]>
Commit | Line | Data |
---|---|---|
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 | .\" | |
247e7a27 | 37 | .\" $OpenBSD: sshd.8,v 1.241 2008/03/27 22:37:57 jmc Exp $ |
38 | .Dd $Mdocdate: April 3 2008 $ | |
3c0ef626 | 39 | .Dt SSHD 8 |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
43 | .Nd OpenSSH SSH daemon | |
44 | .Sh SYNOPSIS | |
45 | .Nm sshd | |
bfe49944 | 46 | .Bk -words |
540d72c3 | 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 | |
e9702f7d | 53 | .Op Fl o Ar option |
3c0ef626 | 54 | .Op Fl p Ar port |
55 | .Op Fl u Ar len | |
bfe49944 | 56 | .Ek |
3c0ef626 | 57 | .Sh DESCRIPTION |
58 | .Nm | |
08822d99 | 59 | (OpenSSH Daemon) is the daemon program for |
3c0ef626 | 60 | .Xr ssh 1 . |
fa0f0f45 | 61 | Together these programs replace |
62 | .Xr rlogin 1 | |
63 | and | |
64 | .Xr rsh 1 , | |
65 | and provide secure encrypted communications between two untrusted hosts | |
3c0ef626 | 66 | over an insecure network. |
3c0ef626 | 67 | .Pp |
68 | .Nm | |
08822d99 | 69 | listens for connections from clients. |
3c0ef626 | 70 | It is normally started at boot from |
71 | .Pa /etc/rc . | |
72 | It forks a new | |
73 | daemon for each incoming connection. | |
74 | The forked daemons handle | |
75 | key exchange, encryption, authentication, command execution, | |
76 | and data exchange. | |
3c0ef626 | 77 | .Pp |
78 | .Nm | |
540d72c3 | 79 | can be configured using command-line options or a configuration file |
80 | (by default | |
08822d99 | 81 | .Xr sshd_config 5 ) ; |
82 | command-line options override values specified in the | |
3c0ef626 | 83 | configuration file. |
3c0ef626 | 84 | .Nm |
85 | rereads its configuration file when it receives a hangup signal, | |
86 | .Dv SIGHUP , | |
30460aeb | 87 | by executing itself with the name and options it was started with, e.g.\& |
3c0ef626 | 88 | .Pa /usr/sbin/sshd . |
89 | .Pp | |
90 | The options are as follows: | |
91 | .Bl -tag -width Ds | |
540d72c3 | 92 | .It Fl 4 |
93 | Forces | |
94 | .Nm | |
95 | to use IPv4 addresses only. | |
96 | .It Fl 6 | |
97 | Forces | |
98 | .Nm | |
99 | to use IPv6 addresses only. | |
3c0ef626 | 100 | .It Fl b Ar bits |
101 | Specifies the number of bits in the ephemeral protocol version 1 | |
102 | server key (default 768). | |
540d72c3 | 103 | .It Fl D |
104 | When this option is specified, | |
105 | .Nm | |
106 | will not detach and does not become a daemon. | |
107 | This allows easy monitoring of | |
108 | .Nm sshd . | |
3c0ef626 | 109 | .It Fl d |
110 | Debug mode. | |
111 | The server sends verbose debug output to the system | |
112 | log, and does not put itself in the background. | |
113 | The server also will not fork and will only process one connection. | |
114 | This option is only intended for debugging for the server. | |
bfe49944 | 115 | Multiple |
116 | .Fl d | |
117 | options increase the debugging level. | |
3c0ef626 | 118 | Maximum is 3. |
119 | .It Fl e | |
120 | When this option is specified, | |
121 | .Nm | |
122 | will send the output to the standard error instead of the system log. | |
fa0f0f45 | 123 | .It Fl f Ar config_file |
3c0ef626 | 124 | Specifies the name of the configuration file. |
125 | The default is | |
e9702f7d | 126 | .Pa /etc/ssh/sshd_config . |
3c0ef626 | 127 | .Nm |
128 | refuses to start if there is no configuration file. | |
129 | .It Fl g Ar login_grace_time | |
130 | Gives the grace time for clients to authenticate themselves (default | |
d03f4262 | 131 | 120 seconds). |
3c0ef626 | 132 | If the client fails to authenticate the user within |
133 | this many seconds, the server disconnects and exits. | |
134 | A value of zero indicates no limit. | |
135 | .It Fl h Ar host_key_file | |
e9702f7d | 136 | Specifies a file from which a host key is read. |
3c0ef626 | 137 | This option must be given if |
138 | .Nm | |
139 | is not run as root (as the normal | |
e9702f7d | 140 | host key files are normally not readable by anyone but root). |
141 | The default is | |
142 | .Pa /etc/ssh/ssh_host_key | |
143 | for protocol version 1, and | |
144 | .Pa /etc/ssh/ssh_host_rsa_key | |
145 | and | |
146 | .Pa /etc/ssh/ssh_host_dsa_key | |
147 | for protocol version 2. | |
3c0ef626 | 148 | It is possible to have multiple host key files for |
149 | the different protocol versions and host key algorithms. | |
150 | .It Fl i | |
151 | Specifies that | |
152 | .Nm | |
bfe49944 | 153 | is being run from |
154 | .Xr inetd 8 . | |
3c0ef626 | 155 | .Nm |
156 | is normally not run | |
157 | from inetd because it needs to generate the server key before it can | |
158 | respond to the client, and this may take tens of seconds. | |
159 | Clients would have to wait too long if the key was regenerated every time. | |
30460aeb | 160 | However, with small key sizes (e.g. 512) using |
3c0ef626 | 161 | .Nm |
162 | from inetd may | |
163 | be feasible. | |
164 | .It Fl k Ar key_gen_time | |
165 | Specifies how often the ephemeral protocol version 1 server key is | |
166 | regenerated (default 3600 seconds, or one hour). | |
167 | The motivation for regenerating the key fairly | |
540d72c3 | 168 | often is that the key is not stored anywhere, and after about an hour |
3c0ef626 | 169 | it becomes impossible to recover the key for decrypting intercepted |
170 | communications even if the machine is cracked into or physically | |
171 | seized. | |
172 | A value of zero indicates that the key will never be regenerated. | |
e9702f7d | 173 | .It Fl o Ar option |
174 | Can be used to give options in the format used in the configuration file. | |
175 | This is useful for specifying options for which there is no separate | |
176 | command-line flag. | |
540d72c3 | 177 | For full details of the options, and their values, see |
178 | .Xr sshd_config 5 . | |
3c0ef626 | 179 | .It Fl p Ar port |
180 | Specifies the port on which the server listens for connections | |
181 | (default 22). | |
e9702f7d | 182 | Multiple port options are permitted. |
08822d99 | 183 | Ports specified in the configuration file with the |
184 | .Cm Port | |
185 | option are ignored when a command-line port is specified. | |
186 | Ports specified using the | |
187 | .Cm ListenAddress | |
188 | option override command-line ports. | |
3c0ef626 | 189 | .It Fl q |
190 | Quiet mode. | |
191 | Nothing is sent to the system log. | |
192 | Normally the beginning, | |
193 | authentication, and termination of each connection is logged. | |
194 | .It Fl t | |
195 | Test mode. | |
196 | Only check the validity of the configuration file and sanity of the keys. | |
e9702f7d | 197 | This is useful for updating |
3c0ef626 | 198 | .Nm |
199 | reliably as configuration options may change. | |
200 | .It Fl u Ar len | |
201 | This option is used to specify the size of the field | |
202 | in the | |
203 | .Li utmp | |
204 | structure that holds the remote host name. | |
205 | If the resolved host name is longer than | |
206 | .Ar len , | |
207 | the dotted decimal value will be used instead. | |
208 | This allows hosts with very long host names that | |
209 | overflow this field to still be uniquely identified. | |
210 | Specifying | |
211 | .Fl u0 | |
212 | indicates that only dotted decimal addresses | |
213 | should be put into the | |
214 | .Pa utmp | |
215 | file. | |
216 | .Fl u0 | |
bfe49944 | 217 | may also be used to prevent |
3c0ef626 | 218 | .Nm |
219 | from making DNS requests unless the authentication | |
220 | mechanism or configuration requires it. | |
221 | Authentication mechanisms that may require DNS include | |
3c0ef626 | 222 | .Cm RhostsRSAAuthentication , |
08822d99 | 223 | .Cm HostbasedAuthentication , |
3c0ef626 | 224 | and using a |
225 | .Cm from="pattern-list" | |
226 | option in a key file. | |
e9702f7d | 227 | Configuration options that require DNS include using a |
228 | USER@HOST pattern in | |
229 | .Cm AllowUsers | |
230 | or | |
231 | .Cm DenyUsers . | |
3c0ef626 | 232 | .El |
08822d99 | 233 | .Sh AUTHENTICATION |
234 | The OpenSSH SSH daemon supports SSH protocols 1 and 2. | |
235 | Both protocols are supported by default, | |
236 | though this can be changed via the | |
237 | .Cm Protocol | |
238 | option in | |
44a053a3 | 239 | .Xr sshd_config 5 . |
08822d99 | 240 | Protocol 2 supports both RSA and DSA keys; |
241 | protocol 1 only supports RSA keys. | |
242 | For both protocols, | |
243 | each host has a host-specific key, | |
244 | normally 2048 bits, | |
245 | used to identify the host. | |
246 | .Pp | |
247 | Forward security for protocol 1 is provided through | |
248 | an additional server key, | |
249 | normally 768 bits, | |
250 | generated when the server starts. | |
251 | This key is normally regenerated every hour if it has been used, and | |
252 | is never stored on disk. | |
253 | Whenever a client connects, the daemon responds with its public | |
254 | host and server keys. | |
255 | The client compares the | |
256 | RSA host key against its own database to verify that it has not changed. | |
257 | The client then generates a 256-bit random number. | |
258 | It encrypts this | |
259 | random number using both the host key and the server key, and sends | |
260 | the encrypted number to the server. | |
261 | Both sides then use this | |
262 | random number as a session key which is used to encrypt all further | |
263 | communications in the session. | |
264 | The rest of the session is encrypted | |
265 | using a conventional cipher, currently Blowfish or 3DES, with 3DES | |
266 | being used by default. | |
267 | The client selects the encryption algorithm | |
268 | to use from those offered by the server. | |
269 | .Pp | |
270 | For protocol 2, | |
271 | forward security is provided through a Diffie-Hellman key agreement. | |
272 | This key agreement results in a shared session key. | |
273 | The rest of the session is encrypted using a symmetric cipher, currently | |
274 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. | |
275 | The client selects the encryption algorithm | |
276 | to use from those offered by the server. | |
277 | Additionally, session integrity is provided | |
278 | through a cryptographic message authentication code | |
fa0f0f45 | 279 | (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160). |
08822d99 | 280 | .Pp |
281 | Finally, the server and the client enter an authentication dialog. | |
282 | The client tries to authenticate itself using | |
283 | host-based authentication, | |
284 | public key authentication, | |
285 | challenge-response authentication, | |
286 | or password authentication. | |
287 | .Pp | |
288 | Regardless of the authentication type, the account is checked to | |
289 | ensure that it is accessible. An account is not accessible if it is | |
290 | locked, listed in | |
291 | .Cm DenyUsers | |
292 | or its group is listed in | |
293 | .Cm DenyGroups | |
294 | \&. The definition of a locked account is system dependant. Some platforms | |
295 | have their own account database (eg AIX) and some modify the passwd field ( | |
296 | .Ql \&*LK\&* | |
297 | on Solaris and UnixWare, | |
298 | .Ql \&* | |
299 | on HP-UX, containing | |
300 | .Ql Nologin | |
301 | on Tru64, | |
302 | a leading | |
303 | .Ql \&*LOCKED\&* | |
304 | on FreeBSD and a leading | |
fa0f0f45 | 305 | .Ql \&! |
306 | on most Linuxes). | |
307 | If there is a requirement to disable password authentication | |
08822d99 | 308 | for the account while allowing still public-key, then the passwd field |
309 | should be set to something other than these values (eg | |
310 | .Ql NP | |
311 | or | |
312 | .Ql \&*NP\&* | |
313 | ). | |
314 | .Pp | |
08822d99 | 315 | If the client successfully authenticates itself, a dialog for |
316 | preparing the session is entered. | |
317 | At this time the client may request | |
318 | things like allocating a pseudo-tty, forwarding X11 connections, | |
319 | forwarding TCP connections, or forwarding the authentication agent | |
320 | connection over the secure channel. | |
321 | .Pp | |
30460aeb | 322 | After this, the client either requests a shell or execution of a command. |
08822d99 | 323 | The sides then enter session mode. |
324 | In this mode, either side may send | |
325 | data at any time, and such data is forwarded to/from the shell or | |
326 | command on the server side, and the user terminal in the client side. | |
327 | .Pp | |
328 | When the user program terminates and all forwarded X11 and other | |
329 | connections have been closed, the server sends command exit status to | |
330 | the client, and both sides exit. | |
3c0ef626 | 331 | .Sh LOGIN PROCESS |
332 | When a user successfully logs in, | |
333 | .Nm | |
334 | does the following: | |
335 | .Bl -enum -offset indent | |
336 | .It | |
337 | If the login is on a tty, and no command has been specified, | |
338 | prints last login time and | |
339 | .Pa /etc/motd | |
340 | (unless prevented in the configuration file or by | |
2ce0bfe4 | 341 | .Pa ~/.hushlogin ; |
3c0ef626 | 342 | see the |
343 | .Sx FILES | |
344 | section). | |
345 | .It | |
346 | If the login is on a tty, records login time. | |
347 | .It | |
348 | Checks | |
349 | .Pa /etc/nologin ; | |
350 | if it exists, prints contents and quits | |
351 | (unless root). | |
352 | .It | |
353 | Changes to run with normal user privileges. | |
354 | .It | |
355 | Sets up basic environment. | |
356 | .It | |
540d72c3 | 357 | Reads the file |
2ce0bfe4 | 358 | .Pa ~/.ssh/environment , |
540d72c3 | 359 | if it exists, and users are allowed to change their environment. |
d03f4262 | 360 | See the |
361 | .Cm PermitUserEnvironment | |
362 | option in | |
363 | .Xr sshd_config 5 . | |
3c0ef626 | 364 | .It |
365 | Changes to user's home directory. | |
366 | .It | |
367 | If | |
2ce0bfe4 | 368 | .Pa ~/.ssh/rc |
3c0ef626 | 369 | exists, runs it; else if |
e9702f7d | 370 | .Pa /etc/ssh/sshrc |
3c0ef626 | 371 | exists, runs |
372 | it; otherwise runs xauth. | |
373 | The | |
374 | .Dq rc | |
375 | files are given the X11 | |
376 | authentication protocol and cookie in standard input. | |
30460aeb | 377 | See |
378 | .Sx SSHRC , | |
379 | below. | |
3c0ef626 | 380 | .It |
381 | Runs user's shell or command. | |
382 | .El | |
30460aeb | 383 | .Sh SSHRC |
384 | If the file | |
385 | .Pa ~/.ssh/rc | |
386 | exists, | |
387 | .Xr sh 1 | |
388 | runs it after reading the | |
389 | environment files but before starting the user's shell or command. | |
390 | It must not produce any output on stdout; stderr must be used | |
391 | instead. | |
392 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
393 | its standard input (and | |
394 | .Ev DISPLAY | |
395 | in its environment). | |
396 | The script must call | |
397 | .Xr xauth 1 | |
398 | because | |
399 | .Nm | |
400 | will not run xauth automatically to add X11 cookies. | |
401 | .Pp | |
402 | The primary purpose of this file is to run any initialization routines | |
403 | which may be needed before the user's home directory becomes | |
404 | accessible; AFS is a particular example of such an environment. | |
405 | .Pp | |
406 | This file will probably contain some initialization code followed by | |
407 | something similar to: | |
408 | .Bd -literal -offset 3n | |
409 | if read proto cookie && [ -n "$DISPLAY" ]; then | |
410 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
411 | # X11UseLocalhost=yes | |
412 | echo add unix:`echo $DISPLAY | | |
413 | cut -c11-` $proto $cookie | |
414 | else | |
415 | # X11UseLocalhost=no | |
416 | echo add $DISPLAY $proto $cookie | |
417 | fi | xauth -q - | |
418 | fi | |
419 | .Ed | |
420 | .Pp | |
421 | If this file does not exist, | |
422 | .Pa /etc/ssh/sshrc | |
423 | is run, and if that | |
424 | does not exist either, xauth is used to add the cookie. | |
3c0ef626 | 425 | .Sh AUTHORIZED_KEYS FILE FORMAT |
3c0ef626 | 426 | .Cm AuthorizedKeysFile |
30460aeb | 427 | specifies the file containing public keys for |
428 | public key authentication; | |
429 | if none is specified, the default is | |
430 | .Pa ~/.ssh/authorized_keys . | |
3c0ef626 | 431 | Each line of the file contains one |
432 | key (empty lines and lines starting with a | |
433 | .Ql # | |
434 | are ignored as | |
435 | comments). | |
30460aeb | 436 | Protocol 1 public keys consist of the following space-separated fields: |
437 | options, bits, exponent, modulus, comment. | |
438 | Protocol 2 public key consist of: | |
439 | options, keytype, base64-encoded key, comment. | |
440 | The options field is optional; | |
441 | its presence is determined by whether the line starts | |
d03f4262 | 442 | with a number or not (the options field never starts with a number). |
30460aeb | 443 | The bits, exponent, modulus, and comment fields give the RSA key for |
3c0ef626 | 444 | protocol version 1; the |
445 | comment field is not used for anything (but may be convenient for the | |
446 | user to identify the key). | |
447 | For protocol version 2 the keytype is | |
448 | .Dq ssh-dss | |
449 | or | |
450 | .Dq ssh-rsa . | |
451 | .Pp | |
452 | Note that lines in this file are usually several hundred bytes long | |
dfddba3d | 453 | (because of the size of the public key encoding) up to a limit of |
454 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA | |
455 | keys up to 16 kilobits. | |
3c0ef626 | 456 | You don't want to type them in; instead, copy the |
457 | .Pa identity.pub , | |
30460aeb | 458 | .Pa id_dsa.pub , |
3c0ef626 | 459 | or the |
460 | .Pa id_rsa.pub | |
461 | file and edit it. | |
462 | .Pp | |
350391c5 | 463 | .Nm |
464 | enforces a minimum RSA key modulus size for protocol 1 | |
465 | and protocol 2 keys of 768 bits. | |
466 | .Pp | |
3c0ef626 | 467 | The options (if present) consist of comma-separated option |
468 | specifications. | |
469 | No spaces are permitted, except within double quotes. | |
470 | The following option specifications are supported (note | |
471 | that option keywords are case-insensitive): | |
472 | .Bl -tag -width Ds | |
3c0ef626 | 473 | .It Cm command="command" |
474 | Specifies that the command is executed whenever this key is used for | |
475 | authentication. | |
476 | The command supplied by the user (if any) is ignored. | |
477 | The command is run on a pty if the client requests a pty; | |
478 | otherwise it is run without a tty. | |
bfe49944 | 479 | If an 8-bit clean channel is required, |
3c0ef626 | 480 | one must not request a pty or should specify |
481 | .Cm no-pty . | |
482 | A quote may be included in the command by quoting it with a backslash. | |
483 | This option might be useful | |
d03f4262 | 484 | to restrict certain public keys to perform just a specific operation. |
3c0ef626 | 485 | An example might be a key that permits remote backups but nothing else. |
08822d99 | 486 | Note that the client may specify TCP and/or X11 |
3c0ef626 | 487 | forwarding unless they are explicitly prohibited. |
30460aeb | 488 | The command originally supplied by the client is available in the |
489 | .Ev SSH_ORIGINAL_COMMAND | |
490 | environment variable. | |
3c0ef626 | 491 | Note that this option applies to shell, command or subsystem execution. |
492 | .It Cm environment="NAME=value" | |
493 | Specifies that the string is to be added to the environment when | |
494 | logging in using this key. | |
495 | Environment variables set this way | |
496 | override other default environment values. | |
497 | Multiple options of this type are permitted. | |
d03f4262 | 498 | Environment processing is disabled by default and is |
499 | controlled via the | |
500 | .Cm PermitUserEnvironment | |
501 | option. | |
3c0ef626 | 502 | This option is automatically disabled if |
503 | .Cm UseLogin | |
504 | is enabled. | |
30460aeb | 505 | .It Cm from="pattern-list" |
506 | Specifies that in addition to public key authentication, the canonical name | |
507 | of the remote host must be present in the comma-separated list of | |
508 | patterns. | |
509 | The purpose | |
510 | of this option is to optionally increase security: public key authentication | |
511 | by itself does not trust the network or name servers or anything (but | |
512 | the key); however, if somebody somehow steals the key, the key | |
513 | permits an intruder to log in from anywhere in the world. | |
514 | This additional option makes using a stolen key more difficult (name | |
515 | servers and/or routers would have to be compromised in addition to | |
516 | just the key). | |
517 | .Pp | |
518 | See | |
519 | .Sx PATTERNS | |
520 | in | |
521 | .Xr ssh_config 5 | |
522 | for more information on patterns. | |
523 | .It Cm no-agent-forwarding | |
524 | Forbids authentication agent forwarding when this key is used for | |
525 | authentication. | |
3c0ef626 | 526 | .It Cm no-port-forwarding |
08822d99 | 527 | Forbids TCP forwarding when this key is used for authentication. |
3c0ef626 | 528 | Any port forward requests by the client will return an error. |
30460aeb | 529 | This might be used, e.g. in connection with the |
3c0ef626 | 530 | .Cm command |
531 | option. | |
30460aeb | 532 | .It Cm no-pty |
533 | Prevents tty allocation (a request to allocate a pty will fail). | |
e74dc197 | 534 | .It Cm no-user-rc |
247e7a27 | 535 | Disables execution of |
e74dc197 | 536 | .Pa ~/.ssh/rc . |
3c0ef626 | 537 | .It Cm no-X11-forwarding |
538 | Forbids X11 forwarding when this key is used for authentication. | |
539 | Any X11 forward requests by the client will return an error. | |
3c0ef626 | 540 | .It Cm permitopen="host:port" |
541 | Limit local | |
542 | .Li ``ssh -L'' | |
543 | port forwarding such that it may only connect to the specified host and | |
544 | port. | |
545 | IPv6 addresses can be specified with an alternative syntax: | |
540d72c3 | 546 | .Ar host Ns / Ns Ar port . |
3c0ef626 | 547 | Multiple |
548 | .Cm permitopen | |
7cac2b65 | 549 | options may be applied separated by commas. |
550 | No pattern matching is performed on the specified hostnames, | |
551 | they must be literal domains or addresses. | |
08822d99 | 552 | .It Cm tunnel="n" |
553 | Force a | |
554 | .Xr tun 4 | |
555 | device on the server. | |
556 | Without this option, the next available device will be used if | |
557 | the client requests a tunnel. | |
3c0ef626 | 558 | .El |
3c0ef626 | 559 | .Pp |
30460aeb | 560 | An example authorized_keys file: |
561 | .Bd -literal -offset 3n | |
562 | # Comments allowed at start of line | |
563 | ssh-rsa AAAAB3Nza...LiPk== user@example.net | |
564 | from="*.sales.example.net,!pc.sales.example.net" ssh-rsa | |
565 | AAAAB2...19Q== john@example.net | |
566 | command="dump /home",no-pty,no-port-forwarding ssh-dss | |
567 | AAAAC3...51R== example.net | |
568 | permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss | |
569 | AAAAB5...21S== | |
570 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== | |
571 | jane@example.net | |
572 | .Ed | |
3c0ef626 | 573 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
574 | The | |
bfe49944 | 575 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 576 | and |
2ce0bfe4 | 577 | .Pa ~/.ssh/known_hosts |
3c0ef626 | 578 | files contain host public keys for all known hosts. |
579 | The global file should | |
580 | be prepared by the administrator (optional), and the per-user file is | |
30460aeb | 581 | maintained automatically: whenever the user connects from an unknown host, |
3c0ef626 | 582 | its key is added to the per-user file. |
583 | .Pp | |
584 | Each line in these files contains the following fields: hostnames, | |
585 | bits, exponent, modulus, comment. | |
586 | The fields are separated by spaces. | |
587 | .Pp | |
7cac2b65 | 588 | Hostnames is a comma-separated list of patterns |
30460aeb | 589 | .Pf ( Ql * |
7cac2b65 | 590 | and |
591 | .Ql \&? | |
592 | act as | |
3c0ef626 | 593 | wildcards); each pattern in turn is matched against the canonical host |
594 | name (when authenticating a client) or against the user-supplied | |
595 | name (when authenticating a server). | |
596 | A pattern may also be preceded by | |
7cac2b65 | 597 | .Ql \&! |
3c0ef626 | 598 | to indicate negation: if the host name matches a negated |
599 | pattern, it is not accepted (by that line) even if it matched another | |
600 | pattern on the line. | |
30460aeb | 601 | A hostname or address may optionally be enclosed within |
602 | .Ql \&[ | |
603 | and | |
604 | .Ql \&] | |
605 | brackets then followed by | |
606 | .Ql \&: | |
607 | and a non-standard port number. | |
3c0ef626 | 608 | .Pp |
dfddba3d | 609 | Alternately, hostnames may be stored in a hashed form which hides host names |
610 | and addresses should the file's contents be disclosed. | |
611 | Hashed hostnames start with a | |
612 | .Ql | | |
613 | character. | |
614 | Only one hashed hostname may appear on a single line and none of the above | |
615 | negation or wildcard operators may be applied. | |
616 | .Pp | |
3c0ef626 | 617 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
30460aeb | 618 | can be obtained, for example, from |
e9702f7d | 619 | .Pa /etc/ssh/ssh_host_key.pub . |
3c0ef626 | 620 | The optional comment field continues to the end of the line, and is not used. |
621 | .Pp | |
622 | Lines starting with | |
623 | .Ql # | |
624 | and empty lines are ignored as comments. | |
625 | .Pp | |
626 | When performing host authentication, authentication is accepted if any | |
627 | matching line has the proper key. | |
628 | It is thus permissible (but not | |
629 | recommended) to have several lines or different host keys for the same | |
630 | names. | |
631 | This will inevitably happen when short forms of host names | |
632 | from different domains are put in the file. | |
633 | It is possible | |
634 | that the files contain conflicting information; authentication is | |
635 | accepted if valid information can be found from either file. | |
636 | .Pp | |
637 | Note that the lines in these files are typically hundreds of characters | |
638 | long, and you definitely don't want to type in the host keys by hand. | |
639 | Rather, generate them by a script | |
640 | or by taking | |
e9702f7d | 641 | .Pa /etc/ssh/ssh_host_key.pub |
3c0ef626 | 642 | and adding the host names at the front. |
30460aeb | 643 | .Pp |
644 | An example ssh_known_hosts file: | |
645 | .Bd -literal -offset 3n | |
646 | # Comments allowed at start of line | |
647 | closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net | |
648 | cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= | |
dfddba3d | 649 | # A hashed hostname |
650 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | |
651 | AAAA1234.....= | |
652 | .Ed | |
3c0ef626 | 653 | .Sh FILES |
30460aeb | 654 | .Bl -tag -width Ds -compact |
655 | .It ~/.hushlogin | |
dfddba3d | 656 | This file is used to suppress printing the last login time and |
657 | .Pa /etc/motd , | |
658 | if | |
659 | .Cm PrintLastLog | |
660 | and | |
661 | .Cm PrintMotd , | |
662 | respectively, | |
663 | are enabled. | |
664 | It does not suppress printing of the banner specified by | |
665 | .Cm Banner . | |
30460aeb | 666 | .Pp |
667 | .It ~/.rhosts | |
668 | This file is used for host-based authentication (see | |
669 | .Xr ssh 1 | |
670 | for more information). | |
671 | On some machines this file may need to be | |
672 | world-readable if the user's home directory is on an NFS partition, | |
673 | because | |
3c0ef626 | 674 | .Nm |
30460aeb | 675 | reads it as root. |
676 | Additionally, this file must be owned by the user, | |
677 | and must not have write permissions for anyone else. | |
678 | The recommended | |
679 | permission for most machines is read/write for the user, and not | |
3c0ef626 | 680 | accessible by others. |
681 | .Pp | |
30460aeb | 682 | .It ~/.shosts |
683 | This file is used in exactly the same way as | |
684 | .Pa .rhosts , | |
685 | but allows host-based authentication without permitting login with | |
686 | rlogin/rsh. | |
687 | .Pp | |
e74dc197 | 688 | .It ~/.ssh/ |
689 | This directory is the default location for all user-specific configuration | |
690 | and authentication information. | |
691 | There is no general requirement to keep the entire contents of this directory | |
692 | secret, but the recommended permissions are read/write/execute for the user, | |
693 | and not accessible by others. | |
694 | .Pp | |
30460aeb | 695 | .It ~/.ssh/authorized_keys |
696 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
697 | The format of this file is described above. | |
698 | The content of the file is not highly sensitive, but the recommended | |
699 | permissions are read/write for the user, and not accessible by others. | |
700 | .Pp | |
701 | If this file, the | |
702 | .Pa ~/.ssh | |
703 | directory, or the user's home directory are writable | |
704 | by other users, then the file could be modified or replaced by unauthorized | |
705 | users. | |
706 | In this case, | |
707 | .Nm | |
708 | will not allow it to be used unless the | |
709 | .Cm StrictModes | |
710 | option has been set to | |
711 | .Dq no . | |
712 | The recommended permissions can be set by executing | |
713 | .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys . | |
714 | .Pp | |
715 | .It ~/.ssh/environment | |
3c0ef626 | 716 | This file is read into the environment at login (if it exists). |
717 | It can only contain empty lines, comment lines (that start with | |
718 | .Ql # ) , | |
719 | and assignment lines of the form name=value. | |
720 | The file should be writable | |
721 | only by the user; it need not be readable by anyone else. | |
d03f4262 | 722 | Environment processing is disabled by default and is |
723 | controlled via the | |
724 | .Cm PermitUserEnvironment | |
725 | option. | |
30460aeb | 726 | .Pp |
727 | .It ~/.ssh/known_hosts | |
728 | Contains a list of host keys for all hosts the user has logged into | |
729 | that are not already in the systemwide list of known host keys. | |
730 | The format of this file is described above. | |
731 | This file should be writable only by root/the owner and | |
732 | can, but need not be, world-readable. | |
733 | .Pp | |
734 | .It ~/.ssh/rc | |
735 | Contains initialization routines to be run before | |
736 | the user's home directory becomes accessible. | |
737 | This file should be writable only by the user, and need not be | |
738 | readable by anyone else. | |
739 | .Pp | |
740 | .It /etc/hosts.allow | |
741 | .It /etc/hosts.deny | |
742 | Access controls that should be enforced by tcp-wrappers are defined here. | |
743 | Further details are described in | |
744 | .Xr hosts_access 5 . | |
745 | .Pp | |
746 | .It /etc/hosts.equiv | |
747 | This file is for host-based authentication (see | |
748 | .Xr ssh 1 ) . | |
749 | It should only be writable by root. | |
750 | .Pp | |
751 | .It /etc/moduli | |
752 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
753 | The file format is described in | |
754 | .Xr moduli 5 . | |
755 | .Pp | |
756 | .It /etc/motd | |
757 | See | |
758 | .Xr motd 5 . | |
759 | .Pp | |
760 | .It /etc/nologin | |
761 | If this file exists, | |
350391c5 | 762 | .Nm |
30460aeb | 763 | refuses to let anyone except root log in. |
764 | The contents of the file | |
765 | are displayed to anyone trying to log in, and non-root connections are | |
766 | refused. | |
767 | The file should be world-readable. | |
3c0ef626 | 768 | .Pp |
30460aeb | 769 | .It /etc/shosts.equiv |
770 | This file is used in exactly the same way as | |
771 | .Pa hosts.equiv , | |
772 | but allows host-based authentication without permitting login with | |
773 | rlogin/rsh. | |
774 | .Pp | |
30460aeb | 775 | .It /etc/ssh/ssh_host_key |
776 | .It /etc/ssh/ssh_host_dsa_key | |
777 | .It /etc/ssh/ssh_host_rsa_key | |
778 | These three files contain the private parts of the host keys. | |
779 | These files should only be owned by root, readable only by root, and not | |
780 | accessible to others. | |
781 | Note that | |
782 | .Nm | |
783 | does not start if these files are group/world-accessible. | |
3c0ef626 | 784 | .Pp |
30460aeb | 785 | .It /etc/ssh/ssh_host_key.pub |
786 | .It /etc/ssh/ssh_host_dsa_key.pub | |
787 | .It /etc/ssh/ssh_host_rsa_key.pub | |
788 | These three files contain the public parts of the host keys. | |
789 | These files should be world-readable but writable only by | |
790 | root. | |
791 | Their contents should match the respective private parts. | |
792 | These files are not | |
793 | really used for anything; they are provided for the convenience of | |
794 | the user so their contents can be copied to known hosts files. | |
795 | These files are created using | |
796 | .Xr ssh-keygen 1 . | |
3c0ef626 | 797 | .Pp |
fa0f0f45 | 798 | .It /etc/ssh/ssh_known_hosts |
799 | Systemwide list of known host keys. | |
800 | This file should be prepared by the | |
801 | system administrator to contain the public host keys of all machines in the | |
802 | organization. | |
803 | The format of this file is described above. | |
804 | This file should be writable only by root/the owner and | |
805 | should be world-readable. | |
806 | .Pp | |
30460aeb | 807 | .It /etc/ssh/sshd_config |
808 | Contains configuration data for | |
809 | .Nm sshd . | |
810 | The file format and configuration options are described in | |
811 | .Xr sshd_config 5 . | |
812 | .Pp | |
813 | .It /etc/ssh/sshrc | |
814 | Similar to | |
815 | .Pa ~/.ssh/rc , | |
816 | it can be used to specify | |
3c0ef626 | 817 | machine-specific login-time initializations globally. |
818 | This file should be writable only by root, and should be world-readable. | |
30460aeb | 819 | .Pp |
820 | .It /var/empty | |
821 | .Xr chroot 2 | |
822 | directory used by | |
823 | .Nm | |
824 | during privilege separation in the pre-authentication phase. | |
825 | The directory should not contain any files and must be owned by root | |
826 | and not group or world-writable. | |
827 | .Pp | |
828 | .It /var/run/sshd.pid | |
829 | Contains the process ID of the | |
830 | .Nm | |
831 | listening for connections (if there are several daemons running | |
832 | concurrently for different ports, this contains the process ID of the one | |
833 | started last). | |
834 | The content of this file is not sensitive; it can be world-readable. | |
3c0ef626 | 835 | .El |
3c0ef626 | 836 | .Sh SEE ALSO |
837 | .Xr scp 1 , | |
838 | .Xr sftp 1 , | |
839 | .Xr ssh 1 , | |
840 | .Xr ssh-add 1 , | |
841 | .Xr ssh-agent 1 , | |
842 | .Xr ssh-keygen 1 , | |
fa0f0f45 | 843 | .Xr ssh-keyscan 1 , |
540d72c3 | 844 | .Xr chroot 2 , |
845 | .Xr hosts_access 5 , | |
3c0ef626 | 846 | .Xr login.conf 5 , |
847 | .Xr moduli 5 , | |
44a053a3 | 848 | .Xr sshd_config 5 , |
540d72c3 | 849 | .Xr inetd 8 , |
3c0ef626 | 850 | .Xr sftp-server 8 |
7cac2b65 | 851 | .Sh AUTHORS |
852 | OpenSSH is a derivative of the original and free | |
853 | ssh 1.2.12 release by Tatu Ylonen. | |
854 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
855 | Theo de Raadt and Dug Song | |
856 | removed many bugs, re-added newer features and | |
857 | created OpenSSH. | |
858 | Markus Friedl contributed the support for SSH | |
859 | protocol versions 1.5 and 2.0. | |
860 | Niels Provos and Markus Friedl contributed support | |
861 | for privilege separation. | |
30460aeb | 862 | .Sh CAVEATS |
863 | System security is not improved unless | |
864 | .Nm rshd , | |
865 | .Nm rlogind , | |
866 | and | |
867 | .Nm rexecd | |
868 | are disabled (thus completely disabling | |
869 | .Xr rlogin | |
870 | and | |
871 | .Xr rsh | |
872 | into the machine). |