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 1.282 2018/09/20 03:28:06 djm Exp $
38 .Dd $Mdocdate: July 28 2020 $
43 .Nd OpenSSH SSH daemon configuration file
46 reads configuration data from
47 .Pa /etc/ssh/sshd_config
48 (or the file specified with
51 The file contains keyword-argument pairs, one per line.
52 For each keyword, the first obtained value will be used.
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
74 for how to configure the client.
77 environment variable is always accepted whenever the client
78 requests a pseudo-terminal as it is required by the protocol.
79 Variables are specified by name, which may contain the wildcard characters
83 Multiple environment variables may be separated by whitespace or spread
87 Be warned that some environment variables could be used to bypass restricted
89 For this reason, care should be taken in the use of this directive.
90 The default is not to accept any environment variables.
92 Specifies which address family should be used by
101 .It Cm AllowAgentForwarding
104 forwarding is permitted.
107 Note that disabling agent forwarding does not improve security
108 unless users are also denied shell access, as they can always install
109 their own forwarders.
111 This keyword can be followed by a list of group name patterns, separated
113 If specified, login is allowed only for users whose primary
114 group or supplementary group list matches one of the patterns.
115 Only group names are valid; a numerical group ID is not recognized.
116 By default, login is allowed for all groups.
117 The allow/deny directives are processed in the following order:
126 for more information on patterns.
127 .It Cm AllowStreamLocalForwarding
128 Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
129 The available options are
134 to allow StreamLocal forwarding,
136 to prevent all StreamLocal forwarding,
138 to allow local (from the perspective of
142 to allow remote forwarding only.
143 Note that disabling StreamLocal forwarding does not improve security unless
144 users are also denied shell access, as they can always install their
146 .It Cm AllowTcpForwarding
147 Specifies whether TCP forwarding is permitted.
148 The available options are
153 to allow TCP forwarding,
155 to prevent all TCP forwarding,
157 to allow local (from the perspective of
161 to allow remote forwarding only.
162 Note that disabling TCP forwarding does not improve security unless
163 users are also denied shell access, as they can always install their
166 This keyword can be followed by a list of user name patterns, separated
168 If specified, login is allowed only for user names that
169 match one of the patterns.
170 Only user names are valid; a numerical user ID is not recognized.
171 By default, login is allowed for all users.
172 If the pattern takes the form USER@HOST then USER and HOST
173 are separately checked, restricting logins to particular
174 users from particular hosts.
175 HOST criteria may additionally contain addresses to match in CIDR
176 address/masklen format.
177 The allow/deny directives are processed in the following order:
186 for more information on patterns.
187 .It Cm AuthenticationMethods
188 Specifies the authentication methods that must be successfully completed
189 for a user to be granted access.
190 This option must be followed by one or more lists of comma-separated
191 authentication method names, or by the single string
193 to indicate the default behaviour of accepting any single authentication
195 If the default is overridden, then successful authentication requires
196 completion of every method in at least one of these lists.
199 .Qq publickey,password publickey,keyboard-interactive
200 would require the user to complete public key authentication, followed by
201 either password or keyboard interactive authentication.
202 Only methods that are next in one or more lists are offered at each stage,
203 so for this example it would not be possible to attempt password or
204 keyboard-interactive authentication before public key.
206 For keyboard interactive authentication it is also possible to
207 restrict authentication to a specific device by appending a
208 colon followed by the device identifier
212 depending on the server configuration.
214 .Qq keyboard-interactive:bsdauth
215 would restrict keyboard interactive authentication to the
219 If the publickey method is listed more than once,
221 verifies that keys that have been used successfully are not reused for
222 subsequent authentications.
224 .Qq publickey,publickey
225 requires successful authentication using two different public keys.
227 Note that each authentication method listed should also be explicitly enabled
228 in the configuration.
230 The available authentication methods are:
231 .Qq gssapi-with-mic ,
233 .Qq keyboard-interactive ,
235 (used for access to password-less accounts when
236 .Cm PermitEmptyPasswords
241 .It Cm AuthorizedKeysCommand
242 Specifies a program to be used to look up the user's public keys.
243 The program must be owned by root, not writable by group or others and
244 specified by an absolute path.
246 .Cm AuthorizedKeysCommand
247 accept the tokens described in the
250 If no arguments are specified then the username of the target user is used.
252 The program should produce on standard output zero or
253 more lines of authorized_keys output (see
258 .Cm AuthorizedKeysCommand
259 does not successfully authenticate
260 and authorize the user then public key authentication continues using the usual
261 .Cm AuthorizedKeysFile
264 .Cm AuthorizedKeysCommand
266 .It Cm AuthorizedKeysCommandUser
267 Specifies the user under whose account the
268 .Cm AuthorizedKeysCommand
270 It is recommended to use a dedicated user that has no other role on the host
271 than running authorized keys commands.
273 .Cm AuthorizedKeysCommand
275 .Cm AuthorizedKeysCommandUser
278 will refuse to start.
279 .It Cm AuthorizedKeysFile
280 Specifies the file that contains the public keys used for user authentication.
281 The format is described in the
282 .Sx AUTHORIZED_KEYS FILE FORMAT
286 .Cm AuthorizedKeysFile
287 accept the tokens described in the
291 .Cm AuthorizedKeysFile
292 is taken to be an absolute path or one relative to the user's home
294 Multiple files may be listed, separated by whitespace.
295 Alternately this option may be set to
297 to skip checking for user keys in files.
299 .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
300 .It Cm AuthorizedPrincipalsCommand
301 Specifies a program to be used to generate the list of allowed
302 certificate principals as per
303 .Cm AuthorizedPrincipalsFile .
304 The program must be owned by root, not writable by group or others and
305 specified by an absolute path.
307 .Cm AuthorizedPrincipalsCommand
308 accept the tokens described in the
311 If no arguments are specified then the username of the target user is used.
313 The program should produce on standard output zero or
315 .Cm AuthorizedPrincipalsFile
318 .Cm AuthorizedPrincipalsCommand
320 .Cm AuthorizedPrincipalsFile
321 is specified, then certificates offered by the client for authentication
322 must contain a principal that is listed.
324 .Cm AuthorizedPrincipalsCommand
326 .It Cm AuthorizedPrincipalsCommandUser
327 Specifies the user under whose account the
328 .Cm AuthorizedPrincipalsCommand
330 It is recommended to use a dedicated user that has no other role on the host
331 than running authorized principals commands.
333 .Cm AuthorizedPrincipalsCommand
335 .Cm AuthorizedPrincipalsCommandUser
338 will refuse to start.
339 .It Cm AuthorizedPrincipalsFile
340 Specifies a file that lists principal names that are accepted for
341 certificate authentication.
342 When using certificates signed by a key listed in
343 .Cm TrustedUserCAKeys ,
344 this file lists names, one of which must appear in the certificate for it
345 to be accepted for authentication.
346 Names are listed one per line preceded by key options (as described in
347 .Sx AUTHORIZED_KEYS FILE FORMAT
350 Empty lines and comments starting with
355 .Cm AuthorizedPrincipalsFile
356 accept the tokens described in the
360 .Cm AuthorizedPrincipalsFile
361 is taken to be an absolute path or one relative to the user's home directory.
364 i.e. not to use a principals file \(en in this case, the username
365 of the user must appear in a certificate's principals list for it to be
369 .Cm AuthorizedPrincipalsFile
370 is only used when authentication proceeds using a CA listed in
371 .Cm TrustedUserCAKeys
372 and is not consulted for certification authorities trusted via
373 .Pa ~/.ssh/authorized_keys ,
376 key option offers a similar facility (see
380 The contents of the specified file are sent to the remote user before
381 authentication is allowed.
384 then no banner is displayed.
385 By default, no banner is displayed.
386 .It Cm CASignatureAlgorithms
387 Specifies which algorithms are allowed for signing of certificates
388 by certificate authorities (CAs).
390 .Bd -literal -offset indent
391 ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
392 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
395 Certificates signed using other algorithms will not be accepted for
396 public key or host-based authentication.
397 .It Cm ChallengeResponseAuthentication
398 Specifies whether challenge-response authentication is allowed (e.g. via
399 PAM or through authentication styles supported in
403 .It Cm ChrootDirectory
404 Specifies the pathname of a directory to
406 to after authentication.
409 checks that all components of the pathname are root-owned directories
410 which are not writable by any other user or group.
413 changes the working directory to the user's home directory.
416 accept the tokens described in the
422 must contain the necessary files and directories to support the
424 For an interactive session this requires at least a shell, typically
437 For file transfer sessions using SFTP
438 no additional configuration of the environment is necessary if the in-process
440 though sessions which use logging may require
442 inside the chroot directory on some operating systems (see
446 For safety, it is very important that the directory hierarchy be
447 prevented from modification by other processes on the system (especially
448 those outside the jail).
449 Misconfiguration can lead to unsafe environments which
458 Specifies the ciphers allowed.
459 Multiple ciphers must be comma-separated.
460 If the specified value begins with a
462 character, then the specified ciphers will be appended to the default set
463 instead of replacing them.
464 If the specified value begins with a
466 character, then the specified ciphers (including wildcards) will be removed
467 from the default set instead of replacing them.
469 The supported ciphers are:
471 .Bl -item -compact -offset indent
487 aes128-gcm@openssh.com
489 aes256-gcm@openssh.com
491 chacha20-poly1305@openssh.com
495 .Bd -literal -offset indent
496 chacha20-poly1305@openssh.com,
497 aes128-ctr,aes192-ctr,aes256-ctr,
498 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
499 aes128-cbc,aes192-cbc,aes256-cbc
502 The list of available ciphers may also be obtained using
504 .It Cm ClientAliveCountMax
505 Sets the number of client alive messages which may be sent without
507 receiving any messages back from the client.
508 If this threshold is reached while client alive messages are being sent,
509 sshd will disconnect the client, terminating the session.
510 It is important to note that the use of client alive messages is very
513 The client alive messages are sent through the encrypted channel
514 and therefore will not be spoofable.
515 The TCP keepalive option enabled by
518 The client alive mechanism is valuable when the client or
519 server depend on knowing when a connection has become inactive.
521 The default value is 3.
523 .Cm ClientAliveInterval
525 .Cm ClientAliveCountMax
526 is left at the default, unresponsive SSH clients
527 will be disconnected after approximately 45 seconds.
528 .It Cm ClientAliveInterval
529 Sets a timeout interval in seconds after which if no data has been received
532 will send a message through the encrypted
533 channel to request a response from the client.
535 is 0, indicating that these messages will not be sent to the client.
537 Specifies whether compression is enabled after
538 the user has authenticated successfully.
542 (a legacy synonym for
549 This keyword can be followed by a list of group name patterns, separated
551 Login is disallowed for users whose primary group or supplementary
552 group list matches one of the patterns.
553 Only group names are valid; a numerical group ID is not recognized.
554 By default, login is allowed for all groups.
555 The allow/deny directives are processed in the following order:
564 for more information on patterns.
566 This keyword can be followed by a list of user name patterns, separated
568 Login is disallowed for user names that match one of the patterns.
569 Only user names are valid; a numerical user ID is not recognized.
570 By default, login is allowed for all users.
571 If the pattern takes the form USER@HOST then USER and HOST
572 are separately checked, restricting logins to particular
573 users from particular hosts.
574 HOST criteria may additionally contain addresses to match in CIDR
575 address/masklen format.
576 The allow/deny directives are processed in the following order:
585 for more information on patterns.
586 .It Cm DisableForwarding
587 Disables all forwarding features, including X11,
590 This option overrides all other forwarding-related options and may
591 simplify restricted configurations.
592 .It Cm ExposeAuthInfo
593 Writes a temporary file containing a list of authentication methods and
594 public credentials (e.g. keys) used to authenticate the user.
595 The location of the file is exposed to the user session through the
597 environment variable.
600 .It Cm FingerprintHash
601 Specifies the hash algorithm used when logging key fingerprints.
609 Forces the execution of the command specified by
611 ignoring any command supplied by the client and
614 The command is invoked by using the user's login shell with the -c option.
615 This applies to shell, command, or subsystem execution.
616 It is most useful inside a
619 The command originally supplied by the client is available in the
620 .Ev SSH_ORIGINAL_COMMAND
621 environment variable.
622 Specifying a command of
624 will force the use of an in-process SFTP server that requires no support
626 .Cm ChrootDirectory .
630 Specifies whether remote hosts are allowed to connect to ports
631 forwarded for the client.
634 binds remote port forwardings to the loopback address.
635 This prevents other remote hosts from connecting to forwarded ports.
637 can be used to specify that sshd
638 should allow remote port forwardings to bind to non-loopback addresses, thus
639 allowing other hosts to connect.
642 to force remote port forwardings to be available to the local host only,
644 to force remote port forwardings to bind to the wildcard address, or
646 to allow the client to select the address to which the forwarding is bound.
649 .It Cm GSSAPIAuthentication
650 Specifies whether user authentication based on GSSAPI is allowed.
653 .It Cm GSSAPICleanupCredentials
654 Specifies whether to automatically destroy the user's credentials cache
658 .It Cm GSSAPIStrictAcceptorCheck
659 Determines whether to be strict about the identity of the GSSAPI acceptor
660 a client authenticates against.
663 then the client must authenticate against the host
664 service on the current hostname.
667 then the client may authenticate against any service key stored in the
668 machine's default store.
669 This facility is provided to assist with operation on multi homed machines.
672 .It Cm HostbasedAcceptedKeyTypes
673 Specifies the key types that will be accepted for hostbased authentication
674 as a list of comma-separated patterns.
675 Alternately if the specified value begins with a
677 character, then the specified key types will be appended to the default set
678 instead of replacing them.
679 If the specified value begins with a
681 character, then the specified key types (including wildcards) will be removed
682 from the default set instead of replacing them.
683 The default for this option is:
684 .Bd -literal -offset 3n
685 ecdsa-sha2-nistp256-cert-v01@openssh.com,
686 ecdsa-sha2-nistp384-cert-v01@openssh.com,
687 ecdsa-sha2-nistp521-cert-v01@openssh.com,
688 ssh-ed25519-cert-v01@openssh.com,
689 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
690 ssh-rsa-cert-v01@openssh.com,
691 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
692 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
695 The list of available key types may also be obtained using
697 .It Cm HostbasedAuthentication
698 Specifies whether rhosts or /etc/hosts.equiv authentication together
699 with successful public key client host authentication is allowed
700 (host-based authentication).
703 .It Cm HostbasedUsesNameFromPacketOnly
704 Specifies whether or not the server will attempt to perform a reverse
705 name lookup when matching the name in the
711 .Cm HostbasedAuthentication .
716 uses the name supplied by the client rather than
717 attempting to resolve the name from the TCP connection itself.
720 .It Cm HostCertificate
721 Specifies a file containing a public host certificate.
722 The certificate's public key must match a private host key already specified
725 The default behaviour of
727 is not to load any certificates.
729 Specifies a file containing a private host key
732 .Pa /etc/ssh/ssh_host_ecdsa_key ,
733 .Pa /etc/ssh/ssh_host_ed25519_key
735 .Pa /etc/ssh/ssh_host_rsa_key .
739 will refuse to use a file if it is group/world-accessible
741 .Cm HostKeyAlgorithms
742 option restricts which of the keys are actually used by
745 It is possible to have multiple host key files.
746 It is also possible to specify public host key files instead.
747 In this case operations on the private key will be delegated
751 Identifies the UNIX-domain socket used to communicate
752 with an agent that has access to the private host keys.
755 is specified, the location of the socket will be read from the
757 environment variable.
758 .It Cm HostKeyAlgorithms
759 Specifies the host key algorithms
760 that the server offers.
761 The default for this option is:
762 .Bd -literal -offset 3n
763 ecdsa-sha2-nistp256-cert-v01@openssh.com,
764 ecdsa-sha2-nistp384-cert-v01@openssh.com,
765 ecdsa-sha2-nistp521-cert-v01@openssh.com,
766 ssh-ed25519-cert-v01@openssh.com,
767 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
768 ssh-rsa-cert-v01@openssh.com,
769 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
770 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
773 The list of available key types may also be obtained using
780 files will not be used in
781 .Cm HostbasedAuthentication .
785 .Pa /etc/ssh/shosts.equiv
789 .It Cm IgnoreUserKnownHosts
792 should ignore the user's
793 .Pa ~/.ssh/known_hosts
795 .Cm HostbasedAuthentication
796 and use only the system-wide known hosts file
797 .Pa /etc/ssh/known_hosts .
801 Specifies the IPv4 type-of-service or DSCP class for the connection.
829 to use the operating system default.
830 This option may take one or two arguments, separated by whitespace.
831 If one argument is specified, it is used as the packet class unconditionally.
832 If two values are specified, the first is automatically selected for
833 interactive sessions and the second for non-interactive sessions.
837 for interactive sessions and
840 for non-interactive sessions.
841 .It Cm KbdInteractiveAuthentication
842 Specifies whether to allow keyboard-interactive authentication.
843 The argument to this keyword must be
847 The default is to use whatever value
848 .Cm ChallengeResponseAuthentication
852 .It Cm KerberosAuthentication
853 Specifies whether the password provided by the user for
854 .Cm PasswordAuthentication
855 will be validated through the Kerberos KDC.
856 To use this option, the server needs a
857 Kerberos servtab which allows the verification of the KDC's identity.
860 .It Cm KerberosGetAFSToken
861 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
862 an AFS token before accessing the user's home directory.
865 .It Cm KerberosOrLocalPasswd
866 If password authentication through Kerberos fails then
867 the password will be validated via any additional local mechanism
872 .It Cm KerberosTicketCleanup
873 Specifies whether to automatically destroy the user's ticket cache
878 Specifies the available KEX (Key Exchange) algorithms.
879 Multiple algorithms must be comma-separated.
880 Alternately if the specified value begins with a
882 character, then the specified methods will be appended to the default set
883 instead of replacing them.
884 If the specified value begins with a
886 character, then the specified methods (including wildcards) will be removed
887 from the default set instead of replacing them.
888 The supported algorithms are:
890 .Bl -item -compact -offset indent
894 curve25519-sha256@libssh.org
896 diffie-hellman-group1-sha1
898 diffie-hellman-group14-sha1
900 diffie-hellman-group14-sha256
902 diffie-hellman-group16-sha512
904 diffie-hellman-group18-sha512
906 diffie-hellman-group-exchange-sha1
908 diffie-hellman-group-exchange-sha256
918 .Bd -literal -offset indent
919 curve25519-sha256,curve25519-sha256@libssh.org,
920 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
921 diffie-hellman-group-exchange-sha256,
922 diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
923 diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
926 The list of available key exchange algorithms may also be obtained using
929 Specifies the local addresses
932 The following forms may be used:
934 .Bl -item -offset indent -compact
938 .Ar hostname | address
940 .Op Cm rdomain Ar domain
946 .Op Cm rdomain Ar domain
950 .Ar IPv4_address : port
952 .Op Cm rdomain Ar domain
956 .Oo Ar hostname | address Oc : Ar port
958 .Op Cm rdomain Ar domain
965 listen in an explicit routing domain.
969 sshd will listen on the address and all
972 The default is to listen on all local addresses on the current default
976 options are permitted.
977 For more information on routing domains, see
979 .It Cm LoginGraceTime
980 The server disconnects after this time if the user has not
981 successfully logged in.
982 If the value is 0, there is no time limit.
983 The default is 120 seconds.
985 Gives the verbosity level that is used when logging messages from
987 The possible values are:
988 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
990 DEBUG and DEBUG1 are equivalent.
991 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
992 Logging with a DEBUG level violates the privacy of users and is not recommended.
994 Specifies the available MAC (message authentication code) algorithms.
995 The MAC algorithm is used for data integrity protection.
996 Multiple algorithms must be comma-separated.
997 If the specified value begins with a
999 character, then the specified algorithms will be appended to the default set
1000 instead of replacing them.
1001 If the specified value begins with a
1003 character, then the specified algorithms (including wildcards) will be removed
1004 from the default set instead of replacing them.
1006 The algorithms that contain
1008 calculate the MAC after encryption (encrypt-then-mac).
1009 These are considered safer and their use recommended.
1010 The supported MACs are:
1012 .Bl -item -compact -offset indent
1028 umac-128@openssh.com
1030 hmac-md5-etm@openssh.com
1032 hmac-md5-96-etm@openssh.com
1034 hmac-sha1-etm@openssh.com
1036 hmac-sha1-96-etm@openssh.com
1038 hmac-sha2-256-etm@openssh.com
1040 hmac-sha2-512-etm@openssh.com
1042 umac-64-etm@openssh.com
1044 umac-128-etm@openssh.com
1048 .Bd -literal -offset indent
1049 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1050 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1051 hmac-sha1-etm@openssh.com,
1052 umac-64@openssh.com,umac-128@openssh.com,
1053 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1056 The list of available MAC algorithms may also be obtained using
1059 Introduces a conditional block.
1060 If all of the criteria on the
1062 line are satisfied, the keywords on the following lines override those
1063 set in the global section of the config file, until either another
1065 line or the end of the file.
1066 If a keyword appears in multiple
1068 blocks that are satisfied, only the first instance of the keyword is
1073 are one or more criteria-pattern pairs or the single token
1075 which matches all criteria.
1076 The available criteria are
1089 on which the connection was received.)
1091 The match patterns may consist of single entries or comma-separated
1092 lists and may use the wildcard and negation operators described in the
1099 criteria may additionally contain addresses to match in CIDR
1100 address/masklen format,
1101 such as 192.0.2.0/24 or 2001:db8::/32.
1102 Note that the mask length provided must be consistent with the address -
1103 it is an error to specify a mask length that is too long for the address
1104 or one with bits set in this host portion of the address.
1105 For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1107 Only a subset of keywords may be used on the lines following a
1110 Available keywords are
1112 .Cm AllowAgentForwarding ,
1114 .Cm AllowStreamLocalForwarding ,
1115 .Cm AllowTcpForwarding ,
1117 .Cm AuthenticationMethods ,
1118 .Cm AuthorizedKeysCommand ,
1119 .Cm AuthorizedKeysCommandUser ,
1120 .Cm AuthorizedKeysFile ,
1121 .Cm AuthorizedPrincipalsCommand ,
1122 .Cm AuthorizedPrincipalsCommandUser ,
1123 .Cm AuthorizedPrincipalsFile ,
1125 .Cm ChrootDirectory ,
1126 .Cm ClientAliveCountMax ,
1127 .Cm ClientAliveInterval ,
1132 .Cm GSSAPIAuthentication ,
1133 .Cm HostbasedAcceptedKeyTypes ,
1134 .Cm HostbasedAuthentication ,
1135 .Cm HostbasedUsesNameFromPacketOnly ,
1137 .Cm KbdInteractiveAuthentication ,
1138 .Cm KerberosAuthentication ,
1142 .Cm PasswordAuthentication ,
1143 .Cm PermitEmptyPasswords ,
1146 .Cm PermitRootLogin ,
1150 .Cm PubkeyAcceptedKeyTypes ,
1151 .Cm PubkeyAuthentication ,
1156 .Cm StreamLocalBindMask ,
1157 .Cm StreamLocalBindUnlink ,
1158 .Cm TrustedUserCAKeys ,
1159 .Cm X11DisplayOffset ,
1162 .Cm X11UseLocalHost .
1164 Specifies the maximum number of authentication attempts permitted per
1166 Once the number of failures reaches half this value,
1167 additional failures are logged.
1170 Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1171 sessions permitted per network connection.
1172 Multiple sessions may be established by clients that support connection
1176 to 1 will effectively disable session multiplexing, whereas setting it to 0
1177 will prevent all shell, login and subsystem sessions while still permitting
1181 Specifies the maximum number of concurrent unauthenticated connections to the
1183 Additional connections will be dropped until authentication succeeds or the
1185 expires for a connection.
1186 The default is 10:30:100.
1188 Alternatively, random early drop can be enabled by specifying
1189 the three colon separated values
1190 start:rate:full (e.g. "10:30:60").
1192 will refuse connection attempts with a probability of rate/100 (30%)
1193 if there are currently start (10) unauthenticated connections.
1194 The probability increases linearly and all connection attempts
1195 are refused if the number of unauthenticated connections reaches full (60).
1196 .It Cm PasswordAuthentication
1197 Specifies whether password authentication is allowed.
1202 .It Cm PermitEmptyPasswords
1203 When password authentication is allowed, it specifies whether the
1204 server allows login to accounts with empty password strings.
1208 Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1209 The listen specification must be one of the following forms:
1211 .Bl -item -offset indent -compact
1224 Multiple permissions may be specified by separating them with whitespace.
1227 can be used to remove all restrictions and permit any listen requests.
1230 can be used to prohibit all listen requests.
1231 The host name may contain wildcards as described in the PATTERNS section in
1235 can also be used in place of a port number to allow all ports.
1236 By default all port forwarding listen requests are permitted.
1239 option may further restrict which addresses may be listened on.
1242 will request a listen host of
1244 if no listen host was specifically requested, and this this name is
1245 treated differently to explicit localhost addresses of
1250 Specifies the destinations to which TCP port forwarding is permitted.
1251 The forwarding specification must be one of the following forms:
1253 .Bl -item -offset indent -compact
1262 .Ar IPv4_addr : port
1267 .Ar \&[ IPv6_addr \&] : port
1271 Multiple forwards may be specified by separating them with whitespace.
1274 can be used to remove all restrictions and permit any forwarding requests.
1277 can be used to prohibit all forwarding requests.
1280 can be used for host or port to allow all hosts or ports, respectively.
1281 By default all port forwarding requests are permitted.
1282 .It Cm PermitRootLogin
1283 Specifies whether root can log in using
1285 The argument must be
1287 .Cm prohibit-password ,
1288 .Cm forced-commands-only ,
1294 .Cm ChallengeResponseAuthentication
1299 this setting may be overridden by the PAM policy.
1301 If this option is set to
1302 .Cm prohibit-password
1303 (or its deprecated alias,
1304 .Cm without-password ) ,
1305 password and keyboard-interactive authentication are disabled for root.
1307 If this option is set to
1308 .Cm forced-commands-only ,
1309 root login with public key authentication will be allowed,
1312 option has been specified
1313 (which may be useful for taking remote backups even if root login is
1314 normally not allowed).
1315 All other authentication methods are disabled for root.
1317 If this option is set to
1319 root is not allowed to log in.
1323 allocation is permitted.
1329 device forwarding is allowed.
1330 The argument must be
1346 Independent of this setting, the permissions of the selected
1348 device must allow access to the user.
1349 .It Cm PermitUserEnvironment
1351 .Pa ~/.ssh/environment
1355 .Pa ~/.ssh/authorized_keys
1361 or a pattern-list specifying which environment variable names to accept
1366 Enabling environment processing may enable users to bypass access
1367 restrictions in some configurations using mechanisms such as
1370 Specifies whether any
1376 Specifies the file that contains the process ID of the
1381 .Pa /var/run/sshd.pid .
1383 Specifies the port number that
1387 Multiple options of this type are permitted.
1393 should print the date and time of the last user login when a user logs
1402 when a user logs in interactively.
1403 (On some systems it is also printed by the shell,
1408 .It Cm PubkeyAcceptedKeyTypes
1409 Specifies the key types that will be accepted for public key authentication
1410 as a list of comma-separated patterns.
1411 Alternately if the specified value begins with a
1413 character, then the specified key types will be appended to the default set
1414 instead of replacing them.
1415 If the specified value begins with a
1417 character, then the specified key types (including wildcards) will be removed
1418 from the default set instead of replacing them.
1419 The default for this option is:
1420 .Bd -literal -offset 3n
1421 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1422 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1423 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1424 ssh-ed25519-cert-v01@openssh.com,
1425 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
1426 ssh-rsa-cert-v01@openssh.com,
1427 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1428 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1431 The list of available key types may also be obtained using
1433 .It Cm PubkeyAuthentication
1434 Specifies whether public key authentication is allowed.
1438 Specifies the maximum amount of data that may be transmitted before the
1439 session key is renegotiated, optionally followed a maximum amount of
1440 time that may pass before the session key is renegotiated.
1441 The first argument is specified in bytes and may have a suffix of
1446 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1447 The default is between
1451 depending on the cipher.
1452 The optional second value is specified in seconds and may use any of the
1453 units documented in the
1456 The default value for
1460 which means that rekeying is performed after the cipher's default amount
1461 of data has been sent or received and no time based rekeying is done.
1463 Specifies revoked public keys file, or
1466 Keys listed in this file will be refused for public key authentication.
1467 Note that if this file is not readable, then public key authentication will
1468 be refused for all users.
1469 Keys may be specified as a text file, listing one public key per line, or as
1470 an OpenSSH Key Revocation List (KRL) as generated by
1472 For more information on KRLs, see the KEY REVOCATION LISTS section in
1475 Specifies an explicit routing domain that is applied after authentication
1477 The user session, as well and any forwarded or listening IP sockets,
1478 will be bound to this
1480 If the routing domain is set to
1482 then the domain in which the incoming connection was received will be applied.
1484 Specifies one or more environment variables to set in child sessions started
1489 The environment value may be quoted (e.g. if it contains whitespace
1491 Environment variables set by
1493 override the default environment and any variables specified by the user
1497 .Cm PermitUserEnvironment .
1498 .It Cm StreamLocalBindMask
1499 Sets the octal file creation mode mask
1501 used when creating a Unix-domain socket file for local or remote
1503 This option is only used for port forwarding to a Unix-domain socket file.
1505 The default value is 0177, which creates a Unix-domain socket file that is
1506 readable and writable only by the owner.
1507 Note that not all operating systems honor the file mode on Unix-domain
1509 .It Cm StreamLocalBindUnlink
1510 Specifies whether to remove an existing Unix-domain socket file for local
1511 or remote port forwarding before creating a new one.
1512 If the socket file already exists and
1513 .Cm StreamLocalBindUnlink
1516 will be unable to forward the port to the Unix-domain socket file.
1517 This option is only used for port forwarding to a Unix-domain socket file.
1519 The argument must be
1528 should check file modes and ownership of the
1529 user's files and home directory before accepting login.
1530 This is normally desirable because novices sometimes accidentally leave their
1531 directory or files world-writable.
1534 Note that this does not apply to
1535 .Cm ChrootDirectory ,
1536 whose permissions and ownership are checked unconditionally.
1538 Configures an external subsystem (e.g. file transfer daemon).
1539 Arguments should be a subsystem name and a command (with optional arguments)
1540 to execute upon subsystem request.
1544 implements the SFTP file transfer subsystem.
1546 Alternately the name
1548 implements an in-process SFTP server.
1549 This may simplify configurations using
1551 to force a different filesystem root on clients.
1553 By default no subsystems are defined.
1554 .It Cm SyslogFacility
1555 Gives the facility code that is used when logging messages from
1557 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1558 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1559 The default is AUTH.
1561 Specifies whether the system should send TCP keepalive messages to the
1563 If they are sent, death of the connection or crash of one
1564 of the machines will be properly noticed.
1565 However, this means that
1566 connections will die if the route is down temporarily, and some people
1568 On the other hand, if TCP keepalives are not sent,
1569 sessions may hang indefinitely on the server, leaving
1571 users and consuming server resources.
1575 (to send TCP keepalive messages), and the server will notice
1576 if the network goes down or the client host crashes.
1577 This avoids infinitely hanging sessions.
1579 To disable TCP keepalive messages, the value should be set to
1581 .It Cm TrustedUserCAKeys
1582 Specifies a file containing public keys of certificate authorities that are
1583 trusted to sign user certificates for authentication, or
1586 Keys are listed one per line; empty lines and comments starting with
1589 If a certificate is presented for authentication and has its signing CA key
1590 listed in this file, then it may be used for authentication for any user
1591 listed in the certificate's principals list.
1592 Note that certificates that lack a list of principals will not be permitted
1593 for authentication using
1594 .Cm TrustedUserCAKeys .
1595 For more details on certificates, see the CERTIFICATES section in
1600 attempts to send authentication success and failure messages
1606 For forward compatibility with an upcoming
1610 alias can be used instead.
1614 should look up the remote host name, and to check that
1615 the resolved host name for the remote IP address maps back to the
1616 very same IP address.
1618 If this option is set to
1620 then only addresses and not host names may be used in
1621 .Pa ~/.ssh/authorized_keys
1631 Enables the Pluggable Authentication Module interface.
1634 this will enable PAM authentication using
1635 .Cm ChallengeResponseAuthentication
1637 .Cm PasswordAuthentication
1638 in addition to PAM account and session module processing for all
1639 authentication types.
1641 Because PAM challenge-response authentication usually serves an equivalent
1642 role to password authentication, you should disable either
1643 .Cm PasswordAuthentication
1645 .Cm ChallengeResponseAuthentication.
1649 is enabled, you will not be able to run
1654 .It Cm VersionAddendum
1655 Optionally specifies additional text to append to the SSH protocol banner
1656 sent by the server upon connection.
1658 .Qq FreeBSD-20180909 .
1661 may be used to disable this.
1662 .It Cm X11DisplayOffset
1663 Specifies the first display number available for
1666 This prevents sshd from interfering with real X11 servers.
1668 .It Cm X11Forwarding
1669 Specifies whether X11 forwarding is permitted.
1670 The argument must be
1677 When X11 forwarding is enabled, there may be additional exposure to
1678 the server and to client displays if the
1680 proxy display is configured to listen on the wildcard address (see
1681 .Cm X11UseLocalhost ) ,
1682 though this is not the default.
1683 Additionally, the authentication spoofing and authentication data
1684 verification and substitution occur on the client side.
1685 The security risk of using X11 forwarding is that the client's X11
1686 display server may be exposed to attack when the SSH client requests
1687 forwarding (see the warnings for
1690 .Xr ssh_config 5 ) .
1691 A system administrator may have a stance in which they want to
1692 protect clients that may expose themselves to attack by unwittingly
1693 requesting X11 forwarding, which can warrant a
1697 Note that disabling X11 forwarding does not prevent users from
1698 forwarding X11 traffic, as users can always install their own forwarders.
1699 .It Cm X11UseLocalhost
1702 should bind the X11 forwarding server to the loopback address or to
1703 the wildcard address.
1705 sshd binds the forwarding server to the loopback address and sets the
1706 hostname part of the
1708 environment variable to
1710 This prevents remote hosts from connecting to the proxy display.
1711 However, some older X11 clients may not function with this
1716 to specify that the forwarding server should be bound to the wildcard
1718 The argument must be
1724 .It Cm XAuthLocation
1725 Specifies the full pathname of the
1731 .Pa /usr/local/bin/xauth .
1735 command-line arguments and configuration file options that specify time
1736 may be expressed using a sequence of the form:
1738 .Ar time Op Ar qualifier ,
1742 is a positive integer value and
1744 is one of the following:
1746 .Bl -tag -width Ds -compact -offset indent
1761 Each member of the sequence is added together to calculate
1762 the total time value.
1764 Time format examples:
1766 .Bl -tag -width Ds -compact -offset indent
1768 600 seconds (10 minutes)
1772 1 hour 30 minutes (90 minutes)
1775 Arguments to some keywords can make use of tokens,
1776 which are expanded at runtime:
1778 .Bl -tag -width XXXX -offset indent -compact
1783 The routing domain in which the incoming connection was received.
1785 The fingerprint of the CA key.
1787 The fingerprint of the key or certificate.
1789 The home directory of the user.
1791 The key ID in the certificate.
1793 The base64-encoded CA key.
1795 The base64-encoded key or certificate for authentication.
1797 The serial number of the certificate.
1799 The type of the CA key.
1801 The key or certificate type.
1803 The numeric user ID of the target user.
1808 .Cm AuthorizedKeysCommand
1809 accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1811 .Cm AuthorizedKeysFile
1812 accepts the tokens %%, %h, %U, and %u.
1814 .Cm AuthorizedPrincipalsCommand
1815 accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1817 .Cm AuthorizedPrincipalsFile
1818 accepts the tokens %%, %h, %U, and %u.
1821 accepts the tokens %%, %h, %U, and %u.
1824 accepts the token %D.
1827 .It Pa /etc/ssh/sshd_config
1828 Contains configuration data for
1830 This file should be writable by root only, but it is recommended
1831 (though not necessary) that it be world-readable.
1838 OpenSSH is a derivative of the original and free
1839 ssh 1.2.12 release by
1841 .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1845 removed many bugs, re-added newer features and
1848 contributed the support for SSH protocol versions 1.5 and 2.0.
1852 contributed support for privilege separation.
projects / FreeBSD / FreeBSD.git / blob