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3c0ef626 | 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 | .\" | |
416fd2a8 | 37 | .\" $OpenBSD: sshd.8,v 1.200 2003/10/08 08:27:36 jmc Exp $ |
3c0ef626 | 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 | |
1c14df9e | 46 | .Bk -words |
416fd2a8 | 47 | .Op Fl 46Ddeiqt |
3c0ef626 | 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 | |
e9a17296 | 53 | .Op Fl o Ar option |
3c0ef626 | 54 | .Op Fl p Ar port |
55 | .Op Fl u Ar len | |
1c14df9e | 56 | .Ek |
3c0ef626 | 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 | |
1c14df9e | 80 | works as follows: |
3c0ef626 | 81 | .Ss SSH protocol version 1 |
3c0ef626 | 82 | Each host has a host-specific RSA key |
83 | (normally 1024 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 | |
1c14df9e | 89 | Whenever a client connects, the daemon responds with its public |
3c0ef626 | 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. | |
416fd2a8 | 93 | The client then generates a 256-bit random number. |
3c0ef626 | 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 | |
416fd2a8 | 108 | .Em .rhosts |
3c0ef626 | 109 | authentication, |
416fd2a8 | 110 | .Em .rhosts |
3c0ef626 | 111 | authentication combined with RSA host |
112 | authentication, RSA challenge-response authentication, or password | |
113 | based authentication. | |
114 | .Pp | |
70791e56 | 115 | Regardless of the authentication type, the account is checked to |
116 | ensure that it is accessible. An account is not accessible if it is | |
117 | locked, listed in | |
118 | .Cm DenyUsers | |
119 | or its group is listed in | |
120 | .Cm DenyGroups | |
121 | \&. The definition of a locked account is system dependant. Some platforms | |
122 | have their own account database (eg AIX) and some modify the passwd field ( | |
123 | .Ql \&*LK\&* | |
124 | on Solaris, | |
125 | .Ql \&* | |
126 | on HP-UX, containing | |
127 | .Ql Nologin | |
128 | on Tru64 and a leading | |
129 | .Ql \&!! | |
130 | on Linux). If there is a requirement to disable password authentication | |
131 | for the account while allowing still public-key, then the passwd field | |
132 | should be set to something other than these values (eg | |
133 | .Ql NP | |
134 | or | |
135 | .Ql \&*NP\&* | |
136 | ). | |
137 | .Pp | |
416fd2a8 | 138 | .Em rhosts |
139 | authentication is normally disabled | |
3c0ef626 | 140 | because it is fundamentally insecure, but can be enabled in the server |
141 | configuration file if desired. | |
142 | System security is not improved unless | |
2980ea68 | 143 | .Nm rshd , |
144 | .Nm rlogind , | |
3c0ef626 | 145 | and |
1c14df9e | 146 | .Nm rexecd |
3c0ef626 | 147 | are disabled (thus completely disabling |
2980ea68 | 148 | .Xr rlogin |
3c0ef626 | 149 | and |
2980ea68 | 150 | .Xr rsh |
3c0ef626 | 151 | into the machine). |
3c0ef626 | 152 | .Ss SSH protocol version 2 |
3c0ef626 | 153 | Version 2 works similarly: |
154 | Each host has a host-specific key (RSA or DSA) used to identify the host. | |
155 | However, when the daemon starts, it does not generate a server key. | |
156 | Forward security is provided through a Diffie-Hellman key agreement. | |
157 | This key agreement results in a shared session key. | |
158 | .Pp | |
159 | The rest of the session is encrypted using a symmetric cipher, currently | |
416fd2a8 | 160 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. |
3c0ef626 | 161 | The client selects the encryption algorithm |
162 | to use from those offered by the server. | |
163 | Additionally, session integrity is provided | |
164 | through a cryptographic message authentication code | |
165 | (hmac-sha1 or hmac-md5). | |
166 | .Pp | |
167 | Protocol version 2 provides a public key based | |
168 | user (PubkeyAuthentication) or | |
169 | client host (HostbasedAuthentication) authentication method, | |
170 | conventional password authentication and challenge response based methods. | |
3c0ef626 | 171 | .Ss Command execution and data forwarding |
3c0ef626 | 172 | If the client successfully authenticates itself, a dialog for |
173 | preparing the session is entered. | |
174 | At this time the client may request | |
175 | things like allocating a pseudo-tty, forwarding X11 connections, | |
176 | forwarding TCP/IP connections, or forwarding the authentication agent | |
177 | connection over the secure channel. | |
178 | .Pp | |
179 | Finally, the client either requests a shell or execution of a command. | |
180 | The sides then enter session mode. | |
181 | In this mode, either side may send | |
182 | data at any time, and such data is forwarded to/from the shell or | |
183 | command on the server side, and the user terminal in the client side. | |
184 | .Pp | |
185 | When the user program terminates and all forwarded X11 and other | |
186 | connections have been closed, the server sends command exit status to | |
187 | the client, and both sides exit. | |
188 | .Pp | |
189 | .Nm | |
416fd2a8 | 190 | can be configured using command-line options or a configuration file |
191 | (by default | |
192 | .Xr sshd_config 5 ) . | |
3c0ef626 | 193 | Command-line options override values specified in the |
194 | configuration file. | |
195 | .Pp | |
196 | .Nm | |
197 | rereads its configuration file when it receives a hangup signal, | |
198 | .Dv SIGHUP , | |
199 | by executing itself with the name it was started as, i.e., | |
200 | .Pa /usr/sbin/sshd . | |
201 | .Pp | |
202 | The options are as follows: | |
203 | .Bl -tag -width Ds | |
416fd2a8 | 204 | .It Fl 4 |
205 | Forces | |
206 | .Nm | |
207 | to use IPv4 addresses only. | |
208 | .It Fl 6 | |
209 | Forces | |
210 | .Nm | |
211 | to use IPv6 addresses only. | |
3c0ef626 | 212 | .It Fl b Ar bits |
213 | Specifies the number of bits in the ephemeral protocol version 1 | |
214 | server key (default 768). | |
416fd2a8 | 215 | .It Fl D |
216 | When this option is specified, | |
217 | .Nm | |
218 | will not detach and does not become a daemon. | |
219 | This allows easy monitoring of | |
220 | .Nm sshd . | |
3c0ef626 | 221 | .It Fl d |
222 | Debug mode. | |
223 | The server sends verbose debug output to the system | |
224 | log, and does not put itself in the background. | |
225 | The server also will not fork and will only process one connection. | |
226 | This option is only intended for debugging for the server. | |
1c14df9e | 227 | Multiple |
228 | .Fl d | |
229 | options increase the debugging level. | |
3c0ef626 | 230 | Maximum is 3. |
231 | .It Fl e | |
232 | When this option is specified, | |
233 | .Nm | |
234 | will send the output to the standard error instead of the system log. | |
235 | .It Fl f Ar configuration_file | |
236 | Specifies the name of the configuration file. | |
237 | The default is | |
e9a17296 | 238 | .Pa /etc/ssh/sshd_config . |
3c0ef626 | 239 | .Nm |
240 | refuses to start if there is no configuration file. | |
241 | .It Fl g Ar login_grace_time | |
242 | Gives the grace time for clients to authenticate themselves (default | |
e54b3d7c | 243 | 120 seconds). |
3c0ef626 | 244 | If the client fails to authenticate the user within |
245 | this many seconds, the server disconnects and exits. | |
246 | A value of zero indicates no limit. | |
247 | .It Fl h Ar host_key_file | |
e9a17296 | 248 | Specifies a file from which a host key is read. |
3c0ef626 | 249 | This option must be given if |
250 | .Nm | |
251 | is not run as root (as the normal | |
e9a17296 | 252 | host key files are normally not readable by anyone but root). |
253 | The default is | |
254 | .Pa /etc/ssh/ssh_host_key | |
255 | for protocol version 1, and | |
256 | .Pa /etc/ssh/ssh_host_rsa_key | |
257 | and | |
258 | .Pa /etc/ssh/ssh_host_dsa_key | |
259 | for protocol version 2. | |
3c0ef626 | 260 | It is possible to have multiple host key files for |
261 | the different protocol versions and host key algorithms. | |
262 | .It Fl i | |
263 | Specifies that | |
264 | .Nm | |
1c14df9e | 265 | is being run from |
266 | .Xr inetd 8 . | |
3c0ef626 | 267 | .Nm |
268 | is normally not run | |
269 | from inetd because it needs to generate the server key before it can | |
270 | respond to the client, and this may take tens of seconds. | |
271 | Clients would have to wait too long if the key was regenerated every time. | |
272 | However, with small key sizes (e.g., 512) using | |
273 | .Nm | |
274 | from inetd may | |
275 | be feasible. | |
276 | .It Fl k Ar key_gen_time | |
277 | Specifies how often the ephemeral protocol version 1 server key is | |
278 | regenerated (default 3600 seconds, or one hour). | |
279 | The motivation for regenerating the key fairly | |
416fd2a8 | 280 | often is that the key is not stored anywhere, and after about an hour |
3c0ef626 | 281 | it becomes impossible to recover the key for decrypting intercepted |
282 | communications even if the machine is cracked into or physically | |
283 | seized. | |
284 | A value of zero indicates that the key will never be regenerated. | |
e9a17296 | 285 | .It Fl o Ar option |
286 | Can be used to give options in the format used in the configuration file. | |
287 | This is useful for specifying options for which there is no separate | |
288 | command-line flag. | |
416fd2a8 | 289 | For full details of the options, and their values, see |
290 | .Xr sshd_config 5 . | |
3c0ef626 | 291 | .It Fl p Ar port |
292 | Specifies the port on which the server listens for connections | |
293 | (default 22). | |
e9a17296 | 294 | Multiple port options are permitted. |
295 | Ports specified in the configuration file are ignored when a | |
296 | command-line port is specified. | |
3c0ef626 | 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. | |
e9a17296 | 305 | This is useful for updating |
3c0ef626 | 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 | |
1c14df9e | 325 | may also be used to prevent |
3c0ef626 | 326 | .Nm |
327 | from making DNS requests unless the authentication | |
328 | mechanism or configuration requires it. | |
329 | Authentication mechanisms that may require DNS include | |
3c0ef626 | 330 | .Cm RhostsRSAAuthentication , |
331 | .Cm HostbasedAuthentication | |
332 | and using a | |
333 | .Cm from="pattern-list" | |
334 | option in a key file. | |
e9a17296 | 335 | Configuration options that require DNS include using a |
336 | USER@HOST pattern in | |
337 | .Cm AllowUsers | |
338 | or | |
339 | .Cm DenyUsers . | |
3c0ef626 | 340 | .El |
341 | .Sh CONFIGURATION FILE | |
342 | .Nm | |
343 | reads configuration data from | |
e9a17296 | 344 | .Pa /etc/ssh/sshd_config |
3c0ef626 | 345 | (or the file specified with |
346 | .Fl f | |
347 | on the command line). | |
ff2d7a98 | 348 | The file format and configuration options are described in |
349 | .Xr sshd_config 5 . | |
3c0ef626 | 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 $HOME/.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 | |
416fd2a8 | 376 | Reads the file |
377 | .Pa $HOME/.ssh/environment , | |
378 | if it exists, and users are allowed to change their environment. | |
e54b3d7c | 379 | See the |
380 | .Cm PermitUserEnvironment | |
381 | option in | |
382 | .Xr sshd_config 5 . | |
3c0ef626 | 383 | .It |
384 | Changes to user's home directory. | |
385 | .It | |
386 | If | |
387 | .Pa $HOME/.ssh/rc | |
388 | exists, runs it; else if | |
e9a17296 | 389 | .Pa /etc/ssh/sshrc |
3c0ef626 | 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 $HOME/.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. | |
e54b3d7c | 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). | |
3c0ef626 | 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 | |
e54b3d7c | 430 | (because of the size of the public key encoding). |
3c0ef626 | 431 | You don't want to type them in; instead, copy the |
432 | .Pa identity.pub , | |
433 | .Pa id_dsa.pub | |
434 | or the | |
435 | .Pa id_rsa.pub | |
436 | file and edit it. | |
437 | .Pp | |
2980ea68 | 438 | .Nm |
439 | enforces a minimum RSA key modulus size for protocol 1 | |
440 | and protocol 2 keys of 768 bits. | |
441 | .Pp | |
3c0ef626 | 442 | The options (if present) consist of comma-separated option |
443 | specifications. | |
444 | No spaces are permitted, except within double quotes. | |
445 | The following option specifications are supported (note | |
446 | that option keywords are case-insensitive): | |
447 | .Bl -tag -width Ds | |
448 | .It Cm from="pattern-list" | |
e54b3d7c | 449 | Specifies that in addition to public key authentication, the canonical name |
3c0ef626 | 450 | of the remote host must be present in the comma-separated list of |
451 | patterns | |
70791e56 | 452 | .Pf ( Ql \&* |
3c0ef626 | 453 | and |
70791e56 | 454 | .Ql \&? |
3c0ef626 | 455 | serve as wildcards). |
456 | The list may also contain | |
457 | patterns negated by prefixing them with | |
70791e56 | 458 | .Ql \&! ; |
3c0ef626 | 459 | if the canonical host name matches a negated pattern, the key is not accepted. |
460 | The purpose | |
e54b3d7c | 461 | of this option is to optionally increase security: public key authentication |
3c0ef626 | 462 | by itself does not trust the network or name servers or anything (but |
463 | the key); however, if somebody somehow steals the key, the key | |
464 | permits an intruder to log in from anywhere in the world. | |
465 | This additional option makes using a stolen key more difficult (name | |
466 | servers and/or routers would have to be compromised in addition to | |
467 | just the key). | |
468 | .It Cm command="command" | |
469 | Specifies that the command is executed whenever this key is used for | |
470 | authentication. | |
471 | The command supplied by the user (if any) is ignored. | |
472 | The command is run on a pty if the client requests a pty; | |
473 | otherwise it is run without a tty. | |
1c14df9e | 474 | If an 8-bit clean channel is required, |
3c0ef626 | 475 | one must not request a pty or should specify |
476 | .Cm no-pty . | |
477 | A quote may be included in the command by quoting it with a backslash. | |
478 | This option might be useful | |
e54b3d7c | 479 | to restrict certain public keys to perform just a specific operation. |
3c0ef626 | 480 | An example might be a key that permits remote backups but nothing else. |
481 | Note that the client may specify TCP/IP and/or X11 | |
482 | forwarding unless they are explicitly prohibited. | |
483 | Note that this option applies to shell, command or subsystem execution. | |
484 | .It Cm environment="NAME=value" | |
485 | Specifies that the string is to be added to the environment when | |
486 | logging in using this key. | |
487 | Environment variables set this way | |
488 | override other default environment values. | |
489 | Multiple options of this type are permitted. | |
e54b3d7c | 490 | Environment processing is disabled by default and is |
491 | controlled via the | |
492 | .Cm PermitUserEnvironment | |
493 | option. | |
3c0ef626 | 494 | This option is automatically disabled if |
495 | .Cm UseLogin | |
496 | is enabled. | |
497 | .It Cm no-port-forwarding | |
498 | Forbids TCP/IP forwarding when this key is used for authentication. | |
499 | Any port forward requests by the client will return an error. | |
500 | This might be used, e.g., in connection with the | |
501 | .Cm command | |
502 | option. | |
503 | .It Cm no-X11-forwarding | |
504 | Forbids X11 forwarding when this key is used for authentication. | |
505 | Any X11 forward requests by the client will return an error. | |
506 | .It Cm no-agent-forwarding | |
507 | Forbids authentication agent forwarding when this key is used for | |
508 | authentication. | |
509 | .It Cm no-pty | |
510 | Prevents tty allocation (a request to allocate a pty will fail). | |
511 | .It Cm permitopen="host:port" | |
512 | Limit local | |
513 | .Li ``ssh -L'' | |
514 | port forwarding such that it may only connect to the specified host and | |
515 | port. | |
516 | IPv6 addresses can be specified with an alternative syntax: | |
416fd2a8 | 517 | .Ar host Ns / Ns Ar port . |
3c0ef626 | 518 | Multiple |
519 | .Cm permitopen | |
70791e56 | 520 | options may be applied separated by commas. |
521 | No pattern matching is performed on the specified hostnames, | |
522 | they must be literal domains or addresses. | |
3c0ef626 | 523 | .El |
524 | .Ss Examples | |
416fd2a8 | 525 | 1024 33 12121...312314325 ylo@foo.bar |
3c0ef626 | 526 | .Pp |
416fd2a8 | 527 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula |
3c0ef626 | 528 | .Pp |
416fd2a8 | 529 | command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi |
3c0ef626 | 530 | .Pp |
416fd2a8 | 531 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323 |
3c0ef626 | 532 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
533 | The | |
1c14df9e | 534 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 535 | and |
536 | .Pa $HOME/.ssh/known_hosts | |
537 | files contain host public keys for all known hosts. | |
538 | The global file should | |
539 | be prepared by the administrator (optional), and the per-user file is | |
540 | maintained automatically: whenever the user connects from an unknown host | |
541 | its key is added to the per-user file. | |
542 | .Pp | |
543 | Each line in these files contains the following fields: hostnames, | |
544 | bits, exponent, modulus, comment. | |
545 | The fields are separated by spaces. | |
546 | .Pp | |
70791e56 | 547 | Hostnames is a comma-separated list of patterns |
548 | .Pf ( Ql \&* | |
549 | and | |
550 | .Ql \&? | |
551 | act as | |
3c0ef626 | 552 | wildcards); each pattern in turn is matched against the canonical host |
553 | name (when authenticating a client) or against the user-supplied | |
554 | name (when authenticating a server). | |
555 | A pattern may also be preceded by | |
70791e56 | 556 | .Ql \&! |
3c0ef626 | 557 | to indicate negation: if the host name matches a negated |
558 | pattern, it is not accepted (by that line) even if it matched another | |
559 | pattern on the line. | |
560 | .Pp | |
561 | Bits, exponent, and modulus are taken directly from the RSA host key; they | |
562 | can be obtained, e.g., from | |
e9a17296 | 563 | .Pa /etc/ssh/ssh_host_key.pub . |
3c0ef626 | 564 | The optional comment field continues to the end of the line, and is not used. |
565 | .Pp | |
566 | Lines starting with | |
567 | .Ql # | |
568 | and empty lines are ignored as comments. | |
569 | .Pp | |
570 | When performing host authentication, authentication is accepted if any | |
571 | matching line has the proper key. | |
572 | It is thus permissible (but not | |
573 | recommended) to have several lines or different host keys for the same | |
574 | names. | |
575 | This will inevitably happen when short forms of host names | |
576 | from different domains are put in the file. | |
577 | It is possible | |
578 | that the files contain conflicting information; authentication is | |
579 | accepted if valid information can be found from either file. | |
580 | .Pp | |
581 | Note that the lines in these files are typically hundreds of characters | |
582 | long, and you definitely don't want to type in the host keys by hand. | |
583 | Rather, generate them by a script | |
584 | or by taking | |
e9a17296 | 585 | .Pa /etc/ssh/ssh_host_key.pub |
3c0ef626 | 586 | and adding the host names at the front. |
587 | .Ss Examples | |
588 | .Bd -literal | |
416fd2a8 | 589 | closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi |
3c0ef626 | 590 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= |
591 | .Ed | |
592 | .Sh FILES | |
593 | .Bl -tag -width Ds | |
e9a17296 | 594 | .It Pa /etc/ssh/sshd_config |
3c0ef626 | 595 | Contains configuration data for |
596 | .Nm sshd . | |
ff2d7a98 | 597 | The file format and configuration options are described in |
598 | .Xr sshd_config 5 . | |
e9a17296 | 599 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
3c0ef626 | 600 | These three files contain the private parts of the host keys. |
601 | These files should only be owned by root, readable only by root, and not | |
602 | accessible to others. | |
603 | Note that | |
604 | .Nm | |
605 | does not start if this file is group/world-accessible. | |
e9a17296 | 606 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub |
3c0ef626 | 607 | These three files contain the public parts of the host keys. |
608 | These files should be world-readable but writable only by | |
609 | root. | |
610 | Their contents should match the respective private parts. | |
611 | These files are not | |
612 | really used for anything; they are provided for the convenience of | |
613 | the user so their contents can be copied to known hosts files. | |
614 | These files are created using | |
615 | .Xr ssh-keygen 1 . | |
616 | .It Pa /etc/moduli | |
617 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
e54b3d7c | 618 | The file format is described in |
619 | .Xr moduli 5 . | |
ff2d7a98 | 620 | .It Pa /var/empty |
621 | .Xr chroot 2 | |
622 | directory used by | |
623 | .Nm | |
624 | during privilege separation in the pre-authentication phase. | |
625 | The directory should not contain any files and must be owned by root | |
626 | and not group or world-writable. | |
3c0ef626 | 627 | .It Pa /var/run/sshd.pid |
628 | Contains the process ID of the | |
629 | .Nm | |
630 | listening for connections (if there are several daemons running | |
ff2d7a98 | 631 | concurrently for different ports, this contains the process ID of the one |
3c0ef626 | 632 | started last). |
633 | The content of this file is not sensitive; it can be world-readable. | |
634 | .It Pa $HOME/.ssh/authorized_keys | |
635 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. | |
636 | This file must be readable by root (which may on some machines imply | |
637 | it being world-readable if the user's home directory resides on an NFS | |
638 | volume). | |
639 | It is recommended that it not be accessible by others. | |
640 | The format of this file is described above. | |
641 | Users will place the contents of their | |
642 | .Pa identity.pub , | |
643 | .Pa id_dsa.pub | |
644 | and/or | |
645 | .Pa id_rsa.pub | |
646 | files into this file, as described in | |
647 | .Xr ssh-keygen 1 . | |
416fd2a8 | 648 | .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts" |
3c0ef626 | 649 | These files are consulted when using rhosts with RSA host |
650 | authentication or protocol version 2 hostbased authentication | |
651 | to check the public key of the host. | |
652 | The key must be listed in one of these files to be accepted. | |
653 | The client uses the same files | |
654 | to verify that it is connecting to the correct remote host. | |
655 | These files should be writable only by root/the owner. | |
e9a17296 | 656 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 657 | should be world-readable, and |
658 | .Pa $HOME/.ssh/known_hosts | |
1c14df9e | 659 | can, but need not be, world-readable. |
3c0ef626 | 660 | .It Pa /etc/nologin |
661 | If this file exists, | |
662 | .Nm | |
663 | refuses to let anyone except root log in. | |
664 | The contents of the file | |
665 | are displayed to anyone trying to log in, and non-root connections are | |
666 | refused. | |
667 | The file should be world-readable. | |
668 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
669 | Access controls that should be enforced by tcp-wrappers are defined here. | |
670 | Further details are described in | |
671 | .Xr hosts_access 5 . | |
672 | .It Pa $HOME/.rhosts | |
673 | This file contains host-username pairs, separated by a space, one per | |
674 | line. | |
675 | The given user on the corresponding host is permitted to log in | |
1c14df9e | 676 | without a password. |
3c0ef626 | 677 | The same file is used by rlogind and rshd. |
678 | The file must | |
679 | be writable only by the user; it is recommended that it not be | |
680 | accessible by others. | |
681 | .Pp | |
416fd2a8 | 682 | It is also possible to use netgroups in the file. |
3c0ef626 | 683 | Either host or user |
684 | name may be of the form +@groupname to specify all hosts or all users | |
685 | in the group. | |
686 | .It Pa $HOME/.shosts | |
687 | For ssh, | |
688 | this file is exactly the same as for | |
689 | .Pa .rhosts . | |
690 | However, this file is | |
691 | not used by rlogin and rshd, so using this permits access using SSH only. | |
692 | .It Pa /etc/hosts.equiv | |
693 | This file is used during | |
416fd2a8 | 694 | .Em rhosts |
3c0ef626 | 695 | authentication. |
696 | In the simplest form, this file contains host names, one per line. | |
697 | Users on | |
698 | those hosts are permitted to log in without a password, provided they | |
699 | have the same user name on both machines. | |
700 | The host name may also be | |
701 | followed by a user name; such users are permitted to log in as | |
702 | .Em any | |
703 | user on this machine (except root). | |
704 | Additionally, the syntax | |
705 | .Dq +@group | |
706 | can be used to specify netgroups. | |
707 | Negated entries start with | |
708 | .Ql \&- . | |
709 | .Pp | |
710 | If the client host/user is successfully matched in this file, login is | |
711 | automatically permitted provided the client and server user names are the | |
712 | same. | |
713 | Additionally, successful RSA host authentication is normally required. | |
714 | This file must be writable only by root; it is recommended | |
715 | that it be world-readable. | |
716 | .Pp | |
717 | .Sy "Warning: It is almost never a good idea to use user names in" | |
718 | .Pa hosts.equiv . | |
719 | Beware that it really means that the named user(s) can log in as | |
720 | .Em anybody , | |
721 | which includes bin, daemon, adm, and other accounts that own critical | |
722 | binaries and directories. | |
723 | Using a user name practically grants the user root access. | |
724 | The only valid use for user names that I can think | |
725 | of is in negative entries. | |
726 | .Pp | |
727 | Note that this warning also applies to rsh/rlogin. | |
728 | .It Pa /etc/shosts.equiv | |
729 | This is processed exactly as | |
730 | .Pa /etc/hosts.equiv . | |
731 | However, this file may be useful in environments that want to run both | |
732 | rsh/rlogin and ssh. | |
733 | .It Pa $HOME/.ssh/environment | |
734 | This file is read into the environment at login (if it exists). | |
735 | It can only contain empty lines, comment lines (that start with | |
736 | .Ql # ) , | |
737 | and assignment lines of the form name=value. | |
738 | The file should be writable | |
739 | only by the user; it need not be readable by anyone else. | |
e54b3d7c | 740 | Environment processing is disabled by default and is |
741 | controlled via the | |
742 | .Cm PermitUserEnvironment | |
743 | option. | |
3c0ef626 | 744 | .It Pa $HOME/.ssh/rc |
1c14df9e | 745 | If this file exists, it is run with |
746 | .Pa /bin/sh | |
747 | after reading the | |
3c0ef626 | 748 | environment files but before starting the user's shell or command. |
2980ea68 | 749 | It must not produce any output on stdout; stderr must be used |
750 | instead. | |
751 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
752 | its standard input (and | |
3c0ef626 | 753 | .Ev DISPLAY |
2980ea68 | 754 | in its environment). |
755 | The script must call | |
3c0ef626 | 756 | .Xr xauth 1 |
2980ea68 | 757 | because |
758 | .Nm | |
759 | will not run xauth automatically to add X11 cookies. | |
3c0ef626 | 760 | .Pp |
761 | The primary purpose of this file is to run any initialization routines | |
762 | which may be needed before the user's home directory becomes | |
763 | accessible; AFS is a particular example of such an environment. | |
764 | .Pp | |
765 | This file will probably contain some initialization code followed by | |
766 | something similar to: | |
767 | .Bd -literal | |
2980ea68 | 768 | if read proto cookie && [ -n "$DISPLAY" ]; then |
769 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
770 | # X11UseLocalhost=yes | |
e54b3d7c | 771 | echo add unix:`echo $DISPLAY | |
2980ea68 | 772 | cut -c11-` $proto $cookie |
773 | else | |
774 | # X11UseLocalhost=no | |
e54b3d7c | 775 | echo add $DISPLAY $proto $cookie |
776 | fi | xauth -q - | |
2980ea68 | 777 | fi |
3c0ef626 | 778 | .Ed |
779 | .Pp | |
780 | If this file does not exist, | |
e9a17296 | 781 | .Pa /etc/ssh/sshrc |
3c0ef626 | 782 | is run, and if that |
2980ea68 | 783 | does not exist either, xauth is used to add the cookie. |
3c0ef626 | 784 | .Pp |
785 | This file should be writable only by the user, and need not be | |
786 | readable by anyone else. | |
e9a17296 | 787 | .It Pa /etc/ssh/sshrc |
3c0ef626 | 788 | Like |
789 | .Pa $HOME/.ssh/rc . | |
790 | This can be used to specify | |
791 | machine-specific login-time initializations globally. | |
792 | This file should be writable only by root, and should be world-readable. | |
793 | .El | |
3c0ef626 | 794 | .Sh SEE ALSO |
795 | .Xr scp 1 , | |
796 | .Xr sftp 1 , | |
797 | .Xr ssh 1 , | |
798 | .Xr ssh-add 1 , | |
799 | .Xr ssh-agent 1 , | |
800 | .Xr ssh-keygen 1 , | |
416fd2a8 | 801 | .Xr chroot 2 , |
802 | .Xr hosts_access 5 , | |
3c0ef626 | 803 | .Xr login.conf 5 , |
804 | .Xr moduli 5 , | |
ff2d7a98 | 805 | .Xr sshd_config 5 , |
416fd2a8 | 806 | .Xr inetd 8 , |
3c0ef626 | 807 | .Xr sftp-server 8 |
808 | .Rs | |
809 | .%A T. Ylonen | |
810 | .%A T. Kivinen | |
811 | .%A M. Saarinen | |
812 | .%A T. Rinne | |
813 | .%A S. Lehtinen | |
814 | .%T "SSH Protocol Architecture" | |
2980ea68 | 815 | .%N draft-ietf-secsh-architecture-12.txt |
816 | .%D January 2002 | |
3c0ef626 | 817 | .%O work in progress material |
818 | .Re | |
819 | .Rs | |
820 | .%A M. Friedl | |
821 | .%A N. Provos | |
822 | .%A W. A. Simpson | |
823 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
2980ea68 | 824 | .%N draft-ietf-secsh-dh-group-exchange-02.txt |
825 | .%D January 2002 | |
3c0ef626 | 826 | .%O work in progress material |
827 | .Re | |
70791e56 | 828 | .Sh AUTHORS |
829 | OpenSSH is a derivative of the original and free | |
830 | ssh 1.2.12 release by Tatu Ylonen. | |
831 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
832 | Theo de Raadt and Dug Song | |
833 | removed many bugs, re-added newer features and | |
834 | created OpenSSH. | |
835 | Markus Friedl contributed the support for SSH | |
836 | protocol versions 1.5 and 2.0. | |
837 | Niels Provos and Markus Friedl contributed support | |
838 | for privilege separation. |