3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: sshd_config.5,v 1.15 2003/03/28 10:11:43 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon configuration file
45 .Bl -tag -width Ds -compact
46 .It Pa /etc/ssh/sshd_config
50 reads configuration data from
51 .Pa /etc/ssh/sshd_config
52 (or the file specified with
55 The file contains keyword-argument pairs, one per line.
58 and empty lines are interpreted as comments.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
64 .It Cm AFSTokenPassing
65 Specifies whether an AFS token may be forwarded to the server.
69 This keyword can be followed by a list of group name patterns, separated
71 If specified, login is allowed only for users whose primary
72 group or supplementary group list matches one of the patterns.
77 wildcards in the patterns.
78 Only group names are valid; a numerical group ID is not recognized.
79 By default, login is allowed for all groups.
81 .It Cm AllowTcpForwarding
82 Specifies whether TCP forwarding is permitted.
85 Note that disabling TCP forwarding does not improve security unless
86 users are also denied shell access, as they can always install their
90 This keyword can be followed by a list of user name patterns, separated
92 If specified, login is allowed only for user names that
93 match one of the patterns.
98 wildcards in the patterns.
99 Only user names are valid; a numerical user ID is not recognized.
100 By default, login is allowed for all users.
101 If the pattern takes the form USER@HOST then USER and HOST
102 are separately checked, restricting logins to particular
103 users from particular hosts.
105 .It Cm AuthorizedKeysFile
106 Specifies the file that contains the public keys that can be used
107 for user authentication.
108 .Cm AuthorizedKeysFile
109 may contain tokens of the form %T which are substituted during connection
110 set-up. The following tokens are defined: %% is replaced by a literal '%',
111 %h is replaced by the home directory of the user being authenticated and
112 %u is replaced by the username of that user.
114 .Cm AuthorizedKeysFile
115 is taken to be an absolute path or one relative to the user's home
118 .Dq .ssh/authorized_keys .
120 In some jurisdictions, sending a warning message before authentication
121 may be relevant for getting legal protection.
122 The contents of the specified file are sent to the remote user before
123 authentication is allowed.
124 This option is only available for protocol version 2.
125 By default, no banner is displayed.
127 .It Cm ChallengeResponseAuthentication
128 Specifies whether challenge response authentication is allowed.
129 All authentication styles from
135 Specifies the ciphers allowed for protocol version 2.
136 Multiple ciphers must be comma-separated.
140 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
141 aes192-cbc,aes256-cbc''
143 .It Cm ClientAliveInterval
144 Sets a timeout interval in seconds after which if no data has been received
147 will send a message through the encrypted
148 channel to request a response from the client.
150 is 0, indicating that these messages will not be sent to the client.
151 This option applies to protocol version 2 only.
152 .It Cm ClientAliveCountMax
153 Sets the number of client alive messages (see above) which may be
156 receiving any messages back from the client. If this threshold is
157 reached while client alive messages are being sent,
159 will disconnect the client, terminating the session. It is important
160 to note that the use of client alive messages is very different from
162 (below). The client alive messages are sent through the
163 encrypted channel and therefore will not be spoofable. The TCP keepalive
166 is spoofable. The client alive mechanism is valuable when the client or
167 server depend on knowing when a connection has become inactive.
169 The default value is 3. If
170 .Cm ClientAliveInterval
171 (above) is set to 15, and
172 .Cm ClientAliveCountMax
173 is left at the default, unresponsive ssh clients
174 will be disconnected after approximately 45 seconds.
176 Specifies whether compression is allowed.
184 This keyword can be followed by a list of group name patterns, separated
186 Login is disallowed for users whose primary group or supplementary
187 group list matches one of the patterns.
192 wildcards in the patterns.
193 Only group names are valid; a numerical group ID is not recognized.
194 By default, login is allowed for all groups.
197 This keyword can be followed by a list of user name patterns, separated
199 Login is disallowed for user names that match one of the patterns.
203 can be used as wildcards in the patterns.
204 Only user names are valid; a numerical user ID is not recognized.
205 By default, login is allowed for all users.
206 If the pattern takes the form USER@HOST then USER and HOST
207 are separately checked, restricting logins to particular
208 users from particular hosts.
210 Specifies whether remote hosts are allowed to connect to ports
211 forwarded for the client.
214 binds remote port forwardings to the loopback address.
215 This prevents other remote hosts from connecting to forwarded ports.
217 can be used to specify that
219 should bind remote port forwardings to the wildcard address,
220 thus allowing remote hosts to connect to forwarded ports.
227 .It Cm HostbasedAuthentication
228 Specifies whether rhosts or /etc/hosts.equiv authentication together
229 with successful public key client host authentication is allowed
230 (hostbased authentication).
231 This option is similar to
232 .Cm RhostsRSAAuthentication
233 and applies to protocol version 2 only.
237 Specifies a file containing a private host key
240 .Pa /etc/ssh/ssh_host_key
241 for protocol version 1, and
242 .Pa /etc/ssh/ssh_host_rsa_key
244 .Pa /etc/ssh/ssh_host_dsa_key
245 for protocol version 2.
248 will refuse to use a file if it is group/world-accessible.
249 It is possible to have multiple host key files.
251 keys are used for version 1 and
255 are used for version 2 of the SSH protocol.
256 .It Cm GssapiAuthentication
257 Specifies whether authentication based on GSSAPI may be used, either using
258 the result of a successful key exchange, or using GSSAPI user
262 Note that this option applies to protocol version 2 only.
263 .It Cm GssapiKeyExchange
264 Specifies whether key exchange based on GSSAPI may be used. When using
265 GSSAPI key exchange the server need not have a host key.
268 Note that this option applies to protocol version 2 only.
269 .It Cm GssapiUseSessionCredCache
270 Specifies whether a unique credentials cache name should be generated per
271 session for storing delegated credentials.
274 Note that this option applies to protocol version 2 only.
281 files will not be used in
282 .Cm RhostsAuthentication ,
283 .Cm RhostsRSAAuthentication
285 .Cm HostbasedAuthentication .
289 .Pa /etc/shosts.equiv
293 .It Cm IgnoreUserKnownHosts
296 should ignore the user's
297 .Pa $HOME/.ssh/known_hosts
299 .Cm RhostsRSAAuthentication
301 .Cm HostbasedAuthentication .
305 Specifies whether the system should send TCP keepalive messages to the
307 If they are sent, death of the connection or crash of one
308 of the machines will be properly noticed.
309 However, this means that
310 connections will die if the route is down temporarily, and some people
312 On the other hand, if keepalives are not sent,
313 sessions may hang indefinitely on the server, leaving
315 users and consuming server resources.
319 (to send keepalives), and the server will notice
320 if the network goes down or the client host crashes.
321 This avoids infinitely hanging sessions.
323 To disable keepalives, the value should be set to
325 .It Cm KerberosAuthentication
326 Specifies whether Kerberos authentication is allowed.
327 This can be in the form of a Kerberos ticket, or if
328 .Cm PasswordAuthentication
329 is yes, the password provided by the user will be validated through
331 To use this option, the server needs a
332 Kerberos servtab which allows the verification of the KDC's identity.
335 .It Cm KerberosOrLocalPasswd
336 If set then if password authentication through Kerberos fails then
337 the password will be validated via any additional local mechanism
342 .It Cm KerberosTgtPassing
343 Specifies whether a Kerberos TGT may be forwarded to the server.
346 as this only works when the Kerberos KDC is actually an AFS kaserver.
347 .It Cm KerberosTicketCleanup
348 Specifies whether to automatically destroy the user's ticket cache
352 .It Cm KeyRegenerationInterval
353 In protocol version 1, the ephemeral server key is automatically regenerated
354 after this many seconds (if it has been used).
355 The purpose of regeneration is to prevent
356 decrypting captured sessions by later breaking into the machine and
358 The key is never stored anywhere.
359 If the value is 0, the key is never regenerated.
360 The default is 3600 (seconds).
362 Specifies the local addresses
365 The following forms may be used:
367 .Bl -item -offset indent -compact
371 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
376 .Ar host No | Ar IPv4_addr No : Ar port
382 .Ar host No | Ar IPv6_addr Oc : Ar port
390 will listen on the address and all prior
392 options specified. The default is to listen on all local
396 options are permitted. Additionally, any
398 options must precede this option for non port qualified addresses.
399 .It Cm LoginGraceTime
400 The server disconnects after this time if the user has not
401 successfully logged in.
402 If the value is 0, there is no time limit.
403 The default is 120 seconds.
405 Gives the verbosity level that is used when logging messages from
407 The possible values are:
408 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
410 DEBUG and DEBUG1 are equivalent.
411 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
412 Logging with a DEBUG level violates the privacy of users and is not recommended.
414 Specifies the available MAC (message authentication code) algorithms.
415 The MAC algorithm is used in protocol version 2
416 for data integrity protection.
417 Multiple algorithms must be comma-separated.
419 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
421 Specifies the maximum number of concurrent unauthenticated connections to the
424 Additional connections will be dropped until authentication succeeds or the
426 expires for a connection.
429 Alternatively, random early drop can be enabled by specifying
430 the three colon separated values
434 will refuse connection attempts with a probability of
437 if there are currently
440 unauthenticated connections.
441 The probability increases linearly and all connection attempts
442 are refused if the number of unauthenticated connections reaches
445 .It Cm PAMAuthenticationViaKbdInt
446 Specifies whether PAM challenge response authentication is allowed. This
447 allows the use of most PAM challenge response authentication modules, but
448 it will allow password authentication regardless of whether
449 .Cm PasswordAuthentication
451 .It Cm PasswordAuthentication
452 Specifies whether password authentication is allowed.
455 .It Cm PermitEmptyPasswords
456 When password authentication is allowed, it specifies whether the
457 server allows login to accounts with empty password strings.
460 .It Cm PermitRootLogin
461 Specifies whether root can login using
465 .Dq without-password ,
466 .Dq forced-commands-only
472 If this option is set to
474 password authentication is disabled for root.
476 If this option is set to
477 .Dq forced-commands-only
478 root login with public key authentication will be allowed,
481 option has been specified
482 (which may be useful for taking remote backups even if root login is
483 normally not allowed). All other authentication methods are disabled
486 If this option is set to
488 root is not allowed to login.
489 .It Cm PermitUserEnvironment
491 .Pa ~/.ssh/environment
495 .Pa ~/.ssh/authorized_keys
500 Enabling environment processing may enable users to bypass access
501 restrictions in some configurations using mechanisms such as
504 Specifies the file that contains the process ID of the
508 .Pa /var/run/sshd.pid .
510 Specifies the port number that
514 Multiple options of this type are permitted.
520 should print the date and time when the user last logged in.
528 when a user logs in interactively.
529 (On some systems it is also printed by the shell,
535 Specifies the protocol versions
538 The possible values are
542 Multiple versions must be comma-separated.
545 Note that the order of the protocol list does not indicate preference,
546 because the client selects among multiple protocol versions offered
552 .It Cm PubkeyAuthentication
553 Specifies whether public key authentication is allowed.
556 Note that this option applies to protocol version 2 only.
557 .It Cm RhostsAuthentication
558 Specifies whether authentication using rhosts or /etc/hosts.equiv
560 Normally, this method should not be permitted because it is insecure.
561 .Cm RhostsRSAAuthentication
563 instead, because it performs RSA-based host authentication in addition
564 to normal rhosts or /etc/hosts.equiv authentication.
567 This option applies to protocol version 1 only.
568 .It Cm RhostsRSAAuthentication
569 Specifies whether rhosts or /etc/hosts.equiv authentication together
570 with successful RSA host authentication is allowed.
573 This option applies to protocol version 1 only.
574 .It Cm RSAAuthentication
575 Specifies whether pure RSA authentication is allowed.
578 This option applies to protocol version 1 only.
580 Defines the number of bits in the ephemeral protocol version 1 server key.
581 The minimum value is 512, and the default is 768.
585 should check file modes and ownership of the
586 user's files and home directory before accepting login.
587 This is normally desirable because novices sometimes accidentally leave their
588 directory or files world-writable.
592 Configures an external subsystem (e.g., file transfer daemon).
593 Arguments should be a subsystem name and a command to execute upon subsystem
599 file transfer subsystem.
600 By default no subsystems are defined.
601 Note that this option applies to protocol version 2 only.
602 .It Cm SyslogFacility
603 Gives the facility code that is used when logging messages from
605 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
606 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
611 is used for interactive login sessions.
616 is never used for remote command execution.
617 Note also, that if this is enabled,
619 will be disabled because
621 does not know how to handle
625 .Cm UsePrivilegeSeparation
626 is specified, it will be disabled after authentication.
627 .It Cm UsePrivilegeSeparation
630 separates privileges by creating an unprivileged child process
631 to deal with incoming network traffic.
632 After successful authentication, another process will be created that has
633 the privilege of the authenticated user.
634 The goal of privilege separation is to prevent privilege
635 escalation by containing any corruption within the unprivileged processes.
638 .It Cm VerifyReverseMapping
641 should try to verify the remote host name and check that
642 the resolved host name for the remote IP address maps back to the
643 very same IP address.
646 .It Cm X11DisplayOffset
647 Specifies the first display number available for
652 from interfering with real X11 servers.
655 Specifies whether X11 forwarding is permitted.
663 When X11 forwarding is enabled, there may be additional exposure to
664 the server and to client displays if the
666 proxy display is configured to listen on the wildcard address (see
668 below), however this is not the default.
669 Additionally, the authentication spoofing and authentication data
670 verification and substitution occur on the client side.
671 The security risk of using X11 forwarding is that the client's X11
672 display server may be exposed to attack when the ssh client requests
673 forwarding (see the warnings for
677 A system administrator may have a stance in which they want to
678 protect clients that may expose themselves to attack by unwittingly
679 requesting X11 forwarding, which can warrant a
683 Note that disabling X11 forwarding does not prevent users from
684 forwarding X11 traffic, as users can always install their own forwarders.
685 X11 forwarding is automatically disabled if
688 .It Cm X11UseLocalhost
691 should bind the X11 forwarding server to the loopback address or to
692 the wildcard address.
695 binds the forwarding server to the loopback address and sets the
698 environment variable to
700 This prevents remote hosts from connecting to the proxy display.
701 However, some older X11 clients may not function with this
706 to specify that the forwarding server should be bound to the wildcard
715 Specifies the full pathname of the
719 .Pa /usr/X11R6/bin/xauth .
724 command-line arguments and configuration file options that specify time
725 may be expressed using a sequence of the form:
727 .Ar time Op Ar qualifier ,
731 is a positive integer value and
733 is one of the following:
735 .Bl -tag -width Ds -compact -offset indent
750 Each member of the sequence is added together to calculate
751 the total time value.
753 Time format examples:
755 .Bl -tag -width Ds -compact -offset indent
757 600 seconds (10 minutes)
761 1 hour 30 minutes (90 minutes)
765 .It Pa /etc/ssh/sshd_config
766 Contains configuration data for
768 This file should be writable by root only, but it is recommended
769 (though not necessary) that it be world-readable.
772 OpenSSH is a derivative of the original and free
773 ssh 1.2.12 release by Tatu Ylonen.
774 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
775 Theo de Raadt and Dug Song
776 removed many bugs, re-added newer features and
778 Markus Friedl contributed the support for SSH
779 protocol versions 1.5 and 2.0.
780 Niels Provos and Markus Friedl contributed support
781 for privilege separation.