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 (e.g. via
180 PAM or though authentication styles supported in
184 .It Cm ChrootDirectory
187 to after authentication.
188 This path, and all its components, must be root-owned directories that are
189 not writable by any other user or group.
192 changes the working directory to the user's home directory.
194 The path may contain the following tokens that are expanded at runtime once
195 the connecting user has been authenticated: %% is replaced by a literal '%',
196 %h is replaced by the home directory of the user being authenticated, and
197 %u is replaced by the username of that user.
201 must contain the necessary files and directories to support the
203 For an interactive session this requires at least a shell, typically
217 For file transfer sessions using
219 no additional configuration of the environment is necessary if the
220 in-process sftp server is used,
221 though sessions which use logging do require
223 inside the chroot directory (see
227 The default is not to
230 Specifies the ciphers allowed for protocol version 2.
231 Multiple ciphers must be comma-separated.
232 The supported ciphers are
247 .Bd -literal -offset 3n
248 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
249 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
252 .It Cm ClientAliveCountMax
253 Sets the number of client alive messages (see below) which may be
256 receiving any messages back from the client.
257 If this threshold is reached while client alive messages are being sent,
258 sshd will disconnect the client, terminating the session.
259 It is important to note that the use of client alive messages is very
263 The client alive messages are sent through the encrypted channel
264 and therefore will not be spoofable.
265 The TCP keepalive option enabled by
268 The client alive mechanism is valuable when the client or
269 server depend on knowing when a connection has become inactive.
271 The default value is 3.
273 .Cm ClientAliveInterval
274 (see below) is set to 15, and
275 .Cm ClientAliveCountMax
276 is left at the default, unresponsive SSH clients
277 will be disconnected after approximately 45 seconds.
278 This option applies to protocol version 2 only.
279 .It Cm ClientAliveInterval
280 Sets a timeout interval in seconds after which if no data has been received
283 will send a message through the encrypted
284 channel to request a response from the client.
286 is 0, indicating that these messages will not be sent to the client.
287 This option applies to protocol version 2 only.
289 Specifies whether compression is allowed, or delayed until
290 the user has authenticated successfully.
299 This keyword can be followed by a list of group name patterns, separated
301 Login is disallowed for users whose primary group or supplementary
302 group list matches one of the patterns.
303 Only group names are valid; a numerical group ID is not recognized.
304 By default, login is allowed for all groups.
305 The allow/deny directives are processed in the following order:
316 for more information on patterns.
318 This keyword can be followed by a list of user name patterns, separated
320 Login is disallowed for user names that match one of the patterns.
321 Only user names are valid; a numerical user ID is not recognized.
322 By default, login is allowed for all users.
323 If the pattern takes the form USER@HOST then USER and HOST
324 are separately checked, restricting logins to particular
325 users from particular hosts.
326 The allow/deny directives are processed in the following order:
337 for more information on patterns.
338 .It Cm DisableUsageStats
339 This keyword can be followed by one of the keywords "true", "enabled", "yes",
340 "on" or "1" to disable reporting of usage metrics. Or it can be set to "false",
341 "disabled", "no", "off", "0" to enable reporting of usage metrics, which is the
343 .Cm GLOBUS_USAGE_OPTOUT
344 environment variable to "1" will also disable the reporting of usage metrics.
345 Disabling reporting of usage metrics will cause the
346 .Cm UsageStatsTargets
347 setting to be ignored.
349 Forces the execution of the command specified by
351 ignoring any command supplied by the client and
354 The command is invoked by using the user's login shell with the -c option.
355 This applies to shell, command, or subsystem execution.
356 It is most useful inside a
359 The command originally supplied by the client is available in the
360 .Ev SSH_ORIGINAL_COMMAND
361 environment variable.
362 Specifying a command of
364 will force the use of an in-process sftp server that requires no support
366 .Cm ChrootDirectory .
368 Specifies whether remote hosts are allowed to connect to ports
369 forwarded for the client.
372 binds remote port forwardings to the loopback address.
373 This prevents other remote hosts from connecting to forwarded ports.
375 can be used to specify that sshd
376 should allow remote port forwardings to bind to non-loopback addresses, thus
377 allowing other hosts to connect.
380 to force remote port forwardings to be available to the local host only,
382 to force remote port forwardings to bind to the wildcard address, or
384 to allow the client to select the address to which the forwarding is bound.
387 .It Cm GSSAPIAuthentication
388 Specifies whether user authentication based on GSSAPI is allowed.
391 Note that this option applies to protocol version 2 only.
392 .It Cm GSSAPIDelegateCredentials
393 Specifies whether delegated credentials are stored in the user's environment.
396 .It Cm GSSAPIKeyExchange
397 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
398 doesn't rely on ssh keys to verify host identity.
401 Note that this option applies to protocol version 2 only.
402 .It Cm GSSAPICleanupCredentials
403 Specifies whether to automatically destroy the user's credentials cache
407 Note that this option applies to protocol version 2 only.
408 .It Cm GSSAPICredentialsPath
409 If specified, the delegated GSSAPI credential is stored in the
410 given path, overwriting any existing credentials.
411 Paths can be specified with syntax similar to the AuthorizedKeysFile
412 option (i.e., accepting %h and %u tokens).
413 When using this option,
414 setting 'GssapiCleanupCredentials no' is recommended,
415 so logging out of one session
416 doesn't remove the credentials in use by another session of
418 Currently only implemented for the GSI mechanism.
419 .It Cm GSIAllowLimitedProxy
420 Specifies whether to accept limited proxy credentials for
424 .It Cm GSSAPIStrictAcceptorCheck
425 Determines whether to be strict about the identity of the GSSAPI acceptor
426 a client authenticates against. If
428 then the client must authenticate against the
430 service on the current hostname. If
432 then the client may authenticate against any service key stored in the
433 machine's default store. This facility is provided to assist with operation
434 on multi homed machines.
437 Note that this option applies only to protocol version 2 GSSAPI connections,
440 may only work with recent Kerberos GSSAPI libraries.
441 .It Cm GSSAPIStoreCredentialsOnRekey
442 Controls whether the user's GSSAPI credentials should be updated following a
443 successful connection rekeying. This option can be used to accepted renewed
444 or updated credentials from a compatible client. The default is
446 .It Cm HostbasedAuthentication
447 Specifies whether rhosts or /etc/hosts.equiv authentication together
448 with successful public key client host authentication is allowed
449 (host-based authentication).
450 This option is similar to
451 .Cm RhostsRSAAuthentication
452 and applies to protocol version 2 only.
455 .It Cm HostbasedUsesNameFromPacketOnly
456 Specifies whether or not the server will attempt to perform a reverse
457 name lookup when matching the name in the
463 .Cm HostbasedAuthentication .
468 uses the name supplied by the client rather than
469 attempting to resolve the name from the TCP connection itself.
473 Specifies a file containing a private host key
476 .Pa /etc/ssh/ssh_host_key
477 for protocol version 1, and
478 .Pa /etc/ssh/ssh_host_rsa_key
480 .Pa /etc/ssh/ssh_host_dsa_key
481 for protocol version 2.
484 will refuse to use a file if it is group/world-accessible.
485 It is possible to have multiple host key files.
487 keys are used for version 1 and
491 are used for version 2 of the SSH protocol.
497 files will not be used in
498 .Cm RhostsRSAAuthentication
500 .Cm HostbasedAuthentication .
504 .Pa /etc/shosts.equiv
508 .It Cm IgnoreUserKnownHosts
511 should ignore the user's
512 .Pa ~/.ssh/known_hosts
514 .Cm RhostsRSAAuthentication
516 .Cm HostbasedAuthentication .
519 .It Cm KerberosAuthentication
520 Specifies whether the password provided by the user for
521 .Cm PasswordAuthentication
522 will be validated through the Kerberos KDC.
523 To use this option, the server needs a
524 Kerberos servtab which allows the verification of the KDC's identity.
527 .It Cm KerberosGetAFSToken
528 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
529 an AFS token before accessing the user's home directory.
532 .It Cm KerberosOrLocalPasswd
533 If password authentication through Kerberos fails then
534 the password will be validated via any additional local mechanism
539 .It Cm KerberosTicketCleanup
540 Specifies whether to automatically destroy the user's ticket cache
544 .It Cm KeyRegenerationInterval
545 In protocol version 1, the ephemeral server key is automatically regenerated
546 after this many seconds (if it has been used).
547 The purpose of regeneration is to prevent
548 decrypting captured sessions by later breaking into the machine and
550 The key is never stored anywhere.
551 If the value is 0, the key is never regenerated.
552 The default is 3600 (seconds).
554 Specifies the local addresses
557 The following forms may be used:
559 .Bl -item -offset indent -compact
563 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
568 .Ar host No | Ar IPv4_addr No : Ar port
574 .Ar host No | Ar IPv6_addr Oc : Ar port
581 sshd will listen on the address and all prior
584 The default is to listen on all local addresses.
587 options are permitted.
590 options must precede this option for non-port qualified addresses.
591 .It Cm LoginGraceTime
592 The server disconnects after this time if the user has not
593 successfully logged in.
594 If the value is 0, there is no time limit.
595 The default is 120 seconds.
597 Gives the verbosity level that is used when logging messages from
599 The possible values are:
600 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
602 DEBUG and DEBUG1 are equivalent.
603 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
604 Logging with a DEBUG level violates the privacy of users and is not recommended.
606 Specifies the available MAC (message authentication code) algorithms.
607 The MAC algorithm is used in protocol version 2
608 for data integrity protection.
609 Multiple algorithms must be comma-separated.
611 .Bd -literal -offset indent
612 hmac-md5,hmac-sha1,umac-64@openssh.com,
613 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
616 Introduces a conditional block.
617 If all of the criteria on the
619 line are satisfied, the keywords on the following lines override those
620 set in the global section of the config file, until either another
622 line or the end of the file.
626 are one or more criteria-pattern pairs.
627 The available criteria are
633 The match patterns may consist of single entries or comma-separated
634 lists and may use the wildcard and negation operators described in the
641 criteria may additionally contain addresses to match in CIDR
642 address/masklen format, e.g.\&
646 Note that the mask length provided must be consistent with the address -
647 it is an error to specify a mask length that is too long for the address
648 or one with bits set in this host portion of the address.
655 Only a subset of keywords may be used on the lines following a
658 Available keywords are
659 .Cm AllowAgentForwarding ,
660 .Cm AllowTcpForwarding ,
662 .Cm ChrootDirectory ,
665 .Cm GSSAPIAuthentication ,
666 .Cm HostbasedAuthentication ,
667 .Cm KbdInteractiveAuthentication ,
668 .Cm KerberosAuthentication ,
671 .Cm PasswordAuthentication ,
672 .Cm PermitEmptyPasswords ,
674 .Cm PermitRootLogin ,
675 .Cm RhostsRSAAuthentication ,
676 .Cm RSAAuthentication ,
677 .Cm X11DisplayOffset ,
680 .Cm X11UseLocalHost .
682 Specifies the maximum number of authentication attempts permitted per
684 Once the number of failures reaches half this value,
685 additional failures are logged.
688 Specifies the maximum number of open sessions permitted per network connection.
691 Specifies the maximum number of concurrent unauthenticated connections to the
693 Additional connections will be dropped until authentication succeeds or the
695 expires for a connection.
698 Alternatively, random early drop can be enabled by specifying
699 the three colon separated values
703 will refuse connection attempts with a probability of
706 if there are currently
709 unauthenticated connections.
710 The probability increases linearly and all connection attempts
711 are refused if the number of unauthenticated connections reaches
714 .It Cm PasswordAuthentication
715 Specifies whether password authentication is allowed.
718 .It Cm PermitEmptyPasswords
719 When password authentication is allowed, it specifies whether the
720 server allows login to accounts with empty password strings.
724 Specifies the destinations to which TCP port forwarding is permitted.
725 The forwarding specification must be one of the following forms:
727 .Bl -item -offset indent -compact
741 .Ar \&[ IPv6_addr \&] : port
745 Multiple forwards may be specified by separating them with whitespace.
748 can be used to remove all restrictions and permit any forwarding requests.
749 By default all port forwarding requests are permitted.
750 .It Cm PermitRootLogin
751 Specifies whether root can log in using
755 .Dq without-password ,
756 .Dq forced-commands-only ,
762 If this option is set to
763 .Dq without-password ,
764 password authentication is disabled for root.
766 If this option is set to
767 .Dq forced-commands-only ,
768 root login with public key authentication will be allowed,
771 option has been specified
772 (which may be useful for taking remote backups even if root login is
773 normally not allowed).
774 All other authentication methods are disabled for root.
776 If this option is set to
778 root is not allowed to log in.
782 device forwarding is allowed.
798 .It Cm PermitUserEnvironment
800 .Pa ~/.ssh/environment
804 .Pa ~/.ssh/authorized_keys
809 Enabling environment processing may enable users to bypass access
810 restrictions in some configurations using mechanisms such as
813 Specifies the file that contains the process ID of the
816 .Pa /var/run/sshd.pid .
818 Specifies the port number that
822 Multiple options of this type are permitted.
828 should print the date and time of the last user login when a user logs
837 when a user logs in interactively.
838 (On some systems it is also printed by the shell,
844 Specifies the protocol versions
847 The possible values are
851 Multiple versions must be comma-separated.
854 Note that the order of the protocol list does not indicate preference,
855 because the client selects among multiple protocol versions offered
861 .It Cm PubkeyAuthentication
862 Specifies whether public key authentication is allowed.
865 Note that this option applies to protocol version 2 only.
866 .It Cm RhostsRSAAuthentication
867 Specifies whether rhosts or /etc/hosts.equiv authentication together
868 with successful RSA host authentication is allowed.
871 This option applies to protocol version 1 only.
872 .It Cm RSAAuthentication
873 Specifies whether pure RSA authentication is allowed.
876 This option applies to protocol version 1 only.
878 Defines the number of bits in the ephemeral protocol version 1 server key.
879 The minimum value is 512, and the default is 1024.
883 should check file modes and ownership of the
884 user's files and home directory before accepting login.
885 This is normally desirable because novices sometimes accidentally leave their
886 directory or files world-writable.
890 Configures an external subsystem (e.g. file transfer daemon).
891 Arguments should be a subsystem name and a command (with optional arguments)
892 to execute upon subsystem request.
898 file transfer subsystem.
902 implements an in-process
905 This may simplify configurations using
907 to force a different filesystem root on clients.
909 By default no subsystems are defined.
910 Note that this option applies to protocol version 2 only.
911 .It Cm SyslogFacility
912 Gives the facility code that is used when logging messages from
914 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
915 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
918 Specifies whether the system should send TCP keepalive messages to the
920 If they are sent, death of the connection or crash of one
921 of the machines will be properly noticed.
922 However, this means that
923 connections will die if the route is down temporarily, and some people
925 On the other hand, if TCP keepalives are not sent,
926 sessions may hang indefinitely on the server, leaving
928 users and consuming server resources.
932 (to send TCP keepalive messages), and the server will notice
933 if the network goes down or the client host crashes.
934 This avoids infinitely hanging sessions.
936 To disable TCP keepalive messages, the value should be set to
938 .It Cm UsageStatsTargets
939 This option can be used to specify the target collector hosts to which usage
940 metrics should be reported. This setting will be ignored if
941 .Cm DisableUsageStats
942 is enabled. Multiple targets can be specified separated by comma(s), but no
943 space(s). Each target specification is of the format
944 .Pa host:port[!tags].
945 Tags control what data elements are reported. The following list specifies
946 the tags for the corresponding data elements.
948 .Bl -item -offset indent -compact
952 - OpenSSH version, reported by default.
957 - SSL version, reported by default.
962 - User authentication method used such as "gssapi-keyex", "gssapi-with-mic", etc. Reported by default.
967 - User authentication mechanism used such as "GSI", "Kerberos", etc. Reported by default.
972 - Client IP address. Not reported by default.
977 - User name. Not reported by default.
982 - User DN. Not reported by default.
985 In addition to the above selected information, the following data are
986 reported to ALL the specified/default target collectors. There's no way to
987 exclude these from being reported other than by disabling the reporting of
988 usage metrics altogether:
996 .Cm Component Data Format version
1003 - IP address of reporting server
1010 - Host name of reporting server
1013 If no tags are specified in a host spec, or the special string
1015 is specified, the tags
1017 are assumed. A site could choose to allow a
1018 different set of data to be reported by specifying a different tag set. The
1024 above are more meant for a local collector that a
1025 site might like to deploy since they could be construed as private information.
1031 By default, Usage Metrics reporting is sent to
1032 .Dq usage-stats.cilogon.org:4810 .
1033 This can be made explicit by specifying
1035 (all by itself) for the
1036 target specification as in:
1038 .Bl -item -offset indent -compact
1040 .Cm UsageStatsTargets
1047 .Cm UsageStatsTargets
1048 is not specified, a comma-separated list of targets
1049 (without any tags specified) if specified in the environment variable
1050 .Cm GLOBUS_USAGE_TARGETS
1056 should look up the remote host name and check that
1057 the resolved host name for the remote IP address maps back to the
1058 very same IP address.
1064 is used for interactive login sessions.
1069 is never used for remote command execution.
1070 Note also, that if this is enabled,
1072 will be disabled because
1074 does not know how to handle
1078 .Cm UsePrivilegeSeparation
1079 is specified, it will be disabled after authentication.
1081 Enables the Pluggable Authentication Module interface.
1084 this will enable PAM authentication using
1085 .Cm ChallengeResponseAuthentication
1087 .Cm PasswordAuthentication
1088 in addition to PAM account and session module processing for all
1089 authentication types.
1091 Because PAM challenge-response authentication usually serves an equivalent
1092 role to password authentication, you should disable either
1093 .Cm PasswordAuthentication
1095 .Cm ChallengeResponseAuthentication.
1099 is enabled, you will not be able to run
1104 .It Cm PermitPAMUserChange
1107 this will enable PAM authentication to change the name of the user being
1108 authenticated. The default is
1110 .It Cm UsePrivilegeSeparation
1113 separates privileges by creating an unprivileged child process
1114 to deal with incoming network traffic.
1115 After successful authentication, another process will be created that has
1116 the privilege of the authenticated user.
1117 The goal of privilege separation is to prevent privilege
1118 escalation by containing any corruption within the unprivileged processes.
1121 .It Cm X11DisplayOffset
1122 Specifies the first display number available for
1125 This prevents sshd from interfering with real X11 servers.
1127 .It Cm X11Forwarding
1128 Specifies whether X11 forwarding is permitted.
1129 The argument must be
1136 When X11 forwarding is enabled, there may be additional exposure to
1137 the server and to client displays if the
1139 proxy display is configured to listen on the wildcard address (see
1141 below), though this is not the default.
1142 Additionally, the authentication spoofing and authentication data
1143 verification and substitution occur on the client side.
1144 The security risk of using X11 forwarding is that the client's X11
1145 display server may be exposed to attack when the SSH client requests
1146 forwarding (see the warnings for
1149 .Xr ssh_config 5 ) .
1150 A system administrator may have a stance in which they want to
1151 protect clients that may expose themselves to attack by unwittingly
1152 requesting X11 forwarding, which can warrant a
1156 Note that disabling X11 forwarding does not prevent users from
1157 forwarding X11 traffic, as users can always install their own forwarders.
1158 X11 forwarding is automatically disabled if
1161 .It Cm X11UseLocalhost
1164 should bind the X11 forwarding server to the loopback address or to
1165 the wildcard address.
1167 sshd binds the forwarding server to the loopback address and sets the
1168 hostname part of the
1170 environment variable to
1172 This prevents remote hosts from connecting to the proxy display.
1173 However, some older X11 clients may not function with this
1178 to specify that the forwarding server should be bound to the wildcard
1180 The argument must be
1186 .It Cm XAuthLocation
1187 Specifies the full pathname of the
1191 .Pa /usr/X11R6/bin/xauth .
1195 command-line arguments and configuration file options that specify time
1196 may be expressed using a sequence of the form:
1198 .Ar time Op Ar qualifier ,
1202 is a positive integer value and
1204 is one of the following:
1206 .Bl -tag -width Ds -compact -offset indent
1221 Each member of the sequence is added together to calculate
1222 the total time value.
1224 Time format examples:
1226 .Bl -tag -width Ds -compact -offset indent
1228 600 seconds (10 minutes)
1232 1 hour 30 minutes (90 minutes)
1236 .It Pa /etc/ssh/sshd_config
1237 Contains configuration data for
1239 This file should be writable by root only, but it is recommended
1240 (though not necessary) that it be world-readable.
1245 OpenSSH is a derivative of the original and free
1246 ssh 1.2.12 release by Tatu Ylonen.
1247 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1248 Theo de Raadt and Dug Song
1249 removed many bugs, re-added newer features and
1251 Markus Friedl contributed the support for SSH
1252 protocol versions 1.5 and 2.0.
1253 Niels Provos and Markus Friedl contributed support
1254 for privilege separation.