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1 | SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) | ||
2 | |||
3 | NAME | ||
4 | sshd_config - OpenSSH SSH daemon configuration file | ||
5 | |||
6 | SYNOPSIS | ||
7 | /etc/ssh/sshd_config | ||
8 | |||
9 | DESCRIPTION | ||
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-argu- | ||
12 | ment pairs, one per line. Lines starting with `#' and empty lines are | ||
13 | interpreted as comments. Arguments may optionally be enclosed in double | ||
14 | quotes (") in order to represent arguments containing spaces. | ||
15 | |||
16 | The possible keywords and their meanings are as follows (note that key- | ||
17 | words 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 envi- | ||
23 | ronment passing is only supported for protocol 2. Variables are | ||
24 | 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 de- | ||
39 | fault is ``yes''. Note that disabling agent forwarding does not | ||
40 | improve security unless users are also denied shell access, as | ||
41 | 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 default is | ||
57 | ``yes''. Note that disabling TCP forwarding does not improve se- | ||
58 | curity unless users are also denied shell access, as they can al- | ||
59 | ways install their own forwarders. | ||
60 | |||
61 | AllowUsers | ||
62 | This keyword can be followed by a list of user name patterns, | ||
63 | separated by spaces. If specified, login is allowed only for us- | ||
64 | er names that match one of the patterns. Only user names are | ||
65 | valid; a numerical user ID is not recognized. By default, login | ||
66 | is allowed for all users. If the pattern takes the form US- | ||
67 | ER@HOST then USER and HOST are separately checked, restricting | ||
68 | logins to particular users from particular hosts. The allow/deny | ||
69 | directives are processed in the following order: DenyUsers, | ||
70 | AllowUsers, DenyGroups, and finally AllowGroups. | ||
71 | |||
72 | See PATTERNS in ssh_config(5) for more information on patterns. | ||
73 | |||
74 | AuthorizedKeysFile | ||
75 | Specifies the file that contains the public keys that can be used | ||
76 | for user authentication. AuthorizedKeysFile may contain tokens | ||
77 | of the form %T which are substituted during connection setup. | ||
78 | The following tokens are defined: %% is replaced by a literal | ||
79 | '%', %h is replaced by the home directory of the user being au- | ||
80 | thenticated, and %u is replaced by the username of that user. | ||
81 | After expansion, AuthorizedKeysFile is taken to be an absolute | ||
82 | path or one relative to the user's home directory. The default | ||
83 | is ``.ssh/authorized_keys''. | ||
84 | |||
85 | Banner The contents of the specified file are sent to the remote user | ||
86 | before authentication is allowed. If the argument is ``none'' | ||
87 | then no banner is displayed. This option is only available for | ||
88 | protocol version 2. By default, no banner is displayed. | ||
89 | |||
90 | ChallengeResponseAuthentication | ||
91 | Specifies whether challenge-response authentication is allowed. | ||
92 | All authentication styles from login.conf(5) are supported. The | ||
93 | default is ``yes''. | ||
94 | |||
95 | ChrootDirectory | ||
96 | Specifies a path to chroot(2) to after authentication. This | ||
97 | path, and all its components, must be root-owned directories that | ||
98 | are not writable by any other user or group. | ||
99 | |||
100 | The path may contain the following tokens that are expanded at | ||
101 | runtime once the connecting user has been authenticated: %% is | ||
102 | replaced by a literal '%', %h is replaced by the home directory | ||
103 | of the user being authenticated, and %u is replaced by the user- | ||
104 | name of that user. | ||
105 | |||
106 | The ChrootDirectory must contain the necessary files and directo- | ||
107 | ries to support the users' session. For an interactive session | ||
108 | this requires at least a shell, typically sh(1), and basic /dev | ||
109 | nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), | ||
110 | arandom(4) and tty(4) devices. For file transfer sessions using | ||
111 | ``sftp'', no additional configuration of the environment is nec- | ||
112 | essary if the in-process sftp server is used (see Subsystem for | ||
113 | details). | ||
114 | |||
115 | The default is not to chroot(2). | ||
116 | |||
117 | Ciphers | ||
118 | Specifies the ciphers allowed for protocol version 2. Multiple | ||
119 | ciphers must be comma-separated. The supported ciphers are | ||
120 | ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', | ||
121 | ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'', | ||
122 | ``arcfour256'', ``arcfour'', ``blowfish-cbc'', and | ||
123 | ``cast128-cbc''. The default is: | ||
124 | |||
125 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | ||
126 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | ||
127 | aes192-ctr,aes256-ctr | ||
128 | |||
129 | ClientAliveCountMax | ||
130 | Sets the number of client alive messages (see below) which may be | ||
131 | sent without sshd(8) receiving any messages back from the client. | ||
132 | If this threshold is reached while client alive messages are be- | ||
133 | ing sent, sshd will disconnect the client, terminating the ses- | ||
134 | sion. It is important to note that the use of client alive mes- | ||
135 | sages is very different from TCPKeepAlive (below). The client | ||
136 | alive messages are sent through the encrypted channel and there- | ||
137 | fore will not be spoofable. The TCP keepalive option enabled by | ||
138 | TCPKeepAlive is spoofable. The client alive mechanism is valu- | ||
139 | able when the client or server depend on knowing when a connec- | ||
140 | tion has become inactive. | ||
141 | |||
142 | The default value is 3. If ClientAliveInterval (see below) is | ||
143 | set to 15, and ClientAliveCountMax is left at the default, unre- | ||
144 | sponsive SSH clients will be disconnected after approximately 45 | ||
145 | seconds. This option applies to protocol version 2 only. | ||
146 | |||
147 | ClientAliveInterval | ||
148 | Sets a timeout interval in seconds after which if no data has | ||
149 | been received from the client, sshd(8) will send a message | ||
150 | through the encrypted channel to request a response from the | ||
151 | client. The default is 0, indicating that these messages will | ||
152 | not be sent to the client. This option applies to protocol ver- | ||
153 | sion 2 only. | ||
154 | |||
155 | Compression | ||
156 | Specifies whether compression is allowed, or delayed until the | ||
157 | user has authenticated successfully. The argument must be | ||
158 | ``yes'', ``delayed'', or ``no''. The default is ``delayed''. | ||
159 | |||
160 | DenyGroups | ||
161 | This keyword can be followed by a list of group name patterns, | ||
162 | separated by spaces. Login is disallowed for users whose primary | ||
163 | group or supplementary group list matches one of the patterns. | ||
164 | Only group names are valid; a numerical group ID is not recog- | ||
165 | nized. By default, login is allowed for all groups. The al- | ||
166 | low/deny directives are processed in the following order: | ||
167 | DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. | ||
168 | |||
169 | See PATTERNS in ssh_config(5) for more information on patterns. | ||
170 | |||
171 | DenyUsers | ||
172 | This keyword can be followed by a list of user name patterns, | ||
173 | separated by spaces. Login is disallowed for user names that | ||
174 | match one of the patterns. Only user names are valid; a numeri- | ||
175 | cal user ID is not recognized. By default, login is allowed for | ||
176 | all users. If the pattern takes the form USER@HOST then USER and | ||
177 | HOST are separately checked, restricting logins to particular | ||
178 | users from particular hosts. The allow/deny directives are pro- | ||
179 | cessed in the following order: DenyUsers, AllowUsers, DenyGroups, | ||
180 | and finally AllowGroups. | ||
181 | |||
182 | See PATTERNS in ssh_config(5) for more information on patterns. | ||
183 | |||
184 | ForceCommand | ||
185 | Forces the execution of the command specified by ForceCommand, | ||
186 | ignoring any command supplied by the client and ~/.ssh/rc if pre- | ||
187 | sent. The command is invoked by using the user's login shell | ||
188 | with the -c option. This applies to shell, command, or subsystem | ||
189 | execution. It is most useful inside a Match block. The command | ||
190 | originally supplied by the client is available in the | ||
191 | SSH_ORIGINAL_COMMAND environment variable. Specifying a command | ||
192 | of ``internal-sftp'' will force the use of an in-process sftp | ||
193 | server that requires no support files when used with | ||
194 | ChrootDirectory. | ||
195 | |||
196 | GatewayPorts | ||
197 | Specifies whether remote hosts are allowed to connect to ports | ||
198 | forwarded for the client. By default, sshd(8) binds remote port | ||
199 | forwardings to the loopback address. This prevents other remote | ||
200 | hosts from connecting to forwarded ports. GatewayPorts can be | ||
201 | used to specify that sshd should allow remote port forwardings to | ||
202 | bind to non-loopback addresses, thus allowing other hosts to con- | ||
203 | nect. The argument may be ``no'' to force remote port forward- | ||
204 | ings to be available to the local host only, ``yes'' to force re- | ||
205 | mote port forwardings to bind to the wildcard address, or | ||
206 | ``clientspecified'' to allow the client to select the address to | ||
207 | which the forwarding is bound. The default is ``no''. | ||
208 | |||
209 | GSSAPIAuthentication | ||
210 | Specifies whether user authentication based on GSSAPI is allowed. | ||
211 | The default is ``no''. Note that this option applies to protocol | ||
212 | version 2 only. | ||
213 | |||
214 | GSSAPICleanupCredentials | ||
215 | Specifies whether to automatically destroy the user's credentials | ||
216 | cache on logout. The default is ``yes''. Note that this option | ||
217 | applies to protocol version 2 only. | ||
218 | |||
219 | HostbasedAuthentication | ||
220 | Specifies whether rhosts or /etc/hosts.equiv authentication to- | ||
221 | gether with successful public key client host authentication is | ||
222 | allowed (host-based authentication). This option is similar to | ||
223 | RhostsRSAAuthentication and applies to protocol version 2 only. | ||
224 | The default is ``no''. | ||
225 | |||
226 | HostbasedUsesNameFromPacketOnly | ||
227 | Specifies whether or not the server will attempt to perform a re- | ||
228 | verse name lookup when matching the name in the ~/.shosts, | ||
229 | ~/.rhosts, and /etc/hosts.equiv files during | ||
230 | HostbasedAuthentication. A setting of ``yes'' means that sshd(8) | ||
231 | uses the name supplied by the client rather than attempting to | ||
232 | resolve the name from the TCP connection itself. The default is | ||
233 | ``no''. | ||
234 | |||
235 | HostKey | ||
236 | Specifies a file containing a private host key used by SSH. The | ||
237 | default is /etc/ssh/ssh_host_key for protocol version 1, and | ||
238 | /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro- | ||
239 | tocol version 2. Note that sshd(8) will refuse to use a file if | ||
240 | it is group/world-accessible. It is possible to have multiple | ||
241 | host key files. ``rsa1'' keys are used for version 1 and ``dsa'' | ||
242 | or ``rsa'' are used for version 2 of the SSH protocol. | ||
243 | |||
244 | IgnoreRhosts | ||
245 | Specifies that .rhosts and .shosts files will not be used in | ||
246 | RhostsRSAAuthentication or HostbasedAuthentication. | ||
247 | |||
248 | /etc/hosts.equiv and /etc/shosts.equiv are still used. The de- | ||
249 | fault is ``yes''. | ||
250 | |||
251 | IgnoreUserKnownHosts | ||
252 | Specifies whether sshd(8) should ignore the user's | ||
253 | ~/.ssh/known_hosts during RhostsRSAAuthentication or | ||
254 | HostbasedAuthentication. The default is ``no''. | ||
255 | |||
256 | KerberosAuthentication | ||
257 | Specifies whether the password provided by the user for | ||
258 | PasswordAuthentication will be validated through the Kerberos | ||
259 | KDC. To use this option, the server needs a Kerberos servtab | ||
260 | which allows the verification of the KDC's identity. The default | ||
261 | is ``no''. | ||
262 | |||
263 | KerberosGetAFSToken | ||
264 | If AFS is active and the user has a Kerberos 5 TGT, attempt to | ||
265 | acquire an AFS token before accessing the user's home directory. | ||
266 | The default is ``no''. | ||
267 | |||
268 | KerberosOrLocalPasswd | ||
269 | If password authentication through Kerberos fails then the pass- | ||
270 | word will be validated via any additional local mechanism such as | ||
271 | /etc/passwd. The default is ``yes''. | ||
272 | |||
273 | KerberosTicketCleanup | ||
274 | Specifies whether to automatically destroy the user's ticket | ||
275 | cache file on logout. The default is ``yes''. | ||
276 | |||
277 | KeyRegenerationInterval | ||
278 | In protocol version 1, the ephemeral server key is automatically | ||
279 | regenerated after this many seconds (if it has been used). The | ||
280 | purpose of regeneration is to prevent decrypting captured ses- | ||
281 | sions by later breaking into the machine and stealing the keys. | ||
282 | The key is never stored anywhere. If the value is 0, the key is | ||
283 | never regenerated. The default is 3600 (seconds). | ||
284 | |||
285 | ListenAddress | ||
286 | Specifies the local addresses sshd(8) should listen on. The fol- | ||
287 | lowing forms may be used: | ||
288 | |||
289 | ListenAddress host|IPv4_addr|IPv6_addr | ||
290 | ListenAddress host|IPv4_addr:port | ||
291 | ListenAddress [host|IPv6_addr]:port | ||
292 | |||
293 | If port is not specified, sshd will listen on the address and all | ||
294 | prior Port options specified. The default is to listen on all | ||
295 | local addresses. Multiple ListenAddress options are permitted. | ||
296 | Additionally, any Port options must precede this option for non- | ||
297 | port qualified addresses. | ||
298 | |||
299 | LoginGraceTime | ||
300 | The server disconnects after this time if the user has not suc- | ||
301 | cessfully logged in. If the value is 0, there is no time limit. | ||
302 | The default is 120 seconds. | ||
303 | |||
304 | LogLevel | ||
305 | Gives the verbosity level that is used when logging messages from | ||
306 | sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, | ||
307 | VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. | ||
308 | DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify | ||
309 | higher levels of debugging output. Logging with a DEBUG level | ||
310 | violates the privacy of users and is not recommended. | ||
311 | |||
312 | MACs Specifies the available MAC (message authentication code) algo- | ||
313 | rithms. The MAC algorithm is used in protocol version 2 for data | ||
314 | integrity protection. Multiple algorithms must be comma-separat- | ||
315 | ed. The default is: | ||
316 | |||
317 | hmac-md5,hmac-sha1,umac-64@openssh.com, | ||
318 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | ||
319 | |||
320 | Match Introduces a conditional block. If all of the criteria on the | ||
321 | Match line are satisfied, the keywords on the following lines | ||
322 | override those set in the global section of the config file, un- | ||
323 | til either another Match line or the end of the file. | ||
324 | |||
325 | The arguments to Match are one or more criteria-pattern pairs. | ||
326 | The available criteria are User, Group, Host, and Address. The | ||
327 | match patterns may consist of single entries or comma-separated | ||
328 | lists and may use the wildcard and negation operators described | ||
329 | in the PATTERNS section of ssh_config(5). | ||
330 | |||
331 | The patterns in an Address criteria may additionally contain ad- | ||
332 | dresses to match in CIDR address/masklen format, e.g. | ||
333 | ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask | ||
334 | length provided must be consistent with the address - it is an | ||
335 | error to specify a mask length that is too long for the address | ||
336 | or one with bits set in this host portion of the address. For | ||
337 | example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. | ||
338 | |||
339 | Only a subset of keywords may be used on the lines following a | ||
340 | Match keyword. Available keywords are AllowTcpForwarding, | ||
341 | Banner, ChrootDirectory, ForceCommand, GatewayPorts, | ||
342 | GSSAPIAuthentication, HostbasedAuthentication, | ||
343 | KbdInteractiveAuthentication, KerberosAuthentication, | ||
344 | MaxAuthTries, MaxSessions, PasswordAuthentication, PermitOpen, | ||
345 | PermitRootLogin, RhostsRSAAuthentication, RSAAuthentication, | ||
346 | X11DisplayOffset, X11Forwarding, and X11UseLocalHost. | ||
347 | |||
348 | MaxAuthTries | ||
349 | Specifies the maximum number of authentication attempts permitted | ||
350 | per connection. Once the number of failures reaches half this | ||
351 | value, additional failures are logged. The default is 6. | ||
352 | |||
353 | MaxSessions | ||
354 | Specifies the maximum number of open sessions permitted per net- | ||
355 | work connection. The default is 10. | ||
356 | |||
357 | MaxStartups | ||
358 | Specifies the maximum number of concurrent unauthenticated con- | ||
359 | nections to the SSH daemon. Additional connections will be | ||
360 | dropped until authentication succeeds or the LoginGraceTime ex- | ||
361 | pires for a connection. The default is 10. | ||
362 | |||
363 | Alternatively, random early drop can be enabled by specifying the | ||
364 | three colon separated values ``start:rate:full'' (e.g. | ||
365 | "10:30:60"). sshd(8) will refuse connection attempts with a | ||
366 | probability of ``rate/100'' (30%) if there are currently | ||
367 | ``start'' (10) unauthenticated connections. The probability in- | ||
368 | creases linearly and all connection attempts are refused if the | ||
369 | number of unauthenticated connections reaches ``full'' (60). | ||
370 | |||
371 | PasswordAuthentication | ||
372 | Specifies whether password authentication is allowed. The de- | ||
373 | fault is ``yes''. | ||
374 | |||
375 | PermitEmptyPasswords | ||
376 | When password authentication is allowed, it specifies whether the | ||
377 | server allows login to accounts with empty password strings. The | ||
378 | default is ``no''. | ||
379 | |||
380 | PermitOpen | ||
381 | Specifies the destinations to which TCP port forwarding is per- | ||
382 | mitted. The forwarding specification must be one of the follow- | ||
383 | ing forms: | ||
384 | |||
385 | PermitOpen host:port | ||
386 | PermitOpen IPv4_addr:port | ||
387 | PermitOpen [IPv6_addr]:port | ||
388 | |||
389 | Multiple forwards may be specified by separating them with | ||
390 | whitespace. An argument of ``any'' can be used to remove all re- | ||
391 | strictions and permit any forwarding requests. By default all | ||
392 | port forwarding requests are permitted. | ||
393 | |||
394 | PermitRootLogin | ||
395 | Specifies whether root can log in using ssh(1). The argument | ||
396 | must be ``yes'', ``without-password'', ``forced-commands-only'', | ||
397 | or ``no''. The default is ``yes''. | ||
398 | |||
399 | If this option is set to ``without-password'', password authenti- | ||
400 | cation is disabled for root. | ||
401 | |||
402 | If this option is set to ``forced-commands-only'', root login | ||
403 | with public key authentication will be allowed, but only if the | ||
404 | command option has been specified (which may be useful for taking | ||
405 | remote backups even if root login is normally not allowed). All | ||
406 | other authentication methods are disabled for root. | ||
407 | |||
408 | If this option is set to ``no'', root is not allowed to log in. | ||
409 | |||
410 | PermitTunnel | ||
411 | Specifies whether tun(4) device forwarding is allowed. The argu- | ||
412 | ment must be ``yes'', ``point-to-point'' (layer 3), ``ethernet'' | ||
413 | (layer 2), or ``no''. Specifying ``yes'' permits both ``point- | ||
414 | to-point'' and ``ethernet''. The default is ``no''. | ||
415 | |||
416 | PermitUserEnvironment | ||
417 | Specifies whether ~/.ssh/environment and environment= options in | ||
418 | ~/.ssh/authorized_keys are processed by sshd(8). The default is | ||
419 | ``no''. Enabling environment processing may enable users to by- | ||
420 | pass access restrictions in some configurations using mechanisms | ||
421 | such as LD_PRELOAD. | ||
422 | |||
423 | PidFile | ||
424 | Specifies the file that contains the process ID of the SSH dae- | ||
425 | mon. The default is /var/run/sshd.pid. | ||
426 | |||
427 | Port Specifies the port number that sshd(8) listens on. The default | ||
428 | is 22. Multiple options of this type are permitted. See also | ||
429 | ListenAddress. | ||
430 | |||
431 | PrintLastLog | ||
432 | Specifies whether sshd(8) should print the date and time of the | ||
433 | last user login when a user logs in interactively. The default | ||
434 | is ``yes''. | ||
435 | |||
436 | PrintMotd | ||
437 | Specifies whether sshd(8) should print /etc/motd when a user logs | ||
438 | in interactively. (On some systems it is also printed by the | ||
439 | shell, /etc/profile, or equivalent.) The default is ``yes''. | ||
440 | |||
441 | Protocol | ||
442 | Specifies the protocol versions sshd(8) supports. The possible | ||
443 | values are `1' and `2'. Multiple versions must be comma-separat- | ||
444 | ed. The default is ``2,1''. Note that the order of the protocol | ||
445 | list does not indicate preference, because the client selects | ||
446 | among multiple protocol versions offered by the server. Specify- | ||
447 | ing ``2,1'' is identical to ``1,2''. | ||
448 | |||
449 | PubkeyAuthentication | ||
450 | Specifies whether public key authentication is allowed. The de- | ||
451 | fault is ``yes''. Note that this option applies to protocol ver- | ||
452 | sion 2 only. | ||
453 | |||
454 | RhostsRSAAuthentication | ||
455 | Specifies whether rhosts or /etc/hosts.equiv authentication to- | ||
456 | gether with successful RSA host authentication is allowed. The | ||
457 | default is ``no''. This option applies to protocol version 1 on- | ||
458 | ly. | ||
459 | |||
460 | RSAAuthentication | ||
461 | Specifies whether pure RSA authentication is allowed. The de- | ||
462 | fault is ``yes''. This option applies to protocol version 1 on- | ||
463 | ly. | ||
464 | |||
465 | ServerKeyBits | ||
466 | Defines the number of bits in the ephemeral protocol version 1 | ||
467 | server key. The minimum value is 512, and the default is 1024. | ||
468 | |||
469 | StrictModes | ||
470 | Specifies whether sshd(8) should check file modes and ownership | ||
471 | of the user's files and home directory before accepting login. | ||
472 | This is normally desirable because novices sometimes accidentally | ||
473 | leave their directory or files world-writable. The default is | ||
474 | ``yes''. | ||
475 | |||
476 | Subsystem | ||
477 | Configures an external subsystem (e.g. file transfer daemon). | ||
478 | Arguments should be a subsystem name and a command (with optional | ||
479 | arguments) to execute upon subsystem request. | ||
480 | |||
481 | The command sftp-server(8) implements the ``sftp'' file transfer | ||
482 | subsystem. | ||
483 | |||
484 | Alternately the name ``internal-sftp'' implements an in-process | ||
485 | ``sftp'' server. This may simplify configurations using | ||
486 | ChrootDirectory to force a different filesystem root on clients. | ||
487 | |||
488 | By default no subsystems are defined. Note that this option ap- | ||
489 | plies to protocol version 2 only. | ||
490 | |||
491 | SyslogFacility | ||
492 | Gives the facility code that is used when logging messages from | ||
493 | sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, | ||
494 | LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de- | ||
495 | fault is AUTH. | ||
496 | |||
497 | TCPKeepAlive | ||
498 | Specifies whether the system should send TCP keepalive messages | ||
499 | to the other side. If they are sent, death of the connection or | ||
500 | crash of one of the machines will be properly noticed. However, | ||
501 | this means that connections will die if the route is down tem- | ||
502 | porarily, and some people find it annoying. On the other hand, | ||
503 | if TCP keepalives are not sent, sessions may hang indefinitely on | ||
504 | the server, leaving ``ghost'' users and consuming server re- | ||
505 | sources. | ||
506 | |||
507 | The default is ``yes'' (to send TCP keepalive messages), and the | ||
508 | server will notice if the network goes down or the client host | ||
509 | crashes. This avoids infinitely hanging sessions. | ||
510 | |||
511 | To disable TCP keepalive messages, the value should be set to | ||
512 | ``no''. | ||
513 | |||
514 | UseDNS Specifies whether sshd(8) should look up the remote host name and | ||
515 | check that the resolved host name for the remote IP address maps | ||
516 | back to the very same IP address. The default is ``yes''. | ||
517 | |||
518 | UseLogin | ||
519 | Specifies whether login(1) is used for interactive login ses- | ||
520 | sions. The default is ``no''. Note that login(1) is never used | ||
521 | for remote command execution. Note also, that if this is en- | ||
522 | abled, X11Forwarding will be disabled because login(1) does not | ||
523 | know how to handle xauth(1) cookies. If UsePrivilegeSeparation | ||
524 | is specified, it will be disabled after authentication. | ||
525 | |||
526 | UsePAM Enables the Pluggable Authentication Module interface. If set to | ||
527 | ``yes'' this will enable PAM authentication using | ||
528 | ChallengeResponseAuthentication and PasswordAuthentication in ad- | ||
529 | dition to PAM account and session module processing for all au- | ||
530 | thentication types. | ||
531 | |||
532 | Because PAM challenge-response authentication usually serves an | ||
533 | equivalent role to password authentication, you should disable | ||
534 | either PasswordAuthentication or ChallengeResponseAuthentication. | ||
535 | |||
536 | If UsePAM is enabled, you will not be able to run sshd(8) as a | ||
537 | non-root user. The default is ``no''. | ||
538 | |||
539 | UsePrivilegeSeparation | ||
540 | Specifies whether sshd(8) separates privileges by creating an un- | ||
541 | privileged child process to deal with incoming network traffic. | ||
542 | After successful authentication, another process will be created | ||
543 | that has the privilege of the authenticated user. The goal of | ||
544 | privilege separation is to prevent privilege escalation by con- | ||
545 | taining any corruption within the unprivileged processes. The | ||
546 | default is ``yes''. | ||
547 | |||
548 | X11DisplayOffset | ||
549 | Specifies the first display number available for sshd(8)'s X11 | ||
550 | forwarding. This prevents sshd from interfering with real X11 | ||
551 | servers. The default is 10. | ||
552 | |||
553 | X11Forwarding | ||
554 | Specifies whether X11 forwarding is permitted. The argument must | ||
555 | be ``yes'' or ``no''. The default is ``no''. | ||
556 | |||
557 | When X11 forwarding is enabled, there may be additional exposure | ||
558 | to the server and to client displays if the sshd(8) proxy display | ||
559 | is configured to listen on the wildcard address (see | ||
560 | X11UseLocalhost below), though this is not the default. Addi- | ||
561 | tionally, the authentication spoofing and authentication data | ||
562 | verification and substitution occur on the client side. The se- | ||
563 | curity risk of using X11 forwarding is that the client's X11 dis- | ||
564 | play server may be exposed to attack when the SSH client requests | ||
565 | forwarding (see the warnings for ForwardX11 in ssh_config(5)). A | ||
566 | system administrator may have a stance in which they want to pro- | ||
567 | tect clients that may expose themselves to attack by unwittingly | ||
568 | requesting X11 forwarding, which can warrant a ``no'' setting. | ||
569 | |||
570 | Note that disabling X11 forwarding does not prevent users from | ||
571 | forwarding X11 traffic, as users can always install their own | ||
572 | forwarders. X11 forwarding is automatically disabled if UseLogin | ||
573 | is enabled. | ||
574 | |||
575 | X11UseLocalhost | ||
576 | Specifies whether sshd(8) should bind the X11 forwarding server | ||
577 | to the loopback address or to the wildcard address. By default, | ||
578 | sshd binds the forwarding server to the loopback address and sets | ||
579 | the hostname part of the DISPLAY environment variable to | ||
580 | ``localhost''. This prevents remote hosts from connecting to the | ||
581 | proxy display. However, some older X11 clients may not function | ||
582 | with this configuration. X11UseLocalhost may be set to ``no'' to | ||
583 | specify that the forwarding server should be bound to the wild- | ||
584 | card address. The argument must be ``yes'' or ``no''. The de- | ||
585 | fault is ``yes''. | ||
586 | |||
587 | XAuthLocation | ||
588 | Specifies the full pathname of the xauth(1) program. The default | ||
589 | is /usr/X11R6/bin/xauth. | ||
590 | |||
591 | TIME FORMATS | ||
592 | sshd(8) command-line arguments and configuration file options that speci- | ||
593 | fy time may be expressed using a sequence of the form: time[qualifier], | ||
594 | where time is a positive integer value and qualifier is one of the fol- | ||
595 | lowing: | ||
596 | |||
597 | <none> seconds | ||
598 | s | S seconds | ||
599 | m | M minutes | ||
600 | h | H hours | ||
601 | d | D days | ||
602 | w | W weeks | ||
603 | |||
604 | Each member of the sequence is added together to calculate the total time | ||
605 | value. | ||
606 | |||
607 | Time format examples: | ||
608 | |||
609 | 600 600 seconds (10 minutes) | ||
610 | 10m 10 minutes | ||
611 | 1h30m 1 hour 30 minutes (90 minutes) | ||
612 | |||
613 | FILES | ||
614 | /etc/ssh/sshd_config | ||
615 | Contains configuration data for sshd(8). This file should be | ||
616 | writable by root only, but it is recommended (though not neces- | ||
617 | sary) that it be world-readable. | ||
618 | |||
619 | SEE ALSO | ||
620 | sshd(8) | ||
621 | |||
622 | AUTHORS | ||
623 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by | ||
624 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo | ||
625 | de Raadt and Dug Song removed many bugs, re-added newer features and cre- | ||
626 | ated OpenSSH. Markus Friedl contributed the support for SSH protocol | ||
627 | versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support | ||
628 | for privilege separation. | ||
629 | |||
630 | OpenBSD 4.4 July 2, 2008 10 | ||