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 (e.g. via
225 PAM or though authentication styles supported in
229 .It Cm ChrootDirectory
230 Specifies the pathname of a directory to
232 to after authentication.
233 All components of the pathname must be root-owned directories that are
234 not writable by any other user or group.
237 changes the working directory to the user's home directory.
239 The pathname may contain the following tokens that are expanded at runtime once
240 the connecting user has been authenticated: %% is replaced by a literal '%',
241 %h is replaced by the home directory of the user being authenticated, and
242 %u is replaced by the username of that user.
246 must contain the necessary files and directories to support the
248 For an interactive session this requires at least a shell, typically
262 For file transfer sessions using
264 no additional configuration of the environment is necessary if the
265 in-process sftp server is used,
266 though sessions which use logging do require
268 inside the chroot directory (see
272 The default is not to
275 Specifies the ciphers allowed for protocol version 2.
276 Multiple ciphers must be comma-separated.
277 The supported ciphers are
292 .Bd -literal -offset 3n
293 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
294 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
297 .It Cm ClientAliveCountMax
298 Sets the number of client alive messages (see below) which may be
301 receiving any messages back from the client.
302 If this threshold is reached while client alive messages are being sent,
303 sshd will disconnect the client, terminating the session.
304 It is important to note that the use of client alive messages is very
308 The client alive messages are sent through the encrypted channel
309 and therefore will not be spoofable.
310 The TCP keepalive option enabled by
313 The client alive mechanism is valuable when the client or
314 server depend on knowing when a connection has become inactive.
316 The default value is 3.
318 .Cm ClientAliveInterval
319 (see below) is set to 15, and
320 .Cm ClientAliveCountMax
321 is left at the default, unresponsive SSH clients
322 will be disconnected after approximately 45 seconds.
323 This option applies to protocol version 2 only.
324 .It Cm ClientAliveInterval
325 Sets a timeout interval in seconds after which if no data has been received
328 will send a message through the encrypted
329 channel to request a response from the client.
331 is 0, indicating that these messages will not be sent to the client.
332 This option applies to protocol version 2 only.
334 Specifies whether compression is allowed, or delayed until
335 the user has authenticated successfully.
344 This keyword can be followed by a list of group name patterns, separated
346 Login is disallowed for users whose primary group or supplementary
347 group list matches one of the patterns.
348 Only group names are valid; a numerical group ID is not recognized.
349 By default, login is allowed for all groups.
350 The allow/deny directives are processed in the following order:
361 for more information on patterns.
363 This keyword can be followed by a list of user name patterns, separated
365 Login is disallowed for user names that match one of the patterns.
366 Only user names are valid; a numerical user ID is not recognized.
367 By default, login is allowed for all users.
368 If the pattern takes the form USER@HOST then USER and HOST
369 are separately checked, restricting logins to particular
370 users from particular hosts.
371 The allow/deny directives are processed in the following order:
382 for more information on patterns.
384 Forces the execution of the command specified by
386 ignoring any command supplied by the client and
389 The command is invoked by using the user's login shell with the -c option.
390 This applies to shell, command, or subsystem execution.
391 It is most useful inside a
394 The command originally supplied by the client is available in the
395 .Ev SSH_ORIGINAL_COMMAND
396 environment variable.
397 Specifying a command of
399 will force the use of an in-process sftp server that requires no support
401 .Cm ChrootDirectory .
403 Specifies whether remote hosts are allowed to connect to ports
404 forwarded for the client.
407 binds remote port forwardings to the loopback address.
408 This prevents other remote hosts from connecting to forwarded ports.
410 can be used to specify that sshd
411 should allow remote port forwardings to bind to non-loopback addresses, thus
412 allowing other hosts to connect.
415 to force remote port forwardings to be available to the local host only,
417 to force remote port forwardings to bind to the wildcard address, or
419 to allow the client to select the address to which the forwarding is bound.
422 .It Cm GSSAPIAuthentication
423 Specifies whether user authentication based on GSSAPI is allowed.
426 Note that this option applies to protocol version 2 only.
427 .It Cm GSSAPIKeyExchange
428 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
429 doesn't rely on ssh keys to verify host identity.
432 Note that this option applies to protocol version 2 only.
433 .It Cm GSSAPICleanupCredentials
434 Specifies whether to automatically destroy the user's credentials cache
438 Note that this option applies to protocol version 2 only.
439 .It Cm GSSAPIStrictAcceptorCheck
440 Determines whether to be strict about the identity of the GSSAPI acceptor
441 a client authenticates against. If
443 then the client must authenticate against the
445 service on the current hostname. If
447 then the client may authenticate against any service key stored in the
448 machine's default store. This facility is provided to assist with operation
449 on multi homed machines.
452 Note that this option applies only to protocol version 2 GSSAPI connections,
455 may only work with recent Kerberos GSSAPI libraries.
456 .It Cm GSSAPIStoreCredentialsOnRekey
457 Controls whether the user's GSSAPI credentials should be updated following a
458 successful connection rekeying. This option can be used to accepted renewed
459 or updated credentials from a compatible client. The default is
461 .It Cm HostbasedAuthentication
462 Specifies whether rhosts or /etc/hosts.equiv authentication together
463 with successful public key client host authentication is allowed
464 (host-based authentication).
465 This option is similar to
466 .Cm RhostsRSAAuthentication
467 and applies to protocol version 2 only.
470 .It Cm HostbasedUsesNameFromPacketOnly
471 Specifies whether or not the server will attempt to perform a reverse
472 name lookup when matching the name in the
478 .Cm HostbasedAuthentication .
483 uses the name supplied by the client rather than
484 attempting to resolve the name from the TCP connection itself.
487 .It Cm HostCertificate
488 Specifies a file containing a public host certificate.
489 The certificate's public key must match a private host key already specified
492 The default behaviour of
494 is not to load any certificates.
496 Specifies a file containing a private host key
499 .Pa /etc/ssh/ssh_host_key
500 for protocol version 1, and
501 .Pa /etc/ssh/ssh_host_dsa_key ,
502 .Pa /etc/ssh/ssh_host_ecdsa_key
504 .Pa /etc/ssh/ssh_host_rsa_key
505 for protocol version 2.
508 will refuse to use a file if it is group/world-accessible.
509 It is possible to have multiple host key files.
511 keys are used for version 1 and
516 are used for version 2 of the SSH protocol.
522 files will not be used in
523 .Cm RhostsRSAAuthentication
525 .Cm HostbasedAuthentication .
529 .Pa /etc/shosts.equiv
533 .It Cm IgnoreUserKnownHosts
536 should ignore the user's
537 .Pa ~/.ssh/known_hosts
539 .Cm RhostsRSAAuthentication
541 .Cm HostbasedAuthentication .
545 Specifies the IPv4 type-of-service or DSCP class for the connection.
572 This option may take one or two arguments, separated by whitespace.
573 If one argument is specified, it is used as the packet class unconditionally.
574 If two values are specified, the first is automatically selected for
575 interactive sessions and the second for non-interactive sessions.
578 for interactive sessions and
580 for non-interactive sessions.
581 .It Cm KerberosAuthentication
582 Specifies whether the password provided by the user for
583 .Cm PasswordAuthentication
584 will be validated through the Kerberos KDC.
585 To use this option, the server needs a
586 Kerberos servtab which allows the verification of the KDC's identity.
589 .It Cm KerberosGetAFSToken
590 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
591 an AFS token before accessing the user's home directory.
594 .It Cm KerberosOrLocalPasswd
595 If password authentication through Kerberos fails then
596 the password will be validated via any additional local mechanism
601 .It Cm KerberosTicketCleanup
602 Specifies whether to automatically destroy the user's ticket cache
607 Specifies the available KEX (Key Exchange) algorithms.
608 Multiple algorithms must be comma-separated.
610 .Dq ecdh-sha2-nistp256 ,
611 .Dq ecdh-sha2-nistp384 ,
612 .Dq ecdh-sha2-nistp521 ,
613 .Dq diffie-hellman-group-exchange-sha256 ,
614 .Dq diffie-hellman-group-exchange-sha1 ,
615 .Dq diffie-hellman-group14-sha1 ,
616 .Dq diffie-hellman-group1-sha1 .
617 .It Cm KeyRegenerationInterval
618 In protocol version 1, the ephemeral server key is automatically regenerated
619 after this many seconds (if it has been used).
620 The purpose of regeneration is to prevent
621 decrypting captured sessions by later breaking into the machine and
623 The key is never stored anywhere.
624 If the value is 0, the key is never regenerated.
625 The default is 3600 (seconds).
627 Specifies the local addresses
630 The following forms may be used:
632 .Bl -item -offset indent -compact
636 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
641 .Ar host No | Ar IPv4_addr No : Ar port
647 .Ar host No | Ar IPv6_addr Oc : Ar port
654 sshd will listen on the address and all prior
657 The default is to listen on all local addresses.
660 options are permitted.
663 options must precede this option for non-port qualified addresses.
664 .It Cm LoginGraceTime
665 The server disconnects after this time if the user has not
666 successfully logged in.
667 If the value is 0, there is no time limit.
668 The default is 120 seconds.
670 Gives the verbosity level that is used when logging messages from
672 The possible values are:
673 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
675 DEBUG and DEBUG1 are equivalent.
676 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
677 Logging with a DEBUG level violates the privacy of users and is not recommended.
679 Specifies the available MAC (message authentication code) algorithms.
680 The MAC algorithm is used in protocol version 2
681 for data integrity protection.
682 Multiple algorithms must be comma-separated.
684 .Bd -literal -offset indent
685 hmac-md5,hmac-sha1,umac-64@openssh.com,
686 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
687 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
691 Introduces a conditional block.
692 If all of the criteria on the
694 line are satisfied, the keywords on the following lines override those
695 set in the global section of the config file, until either another
697 line or the end of the file.
701 are one or more criteria-pattern pairs.
702 The available criteria are
708 The match patterns may consist of single entries or comma-separated
709 lists and may use the wildcard and negation operators described in the
716 criteria may additionally contain addresses to match in CIDR
717 address/masklen format, e.g.\&
721 Note that the mask length provided must be consistent with the address -
722 it is an error to specify a mask length that is too long for the address
723 or one with bits set in this host portion of the address.
730 Only a subset of keywords may be used on the lines following a
733 Available keywords are
734 .Cm AllowAgentForwarding ,
735 .Cm AllowTcpForwarding ,
736 .Cm AuthorizedKeysFile ,
737 .Cm AuthorizedPrincipalsFile ,
739 .Cm ChrootDirectory ,
742 .Cm GSSAPIAuthentication ,
743 .Cm HostbasedAuthentication ,
744 .Cm HostbasedUsesNameFromPacketOnly ,
745 .Cm KbdInteractiveAuthentication ,
746 .Cm KerberosAuthentication ,
749 .Cm PasswordAuthentication ,
750 .Cm PermitEmptyPasswords ,
752 .Cm PermitRootLogin ,
754 .Cm PubkeyAuthentication ,
755 .Cm RhostsRSAAuthentication ,
756 .Cm RSAAuthentication ,
757 .Cm X11DisplayOffset ,
760 .Cm X11UseLocalHost .
762 Specifies the maximum number of authentication attempts permitted per
764 Once the number of failures reaches half this value,
765 additional failures are logged.
768 Specifies the maximum number of open sessions permitted per network connection.
771 Specifies the maximum number of concurrent unauthenticated connections to the
773 Additional connections will be dropped until authentication succeeds or the
775 expires for a connection.
778 Alternatively, random early drop can be enabled by specifying
779 the three colon separated values
783 will refuse connection attempts with a probability of
786 if there are currently
789 unauthenticated connections.
790 The probability increases linearly and all connection attempts
791 are refused if the number of unauthenticated connections reaches
794 .It Cm PasswordAuthentication
795 Specifies whether password authentication is allowed.
798 .It Cm PermitEmptyPasswords
799 When password authentication is allowed, it specifies whether the
800 server allows login to accounts with empty password strings.
804 Specifies the destinations to which TCP port forwarding is permitted.
805 The forwarding specification must be one of the following forms:
807 .Bl -item -offset indent -compact
821 .Ar \&[ IPv6_addr \&] : port
825 Multiple forwards may be specified by separating them with whitespace.
828 can be used to remove all restrictions and permit any forwarding requests.
829 By default all port forwarding requests are permitted.
830 .It Cm PermitRootLogin
831 Specifies whether root can log in using
835 .Dq without-password ,
836 .Dq forced-commands-only ,
842 If this option is set to
843 .Dq without-password ,
844 password authentication is disabled for root.
846 If this option is set to
847 .Dq forced-commands-only ,
848 root login with public key authentication will be allowed,
851 option has been specified
852 (which may be useful for taking remote backups even if root login is
853 normally not allowed).
854 All other authentication methods are disabled for root.
856 If this option is set to
858 root is not allowed to log in.
862 device forwarding is allowed.
878 .It Cm PermitUserEnvironment
880 .Pa ~/.ssh/environment
884 .Pa ~/.ssh/authorized_keys
889 Enabling environment processing may enable users to bypass access
890 restrictions in some configurations using mechanisms such as
893 Specifies the file that contains the process ID of the
896 .Pa /var/run/sshd.pid .
898 Specifies the port number that
902 Multiple options of this type are permitted.
908 should print the date and time of the last user login when a user logs
917 when a user logs in interactively.
918 (On some systems it is also printed by the shell,
924 Specifies the protocol versions
927 The possible values are
931 Multiple versions must be comma-separated.
934 Note that the order of the protocol list does not indicate preference,
935 because the client selects among multiple protocol versions offered
941 .It Cm PubkeyAuthentication
942 Specifies whether public key authentication is allowed.
945 Note that this option applies to protocol version 2 only.
947 Specifies a list of revoked public keys.
948 Keys listed in this file will be refused for public key authentication.
949 Note that if this file is not readable, then public key authentication will
950 be refused for all users.
951 .It Cm RhostsRSAAuthentication
952 Specifies whether rhosts or /etc/hosts.equiv authentication together
953 with successful RSA host authentication is allowed.
956 This option applies to protocol version 1 only.
957 .It Cm RSAAuthentication
958 Specifies whether pure RSA authentication is allowed.
961 This option applies to protocol version 1 only.
963 Defines the number of bits in the ephemeral protocol version 1 server key.
964 The minimum value is 512, and the default is 1024.
968 should check file modes and ownership of the
969 user's files and home directory before accepting login.
970 This is normally desirable because novices sometimes accidentally leave their
971 directory or files world-writable.
974 Note that this does not apply to
975 .Cm ChrootDirectory ,
976 whose permissions and ownership are checked unconditionally.
978 Configures an external subsystem (e.g. file transfer daemon).
979 Arguments should be a subsystem name and a command (with optional arguments)
980 to execute upon subsystem request.
986 file transfer subsystem.
990 implements an in-process
993 This may simplify configurations using
995 to force a different filesystem root on clients.
997 By default no subsystems are defined.
998 Note that this option applies to protocol version 2 only.
999 .It Cm SyslogFacility
1000 Gives the facility code that is used when logging messages from
1002 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1003 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1004 The default is AUTH.
1006 Specifies whether the system should send TCP keepalive messages to the
1008 If they are sent, death of the connection or crash of one
1009 of the machines will be properly noticed.
1010 However, this means that
1011 connections will die if the route is down temporarily, and some people
1013 On the other hand, if TCP keepalives are not sent,
1014 sessions may hang indefinitely on the server, leaving
1016 users and consuming server resources.
1020 (to send TCP keepalive messages), and the server will notice
1021 if the network goes down or the client host crashes.
1022 This avoids infinitely hanging sessions.
1024 To disable TCP keepalive messages, the value should be set to
1026 .It Cm TrustedUserCAKeys
1027 Specifies a file containing public keys of certificate authorities that are
1028 trusted to sign user certificates for authentication.
1029 Keys are listed one per line; empty lines and comments starting with
1032 If a certificate is presented for authentication and has its signing CA key
1033 listed in this file, then it may be used for authentication for any user
1034 listed in the certificate's principals list.
1035 Note that certificates that lack a list of principals will not be permitted
1036 for authentication using
1037 .Cm TrustedUserCAKeys .
1038 For more details on certificates, see the
1045 should look up the remote host name and check that
1046 the resolved host name for the remote IP address maps back to the
1047 very same IP address.
1053 is used for interactive login sessions.
1058 is never used for remote command execution.
1059 Note also, that if this is enabled,
1061 will be disabled because
1063 does not know how to handle
1067 .Cm UsePrivilegeSeparation
1068 is specified, it will be disabled after authentication.
1070 Enables the Pluggable Authentication Module interface.
1073 this will enable PAM authentication using
1074 .Cm ChallengeResponseAuthentication
1076 .Cm PasswordAuthentication
1077 in addition to PAM account and session module processing for all
1078 authentication types.
1080 Because PAM challenge-response authentication usually serves an equivalent
1081 role to password authentication, you should disable either
1082 .Cm PasswordAuthentication
1084 .Cm ChallengeResponseAuthentication.
1088 is enabled, you will not be able to run
1093 .It Cm UsePrivilegeSeparation
1096 separates privileges by creating an unprivileged child process
1097 to deal with incoming network traffic.
1098 After successful authentication, another process will be created that has
1099 the privilege of the authenticated user.
1100 The goal of privilege separation is to prevent privilege
1101 escalation by containing any corruption within the unprivileged processes.
1105 .Cm UsePrivilegeSeparation
1108 then the pre-authentication unprivileged process is subject to additional
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/X11R6/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.