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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 | .\" |
84fc17bf | 37 | .\" $OpenBSD: sshd.8,v 1.116 2001/04/15 19:41:21 stevesk Exp $ |
bf740959 | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
2c86906e | 43 | .Nd OpenSSH ssh daemon |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm sshd | |
ff14faf1 | 46 | .Op Fl deiqD46 |
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 | |
52 | .Op Fl p Ar port | |
c345cf9d | 53 | .Op Fl u Ar len |
5260325f | 54 | .Op Fl V Ar client_protocol_id |
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 , | |
121 | .Xr rexecd 8 , | |
122 | and | |
123 | .Xr rexd 8 | |
124 | are disabled (thus completely disabling | |
125 | .Xr rlogin 1 | |
126 | and | |
127 | .Xr rsh 1 | |
3189621b | 128 | into the machine). |
bf740959 | 129 | .Pp |
1d1ffb87 | 130 | .Ss SSH protocol version 2 |
131 | .Pp | |
c345cf9d | 132 | Version 2 works similarly: |
1d1ffb87 | 133 | Each host has a host-specific DSA key used to identify the host. |
134 | However, when the daemon starts, it does not generate a server key. | |
135 | Forward security is provided through a Diffie-Hellman key agreement. | |
136 | This key agreement results in a shared session key. | |
51c251f0 | 137 | The rest of the session is encrypted using a symmetric cipher, currently |
138 | Blowfish, 3DES, CAST128, Arcfour, 128 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 | |
fa08c86b | 146 | user authentication method (PubkeyAuthentication) |
1d1ffb87 | 147 | and conventional password authentication. |
148 | .Pp | |
149 | .Ss Command execution and data forwarding | |
150 | .Pp | |
bf740959 | 151 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 152 | preparing the session is entered. |
153 | At this time the client may request | |
bf740959 | 154 | things like allocating a pseudo-tty, forwarding X11 connections, |
155 | forwarding TCP/IP connections, or forwarding the authentication agent | |
156 | connection over the secure channel. | |
157 | .Pp | |
158 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 159 | The sides then enter session mode. |
160 | In this mode, either side may send | |
bf740959 | 161 | data at any time, and such data is forwarded to/from the shell or |
162 | command on the server side, and the user terminal in the client side. | |
163 | .Pp | |
164 | When the user program terminates and all forwarded X11 and other | |
165 | connections have been closed, the server sends command exit status to | |
166 | the client, and both sides exit. | |
167 | .Pp | |
168 | .Nm | |
169 | can be configured using command-line options or a configuration | |
4fe2af09 | 170 | file. |
171 | Command-line options override values specified in the | |
bf740959 | 172 | configuration file. |
173 | .Pp | |
9d6b7add | 174 | .Nm |
175 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 176 | .Dv SIGHUP , |
177 | by executing itself with the name it was started as, ie. | |
178 | .Pa /usr/sbin/sshd . | |
9d6b7add | 179 | .Pp |
bf740959 | 180 | The options are as follows: |
181 | .Bl -tag -width Ds | |
182 | .It Fl b Ar bits | |
183 | Specifies the number of bits in the server key (default 768). | |
184 | .Pp | |
185 | .It Fl d | |
4fe2af09 | 186 | Debug mode. |
187 | The server sends verbose debug output to the system | |
188 | log, and does not put itself in the background. | |
189 | The server also will not fork and will only process one connection. | |
190 | This option is only intended for debugging for the server. | |
94ec8c6b | 191 | Multiple -d options increases the debugging level. |
192 | Maximum is 3. | |
bf740959 | 193 | .It Fl f Ar configuration_file |
4fe2af09 | 194 | Specifies the name of the configuration file. |
195 | The default is | |
5f4fdfae | 196 | .Pa /etc/sshd_config . |
bf740959 | 197 | .Nm |
198 | refuses to start if there is no configuration file. | |
199 | .It Fl g Ar login_grace_time | |
200 | Gives the grace time for clients to authenticate themselves (default | |
ee5e6612 | 201 | 600 seconds). |
4fe2af09 | 202 | If the client fails to authenticate the user within |
203 | this many seconds, the server disconnects and exits. | |
204 | A value of zero indicates no limit. | |
bf740959 | 205 | .It Fl h Ar host_key_file |
8abcdba4 | 206 | Specifies the file from which the host key is read (default |
5f4fdfae | 207 | .Pa /etc/ssh_host_key ) . |
bf740959 | 208 | This option must be given if |
209 | .Nm | |
210 | is not run as root (as the normal | |
211 | host file is normally not readable by anyone but root). | |
8abcdba4 | 212 | It is possible to have multiple host key files for |
213 | the different protocol versions. | |
bf740959 | 214 | .It Fl i |
215 | Specifies that | |
216 | .Nm | |
f54651ce | 217 | is being run from inetd. |
bf740959 | 218 | .Nm |
219 | is normally not run | |
220 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 221 | respond to the client, and this may take tens of seconds. |
222 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 223 | However, with small key sizes (e.g., 512) using |
bf740959 | 224 | .Nm |
225 | from inetd may | |
226 | be feasible. | |
227 | .It Fl k Ar key_gen_time | |
228 | Specifies how often the server key is regenerated (default 3600 | |
4fe2af09 | 229 | seconds, or one hour). |
230 | The motivation for regenerating the key fairly | |
bf740959 | 231 | often is that the key is not stored anywhere, and after about an hour, |
232 | it becomes impossible to recover the key for decrypting intercepted | |
233 | communications even if the machine is cracked into or physically | |
4fe2af09 | 234 | seized. |
235 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 236 | .It Fl p Ar port |
237 | Specifies the port on which the server listens for connections | |
238 | (default 22). | |
239 | .It Fl q | |
4fe2af09 | 240 | Quiet mode. |
241 | Nothing is sent to the system log. | |
242 | Normally the beginning, | |
bf740959 | 243 | authentication, and termination of each connection is logged. |
c345cf9d | 244 | .It Fl u Ar len |
245 | This option is used to specify the size of the field | |
246 | in the | |
247 | .Li utmp | |
248 | structure that holds the remote host name. | |
249 | If the resolved host name is longer than | |
250 | .Ar len , | |
251 | the dotted decimal value will be used instead. | |
252 | This allows hosts with very long host names that | |
253 | overflow this field to still be uniquely identified. | |
254 | Specifying | |
255 | .Fl u0 | |
256 | indicates that only dotted decimal addresses | |
257 | should be put into the | |
258 | .Pa utmp | |
259 | file. | |
8abcdba4 | 260 | .It Fl D |
261 | When this option is specified | |
262 | .Nm | |
263 | will not detach and does not become a daemon. | |
264 | This allows easy monitoring of | |
265 | .Nm sshd . | |
ff14faf1 | 266 | .It Fl e |
267 | When this option is specified, | |
268 | .Nm | |
269 | will send the output to the standard error instead of the system log. | |
5260325f | 270 | .It Fl V Ar client_protocol_id |
94ec8c6b | 271 | SSH-2 compatibility mode. |
3189621b | 272 | When this option is specified |
5260325f | 273 | .Nm |
3189621b | 274 | assumes the client has sent the supplied version string |
5260325f | 275 | and skips the |
276 | Protocol Version Identification Exchange. | |
94ec8c6b | 277 | This option is not intended to be called directly. |
48e671d5 | 278 | .It Fl 4 |
279 | Forces | |
280 | .Nm | |
281 | to use IPv4 addresses only. | |
282 | .It Fl 6 | |
283 | Forces | |
284 | .Nm | |
285 | to use IPv6 addresses only. | |
bf740959 | 286 | .El |
287 | .Sh CONFIGURATION FILE | |
288 | .Nm | |
f54651ce | 289 | reads configuration data from |
5f4fdfae | 290 | .Pa /etc/sshd_config |
bf740959 | 291 | (or the file specified with |
292 | .Fl f | |
4fe2af09 | 293 | on the command line). |
294 | The file contains keyword-value pairs, one per line. | |
295 | Lines starting with | |
bf740959 | 296 | .Ql # |
297 | and empty lines are interpreted as comments. | |
298 | .Pp | |
299 | The following keywords are possible. | |
300 | .Bl -tag -width Ds | |
301 | .It Cm AFSTokenPassing | |
4fe2af09 | 302 | Specifies whether an AFS token may be forwarded to the server. |
303 | Default is | |
bf740959 | 304 | .Dq yes . |
305 | .It Cm AllowGroups | |
76896b4e | 306 | This keyword can be followed by a list of group names, separated |
4fe2af09 | 307 | by spaces. |
308 | If specified, login is allowed only for users whose primary | |
c6a69271 | 309 | group or supplementary group list matches one of the patterns. |
bf740959 | 310 | .Ql \&* |
311 | and | |
312 | .Ql ? | |
313 | can be used as | |
4fe2af09 | 314 | wildcards in the patterns. |
c345cf9d | 315 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 316 | By default login is allowed regardless of the group list. |
bf740959 | 317 | .Pp |
33de75a3 | 318 | .It Cm AllowTcpForwarding |
319 | Specifies whether TCP forwarding is permitted. | |
320 | The default is | |
321 | .Dq yes . | |
322 | Note that disabling TCP forwarding does not improve security unless | |
323 | users are also denied shell access, as they can always install their | |
324 | own forwarders. | |
325 | .Pp | |
bf740959 | 326 | .It Cm AllowUsers |
76896b4e | 327 | This keyword can be followed by a list of user names, separated |
4fe2af09 | 328 | by spaces. |
329 | If specified, login is allowed only for users names that | |
bf740959 | 330 | match one of the patterns. |
331 | .Ql \&* | |
332 | and | |
333 | .Ql ? | |
334 | can be used as | |
4fe2af09 | 335 | wildcards in the patterns. |
c345cf9d | 336 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 337 | By default login is allowed regardless of the user name. |
bf740959 | 338 | .Pp |
eea39c02 | 339 | .It Cm Banner |
340 | In some jurisdictions, sending a warning message before authentication | |
341 | may be relevant for getting legal protection. | |
342 | The contents of the specified file are sent to the remote user before | |
343 | authentication is allowed. | |
344 | This option is only available for protocol version 2. | |
345 | .Pp | |
657297ff | 346 | .It Cm ChallengeResponseAuthentication |
347 | Specifies whether | |
348 | challenge response | |
349 | authentication is allowed. | |
350 | Currently there is only support for | |
351 | .Xr skey 1 | |
352 | authentication. | |
353 | The default is | |
354 | .Dq yes . | |
f54651ce | 355 | .It Cm Ciphers |
356 | Specifies the ciphers allowed for protocol version 2. | |
357 | Multiple ciphers must be comma-separated. | |
358 | The default is | |
7a6c39a3 | 359 | .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. |
bf740959 | 360 | .It Cm CheckMail |
361 | Specifies whether | |
362 | .Nm | |
363 | should check for new mail for interactive logins. | |
364 | The default is | |
365 | .Dq no . | |
3ffc6336 | 366 | .It Cm ClientAliveInterval |
367 | Sets a timeout interval in seconds after which if no data has been received | |
368 | from the client, | |
369 | .Nm | |
370 | will send a message through the encrypted | |
84fc17bf | 371 | channel to request a response from the client. |
372 | The default | |
3ffc6336 | 373 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 374 | This option applies to protocol version 2 only. |
3ffc6336 | 375 | .It Cm ClientAliveCountMax |
376 | Sets the number of client alive messages (see above) which may be | |
377 | sent without | |
378 | .Nm | |
379 | receiving any messages back from the client. If this threshold is | |
380 | reached while client alive messages are being sent, | |
381 | .Nm | |
382 | will disconnect the client, terminating the session. It is important | |
383 | to note that the use of client alive messages is very different from | |
84fc17bf | 384 | .Cm Keepalive |
385 | (below). The client alive messages are sent through the | |
3ffc6336 | 386 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 387 | option enabled by |
388 | .Cm Keepalive | |
389 | is spoofable. You want to use the client | |
3ffc6336 | 390 | alive mechanism when you are basing something important on |
391 | clients having an active connection to the server. | |
84fc17bf | 392 | .Pp |
393 | The default value is 3. If you set | |
394 | .Cm ClientAliveInterval | |
3ffc6336 | 395 | (above) to 15, and leave this value at the default, unresponsive ssh clients |
396 | will be disconnected after approximately 45 seconds. | |
bf740959 | 397 | .It Cm DenyGroups |
398 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 399 | by spaces. |
c6a69271 | 400 | Users whose primary group or supplementary group list matches |
401 | one of the patterns aren't allowed to log in. | |
bf740959 | 402 | .Ql \&* |
403 | and | |
404 | .Ql ? | |
405 | can be used as | |
4fe2af09 | 406 | wildcards in the patterns. |
c345cf9d | 407 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 408 | By default login is allowed regardless of the group list. |
bf740959 | 409 | .Pp |
410 | .It Cm DenyUsers | |
411 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 412 | by spaces. |
413 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 414 | .Ql \&* |
415 | and | |
416 | .Ql ? | |
4fe2af09 | 417 | can be used as wildcards in the patterns. |
c345cf9d | 418 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 419 | By default login is allowed regardless of the user name. |
1d1ffb87 | 420 | .It Cm GatewayPorts |
421 | Specifies whether remote hosts are allowed to connect to ports | |
422 | forwarded for the client. | |
423 | The argument must be | |
424 | .Dq yes | |
425 | or | |
426 | .Dq no . | |
427 | The default is | |
428 | .Dq no . | |
bf740959 | 429 | .It Cm HostKey |
fa08c86b | 430 | Specifies the file containing the private host keys (default |
1d1ffb87 | 431 | .Pa /etc/ssh_host_key ) |
fa08c86b | 432 | used by SSH protocol versions 1 and 2. |
bf740959 | 433 | Note that |
434 | .Nm | |
42f11eb2 | 435 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 436 | It is possible to have multiple host key files. |
437 | .Dq rsa1 | |
438 | keys are used for version 1 and | |
439 | .Dq dsa | |
440 | or | |
441 | .Dq rsa | |
442 | are used for version 2 of the SSH protocol. | |
bf740959 | 443 | .It Cm IgnoreRhosts |
c8d54615 | 444 | Specifies that |
445 | .Pa .rhosts | |
f54651ce | 446 | and |
c8d54615 | 447 | .Pa .shosts |
448 | files will not be used in authentication. | |
bf740959 | 449 | .Pa /etc/hosts.equiv |
450 | and | |
f54651ce | 451 | .Pa /etc/shosts.equiv |
4fe2af09 | 452 | are still used. |
f54651ce | 453 | The default is |
c8d54615 | 454 | .Dq yes . |
b4748e2f | 455 | .It Cm IgnoreUserKnownHosts |
456 | Specifies whether | |
457 | .Nm | |
458 | should ignore the user's | |
459 | .Pa $HOME/.ssh/known_hosts | |
460 | during | |
461 | .Cm RhostsRSAAuthentication . | |
462 | The default is | |
463 | .Dq no . | |
bf740959 | 464 | .It Cm KeepAlive |
465 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 466 | other side. |
467 | If they are sent, death of the connection or crash of one | |
468 | of the machines will be properly noticed. | |
469 | However, this means that | |
bf740959 | 470 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 471 | find it annoying. |
d0c832f3 | 472 | On the other hand, if keepalives are not sent, |
bf740959 | 473 | sessions may hang indefinitely on the server, leaving |
474 | .Dq ghost | |
475 | users and consuming server resources. | |
476 | .Pp | |
477 | The default is | |
478 | .Dq yes | |
479 | (to send keepalives), and the server will notice | |
4fe2af09 | 480 | if the network goes down or the client host reboots. |
481 | This avoids infinitely hanging sessions. | |
bf740959 | 482 | .Pp |
483 | To disable keepalives, the value should be set to | |
484 | .Dq no | |
485 | in both the server and the client configuration files. | |
486 | .It Cm KerberosAuthentication | |
4fe2af09 | 487 | Specifies whether Kerberos authentication is allowed. |
488 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 489 | .Cm PasswordAuthentication |
490 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 491 | the Kerberos KDC. |
492 | To use this option, the server needs a | |
c345cf9d | 493 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 494 | Default is |
bf740959 | 495 | .Dq yes . |
496 | .It Cm KerberosOrLocalPasswd | |
497 | If set then if password authentication through Kerberos fails then | |
498 | the password will be validated via any additional local mechanism | |
499 | such as | |
188adeb2 | 500 | .Pa /etc/passwd . |
4fe2af09 | 501 | Default is |
bf740959 | 502 | .Dq yes . |
503 | .It Cm KerberosTgtPassing | |
504 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 505 | Default is |
bf740959 | 506 | .Dq no , |
507 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
508 | .It Cm KerberosTicketCleanup | |
509 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 510 | file on logout. |
511 | Default is | |
bf740959 | 512 | .Dq yes . |
513 | .It Cm KeyRegenerationInterval | |
514 | The server key is automatically regenerated after this many seconds | |
4fe2af09 | 515 | (if it has been used). |
516 | The purpose of regeneration is to prevent | |
bf740959 | 517 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 518 | stealing the keys. |
519 | The key is never stored anywhere. | |
520 | If the value is 0, the key is never regenerated. | |
521 | The default is 3600 (seconds). | |
bf740959 | 522 | .It Cm ListenAddress |
6e9944b8 | 523 | Specifies the local addresses |
b8a297f1 | 524 | .Xr sshd 8 |
bf740959 | 525 | should listen on. |
6e9944b8 | 526 | The following forms may be used: |
527 | .Pp | |
528 | .Bl -item -offset indent -compact | |
529 | .It | |
530 | .Cm ListenAddress | |
b8a297f1 | 531 | .Sm off |
532 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
533 | .Sm on | |
6e9944b8 | 534 | .It |
535 | .Cm ListenAddress | |
b8a297f1 | 536 | .Sm off |
537 | .Ar host No | Ar IPv4_addr No : Ar port | |
538 | .Sm on | |
6e9944b8 | 539 | .It |
540 | .Cm ListenAddress | |
b8a297f1 | 541 | .Sm off |
542 | .Oo | |
543 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
544 | .Sm on | |
6e9944b8 | 545 | .El |
546 | .Pp | |
547 | If | |
b8a297f1 | 548 | .Ar port |
6e9944b8 | 549 | is not specified, |
b8a297f1 | 550 | .Xr sshd 8 |
6e9944b8 | 551 | will listen on the address and all prior |
552 | .Cm Port | |
553 | options specified. The default is to listen on all local | |
554 | addresses. Multiple | |
555 | .Cm ListenAddress | |
556 | options are permitted. Additionally, any | |
557 | .Cm Port | |
558 | options must precede this option for non port qualified addresses. | |
bf740959 | 559 | .It Cm LoginGraceTime |
560 | The server disconnects after this time if the user has not | |
4fe2af09 | 561 | successfully logged in. |
562 | If the value is 0, there is no time limit. | |
bf740959 | 563 | The default is 600 (seconds). |
6a17f9c2 | 564 | .It Cm LogLevel |
565 | Gives the verbosity level that is used when logging messages from | |
566 | .Nm sshd . | |
567 | The possible values are: | |
59c97189 | 568 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
569 | The default is INFO. | |
6a17f9c2 | 570 | Logging with level DEBUG violates the privacy of users |
571 | and is not recommended. | |
b2552997 | 572 | .It Cm MACs |
573 | Specifies the available MAC (message authentication code) algorithms. | |
574 | The MAC algorithm is used in protocol version 2 | |
575 | for data integrity protection. | |
576 | Multiple algorithms must be comma-separated. | |
577 | The default is | |
578 | .Pp | |
579 | .Bd -literal | |
7a6c39a3 | 580 | ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com, |
b2552997 | 581 | hmac-sha1-96,hmac-md5-96'' |
582 | .Ed | |
089fbbd2 | 583 | .It Cm MaxStartups |
584 | Specifies the maximum number of concurrent unauthenticated connections to the | |
585 | .Nm | |
586 | daemon. | |
587 | Additional connections will be dropped until authentication succeeds or the | |
588 | .Cm LoginGraceTime | |
589 | expires for a connection. | |
590 | The default is 10. | |
c345cf9d | 591 | .Pp |
592 | Alternatively, random early drop can be enabled by specifying | |
593 | the three colon separated values | |
594 | .Dq start:rate:full | |
94ec8c6b | 595 | (e.g., "10:30:60"). |
c345cf9d | 596 | .Nm |
6f37606e | 597 | will refuse connection attempts with a probability of |
c345cf9d | 598 | .Dq rate/100 |
599 | (30%) | |
600 | if there are currently | |
601 | .Dq start | |
602 | (10) | |
603 | unauthenticated connections. | |
6f37606e | 604 | The probability increases linearly and all connection attempts |
c345cf9d | 605 | are refused if the number of unauthenticated connections reaches |
606 | .Dq full | |
607 | (60). | |
bf740959 | 608 | .It Cm PasswordAuthentication |
609 | Specifies whether password authentication is allowed. | |
610 | The default is | |
611 | .Dq yes . | |
c345cf9d | 612 | Note that this option applies to both protocol versions 1 and 2. |
bf740959 | 613 | .It Cm PermitEmptyPasswords |
614 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 615 | server allows login to accounts with empty password strings. |
616 | The default is | |
c8d54615 | 617 | .Dq no . |
bf740959 | 618 | .It Cm PermitRootLogin |
46f23b8d | 619 | Specifies whether root can login using |
bf740959 | 620 | .Xr ssh 1 . |
621 | The argument must be | |
622 | .Dq yes , | |
15853e93 | 623 | .Dq without-password , |
624 | .Dq forced-commands-only | |
bf740959 | 625 | or |
626 | .Dq no . | |
627 | The default is | |
628 | .Dq yes . | |
15853e93 | 629 | .Pp |
630 | If this option is set to | |
bf740959 | 631 | .Dq without-password |
15853e93 | 632 | password authentication is disabled for root. |
bf740959 | 633 | .Pp |
15853e93 | 634 | If this option is set to |
635 | .Dq forced-commands-only | |
636 | root login with public key authentication will be allowed, | |
637 | but only if the | |
bf740959 | 638 | .Ar command |
15853e93 | 639 | option has been specified |
bf740959 | 640 | (which may be useful for taking remote backups even if root login is |
15853e93 | 641 | normally not allowed). All other authentication methods are disabled |
642 | for root. | |
46f23b8d | 643 | .Pp |
644 | If this option is set to | |
645 | .Dq no | |
646 | root is not allowed to login. | |
0fbe8c74 | 647 | .It Cm PidFile |
648 | Specifies the file that contains the process identifier of the | |
649 | .Nm | |
650 | daemon. | |
651 | The default is | |
652 | .Pa /var/run/sshd.pid . | |
bf740959 | 653 | .It Cm Port |
654 | Specifies the port number that | |
655 | .Nm | |
4fe2af09 | 656 | listens on. |
657 | The default is 22. | |
48e671d5 | 658 | Multiple options of this type are permitted. |
4f4648f9 | 659 | .It Cm PrintLastLog |
660 | Specifies whether | |
661 | .Nm | |
662 | should print the date and time when the user last logged in. | |
663 | The default is | |
664 | .Dq yes . | |
bf740959 | 665 | .It Cm PrintMotd |
666 | Specifies whether | |
667 | .Nm | |
f54651ce | 668 | should print |
bf740959 | 669 | .Pa /etc/motd |
4fe2af09 | 670 | when a user logs in interactively. |
671 | (On some systems it is also printed by the shell, | |
bf740959 | 672 | .Pa /etc/profile , |
4fe2af09 | 673 | or equivalent.) |
674 | The default is | |
bf740959 | 675 | .Dq yes . |
f54651ce | 676 | .It Cm Protocol |
677 | Specifies the protocol versions | |
678 | .Nm | |
679 | should support. | |
680 | The possible values are | |
681 | .Dq 1 | |
682 | and | |
683 | .Dq 2 . | |
684 | Multiple versions must be comma-separated. | |
685 | The default is | |
686 | .Dq 1 . | |
657297ff | 687 | .It Cm PubkeyAuthentication |
688 | Specifies whether public key authentication is allowed. | |
689 | The default is | |
690 | .Dq yes . | |
691 | Note that this option applies to protocol version 2 only. | |
61e96248 | 692 | .It Cm ReverseMappingCheck |
693 | Specifies whether | |
694 | .Nm | |
695 | should try to verify the remote host name and check that | |
696 | the resolved host name for the remote IP address maps back to the | |
697 | very same IP address. | |
698 | The default is | |
699 | .Dq no . | |
bf740959 | 700 | .It Cm RhostsAuthentication |
701 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 702 | files is sufficient. |
703 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 704 | .Cm RhostsRSAAuthentication |
705 | should be used | |
706 | instead, because it performs RSA-based host authentication in addition | |
707 | to normal rhosts or /etc/hosts.equiv authentication. | |
708 | The default is | |
709 | .Dq no . | |
710 | .It Cm RhostsRSAAuthentication | |
711 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 712 | with successful RSA host authentication is allowed. |
713 | The default is | |
c8d54615 | 714 | .Dq no . |
bf740959 | 715 | .It Cm RSAAuthentication |
4fe2af09 | 716 | Specifies whether pure RSA authentication is allowed. |
717 | The default is | |
bf740959 | 718 | .Dq yes . |
1d1ffb87 | 719 | Note that this option applies to protocol version 1 only. |
bf740959 | 720 | .It Cm ServerKeyBits |
4fe2af09 | 721 | Defines the number of bits in the server key. |
722 | The minimum value is 512, and the default is 768. | |
bf740959 | 723 | .It Cm StrictModes |
724 | Specifies whether | |
725 | .Nm | |
726 | should check file modes and ownership of the | |
4fe2af09 | 727 | user's files and home directory before accepting login. |
728 | This is normally desirable because novices sometimes accidentally leave their | |
729 | directory or files world-writable. | |
730 | The default is | |
bf740959 | 731 | .Dq yes . |
38c295d6 | 732 | .It Cm Subsystem |
94ec8c6b | 733 | Configures an external subsystem (e.g., file transfer daemon). |
734 | Arguments should be a subsystem name and a command to execute upon subsystem | |
735 | request. | |
b5e300c2 | 736 | The command |
737 | .Xr sftp-server 8 | |
738 | implements the | |
739 | .Dq sftp | |
740 | file transfer subsystem. | |
38c295d6 | 741 | By default no subsystems are defined. |
742 | Note that this option applies to protocol version 2 only. | |
bf740959 | 743 | .It Cm SyslogFacility |
744 | Gives the facility code that is used when logging messages from | |
745 | .Nm sshd . | |
746 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 747 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
748 | The default is AUTH. | |
bf740959 | 749 | .It Cm UseLogin |
750 | Specifies whether | |
751 | .Xr login 1 | |
fa649821 | 752 | is used for interactive login sessions. |
753 | Note that | |
754 | .Xr login 1 | |
c345cf9d | 755 | is never used for remote command execution. |
4fe2af09 | 756 | The default is |
bf740959 | 757 | .Dq no . |
bf740959 | 758 | .It Cm X11DisplayOffset |
759 | Specifies the first display number available for | |
760 | .Nm sshd Ns 's | |
4fe2af09 | 761 | X11 forwarding. |
762 | This prevents | |
bf740959 | 763 | .Nm |
764 | from interfering with real X11 servers. | |
c8d54615 | 765 | The default is 10. |
7b2ea3a1 | 766 | .It Cm X11Forwarding |
4fe2af09 | 767 | Specifies whether X11 forwarding is permitted. |
768 | The default is | |
c8d54615 | 769 | .Dq no . |
7b2ea3a1 | 770 | Note that disabling X11 forwarding does not improve security in any |
771 | way, as users can always install their own forwarders. | |
fa649821 | 772 | .It Cm XAuthLocation |
773 | Specifies the location of the | |
774 | .Xr xauth 1 | |
775 | program. | |
776 | The default is | |
777 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 778 | .El |
779 | .Sh LOGIN PROCESS | |
780 | When a user successfully logs in, | |
781 | .Nm | |
782 | does the following: | |
783 | .Bl -enum -offset indent | |
784 | .It | |
785 | If the login is on a tty, and no command has been specified, | |
f54651ce | 786 | prints last login time and |
bf740959 | 787 | .Pa /etc/motd |
788 | (unless prevented in the configuration file or by | |
789 | .Pa $HOME/.hushlogin ; | |
790 | see the | |
f54651ce | 791 | .Sx FILES |
bf740959 | 792 | section). |
793 | .It | |
794 | If the login is on a tty, records login time. | |
795 | .It | |
796 | Checks | |
797 | .Pa /etc/nologin ; | |
798 | if it exists, prints contents and quits | |
799 | (unless root). | |
800 | .It | |
801 | Changes to run with normal user privileges. | |
802 | .It | |
803 | Sets up basic environment. | |
804 | .It | |
805 | Reads | |
806 | .Pa $HOME/.ssh/environment | |
807 | if it exists. | |
808 | .It | |
809 | Changes to user's home directory. | |
810 | .It | |
811 | If | |
812 | .Pa $HOME/.ssh/rc | |
813 | exists, runs it; else if | |
5f4fdfae | 814 | .Pa /etc/sshrc |
bf740959 | 815 | exists, runs |
4fe2af09 | 816 | it; otherwise runs xauth. |
817 | The | |
bf740959 | 818 | .Dq rc |
819 | files are given the X11 | |
820 | authentication protocol and cookie in standard input. | |
821 | .It | |
822 | Runs user's shell or command. | |
823 | .El | |
824 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
f54651ce | 825 | The |
bf740959 | 826 | .Pa $HOME/.ssh/authorized_keys |
827 | file lists the RSA keys that are | |
1d1ffb87 | 828 | permitted for RSA authentication in SSH protocols 1.3 and 1.5 |
d0c832f3 | 829 | Similarly, the |
1d1ffb87 | 830 | .Pa $HOME/.ssh/authorized_keys2 |
8abcdba4 | 831 | file lists the DSA and RSA keys that are |
832 | permitted for public key authentication (PubkeyAuthentication) | |
833 | in SSH protocol 2.0. | |
834 | .Pp | |
4fe2af09 | 835 | Each line of the file contains one |
bf740959 | 836 | key (empty lines and lines starting with a |
837 | .Ql # | |
838 | are ignored as | |
4fe2af09 | 839 | comments). |
8abcdba4 | 840 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 841 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 842 | Each protocol version 2 public key consists of: |
843 | options, keytype, base64 encoded key, comment. | |
844 | The options fields | |
845 | are optional; its presence is determined by whether the line starts | |
bf740959 | 846 | with a number or not (the option field never starts with a number). |
8abcdba4 | 847 | The bits, exponent, modulus and comment fields give the RSA key for |
848 | protocol version 1; the | |
bf740959 | 849 | comment field is not used for anything (but may be convenient for the |
850 | user to identify the key). | |
8abcdba4 | 851 | For protocol version 2 the keytype is |
852 | .Dq ssh-dss | |
853 | or | |
854 | .Dq ssh-rsa . | |
bf740959 | 855 | .Pp |
856 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 857 | (because of the size of the RSA key modulus). |
858 | You don't want to type them in; instead, copy the | |
c0ecc314 | 859 | .Pa identity.pub , |
8abcdba4 | 860 | .Pa id_dsa.pub |
c0ecc314 | 861 | or the |
862 | .Pa id_rsa.pub | |
bf740959 | 863 | file and edit it. |
864 | .Pp | |
c345cf9d | 865 | The options (if present) consist of comma-separated option |
4fe2af09 | 866 | specifications. |
867 | No spaces are permitted, except within double quotes. | |
bf740959 | 868 | The following option specifications are supported: |
869 | .Bl -tag -width Ds | |
870 | .It Cm from="pattern-list" | |
871 | Specifies that in addition to RSA authentication, the canonical name | |
872 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 873 | patterns |
874 | .Pf ( Ql * | |
875 | and | |
876 | .Ql ? | |
877 | serve as wildcards). | |
878 | The list may also contain | |
879 | patterns negated by prefixing them with | |
880 | .Ql ! ; | |
881 | if the canonical host name matches a negated pattern, the key is not accepted. | |
882 | The purpose | |
bf740959 | 883 | of this option is to optionally increase security: RSA authentication |
884 | by itself does not trust the network or name servers or anything (but | |
885 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 886 | permits an intruder to log in from anywhere in the world. |
887 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 888 | servers and/or routers would have to be compromised in addition to |
889 | just the key). | |
890 | .It Cm command="command" | |
891 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 892 | authentication. |
893 | The command supplied by the user (if any) is ignored. | |
bf740959 | 894 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 895 | otherwise it is run without a tty. |
61e96248 | 896 | Note that if you want a 8-bit clean channel, |
897 | you must not request a pty or should specify | |
898 | .Cm no-pty . | |
4fe2af09 | 899 | A quote may be included in the command by quoting it with a backslash. |
900 | This option might be useful | |
901 | to restrict certain RSA keys to perform just a specific operation. | |
902 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 903 | Note that the client may specify TCP/IP and/or X11 |
904 | forwarding unless they are explicitly prohibited. | |
bf740959 | 905 | .It Cm environment="NAME=value" |
906 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 907 | logging in using this key. |
908 | Environment variables set this way | |
909 | override other default environment values. | |
910 | Multiple options of this type are permitted. | |
bf740959 | 911 | .It Cm no-port-forwarding |
912 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 913 | Any port forward requests by the client will return an error. |
914 | This might be used, e.g., in connection with the | |
bf740959 | 915 | .Cm command |
916 | option. | |
917 | .It Cm no-X11-forwarding | |
918 | Forbids X11 forwarding when this key is used for authentication. | |
919 | Any X11 forward requests by the client will return an error. | |
920 | .It Cm no-agent-forwarding | |
921 | Forbids authentication agent forwarding when this key is used for | |
922 | authentication. | |
923 | .It Cm no-pty | |
924 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 925 | .It Cm permitopen="host:port" |
926 | Limit local | |
927 | .Li ``ssh -L'' | |
b2ae83b8 | 928 | port forwarding such that it may only connect to the specified host and |
dc504afd | 929 | port. Multiple |
930 | .Cm permitopen | |
b2ae83b8 | 931 | options may be applied separated by commas. No pattern matching is |
dc504afd | 932 | performed on the specified hostnames, they must be literal domains or |
933 | addresses. | |
bf740959 | 934 | .El |
935 | .Ss Examples | |
936 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
937 | .Pp | |
938 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
939 | .Pp | |
940 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 941 | .Pp |
942 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 943 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 944 | The |
1d1ffb87 | 945 | .Pa /etc/ssh_known_hosts , |
946 | .Pa /etc/ssh_known_hosts2 , | |
947 | .Pa $HOME/.ssh/known_hosts , | |
f54651ce | 948 | and |
1d1ffb87 | 949 | .Pa $HOME/.ssh/known_hosts2 |
4fe2af09 | 950 | files contain host public keys for all known hosts. |
951 | The global file should | |
952 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 953 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 954 | its key is added to the per-user file. |
bf740959 | 955 | .Pp |
956 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 957 | bits, exponent, modulus, comment. |
958 | The fields are separated by spaces. | |
bf740959 | 959 | .Pp |
960 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
961 | wildcards); each pattern in turn is matched against the canonical host | |
962 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 963 | name (when authenticating a server). |
964 | A pattern may also be preceded by | |
bf740959 | 965 | .Ql ! |
966 | to indicate negation: if the host name matches a negated | |
967 | pattern, it is not accepted (by that line) even if it matched another | |
968 | pattern on the line. | |
969 | .Pp | |
1d1ffb87 | 970 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 971 | can be obtained, e.g., from |
5f4fdfae | 972 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 973 | The optional comment field continues to the end of the line, and is not used. |
974 | .Pp | |
975 | Lines starting with | |
976 | .Ql # | |
977 | and empty lines are ignored as comments. | |
978 | .Pp | |
979 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 980 | matching line has the proper key. |
981 | It is thus permissible (but not | |
bf740959 | 982 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 983 | names. |
984 | This will inevitably happen when short forms of host names | |
985 | from different domains are put in the file. | |
986 | It is possible | |
bf740959 | 987 | that the files contain conflicting information; authentication is |
988 | accepted if valid information can be found from either file. | |
989 | .Pp | |
990 | Note that the lines in these files are typically hundreds of characters | |
991 | long, and you definitely don't want to type in the host keys by hand. | |
992 | Rather, generate them by a script | |
f54651ce | 993 | or by taking |
5f4fdfae | 994 | .Pa /etc/ssh_host_key.pub |
bf740959 | 995 | and adding the host names at the front. |
996 | .Ss Examples | |
997 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
998 | .Sh FILES | |
999 | .Bl -tag -width Ds | |
5f4fdfae | 1000 | .It Pa /etc/sshd_config |
bf740959 | 1001 | Contains configuration data for |
1002 | .Nm sshd . | |
1003 | This file should be writable by root only, but it is recommended | |
1004 | (though not necessary) that it be world-readable. | |
b8dc87d3 | 1005 | .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
1006 | These three files contain the private parts of the | |
1007 | (SSH1, SSH2 DSA, and SSH2 RSA) host keys. | |
1008 | These files should only be owned by root, readable only by root, and not | |
bf740959 | 1009 | accessible to others. |
1010 | Note that | |
1011 | .Nm | |
1012 | does not start if this file is group/world-accessible. | |
b8dc87d3 | 1013 | .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
672f212f | 1014 | These three files contain the public parts of the |
b8dc87d3 | 1015 | (SSH1, SSH2 DSA, and SSH2 RSA) host keys. |
1016 | These files should be world-readable but writable only by | |
4fe2af09 | 1017 | root. |
b8dc87d3 | 1018 | Their contents should match the respective private parts. |
1019 | These files are not | |
1020 | really used for anything; they are provided for the convenience of | |
1021 | the user so their contents can be copied to known hosts files. | |
1022 | These files are created using | |
bf740959 | 1023 | .Xr ssh-keygen 1 . |
c523303b | 1024 | .It Pa /etc/primes |
1025 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
5f4fdfae | 1026 | .It Pa /var/run/sshd.pid |
bf740959 | 1027 | Contains the process ID of the |
1028 | .Nm | |
1029 | listening for connections (if there are several daemons running | |
1030 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1031 | started last). |
c345cf9d | 1032 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1033 | .It Pa $HOME/.ssh/authorized_keys |
1034 | Lists the RSA keys that can be used to log into the user's account. | |
1035 | This file must be readable by root (which may on some machines imply | |
1036 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 1037 | volume). |
1038 | It is recommended that it not be accessible by others. | |
1039 | The format of this file is described above. | |
1d1ffb87 | 1040 | Users will place the contents of their |
1041 | .Pa identity.pub | |
1042 | files into this file, as described in | |
1043 | .Xr ssh-keygen 1 . | |
1044 | .It Pa $HOME/.ssh/authorized_keys2 | |
1045 | Lists the DSA keys that can be used to log into the user's account. | |
1046 | This file must be readable by root (which may on some machines imply | |
1047 | it being world-readable if the user's home directory resides on an NFS | |
1048 | volume). | |
1049 | It is recommended that it not be accessible by others. | |
1050 | The format of this file is described above. | |
1051 | Users will place the contents of their | |
1052 | .Pa id_dsa.pub | |
c0ecc314 | 1053 | and/or |
1054 | .Pa id_rsa.pub | |
1d1ffb87 | 1055 | files into this file, as described in |
1056 | .Xr ssh-keygen 1 . | |
5f4fdfae | 1057 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1058 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 1059 | authentication to check the public key of the host. |
1060 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 1061 | The client uses the same files |
e91c60f2 | 1062 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1063 | These files should be writable only by root/the owner. |
5f4fdfae | 1064 | .Pa /etc/ssh_known_hosts |
bf740959 | 1065 | should be world-readable, and |
1066 | .Pa $HOME/.ssh/known_hosts | |
1067 | can but need not be world-readable. | |
1068 | .It Pa /etc/nologin | |
f54651ce | 1069 | If this file exists, |
bf740959 | 1070 | .Nm |
4fe2af09 | 1071 | refuses to let anyone except root log in. |
1072 | The contents of the file | |
bf740959 | 1073 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1074 | refused. |
1075 | The file should be world-readable. | |
bf740959 | 1076 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
1077 | If compiled with | |
1078 | .Sy LIBWRAP | |
1079 | support, tcp-wrappers access controls may be defined here as described in | |
1080 | .Xr hosts_access 5 . | |
1081 | .It Pa $HOME/.rhosts | |
1082 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1083 | line. |
1084 | The given user on the corresponding host is permitted to log in | |
1085 | without password. | |
1086 | The same file is used by rlogind and rshd. | |
bf740959 | 1087 | The file must |
1088 | be writable only by the user; it is recommended that it not be | |
1089 | accessible by others. | |
1090 | .Pp | |
4fe2af09 | 1091 | If is also possible to use netgroups in the file. |
1092 | Either host or user | |
bf740959 | 1093 | name may be of the form +@groupname to specify all hosts or all users |
1094 | in the group. | |
1095 | .It Pa $HOME/.shosts | |
1096 | For ssh, | |
1097 | this file is exactly the same as for | |
1098 | .Pa .rhosts . | |
1099 | However, this file is | |
1100 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1101 | .It Pa /etc/hosts.equiv |
bf740959 | 1102 | This file is used during |
1103 | .Pa .rhosts | |
4fe2af09 | 1104 | authentication. |
1105 | In the simplest form, this file contains host names, one per line. | |
1106 | Users on | |
bf740959 | 1107 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1108 | have the same user name on both machines. |
1109 | The host name may also be | |
bf740959 | 1110 | followed by a user name; such users are permitted to log in as |
1111 | .Em any | |
4fe2af09 | 1112 | user on this machine (except root). |
1113 | Additionally, the syntax | |
bf740959 | 1114 | .Dq +@group |
4fe2af09 | 1115 | can be used to specify netgroups. |
1116 | Negated entries start with | |
bf740959 | 1117 | .Ql \&- . |
1118 | .Pp | |
1119 | If the client host/user is successfully matched in this file, login is | |
1120 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1121 | same. |
1122 | Additionally, successful RSA host authentication is normally required. | |
1123 | This file must be writable only by root; it is recommended | |
bf740959 | 1124 | that it be world-readable. |
1125 | .Pp | |
1126 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1127 | .Pa hosts.equiv . | |
1128 | Beware that it really means that the named user(s) can log in as | |
1129 | .Em anybody , | |
1130 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1131 | binaries and directories. |
1132 | Using a user name practically grants the user root access. | |
1133 | The only valid use for user names that I can think | |
bf740959 | 1134 | of is in negative entries. |
1135 | .Pp | |
1136 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1137 | .It Pa /etc/shosts.equiv |
bf740959 | 1138 | This is processed exactly as |
1139 | .Pa /etc/hosts.equiv . | |
1140 | However, this file may be useful in environments that want to run both | |
1141 | rsh/rlogin and ssh. | |
1142 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1143 | This file is read into the environment at login (if it exists). |
1144 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1145 | .Ql # ) , |
4fe2af09 | 1146 | and assignment lines of the form name=value. |
1147 | The file should be writable | |
bf740959 | 1148 | only by the user; it need not be readable by anyone else. |
1149 | .It Pa $HOME/.ssh/rc | |
1150 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1151 | environment files but before starting the user's shell or command. |
1152 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 1153 | standard input (and |
1154 | .Ev DISPLAY | |
4fe2af09 | 1155 | in environment). |
1156 | This must call | |
bf740959 | 1157 | .Xr xauth 1 |
1158 | in that case. | |
1159 | .Pp | |
1160 | The primary purpose of this file is to run any initialization routines | |
1161 | which may be needed before the user's home directory becomes | |
1162 | accessible; AFS is a particular example of such an environment. | |
1163 | .Pp | |
1164 | This file will probably contain some initialization code followed by | |
1165 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
1166 | $proto $cookie | xauth -q -; fi". | |
1167 | .Pp | |
1168 | If this file does not exist, | |
5f4fdfae | 1169 | .Pa /etc/sshrc |
bf740959 | 1170 | is run, and if that |
1171 | does not exist either, xauth is used to store the cookie. | |
1172 | .Pp | |
1173 | This file should be writable only by the user, and need not be | |
1174 | readable by anyone else. | |
5f4fdfae | 1175 | .It Pa /etc/sshrc |
bf740959 | 1176 | Like |
1177 | .Pa $HOME/.ssh/rc . | |
1178 | This can be used to specify | |
4fe2af09 | 1179 | machine-specific login-time initializations globally. |
1180 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1181 | .El |
fa08c86b | 1182 | .Sh AUTHORS |
7f5c4295 | 1183 | OpenSSH is a derivative of the original and free |
1184 | ssh 1.2.12 release by Tatu Ylonen. | |
1185 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1186 | Theo de Raadt and Dug Song | |
1187 | removed many bugs, re-added newer features and | |
1188 | created OpenSSH. | |
1189 | Markus Friedl contributed the support for SSH | |
1190 | protocol versions 1.5 and 2.0. | |
bf740959 | 1191 | .Sh SEE ALSO |
bf740959 | 1192 | .Xr scp 1 , |
61e96248 | 1193 | .Xr sftp 1 , |
b5e300c2 | 1194 | .Xr sftp-server 8 , |
bf740959 | 1195 | .Xr ssh 1 , |
1196 | .Xr ssh-add 1 , | |
1197 | .Xr ssh-agent 1 , | |
1198 | .Xr ssh-keygen 1 , | |
7368a6c8 | 1199 | .Xr rlogin 1 , |
1200 | .Xr rsh 1 |