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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 | .\" |
75653d3e | 37 | .\" $OpenBSD: sshd.8,v 1.185 2002/06/22 16:32:54 stevesk 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 | |
2043936f | 46 | .Op Fl deiqtD46 |
bf740959 | 47 | .Op Fl b Ar bits |
48 | .Op Fl f Ar config_file | |
49 | .Op Fl g Ar login_grace_time | |
50 | .Op Fl h Ar host_key_file | |
51 | .Op Fl k Ar key_gen_time | |
2717fa0f | 52 | .Op Fl o Ar option |
bf740959 | 53 | .Op Fl p Ar port |
c345cf9d | 54 | .Op Fl u Ar len |
f54651ce | 55 | .Sh DESCRIPTION |
bf740959 | 56 | .Nm |
2c86906e | 57 | (SSH Daemon) is the daemon program for |
bf740959 | 58 | .Xr ssh 1 . |
3189621b | 59 | Together these programs replace rlogin and rsh, and |
bf740959 | 60 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 61 | over an insecure network. |
62 | The programs are intended to be as easy to | |
bf740959 | 63 | install and use as possible. |
64 | .Pp | |
65 | .Nm | |
4fe2af09 | 66 | is the daemon that 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. | |
1d1ffb87 | 74 | This implementation of |
75 | .Nm | |
76 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 77 | .Nm |
4fe2af09 | 78 | works as follows. |
1d1ffb87 | 79 | .Pp |
80 | .Ss SSH protocol version 1 | |
81 | .Pp | |
4fe2af09 | 82 | Each host has a host-specific RSA key |
83 | (normally 1024 bits) used to identify the host. | |
84 | Additionally, when | |
bf740959 | 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 | |
3189621b | 89 | Whenever a client connects the daemon responds with its public |
90 | host and server keys. | |
4fe2af09 | 91 | The client compares the |
1d1ffb87 | 92 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 93 | The client then generates a 256 bit random number. |
94 | It encrypts this | |
bf740959 | 95 | random number using both the host key and the server key, and sends |
4fe2af09 | 96 | the encrypted number to the server. |
3189621b | 97 | Both sides then use this |
bf740959 | 98 | random number as a session key which is used to encrypt all further |
4fe2af09 | 99 | communications in the session. |
100 | The rest of the session is encrypted | |
3189621b | 101 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 102 | being used by default. |
4fe2af09 | 103 | The client selects the encryption algorithm |
bf740959 | 104 | to use from those offered by the server. |
105 | .Pp | |
4fe2af09 | 106 | Next, the server and the client enter an authentication dialog. |
107 | The client tries to authenticate itself using | |
bf740959 | 108 | .Pa .rhosts |
109 | authentication, | |
110 | .Pa .rhosts | |
111 | authentication combined with RSA host | |
112 | authentication, RSA challenge-response authentication, or password | |
113 | based authentication. | |
114 | .Pp | |
115 | Rhosts authentication is normally disabled | |
116 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 117 | configuration file if desired. |
118 | System security is not improved unless | |
7e276482 | 119 | .Nm rshd , |
120 | .Nm rlogind , | |
bf740959 | 121 | and |
7e276482 | 122 | .Xr rexecd |
bf740959 | 123 | are disabled (thus completely disabling |
7e276482 | 124 | .Xr rlogin |
bf740959 | 125 | and |
7e276482 | 126 | .Xr rsh |
3189621b | 127 | into the machine). |
bf740959 | 128 | .Pp |
1d1ffb87 | 129 | .Ss SSH protocol version 2 |
130 | .Pp | |
c345cf9d | 131 | Version 2 works similarly: |
7f19f8bb | 132 | Each host has a host-specific key (RSA or DSA) used to identify the host. |
1d1ffb87 | 133 | However, when the daemon starts, it does not generate a server key. |
134 | Forward security is provided through a Diffie-Hellman key agreement. | |
135 | This key agreement results in a shared session key. | |
da89cf4d | 136 | .Pp |
51c251f0 | 137 | The rest of the session is encrypted using a symmetric cipher, currently |
da89cf4d | 138 | 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. |
1d1ffb87 | 139 | The client selects the encryption algorithm |
140 | to use from those offered by the server. | |
141 | Additionally, session integrity is provided | |
d0c832f3 | 142 | through a cryptographic message authentication code |
1d1ffb87 | 143 | (hmac-sha1 or hmac-md5). |
144 | .Pp | |
145 | Protocol version 2 provides a public key based | |
da89cf4d | 146 | user (PubkeyAuthentication) or |
147 | client host (HostbasedAuthentication) authentication method, | |
148 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 149 | .Pp |
150 | .Ss Command execution and data forwarding | |
151 | .Pp | |
bf740959 | 152 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 153 | preparing the session is entered. |
154 | At this time the client may request | |
bf740959 | 155 | things like allocating a pseudo-tty, forwarding X11 connections, |
156 | forwarding TCP/IP connections, or forwarding the authentication agent | |
157 | connection over the secure channel. | |
158 | .Pp | |
159 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 160 | The sides then enter session mode. |
161 | In this mode, either side may send | |
bf740959 | 162 | data at any time, and such data is forwarded to/from the shell or |
163 | command on the server side, and the user terminal in the client side. | |
164 | .Pp | |
165 | When the user program terminates and all forwarded X11 and other | |
166 | connections have been closed, the server sends command exit status to | |
167 | the client, and both sides exit. | |
168 | .Pp | |
169 | .Nm | |
170 | can be configured using command-line options or a configuration | |
4fe2af09 | 171 | file. |
172 | Command-line options override values specified in the | |
bf740959 | 173 | configuration file. |
174 | .Pp | |
9d6b7add | 175 | .Nm |
176 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 177 | .Dv SIGHUP , |
a5a2da3b | 178 | by executing itself with the name it was started as, i.e., |
409edaba | 179 | .Pa /usr/sbin/sshd . |
9d6b7add | 180 | .Pp |
bf740959 | 181 | The options are as follows: |
182 | .Bl -tag -width Ds | |
183 | .It Fl b Ar bits | |
da89cf4d | 184 | Specifies the number of bits in the ephemeral protocol version 1 |
185 | server key (default 768). | |
bf740959 | 186 | .It Fl d |
4fe2af09 | 187 | Debug mode. |
188 | The server sends verbose debug output to the system | |
189 | log, and does not put itself in the background. | |
190 | The server also will not fork and will only process one connection. | |
191 | This option is only intended for debugging for the server. | |
da89cf4d | 192 | Multiple -d options increase the debugging level. |
94ec8c6b | 193 | Maximum is 3. |
da89cf4d | 194 | .It Fl e |
195 | When this option is specified, | |
196 | .Nm | |
197 | will send the output to the standard error instead of the system log. | |
bf740959 | 198 | .It Fl f Ar configuration_file |
4fe2af09 | 199 | Specifies the name of the configuration file. |
200 | The default is | |
2a8a6488 | 201 | .Pa /etc/ssh/sshd_config . |
bf740959 | 202 | .Nm |
203 | refuses to start if there is no configuration file. | |
204 | .It Fl g Ar login_grace_time | |
205 | Gives the grace time for clients to authenticate themselves (default | |
ee5e6612 | 206 | 600 seconds). |
4fe2af09 | 207 | If the client fails to authenticate the user within |
208 | this many seconds, the server disconnects and exits. | |
209 | A value of zero indicates no limit. | |
bf740959 | 210 | .It Fl h Ar host_key_file |
0f84fe37 | 211 | Specifies a file from which a host key is read. |
bf740959 | 212 | This option must be given if |
213 | .Nm | |
214 | is not run as root (as the normal | |
0f84fe37 | 215 | host key files are normally not readable by anyone but root). |
216 | The default is | |
2a8a6488 | 217 | .Pa /etc/ssh/ssh_host_key |
0f84fe37 | 218 | for protocol version 1, and |
2a8a6488 | 219 | .Pa /etc/ssh/ssh_host_rsa_key |
0f84fe37 | 220 | and |
2a8a6488 | 221 | .Pa /etc/ssh/ssh_host_dsa_key |
0f84fe37 | 222 | for protocol version 2. |
8abcdba4 | 223 | It is possible to have multiple host key files for |
da89cf4d | 224 | the different protocol versions and host key algorithms. |
bf740959 | 225 | .It Fl i |
226 | Specifies that | |
227 | .Nm | |
f54651ce | 228 | is being run from inetd. |
bf740959 | 229 | .Nm |
230 | is normally not run | |
231 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 232 | respond to the client, and this may take tens of seconds. |
233 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 234 | However, with small key sizes (e.g., 512) using |
bf740959 | 235 | .Nm |
236 | from inetd may | |
237 | be feasible. | |
238 | .It Fl k Ar key_gen_time | |
da89cf4d | 239 | Specifies how often the ephemeral protocol version 1 server key is |
240 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 241 | The motivation for regenerating the key fairly |
bf740959 | 242 | often is that the key is not stored anywhere, and after about an hour, |
243 | it becomes impossible to recover the key for decrypting intercepted | |
244 | communications even if the machine is cracked into or physically | |
4fe2af09 | 245 | seized. |
246 | A value of zero indicates that the key will never be regenerated. | |
2717fa0f | 247 | .It Fl o Ar option |
248 | Can be used to give options in the format used in the configuration file. | |
249 | This is useful for specifying options for which there is no separate | |
250 | command-line flag. | |
bf740959 | 251 | .It Fl p Ar port |
252 | Specifies the port on which the server listens for connections | |
253 | (default 22). | |
135113a3 | 254 | Multiple port options are permitted. |
255 | Ports specified in the configuration file are ignored when a | |
256 | command-line port is specified. | |
bf740959 | 257 | .It Fl q |
4fe2af09 | 258 | Quiet mode. |
259 | Nothing is sent to the system log. | |
260 | Normally the beginning, | |
bf740959 | 261 | authentication, and termination of each connection is logged. |
f87f09aa | 262 | .It Fl t |
263 | Test mode. | |
264 | Only check the validity of the configuration file and sanity of the keys. | |
184eed6a | 265 | This is useful for updating |
f87f09aa | 266 | .Nm |
267 | reliably as configuration options may change. | |
c345cf9d | 268 | .It Fl u Ar len |
269 | This option is used to specify the size of the field | |
270 | in the | |
271 | .Li utmp | |
272 | structure that holds the remote host name. | |
273 | If the resolved host name is longer than | |
274 | .Ar len , | |
275 | the dotted decimal value will be used instead. | |
276 | This allows hosts with very long host names that | |
277 | overflow this field to still be uniquely identified. | |
278 | Specifying | |
279 | .Fl u0 | |
280 | indicates that only dotted decimal addresses | |
281 | should be put into the | |
282 | .Pa utmp | |
283 | file. | |
e675b851 | 284 | .Fl u0 |
285 | is also be used to prevent | |
286 | .Nm | |
287 | from making DNS requests unless the authentication | |
288 | mechanism or configuration requires it. | |
289 | Authentication mechanisms that may require DNS include | |
290 | .Cm RhostsAuthentication , | |
291 | .Cm RhostsRSAAuthentication , | |
292 | .Cm HostbasedAuthentication | |
293 | and using a | |
294 | .Cm from="pattern-list" | |
295 | option in a key file. | |
f464aad8 | 296 | Configuration options that require DNS include using a |
297 | USER@HOST pattern in | |
298 | .Cm AllowUsers | |
299 | or | |
300 | .Cm DenyUsers . | |
8abcdba4 | 301 | .It Fl D |
302 | When this option is specified | |
303 | .Nm | |
304 | will not detach and does not become a daemon. | |
305 | This allows easy monitoring of | |
306 | .Nm sshd . | |
48e671d5 | 307 | .It Fl 4 |
308 | Forces | |
309 | .Nm | |
310 | to use IPv4 addresses only. | |
311 | .It Fl 6 | |
312 | Forces | |
313 | .Nm | |
314 | to use IPv6 addresses only. | |
bf740959 | 315 | .El |
316 | .Sh CONFIGURATION FILE | |
317 | .Nm | |
f54651ce | 318 | reads configuration data from |
2a8a6488 | 319 | .Pa /etc/ssh/sshd_config |
bf740959 | 320 | (or the file specified with |
321 | .Fl f | |
4fe2af09 | 322 | on the command line). |
588df31a | 323 | The file format and configuration options are described in |
324 | .Xr sshd_config 5 . | |
bf740959 | 325 | .Sh LOGIN PROCESS |
326 | When a user successfully logs in, | |
327 | .Nm | |
328 | does the following: | |
329 | .Bl -enum -offset indent | |
330 | .It | |
331 | If the login is on a tty, and no command has been specified, | |
f54651ce | 332 | prints last login time and |
bf740959 | 333 | .Pa /etc/motd |
334 | (unless prevented in the configuration file or by | |
335 | .Pa $HOME/.hushlogin ; | |
336 | see the | |
f54651ce | 337 | .Sx FILES |
bf740959 | 338 | section). |
339 | .It | |
340 | If the login is on a tty, records login time. | |
341 | .It | |
342 | Checks | |
343 | .Pa /etc/nologin ; | |
344 | if it exists, prints contents and quits | |
345 | (unless root). | |
346 | .It | |
347 | Changes to run with normal user privileges. | |
348 | .It | |
349 | Sets up basic environment. | |
350 | .It | |
351 | Reads | |
352 | .Pa $HOME/.ssh/environment | |
353 | if it exists. | |
354 | .It | |
355 | Changes to user's home directory. | |
356 | .It | |
357 | If | |
358 | .Pa $HOME/.ssh/rc | |
359 | exists, runs it; else if | |
af98ced9 | 360 | .Pa /etc/ssh/sshrc |
bf740959 | 361 | exists, runs |
4fe2af09 | 362 | it; otherwise runs xauth. |
363 | The | |
bf740959 | 364 | .Dq rc |
365 | files are given the X11 | |
366 | authentication protocol and cookie in standard input. | |
367 | .It | |
368 | Runs user's shell or command. | |
369 | .El | |
370 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 371 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 372 | is the default file that lists the public keys that are |
373 | permitted for RSA authentication in protocol version 1 | |
374 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 375 | in protocol version 2. |
96a7b0cc | 376 | .Cm AuthorizedKeysFile |
c8445989 | 377 | may be used to specify an alternative file. |
8abcdba4 | 378 | .Pp |
4fe2af09 | 379 | Each line of the file contains one |
bf740959 | 380 | key (empty lines and lines starting with a |
381 | .Ql # | |
382 | are ignored as | |
4fe2af09 | 383 | comments). |
8abcdba4 | 384 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 385 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 386 | Each protocol version 2 public key consists of: |
387 | options, keytype, base64 encoded key, comment. | |
388 | The options fields | |
389 | are optional; its presence is determined by whether the line starts | |
bf740959 | 390 | with a number or not (the option field never starts with a number). |
8abcdba4 | 391 | The bits, exponent, modulus and comment fields give the RSA key for |
392 | protocol version 1; the | |
bf740959 | 393 | comment field is not used for anything (but may be convenient for the |
394 | user to identify the key). | |
8abcdba4 | 395 | For protocol version 2 the keytype is |
396 | .Dq ssh-dss | |
397 | or | |
398 | .Dq ssh-rsa . | |
bf740959 | 399 | .Pp |
400 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 401 | (because of the size of the RSA key modulus). |
402 | You don't want to type them in; instead, copy the | |
c0ecc314 | 403 | .Pa identity.pub , |
8abcdba4 | 404 | .Pa id_dsa.pub |
c0ecc314 | 405 | or the |
406 | .Pa id_rsa.pub | |
bf740959 | 407 | file and edit it. |
408 | .Pp | |
3bc822df | 409 | .Nm |
410 | enforces a minimum RSA key modulus size for protocol 1 | |
411 | and protocol 2 keys of 768 bits. | |
412 | .Pp | |
c345cf9d | 413 | The options (if present) consist of comma-separated option |
4fe2af09 | 414 | specifications. |
415 | No spaces are permitted, except within double quotes. | |
54bf768d | 416 | The following option specifications are supported (note |
417 | that option keywords are case-insensitive): | |
bf740959 | 418 | .Bl -tag -width Ds |
419 | .It Cm from="pattern-list" | |
420 | Specifies that in addition to RSA authentication, the canonical name | |
421 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 422 | patterns |
423 | .Pf ( Ql * | |
424 | and | |
425 | .Ql ? | |
426 | serve as wildcards). | |
427 | The list may also contain | |
428 | patterns negated by prefixing them with | |
429 | .Ql ! ; | |
430 | if the canonical host name matches a negated pattern, the key is not accepted. | |
431 | The purpose | |
bf740959 | 432 | of this option is to optionally increase security: RSA authentication |
433 | by itself does not trust the network or name servers or anything (but | |
434 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 435 | permits an intruder to log in from anywhere in the world. |
436 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 437 | servers and/or routers would have to be compromised in addition to |
438 | just the key). | |
439 | .It Cm command="command" | |
440 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 441 | authentication. |
442 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 443 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 444 | otherwise it is run without a tty. |
91789042 | 445 | If a 8-bit clean channel is required, |
446 | one must not request a pty or should specify | |
61e96248 | 447 | .Cm no-pty . |
4fe2af09 | 448 | A quote may be included in the command by quoting it with a backslash. |
449 | This option might be useful | |
450 | to restrict certain RSA keys to perform just a specific operation. | |
451 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 452 | Note that the client may specify TCP/IP and/or X11 |
453 | forwarding unless they are explicitly prohibited. | |
4cdbc654 | 454 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 455 | .It Cm environment="NAME=value" |
456 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 457 | logging in using this key. |
458 | Environment variables set this way | |
459 | override other default environment values. | |
460 | Multiple options of this type are permitted. | |
2548961d | 461 | This option is automatically disabled if |
462 | .Cm UseLogin | |
463 | is enabled. | |
bf740959 | 464 | .It Cm no-port-forwarding |
465 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 466 | Any port forward requests by the client will return an error. |
467 | This might be used, e.g., in connection with the | |
bf740959 | 468 | .Cm command |
469 | option. | |
470 | .It Cm no-X11-forwarding | |
471 | Forbids X11 forwarding when this key is used for authentication. | |
472 | Any X11 forward requests by the client will return an error. | |
473 | .It Cm no-agent-forwarding | |
474 | Forbids authentication agent forwarding when this key is used for | |
475 | authentication. | |
476 | .It Cm no-pty | |
477 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 478 | .It Cm permitopen="host:port" |
3730bb22 | 479 | Limit local |
dc504afd | 480 | .Li ``ssh -L'' |
b2ae83b8 | 481 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 482 | port. |
483 | IPv6 addresses can be specified with an alternative syntax: | |
484 | .Ar host/port . | |
485 | Multiple | |
dc504afd | 486 | .Cm permitopen |
3730bb22 | 487 | options may be applied separated by commas. No pattern matching is |
488 | performed on the specified hostnames, they must be literal domains or | |
dc504afd | 489 | addresses. |
bf740959 | 490 | .El |
491 | .Ss Examples | |
492 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
493 | .Pp | |
494 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
495 | .Pp | |
496 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 497 | .Pp |
498 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 499 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 500 | The |
2a8a6488 | 501 | .Pa /etc/ssh/ssh_known_hosts , |
f54651ce | 502 | and |
f49bc4f7 | 503 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 504 | files contain host public keys for all known hosts. |
505 | The global file should | |
506 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 507 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 508 | its key is added to the per-user file. |
bf740959 | 509 | .Pp |
510 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 511 | bits, exponent, modulus, comment. |
512 | The fields are separated by spaces. | |
bf740959 | 513 | .Pp |
514 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
515 | wildcards); each pattern in turn is matched against the canonical host | |
516 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 517 | name (when authenticating a server). |
518 | A pattern may also be preceded by | |
bf740959 | 519 | .Ql ! |
520 | to indicate negation: if the host name matches a negated | |
521 | pattern, it is not accepted (by that line) even if it matched another | |
522 | pattern on the line. | |
523 | .Pp | |
1d1ffb87 | 524 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 525 | can be obtained, e.g., from |
2a8a6488 | 526 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 527 | The optional comment field continues to the end of the line, and is not used. |
528 | .Pp | |
529 | Lines starting with | |
530 | .Ql # | |
531 | and empty lines are ignored as comments. | |
532 | .Pp | |
533 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 534 | matching line has the proper key. |
535 | It is thus permissible (but not | |
bf740959 | 536 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 537 | names. |
538 | This will inevitably happen when short forms of host names | |
539 | from different domains are put in the file. | |
540 | It is possible | |
bf740959 | 541 | that the files contain conflicting information; authentication is |
542 | accepted if valid information can be found from either file. | |
543 | .Pp | |
544 | Note that the lines in these files are typically hundreds of characters | |
545 | long, and you definitely don't want to type in the host keys by hand. | |
546 | Rather, generate them by a script | |
f54651ce | 547 | or by taking |
2a8a6488 | 548 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 549 | and adding the host names at the front. |
550 | .Ss Examples | |
da89cf4d | 551 | .Bd -literal |
552 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
553 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
554 | .Ed | |
bf740959 | 555 | .Sh FILES |
556 | .Bl -tag -width Ds | |
2a8a6488 | 557 | .It Pa /etc/ssh/sshd_config |
bf740959 | 558 | Contains configuration data for |
559 | .Nm sshd . | |
588df31a | 560 | The file format and configuration options are described in |
561 | .Xr sshd_config 5 . | |
2a8a6488 | 562 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
da89cf4d | 563 | These three files contain the private parts of the host keys. |
b8dc87d3 | 564 | These files should only be owned by root, readable only by root, and not |
bf740959 | 565 | accessible to others. |
566 | Note that | |
567 | .Nm | |
568 | does not start if this file is group/world-accessible. | |
2a8a6488 | 569 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub |
da89cf4d | 570 | These three files contain the public parts of the host keys. |
b8dc87d3 | 571 | These files should be world-readable but writable only by |
4fe2af09 | 572 | root. |
b8dc87d3 | 573 | Their contents should match the respective private parts. |
574 | These files are not | |
575 | really used for anything; they are provided for the convenience of | |
576 | the user so their contents can be copied to known hosts files. | |
577 | These files are created using | |
bf740959 | 578 | .Xr ssh-keygen 1 . |
e2432638 | 579 | .It Pa /etc/moduli |
c523303b | 580 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
75653d3e | 581 | .It Pa /var/empty |
582 | .Xr chroot 2 | |
583 | directory used by | |
584 | .Nm | |
585 | during privilege separation in the pre-authentication phase. | |
586 | The directory should not contain any files and must be owned by root | |
587 | and not group or world-writable. | |
5f4fdfae | 588 | .It Pa /var/run/sshd.pid |
bf740959 | 589 | Contains the process ID of the |
590 | .Nm | |
591 | listening for connections (if there are several daemons running | |
592 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 593 | started last). |
c345cf9d | 594 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 595 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 596 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 597 | This file must be readable by root (which may on some machines imply |
598 | it being world-readable if the user's home directory resides on an NFS | |
599 | volume). | |
600 | It is recommended that it not be accessible by others. | |
601 | The format of this file is described above. | |
602 | Users will place the contents of their | |
96a7b0cc | 603 | .Pa identity.pub , |
1d1ffb87 | 604 | .Pa id_dsa.pub |
c0ecc314 | 605 | and/or |
606 | .Pa id_rsa.pub | |
1d1ffb87 | 607 | files into this file, as described in |
608 | .Xr ssh-keygen 1 . | |
2a8a6488 | 609 | .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 610 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 611 | authentication or protocol version 2 hostbased authentication |
612 | to check the public key of the host. | |
4fe2af09 | 613 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 614 | The client uses the same files |
e91c60f2 | 615 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 616 | These files should be writable only by root/the owner. |
2a8a6488 | 617 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 618 | should be world-readable, and |
619 | .Pa $HOME/.ssh/known_hosts | |
620 | can but need not be world-readable. | |
621 | .It Pa /etc/nologin | |
f54651ce | 622 | If this file exists, |
bf740959 | 623 | .Nm |
4fe2af09 | 624 | refuses to let anyone except root log in. |
625 | The contents of the file | |
bf740959 | 626 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 627 | refused. |
628 | The file should be world-readable. | |
bf740959 | 629 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
5b263aae | 630 | Access controls that should be enforced by tcp-wrappers are defined here. |
631 | Further details are described in | |
bf740959 | 632 | .Xr hosts_access 5 . |
633 | .It Pa $HOME/.rhosts | |
634 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 635 | line. |
636 | The given user on the corresponding host is permitted to log in | |
637 | without password. | |
638 | The same file is used by rlogind and rshd. | |
bf740959 | 639 | The file must |
640 | be writable only by the user; it is recommended that it not be | |
641 | accessible by others. | |
642 | .Pp | |
4fe2af09 | 643 | If is also possible to use netgroups in the file. |
644 | Either host or user | |
bf740959 | 645 | name may be of the form +@groupname to specify all hosts or all users |
646 | in the group. | |
647 | .It Pa $HOME/.shosts | |
648 | For ssh, | |
649 | this file is exactly the same as for | |
650 | .Pa .rhosts . | |
651 | However, this file is | |
652 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 653 | .It Pa /etc/hosts.equiv |
bf740959 | 654 | This file is used during |
655 | .Pa .rhosts | |
4fe2af09 | 656 | authentication. |
657 | In the simplest form, this file contains host names, one per line. | |
658 | Users on | |
bf740959 | 659 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 660 | have the same user name on both machines. |
661 | The host name may also be | |
bf740959 | 662 | followed by a user name; such users are permitted to log in as |
663 | .Em any | |
4fe2af09 | 664 | user on this machine (except root). |
665 | Additionally, the syntax | |
bf740959 | 666 | .Dq +@group |
4fe2af09 | 667 | can be used to specify netgroups. |
668 | Negated entries start with | |
bf740959 | 669 | .Ql \&- . |
670 | .Pp | |
671 | If the client host/user is successfully matched in this file, login is | |
672 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 673 | same. |
674 | Additionally, successful RSA host authentication is normally required. | |
675 | This file must be writable only by root; it is recommended | |
bf740959 | 676 | that it be world-readable. |
677 | .Pp | |
678 | .Sy "Warning: It is almost never a good idea to use user names in" | |
679 | .Pa hosts.equiv . | |
680 | Beware that it really means that the named user(s) can log in as | |
681 | .Em anybody , | |
682 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 683 | binaries and directories. |
684 | Using a user name practically grants the user root access. | |
685 | The only valid use for user names that I can think | |
bf740959 | 686 | of is in negative entries. |
687 | .Pp | |
688 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 689 | .It Pa /etc/shosts.equiv |
bf740959 | 690 | This is processed exactly as |
691 | .Pa /etc/hosts.equiv . | |
692 | However, this file may be useful in environments that want to run both | |
693 | rsh/rlogin and ssh. | |
694 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 695 | This file is read into the environment at login (if it exists). |
696 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 697 | .Ql # ) , |
4fe2af09 | 698 | and assignment lines of the form name=value. |
699 | The file should be writable | |
bf740959 | 700 | only by the user; it need not be readable by anyone else. |
701 | .It Pa $HOME/.ssh/rc | |
702 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 703 | environment files but before starting the user's shell or command. |
d4c6ddff | 704 | It must not produce any output on stdout; stderr must be used |
705 | instead. | |
706 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
707 | its standard input (and | |
bf740959 | 708 | .Ev DISPLAY |
d4c6ddff | 709 | in its environment). |
710 | The script must call | |
bf740959 | 711 | .Xr xauth 1 |
d4c6ddff | 712 | because |
713 | .Nm | |
714 | will not run xauth automatically to add X11 cookies. | |
bf740959 | 715 | .Pp |
716 | The primary purpose of this file is to run any initialization routines | |
717 | which may be needed before the user's home directory becomes | |
718 | accessible; AFS is a particular example of such an environment. | |
719 | .Pp | |
720 | This file will probably contain some initialization code followed by | |
da89cf4d | 721 | something similar to: |
722 | .Bd -literal | |
d4c6ddff | 723 | if read proto cookie && [ -n "$DISPLAY" ]; then |
724 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
725 | # X11UseLocalhost=yes | |
726 | xauth add unix:`echo $DISPLAY | | |
727 | cut -c11-` $proto $cookie | |
728 | else | |
729 | # X11UseLocalhost=no | |
730 | xauth add $DISPLAY $proto $cookie | |
da89cf4d | 731 | fi |
d4c6ddff | 732 | fi |
da89cf4d | 733 | .Ed |
bf740959 | 734 | .Pp |
735 | If this file does not exist, | |
2a8a6488 | 736 | .Pa /etc/ssh/sshrc |
bf740959 | 737 | is run, and if that |
d4c6ddff | 738 | does not exist either, xauth is used to add the cookie. |
bf740959 | 739 | .Pp |
740 | This file should be writable only by the user, and need not be | |
741 | readable by anyone else. | |
2a8a6488 | 742 | .It Pa /etc/ssh/sshrc |
bf740959 | 743 | Like |
744 | .Pa $HOME/.ssh/rc . | |
745 | This can be used to specify | |
4fe2af09 | 746 | machine-specific login-time initializations globally. |
747 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 748 | .El |
fa08c86b | 749 | .Sh AUTHORS |
7f5c4295 | 750 | OpenSSH is a derivative of the original and free |
751 | ssh 1.2.12 release by Tatu Ylonen. | |
752 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
753 | Theo de Raadt and Dug Song | |
754 | removed many bugs, re-added newer features and | |
755 | created OpenSSH. | |
756 | Markus Friedl contributed the support for SSH | |
757 | protocol versions 1.5 and 2.0. | |
ce19ff48 | 758 | Niels Provos and Markus Friedl contributed support |
759 | for privilege separation. | |
bf740959 | 760 | .Sh SEE ALSO |
bf740959 | 761 | .Xr scp 1 , |
61e96248 | 762 | .Xr sftp 1 , |
bf740959 | 763 | .Xr ssh 1 , |
764 | .Xr ssh-add 1 , | |
765 | .Xr ssh-agent 1 , | |
766 | .Xr ssh-keygen 1 , | |
248bad82 | 767 | .Xr login.conf 5 , |
768 | .Xr moduli 5 , | |
588df31a | 769 | .Xr sshd_config 5 , |
a5a2da3b | 770 | .Xr sftp-server 8 |
2cad6cef | 771 | .Rs |
772 | .%A T. Ylonen | |
773 | .%A T. Kivinen | |
774 | .%A M. Saarinen | |
775 | .%A T. Rinne | |
776 | .%A S. Lehtinen | |
777 | .%T "SSH Protocol Architecture" | |
17f5e68a | 778 | .%N draft-ietf-secsh-architecture-12.txt |
779 | .%D January 2002 | |
2cad6cef | 780 | .%O work in progress material |
781 | .Re | |
da89cf4d | 782 | .Rs |
783 | .%A M. Friedl | |
784 | .%A N. Provos | |
785 | .%A W. A. Simpson | |
786 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
17f5e68a | 787 | .%N draft-ietf-secsh-dh-group-exchange-02.txt |
788 | .%D January 2002 | |
da89cf4d | 789 | .%O work in progress material |
790 | .Re |