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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 | .\" |
6691e41b | 37 | .\" $OpenBSD: sshd.8,v 1.194 2003/01/31 21:54:40 jmc 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 |
2043936f | 47 | .Op Fl deiqtD46 |
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 |
2c86906e | 59 | (SSH 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. |
64 | The programs are intended to be as easy to | |
bf740959 | 65 | install and use as possible. |
66 | .Pp | |
67 | .Nm | |
4fe2af09 | 68 | is the daemon that listens for connections from clients. |
f54651ce | 69 | It is normally started at boot from |
bf740959 | 70 | .Pa /etc/rc . |
71 | It forks a new | |
4fe2af09 | 72 | daemon for each incoming connection. |
73 | The forked daemons handle | |
bf740959 | 74 | key exchange, encryption, authentication, command execution, |
75 | and data exchange. | |
1d1ffb87 | 76 | This implementation of |
77 | .Nm | |
78 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 79 | .Nm |
6691e41b | 80 | works as follows: |
1d1ffb87 | 81 | .Pp |
82 | .Ss SSH protocol version 1 | |
83 | .Pp | |
4fe2af09 | 84 | Each host has a host-specific RSA key |
85 | (normally 1024 bits) used to identify the host. | |
86 | Additionally, when | |
bf740959 | 87 | the daemon starts, it generates a server RSA key (normally 768 bits). |
88 | This key is normally regenerated every hour if it has been used, and | |
89 | is never stored on disk. | |
90 | .Pp | |
6691e41b | 91 | Whenever a client connects, the daemon responds with its public |
3189621b | 92 | host and server keys. |
4fe2af09 | 93 | The client compares the |
1d1ffb87 | 94 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 95 | The client then generates a 256 bit random number. |
96 | It encrypts this | |
bf740959 | 97 | random number using both the host key and the server key, and sends |
4fe2af09 | 98 | the encrypted number to the server. |
3189621b | 99 | Both sides then use this |
bf740959 | 100 | random number as a session key which is used to encrypt all further |
4fe2af09 | 101 | communications in the session. |
102 | The rest of the session is encrypted | |
3189621b | 103 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 104 | being used by default. |
4fe2af09 | 105 | The client selects the encryption algorithm |
bf740959 | 106 | to use from those offered by the server. |
107 | .Pp | |
4fe2af09 | 108 | Next, the server and the client enter an authentication dialog. |
109 | The client tries to authenticate itself using | |
bf740959 | 110 | .Pa .rhosts |
111 | authentication, | |
112 | .Pa .rhosts | |
113 | authentication combined with RSA host | |
114 | authentication, RSA challenge-response authentication, or password | |
115 | based authentication. | |
116 | .Pp | |
117 | Rhosts authentication is normally disabled | |
118 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 119 | configuration file if desired. |
120 | System security is not improved unless | |
7e276482 | 121 | .Nm rshd , |
122 | .Nm rlogind , | |
bf740959 | 123 | and |
6691e41b | 124 | .Nm rexecd |
bf740959 | 125 | are disabled (thus completely disabling |
7e276482 | 126 | .Xr rlogin |
bf740959 | 127 | and |
7e276482 | 128 | .Xr rsh |
3189621b | 129 | into the machine). |
bf740959 | 130 | .Pp |
1d1ffb87 | 131 | .Ss SSH protocol version 2 |
132 | .Pp | |
c345cf9d | 133 | Version 2 works similarly: |
7f19f8bb | 134 | Each host has a host-specific key (RSA or DSA) used to identify the host. |
1d1ffb87 | 135 | However, when the daemon starts, it does not generate a server key. |
136 | Forward security is provided through a Diffie-Hellman key agreement. | |
137 | This key agreement results in a shared session key. | |
da89cf4d | 138 | .Pp |
51c251f0 | 139 | The rest of the session is encrypted using a symmetric cipher, currently |
da89cf4d | 140 | 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. |
1d1ffb87 | 141 | The client selects the encryption algorithm |
142 | to use from those offered by the server. | |
143 | Additionally, session integrity is provided | |
d0c832f3 | 144 | through a cryptographic message authentication code |
1d1ffb87 | 145 | (hmac-sha1 or hmac-md5). |
146 | .Pp | |
147 | Protocol version 2 provides a public key based | |
da89cf4d | 148 | user (PubkeyAuthentication) or |
149 | client host (HostbasedAuthentication) authentication method, | |
150 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 151 | .Pp |
152 | .Ss Command execution and data forwarding | |
153 | .Pp | |
bf740959 | 154 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 155 | preparing the session is entered. |
156 | At this time the client may request | |
bf740959 | 157 | things like allocating a pseudo-tty, forwarding X11 connections, |
158 | forwarding TCP/IP connections, or forwarding the authentication agent | |
159 | connection over the secure channel. | |
160 | .Pp | |
161 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 162 | The sides then enter session mode. |
163 | In this mode, either side may send | |
bf740959 | 164 | data at any time, and such data is forwarded to/from the shell or |
165 | command on the server side, and the user terminal in the client side. | |
166 | .Pp | |
167 | When the user program terminates and all forwarded X11 and other | |
168 | connections have been closed, the server sends command exit status to | |
169 | the client, and both sides exit. | |
170 | .Pp | |
171 | .Nm | |
172 | can be configured using command-line options or a configuration | |
4fe2af09 | 173 | file. |
174 | Command-line options override values specified in the | |
bf740959 | 175 | configuration file. |
176 | .Pp | |
9d6b7add | 177 | .Nm |
178 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 179 | .Dv SIGHUP , |
a5a2da3b | 180 | by executing itself with the name it was started as, i.e., |
409edaba | 181 | .Pa /usr/sbin/sshd . |
9d6b7add | 182 | .Pp |
bf740959 | 183 | The options are as follows: |
184 | .Bl -tag -width Ds | |
185 | .It Fl b Ar bits | |
da89cf4d | 186 | Specifies the number of bits in the ephemeral protocol version 1 |
187 | server key (default 768). | |
bf740959 | 188 | .It Fl d |
4fe2af09 | 189 | Debug mode. |
190 | The server sends verbose debug output to the system | |
191 | log, and does not put itself in the background. | |
192 | The server also will not fork and will only process one connection. | |
193 | This option is only intended for debugging for the server. | |
6691e41b | 194 | Multiple |
195 | .Fl d | |
196 | options increase the debugging level. | |
94ec8c6b | 197 | Maximum is 3. |
da89cf4d | 198 | .It Fl e |
199 | When this option is specified, | |
200 | .Nm | |
201 | will send the output to the standard error instead of the system log. | |
bf740959 | 202 | .It Fl f Ar configuration_file |
4fe2af09 | 203 | Specifies the name of the configuration file. |
204 | The default is | |
2a8a6488 | 205 | .Pa /etc/ssh/sshd_config . |
bf740959 | 206 | .Nm |
207 | refuses to start if there is no configuration file. | |
208 | .It Fl g Ar login_grace_time | |
209 | Gives the grace time for clients to authenticate themselves (default | |
3445ca02 | 210 | 120 seconds). |
4fe2af09 | 211 | If the client fails to authenticate the user within |
212 | this many seconds, the server disconnects and exits. | |
213 | A value of zero indicates no limit. | |
bf740959 | 214 | .It Fl h Ar host_key_file |
0f84fe37 | 215 | Specifies a file from which a host key is read. |
bf740959 | 216 | This option must be given if |
217 | .Nm | |
218 | is not run as root (as the normal | |
0f84fe37 | 219 | host key files are normally not readable by anyone but root). |
220 | The default is | |
2a8a6488 | 221 | .Pa /etc/ssh/ssh_host_key |
0f84fe37 | 222 | for protocol version 1, and |
2a8a6488 | 223 | .Pa /etc/ssh/ssh_host_rsa_key |
0f84fe37 | 224 | and |
2a8a6488 | 225 | .Pa /etc/ssh/ssh_host_dsa_key |
0f84fe37 | 226 | for protocol version 2. |
8abcdba4 | 227 | It is possible to have multiple host key files for |
da89cf4d | 228 | the different protocol versions and host key algorithms. |
bf740959 | 229 | .It Fl i |
230 | Specifies that | |
231 | .Nm | |
6691e41b | 232 | is being run from |
233 | .Xr inetd 8 . | |
bf740959 | 234 | .Nm |
235 | is normally not run | |
236 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 237 | respond to the client, and this may take tens of seconds. |
238 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 239 | However, with small key sizes (e.g., 512) using |
bf740959 | 240 | .Nm |
241 | from inetd may | |
242 | be feasible. | |
243 | .It Fl k Ar key_gen_time | |
da89cf4d | 244 | Specifies how often the ephemeral protocol version 1 server key is |
245 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 246 | The motivation for regenerating the key fairly |
bf740959 | 247 | often is that the key is not stored anywhere, and after about an hour, |
248 | it becomes impossible to recover the key for decrypting intercepted | |
249 | communications even if the machine is cracked into or physically | |
4fe2af09 | 250 | seized. |
251 | A value of zero indicates that the key will never be regenerated. | |
2717fa0f | 252 | .It Fl o Ar option |
253 | Can be used to give options in the format used in the configuration file. | |
254 | This is useful for specifying options for which there is no separate | |
255 | command-line flag. | |
bf740959 | 256 | .It Fl p Ar port |
257 | Specifies the port on which the server listens for connections | |
258 | (default 22). | |
135113a3 | 259 | Multiple port options are permitted. |
260 | Ports specified in the configuration file are ignored when a | |
261 | command-line port is specified. | |
bf740959 | 262 | .It Fl q |
4fe2af09 | 263 | Quiet mode. |
264 | Nothing is sent to the system log. | |
265 | Normally the beginning, | |
bf740959 | 266 | authentication, and termination of each connection is logged. |
f87f09aa | 267 | .It Fl t |
268 | Test mode. | |
269 | Only check the validity of the configuration file and sanity of the keys. | |
184eed6a | 270 | This is useful for updating |
f87f09aa | 271 | .Nm |
272 | reliably as configuration options may change. | |
c345cf9d | 273 | .It Fl u Ar len |
274 | This option is used to specify the size of the field | |
275 | in the | |
276 | .Li utmp | |
277 | structure that holds the remote host name. | |
278 | If the resolved host name is longer than | |
279 | .Ar len , | |
280 | the dotted decimal value will be used instead. | |
281 | This allows hosts with very long host names that | |
282 | overflow this field to still be uniquely identified. | |
283 | Specifying | |
284 | .Fl u0 | |
285 | indicates that only dotted decimal addresses | |
286 | should be put into the | |
287 | .Pa utmp | |
288 | file. | |
e675b851 | 289 | .Fl u0 |
6691e41b | 290 | may also be used to prevent |
e675b851 | 291 | .Nm |
292 | from making DNS requests unless the authentication | |
293 | mechanism or configuration requires it. | |
294 | Authentication mechanisms that may require DNS include | |
295 | .Cm RhostsAuthentication , | |
296 | .Cm RhostsRSAAuthentication , | |
297 | .Cm HostbasedAuthentication | |
298 | and using a | |
299 | .Cm from="pattern-list" | |
300 | option in a key file. | |
f464aad8 | 301 | Configuration options that require DNS include using a |
302 | USER@HOST pattern in | |
303 | .Cm AllowUsers | |
304 | or | |
305 | .Cm DenyUsers . | |
8abcdba4 | 306 | .It Fl D |
307 | When this option is specified | |
308 | .Nm | |
309 | will not detach and does not become a daemon. | |
310 | This allows easy monitoring of | |
311 | .Nm sshd . | |
48e671d5 | 312 | .It Fl 4 |
313 | Forces | |
314 | .Nm | |
315 | to use IPv4 addresses only. | |
316 | .It Fl 6 | |
317 | Forces | |
318 | .Nm | |
319 | to use IPv6 addresses only. | |
bf740959 | 320 | .El |
321 | .Sh CONFIGURATION FILE | |
322 | .Nm | |
f54651ce | 323 | reads configuration data from |
2a8a6488 | 324 | .Pa /etc/ssh/sshd_config |
bf740959 | 325 | (or the file specified with |
326 | .Fl f | |
4fe2af09 | 327 | on the command line). |
588df31a | 328 | The file format and configuration options are described in |
329 | .Xr sshd_config 5 . | |
bf740959 | 330 | .Sh LOGIN PROCESS |
331 | When a user successfully logs in, | |
332 | .Nm | |
333 | does the following: | |
334 | .Bl -enum -offset indent | |
335 | .It | |
336 | If the login is on a tty, and no command has been specified, | |
f54651ce | 337 | prints last login time and |
bf740959 | 338 | .Pa /etc/motd |
339 | (unless prevented in the configuration file or by | |
340 | .Pa $HOME/.hushlogin ; | |
341 | see the | |
f54651ce | 342 | .Sx FILES |
bf740959 | 343 | section). |
344 | .It | |
345 | If the login is on a tty, records login time. | |
346 | .It | |
347 | Checks | |
348 | .Pa /etc/nologin ; | |
349 | if it exists, prints contents and quits | |
350 | (unless root). | |
351 | .It | |
352 | Changes to run with normal user privileges. | |
353 | .It | |
354 | Sets up basic environment. | |
355 | .It | |
356 | Reads | |
357 | .Pa $HOME/.ssh/environment | |
6a342527 | 358 | if it exists and users are allowed to change their environment. |
35453849 | 359 | See the |
6a342527 | 360 | .Cm PermitUserEnvironment |
35453849 | 361 | option in |
6a342527 | 362 | .Xr sshd_config 5 . |
bf740959 | 363 | .It |
364 | Changes to user's home directory. | |
365 | .It | |
366 | If | |
367 | .Pa $HOME/.ssh/rc | |
368 | exists, runs it; else if | |
af98ced9 | 369 | .Pa /etc/ssh/sshrc |
bf740959 | 370 | exists, runs |
4fe2af09 | 371 | it; otherwise runs xauth. |
372 | The | |
bf740959 | 373 | .Dq rc |
374 | files are given the X11 | |
375 | authentication protocol and cookie in standard input. | |
376 | .It | |
377 | Runs user's shell or command. | |
378 | .El | |
379 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 380 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 381 | is the default file that lists the public keys that are |
382 | permitted for RSA authentication in protocol version 1 | |
383 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 384 | in protocol version 2. |
96a7b0cc | 385 | .Cm AuthorizedKeysFile |
c8445989 | 386 | may be used to specify an alternative file. |
8abcdba4 | 387 | .Pp |
4fe2af09 | 388 | Each line of the file contains one |
bf740959 | 389 | key (empty lines and lines starting with a |
390 | .Ql # | |
391 | are ignored as | |
4fe2af09 | 392 | comments). |
8abcdba4 | 393 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 394 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 395 | Each protocol version 2 public key consists of: |
396 | options, keytype, base64 encoded key, comment. | |
755c4339 | 397 | The options field |
398 | is optional; its presence is determined by whether the line starts | |
399 | with a number or not (the options field never starts with a number). | |
8abcdba4 | 400 | The bits, exponent, modulus and comment fields give the RSA key for |
401 | protocol version 1; the | |
bf740959 | 402 | comment field is not used for anything (but may be convenient for the |
403 | user to identify the key). | |
8abcdba4 | 404 | For protocol version 2 the keytype is |
405 | .Dq ssh-dss | |
406 | or | |
407 | .Dq ssh-rsa . | |
bf740959 | 408 | .Pp |
409 | Note that lines in this file are usually several hundred bytes long | |
755c4339 | 410 | (because of the size of the public key encoding). |
4fe2af09 | 411 | You don't want to type them in; instead, copy the |
c0ecc314 | 412 | .Pa identity.pub , |
8abcdba4 | 413 | .Pa id_dsa.pub |
c0ecc314 | 414 | or the |
415 | .Pa id_rsa.pub | |
bf740959 | 416 | file and edit it. |
417 | .Pp | |
3bc822df | 418 | .Nm |
419 | enforces a minimum RSA key modulus size for protocol 1 | |
420 | and protocol 2 keys of 768 bits. | |
421 | .Pp | |
c345cf9d | 422 | The options (if present) consist of comma-separated option |
4fe2af09 | 423 | specifications. |
424 | No spaces are permitted, except within double quotes. | |
54bf768d | 425 | The following option specifications are supported (note |
426 | that option keywords are case-insensitive): | |
bf740959 | 427 | .Bl -tag -width Ds |
428 | .It Cm from="pattern-list" | |
755c4339 | 429 | Specifies that in addition to public key authentication, the canonical name |
bf740959 | 430 | of the remote host must be present in the comma-separated list of |
4fe2af09 | 431 | patterns |
432 | .Pf ( Ql * | |
433 | and | |
434 | .Ql ? | |
435 | serve as wildcards). | |
436 | The list may also contain | |
437 | patterns negated by prefixing them with | |
438 | .Ql ! ; | |
439 | if the canonical host name matches a negated pattern, the key is not accepted. | |
440 | The purpose | |
755c4339 | 441 | of this option is to optionally increase security: public key authentication |
bf740959 | 442 | by itself does not trust the network or name servers or anything (but |
443 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 444 | permits an intruder to log in from anywhere in the world. |
445 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 446 | servers and/or routers would have to be compromised in addition to |
447 | just the key). | |
448 | .It Cm command="command" | |
449 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 450 | authentication. |
451 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 452 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 453 | otherwise it is run without a tty. |
6691e41b | 454 | If an 8-bit clean channel is required, |
91789042 | 455 | one must not request a pty or should specify |
61e96248 | 456 | .Cm no-pty . |
4fe2af09 | 457 | A quote may be included in the command by quoting it with a backslash. |
458 | This option might be useful | |
755c4339 | 459 | to restrict certain public keys to perform just a specific operation. |
4fe2af09 | 460 | An example might be a key that permits remote backups but nothing else. |
d0c832f3 | 461 | Note that the client may specify TCP/IP and/or X11 |
462 | forwarding unless they are explicitly prohibited. | |
4cdbc654 | 463 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 464 | .It Cm environment="NAME=value" |
465 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 466 | logging in using this key. |
467 | Environment variables set this way | |
468 | override other default environment values. | |
469 | Multiple options of this type are permitted. | |
35453849 | 470 | Environment processing is disabled by default and is |
471 | controlled via the | |
472 | .Cm PermitUserEnvironment | |
473 | option. | |
2548961d | 474 | This option is automatically disabled if |
475 | .Cm UseLogin | |
476 | is enabled. | |
bf740959 | 477 | .It Cm no-port-forwarding |
478 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 479 | Any port forward requests by the client will return an error. |
480 | This might be used, e.g., in connection with the | |
bf740959 | 481 | .Cm command |
482 | option. | |
483 | .It Cm no-X11-forwarding | |
484 | Forbids X11 forwarding when this key is used for authentication. | |
485 | Any X11 forward requests by the client will return an error. | |
486 | .It Cm no-agent-forwarding | |
487 | Forbids authentication agent forwarding when this key is used for | |
488 | authentication. | |
489 | .It Cm no-pty | |
490 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 491 | .It Cm permitopen="host:port" |
3730bb22 | 492 | Limit local |
dc504afd | 493 | .Li ``ssh -L'' |
b2ae83b8 | 494 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 495 | port. |
496 | IPv6 addresses can be specified with an alternative syntax: | |
497 | .Ar host/port . | |
498 | Multiple | |
dc504afd | 499 | .Cm permitopen |
3730bb22 | 500 | options may be applied separated by commas. No pattern matching is |
501 | performed on the specified hostnames, they must be literal domains or | |
dc504afd | 502 | addresses. |
bf740959 | 503 | .El |
504 | .Ss Examples | |
505 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
506 | .Pp | |
507 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
508 | .Pp | |
509 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 510 | .Pp |
511 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 512 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 513 | The |
6691e41b | 514 | .Pa /etc/ssh/ssh_known_hosts |
f54651ce | 515 | and |
f49bc4f7 | 516 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 517 | files contain host public keys for all known hosts. |
518 | The global file should | |
519 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 520 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 521 | its key is added to the per-user file. |
bf740959 | 522 | .Pp |
523 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 524 | bits, exponent, modulus, comment. |
525 | The fields are separated by spaces. | |
bf740959 | 526 | .Pp |
527 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
528 | wildcards); each pattern in turn is matched against the canonical host | |
529 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 530 | name (when authenticating a server). |
531 | A pattern may also be preceded by | |
bf740959 | 532 | .Ql ! |
533 | to indicate negation: if the host name matches a negated | |
534 | pattern, it is not accepted (by that line) even if it matched another | |
535 | pattern on the line. | |
536 | .Pp | |
1d1ffb87 | 537 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 538 | can be obtained, e.g., from |
2a8a6488 | 539 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 540 | The optional comment field continues to the end of the line, and is not used. |
541 | .Pp | |
542 | Lines starting with | |
543 | .Ql # | |
544 | and empty lines are ignored as comments. | |
545 | .Pp | |
546 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 547 | matching line has the proper key. |
548 | It is thus permissible (but not | |
bf740959 | 549 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 550 | names. |
551 | This will inevitably happen when short forms of host names | |
552 | from different domains are put in the file. | |
553 | It is possible | |
bf740959 | 554 | that the files contain conflicting information; authentication is |
555 | accepted if valid information can be found from either file. | |
556 | .Pp | |
557 | Note that the lines in these files are typically hundreds of characters | |
558 | long, and you definitely don't want to type in the host keys by hand. | |
559 | Rather, generate them by a script | |
f54651ce | 560 | or by taking |
2a8a6488 | 561 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 562 | and adding the host names at the front. |
563 | .Ss Examples | |
da89cf4d | 564 | .Bd -literal |
565 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
566 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
567 | .Ed | |
bf740959 | 568 | .Sh FILES |
569 | .Bl -tag -width Ds | |
2a8a6488 | 570 | .It Pa /etc/ssh/sshd_config |
bf740959 | 571 | Contains configuration data for |
572 | .Nm sshd . | |
588df31a | 573 | The file format and configuration options are described in |
574 | .Xr sshd_config 5 . | |
2a8a6488 | 575 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
da89cf4d | 576 | These three files contain the private parts of the host keys. |
b8dc87d3 | 577 | These files should only be owned by root, readable only by root, and not |
bf740959 | 578 | accessible to others. |
579 | Note that | |
580 | .Nm | |
581 | does not start if this file is group/world-accessible. | |
2a8a6488 | 582 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub |
da89cf4d | 583 | These three files contain the public parts of the host keys. |
b8dc87d3 | 584 | These files should be world-readable but writable only by |
4fe2af09 | 585 | root. |
b8dc87d3 | 586 | Their contents should match the respective private parts. |
587 | These files are not | |
588 | really used for anything; they are provided for the convenience of | |
589 | the user so their contents can be copied to known hosts files. | |
590 | These files are created using | |
bf740959 | 591 | .Xr ssh-keygen 1 . |
e2432638 | 592 | .It Pa /etc/moduli |
c523303b | 593 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
f1dcc34e | 594 | The file format is described in |
595 | .Xr moduli 5 . | |
75653d3e | 596 | .It Pa /var/empty |
597 | .Xr chroot 2 | |
598 | directory used by | |
599 | .Nm | |
600 | during privilege separation in the pre-authentication phase. | |
601 | The directory should not contain any files and must be owned by root | |
602 | and not group or world-writable. | |
5f4fdfae | 603 | .It Pa /var/run/sshd.pid |
bf740959 | 604 | Contains the process ID of the |
605 | .Nm | |
606 | listening for connections (if there are several daemons running | |
baa08b92 | 607 | concurrently for different ports, this contains the process ID of the one |
4fe2af09 | 608 | started last). |
c345cf9d | 609 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 610 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 611 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 612 | This file must be readable by root (which may on some machines imply |
613 | it being world-readable if the user's home directory resides on an NFS | |
614 | volume). | |
615 | It is recommended that it not be accessible by others. | |
616 | The format of this file is described above. | |
617 | Users will place the contents of their | |
96a7b0cc | 618 | .Pa identity.pub , |
1d1ffb87 | 619 | .Pa id_dsa.pub |
c0ecc314 | 620 | and/or |
621 | .Pa id_rsa.pub | |
1d1ffb87 | 622 | files into this file, as described in |
623 | .Xr ssh-keygen 1 . | |
2a8a6488 | 624 | .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 625 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 626 | authentication or protocol version 2 hostbased authentication |
627 | to check the public key of the host. | |
4fe2af09 | 628 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 629 | The client uses the same files |
e91c60f2 | 630 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 631 | These files should be writable only by root/the owner. |
2a8a6488 | 632 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 633 | should be world-readable, and |
634 | .Pa $HOME/.ssh/known_hosts | |
6691e41b | 635 | can, but need not be, world-readable. |
bf740959 | 636 | .It Pa /etc/nologin |
f54651ce | 637 | If this file exists, |
bf740959 | 638 | .Nm |
4fe2af09 | 639 | refuses to let anyone except root log in. |
640 | The contents of the file | |
bf740959 | 641 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 642 | refused. |
643 | The file should be world-readable. | |
bf740959 | 644 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
5b263aae | 645 | Access controls that should be enforced by tcp-wrappers are defined here. |
646 | Further details are described in | |
bf740959 | 647 | .Xr hosts_access 5 . |
648 | .It Pa $HOME/.rhosts | |
649 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 650 | line. |
651 | The given user on the corresponding host is permitted to log in | |
6691e41b | 652 | without a password. |
4fe2af09 | 653 | The same file is used by rlogind and rshd. |
bf740959 | 654 | The file must |
655 | be writable only by the user; it is recommended that it not be | |
656 | accessible by others. | |
657 | .Pp | |
4fe2af09 | 658 | If is also possible to use netgroups in the file. |
659 | Either host or user | |
bf740959 | 660 | name may be of the form +@groupname to specify all hosts or all users |
661 | in the group. | |
662 | .It Pa $HOME/.shosts | |
663 | For ssh, | |
664 | this file is exactly the same as for | |
665 | .Pa .rhosts . | |
666 | However, this file is | |
667 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 668 | .It Pa /etc/hosts.equiv |
bf740959 | 669 | This file is used during |
670 | .Pa .rhosts | |
4fe2af09 | 671 | authentication. |
672 | In the simplest form, this file contains host names, one per line. | |
673 | Users on | |
bf740959 | 674 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 675 | have the same user name on both machines. |
676 | The host name may also be | |
bf740959 | 677 | followed by a user name; such users are permitted to log in as |
678 | .Em any | |
4fe2af09 | 679 | user on this machine (except root). |
680 | Additionally, the syntax | |
bf740959 | 681 | .Dq +@group |
4fe2af09 | 682 | can be used to specify netgroups. |
683 | Negated entries start with | |
bf740959 | 684 | .Ql \&- . |
685 | .Pp | |
686 | If the client host/user is successfully matched in this file, login is | |
687 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 688 | same. |
689 | Additionally, successful RSA host authentication is normally required. | |
690 | This file must be writable only by root; it is recommended | |
bf740959 | 691 | that it be world-readable. |
692 | .Pp | |
693 | .Sy "Warning: It is almost never a good idea to use user names in" | |
694 | .Pa hosts.equiv . | |
695 | Beware that it really means that the named user(s) can log in as | |
696 | .Em anybody , | |
697 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 698 | binaries and directories. |
699 | Using a user name practically grants the user root access. | |
700 | The only valid use for user names that I can think | |
bf740959 | 701 | of is in negative entries. |
702 | .Pp | |
703 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 704 | .It Pa /etc/shosts.equiv |
bf740959 | 705 | This is processed exactly as |
706 | .Pa /etc/hosts.equiv . | |
707 | However, this file may be useful in environments that want to run both | |
708 | rsh/rlogin and ssh. | |
709 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 710 | This file is read into the environment at login (if it exists). |
711 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 712 | .Ql # ) , |
4fe2af09 | 713 | and assignment lines of the form name=value. |
714 | The file should be writable | |
bf740959 | 715 | only by the user; it need not be readable by anyone else. |
35453849 | 716 | Environment processing is disabled by default and is |
717 | controlled via the | |
718 | .Cm PermitUserEnvironment | |
719 | option. | |
bf740959 | 720 | .It Pa $HOME/.ssh/rc |
6691e41b | 721 | If this file exists, it is run with |
722 | .Pa /bin/sh | |
723 | after reading the | |
4fe2af09 | 724 | environment files but before starting the user's shell or command. |
d4c6ddff | 725 | It must not produce any output on stdout; stderr must be used |
726 | instead. | |
727 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
728 | its standard input (and | |
bf740959 | 729 | .Ev DISPLAY |
d4c6ddff | 730 | in its environment). |
731 | The script must call | |
bf740959 | 732 | .Xr xauth 1 |
d4c6ddff | 733 | because |
734 | .Nm | |
735 | will not run xauth automatically to add X11 cookies. | |
bf740959 | 736 | .Pp |
737 | The primary purpose of this file is to run any initialization routines | |
738 | which may be needed before the user's home directory becomes | |
739 | accessible; AFS is a particular example of such an environment. | |
740 | .Pp | |
741 | This file will probably contain some initialization code followed by | |
da89cf4d | 742 | something similar to: |
743 | .Bd -literal | |
d4c6ddff | 744 | if read proto cookie && [ -n "$DISPLAY" ]; then |
745 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
746 | # X11UseLocalhost=yes | |
cb11b555 | 747 | echo add unix:`echo $DISPLAY | |
d4c6ddff | 748 | cut -c11-` $proto $cookie |
749 | else | |
750 | # X11UseLocalhost=no | |
cb11b555 | 751 | echo add $DISPLAY $proto $cookie |
752 | fi | xauth -q - | |
d4c6ddff | 753 | fi |
da89cf4d | 754 | .Ed |
bf740959 | 755 | .Pp |
756 | If this file does not exist, | |
2a8a6488 | 757 | .Pa /etc/ssh/sshrc |
bf740959 | 758 | is run, and if that |
d4c6ddff | 759 | does not exist either, xauth is used to add the cookie. |
bf740959 | 760 | .Pp |
761 | This file should be writable only by the user, and need not be | |
762 | readable by anyone else. | |
2a8a6488 | 763 | .It Pa /etc/ssh/sshrc |
bf740959 | 764 | Like |
765 | .Pa $HOME/.ssh/rc . | |
766 | This can be used to specify | |
4fe2af09 | 767 | machine-specific login-time initializations globally. |
768 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 769 | .El |
fa08c86b | 770 | .Sh AUTHORS |
7f5c4295 | 771 | OpenSSH is a derivative of the original and free |
772 | ssh 1.2.12 release by Tatu Ylonen. | |
773 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
774 | Theo de Raadt and Dug Song | |
775 | removed many bugs, re-added newer features and | |
776 | created OpenSSH. | |
777 | Markus Friedl contributed the support for SSH | |
778 | protocol versions 1.5 and 2.0. | |
ce19ff48 | 779 | Niels Provos and Markus Friedl contributed support |
780 | for privilege separation. | |
bf740959 | 781 | .Sh SEE ALSO |
bf740959 | 782 | .Xr scp 1 , |
61e96248 | 783 | .Xr sftp 1 , |
bf740959 | 784 | .Xr ssh 1 , |
785 | .Xr ssh-add 1 , | |
786 | .Xr ssh-agent 1 , | |
787 | .Xr ssh-keygen 1 , | |
248bad82 | 788 | .Xr login.conf 5 , |
789 | .Xr moduli 5 , | |
588df31a | 790 | .Xr sshd_config 5 , |
a5a2da3b | 791 | .Xr sftp-server 8 |
2cad6cef | 792 | .Rs |
793 | .%A T. Ylonen | |
794 | .%A T. Kivinen | |
795 | .%A M. Saarinen | |
796 | .%A T. Rinne | |
797 | .%A S. Lehtinen | |
798 | .%T "SSH Protocol Architecture" | |
17f5e68a | 799 | .%N draft-ietf-secsh-architecture-12.txt |
800 | .%D January 2002 | |
2cad6cef | 801 | .%O work in progress material |
802 | .Re | |
da89cf4d | 803 | .Rs |
804 | .%A M. Friedl | |
805 | .%A N. Provos | |
806 | .%A W. A. Simpson | |
807 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
17f5e68a | 808 | .%N draft-ietf-secsh-dh-group-exchange-02.txt |
809 | .%D January 2002 | |
da89cf4d | 810 | .%O work in progress material |
811 | .Re |