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 1.135 2011/08/02 01:22:11 djm Exp $
37 .Dd $Mdocdate: August 2 2011 $
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
201 The default is not to use a principals file \(en in this case, the username
202 of the user must appear in a certificate's principals list for it to be
205 .Cm AuthorizedPrincipalsFile
206 is only used when authentication proceeds using a CA listed in
207 .Cm TrustedUserCAKeys
208 and is not consulted for certification authorities trusted via
209 .Pa ~/.ssh/authorized_keys ,
212 key option offers a similar facility (see
216 The contents of the specified file are sent to the remote user before
217 authentication is allowed.
220 then no banner is displayed.
221 This option is only available for protocol version 2.
222 By default, no banner is displayed.
223 .It Cm ChallengeResponseAuthentication
224 Specifies whether challenge-response authentication is allowed.
227 this controls the use of PAM (see
230 Note that this affects the effectiveness of the
231 .Cm PasswordAuthentication
237 .It Cm ChrootDirectory
238 Specifies the pathname of a directory to
240 to after authentication.
241 All components of the pathname must be root-owned directories that are
242 not writable by any other user or group.
245 changes the working directory to the user's home directory.
247 The pathname may contain the following tokens that are expanded at runtime once
248 the connecting user has been authenticated: %% is replaced by a literal '%',
249 %h is replaced by the home directory of the user being authenticated, and
250 %u is replaced by the username of that user.
254 must contain the necessary files and directories to support the
256 For an interactive session this requires at least a shell, typically
270 For file transfer sessions using
272 no additional configuration of the environment is necessary if the
273 in-process sftp server is used,
274 though sessions which use logging do require
276 inside the chroot directory (see
280 The default is not to
283 Specifies the ciphers allowed for protocol version 2.
284 Multiple ciphers must be comma-separated.
285 The supported ciphers are
300 .Bd -literal -offset 3n
301 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
302 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
305 .It Cm ClientAliveCountMax
306 Sets the number of client alive messages (see below) which may be
309 receiving any messages back from the client.
310 If this threshold is reached while client alive messages are being sent,
311 sshd will disconnect the client, terminating the session.
312 It is important to note that the use of client alive messages is very
316 The client alive messages are sent through the encrypted channel
317 and therefore will not be spoofable.
318 The TCP keepalive option enabled by
321 The client alive mechanism is valuable when the client or
322 server depend on knowing when a connection has become inactive.
324 The default value is 3.
326 .Cm ClientAliveInterval
327 (see below) is set to 15, and
328 .Cm ClientAliveCountMax
329 is left at the default, unresponsive SSH clients
330 will be disconnected after approximately 45 seconds.
331 This option applies to protocol version 2 only.
332 .It Cm ClientAliveInterval
333 Sets a timeout interval in seconds after which if no data has been received
336 will send a message through the encrypted
337 channel to request a response from the client.
339 is 0, indicating that these messages will not be sent to the client.
340 This option applies to protocol version 2 only.
342 Specifies whether compression is allowed, or delayed until
343 the user has authenticated successfully.
352 This keyword can be followed by a list of group name patterns, separated
354 Login is disallowed for users whose primary group or supplementary
355 group list matches one of the patterns.
356 Only group names are valid; a numerical group ID is not recognized.
357 By default, login is allowed for all groups.
358 The allow/deny directives are processed in the following order:
369 for more information on patterns.
371 This keyword can be followed by a list of user name patterns, separated
373 Login is disallowed for user names that match one of the patterns.
374 Only user names are valid; a numerical user ID is not recognized.
375 By default, login is allowed for all users.
376 If the pattern takes the form USER@HOST then USER and HOST
377 are separately checked, restricting logins to particular
378 users from particular hosts.
379 The allow/deny directives are processed in the following order:
390 for more information on patterns.
392 Forces the execution of the command specified by
394 ignoring any command supplied by the client and
397 The command is invoked by using the user's login shell with the -c option.
398 This applies to shell, command, or subsystem execution.
399 It is most useful inside a
402 The command originally supplied by the client is available in the
403 .Ev SSH_ORIGINAL_COMMAND
404 environment variable.
405 Specifying a command of
407 will force the use of an in-process sftp server that requires no support
409 .Cm ChrootDirectory .
411 Specifies whether remote hosts are allowed to connect to ports
412 forwarded for the client.
415 binds remote port forwardings to the loopback address.
416 This prevents other remote hosts from connecting to forwarded ports.
418 can be used to specify that sshd
419 should allow remote port forwardings to bind to non-loopback addresses, thus
420 allowing other hosts to connect.
423 to force remote port forwardings to be available to the local host only,
425 to force remote port forwardings to bind to the wildcard address, or
427 to allow the client to select the address to which the forwarding is bound.
430 .It Cm GSSAPIAuthentication
431 Specifies whether user authentication based on GSSAPI is allowed.
434 Note that this option applies to protocol version 2 only.
435 .It Cm GSSAPICleanupCredentials
436 Specifies whether to automatically destroy the user's credentials cache
440 Note that this option applies to protocol version 2 only.
441 .It Cm HostbasedAuthentication
442 Specifies whether rhosts or /etc/hosts.equiv authentication together
443 with successful public key client host authentication is allowed
444 (host-based authentication).
445 This option is similar to
446 .Cm RhostsRSAAuthentication
447 and applies to protocol version 2 only.
450 .It Cm HostbasedUsesNameFromPacketOnly
451 Specifies whether or not the server will attempt to perform a reverse
452 name lookup when matching the name in the
458 .Cm HostbasedAuthentication .
463 uses the name supplied by the client rather than
464 attempting to resolve the name from the TCP connection itself.
467 .It Cm HostCertificate
468 Specifies a file containing a public host certificate.
469 The certificate's public key must match a private host key already specified
472 The default behaviour of
474 is not to load any certificates.
476 Specifies a file containing a private host key
479 .Pa /etc/ssh/ssh_host_key
480 for protocol version 1, and
481 .Pa /etc/ssh/ssh_host_dsa_key ,
482 .Pa /etc/ssh/ssh_host_ecdsa_key
484 .Pa /etc/ssh/ssh_host_rsa_key
485 for protocol version 2.
488 will refuse to use a file if it is group/world-accessible.
489 It is possible to have multiple host key files.
491 keys are used for version 1 and
496 are used for version 2 of the SSH protocol.
502 files will not be used in
503 .Cm RhostsRSAAuthentication
505 .Cm HostbasedAuthentication .
509 .Pa /etc/ssh/shosts.equiv
513 .It Cm IgnoreUserKnownHosts
516 should ignore the user's
517 .Pa ~/.ssh/known_hosts
519 .Cm RhostsRSAAuthentication
521 .Cm HostbasedAuthentication .
525 Specifies the IPv4 type-of-service or DSCP class for the connection.
552 This option may take one or two arguments, separated by whitespace.
553 If one argument is specified, it is used as the packet class unconditionally.
554 If two values are specified, the first is automatically selected for
555 interactive sessions and the second for non-interactive sessions.
558 for interactive sessions and
560 for non-interactive sessions.
561 .It Cm KerberosAuthentication
562 Specifies whether the password provided by the user for
563 .Cm PasswordAuthentication
564 will be validated through the Kerberos KDC.
565 To use this option, the server needs a
566 Kerberos servtab which allows the verification of the KDC's identity.
569 .It Cm KerberosGetAFSToken
570 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
571 an AFS token before accessing the user's home directory.
574 .It Cm KerberosOrLocalPasswd
575 If password authentication through Kerberos fails then
576 the password will be validated via any additional local mechanism
581 .It Cm KerberosTicketCleanup
582 Specifies whether to automatically destroy the user's ticket cache
587 Specifies the available KEX (Key Exchange) algorithms.
588 Multiple algorithms must be comma-separated.
590 .Dq ecdh-sha2-nistp256 ,
591 .Dq ecdh-sha2-nistp384 ,
592 .Dq ecdh-sha2-nistp521 ,
593 .Dq diffie-hellman-group-exchange-sha256 ,
594 .Dq diffie-hellman-group-exchange-sha1 ,
595 .Dq diffie-hellman-group14-sha1 ,
596 .Dq diffie-hellman-group1-sha1 .
597 .It Cm KeyRegenerationInterval
598 In protocol version 1, the ephemeral server key is automatically regenerated
599 after this many seconds (if it has been used).
600 The purpose of regeneration is to prevent
601 decrypting captured sessions by later breaking into the machine and
603 The key is never stored anywhere.
604 If the value is 0, the key is never regenerated.
605 The default is 3600 (seconds).
607 Specifies the local addresses
610 The following forms may be used:
612 .Bl -item -offset indent -compact
616 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
621 .Ar host No | Ar IPv4_addr No : Ar port
627 .Ar host No | Ar IPv6_addr Oc : Ar port
634 sshd will listen on the address and all prior
637 The default is to listen on all local addresses.
640 options are permitted.
643 options must precede this option for non-port qualified addresses.
644 .It Cm LoginGraceTime
645 The server disconnects after this time if the user has not
646 successfully logged in.
647 If the value is 0, there is no time limit.
648 The default is 120 seconds.
650 Gives the verbosity level that is used when logging messages from
652 The possible values are:
653 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
655 DEBUG and DEBUG1 are equivalent.
656 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
657 Logging with a DEBUG level violates the privacy of users and is not recommended.
659 Specifies the available MAC (message authentication code) algorithms.
660 The MAC algorithm is used in protocol version 2
661 for data integrity protection.
662 Multiple algorithms must be comma-separated.
664 .Bd -literal -offset indent
665 hmac-md5,hmac-sha1,umac-64@openssh.com,
666 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
667 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
671 Introduces a conditional block.
672 If all of the criteria on the
674 line are satisfied, the keywords on the following lines override those
675 set in the global section of the config file, until either another
677 line or the end of the file.
681 are one or more criteria-pattern pairs.
682 The available criteria are
688 The match patterns may consist of single entries or comma-separated
689 lists and may use the wildcard and negation operators described in the
696 criteria may additionally contain addresses to match in CIDR
697 address/masklen format, e.g.\&
701 Note that the mask length provided must be consistent with the address -
702 it is an error to specify a mask length that is too long for the address
703 or one with bits set in this host portion of the address.
710 Only a subset of keywords may be used on the lines following a
713 Available keywords are
714 .Cm AllowAgentForwarding ,
715 .Cm AllowTcpForwarding ,
716 .Cm AuthorizedKeysFile ,
717 .Cm AuthorizedPrincipalsFile ,
719 .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.
779 .Cm ChallengeResponseAuthentication
785 and the PAM authentication policy for
789 password authentication will be allowed through the challenge-response
790 mechanism regardless of the value of
791 .Cm PasswordAuthentication .
792 .It Cm PermitEmptyPasswords
793 When password authentication is allowed, it specifies whether the
794 server allows login to accounts with empty password strings.
798 Specifies the destinations to which TCP port forwarding is permitted.
799 The forwarding specification must be one of the following forms:
801 .Bl -item -offset indent -compact
815 .Ar \&[ IPv6_addr \&] : port
819 Multiple forwards may be specified by separating them with whitespace.
822 can be used to remove all restrictions and permit any forwarding requests.
823 By default all port forwarding requests are permitted.
824 .It Cm PermitRootLogin
825 Specifies whether root can log in using
829 .Dq without-password ,
830 .Dq forced-commands-only ,
836 .Cm ChallengeResponseAuthentication
839 the root user may be allowed in with its password even if
840 .Cm PermitRootLogin is set to
841 .Dq without-password .
843 If this option is set to
844 .Dq without-password ,
845 password authentication is disabled for root.
847 If this option is set to
848 .Dq forced-commands-only ,
849 root login with public key authentication will be allowed,
852 option has been specified
853 (which may be useful for taking remote backups even if root login is
854 normally not allowed).
855 All other authentication methods are disabled for root.
857 If this option is set to
859 root is not allowed to log in.
863 device forwarding is allowed.
879 .It Cm PermitUserEnvironment
881 .Pa ~/.ssh/environment
885 .Pa ~/.ssh/authorized_keys
890 Enabling environment processing may enable users to bypass access
891 restrictions in some configurations using mechanisms such as
894 Specifies the file that contains the process ID of the
897 .Pa /var/run/sshd.pid .
899 Specifies the port number that
903 Multiple options of this type are permitted.
909 should print the date and time of the last user login when a user logs
918 when a user logs in interactively.
919 (On some systems it is also printed by the shell,
925 Specifies the protocol versions
928 The possible values are
932 Multiple versions must be comma-separated.
935 Note that the order of the protocol list does not indicate preference,
936 because the client selects among multiple protocol versions offered
942 .It Cm PubkeyAuthentication
943 Specifies whether public key authentication is allowed.
946 Note that this option applies to protocol version 2 only.
948 Specifies a list of revoked public keys.
949 Keys listed in this file will be refused for public key authentication.
950 Note that if this file is not readable, then public key authentication will
951 be refused for all users.
952 .It Cm RhostsRSAAuthentication
953 Specifies whether rhosts or
955 authentication together
956 with successful RSA host authentication is allowed.
959 This option applies to protocol version 1 only.
960 .It Cm RSAAuthentication
961 Specifies whether pure RSA authentication is allowed.
964 This option applies to protocol version 1 only.
966 Defines the number of bits in the ephemeral protocol version 1 server key.
967 The minimum value is 512, and the default is 1024.
971 should check file modes and ownership of the
972 user's files and home directory before accepting login.
973 This is normally desirable because novices sometimes accidentally leave their
974 directory or files world-writable.
977 Note that this does not apply to
978 .Cm ChrootDirectory ,
979 whose permissions and ownership are checked unconditionally.
981 Configures an external subsystem (e.g. file transfer daemon).
982 Arguments should be a subsystem name and a command (with optional arguments)
983 to execute upon subsystem request.
989 file transfer subsystem.
993 implements an in-process
996 This may simplify configurations using
998 to force a different filesystem root on clients.
1000 By default no subsystems are defined.
1001 Note that this option applies to protocol version 2 only.
1002 .It Cm SyslogFacility
1003 Gives the facility code that is used when logging messages from
1005 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1006 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1007 The default is AUTH.
1009 Specifies whether the system should send TCP keepalive messages to the
1011 If they are sent, death of the connection or crash of one
1012 of the machines will be properly noticed.
1013 However, this means that
1014 connections will die if the route is down temporarily, and some people
1016 On the other hand, if TCP keepalives are not sent,
1017 sessions may hang indefinitely on the server, leaving
1019 users and consuming server resources.
1023 (to send TCP keepalive messages), and the server will notice
1024 if the network goes down or the client host crashes.
1025 This avoids infinitely hanging sessions.
1027 To disable TCP keepalive messages, the value should be set to
1029 .It Cm TrustedUserCAKeys
1030 Specifies a file containing public keys of certificate authorities that are
1031 trusted to sign user certificates for authentication.
1032 Keys are listed one per line; empty lines and comments starting with
1035 If a certificate is presented for authentication and has its signing CA key
1036 listed in this file, then it may be used for authentication for any user
1037 listed in the certificate's principals list.
1038 Note that certificates that lack a list of principals will not be permitted
1039 for authentication using
1040 .Cm TrustedUserCAKeys .
1041 For more details on certificates, see the
1048 should look up the remote host name and check that
1049 the resolved host name for the remote IP address maps back to the
1050 very same IP address.
1056 is used for interactive login sessions.
1061 is never used for remote command execution.
1062 Note also, that if this is enabled,
1064 will be disabled because
1066 does not know how to handle
1070 .Cm UsePrivilegeSeparation
1071 is specified, it will be disabled after authentication.
1073 Enables the Pluggable Authentication Module interface.
1076 this will enable PAM authentication using
1077 .Cm ChallengeResponseAuthentication
1079 .Cm PasswordAuthentication
1080 in addition to PAM account and session module processing for all
1081 authentication types.
1083 Because PAM challenge-response authentication usually serves an equivalent
1084 role to password authentication, you should disable either
1085 .Cm PasswordAuthentication
1087 .Cm ChallengeResponseAuthentication.
1091 is enabled, you will not be able to run
1096 .It Cm UsePrivilegeSeparation
1099 separates privileges by creating an unprivileged child process
1100 to deal with incoming network traffic.
1101 After successful authentication, another process will be created that has
1102 the privilege of the authenticated user.
1103 The goal of privilege separation is to prevent privilege
1104 escalation by containing any corruption within the unprivileged processes.
1108 .Cm UsePrivilegeSeparation
1111 then the pre-authentication unprivileged process is subject to additional
1113 .It Cm VersionAddendum
1114 Specifies a string to append to the regular version string to identify
1115 OS- or site-specific modifications.
1117 .Dq DragonFly-20110920 .
1118 .It Cm X11DisplayOffset
1119 Specifies the first display number available for
1122 This prevents sshd from interfering with real X11 servers.
1124 .It Cm X11Forwarding
1125 Specifies whether X11 forwarding is permitted.
1126 The argument must be
1133 When X11 forwarding is enabled, there may be additional exposure to
1134 the server and to client displays if the
1136 proxy display is configured to listen on the wildcard address (see
1138 below), though this is not the default.
1139 Additionally, the authentication spoofing and authentication data
1140 verification and substitution occur on the client side.
1141 The security risk of using X11 forwarding is that the client's X11
1142 display server may be exposed to attack when the SSH client requests
1143 forwarding (see the warnings for
1146 .Xr ssh_config 5 ) .
1147 A system administrator may have a stance in which they want to
1148 protect clients that may expose themselves to attack by unwittingly
1149 requesting X11 forwarding, which can warrant a
1153 Note that disabling X11 forwarding does not prevent users from
1154 forwarding X11 traffic, as users can always install their own forwarders.
1155 X11 forwarding is automatically disabled if
1158 .It Cm X11UseLocalhost
1161 should bind the X11 forwarding server to the loopback address or to
1162 the wildcard address.
1164 sshd binds the forwarding server to the loopback address and sets the
1165 hostname part of the
1167 environment variable to
1169 This prevents remote hosts from connecting to the proxy display.
1170 However, some older X11 clients may not function with this
1175 to specify that the forwarding server should be bound to the wildcard
1177 The argument must be
1183 .It Cm XAuthLocation
1184 Specifies the full pathname of the
1188 .Pa /usr/X11R6/bin/xauth .
1192 command-line arguments and configuration file options that specify time
1193 may be expressed using a sequence of the form:
1195 .Ar time Op Ar qualifier ,
1199 is a positive integer value and
1201 is one of the following:
1203 .Bl -tag -width Ds -compact -offset indent
1218 Each member of the sequence is added together to calculate
1219 the total time value.
1221 Time format examples:
1223 .Bl -tag -width Ds -compact -offset indent
1225 600 seconds (10 minutes)
1229 1 hour 30 minutes (90 minutes)
1233 .It Pa /etc/ssh/sshd_config
1234 Contains configuration data for
1236 This file should be writable by root only, but it is recommended
1237 (though not necessary) that it be world-readable.
1242 OpenSSH is a derivative of the original and free
1243 ssh 1.2.12 release by Tatu Ylonen.
1244 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1245 Theo de Raadt and Dug Song
1246 removed many bugs, re-added newer features and
1248 Markus Friedl contributed the support for SSH
1249 protocol versions 1.5 and 2.0.
1250 Niels Provos and Markus Friedl contributed support
1251 for privilege separation.