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 $
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
202 The default is not to use a principals file \(en in this case, the username
203 of the user must appear in a certificate's principals list for it to be
206 .Cm AuthorizedPrincipalsFile
207 is only used when authentication proceeds using a CA listed in
208 .Cm TrustedUserCAKeys
209 and is not consulted for certification authorities trusted via
210 .Pa ~/.ssh/authorized_keys ,
213 key option offers a similar facility (see
217 The contents of the specified file are sent to the remote user before
218 authentication is allowed.
221 then no banner is displayed.
222 This option is only available for protocol version 2.
223 By default, no banner is displayed.
224 .It Cm ChallengeResponseAuthentication
225 Specifies whether challenge-response authentication is allowed (e.g. via
226 PAM or though authentication styles supported in
230 .It Cm ChrootDirectory
231 Specifies the pathname of a directory to
233 to after authentication.
234 All components of the pathname must be root-owned directories that are
235 not writable by any other user or group.
238 changes the working directory to the user's home directory.
240 The pathname may contain the following tokens that are expanded at runtime once
241 the connecting user has been authenticated: %% is replaced by a literal '%',
242 %h is replaced by the home directory of the user being authenticated, and
243 %u is replaced by the username of that user.
247 must contain the necessary files and directories to support the
249 For an interactive session this requires at least a shell, typically
263 For file transfer sessions using
265 no additional configuration of the environment is necessary if the
266 in-process sftp server is used,
267 though sessions which use logging do require
269 inside the chroot directory (see
273 The default is not to
276 Specifies the ciphers allowed for protocol version 2.
277 Multiple ciphers must be comma-separated.
278 The supported ciphers are
293 .Bd -literal -offset 3n
294 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
295 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
298 .It Cm ClientAliveCountMax
299 Sets the number of client alive messages (see below) which may be
302 receiving any messages back from the client.
303 If this threshold is reached while client alive messages are being sent,
304 sshd will disconnect the client, terminating the session.
305 It is important to note that the use of client alive messages is very
309 The client alive messages are sent through the encrypted channel
310 and therefore will not be spoofable.
311 The TCP keepalive option enabled by
314 The client alive mechanism is valuable when the client or
315 server depend on knowing when a connection has become inactive.
317 The default value is 3.
319 .Cm ClientAliveInterval
320 (see below) is set to 15, and
321 .Cm ClientAliveCountMax
322 is left at the default, unresponsive SSH clients
323 will be disconnected after approximately 45 seconds.
324 This option applies to protocol version 2 only.
325 .It Cm ClientAliveInterval
326 Sets a timeout interval in seconds after which if no data has been received
329 will send a message through the encrypted
330 channel to request a response from the client.
332 is 0, indicating that these messages will not be sent to the client.
333 This option applies to protocol version 2 only.
335 Specifies whether compression is allowed, or delayed until
336 the user has authenticated successfully.
345 This keyword can be followed by a list of group name patterns, separated
347 Login is disallowed for users whose primary group or supplementary
348 group list matches one of the patterns.
349 Only group names are valid; a numerical group ID is not recognized.
350 By default, login is allowed for all groups.
351 The allow/deny directives are processed in the following order:
362 for more information on patterns.
364 This keyword can be followed by a list of user name patterns, separated
366 Login is disallowed for user names that match one of the patterns.
367 Only user names are valid; a numerical user ID is not recognized.
368 By default, login is allowed for all users.
369 If the pattern takes the form USER@HOST then USER and HOST
370 are separately checked, restricting logins to particular
371 users from particular hosts.
372 The allow/deny directives are processed in the following order:
383 for more information on patterns.
385 Forces the execution of the command specified by
387 ignoring any command supplied by the client and
390 The command is invoked by using the user's login shell with the -c option.
391 This applies to shell, command, or subsystem execution.
392 It is most useful inside a
395 The command originally supplied by the client is available in the
396 .Ev SSH_ORIGINAL_COMMAND
397 environment variable.
398 Specifying a command of
400 will force the use of an in-process sftp server that requires no support
402 .Cm ChrootDirectory .
404 Specifies whether remote hosts are allowed to connect to ports
405 forwarded for the client.
408 binds remote port forwardings to the loopback address.
409 This prevents other remote hosts from connecting to forwarded ports.
411 can be used to specify that sshd
412 should allow remote port forwardings to bind to non-loopback addresses, thus
413 allowing other hosts to connect.
416 to force remote port forwardings to be available to the local host only,
418 to force remote port forwardings to bind to the wildcard address, or
420 to allow the client to select the address to which the forwarding is bound.
423 .It Cm GSSAPIAuthentication
424 Specifies whether user authentication based on GSSAPI is allowed.
427 Note that this option applies to protocol version 2 only.
428 .It Cm GSSAPICleanupCredentials
429 Specifies whether to automatically destroy the user's credentials cache
433 Note that this option applies to protocol version 2 only.
434 .It Cm HostbasedAuthentication
435 Specifies whether rhosts or /etc/hosts.equiv authentication together
436 with successful public key client host authentication is allowed
437 (host-based authentication).
438 This option is similar to
439 .Cm RhostsRSAAuthentication
440 and applies to protocol version 2 only.
443 .It Cm HostbasedUsesNameFromPacketOnly
444 Specifies whether or not the server will attempt to perform a reverse
445 name lookup when matching the name in the
451 .Cm HostbasedAuthentication .
456 uses the name supplied by the client rather than
457 attempting to resolve the name from the TCP connection itself.
460 .It Cm HostCertificate
461 Specifies a file containing a public host certificate.
462 The certificate's public key must match a private host key already specified
465 The default behaviour of
467 is not to load any certificates.
469 Specifies a file containing a private host key
472 .Pa /etc/ssh/ssh_host_key
473 for protocol version 1, and
474 .Pa /etc/ssh/ssh_host_dsa_key ,
475 .Pa /etc/ssh/ssh_host_ecdsa_key
477 .Pa /etc/ssh/ssh_host_rsa_key
478 for protocol version 2.
481 will refuse to use a file if it is group/world-accessible.
482 It is possible to have multiple host key files.
484 keys are used for version 1 and
489 are used for version 2 of the SSH protocol.
495 files will not be used in
496 .Cm RhostsRSAAuthentication
498 .Cm HostbasedAuthentication .
502 .Pa /etc/ssh/shosts.equiv
506 .It Cm IgnoreUserKnownHosts
509 should ignore the user's
510 .Pa ~/.ssh/known_hosts
512 .Cm RhostsRSAAuthentication
514 .Cm HostbasedAuthentication .
518 Specifies the IPv4 type-of-service or DSCP class for the connection.
545 This option may take one or two arguments, separated by whitespace.
546 If one argument is specified, it is used as the packet class unconditionally.
547 If two values are specified, the first is automatically selected for
548 interactive sessions and the second for non-interactive sessions.
551 for interactive sessions and
553 for non-interactive sessions.
554 .It Cm KerberosAuthentication
555 Specifies whether the password provided by the user for
556 .Cm PasswordAuthentication
557 will be validated through the Kerberos KDC.
558 To use this option, the server needs a
559 Kerberos servtab which allows the verification of the KDC's identity.
562 .It Cm KerberosGetAFSToken
563 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
564 an AFS token before accessing the user's home directory.
567 .It Cm KerberosOrLocalPasswd
568 If password authentication through Kerberos fails then
569 the password will be validated via any additional local mechanism
574 .It Cm KerberosTicketCleanup
575 Specifies whether to automatically destroy the user's ticket cache
580 Specifies the available KEX (Key Exchange) algorithms.
581 Multiple algorithms must be comma-separated.
583 .Dq ecdh-sha2-nistp256 ,
584 .Dq ecdh-sha2-nistp384 ,
585 .Dq ecdh-sha2-nistp521 ,
586 .Dq diffie-hellman-group-exchange-sha256 ,
587 .Dq diffie-hellman-group-exchange-sha1 ,
588 .Dq diffie-hellman-group14-sha1 ,
589 .Dq diffie-hellman-group1-sha1 .
590 .It Cm KeyRegenerationInterval
591 In protocol version 1, the ephemeral server key is automatically regenerated
592 after this many seconds (if it has been used).
593 The purpose of regeneration is to prevent
594 decrypting captured sessions by later breaking into the machine and
596 The key is never stored anywhere.
597 If the value is 0, the key is never regenerated.
598 The default is 3600 (seconds).
600 Specifies the local addresses
603 The following forms may be used:
605 .Bl -item -offset indent -compact
609 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
614 .Ar host No | Ar IPv4_addr No : Ar port
620 .Ar host No | Ar IPv6_addr Oc : Ar port
627 sshd will listen on the address and all prior
630 The default is to listen on all local addresses.
633 options are permitted.
636 options must precede this option for non-port qualified addresses.
637 .It Cm LoginGraceTime
638 The server disconnects after this time if the user has not
639 successfully logged in.
640 If the value is 0, there is no time limit.
641 The default is 120 seconds.
643 Gives the verbosity level that is used when logging messages from
645 The possible values are:
646 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
648 DEBUG and DEBUG1 are equivalent.
649 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
650 Logging with a DEBUG level violates the privacy of users and is not recommended.
652 Specifies the available MAC (message authentication code) algorithms.
653 The MAC algorithm is used in protocol version 2
654 for data integrity protection.
655 Multiple algorithms must be comma-separated.
657 .Bd -literal -offset indent
658 hmac-md5,hmac-sha1,umac-64@openssh.com,
659 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
660 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
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
681 The match patterns may consist of single entries or comma-separated
682 lists and may use the wildcard and negation operators described in the
689 criteria may additionally contain addresses to match in CIDR
690 address/masklen format, e.g.\&
694 Note that the mask length provided must be consistent with the address -
695 it is an error to specify a mask length that is too long for the address
696 or one with bits set in this host portion of the address.
703 Only a subset of keywords may be used on the lines following a
706 Available keywords are
707 .Cm AllowAgentForwarding ,
708 .Cm AllowTcpForwarding ,
709 .Cm AuthorizedKeysFile ,
710 .Cm AuthorizedPrincipalsFile ,
712 .Cm ChrootDirectory ,
715 .Cm GSSAPIAuthentication ,
716 .Cm HostbasedAuthentication ,
717 .Cm HostbasedUsesNameFromPacketOnly ,
718 .Cm KbdInteractiveAuthentication ,
719 .Cm KerberosAuthentication ,
722 .Cm PasswordAuthentication ,
723 .Cm PermitEmptyPasswords ,
725 .Cm PermitRootLogin ,
727 .Cm PubkeyAuthentication ,
728 .Cm RhostsRSAAuthentication ,
729 .Cm RSAAuthentication ,
730 .Cm X11DisplayOffset ,
733 .Cm X11UseLocalHost .
735 Specifies the maximum number of authentication attempts permitted per
737 Once the number of failures reaches half this value,
738 additional failures are logged.
741 Specifies the maximum number of open sessions permitted per network connection.
744 Specifies the maximum number of concurrent unauthenticated connections to the
746 Additional connections will be dropped until authentication succeeds or the
748 expires for a connection.
751 Alternatively, random early drop can be enabled by specifying
752 the three colon separated values
756 will refuse connection attempts with a probability of
759 if there are currently
762 unauthenticated connections.
763 The probability increases linearly and all connection attempts
764 are refused if the number of unauthenticated connections reaches
767 .It Cm PasswordAuthentication
768 Specifies whether password authentication is allowed.
773 .It Cm PermitEmptyPasswords
774 When password authentication is allowed, it specifies whether the
775 server allows login to accounts with empty password strings.
779 Specifies the destinations to which TCP port forwarding is permitted.
780 The forwarding specification must be one of the following forms:
782 .Bl -item -offset indent -compact
796 .Ar \&[ IPv6_addr \&] : port
800 Multiple forwards may be specified by separating them with whitespace.
803 can be used to remove all restrictions and permit any forwarding requests.
804 By default all port forwarding requests are permitted.
805 .It Cm PermitRootLogin
806 Specifies whether root can log in using
810 .Dq without-password ,
811 .Dq forced-commands-only ,
817 .Cm ChallengeResponseAuthentication
820 the root user may be allowed in with its password even if
821 .Cm PermitRootLogin is set to
822 .Dq without-password .
824 If this option is set to
825 .Dq without-password ,
826 password authentication is disabled for root.
828 If this option is set to
829 .Dq forced-commands-only ,
830 root login with public key authentication will be allowed,
833 option has been specified
834 (which may be useful for taking remote backups even if root login is
835 normally not allowed).
836 All other authentication methods are disabled for root.
838 If this option is set to
840 root is not allowed to log in.
844 device forwarding is allowed.
860 .It Cm PermitUserEnvironment
862 .Pa ~/.ssh/environment
866 .Pa ~/.ssh/authorized_keys
871 Enabling environment processing may enable users to bypass access
872 restrictions in some configurations using mechanisms such as
875 Specifies the file that contains the process ID of the
878 .Pa /var/run/sshd.pid .
880 Specifies the port number that
884 Multiple options of this type are permitted.
890 should print the date and time of the last user login when a user logs
899 when a user logs in interactively.
900 (On some systems it is also printed by the shell,
906 Specifies the protocol versions
909 The possible values are
913 Multiple versions must be comma-separated.
916 Note that the order of the protocol list does not indicate preference,
917 because the client selects among multiple protocol versions offered
923 .It Cm PubkeyAuthentication
924 Specifies whether public key authentication is allowed.
927 Note that this option applies to protocol version 2 only.
929 Specifies a list of revoked public keys.
930 Keys listed in this file will be refused for public key authentication.
931 Note that if this file is not readable, then public key authentication will
932 be refused for all users.
933 .It Cm RhostsRSAAuthentication
934 Specifies whether rhosts or
936 authentication together
937 with successful RSA host authentication is allowed.
940 This option applies to protocol version 1 only.
941 .It Cm RSAAuthentication
942 Specifies whether pure RSA authentication is allowed.
945 This option applies to protocol version 1 only.
947 Defines the number of bits in the ephemeral protocol version 1 server key.
948 The minimum value is 512, and the default is 1024.
952 should check file modes and ownership of the
953 user's files and home directory before accepting login.
954 This is normally desirable because novices sometimes accidentally leave their
955 directory or files world-writable.
958 Note that this does not apply to
959 .Cm ChrootDirectory ,
960 whose permissions and ownership are checked unconditionally.
962 Configures an external subsystem (e.g. file transfer daemon).
963 Arguments should be a subsystem name and a command (with optional arguments)
964 to execute upon subsystem request.
970 file transfer subsystem.
974 implements an in-process
977 This may simplify configurations using
979 to force a different filesystem root on clients.
981 By default no subsystems are defined.
982 Note that this option applies to protocol version 2 only.
983 .It Cm SyslogFacility
984 Gives the facility code that is used when logging messages from
986 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
987 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
990 Specifies whether the system should send TCP keepalive messages to the
992 If they are sent, death of the connection or crash of one
993 of the machines will be properly noticed.
994 However, this means that
995 connections will die if the route is down temporarily, and some people
997 On the other hand, if TCP keepalives are not sent,
998 sessions may hang indefinitely on the server, leaving
1000 users and consuming server resources.
1004 (to send TCP keepalive messages), and the server will notice
1005 if the network goes down or the client host crashes.
1006 This avoids infinitely hanging sessions.
1008 To disable TCP keepalive messages, the value should be set to
1010 .It Cm TrustedUserCAKeys
1011 Specifies a file containing public keys of certificate authorities that are
1012 trusted to sign user certificates for authentication.
1013 Keys are listed one per line; empty lines and comments starting with
1016 If a certificate is presented for authentication and has its signing CA key
1017 listed in this file, then it may be used for authentication for any user
1018 listed in the certificate's principals list.
1019 Note that certificates that lack a list of principals will not be permitted
1020 for authentication using
1021 .Cm TrustedUserCAKeys .
1022 For more details on certificates, see the
1029 should look up the remote host name and check that
1030 the resolved host name for the remote IP address maps back to the
1031 very same IP address.
1037 is used for interactive login sessions.
1042 is never used for remote command execution.
1043 Note also, that if this is enabled,
1045 will be disabled because
1047 does not know how to handle
1051 .Cm UsePrivilegeSeparation
1052 is specified, it will be disabled after authentication.
1054 Enables the Pluggable Authentication Module interface.
1057 this will enable PAM authentication using
1058 .Cm ChallengeResponseAuthentication
1060 .Cm PasswordAuthentication
1061 in addition to PAM account and session module processing for all
1062 authentication types.
1064 Because PAM challenge-response authentication usually serves an equivalent
1065 role to password authentication, you should disable either
1066 .Cm PasswordAuthentication
1068 .Cm ChallengeResponseAuthentication.
1072 is enabled, you will not be able to run
1077 .It Cm UsePrivilegeSeparation
1080 separates privileges by creating an unprivileged child process
1081 to deal with incoming network traffic.
1082 After successful authentication, another process will be created that has
1083 the privilege of the authenticated user.
1084 The goal of privilege separation is to prevent privilege
1085 escalation by containing any corruption within the unprivileged processes.
1089 .Cm UsePrivilegeSeparation
1092 then the pre-authentication unprivileged process is subject to additional
1094 .It Cm VersionAddendum
1095 Specifies a string to append to the regular version string to identify
1096 OS- or site-specific modifications.
1098 .Dq FreeBSD-20111001 .
1099 .It Cm X11DisplayOffset
1100 Specifies the first display number available for
1103 This prevents sshd from interfering with real X11 servers.
1105 .It Cm X11Forwarding
1106 Specifies whether X11 forwarding is permitted.
1107 The argument must be
1114 When X11 forwarding is enabled, there may be additional exposure to
1115 the server and to client displays if the
1117 proxy display is configured to listen on the wildcard address (see
1119 below), though this is not the default.
1120 Additionally, the authentication spoofing and authentication data
1121 verification and substitution occur on the client side.
1122 The security risk of using X11 forwarding is that the client's X11
1123 display server may be exposed to attack when the SSH client requests
1124 forwarding (see the warnings for
1127 .Xr ssh_config 5 ) .
1128 A system administrator may have a stance in which they want to
1129 protect clients that may expose themselves to attack by unwittingly
1130 requesting X11 forwarding, which can warrant a
1134 Note that disabling X11 forwarding does not prevent users from
1135 forwarding X11 traffic, as users can always install their own forwarders.
1136 X11 forwarding is automatically disabled if
1139 .It Cm X11UseLocalhost
1142 should bind the X11 forwarding server to the loopback address or to
1143 the wildcard address.
1145 sshd binds the forwarding server to the loopback address and sets the
1146 hostname part of the
1148 environment variable to
1150 This prevents remote hosts from connecting to the proxy display.
1151 However, some older X11 clients may not function with this
1156 to specify that the forwarding server should be bound to the wildcard
1158 The argument must be
1164 .It Cm XAuthLocation
1165 Specifies the full pathname of the
1169 .Pa /usr/local/bin/xauth .
1173 command-line arguments and configuration file options that specify time
1174 may be expressed using a sequence of the form:
1176 .Ar time Op Ar qualifier ,
1180 is a positive integer value and
1182 is one of the following:
1184 .Bl -tag -width Ds -compact -offset indent
1199 Each member of the sequence is added together to calculate
1200 the total time value.
1202 Time format examples:
1204 .Bl -tag -width Ds -compact -offset indent
1206 600 seconds (10 minutes)
1210 1 hour 30 minutes (90 minutes)
1214 .It Pa /etc/ssh/sshd_config
1215 Contains configuration data for
1217 This file should be writable by root only, but it is recommended
1218 (though not necessary) that it be world-readable.
1223 OpenSSH is a derivative of the original and free
1224 ssh 1.2.12 release by Tatu Ylonen.
1225 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1226 Theo de Raadt and Dug Song
1227 removed many bugs, re-added newer features and
1229 Markus Friedl contributed the support for SSH
1230 protocol versions 1.5 and 2.0.
1231 Niels Provos and Markus Friedl contributed support
1232 for privilege separation.