
1089 1 hour 30 minutes (90 minutes)
1093 .It Pa /etc/ssh/sshd_config
1094 Contains configuration data for
1096 This file should be writable by root only, but it is recommended
1097 (though not necessary) that it be world-readable.
1102 OpenSSH is a derivative of the original and free
1103 ssh 1.2.12 release by Tatu Ylonen.
1104 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1105 Theo de Raadt and Dug Song
1106 removed many bugs, re-added newer features and
1108 Markus Friedl contributed the support for SSH
1109 protocol versions 1.5 and 2.0.
1110 Niels Provos and Markus Friedl contributed support
1111 for privilege separation.