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1SSHD_CONFIG(5) File Formats 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 AllowStreamLocalForwarding
66 Specifies whether StreamLocal (Unix-domain socket) forwarding is
67 permitted. The available options are ``yes'' or ``all'' to allow
68 StreamLocal forwarding, ``no'' to prevent all StreamLocal
69 forwarding, ``local'' to allow local (from the perspective of
70 ssh(1)) forwarding only or ``remote'' to allow remote forwarding
71 only. The default is ``yes''. Note that disabling StreamLocal
72 forwarding does not improve security unless users are also denied
73 shell access, as they can always install their own forwarders.
74
75 AllowUsers
76 This keyword can be followed by a list of user name patterns,
77 separated by spaces. If specified, login is allowed only for
78 user names that match one of the patterns. Only user names are
79 valid; a numerical user ID is not recognized. By default, login
80 is allowed for all users. If the pattern takes the form
81 USER@HOST then USER and HOST are separately checked, restricting
82 logins to particular users from particular hosts. The allow/deny
83 directives are processed in the following order: DenyUsers,
84 AllowUsers, DenyGroups, and finally AllowGroups.
85
86 See PATTERNS in ssh_config(5) for more information on patterns.
87
88 AuthenticationMethods
89 Specifies the authentication methods that must be successfully
90 completed for a user to be granted access. This option must be
91 followed by one or more comma-separated lists of authentication
92 method names. Successful authentication requires completion of
93 every method in at least one of these lists.
94
95 For example, an argument of ``publickey,password
96 publickey,keyboard-interactive'' would require the user to
97 complete public key authentication, followed by either password
98 or keyboard interactive authentication. Only methods that are
99 next in one or more lists are offered at each stage, so for this
100 example, it would not be possible to attempt password or
101 keyboard-interactive authentication before public key.
102
103 For keyboard interactive authentication it is also possible to
104 restrict authentication to a specific device by appending a colon
105 followed by the device identifier ``bsdauth'', ``pam'', or
106 ``skey'', depending on the server configuration. For example,
107 ``keyboard-interactive:bsdauth'' would restrict keyboard
108 interactive authentication to the ``bsdauth'' device.
109
110 This option is only available for SSH protocol 2 and will yield a
111 fatal error if enabled if protocol 1 is also enabled. Note that
112 each authentication method listed should also be explicitly
113 enabled in the configuration. The default is not to require
114 multiple authentication; successful completion of a single
115 authentication method is sufficient.
116
117 AuthorizedKeysCommand
118 Specifies a program to be used to look up the user's public keys.
119 The program must be owned by root and not writable by group or
120 others. It will be invoked with a single argument of the
121 username being authenticated, and should produce on standard
122 output zero or more lines of authorized_keys output (see
123 AUTHORIZED_KEYS in sshd(8)). If a key supplied by
124 AuthorizedKeysCommand does not successfully authenticate and
125 authorize the user then public key authentication continues using
126 the usual AuthorizedKeysFile files. By default, no
127 AuthorizedKeysCommand is run.
128
129 AuthorizedKeysCommandUser
130 Specifies the user under whose account the AuthorizedKeysCommand
131 is run. It is recommended to use a dedicated user that has no
132 other role on the host than running authorized keys commands.
133
134 AuthorizedKeysFile
135 Specifies the file that contains the public keys that can be used
136 for user authentication. The format is described in the
137 AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
138 AuthorizedKeysFile may contain tokens of the form %T which are
139 substituted during connection setup. The following tokens are
140 defined: %% is replaced by a literal '%', %h is replaced by the
141 home directory of the user being authenticated, and %u is
142 replaced by the username of that user. After expansion,
143 AuthorizedKeysFile is taken to be an absolute path or one
144 relative to the user's home directory. Multiple files may be
145 listed, separated by whitespace. The default is
146 ``.ssh/authorized_keys .ssh/authorized_keys2''.
147
148 AuthorizedPrincipalsFile
149 Specifies a file that lists principal names that are accepted for
150 certificate authentication. When using certificates signed by a
151 key listed in TrustedUserCAKeys, this file lists names, one of
152 which must appear in the certificate for it to be accepted for
153 authentication. Names are listed one per line preceded by key
154 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
155 Empty lines and comments starting with `#' are ignored.
156
157 AuthorizedPrincipalsFile may contain tokens of the form %T which
158 are substituted during connection setup. The following tokens
159 are defined: %% is replaced by a literal '%', %h is replaced by
160 the home directory of the user being authenticated, and %u is
161 replaced by the username of that user. After expansion,
162 AuthorizedPrincipalsFile is taken to be an absolute path or one
163 relative to the user's home directory.
164
165 The default is ``none'', i.e. not to use a principals file - in
166 this case, the username of the user must appear in a
167 certificate's principals list for it to be accepted. Note that
168 AuthorizedPrincipalsFile is only used when authentication
169 proceeds using a CA listed in TrustedUserCAKeys and is not
170 consulted for certification authorities trusted via
171 ~/.ssh/authorized_keys, though the principals= key option offers
172 a similar facility (see sshd(8) for details).
173
174 Banner The contents of the specified file are sent to the remote user
175 before authentication is allowed. If the argument is ``none''
176 then no banner is displayed. This option is only available for
177 protocol version 2. By default, no banner is displayed.
178
179 ChallengeResponseAuthentication
180 Specifies whether challenge-response authentication is allowed
181 (e.g. via PAM or through authentication styles supported in
182 login.conf(5)) The default is ``yes''.
183
184 ChrootDirectory
185 Specifies the pathname of a directory to chroot(2) to after
186 authentication. All components of the pathname must be root-
187 owned directories that are not writable by any other user or
188 group. After the chroot, sshd(8) changes the working directory
189 to the user's home directory.
190
191 The pathname may contain the following tokens that are expanded
192 at runtime once the connecting user has been authenticated: %% is
193 replaced by a literal '%', %h is replaced by the home directory
194 of the user being authenticated, and %u is replaced by the
195 username of that user.
196
197 The ChrootDirectory must contain the necessary files and
198 directories to support the user's session. For an interactive
199 session this requires at least a shell, typically sh(1), and
200 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
201 stderr(4), arandom(4) and tty(4) devices. For file transfer
202 sessions using ``sftp'', no additional configuration of the
203 environment is necessary if the in-process sftp server is used,
204 though sessions which use logging may require /dev/log inside the
205 chroot directory on some operating systems (see sftp-server(8)
206 for details).
207
208 The default is not to chroot(2).
209
210 Ciphers
211 Specifies the ciphers allowed for protocol version 2. Multiple
212 ciphers must be comma-separated. The supported ciphers are:
213
214 3des-cbc
215 aes128-cbc
216 aes192-cbc
217 aes256-cbc
218 aes128-ctr
219 aes192-ctr
220 aes256-ctr
221 aes128-gcm@openssh.com
222 aes256-gcm@openssh.com
223 arcfour
224 arcfour128
225 arcfour256
226 blowfish-cbc
227 cast128-cbc
228 chacha20-poly1305@openssh.com
229
230 The default is:
231
232 aes128-ctr,aes192-ctr,aes256-ctr,
233 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
234 chacha20-poly1305@openssh.com
235
236 The list of available ciphers may also be obtained using the -Q
237 option of ssh(1).
238
239 ClientAliveCountMax
240 Sets the number of client alive messages (see below) which may be
241 sent without sshd(8) receiving any messages back from the client.
242 If this threshold is reached while client alive messages are
243 being sent, sshd will disconnect the client, terminating the
244 session. It is important to note that the use of client alive
245 messages is very different from TCPKeepAlive (below). The client
246 alive messages are sent through the encrypted channel and
247 therefore will not be spoofable. The TCP keepalive option
248 enabled by TCPKeepAlive is spoofable. The client alive mechanism
249 is valuable when the client or server depend on knowing when a
250 connection has become inactive.
251
252 The default value is 3. If ClientAliveInterval (see below) is
253 set to 15, and ClientAliveCountMax is left at the default,
254 unresponsive SSH clients will be disconnected after approximately
255 45 seconds. This option applies to protocol version 2 only.
256
257 ClientAliveInterval
258 Sets a timeout interval in seconds after which if no data has
259 been received from the client, sshd(8) will send a message
260 through the encrypted channel to request a response from the
261 client. The default is 0, indicating that these messages will
262 not be sent to the client. This option applies to protocol
263 version 2 only.
264
265 Compression
266 Specifies whether compression is allowed, or delayed until the
267 user has authenticated successfully. The argument must be
268 ``yes'', ``delayed'', or ``no''. The default is ``delayed''.
269
270 DenyGroups
271 This keyword can be followed by a list of group name patterns,
272 separated by spaces. Login is disallowed for users whose primary
273 group or supplementary group list matches one of the patterns.
274 Only group names are valid; a numerical group ID is not
275 recognized. By default, login is allowed for all groups. The
276 allow/deny directives are processed in the following order:
277 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
278
279 See PATTERNS in ssh_config(5) for more information on patterns.
280
281 DenyUsers
282 This keyword can be followed by a list of user name patterns,
283 separated by spaces. Login is disallowed for user names that
284 match one of the patterns. Only user names are valid; a
285 numerical user ID is not recognized. By default, login is
286 allowed for all users. If the pattern takes the form USER@HOST
287 then USER and HOST are separately checked, restricting logins to
288 particular users from particular hosts. The allow/deny
289 directives are processed in the following order: DenyUsers,
290 AllowUsers, DenyGroups, and finally AllowGroups.
291
292 See PATTERNS in ssh_config(5) for more information on patterns.
293
294 ForceCommand
295 Forces the execution of the command specified by ForceCommand,
296 ignoring any command supplied by the client and ~/.ssh/rc if
297 present. The command is invoked by using the user's login shell
298 with the -c option. This applies to shell, command, or subsystem
299 execution. It is most useful inside a Match block. The command
300 originally supplied by the client is available in the
301 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
302 of ``internal-sftp'' will force the use of an in-process sftp
303 server that requires no support files when used with
304 ChrootDirectory.
305
306 GatewayPorts
307 Specifies whether remote hosts are allowed to connect to ports
308 forwarded for the client. By default, sshd(8) binds remote port
309 forwardings to the loopback address. This prevents other remote
310 hosts from connecting to forwarded ports. GatewayPorts can be
311 used to specify that sshd should allow remote port forwardings to
312 bind to non-loopback addresses, thus allowing other hosts to
313 connect. The argument may be ``no'' to force remote port
314 forwardings to be available to the local host only, ``yes'' to
315 force remote port forwardings to bind to the wildcard address, or
316 ``clientspecified'' to allow the client to select the address to
317 which the forwarding is bound. The default is ``no''.
318
319 GSSAPIAuthentication
320 Specifies whether user authentication based on GSSAPI is allowed.
321 The default is ``no''. Note that this option applies to protocol
322 version 2 only.
323
324 GSSAPICleanupCredentials
325 Specifies whether to automatically destroy the user's credentials
326 cache on logout. The default is ``yes''. Note that this option
327 applies to protocol version 2 only.
328
329 HostbasedAuthentication
330 Specifies whether rhosts or /etc/hosts.equiv authentication
331 together with successful public key client host authentication is
332 allowed (host-based authentication). This option is similar to
333 RhostsRSAAuthentication and applies to protocol version 2 only.
334 The default is ``no''.
335
336 HostbasedUsesNameFromPacketOnly
337 Specifies whether or not the server will attempt to perform a
338 reverse name lookup when matching the name in the ~/.shosts,
339 ~/.rhosts, and /etc/hosts.equiv files during
340 HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
341 uses the name supplied by the client rather than attempting to
342 resolve the name from the TCP connection itself. The default is
343 ``no''.
344
345 HostCertificate
346 Specifies a file containing a public host certificate. The
347 certificate's public key must match a private host key already
348 specified by HostKey. The default behaviour of sshd(8) is not to
349 load any certificates.
350
351 HostKey
352 Specifies a file containing a private host key used by SSH. The
353 default is /etc/ssh/ssh_host_key for protocol version 1, and
354 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
355 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
356 protocol version 2. Note that sshd(8) will refuse to use a file
357 if it is group/world-accessible. It is possible to have multiple
358 host key files. ``rsa1'' keys are used for version 1 and
359 ``dsa'', ``ecdsa'', ``ed25519'' or ``rsa'' are used for version 2
360 of the SSH protocol. It is also possible to specify public host
361 key files instead. In this case operations on the private key
362 will be delegated to an ssh-agent(1).
363
364 HostKeyAgent
365 Identifies the UNIX-domain socket used to communicate with an
366 agent that has access to the private host keys. If
367 ``SSH_AUTH_SOCK'' is specified, the location of the socket will
368 be read from the SSH_AUTH_SOCK environment variable.
369
370 IgnoreRhosts
371 Specifies that .rhosts and .shosts files will not be used in
372 RhostsRSAAuthentication or HostbasedAuthentication.
373
374 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
375 default is ``yes''.
376
377 IgnoreUserKnownHosts
378 Specifies whether sshd(8) should ignore the user's
379 ~/.ssh/known_hosts during RhostsRSAAuthentication or
380 HostbasedAuthentication. The default is ``no''.
381
382 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
383 connection. Accepted values are ``af11'', ``af12'', ``af13'',
384 ``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'',
385 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'',
386 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
387 ``throughput'', ``reliability'', or a numeric value. This option
388 may take one or two arguments, separated by whitespace. If one
389 argument is specified, it is used as the packet class
390 unconditionally. If two values are specified, the first is
391 automatically selected for interactive sessions and the second
392 for non-interactive sessions. The default is ``lowdelay'' for
393 interactive sessions and ``throughput'' for non-interactive
394 sessions.
395
396 KbdInteractiveAuthentication
397 Specifies whether to allow keyboard-interactive authentication.
398 The argument to this keyword must be ``yes'' or ``no''. The
399 default is to use whatever value ChallengeResponseAuthentication
400 is set to (by default ``yes'').
401
402 KerberosAuthentication
403 Specifies whether the password provided by the user for
404 PasswordAuthentication will be validated through the Kerberos
405 KDC. To use this option, the server needs a Kerberos servtab
406 which allows the verification of the KDC's identity. The default
407 is ``no''.
408
409 KerberosGetAFSToken
410 If AFS is active and the user has a Kerberos 5 TGT, attempt to
411 acquire an AFS token before accessing the user's home directory.
412 The default is ``no''.
413
414 KerberosOrLocalPasswd
415 If password authentication through Kerberos fails then the
416 password will be validated via any additional local mechanism
417 such as /etc/passwd. The default is ``yes''.
418
419 KerberosTicketCleanup
420 Specifies whether to automatically destroy the user's ticket
421 cache file on logout. The default is ``yes''.
422
423 KexAlgorithms
424 Specifies the available KEX (Key Exchange) algorithms. Multiple
425 algorithms must be comma-separated. The supported algorithms
426 are:
427
428 curve25519-sha256@libssh.org
429 diffie-hellman-group1-sha1
430 diffie-hellman-group14-sha1
431 diffie-hellman-group-exchange-sha1
432 diffie-hellman-group-exchange-sha256
433 ecdh-sha2-nistp256
434 ecdh-sha2-nistp384
435 ecdh-sha2-nistp521
436
437 The default is:
438
439 curve25519-sha256@libssh.org,
440 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
441 diffie-hellman-group-exchange-sha256,
442 diffie-hellman-group14-sha1
443
444 KeyRegenerationInterval
445 In protocol version 1, the ephemeral server key is automatically
446 regenerated after this many seconds (if it has been used). The
447 purpose of regeneration is to prevent decrypting captured
448 sessions by later breaking into the machine and stealing the
449 keys. The key is never stored anywhere. If the value is 0, the
450 key is never regenerated. The default is 3600 (seconds).
451
452 ListenAddress
453 Specifies the local addresses sshd(8) should listen on. The
454 following forms may be used:
455
456 ListenAddress host|IPv4_addr|IPv6_addr
457 ListenAddress host|IPv4_addr:port
458 ListenAddress [host|IPv6_addr]:port
459
460 If port is not specified, sshd will listen on the address and all
461 prior Port options specified. The default is to listen on all
462 local addresses. Multiple ListenAddress options are permitted.
463 Additionally, any Port options must precede this option for non-
464 port qualified addresses.
465
466 LoginGraceTime
467 The server disconnects after this time if the user has not
468 successfully logged in. If the value is 0, there is no time
469 limit. The default is 120 seconds.
470
471 LogLevel
472 Gives the verbosity level that is used when logging messages from
473 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
474 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
475 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
476 higher levels of debugging output. Logging with a DEBUG level
477 violates the privacy of users and is not recommended.
478
479 MACs Specifies the available MAC (message authentication code)
480 algorithms. The MAC algorithm is used in protocol version 2 for
481 data integrity protection. Multiple algorithms must be comma-
482 separated. The algorithms that contain ``-etm'' calculate the
483 MAC after encryption (encrypt-then-mac). These are considered
484 safer and their use recommended. The supported MACs are:
485
486 hmac-md5
487 hmac-md5-96
488 hmac-ripemd160
489 hmac-sha1
490 hmac-sha1-96
491 hmac-sha2-256
492 hmac-sha2-512
493 umac-64@openssh.com
494 umac-128@openssh.com
495 hmac-md5-etm@openssh.com
496 hmac-md5-96-etm@openssh.com
497 hmac-ripemd160-etm@openssh.com
498 hmac-sha1-etm@openssh.com
499 hmac-sha1-96-etm@openssh.com
500 hmac-sha2-256-etm@openssh.com
501 hmac-sha2-512-etm@openssh.com
502 umac-64-etm@openssh.com
503 umac-128-etm@openssh.com
504
505 The default is:
506
507 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
508 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
509 umac-64@openssh.com,umac-128@openssh.com,
510 hmac-sha2-256,hmac-sha2-512
511
512 Match Introduces a conditional block. If all of the criteria on the
513 Match line are satisfied, the keywords on the following lines
514 override those set in the global section of the config file,
515 until either another Match line or the end of the file. If a
516 keyword appears in multiple Match blocks that are satisified,
517 only the first instance of the keyword is applied.
518
519 The arguments to Match are one or more criteria-pattern pairs or
520 the single token All which matches all criteria. The available
521 criteria are User, Group, Host, LocalAddress, LocalPort, and
522 Address. The match patterns may consist of single entries or
523 comma-separated lists and may use the wildcard and negation
524 operators described in the PATTERNS section of ssh_config(5).
525
526 The patterns in an Address criteria may additionally contain
527 addresses to match in CIDR address/masklen format, e.g.
528 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
529 length provided must be consistent with the address - it is an
530 error to specify a mask length that is too long for the address
531 or one with bits set in this host portion of the address. For
532 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
533
534 Only a subset of keywords may be used on the lines following a
535 Match keyword. Available keywords are AcceptEnv,
536 AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
537 AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
538 AuthorizedKeysCommandUser, AuthorizedKeysFile,
539 AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
540 DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
541 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
542 KbdInteractiveAuthentication, KerberosAuthentication,
543 MaxAuthTries, MaxSessions, PasswordAuthentication,
544 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
545 PermitTunnel, PermitUserRC, PubkeyAuthentication, RekeyLimit,
546 RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
547 X11Forwarding and X11UseLocalHost.
548
549 MaxAuthTries
550 Specifies the maximum number of authentication attempts permitted
551 per connection. Once the number of failures reaches half this
552 value, additional failures are logged. The default is 6.
553
554 MaxSessions
555 Specifies the maximum number of open sessions permitted per
556 network connection. The default is 10.
557
558 MaxStartups
559 Specifies the maximum number of concurrent unauthenticated
560 connections to the SSH daemon. Additional connections will be
561 dropped until authentication succeeds or the LoginGraceTime
562 expires for a connection. The default is 10:30:100.
563
564 Alternatively, random early drop can be enabled by specifying the
565 three colon separated values ``start:rate:full'' (e.g.
566 "10:30:60"). sshd(8) will refuse connection attempts with a
567 probability of ``rate/100'' (30%) if there are currently
568 ``start'' (10) unauthenticated connections. The probability
569 increases linearly and all connection attempts are refused if the
570 number of unauthenticated connections reaches ``full'' (60).
571
572 PasswordAuthentication
573 Specifies whether password authentication is allowed. The
574 default is ``yes''.
575
576 PermitEmptyPasswords
577 When password authentication is allowed, it specifies whether the
578 server allows login to accounts with empty password strings. The
579 default is ``no''.
580
581 PermitOpen
582 Specifies the destinations to which TCP port forwarding is
583 permitted. The forwarding specification must be one of the
584 following forms:
585
586 PermitOpen host:port
587 PermitOpen IPv4_addr:port
588 PermitOpen [IPv6_addr]:port
589
590 Multiple forwards may be specified by separating them with
591 whitespace. An argument of ``any'' can be used to remove all
592 restrictions and permit any forwarding requests. An argument of
593 ``none'' can be used to prohibit all forwarding requests. By
594 default all port forwarding requests are permitted.
595
596 PermitRootLogin
597 Specifies whether root can log in using ssh(1). The argument
598 must be ``yes'', ``without-password'', ``forced-commands-only'',
599 or ``no''. The default is ``yes''.
600
601 If this option is set to ``without-password'', password
602 authentication is disabled for root.
603
604 If this option is set to ``forced-commands-only'', root login
605 with public key authentication will be allowed, but only if the
606 command option has been specified (which may be useful for taking
607 remote backups even if root login is normally not allowed). All
608 other authentication methods are disabled for root.
609
610 If this option is set to ``no'', root is not allowed to log in.
611
612 PermitTunnel
613 Specifies whether tun(4) device forwarding is allowed. The
614 argument must be ``yes'', ``point-to-point'' (layer 3),
615 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits
616 both ``point-to-point'' and ``ethernet''. The default is ``no''.
617
618 PermitTTY
619 Specifies whether pty(4) allocation is permitted. The default is
620 ``yes''.
621
622 PermitUserEnvironment
623 Specifies whether ~/.ssh/environment and environment= options in
624 ~/.ssh/authorized_keys are processed by sshd(8). The default is
625 ``no''. Enabling environment processing may enable users to
626 bypass access restrictions in some configurations using
627 mechanisms such as LD_PRELOAD.
628
629 PermitUserRC
630 Specifies whether any ~/.ssh/rc file is executed. The default is
631 ``yes''.
632
633 PidFile
634 Specifies the file that contains the process ID of the SSH
635 daemon. The default is /var/run/sshd.pid.
636
637 Port Specifies the port number that sshd(8) listens on. The default
638 is 22. Multiple options of this type are permitted. See also
639 ListenAddress.
640
641 PrintLastLog
642 Specifies whether sshd(8) should print the date and time of the
643 last user login when a user logs in interactively. The default
644 is ``yes''.
645
646 PrintMotd
647 Specifies whether sshd(8) should print /etc/motd when a user logs
648 in interactively. (On some systems it is also printed by the
649 shell, /etc/profile, or equivalent.) The default is ``yes''.
650
651 Protocol
652 Specifies the protocol versions sshd(8) supports. The possible
653 values are `1' and `2'. Multiple versions must be comma-
654 separated. The default is `2'. Note that the order of the
655 protocol list does not indicate preference, because the client
656 selects among multiple protocol versions offered by the server.
657 Specifying ``2,1'' is identical to ``1,2''.
658
659 PubkeyAuthentication
660 Specifies whether public key authentication is allowed. The
661 default is ``yes''. Note that this option applies to protocol
662 version 2 only.
663
664 RekeyLimit
665 Specifies the maximum amount of data that may be transmitted
666 before the session key is renegotiated, optionally followed a
667 maximum amount of time that may pass before the session key is
668 renegotiated. The first argument is specified in bytes and may
669 have a suffix of `K', `M', or `G' to indicate Kilobytes,
670 Megabytes, or Gigabytes, respectively. The default is between
671 `1G' and `4G', depending on the cipher. The optional second
672 value is specified in seconds and may use any of the units
673 documented in the TIME FORMATS section. The default value for
674 RekeyLimit is ``default none'', which means that rekeying is
675 performed after the cipher's default amount of data has been sent
676 or received and no time based rekeying is done. This option
677 applies to protocol version 2 only.
678
679 RevokedKeys
680 Specifies revoked public keys. Keys listed in this file will be
681 refused for public key authentication. Note that if this file is
682 not readable, then public key authentication will be refused for
683 all users. Keys may be specified as a text file, listing one
684 public key per line, or as an OpenSSH Key Revocation List (KRL)
685 as generated by ssh-keygen(1). For more information on KRLs, see
686 the KEY REVOCATION LISTS section in ssh-keygen(1).
687
688 RhostsRSAAuthentication
689 Specifies whether rhosts or /etc/hosts.equiv authentication
690 together with successful RSA host authentication is allowed. The
691 default is ``no''. This option applies to protocol version 1
692 only.
693
694 RSAAuthentication
695 Specifies whether pure RSA authentication is allowed. The
696 default is ``yes''. This option applies to protocol version 1
697 only.
698
699 ServerKeyBits
700 Defines the number of bits in the ephemeral protocol version 1
701 server key. The minimum value is 512, and the default is 1024.
702
703 StreamLocalBindMask
704 Sets the octal file creation mode mask (umask) used when creating
705 a Unix-domain socket file for local or remote port forwarding.
706 This option is only used for port forwarding to a Unix-domain
707 socket file.
708
709 The default value is 0177, which creates a Unix-domain socket
710 file that is readable and writable only by the owner. Note that
711 not all operating systems honor the file mode on Unix-domain
712 socket files.
713
714 StreamLocalBindUnlink
715 Specifies whether to remove an existing Unix-domain socket file
716 for local or remote port forwarding before creating a new one.
717 If the socket file already exists and StreamLocalBindUnlink is
718 not enabled, sshd will be unable to forward the port to the Unix-
719 domain socket file. This option is only used for port forwarding
720 to a Unix-domain socket file.
721
722 The argument must be ``yes'' or ``no''. The default is ``no''.
723
724 StrictModes
725 Specifies whether sshd(8) should check file modes and ownership
726 of the user's files and home directory before accepting login.
727 This is normally desirable because novices sometimes accidentally
728 leave their directory or files world-writable. The default is
729 ``yes''. Note that this does not apply to ChrootDirectory, whose
730 permissions and ownership are checked unconditionally.
731
732 Subsystem
733 Configures an external subsystem (e.g. file transfer daemon).
734 Arguments should be a subsystem name and a command (with optional
735 arguments) to execute upon subsystem request.
736
737 The command sftp-server(8) implements the ``sftp'' file transfer
738 subsystem.
739
740 Alternately the name ``internal-sftp'' implements an in-process
741 ``sftp'' server. This may simplify configurations using
742 ChrootDirectory to force a different filesystem root on clients.
743
744 By default no subsystems are defined. Note that this option
745 applies to protocol version 2 only.
746
747 SyslogFacility
748 Gives the facility code that is used when logging messages from
749 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
750 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
751 default is AUTH.
752
753 TCPKeepAlive
754 Specifies whether the system should send TCP keepalive messages
755 to the other side. If they are sent, death of the connection or
756 crash of one of the machines will be properly noticed. However,
757 this means that connections will die if the route is down
758 temporarily, and some people find it annoying. On the other
759 hand, if TCP keepalives are not sent, sessions may hang
760 indefinitely on the server, leaving ``ghost'' users and consuming
761 server resources.
762
763 The default is ``yes'' (to send TCP keepalive messages), and the
764 server will notice if the network goes down or the client host
765 crashes. This avoids infinitely hanging sessions.
766
767 To disable TCP keepalive messages, the value should be set to
768 ``no''.
769
770 TrustedUserCAKeys
771 Specifies a file containing public keys of certificate
772 authorities that are trusted to sign user certificates for
773 authentication. Keys are listed one per line; empty lines and
774 comments starting with `#' are allowed. If a certificate is
775 presented for authentication and has its signing CA key listed in
776 this file, then it may be used for authentication for any user
777 listed in the certificate's principals list. Note that
778 certificates that lack a list of principals will not be permitted
779 for authentication using TrustedUserCAKeys. For more details on
780 certificates, see the CERTIFICATES section in ssh-keygen(1).
781
782 UseDNS Specifies whether sshd(8) should look up the remote host name and
783 check that the resolved host name for the remote IP address maps
784 back to the very same IP address. The default is ``yes''.
785
786 UseLogin
787 Specifies whether login(1) is used for interactive login
788 sessions. The default is ``no''. Note that login(1) is never
789 used for remote command execution. Note also, that if this is
790 enabled, X11Forwarding will be disabled because login(1) does not
791 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
792 is specified, it will be disabled after authentication.
793
794 UsePAM Enables the Pluggable Authentication Module interface. If set to
795 ``yes'' this will enable PAM authentication using
796 ChallengeResponseAuthentication and PasswordAuthentication in
797 addition to PAM account and session module processing for all
798 authentication types.
799
800 Because PAM challenge-response authentication usually serves an
801 equivalent role to password authentication, you should disable
802 either PasswordAuthentication or ChallengeResponseAuthentication.
803
804 If UsePAM is enabled, you will not be able to run sshd(8) as a
805 non-root user. The default is ``no''.
806
807 UsePrivilegeSeparation
808 Specifies whether sshd(8) separates privileges by creating an
809 unprivileged child process to deal with incoming network traffic.
810 After successful authentication, another process will be created
811 that has the privilege of the authenticated user. The goal of
812 privilege separation is to prevent privilege escalation by
813 containing any corruption within the unprivileged processes. The
814 default is ``yes''. If UsePrivilegeSeparation is set to
815 ``sandbox'' then the pre-authentication unprivileged process is
816 subject to additional restrictions.
817
818 VersionAddendum
819 Optionally specifies additional text to append to the SSH
820 protocol banner sent by the server upon connection. The default
821 is ``none''.
822
823 X11DisplayOffset
824 Specifies the first display number available for sshd(8)'s X11
825 forwarding. This prevents sshd from interfering with real X11
826 servers. The default is 10.
827
828 X11Forwarding
829 Specifies whether X11 forwarding is permitted. The argument must
830 be ``yes'' or ``no''. The default is ``no''.
831
832 When X11 forwarding is enabled, there may be additional exposure
833 to the server and to client displays if the sshd(8) proxy display
834 is configured to listen on the wildcard address (see
835 X11UseLocalhost below), though this is not the default.
836 Additionally, the authentication spoofing and authentication data
837 verification and substitution occur on the client side. The
838 security risk of using X11 forwarding is that the client's X11
839 display server may be exposed to attack when the SSH client
840 requests forwarding (see the warnings for ForwardX11 in
841 ssh_config(5)). A system administrator may have a stance in
842 which they want to protect clients that may expose themselves to
843 attack by unwittingly requesting X11 forwarding, which can
844 warrant a ``no'' setting.
845
846 Note that disabling X11 forwarding does not prevent users from
847 forwarding X11 traffic, as users can always install their own
848 forwarders. X11 forwarding is automatically disabled if UseLogin
849 is enabled.
850
851 X11UseLocalhost
852 Specifies whether sshd(8) should bind the X11 forwarding server
853 to the loopback address or to the wildcard address. By default,
854 sshd binds the forwarding server to the loopback address and sets
855 the hostname part of the DISPLAY environment variable to
856 ``localhost''. This prevents remote hosts from connecting to the
857 proxy display. However, some older X11 clients may not function
858 with this configuration. X11UseLocalhost may be set to ``no'' to
859 specify that the forwarding server should be bound to the
860 wildcard address. The argument must be ``yes'' or ``no''. The
861 default is ``yes''.
862
863 XAuthLocation
864 Specifies the full pathname of the xauth(1) program. The default
865 is /usr/X11R6/bin/xauth.
866
867TIME FORMATS
868 sshd(8) command-line arguments and configuration file options that
869 specify time may be expressed using a sequence of the form:
870 time[qualifier], where time is a positive integer value and qualifier is
871 one of the following:
872
873 <none> seconds
874 s | S seconds
875 m | M minutes
876 h | H hours
877 d | D days
878 w | W weeks
879
880 Each member of the sequence is added together to calculate the total time
881 value.
882
883 Time format examples:
884
885 600 600 seconds (10 minutes)
886 10m 10 minutes
887 1h30m 1 hour 30 minutes (90 minutes)
888
889FILES
890 /etc/ssh/sshd_config
891 Contains configuration data for sshd(8). This file should be
892 writable by root only, but it is recommended (though not
893 necessary) that it be world-readable.
894
895SEE ALSO
896 sshd(8)
897
898AUTHORS
899 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
900 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
901 de Raadt and Dug Song removed many bugs, re-added newer features and
902 created OpenSSH. Markus Friedl contributed the support for SSH protocol
903 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
904 for privilege separation.
905
906OpenBSD 5.6 July 28, 2014 OpenBSD 5.6
generated by cgit v1.2.3 (git 2.39.1) at 2024-06-27 09:43:45 +0000