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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 1.210 2005/12/21 22:44:26 stevesk 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 | (SSH 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 | The programs are intended to be as easy to | |
65 | install and use as possible. | |
66 | .Pp | |
67 | .Nm | |
68 | is the daemon that listens for connections from clients. | |
69 | It is normally started at boot from | |
70 | .Pa /etc/rc . | |
71 | It forks a new | |
72 | daemon for each incoming connection. | |
73 | The forked daemons handle | |
74 | key exchange, encryption, authentication, command execution, | |
75 | and data exchange. | |
76 | This implementation of | |
77 | .Nm | |
78 | supports both SSH protocol version 1 and 2 simultaneously. | |
79 | .Nm | |
80 | works as follows: | |
81 | .Ss SSH protocol version 1 | |
82 | Each host has a host-specific RSA key | |
83 | (normally 2048 bits) used to identify the host. | |
84 | Additionally, when | |
85 | the daemon starts, it generates a server RSA key (normally 768 bits). | |
86 | This key is normally regenerated every hour if it has been used, and | |
87 | is never stored on disk. | |
88 | .Pp | |
89 | Whenever a client connects, the daemon responds with its public | |
90 | host and server keys. | |
91 | The client compares the | |
92 | RSA host key against its own database to verify that it has not changed. | |
93 | The client then generates a 256-bit random number. | |
94 | It encrypts this | |
95 | random number using both the host key and the server key, and sends | |
96 | the encrypted number to the server. | |
97 | Both sides then use this | |
98 | random number as a session key which is used to encrypt all further | |
99 | communications in the session. | |
100 | The rest of the session is encrypted | |
101 | using a conventional cipher, currently Blowfish or 3DES, with 3DES | |
102 | being used by default. | |
103 | The client selects the encryption algorithm | |
104 | to use from those offered by the server. | |
105 | .Pp | |
106 | Next, the server and the client enter an authentication dialog. | |
107 | The client tries to authenticate itself using | |
108 | .Em .rhosts | |
109 | authentication combined with RSA host | |
110 | authentication, RSA challenge-response authentication, or password | |
111 | based authentication. | |
112 | .Pp | |
113 | Regardless of the authentication type, the account is checked to | |
114 | ensure that it is accessible. An account is not accessible if it is | |
115 | locked, listed in | |
116 | .Cm DenyUsers | |
117 | or its group is listed in | |
118 | .Cm DenyGroups | |
119 | \&. The definition of a locked account is system dependant. Some platforms | |
120 | have their own account database (eg AIX) and some modify the passwd field ( | |
121 | .Ql \&*LK\&* | |
122 | on Solaris and UnixWare, | |
123 | .Ql \&* | |
124 | on HP-UX, containing | |
125 | .Ql Nologin | |
126 | on Tru64, | |
127 | a leading | |
128 | .Ql \&*LOCKED\&* | |
129 | on FreeBSD and a leading | |
130 | .Ql \&!! | |
131 | on Linux). If there is a requirement to disable password authentication | |
132 | for the account while allowing still public-key, then the passwd field | |
133 | should be set to something other than these values (eg | |
134 | .Ql NP | |
135 | or | |
136 | .Ql \&*NP\&* | |
137 | ). | |
138 | .Pp | |
139 | .Nm rshd , | |
140 | .Nm rlogind , | |
141 | and | |
142 | .Nm rexecd | |
143 | are disabled (thus completely disabling | |
144 | .Xr rlogin | |
145 | and | |
146 | .Xr rsh | |
147 | into the machine). | |
148 | .Ss SSH protocol version 2 | |
149 | Version 2 works similarly: | |
150 | Each host has a host-specific key (RSA or DSA) used to identify the host. | |
151 | However, when the daemon starts, it does not generate a server key. | |
152 | Forward security is provided through a Diffie-Hellman key agreement. | |
153 | This key agreement results in a shared session key. | |
154 | .Pp | |
155 | The rest of the session is encrypted using a symmetric cipher, currently | |
156 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. | |
157 | The client selects the encryption algorithm | |
158 | to use from those offered by the server. | |
159 | Additionally, session integrity is provided | |
160 | through a cryptographic message authentication code | |
161 | (hmac-sha1 or hmac-md5). | |
162 | .Pp | |
163 | Protocol version 2 provides a public key based | |
164 | user (PubkeyAuthentication) or | |
165 | client host (HostbasedAuthentication) authentication method, | |
166 | conventional password authentication and challenge response based methods. | |
167 | .Ss Command execution and data forwarding | |
168 | If the client successfully authenticates itself, a dialog for | |
169 | preparing the session is entered. | |
170 | At this time the client may request | |
171 | things like allocating a pseudo-tty, forwarding X11 connections, | |
172 | forwarding TCP/IP connections, or forwarding the authentication agent | |
173 | connection over the secure channel. | |
174 | .Pp | |
175 | Finally, the client either requests a shell or execution of a command. | |
176 | The sides then enter session mode. | |
177 | In this mode, either side may send | |
178 | data at any time, and such data is forwarded to/from the shell or | |
179 | command on the server side, and the user terminal in the client side. | |
180 | .Pp | |
181 | When the user program terminates and all forwarded X11 and other | |
182 | connections have been closed, the server sends command exit status to | |
183 | the client, and both sides exit. | |
184 | .Pp | |
185 | .Nm | |
186 | can be configured using command-line options or a configuration file | |
187 | (by default | |
188 | .Xr sshd_config 5 ) . | |
189 | Command-line options override values specified in the | |
190 | configuration file. | |
191 | .Pp | |
192 | .Nm | |
193 | rereads its configuration file when it receives a hangup signal, | |
194 | .Dv SIGHUP , | |
195 | by executing itself with the name and options it was started with, e.g., | |
196 | .Pa /usr/sbin/sshd . | |
197 | .Pp | |
198 | The options are as follows: | |
199 | .Bl -tag -width Ds | |
200 | .It Fl 4 | |
201 | Forces | |
202 | .Nm | |
203 | to use IPv4 addresses only. | |
204 | .It Fl 6 | |
205 | Forces | |
206 | .Nm | |
207 | to use IPv6 addresses only. | |
208 | .It Fl b Ar bits | |
209 | Specifies the number of bits in the ephemeral protocol version 1 | |
210 | server key (default 768). | |
211 | .It Fl D | |
212 | When this option is specified, | |
213 | .Nm | |
214 | will not detach and does not become a daemon. | |
215 | This allows easy monitoring of | |
216 | .Nm sshd . | |
217 | .It Fl d | |
218 | Debug mode. | |
219 | The server sends verbose debug output to the system | |
220 | log, and does not put itself in the background. | |
221 | The server also will not fork and will only process one connection. | |
222 | This option is only intended for debugging for the server. | |
223 | Multiple | |
224 | .Fl d | |
225 | options increase the debugging level. | |
226 | Maximum is 3. | |
227 | .It Fl e | |
228 | When this option is specified, | |
229 | .Nm | |
230 | will send the output to the standard error instead of the system log. | |
231 | .It Fl f Ar configuration_file | |
232 | Specifies the name of the configuration file. | |
233 | The default is | |
234 | .Pa /etc/ssh/sshd_config . | |
235 | .Nm | |
236 | refuses to start if there is no configuration file. | |
237 | .It Fl g Ar login_grace_time | |
238 | Gives the grace time for clients to authenticate themselves (default | |
239 | 120 seconds). | |
240 | If the client fails to authenticate the user within | |
241 | this many seconds, the server disconnects and exits. | |
242 | A value of zero indicates no limit. | |
243 | .It Fl h Ar host_key_file | |
244 | Specifies a file from which a host key is read. | |
245 | This option must be given if | |
246 | .Nm | |
247 | is not run as root (as the normal | |
248 | host key files are normally not readable by anyone but root). | |
249 | The default is | |
250 | .Pa /etc/ssh/ssh_host_key | |
251 | for protocol version 1, and | |
252 | .Pa /etc/ssh/ssh_host_rsa_key | |
253 | and | |
254 | .Pa /etc/ssh/ssh_host_dsa_key | |
255 | for protocol version 2. | |
256 | It is possible to have multiple host key files for | |
257 | the different protocol versions and host key algorithms. | |
258 | .It Fl i | |
259 | Specifies that | |
260 | .Nm | |
261 | is being run from | |
262 | .Xr inetd 8 . | |
263 | .Nm | |
264 | is normally not run | |
265 | from inetd because it needs to generate the server key before it can | |
266 | respond to the client, and this may take tens of seconds. | |
267 | Clients would have to wait too long if the key was regenerated every time. | |
268 | However, with small key sizes (e.g., 512) using | |
269 | .Nm | |
270 | from inetd may | |
271 | be feasible. | |
272 | .It Fl k Ar key_gen_time | |
273 | Specifies how often the ephemeral protocol version 1 server key is | |
274 | regenerated (default 3600 seconds, or one hour). | |
275 | The motivation for regenerating the key fairly | |
276 | often is that the key is not stored anywhere, and after about an hour | |
277 | it becomes impossible to recover the key for decrypting intercepted | |
278 | communications even if the machine is cracked into or physically | |
279 | seized. | |
280 | A value of zero indicates that the key will never be regenerated. | |
281 | .It Fl o Ar option | |
282 | Can be used to give options in the format used in the configuration file. | |
283 | This is useful for specifying options for which there is no separate | |
284 | command-line flag. | |
285 | For full details of the options, and their values, see | |
286 | .Xr sshd_config 5 . | |
287 | .It Fl p Ar port | |
288 | Specifies the port on which the server listens for connections | |
289 | (default 22). | |
290 | Multiple port options are permitted. | |
291 | Ports specified in the configuration file with the | |
292 | .Cm Port | |
293 | option are ignored when a command-line port is specified. | |
294 | Ports specified using the | |
295 | .Cm ListenAddress | |
296 | option override command-line ports. | |
297 | .It Fl q | |
298 | Quiet mode. | |
299 | Nothing is sent to the system log. | |
300 | Normally the beginning, | |
301 | authentication, and termination of each connection is logged. | |
302 | .It Fl t | |
303 | Test mode. | |
304 | Only check the validity of the configuration file and sanity of the keys. | |
305 | This is useful for updating | |
306 | .Nm | |
307 | reliably as configuration options may change. | |
308 | .It Fl u Ar len | |
309 | This option is used to specify the size of the field | |
310 | in the | |
311 | .Li utmp | |
312 | structure that holds the remote host name. | |
313 | If the resolved host name is longer than | |
314 | .Ar len , | |
315 | the dotted decimal value will be used instead. | |
316 | This allows hosts with very long host names that | |
317 | overflow this field to still be uniquely identified. | |
318 | Specifying | |
319 | .Fl u0 | |
320 | indicates that only dotted decimal addresses | |
321 | should be put into the | |
322 | .Pa utmp | |
323 | file. | |
324 | .Fl u0 | |
325 | may also be used to prevent | |
326 | .Nm | |
327 | from making DNS requests unless the authentication | |
328 | mechanism or configuration requires it. | |
329 | Authentication mechanisms that may require DNS include | |
330 | .Cm RhostsRSAAuthentication , | |
331 | .Cm HostbasedAuthentication | |
332 | and using a | |
333 | .Cm from="pattern-list" | |
334 | option in a key file. | |
335 | Configuration options that require DNS include using a | |
336 | USER@HOST pattern in | |
337 | .Cm AllowUsers | |
338 | or | |
339 | .Cm DenyUsers . | |
340 | .El | |
341 | .Sh CONFIGURATION FILE | |
342 | .Nm | |
343 | reads configuration data from | |
344 | .Pa /etc/ssh/sshd_config | |
345 | (or the file specified with | |
346 | .Fl f | |
347 | on the command line). | |
348 | The file format and configuration options are described in | |
349 | .Xr sshd_config 5 . | |
350 | .Sh LOGIN PROCESS | |
351 | When a user successfully logs in, | |
352 | .Nm | |
353 | does the following: | |
354 | .Bl -enum -offset indent | |
355 | .It | |
356 | If the login is on a tty, and no command has been specified, | |
357 | prints last login time and | |
358 | .Pa /etc/motd | |
359 | (unless prevented in the configuration file or by | |
360 | .Pa ~/.hushlogin ; | |
361 | see the | |
362 | .Sx FILES | |
363 | section). | |
364 | .It | |
365 | If the login is on a tty, records login time. | |
366 | .It | |
367 | Checks | |
368 | .Pa /etc/nologin ; | |
369 | if it exists, prints contents and quits | |
370 | (unless root). | |
371 | .It | |
372 | Changes to run with normal user privileges. | |
373 | .It | |
374 | Sets up basic environment. | |
375 | .It | |
376 | Reads the file | |
377 | .Pa ~/.ssh/environment , | |
378 | if it exists, and users are allowed to change their environment. | |
379 | See the | |
380 | .Cm PermitUserEnvironment | |
381 | option in | |
382 | .Xr sshd_config 5 . | |
383 | .It | |
384 | Changes to user's home directory. | |
385 | .It | |
386 | If | |
387 | .Pa ~/.ssh/rc | |
388 | exists, runs it; else if | |
389 | .Pa /etc/ssh/sshrc | |
390 | exists, runs | |
391 | it; otherwise runs xauth. | |
392 | The | |
393 | .Dq rc | |
394 | files are given the X11 | |
395 | authentication protocol and cookie in standard input. | |
396 | .It | |
397 | Runs user's shell or command. | |
398 | .El | |
399 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
400 | .Pa ~/.ssh/authorized_keys | |
401 | is the default file that lists the public keys that are | |
402 | permitted for RSA authentication in protocol version 1 | |
403 | and for public key authentication (PubkeyAuthentication) | |
404 | in protocol version 2. | |
405 | .Cm AuthorizedKeysFile | |
406 | may be used to specify an alternative file. | |
407 | .Pp | |
408 | Each line of the file contains one | |
409 | key (empty lines and lines starting with a | |
410 | .Ql # | |
411 | are ignored as | |
412 | comments). | |
413 | Each RSA public key consists of the following fields, separated by | |
414 | spaces: options, bits, exponent, modulus, comment. | |
415 | Each protocol version 2 public key consists of: | |
416 | options, keytype, base64 encoded key, comment. | |
417 | The options field | |
418 | is optional; its presence is determined by whether the line starts | |
419 | with a number or not (the options field never starts with a number). | |
420 | The bits, exponent, modulus and comment fields give the RSA key for | |
421 | protocol version 1; the | |
422 | comment field is not used for anything (but may be convenient for the | |
423 | user to identify the key). | |
424 | For protocol version 2 the keytype is | |
425 | .Dq ssh-dss | |
426 | or | |
427 | .Dq ssh-rsa . | |
428 | .Pp | |
429 | Note that lines in this file are usually several hundred bytes long | |
430 | (because of the size of the public key encoding) up to a limit of | |
431 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA | |
432 | keys up to 16 kilobits. | |
433 | You don't want to type them in; instead, copy the | |
434 | .Pa identity.pub , | |
435 | .Pa id_dsa.pub | |
436 | or the | |
437 | .Pa id_rsa.pub | |
438 | file and edit it. | |
439 | .Pp | |
440 | .Nm | |
441 | enforces a minimum RSA key modulus size for protocol 1 | |
442 | and protocol 2 keys of 768 bits. | |
443 | .Pp | |
444 | The options (if present) consist of comma-separated option | |
445 | specifications. | |
446 | No spaces are permitted, except within double quotes. | |
447 | The following option specifications are supported (note | |
448 | that option keywords are case-insensitive): | |
449 | .Bl -tag -width Ds | |
450 | .It Cm from="pattern-list" | |
451 | Specifies that in addition to public key authentication, the canonical name | |
452 | of the remote host must be present in the comma-separated list of | |
453 | patterns | |
454 | .Pf ( Ql \&* | |
455 | and | |
456 | .Ql \&? | |
457 | serve as wildcards). | |
458 | The list may also contain | |
459 | patterns negated by prefixing them with | |
460 | .Ql \&! ; | |
461 | if the canonical host name matches a negated pattern, the key is not accepted. | |
462 | The purpose | |
463 | of this option is to optionally increase security: public key authentication | |
464 | by itself does not trust the network or name servers or anything (but | |
465 | the key); however, if somebody somehow steals the key, the key | |
466 | permits an intruder to log in from anywhere in the world. | |
467 | This additional option makes using a stolen key more difficult (name | |
468 | servers and/or routers would have to be compromised in addition to | |
469 | just the key). | |
470 | .It Cm command="command" | |
471 | Specifies that the command is executed whenever this key is used for | |
472 | authentication. | |
473 | The command supplied by the user (if any) is ignored. | |
474 | The command is run on a pty if the client requests a pty; | |
475 | otherwise it is run without a tty. | |
476 | If an 8-bit clean channel is required, | |
477 | one must not request a pty or should specify | |
478 | .Cm no-pty . | |
479 | A quote may be included in the command by quoting it with a backslash. | |
480 | This option might be useful | |
481 | to restrict certain public keys to perform just a specific operation. | |
482 | An example might be a key that permits remote backups but nothing else. | |
483 | Note that the client may specify TCP/IP and/or X11 | |
484 | forwarding unless they are explicitly prohibited. | |
485 | Note that this option applies to shell, command or subsystem execution. | |
486 | .It Cm environment="NAME=value" | |
487 | Specifies that the string is to be added to the environment when | |
488 | logging in using this key. | |
489 | Environment variables set this way | |
490 | override other default environment values. | |
491 | Multiple options of this type are permitted. | |
492 | Environment processing is disabled by default and is | |
493 | controlled via the | |
494 | .Cm PermitUserEnvironment | |
495 | option. | |
496 | This option is automatically disabled if | |
497 | .Cm UseLogin | |
498 | is enabled. | |
499 | .It Cm no-port-forwarding | |
500 | Forbids TCP/IP forwarding when this key is used for authentication. | |
501 | Any port forward requests by the client will return an error. | |
502 | This might be used, e.g., in connection with the | |
503 | .Cm command | |
504 | option. | |
505 | .It Cm no-X11-forwarding | |
506 | Forbids X11 forwarding when this key is used for authentication. | |
507 | Any X11 forward requests by the client will return an error. | |
508 | .It Cm no-agent-forwarding | |
509 | Forbids authentication agent forwarding when this key is used for | |
510 | authentication. | |
511 | .It Cm no-pty | |
512 | Prevents tty allocation (a request to allocate a pty will fail). | |
513 | .It Cm permitopen="host:port" | |
514 | Limit local | |
515 | .Li ``ssh -L'' | |
516 | port forwarding such that it may only connect to the specified host and | |
517 | port. | |
518 | IPv6 addresses can be specified with an alternative syntax: | |
519 | .Ar host Ns / Ns Ar port . | |
520 | Multiple | |
521 | .Cm permitopen | |
522 | options may be applied separated by commas. | |
523 | No pattern matching is performed on the specified hostnames, | |
524 | they must be literal domains or addresses. | |
525 | .It Cm tunnel="n" | |
526 | Force a | |
527 | .Xr tun 4 | |
528 | device on the server. | |
529 | Without this option, the next available device will be used if | |
530 | the client requests a tunnel. | |
531 | .El | |
532 | .Ss Examples | |
533 | 1024 33 12121...312314325 ylo@foo.bar | |
534 | .Pp | |
535 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula | |
536 | .Pp | |
537 | command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi | |
538 | .Pp | |
539 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323 | |
540 | .Pp | |
541 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org | |
542 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
543 | The | |
544 | .Pa /etc/ssh/ssh_known_hosts | |
545 | and | |
546 | .Pa ~/.ssh/known_hosts | |
547 | files contain host public keys for all known hosts. | |
548 | The global file should | |
549 | be prepared by the administrator (optional), and the per-user file is | |
550 | maintained automatically: whenever the user connects from an unknown host | |
551 | its key is added to the per-user file. | |
552 | .Pp | |
553 | Each line in these files contains the following fields: hostnames, | |
554 | bits, exponent, modulus, comment. | |
555 | The fields are separated by spaces. | |
556 | .Pp | |
557 | Hostnames is a comma-separated list of patterns | |
558 | .Pf ( Ql \&* | |
559 | and | |
560 | .Ql \&? | |
561 | act as | |
562 | wildcards); each pattern in turn is matched against the canonical host | |
563 | name (when authenticating a client) or against the user-supplied | |
564 | name (when authenticating a server). | |
565 | A pattern may also be preceded by | |
566 | .Ql \&! | |
567 | to indicate negation: if the host name matches a negated | |
568 | pattern, it is not accepted (by that line) even if it matched another | |
569 | pattern on the line. | |
570 | .Pp | |
571 | Alternately, hostnames may be stored in a hashed form which hides host names | |
572 | and addresses should the file's contents be disclosed. | |
573 | Hashed hostnames start with a | |
574 | .Ql | | |
575 | character. | |
576 | Only one hashed hostname may appear on a single line and none of the above | |
577 | negation or wildcard operators may be applied. | |
578 | .Pp | |
579 | Bits, exponent, and modulus are taken directly from the RSA host key; they | |
580 | can be obtained, e.g., from | |
581 | .Pa /etc/ssh/ssh_host_key.pub . | |
582 | The optional comment field continues to the end of the line, and is not used. | |
583 | .Pp | |
584 | Lines starting with | |
585 | .Ql # | |
586 | and empty lines are ignored as comments. | |
587 | .Pp | |
588 | When performing host authentication, authentication is accepted if any | |
589 | matching line has the proper key. | |
590 | It is thus permissible (but not | |
591 | recommended) to have several lines or different host keys for the same | |
592 | names. | |
593 | This will inevitably happen when short forms of host names | |
594 | from different domains are put in the file. | |
595 | It is possible | |
596 | that the files contain conflicting information; authentication is | |
597 | accepted if valid information can be found from either file. | |
598 | .Pp | |
599 | Note that the lines in these files are typically hundreds of characters | |
600 | long, and you definitely don't want to type in the host keys by hand. | |
601 | Rather, generate them by a script | |
602 | or by taking | |
603 | .Pa /etc/ssh/ssh_host_key.pub | |
604 | and adding the host names at the front. | |
605 | .Ss Examples | |
606 | .Bd -literal | |
607 | closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi | |
608 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
609 | .Ed | |
610 | .Bd -literal | |
611 | # A hashed hostname | |
612 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | |
613 | AAAA1234.....= | |
614 | .Ed | |
615 | .Sh FILES | |
616 | .Bl -tag -width Ds | |
617 | .It Pa /etc/ssh/sshd_config | |
618 | Contains configuration data for | |
619 | .Nm sshd . | |
620 | The file format and configuration options are described in | |
621 | .Xr sshd_config 5 . | |
622 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key | |
623 | These three files contain the private parts of the host keys. | |
624 | These files should only be owned by root, readable only by root, and not | |
625 | accessible to others. | |
626 | Note that | |
627 | .Nm | |
628 | does not start if this file is group/world-accessible. | |
629 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub | |
630 | These three files contain the public parts of the host keys. | |
631 | These files should be world-readable but writable only by | |
632 | root. | |
633 | Their contents should match the respective private parts. | |
634 | These files are not | |
635 | really used for anything; they are provided for the convenience of | |
636 | the user so their contents can be copied to known hosts files. | |
637 | These files are created using | |
638 | .Xr ssh-keygen 1 . | |
639 | .It Pa /etc/moduli | |
640 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
641 | The file format is described in | |
642 | .Xr moduli 5 . | |
643 | .It Pa /var/empty | |
644 | .Xr chroot 2 | |
645 | directory used by | |
646 | .Nm | |
647 | during privilege separation in the pre-authentication phase. | |
648 | The directory should not contain any files and must be owned by root | |
649 | and not group or world-writable. | |
650 | .It Pa /var/run/sshd.pid | |
651 | Contains the process ID of the | |
652 | .Nm | |
653 | listening for connections (if there are several daemons running | |
654 | concurrently for different ports, this contains the process ID of the one | |
655 | started last). | |
656 | The content of this file is not sensitive; it can be world-readable. | |
657 | .It Pa ~/.ssh/authorized_keys | |
658 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. | |
659 | This file must be readable by root (which may on some machines imply | |
660 | it being world-readable if the user's home directory resides on an NFS | |
661 | volume). | |
662 | It is recommended that it not be accessible by others. | |
663 | The format of this file is described above. | |
664 | Users will place the contents of their | |
665 | .Pa identity.pub , | |
666 | .Pa id_dsa.pub | |
667 | and/or | |
668 | .Pa id_rsa.pub | |
669 | files into this file, as described in | |
670 | .Xr ssh-keygen 1 . | |
671 | .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts" | |
672 | These files are consulted when using rhosts with RSA host | |
673 | authentication or protocol version 2 hostbased authentication | |
674 | to check the public key of the host. | |
675 | The key must be listed in one of these files to be accepted. | |
676 | The client uses the same files | |
677 | to verify that it is connecting to the correct remote host. | |
678 | These files should be writable only by root/the owner. | |
679 | .Pa /etc/ssh/ssh_known_hosts | |
680 | should be world-readable, and | |
681 | .Pa ~/.ssh/known_hosts | |
682 | can, but need not be, world-readable. | |
683 | .It Pa /etc/motd | |
684 | See | |
685 | .Xr motd 5 . | |
686 | .It Pa ~/.hushlogin | |
687 | This file is used to suppress printing the last login time and | |
688 | .Pa /etc/motd , | |
689 | if | |
690 | .Cm PrintLastLog | |
691 | and | |
692 | .Cm PrintMotd , | |
693 | respectively, | |
694 | are enabled. | |
695 | It does not suppress printing of the banner specified by | |
696 | .Cm Banner . | |
697 | .It Pa /etc/nologin | |
698 | If this file exists, | |
699 | .Nm | |
700 | refuses to let anyone except root log in. | |
701 | The contents of the file | |
702 | are displayed to anyone trying to log in, and non-root connections are | |
703 | refused. | |
704 | The file should be world-readable. | |
705 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
706 | Access controls that should be enforced by tcp-wrappers are defined here. | |
707 | Further details are described in | |
708 | .Xr hosts_access 5 . | |
709 | .It Pa ~/.rhosts | |
710 | This file is used during | |
711 | .Cm RhostsRSAAuthentication | |
712 | and | |
713 | .Cm HostbasedAuthentication | |
714 | and contains host-username pairs, separated by a space, one per | |
715 | line. | |
716 | The given user on the corresponding host is permitted to log in | |
717 | without a password. | |
718 | The same file is used by rlogind and rshd. | |
719 | The file must | |
720 | be writable only by the user; it is recommended that it not be | |
721 | accessible by others. | |
722 | .Pp | |
723 | It is also possible to use netgroups in the file. | |
724 | Either host or user | |
725 | name may be of the form +@groupname to specify all hosts or all users | |
726 | in the group. | |
727 | .It Pa ~/.shosts | |
728 | For ssh, | |
729 | this file is exactly the same as for | |
730 | .Pa .rhosts . | |
731 | However, this file is | |
732 | not used by rlogin and rshd, so using this permits access using SSH only. | |
733 | .It Pa /etc/hosts.equiv | |
734 | This file is used during | |
735 | .Cm RhostsRSAAuthentication | |
736 | and | |
737 | .Cm HostbasedAuthentication | |
738 | authentication. | |
739 | In the simplest form, this file contains host names, one per line. | |
740 | Users on | |
741 | those hosts are permitted to log in without a password, provided they | |
742 | have the same user name on both machines. | |
743 | The host name may also be | |
744 | followed by a user name; such users are permitted to log in as | |
745 | .Em any | |
746 | user on this machine (except root). | |
747 | Additionally, the syntax | |
748 | .Dq +@group | |
749 | can be used to specify netgroups. | |
750 | Negated entries start with | |
751 | .Ql \&- . | |
752 | .Pp | |
753 | If the client host/user is successfully matched in this file, login is | |
754 | automatically permitted provided the client and server user names are the | |
755 | same. | |
756 | Additionally, successful client host key authentication is required. | |
757 | This file must be writable only by root; it is recommended | |
758 | that it be world-readable. | |
759 | .Pp | |
760 | .Sy "Warning: It is almost never a good idea to use user names in" | |
761 | .Pa hosts.equiv . | |
762 | Beware that it really means that the named user(s) can log in as | |
763 | .Em anybody , | |
764 | which includes bin, daemon, adm, and other accounts that own critical | |
765 | binaries and directories. | |
766 | Using a user name practically grants the user root access. | |
767 | The only valid use for user names that I can think | |
768 | of is in negative entries. | |
769 | .Pp | |
770 | Note that this warning also applies to rsh/rlogin. | |
771 | .It Pa /etc/shosts.equiv | |
772 | This is processed exactly as | |
773 | .Pa /etc/hosts.equiv . | |
774 | However, this file may be useful in environments that want to run both | |
775 | rsh/rlogin and ssh. | |
776 | .It Pa ~/.ssh/environment | |
777 | This file is read into the environment at login (if it exists). | |
778 | It can only contain empty lines, comment lines (that start with | |
779 | .Ql # ) , | |
780 | and assignment lines of the form name=value. | |
781 | The file should be writable | |
782 | only by the user; it need not be readable by anyone else. | |
783 | Environment processing is disabled by default and is | |
784 | controlled via the | |
785 | .Cm PermitUserEnvironment | |
786 | option. | |
787 | .It Pa ~/.ssh/rc | |
788 | If this file exists, it is run with | |
789 | .Pa /bin/sh | |
790 | after reading the | |
791 | environment files but before starting the user's shell or command. | |
792 | It must not produce any output on stdout; stderr must be used | |
793 | instead. | |
794 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
795 | its standard input (and | |
796 | .Ev DISPLAY | |
797 | in its environment). | |
798 | The script must call | |
799 | .Xr xauth 1 | |
800 | because | |
801 | .Nm | |
802 | will not run xauth automatically to add X11 cookies. | |
803 | .Pp | |
804 | The primary purpose of this file is to run any initialization routines | |
805 | which may be needed before the user's home directory becomes | |
806 | accessible; AFS is a particular example of such an environment. | |
807 | .Pp | |
808 | This file will probably contain some initialization code followed by | |
809 | something similar to: | |
810 | .Bd -literal | |
811 | if read proto cookie && [ -n "$DISPLAY" ]; then | |
812 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
813 | # X11UseLocalhost=yes | |
814 | echo add unix:`echo $DISPLAY | | |
815 | cut -c11-` $proto $cookie | |
816 | else | |
817 | # X11UseLocalhost=no | |
818 | echo add $DISPLAY $proto $cookie | |
819 | fi | xauth -q - | |
820 | fi | |
821 | .Ed | |
822 | .Pp | |
823 | If this file does not exist, | |
824 | .Pa /etc/ssh/sshrc | |
825 | is run, and if that | |
826 | does not exist either, xauth is used to add the cookie. | |
827 | .Pp | |
828 | This file should be writable only by the user, and need not be | |
829 | readable by anyone else. | |
830 | .It Pa /etc/ssh/sshrc | |
831 | Like | |
832 | .Pa ~/.ssh/rc . | |
833 | This can be used to specify | |
834 | machine-specific login-time initializations globally. | |
835 | This file should be writable only by root, and should be world-readable. | |
836 | .El | |
837 | .Sh SEE ALSO | |
838 | .Xr scp 1 , | |
839 | .Xr sftp 1 , | |
840 | .Xr ssh 1 , | |
841 | .Xr ssh-add 1 , | |
842 | .Xr ssh-agent 1 , | |
843 | .Xr ssh-keygen 1 , | |
844 | .Xr chroot 2 , | |
845 | .Xr hosts_access 5 , | |
846 | .Xr login.conf 5 , | |
847 | .Xr moduli 5 , | |
848 | .Xr sshd_config 5 , | |
849 | .Xr inetd 8 , | |
850 | .Xr sftp-server 8 | |
851 | .Rs | |
852 | .%A T. Ylonen | |
853 | .%A T. Kivinen | |
854 | .%A M. Saarinen | |
855 | .%A T. Rinne | |
856 | .%A S. Lehtinen | |
857 | .%T "SSH Protocol Architecture" | |
858 | .%N draft-ietf-secsh-architecture-12.txt | |
859 | .%D January 2002 | |
860 | .%O work in progress material | |
861 | .Re | |
862 | .Rs | |
863 | .%A M. Friedl | |
864 | .%A N. Provos | |
865 | .%A W. A. Simpson | |
866 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
867 | .%N draft-ietf-secsh-dh-group-exchange-02.txt | |
868 | .%D January 2002 | |
869 | .%O work in progress material | |
870 | .Re | |
871 | .Sh AUTHORS | |
872 | OpenSSH is a derivative of the original and free | |
873 | ssh 1.2.12 release by Tatu Ylonen. | |
874 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
875 | Theo de Raadt and Dug Song | |
876 | removed many bugs, re-added newer features and | |
877 | created OpenSSH. | |
878 | Markus Friedl contributed the support for SSH | |
879 | protocol versions 1.5 and 2.0. | |
880 | Niels Provos and Markus Friedl contributed support | |
881 | for privilege separation. |