2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: sshd_config.5,v 1.144 2012/06/29 13:57:25 naddy 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
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 The format is described in the
159 .Sx AUTHORIZED_KEYS FILE FORMAT
162 .Cm AuthorizedKeysFile
163 may contain tokens of the form %T which are substituted during connection
165 The following tokens are defined: %% is replaced by a literal '%',
166 %h is replaced by the home directory of the user being authenticated, and
167 %u is replaced by the username of that user.
169 .Cm AuthorizedKeysFile
170 is taken to be an absolute path or one relative to the user's home
172 Multiple files may be listed, separated by whitespace.
174 .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
175 .It Cm AuthorizedPrincipalsFile
176 Specifies a file that lists principal names that are accepted for
177 certificate authentication.
178 When using certificates signed by a key listed in
179 .Cm TrustedUserCAKeys ,
180 this file lists names, one of which must appear in the certificate for it
181 to be accepted for authentication.
182 Names are listed one per line preceded by key options (as described
184 .Sx AUTHORIZED_KEYS FILE FORMAT
187 Empty lines and comments starting with
191 .Cm AuthorizedPrincipalsFile
192 may contain tokens of the form %T which are substituted during connection
194 The following tokens are defined: %% is replaced by a literal '%',
195 %h is replaced by the home directory of the user being authenticated, and
196 %u is replaced by the username of that user.
198 .Cm AuthorizedPrincipalsFile
199 is taken to be an absolute path or one relative to the user's home
204 i.e. not to use a principals file \(en in this case, the username
205 of the user must appear in a certificate's principals list for it to be
208 .Cm AuthorizedPrincipalsFile
209 is only used when authentication proceeds using a CA listed in
210 .Cm TrustedUserCAKeys
211 and is not consulted for certification authorities trusted via
212 .Pa ~/.ssh/authorized_keys ,
215 key option offers a similar facility (see
219 The contents of the specified file are sent to the remote user before
220 authentication is allowed.
223 then no banner is displayed.
224 This option is only available for protocol version 2.
225 By default, no banner is displayed.
226 .It Cm ChallengeResponseAuthentication
227 Specifies whether challenge-response authentication is allowed (e.g. via
228 PAM or though authentication styles supported in
232 .It Cm ChrootDirectory
233 Specifies the pathname of a directory to
235 to after authentication.
236 All components of the pathname must be root-owned directories that are
237 not writable by any other user or group.
240 changes the working directory to the user's home directory.
242 The pathname may contain the following tokens that are expanded at runtime once
243 the connecting user has been authenticated: %% is replaced by a literal '%',
244 %h is replaced by the home directory of the user being authenticated, and
245 %u is replaced by the username of that user.
249 must contain the necessary files and directories to support the
251 For an interactive session this requires at least a shell, typically
265 For file transfer sessions using
267 no additional configuration of the environment is necessary if the
268 in-process sftp server is used,
269 though sessions which use logging do require
271 inside the chroot directory (see
275 The default is not to
278 Specifies the ciphers allowed for protocol version 2.
279 Multiple ciphers must be comma-separated.
280 The supported ciphers are
295 .Bd -literal -offset 3n
296 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
297 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
300 .It Cm ClientAliveCountMax
301 Sets the number of client alive messages (see below) which may be
304 receiving any messages back from the client.
305 If this threshold is reached while client alive messages are being sent,
306 sshd will disconnect the client, terminating the session.
307 It is important to note that the use of client alive messages is very
311 The client alive messages are sent through the encrypted channel
312 and therefore will not be spoofable.
313 The TCP keepalive option enabled by
316 The client alive mechanism is valuable when the client or
317 server depend on knowing when a connection has become inactive.
319 The default value is 3.
321 .Cm ClientAliveInterval
322 (see below) is set to 15, and
323 .Cm ClientAliveCountMax
324 is left at the default, unresponsive SSH clients
325 will be disconnected after approximately 45 seconds.
326 This option applies to protocol version 2 only.
327 .It Cm ClientAliveInterval
328 Sets a timeout interval in seconds after which if no data has been received
331 will send a message through the encrypted
332 channel to request a response from the client.
334 is 0, indicating that these messages will not be sent to the client.
335 This option applies to protocol version 2 only.
337 Specifies whether compression is allowed, or delayed until
338 the user has authenticated successfully.
347 This keyword can be followed by a list of group name patterns, separated
349 Login is disallowed for users whose primary group or supplementary
350 group list matches one of the patterns.
351 Only group names are valid; a numerical group ID is not recognized.
352 By default, login is allowed for all groups.
353 The allow/deny directives are processed in the following order:
364 for more information on patterns.
366 This keyword can be followed by a list of user name patterns, separated
368 Login is disallowed for user names that match one of the patterns.
369 Only user names are valid; a numerical user ID is not recognized.
370 By default, login is allowed for all users.
371 If the pattern takes the form USER@HOST then USER and HOST
372 are separately checked, restricting logins to particular
373 users from particular hosts.
374 The allow/deny directives are processed in the following order:
385 for more information on patterns.
387 Forces the execution of the command specified by
389 ignoring any command supplied by the client and
392 The command is invoked by using the user's login shell with the -c option.
393 This applies to shell, command, or subsystem execution.
394 It is most useful inside a
397 The command originally supplied by the client is available in the
398 .Ev SSH_ORIGINAL_COMMAND
399 environment variable.
400 Specifying a command of
402 will force the use of an in-process sftp server that requires no support
404 .Cm ChrootDirectory .
406 Specifies whether remote hosts are allowed to connect to ports
407 forwarded for the client.
410 binds remote port forwardings to the loopback address.
411 This prevents other remote hosts from connecting to forwarded ports.
413 can be used to specify that sshd
414 should allow remote port forwardings to bind to non-loopback addresses, thus
415 allowing other hosts to connect.
418 to force remote port forwardings to be available to the local host only,
420 to force remote port forwardings to bind to the wildcard address, or
422 to allow the client to select the address to which the forwarding is bound.
425 .It Cm GSSAPIAuthentication
426 Specifies whether user authentication based on GSSAPI is allowed.
429 Note that this option applies to protocol version 2 only.
430 .It Cm GSSAPICleanupCredentials
431 Specifies whether to automatically destroy the user's credentials cache
435 Note that this option applies to protocol version 2 only.
436 .It Cm HostbasedAuthentication
437 Specifies whether rhosts or /etc/hosts.equiv authentication together
438 with successful public key client host authentication is allowed
439 (host-based authentication).
440 This option is similar to
441 .Cm RhostsRSAAuthentication
442 and applies to protocol version 2 only.
445 .It Cm HostbasedUsesNameFromPacketOnly
446 Specifies whether or not the server will attempt to perform a reverse
447 name lookup when matching the name in the
453 .Cm HostbasedAuthentication .
458 uses the name supplied by the client rather than
459 attempting to resolve the name from the TCP connection itself.
462 .It Cm HostCertificate
463 Specifies a file containing a public host certificate.
464 The certificate's public key must match a private host key already specified
467 The default behaviour of
469 is not to load any certificates.
471 Specifies a file containing a private host key
474 .Pa /etc/ssh/ssh_host_key
475 for protocol version 1, and
476 .Pa /etc/ssh/ssh_host_dsa_key ,
477 .Pa /etc/ssh/ssh_host_ecdsa_key
479 .Pa /etc/ssh/ssh_host_rsa_key
480 for protocol version 2.
483 will refuse to use a file if it is group/world-accessible.
484 It is possible to have multiple host key files.
486 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/ssh/shosts.equiv
508 .It Cm IgnoreUserKnownHosts
511 should ignore the user's
512 .Pa ~/.ssh/known_hosts
514 .Cm RhostsRSAAuthentication
516 .Cm HostbasedAuthentication .
520 Specifies the IPv4 type-of-service or DSCP class for the connection.
547 This option may take one or two arguments, separated by whitespace.
548 If one argument is specified, it is used as the packet class unconditionally.
549 If two values are specified, the first is automatically selected for
550 interactive sessions and the second for non-interactive sessions.
553 for interactive sessions and
555 for non-interactive sessions.
556 .It Cm KerberosAuthentication
557 Specifies whether the password provided by the user for
558 .Cm PasswordAuthentication
559 will be validated through the Kerberos KDC.
560 To use this option, the server needs a
561 Kerberos servtab which allows the verification of the KDC's identity.
564 .It Cm KerberosGetAFSToken
565 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
566 an AFS token before accessing the user's home directory.
569 .It Cm KerberosOrLocalPasswd
570 If password authentication through Kerberos fails then
571 the password will be validated via any additional local mechanism
576 .It Cm KerberosTicketCleanup
577 Specifies whether to automatically destroy the user's ticket cache
582 Specifies the available KEX (Key Exchange) algorithms.
583 Multiple algorithms must be comma-separated.
585 .Dq ecdh-sha2-nistp256 ,
586 .Dq ecdh-sha2-nistp384 ,
587 .Dq ecdh-sha2-nistp521 ,
588 .Dq diffie-hellman-group-exchange-sha256 ,
589 .Dq diffie-hellman-group-exchange-sha1 ,
590 .Dq diffie-hellman-group14-sha1 ,
591 .Dq diffie-hellman-group1-sha1 .
592 .It Cm KeyRegenerationInterval
593 In protocol version 1, the ephemeral server key is automatically regenerated
594 after this many seconds (if it has been used).
595 The purpose of regeneration is to prevent
596 decrypting captured sessions by later breaking into the machine and
598 The key is never stored anywhere.
599 If the value is 0, the key is never regenerated.
600 The default is 3600 (seconds).
602 Specifies the local addresses
605 The following forms may be used:
607 .Bl -item -offset indent -compact
611 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
616 .Ar host No | Ar IPv4_addr No : Ar port
622 .Ar host No | Ar IPv6_addr Oc : Ar port
629 sshd will listen on the address and all prior
632 The default is to listen on all local addresses.
635 options are permitted.
638 options must precede this option for non-port qualified addresses.
639 .It Cm LoginGraceTime
640 The server disconnects after this time if the user has not
641 successfully logged in.
642 If the value is 0, there is no time limit.
643 The default is 120 seconds.
645 Gives the verbosity level that is used when logging messages from
647 The possible values are:
648 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
650 DEBUG and DEBUG1 are equivalent.
651 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
652 Logging with a DEBUG level violates the privacy of users and is not recommended.
654 Specifies the available MAC (message authentication code) algorithms.
655 The MAC algorithm is used in protocol version 2
656 for data integrity protection.
657 Multiple algorithms must be comma-separated.
659 .Bd -literal -offset indent
660 hmac-md5,hmac-sha1,umac-64@openssh.com,
661 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
662 hmac-sha1-96,hmac-md5-96
665 Introduces a conditional block.
666 If all of the criteria on the
668 line are satisfied, the keywords on the following lines override those
669 set in the global section of the config file, until either another
671 line or the end of the file.
675 are one or more criteria-pattern pairs.
676 The available criteria are
684 The match patterns may consist of single entries or comma-separated
685 lists and may use the wildcard and negation operators described in the
692 criteria may additionally contain addresses to match in CIDR
693 address/masklen format, e.g.\&
697 Note that the mask length provided must be consistent with the address -
698 it is an error to specify a mask length that is too long for the address
699 or one with bits set in this host portion of the address.
706 Only a subset of keywords may be used on the lines following a
709 Available keywords are
711 .Cm AllowAgentForwarding ,
713 .Cm AllowTcpForwarding ,
715 .Cm AuthorizedKeysFile ,
716 .Cm AuthorizedPrincipalsFile ,
718 .Cm ChrootDirectory ,
723 .Cm GSSAPIAuthentication ,
724 .Cm HostbasedAuthentication ,
725 .Cm HostbasedUsesNameFromPacketOnly ,
726 .Cm KbdInteractiveAuthentication ,
727 .Cm KerberosAuthentication ,
730 .Cm PasswordAuthentication ,
731 .Cm PermitEmptyPasswords ,
733 .Cm PermitRootLogin ,
735 .Cm PubkeyAuthentication ,
736 .Cm RhostsRSAAuthentication ,
737 .Cm RSAAuthentication ,
738 .Cm X11DisplayOffset ,
741 .Cm X11UseLocalHost .
743 Specifies the maximum number of authentication attempts permitted per
745 Once the number of failures reaches half this value,
746 additional failures are logged.
749 Specifies the maximum number of open sessions permitted per network connection.
752 Specifies the maximum number of concurrent unauthenticated connections to the
754 Additional connections will be dropped until authentication succeeds or the
756 expires for a connection.
759 Alternatively, random early drop can be enabled by specifying
760 the three colon separated values
764 will refuse connection attempts with a probability of
767 if there are currently
770 unauthenticated connections.
771 The probability increases linearly and all connection attempts
772 are refused if the number of unauthenticated connections reaches
775 .It Cm PasswordAuthentication
776 Specifies whether password authentication is allowed.
781 .It Cm PermitEmptyPasswords
782 When password authentication is allowed, it specifies whether the
783 server allows login to accounts with empty password strings.
787 Specifies the destinations to which TCP port forwarding is permitted.
788 The forwarding specification must be one of the following forms:
790 .Bl -item -offset indent -compact
804 .Ar \&[ IPv6_addr \&] : port
808 Multiple forwards may be specified by separating them with whitespace.
811 can be used to remove all restrictions and permit any forwarding requests.
814 can be used to prohibit all forwarding requests.
815 By default all port forwarding requests are permitted.
816 .It Cm PermitRootLogin
817 Specifies whether root can log in using
821 .Dq without-password ,
822 .Dq forced-commands-only ,
828 .Cm ChallengeResponseAuthentication
831 the root user may be allowed in with its password even if
832 .Cm PermitRootLogin is set to
833 .Dq without-password .
835 If this option is set to
836 .Dq without-password ,
837 password authentication is disabled for root.
839 If this option is set to
840 .Dq forced-commands-only ,
841 root login with public key authentication will be allowed,
844 option has been specified
845 (which may be useful for taking remote backups even if root login is
846 normally not allowed).
847 All other authentication methods are disabled for root.
849 If this option is set to
851 root is not allowed to log in.
855 device forwarding is allowed.
871 .It Cm PermitUserEnvironment
873 .Pa ~/.ssh/environment
877 .Pa ~/.ssh/authorized_keys
882 Enabling environment processing may enable users to bypass access
883 restrictions in some configurations using mechanisms such as
886 Specifies the file that contains the process ID of the
889 .Pa /var/run/sshd.pid .
891 Specifies the port number that
895 Multiple options of this type are permitted.
901 should print the date and time of the last user login when a user logs
910 when a user logs in interactively.
911 (On some systems it is also printed by the shell,
917 Specifies the protocol versions
920 The possible values are
924 Multiple versions must be comma-separated.
927 Note that the order of the protocol list does not indicate preference,
928 because the client selects among multiple protocol versions offered
934 .It Cm PubkeyAuthentication
935 Specifies whether public key authentication is allowed.
938 Note that this option applies to protocol version 2 only.
940 Specifies a list of revoked public keys.
941 Keys listed in this file will be refused for public key authentication.
942 Note that if this file is not readable, then public key authentication will
943 be refused for all users.
944 .It Cm RhostsRSAAuthentication
945 Specifies whether rhosts or
947 authentication together
948 with successful RSA host authentication is allowed.
951 This option applies to protocol version 1 only.
952 .It Cm RSAAuthentication
953 Specifies whether pure RSA authentication is allowed.
956 This option applies to protocol version 1 only.
958 Defines the number of bits in the ephemeral protocol version 1 server key.
959 The minimum value is 512, and the default is 1024.
963 should check file modes and ownership of the
964 user's files and home directory before accepting login.
965 This is normally desirable because novices sometimes accidentally leave their
966 directory or files world-writable.
969 Note that this does not apply to
970 .Cm ChrootDirectory ,
971 whose permissions and ownership are checked unconditionally.
973 Configures an external subsystem (e.g. file transfer daemon).
974 Arguments should be a subsystem name and a command (with optional arguments)
975 to execute upon subsystem request.
981 file transfer subsystem.
985 implements an in-process
988 This may simplify configurations using
990 to force a different filesystem root on clients.
992 By default no subsystems are defined.
993 Note that this option applies to protocol version 2 only.
994 .It Cm SyslogFacility
995 Gives the facility code that is used when logging messages from
997 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
998 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1001 Specifies whether the system should send TCP keepalive messages to the
1003 If they are sent, death of the connection or crash of one
1004 of the machines will be properly noticed.
1005 However, this means that
1006 connections will die if the route is down temporarily, and some people
1008 On the other hand, if TCP keepalives are not sent,
1009 sessions may hang indefinitely on the server, leaving
1011 users and consuming server resources.
1015 (to send TCP keepalive messages), and the server will notice
1016 if the network goes down or the client host crashes.
1017 This avoids infinitely hanging sessions.
1019 To disable TCP keepalive messages, the value should be set to
1021 .It Cm TrustedUserCAKeys
1022 Specifies a file containing public keys of certificate authorities that are
1023 trusted to sign user certificates for authentication.
1024 Keys are listed one per line; empty lines and comments starting with
1027 If a certificate is presented for authentication and has its signing CA key
1028 listed in this file, then it may be used for authentication for any user
1029 listed in the certificate's principals list.
1030 Note that certificates that lack a list of principals will not be permitted
1031 for authentication using
1032 .Cm TrustedUserCAKeys .
1033 For more details on certificates, see the
1040 should look up the remote host name and check that
1041 the resolved host name for the remote IP address maps back to the
1042 very same IP address.
1048 is used for interactive login sessions.
1053 is never used for remote command execution.
1054 Note also, that if this is enabled,
1056 will be disabled because
1058 does not know how to handle
1062 .Cm UsePrivilegeSeparation
1063 is specified, it will be disabled after authentication.
1065 Enables the Pluggable Authentication Module interface.
1068 this will enable PAM authentication using
1069 .Cm ChallengeResponseAuthentication
1071 .Cm PasswordAuthentication
1072 in addition to PAM account and session module processing for all
1073 authentication types.
1075 Because PAM challenge-response authentication usually serves an equivalent
1076 role to password authentication, you should disable either
1077 .Cm PasswordAuthentication
1079 .Cm ChallengeResponseAuthentication.
1083 is enabled, you will not be able to run
1088 .It Cm UsePrivilegeSeparation
1091 separates privileges by creating an unprivileged child process
1092 to deal with incoming network traffic.
1093 After successful authentication, another process will be created that has
1094 the privilege of the authenticated user.
1095 The goal of privilege separation is to prevent privilege
1096 escalation by containing any corruption within the unprivileged processes.
1100 .Cm UsePrivilegeSeparation
1103 then the pre-authentication unprivileged process is subject to additional
1105 .It Cm VersionAddendum
1106 Optionally specifies additional text to append to the SSH protocol banner
1107 sent by the server upon connection.
1109 .Dq FreeBSD-20120901 .
1110 .It Cm X11DisplayOffset
1111 Specifies the first display number available for
1114 This prevents sshd from interfering with real X11 servers.
1116 .It Cm X11Forwarding
1117 Specifies whether X11 forwarding is permitted.
1118 The argument must be
1125 When X11 forwarding is enabled, there may be additional exposure to
1126 the server and to client displays if the
1128 proxy display is configured to listen on the wildcard address (see
1130 below), though this is not the default.
1131 Additionally, the authentication spoofing and authentication data
1132 verification and substitution occur on the client side.
1133 The security risk of using X11 forwarding is that the client's X11
1134 display server may be exposed to attack when the SSH client requests
1135 forwarding (see the warnings for
1138 .Xr ssh_config 5 ) .
1139 A system administrator may have a stance in which they want to
1140 protect clients that may expose themselves to attack by unwittingly
1141 requesting X11 forwarding, which can warrant a
1145 Note that disabling X11 forwarding does not prevent users from
1146 forwarding X11 traffic, as users can always install their own forwarders.
1147 X11 forwarding is automatically disabled if
1150 .It Cm X11UseLocalhost
1153 should bind the X11 forwarding server to the loopback address or to
1154 the wildcard address.
1156 sshd binds the forwarding server to the loopback address and sets the
1157 hostname part of the
1159 environment variable to
1161 This prevents remote hosts from connecting to the proxy display.
1162 However, some older X11 clients may not function with this
1167 to specify that the forwarding server should be bound to the wildcard
1169 The argument must be
1175 .It Cm XAuthLocation
1176 Specifies the full pathname of the
1180 .Pa /usr/local/bin/xauth .
1184 command-line arguments and configuration file options that specify time
1185 may be expressed using a sequence of the form:
1187 .Ar time Op Ar qualifier ,
1191 is a positive integer value and
1193 is one of the following:
1195 .Bl -tag -width Ds -compact -offset indent
1210 Each member of the sequence is added together to calculate
1211 the total time value.
1213 Time format examples:
1215 .Bl -tag -width Ds -compact -offset indent
1217 600 seconds (10 minutes)
1221 1 hour 30 minutes (90 minutes)
1225 .It Pa /etc/ssh/sshd_config
1226 Contains configuration data for
1228 This file should be writable by root only, but it is recommended
1229 (though not necessary) that it be world-readable.
1234 OpenSSH is a derivative of the original and free
1235 ssh 1.2.12 release by Tatu Ylonen.
1236 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1237 Theo de Raadt and Dug Song
1238 removed many bugs, re-added newer features and
1240 Markus Friedl contributed the support for SSH
1241 protocol versions 1.5 and 2.0.
1242 Niels Provos and Markus Friedl contributed support
1243 for privilege separation.