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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 | .\" |
1642b888 | 37 | .\" $OpenBSD: sshd.8,v 1.179 2002/05/04 02:39:35 deraadt 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 | |
bf740959 | 119 | .Xr rshd 8 , |
120 | .Xr rlogind 8 , | |
bf740959 | 121 | and |
9495bfc5 | 122 | .Xr rexecd 8 |
bf740959 | 123 | are disabled (thus completely disabling |
124 | .Xr rlogin 1 | |
125 | and | |
126 | .Xr rsh 1 | |
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). |
54bf768d | 323 | The file contains keyword-argument pairs, one per line. |
4fe2af09 | 324 | Lines starting with |
bf740959 | 325 | .Ql # |
326 | and empty lines are interpreted as comments. | |
327 | .Pp | |
54bf768d | 328 | The possible |
329 | keywords and their meanings are as follows (note that | |
330 | keywords are case-insensitive and arguments are case-sensitive): | |
bf740959 | 331 | .Bl -tag -width Ds |
332 | .It Cm AFSTokenPassing | |
4fe2af09 | 333 | Specifies whether an AFS token may be forwarded to the server. |
334 | Default is | |
22d0faff | 335 | .Dq no . |
bf740959 | 336 | .It Cm AllowGroups |
dfafef8f | 337 | This keyword can be followed by a list of group name patterns, separated |
4fe2af09 | 338 | by spaces. |
339 | If specified, login is allowed only for users whose primary | |
c6a69271 | 340 | group or supplementary group list matches one of the patterns. |
bf740959 | 341 | .Ql \&* |
342 | and | |
343 | .Ql ? | |
344 | can be used as | |
4fe2af09 | 345 | wildcards in the patterns. |
91789042 | 346 | Only group names are valid; a numerical group ID is not recognized. |
dfafef8f | 347 | By default, login is allowed for all groups. |
bf740959 | 348 | .Pp |
33de75a3 | 349 | .It Cm AllowTcpForwarding |
350 | Specifies whether TCP forwarding is permitted. | |
351 | The default is | |
352 | .Dq yes . | |
353 | Note that disabling TCP forwarding does not improve security unless | |
354 | users are also denied shell access, as they can always install their | |
355 | own forwarders. | |
356 | .Pp | |
bf740959 | 357 | .It Cm AllowUsers |
dfafef8f | 358 | This keyword can be followed by a list of user name patterns, separated |
4fe2af09 | 359 | by spaces. |
360 | If specified, login is allowed only for users names that | |
bf740959 | 361 | match one of the patterns. |
362 | .Ql \&* | |
363 | and | |
364 | .Ql ? | |
365 | can be used as | |
4fe2af09 | 366 | wildcards in the patterns. |
91789042 | 367 | Only user names are valid; a numerical user ID is not recognized. |
dfafef8f | 368 | By default, login is allowed for all users. |
80f8f24f | 369 | If the pattern takes the form USER@HOST then USER and HOST |
91789042 | 370 | are separately checked, restricting logins to particular |
80f8f24f | 371 | users from particular hosts. |
bf740959 | 372 | .Pp |
c8445989 | 373 | .It Cm AuthorizedKeysFile |
7f19f8bb | 374 | Specifies the file that contains the public keys that can be used |
375 | for user authentication. | |
c8445989 | 376 | .Cm AuthorizedKeysFile |
377 | may contain tokens of the form %T which are substituted during connection | |
ae897d7c | 378 | set-up. The following tokens are defined: %% is replaced by a literal '%', |
c8445989 | 379 | %h is replaced by the home directory of the user being authenticated and |
380 | %u is replaced by the username of that user. | |
381 | After expansion, | |
382 | .Cm AuthorizedKeysFile | |
f740438e | 383 | is taken to be an absolute path or one relative to the user's home |
c8445989 | 384 | directory. |
385 | The default is | |
cb362b5e | 386 | .Dq .ssh/authorized_keys . |
eea39c02 | 387 | .It Cm Banner |
388 | In some jurisdictions, sending a warning message before authentication | |
389 | may be relevant for getting legal protection. | |
390 | The contents of the specified file are sent to the remote user before | |
391 | authentication is allowed. | |
392 | This option is only available for protocol version 2. | |
fffbaee2 | 393 | By default, no banner is displayed. |
eea39c02 | 394 | .Pp |
657297ff | 395 | .It Cm ChallengeResponseAuthentication |
248bad82 | 396 | Specifies whether challenge response authentication is allowed. |
397 | All authentication styles from | |
398 | .Xr login.conf 5 | |
399 | are supported. | |
657297ff | 400 | The default is |
401 | .Dq yes . | |
a18395da | 402 | .It Cm Ciphers |
403 | Specifies the ciphers allowed for protocol version 2. | |
404 | Multiple ciphers must be comma-separated. | |
405 | The default is | |
ba218fbe | 406 | .Pp |
407 | .Bd -literal | |
408 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
409 | aes192-cbc,aes256-cbc'' | |
410 | .Ed | |
3ffc6336 | 411 | .It Cm ClientAliveInterval |
412 | Sets a timeout interval in seconds after which if no data has been received | |
3730bb22 | 413 | from the client, |
3ffc6336 | 414 | .Nm |
415 | will send a message through the encrypted | |
84fc17bf | 416 | channel to request a response from the client. |
417 | The default | |
3ffc6336 | 418 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 419 | This option applies to protocol version 2 only. |
3ffc6336 | 420 | .It Cm ClientAliveCountMax |
421 | Sets the number of client alive messages (see above) which may be | |
422 | sent without | |
423 | .Nm | |
424 | receiving any messages back from the client. If this threshold is | |
3730bb22 | 425 | reached while client alive messages are being sent, |
3ffc6336 | 426 | .Nm |
427 | will disconnect the client, terminating the session. It is important | |
3730bb22 | 428 | to note that the use of client alive messages is very different from |
713d61f7 | 429 | .Cm KeepAlive |
84fc17bf | 430 | (below). The client alive messages are sent through the |
3ffc6336 | 431 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 432 | option enabled by |
713d61f7 | 433 | .Cm KeepAlive |
91789042 | 434 | is spoofable. The client alive mechanism is valuable when the client or |
435 | server depend on knowing when a connection has become inactive. | |
84fc17bf | 436 | .Pp |
91789042 | 437 | The default value is 3. If |
84fc17bf | 438 | .Cm ClientAliveInterval |
91789042 | 439 | (above) is set to 15, and |
fc7eca52 | 440 | .Cm ClientAliveCountMax |
441 | is left at the default, unresponsive ssh clients | |
3730bb22 | 442 | will be disconnected after approximately 45 seconds. |
bf740959 | 443 | .It Cm DenyGroups |
dfafef8f | 444 | This keyword can be followed by a list of group name patterns, separated |
4fe2af09 | 445 | by spaces. |
dfafef8f | 446 | Login is disallowed for users whose primary group or supplementary |
447 | group list matches one of the patterns. | |
bf740959 | 448 | .Ql \&* |
449 | and | |
450 | .Ql ? | |
451 | can be used as | |
4fe2af09 | 452 | wildcards in the patterns. |
91789042 | 453 | Only group names are valid; a numerical group ID is not recognized. |
dfafef8f | 454 | By default, login is allowed for all groups. |
bf740959 | 455 | .Pp |
456 | .It Cm DenyUsers | |
dfafef8f | 457 | This keyword can be followed by a list of user name patterns, separated |
4fe2af09 | 458 | by spaces. |
459 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 460 | .Ql \&* |
461 | and | |
462 | .Ql ? | |
4fe2af09 | 463 | can be used as wildcards in the patterns. |
91789042 | 464 | Only user names are valid; a numerical user ID is not recognized. |
dfafef8f | 465 | By default, login is allowed for all users. |
ebb1bf1a | 466 | If the pattern takes the form USER@HOST then USER and HOST |
467 | are separately checked, restricting logins to particular | |
468 | users from particular hosts. | |
1d1ffb87 | 469 | .It Cm GatewayPorts |
470 | Specifies whether remote hosts are allowed to connect to ports | |
471 | forwarded for the client. | |
70068acc | 472 | By default, |
473 | .Nm | |
474 | binds remote port forwardings to the loopback addresss. This | |
475 | prevents other remote hosts from connecting to forwarded ports. | |
476 | .Cm GatewayPorts | |
477 | can be used to specify that | |
478 | .Nm | |
479 | should bind remote port forwardings to the wildcard address, | |
480 | thus allowing remote hosts to connect to forwarded ports. | |
1d1ffb87 | 481 | The argument must be |
482 | .Dq yes | |
483 | or | |
484 | .Dq no . | |
485 | The default is | |
486 | .Dq no . | |
da89cf4d | 487 | .It Cm HostbasedAuthentication |
488 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
489 | with successful public key client host authentication is allowed | |
490 | (hostbased authentication). | |
491 | This option is similar to | |
492 | .Cm RhostsRSAAuthentication | |
493 | and applies to protocol version 2 only. | |
494 | The default is | |
495 | .Dq no . | |
bf740959 | 496 | .It Cm HostKey |
0f84fe37 | 497 | Specifies a file containing a private host key |
498 | used by SSH. | |
499 | The default is | |
2a8a6488 | 500 | .Pa /etc/ssh/ssh_host_key |
0f84fe37 | 501 | for protocol version 1, and |
2a8a6488 | 502 | .Pa /etc/ssh/ssh_host_rsa_key |
0f84fe37 | 503 | and |
2a8a6488 | 504 | .Pa /etc/ssh/ssh_host_dsa_key |
0f84fe37 | 505 | for protocol version 2. |
bf740959 | 506 | Note that |
507 | .Nm | |
42f11eb2 | 508 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 509 | It is possible to have multiple host key files. |
510 | .Dq rsa1 | |
511 | keys are used for version 1 and | |
512 | .Dq dsa | |
513 | or | |
514 | .Dq rsa | |
515 | are used for version 2 of the SSH protocol. | |
bf740959 | 516 | .It Cm IgnoreRhosts |
c8d54615 | 517 | Specifies that |
518 | .Pa .rhosts | |
f54651ce | 519 | and |
c8d54615 | 520 | .Pa .shosts |
da89cf4d | 521 | files will not be used in |
522 | .Cm RhostsAuthentication , | |
523 | .Cm RhostsRSAAuthentication | |
524 | or | |
525 | .Cm HostbasedAuthentication . | |
526 | .Pp | |
bf740959 | 527 | .Pa /etc/hosts.equiv |
528 | and | |
f54651ce | 529 | .Pa /etc/shosts.equiv |
4fe2af09 | 530 | are still used. |
f54651ce | 531 | The default is |
c8d54615 | 532 | .Dq yes . |
b4748e2f | 533 | .It Cm IgnoreUserKnownHosts |
534 | Specifies whether | |
535 | .Nm | |
536 | should ignore the user's | |
537 | .Pa $HOME/.ssh/known_hosts | |
538 | during | |
da89cf4d | 539 | .Cm RhostsRSAAuthentication |
540 | or | |
541 | .Cm HostbasedAuthentication . | |
b4748e2f | 542 | The default is |
543 | .Dq no . | |
bf740959 | 544 | .It Cm KeepAlive |
3b5a1b05 | 545 | Specifies whether the system should send TCP keepalive messages to the |
4fe2af09 | 546 | other side. |
547 | If they are sent, death of the connection or crash of one | |
548 | of the machines will be properly noticed. | |
549 | However, this means that | |
bf740959 | 550 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 551 | find it annoying. |
d0c832f3 | 552 | On the other hand, if keepalives are not sent, |
bf740959 | 553 | sessions may hang indefinitely on the server, leaving |
554 | .Dq ghost | |
555 | users and consuming server resources. | |
556 | .Pp | |
557 | The default is | |
558 | .Dq yes | |
559 | (to send keepalives), and the server will notice | |
3b5a1b05 | 560 | if the network goes down or the client host crashes. |
4fe2af09 | 561 | This avoids infinitely hanging sessions. |
bf740959 | 562 | .Pp |
563 | To disable keepalives, the value should be set to | |
3b5a1b05 | 564 | .Dq no . |
bf740959 | 565 | .It Cm KerberosAuthentication |
4fe2af09 | 566 | Specifies whether Kerberos authentication is allowed. |
567 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 568 | .Cm PasswordAuthentication |
569 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 570 | the Kerberos KDC. |
571 | To use this option, the server needs a | |
c345cf9d | 572 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 573 | Default is |
eadc806d | 574 | .Dq no . |
bf740959 | 575 | .It Cm KerberosOrLocalPasswd |
576 | If set then if password authentication through Kerberos fails then | |
577 | the password will be validated via any additional local mechanism | |
578 | such as | |
188adeb2 | 579 | .Pa /etc/passwd . |
4fe2af09 | 580 | Default is |
bf740959 | 581 | .Dq yes . |
582 | .It Cm KerberosTgtPassing | |
583 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 584 | Default is |
bf740959 | 585 | .Dq no , |
586 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
587 | .It Cm KerberosTicketCleanup | |
588 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 589 | file on logout. |
590 | Default is | |
bf740959 | 591 | .Dq yes . |
592 | .It Cm KeyRegenerationInterval | |
da89cf4d | 593 | In protocol version 1, the ephemeral server key is automatically regenerated |
594 | after this many seconds (if it has been used). | |
4fe2af09 | 595 | The purpose of regeneration is to prevent |
bf740959 | 596 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 597 | stealing the keys. |
598 | The key is never stored anywhere. | |
599 | If the value is 0, the key is never regenerated. | |
600 | The default is 3600 (seconds). | |
bf740959 | 601 | .It Cm ListenAddress |
6e9944b8 | 602 | Specifies the local addresses |
da89cf4d | 603 | .Nm |
bf740959 | 604 | should listen on. |
6e9944b8 | 605 | The following forms may be used: |
606 | .Pp | |
607 | .Bl -item -offset indent -compact | |
608 | .It | |
609 | .Cm ListenAddress | |
b8a297f1 | 610 | .Sm off |
611 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
612 | .Sm on | |
6e9944b8 | 613 | .It |
614 | .Cm ListenAddress | |
b8a297f1 | 615 | .Sm off |
616 | .Ar host No | Ar IPv4_addr No : Ar port | |
617 | .Sm on | |
6e9944b8 | 618 | .It |
619 | .Cm ListenAddress | |
b8a297f1 | 620 | .Sm off |
621 | .Oo | |
622 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
623 | .Sm on | |
6e9944b8 | 624 | .El |
625 | .Pp | |
626 | If | |
b8a297f1 | 627 | .Ar port |
6e9944b8 | 628 | is not specified, |
da89cf4d | 629 | .Nm |
6e9944b8 | 630 | will listen on the address and all prior |
631 | .Cm Port | |
632 | options specified. The default is to listen on all local | |
633 | addresses. Multiple | |
634 | .Cm ListenAddress | |
635 | options are permitted. Additionally, any | |
636 | .Cm Port | |
637 | options must precede this option for non port qualified addresses. | |
bf740959 | 638 | .It Cm LoginGraceTime |
639 | The server disconnects after this time if the user has not | |
4fe2af09 | 640 | successfully logged in. |
641 | If the value is 0, there is no time limit. | |
bf740959 | 642 | The default is 600 (seconds). |
6a17f9c2 | 643 | .It Cm LogLevel |
644 | Gives the verbosity level that is used when logging messages from | |
645 | .Nm sshd . | |
646 | The possible values are: | |
60015649 | 647 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. |
648 | The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 | |
649 | and DEBUG3 each specify higher levels of debugging output. | |
650 | Logging with a DEBUG level violates the privacy of users | |
6a17f9c2 | 651 | and is not recommended. |
b2552997 | 652 | .It Cm MACs |
653 | Specifies the available MAC (message authentication code) algorithms. | |
654 | The MAC algorithm is used in protocol version 2 | |
655 | for data integrity protection. | |
656 | Multiple algorithms must be comma-separated. | |
657 | The default is | |
47bf6266 | 658 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
089fbbd2 | 659 | .It Cm MaxStartups |
660 | Specifies the maximum number of concurrent unauthenticated connections to the | |
661 | .Nm | |
662 | daemon. | |
663 | Additional connections will be dropped until authentication succeeds or the | |
664 | .Cm LoginGraceTime | |
665 | expires for a connection. | |
666 | The default is 10. | |
c345cf9d | 667 | .Pp |
668 | Alternatively, random early drop can be enabled by specifying | |
669 | the three colon separated values | |
670 | .Dq start:rate:full | |
94ec8c6b | 671 | (e.g., "10:30:60"). |
c345cf9d | 672 | .Nm |
6f37606e | 673 | will refuse connection attempts with a probability of |
c345cf9d | 674 | .Dq rate/100 |
675 | (30%) | |
676 | if there are currently | |
677 | .Dq start | |
678 | (10) | |
679 | unauthenticated connections. | |
6f37606e | 680 | The probability increases linearly and all connection attempts |
c345cf9d | 681 | are refused if the number of unauthenticated connections reaches |
682 | .Dq full | |
683 | (60). | |
10f72868 | 684 | .It Cm PAMAuthenticationViaKbdInt |
685 | Specifies whether PAM challenge response authentication is allowed. This | |
686 | allows the use of most PAM challenge response authentication modules, but | |
687 | it will allow password authentication regardless of whether | |
688 | .Cm PasswordAuthentication | |
689 | is disabled. | |
690 | The default is | |
691 | .Dq no . | |
bf740959 | 692 | .It Cm PasswordAuthentication |
693 | Specifies whether password authentication is allowed. | |
694 | The default is | |
695 | .Dq yes . | |
696 | .It Cm PermitEmptyPasswords | |
697 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 698 | server allows login to accounts with empty password strings. |
699 | The default is | |
c8d54615 | 700 | .Dq no . |
bf740959 | 701 | .It Cm PermitRootLogin |
46f23b8d | 702 | Specifies whether root can login using |
bf740959 | 703 | .Xr ssh 1 . |
704 | The argument must be | |
705 | .Dq yes , | |
15853e93 | 706 | .Dq without-password , |
707 | .Dq forced-commands-only | |
bf740959 | 708 | or |
709 | .Dq no . | |
710 | The default is | |
711 | .Dq yes . | |
15853e93 | 712 | .Pp |
713 | If this option is set to | |
bf740959 | 714 | .Dq without-password |
15853e93 | 715 | password authentication is disabled for root. |
bf740959 | 716 | .Pp |
15853e93 | 717 | If this option is set to |
718 | .Dq forced-commands-only | |
719 | root login with public key authentication will be allowed, | |
720 | but only if the | |
bf740959 | 721 | .Ar command |
15853e93 | 722 | option has been specified |
bf740959 | 723 | (which may be useful for taking remote backups even if root login is |
15853e93 | 724 | normally not allowed). All other authentication methods are disabled |
725 | for root. | |
46f23b8d | 726 | .Pp |
727 | If this option is set to | |
728 | .Dq no | |
729 | root is not allowed to login. | |
0fbe8c74 | 730 | .It Cm PidFile |
731 | Specifies the file that contains the process identifier of the | |
732 | .Nm | |
733 | daemon. | |
734 | The default is | |
735 | .Pa /var/run/sshd.pid . | |
bf740959 | 736 | .It Cm Port |
737 | Specifies the port number that | |
738 | .Nm | |
4fe2af09 | 739 | listens on. |
740 | The default is 22. | |
48e671d5 | 741 | Multiple options of this type are permitted. |
da89cf4d | 742 | See also |
743 | .Cm ListenAddress . | |
4f4648f9 | 744 | .It Cm PrintLastLog |
745 | Specifies whether | |
746 | .Nm | |
747 | should print the date and time when the user last logged in. | |
748 | The default is | |
749 | .Dq yes . | |
bf740959 | 750 | .It Cm PrintMotd |
751 | Specifies whether | |
752 | .Nm | |
f54651ce | 753 | should print |
bf740959 | 754 | .Pa /etc/motd |
4fe2af09 | 755 | when a user logs in interactively. |
756 | (On some systems it is also printed by the shell, | |
bf740959 | 757 | .Pa /etc/profile , |
4fe2af09 | 758 | or equivalent.) |
759 | The default is | |
bf740959 | 760 | .Dq yes . |
f54651ce | 761 | .It Cm Protocol |
762 | Specifies the protocol versions | |
763 | .Nm | |
764 | should support. | |
765 | The possible values are | |
766 | .Dq 1 | |
767 | and | |
768 | .Dq 2 . | |
769 | Multiple versions must be comma-separated. | |
770 | The default is | |
6023325e | 771 | .Dq 2,1 . |
657297ff | 772 | .It Cm PubkeyAuthentication |
773 | Specifies whether public key authentication is allowed. | |
774 | The default is | |
775 | .Dq yes . | |
776 | Note that this option applies to protocol version 2 only. | |
bf740959 | 777 | .It Cm RhostsAuthentication |
778 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 779 | files is sufficient. |
780 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 781 | .Cm RhostsRSAAuthentication |
782 | should be used | |
783 | instead, because it performs RSA-based host authentication in addition | |
784 | to normal rhosts or /etc/hosts.equiv authentication. | |
785 | The default is | |
786 | .Dq no . | |
da89cf4d | 787 | This option applies to protocol version 1 only. |
bf740959 | 788 | .It Cm RhostsRSAAuthentication |
789 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 790 | with successful RSA host authentication is allowed. |
791 | The default is | |
c8d54615 | 792 | .Dq no . |
da89cf4d | 793 | This option applies to protocol version 1 only. |
bf740959 | 794 | .It Cm RSAAuthentication |
4fe2af09 | 795 | Specifies whether pure RSA authentication is allowed. |
796 | The default is | |
bf740959 | 797 | .Dq yes . |
da89cf4d | 798 | This option applies to protocol version 1 only. |
bf740959 | 799 | .It Cm ServerKeyBits |
da89cf4d | 800 | Defines the number of bits in the ephemeral protocol version 1 server key. |
4fe2af09 | 801 | The minimum value is 512, and the default is 768. |
bf740959 | 802 | .It Cm StrictModes |
803 | Specifies whether | |
804 | .Nm | |
805 | should check file modes and ownership of the | |
4fe2af09 | 806 | user's files and home directory before accepting login. |
807 | This is normally desirable because novices sometimes accidentally leave their | |
808 | directory or files world-writable. | |
809 | The default is | |
bf740959 | 810 | .Dq yes . |
38c295d6 | 811 | .It Cm Subsystem |
94ec8c6b | 812 | Configures an external subsystem (e.g., file transfer daemon). |
813 | Arguments should be a subsystem name and a command to execute upon subsystem | |
814 | request. | |
b5e300c2 | 815 | The command |
816 | .Xr sftp-server 8 | |
817 | implements the | |
818 | .Dq sftp | |
819 | file transfer subsystem. | |
38c295d6 | 820 | By default no subsystems are defined. |
821 | Note that this option applies to protocol version 2 only. | |
bf740959 | 822 | .It Cm SyslogFacility |
823 | Gives the facility code that is used when logging messages from | |
824 | .Nm sshd . | |
825 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 826 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
827 | The default is AUTH. | |
bf740959 | 828 | .It Cm UseLogin |
829 | Specifies whether | |
830 | .Xr login 1 | |
fa649821 | 831 | is used for interactive login sessions. |
ff027d84 | 832 | The default is |
833 | .Dq no . | |
fa649821 | 834 | Note that |
835 | .Xr login 1 | |
c345cf9d | 836 | is never used for remote command execution. |
3730bb22 | 837 | Note also, that if this is enabled, |
838 | .Cm X11Forwarding | |
ff027d84 | 839 | will be disabled because |
840 | .Xr login 1 | |
841 | does not know how to handle | |
3730bb22 | 842 | .Xr xauth 1 |
70aa9ff4 | 843 | cookies. If |
844 | .Cm UsePrivilegeSeparation | |
845 | is specified, it will be disabled after authentication. | |
846 | .It Cm UsePrivilegeSeparation | |
847 | Specifies whether | |
848 | .Nm | |
849 | separated privileges by creating an unprivileged child process | |
850 | to deal with incoming network traffic. After successful authentication, | |
851 | another process will be created that has the privilege of the authenticated | |
852 | user. The goal of privilege separation is to prevent privilege | |
853 | escalation by containing any corruption within the unprivileged processes. | |
854 | The default is | |
1642b888 | 855 | .Dq yes . |
bf4c5edc | 856 | .It Cm VerifyReverseMapping |
857 | Specifies whether | |
858 | .Nm | |
859 | should try to verify the remote host name and check that | |
860 | the resolved host name for the remote IP address maps back to the | |
861 | very same IP address. | |
862 | The default is | |
863 | .Dq no . | |
bf740959 | 864 | .It Cm X11DisplayOffset |
865 | Specifies the first display number available for | |
866 | .Nm sshd Ns 's | |
4fe2af09 | 867 | X11 forwarding. |
868 | This prevents | |
bf740959 | 869 | .Nm |
870 | from interfering with real X11 servers. | |
c8d54615 | 871 | The default is 10. |
7b2ea3a1 | 872 | .It Cm X11Forwarding |
4fe2af09 | 873 | Specifies whether X11 forwarding is permitted. |
874 | The default is | |
c8d54615 | 875 | .Dq no . |
7b2ea3a1 | 876 | Note that disabling X11 forwarding does not improve security in any |
877 | way, as users can always install their own forwarders. | |
3730bb22 | 878 | X11 forwarding is automatically disabled if |
879 | .Cm UseLogin | |
880 | is enabled. | |
e6e573bd | 881 | .It Cm X11UseLocalhost |
882 | Specifies whether | |
883 | .Nm | |
884 | should bind the X11 forwarding server to the loopback address or to | |
885 | the wildcard address. By default, | |
886 | .Nm | |
887 | binds the forwarding server to the loopback address and sets the | |
888 | hostname part of the | |
889 | .Ev DISPLAY | |
890 | environment variable to | |
891 | .Dq localhost . | |
892 | This prevents remote hosts from connecting to the fake display. | |
893 | However, some older X11 clients may not function with this | |
894 | configuration. | |
895 | .Cm X11UseLocalhost | |
896 | may be set to | |
897 | .Dq no | |
898 | to specify that the forwarding server should be bound to the wildcard | |
899 | address. | |
900 | The argument must be | |
901 | .Dq yes | |
902 | or | |
903 | .Dq no . | |
904 | The default is | |
905 | .Dq yes . | |
fa649821 | 906 | .It Cm XAuthLocation |
907 | Specifies the location of the | |
908 | .Xr xauth 1 | |
909 | program. | |
910 | The default is | |
911 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 912 | .El |
e2b1fb42 | 913 | .Ss Time Formats |
914 | .Pp | |
915 | .Nm | |
916 | command-line arguments and configuration file options that specify time | |
917 | may be expressed using a sequence of the form: | |
918 | .Sm off | |
919 | .Ar time Oo Ar qualifier Oc , | |
920 | .Sm on | |
921 | where | |
922 | .Ar time | |
923 | is a positive integer value and | |
924 | .Ar qualifier | |
925 | is one of the following: | |
926 | .Pp | |
927 | .Bl -tag -width Ds -compact -offset indent | |
928 | .It Cm <none> | |
929 | seconds | |
930 | .It Cm s | Cm S | |
931 | seconds | |
932 | .It Cm m | Cm M | |
933 | minutes | |
934 | .It Cm h | Cm H | |
935 | hours | |
936 | .It Cm d | Cm D | |
937 | days | |
938 | .It Cm w | Cm W | |
939 | weeks | |
940 | .El | |
941 | .Pp | |
942 | Each member of the sequence is added together to calculate | |
943 | the total time value. | |
944 | .Pp | |
945 | Time format examples: | |
946 | .Pp | |
947 | .Bl -tag -width Ds -compact -offset indent | |
948 | .It 600 | |
949 | 600 seconds (10 minutes) | |
950 | .It 10m | |
951 | 10 minutes | |
952 | .It 1h30m | |
953 | 1 hour 30 minutes (90 minutes) | |
954 | .El | |
bf740959 | 955 | .Sh LOGIN PROCESS |
956 | When a user successfully logs in, | |
957 | .Nm | |
958 | does the following: | |
959 | .Bl -enum -offset indent | |
960 | .It | |
961 | If the login is on a tty, and no command has been specified, | |
f54651ce | 962 | prints last login time and |
bf740959 | 963 | .Pa /etc/motd |
964 | (unless prevented in the configuration file or by | |
965 | .Pa $HOME/.hushlogin ; | |
966 | see the | |
f54651ce | 967 | .Sx FILES |
bf740959 | 968 | section). |
969 | .It | |
970 | If the login is on a tty, records login time. | |
971 | .It | |
972 | Checks | |
973 | .Pa /etc/nologin ; | |
974 | if it exists, prints contents and quits | |
975 | (unless root). | |
976 | .It | |
977 | Changes to run with normal user privileges. | |
978 | .It | |
979 | Sets up basic environment. | |
980 | .It | |
981 | Reads | |
982 | .Pa $HOME/.ssh/environment | |
983 | if it exists. | |
984 | .It | |
985 | Changes to user's home directory. | |
986 | .It | |
987 | If | |
988 | .Pa $HOME/.ssh/rc | |
989 | exists, runs it; else if | |
af98ced9 | 990 | .Pa /etc/ssh/sshrc |
bf740959 | 991 | exists, runs |
4fe2af09 | 992 | it; otherwise runs xauth. |
993 | The | |
bf740959 | 994 | .Dq rc |
995 | files are given the X11 | |
996 | authentication protocol and cookie in standard input. | |
997 | .It | |
998 | Runs user's shell or command. | |
999 | .El | |
1000 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 1001 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 1002 | is the default file that lists the public keys that are |
1003 | permitted for RSA authentication in protocol version 1 | |
1004 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 1005 | in protocol version 2. |
96a7b0cc | 1006 | .Cm AuthorizedKeysFile |
c8445989 | 1007 | may be used to specify an alternative file. |
8abcdba4 | 1008 | .Pp |
4fe2af09 | 1009 | Each line of the file contains one |
bf740959 | 1010 | key (empty lines and lines starting with a |
1011 | .Ql # | |
1012 | are ignored as | |
4fe2af09 | 1013 | comments). |
8abcdba4 | 1014 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 1015 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 1016 | Each protocol version 2 public key consists of: |
1017 | options, keytype, base64 encoded key, comment. | |
1018 | The options fields | |
1019 | are optional; its presence is determined by whether the line starts | |
bf740959 | 1020 | with a number or not (the option field never starts with a number). |
8abcdba4 | 1021 | The bits, exponent, modulus and comment fields give the RSA key for |
1022 | protocol version 1; the | |
bf740959 | 1023 | comment field is not used for anything (but may be convenient for the |
1024 | user to identify the key). | |
8abcdba4 | 1025 | For protocol version 2 the keytype is |
1026 | .Dq ssh-dss | |
1027 | or | |
1028 | .Dq ssh-rsa . | |
bf740959 | 1029 | .Pp |
1030 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 1031 | (because of the size of the RSA key modulus). |
1032 | You don't want to type them in; instead, copy the | |
c0ecc314 | 1033 | .Pa identity.pub , |
8abcdba4 | 1034 | .Pa id_dsa.pub |
c0ecc314 | 1035 | or the |
1036 | .Pa id_rsa.pub | |
bf740959 | 1037 | file and edit it. |
1038 | .Pp | |
3bc822df | 1039 | .Nm |
1040 | enforces a minimum RSA key modulus size for protocol 1 | |
1041 | and protocol 2 keys of 768 bits. | |
1042 | .Pp | |
c345cf9d | 1043 | The options (if present) consist of comma-separated option |
4fe2af09 | 1044 | specifications. |
1045 | No spaces are permitted, except within double quotes. | |
54bf768d | 1046 | The following option specifications are supported (note |
1047 | that option keywords are case-insensitive): | |
bf740959 | 1048 | .Bl -tag -width Ds |
1049 | .It Cm from="pattern-list" | |
1050 | Specifies that in addition to RSA authentication, the canonical name | |
1051 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 1052 | patterns |
1053 | .Pf ( Ql * | |
1054 | and | |
1055 | .Ql ? | |
1056 | serve as wildcards). | |
1057 | The list may also contain | |
1058 | patterns negated by prefixing them with | |
1059 | .Ql ! ; | |
1060 | if the canonical host name matches a negated pattern, the key is not accepted. | |
1061 | The purpose | |
bf740959 | 1062 | of this option is to optionally increase security: RSA authentication |
1063 | by itself does not trust the network or name servers or anything (but | |
1064 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 1065 | permits an intruder to log in from anywhere in the world. |
1066 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 1067 | servers and/or routers would have to be compromised in addition to |
1068 | just the key). | |
1069 | .It Cm command="command" | |
1070 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 1071 | authentication. |
1072 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 1073 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 1074 | otherwise it is run without a tty. |
91789042 | 1075 | If a 8-bit clean channel is required, |
1076 | one must not request a pty or should specify | |
61e96248 | 1077 | .Cm no-pty . |
4fe2af09 | 1078 | A quote may be included in the command by quoting it with a backslash. |
1079 | This option might be useful | |
1080 | to restrict certain RSA keys to perform just a specific operation. | |
1081 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 1082 | Note that the client may specify TCP/IP and/or X11 |
1083 | forwarding unless they are explicitly prohibited. | |
4cdbc654 | 1084 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 1085 | .It Cm environment="NAME=value" |
1086 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 1087 | logging in using this key. |
1088 | Environment variables set this way | |
1089 | override other default environment values. | |
1090 | Multiple options of this type are permitted. | |
2548961d | 1091 | This option is automatically disabled if |
1092 | .Cm UseLogin | |
1093 | is enabled. | |
bf740959 | 1094 | .It Cm no-port-forwarding |
1095 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 1096 | Any port forward requests by the client will return an error. |
1097 | This might be used, e.g., in connection with the | |
bf740959 | 1098 | .Cm command |
1099 | option. | |
1100 | .It Cm no-X11-forwarding | |
1101 | Forbids X11 forwarding when this key is used for authentication. | |
1102 | Any X11 forward requests by the client will return an error. | |
1103 | .It Cm no-agent-forwarding | |
1104 | Forbids authentication agent forwarding when this key is used for | |
1105 | authentication. | |
1106 | .It Cm no-pty | |
1107 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 1108 | .It Cm permitopen="host:port" |
3730bb22 | 1109 | Limit local |
dc504afd | 1110 | .Li ``ssh -L'' |
b2ae83b8 | 1111 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 1112 | port. |
1113 | IPv6 addresses can be specified with an alternative syntax: | |
1114 | .Ar host/port . | |
1115 | Multiple | |
dc504afd | 1116 | .Cm permitopen |
3730bb22 | 1117 | options may be applied separated by commas. No pattern matching is |
1118 | performed on the specified hostnames, they must be literal domains or | |
dc504afd | 1119 | addresses. |
bf740959 | 1120 | .El |
1121 | .Ss Examples | |
1122 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
1123 | .Pp | |
1124 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
1125 | .Pp | |
1126 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 1127 | .Pp |
1128 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 1129 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 1130 | The |
2a8a6488 | 1131 | .Pa /etc/ssh/ssh_known_hosts , |
f54651ce | 1132 | and |
f49bc4f7 | 1133 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 1134 | files contain host public keys for all known hosts. |
1135 | The global file should | |
1136 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 1137 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 1138 | its key is added to the per-user file. |
bf740959 | 1139 | .Pp |
1140 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 1141 | bits, exponent, modulus, comment. |
1142 | The fields are separated by spaces. | |
bf740959 | 1143 | .Pp |
1144 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
1145 | wildcards); each pattern in turn is matched against the canonical host | |
1146 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 1147 | name (when authenticating a server). |
1148 | A pattern may also be preceded by | |
bf740959 | 1149 | .Ql ! |
1150 | to indicate negation: if the host name matches a negated | |
1151 | pattern, it is not accepted (by that line) even if it matched another | |
1152 | pattern on the line. | |
1153 | .Pp | |
1d1ffb87 | 1154 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 1155 | can be obtained, e.g., from |
2a8a6488 | 1156 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 1157 | The optional comment field continues to the end of the line, and is not used. |
1158 | .Pp | |
1159 | Lines starting with | |
1160 | .Ql # | |
1161 | and empty lines are ignored as comments. | |
1162 | .Pp | |
1163 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 1164 | matching line has the proper key. |
1165 | It is thus permissible (but not | |
bf740959 | 1166 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 1167 | names. |
1168 | This will inevitably happen when short forms of host names | |
1169 | from different domains are put in the file. | |
1170 | It is possible | |
bf740959 | 1171 | that the files contain conflicting information; authentication is |
1172 | accepted if valid information can be found from either file. | |
1173 | .Pp | |
1174 | Note that the lines in these files are typically hundreds of characters | |
1175 | long, and you definitely don't want to type in the host keys by hand. | |
1176 | Rather, generate them by a script | |
f54651ce | 1177 | or by taking |
2a8a6488 | 1178 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 1179 | and adding the host names at the front. |
1180 | .Ss Examples | |
da89cf4d | 1181 | .Bd -literal |
1182 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
1183 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
1184 | .Ed | |
bf740959 | 1185 | .Sh FILES |
1186 | .Bl -tag -width Ds | |
2a8a6488 | 1187 | .It Pa /etc/ssh/sshd_config |
bf740959 | 1188 | Contains configuration data for |
1189 | .Nm sshd . | |
1190 | This file should be writable by root only, but it is recommended | |
1191 | (though not necessary) that it be world-readable. | |
2a8a6488 | 1192 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
da89cf4d | 1193 | These three files contain the private parts of the host keys. |
b8dc87d3 | 1194 | These files should only be owned by root, readable only by root, and not |
bf740959 | 1195 | accessible to others. |
1196 | Note that | |
1197 | .Nm | |
1198 | does not start if this file is group/world-accessible. | |
2a8a6488 | 1199 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub |
da89cf4d | 1200 | These three files contain the public parts of the host keys. |
b8dc87d3 | 1201 | These files should be world-readable but writable only by |
4fe2af09 | 1202 | root. |
b8dc87d3 | 1203 | Their contents should match the respective private parts. |
1204 | These files are not | |
1205 | really used for anything; they are provided for the convenience of | |
1206 | the user so their contents can be copied to known hosts files. | |
1207 | These files are created using | |
bf740959 | 1208 | .Xr ssh-keygen 1 . |
e2432638 | 1209 | .It Pa /etc/moduli |
c523303b | 1210 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
5f4fdfae | 1211 | .It Pa /var/run/sshd.pid |
bf740959 | 1212 | Contains the process ID of the |
1213 | .Nm | |
1214 | listening for connections (if there are several daemons running | |
1215 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1216 | started last). |
c345cf9d | 1217 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1218 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 1219 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 1220 | This file must be readable by root (which may on some machines imply |
1221 | it being world-readable if the user's home directory resides on an NFS | |
1222 | volume). | |
1223 | It is recommended that it not be accessible by others. | |
1224 | The format of this file is described above. | |
1225 | Users will place the contents of their | |
96a7b0cc | 1226 | .Pa identity.pub , |
1d1ffb87 | 1227 | .Pa id_dsa.pub |
c0ecc314 | 1228 | and/or |
1229 | .Pa id_rsa.pub | |
1d1ffb87 | 1230 | files into this file, as described in |
1231 | .Xr ssh-keygen 1 . | |
2a8a6488 | 1232 | .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1233 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 1234 | authentication or protocol version 2 hostbased authentication |
1235 | to check the public key of the host. | |
4fe2af09 | 1236 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 1237 | The client uses the same files |
e91c60f2 | 1238 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1239 | These files should be writable only by root/the owner. |
2a8a6488 | 1240 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 1241 | should be world-readable, and |
1242 | .Pa $HOME/.ssh/known_hosts | |
1243 | can but need not be world-readable. | |
1244 | .It Pa /etc/nologin | |
f54651ce | 1245 | If this file exists, |
bf740959 | 1246 | .Nm |
4fe2af09 | 1247 | refuses to let anyone except root log in. |
1248 | The contents of the file | |
bf740959 | 1249 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1250 | refused. |
1251 | The file should be world-readable. | |
bf740959 | 1252 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
5b263aae | 1253 | Access controls that should be enforced by tcp-wrappers are defined here. |
1254 | Further details are described in | |
bf740959 | 1255 | .Xr hosts_access 5 . |
1256 | .It Pa $HOME/.rhosts | |
1257 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1258 | line. |
1259 | The given user on the corresponding host is permitted to log in | |
1260 | without password. | |
1261 | The same file is used by rlogind and rshd. | |
bf740959 | 1262 | The file must |
1263 | be writable only by the user; it is recommended that it not be | |
1264 | accessible by others. | |
1265 | .Pp | |
4fe2af09 | 1266 | If is also possible to use netgroups in the file. |
1267 | Either host or user | |
bf740959 | 1268 | name may be of the form +@groupname to specify all hosts or all users |
1269 | in the group. | |
1270 | .It Pa $HOME/.shosts | |
1271 | For ssh, | |
1272 | this file is exactly the same as for | |
1273 | .Pa .rhosts . | |
1274 | However, this file is | |
1275 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1276 | .It Pa /etc/hosts.equiv |
bf740959 | 1277 | This file is used during |
1278 | .Pa .rhosts | |
4fe2af09 | 1279 | authentication. |
1280 | In the simplest form, this file contains host names, one per line. | |
1281 | Users on | |
bf740959 | 1282 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1283 | have the same user name on both machines. |
1284 | The host name may also be | |
bf740959 | 1285 | followed by a user name; such users are permitted to log in as |
1286 | .Em any | |
4fe2af09 | 1287 | user on this machine (except root). |
1288 | Additionally, the syntax | |
bf740959 | 1289 | .Dq +@group |
4fe2af09 | 1290 | can be used to specify netgroups. |
1291 | Negated entries start with | |
bf740959 | 1292 | .Ql \&- . |
1293 | .Pp | |
1294 | If the client host/user is successfully matched in this file, login is | |
1295 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1296 | same. |
1297 | Additionally, successful RSA host authentication is normally required. | |
1298 | This file must be writable only by root; it is recommended | |
bf740959 | 1299 | that it be world-readable. |
1300 | .Pp | |
1301 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1302 | .Pa hosts.equiv . | |
1303 | Beware that it really means that the named user(s) can log in as | |
1304 | .Em anybody , | |
1305 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1306 | binaries and directories. |
1307 | Using a user name practically grants the user root access. | |
1308 | The only valid use for user names that I can think | |
bf740959 | 1309 | of is in negative entries. |
1310 | .Pp | |
1311 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1312 | .It Pa /etc/shosts.equiv |
bf740959 | 1313 | This is processed exactly as |
1314 | .Pa /etc/hosts.equiv . | |
1315 | However, this file may be useful in environments that want to run both | |
1316 | rsh/rlogin and ssh. | |
1317 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1318 | This file is read into the environment at login (if it exists). |
1319 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1320 | .Ql # ) , |
4fe2af09 | 1321 | and assignment lines of the form name=value. |
1322 | The file should be writable | |
bf740959 | 1323 | only by the user; it need not be readable by anyone else. |
1324 | .It Pa $HOME/.ssh/rc | |
1325 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1326 | environment files but before starting the user's shell or command. |
d4c6ddff | 1327 | It must not produce any output on stdout; stderr must be used |
1328 | instead. | |
1329 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
1330 | its standard input (and | |
bf740959 | 1331 | .Ev DISPLAY |
d4c6ddff | 1332 | in its environment). |
1333 | The script must call | |
bf740959 | 1334 | .Xr xauth 1 |
d4c6ddff | 1335 | because |
1336 | .Nm | |
1337 | will not run xauth automatically to add X11 cookies. | |
bf740959 | 1338 | .Pp |
1339 | The primary purpose of this file is to run any initialization routines | |
1340 | which may be needed before the user's home directory becomes | |
1341 | accessible; AFS is a particular example of such an environment. | |
1342 | .Pp | |
1343 | This file will probably contain some initialization code followed by | |
da89cf4d | 1344 | something similar to: |
1345 | .Bd -literal | |
d4c6ddff | 1346 | if read proto cookie && [ -n "$DISPLAY" ]; then |
1347 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
1348 | # X11UseLocalhost=yes | |
1349 | xauth add unix:`echo $DISPLAY | | |
1350 | cut -c11-` $proto $cookie | |
1351 | else | |
1352 | # X11UseLocalhost=no | |
1353 | xauth add $DISPLAY $proto $cookie | |
da89cf4d | 1354 | fi |
d4c6ddff | 1355 | fi |
da89cf4d | 1356 | .Ed |
bf740959 | 1357 | .Pp |
1358 | If this file does not exist, | |
2a8a6488 | 1359 | .Pa /etc/ssh/sshrc |
bf740959 | 1360 | is run, and if that |
d4c6ddff | 1361 | does not exist either, xauth is used to add the cookie. |
bf740959 | 1362 | .Pp |
1363 | This file should be writable only by the user, and need not be | |
1364 | readable by anyone else. | |
2a8a6488 | 1365 | .It Pa /etc/ssh/sshrc |
bf740959 | 1366 | Like |
1367 | .Pa $HOME/.ssh/rc . | |
1368 | This can be used to specify | |
4fe2af09 | 1369 | machine-specific login-time initializations globally. |
1370 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1371 | .El |
fa08c86b | 1372 | .Sh AUTHORS |
7f5c4295 | 1373 | OpenSSH is a derivative of the original and free |
1374 | ssh 1.2.12 release by Tatu Ylonen. | |
1375 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1376 | Theo de Raadt and Dug Song | |
1377 | removed many bugs, re-added newer features and | |
1378 | created OpenSSH. | |
1379 | Markus Friedl contributed the support for SSH | |
1380 | protocol versions 1.5 and 2.0. | |
ce19ff48 | 1381 | Niels Provos and Markus Friedl contributed support |
1382 | for privilege separation. | |
bf740959 | 1383 | .Sh SEE ALSO |
bf740959 | 1384 | .Xr scp 1 , |
61e96248 | 1385 | .Xr sftp 1 , |
bf740959 | 1386 | .Xr ssh 1 , |
1387 | .Xr ssh-add 1 , | |
1388 | .Xr ssh-agent 1 , | |
1389 | .Xr ssh-keygen 1 , | |
248bad82 | 1390 | .Xr login.conf 5 , |
1391 | .Xr moduli 5 , | |
a5a2da3b | 1392 | .Xr sftp-server 8 |
2cad6cef | 1393 | .Rs |
1394 | .%A T. Ylonen | |
1395 | .%A T. Kivinen | |
1396 | .%A M. Saarinen | |
1397 | .%A T. Rinne | |
1398 | .%A S. Lehtinen | |
1399 | .%T "SSH Protocol Architecture" | |
17f5e68a | 1400 | .%N draft-ietf-secsh-architecture-12.txt |
1401 | .%D January 2002 | |
2cad6cef | 1402 | .%O work in progress material |
1403 | .Re | |
da89cf4d | 1404 | .Rs |
1405 | .%A M. Friedl | |
1406 | .%A N. Provos | |
1407 | .%A W. A. Simpson | |
1408 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
17f5e68a | 1409 | .%N draft-ietf-secsh-dh-group-exchange-02.txt |
1410 | .%D January 2002 | |
da89cf4d | 1411 | .%O work in progress material |
1412 | .Re |