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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
bf740959 | 3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
bf740959 | 4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
5 | .\" All rights reserved | |
6 | .\" | |
bcbf86ec | 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 | .\" | |
f3c7c613 | 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. | |
bcbf86ec | 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. | |
bf740959 | 25 | .\" |
bcbf86ec | 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. | |
bf740959 | 36 | .\" |
d4530052 | 37 | .\" $OpenBSD: sshd.8,v 1.231 2006/07/10 12:46:52 dtucker Exp $ |
bf740959 | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
da89cf4d | 43 | .Nd OpenSSH SSH daemon |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm sshd | |
6691e41b | 46 | .Bk -words |
433e60ac | 47 | .Op Fl 46Ddeiqt |
bf740959 | 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 | |
2717fa0f | 53 | .Op Fl o Ar option |
bf740959 | 54 | .Op Fl p Ar port |
c345cf9d | 55 | .Op Fl u Ar len |
6691e41b | 56 | .Ek |
f54651ce | 57 | .Sh DESCRIPTION |
bf740959 | 58 | .Nm |
ec63d7ce | 59 | (OpenSSH Daemon) is the daemon program for |
bf740959 | 60 | .Xr ssh 1 . |
3189621b | 61 | Together these programs replace rlogin and rsh, and |
bf740959 | 62 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 63 | over an insecure network. |
bf740959 | 64 | .Pp |
65 | .Nm | |
ec63d7ce | 66 | listens for connections from clients. |
f54651ce | 67 | It is normally started at boot from |
bf740959 | 68 | .Pa /etc/rc . |
69 | It forks a new | |
4fe2af09 | 70 | daemon for each incoming connection. |
71 | The forked daemons handle | |
bf740959 | 72 | key exchange, encryption, authentication, command execution, |
73 | and data exchange. | |
bf740959 | 74 | .Pp |
75 | .Nm | |
433e60ac | 76 | can be configured using command-line options or a configuration file |
77 | (by default | |
ec63d7ce | 78 | .Xr sshd_config 5 ) ; |
79 | command-line options override values specified in the | |
bf740959 | 80 | configuration file. |
9d6b7add | 81 | .Nm |
82 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 83 | .Dv SIGHUP , |
f09ffbdb | 84 | by executing itself with the name and options it was started with, e.g.\& |
409edaba | 85 | .Pa /usr/sbin/sshd . |
9d6b7add | 86 | .Pp |
bf740959 | 87 | The options are as follows: |
88 | .Bl -tag -width Ds | |
433e60ac | 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. | |
bf740959 | 97 | .It Fl b Ar bits |
da89cf4d | 98 | Specifies the number of bits in the ephemeral protocol version 1 |
99 | server key (default 768). | |
433e60ac | 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 . | |
bf740959 | 106 | .It Fl d |
4fe2af09 | 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. | |
6691e41b | 112 | Multiple |
113 | .Fl d | |
114 | options increase the debugging level. | |
94ec8c6b | 115 | Maximum is 3. |
da89cf4d | 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. | |
bf740959 | 120 | .It Fl f Ar configuration_file |
4fe2af09 | 121 | Specifies the name of the configuration file. |
122 | The default is | |
2a8a6488 | 123 | .Pa /etc/ssh/sshd_config . |
bf740959 | 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 | |
3445ca02 | 128 | 120 seconds). |
4fe2af09 | 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. | |
bf740959 | 132 | .It Fl h Ar host_key_file |
0f84fe37 | 133 | Specifies a file from which a host key is read. |
bf740959 | 134 | This option must be given if |
135 | .Nm | |
136 | is not run as root (as the normal | |
0f84fe37 | 137 | host key files are normally not readable by anyone but root). |
138 | The default is | |
2a8a6488 | 139 | .Pa /etc/ssh/ssh_host_key |
0f84fe37 | 140 | for protocol version 1, and |
2a8a6488 | 141 | .Pa /etc/ssh/ssh_host_rsa_key |
0f84fe37 | 142 | and |
2a8a6488 | 143 | .Pa /etc/ssh/ssh_host_dsa_key |
0f84fe37 | 144 | for protocol version 2. |
8abcdba4 | 145 | It is possible to have multiple host key files for |
da89cf4d | 146 | the different protocol versions and host key algorithms. |
bf740959 | 147 | .It Fl i |
148 | Specifies that | |
149 | .Nm | |
6691e41b | 150 | is being run from |
151 | .Xr inetd 8 . | |
bf740959 | 152 | .Nm |
153 | is normally not run | |
154 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 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. | |
f09ffbdb | 157 | However, with small key sizes (e.g. 512) using |
bf740959 | 158 | .Nm |
159 | from inetd may | |
160 | be feasible. | |
161 | .It Fl k Ar key_gen_time | |
da89cf4d | 162 | Specifies how often the ephemeral protocol version 1 server key is |
163 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 164 | The motivation for regenerating the key fairly |
433e60ac | 165 | often is that the key is not stored anywhere, and after about an hour |
bf740959 | 166 | it becomes impossible to recover the key for decrypting intercepted |
167 | communications even if the machine is cracked into or physically | |
4fe2af09 | 168 | seized. |
169 | A value of zero indicates that the key will never be regenerated. | |
2717fa0f | 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. | |
433e60ac | 174 | For full details of the options, and their values, see |
175 | .Xr sshd_config 5 . | |
bf740959 | 176 | .It Fl p Ar port |
177 | Specifies the port on which the server listens for connections | |
178 | (default 22). | |
135113a3 | 179 | Multiple port options are permitted. |
6cd6c442 | 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. | |
bf740959 | 186 | .It Fl q |
4fe2af09 | 187 | Quiet mode. |
188 | Nothing is sent to the system log. | |
189 | Normally the beginning, | |
bf740959 | 190 | authentication, and termination of each connection is logged. |
f87f09aa | 191 | .It Fl t |
192 | Test mode. | |
193 | Only check the validity of the configuration file and sanity of the keys. | |
184eed6a | 194 | This is useful for updating |
f87f09aa | 195 | .Nm |
196 | reliably as configuration options may change. | |
c345cf9d | 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. | |
e675b851 | 213 | .Fl u0 |
6691e41b | 214 | may also be used to prevent |
e675b851 | 215 | .Nm |
216 | from making DNS requests unless the authentication | |
217 | mechanism or configuration requires it. | |
218 | Authentication mechanisms that may require DNS include | |
e675b851 | 219 | .Cm RhostsRSAAuthentication , |
170c69ba | 220 | .Cm HostbasedAuthentication , |
e675b851 | 221 | and using a |
222 | .Cm from="pattern-list" | |
223 | option in a key file. | |
f464aad8 | 224 | Configuration options that require DNS include using a |
225 | USER@HOST pattern in | |
226 | .Cm AllowUsers | |
227 | or | |
228 | .Cm DenyUsers . | |
bf740959 | 229 | .El |
f3906047 | 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. | |
ec63d7ce | 243 | .Pp |
f3906047 | 244 | Forward security for protocol 1 is provided through |
245 | an additional server key, | |
246 | normally 768 bits, | |
247 | generated when the server starts. | |
ec63d7ce | 248 | This key is normally regenerated every hour if it has been used, and |
249 | is never stored on disk. | |
ec63d7ce | 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 | |
f3906047 | 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. | |
ec63d7ce | 279 | The client tries to authenticate itself using |
f3906047 | 280 | host-based authentication, |
281 | public key authentication, | |
282 | challenge-response authentication, | |
283 | or password authentication. | |
ec63d7ce | 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 | |
ec63d7ce | 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 | |
4eb67845 | 318 | After this, the client either requests a shell or execution of a command. |
ec63d7ce | 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. | |
bf740959 | 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, | |
f54651ce | 334 | prints last login time and |
bf740959 | 335 | .Pa /etc/motd |
336 | (unless prevented in the configuration file or by | |
140e3e97 | 337 | .Pa ~/.hushlogin ; |
bf740959 | 338 | see the |
f54651ce | 339 | .Sx FILES |
bf740959 | 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 | |
433e60ac | 353 | Reads the file |
140e3e97 | 354 | .Pa ~/.ssh/environment , |
433e60ac | 355 | if it exists, and users are allowed to change their environment. |
35453849 | 356 | See the |
6a342527 | 357 | .Cm PermitUserEnvironment |
35453849 | 358 | option in |
6a342527 | 359 | .Xr sshd_config 5 . |
bf740959 | 360 | .It |
361 | Changes to user's home directory. | |
362 | .It | |
363 | If | |
140e3e97 | 364 | .Pa ~/.ssh/rc |
bf740959 | 365 | exists, runs it; else if |
af98ced9 | 366 | .Pa /etc/ssh/sshrc |
bf740959 | 367 | exists, runs |
4fe2af09 | 368 | it; otherwise runs xauth. |
369 | The | |
bf740959 | 370 | .Dq rc |
371 | files are given the X11 | |
372 | authentication protocol and cookie in standard input. | |
f3d2d92e | 373 | See |
374 | .Sx SSHRC , | |
375 | below. | |
bf740959 | 376 | .It |
377 | Runs user's shell or command. | |
378 | .El | |
f3d2d92e | 379 | .Sh SSHRC |
380 | If the file | |
381 | .Pa ~/.ssh/rc | |
382 | exists, | |
383 | .Xr sh 1 | |
384 | runs it after reading the | |
385 | environment files but before starting the user's shell or command. | |
386 | It must not produce any output on stdout; stderr must be used | |
387 | instead. | |
388 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
389 | its standard input (and | |
390 | .Ev DISPLAY | |
391 | in its environment). | |
392 | The script must call | |
393 | .Xr xauth 1 | |
394 | because | |
395 | .Nm | |
396 | will not run xauth automatically to add X11 cookies. | |
397 | .Pp | |
398 | The primary purpose of this file is to run any initialization routines | |
399 | which may be needed before the user's home directory becomes | |
400 | accessible; AFS is a particular example of such an environment. | |
401 | .Pp | |
402 | This file will probably contain some initialization code followed by | |
403 | something similar to: | |
404 | .Bd -literal -offset 3n | |
405 | if read proto cookie && [ -n "$DISPLAY" ]; then | |
406 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
407 | # X11UseLocalhost=yes | |
408 | echo add unix:`echo $DISPLAY | | |
409 | cut -c11-` $proto $cookie | |
410 | else | |
411 | # X11UseLocalhost=no | |
412 | echo add $DISPLAY $proto $cookie | |
413 | fi | xauth -q - | |
414 | fi | |
415 | .Ed | |
416 | .Pp | |
417 | If this file does not exist, | |
418 | .Pa /etc/ssh/sshrc | |
419 | is run, and if that | |
420 | does not exist either, xauth is used to add the cookie. | |
bf740959 | 421 | .Sh AUTHORIZED_KEYS FILE FORMAT |
96a7b0cc | 422 | .Cm AuthorizedKeysFile |
010c04e5 | 423 | specifies the file containing public keys for |
424 | public key authentication; | |
425 | if none is specified, the default is | |
426 | .Pa ~/.ssh/authorized_keys . | |
4fe2af09 | 427 | Each line of the file contains one |
bf740959 | 428 | key (empty lines and lines starting with a |
429 | .Ql # | |
430 | are ignored as | |
4fe2af09 | 431 | comments). |
010c04e5 | 432 | Protocol 1 public keys consist of the following space-separated fields: |
433 | options, bits, exponent, modulus, comment. | |
434 | Protocol 2 public key consist of: | |
435 | options, keytype, base64-encoded key, comment. | |
436 | The options field is optional; | |
437 | its presence is determined by whether the line starts | |
755c4339 | 438 | with a number or not (the options field never starts with a number). |
010c04e5 | 439 | The bits, exponent, modulus, and comment fields give the RSA key for |
8abcdba4 | 440 | protocol version 1; the |
bf740959 | 441 | comment field is not used for anything (but may be convenient for the |
442 | user to identify the key). | |
8abcdba4 | 443 | For protocol version 2 the keytype is |
444 | .Dq ssh-dss | |
445 | or | |
446 | .Dq ssh-rsa . | |
bf740959 | 447 | .Pp |
448 | Note that lines in this file are usually several hundred bytes long | |
ea067773 | 449 | (because of the size of the public key encoding) up to a limit of |
450 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA | |
451 | keys up to 16 kilobits. | |
4fe2af09 | 452 | You don't want to type them in; instead, copy the |
c0ecc314 | 453 | .Pa identity.pub , |
010c04e5 | 454 | .Pa id_dsa.pub , |
c0ecc314 | 455 | or the |
456 | .Pa id_rsa.pub | |
bf740959 | 457 | file and edit it. |
458 | .Pp | |
3bc822df | 459 | .Nm |
460 | enforces a minimum RSA key modulus size for protocol 1 | |
461 | and protocol 2 keys of 768 bits. | |
462 | .Pp | |
c345cf9d | 463 | The options (if present) consist of comma-separated option |
4fe2af09 | 464 | specifications. |
465 | No spaces are permitted, except within double quotes. | |
54bf768d | 466 | The following option specifications are supported (note |
467 | that option keywords are case-insensitive): | |
bf740959 | 468 | .Bl -tag -width Ds |
bf740959 | 469 | .It Cm command="command" |
470 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 471 | authentication. |
472 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 473 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 474 | otherwise it is run without a tty. |
6691e41b | 475 | If an 8-bit clean channel is required, |
91789042 | 476 | one must not request a pty or should specify |
61e96248 | 477 | .Cm no-pty . |
4fe2af09 | 478 | A quote may be included in the command by quoting it with a backslash. |
479 | This option might be useful | |
755c4339 | 480 | to restrict certain public keys to perform just a specific operation. |
4fe2af09 | 481 | An example might be a key that permits remote backups but nothing else. |
e5d4cfad | 482 | Note that the client may specify TCP and/or X11 |
d0c832f3 | 483 | forwarding unless they are explicitly prohibited. |
4cdbc654 | 484 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 485 | .It Cm environment="NAME=value" |
486 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 487 | logging in using this key. |
488 | Environment variables set this way | |
489 | override other default environment values. | |
490 | Multiple options of this type are permitted. | |
35453849 | 491 | Environment processing is disabled by default and is |
492 | controlled via the | |
493 | .Cm PermitUserEnvironment | |
494 | option. | |
2548961d | 495 | This option is automatically disabled if |
496 | .Cm UseLogin | |
497 | is enabled. | |
9ed4bd80 | 498 | .It Cm from="pattern-list" |
499 | Specifies that in addition to public key authentication, the canonical name | |
500 | of the remote host must be present in the comma-separated list of | |
4cb23985 | 501 | patterns. |
9ed4bd80 | 502 | The purpose |
503 | of this option is to optionally increase security: public key authentication | |
504 | by itself does not trust the network or name servers or anything (but | |
505 | the key); however, if somebody somehow steals the key, the key | |
506 | permits an intruder to log in from anywhere in the world. | |
507 | This additional option makes using a stolen key more difficult (name | |
508 | servers and/or routers would have to be compromised in addition to | |
509 | just the key). | |
4cb23985 | 510 | .Pp |
511 | See | |
512 | .Sx PATTERNS | |
513 | in | |
514 | .Xr ssh_config 5 | |
515 | for more information on patterns. | |
9ed4bd80 | 516 | .It Cm no-agent-forwarding |
517 | Forbids authentication agent forwarding when this key is used for | |
518 | authentication. | |
bf740959 | 519 | .It Cm no-port-forwarding |
e5d4cfad | 520 | Forbids TCP forwarding when this key is used for authentication. |
4fe2af09 | 521 | Any port forward requests by the client will return an error. |
f09ffbdb | 522 | This might be used, e.g. in connection with the |
bf740959 | 523 | .Cm command |
524 | option. | |
9ed4bd80 | 525 | .It Cm no-pty |
526 | Prevents tty allocation (a request to allocate a pty will fail). | |
bf740959 | 527 | .It Cm no-X11-forwarding |
528 | Forbids X11 forwarding when this key is used for authentication. | |
529 | Any X11 forward requests by the client will return an error. | |
dc504afd | 530 | .It Cm permitopen="host:port" |
3730bb22 | 531 | Limit local |
dc504afd | 532 | .Li ``ssh -L'' |
b2ae83b8 | 533 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 534 | port. |
535 | IPv6 addresses can be specified with an alternative syntax: | |
433e60ac | 536 | .Ar host Ns / Ns Ar port . |
ed787d14 | 537 | Multiple |
dc504afd | 538 | .Cm permitopen |
3cbc677d | 539 | options may be applied separated by commas. |
540 | No pattern matching is performed on the specified hostnames, | |
541 | they must be literal domains or addresses. | |
d20f3c9e | 542 | .It Cm tunnel="n" |
543 | Force a | |
544 | .Xr tun 4 | |
545 | device on the server. | |
546 | Without this option, the next available device will be used if | |
547 | the client requests a tunnel. | |
bf740959 | 548 | .El |
bf740959 | 549 | .Pp |
1e0fcfc6 | 550 | An example authorized_keys file: |
481e991f | 551 | .Bd -literal -offset 3n |
1e0fcfc6 | 552 | # Comments allowed at start of line |
553 | ssh-rsa AAAAB3Nza...LiPk== user@example.net | |
481e991f | 554 | from="*.sales.example.net,!pc.sales.example.net" ssh-rsa |
555 | AAAAB2...19Q== john@example.net | |
556 | command="dump /home",no-pty,no-port-forwarding ssh-dss | |
557 | AAAAC3...51R== example.net | |
558 | permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss | |
559 | AAAAB5...21S== | |
560 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== | |
561 | jane@example.net | |
1e0fcfc6 | 562 | .Ed |
bf740959 | 563 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 564 | The |
6691e41b | 565 | .Pa /etc/ssh/ssh_known_hosts |
f54651ce | 566 | and |
140e3e97 | 567 | .Pa ~/.ssh/known_hosts |
4fe2af09 | 568 | files contain host public keys for all known hosts. |
569 | The global file should | |
570 | be prepared by the administrator (optional), and the per-user file is | |
2881e0e9 | 571 | maintained automatically: whenever the user connects from an unknown host, |
4fe2af09 | 572 | its key is added to the per-user file. |
bf740959 | 573 | .Pp |
574 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 575 | bits, exponent, modulus, comment. |
576 | The fields are separated by spaces. | |
bf740959 | 577 | .Pp |
09dc8896 | 578 | Hostnames is a comma-separated list of patterns |
2881e0e9 | 579 | .Pf ( Ql * |
3cbc677d | 580 | and |
581 | .Ql \&? | |
9a26a6e2 | 582 | act as |
bf740959 | 583 | wildcards); each pattern in turn is matched against the canonical host |
584 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 585 | name (when authenticating a server). |
586 | A pattern may also be preceded by | |
9a26a6e2 | 587 | .Ql \&! |
bf740959 | 588 | to indicate negation: if the host name matches a negated |
589 | pattern, it is not accepted (by that line) even if it matched another | |
590 | pattern on the line. | |
d4530052 | 591 | A hostname or address may optionally be enclosed within |
592 | .Ql \&[ | |
593 | and | |
594 | .Ql \&] | |
595 | brackets then followed by | |
596 | .Ql \&: | |
597 | and and a non-standard port number. | |
bf740959 | 598 | .Pp |
5c63c2ab | 599 | Alternately, hostnames may be stored in a hashed form which hides host names |
c79ae9fd | 600 | and addresses should the file's contents be disclosed. |
601 | Hashed hostnames start with a | |
602 | .Ql | | |
5c63c2ab | 603 | character. |
604 | Only one hashed hostname may appear on a single line and none of the above | |
605 | negation or wildcard operators may be applied. | |
606 | .Pp | |
1d1ffb87 | 607 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
2881e0e9 | 608 | can be obtained, for example, from |
2a8a6488 | 609 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 610 | The optional comment field continues to the end of the line, and is not used. |
611 | .Pp | |
612 | Lines starting with | |
613 | .Ql # | |
614 | and empty lines are ignored as comments. | |
615 | .Pp | |
616 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 617 | matching line has the proper key. |
618 | It is thus permissible (but not | |
bf740959 | 619 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 620 | names. |
621 | This will inevitably happen when short forms of host names | |
622 | from different domains are put in the file. | |
623 | It is possible | |
bf740959 | 624 | that the files contain conflicting information; authentication is |
625 | accepted if valid information can be found from either file. | |
626 | .Pp | |
627 | Note that the lines in these files are typically hundreds of characters | |
628 | long, and you definitely don't want to type in the host keys by hand. | |
629 | Rather, generate them by a script | |
f54651ce | 630 | or by taking |
2a8a6488 | 631 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 632 | and adding the host names at the front. |
9d4b0313 | 633 | .Pp |
634 | An example ssh_known_hosts file: | |
635 | .Bd -literal -offset 3n | |
636 | # Comments allowed at start of line | |
637 | closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net | |
638 | cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= | |
5c63c2ab | 639 | # A hashed hostname |
640 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | |
641 | AAAA1234.....= | |
642 | .Ed | |
bf740959 | 643 | .Sh FILES |
10fa86ae | 644 | .Bl -tag -width Ds -compact |
645 | .It ~/.hushlogin | |
a333272d | 646 | This file is used to suppress printing the last login time and |
647 | .Pa /etc/motd , | |
648 | if | |
649 | .Cm PrintLastLog | |
650 | and | |
651 | .Cm PrintMotd , | |
652 | respectively, | |
653 | are enabled. | |
654 | It does not suppress printing of the banner specified by | |
655 | .Cm Banner . | |
10fa86ae | 656 | .Pp |
657 | .It ~/.rhosts | |
00e7c607 | 658 | This file is used for host-based authentication (see |
659 | .Xr ssh 1 | |
660 | for more information). | |
661 | On some machines this file may need to be | |
662 | world-readable if the user's home directory is on an NFS partition, | |
663 | because | |
664 | .Nm | |
665 | reads it as root. | |
666 | Additionally, this file must be owned by the user, | |
667 | and must not have write permissions for anyone else. | |
668 | The recommended | |
669 | permission for most machines is read/write for the user, and not | |
bf740959 | 670 | accessible by others. |
671 | .Pp | |
10fa86ae | 672 | .It ~/.shosts |
00e7c607 | 673 | This file is used in exactly the same way as |
674 | .Pa .rhosts , | |
675 | but allows host-based authentication without permitting login with | |
676 | rlogin/rsh. | |
bf740959 | 677 | .Pp |
10fa86ae | 678 | .It ~/.ssh/authorized_keys |
00e7c607 | 679 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. |
10fa86ae | 680 | The format of this file is described above. |
00e7c607 | 681 | This file is not highly sensitive, but the recommended |
682 | permissions are read/write for the user, and not accessible by others. | |
bf740959 | 683 | .Pp |
10fa86ae | 684 | .It ~/.ssh/environment |
4fe2af09 | 685 | This file is read into the environment at login (if it exists). |
686 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 687 | .Ql # ) , |
4fe2af09 | 688 | and assignment lines of the form name=value. |
689 | The file should be writable | |
bf740959 | 690 | only by the user; it need not be readable by anyone else. |
35453849 | 691 | Environment processing is disabled by default and is |
692 | controlled via the | |
693 | .Cm PermitUserEnvironment | |
694 | option. | |
10fa86ae | 695 | .Pp |
696 | .It ~/.ssh/known_hosts | |
00e7c607 | 697 | Contains a list of host keys for all hosts the user has logged into |
698 | that are not already in the systemwide list of known host keys. | |
699 | The format of this file is described above. | |
700 | This file should be writable only by root/the owner and | |
10fa86ae | 701 | can, but need not be, world-readable. |
702 | .Pp | |
703 | .It ~/.ssh/rc | |
f3d2d92e | 704 | Contains initialization routines to be run before |
705 | the user's home directory becomes accessible. | |
bf740959 | 706 | This file should be writable only by the user, and need not be |
707 | readable by anyone else. | |
10fa86ae | 708 | .Pp |
709 | .It /etc/hosts.allow | |
710 | .It /etc/hosts.deny | |
711 | Access controls that should be enforced by tcp-wrappers are defined here. | |
712 | Further details are described in | |
713 | .Xr hosts_access 5 . | |
714 | .Pp | |
715 | .It /etc/hosts.equiv | |
e56bbe08 | 716 | This file is for host-based authentication (see |
717 | .Xr ssh 1 ) . | |
718 | It should only be writable by root. | |
10fa86ae | 719 | .Pp |
720 | .It /etc/moduli | |
721 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
722 | The file format is described in | |
723 | .Xr moduli 5 . | |
724 | .Pp | |
725 | .It /etc/motd | |
726 | See | |
727 | .Xr motd 5 . | |
728 | .Pp | |
729 | .It /etc/nologin | |
730 | If this file exists, | |
731 | .Nm | |
732 | refuses to let anyone except root log in. | |
733 | The contents of the file | |
734 | are displayed to anyone trying to log in, and non-root connections are | |
735 | refused. | |
736 | The file should be world-readable. | |
737 | .Pp | |
738 | .It /etc/shosts.equiv | |
e56bbe08 | 739 | This file is used in exactly the same way as |
740 | .Pa hosts.equiv , | |
741 | but allows host-based authentication without permitting login with | |
742 | rlogin/rsh. | |
10fa86ae | 743 | .Pp |
00e7c607 | 744 | .It /etc/ssh/ssh_known_hosts |
745 | Systemwide list of known host keys. | |
746 | This file should be prepared by the | |
747 | system administrator to contain the public host keys of all machines in the | |
748 | organization. | |
749 | The format of this file is described above. | |
750 | This file should be writable only by root/the owner and | |
751 | should be world-readable. | |
752 | .Pp | |
10fa86ae | 753 | .It /etc/ssh/ssh_host_key |
754 | .It /etc/ssh/ssh_host_dsa_key | |
755 | .It /etc/ssh/ssh_host_rsa_key | |
756 | These three files contain the private parts of the host keys. | |
757 | These files should only be owned by root, readable only by root, and not | |
758 | accessible to others. | |
759 | Note that | |
760 | .Nm | |
3783659a | 761 | does not start if these files are group/world-accessible. |
10fa86ae | 762 | .Pp |
763 | .It /etc/ssh/ssh_host_key.pub | |
764 | .It /etc/ssh/ssh_host_dsa_key.pub | |
765 | .It /etc/ssh/ssh_host_rsa_key.pub | |
766 | These three files contain the public parts of the host keys. | |
767 | These files should be world-readable but writable only by | |
768 | root. | |
769 | Their contents should match the respective private parts. | |
770 | These files are not | |
771 | really used for anything; they are provided for the convenience of | |
772 | the user so their contents can be copied to known hosts files. | |
773 | These files are created using | |
774 | .Xr ssh-keygen 1 . | |
775 | .Pp | |
776 | .It /etc/ssh/sshd_config | |
777 | Contains configuration data for | |
778 | .Nm sshd . | |
779 | The file format and configuration options are described in | |
780 | .Xr sshd_config 5 . | |
781 | .Pp | |
782 | .It /etc/ssh/sshrc | |
f3d2d92e | 783 | Similar to |
784 | .Pa ~/.ssh/rc , | |
785 | it can be used to specify | |
4fe2af09 | 786 | machine-specific login-time initializations globally. |
787 | This file should be writable only by root, and should be world-readable. | |
10fa86ae | 788 | .Pp |
789 | .It /var/empty | |
790 | .Xr chroot 2 | |
791 | directory used by | |
792 | .Nm | |
793 | during privilege separation in the pre-authentication phase. | |
794 | The directory should not contain any files and must be owned by root | |
795 | and not group or world-writable. | |
796 | .Pp | |
797 | .It /var/run/sshd.pid | |
798 | Contains the process ID of the | |
799 | .Nm | |
800 | listening for connections (if there are several daemons running | |
801 | concurrently for different ports, this contains the process ID of the one | |
802 | started last). | |
803 | The content of this file is not sensitive; it can be world-readable. | |
089fbbd2 | 804 | .El |
bf740959 | 805 | .Sh SEE ALSO |
bf740959 | 806 | .Xr scp 1 , |
61e96248 | 807 | .Xr sftp 1 , |
bf740959 | 808 | .Xr ssh 1 , |
809 | .Xr ssh-add 1 , | |
810 | .Xr ssh-agent 1 , | |
811 | .Xr ssh-keygen 1 , | |
433e60ac | 812 | .Xr chroot 2 , |
813 | .Xr hosts_access 5 , | |
248bad82 | 814 | .Xr login.conf 5 , |
815 | .Xr moduli 5 , | |
588df31a | 816 | .Xr sshd_config 5 , |
433e60ac | 817 | .Xr inetd 8 , |
a5a2da3b | 818 | .Xr sftp-server 8 |
be193d89 | 819 | .Sh AUTHORS |
820 | OpenSSH is a derivative of the original and free | |
821 | ssh 1.2.12 release by Tatu Ylonen. | |
822 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
823 | Theo de Raadt and Dug Song | |
824 | removed many bugs, re-added newer features and | |
825 | created OpenSSH. | |
826 | Markus Friedl contributed the support for SSH | |
827 | protocol versions 1.5 and 2.0. | |
828 | Niels Provos and Markus Friedl contributed support | |
829 | for privilege separation. | |
4eb67845 | 830 | .Sh CAVEATS |
831 | System security is not improved unless | |
832 | .Nm rshd , | |
833 | .Nm rlogind , | |
834 | and | |
835 | .Nm rexecd | |
836 | are disabled (thus completely disabling | |
837 | .Xr rlogin | |
838 | and | |
839 | .Xr rsh | |
840 | into the machine). |