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1SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5)
2
3NAME
4 sshd_config - OpenSSH SSH daemon configuration file
5
6SYNOPSIS
7 /etc/ssh/sshd_config
8
9DESCRIPTION
10 sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
11 specified with -f on the command line). The file contains keyword-
12 argument pairs, one per line. Lines starting with `#' and empty lines
13 are interpreted as comments. Arguments may optionally be enclosed in
14 double quotes (") in order to represent arguments containing spaces.
15
16 The possible keywords and their meanings are as follows (note that
17 keywords are case-insensitive and arguments are case-sensitive):
18
19 AcceptEnv
20 Specifies what environment variables sent by the client will be
21 copied into the session's environ(7). See SendEnv in
22 ssh_config(5) for how to configure the client. Note that
23 environment passing is only supported for protocol 2. Variables
24 are specified by name, which may contain the wildcard characters
25 `*' and `?'. Multiple environment variables may be separated by
26 whitespace or spread across multiple AcceptEnv directives. Be
27 warned that some environment variables could be used to bypass
28 restricted user environments. For this reason, care should be
29 taken in the use of this directive. The default is not to accept
30 any environment variables.
31
32 AddressFamily
33 Specifies which address family should be used by sshd(8). Valid
34 arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
35 (use IPv6 only). The default is ``any''.
36
37 AllowAgentForwarding
38 Specifies whether ssh-agent(1) forwarding is permitted. The
39 default is ``yes''. Note that disabling agent forwarding does
40 not improve security unless users are also denied shell access,
41 as they can always install their own forwarders.
42
43 AllowGroups
44 This keyword can be followed by a list of group name patterns,
45 separated by spaces. If specified, login is allowed only for
46 users whose primary group or supplementary group list matches one
47 of the patterns. Only group names are valid; a numerical group
48 ID is not recognized. By default, login is allowed for all
49 groups. The allow/deny directives are processed in the following
50 order: DenyUsers, AllowUsers, DenyGroups, and finally
51 AllowGroups.
52
53 See PATTERNS in ssh_config(5) for more information on patterns.
54
55 AllowTcpForwarding
56 Specifies whether TCP forwarding is permitted. The available
57 options are ``yes'' or ``all'' to allow TCP forwarding, ``no'' to
58 prevent all TCP forwarding, ``local'' to allow local (from the
59 perspective of ssh(1)) forwarding only or ``remote'' to allow
60 remote forwarding only. The default is ``yes''. Note that
61 disabling TCP forwarding does not improve security unless users
62 are also denied shell access, as they can always install their
63 own forwarders.
64
65 AllowUsers
66 This keyword can be followed by a list of user name patterns,
67 separated by spaces. If specified, login is allowed only for
68 user names that match one of the patterns. Only user names are
69 valid; a numerical user ID is not recognized. By default, login
70 is allowed for all users. If the pattern takes the form
71 USER@HOST then USER and HOST are separately checked, restricting
72 logins to particular users from particular hosts. The allow/deny
73 directives are processed in the following order: DenyUsers,
74 AllowUsers, DenyGroups, and finally AllowGroups.
75
76 See PATTERNS in ssh_config(5) for more information on patterns.
77
78 AuthenticationMethods
79 Specifies the authentication methods that must be successfully
80 completed for a user to be granted access. This option must be
81 followed by one or more comma-separated lists of authentication
82 method names. Successful authentication requires completion of
83 every method in at least one of these lists.
84
85 For example, an argument of ``publickey,password
86 publickey,keyboard-interactive'' would require the user to
87 complete public key authentication, followed by either password
88 or keyboard interactive authentication. Only methods that are
89 next in one or more lists are offered at each stage, so for this
90 example, it would not be possible to attempt password or
91 keyboard-interactive authentication before public key.
92
93 For keyboard interactive authentication it is also possible to
94 restrict authentication to a specific device by appending a colon
95 followed by the device identifier ``bsdauth'', ``pam'', or
96 ``skey'', depending on the server configuration. For example,
97 ``keyboard-interactive:bsdauth'' would restrict keyboard
98 interactive authentication to the ``bsdauth'' device.
99
100 This option is only available for SSH protocol 2 and will yield a
101 fatal error if enabled if protocol 1 is also enabled. Note that
102 each authentication method listed should also be explicitly
103 enabled in the configuration. The default is not to require
104 multiple authentication; successful completion of a single
105 authentication method is sufficient.
106
107 AuthorizedKeysCommand
108 Specifies a program to be used to look up the user's public keys.
109 The program must be owned by root and not writable by group or
110 others. It will be invoked with a single argument of the
111 username being authenticated, and should produce on standard
112 output zero or more lines of authorized_keys output (see
113 AUTHORIZED_KEYS in sshd(8)). If a key supplied by
114 AuthorizedKeysCommand does not successfully authenticate and
115 authorize the user then public key authentication continues using
116 the usual AuthorizedKeysFile files. By default, no
117 AuthorizedKeysCommand is run.
118
119 AuthorizedKeysCommandUser
120 Specifies the user under whose account the AuthorizedKeysCommand
121 is run. It is recommended to use a dedicated user that has no
122 other role on the host than running authorized keys commands.
123
124 AuthorizedKeysFile
125 Specifies the file that contains the public keys that can be used
126 for user authentication. The format is described in the
127 AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
128 AuthorizedKeysFile may contain tokens of the form %T which are
129 substituted during connection setup. The following tokens are
130 defined: %% is replaced by a literal '%', %h is replaced by the
131 home directory of the user being authenticated, and %u is
132 replaced by the username of that user. After expansion,
133 AuthorizedKeysFile is taken to be an absolute path or one
134 relative to the user's home directory. Multiple files may be
135 listed, separated by whitespace. The default is
136 ``.ssh/authorized_keys .ssh/authorized_keys2''.
137
138 AuthorizedPrincipalsFile
139 Specifies a file that lists principal names that are accepted for
140 certificate authentication. When using certificates signed by a
141 key listed in TrustedUserCAKeys, this file lists names, one of
142 which must appear in the certificate for it to be accepted for
143 authentication. Names are listed one per line preceded by key
144 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
145 Empty lines and comments starting with `#' are ignored.
146
147 AuthorizedPrincipalsFile may contain tokens of the form %T which
148 are substituted during connection setup. The following tokens
149 are defined: %% is replaced by a literal '%', %h is replaced by
150 the home directory of the user being authenticated, and %u is
151 replaced by the username of that user. After expansion,
152 AuthorizedPrincipalsFile is taken to be an absolute path or one
153 relative to the user's home directory.
154
155 The default is ``none'', i.e. not to use a principals file - in
156 this case, the username of the user must appear in a
157 certificate's principals list for it to be accepted. Note that
158 AuthorizedPrincipalsFile is only used when authentication
159 proceeds using a CA listed in TrustedUserCAKeys and is not
160 consulted for certification authorities trusted via
161 ~/.ssh/authorized_keys, though the principals= key option offers
162 a similar facility (see sshd(8) for details).
163
164 Banner The contents of the specified file are sent to the remote user
165 before authentication is allowed. If the argument is ``none''
166 then no banner is displayed. This option is only available for
167 protocol version 2. By default, no banner is displayed.
168
169 ChallengeResponseAuthentication
170 Specifies whether challenge-response authentication is allowed
171 (e.g. via PAM or though authentication styles supported in
172 login.conf(5)) The default is ``yes''.
173
174 ChrootDirectory
175 Specifies the pathname of a directory to chroot(2) to after
176 authentication. All components of the pathname must be root-
177 owned directories that are not writable by any other user or
178 group. After the chroot, sshd(8) changes the working directory
179 to the user's home directory.
180
181 The pathname may contain the following tokens that are expanded
182 at runtime once the connecting user has been authenticated: %% is
183 replaced by a literal '%', %h is replaced by the home directory
184 of the user being authenticated, and %u is replaced by the
185 username of that user.
186
187 The ChrootDirectory must contain the necessary files and
188 directories to support the user's session. For an interactive
189 session this requires at least a shell, typically sh(1), and
190 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
191 stderr(4), arandom(4) and tty(4) devices. For file transfer
192 sessions using ``sftp'', no additional configuration of the
193 environment is necessary if the in-process sftp server is used,
194 though sessions which use logging do require /dev/log inside the
195 chroot directory (see sftp-server(8) for details).
196
197 The default is not to chroot(2).
198
199 Ciphers
200 Specifies the ciphers allowed for protocol version 2. Multiple
201 ciphers must be comma-separated. The supported ciphers are
202 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
203 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
204 ``aes128-gcm@openssh.com'', ``aes256-gcm@openssh.com'',
205 ``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
206 and ``cast128-cbc''. The default is:
207
208 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
209 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
210 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
211 aes256-cbc,arcfour
212
213 ClientAliveCountMax
214 Sets the number of client alive messages (see below) which may be
215 sent without sshd(8) receiving any messages back from the client.
216 If this threshold is reached while client alive messages are
217 being sent, sshd will disconnect the client, terminating the
218 session. It is important to note that the use of client alive
219 messages is very different from TCPKeepAlive (below). The client
220 alive messages are sent through the encrypted channel and
221 therefore will not be spoofable. The TCP keepalive option
222 enabled by TCPKeepAlive is spoofable. The client alive mechanism
223 is valuable when the client or server depend on knowing when a
224 connection has become inactive.
225
226 The default value is 3. If ClientAliveInterval (see below) is
227 set to 15, and ClientAliveCountMax is left at the default,
228 unresponsive SSH clients will be disconnected after approximately
229 45 seconds. This option applies to protocol version 2 only.
230
231 ClientAliveInterval
232 Sets a timeout interval in seconds after which if no data has
233 been received from the client, sshd(8) will send a message
234 through the encrypted channel to request a response from the
235 client. The default is 0, indicating that these messages will
236 not be sent to the client. This option applies to protocol
237 version 2 only.
238
239 Compression
240 Specifies whether compression is allowed, or delayed until the
241 user has authenticated successfully. The argument must be
242 ``yes'', ``delayed'', or ``no''. The default is ``delayed''.
243
244 DenyGroups
245 This keyword can be followed by a list of group name patterns,
246 separated by spaces. Login is disallowed for users whose primary
247 group or supplementary group list matches one of the patterns.
248 Only group names are valid; a numerical group ID is not
249 recognized. By default, login is allowed for all groups. The
250 allow/deny directives are processed in the following order:
251 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
252
253 See PATTERNS in ssh_config(5) for more information on patterns.
254
255 DenyUsers
256 This keyword can be followed by a list of user name patterns,
257 separated by spaces. Login is disallowed for user names that
258 match one of the patterns. Only user names are valid; a
259 numerical user ID is not recognized. By default, login is
260 allowed for all users. If the pattern takes the form USER@HOST
261 then USER and HOST are separately checked, restricting logins to
262 particular users from particular hosts. The allow/deny
263 directives are processed in the following order: DenyUsers,
264 AllowUsers, DenyGroups, and finally AllowGroups.
265
266 See PATTERNS in ssh_config(5) for more information on patterns.
267
268 ForceCommand
269 Forces the execution of the command specified by ForceCommand,
270 ignoring any command supplied by the client and ~/.ssh/rc if
271 present. The command is invoked by using the user's login shell
272 with the -c option. This applies to shell, command, or subsystem
273 execution. It is most useful inside a Match block. The command
274 originally supplied by the client is available in the
275 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
276 of ``internal-sftp'' will force the use of an in-process sftp
277 server that requires no support files when used with
278 ChrootDirectory.
279
280 GatewayPorts
281 Specifies whether remote hosts are allowed to connect to ports
282 forwarded for the client. By default, sshd(8) binds remote port
283 forwardings to the loopback address. This prevents other remote
284 hosts from connecting to forwarded ports. GatewayPorts can be
285 used to specify that sshd should allow remote port forwardings to
286 bind to non-loopback addresses, thus allowing other hosts to
287 connect. The argument may be ``no'' to force remote port
288 forwardings to be available to the local host only, ``yes'' to
289 force remote port forwardings to bind to the wildcard address, or
290 ``clientspecified'' to allow the client to select the address to
291 which the forwarding is bound. The default is ``no''.
292
293 GSSAPIAuthentication
294 Specifies whether user authentication based on GSSAPI is allowed.
295 The default is ``no''. Note that this option applies to protocol
296 version 2 only.
297
298 GSSAPICleanupCredentials
299 Specifies whether to automatically destroy the user's credentials
300 cache on logout. The default is ``yes''. Note that this option
301 applies to protocol version 2 only.
302
303 HostbasedAuthentication
304 Specifies whether rhosts or /etc/hosts.equiv authentication
305 together with successful public key client host authentication is
306 allowed (host-based authentication). This option is similar to
307 RhostsRSAAuthentication and applies to protocol version 2 only.
308 The default is ``no''.
309
310 HostbasedUsesNameFromPacketOnly
311 Specifies whether or not the server will attempt to perform a
312 reverse name lookup when matching the name in the ~/.shosts,
313 ~/.rhosts, and /etc/hosts.equiv files during
314 HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
315 uses the name supplied by the client rather than attempting to
316 resolve the name from the TCP connection itself. The default is
317 ``no''.
318
319 HostCertificate
320 Specifies a file containing a public host certificate. The
321 certificate's public key must match a private host key already
322 specified by HostKey. The default behaviour of sshd(8) is not to
323 load any certificates.
324
325 HostKey
326 Specifies a file containing a private host key used by SSH. The
327 default is /etc/ssh/ssh_host_key for protocol version 1, and
328 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
329 /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that
330 sshd(8) will refuse to use a file if it is group/world-
331 accessible. It is possible to have multiple host key files.
332 ``rsa1'' keys are used for version 1 and ``dsa'', ``ecdsa'' or
333 ``rsa'' are used for version 2 of the SSH protocol. It is also
334 possible to specify public host key files instead. In this case
335 operations on the private key will be delegated to an
336 ssh-agent(1).
337
338 HostKeyAgent
339 Identifies the UNIX-domain socket used to communicate with an
340 agent that has access to the private host keys. If
341 ``SSH_AUTH_SOCK'' is specified, the location of the socket will
342 be read from the SSH_AUTH_SOCK environment variable.
343
344 IgnoreRhosts
345 Specifies that .rhosts and .shosts files will not be used in
346 RhostsRSAAuthentication or HostbasedAuthentication.
347
348 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
349 default is ``yes''.
350
351 IgnoreUserKnownHosts
352 Specifies whether sshd(8) should ignore the user's
353 ~/.ssh/known_hosts during RhostsRSAAuthentication or
354 HostbasedAuthentication. The default is ``no''.
355
356 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
357 connection. Accepted values are ``af11'', ``af12'', ``af13'',
358 ``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'',
359 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'',
360 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
361 ``throughput'', ``reliability'', or a numeric value. This option
362 may take one or two arguments, separated by whitespace. If one
363 argument is specified, it is used as the packet class
364 unconditionally. If two values are specified, the first is
365 automatically selected for interactive sessions and the second
366 for non-interactive sessions. The default is ``lowdelay'' for
367 interactive sessions and ``throughput'' for non-interactive
368 sessions.
369
370 KerberosAuthentication
371 Specifies whether the password provided by the user for
372 PasswordAuthentication will be validated through the Kerberos
373 KDC. To use this option, the server needs a Kerberos servtab
374 which allows the verification of the KDC's identity. The default
375 is ``no''.
376
377 KerberosGetAFSToken
378 If AFS is active and the user has a Kerberos 5 TGT, attempt to
379 acquire an AFS token before accessing the user's home directory.
380 The default is ``no''.
381
382 KerberosOrLocalPasswd
383 If password authentication through Kerberos fails then the
384 password will be validated via any additional local mechanism
385 such as /etc/passwd. The default is ``yes''.
386
387 KerberosTicketCleanup
388 Specifies whether to automatically destroy the user's ticket
389 cache file on logout. The default is ``yes''.
390
391 KexAlgorithms
392 Specifies the available KEX (Key Exchange) algorithms. Multiple
393 algorithms must be comma-separated. The default is
394 ``ecdh-sha2-nistp256'', ``ecdh-sha2-nistp384'',
395 ``ecdh-sha2-nistp521'', ``diffie-hellman-group-exchange-sha256'',
396 ``diffie-hellman-group-exchange-sha1'',
397 ``diffie-hellman-group14-sha1'', ``diffie-hellman-group1-sha1''.
398
399 KeyRegenerationInterval
400 In protocol version 1, the ephemeral server key is automatically
401 regenerated after this many seconds (if it has been used). The
402 purpose of regeneration is to prevent decrypting captured
403 sessions by later breaking into the machine and stealing the
404 keys. The key is never stored anywhere. If the value is 0, the
405 key is never regenerated. The default is 3600 (seconds).
406
407 ListenAddress
408 Specifies the local addresses sshd(8) should listen on. The
409 following forms may be used:
410
411 ListenAddress host|IPv4_addr|IPv6_addr
412 ListenAddress host|IPv4_addr:port
413 ListenAddress [host|IPv6_addr]:port
414
415 If port is not specified, sshd will listen on the address and all
416 prior Port options specified. The default is to listen on all
417 local addresses. Multiple ListenAddress options are permitted.
418 Additionally, any Port options must precede this option for non-
419 port qualified addresses.
420
421 LoginGraceTime
422 The server disconnects after this time if the user has not
423 successfully logged in. If the value is 0, there is no time
424 limit. The default is 120 seconds.
425
426 LogLevel
427 Gives the verbosity level that is used when logging messages from
428 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
429 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
430 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
431 higher levels of debugging output. Logging with a DEBUG level
432 violates the privacy of users and is not recommended.
433
434 MACs Specifies the available MAC (message authentication code)
435 algorithms. The MAC algorithm is used in protocol version 2 for
436 data integrity protection. Multiple algorithms must be comma-
437 separated. The algorithms that contain ``-etm'' calculate the
438 MAC after encryption (encrypt-then-mac). These are considered
439 safer and their use recommended. The default is:
440
441 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
442 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
443 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
444 hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
445 hmac-md5-96-etm@openssh.com,
446 hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
447 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
448 hmac-sha1-96,hmac-md5-96
449
450 Match Introduces a conditional block. If all of the criteria on the
451 Match line are satisfied, the keywords on the following lines
452 override those set in the global section of the config file,
453 until either another Match line or the end of the file.
454
455 The arguments to Match are one or more criteria-pattern pairs.
456 The available criteria are User, Group, Host, LocalAddress,
457 LocalPort, and Address. The match patterns may consist of single
458 entries or comma-separated lists and may use the wildcard and
459 negation operators described in the PATTERNS section of
460 ssh_config(5).
461
462 The patterns in an Address criteria may additionally contain
463 addresses to match in CIDR address/masklen format, e.g.
464 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
465 length provided must be consistent with the address - it is an
466 error to specify a mask length that is too long for the address
467 or one with bits set in this host portion of the address. For
468 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
469
470 Only a subset of keywords may be used on the lines following a
471 Match keyword. Available keywords are AcceptEnv,
472 AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
473 AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
474 AuthorizedKeysCommandUser, AuthorizedKeysFile,
475 AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
476 DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
477 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
478 KbdInteractiveAuthentication, KerberosAuthentication,
479 MaxAuthTries, MaxSessions, PasswordAuthentication,
480 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTunnel,
481 PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication,
482 RSAAuthentication, X11DisplayOffset, X11Forwarding and
483 X11UseLocalHost.
484
485 MaxAuthTries
486 Specifies the maximum number of authentication attempts permitted
487 per connection. Once the number of failures reaches half this
488 value, additional failures are logged. The default is 6.
489
490 MaxSessions
491 Specifies the maximum number of open sessions permitted per
492 network connection. The default is 10.
493
494 MaxStartups
495 Specifies the maximum number of concurrent unauthenticated
496 connections to the SSH daemon. Additional connections will be
497 dropped until authentication succeeds or the LoginGraceTime
498 expires for a connection. The default is 10:30:100.
499
500 Alternatively, random early drop can be enabled by specifying the
501 three colon separated values ``start:rate:full'' (e.g.
502 "10:30:60"). sshd(8) will refuse connection attempts with a
503 probability of ``rate/100'' (30%) if there are currently
504 ``start'' (10) unauthenticated connections. The probability
505 increases linearly and all connection attempts are refused if the
506 number of unauthenticated connections reaches ``full'' (60).
507
508 PasswordAuthentication
509 Specifies whether password authentication is allowed. The
510 default is ``yes''.
511
512 PermitEmptyPasswords
513 When password authentication is allowed, it specifies whether the
514 server allows login to accounts with empty password strings. The
515 default is ``no''.
516
517 PermitOpen
518 Specifies the destinations to which TCP port forwarding is
519 permitted. The forwarding specification must be one of the
520 following forms:
521
522 PermitOpen host:port
523 PermitOpen IPv4_addr:port
524 PermitOpen [IPv6_addr]:port
525
526 Multiple forwards may be specified by separating them with
527 whitespace. An argument of ``any'' can be used to remove all
528 restrictions and permit any forwarding requests. An argument of
529 ``none'' can be used to prohibit all forwarding requests. By
530 default all port forwarding requests are permitted.
531
532 PermitRootLogin
533 Specifies whether root can log in using ssh(1). The argument
534 must be ``yes'', ``without-password'', ``forced-commands-only'',
535 or ``no''. The default is ``yes''.
536
537 If this option is set to ``without-password'', password
538 authentication is disabled for root.
539
540 If this option is set to ``forced-commands-only'', root login
541 with public key authentication will be allowed, but only if the
542 command option has been specified (which may be useful for taking
543 remote backups even if root login is normally not allowed). All
544 other authentication methods are disabled for root.
545
546 If this option is set to ``no'', root is not allowed to log in.
547
548 PermitTunnel
549 Specifies whether tun(4) device forwarding is allowed. The
550 argument must be ``yes'', ``point-to-point'' (layer 3),
551 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits
552 both ``point-to-point'' and ``ethernet''. The default is ``no''.
553
554 PermitUserEnvironment
555 Specifies whether ~/.ssh/environment and environment= options in
556 ~/.ssh/authorized_keys are processed by sshd(8). The default is
557 ``no''. Enabling environment processing may enable users to
558 bypass access restrictions in some configurations using
559 mechanisms such as LD_PRELOAD.
560
561 PidFile
562 Specifies the file that contains the process ID of the SSH
563 daemon. The default is /var/run/sshd.pid.
564
565 Port Specifies the port number that sshd(8) listens on. The default
566 is 22. Multiple options of this type are permitted. See also
567 ListenAddress.
568
569 PrintLastLog
570 Specifies whether sshd(8) should print the date and time of the
571 last user login when a user logs in interactively. The default
572 is ``yes''.
573
574 PrintMotd
575 Specifies whether sshd(8) should print /etc/motd when a user logs
576 in interactively. (On some systems it is also printed by the
577 shell, /etc/profile, or equivalent.) The default is ``yes''.
578
579 Protocol
580 Specifies the protocol versions sshd(8) supports. The possible
581 values are `1' and `2'. Multiple versions must be comma-
582 separated. The default is `2'. Note that the order of the
583 protocol list does not indicate preference, because the client
584 selects among multiple protocol versions offered by the server.
585 Specifying ``2,1'' is identical to ``1,2''.
586
587 PubkeyAuthentication
588 Specifies whether public key authentication is allowed. The
589 default is ``yes''. Note that this option applies to protocol
590 version 2 only.
591
592 RekeyLimit
593 Specifies the maximum amount of data that may be transmitted
594 before the session key is renegotiated, optionally followed a
595 maximum amount of time that may pass before the session key is
596 renegotiated. The first argument is specified in bytes and may
597 have a suffix of `K', `M', or `G' to indicate Kilobytes,
598 Megabytes, or Gigabytes, respectively. The default is between
599 `1G' and `4G', depending on the cipher. The optional second
600 value is specified in seconds and may use any of the units
601 documented in the TIME FORMATS section. The default value for
602 RekeyLimit is ``default none'', which means that rekeying is
603 performed after the cipher's default amount of data has been sent
604 or received and no time based rekeying is done. This option
605 applies to protocol version 2 only.
606
607 RevokedKeys
608 Specifies revoked public keys. Keys listed in this file will be
609 refused for public key authentication. Note that if this file is
610 not readable, then public key authentication will be refused for
611 all users. Keys may be specified as a text file, listing one
612 public key per line, or as an OpenSSH Key Revocation List (KRL)
613 as generated by ssh-keygen(1). For more information on KRLs, see
614 the KEY REVOCATION LISTS section in ssh-keygen(1).
615
616 RhostsRSAAuthentication
617 Specifies whether rhosts or /etc/hosts.equiv authentication
618 together with successful RSA host authentication is allowed. The
619 default is ``no''. This option applies to protocol version 1
620 only.
621
622 RSAAuthentication
623 Specifies whether pure RSA authentication is allowed. The
624 default is ``yes''. This option applies to protocol version 1
625 only.
626
627 ServerKeyBits
628 Defines the number of bits in the ephemeral protocol version 1
629 server key. The minimum value is 512, and the default is 1024.
630
631 StrictModes
632 Specifies whether sshd(8) should check file modes and ownership
633 of the user's files and home directory before accepting login.
634 This is normally desirable because novices sometimes accidentally
635 leave their directory or files world-writable. The default is
636 ``yes''. Note that this does not apply to ChrootDirectory, whose
637 permissions and ownership are checked unconditionally.
638
639 Subsystem
640 Configures an external subsystem (e.g. file transfer daemon).
641 Arguments should be a subsystem name and a command (with optional
642 arguments) to execute upon subsystem request.
643
644 The command sftp-server(8) implements the ``sftp'' file transfer
645 subsystem.
646
647 Alternately the name ``internal-sftp'' implements an in-process
648 ``sftp'' server. This may simplify configurations using
649 ChrootDirectory to force a different filesystem root on clients.
650
651 By default no subsystems are defined. Note that this option
652 applies to protocol version 2 only.
653
654 SyslogFacility
655 Gives the facility code that is used when logging messages from
656 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
657 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
658 default is AUTH.
659
660 TCPKeepAlive
661 Specifies whether the system should send TCP keepalive messages
662 to the other side. If they are sent, death of the connection or
663 crash of one of the machines will be properly noticed. However,
664 this means that connections will die if the route is down
665 temporarily, and some people find it annoying. On the other
666 hand, if TCP keepalives are not sent, sessions may hang
667 indefinitely on the server, leaving ``ghost'' users and consuming
668 server resources.
669
670 The default is ``yes'' (to send TCP keepalive messages), and the
671 server will notice if the network goes down or the client host
672 crashes. This avoids infinitely hanging sessions.
673
674 To disable TCP keepalive messages, the value should be set to
675 ``no''.
676
677 TrustedUserCAKeys
678 Specifies a file containing public keys of certificate
679 authorities that are trusted to sign user certificates for
680 authentication. Keys are listed one per line; empty lines and
681 comments starting with `#' are allowed. If a certificate is
682 presented for authentication and has its signing CA key listed in
683 this file, then it may be used for authentication for any user
684 listed in the certificate's principals list. Note that
685 certificates that lack a list of principals will not be permitted
686 for authentication using TrustedUserCAKeys. For more details on
687 certificates, see the CERTIFICATES section in ssh-keygen(1).
688
689 UseDNS Specifies whether sshd(8) should look up the remote host name and
690 check that the resolved host name for the remote IP address maps
691 back to the very same IP address. The default is ``yes''.
692
693 UseLogin
694 Specifies whether login(1) is used for interactive login
695 sessions. The default is ``no''. Note that login(1) is never
696 used for remote command execution. Note also, that if this is
697 enabled, X11Forwarding will be disabled because login(1) does not
698 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
699 is specified, it will be disabled after authentication.
700
701 UsePAM Enables the Pluggable Authentication Module interface. If set to
702 ``yes'' this will enable PAM authentication using
703 ChallengeResponseAuthentication and PasswordAuthentication in
704 addition to PAM account and session module processing for all
705 authentication types.
706
707 Because PAM challenge-response authentication usually serves an
708 equivalent role to password authentication, you should disable
709 either PasswordAuthentication or ChallengeResponseAuthentication.
710
711 If UsePAM is enabled, you will not be able to run sshd(8) as a
712 non-root user. The default is ``no''.
713
714 UsePrivilegeSeparation
715 Specifies whether sshd(8) separates privileges by creating an
716 unprivileged child process to deal with incoming network traffic.
717 After successful authentication, another process will be created
718 that has the privilege of the authenticated user. The goal of
719 privilege separation is to prevent privilege escalation by
720 containing any corruption within the unprivileged processes. The
721 default is ``yes''. If UsePrivilegeSeparation is set to
722 ``sandbox'' then the pre-authentication unprivileged process is
723 subject to additional restrictions.
724
725 VersionAddendum
726 Optionally specifies additional text to append to the SSH
727 protocol banner sent by the server upon connection. The default
728 is ``none''.
729
730 X11DisplayOffset
731 Specifies the first display number available for sshd(8)'s X11
732 forwarding. This prevents sshd from interfering with real X11
733 servers. The default is 10.
734
735 X11Forwarding
736 Specifies whether X11 forwarding is permitted. The argument must
737 be ``yes'' or ``no''. The default is ``no''.
738
739 When X11 forwarding is enabled, there may be additional exposure
740 to the server and to client displays if the sshd(8) proxy display
741 is configured to listen on the wildcard address (see
742 X11UseLocalhost below), though this is not the default.
743 Additionally, the authentication spoofing and authentication data
744 verification and substitution occur on the client side. The
745 security risk of using X11 forwarding is that the client's X11
746 display server may be exposed to attack when the SSH client
747 requests forwarding (see the warnings for ForwardX11 in
748 ssh_config(5)). A system administrator may have a stance in
749 which they want to protect clients that may expose themselves to
750 attack by unwittingly requesting X11 forwarding, which can
751 warrant a ``no'' setting.
752
753 Note that disabling X11 forwarding does not prevent users from
754 forwarding X11 traffic, as users can always install their own
755 forwarders. X11 forwarding is automatically disabled if UseLogin
756 is enabled.
757
758 X11UseLocalhost
759 Specifies whether sshd(8) should bind the X11 forwarding server
760 to the loopback address or to the wildcard address. By default,
761 sshd binds the forwarding server to the loopback address and sets
762 the hostname part of the DISPLAY environment variable to
763 ``localhost''. This prevents remote hosts from connecting to the
764 proxy display. However, some older X11 clients may not function
765 with this configuration. X11UseLocalhost may be set to ``no'' to
766 specify that the forwarding server should be bound to the
767 wildcard address. The argument must be ``yes'' or ``no''. The
768 default is ``yes''.
769
770 XAuthLocation
771 Specifies the full pathname of the xauth(1) program. The default
772 is /usr/X11R6/bin/xauth.
773
774TIME FORMATS
775 sshd(8) command-line arguments and configuration file options that
776 specify time may be expressed using a sequence of the form:
777 time[qualifier], where time is a positive integer value and qualifier is
778 one of the following:
779
780 <none> seconds
781 s | S seconds
782 m | M minutes
783 h | H hours
784 d | D days
785 w | W weeks
786
787 Each member of the sequence is added together to calculate the total time
788 value.
789
790 Time format examples:
791
792 600 600 seconds (10 minutes)
793 10m 10 minutes
794 1h30m 1 hour 30 minutes (90 minutes)
795
796FILES
797 /etc/ssh/sshd_config
798 Contains configuration data for sshd(8). This file should be
799 writable by root only, but it is recommended (though not
800 necessary) that it be world-readable.
801
802SEE ALSO
803 sshd(8)
804
805AUTHORS
806 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
807 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
808 de Raadt and Dug Song removed many bugs, re-added newer features and
809 created OpenSSH. Markus Friedl contributed the support for SSH protocol
810 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
811 for privilege separation.
812
813OpenBSD 5.4 July 19, 2013 OpenBSD 5.4
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-09 21:45:24 +0000