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 1.284 2019/03/22 20:58:34 jmc Exp $
37 .Dd $Mdocdate: March 22 2019 $
42 .Nd OpenSSH SSH daemon configuration file
45 reads configuration data from
46 .Pa /etc/ssh/sshd_config
47 (or the file specified with
50 The file contains keyword-argument pairs, one per line.
51 For each keyword, the first obtained value will be used.
54 and empty lines are interpreted as comments.
55 Arguments may optionally be enclosed in double quotes
57 in order to represent arguments containing spaces.
60 keywords and their meanings are as follows (note that
61 keywords are case-insensitive and arguments are case-sensitive):
64 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
76 environment variable is always accepted whenever the client
77 requests a pseudo-terminal as it is required by the protocol.
78 Variables are specified by name, which may contain the wildcard characters
82 Multiple environment variables may be separated by whitespace or spread
86 Be warned that some environment variables could be used to bypass restricted
88 For this reason, care should be taken in the use of this directive.
89 The default is not to accept any environment variables.
91 Specifies which address family should be used by
100 .It Cm AllowAgentForwarding
103 forwarding is permitted.
106 Note that disabling agent forwarding does not improve security
107 unless users are also denied shell access, as they can always install
108 their own forwarders.
110 This keyword can be followed by a list of group name patterns, separated
112 If specified, login is allowed only for users whose primary
113 group or supplementary group list matches one of the patterns.
114 Only group names are valid; a numerical group ID is not recognized.
115 By default, login is allowed for all groups.
116 The allow/deny directives are processed in the following order:
125 for more information on patterns.
126 .It Cm AllowStreamLocalForwarding
127 Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
128 The available options are
133 to allow StreamLocal forwarding,
135 to prevent all StreamLocal forwarding,
137 to allow local (from the perspective of
141 to allow remote forwarding only.
142 Note that disabling StreamLocal forwarding does not improve security unless
143 users are also denied shell access, as they can always install their
145 .It Cm AllowTcpForwarding
146 Specifies whether TCP forwarding is permitted.
147 The available options are
152 to allow TCP forwarding,
154 to prevent all TCP forwarding,
156 to allow local (from the perspective of
160 to allow remote forwarding only.
161 Note that disabling TCP forwarding does not improve security unless
162 users are also denied shell access, as they can always install their
165 This keyword can be followed by a list of user name patterns, separated
167 If specified, login is allowed only for user names that
168 match one of the patterns.
169 Only user names are valid; a numerical user ID is not recognized.
170 By default, login is allowed for all users.
171 If the pattern takes the form USER@HOST then USER and HOST
172 are separately checked, restricting logins to particular
173 users from particular hosts.
174 HOST criteria may additionally contain addresses to match in CIDR
175 address/masklen format.
176 The allow/deny directives are processed in the following order:
185 for more information on patterns.
186 .It Cm AuthenticationMethods
187 Specifies the authentication methods that must be successfully completed
188 for a user to be granted access.
189 This option must be followed by one or more lists of comma-separated
190 authentication method names, or by the single string
192 to indicate the default behaviour of accepting any single authentication
194 If the default is overridden, then successful authentication requires
195 completion of every method in at least one of these lists.
198 .Qq publickey,password publickey,keyboard-interactive
199 would require the user to complete public key authentication, followed by
200 either password or keyboard interactive authentication.
201 Only methods that are next in one or more lists are offered at each stage,
202 so for this example it would not be possible to attempt password or
203 keyboard-interactive authentication before public key.
205 For keyboard interactive authentication it is also possible to
206 restrict authentication to a specific device by appending a
207 colon followed by the device identifier
211 depending on the server configuration.
213 .Qq keyboard-interactive:bsdauth
214 would restrict keyboard interactive authentication to the
218 If the publickey method is listed more than once,
220 verifies that keys that have been used successfully are not reused for
221 subsequent authentications.
223 .Qq publickey,publickey
224 requires successful authentication using two different public keys.
226 Note that each authentication method listed should also be explicitly enabled
227 in the configuration.
229 The available authentication methods are:
230 .Qq gssapi-with-mic ,
232 .Qq keyboard-interactive ,
234 (used for access to password-less accounts when
235 .Cm PermitEmptyPasswords
240 .It Cm AuthorizedKeysCommand
241 Specifies a program to be used to look up the user's public keys.
242 The program must be owned by root, not writable by group or others and
243 specified by an absolute path.
245 .Cm AuthorizedKeysCommand
246 accept the tokens described in the
249 If no arguments are specified then the username of the target user is used.
251 The program should produce on standard output zero or
252 more lines of authorized_keys output (see
257 .Cm AuthorizedKeysCommand
258 does not successfully authenticate
259 and authorize the user then public key authentication continues using the usual
260 .Cm AuthorizedKeysFile
263 .Cm AuthorizedKeysCommand
265 .It Cm AuthorizedKeysCommandUser
266 Specifies the user under whose account the
267 .Cm AuthorizedKeysCommand
269 It is recommended to use a dedicated user that has no other role on the host
270 than running authorized keys commands.
272 .Cm AuthorizedKeysCommand
274 .Cm AuthorizedKeysCommandUser
277 will refuse to start.
278 .It Cm AuthorizedKeysFile
279 Specifies the file that contains the public keys used for user authentication.
280 The format is described in the
281 .Sx AUTHORIZED_KEYS FILE FORMAT
285 .Cm AuthorizedKeysFile
286 accept the tokens described in the
290 .Cm AuthorizedKeysFile
291 is taken to be an absolute path or one relative to the user's home
293 Multiple files may be listed, separated by whitespace.
294 Alternately this option may be set to
296 to skip checking for user keys in files.
298 .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
299 .It Cm AuthorizedPrincipalsCommand
300 Specifies a program to be used to generate the list of allowed
301 certificate principals as per
302 .Cm AuthorizedPrincipalsFile .
303 The program must be owned by root, not writable by group or others and
304 specified by an absolute path.
306 .Cm AuthorizedPrincipalsCommand
307 accept the tokens described in the
310 If no arguments are specified then the username of the target user is used.
312 The program should produce on standard output zero or
314 .Cm AuthorizedPrincipalsFile
317 .Cm AuthorizedPrincipalsCommand
319 .Cm AuthorizedPrincipalsFile
320 is specified, then certificates offered by the client for authentication
321 must contain a principal that is listed.
323 .Cm AuthorizedPrincipalsCommand
325 .It Cm AuthorizedPrincipalsCommandUser
326 Specifies the user under whose account the
327 .Cm AuthorizedPrincipalsCommand
329 It is recommended to use a dedicated user that has no other role on the host
330 than running authorized principals commands.
332 .Cm AuthorizedPrincipalsCommand
334 .Cm AuthorizedPrincipalsCommandUser
337 will refuse to start.
338 .It Cm AuthorizedPrincipalsFile
339 Specifies a file that lists principal names that are accepted for
340 certificate authentication.
341 When using certificates signed by a key listed in
342 .Cm TrustedUserCAKeys ,
343 this file lists names, one of which must appear in the certificate for it
344 to be accepted for authentication.
345 Names are listed one per line preceded by key options (as described in
346 .Sx AUTHORIZED_KEYS FILE FORMAT
349 Empty lines and comments starting with
354 .Cm AuthorizedPrincipalsFile
355 accept the tokens described in the
359 .Cm AuthorizedPrincipalsFile
360 is taken to be an absolute path or one relative to the user's home directory.
363 i.e. not to use a principals file \(en in this case, the username
364 of the user must appear in a certificate's principals list for it to be
368 .Cm AuthorizedPrincipalsFile
369 is only used when authentication proceeds using a CA listed in
370 .Cm TrustedUserCAKeys
371 and is not consulted for certification authorities trusted via
372 .Pa ~/.ssh/authorized_keys ,
375 key option offers a similar facility (see
379 The contents of the specified file are sent to the remote user before
380 authentication is allowed.
383 then no banner is displayed.
384 By default, no banner is displayed.
385 .It Cm CASignatureAlgorithms
386 Specifies which algorithms are allowed for signing of certificates
387 by certificate authorities (CAs).
389 .Bd -literal -offset indent
390 ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
391 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
394 Certificates signed using other algorithms will not be accepted for
395 public key or host-based authentication.
396 .It Cm ChallengeResponseAuthentication
397 Specifies whether challenge-response authentication is allowed (e.g. via
398 PAM or through authentication styles supported in
402 .It Cm ChrootDirectory
403 Specifies the pathname of a directory to
405 to after authentication.
408 checks that all components of the pathname are root-owned directories
409 which are not writable by any other user or group.
412 changes the working directory to the user's home directory.
415 accept the tokens described in the
421 must contain the necessary files and directories to support the
423 For an interactive session this requires at least a shell, typically
436 For file transfer sessions using SFTP
437 no additional configuration of the environment is necessary if the in-process
439 though sessions which use logging may require
441 inside the chroot directory on some operating systems (see
445 For safety, it is very important that the directory hierarchy be
446 prevented from modification by other processes on the system (especially
447 those outside the jail).
448 Misconfiguration can lead to unsafe environments which
457 Specifies the ciphers allowed.
458 Multiple ciphers must be comma-separated.
459 If the specified value begins with a
461 character, then the specified ciphers will be appended to the default set
462 instead of replacing them.
463 If the specified value begins with a
465 character, then the specified ciphers (including wildcards) will be removed
466 from the default set instead of replacing them.
468 The supported ciphers are:
470 .Bl -item -compact -offset indent
486 aes128-gcm@openssh.com
488 aes256-gcm@openssh.com
490 chacha20-poly1305@openssh.com
494 .Bd -literal -offset indent
495 chacha20-poly1305@openssh.com,
496 aes128-ctr,aes192-ctr,aes256-ctr,
497 aes128-gcm@openssh.com,aes256-gcm@openssh.com
500 The list of available ciphers may also be obtained using
502 .It Cm ClientAliveCountMax
503 Sets the number of client alive messages which may be sent without
505 receiving any messages back from the client.
506 If this threshold is reached while client alive messages are being sent,
507 sshd will disconnect the client, terminating the session.
508 It is important to note that the use of client alive messages is very
511 The client alive messages are sent through the encrypted channel
512 and therefore will not be spoofable.
513 The TCP keepalive option enabled by
516 The client alive mechanism is valuable when the client or
517 server depend on knowing when a connection has become inactive.
519 The default value is 3.
521 .Cm ClientAliveInterval
523 .Cm ClientAliveCountMax
524 is left at the default, unresponsive SSH clients
525 will be disconnected after approximately 45 seconds.
526 .It Cm ClientAliveInterval
527 Sets a timeout interval in seconds after which if no data has been received
530 will send a message through the encrypted
531 channel to request a response from the client.
533 is 0, indicating that these messages will not be sent to the client.
535 Specifies whether compression is enabled after
536 the user has authenticated successfully.
540 (a legacy synonym for
547 This keyword can be followed by a list of group name patterns, separated
549 Login is disallowed for users whose primary group or supplementary
550 group list matches one of the patterns.
551 Only group names are valid; a numerical group ID is not recognized.
552 By default, login is allowed for all groups.
553 The allow/deny directives are processed in the following order:
562 for more information on patterns.
564 This keyword can be followed by a list of user name patterns, separated
566 Login is disallowed for user names that match one of the patterns.
567 Only user names are valid; a numerical user ID is not recognized.
568 By default, login is allowed for all users.
569 If the pattern takes the form USER@HOST then USER and HOST
570 are separately checked, restricting logins to particular
571 users from particular hosts.
572 HOST criteria may additionally contain addresses to match in CIDR
573 address/masklen format.
574 The allow/deny directives are processed in the following order:
583 for more information on patterns.
584 .It Cm DisableForwarding
585 Disables all forwarding features, including X11,
588 This option overrides all other forwarding-related options and may
589 simplify restricted configurations.
590 .It Cm ExposeAuthInfo
591 Writes a temporary file containing a list of authentication methods and
592 public credentials (e.g. keys) used to authenticate the user.
593 The location of the file is exposed to the user session through the
595 environment variable.
598 .It Cm FingerprintHash
599 Specifies the hash algorithm used when logging key fingerprints.
607 Forces the execution of the command specified by
609 ignoring any command supplied by the client and
612 The command is invoked by using the user's login shell with the -c option.
613 This applies to shell, command, or subsystem execution.
614 It is most useful inside a
617 The command originally supplied by the client is available in the
618 .Ev SSH_ORIGINAL_COMMAND
619 environment variable.
620 Specifying a command of
622 will force the use of an in-process SFTP server that requires no support
624 .Cm ChrootDirectory .
628 Specifies whether remote hosts are allowed to connect to ports
629 forwarded for the client.
632 binds remote port forwardings to the loopback address.
633 This prevents other remote hosts from connecting to forwarded ports.
635 can be used to specify that sshd
636 should allow remote port forwardings to bind to non-loopback addresses, thus
637 allowing other hosts to connect.
640 to force remote port forwardings to be available to the local host only,
642 to force remote port forwardings to bind to the wildcard address, or
644 to allow the client to select the address to which the forwarding is bound.
647 .It Cm GSSAPIAuthentication
648 Specifies whether user authentication based on GSSAPI is allowed.
651 .It Cm GSSAPICleanupCredentials
652 Specifies whether to automatically destroy the user's credentials cache
656 .It Cm GSSAPIStrictAcceptorCheck
657 Determines whether to be strict about the identity of the GSSAPI acceptor
658 a client authenticates against.
661 then the client must authenticate against the host
662 service on the current hostname.
665 then the client may authenticate against any service key stored in the
666 machine's default store.
667 This facility is provided to assist with operation on multi homed machines.
670 .It Cm HostbasedAcceptedKeyTypes
671 Specifies the key types that will be accepted for hostbased authentication
672 as a list of comma-separated patterns.
673 Alternately if the specified value begins with a
675 character, then the specified key types will be appended to the default set
676 instead of replacing them.
677 If the specified value begins with a
679 character, then the specified key types (including wildcards) will be removed
680 from the default set instead of replacing them.
681 The default for this option is:
682 .Bd -literal -offset 3n
683 ecdsa-sha2-nistp256-cert-v01@openssh.com,
684 ecdsa-sha2-nistp384-cert-v01@openssh.com,
685 ecdsa-sha2-nistp521-cert-v01@openssh.com,
686 ssh-ed25519-cert-v01@openssh.com,
687 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
688 ssh-rsa-cert-v01@openssh.com,
689 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
690 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
693 The list of available key types may also be obtained using
695 .It Cm HostbasedAuthentication
696 Specifies whether rhosts or /etc/hosts.equiv authentication together
697 with successful public key client host authentication is allowed
698 (host-based authentication).
701 .It Cm HostbasedUsesNameFromPacketOnly
702 Specifies whether or not the server will attempt to perform a reverse
703 name lookup when matching the name in the
709 .Cm HostbasedAuthentication .
714 uses the name supplied by the client rather than
715 attempting to resolve the name from the TCP connection itself.
718 .It Cm HostCertificate
719 Specifies a file containing a public host certificate.
720 The certificate's public key must match a private host key already specified
723 The default behaviour of
725 is not to load any certificates.
727 Specifies a file containing a private host key
730 .Pa /etc/ssh/ssh_host_ecdsa_key ,
731 .Pa /etc/ssh/ssh_host_ed25519_key
733 .Pa /etc/ssh/ssh_host_rsa_key .
737 will refuse to use a file if it is group/world-accessible
739 .Cm HostKeyAlgorithms
740 option restricts which of the keys are actually used by
743 It is possible to have multiple host key files.
744 It is also possible to specify public host key files instead.
745 In this case operations on the private key will be delegated
749 Identifies the UNIX-domain socket used to communicate
750 with an agent that has access to the private host keys.
753 is specified, the location of the socket will be read from the
755 environment variable.
756 .It Cm HostKeyAlgorithms
757 Specifies the host key algorithms
758 that the server offers.
759 The default for this option is:
760 .Bd -literal -offset 3n
761 ecdsa-sha2-nistp256-cert-v01@openssh.com,
762 ecdsa-sha2-nistp384-cert-v01@openssh.com,
763 ecdsa-sha2-nistp521-cert-v01@openssh.com,
764 ssh-ed25519-cert-v01@openssh.com,
765 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
766 ssh-rsa-cert-v01@openssh.com,
767 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
768 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
771 The list of available key types may also be obtained using
778 files will not be used in
779 .Cm HostbasedAuthentication .
783 .Pa /etc/shosts.equiv
787 .It Cm IgnoreUserKnownHosts
790 should ignore the user's
791 .Pa ~/.ssh/known_hosts
793 .Cm HostbasedAuthentication
794 and use only the system-wide known hosts file
795 .Pa /etc/ssh/known_hosts .
799 Specifies the IPv4 type-of-service or DSCP class for the connection.
827 to use the operating system default.
828 This option may take one or two arguments, separated by whitespace.
829 If one argument is specified, it is used as the packet class unconditionally.
830 If two values are specified, the first is automatically selected for
831 interactive sessions and the second for non-interactive sessions.
835 for interactive sessions and
838 for non-interactive sessions.
839 .It Cm KbdInteractiveAuthentication
840 Specifies whether to allow keyboard-interactive authentication.
841 The argument to this keyword must be
845 The default is to use whatever value
846 .Cm ChallengeResponseAuthentication
850 .It Cm KerberosAuthentication
851 Specifies whether the password provided by the user for
852 .Cm PasswordAuthentication
853 will be validated through the Kerberos KDC.
854 To use this option, the server needs a
855 Kerberos servtab which allows the verification of the KDC's identity.
858 .It Cm KerberosGetAFSToken
859 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
860 an AFS token before accessing the user's home directory.
863 .It Cm KerberosOrLocalPasswd
864 If password authentication through Kerberos fails then
865 the password will be validated via any additional local mechanism
870 .It Cm KerberosTicketCleanup
871 Specifies whether to automatically destroy the user's ticket cache
876 Specifies the available KEX (Key Exchange) algorithms.
877 Multiple algorithms must be comma-separated.
878 Alternately if the specified value begins with a
880 character, then the specified methods will be appended to the default set
881 instead of replacing them.
882 If the specified value begins with a
884 character, then the specified methods (including wildcards) will be removed
885 from the default set instead of replacing them.
886 The supported algorithms are:
888 .Bl -item -compact -offset indent
892 curve25519-sha256@libssh.org
894 diffie-hellman-group1-sha1
896 diffie-hellman-group14-sha1
898 diffie-hellman-group14-sha256
900 diffie-hellman-group16-sha512
902 diffie-hellman-group18-sha512
904 diffie-hellman-group-exchange-sha1
906 diffie-hellman-group-exchange-sha256
916 .Bd -literal -offset indent
917 curve25519-sha256,curve25519-sha256@libssh.org,
918 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
919 diffie-hellman-group-exchange-sha256,
920 diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
921 diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
924 The list of available key exchange algorithms may also be obtained using
927 Specifies the local addresses
930 The following forms may be used:
932 .Bl -item -offset indent -compact
936 .Ar hostname | address
938 .Op Cm rdomain Ar domain
944 .Op Cm rdomain Ar domain
948 .Ar IPv4_address : port
950 .Op Cm rdomain Ar domain
954 .Oo Ar hostname | address Oc : Ar port
956 .Op Cm rdomain Ar domain
963 listen in an explicit routing domain.
967 sshd will listen on the address and all
970 The default is to listen on all local addresses on the current default
974 options are permitted.
975 For more information on routing domains, see
977 .It Cm LoginGraceTime
978 The server disconnects after this time if the user has not
979 successfully logged in.
980 If the value is 0, there is no time limit.
981 The default is 120 seconds.
983 Gives the verbosity level that is used when logging messages from
985 The possible values are:
986 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
988 DEBUG and DEBUG1 are equivalent.
989 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
990 Logging with a DEBUG level violates the privacy of users and is not recommended.
992 Specifies the available MAC (message authentication code) algorithms.
993 The MAC algorithm is used for data integrity protection.
994 Multiple algorithms must be comma-separated.
995 If the specified value begins with a
997 character, then the specified algorithms will be appended to the default set
998 instead of replacing them.
999 If the specified value begins with a
1001 character, then the specified algorithms (including wildcards) will be removed
1002 from the default set instead of replacing them.
1004 The algorithms that contain
1006 calculate the MAC after encryption (encrypt-then-mac).
1007 These are considered safer and their use recommended.
1008 The supported MACs are:
1010 .Bl -item -compact -offset indent
1026 umac-128@openssh.com
1028 hmac-md5-etm@openssh.com
1030 hmac-md5-96-etm@openssh.com
1032 hmac-sha1-etm@openssh.com
1034 hmac-sha1-96-etm@openssh.com
1036 hmac-sha2-256-etm@openssh.com
1038 hmac-sha2-512-etm@openssh.com
1040 umac-64-etm@openssh.com
1042 umac-128-etm@openssh.com
1046 .Bd -literal -offset indent
1047 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1048 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1049 hmac-sha1-etm@openssh.com,
1050 umac-64@openssh.com,umac-128@openssh.com,
1051 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1054 The list of available MAC algorithms may also be obtained using
1057 Introduces a conditional block.
1058 If all of the criteria on the
1060 line are satisfied, the keywords on the following lines override those
1061 set in the global section of the config file, until either another
1063 line or the end of the file.
1064 If a keyword appears in multiple
1066 blocks that are satisfied, only the first instance of the keyword is
1071 are one or more criteria-pattern pairs or the single token
1073 which matches all criteria.
1074 The available criteria are
1087 on which the connection was received).
1089 The match patterns may consist of single entries or comma-separated
1090 lists and may use the wildcard and negation operators described in the
1097 criteria may additionally contain addresses to match in CIDR
1098 address/masklen format,
1099 such as 192.0.2.0/24 or 2001:db8::/32.
1100 Note that the mask length provided must be consistent with the address -
1101 it is an error to specify a mask length that is too long for the address
1102 or one with bits set in this host portion of the address.
1103 For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1105 Only a subset of keywords may be used on the lines following a
1108 Available keywords are
1110 .Cm AllowAgentForwarding ,
1112 .Cm AllowStreamLocalForwarding ,
1113 .Cm AllowTcpForwarding ,
1115 .Cm AuthenticationMethods ,
1116 .Cm AuthorizedKeysCommand ,
1117 .Cm AuthorizedKeysCommandUser ,
1118 .Cm AuthorizedKeysFile ,
1119 .Cm AuthorizedPrincipalsCommand ,
1120 .Cm AuthorizedPrincipalsCommandUser ,
1121 .Cm AuthorizedPrincipalsFile ,
1123 .Cm ChrootDirectory ,
1124 .Cm ClientAliveCountMax ,
1125 .Cm ClientAliveInterval ,
1130 .Cm GSSAPIAuthentication ,
1131 .Cm HostbasedAcceptedKeyTypes ,
1132 .Cm HostbasedAuthentication ,
1133 .Cm HostbasedUsesNameFromPacketOnly ,
1135 .Cm KbdInteractiveAuthentication ,
1136 .Cm KerberosAuthentication ,
1140 .Cm PasswordAuthentication ,
1141 .Cm PermitEmptyPasswords ,
1144 .Cm PermitRootLogin ,
1148 .Cm PubkeyAcceptedKeyTypes ,
1149 .Cm PubkeyAuthentication ,
1154 .Cm StreamLocalBindMask ,
1155 .Cm StreamLocalBindUnlink ,
1156 .Cm TrustedUserCAKeys ,
1157 .Cm X11DisplayOffset ,
1160 .Cm X11UseLocalHost .
1162 Specifies the maximum number of authentication attempts permitted per
1164 Once the number of failures reaches half this value,
1165 additional failures are logged.
1168 Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1169 sessions permitted per network connection.
1170 Multiple sessions may be established by clients that support connection
1174 to 1 will effectively disable session multiplexing, whereas setting it to 0
1175 will prevent all shell, login and subsystem sessions while still permitting
1179 Specifies the maximum number of concurrent unauthenticated connections to the
1181 Additional connections will be dropped until authentication succeeds or the
1183 expires for a connection.
1184 The default is 10:30:100.
1186 Alternatively, random early drop can be enabled by specifying
1187 the three colon separated values
1188 start:rate:full (e.g. "10:30:60").
1190 will refuse connection attempts with a probability of rate/100 (30%)
1191 if there are currently start (10) unauthenticated connections.
1192 The probability increases linearly and all connection attempts
1193 are refused if the number of unauthenticated connections reaches full (60).
1194 .It Cm PasswordAuthentication
1195 Specifies whether password authentication is allowed.
1198 .It Cm PermitEmptyPasswords
1199 When password authentication is allowed, it specifies whether the
1200 server allows login to accounts with empty password strings.
1204 Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1205 The listen specification must be one of the following forms:
1207 .Bl -item -offset indent -compact
1220 Multiple permissions may be specified by separating them with whitespace.
1223 can be used to remove all restrictions and permit any listen requests.
1226 can be used to prohibit all listen requests.
1227 The host name may contain wildcards as described in the PATTERNS section in
1231 can also be used in place of a port number to allow all ports.
1232 By default all port forwarding listen requests are permitted.
1235 option may further restrict which addresses may be listened on.
1238 will request a listen host of
1240 if no listen host was specifically requested, and this name is
1241 treated differently to explicit localhost addresses of
1246 Specifies the destinations to which TCP port forwarding is permitted.
1247 The forwarding specification must be one of the following forms:
1249 .Bl -item -offset indent -compact
1258 .Ar IPv4_addr : port
1263 .Ar \&[ IPv6_addr \&] : port
1267 Multiple forwards may be specified by separating them with whitespace.
1270 can be used to remove all restrictions and permit any forwarding requests.
1273 can be used to prohibit all forwarding requests.
1276 can be used for host or port to allow all hosts or ports, respectively.
1277 By default all port forwarding requests are permitted.
1278 .It Cm PermitRootLogin
1279 Specifies whether root can log in using
1281 The argument must be
1283 .Cm prohibit-password ,
1284 .Cm forced-commands-only ,
1288 .Cm prohibit-password .
1290 If this option is set to
1291 .Cm prohibit-password
1292 (or its deprecated alias,
1293 .Cm without-password ) ,
1294 password and keyboard-interactive authentication are disabled for root.
1296 If this option is set to
1297 .Cm forced-commands-only ,
1298 root login with public key authentication will be allowed,
1301 option has been specified
1302 (which may be useful for taking remote backups even if root login is
1303 normally not allowed).
1304 All other authentication methods are disabled for root.
1306 If this option is set to
1308 root is not allowed to log in.
1312 allocation is permitted.
1318 device forwarding is allowed.
1319 The argument must be
1335 Independent of this setting, the permissions of the selected
1337 device must allow access to the user.
1338 .It Cm PermitUserEnvironment
1340 .Pa ~/.ssh/environment
1344 .Pa ~/.ssh/authorized_keys
1350 or a pattern-list specifying which environment variable names to accept
1355 Enabling environment processing may enable users to bypass access
1356 restrictions in some configurations using mechanisms such as
1359 Specifies whether any
1365 Specifies the file that contains the process ID of the
1370 .Pa /var/run/sshd.pid .
1372 Specifies the port number that
1376 Multiple options of this type are permitted.
1382 should print the date and time of the last user login when a user logs
1391 when a user logs in interactively.
1392 (On some systems it is also printed by the shell,
1397 .It Cm PubkeyAcceptedKeyTypes
1398 Specifies the key types that will be accepted for public key authentication
1399 as a list of comma-separated patterns.
1400 Alternately if the specified value begins with a
1402 character, then the specified key types will be appended to the default set
1403 instead of replacing them.
1404 If the specified value begins with a
1406 character, then the specified key types (including wildcards) will be removed
1407 from the default set instead of replacing them.
1408 The default for this option is:
1409 .Bd -literal -offset 3n
1410 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1411 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1412 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1413 ssh-ed25519-cert-v01@openssh.com,
1414 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
1415 ssh-rsa-cert-v01@openssh.com,
1416 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1417 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1420 The list of available key types may also be obtained using
1422 .It Cm PubkeyAuthentication
1423 Specifies whether public key authentication is allowed.
1427 Specifies the maximum amount of data that may be transmitted before the
1428 session key is renegotiated, optionally followed a maximum amount of
1429 time that may pass before the session key is renegotiated.
1430 The first argument is specified in bytes and may have a suffix of
1435 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1436 The default is between
1440 depending on the cipher.
1441 The optional second value is specified in seconds and may use any of the
1442 units documented in the
1445 The default value for
1449 which means that rekeying is performed after the cipher's default amount
1450 of data has been sent or received and no time based rekeying is done.
1452 Specifies revoked public keys file, or
1455 Keys listed in this file will be refused for public key authentication.
1456 Note that if this file is not readable, then public key authentication will
1457 be refused for all users.
1458 Keys may be specified as a text file, listing one public key per line, or as
1459 an OpenSSH Key Revocation List (KRL) as generated by
1461 For more information on KRLs, see the KEY REVOCATION LISTS section in
1464 Specifies an explicit routing domain that is applied after authentication
1466 The user session, as well and any forwarded or listening IP sockets,
1467 will be bound to this
1469 If the routing domain is set to
1471 then the domain in which the incoming connection was received will be applied.
1473 Specifies one or more environment variables to set in child sessions started
1478 The environment value may be quoted (e.g. if it contains whitespace
1480 Environment variables set by
1482 override the default environment and any variables specified by the user
1486 .Cm PermitUserEnvironment .
1487 .It Cm StreamLocalBindMask
1488 Sets the octal file creation mode mask
1490 used when creating a Unix-domain socket file for local or remote
1492 This option is only used for port forwarding to a Unix-domain socket file.
1494 The default value is 0177, which creates a Unix-domain socket file that is
1495 readable and writable only by the owner.
1496 Note that not all operating systems honor the file mode on Unix-domain
1498 .It Cm StreamLocalBindUnlink
1499 Specifies whether to remove an existing Unix-domain socket file for local
1500 or remote port forwarding before creating a new one.
1501 If the socket file already exists and
1502 .Cm StreamLocalBindUnlink
1505 will be unable to forward the port to the Unix-domain socket file.
1506 This option is only used for port forwarding to a Unix-domain socket file.
1508 The argument must be
1517 should check file modes and ownership of the
1518 user's files and home directory before accepting login.
1519 This is normally desirable because novices sometimes accidentally leave their
1520 directory or files world-writable.
1523 Note that this does not apply to
1524 .Cm ChrootDirectory ,
1525 whose permissions and ownership are checked unconditionally.
1527 Configures an external subsystem (e.g. file transfer daemon).
1528 Arguments should be a subsystem name and a command (with optional arguments)
1529 to execute upon subsystem request.
1533 implements the SFTP file transfer subsystem.
1535 Alternately the name
1537 implements an in-process SFTP server.
1538 This may simplify configurations using
1540 to force a different filesystem root on clients.
1542 By default no subsystems are defined.
1543 .It Cm SyslogFacility
1544 Gives the facility code that is used when logging messages from
1546 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1547 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1548 The default is AUTH.
1550 Specifies whether the system should send TCP keepalive messages to the
1552 If they are sent, death of the connection or crash of one
1553 of the machines will be properly noticed.
1554 However, this means that
1555 connections will die if the route is down temporarily, and some people
1557 On the other hand, if TCP keepalives are not sent,
1558 sessions may hang indefinitely on the server, leaving
1560 users and consuming server resources.
1564 (to send TCP keepalive messages), and the server will notice
1565 if the network goes down or the client host crashes.
1566 This avoids infinitely hanging sessions.
1568 To disable TCP keepalive messages, the value should be set to
1570 .It Cm TrustedUserCAKeys
1571 Specifies a file containing public keys of certificate authorities that are
1572 trusted to sign user certificates for authentication, or
1575 Keys are listed one per line; empty lines and comments starting with
1578 If a certificate is presented for authentication and has its signing CA key
1579 listed in this file, then it may be used for authentication for any user
1580 listed in the certificate's principals list.
1581 Note that certificates that lack a list of principals will not be permitted
1582 for authentication using
1583 .Cm TrustedUserCAKeys .
1584 For more details on certificates, see the CERTIFICATES section in
1589 should look up the remote host name, and to check that
1590 the resolved host name for the remote IP address maps back to the
1591 very same IP address.
1593 If this option is set to
1595 (the default) then only addresses and not host names may be used in
1596 .Pa ~/.ssh/authorized_keys
1604 Enables the Pluggable Authentication Module interface.
1607 this will enable PAM authentication using
1608 .Cm ChallengeResponseAuthentication
1610 .Cm PasswordAuthentication
1611 in addition to PAM account and session module processing for all
1612 authentication types.
1614 Because PAM challenge-response authentication usually serves an equivalent
1615 role to password authentication, you should disable either
1616 .Cm PasswordAuthentication
1618 .Cm ChallengeResponseAuthentication.
1622 is enabled, you will not be able to run
1627 .It Cm VersionAddendum
1628 Optionally specifies additional text to append to the SSH protocol banner
1629 sent by the server upon connection.
1632 .It Cm X11DisplayOffset
1633 Specifies the first display number available for
1636 This prevents sshd from interfering with real X11 servers.
1638 .It Cm X11Forwarding
1639 Specifies whether X11 forwarding is permitted.
1640 The argument must be
1647 When X11 forwarding is enabled, there may be additional exposure to
1648 the server and to client displays if the
1650 proxy display is configured to listen on the wildcard address (see
1651 .Cm X11UseLocalhost ) ,
1652 though this is not the default.
1653 Additionally, the authentication spoofing and authentication data
1654 verification and substitution occur on the client side.
1655 The security risk of using X11 forwarding is that the client's X11
1656 display server may be exposed to attack when the SSH client requests
1657 forwarding (see the warnings for
1660 .Xr ssh_config 5 ) .
1661 A system administrator may have a stance in which they want to
1662 protect clients that may expose themselves to attack by unwittingly
1663 requesting X11 forwarding, which can warrant a
1667 Note that disabling X11 forwarding does not prevent users from
1668 forwarding X11 traffic, as users can always install their own forwarders.
1669 .It Cm X11UseLocalhost
1672 should bind the X11 forwarding server to the loopback address or to
1673 the wildcard address.
1675 sshd binds the forwarding server to the loopback address and sets the
1676 hostname part of the
1678 environment variable to
1680 This prevents remote hosts from connecting to the proxy display.
1681 However, some older X11 clients may not function with this
1686 to specify that the forwarding server should be bound to the wildcard
1688 The argument must be
1694 .It Cm XAuthLocation
1695 Specifies the full pathname of the
1701 .Pa /usr/X11R6/bin/xauth .
1705 command-line arguments and configuration file options that specify time
1706 may be expressed using a sequence of the form:
1708 .Ar time Op Ar qualifier ,
1712 is a positive integer value and
1714 is one of the following:
1716 .Bl -tag -width Ds -compact -offset indent
1731 Each member of the sequence is added together to calculate
1732 the total time value.
1734 Time format examples:
1736 .Bl -tag -width Ds -compact -offset indent
1738 600 seconds (10 minutes)
1742 1 hour 30 minutes (90 minutes)
1745 Arguments to some keywords can make use of tokens,
1746 which are expanded at runtime:
1748 .Bl -tag -width XXXX -offset indent -compact
1753 The routing domain in which the incoming connection was received.
1755 The fingerprint of the CA key.
1757 The fingerprint of the key or certificate.
1759 The home directory of the user.
1761 The key ID in the certificate.
1763 The base64-encoded CA key.
1765 The base64-encoded key or certificate for authentication.
1767 The serial number of the certificate.
1769 The type of the CA key.
1771 The key or certificate type.
1773 The numeric user ID of the target user.
1778 .Cm AuthorizedKeysCommand
1779 accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1781 .Cm AuthorizedKeysFile
1782 accepts the tokens %%, %h, %U, and %u.
1784 .Cm AuthorizedPrincipalsCommand
1785 accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1787 .Cm AuthorizedPrincipalsFile
1788 accepts the tokens %%, %h, %U, and %u.
1791 accepts the tokens %%, %h, %U, and %u.
1794 accepts the token %D.
1797 .It Pa /etc/ssh/sshd_config
1798 Contains configuration data for
1800 This file should be writable by root only, but it is recommended
1801 (though not necessary) that it be world-readable.
1808 OpenSSH is a derivative of the original and free
1809 ssh 1.2.12 release by
1811 .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1815 removed many bugs, re-added newer features and
1818 contributed the support for SSH protocol versions 1.5 and 2.0.
1822 contributed support for privilege separation.