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 $
37 .Dd $Mdocdate: June 29 2012 $
42 .Nd OpenSSH SSH daemon configuration file
44 .Nm /etc/ssh/sshd_config
47 reads configuration data from
48 .Pa /etc/ssh/sshd_config
49 (or the file specified with
52 The file contains keyword-argument pairs, one per line.
55 and empty lines are interpreted as comments.
56 Arguments may optionally be enclosed in double quotes
58 in order to represent arguments containing spaces.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
65 Specifies what environment variables sent by the client will be copied into
72 for how to configure the client.
73 Note that environment passing is only supported for protocol 2.
74 Variables are specified by name, which may contain the wildcard characters
78 Multiple environment variables may be separated by whitespace or spread
82 Be warned that some environment variables could be used to bypass restricted
84 For this reason, care should be taken in the use of this directive.
85 The default is not to accept any environment variables.
87 Specifies which address family should be used by
97 .It Cm AllowAgentForwarding
100 forwarding is permitted.
103 Note that disabling agent forwarding does not improve security
104 unless users are also denied shell access, as they can always install
105 their own forwarders.
107 This keyword can be followed by a list of group name patterns, separated
109 If specified, login is allowed only for users whose primary
110 group or supplementary group list matches one of the patterns.
111 Only group names are valid; a numerical group ID is not recognized.
112 By default, login is allowed for all groups.
113 The allow/deny directives are processed in the following order:
124 for more information on patterns.
125 .It Cm AllowTcpForwarding
126 Specifies whether TCP forwarding is permitted.
129 Note that disabling TCP forwarding does not improve security unless
130 users are also denied shell access, as they can always install their
133 This keyword can be followed by a list of user name patterns, separated
135 If specified, login is allowed only for user names that
136 match one of the patterns.
137 Only user names are valid; a numerical user ID is not recognized.
138 By default, login is allowed for all users.
139 If the pattern takes the form USER@HOST then USER and HOST
140 are separately checked, restricting logins to particular
141 users from particular hosts.
142 The allow/deny directives are processed in the following order:
153 for more information on patterns.
154 .It Cm AuthorizedKeysFile
155 Specifies the file that contains the public keys that can be used
156 for user authentication.
157 The format is described in the
158 .Sx AUTHORIZED_KEYS FILE FORMAT
161 .Cm AuthorizedKeysFile
162 may contain tokens of the form %T which are substituted during connection
164 The following tokens are defined: %% is replaced by a literal '%',
165 %h is replaced by the home directory of the user being authenticated, and
166 %u is replaced by the username of that user.
168 .Cm AuthorizedKeysFile
169 is taken to be an absolute path or one relative to the user's home
171 Multiple files may be listed, separated by whitespace.
173 .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
174 .It Cm AuthorizedPrincipalsFile
175 Specifies a file that lists principal names that are accepted for
176 certificate authentication.
177 When using certificates signed by a key listed in
178 .Cm TrustedUserCAKeys ,
179 this file lists names, one of which must appear in the certificate for it
180 to be accepted for authentication.
181 Names are listed one per line preceded by key options (as described
183 .Sx AUTHORIZED_KEYS FILE FORMAT
186 Empty lines and comments starting with
190 .Cm AuthorizedPrincipalsFile
191 may contain tokens of the form %T which are substituted during connection
193 The following tokens are defined: %% 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.
197 .Cm AuthorizedPrincipalsFile
198 is taken to be an absolute path or one relative to the user's home
203 i.e. not to use a principals file \(en in this case, the username
204 of the user must appear in a certificate's principals list for it to be
207 .Cm AuthorizedPrincipalsFile
208 is only used when authentication proceeds using a CA listed in
209 .Cm TrustedUserCAKeys
210 and is not consulted for certification authorities trusted via
211 .Pa ~/.ssh/authorized_keys ,
214 key option offers a similar facility (see
218 The contents of the specified file are sent to the remote user before
219 authentication is allowed.
222 then no banner is displayed.
223 This option is only available for protocol version 2.
224 By default, no banner is displayed.
225 .It Cm ChallengeResponseAuthentication
226 Specifies whether challenge-response authentication is allowed (e.g. via
227 PAM or though authentication styles supported in
231 .It Cm ChrootDirectory
232 Specifies the pathname of a directory to
234 to after authentication.
235 All components of the pathname must be root-owned directories that are
236 not writable by any other user or group.
239 changes the working directory to the user's home directory.
241 The pathname may contain the following tokens that are expanded at runtime once
242 the connecting user has been authenticated: %% is replaced by a literal '%',
243 %h is replaced by the home directory of the user being authenticated, and
244 %u is replaced by the username of that user.
248 must contain the necessary files and directories to support the
250 For an interactive session this requires at least a shell, typically
264 For file transfer sessions using
266 no additional configuration of the environment is necessary if the
267 in-process sftp server is used,
268 though sessions which use logging do require
270 inside the chroot directory (see
274 The default is not to
277 Specifies the ciphers allowed for protocol version 2.
278 Multiple ciphers must be comma-separated.
279 The supported ciphers are
294 .Bd -literal -offset 3n
295 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
296 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
299 .It Cm ClientAliveCountMax
300 Sets the number of client alive messages (see below) which may be
303 receiving any messages back from the client.
304 If this threshold is reached while client alive messages are being sent,
305 sshd will disconnect the client, terminating the session.
306 It is important to note that the use of client alive messages is very
310 The client alive messages are sent through the encrypted channel
311 and therefore will not be spoofable.
312 The TCP keepalive option enabled by
315 The client alive mechanism is valuable when the client or
316 server depend on knowing when a connection has become inactive.
318 The default value is 3.
320 .Cm ClientAliveInterval
321 (see below) is set to 15, and
322 .Cm ClientAliveCountMax
323 is left at the default, unresponsive SSH clients
324 will be disconnected after approximately 45 seconds.
325 This option applies to protocol version 2 only.
326 .It Cm ClientAliveInterval
327 Sets a timeout interval in seconds after which if no data has been received
330 will send a message through the encrypted
331 channel to request a response from the client.
333 is 0, indicating that these messages will not be sent to the client.
334 This option applies to protocol version 2 only.
336 Specifies whether compression is allowed, or delayed until
337 the user has authenticated successfully.
346 This keyword can be followed by a list of group name patterns, separated
348 Login is disallowed for users whose primary group or supplementary
349 group list matches one of the patterns.
350 Only group names are valid; a numerical group ID is not recognized.
351 By default, login is allowed for all groups.
352 The allow/deny directives are processed in the following order:
363 for more information on patterns.
365 This keyword can be followed by a list of user name patterns, separated
367 Login is disallowed for user names that match one of the patterns.
368 Only user names are valid; a numerical user ID is not recognized.
369 By default, login is allowed for all users.
370 If the pattern takes the form USER@HOST then USER and HOST
371 are separately checked, restricting logins to particular
372 users from particular hosts.
373 The allow/deny directives are processed in the following order:
384 for more information on patterns.
386 Forces the execution of the command specified by
388 ignoring any command supplied by the client and
391 The command is invoked by using the user's login shell with the -c option.
392 This applies to shell, command, or subsystem execution.
393 It is most useful inside a
396 The command originally supplied by the client is available in the
397 .Ev SSH_ORIGINAL_COMMAND
398 environment variable.
399 Specifying a command of
401 will force the use of an in-process sftp server that requires no support
403 .Cm ChrootDirectory .
405 Specifies whether remote hosts are allowed to connect to ports
406 forwarded for the client.
409 binds remote port forwardings to the loopback address.
410 This prevents other remote hosts from connecting to forwarded ports.
412 can be used to specify that sshd
413 should allow remote port forwardings to bind to non-loopback addresses, thus
414 allowing other hosts to connect.
417 to force remote port forwardings to be available to the local host only,
419 to force remote port forwardings to bind to the wildcard address, or
421 to allow the client to select the address to which the forwarding is bound.
424 .It Cm GSSAPIAuthentication
425 Specifies whether user authentication based on GSSAPI is allowed.
428 Note that this option applies to protocol version 2 only.
429 .It Cm GSSAPICleanupCredentials
430 Specifies whether to automatically destroy the user's credentials cache
434 Note that this option applies to protocol version 2 only.
435 .It Cm HostbasedAuthentication
436 Specifies whether rhosts or /etc/hosts.equiv authentication together
437 with successful public key client host authentication is allowed
438 (host-based authentication).
439 This option is similar to
440 .Cm RhostsRSAAuthentication
441 and applies to protocol version 2 only.
444 .It Cm HostbasedUsesNameFromPacketOnly
445 Specifies whether or not the server will attempt to perform a reverse
446 name lookup when matching the name in the
452 .Cm HostbasedAuthentication .
457 uses the name supplied by the client rather than
458 attempting to resolve the name from the TCP connection itself.
461 .It Cm HostCertificate
462 Specifies a file containing a public host certificate.
463 The certificate's public key must match a private host key already specified
466 The default behaviour of
468 is not to load any certificates.
470 Specifies a file containing a private host key
473 .Pa /etc/ssh/ssh_host_key
474 for protocol version 1, and
475 .Pa /etc/ssh/ssh_host_dsa_key ,
476 .Pa /etc/ssh/ssh_host_ecdsa_key
478 .Pa /etc/ssh/ssh_host_rsa_key
479 for protocol version 2.
482 will refuse to use a file if it is group/world-accessible.
483 It is possible to have multiple host key files.
485 keys are used for version 1 and
490 are used for version 2 of the SSH protocol.
496 files will not be used in
497 .Cm RhostsRSAAuthentication
499 .Cm HostbasedAuthentication .
503 .Pa /etc/shosts.equiv
507 .It Cm IgnoreUserKnownHosts
510 should ignore the user's
511 .Pa ~/.ssh/known_hosts
513 .Cm RhostsRSAAuthentication
515 .Cm HostbasedAuthentication .
519 Specifies the IPv4 type-of-service or DSCP class for the connection.
546 This option may take one or two arguments, separated by whitespace.
547 If one argument is specified, it is used as the packet class unconditionally.
548 If two values are specified, the first is automatically selected for
549 interactive sessions and the second for non-interactive sessions.
552 for interactive sessions and
554 for non-interactive sessions.
555 .It Cm KerberosAuthentication
556 Specifies whether the password provided by the user for
557 .Cm PasswordAuthentication
558 will be validated through the Kerberos KDC.
559 To use this option, the server needs a
560 Kerberos servtab which allows the verification of the KDC's identity.
563 .It Cm KerberosGetAFSToken
564 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
565 an AFS token before accessing the user's home directory.
568 .It Cm KerberosOrLocalPasswd
569 If password authentication through Kerberos fails then
570 the password will be validated via any additional local mechanism
575 .It Cm KerberosTicketCleanup
576 Specifies whether to automatically destroy the user's ticket cache
581 Specifies the available KEX (Key Exchange) algorithms.
582 Multiple algorithms must be comma-separated.
584 .Dq ecdh-sha2-nistp256 ,
585 .Dq ecdh-sha2-nistp384 ,
586 .Dq ecdh-sha2-nistp521 ,
587 .Dq diffie-hellman-group-exchange-sha256 ,
588 .Dq diffie-hellman-group-exchange-sha1 ,
589 .Dq diffie-hellman-group14-sha1 ,
590 .Dq diffie-hellman-group1-sha1 .
591 .It Cm KeyRegenerationInterval
592 In protocol version 1, the ephemeral server key is automatically regenerated
593 after this many seconds (if it has been used).
594 The purpose of regeneration is to prevent
595 decrypting captured sessions by later breaking into the machine and
597 The key is never stored anywhere.
598 If the value is 0, the key is never regenerated.
599 The default is 3600 (seconds).
601 Specifies the local addresses
604 The following forms may be used:
606 .Bl -item -offset indent -compact
610 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
615 .Ar host No | Ar IPv4_addr No : Ar port
621 .Ar host No | Ar IPv6_addr Oc : Ar port
628 sshd will listen on the address and all prior
631 The default is to listen on all local addresses.
634 options are permitted.
637 options must precede this option for non-port qualified addresses.
638 .It Cm LoginGraceTime
639 The server disconnects after this time if the user has not
640 successfully logged in.
641 If the value is 0, there is no time limit.
642 The default is 120 seconds.
644 Gives the verbosity level that is used when logging messages from
646 The possible values are:
647 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
649 DEBUG and DEBUG1 are equivalent.
650 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
651 Logging with a DEBUG level violates the privacy of users and is not recommended.
653 Specifies the available MAC (message authentication code) algorithms.
654 The MAC algorithm is used in protocol version 2
655 for data integrity protection.
656 Multiple algorithms must be comma-separated.
658 .Bd -literal -offset indent
659 hmac-md5,hmac-sha1,umac-64@openssh.com,
660 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
661 hmac-sha1-96,hmac-md5-96
664 Introduces a conditional block.
665 If all of the criteria on the
667 line are satisfied, the keywords on the following lines override those
668 set in the global section of the config file, until either another
670 line or the end of the file.
674 are one or more criteria-pattern pairs.
675 The available criteria are
683 The match patterns may consist of single entries or comma-separated
684 lists and may use the wildcard and negation operators described in the
691 criteria may additionally contain addresses to match in CIDR
692 address/masklen format, e.g.\&
696 Note that the mask length provided must be consistent with the address -
697 it is an error to specify a mask length that is too long for the address
698 or one with bits set in this host portion of the address.
705 Only a subset of keywords may be used on the lines following a
708 Available keywords are
710 .Cm AllowAgentForwarding ,
712 .Cm AllowTcpForwarding ,
714 .Cm AuthorizedKeysFile ,
715 .Cm AuthorizedPrincipalsFile ,
717 .Cm ChrootDirectory ,
722 .Cm GSSAPIAuthentication ,
723 .Cm HostbasedAuthentication ,
724 .Cm HostbasedUsesNameFromPacketOnly ,
725 .Cm KbdInteractiveAuthentication ,
726 .Cm KerberosAuthentication ,
729 .Cm PasswordAuthentication ,
730 .Cm PermitEmptyPasswords ,
732 .Cm PermitRootLogin ,
734 .Cm PubkeyAuthentication ,
735 .Cm RhostsRSAAuthentication ,
736 .Cm RSAAuthentication ,
737 .Cm X11DisplayOffset ,
740 .Cm X11UseLocalHost .
742 Specifies the maximum number of authentication attempts permitted per
744 Once the number of failures reaches half this value,
745 additional failures are logged.
748 Specifies the maximum number of open sessions permitted per network connection.
751 Specifies the maximum number of concurrent unauthenticated connections to the
753 Additional connections will be dropped until authentication succeeds or the
755 expires for a connection.
758 Alternatively, random early drop can be enabled by specifying
759 the three colon separated values
763 will refuse connection attempts with a probability of
766 if there are currently
769 unauthenticated connections.
770 The probability increases linearly and all connection attempts
771 are refused if the number of unauthenticated connections reaches
774 .It Cm PasswordAuthentication
775 Specifies whether password authentication is allowed.
778 .It Cm PermitEmptyPasswords
779 When password authentication is allowed, it specifies whether the
780 server allows login to accounts with empty password strings.
784 Specifies the destinations to which TCP port forwarding is permitted.
785 The forwarding specification must be one of the following forms:
787 .Bl -item -offset indent -compact
801 .Ar \&[ IPv6_addr \&] : port
805 Multiple forwards may be specified by separating them with whitespace.
808 can be used to remove all restrictions and permit any forwarding requests.
811 can be used to prohibit all forwarding requests.
812 By default all port forwarding requests are permitted.
813 .It Cm PermitRootLogin
814 Specifies whether root can log in using
818 .Dq without-password ,
819 .Dq forced-commands-only ,
825 If this option is set to
826 .Dq without-password ,
827 password authentication is disabled for root.
829 If this option is set to
830 .Dq forced-commands-only ,
831 root login with public key authentication will be allowed,
834 option has been specified
835 (which may be useful for taking remote backups even if root login is
836 normally not allowed).
837 All other authentication methods are disabled for root.
839 If this option is set to
841 root is not allowed to log in.
845 device forwarding is allowed.
861 .It Cm PermitUserEnvironment
863 .Pa ~/.ssh/environment
867 .Pa ~/.ssh/authorized_keys
872 Enabling environment processing may enable users to bypass access
873 restrictions in some configurations using mechanisms such as
876 Specifies the file that contains the process ID of the
879 .Pa /var/run/sshd.pid .
881 Specifies the port number that
885 Multiple options of this type are permitted.
891 should print the date and time of the last user login when a user logs
900 when a user logs in interactively.
901 (On some systems it is also printed by the shell,
907 Specifies the protocol versions
910 The possible values are
914 Multiple versions must be comma-separated.
917 Note that the order of the protocol list does not indicate preference,
918 because the client selects among multiple protocol versions offered
924 .It Cm PubkeyAuthentication
925 Specifies whether public key authentication is allowed.
928 Note that this option applies to protocol version 2 only.
930 Specifies a list of revoked public keys.
931 Keys listed in this file will be refused for public key authentication.
932 Note that if this file is not readable, then public key authentication will
933 be refused for all users.
934 .It Cm RhostsRSAAuthentication
935 Specifies whether rhosts or /etc/hosts.equiv authentication together
936 with successful RSA host authentication is allowed.
939 This option applies to protocol version 1 only.
940 .It Cm RSAAuthentication
941 Specifies whether pure RSA authentication is allowed.
944 This option applies to protocol version 1 only.
946 Defines the number of bits in the ephemeral protocol version 1 server key.
947 The minimum value is 512, and the default is 1024.
951 should check file modes and ownership of the
952 user's files and home directory before accepting login.
953 This is normally desirable because novices sometimes accidentally leave their
954 directory or files world-writable.
957 Note that this does not apply to
958 .Cm ChrootDirectory ,
959 whose permissions and ownership are checked unconditionally.
961 Configures an external subsystem (e.g. file transfer daemon).
962 Arguments should be a subsystem name and a command (with optional arguments)
963 to execute upon subsystem request.
969 file transfer subsystem.
973 implements an in-process
976 This may simplify configurations using
978 to force a different filesystem root on clients.
980 By default no subsystems are defined.
981 Note that this option applies to protocol version 2 only.
982 .It Cm SyslogFacility
983 Gives the facility code that is used when logging messages from
985 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
986 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
989 Specifies whether the system should send TCP keepalive messages to the
991 If they are sent, death of the connection or crash of one
992 of the machines will be properly noticed.
993 However, this means that
994 connections will die if the route is down temporarily, and some people
996 On the other hand, if TCP keepalives are not sent,
997 sessions may hang indefinitely on the server, leaving
999 users and consuming server resources.
1003 (to send TCP keepalive messages), and the server will notice
1004 if the network goes down or the client host crashes.
1005 This avoids infinitely hanging sessions.
1007 To disable TCP keepalive messages, the value should be set to
1009 .It Cm TrustedUserCAKeys
1010 Specifies a file containing public keys of certificate authorities that are
1011 trusted to sign user certificates for authentication.
1012 Keys are listed one per line; empty lines and comments starting with
1015 If a certificate is presented for authentication and has its signing CA key
1016 listed in this file, then it may be used for authentication for any user
1017 listed in the certificate's principals list.
1018 Note that certificates that lack a list of principals will not be permitted
1019 for authentication using
1020 .Cm TrustedUserCAKeys .
1021 For more details on certificates, see the
1028 should look up the remote host name and check that
1029 the resolved host name for the remote IP address maps back to the
1030 very same IP address.
1036 is used for interactive login sessions.
1041 is never used for remote command execution.
1042 Note also, that if this is enabled,
1044 will be disabled because
1046 does not know how to handle
1050 .Cm UsePrivilegeSeparation
1051 is specified, it will be disabled after authentication.
1053 Enables the Pluggable Authentication Module interface.
1056 this will enable PAM authentication using
1057 .Cm ChallengeResponseAuthentication
1059 .Cm PasswordAuthentication
1060 in addition to PAM account and session module processing for all
1061 authentication types.
1063 Because PAM challenge-response authentication usually serves an equivalent
1064 role to password authentication, you should disable either
1065 .Cm PasswordAuthentication
1067 .Cm ChallengeResponseAuthentication.
1071 is enabled, you will not be able to run
1076 .It Cm UsePrivilegeSeparation
1079 separates privileges by creating an unprivileged child process
1080 to deal with incoming network traffic.
1081 After successful authentication, another process will be created that has
1082 the privilege of the authenticated user.
1083 The goal of privilege separation is to prevent privilege
1084 escalation by containing any corruption within the unprivileged processes.
1088 .Cm UsePrivilegeSeparation
1091 then the pre-authentication unprivileged process is subject to additional
1093 .It Cm VersionAddendum
1094 Optionally specifies additional text to append to the SSH protocol banner
1095 sent by the server upon connection.
1098 .It Cm X11DisplayOffset
1099 Specifies the first display number available for
1102 This prevents sshd from interfering with real X11 servers.
1104 .It Cm X11Forwarding
1105 Specifies whether X11 forwarding is permitted.
1106 The argument must be
1113 When X11 forwarding is enabled, there may be additional exposure to
1114 the server and to client displays if the
1116 proxy display is configured to listen on the wildcard address (see
1118 below), though this is not the default.
1119 Additionally, the authentication spoofing and authentication data
1120 verification and substitution occur on the client side.
1121 The security risk of using X11 forwarding is that the client's X11
1122 display server may be exposed to attack when the SSH client requests
1123 forwarding (see the warnings for
1126 .Xr ssh_config 5 ) .
1127 A system administrator may have a stance in which they want to
1128 protect clients that may expose themselves to attack by unwittingly
1129 requesting X11 forwarding, which can warrant a
1133 Note that disabling X11 forwarding does not prevent users from
1134 forwarding X11 traffic, as users can always install their own forwarders.
1135 X11 forwarding is automatically disabled if
1138 .It Cm X11UseLocalhost
1141 should bind the X11 forwarding server to the loopback address or to
1142 the wildcard address.
1144 sshd binds the forwarding server to the loopback address and sets the
1145 hostname part of the
1147 environment variable to
1149 This prevents remote hosts from connecting to the proxy display.
1150 However, some older X11 clients may not function with this
1155 to specify that the forwarding server should be bound to the wildcard
1157 The argument must be
1163 .It Cm XAuthLocation
1164 Specifies the full pathname of the
1168 .Pa /usr/X11R6/bin/xauth .
1172 command-line arguments and configuration file options that specify time
1173 may be expressed using a sequence of the form:
1175 .Ar time Op Ar qualifier ,
1179 is a positive integer value and
1181 is one of the following:
1183 .Bl -tag -width Ds -compact -offset indent
1198 Each member of the sequence is added together to calculate
1199 the total time value.
1201 Time format examples:
1203 .Bl -tag -width Ds -compact -offset indent
1205 600 seconds (10 minutes)
1209 1 hour 30 minutes (90 minutes)
1213 .It Pa /etc/ssh/sshd_config
1214 Contains configuration data for
1216 This file should be writable by root only, but it is recommended
1217 (though not necessary) that it be world-readable.
1222 OpenSSH is a derivative of the original and free
1223 ssh 1.2.12 release by Tatu Ylonen.
1224 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1225 Theo de Raadt and Dug Song
1226 removed many bugs, re-added newer features and
1228 Markus Friedl contributed the support for SSH
1229 protocol versions 1.5 and 2.0.
1230 Niels Provos and Markus Friedl contributed support
1231 for privilege separation.