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 1.194 2015/02/20 23:46:01 djm Exp $
37 .Dd $Mdocdate: February 20 2015 $
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:
122 for more information on patterns.
123 .It Cm AllowTcpForwarding
124 Specifies whether TCP forwarding is permitted.
125 The available options are
129 to allow TCP forwarding,
131 to prevent all TCP forwarding,
133 to allow local (from the perspective of
137 to allow remote forwarding only.
140 Note that disabling TCP forwarding does not improve security unless
141 users are also denied shell access, as they can always install their
143 .It Cm AllowStreamLocalForwarding
144 Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
145 The available options are
149 to allow StreamLocal forwarding,
151 to prevent all StreamLocal forwarding,
153 to allow local (from the perspective of
157 to allow remote forwarding only.
160 Note that disabling StreamLocal forwarding does not improve security unless
161 users are also denied shell access, as they can always install their
164 This keyword can be followed by a list of user name patterns, separated
166 If specified, login is allowed only for user names that
167 match one of the patterns.
168 Only user names are valid; a numerical user ID is not recognized.
169 By default, login is allowed for all users.
170 If the pattern takes the form USER@HOST then USER and HOST
171 are separately checked, restricting logins to particular
172 users from particular hosts.
173 The allow/deny directives are processed in the following order:
182 for more information on patterns.
183 .It Cm AuthenticationMethods
184 Specifies the authentication methods that must be successfully completed
185 for a user to be granted access.
186 This option must be followed by one or more comma-separated lists of
187 authentication method names.
188 Successful authentication requires completion of every method in at least
191 For example, an argument of
192 .Dq publickey,password publickey,keyboard-interactive
193 would require the user to complete public key authentication, followed by
194 either password or keyboard interactive authentication.
195 Only methods that are next in one or more lists are offered at each stage,
196 so for this example, it would not be possible to attempt password or
197 keyboard-interactive authentication before public key.
199 For keyboard interactive authentication it is also possible to
200 restrict authentication to a specific device by appending a
201 colon followed by the device identifier
206 depending on the server configuration.
208 .Dq keyboard-interactive:bsdauth
209 would restrict keyboard interactive authentication to the
215 method is listed more than once,
217 verifies that keys that have been used successfully are not reused for
218 subsequent authentications.
220 .Cm AuthenticationMethods
222 .Dq publickey,publickey
223 will require successful authentication using two different public keys.
225 This option is only available for SSH protocol 2 and will yield a fatal
226 error if enabled if protocol 1 is also enabled.
227 Note that each authentication method listed should also be explicitly enabled
228 in the configuration.
229 The default is not to require multiple authentication; successful completion
230 of a single authentication method is sufficient.
231 .It Cm AuthorizedKeysCommand
232 Specifies a program to be used to look up the user's public keys.
233 The program must be owned by root and not writable by group or others.
234 It will be invoked with a single argument of the username
235 being authenticated, and should produce on standard output zero or
236 more lines of authorized_keys output (see AUTHORIZED_KEYS in
238 If a key supplied by AuthorizedKeysCommand does not successfully authenticate
239 and authorize the user then public key authentication continues using the usual
240 .Cm AuthorizedKeysFile
242 By default, no AuthorizedKeysCommand is run.
243 .It Cm AuthorizedKeysCommandUser
244 Specifies the user under whose account the AuthorizedKeysCommand is run.
245 It is recommended to use a dedicated user that has no other role on the host
246 than running authorized keys commands.
248 .Cm AuthorizedKeysCommand
250 .Cm AuthorizedKeysCommandUser
253 will refuse to start.
254 .It Cm AuthorizedKeysFile
255 Specifies the file that contains the public keys that can be used
256 for user authentication.
257 The format is described in the
258 AUTHORIZED_KEYS FILE FORMAT
261 .Cm AuthorizedKeysFile
262 may contain tokens of the form %T which are substituted during connection
264 The following tokens are defined: %% is replaced by a literal '%',
265 %h is replaced by the home directory of the user being authenticated, and
266 %u is replaced by the username of that user.
268 .Cm AuthorizedKeysFile
269 is taken to be an absolute path or one relative to the user's home
271 Multiple files may be listed, separated by whitespace.
273 .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
274 .It Cm AuthorizedPrincipalsFile
275 Specifies a file that lists principal names that are accepted for
276 certificate authentication.
277 When using certificates signed by a key listed in
278 .Cm TrustedUserCAKeys ,
279 this file lists names, one of which must appear in the certificate for it
280 to be accepted for authentication.
281 Names are listed one per line preceded by key options (as described
282 in AUTHORIZED_KEYS FILE FORMAT in
284 Empty lines and comments starting with
288 .Cm AuthorizedPrincipalsFile
289 may contain tokens of the form %T which are substituted during connection
291 The following tokens are defined: %% is replaced by a literal '%',
292 %h is replaced by the home directory of the user being authenticated, and
293 %u is replaced by the username of that user.
295 .Cm AuthorizedPrincipalsFile
296 is taken to be an absolute path or one relative to the user's home
301 i.e. not to use a principals file \(en in this case, the username
302 of the user must appear in a certificate's principals list for it to be
305 .Cm AuthorizedPrincipalsFile
306 is only used when authentication proceeds using a CA listed in
307 .Cm TrustedUserCAKeys
308 and is not consulted for certification authorities trusted via
309 .Pa ~/.ssh/authorized_keys ,
312 key option offers a similar facility (see
316 The contents of the specified file are sent to the remote user before
317 authentication is allowed.
320 then no banner is displayed.
321 This option is only available for protocol version 2.
322 By default, no banner is displayed.
323 .It Cm ChallengeResponseAuthentication
324 Specifies whether challenge-response authentication is allowed (e.g. via
325 PAM or through authentication styles supported in
329 .It Cm ChrootDirectory
330 Specifies the pathname of a directory to
332 to after authentication.
335 checks that all components of the pathname are root-owned directories
336 which are not writable by any other user or group.
339 changes the working directory to the user's home directory.
341 The pathname may contain the following tokens that are expanded at runtime once
342 the connecting user has been authenticated: %% is replaced by a literal '%',
343 %h is replaced by the home directory of the user being authenticated, and
344 %u is replaced by the username of that user.
348 must contain the necessary files and directories to support the
350 For an interactive session this requires at least a shell, typically
363 For file transfer sessions using
365 no additional configuration of the environment is necessary if the
366 in-process sftp server is used,
367 though sessions which use logging may require
369 inside the chroot directory on some operating systems (see
373 For safety, it is very important that the directory hierarchy be
374 prevented from modification by other processes on the system (especially
375 those outside the jail).
376 Misconfiguration can lead to unsafe environments which
380 The default is not to
383 Specifies the ciphers allowed for protocol version 2.
384 Multiple ciphers must be comma-separated.
385 The supported ciphers are:
387 .Bl -item -compact -offset indent
403 aes128-gcm@openssh.com
405 aes256-gcm@openssh.com
417 chacha20-poly1305@openssh.com
421 .Bd -literal -offset indent
422 aes128-ctr,aes192-ctr,aes256-ctr,
423 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
424 chacha20-poly1305@openssh.com
427 The list of available ciphers may also be obtained using the
433 .It Cm ClientAliveCountMax
434 Sets the number of client alive messages (see below) which may be
437 receiving any messages back from the client.
438 If this threshold is reached while client alive messages are being sent,
439 sshd will disconnect the client, terminating the session.
440 It is important to note that the use of client alive messages is very
444 The client alive messages are sent through the encrypted channel
445 and therefore will not be spoofable.
446 The TCP keepalive option enabled by
449 The client alive mechanism is valuable when the client or
450 server depend on knowing when a connection has become inactive.
452 The default value is 3.
454 .Cm ClientAliveInterval
455 (see below) is set to 15, and
456 .Cm ClientAliveCountMax
457 is left at the default, unresponsive SSH clients
458 will be disconnected after approximately 45 seconds.
459 This option applies to protocol version 2 only.
460 .It Cm ClientAliveInterval
461 Sets a timeout interval in seconds after which if no data has been received
464 will send a message through the encrypted
465 channel to request a response from the client.
467 is 0, indicating that these messages will not be sent to the client.
468 This option applies to protocol version 2 only.
470 Specifies whether compression is allowed, or delayed until
471 the user has authenticated successfully.
480 This keyword can be followed by a list of group name patterns, separated
482 Login is disallowed for users whose primary group or supplementary
483 group list matches one of the patterns.
484 Only group names are valid; a numerical group ID is not recognized.
485 By default, login is allowed for all groups.
486 The allow/deny directives are processed in the following order:
495 for more information on patterns.
497 This keyword can be followed by a list of user name patterns, separated
499 Login is disallowed for user names that match one of the patterns.
500 Only user names are valid; a numerical user ID is not recognized.
501 By default, login is allowed for all users.
502 If the pattern takes the form USER@HOST then USER and HOST
503 are separately checked, restricting logins to particular
504 users from particular hosts.
505 The allow/deny directives are processed in the following order:
514 for more information on patterns.
515 .It Cm FingerprintHash
516 Specifies the hash algorithm used when logging key fingerprints.
524 Forces the execution of the command specified by
526 ignoring any command supplied by the client and
529 The command is invoked by using the user's login shell with the -c option.
530 This applies to shell, command, or subsystem execution.
531 It is most useful inside a
534 The command originally supplied by the client is available in the
535 .Ev SSH_ORIGINAL_COMMAND
536 environment variable.
537 Specifying a command of
539 will force the use of an in-process sftp server that requires no support
541 .Cm ChrootDirectory .
543 Specifies whether remote hosts are allowed to connect to ports
544 forwarded for the client.
547 binds remote port forwardings to the loopback address.
548 This prevents other remote hosts from connecting to forwarded ports.
550 can be used to specify that sshd
551 should allow remote port forwardings to bind to non-loopback addresses, thus
552 allowing other hosts to connect.
555 to force remote port forwardings to be available to the local host only,
557 to force remote port forwardings to bind to the wildcard address, or
559 to allow the client to select the address to which the forwarding is bound.
562 .It Cm GSSAPIAuthentication
563 Specifies whether user authentication based on GSSAPI is allowed.
566 Note that this option applies to protocol version 2 only.
567 .It Cm GSSAPICleanupCredentials
568 Specifies whether to automatically destroy the user's credentials cache
572 Note that this option applies to protocol version 2 only.
573 .It Cm HostbasedAcceptedKeyTypes
574 Specifies the key types that will be accepted for hostbased authentication
575 as a comma-separated pattern list.
578 will allow all key types.
583 may be used to list supported key types.
584 .It Cm HostbasedAuthentication
585 Specifies whether rhosts or /etc/hosts.equiv authentication together
586 with successful public key client host authentication is allowed
587 (host-based authentication).
588 This option is similar to
589 .Cm RhostsRSAAuthentication
590 and applies to protocol version 2 only.
593 .It Cm HostbasedUsesNameFromPacketOnly
594 Specifies whether or not the server will attempt to perform a reverse
595 name lookup when matching the name in the
601 .Cm HostbasedAuthentication .
606 uses the name supplied by the client rather than
607 attempting to resolve the name from the TCP connection itself.
610 .It Cm HostCertificate
611 Specifies a file containing a public host certificate.
612 The certificate's public key must match a private host key already specified
615 The default behaviour of
617 is not to load any certificates.
619 Specifies a file containing a private host key
622 .Pa /etc/ssh/ssh_host_key
623 for protocol version 1, and
624 .Pa /etc/ssh/ssh_host_dsa_key ,
625 .Pa /etc/ssh/ssh_host_ecdsa_key ,
626 .Pa /etc/ssh/ssh_host_ed25519_key
628 .Pa /etc/ssh/ssh_host_rsa_key
629 for protocol version 2.
632 will refuse to use a file if it is group/world-accessible.
633 It is possible to have multiple host key files.
635 keys are used for version 1 and
641 are used for version 2 of the SSH protocol.
642 It is also possible to specify public host key files instead.
643 In this case operations on the private key will be delegated
647 Identifies the UNIX-domain socket used to communicate
648 with an agent that has access to the private host keys.
651 is specified, the location of the socket will be read from the
653 environment variable.
659 files will not be used in
660 .Cm RhostsRSAAuthentication
662 .Cm HostbasedAuthentication .
666 .Pa /etc/shosts.equiv
670 .It Cm IgnoreUserKnownHosts
673 should ignore the user's
674 .Pa ~/.ssh/known_hosts
676 .Cm RhostsRSAAuthentication
678 .Cm HostbasedAuthentication .
682 Specifies the IPv4 type-of-service or DSCP class for the connection.
709 This option may take one or two arguments, separated by whitespace.
710 If one argument is specified, it is used as the packet class unconditionally.
711 If two values are specified, the first is automatically selected for
712 interactive sessions and the second for non-interactive sessions.
715 for interactive sessions and
717 for non-interactive sessions.
718 .It Cm KbdInteractiveAuthentication
719 Specifies whether to allow keyboard-interactive authentication.
720 The argument to this keyword must be
724 The default is to use whatever value
725 .Cm ChallengeResponseAuthentication
729 .It Cm KerberosAuthentication
730 Specifies whether the password provided by the user for
731 .Cm PasswordAuthentication
732 will be validated through the Kerberos KDC.
733 To use this option, the server needs a
734 Kerberos servtab which allows the verification of the KDC's identity.
737 .It Cm KerberosGetAFSToken
738 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
739 an AFS token before accessing the user's home directory.
742 .It Cm KerberosOrLocalPasswd
743 If password authentication through Kerberos fails then
744 the password will be validated via any additional local mechanism
749 .It Cm KerberosTicketCleanup
750 Specifies whether to automatically destroy the user's ticket cache
755 Specifies the available KEX (Key Exchange) algorithms.
756 Multiple algorithms must be comma-separated.
757 The supported algorithms are:
759 .Bl -item -compact -offset indent
761 curve25519-sha256@libssh.org
763 diffie-hellman-group1-sha1
765 diffie-hellman-group14-sha1
767 diffie-hellman-group-exchange-sha1
769 diffie-hellman-group-exchange-sha256
779 .Bd -literal -offset indent
780 curve25519-sha256@libssh.org,
781 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
782 diffie-hellman-group-exchange-sha256,
783 diffie-hellman-group14-sha1
786 The list of available key exchange algorithms may also be obtained using the
792 .It Cm KeyRegenerationInterval
793 In protocol version 1, the ephemeral server key is automatically regenerated
794 after this many seconds (if it has been used).
795 The purpose of regeneration is to prevent
796 decrypting captured sessions by later breaking into the machine and
798 The key is never stored anywhere.
799 If the value is 0, the key is never regenerated.
800 The default is 3600 (seconds).
802 Specifies the local addresses
805 The following forms may be used:
807 .Bl -item -offset indent -compact
811 .Ar host | Ar IPv4_addr | Ar IPv6_addr
816 .Ar host | Ar IPv4_addr : Ar port
822 .Ar host | Ar IPv6_addr Oc : Ar port
829 sshd will listen on the address and all prior
832 The default is to listen on all local addresses.
835 options are permitted.
838 options must precede this option for non-port qualified addresses.
839 .It Cm LoginGraceTime
840 The server disconnects after this time if the user has not
841 successfully logged in.
842 If the value is 0, there is no time limit.
843 The default is 120 seconds.
845 Gives the verbosity level that is used when logging messages from
847 The possible values are:
848 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
850 DEBUG and DEBUG1 are equivalent.
851 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
852 Logging with a DEBUG level violates the privacy of users and is not recommended.
854 Specifies the available MAC (message authentication code) algorithms.
855 The MAC algorithm is used in protocol version 2
856 for data integrity protection.
857 Multiple algorithms must be comma-separated.
858 The algorithms that contain
860 calculate the MAC after encryption (encrypt-then-mac).
861 These are considered safer and their use recommended.
862 The supported MACs are:
864 .Bl -item -compact -offset indent
884 hmac-md5-etm@openssh.com
886 hmac-md5-96-etm@openssh.com
888 hmac-ripemd160-etm@openssh.com
890 hmac-sha1-etm@openssh.com
892 hmac-sha1-96-etm@openssh.com
894 hmac-sha2-256-etm@openssh.com
896 hmac-sha2-512-etm@openssh.com
898 umac-64-etm@openssh.com
900 umac-128-etm@openssh.com
904 .Bd -literal -offset indent
905 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
906 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
907 umac-64@openssh.com,umac-128@openssh.com,
908 hmac-sha2-256,hmac-sha2-512
911 The list of available MAC algorithms may also be obtained using the
918 Introduces a conditional block.
919 If all of the criteria on the
921 line are satisfied, the keywords on the following lines override those
922 set in the global section of the config file, until either another
924 line or the end of the file.
925 If a keyword appears in multiple
927 blocks that are satisfied, only the first instance of the keyword is
932 are one or more criteria-pattern pairs or the single token
934 which matches all criteria.
935 The available criteria are
943 The match patterns may consist of single entries or comma-separated
944 lists and may use the wildcard and negation operators described in the
950 criteria may additionally contain addresses to match in CIDR
951 address/masklen format, e.g.\&
955 Note that the mask length provided must be consistent with the address -
956 it is an error to specify a mask length that is too long for the address
957 or one with bits set in this host portion of the address.
964 Only a subset of keywords may be used on the lines following a
967 Available keywords are
969 .Cm AllowAgentForwarding ,
971 .Cm AllowStreamLocalForwarding ,
972 .Cm AllowTcpForwarding ,
974 .Cm AuthenticationMethods ,
975 .Cm AuthorizedKeysCommand ,
976 .Cm AuthorizedKeysCommandUser ,
977 .Cm AuthorizedKeysFile ,
978 .Cm AuthorizedPrincipalsFile ,
980 .Cm ChrootDirectory ,
985 .Cm GSSAPIAuthentication ,
986 .Cm HostbasedAcceptedKeyTypes ,
987 .Cm HostbasedAuthentication ,
988 .Cm HostbasedUsesNameFromPacketOnly ,
990 .Cm KbdInteractiveAuthentication ,
991 .Cm KerberosAuthentication ,
994 .Cm PasswordAuthentication ,
995 .Cm PermitEmptyPasswords ,
997 .Cm PermitRootLogin ,
1001 .Cm PubkeyAcceptedKeyTypes ,
1002 .Cm PubkeyAuthentication ,
1005 .Cm RhostsRSAAuthentication ,
1006 .Cm RSAAuthentication ,
1007 .Cm StreamLocalBindMask ,
1008 .Cm StreamLocalBindUnlink ,
1009 .Cm TrustedUserCAKeys ,
1010 .Cm X11DisplayOffset ,
1013 .Cm X11UseLocalHost .
1015 Specifies the maximum number of authentication attempts permitted per
1017 Once the number of failures reaches half this value,
1018 additional failures are logged.
1021 Specifies the maximum number of open sessions permitted per network connection.
1024 Specifies the maximum number of concurrent unauthenticated connections to the
1026 Additional connections will be dropped until authentication succeeds or the
1028 expires for a connection.
1029 The default is 10:30:100.
1031 Alternatively, random early drop can be enabled by specifying
1032 the three colon separated values
1036 will refuse connection attempts with a probability of
1039 if there are currently
1042 unauthenticated connections.
1043 The probability increases linearly and all connection attempts
1044 are refused if the number of unauthenticated connections reaches
1047 .It Cm PasswordAuthentication
1048 Specifies whether password authentication is allowed.
1051 .It Cm PermitEmptyPasswords
1052 When password authentication is allowed, it specifies whether the
1053 server allows login to accounts with empty password strings.
1057 Specifies the destinations to which TCP port forwarding is permitted.
1058 The forwarding specification must be one of the following forms:
1060 .Bl -item -offset indent -compact
1069 .Ar IPv4_addr : port
1074 .Ar \&[ IPv6_addr \&] : port
1078 Multiple forwards may be specified by separating them with whitespace.
1081 can be used to remove all restrictions and permit any forwarding requests.
1084 can be used to prohibit all forwarding requests.
1085 By default all port forwarding requests are permitted.
1086 .It Cm PermitRootLogin
1087 Specifies whether root can log in using
1089 The argument must be
1091 .Dq without-password ,
1092 .Dq forced-commands-only ,
1098 If this option is set to
1099 .Dq without-password ,
1100 password authentication is disabled for root.
1102 If this option is set to
1103 .Dq forced-commands-only ,
1104 root login with public key authentication will be allowed,
1107 option has been specified
1108 (which may be useful for taking remote backups even if root login is
1109 normally not allowed).
1110 All other authentication methods are disabled for root.
1112 If this option is set to
1114 root is not allowed to log in.
1118 device forwarding is allowed.
1119 The argument must be
1135 Independent of this setting, the permissions of the selected
1137 device must allow access to the user.
1141 allocation is permitted.
1144 .It Cm PermitUserEnvironment
1146 .Pa ~/.ssh/environment
1150 .Pa ~/.ssh/authorized_keys
1155 Enabling environment processing may enable users to bypass access
1156 restrictions in some configurations using mechanisms such as
1159 Specifies whether any
1165 Specifies the file that contains the process ID of the
1168 .Pa /var/run/sshd.pid .
1170 Specifies the port number that
1174 Multiple options of this type are permitted.
1180 should print the date and time of the last user login when a user logs
1189 when a user logs in interactively.
1190 (On some systems it is also printed by the shell,
1196 Specifies the protocol versions
1199 The possible values are
1203 Multiple versions must be comma-separated.
1206 Note that the order of the protocol list does not indicate preference,
1207 because the client selects among multiple protocol versions offered
1213 .It Cm PubkeyAcceptedKeyTypes
1214 Specifies the key types that will be accepted for public key authentication
1215 as a comma-separated pattern list.
1218 will allow all key types.
1223 may be used to list supported key types.
1224 .It Cm PubkeyAuthentication
1225 Specifies whether public key authentication is allowed.
1228 Note that this option applies to protocol version 2 only.
1230 Specifies the maximum amount of data that may be transmitted before the
1231 session key is renegotiated, optionally followed a maximum amount of
1232 time that may pass before the session key is renegotiated.
1233 The first argument is specified in bytes and may have a suffix of
1238 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1239 The default is between
1243 depending on the cipher.
1244 The optional second value is specified in seconds and may use any of the
1245 units documented in the
1248 The default value for
1252 which means that rekeying is performed after the cipher's default amount
1253 of data has been sent or received and no time based rekeying is done.
1254 This option applies to protocol version 2 only.
1256 Specifies revoked public keys.
1257 Keys listed in this file will be refused for public key authentication.
1258 Note that if this file is not readable, then public key authentication will
1259 be refused for all users.
1260 Keys may be specified as a text file, listing one public key per line, or as
1261 an OpenSSH Key Revocation List (KRL) as generated by
1263 For more information on KRLs, see the KEY REVOCATION LISTS section in
1265 .It Cm RhostsRSAAuthentication
1266 Specifies whether rhosts or /etc/hosts.equiv authentication together
1267 with successful RSA host authentication is allowed.
1270 This option applies to protocol version 1 only.
1271 .It Cm RSAAuthentication
1272 Specifies whether pure RSA authentication is allowed.
1275 This option applies to protocol version 1 only.
1276 .It Cm ServerKeyBits
1277 Defines the number of bits in the ephemeral protocol version 1 server key.
1278 The minimum value is 512, and the default is 1024.
1279 .It Cm StreamLocalBindMask
1280 Sets the octal file creation mode mask
1282 used when creating a Unix-domain socket file for local or remote
1284 This option is only used for port forwarding to a Unix-domain socket file.
1286 The default value is 0177, which creates a Unix-domain socket file that is
1287 readable and writable only by the owner.
1288 Note that not all operating systems honor the file mode on Unix-domain
1290 .It Cm StreamLocalBindUnlink
1291 Specifies whether to remove an existing Unix-domain socket file for local
1292 or remote port forwarding before creating a new one.
1293 If the socket file already exists and
1294 .Cm StreamLocalBindUnlink
1297 will be unable to forward the port to the Unix-domain socket file.
1298 This option is only used for port forwarding to a Unix-domain socket file.
1300 The argument must be
1309 should check file modes and ownership of the
1310 user's files and home directory before accepting login.
1311 This is normally desirable because novices sometimes accidentally leave their
1312 directory or files world-writable.
1315 Note that this does not apply to
1316 .Cm ChrootDirectory ,
1317 whose permissions and ownership are checked unconditionally.
1319 Configures an external subsystem (e.g. file transfer daemon).
1320 Arguments should be a subsystem name and a command (with optional arguments)
1321 to execute upon subsystem request.
1327 file transfer subsystem.
1329 Alternately the name
1331 implements an in-process
1334 This may simplify configurations using
1336 to force a different filesystem root on clients.
1338 By default no subsystems are defined.
1339 Note that this option applies to protocol version 2 only.
1340 .It Cm SyslogFacility
1341 Gives the facility code that is used when logging messages from
1343 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1344 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1345 The default is AUTH.
1347 Specifies whether the system should send TCP keepalive messages to the
1349 If they are sent, death of the connection or crash of one
1350 of the machines will be properly noticed.
1351 However, this means that
1352 connections will die if the route is down temporarily, and some people
1354 On the other hand, if TCP keepalives are not sent,
1355 sessions may hang indefinitely on the server, leaving
1357 users and consuming server resources.
1361 (to send TCP keepalive messages), and the server will notice
1362 if the network goes down or the client host crashes.
1363 This avoids infinitely hanging sessions.
1365 To disable TCP keepalive messages, the value should be set to
1367 .It Cm TrustedUserCAKeys
1368 Specifies a file containing public keys of certificate authorities that are
1369 trusted to sign user certificates for authentication.
1370 Keys are listed one per line; empty lines and comments starting with
1373 If a certificate is presented for authentication and has its signing CA key
1374 listed in this file, then it may be used for authentication for any user
1375 listed in the certificate's principals list.
1376 Note that certificates that lack a list of principals will not be permitted
1377 for authentication using
1378 .Cm TrustedUserCAKeys .
1379 For more details on certificates, see the CERTIFICATES section in
1384 should look up the remote host name and check that
1385 the resolved host name for the remote IP address maps back to the
1386 very same IP address.
1392 is used for interactive login sessions.
1397 is never used for remote command execution.
1398 Note also, that if this is enabled,
1400 will be disabled because
1402 does not know how to handle
1406 .Cm UsePrivilegeSeparation
1407 is specified, it will be disabled after authentication.
1409 Enables the Pluggable Authentication Module interface.
1412 this will enable PAM authentication using
1413 .Cm ChallengeResponseAuthentication
1415 .Cm PasswordAuthentication
1416 in addition to PAM account and session module processing for all
1417 authentication types.
1419 Because PAM challenge-response authentication usually serves an equivalent
1420 role to password authentication, you should disable either
1421 .Cm PasswordAuthentication
1423 .Cm ChallengeResponseAuthentication.
1427 is enabled, you will not be able to run
1432 .It Cm UsePrivilegeSeparation
1435 separates privileges by creating an unprivileged child process
1436 to deal with incoming network traffic.
1437 After successful authentication, another process will be created that has
1438 the privilege of the authenticated user.
1439 The goal of privilege separation is to prevent privilege
1440 escalation by containing any corruption within the unprivileged processes.
1444 .Cm UsePrivilegeSeparation
1447 then the pre-authentication unprivileged process is subject to additional
1449 .It Cm VersionAddendum
1450 Optionally specifies additional text to append to the SSH protocol banner
1451 sent by the server upon connection.
1454 .It Cm X11DisplayOffset
1455 Specifies the first display number available for
1458 This prevents sshd from interfering with real X11 servers.
1460 .It Cm X11Forwarding
1461 Specifies whether X11 forwarding is permitted.
1462 The argument must be
1469 When X11 forwarding is enabled, there may be additional exposure to
1470 the server and to client displays if the
1472 proxy display is configured to listen on the wildcard address (see
1474 below), though this is not the default.
1475 Additionally, the authentication spoofing and authentication data
1476 verification and substitution occur on the client side.
1477 The security risk of using X11 forwarding is that the client's X11
1478 display server may be exposed to attack when the SSH client requests
1479 forwarding (see the warnings for
1482 .Xr ssh_config 5 ) .
1483 A system administrator may have a stance in which they want to
1484 protect clients that may expose themselves to attack by unwittingly
1485 requesting X11 forwarding, which can warrant a
1489 Note that disabling X11 forwarding does not prevent users from
1490 forwarding X11 traffic, as users can always install their own forwarders.
1491 X11 forwarding is automatically disabled if
1494 .It Cm X11UseLocalhost
1497 should bind the X11 forwarding server to the loopback address or to
1498 the wildcard address.
1500 sshd binds the forwarding server to the loopback address and sets the
1501 hostname part of the
1503 environment variable to
1505 This prevents remote hosts from connecting to the proxy display.
1506 However, some older X11 clients may not function with this
1511 to specify that the forwarding server should be bound to the wildcard
1513 The argument must be
1519 .It Cm XAuthLocation
1520 Specifies the full pathname of the
1524 .Pa /usr/X11R6/bin/xauth .
1528 command-line arguments and configuration file options that specify time
1529 may be expressed using a sequence of the form:
1531 .Ar time Op Ar qualifier ,
1535 is a positive integer value and
1537 is one of the following:
1539 .Bl -tag -width Ds -compact -offset indent
1554 Each member of the sequence is added together to calculate
1555 the total time value.
1557 Time format examples:
1559 .Bl -tag -width Ds -compact -offset indent
1561 600 seconds (10 minutes)
1565 1 hour 30 minutes (90 minutes)
1569 .It Pa /etc/ssh/sshd_config
1570 Contains configuration data for
1572 This file should be writable by root only, but it is recommended
1573 (though not necessary) that it be world-readable.
1578 OpenSSH is a derivative of the original and free
1579 ssh 1.2.12 release by Tatu Ylonen.
1580 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1581 Theo de Raadt and Dug Song
1582 removed many bugs, re-added newer features and
1584 Markus Friedl contributed the support for SSH
1585 protocol versions 1.5 and 2.0.
1586 Niels Provos and Markus Friedl contributed support
1587 for privilege separation.