Update files for OpenSSH-5.9p1 import.
[dragonfly.git] / crypto / openssh / sshd_config.5
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1.\"
2.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4.\" All rights reserved
5.\"
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".
11.\"
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.
15.\"
16.\" Redistribution and use in source and binary forms, with or without
17.\" modification, are permitted provided that the following conditions
18.\" are met:
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.
24.\"
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.
35.\"
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36.\" $OpenBSD: sshd_config.5,v 1.135 2011/08/02 01:22:11 djm Exp $
37.Dd $Mdocdate: August 2 2011 $
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38.Dt SSHD_CONFIG 5
39.Os
40.Sh NAME
41.Nm sshd_config
42.Nd OpenSSH SSH daemon configuration file
43.Sh SYNOPSIS
44.Nm /etc/ssh/sshd_config
45.Sh DESCRIPTION
46.Xr sshd 8
47reads configuration data from
48.Pa /etc/ssh/sshd_config
49(or the file specified with
50.Fl f
51on the command line).
52The file contains keyword-argument pairs, one per line.
53Lines starting with
54.Ql #
55and empty lines are interpreted as comments.
56Arguments may optionally be enclosed in double quotes
57.Pq \&"
58in order to represent arguments containing spaces.
59.Pp
60The possible
61keywords and their meanings are as follows (note that
62keywords are case-insensitive and arguments are case-sensitive):
63.Bl -tag -width Ds
64.It Cm AcceptEnv
65Specifies what environment variables sent by the client will be copied into
66the session's
67.Xr environ 7 .
68See
69.Cm SendEnv
70in
71.Xr ssh_config 5
72for how to configure the client.
73Note that environment passing is only supported for protocol 2.
74Variables are specified by name, which may contain the wildcard characters
75.Ql *
76and
77.Ql \&? .
78Multiple environment variables may be separated by whitespace or spread
79across multiple
80.Cm AcceptEnv
81directives.
82Be warned that some environment variables could be used to bypass restricted
83user environments.
84For this reason, care should be taken in the use of this directive.
85The default is not to accept any environment variables.
86.It Cm AddressFamily
87Specifies which address family should be used by
88.Xr sshd 8 .
89Valid arguments are
90.Dq any ,
91.Dq inet
92(use IPv4 only), or
93.Dq inet6
94(use IPv6 only).
95The default is
96.Dq any .
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97.It Cm AllowAgentForwarding
98Specifies whether
99.Xr ssh-agent 1
100forwarding is permitted.
101The default is
102.Dq yes .
103Note that disabling agent forwarding does not improve security
104unless users are also denied shell access, as they can always install
105their own forwarders.
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106.It Cm AllowGroups
107This keyword can be followed by a list of group name patterns, separated
108by spaces.
109If specified, login is allowed only for users whose primary
110group or supplementary group list matches one of the patterns.
111Only group names are valid; a numerical group ID is not recognized.
112By default, login is allowed for all groups.
113The allow/deny directives are processed in the following order:
114.Cm DenyUsers ,
115.Cm AllowUsers ,
116.Cm DenyGroups ,
117and finally
118.Cm AllowGroups .
119.Pp
120See
121.Sx PATTERNS
122in
123.Xr ssh_config 5
124for more information on patterns.
125.It Cm AllowTcpForwarding
126Specifies whether TCP forwarding is permitted.
127The default is
128.Dq yes .
129Note that disabling TCP forwarding does not improve security unless
130users are also denied shell access, as they can always install their
131own forwarders.
132.It Cm AllowUsers
133This keyword can be followed by a list of user name patterns, separated
134by spaces.
135If specified, login is allowed only for user names that
136match one of the patterns.
137Only user names are valid; a numerical user ID is not recognized.
138By default, login is allowed for all users.
139If the pattern takes the form USER@HOST then USER and HOST
140are separately checked, restricting logins to particular
141users from particular hosts.
142The allow/deny directives are processed in the following order:
143.Cm DenyUsers ,
144.Cm AllowUsers ,
145.Cm DenyGroups ,
146and finally
147.Cm AllowGroups .
148.Pp
149See
150.Sx PATTERNS
151in
152.Xr ssh_config 5
153for more information on patterns.
154.It Cm AuthorizedKeysFile
155Specifies the file that contains the public keys that can be used
156for user authentication.
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157The format is described in the
158.Sx AUTHORIZED_KEYS FILE FORMAT
159section of
160.Xr sshd 8 .
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161.Cm AuthorizedKeysFile
162may contain tokens of the form %T which are substituted during connection
163setup.
164The following tokens are defined: %% is replaced by a literal '%',
165%h is replaced by the home directory of the user being authenticated, and
166%u is replaced by the username of that user.
167After expansion,
168.Cm AuthorizedKeysFile
169is taken to be an absolute path or one relative to the user's home
170directory.
1c188a7f 171Multiple files may be listed, separated by whitespace.
16c343f1 172The default is
1c188a7f 173.Dq .ssh/authorized_keys .ssh/authorized_keys2 .
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174.It Cm AuthorizedPrincipalsFile
175Specifies a file that lists principal names that are accepted for
176certificate authentication.
177When using certificates signed by a key listed in
178.Cm TrustedUserCAKeys ,
179this file lists names, one of which must appear in the certificate for it
180to be accepted for authentication.
181Names are listed one per line preceded by key options (as described
182in
183.Sx AUTHORIZED_KEYS FILE FORMAT
184in
185.Xr sshd 8 ) .
186Empty lines and comments starting with
187.Ql #
188are ignored.
189.Pp
190.Cm AuthorizedPrincipalsFile
191may contain tokens of the form %T which are substituted during connection
192setup.
193The following tokens are defined: %% is replaced by a literal '%',
194%h is replaced by the home directory of the user being authenticated, and
195%u is replaced by the username of that user.
196After expansion,
197.Cm AuthorizedPrincipalsFile
198is taken to be an absolute path or one relative to the user's home
199directory.
200.Pp
201The default is not to use a principals file \(en in this case, the username
202of the user must appear in a certificate's principals list for it to be
203accepted.
204Note that
205.Cm AuthorizedPrincipalsFile
206is only used when authentication proceeds using a CA listed in
207.Cm TrustedUserCAKeys
208and is not consulted for certification authorities trusted via
209.Pa ~/.ssh/authorized_keys ,
210though the
211.Cm principals=
212key option offers a similar facility (see
213.Xr sshd 8
214for details).
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215.It Cm Banner
216The contents of the specified file are sent to the remote user before
217authentication is allowed.
218If the argument is
219.Dq none
220then no banner is displayed.
221This option is only available for protocol version 2.
222By default, no banner is displayed.
223.It Cm ChallengeResponseAuthentication
224Specifies whether challenge-response authentication is allowed.
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225Specifically, in
226.Dx ,
227this controls the use of PAM (see
228.Xr pam 3 )
229for authentication.
230Note that this affects the effectiveness of the
231.Cm PasswordAuthentication
232and
233.Cm PermitRootLogin
234variables.
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235The default is
236.Dq yes .
237.It Cm ChrootDirectory
856ea928 238Specifies the pathname of a directory to
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239.Xr chroot 2
240to after authentication.
856ea928 241All components of the pathname must be root-owned directories that are
16c343f1 242not writable by any other user or group.
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243After the chroot,
244.Xr sshd 8
245changes the working directory to the user's home directory.
16c343f1 246.Pp
856ea928 247The pathname may contain the following tokens that are expanded at runtime once
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248the connecting user has been authenticated: %% is replaced by a literal '%',
249%h is replaced by the home directory of the user being authenticated, and
250%u is replaced by the username of that user.
251.Pp
252The
253.Cm ChrootDirectory
254must contain the necessary files and directories to support the
40c002af 255user's session.
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256For an interactive session this requires at least a shell, typically
257.Xr sh 1 ,
258and basic
259.Pa /dev
260nodes such as
261.Xr null 4 ,
262.Xr zero 4 ,
263.Xr stdin 4 ,
264.Xr stdout 4 ,
265.Xr stderr 4 ,
266.Xr arandom 4
267and
268.Xr tty 4
269devices.
270For file transfer sessions using
271.Dq sftp ,
272no additional configuration of the environment is necessary if the
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273in-process sftp server is used,
274though sessions which use logging do require
275.Pa /dev/log
276inside the chroot directory (see
277.Xr sftp-server 8
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278for details).
279.Pp
280The default is not to
281.Xr chroot 2 .
282.It Cm Ciphers
283Specifies the ciphers allowed for protocol version 2.
284Multiple ciphers must be comma-separated.
285The supported ciphers are
286.Dq 3des-cbc ,
287.Dq aes128-cbc ,
288.Dq aes192-cbc ,
289.Dq aes256-cbc ,
290.Dq aes128-ctr ,
291.Dq aes192-ctr ,
292.Dq aes256-ctr ,
293.Dq arcfour128 ,
294.Dq arcfour256 ,
295.Dq arcfour ,
296.Dq blowfish-cbc ,
297and
298.Dq cast128-cbc .
299The default is:
300.Bd -literal -offset 3n
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301aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
302aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
303aes256-cbc,arcfour
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304.Ed
305.It Cm ClientAliveCountMax
306Sets the number of client alive messages (see below) which may be
307sent without
308.Xr sshd 8
309receiving any messages back from the client.
310If this threshold is reached while client alive messages are being sent,
311sshd will disconnect the client, terminating the session.
312It is important to note that the use of client alive messages is very
313different from
314.Cm TCPKeepAlive
315(below).
316The client alive messages are sent through the encrypted channel
317and therefore will not be spoofable.
318The TCP keepalive option enabled by
319.Cm TCPKeepAlive
320is spoofable.
321The client alive mechanism is valuable when the client or
322server depend on knowing when a connection has become inactive.
323.Pp
324The default value is 3.
325If
326.Cm ClientAliveInterval
327(see below) is set to 15, and
328.Cm ClientAliveCountMax
329is left at the default, unresponsive SSH clients
330will be disconnected after approximately 45 seconds.
331This option applies to protocol version 2 only.
332.It Cm ClientAliveInterval
333Sets a timeout interval in seconds after which if no data has been received
334from the client,
335.Xr sshd 8
336will send a message through the encrypted
337channel to request a response from the client.
338The default
339is 0, indicating that these messages will not be sent to the client.
340This option applies to protocol version 2 only.
341.It Cm Compression
342Specifies whether compression is allowed, or delayed until
343the user has authenticated successfully.
344The argument must be
345.Dq yes ,
346.Dq delayed ,
347or
348.Dq no .
349The default is
350.Dq delayed .
351.It Cm DenyGroups
352This keyword can be followed by a list of group name patterns, separated
353by spaces.
354Login is disallowed for users whose primary group or supplementary
355group list matches one of the patterns.
356Only group names are valid; a numerical group ID is not recognized.
357By default, login is allowed for all groups.
358The allow/deny directives are processed in the following order:
359.Cm DenyUsers ,
360.Cm AllowUsers ,
361.Cm DenyGroups ,
362and finally
363.Cm AllowGroups .
364.Pp
365See
366.Sx PATTERNS
367in
368.Xr ssh_config 5
369for more information on patterns.
370.It Cm DenyUsers
371This keyword can be followed by a list of user name patterns, separated
372by spaces.
373Login is disallowed for user names that match one of the patterns.
374Only user names are valid; a numerical user ID is not recognized.
375By default, login is allowed for all users.
376If the pattern takes the form USER@HOST then USER and HOST
377are separately checked, restricting logins to particular
378users from particular hosts.
379The allow/deny directives are processed in the following order:
380.Cm DenyUsers ,
381.Cm AllowUsers ,
382.Cm DenyGroups ,
383and finally
384.Cm AllowGroups .
385.Pp
386See
387.Sx PATTERNS
388in
389.Xr ssh_config 5
390for more information on patterns.
391.It Cm ForceCommand
392Forces the execution of the command specified by
393.Cm ForceCommand ,
394ignoring any command supplied by the client and
395.Pa ~/.ssh/rc
396if present.
397The command is invoked by using the user's login shell with the -c option.
398This applies to shell, command, or subsystem execution.
399It is most useful inside a
400.Cm Match
401block.
402The command originally supplied by the client is available in the
403.Ev SSH_ORIGINAL_COMMAND
404environment variable.
405Specifying a command of
406.Dq internal-sftp
407will force the use of an in-process sftp server that requires no support
408files when used with
409.Cm ChrootDirectory .
410.It Cm GatewayPorts
411Specifies whether remote hosts are allowed to connect to ports
412forwarded for the client.
413By default,
414.Xr sshd 8
415binds remote port forwardings to the loopback address.
416This prevents other remote hosts from connecting to forwarded ports.
417.Cm GatewayPorts
418can be used to specify that sshd
419should allow remote port forwardings to bind to non-loopback addresses, thus
420allowing other hosts to connect.
421The argument may be
422.Dq no
423to force remote port forwardings to be available to the local host only,
424.Dq yes
425to force remote port forwardings to bind to the wildcard address, or
426.Dq clientspecified
427to allow the client to select the address to which the forwarding is bound.
428The default is
429.Dq no .
430.It Cm GSSAPIAuthentication
431Specifies whether user authentication based on GSSAPI is allowed.
432The default is
433.Dq no .
434Note that this option applies to protocol version 2 only.
435.It Cm GSSAPICleanupCredentials
436Specifies whether to automatically destroy the user's credentials cache
437on logout.
438The default is
439.Dq yes .
440Note that this option applies to protocol version 2 only.
441.It Cm HostbasedAuthentication
442Specifies whether rhosts or /etc/hosts.equiv authentication together
443with successful public key client host authentication is allowed
444(host-based authentication).
445This option is similar to
446.Cm RhostsRSAAuthentication
447and applies to protocol version 2 only.
448The default is
449.Dq no .
450.It Cm HostbasedUsesNameFromPacketOnly
451Specifies whether or not the server will attempt to perform a reverse
452name lookup when matching the name in the
453.Pa ~/.shosts ,
454.Pa ~/.rhosts ,
455and
456.Pa /etc/hosts.equiv
457files during
458.Cm HostbasedAuthentication .
459A setting of
460.Dq yes
461means that
462.Xr sshd 8
463uses the name supplied by the client rather than
464attempting to resolve the name from the TCP connection itself.
465The default is
466.Dq no .
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467.It Cm HostCertificate
468Specifies a file containing a public host certificate.
469The certificate's public key must match a private host key already specified
470by
471.Cm HostKey .
472The default behaviour of
473.Xr sshd 8
474is not to load any certificates.
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475.It Cm HostKey
476Specifies a file containing a private host key
477used by SSH.
478The default is
479.Pa /etc/ssh/ssh_host_key
480for protocol version 1, and
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481.Pa /etc/ssh/ssh_host_dsa_key ,
482.Pa /etc/ssh/ssh_host_ecdsa_key
c8c467ee 483and
9f304aaf 484.Pa /etc/ssh/ssh_host_rsa_key
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485for protocol version 2.
486Note that
487.Xr sshd 8
488will refuse to use a file if it is group/world-accessible.
489It is possible to have multiple host key files.
490.Dq rsa1
491keys are used for version 1 and
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492.Dq dsa ,
493.Dq ecdsa
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494or
495.Dq rsa
496are used for version 2 of the SSH protocol.
497.It Cm IgnoreRhosts
498Specifies that
499.Pa .rhosts
500and
501.Pa .shosts
502files will not be used in
503.Cm RhostsRSAAuthentication
504or
505.Cm HostbasedAuthentication .
506.Pp
507.Pa /etc/hosts.equiv
508and
40948e2d 509.Pa /etc/ssh/shosts.equiv
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510are still used.
511The default is
512.Dq yes .
513.It Cm IgnoreUserKnownHosts
514Specifies whether
515.Xr sshd 8
516should ignore the user's
517.Pa ~/.ssh/known_hosts
518during
519.Cm RhostsRSAAuthentication
520or
521.Cm HostbasedAuthentication .
522The default is
523.Dq no .
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524.It Cm IPQoS
525Specifies the IPv4 type-of-service or DSCP class for the connection.
526Accepted values are
527.Dq af11 ,
528.Dq af12 ,
529.Dq af13 ,
530.Dq af14 ,
531.Dq af22 ,
532.Dq af23 ,
533.Dq af31 ,
534.Dq af32 ,
535.Dq af33 ,
536.Dq af41 ,
537.Dq af42 ,
538.Dq af43 ,
539.Dq cs0 ,
540.Dq cs1 ,
541.Dq cs2 ,
542.Dq cs3 ,
543.Dq cs4 ,
544.Dq cs5 ,
545.Dq cs6 ,
546.Dq cs7 ,
547.Dq ef ,
548.Dq lowdelay ,
549.Dq throughput ,
550.Dq reliability ,
551or a numeric value.
552This option may take one or two arguments, separated by whitespace.
553If one argument is specified, it is used as the packet class unconditionally.
554If two values are specified, the first is automatically selected for
555interactive sessions and the second for non-interactive sessions.
556The default is
557.Dq lowdelay
558for interactive sessions and
559.Dq throughput
560for non-interactive sessions.
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561.It Cm KerberosAuthentication
562Specifies whether the password provided by the user for
563.Cm PasswordAuthentication
564will be validated through the Kerberos KDC.
565To use this option, the server needs a
566Kerberos servtab which allows the verification of the KDC's identity.
567The default is
568.Dq no .
569.It Cm KerberosGetAFSToken
570If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
571an AFS token before accessing the user's home directory.
572The default is
573.Dq no .
574.It Cm KerberosOrLocalPasswd
575If password authentication through Kerberos fails then
576the password will be validated via any additional local mechanism
577such as
578.Pa /etc/passwd .
579The default is
580.Dq yes .
581.It Cm KerberosTicketCleanup
582Specifies whether to automatically destroy the user's ticket cache
583file on logout.
584The default is
585.Dq yes .
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586.It Cm KexAlgorithms
587Specifies the available KEX (Key Exchange) algorithms.
588Multiple algorithms must be comma-separated.
589The default is
590.Dq ecdh-sha2-nistp256 ,
591.Dq ecdh-sha2-nistp384 ,
592.Dq ecdh-sha2-nistp521 ,
593.Dq diffie-hellman-group-exchange-sha256 ,
594.Dq diffie-hellman-group-exchange-sha1 ,
595.Dq diffie-hellman-group14-sha1 ,
596.Dq diffie-hellman-group1-sha1 .
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597.It Cm KeyRegenerationInterval
598In protocol version 1, the ephemeral server key is automatically regenerated
599after this many seconds (if it has been used).
600The purpose of regeneration is to prevent
601decrypting captured sessions by later breaking into the machine and
602stealing the keys.
603The key is never stored anywhere.
604If the value is 0, the key is never regenerated.
605The default is 3600 (seconds).
606.It Cm ListenAddress
607Specifies the local addresses
608.Xr sshd 8
609should listen on.
610The following forms may be used:
611.Pp
612.Bl -item -offset indent -compact
613.It
614.Cm ListenAddress
615.Sm off
616.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
617.Sm on
618.It
619.Cm ListenAddress
620.Sm off
621.Ar host No | Ar IPv4_addr No : Ar port
622.Sm on
623.It
624.Cm ListenAddress
625.Sm off
626.Oo
627.Ar host No | Ar IPv6_addr Oc : Ar port
628.Sm on
629.El
630.Pp
631If
632.Ar port
633is not specified,
634sshd will listen on the address and all prior
635.Cm Port
636options specified.
637The default is to listen on all local addresses.
638Multiple
639.Cm ListenAddress
640options are permitted.
641Additionally, any
642.Cm Port
643options must precede this option for non-port qualified addresses.
644.It Cm LoginGraceTime
645The server disconnects after this time if the user has not
646successfully logged in.
647If the value is 0, there is no time limit.
648The default is 120 seconds.
649.It Cm LogLevel
650Gives the verbosity level that is used when logging messages from
651.Xr sshd 8 .
652The possible values are:
653QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
654The default is INFO.
655DEBUG and DEBUG1 are equivalent.
656DEBUG2 and DEBUG3 each specify higher levels of debugging output.
657Logging with a DEBUG level violates the privacy of users and is not recommended.
658.It Cm MACs
659Specifies the available MAC (message authentication code) algorithms.
660The MAC algorithm is used in protocol version 2
661for data integrity protection.
662Multiple algorithms must be comma-separated.
663The default is:
664.Bd -literal -offset indent
665hmac-md5,hmac-sha1,umac-64@openssh.com,
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666hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
667hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
668hmac-sha2-512-96
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669.Ed
670.It Cm Match
671Introduces a conditional block.
672If all of the criteria on the
673.Cm Match
674line are satisfied, the keywords on the following lines override those
675set in the global section of the config file, until either another
676.Cm Match
677line or the end of the file.
c8c467ee 678.Pp
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679The arguments to
680.Cm Match
681are one or more criteria-pattern pairs.
682The available criteria are
683.Cm User ,
684.Cm Group ,
685.Cm Host ,
686and
687.Cm Address .
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688The match patterns may consist of single entries or comma-separated
689lists and may use the wildcard and negation operators described in the
690.Sx PATTERNS
691section of
692.Xr ssh_config 5 .
693.Pp
694The patterns in an
695.Cm Address
696criteria may additionally contain addresses to match in CIDR
697address/masklen format, e.g.\&
698.Dq 192.0.2.0/24
699or
700.Dq 3ffe:ffff::/32 .
701Note that the mask length provided must be consistent with the address -
702it is an error to specify a mask length that is too long for the address
703or one with bits set in this host portion of the address.
704For example,
705.Dq 192.0.2.0/33
706and
707.Dq 192.0.2.0/8
708respectively.
709.Pp
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710Only a subset of keywords may be used on the lines following a
711.Cm Match
712keyword.
713Available keywords are
cb5eb4f1 714.Cm AllowAgentForwarding ,
16c343f1 715.Cm AllowTcpForwarding ,
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716.Cm AuthorizedKeysFile ,
717.Cm AuthorizedPrincipalsFile ,
16c343f1 718.Cm Banner ,
c8c467ee 719.Cm ChrootDirectory ,
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720.Cm ForceCommand ,
721.Cm GatewayPorts ,
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722.Cm GSSAPIAuthentication ,
723.Cm HostbasedAuthentication ,
856ea928 724.Cm HostbasedUsesNameFromPacketOnly ,
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725.Cm KbdInteractiveAuthentication ,
726.Cm KerberosAuthentication ,
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727.Cm MaxAuthTries ,
728.Cm MaxSessions ,
16c343f1 729.Cm PasswordAuthentication ,
cb5eb4f1 730.Cm PermitEmptyPasswords ,
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731.Cm PermitOpen ,
732.Cm PermitRootLogin ,
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733.Cm PermitTunnel ,
734.Cm PubkeyAuthentication ,
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735.Cm RhostsRSAAuthentication ,
736.Cm RSAAuthentication ,
737.Cm X11DisplayOffset ,
cb5eb4f1 738.Cm X11Forwarding
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739and
740.Cm X11UseLocalHost .
741.It Cm MaxAuthTries
742Specifies the maximum number of authentication attempts permitted per
743connection.
744Once the number of failures reaches half this value,
745additional failures are logged.
746The default is 6.
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747.It Cm MaxSessions
748Specifies the maximum number of open sessions permitted per network connection.
749The default is 10.
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750.It Cm MaxStartups
751Specifies the maximum number of concurrent unauthenticated connections to the
752SSH daemon.
753Additional connections will be dropped until authentication succeeds or the
754.Cm LoginGraceTime
755expires for a connection.
756The default is 10.
757.Pp
758Alternatively, random early drop can be enabled by specifying
759the three colon separated values
760.Dq start:rate:full
761(e.g. "10:30:60").
762.Xr sshd 8
763will refuse connection attempts with a probability of
764.Dq rate/100
765(30%)
766if there are currently
767.Dq start
768(10)
769unauthenticated connections.
770The probability increases linearly and all connection attempts
771are refused if the number of unauthenticated connections reaches
772.Dq full
773(60).
774.It Cm PasswordAuthentication
775Specifies whether password authentication is allowed.
776The default is
777.Dq yes .
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778Note that if
779.Cm ChallengeResponseAuthentication
780is
781.Dq yes ,
782.Cm UsePAM
783is
784.Dq yes ,
785and the PAM authentication policy for
786.Nm sshd
787includes
788.Xr pam_unix 8 ,
789password authentication will be allowed through the challenge-response
790mechanism regardless of the value of
791.Cm PasswordAuthentication .
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792.It Cm PermitEmptyPasswords
793When password authentication is allowed, it specifies whether the
794server allows login to accounts with empty password strings.
795The default is
796.Dq no .
797.It Cm PermitOpen
798Specifies the destinations to which TCP port forwarding is permitted.
799The forwarding specification must be one of the following forms:
800.Pp
801.Bl -item -offset indent -compact
802.It
803.Cm PermitOpen
804.Sm off
805.Ar host : port
806.Sm on
807.It
808.Cm PermitOpen
809.Sm off
810.Ar IPv4_addr : port
811.Sm on
812.It
813.Cm PermitOpen
814.Sm off
815.Ar \&[ IPv6_addr \&] : port
816.Sm on
817.El
818.Pp
819Multiple forwards may be specified by separating them with whitespace.
820An argument of
821.Dq any
822can be used to remove all restrictions and permit any forwarding requests.
823By default all port forwarding requests are permitted.
824.It Cm PermitRootLogin
825Specifies whether root can log in using
826.Xr ssh 1 .
827The argument must be
828.Dq yes ,
829.Dq without-password ,
830.Dq forced-commands-only ,
831or
832.Dq no .
833The default is
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834.Dq no .
835Note that if
836.Cm ChallengeResponseAuthentication
837is
838.Dq yes ,
839the root user may be allowed in with its password even if
840.Cm PermitRootLogin is set to
841.Dq without-password .
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842.Pp
843If this option is set to
844.Dq without-password ,
845password authentication is disabled for root.
846.Pp
847If this option is set to
848.Dq forced-commands-only ,
849root login with public key authentication will be allowed,
850but only if the
851.Ar command
852option has been specified
853(which may be useful for taking remote backups even if root login is
854normally not allowed).
855All other authentication methods are disabled for root.
856.Pp
857If this option is set to
858.Dq no ,
859root is not allowed to log in.
860.It Cm PermitTunnel
861Specifies whether
862.Xr tun 4
863device forwarding is allowed.
864The argument must be
865.Dq yes ,
866.Dq point-to-point
867(layer 3),
868.Dq ethernet
869(layer 2), or
870.Dq no .
871Specifying
872.Dq yes
873permits both
874.Dq point-to-point
875and
876.Dq ethernet .
877The default is
878.Dq no .
879.It Cm PermitUserEnvironment
880Specifies whether
881.Pa ~/.ssh/environment
882and
883.Cm environment=
884options in
885.Pa ~/.ssh/authorized_keys
886are processed by
887.Xr sshd 8 .
888The default is
889.Dq no .
890Enabling environment processing may enable users to bypass access
891restrictions in some configurations using mechanisms such as
892.Ev LD_PRELOAD .
893.It Cm PidFile
894Specifies the file that contains the process ID of the
895SSH daemon.
896The default is
897.Pa /var/run/sshd.pid .
898.It Cm Port
899Specifies the port number that
900.Xr sshd 8
901listens on.
902The default is 22.
903Multiple options of this type are permitted.
904See also
905.Cm ListenAddress .
906.It Cm PrintLastLog
907Specifies whether
908.Xr sshd 8
909should print the date and time of the last user login when a user logs
910in interactively.
911The default is
912.Dq yes .
913.It Cm PrintMotd
914Specifies whether
915.Xr sshd 8
916should print
917.Pa /etc/motd
918when a user logs in interactively.
919(On some systems it is also printed by the shell,
920.Pa /etc/profile ,
921or equivalent.)
922The default is
923.Dq yes .
924.It Cm Protocol
925Specifies the protocol versions
926.Xr sshd 8
927supports.
928The possible values are
929.Sq 1
930and
931.Sq 2 .
932Multiple versions must be comma-separated.
933The default is
856ea928 934.Sq 2 .
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935Note that the order of the protocol list does not indicate preference,
936because the client selects among multiple protocol versions offered
937by the server.
938Specifying
939.Dq 2,1
940is identical to
941.Dq 1,2 .
942.It Cm PubkeyAuthentication
943Specifies whether public key authentication is allowed.
944The default is
945.Dq yes .
946Note that this option applies to protocol version 2 only.
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947.It Cm RevokedKeys
948Specifies a list of revoked public keys.
949Keys listed in this file will be refused for public key authentication.
950Note that if this file is not readable, then public key authentication will
951be refused for all users.
16c343f1 952.It Cm RhostsRSAAuthentication
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953Specifies whether rhosts or
954.Pa /etc/hosts.equiv
955authentication together
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956with successful RSA host authentication is allowed.
957The default is
958.Dq no .
959This option applies to protocol version 1 only.
960.It Cm RSAAuthentication
961Specifies whether pure RSA authentication is allowed.
962The default is
963.Dq yes .
964This option applies to protocol version 1 only.
965.It Cm ServerKeyBits
966Defines the number of bits in the ephemeral protocol version 1 server key.
c8c467ee 967The minimum value is 512, and the default is 1024.
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968.It Cm StrictModes
969Specifies whether
970.Xr sshd 8
971should check file modes and ownership of the
972user's files and home directory before accepting login.
973This is normally desirable because novices sometimes accidentally leave their
974directory or files world-writable.
975The default is
976.Dq yes .
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977Note that this does not apply to
978.Cm ChrootDirectory ,
979whose permissions and ownership are checked unconditionally.
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980.It Cm Subsystem
981Configures an external subsystem (e.g. file transfer daemon).
982Arguments should be a subsystem name and a command (with optional arguments)
983to execute upon subsystem request.
984.Pp
985The command
986.Xr sftp-server 8
987implements the
988.Dq sftp
989file transfer subsystem.
990.Pp
991Alternately the name
992.Dq internal-sftp
993implements an in-process
994.Dq sftp
995server.
996This may simplify configurations using
997.Cm ChrootDirectory
998to force a different filesystem root on clients.
999.Pp
1000By default no subsystems are defined.
1001Note that this option applies to protocol version 2 only.
1002.It Cm SyslogFacility
1003Gives the facility code that is used when logging messages from
1004.Xr sshd 8 .
1005The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1006LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1007The default is AUTH.
1008.It Cm TCPKeepAlive
1009Specifies whether the system should send TCP keepalive messages to the
1010other side.
1011If they are sent, death of the connection or crash of one
1012of the machines will be properly noticed.
1013However, this means that
1014connections will die if the route is down temporarily, and some people
1015find it annoying.
1016On the other hand, if TCP keepalives are not sent,
1017sessions may hang indefinitely on the server, leaving
1018.Dq ghost
1019users and consuming server resources.
1020.Pp
1021The default is
1022.Dq yes
1023(to send TCP keepalive messages), and the server will notice
1024if the network goes down or the client host crashes.
1025This avoids infinitely hanging sessions.
1026.Pp
1027To disable TCP keepalive messages, the value should be set to
1028.Dq no .
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1029.It Cm TrustedUserCAKeys
1030Specifies a file containing public keys of certificate authorities that are
1031trusted to sign user certificates for authentication.
1032Keys are listed one per line; empty lines and comments starting with
1033.Ql #
1034are allowed.
1035If a certificate is presented for authentication and has its signing CA key
1036listed in this file, then it may be used for authentication for any user
1037listed in the certificate's principals list.
1038Note that certificates that lack a list of principals will not be permitted
1039for authentication using
1040.Cm TrustedUserCAKeys .
1041For more details on certificates, see the
1042.Sx CERTIFICATES
1043section in
1044.Xr ssh-keygen 1 .
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1045.It Cm UseDNS
1046Specifies whether
1047.Xr sshd 8
1048should look up the remote host name and check that
1049the resolved host name for the remote IP address maps back to the
1050very same IP address.
1051The default is
1052.Dq yes .
1053.It Cm UseLogin
1054Specifies whether
1055.Xr login 1
1056is used for interactive login sessions.
1057The default is
1058.Dq no .
1059Note that
1060.Xr login 1
1061is never used for remote command execution.
1062Note also, that if this is enabled,
1063.Cm X11Forwarding
1064will be disabled because
1065.Xr login 1
1066does not know how to handle
1067.Xr xauth 1
1068cookies.
1069If
1070.Cm UsePrivilegeSeparation
1071is specified, it will be disabled after authentication.
1072.It Cm UsePAM
1073Enables the Pluggable Authentication Module interface.
1074If set to
1075.Dq yes
1076this will enable PAM authentication using
1077.Cm ChallengeResponseAuthentication
1078and
1079.Cm PasswordAuthentication
1080in addition to PAM account and session module processing for all
1081authentication types.
1082.Pp
1083Because PAM challenge-response authentication usually serves an equivalent
1084role to password authentication, you should disable either
1085.Cm PasswordAuthentication
1086or
1087.Cm ChallengeResponseAuthentication.
1088.Pp
1089If
1090.Cm UsePAM
1091is enabled, you will not be able to run
1092.Xr sshd 8
1093as a non-root user.
1094The default is
1095.Dq no .
1096.It Cm UsePrivilegeSeparation
1097Specifies whether
1098.Xr sshd 8
1099separates privileges by creating an unprivileged child process
1100to deal with incoming network traffic.
1101After successful authentication, another process will be created that has
1102the privilege of the authenticated user.
1103The goal of privilege separation is to prevent privilege
1104escalation by containing any corruption within the unprivileged processes.
1105The default is
1106.Dq yes .
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1107If
1108.Cm UsePrivilegeSeparation
1109is set to
1110.Dq sandbox
1111then the pre-authentication unprivileged process is subject to additional
1112restrictions.
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1113.It Cm VersionAddendum
1114Specifies a string to append to the regular version string to identify
1115OS- or site-specific modifications.
1116The default is
fa7cb8ce 1117.Dq DragonFly-20110920 .
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1118.It Cm X11DisplayOffset
1119Specifies the first display number available for
1120.Xr sshd 8 Ns 's
1121X11 forwarding.
1122This prevents sshd from interfering with real X11 servers.
1123The default is 10.
1124.It Cm X11Forwarding
1125Specifies whether X11 forwarding is permitted.
1126The argument must be
1127.Dq yes
1128or
1129.Dq no .
1130The default is
40948e2d 1131.Dq yes .
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1132.Pp
1133When X11 forwarding is enabled, there may be additional exposure to
1134the server and to client displays if the
1135.Xr sshd 8
1136proxy display is configured to listen on the wildcard address (see
1137.Cm X11UseLocalhost
1138below), though this is not the default.
1139Additionally, the authentication spoofing and authentication data
1140verification and substitution occur on the client side.
1141The security risk of using X11 forwarding is that the client's X11
1142display server may be exposed to attack when the SSH client requests
1143forwarding (see the warnings for
1144.Cm ForwardX11
1145in
1146.Xr ssh_config 5 ) .
1147A system administrator may have a stance in which they want to
1148protect clients that may expose themselves to attack by unwittingly
1149requesting X11 forwarding, which can warrant a
1150.Dq no
1151setting.
1152.Pp
1153Note that disabling X11 forwarding does not prevent users from
1154forwarding X11 traffic, as users can always install their own forwarders.
1155X11 forwarding is automatically disabled if
1156.Cm UseLogin
1157is enabled.
1158.It Cm X11UseLocalhost
1159Specifies whether
1160.Xr sshd 8
1161should bind the X11 forwarding server to the loopback address or to
1162the wildcard address.
1163By default,
1164sshd binds the forwarding server to the loopback address and sets the
1165hostname part of the
1166.Ev DISPLAY
1167environment variable to
1168.Dq localhost .
1169This prevents remote hosts from connecting to the proxy display.
1170However, some older X11 clients may not function with this
1171configuration.
1172.Cm X11UseLocalhost
1173may be set to
1174.Dq no
1175to specify that the forwarding server should be bound to the wildcard
1176address.
1177The argument must be
1178.Dq yes
1179or
1180.Dq no .
1181The default is
1182.Dq yes .
1183.It Cm XAuthLocation
1184Specifies the full pathname of the
1185.Xr xauth 1
1186program.
1187The default is
1188.Pa /usr/X11R6/bin/xauth .
1189.El
1190.Sh TIME FORMATS
1191.Xr sshd 8
1192command-line arguments and configuration file options that specify time
1193may be expressed using a sequence of the form:
1194.Sm off
1195.Ar time Op Ar qualifier ,
1196.Sm on
1197where
1198.Ar time
1199is a positive integer value and
1200.Ar qualifier
1201is one of the following:
1202.Pp
1203.Bl -tag -width Ds -compact -offset indent
1204.It Aq Cm none
1205seconds
1206.It Cm s | Cm S
1207seconds
1208.It Cm m | Cm M
1209minutes
1210.It Cm h | Cm H
1211hours
1212.It Cm d | Cm D
1213days
1214.It Cm w | Cm W
1215weeks
1216.El
1217.Pp
1218Each member of the sequence is added together to calculate
1219the total time value.
1220.Pp
1221Time format examples:
1222.Pp
1223.Bl -tag -width Ds -compact -offset indent
1224.It 600
1225600 seconds (10 minutes)
1226.It 10m
122710 minutes
1228.It 1h30m
12291 hour 30 minutes (90 minutes)
1230.El
1231.Sh FILES
1232.Bl -tag -width Ds
1233.It Pa /etc/ssh/sshd_config
1234Contains configuration data for
1235.Xr sshd 8 .
1236This file should be writable by root only, but it is recommended
1237(though not necessary) that it be world-readable.
1238.El
1239.Sh SEE ALSO
1240.Xr sshd 8
1241.Sh AUTHORS
1242OpenSSH is a derivative of the original and free
1243ssh 1.2.12 release by Tatu Ylonen.
1244Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1245Theo de Raadt and Dug Song
1246removed many bugs, re-added newer features and
1247created OpenSSH.
1248Markus Friedl contributed the support for SSH
1249protocol versions 1.5 and 2.0.
1250Niels Provos and Markus Friedl contributed support
1251for privilege separation.