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 1.106 2009/04/21 15:13:17 stevesk Exp $
38 .Dd $Mdocdate: April 21 2009 $
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.
182 this controls the use of PAM (see
185 Note that this affects the effectiveness of the
186 .Cm PasswordAuthentication
192 .It Cm ChrootDirectory
195 to after authentication.
196 This path, and all its components, must be root-owned directories that are
197 not writable by any other user or group.
200 changes the working directory to the user's home directory.
202 The path may contain the following tokens that are expanded at runtime once
203 the connecting user has been authenticated: %% is replaced by a literal '%',
204 %h is replaced by the home directory of the user being authenticated, and
205 %u is replaced by the username of that user.
209 must contain the necessary files and directories to support the
211 For an interactive session this requires at least a shell, typically
225 For file transfer sessions using
227 no additional configuration of the environment is necessary if the
228 in-process sftp server is used,
229 though sessions which use logging do require
231 inside the chroot directory (see
235 The default is not to
238 Specifies the ciphers allowed for protocol version 2.
239 Multiple ciphers must be comma-separated.
240 The supported ciphers are
255 .Bd -literal -offset 3n
256 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
257 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
260 .It Cm ClientAliveCountMax
261 Sets the number of client alive messages (see below) which may be
264 receiving any messages back from the client.
265 If this threshold is reached while client alive messages are being sent,
266 sshd will disconnect the client, terminating the session.
267 It is important to note that the use of client alive messages is very
271 The client alive messages are sent through the encrypted channel
272 and therefore will not be spoofable.
273 The TCP keepalive option enabled by
276 The client alive mechanism is valuable when the client or
277 server depend on knowing when a connection has become inactive.
279 The default value is 3.
281 .Cm ClientAliveInterval
282 (see below) is set to 15, and
283 .Cm ClientAliveCountMax
284 is left at the default, unresponsive SSH clients
285 will be disconnected after approximately 45 seconds.
286 This option applies to protocol version 2 only.
287 .It Cm ClientAliveInterval
288 Sets a timeout interval in seconds after which if no data has been received
291 will send a message through the encrypted
292 channel to request a response from the client.
294 is 0, indicating that these messages will not be sent to the client.
295 This option applies to protocol version 2 only.
297 Specifies whether compression is allowed, or delayed until
298 the user has authenticated successfully.
307 This keyword can be followed by a list of group name patterns, separated
309 Login is disallowed for users whose primary group or supplementary
310 group list matches one of the patterns.
311 Only group names are valid; a numerical group ID is not recognized.
312 By default, login is allowed for all groups.
313 The allow/deny directives are processed in the following order:
324 for more information on patterns.
326 This keyword can be followed by a list of user name patterns, separated
328 Login is disallowed for user names that match one of the patterns.
329 Only user names are valid; a numerical user ID is not recognized.
330 By default, login is allowed for all users.
331 If the pattern takes the form USER@HOST then USER and HOST
332 are separately checked, restricting logins to particular
333 users from particular hosts.
334 The allow/deny directives are processed in the following order:
345 for more information on patterns.
347 Forces the execution of the command specified by
349 ignoring any command supplied by the client and
352 The command is invoked by using the user's login shell with the -c option.
353 This applies to shell, command, or subsystem execution.
354 It is most useful inside a
357 The command originally supplied by the client is available in the
358 .Ev SSH_ORIGINAL_COMMAND
359 environment variable.
360 Specifying a command of
362 will force the use of an in-process sftp server that requires no support
364 .Cm ChrootDirectory .
366 Specifies whether remote hosts are allowed to connect to ports
367 forwarded for the client.
370 binds remote port forwardings to the loopback address.
371 This prevents other remote hosts from connecting to forwarded ports.
373 can be used to specify that sshd
374 should allow remote port forwardings to bind to non-loopback addresses, thus
375 allowing other hosts to connect.
378 to force remote port forwardings to be available to the local host only,
380 to force remote port forwardings to bind to the wildcard address, or
382 to allow the client to select the address to which the forwarding is bound.
385 .It Cm GSSAPIAuthentication
386 Specifies whether user authentication based on GSSAPI is allowed.
389 Note that this option applies to protocol version 2 only.
390 .It Cm GSSAPICleanupCredentials
391 Specifies whether to automatically destroy the user's credentials cache
395 Note that this option applies to protocol version 2 only.
396 .It Cm HostbasedAuthentication
397 Specifies whether rhosts or /etc/hosts.equiv authentication together
398 with successful public key client host authentication is allowed
399 (host-based authentication).
400 This option is similar to
401 .Cm RhostsRSAAuthentication
402 and applies to protocol version 2 only.
405 .It Cm HostbasedUsesNameFromPacketOnly
406 Specifies whether or not the server will attempt to perform a reverse
407 name lookup when matching the name in the
413 .Cm HostbasedAuthentication .
418 uses the name supplied by the client rather than
419 attempting to resolve the name from the TCP connection itself.
423 Specifies a file containing a private host key
426 .Pa /etc/ssh/ssh_host_key
427 for protocol version 1, and
428 .Pa /etc/ssh/ssh_host_rsa_key
430 .Pa /etc/ssh/ssh_host_dsa_key
431 for protocol version 2.
434 will refuse to use a file if it is group/world-accessible.
435 It is possible to have multiple host key files.
437 keys are used for version 1 and
441 are used for version 2 of the SSH protocol.
447 files will not be used in
448 .Cm RhostsRSAAuthentication
450 .Cm HostbasedAuthentication .
454 .Pa /etc/ssh/shosts.equiv
458 .It Cm IgnoreUserKnownHosts
461 should ignore the user's
462 .Pa ~/.ssh/known_hosts
464 .Cm RhostsRSAAuthentication
466 .Cm HostbasedAuthentication .
469 .It Cm KerberosAuthentication
470 Specifies whether the password provided by the user for
471 .Cm PasswordAuthentication
472 will be validated through the Kerberos KDC.
473 To use this option, the server needs a
474 Kerberos servtab which allows the verification of the KDC's identity.
477 .It Cm KerberosGetAFSToken
478 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
479 an AFS token before accessing the user's home directory.
482 .It Cm KerberosOrLocalPasswd
483 If password authentication through Kerberos fails then
484 the password will be validated via any additional local mechanism
489 .It Cm KerberosTicketCleanup
490 Specifies whether to automatically destroy the user's ticket cache
494 .It Cm KeyRegenerationInterval
495 In protocol version 1, the ephemeral server key is automatically regenerated
496 after this many seconds (if it has been used).
497 The purpose of regeneration is to prevent
498 decrypting captured sessions by later breaking into the machine and
500 The key is never stored anywhere.
501 If the value is 0, the key is never regenerated.
502 The default is 3600 (seconds).
504 Specifies the local addresses
507 The following forms may be used:
509 .Bl -item -offset indent -compact
513 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
518 .Ar host No | Ar IPv4_addr No : Ar port
524 .Ar host No | Ar IPv6_addr Oc : Ar port
531 sshd will listen on the address and all prior
534 The default is to listen on all local addresses.
537 options are permitted.
540 options must precede this option for non-port qualified addresses.
541 .It Cm LoginGraceTime
542 The server disconnects after this time if the user has not
543 successfully logged in.
544 If the value is 0, there is no time limit.
545 The default is 120 seconds.
547 Gives the verbosity level that is used when logging messages from
549 The possible values are:
550 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
552 DEBUG and DEBUG1 are equivalent.
553 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
554 Logging with a DEBUG level violates the privacy of users and is not recommended.
556 Specifies the available MAC (message authentication code) algorithms.
557 The MAC algorithm is used in protocol version 2
558 for data integrity protection.
559 Multiple algorithms must be comma-separated.
561 .Bd -literal -offset indent
562 hmac-md5,hmac-sha1,umac-64@openssh.com,
563 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
566 Introduces a conditional block.
567 If all of the criteria on the
569 line are satisfied, the keywords on the following lines override those
570 set in the global section of the config file, until either another
572 line or the end of the file.
576 are one or more criteria-pattern pairs.
577 The available criteria are
583 The match patterns may consist of single entries or comma-separated
584 lists and may use the wildcard and negation operators described in the
591 criteria may additionally contain addresses to match in CIDR
592 address/masklen format, e.g.\&
596 Note that the mask length provided must be consistent with the address -
597 it is an error to specify a mask length that is too long for the address
598 or one with bits set in this host portion of the address.
605 Only a subset of keywords may be used on the lines following a
608 Available keywords are
609 .Cm AllowAgentForwarding ,
610 .Cm AllowTcpForwarding ,
612 .Cm ChrootDirectory ,
615 .Cm GSSAPIAuthentication ,
616 .Cm HostbasedAuthentication ,
617 .Cm KbdInteractiveAuthentication ,
618 .Cm KerberosAuthentication ,
621 .Cm PasswordAuthentication ,
622 .Cm PermitEmptyPasswords ,
624 .Cm PermitRootLogin ,
625 .Cm RhostsRSAAuthentication ,
626 .Cm RSAAuthentication ,
627 .Cm X11DisplayOffset ,
630 .Cm X11UseLocalHost .
632 Specifies the maximum number of authentication attempts permitted per
634 Once the number of failures reaches half this value,
635 additional failures are logged.
638 Specifies the maximum number of open sessions permitted per network connection.
641 Specifies the maximum number of concurrent unauthenticated connections to the
643 Additional connections will be dropped until authentication succeeds or the
645 expires for a connection.
648 Alternatively, random early drop can be enabled by specifying
649 the three colon separated values
653 will refuse connection attempts with a probability of
656 if there are currently
659 unauthenticated connections.
660 The probability increases linearly and all connection attempts
661 are refused if the number of unauthenticated connections reaches
664 .It Cm PasswordAuthentication
665 Specifies whether password authentication is allowed.
669 .Cm ChallengeResponseAuthentication
675 and the PAM authentication policy for
679 password authentication will be allowed through the challenge-response
680 mechanism regardless of the value of
681 .Cm PasswordAuthentication .
682 .It Cm PermitEmptyPasswords
683 When password authentication is allowed, it specifies whether the
684 server allows login to accounts with empty password strings.
688 Specifies the destinations to which TCP port forwarding is permitted.
689 The forwarding specification must be one of the following forms:
691 .Bl -item -offset indent -compact
705 .Ar \&[ IPv6_addr \&] : port
709 Multiple forwards may be specified by separating them with whitespace.
712 can be used to remove all restrictions and permit any forwarding requests.
713 By default all port forwarding requests are permitted.
714 .It Cm PermitRootLogin
715 Specifies whether root can log in using
719 .Dq without-password ,
720 .Dq forced-commands-only ,
726 .Cm ChallengeResponseAuthentication
729 the root user may be allowed in with its password even if
730 .Cm PermitRootLogin is set to
731 .Dq without-password .
733 If this option is set to
734 .Dq without-password ,
735 password authentication is disabled for root.
737 If this option is set to
738 .Dq forced-commands-only ,
739 root login with public key authentication will be allowed,
742 option has been specified
743 (which may be useful for taking remote backups even if root login is
744 normally not allowed).
745 All other authentication methods are disabled for root.
747 If this option is set to
749 root is not allowed to log in.
753 device forwarding is allowed.
769 .It Cm PermitUserEnvironment
771 .Pa ~/.ssh/environment
775 .Pa ~/.ssh/authorized_keys
780 Enabling environment processing may enable users to bypass access
781 restrictions in some configurations using mechanisms such as
784 Specifies the file that contains the process ID of the
787 .Pa /var/run/sshd.pid .
789 Specifies the port number that
793 Multiple options of this type are permitted.
799 should print the date and time of the last user login when a user logs
808 when a user logs in interactively.
809 (On some systems it is also printed by the shell,
815 Specifies the protocol versions
818 The possible values are
822 Multiple versions must be comma-separated.
825 Note that the order of the protocol list does not indicate preference,
826 because the client selects among multiple protocol versions offered
832 .It Cm PubkeyAuthentication
833 Specifies whether public key authentication is allowed.
836 Note that this option applies to protocol version 2 only.
837 .It Cm RhostsRSAAuthentication
838 Specifies whether rhosts or
840 authentication together
841 with successful RSA host authentication is allowed.
844 This option applies to protocol version 1 only.
845 .It Cm RSAAuthentication
846 Specifies whether pure RSA authentication is allowed.
849 This option applies to protocol version 1 only.
851 Defines the number of bits in the ephemeral protocol version 1 server key.
852 The minimum value is 512, and the default is 1024.
856 should check file modes and ownership of the
857 user's files and home directory before accepting login.
858 This is normally desirable because novices sometimes accidentally leave their
859 directory or files world-writable.
863 Configures an external subsystem (e.g. file transfer daemon).
864 Arguments should be a subsystem name and a command (with optional arguments)
865 to execute upon subsystem request.
871 file transfer subsystem.
875 implements an in-process
878 This may simplify configurations using
880 to force a different filesystem root on clients.
882 By default no subsystems are defined.
883 Note that this option applies to protocol version 2 only.
884 .It Cm SyslogFacility
885 Gives the facility code that is used when logging messages from
887 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
888 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
891 Specifies whether the system should send TCP keepalive messages to the
893 If they are sent, death of the connection or crash of one
894 of the machines will be properly noticed.
895 However, this means that
896 connections will die if the route is down temporarily, and some people
898 On the other hand, if TCP keepalives are not sent,
899 sessions may hang indefinitely on the server, leaving
901 users and consuming server resources.
905 (to send TCP keepalive messages), and the server will notice
906 if the network goes down or the client host crashes.
907 This avoids infinitely hanging sessions.
909 To disable TCP keepalive messages, the value should be set to
914 should look up the remote host name and check that
915 the resolved host name for the remote IP address maps back to the
916 very same IP address.
922 is used for interactive login sessions.
927 is never used for remote command execution.
928 Note also, that if this is enabled,
930 will be disabled because
932 does not know how to handle
936 .Cm UsePrivilegeSeparation
937 is specified, it will be disabled after authentication.
939 Enables the Pluggable Authentication Module interface.
942 this will enable PAM authentication using
943 .Cm ChallengeResponseAuthentication
945 .Cm PasswordAuthentication
946 in addition to PAM account and session module processing for all
947 authentication types.
949 Because PAM challenge-response authentication usually serves an equivalent
950 role to password authentication, you should disable either
951 .Cm PasswordAuthentication
953 .Cm ChallengeResponseAuthentication.
957 is enabled, you will not be able to run
962 .It Cm UsePrivilegeSeparation
965 separates privileges by creating an unprivileged child process
966 to deal with incoming network traffic.
967 After successful authentication, another process will be created that has
968 the privilege of the authenticated user.
969 The goal of privilege separation is to prevent privilege
970 escalation by containing any corruption within the unprivileged processes.
973 .It Cm VersionAddendum
974 Specifies a string to append to the regular version string to identify
975 OS- or site-specific modifications.
977 .Dq DragonFly-20090620 .
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.
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