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.
62 package sets several options as standard in
63 .Pa /etc/ssh/sshd_config
64 which are not the default in
66 The exact list depends on whether the package was installed fresh or
67 upgraded from various possible previous versions, but includes at least the
70 .Bl -bullet -offset indent -compact
74 .Cm ChallengeResponseAuthentication No no
76 .Cm X11Forwarding No yes
80 .Cm AcceptEnv No LANG LC_*
82 .Cm Subsystem No sftp /usr/lib/openssh/sftp-server
88 keywords and their meanings are as follows (note that
89 keywords are case-insensitive and arguments are case-sensitive):
92 Specifies what environment variables sent by the client will be copied into
99 for how to configure the client.
100 Note that environment passing is only supported for protocol 2.
101 Variables are specified by name, which may contain the wildcard characters
105 Multiple environment variables may be separated by whitespace or spread
109 Be warned that some environment variables could be used to bypass restricted
111 For this reason, care should be taken in the use of this directive.
112 The default is not to accept any environment variables.
114 Specifies which address family should be used by
124 .It Cm AllowAgentForwarding
127 forwarding is permitted.
130 Note that disabling agent forwarding does not improve security
131 unless users are also denied shell access, as they can always install
132 their own forwarders.
134 This keyword can be followed by a list of group name patterns, separated
136 If specified, login is allowed only for users whose primary
137 group or supplementary group list matches one of the patterns.
138 Only group names are valid; a numerical group ID is not recognized.
139 By default, login is allowed for all groups.
140 The allow/deny directives are processed in the following order:
151 for more information on patterns.
152 .It Cm AllowTcpForwarding
153 Specifies whether TCP forwarding is permitted.
156 Note that disabling TCP forwarding does not improve security unless
157 users are also denied shell access, as they can always install their
160 This keyword can be followed by a list of user name patterns, separated
162 If specified, login is allowed only for user names that
163 match one of the patterns.
164 Only user names are valid; a numerical user ID is not recognized.
165 By default, login is allowed for all users.
166 If the pattern takes the form USER@HOST then USER and HOST
167 are separately checked, restricting logins to particular
168 users from particular hosts.
169 The allow/deny directives are processed in the following order:
180 for more information on patterns.
181 .It Cm AuthorizedKeysFile
182 Specifies the file that contains the public keys that can be used
183 for user authentication.
184 The format is described in the
185 .Sx AUTHORIZED_KEYS FILE FORMAT
188 .Cm AuthorizedKeysFile
189 may contain tokens of the form %T which are substituted during connection
191 The following tokens are defined: %% is replaced by a literal '%',
192 %h is replaced by the home directory of the user being authenticated, and
193 %u is replaced by the username of that user.
195 .Cm AuthorizedKeysFile
196 is taken to be an absolute path or one relative to the user's home
198 Multiple files may be listed, separated by whitespace.
200 .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
201 .It Cm AuthorizedPrincipalsFile
202 Specifies a file that lists principal names that are accepted for
203 certificate authentication.
204 When using certificates signed by a key listed in
205 .Cm TrustedUserCAKeys ,
206 this file lists names, one of which must appear in the certificate for it
207 to be accepted for authentication.
208 Names are listed one per line preceded by key options (as described
210 .Sx AUTHORIZED_KEYS FILE FORMAT
213 Empty lines and comments starting with
217 .Cm AuthorizedPrincipalsFile
218 may contain tokens of the form %T which are substituted during connection
220 The following tokens are defined: %% is replaced by a literal '%',
221 %h is replaced by the home directory of the user being authenticated, and
222 %u is replaced by the username of that user.
224 .Cm AuthorizedPrincipalsFile
225 is taken to be an absolute path or one relative to the user's home
228 The default is not to use a principals file \(en in this case, the username
229 of the user must appear in a certificate's principals list for it to be
232 .Cm AuthorizedPrincipalsFile
233 is only used when authentication proceeds using a CA listed in
234 .Cm TrustedUserCAKeys
235 and is not consulted for certification authorities trusted via
236 .Pa ~/.ssh/authorized_keys ,
239 key option offers a similar facility (see
243 The contents of the specified file are sent to the remote user before
244 authentication is allowed.
247 then no banner is displayed.
248 This option is only available for protocol version 2.
249 By default, no banner is displayed.
250 .It Cm ChallengeResponseAuthentication
251 Specifies whether challenge-response authentication is allowed (e.g. via
255 .It Cm ChrootDirectory
256 Specifies the pathname of a directory to
258 to after authentication.
259 All components of the pathname must be root-owned directories that are
260 not writable by any other user or group.
263 changes the working directory to the user's home directory.
265 The pathname may contain the following tokens that are expanded at runtime once
266 the connecting user has been authenticated: %% is replaced by a literal '%',
267 %h is replaced by the home directory of the user being authenticated, and
268 %u is replaced by the username of that user.
272 must contain the necessary files and directories to support the
274 For an interactive session this requires at least a shell, typically
288 For file transfer sessions using
290 no additional configuration of the environment is necessary if the
291 in-process sftp server is used,
292 though sessions which use logging do require
294 inside the chroot directory (see
298 The default is not to
301 Specifies the ciphers allowed for protocol version 2.
302 Multiple ciphers must be comma-separated.
303 The supported ciphers are
318 .Bd -literal -offset 3n
319 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
320 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
323 .It Cm ClientAliveCountMax
324 Sets the number of client alive messages (see below) which may be
327 receiving any messages back from the client.
328 If this threshold is reached while client alive messages are being sent,
329 sshd will disconnect the client, terminating the session.
330 It is important to note that the use of client alive messages is very
334 The client alive messages are sent through the encrypted channel
335 and therefore will not be spoofable.
336 The TCP keepalive option enabled by
339 The client alive mechanism is valuable when the client or
340 server depend on knowing when a connection has become inactive.
342 The default value is 3.
344 .Cm ClientAliveInterval
345 (see below) is set to 15, and
346 .Cm ClientAliveCountMax
347 is left at the default, unresponsive SSH clients
348 will be disconnected after approximately 45 seconds.
349 This option applies to protocol version 2 only.
350 .It Cm ClientAliveInterval
351 Sets a timeout interval in seconds after which if no data has been received
354 will send a message through the encrypted
355 channel to request a response from the client.
357 is 0, indicating that these messages will not be sent to the client.
358 This option applies to protocol version 2 only.
360 Specifies whether compression is allowed, or delayed until
361 the user has authenticated successfully.
370 Specifies whether the distribution-specified extra version suffix is
371 included during initial protocol handshake.
375 This keyword can be followed by a list of group name patterns, separated
377 Login is disallowed for users whose primary group or supplementary
378 group list matches one of the patterns.
379 Only group names are valid; a numerical group ID is not recognized.
380 By default, login is allowed for all groups.
381 The allow/deny directives are processed in the following order:
392 for more information on patterns.
394 This keyword can be followed by a list of user name patterns, separated
396 Login is disallowed for user names that match one of the patterns.
397 Only user names are valid; a numerical user ID is not recognized.
398 By default, login is allowed for all users.
399 If the pattern takes the form USER@HOST then USER and HOST
400 are separately checked, restricting logins to particular
401 users from particular hosts.
402 The allow/deny directives are processed in the following order:
413 for more information on patterns.
415 Forces the execution of the command specified by
417 ignoring any command supplied by the client and
420 The command is invoked by using the user's login shell with the -c option.
421 This applies to shell, command, or subsystem execution.
422 It is most useful inside a
425 The command originally supplied by the client is available in the
426 .Ev SSH_ORIGINAL_COMMAND
427 environment variable.
428 Specifying a command of
430 will force the use of an in-process sftp server that requires no support
432 .Cm ChrootDirectory .
434 Specifies whether remote hosts are allowed to connect to ports
435 forwarded for the client.
438 binds remote port forwardings to the loopback address.
439 This prevents other remote hosts from connecting to forwarded ports.
441 can be used to specify that sshd
442 should allow remote port forwardings to bind to non-loopback addresses, thus
443 allowing other hosts to connect.
446 to force remote port forwardings to be available to the local host only,
448 to force remote port forwardings to bind to the wildcard address, or
450 to allow the client to select the address to which the forwarding is bound.
453 .It Cm GSSAPIAuthentication
454 Specifies whether user authentication based on GSSAPI is allowed.
457 Note that this option applies to protocol version 2 only.
458 .It Cm GSSAPIKeyExchange
459 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
460 doesn't rely on ssh keys to verify host identity.
463 Note that this option applies to protocol version 2 only.
464 .It Cm GSSAPICleanupCredentials
465 Specifies whether to automatically destroy the user's credentials cache
469 Note that this option applies to protocol version 2 only.
470 .It Cm GSSAPIRequireMIC
471 Specifies whether to permit authentication using GSS-API mechanisms
472 and/or contexts that do not support per-message integrity protection.
475 then the server will fail an otherwise valid gssapi-with-mic authentication
476 if per-message integrity protection is not supported.
479 Note that this option applies to protocol version 2 only.
480 .It Cm GSSAPIStrictAcceptorCheck
481 Determines whether to be strict about the identity of the GSSAPI acceptor
482 a client authenticates against. If
484 then the client must authenticate against the
486 service on the current hostname. If
488 then the client may authenticate against any service key stored in the
489 machine's default store. This facility is provided to assist with operation
490 on multi homed machines.
493 Note that this option applies only to protocol version 2 GSSAPI connections,
496 may only work with recent Kerberos GSSAPI libraries.
497 .It Cm GSSAPIStoreCredentialsOnRekey
498 Controls whether the user's GSSAPI credentials should be updated following a
499 successful connection rekeying. This option can be used to accepted renewed
500 or updated credentials from a compatible client. The default is
502 .It Cm HostbasedAuthentication
503 Specifies whether rhosts or /etc/hosts.equiv authentication together
504 with successful public key client host authentication is allowed
505 (host-based authentication).
506 This option is similar to
507 .Cm RhostsRSAAuthentication
508 and applies to protocol version 2 only.
511 .It Cm HostbasedUsesNameFromPacketOnly
512 Specifies whether or not the server will attempt to perform a reverse
513 name lookup when matching the name in the
519 .Cm HostbasedAuthentication .
524 uses the name supplied by the client rather than
525 attempting to resolve the name from the TCP connection itself.
528 .It Cm HostCertificate
529 Specifies a file containing a public host certificate.
530 The certificate's public key must match a private host key already specified
533 The default behaviour of
535 is not to load any certificates.
537 Specifies a file containing a private host key
540 .Pa /etc/ssh/ssh_host_key
541 for protocol version 1, and
542 .Pa /etc/ssh/ssh_host_dsa_key ,
543 .Pa /etc/ssh/ssh_host_ecdsa_key
545 .Pa /etc/ssh/ssh_host_rsa_key
546 for protocol version 2.
549 will refuse to use a file if it is group/world-accessible.
550 It is possible to have multiple host key files.
552 keys are used for version 1 and
557 are used for version 2 of the SSH protocol.
563 files will not be used in
564 .Cm RhostsRSAAuthentication
566 .Cm HostbasedAuthentication .
570 .Pa /etc/shosts.equiv
574 .It Cm IgnoreUserKnownHosts
577 should ignore the user's
578 .Pa ~/.ssh/known_hosts
580 .Cm RhostsRSAAuthentication
582 .Cm HostbasedAuthentication .
586 Specifies the IPv4 type-of-service or DSCP class for the connection.
613 This option may take one or two arguments, separated by whitespace.
614 If one argument is specified, it is used as the packet class unconditionally.
615 If two values are specified, the first is automatically selected for
616 interactive sessions and the second for non-interactive sessions.
619 for interactive sessions and
621 for non-interactive sessions.
622 .It Cm KerberosAuthentication
623 Specifies whether the password provided by the user for
624 .Cm PasswordAuthentication
625 will be validated through the Kerberos KDC.
626 To use this option, the server needs a
627 Kerberos servtab which allows the verification of the KDC's identity.
630 .It Cm KerberosGetAFSToken
631 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
632 an AFS token before accessing the user's home directory.
635 .It Cm KerberosOrLocalPasswd
636 If password authentication through Kerberos fails then
637 the password will be validated via any additional local mechanism
642 .It Cm KerberosTicketCleanup
643 Specifies whether to automatically destroy the user's ticket cache
648 Specifies the available KEX (Key Exchange) algorithms.
649 Multiple algorithms must be comma-separated.
651 .Dq ecdh-sha2-nistp256 ,
652 .Dq ecdh-sha2-nistp384 ,
653 .Dq ecdh-sha2-nistp521 ,
654 .Dq diffie-hellman-group-exchange-sha256 ,
655 .Dq diffie-hellman-group-exchange-sha1 ,
656 .Dq diffie-hellman-group14-sha1 ,
657 .Dq diffie-hellman-group1-sha1 .
658 .It Cm KeyRegenerationInterval
659 In protocol version 1, the ephemeral server key is automatically regenerated
660 after this many seconds (if it has been used).
661 The purpose of regeneration is to prevent
662 decrypting captured sessions by later breaking into the machine and
664 The key is never stored anywhere.
665 If the value is 0, the key is never regenerated.
666 The default is 3600 (seconds).
668 Specifies the local addresses
671 The following forms may be used:
673 .Bl -item -offset indent -compact
677 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
682 .Ar host No | Ar IPv4_addr No : Ar port
688 .Ar host No | Ar IPv6_addr Oc : Ar port
695 sshd will listen on the address and all prior
698 The default is to listen on all local addresses.
701 options are permitted.
704 options must precede this option for non-port qualified addresses.
705 .It Cm LoginGraceTime
706 The server disconnects after this time if the user has not
707 successfully logged in.
708 If the value is 0, there is no time limit.
709 The default is 120 seconds.
711 Gives the verbosity level that is used when logging messages from
713 The possible values are:
714 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
716 DEBUG and DEBUG1 are equivalent.
717 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
718 Logging with a DEBUG level violates the privacy of users and is not recommended.
720 Specifies the available MAC (message authentication code) algorithms.
721 The MAC algorithm is used in protocol version 2
722 for data integrity protection.
723 Multiple algorithms must be comma-separated.
725 .Bd -literal -offset indent
726 hmac-md5,hmac-sha1,umac-64@openssh.com,
727 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
728 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
732 Introduces a conditional block.
733 If all of the criteria on the
735 line are satisfied, the keywords on the following lines override those
736 set in the global section of the config file, until either another
738 line or the end of the file.
742 are one or more criteria-pattern pairs.
743 The available criteria are
749 The match patterns may consist of single entries or comma-separated
750 lists and may use the wildcard and negation operators described in the
757 criteria may additionally contain addresses to match in CIDR
758 address/masklen format, e.g.\&
762 Note that the mask length provided must be consistent with the address -
763 it is an error to specify a mask length that is too long for the address
764 or one with bits set in this host portion of the address.
771 Only a subset of keywords may be used on the lines following a
774 Available keywords are
775 .Cm AllowAgentForwarding ,
776 .Cm AllowTcpForwarding ,
777 .Cm AuthorizedKeysFile ,
778 .Cm AuthorizedPrincipalsFile ,
780 .Cm ChrootDirectory ,
783 .Cm GSSAPIAuthentication ,
784 .Cm HostbasedAuthentication ,
785 .Cm HostbasedUsesNameFromPacketOnly ,
786 .Cm KbdInteractiveAuthentication ,
787 .Cm KerberosAuthentication ,
790 .Cm PasswordAuthentication ,
791 .Cm PermitEmptyPasswords ,
793 .Cm PermitRootLogin ,
795 .Cm PubkeyAuthentication ,
796 .Cm RhostsRSAAuthentication ,
797 .Cm RSAAuthentication ,
798 .Cm X11DisplayOffset ,
801 .Cm X11UseLocalHost .
803 Specifies the maximum number of authentication attempts permitted per
805 Once the number of failures reaches half this value,
806 additional failures are logged.
809 Specifies the maximum number of open sessions permitted per network connection.
812 Specifies the maximum number of concurrent unauthenticated connections to the
814 Additional connections will be dropped until authentication succeeds or the
816 expires for a connection.
819 Alternatively, random early drop can be enabled by specifying
820 the three colon separated values
824 will refuse connection attempts with a probability of
827 if there are currently
830 unauthenticated connections.
831 The probability increases linearly and all connection attempts
832 are refused if the number of unauthenticated connections reaches
835 .It Cm PasswordAuthentication
836 Specifies whether password authentication is allowed.
839 .It Cm PermitBlacklistedKeys
842 should allow keys recorded in its blacklist of known-compromised keys (see
843 .Xr ssh-vulnkey 1 ) .
846 then attempts to authenticate with compromised keys will be logged but
850 then attempts to authenticate with compromised keys will be rejected.
853 .It Cm PermitEmptyPasswords
854 When password authentication is allowed, it specifies whether the
855 server allows login to accounts with empty password strings.
859 Specifies the destinations to which TCP port forwarding is permitted.
860 The forwarding specification must be one of the following forms:
862 .Bl -item -offset indent -compact
876 .Ar \&[ IPv6_addr \&] : port
880 Multiple forwards may be specified by separating them with whitespace.
883 can be used to remove all restrictions and permit any forwarding requests.
884 By default all port forwarding requests are permitted.
885 .It Cm PermitRootLogin
886 Specifies whether root can log in using
890 .Dq without-password ,
891 .Dq forced-commands-only ,
897 If this option is set to
898 .Dq without-password ,
899 password authentication is disabled for root.
901 If this option is set to
902 .Dq forced-commands-only ,
903 root login with public key authentication will be allowed,
906 option has been specified
907 (which may be useful for taking remote backups even if root login is
908 normally not allowed).
909 All other authentication methods are disabled for root.
911 If this option is set to
913 root is not allowed to log in.
917 device forwarding is allowed.
933 .It Cm PermitUserEnvironment
935 .Pa ~/.ssh/environment
939 .Pa ~/.ssh/authorized_keys
944 Enabling environment processing may enable users to bypass access
945 restrictions in some configurations using mechanisms such as
948 Specifies the file that contains the process ID of the
951 .Pa /var/run/sshd.pid .
953 Specifies the port number that
957 Multiple options of this type are permitted.
963 should print the date and time of the last user login when a user logs
972 when a user logs in interactively.
973 (On some systems it is also printed by the shell,
979 Specifies the protocol versions
982 The possible values are
986 Multiple versions must be comma-separated.
989 Note that the order of the protocol list does not indicate preference,
990 because the client selects among multiple protocol versions offered
996 .It Cm PubkeyAuthentication
997 Specifies whether public key authentication is allowed.
1000 Note that this option applies to protocol version 2 only.
1002 Specifies a list of revoked public keys.
1003 Keys listed in this file will be refused for public key authentication.
1004 Note that if this file is not readable, then public key authentication will
1005 be refused for all users.
1006 .It Cm RhostsRSAAuthentication
1007 Specifies whether rhosts or /etc/hosts.equiv authentication together
1008 with successful RSA host authentication is allowed.
1011 This option applies to protocol version 1 only.
1012 .It Cm RSAAuthentication
1013 Specifies whether pure RSA authentication is allowed.
1016 This option applies to protocol version 1 only.
1017 .It Cm ServerKeyBits
1018 Defines the number of bits in the ephemeral protocol version 1 server key.
1019 The minimum value is 512, and the default is 1024.
1023 should check file modes and ownership of the
1024 user's files and home directory before accepting login.
1025 This is normally desirable because novices sometimes accidentally leave their
1026 directory or files world-writable.
1029 Note that this does not apply to
1030 .Cm ChrootDirectory ,
1031 whose permissions and ownership are checked unconditionally.
1033 Configures an external subsystem (e.g. file transfer daemon).
1034 Arguments should be a subsystem name and a command (with optional arguments)
1035 to execute upon subsystem request.
1041 file transfer subsystem.
1043 Alternately the name
1045 implements an in-process
1048 This may simplify configurations using
1050 to force a different filesystem root on clients.
1052 By default no subsystems are defined.
1053 Note that this option applies to protocol version 2 only.
1054 .It Cm SyslogFacility
1055 Gives the facility code that is used when logging messages from
1057 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1058 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1059 The default is AUTH.
1061 Specifies whether the system should send TCP keepalive messages to the
1063 If they are sent, death of the connection or crash of one
1064 of the machines will be properly noticed.
1065 However, this means that
1066 connections will die if the route is down temporarily, and some people
1068 On the other hand, if TCP keepalives are not sent,
1069 sessions may hang indefinitely on the server, leaving
1071 users and consuming server resources.
1075 (to send TCP keepalive messages), and the server will notice
1076 if the network goes down or the client host crashes.
1077 This avoids infinitely hanging sessions.
1079 To disable TCP keepalive messages, the value should be set to
1082 This option was formerly called
1084 .It Cm TrustedUserCAKeys
1085 Specifies a file containing public keys of certificate authorities that are
1086 trusted to sign user certificates for authentication.
1087 Keys are listed one per line; empty lines and comments starting with
1090 If a certificate is presented for authentication and has its signing CA key
1091 listed in this file, then it may be used for authentication for any user
1092 listed in the certificate's principals list.
1093 Note that certificates that lack a list of principals will not be permitted
1094 for authentication using
1095 .Cm TrustedUserCAKeys .
1096 For more details on certificates, see the
1103 should look up the remote host name and check that
1104 the resolved host name for the remote IP address maps back to the
1105 very same IP address.
1111 is used for interactive login sessions.
1116 is never used for remote command execution.
1117 Note also, that if this is enabled,
1119 will be disabled because
1121 does not know how to handle
1125 .Cm UsePrivilegeSeparation
1126 is specified, it will be disabled after authentication.
1128 Enables the Pluggable Authentication Module interface.
1131 this will enable PAM authentication using
1132 .Cm ChallengeResponseAuthentication
1134 .Cm PasswordAuthentication
1135 in addition to PAM account and session module processing for all
1136 authentication types.
1138 Because PAM challenge-response authentication usually serves an equivalent
1139 role to password authentication, you should disable either
1140 .Cm PasswordAuthentication
1142 .Cm ChallengeResponseAuthentication.
1146 is enabled, you will not be able to run
1151 .It Cm UsePrivilegeSeparation
1154 separates privileges by creating an unprivileged child process
1155 to deal with incoming network traffic.
1156 After successful authentication, another process will be created that has
1157 the privilege of the authenticated user.
1158 The goal of privilege separation is to prevent privilege
1159 escalation by containing any corruption within the unprivileged processes.
1163 .Cm UsePrivilegeSeparation
1166 then the pre-authentication unprivileged process is subject to additional
1168 .It Cm X11DisplayOffset
1169 Specifies the first display number available for
1172 This prevents sshd from interfering with real X11 servers.
1174 .It Cm X11Forwarding
1175 Specifies whether X11 forwarding is permitted.
1176 The argument must be
1183 When X11 forwarding is enabled, there may be additional exposure to
1184 the server and to client displays if the
1186 proxy display is configured to listen on the wildcard address (see
1188 below), though this is not the default.
1189 Additionally, the authentication spoofing and authentication data
1190 verification and substitution occur on the client side.
1191 The security risk of using X11 forwarding is that the client's X11
1192 display server may be exposed to attack when the SSH client requests
1193 forwarding (see the warnings for
1196 .Xr ssh_config 5 ) .
1197 A system administrator may have a stance in which they want to
1198 protect clients that may expose themselves to attack by unwittingly
1199 requesting X11 forwarding, which can warrant a
1203 Note that disabling X11 forwarding does not prevent users from
1204 forwarding X11 traffic, as users can always install their own forwarders.
1205 X11 forwarding is automatically disabled if
1208 .It Cm X11UseLocalhost
1211 should bind the X11 forwarding server to the loopback address or to
1212 the wildcard address.
1214 sshd binds the forwarding server to the loopback address and sets the
1215 hostname part of the
1217 environment variable to
1219 This prevents remote hosts from connecting to the proxy display.
1220 However, some older X11 clients may not function with this
1225 to specify that the forwarding server should be bound to the wildcard
1227 The argument must be
1233 .It Cm XAuthLocation
1234 Specifies the full pathname of the
1238 .Pa /usr/X11R6/bin/xauth .
1242 command-line arguments and configuration file options that specify time
1243 may be expressed using a sequence of the form:
1245 .Ar time Op Ar qualifier ,
1249 is a positive integer value and
1251 is one of the following:
1253 .Bl -tag -width Ds -compact -offset indent
1268 Each member of the sequence is added together to calculate
1269 the total time value.
1271 Time format examples:
1273 .Bl -tag -width Ds -compact -offset indent
1275 600 seconds (10 minutes)
1279 1 hour 30 minutes (90 minutes)
1283 .It Pa /etc/ssh/sshd_config
1284 Contains configuration data for
1286 This file should be writable by root only, but it is recommended
1287 (though not necessary) that it be world-readable.
1292 OpenSSH is a derivative of the original and free
1293 ssh 1.2.12 release by Tatu Ylonen.
1294 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1295 Theo de Raadt and Dug Song
1296 removed many bugs, re-added newer features and
1298 Markus Friedl contributed the support for SSH
1299 protocol versions 1.5 and 2.0.
1300 Niels Provos and Markus Friedl contributed support
1301 for privilege separation.