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 1.29 2004/03/08 10:18:57 dtucker 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):
65
This keyword can be followed by a list of group name patterns, separated
67
If specified, login is allowed only for users whose primary
68
group or supplementary group list matches one of the patterns.
73
wildcards in the patterns.
74
Only group names are valid; a numerical group ID is not recognized.
75
By default, login is allowed for all groups.
77
.It Cm AllowTcpForwarding
78
Specifies whether TCP forwarding is permitted.
81
Note that disabling TCP forwarding does not improve security unless
82
users are also denied shell access, as they can always install their
86
This keyword can be followed by a list of user name patterns, separated
88
If specified, login is allowed only for user names that
89
match one of the patterns.
94
wildcards in the patterns.
95
Only user names are valid; a numerical user ID is not recognized.
96
By default, login is allowed for all users.
97
If the pattern takes the form USER@HOST then USER and HOST
98
are separately checked, restricting logins to particular
99
users from particular hosts.
101
.It Cm AuthorizedKeysFile
102
Specifies the file that contains the public keys that can be used
103
for user authentication.
104
.Cm AuthorizedKeysFile
105
may contain tokens of the form %T which are substituted during connection
107
The following tokens are defined: %% is replaced by a literal '%',
108
%h is replaced by the home directory of the user being authenticated and
109
%u is replaced by the username of that user.
111
.Cm AuthorizedKeysFile
112
is taken to be an absolute path or one relative to the user's home
115
.Dq .ssh/authorized_keys .
117
In some jurisdictions, sending a warning message before authentication
118
may be relevant for getting legal protection.
119
The contents of the specified file are sent to the remote user before
120
authentication is allowed.
121
This option is only available for protocol version 2.
122
By default, no banner is displayed.
124
.It Cm ChallengeResponseAuthentication
125
Specifies whether challenge response authentication is allowed.
126
All authentication styles from
132
Specifies the ciphers allowed for protocol version 2.
133
Multiple ciphers must be comma-separated.
137
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
138
aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
140
.It Cm ClientAliveInterval
141
Sets a timeout interval in seconds after which if no data has been received
144
will send a message through the encrypted
145
channel to request a response from the client.
147
is 0, indicating that these messages will not be sent to the client.
148
This option applies to protocol version 2 only.
149
.It Cm ClientAliveCountMax
150
Sets the number of client alive messages (see above) which may be
153
receiving any messages back from the client.
154
If this threshold is reached while client alive messages are being sent,
156
will disconnect the client, terminating the session.
157
It is important to note that the use of client alive messages is very
161
The client alive messages are sent through the encrypted channel
162
and therefore will not be spoofable.
163
The TCP keepalive option enabled by
166
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.
171
.Cm ClientAliveInterval
172
(above) is set to 15, and
173
.Cm ClientAliveCountMax
174
is left at the default, unresponsive ssh clients
175
will be disconnected after approximately 45 seconds.
177
Specifies whether compression is allowed.
185
This keyword can be followed by a list of group name patterns, separated
187
Login is disallowed for users whose primary group or supplementary
188
group list matches one of the patterns.
193
wildcards in the patterns.
194
Only group names are valid; a numerical group ID is not recognized.
195
By default, login is allowed for all groups.
198
This keyword can be followed by a list of user name patterns, separated
200
Login is disallowed for user names that match one of the patterns.
204
can be used as wildcards in the patterns.
205
Only user names are valid; a numerical user ID is not recognized.
206
By default, login is allowed for all users.
207
If the pattern takes the form USER@HOST then USER and HOST
208
are separately checked, restricting logins to particular
209
users from particular hosts.
211
Specifies whether remote hosts are allowed to connect to ports
212
forwarded for the client.
215
binds remote port forwardings to the loopback address.
216
This prevents other remote hosts from connecting to forwarded ports.
218
can be used to specify that
220
should bind remote port forwardings to the wildcard address,
221
thus allowing remote hosts to connect to forwarded ports.
228
.It Cm GSSAPIAuthentication
229
Specifies whether user authentication based on GSSAPI is allowed.
232
Note that this option applies to protocol version 2 only.
233
.It Cm GSSAPICleanupCredentials
234
Specifies whether to automatically destroy the user's credentials cache
238
Note that this option applies to protocol version 2 only.
239
.It Cm HostbasedAuthentication
240
Specifies whether rhosts or /etc/hosts.equiv authentication together
241
with successful public key client host authentication is allowed
242
(hostbased authentication).
243
This option is similar to
244
.Cm RhostsRSAAuthentication
245
and applies to protocol version 2 only.
249
Specifies a file containing a private host key
252
.Pa /etc/ssh/ssh_host_key
253
for protocol version 1, and
254
.Pa /etc/ssh/ssh_host_rsa_key
256
.Pa /etc/ssh/ssh_host_dsa_key
257
for protocol version 2.
260
will refuse to use a file if it is group/world-accessible.
261
It is possible to have multiple host key files.
263
keys are used for version 1 and
267
are used for version 2 of the SSH protocol.
273
files will not be used in
274
.Cm RhostsRSAAuthentication
276
.Cm HostbasedAuthentication .
280
.Pa /etc/shosts.equiv
284
.It Cm IgnoreUserKnownHosts
287
should ignore the user's
288
.Pa $HOME/.ssh/known_hosts
290
.Cm RhostsRSAAuthentication
292
.Cm HostbasedAuthentication .
295
.It Cm KerberosAuthentication
296
Specifies whether the password provided by the user for
297
.Cm PasswordAuthentication
298
will be validated through the Kerberos KDC.
299
To use this option, the server needs a
300
Kerberos servtab which allows the verification of the KDC's identity.
303
.It Cm KerberosGetAFSToken
304
If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
305
an AFS token before accessing the user's home directory.
308
.It Cm KerberosOrLocalPasswd
309
If set then if password authentication through Kerberos fails then
310
the password will be validated via any additional local mechanism
315
.It Cm KerberosTicketCleanup
316
Specifies whether to automatically destroy the user's ticket cache
320
.It Cm KeyRegenerationInterval
321
In protocol version 1, the ephemeral server key is automatically regenerated
322
after this many seconds (if it has been used).
323
The purpose of regeneration is to prevent
324
decrypting captured sessions by later breaking into the machine and
326
The key is never stored anywhere.
327
If the value is 0, the key is never regenerated.
328
The default is 3600 (seconds).
330
Specifies the local addresses
333
The following forms may be used:
335
.Bl -item -offset indent -compact
339
.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
344
.Ar host No | Ar IPv4_addr No : Ar port
350
.Ar host No | Ar IPv6_addr Oc : Ar port
358
will listen on the address and all prior
361
The default is to listen on all local addresses.
364
options are permitted.
367
options must precede this option for non port qualified addresses.
368
.It Cm LoginGraceTime
369
The server disconnects after this time if the user has not
370
successfully logged in.
371
If the value is 0, there is no time limit.
372
The default is 120 seconds.
374
Gives the verbosity level that is used when logging messages from
376
The possible values are:
377
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
379
DEBUG and DEBUG1 are equivalent.
380
DEBUG2 and DEBUG3 each specify higher levels of debugging output.
381
Logging with a DEBUG level violates the privacy of users and is not recommended.
383
Specifies the available MAC (message authentication code) algorithms.
384
The MAC algorithm is used in protocol version 2
385
for data integrity protection.
386
Multiple algorithms must be comma-separated.
388
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
390
Specifies the maximum number of concurrent unauthenticated connections to the
393
Additional connections will be dropped until authentication succeeds or the
395
expires for a connection.
398
Alternatively, random early drop can be enabled by specifying
399
the three colon separated values
403
will refuse connection attempts with a probability of
406
if there are currently
409
unauthenticated connections.
410
The probability increases linearly and all connection attempts
411
are refused if the number of unauthenticated connections reaches
414
.It Cm PasswordAuthentication
415
Specifies whether password authentication is allowed.
418
.It Cm PermitEmptyPasswords
419
When password authentication is allowed, it specifies whether the
420
server allows login to accounts with empty password strings.
423
.It Cm PermitRootLogin
424
Specifies whether root can login using
428
.Dq without-password ,
429
.Dq forced-commands-only
435
If this option is set to
437
password authentication is disabled for root. Note that other authentication
438
methods (e.g., keyboard-interactive/PAM) may still allow root to login using
441
If this option is set to
442
.Dq forced-commands-only
443
root login with public key authentication will be allowed,
446
option has been specified
447
(which may be useful for taking remote backups even if root login is
448
normally not allowed).
449
All other authentication methods are disabled for root.
451
If this option is set to
453
root is not allowed to login.
454
.It Cm PermitUserEnvironment
456
.Pa ~/.ssh/environment
460
.Pa ~/.ssh/authorized_keys
465
Enabling environment processing may enable users to bypass access
466
restrictions in some configurations using mechanisms such as
469
Specifies the file that contains the process ID of the
473
.Pa /var/run/sshd.pid .
475
Specifies the port number that
479
Multiple options of this type are permitted.
485
should print the date and time when the user last logged in.
493
when a user logs in interactively.
494
(On some systems it is also printed by the shell,
500
Specifies the protocol versions
503
The possible values are
507
Multiple versions must be comma-separated.
510
Note that the order of the protocol list does not indicate preference,
511
because the client selects among multiple protocol versions offered
517
.It Cm PubkeyAuthentication
518
Specifies whether public key authentication is allowed.
521
Note that this option applies to protocol version 2 only.
522
.It Cm RhostsRSAAuthentication
523
Specifies whether rhosts or /etc/hosts.equiv authentication together
524
with successful RSA host authentication is allowed.
527
This option applies to protocol version 1 only.
528
.It Cm RSAAuthentication
529
Specifies whether pure RSA authentication is allowed.
532
This option applies to protocol version 1 only.
534
Defines the number of bits in the ephemeral protocol version 1 server key.
535
The minimum value is 512, and the default is 768.
539
should check file modes and ownership of the
540
user's files and home directory before accepting login.
541
This is normally desirable because novices sometimes accidentally leave their
542
directory or files world-writable.
546
Configures an external subsystem (e.g., file transfer daemon).
547
Arguments should be a subsystem name and a command to execute upon subsystem
553
file transfer subsystem.
554
By default no subsystems are defined.
555
Note that this option applies to protocol version 2 only.
556
.It Cm SyslogFacility
557
Gives the facility code that is used when logging messages from
559
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
560
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
563
Specifies whether the system should send TCP keepalive messages to the
565
If they are sent, death of the connection or crash of one
566
of the machines will be properly noticed.
567
However, this means that
568
connections will die if the route is down temporarily, and some people
570
On the other hand, if TCP keepalives are not sent,
571
sessions may hang indefinitely on the server, leaving
573
users and consuming server resources.
577
(to send TCP keepalive messages), and the server will notice
578
if the network goes down or the client host crashes.
579
This avoids infinitely hanging sessions.
581
To disable TCP keepalive messages, the value should be set to
586
should lookup the remote host name and check that
587
the resolved host name for the remote IP address maps back to the
588
very same IP address.
594
is used for interactive login sessions.
599
is never used for remote command execution.
600
Note also, that if this is enabled,
602
will be disabled because
604
does not know how to handle
608
.Cm UsePrivilegeSeparation
609
is specified, it will be disabled after authentication.
611
Enables PAM authentication (via challenge-response) and session set up.
612
If you enable this, you should probably disable
613
.Cm PasswordAuthentication .
616
then you will not be able to run sshd as a non-root user. The default is
618
.It Cm UsePrivilegeSeparation
621
separates privileges by creating an unprivileged child process
622
to deal with incoming network traffic.
623
After successful authentication, another process will be created that has
624
the privilege of the authenticated user.
625
The goal of privilege separation is to prevent privilege
626
escalation by containing any corruption within the unprivileged processes.
629
.It Cm X11DisplayOffset
630
Specifies the first display number available for
635
from interfering with real X11 servers.
638
Specifies whether X11 forwarding is permitted.
646
When X11 forwarding is enabled, there may be additional exposure to
647
the server and to client displays if the
649
proxy display is configured to listen on the wildcard address (see
651
below), however this is not the default.
652
Additionally, the authentication spoofing and authentication data
653
verification and substitution occur on the client side.
654
The security risk of using X11 forwarding is that the client's X11
655
display server may be exposed to attack when the ssh client requests
656
forwarding (see the warnings for
660
A system administrator may have a stance in which they want to
661
protect clients that may expose themselves to attack by unwittingly
662
requesting X11 forwarding, which can warrant a
666
Note that disabling X11 forwarding does not prevent users from
667
forwarding X11 traffic, as users can always install their own forwarders.
668
X11 forwarding is automatically disabled if
671
.It Cm X11UseLocalhost
674
should bind the X11 forwarding server to the loopback address or to
675
the wildcard address.
678
binds the forwarding server to the loopback address and sets the
681
environment variable to
683
This prevents remote hosts from connecting to the proxy display.
684
However, some older X11 clients may not function with this
689
to specify that the forwarding server should be bound to the wildcard
698
Specifies the full pathname of the
702
.Pa /usr/X11R6/bin/xauth .
706
command-line arguments and configuration file options that specify time
707
may be expressed using a sequence of the form:
709
.Ar time Op Ar qualifier ,
713
is a positive integer value and
715
is one of the following:
717
.Bl -tag -width Ds -compact -offset indent
732
Each member of the sequence is added together to calculate
733
the total time value.
735
Time format examples:
737
.Bl -tag -width Ds -compact -offset indent
739
600 seconds (10 minutes)
743
1 hour 30 minutes (90 minutes)
747
.It Pa /etc/ssh/sshd_config
748
Contains configuration data for
750
This file should be writable by root only, but it is recommended
751
(though not necessary) that it be world-readable.
756
OpenSSH is a derivative of the original and free
757
ssh 1.2.12 release by Tatu Ylonen.
758
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
759
Theo de Raadt and Dug Song
760
removed many bugs, re-added newer features and
762
Markus Friedl contributed the support for SSH
763
protocol versions 1.5 and 2.0.
764
Niels Provos and Markus Friedl contributed support
765
for privilege separation.
added
removed
sshd_config.5
