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1 | .\" -*- nroff -*- | |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
37 | .\" $OpenBSD: sshd.8,v 1.221 2006/02/13 11:02:26 jmc Exp $ | |
38 | .Dd September 25, 1999 | |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
43 | .Nd OpenSSH SSH daemon | |
44 | .Sh SYNOPSIS | |
45 | .Nm sshd | |
46 | .Bk -words | |
47 | .Op Fl 46Ddeiqt | |
48 | .Op Fl b Ar bits | |
49 | .Op Fl f Ar config_file | |
50 | .Op Fl g Ar login_grace_time | |
51 | .Op Fl h Ar host_key_file | |
52 | .Op Fl k Ar key_gen_time | |
53 | .Op Fl o Ar option | |
54 | .Op Fl p Ar port | |
55 | .Op Fl u Ar len | |
56 | .Ek | |
57 | .Sh DESCRIPTION | |
58 | .Nm | |
59 | (OpenSSH Daemon) is the daemon program for | |
60 | .Xr ssh 1 . | |
61 | Together these programs replace rlogin and rsh, and | |
62 | provide secure encrypted communications between two untrusted hosts | |
63 | over an insecure network. | |
64 | .Pp | |
65 | .Nm | |
66 | listens for connections from clients. | |
67 | It is normally started at boot from | |
68 | .Pa /etc/rc . | |
69 | It forks a new | |
70 | daemon for each incoming connection. | |
71 | The forked daemons handle | |
72 | key exchange, encryption, authentication, command execution, | |
73 | and data exchange. | |
74 | .Pp | |
75 | .Nm | |
76 | can be configured using command-line options or a configuration file | |
77 | (by default | |
78 | .Xr sshd_config 5 ) ; | |
79 | command-line options override values specified in the | |
80 | configuration file. | |
81 | .Nm | |
82 | rereads its configuration file when it receives a hangup signal, | |
83 | .Dv SIGHUP , | |
84 | by executing itself with the name and options it was started with, e.g., | |
85 | .Pa /usr/sbin/sshd . | |
86 | .Pp | |
87 | The options are as follows: | |
88 | .Bl -tag -width Ds | |
89 | .It Fl 4 | |
90 | Forces | |
91 | .Nm | |
92 | to use IPv4 addresses only. | |
93 | .It Fl 6 | |
94 | Forces | |
95 | .Nm | |
96 | to use IPv6 addresses only. | |
97 | .It Fl b Ar bits | |
98 | Specifies the number of bits in the ephemeral protocol version 1 | |
99 | server key (default 768). | |
100 | .It Fl D | |
101 | When this option is specified, | |
102 | .Nm | |
103 | will not detach and does not become a daemon. | |
104 | This allows easy monitoring of | |
105 | .Nm sshd . | |
106 | .It Fl d | |
107 | Debug mode. | |
108 | The server sends verbose debug output to the system | |
109 | log, and does not put itself in the background. | |
110 | The server also will not fork and will only process one connection. | |
111 | This option is only intended for debugging for the server. | |
112 | Multiple | |
113 | .Fl d | |
114 | options increase the debugging level. | |
115 | Maximum is 3. | |
116 | .It Fl e | |
117 | When this option is specified, | |
118 | .Nm | |
119 | will send the output to the standard error instead of the system log. | |
120 | .It Fl f Ar configuration_file | |
121 | Specifies the name of the configuration file. | |
122 | The default is | |
123 | .Pa /etc/ssh/sshd_config . | |
124 | .Nm | |
125 | refuses to start if there is no configuration file. | |
126 | .It Fl g Ar login_grace_time | |
127 | Gives the grace time for clients to authenticate themselves (default | |
128 | 120 seconds). | |
129 | If the client fails to authenticate the user within | |
130 | this many seconds, the server disconnects and exits. | |
131 | A value of zero indicates no limit. | |
132 | .It Fl h Ar host_key_file | |
133 | Specifies a file from which a host key is read. | |
134 | This option must be given if | |
135 | .Nm | |
136 | is not run as root (as the normal | |
137 | host key files are normally not readable by anyone but root). | |
138 | The default is | |
139 | .Pa /etc/ssh/ssh_host_key | |
140 | for protocol version 1, and | |
141 | .Pa /etc/ssh/ssh_host_rsa_key | |
142 | and | |
143 | .Pa /etc/ssh/ssh_host_dsa_key | |
144 | for protocol version 2. | |
145 | It is possible to have multiple host key files for | |
146 | the different protocol versions and host key algorithms. | |
147 | .It Fl i | |
148 | Specifies that | |
149 | .Nm | |
150 | is being run from | |
151 | .Xr inetd 8 . | |
152 | .Nm | |
153 | is normally not run | |
154 | from inetd because it needs to generate the server key before it can | |
155 | respond to the client, and this may take tens of seconds. | |
156 | Clients would have to wait too long if the key was regenerated every time. | |
157 | However, with small key sizes (e.g., 512) using | |
158 | .Nm | |
159 | from inetd may | |
160 | be feasible. | |
161 | .It Fl k Ar key_gen_time | |
162 | Specifies how often the ephemeral protocol version 1 server key is | |
163 | regenerated (default 3600 seconds, or one hour). | |
164 | The motivation for regenerating the key fairly | |
165 | often is that the key is not stored anywhere, and after about an hour | |
166 | it becomes impossible to recover the key for decrypting intercepted | |
167 | communications even if the machine is cracked into or physically | |
168 | seized. | |
169 | A value of zero indicates that the key will never be regenerated. | |
170 | .It Fl o Ar option | |
171 | Can be used to give options in the format used in the configuration file. | |
172 | This is useful for specifying options for which there is no separate | |
173 | command-line flag. | |
174 | For full details of the options, and their values, see | |
175 | .Xr sshd_config 5 . | |
176 | .It Fl p Ar port | |
177 | Specifies the port on which the server listens for connections | |
178 | (default 22). | |
179 | Multiple port options are permitted. | |
180 | Ports specified in the configuration file with the | |
181 | .Cm Port | |
182 | option are ignored when a command-line port is specified. | |
183 | Ports specified using the | |
184 | .Cm ListenAddress | |
185 | option override command-line ports. | |
186 | .It Fl q | |
187 | Quiet mode. | |
188 | Nothing is sent to the system log. | |
189 | Normally the beginning, | |
190 | authentication, and termination of each connection is logged. | |
191 | .It Fl t | |
192 | Test mode. | |
193 | Only check the validity of the configuration file and sanity of the keys. | |
194 | This is useful for updating | |
195 | .Nm | |
196 | reliably as configuration options may change. | |
197 | .It Fl u Ar len | |
198 | This option is used to specify the size of the field | |
199 | in the | |
200 | .Li utmp | |
201 | structure that holds the remote host name. | |
202 | If the resolved host name is longer than | |
203 | .Ar len , | |
204 | the dotted decimal value will be used instead. | |
205 | This allows hosts with very long host names that | |
206 | overflow this field to still be uniquely identified. | |
207 | Specifying | |
208 | .Fl u0 | |
209 | indicates that only dotted decimal addresses | |
210 | should be put into the | |
211 | .Pa utmp | |
212 | file. | |
213 | .Fl u0 | |
214 | may also be used to prevent | |
215 | .Nm | |
216 | from making DNS requests unless the authentication | |
217 | mechanism or configuration requires it. | |
218 | Authentication mechanisms that may require DNS include | |
219 | .Cm RhostsRSAAuthentication , | |
220 | .Cm HostbasedAuthentication , | |
221 | and using a | |
222 | .Cm from="pattern-list" | |
223 | option in a key file. | |
224 | Configuration options that require DNS include using a | |
225 | USER@HOST pattern in | |
226 | .Cm AllowUsers | |
227 | or | |
228 | .Cm DenyUsers . | |
229 | .El | |
230 | .Sh AUTHENTICATION | |
231 | The OpenSSH SSH daemon supports SSH protocols 1 and 2. | |
232 | Both protocols are supported by default, | |
233 | though this can be changed via the | |
234 | .Cm Protocol | |
235 | option in | |
236 | .Xr sshd_config 5 . | |
237 | Protocol 2 supports both RSA and DSA keys; | |
238 | protocol 1 only supports RSA keys. | |
239 | For both protocols, | |
240 | each host has a host-specific key, | |
241 | normally 2048 bits, | |
242 | used to identify the host. | |
243 | .Pp | |
244 | Forward security for protocol 1 is provided through | |
245 | an additional server key, | |
246 | normally 768 bits, | |
247 | generated when the server starts. | |
248 | This key is normally regenerated every hour if it has been used, and | |
249 | is never stored on disk. | |
250 | Whenever a client connects, the daemon responds with its public | |
251 | host and server keys. | |
252 | The client compares the | |
253 | RSA host key against its own database to verify that it has not changed. | |
254 | The client then generates a 256-bit random number. | |
255 | It encrypts this | |
256 | random number using both the host key and the server key, and sends | |
257 | the encrypted number to the server. | |
258 | Both sides then use this | |
259 | random number as a session key which is used to encrypt all further | |
260 | communications in the session. | |
261 | The rest of the session is encrypted | |
262 | using a conventional cipher, currently Blowfish or 3DES, with 3DES | |
263 | being used by default. | |
264 | The client selects the encryption algorithm | |
265 | to use from those offered by the server. | |
266 | .Pp | |
267 | For protocol 2, | |
268 | forward security is provided through a Diffie-Hellman key agreement. | |
269 | This key agreement results in a shared session key. | |
270 | The rest of the session is encrypted using a symmetric cipher, currently | |
271 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. | |
272 | The client selects the encryption algorithm | |
273 | to use from those offered by the server. | |
274 | Additionally, session integrity is provided | |
275 | through a cryptographic message authentication code | |
276 | (hmac-sha1 or hmac-md5). | |
277 | .Pp | |
278 | Finally, the server and the client enter an authentication dialog. | |
279 | The client tries to authenticate itself using | |
280 | host-based authentication, | |
281 | public key authentication, | |
282 | challenge-response authentication, | |
283 | or password authentication. | |
284 | .Pp | |
285 | Regardless of the authentication type, the account is checked to | |
286 | ensure that it is accessible. An account is not accessible if it is | |
287 | locked, listed in | |
288 | .Cm DenyUsers | |
289 | or its group is listed in | |
290 | .Cm DenyGroups | |
291 | \&. The definition of a locked account is system dependant. Some platforms | |
292 | have their own account database (eg AIX) and some modify the passwd field ( | |
293 | .Ql \&*LK\&* | |
294 | on Solaris and UnixWare, | |
295 | .Ql \&* | |
296 | on HP-UX, containing | |
297 | .Ql Nologin | |
298 | on Tru64, | |
299 | a leading | |
300 | .Ql \&*LOCKED\&* | |
301 | on FreeBSD and a leading | |
302 | .Ql \&!! | |
303 | on Linux). If there is a requirement to disable password authentication | |
304 | for the account while allowing still public-key, then the passwd field | |
305 | should be set to something other than these values (eg | |
306 | .Ql NP | |
307 | or | |
308 | .Ql \&*NP\&* | |
309 | ). | |
310 | .Pp | |
311 | If the client successfully authenticates itself, a dialog for | |
312 | preparing the session is entered. | |
313 | At this time the client may request | |
314 | things like allocating a pseudo-tty, forwarding X11 connections, | |
315 | forwarding TCP connections, or forwarding the authentication agent | |
316 | connection over the secure channel. | |
317 | .Pp | |
318 | After this, the client either requests a shell or execution of a command. | |
319 | The sides then enter session mode. | |
320 | In this mode, either side may send | |
321 | data at any time, and such data is forwarded to/from the shell or | |
322 | command on the server side, and the user terminal in the client side. | |
323 | .Pp | |
324 | When the user program terminates and all forwarded X11 and other | |
325 | connections have been closed, the server sends command exit status to | |
326 | the client, and both sides exit. | |
327 | .Sh LOGIN PROCESS | |
328 | When a user successfully logs in, | |
329 | .Nm | |
330 | does the following: | |
331 | .Bl -enum -offset indent | |
332 | .It | |
333 | If the login is on a tty, and no command has been specified, | |
334 | prints last login time and | |
335 | .Pa /etc/motd | |
336 | (unless prevented in the configuration file or by | |
337 | .Pa ~/.hushlogin ; | |
338 | see the | |
339 | .Sx FILES | |
340 | section). | |
341 | .It | |
342 | If the login is on a tty, records login time. | |
343 | .It | |
344 | Checks | |
345 | .Pa /etc/nologin ; | |
346 | if it exists, prints contents and quits | |
347 | (unless root). | |
348 | .It | |
349 | Changes to run with normal user privileges. | |
350 | .It | |
351 | Sets up basic environment. | |
352 | .It | |
353 | Reads the file | |
354 | .Pa ~/.ssh/environment , | |
355 | if it exists, and users are allowed to change their environment. | |
356 | See the | |
357 | .Cm PermitUserEnvironment | |
358 | option in | |
359 | .Xr sshd_config 5 . | |
360 | .It | |
361 | Changes to user's home directory. | |
362 | .It | |
363 | If | |
364 | .Pa ~/.ssh/rc | |
365 | exists, runs it; else if | |
366 | .Pa /etc/ssh/sshrc | |
367 | exists, runs | |
368 | it; otherwise runs xauth. | |
369 | The | |
370 | .Dq rc | |
371 | files are given the X11 | |
372 | authentication protocol and cookie in standard input. | |
373 | .It | |
374 | Runs user's shell or command. | |
375 | .El | |
376 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
377 | .Cm AuthorizedKeysFile | |
378 | specifies the file containing public keys for | |
379 | public key authentication; | |
380 | if none is specified, the default is | |
381 | .Pa ~/.ssh/authorized_keys . | |
382 | Each line of the file contains one | |
383 | key (empty lines and lines starting with a | |
384 | .Ql # | |
385 | are ignored as | |
386 | comments). | |
387 | Protocol 1 public keys consist of the following space-separated fields: | |
388 | options, bits, exponent, modulus, comment. | |
389 | Protocol 2 public key consist of: | |
390 | options, keytype, base64-encoded key, comment. | |
391 | The options field is optional; | |
392 | its presence is determined by whether the line starts | |
393 | with a number or not (the options field never starts with a number). | |
394 | The bits, exponent, modulus, and comment fields give the RSA key for | |
395 | protocol version 1; the | |
396 | comment field is not used for anything (but may be convenient for the | |
397 | user to identify the key). | |
398 | For protocol version 2 the keytype is | |
399 | .Dq ssh-dss | |
400 | or | |
401 | .Dq ssh-rsa . | |
402 | .Pp | |
403 | Note that lines in this file are usually several hundred bytes long | |
404 | (because of the size of the public key encoding) up to a limit of | |
405 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA | |
406 | keys up to 16 kilobits. | |
407 | You don't want to type them in; instead, copy the | |
408 | .Pa identity.pub , | |
409 | .Pa id_dsa.pub , | |
410 | or the | |
411 | .Pa id_rsa.pub | |
412 | file and edit it. | |
413 | .Pp | |
414 | .Nm | |
415 | enforces a minimum RSA key modulus size for protocol 1 | |
416 | and protocol 2 keys of 768 bits. | |
417 | .Pp | |
418 | The options (if present) consist of comma-separated option | |
419 | specifications. | |
420 | No spaces are permitted, except within double quotes. | |
421 | The following option specifications are supported (note | |
422 | that option keywords are case-insensitive): | |
423 | .Bl -tag -width Ds | |
424 | .It Cm command="command" | |
425 | Specifies that the command is executed whenever this key is used for | |
426 | authentication. | |
427 | The command supplied by the user (if any) is ignored. | |
428 | The command is run on a pty if the client requests a pty; | |
429 | otherwise it is run without a tty. | |
430 | If an 8-bit clean channel is required, | |
431 | one must not request a pty or should specify | |
432 | .Cm no-pty . | |
433 | A quote may be included in the command by quoting it with a backslash. | |
434 | This option might be useful | |
435 | to restrict certain public keys to perform just a specific operation. | |
436 | An example might be a key that permits remote backups but nothing else. | |
437 | Note that the client may specify TCP and/or X11 | |
438 | forwarding unless they are explicitly prohibited. | |
439 | Note that this option applies to shell, command or subsystem execution. | |
440 | .It Cm environment="NAME=value" | |
441 | Specifies that the string is to be added to the environment when | |
442 | logging in using this key. | |
443 | Environment variables set this way | |
444 | override other default environment values. | |
445 | Multiple options of this type are permitted. | |
446 | Environment processing is disabled by default and is | |
447 | controlled via the | |
448 | .Cm PermitUserEnvironment | |
449 | option. | |
450 | This option is automatically disabled if | |
451 | .Cm UseLogin | |
452 | is enabled. | |
453 | .It Cm from="pattern-list" | |
454 | Specifies that in addition to public key authentication, the canonical name | |
455 | of the remote host must be present in the comma-separated list of | |
456 | patterns | |
457 | .Pf ( Ql \&* | |
458 | and | |
459 | .Ql \&? | |
460 | serve as wildcards). | |
461 | The list may also contain | |
462 | patterns negated by prefixing them with | |
463 | .Ql \&! ; | |
464 | if the canonical host name matches a negated pattern, the key is not accepted. | |
465 | The purpose | |
466 | of this option is to optionally increase security: public key authentication | |
467 | by itself does not trust the network or name servers or anything (but | |
468 | the key); however, if somebody somehow steals the key, the key | |
469 | permits an intruder to log in from anywhere in the world. | |
470 | This additional option makes using a stolen key more difficult (name | |
471 | servers and/or routers would have to be compromised in addition to | |
472 | just the key). | |
473 | .It Cm no-agent-forwarding | |
474 | Forbids authentication agent forwarding when this key is used for | |
475 | authentication. | |
476 | .It Cm no-port-forwarding | |
477 | Forbids TCP forwarding when this key is used for authentication. | |
478 | Any port forward requests by the client will return an error. | |
479 | This might be used, e.g., in connection with the | |
480 | .Cm command | |
481 | option. | |
482 | .It Cm no-pty | |
483 | Prevents tty allocation (a request to allocate a pty will fail). | |
484 | .It Cm no-X11-forwarding | |
485 | Forbids X11 forwarding when this key is used for authentication. | |
486 | Any X11 forward requests by the client will return an error. | |
487 | .It Cm permitopen="host:port" | |
488 | Limit local | |
489 | .Li ``ssh -L'' | |
490 | port forwarding such that it may only connect to the specified host and | |
491 | port. | |
492 | IPv6 addresses can be specified with an alternative syntax: | |
493 | .Ar host Ns / Ns Ar port . | |
494 | Multiple | |
495 | .Cm permitopen | |
496 | options may be applied separated by commas. | |
497 | No pattern matching is performed on the specified hostnames, | |
498 | they must be literal domains or addresses. | |
499 | .It Cm tunnel="n" | |
500 | Force a | |
501 | .Xr tun 4 | |
502 | device on the server. | |
503 | Without this option, the next available device will be used if | |
504 | the client requests a tunnel. | |
505 | .El | |
506 | .Pp | |
507 | An example authorized_keys file: | |
508 | .Bd -literal | |
509 | # Comments allowed at start of line | |
510 | ssh-rsa AAAAB3Nza...LiPk== user@example.net | |
511 | from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net | |
512 | command="dump /home",no-pty,no-port-forwarding ssh-dss AAAAC3...51R== example.net | |
513 | permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss AAAAB5...21S== | |
514 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== jane@example.net | |
515 | .Ed | |
516 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
517 | The | |
518 | .Pa /etc/ssh/ssh_known_hosts | |
519 | and | |
520 | .Pa ~/.ssh/known_hosts | |
521 | files contain host public keys for all known hosts. | |
522 | The global file should | |
523 | be prepared by the administrator (optional), and the per-user file is | |
524 | maintained automatically: whenever the user connects from an unknown host, | |
525 | its key is added to the per-user file. | |
526 | .Pp | |
527 | Each line in these files contains the following fields: hostnames, | |
528 | bits, exponent, modulus, comment. | |
529 | The fields are separated by spaces. | |
530 | .Pp | |
531 | Hostnames is a comma-separated list of patterns | |
532 | .Pf ( Ql * | |
533 | and | |
534 | .Ql \&? | |
535 | act as | |
536 | wildcards); each pattern in turn is matched against the canonical host | |
537 | name (when authenticating a client) or against the user-supplied | |
538 | name (when authenticating a server). | |
539 | A pattern may also be preceded by | |
540 | .Ql \&! | |
541 | to indicate negation: if the host name matches a negated | |
542 | pattern, it is not accepted (by that line) even if it matched another | |
543 | pattern on the line. | |
544 | .Pp | |
545 | Alternately, hostnames may be stored in a hashed form which hides host names | |
546 | and addresses should the file's contents be disclosed. | |
547 | Hashed hostnames start with a | |
548 | .Ql | | |
549 | character. | |
550 | Only one hashed hostname may appear on a single line and none of the above | |
551 | negation or wildcard operators may be applied. | |
552 | .Pp | |
553 | Bits, exponent, and modulus are taken directly from the RSA host key; they | |
554 | can be obtained, for example, from | |
555 | .Pa /etc/ssh/ssh_host_key.pub . | |
556 | The optional comment field continues to the end of the line, and is not used. | |
557 | .Pp | |
558 | Lines starting with | |
559 | .Ql # | |
560 | and empty lines are ignored as comments. | |
561 | .Pp | |
562 | When performing host authentication, authentication is accepted if any | |
563 | matching line has the proper key. | |
564 | It is thus permissible (but not | |
565 | recommended) to have several lines or different host keys for the same | |
566 | names. | |
567 | This will inevitably happen when short forms of host names | |
568 | from different domains are put in the file. | |
569 | It is possible | |
570 | that the files contain conflicting information; authentication is | |
571 | accepted if valid information can be found from either file. | |
572 | .Pp | |
573 | Note that the lines in these files are typically hundreds of characters | |
574 | long, and you definitely don't want to type in the host keys by hand. | |
575 | Rather, generate them by a script | |
576 | or by taking | |
577 | .Pa /etc/ssh/ssh_host_key.pub | |
578 | and adding the host names at the front. | |
579 | .Pp | |
580 | An example ssh_known_hosts file: | |
581 | .Bd -literal -offset 3n | |
582 | # Comments allowed at start of line | |
583 | closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net | |
584 | cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= | |
585 | # A hashed hostname | |
586 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | |
587 | AAAA1234.....= | |
588 | .Ed | |
589 | .Sh FILES | |
590 | .Bl -tag -width Ds | |
591 | .It Pa /etc/ssh/sshd_config | |
592 | Contains configuration data for | |
593 | .Nm sshd . | |
594 | The file format and configuration options are described in | |
595 | .Xr sshd_config 5 . | |
596 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key | |
597 | These three files contain the private parts of the host keys. | |
598 | These files should only be owned by root, readable only by root, and not | |
599 | accessible to others. | |
600 | Note that | |
601 | .Nm | |
602 | does not start if this file is group/world-accessible. | |
603 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub | |
604 | These three files contain the public parts of the host keys. | |
605 | These files should be world-readable but writable only by | |
606 | root. | |
607 | Their contents should match the respective private parts. | |
608 | These files are not | |
609 | really used for anything; they are provided for the convenience of | |
610 | the user so their contents can be copied to known hosts files. | |
611 | These files are created using | |
612 | .Xr ssh-keygen 1 . | |
613 | .It Pa /etc/moduli | |
614 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
615 | The file format is described in | |
616 | .Xr moduli 5 . | |
617 | .It Pa /var/empty | |
618 | .Xr chroot 2 | |
619 | directory used by | |
620 | .Nm | |
621 | during privilege separation in the pre-authentication phase. | |
622 | The directory should not contain any files and must be owned by root | |
623 | and not group or world-writable. | |
624 | .It Pa /var/run/sshd.pid | |
625 | Contains the process ID of the | |
626 | .Nm | |
627 | listening for connections (if there are several daemons running | |
628 | concurrently for different ports, this contains the process ID of the one | |
629 | started last). | |
630 | The content of this file is not sensitive; it can be world-readable. | |
631 | .It Pa ~/.ssh/authorized_keys | |
632 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. | |
633 | This file must be readable by root (which may on some machines imply | |
634 | it being world-readable if the user's home directory resides on an NFS | |
635 | volume). | |
636 | It is recommended that it not be accessible by others. | |
637 | The format of this file is described above. | |
638 | Users will place the contents of their | |
639 | .Pa identity.pub , | |
640 | .Pa id_dsa.pub | |
641 | and/or | |
642 | .Pa id_rsa.pub | |
643 | files into this file, as described in | |
644 | .Xr ssh-keygen 1 . | |
645 | .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts" | |
646 | These files are consulted when using rhosts with RSA host | |
647 | authentication or protocol version 2 hostbased authentication | |
648 | to check the public key of the host. | |
649 | The key must be listed in one of these files to be accepted. | |
650 | The client uses the same files | |
651 | to verify that it is connecting to the correct remote host. | |
652 | These files should be writable only by root/the owner. | |
653 | .Pa /etc/ssh/ssh_known_hosts | |
654 | should be world-readable, and | |
655 | .Pa ~/.ssh/known_hosts | |
656 | can, but need not be, world-readable. | |
657 | .It Pa /etc/motd | |
658 | See | |
659 | .Xr motd 5 . | |
660 | .It Pa ~/.hushlogin | |
661 | This file is used to suppress printing the last login time and | |
662 | .Pa /etc/motd , | |
663 | if | |
664 | .Cm PrintLastLog | |
665 | and | |
666 | .Cm PrintMotd , | |
667 | respectively, | |
668 | are enabled. | |
669 | It does not suppress printing of the banner specified by | |
670 | .Cm Banner . | |
671 | .It Pa /etc/nologin | |
672 | If this file exists, | |
673 | .Nm | |
674 | refuses to let anyone except root log in. | |
675 | The contents of the file | |
676 | are displayed to anyone trying to log in, and non-root connections are | |
677 | refused. | |
678 | The file should be world-readable. | |
679 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
680 | Access controls that should be enforced by tcp-wrappers are defined here. | |
681 | Further details are described in | |
682 | .Xr hosts_access 5 . | |
683 | .It Pa ~/.rhosts | |
684 | This file is used during | |
685 | .Cm RhostsRSAAuthentication | |
686 | and | |
687 | .Cm HostbasedAuthentication | |
688 | and contains host-username pairs, separated by a space, one per | |
689 | line. | |
690 | The given user on the corresponding host is permitted to log in | |
691 | without a password. | |
692 | The same file is used by rlogind and rshd. | |
693 | The file must | |
694 | be writable only by the user; it is recommended that it not be | |
695 | accessible by others. | |
696 | .Pp | |
697 | It is also possible to use netgroups in the file. | |
698 | Either host or user | |
699 | name may be of the form +@groupname to specify all hosts or all users | |
700 | in the group. | |
701 | .It Pa ~/.shosts | |
702 | For ssh, | |
703 | this file is exactly the same as for | |
704 | .Pa .rhosts . | |
705 | However, this file is | |
706 | not used by rlogin and rshd, so using this permits access using SSH only. | |
707 | .It Pa /etc/hosts.equiv | |
708 | This file is used during | |
709 | .Cm RhostsRSAAuthentication | |
710 | and | |
711 | .Cm HostbasedAuthentication | |
712 | authentication. | |
713 | In the simplest form, this file contains host names, one per line. | |
714 | Users on | |
715 | those hosts are permitted to log in without a password, provided they | |
716 | have the same user name on both machines. | |
717 | The host name may also be | |
718 | followed by a user name; such users are permitted to log in as | |
719 | .Em any | |
720 | user on this machine (except root). | |
721 | Additionally, the syntax | |
722 | .Dq +@group | |
723 | can be used to specify netgroups. | |
724 | Negated entries start with | |
725 | .Ql \&- . | |
726 | .Pp | |
727 | If the client host/user is successfully matched in this file, login is | |
728 | automatically permitted provided the client and server user names are the | |
729 | same. | |
730 | Additionally, successful client host key authentication is required. | |
731 | This file must be writable only by root; it is recommended | |
732 | that it be world-readable. | |
733 | .Pp | |
734 | .Sy "Warning: It is almost never a good idea to use user names in" | |
735 | .Pa hosts.equiv . | |
736 | Beware that it really means that the named user(s) can log in as | |
737 | .Em anybody , | |
738 | which includes bin, daemon, adm, and other accounts that own critical | |
739 | binaries and directories. | |
740 | Using a user name practically grants the user root access. | |
741 | The only valid use for user names that I can think | |
742 | of is in negative entries. | |
743 | .Pp | |
744 | Note that this warning also applies to rsh/rlogin. | |
745 | .It Pa /etc/shosts.equiv | |
746 | This is processed exactly as | |
747 | .Pa /etc/hosts.equiv . | |
748 | However, this file may be useful in environments that want to run both | |
749 | rsh/rlogin and ssh. | |
750 | .It Pa ~/.ssh/environment | |
751 | This file is read into the environment at login (if it exists). | |
752 | It can only contain empty lines, comment lines (that start with | |
753 | .Ql # ) , | |
754 | and assignment lines of the form name=value. | |
755 | The file should be writable | |
756 | only by the user; it need not be readable by anyone else. | |
757 | Environment processing is disabled by default and is | |
758 | controlled via the | |
759 | .Cm PermitUserEnvironment | |
760 | option. | |
761 | .It Pa ~/.ssh/rc | |
762 | If this file exists, it is run with | |
763 | .Pa /bin/sh | |
764 | after reading the | |
765 | environment files but before starting the user's shell or command. | |
766 | It must not produce any output on stdout; stderr must be used | |
767 | instead. | |
768 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
769 | its standard input (and | |
770 | .Ev DISPLAY | |
771 | in its environment). | |
772 | The script must call | |
773 | .Xr xauth 1 | |
774 | because | |
775 | .Nm | |
776 | will not run xauth automatically to add X11 cookies. | |
777 | .Pp | |
778 | The primary purpose of this file is to run any initialization routines | |
779 | which may be needed before the user's home directory becomes | |
780 | accessible; AFS is a particular example of such an environment. | |
781 | .Pp | |
782 | This file will probably contain some initialization code followed by | |
783 | something similar to: | |
784 | .Bd -literal | |
785 | if read proto cookie && [ -n "$DISPLAY" ]; then | |
786 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
787 | # X11UseLocalhost=yes | |
788 | echo add unix:`echo $DISPLAY | | |
789 | cut -c11-` $proto $cookie | |
790 | else | |
791 | # X11UseLocalhost=no | |
792 | echo add $DISPLAY $proto $cookie | |
793 | fi | xauth -q - | |
794 | fi | |
795 | .Ed | |
796 | .Pp | |
797 | If this file does not exist, | |
798 | .Pa /etc/ssh/sshrc | |
799 | is run, and if that | |
800 | does not exist either, xauth is used to add the cookie. | |
801 | .Pp | |
802 | This file should be writable only by the user, and need not be | |
803 | readable by anyone else. | |
804 | .It Pa /etc/ssh/sshrc | |
805 | Like | |
806 | .Pa ~/.ssh/rc . | |
807 | This can be used to specify | |
808 | machine-specific login-time initializations globally. | |
809 | This file should be writable only by root, and should be world-readable. | |
810 | .El | |
811 | .Sh SEE ALSO | |
812 | .Xr scp 1 , | |
813 | .Xr sftp 1 , | |
814 | .Xr ssh 1 , | |
815 | .Xr ssh-add 1 , | |
816 | .Xr ssh-agent 1 , | |
817 | .Xr ssh-keygen 1 , | |
818 | .Xr chroot 2 , | |
819 | .Xr hosts_access 5 , | |
820 | .Xr login.conf 5 , | |
821 | .Xr moduli 5 , | |
822 | .Xr sshd_config 5 , | |
823 | .Xr inetd 8 , | |
824 | .Xr sftp-server 8 | |
825 | .Rs | |
826 | .%A T. Ylonen | |
827 | .%A T. Kivinen | |
828 | .%A M. Saarinen | |
829 | .%A T. Rinne | |
830 | .%A S. Lehtinen | |
831 | .%T "SSH Protocol Architecture" | |
832 | .%N draft-ietf-secsh-architecture-12.txt | |
833 | .%D January 2002 | |
834 | .%O work in progress material | |
835 | .Re | |
836 | .Rs | |
837 | .%A M. Friedl | |
838 | .%A N. Provos | |
839 | .%A W. A. Simpson | |
840 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
841 | .%N draft-ietf-secsh-dh-group-exchange-02.txt | |
842 | .%D January 2002 | |
843 | .%O work in progress material | |
844 | .Re | |
845 | .Sh AUTHORS | |
846 | OpenSSH is a derivative of the original and free | |
847 | ssh 1.2.12 release by Tatu Ylonen. | |
848 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
849 | Theo de Raadt and Dug Song | |
850 | removed many bugs, re-added newer features and | |
851 | created OpenSSH. | |
852 | Markus Friedl contributed the support for SSH | |
853 | protocol versions 1.5 and 2.0. | |
854 | Niels Provos and Markus Friedl contributed support | |
855 | for privilege separation. | |
856 | .Sh CAVEATS | |
857 | System security is not improved unless | |
858 | .Nm rshd , | |
859 | .Nm rlogind , | |
860 | and | |
861 | .Nm rexecd | |
862 | are disabled (thus completely disabling | |
863 | .Xr rlogin | |
864 | and | |
865 | .Xr rsh | |
866 | into the machine). |