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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 | .\" |
7d747e89 | 37 | .\" $OpenBSD: sshd.8,v 1.132 2001/06/23 03:03:59 markus 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 | |
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. | |
da89cf4d | 137 | .Pp |
51c251f0 | 138 | The rest of the session is encrypted using a symmetric cipher, currently |
da89cf4d | 139 | 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. |
1d1ffb87 | 140 | The client selects the encryption algorithm |
141 | to use from those offered by the server. | |
142 | Additionally, session integrity is provided | |
d0c832f3 | 143 | through a cryptographic message authentication code |
1d1ffb87 | 144 | (hmac-sha1 or hmac-md5). |
145 | .Pp | |
146 | Protocol version 2 provides a public key based | |
da89cf4d | 147 | user (PubkeyAuthentication) or |
148 | client host (HostbasedAuthentication) authentication method, | |
149 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 150 | .Pp |
151 | .Ss Command execution and data forwarding | |
152 | .Pp | |
bf740959 | 153 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 154 | preparing the session is entered. |
155 | At this time the client may request | |
bf740959 | 156 | things like allocating a pseudo-tty, forwarding X11 connections, |
157 | forwarding TCP/IP connections, or forwarding the authentication agent | |
158 | connection over the secure channel. | |
159 | .Pp | |
160 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 161 | The sides then enter session mode. |
162 | In this mode, either side may send | |
bf740959 | 163 | data at any time, and such data is forwarded to/from the shell or |
164 | command on the server side, and the user terminal in the client side. | |
165 | .Pp | |
166 | When the user program terminates and all forwarded X11 and other | |
167 | connections have been closed, the server sends command exit status to | |
168 | the client, and both sides exit. | |
169 | .Pp | |
170 | .Nm | |
171 | can be configured using command-line options or a configuration | |
4fe2af09 | 172 | file. |
173 | Command-line options override values specified in the | |
bf740959 | 174 | configuration file. |
175 | .Pp | |
9d6b7add | 176 | .Nm |
177 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 178 | .Dv SIGHUP , |
a5a2da3b | 179 | by executing itself with the name it was started as, i.e., |
409edaba | 180 | .Pa /usr/sbin/sshd . |
9d6b7add | 181 | .Pp |
bf740959 | 182 | The options are as follows: |
183 | .Bl -tag -width Ds | |
184 | .It Fl b Ar bits | |
da89cf4d | 185 | Specifies the number of bits in the ephemeral protocol version 1 |
186 | server key (default 768). | |
bf740959 | 187 | .Pp |
188 | .It Fl d | |
4fe2af09 | 189 | Debug mode. |
190 | The server sends verbose debug output to the system | |
191 | log, and does not put itself in the background. | |
192 | The server also will not fork and will only process one connection. | |
193 | This option is only intended for debugging for the server. | |
da89cf4d | 194 | Multiple -d options increase the debugging level. |
94ec8c6b | 195 | Maximum is 3. |
da89cf4d | 196 | .It Fl e |
197 | When this option is specified, | |
198 | .Nm | |
199 | will send the output to the standard error instead of the system log. | |
bf740959 | 200 | .It Fl f Ar configuration_file |
4fe2af09 | 201 | Specifies the name of the configuration file. |
202 | The default is | |
5f4fdfae | 203 | .Pa /etc/sshd_config . |
bf740959 | 204 | .Nm |
205 | refuses to start if there is no configuration file. | |
206 | .It Fl g Ar login_grace_time | |
207 | Gives the grace time for clients to authenticate themselves (default | |
ee5e6612 | 208 | 600 seconds). |
4fe2af09 | 209 | If the client fails to authenticate the user within |
210 | this many seconds, the server disconnects and exits. | |
211 | A value of zero indicates no limit. | |
bf740959 | 212 | .It Fl h Ar host_key_file |
8abcdba4 | 213 | Specifies the file from which the host key is read (default |
5f4fdfae | 214 | .Pa /etc/ssh_host_key ) . |
bf740959 | 215 | This option must be given if |
216 | .Nm | |
217 | is not run as root (as the normal | |
218 | host file is normally not readable by anyone but root). | |
8abcdba4 | 219 | It is possible to have multiple host key files for |
da89cf4d | 220 | the different protocol versions and host key algorithms. |
bf740959 | 221 | .It Fl i |
222 | Specifies that | |
223 | .Nm | |
f54651ce | 224 | is being run from inetd. |
bf740959 | 225 | .Nm |
226 | is normally not run | |
227 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 228 | respond to the client, and this may take tens of seconds. |
229 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 230 | However, with small key sizes (e.g., 512) using |
bf740959 | 231 | .Nm |
232 | from inetd may | |
233 | be feasible. | |
234 | .It Fl k Ar key_gen_time | |
da89cf4d | 235 | Specifies how often the ephemeral protocol version 1 server key is |
236 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 237 | The motivation for regenerating the key fairly |
bf740959 | 238 | often is that the key is not stored anywhere, and after about an hour, |
239 | it becomes impossible to recover the key for decrypting intercepted | |
240 | communications even if the machine is cracked into or physically | |
4fe2af09 | 241 | seized. |
242 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 243 | .It Fl p Ar port |
244 | Specifies the port on which the server listens for connections | |
245 | (default 22). | |
246 | .It Fl q | |
4fe2af09 | 247 | Quiet mode. |
248 | Nothing is sent to the system log. | |
249 | Normally the beginning, | |
bf740959 | 250 | authentication, and termination of each connection is logged. |
c345cf9d | 251 | .It Fl u Ar len |
252 | This option is used to specify the size of the field | |
253 | in the | |
254 | .Li utmp | |
255 | structure that holds the remote host name. | |
256 | If the resolved host name is longer than | |
257 | .Ar len , | |
258 | the dotted decimal value will be used instead. | |
259 | This allows hosts with very long host names that | |
260 | overflow this field to still be uniquely identified. | |
261 | Specifying | |
262 | .Fl u0 | |
263 | indicates that only dotted decimal addresses | |
264 | should be put into the | |
265 | .Pa utmp | |
266 | file. | |
8abcdba4 | 267 | .It Fl D |
268 | When this option is specified | |
269 | .Nm | |
270 | will not detach and does not become a daemon. | |
271 | This allows easy monitoring of | |
272 | .Nm sshd . | |
48e671d5 | 273 | .It Fl 4 |
274 | Forces | |
275 | .Nm | |
276 | to use IPv4 addresses only. | |
277 | .It Fl 6 | |
278 | Forces | |
279 | .Nm | |
280 | to use IPv6 addresses only. | |
bf740959 | 281 | .El |
282 | .Sh CONFIGURATION FILE | |
283 | .Nm | |
f54651ce | 284 | reads configuration data from |
5f4fdfae | 285 | .Pa /etc/sshd_config |
bf740959 | 286 | (or the file specified with |
287 | .Fl f | |
4fe2af09 | 288 | on the command line). |
289 | The file contains keyword-value pairs, one per line. | |
290 | Lines starting with | |
bf740959 | 291 | .Ql # |
292 | and empty lines are interpreted as comments. | |
293 | .Pp | |
294 | The following keywords are possible. | |
295 | .Bl -tag -width Ds | |
296 | .It Cm AFSTokenPassing | |
4fe2af09 | 297 | Specifies whether an AFS token may be forwarded to the server. |
298 | Default is | |
bf740959 | 299 | .Dq yes . |
300 | .It Cm AllowGroups | |
76896b4e | 301 | This keyword can be followed by a list of group names, separated |
4fe2af09 | 302 | by spaces. |
303 | If specified, login is allowed only for users whose primary | |
c6a69271 | 304 | group or supplementary group list matches one of the patterns. |
bf740959 | 305 | .Ql \&* |
306 | and | |
307 | .Ql ? | |
308 | can be used as | |
4fe2af09 | 309 | wildcards in the patterns. |
c345cf9d | 310 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 311 | By default login is allowed regardless of the group list. |
bf740959 | 312 | .Pp |
33de75a3 | 313 | .It Cm AllowTcpForwarding |
314 | Specifies whether TCP forwarding is permitted. | |
315 | The default is | |
316 | .Dq yes . | |
317 | Note that disabling TCP forwarding does not improve security unless | |
318 | users are also denied shell access, as they can always install their | |
319 | own forwarders. | |
320 | .Pp | |
bf740959 | 321 | .It Cm AllowUsers |
76896b4e | 322 | This keyword can be followed by a list of user names, separated |
4fe2af09 | 323 | by spaces. |
324 | If specified, login is allowed only for users names that | |
bf740959 | 325 | match one of the patterns. |
326 | .Ql \&* | |
327 | and | |
328 | .Ql ? | |
329 | can be used as | |
4fe2af09 | 330 | wildcards in the patterns. |
c345cf9d | 331 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 332 | By default login is allowed regardless of the user name. |
bf740959 | 333 | .Pp |
c8445989 | 334 | .It Cm AuthorizedKeysFile |
335 | Specifies the file that contains the public RSA keys that can be used | |
336 | for RSA authentication in protocol version 1. | |
337 | .Cm AuthorizedKeysFile | |
338 | may contain tokens of the form %T which are substituted during connection | |
339 | set-up. The following tokens are defined; %% is replaces by a literal '%', | |
340 | %h is replaced by the home directory of the user being authenticated and | |
341 | %u is replaced by the username of that user. | |
342 | After expansion, | |
343 | .Cm AuthorizedKeysFile | |
f740438e | 344 | is taken to be an absolute path or one relative to the user's home |
c8445989 | 345 | directory. |
346 | The default is | |
347 | .Dq .ssh/authorized_keys | |
eea39c02 | 348 | .It Cm Banner |
349 | In some jurisdictions, sending a warning message before authentication | |
350 | may be relevant for getting legal protection. | |
351 | The contents of the specified file are sent to the remote user before | |
352 | authentication is allowed. | |
353 | This option is only available for protocol version 2. | |
354 | .Pp | |
657297ff | 355 | .It Cm ChallengeResponseAuthentication |
356 | Specifies whether | |
357 | challenge response | |
358 | authentication is allowed. | |
359 | Currently there is only support for | |
360 | .Xr skey 1 | |
361 | authentication. | |
362 | The default is | |
363 | .Dq yes . | |
bf740959 | 364 | .It Cm CheckMail |
365 | Specifies whether | |
366 | .Nm | |
367 | should check for new mail for interactive logins. | |
368 | The default is | |
369 | .Dq no . | |
a18395da | 370 | .It Cm Ciphers |
371 | Specifies the ciphers allowed for protocol version 2. | |
372 | Multiple ciphers must be comma-separated. | |
373 | The default is | |
374 | .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. | |
3ffc6336 | 375 | .It Cm ClientAliveInterval |
376 | Sets a timeout interval in seconds after which if no data has been received | |
377 | from the client, | |
378 | .Nm | |
379 | will send a message through the encrypted | |
84fc17bf | 380 | channel to request a response from the client. |
381 | The default | |
3ffc6336 | 382 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 383 | This option applies to protocol version 2 only. |
3ffc6336 | 384 | .It Cm ClientAliveCountMax |
385 | Sets the number of client alive messages (see above) which may be | |
386 | sent without | |
387 | .Nm | |
388 | receiving any messages back from the client. If this threshold is | |
389 | reached while client alive messages are being sent, | |
390 | .Nm | |
391 | will disconnect the client, terminating the session. It is important | |
392 | to note that the use of client alive messages is very different from | |
84fc17bf | 393 | .Cm Keepalive |
394 | (below). The client alive messages are sent through the | |
3ffc6336 | 395 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 396 | option enabled by |
397 | .Cm Keepalive | |
398 | is spoofable. You want to use the client | |
3ffc6336 | 399 | alive mechanism when you are basing something important on |
400 | clients having an active connection to the server. | |
84fc17bf | 401 | .Pp |
402 | The default value is 3. If you set | |
403 | .Cm ClientAliveInterval | |
3ffc6336 | 404 | (above) to 15, and leave this value at the default, unresponsive ssh clients |
405 | will be disconnected after approximately 45 seconds. | |
bf740959 | 406 | .It Cm DenyGroups |
407 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 408 | by spaces. |
c6a69271 | 409 | Users whose primary group or supplementary group list matches |
410 | one of the patterns aren't allowed to log in. | |
bf740959 | 411 | .Ql \&* |
412 | and | |
413 | .Ql ? | |
414 | can be used as | |
4fe2af09 | 415 | wildcards in the patterns. |
c345cf9d | 416 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 417 | By default login is allowed regardless of the group list. |
bf740959 | 418 | .Pp |
419 | .It Cm DenyUsers | |
420 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 421 | by spaces. |
422 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 423 | .Ql \&* |
424 | and | |
425 | .Ql ? | |
4fe2af09 | 426 | can be used as wildcards in the patterns. |
c345cf9d | 427 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 428 | By default login is allowed regardless of the user name. |
1d1ffb87 | 429 | .It Cm GatewayPorts |
430 | Specifies whether remote hosts are allowed to connect to ports | |
431 | forwarded for the client. | |
432 | The argument must be | |
433 | .Dq yes | |
434 | or | |
435 | .Dq no . | |
436 | The default is | |
437 | .Dq no . | |
da89cf4d | 438 | .It Cm HostbasedAuthentication |
439 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
440 | with successful public key client host authentication is allowed | |
441 | (hostbased authentication). | |
442 | This option is similar to | |
443 | .Cm RhostsRSAAuthentication | |
444 | and applies to protocol version 2 only. | |
445 | The default is | |
446 | .Dq no . | |
bf740959 | 447 | .It Cm HostKey |
fa08c86b | 448 | Specifies the file containing the private host keys (default |
1d1ffb87 | 449 | .Pa /etc/ssh_host_key ) |
fa08c86b | 450 | used by SSH protocol versions 1 and 2. |
bf740959 | 451 | Note that |
452 | .Nm | |
42f11eb2 | 453 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 454 | It is possible to have multiple host key files. |
455 | .Dq rsa1 | |
456 | keys are used for version 1 and | |
457 | .Dq dsa | |
458 | or | |
459 | .Dq rsa | |
460 | are used for version 2 of the SSH protocol. | |
bf740959 | 461 | .It Cm IgnoreRhosts |
c8d54615 | 462 | Specifies that |
463 | .Pa .rhosts | |
f54651ce | 464 | and |
c8d54615 | 465 | .Pa .shosts |
da89cf4d | 466 | files will not be used in |
467 | .Cm RhostsAuthentication , | |
468 | .Cm RhostsRSAAuthentication | |
469 | or | |
470 | .Cm HostbasedAuthentication . | |
471 | .Pp | |
bf740959 | 472 | .Pa /etc/hosts.equiv |
473 | and | |
f54651ce | 474 | .Pa /etc/shosts.equiv |
4fe2af09 | 475 | are still used. |
f54651ce | 476 | The default is |
c8d54615 | 477 | .Dq yes . |
b4748e2f | 478 | .It Cm IgnoreUserKnownHosts |
479 | Specifies whether | |
480 | .Nm | |
481 | should ignore the user's | |
482 | .Pa $HOME/.ssh/known_hosts | |
483 | during | |
da89cf4d | 484 | .Cm RhostsRSAAuthentication |
485 | or | |
486 | .Cm HostbasedAuthentication . | |
b4748e2f | 487 | The default is |
488 | .Dq no . | |
bf740959 | 489 | .It Cm KeepAlive |
490 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 491 | other side. |
492 | If they are sent, death of the connection or crash of one | |
493 | of the machines will be properly noticed. | |
494 | However, this means that | |
bf740959 | 495 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 496 | find it annoying. |
d0c832f3 | 497 | On the other hand, if keepalives are not sent, |
bf740959 | 498 | sessions may hang indefinitely on the server, leaving |
499 | .Dq ghost | |
500 | users and consuming server resources. | |
501 | .Pp | |
502 | The default is | |
503 | .Dq yes | |
504 | (to send keepalives), and the server will notice | |
4fe2af09 | 505 | if the network goes down or the client host reboots. |
506 | This avoids infinitely hanging sessions. | |
bf740959 | 507 | .Pp |
508 | To disable keepalives, the value should be set to | |
509 | .Dq no | |
510 | in both the server and the client configuration files. | |
511 | .It Cm KerberosAuthentication | |
4fe2af09 | 512 | Specifies whether Kerberos authentication is allowed. |
513 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 514 | .Cm PasswordAuthentication |
515 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 516 | the Kerberos KDC. |
517 | To use this option, the server needs a | |
c345cf9d | 518 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 519 | Default is |
bf740959 | 520 | .Dq yes . |
521 | .It Cm KerberosOrLocalPasswd | |
522 | If set then if password authentication through Kerberos fails then | |
523 | the password will be validated via any additional local mechanism | |
524 | such as | |
188adeb2 | 525 | .Pa /etc/passwd . |
4fe2af09 | 526 | Default is |
bf740959 | 527 | .Dq yes . |
528 | .It Cm KerberosTgtPassing | |
529 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 530 | Default is |
bf740959 | 531 | .Dq no , |
532 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
533 | .It Cm KerberosTicketCleanup | |
534 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 535 | file on logout. |
536 | Default is | |
bf740959 | 537 | .Dq yes . |
538 | .It Cm KeyRegenerationInterval | |
da89cf4d | 539 | In protocol version 1, the ephemeral server key is automatically regenerated |
540 | after this many seconds (if it has been used). | |
4fe2af09 | 541 | The purpose of regeneration is to prevent |
bf740959 | 542 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 543 | stealing the keys. |
544 | The key is never stored anywhere. | |
545 | If the value is 0, the key is never regenerated. | |
546 | The default is 3600 (seconds). | |
bf740959 | 547 | .It Cm ListenAddress |
6e9944b8 | 548 | Specifies the local addresses |
da89cf4d | 549 | .Nm |
bf740959 | 550 | should listen on. |
6e9944b8 | 551 | The following forms may be used: |
552 | .Pp | |
553 | .Bl -item -offset indent -compact | |
554 | .It | |
555 | .Cm ListenAddress | |
b8a297f1 | 556 | .Sm off |
557 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
558 | .Sm on | |
6e9944b8 | 559 | .It |
560 | .Cm ListenAddress | |
b8a297f1 | 561 | .Sm off |
562 | .Ar host No | Ar IPv4_addr No : Ar port | |
563 | .Sm on | |
6e9944b8 | 564 | .It |
565 | .Cm ListenAddress | |
b8a297f1 | 566 | .Sm off |
567 | .Oo | |
568 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
569 | .Sm on | |
6e9944b8 | 570 | .El |
571 | .Pp | |
572 | If | |
b8a297f1 | 573 | .Ar port |
6e9944b8 | 574 | is not specified, |
da89cf4d | 575 | .Nm |
6e9944b8 | 576 | will listen on the address and all prior |
577 | .Cm Port | |
578 | options specified. The default is to listen on all local | |
579 | addresses. Multiple | |
580 | .Cm ListenAddress | |
581 | options are permitted. Additionally, any | |
582 | .Cm Port | |
583 | options must precede this option for non port qualified addresses. | |
bf740959 | 584 | .It Cm LoginGraceTime |
585 | The server disconnects after this time if the user has not | |
4fe2af09 | 586 | successfully logged in. |
587 | If the value is 0, there is no time limit. | |
bf740959 | 588 | The default is 600 (seconds). |
6a17f9c2 | 589 | .It Cm LogLevel |
590 | Gives the verbosity level that is used when logging messages from | |
591 | .Nm sshd . | |
592 | The possible values are: | |
59c97189 | 593 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
594 | The default is INFO. | |
6a17f9c2 | 595 | Logging with level DEBUG violates the privacy of users |
596 | and is not recommended. | |
b2552997 | 597 | .It Cm MACs |
598 | Specifies the available MAC (message authentication code) algorithms. | |
599 | The MAC algorithm is used in protocol version 2 | |
600 | for data integrity protection. | |
601 | Multiple algorithms must be comma-separated. | |
602 | The default is | |
47bf6266 | 603 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
089fbbd2 | 604 | .It Cm MaxStartups |
605 | Specifies the maximum number of concurrent unauthenticated connections to the | |
606 | .Nm | |
607 | daemon. | |
608 | Additional connections will be dropped until authentication succeeds or the | |
609 | .Cm LoginGraceTime | |
610 | expires for a connection. | |
611 | The default is 10. | |
c345cf9d | 612 | .Pp |
613 | Alternatively, random early drop can be enabled by specifying | |
614 | the three colon separated values | |
615 | .Dq start:rate:full | |
94ec8c6b | 616 | (e.g., "10:30:60"). |
c345cf9d | 617 | .Nm |
6f37606e | 618 | will refuse connection attempts with a probability of |
c345cf9d | 619 | .Dq rate/100 |
620 | (30%) | |
621 | if there are currently | |
622 | .Dq start | |
623 | (10) | |
624 | unauthenticated connections. | |
6f37606e | 625 | The probability increases linearly and all connection attempts |
c345cf9d | 626 | are refused if the number of unauthenticated connections reaches |
627 | .Dq full | |
628 | (60). | |
10f72868 | 629 | .It Cm PAMAuthenticationViaKbdInt |
630 | Specifies whether PAM challenge response authentication is allowed. This | |
631 | allows the use of most PAM challenge response authentication modules, but | |
632 | it will allow password authentication regardless of whether | |
633 | .Cm PasswordAuthentication | |
634 | is disabled. | |
635 | The default is | |
636 | .Dq no . | |
bf740959 | 637 | .It Cm PasswordAuthentication |
638 | Specifies whether password authentication is allowed. | |
639 | The default is | |
640 | .Dq yes . | |
641 | .It Cm PermitEmptyPasswords | |
642 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 643 | server allows login to accounts with empty password strings. |
644 | The default is | |
c8d54615 | 645 | .Dq no . |
bf740959 | 646 | .It Cm PermitRootLogin |
46f23b8d | 647 | Specifies whether root can login using |
bf740959 | 648 | .Xr ssh 1 . |
649 | The argument must be | |
650 | .Dq yes , | |
15853e93 | 651 | .Dq without-password , |
652 | .Dq forced-commands-only | |
bf740959 | 653 | or |
654 | .Dq no . | |
655 | The default is | |
656 | .Dq yes . | |
15853e93 | 657 | .Pp |
658 | If this option is set to | |
bf740959 | 659 | .Dq without-password |
15853e93 | 660 | password authentication is disabled for root. |
bf740959 | 661 | .Pp |
15853e93 | 662 | If this option is set to |
663 | .Dq forced-commands-only | |
664 | root login with public key authentication will be allowed, | |
665 | but only if the | |
bf740959 | 666 | .Ar command |
15853e93 | 667 | option has been specified |
bf740959 | 668 | (which may be useful for taking remote backups even if root login is |
15853e93 | 669 | normally not allowed). All other authentication methods are disabled |
670 | for root. | |
46f23b8d | 671 | .Pp |
672 | If this option is set to | |
673 | .Dq no | |
674 | root is not allowed to login. | |
0fbe8c74 | 675 | .It Cm PidFile |
676 | Specifies the file that contains the process identifier of the | |
677 | .Nm | |
678 | daemon. | |
679 | The default is | |
680 | .Pa /var/run/sshd.pid . | |
bf740959 | 681 | .It Cm Port |
682 | Specifies the port number that | |
683 | .Nm | |
4fe2af09 | 684 | listens on. |
685 | The default is 22. | |
48e671d5 | 686 | Multiple options of this type are permitted. |
da89cf4d | 687 | See also |
688 | .Cm ListenAddress . | |
4f4648f9 | 689 | .It Cm PrintLastLog |
690 | Specifies whether | |
691 | .Nm | |
692 | should print the date and time when the user last logged in. | |
693 | The default is | |
694 | .Dq yes . | |
bf740959 | 695 | .It Cm PrintMotd |
696 | Specifies whether | |
697 | .Nm | |
f54651ce | 698 | should print |
bf740959 | 699 | .Pa /etc/motd |
4fe2af09 | 700 | when a user logs in interactively. |
701 | (On some systems it is also printed by the shell, | |
bf740959 | 702 | .Pa /etc/profile , |
4fe2af09 | 703 | or equivalent.) |
704 | The default is | |
bf740959 | 705 | .Dq yes . |
f54651ce | 706 | .It Cm Protocol |
707 | Specifies the protocol versions | |
708 | .Nm | |
709 | should support. | |
710 | The possible values are | |
711 | .Dq 1 | |
712 | and | |
713 | .Dq 2 . | |
714 | Multiple versions must be comma-separated. | |
715 | The default is | |
6023325e | 716 | .Dq 2,1 . |
657297ff | 717 | .It Cm PubkeyAuthentication |
718 | Specifies whether public key authentication is allowed. | |
719 | The default is | |
720 | .Dq yes . | |
721 | Note that this option applies to protocol version 2 only. | |
61e96248 | 722 | .It Cm ReverseMappingCheck |
723 | Specifies whether | |
724 | .Nm | |
725 | should try to verify the remote host name and check that | |
726 | the resolved host name for the remote IP address maps back to the | |
727 | very same IP address. | |
728 | The default is | |
729 | .Dq no . | |
bf740959 | 730 | .It Cm RhostsAuthentication |
731 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 732 | files is sufficient. |
733 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 734 | .Cm RhostsRSAAuthentication |
735 | should be used | |
736 | instead, because it performs RSA-based host authentication in addition | |
737 | to normal rhosts or /etc/hosts.equiv authentication. | |
738 | The default is | |
739 | .Dq no . | |
da89cf4d | 740 | This option applies to protocol version 1 only. |
bf740959 | 741 | .It Cm RhostsRSAAuthentication |
742 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 743 | with successful RSA host authentication is allowed. |
744 | The default is | |
c8d54615 | 745 | .Dq no . |
da89cf4d | 746 | This option applies to protocol version 1 only. |
bf740959 | 747 | .It Cm RSAAuthentication |
4fe2af09 | 748 | Specifies whether pure RSA authentication is allowed. |
749 | The default is | |
bf740959 | 750 | .Dq yes . |
da89cf4d | 751 | This option applies to protocol version 1 only. |
bf740959 | 752 | .It Cm ServerKeyBits |
da89cf4d | 753 | Defines the number of bits in the ephemeral protocol version 1 server key. |
4fe2af09 | 754 | The minimum value is 512, and the default is 768. |
bf740959 | 755 | .It Cm StrictModes |
756 | Specifies whether | |
757 | .Nm | |
758 | should check file modes and ownership of the | |
4fe2af09 | 759 | user's files and home directory before accepting login. |
760 | This is normally desirable because novices sometimes accidentally leave their | |
761 | directory or files world-writable. | |
762 | The default is | |
bf740959 | 763 | .Dq yes . |
38c295d6 | 764 | .It Cm Subsystem |
94ec8c6b | 765 | Configures an external subsystem (e.g., file transfer daemon). |
766 | Arguments should be a subsystem name and a command to execute upon subsystem | |
767 | request. | |
b5e300c2 | 768 | The command |
769 | .Xr sftp-server 8 | |
770 | implements the | |
771 | .Dq sftp | |
772 | file transfer subsystem. | |
38c295d6 | 773 | By default no subsystems are defined. |
774 | Note that this option applies to protocol version 2 only. | |
bf740959 | 775 | .It Cm SyslogFacility |
776 | Gives the facility code that is used when logging messages from | |
777 | .Nm sshd . | |
778 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 779 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
780 | The default is AUTH. | |
bf740959 | 781 | .It Cm UseLogin |
782 | Specifies whether | |
783 | .Xr login 1 | |
fa649821 | 784 | is used for interactive login sessions. |
ff027d84 | 785 | The default is |
786 | .Dq no . | |
fa649821 | 787 | Note that |
788 | .Xr login 1 | |
c345cf9d | 789 | is never used for remote command execution. |
ff027d84 | 790 | Note also, that if this is enabled, |
791 | .Cm X11Forwarding | |
792 | will be disabled because | |
793 | .Xr login 1 | |
794 | does not know how to handle | |
795 | .Xr xauth 1 | |
796 | cookies. | |
bf740959 | 797 | .It Cm X11DisplayOffset |
798 | Specifies the first display number available for | |
799 | .Nm sshd Ns 's | |
4fe2af09 | 800 | X11 forwarding. |
801 | This prevents | |
bf740959 | 802 | .Nm |
803 | from interfering with real X11 servers. | |
c8d54615 | 804 | The default is 10. |
7b2ea3a1 | 805 | .It Cm X11Forwarding |
4fe2af09 | 806 | Specifies whether X11 forwarding is permitted. |
807 | The default is | |
c8d54615 | 808 | .Dq no . |
7b2ea3a1 | 809 | Note that disabling X11 forwarding does not improve security in any |
810 | way, as users can always install their own forwarders. | |
ff027d84 | 811 | X11 forwarding is automatically disabled if |
812 | .Cm UseLogin | |
813 | is enabled. | |
fa649821 | 814 | .It Cm XAuthLocation |
815 | Specifies the location of the | |
816 | .Xr xauth 1 | |
817 | program. | |
818 | The default is | |
819 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 820 | .El |
e2b1fb42 | 821 | .Ss Time Formats |
822 | .Pp | |
823 | .Nm | |
824 | command-line arguments and configuration file options that specify time | |
825 | may be expressed using a sequence of the form: | |
826 | .Sm off | |
827 | .Ar time Oo Ar qualifier Oc , | |
828 | .Sm on | |
829 | where | |
830 | .Ar time | |
831 | is a positive integer value and | |
832 | .Ar qualifier | |
833 | is one of the following: | |
834 | .Pp | |
835 | .Bl -tag -width Ds -compact -offset indent | |
836 | .It Cm <none> | |
837 | seconds | |
838 | .It Cm s | Cm S | |
839 | seconds | |
840 | .It Cm m | Cm M | |
841 | minutes | |
842 | .It Cm h | Cm H | |
843 | hours | |
844 | .It Cm d | Cm D | |
845 | days | |
846 | .It Cm w | Cm W | |
847 | weeks | |
848 | .El | |
849 | .Pp | |
850 | Each member of the sequence is added together to calculate | |
851 | the total time value. | |
852 | .Pp | |
853 | Time format examples: | |
854 | .Pp | |
855 | .Bl -tag -width Ds -compact -offset indent | |
856 | .It 600 | |
857 | 600 seconds (10 minutes) | |
858 | .It 10m | |
859 | 10 minutes | |
860 | .It 1h30m | |
861 | 1 hour 30 minutes (90 minutes) | |
862 | .El | |
bf740959 | 863 | .Sh LOGIN PROCESS |
864 | When a user successfully logs in, | |
865 | .Nm | |
866 | does the following: | |
867 | .Bl -enum -offset indent | |
868 | .It | |
869 | If the login is on a tty, and no command has been specified, | |
f54651ce | 870 | prints last login time and |
bf740959 | 871 | .Pa /etc/motd |
872 | (unless prevented in the configuration file or by | |
873 | .Pa $HOME/.hushlogin ; | |
874 | see the | |
f54651ce | 875 | .Sx FILES |
bf740959 | 876 | section). |
877 | .It | |
878 | If the login is on a tty, records login time. | |
879 | .It | |
880 | Checks | |
881 | .Pa /etc/nologin ; | |
882 | if it exists, prints contents and quits | |
883 | (unless root). | |
884 | .It | |
885 | Changes to run with normal user privileges. | |
886 | .It | |
887 | Sets up basic environment. | |
888 | .It | |
889 | Reads | |
890 | .Pa $HOME/.ssh/environment | |
891 | if it exists. | |
892 | .It | |
893 | Changes to user's home directory. | |
894 | .It | |
895 | If | |
896 | .Pa $HOME/.ssh/rc | |
897 | exists, runs it; else if | |
5f4fdfae | 898 | .Pa /etc/sshrc |
bf740959 | 899 | exists, runs |
4fe2af09 | 900 | it; otherwise runs xauth. |
901 | The | |
bf740959 | 902 | .Dq rc |
903 | files are given the X11 | |
904 | authentication protocol and cookie in standard input. | |
905 | .It | |
906 | Runs user's shell or command. | |
907 | .El | |
908 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 909 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 910 | is the default file that lists the public keys that are |
911 | permitted for RSA authentication in protocol version 1 | |
912 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 913 | in protocol version 2. |
96a7b0cc | 914 | .Cm AuthorizedKeysFile |
c8445989 | 915 | may be used to specify an alternative file. |
8abcdba4 | 916 | .Pp |
4fe2af09 | 917 | Each line of the file contains one |
bf740959 | 918 | key (empty lines and lines starting with a |
919 | .Ql # | |
920 | are ignored as | |
4fe2af09 | 921 | comments). |
8abcdba4 | 922 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 923 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 924 | Each protocol version 2 public key consists of: |
925 | options, keytype, base64 encoded key, comment. | |
926 | The options fields | |
927 | are optional; its presence is determined by whether the line starts | |
bf740959 | 928 | with a number or not (the option field never starts with a number). |
8abcdba4 | 929 | The bits, exponent, modulus and comment fields give the RSA key for |
930 | protocol version 1; the | |
bf740959 | 931 | comment field is not used for anything (but may be convenient for the |
932 | user to identify the key). | |
8abcdba4 | 933 | For protocol version 2 the keytype is |
934 | .Dq ssh-dss | |
935 | or | |
936 | .Dq ssh-rsa . | |
bf740959 | 937 | .Pp |
938 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 939 | (because of the size of the RSA key modulus). |
940 | You don't want to type them in; instead, copy the | |
c0ecc314 | 941 | .Pa identity.pub , |
8abcdba4 | 942 | .Pa id_dsa.pub |
c0ecc314 | 943 | or the |
944 | .Pa id_rsa.pub | |
bf740959 | 945 | file and edit it. |
946 | .Pp | |
c345cf9d | 947 | The options (if present) consist of comma-separated option |
4fe2af09 | 948 | specifications. |
949 | No spaces are permitted, except within double quotes. | |
bf740959 | 950 | The following option specifications are supported: |
951 | .Bl -tag -width Ds | |
952 | .It Cm from="pattern-list" | |
953 | Specifies that in addition to RSA authentication, the canonical name | |
954 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 955 | patterns |
956 | .Pf ( Ql * | |
957 | and | |
958 | .Ql ? | |
959 | serve as wildcards). | |
960 | The list may also contain | |
961 | patterns negated by prefixing them with | |
962 | .Ql ! ; | |
963 | if the canonical host name matches a negated pattern, the key is not accepted. | |
964 | The purpose | |
bf740959 | 965 | of this option is to optionally increase security: RSA authentication |
966 | by itself does not trust the network or name servers or anything (but | |
967 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 968 | permits an intruder to log in from anywhere in the world. |
969 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 970 | servers and/or routers would have to be compromised in addition to |
971 | just the key). | |
972 | .It Cm command="command" | |
973 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 974 | authentication. |
975 | The command supplied by the user (if any) is ignored. | |
bf740959 | 976 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 977 | otherwise it is run without a tty. |
61e96248 | 978 | Note that if you want a 8-bit clean channel, |
979 | you must not request a pty or should specify | |
980 | .Cm no-pty . | |
4fe2af09 | 981 | A quote may be included in the command by quoting it with a backslash. |
982 | This option might be useful | |
983 | to restrict certain RSA keys to perform just a specific operation. | |
984 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 985 | Note that the client may specify TCP/IP and/or X11 |
986 | forwarding unless they are explicitly prohibited. | |
bf740959 | 987 | .It Cm environment="NAME=value" |
988 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 989 | logging in using this key. |
990 | Environment variables set this way | |
991 | override other default environment values. | |
992 | Multiple options of this type are permitted. | |
bf740959 | 993 | .It Cm no-port-forwarding |
994 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 995 | Any port forward requests by the client will return an error. |
996 | This might be used, e.g., in connection with the | |
bf740959 | 997 | .Cm command |
998 | option. | |
999 | .It Cm no-X11-forwarding | |
1000 | Forbids X11 forwarding when this key is used for authentication. | |
1001 | Any X11 forward requests by the client will return an error. | |
1002 | .It Cm no-agent-forwarding | |
1003 | Forbids authentication agent forwarding when this key is used for | |
1004 | authentication. | |
1005 | .It Cm no-pty | |
1006 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 1007 | .It Cm permitopen="host:port" |
1008 | Limit local | |
1009 | .Li ``ssh -L'' | |
b2ae83b8 | 1010 | port forwarding such that it may only connect to the specified host and |
dc504afd | 1011 | port. Multiple |
1012 | .Cm permitopen | |
b2ae83b8 | 1013 | options may be applied separated by commas. No pattern matching is |
dc504afd | 1014 | performed on the specified hostnames, they must be literal domains or |
1015 | addresses. | |
bf740959 | 1016 | .El |
1017 | .Ss Examples | |
1018 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
1019 | .Pp | |
1020 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
1021 | .Pp | |
1022 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 1023 | .Pp |
1024 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 1025 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 1026 | The |
1d1ffb87 | 1027 | .Pa /etc/ssh_known_hosts , |
f54651ce | 1028 | and |
f49bc4f7 | 1029 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 1030 | files contain host public keys for all known hosts. |
1031 | The global file should | |
1032 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 1033 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 1034 | its key is added to the per-user file. |
bf740959 | 1035 | .Pp |
1036 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 1037 | bits, exponent, modulus, comment. |
1038 | The fields are separated by spaces. | |
bf740959 | 1039 | .Pp |
1040 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
1041 | wildcards); each pattern in turn is matched against the canonical host | |
1042 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 1043 | name (when authenticating a server). |
1044 | A pattern may also be preceded by | |
bf740959 | 1045 | .Ql ! |
1046 | to indicate negation: if the host name matches a negated | |
1047 | pattern, it is not accepted (by that line) even if it matched another | |
1048 | pattern on the line. | |
1049 | .Pp | |
1d1ffb87 | 1050 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 1051 | can be obtained, e.g., from |
5f4fdfae | 1052 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 1053 | The optional comment field continues to the end of the line, and is not used. |
1054 | .Pp | |
1055 | Lines starting with | |
1056 | .Ql # | |
1057 | and empty lines are ignored as comments. | |
1058 | .Pp | |
1059 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 1060 | matching line has the proper key. |
1061 | It is thus permissible (but not | |
bf740959 | 1062 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 1063 | names. |
1064 | This will inevitably happen when short forms of host names | |
1065 | from different domains are put in the file. | |
1066 | It is possible | |
bf740959 | 1067 | that the files contain conflicting information; authentication is |
1068 | accepted if valid information can be found from either file. | |
1069 | .Pp | |
1070 | Note that the lines in these files are typically hundreds of characters | |
1071 | long, and you definitely don't want to type in the host keys by hand. | |
1072 | Rather, generate them by a script | |
f54651ce | 1073 | or by taking |
5f4fdfae | 1074 | .Pa /etc/ssh_host_key.pub |
bf740959 | 1075 | and adding the host names at the front. |
1076 | .Ss Examples | |
da89cf4d | 1077 | .Bd -literal |
1078 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
1079 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
1080 | .Ed | |
bf740959 | 1081 | .Sh FILES |
1082 | .Bl -tag -width Ds | |
5f4fdfae | 1083 | .It Pa /etc/sshd_config |
bf740959 | 1084 | Contains configuration data for |
1085 | .Nm sshd . | |
1086 | This file should be writable by root only, but it is recommended | |
1087 | (though not necessary) that it be world-readable. | |
b8dc87d3 | 1088 | .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
da89cf4d | 1089 | These three files contain the private parts of the host keys. |
b8dc87d3 | 1090 | These files should only be owned by root, readable only by root, and not |
bf740959 | 1091 | accessible to others. |
1092 | Note that | |
1093 | .Nm | |
1094 | does not start if this file is group/world-accessible. | |
b8dc87d3 | 1095 | .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
da89cf4d | 1096 | These three files contain the public parts of the host keys. |
b8dc87d3 | 1097 | These files should be world-readable but writable only by |
4fe2af09 | 1098 | root. |
b8dc87d3 | 1099 | Their contents should match the respective private parts. |
1100 | These files are not | |
1101 | really used for anything; they are provided for the convenience of | |
1102 | the user so their contents can be copied to known hosts files. | |
1103 | These files are created using | |
bf740959 | 1104 | .Xr ssh-keygen 1 . |
e2432638 | 1105 | .It Pa /etc/moduli |
c523303b | 1106 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
5f4fdfae | 1107 | .It Pa /var/run/sshd.pid |
bf740959 | 1108 | Contains the process ID of the |
1109 | .Nm | |
1110 | listening for connections (if there are several daemons running | |
1111 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1112 | started last). |
c345cf9d | 1113 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1114 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 1115 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 1116 | This file must be readable by root (which may on some machines imply |
1117 | it being world-readable if the user's home directory resides on an NFS | |
1118 | volume). | |
1119 | It is recommended that it not be accessible by others. | |
1120 | The format of this file is described above. | |
1121 | Users will place the contents of their | |
96a7b0cc | 1122 | .Pa identity.pub , |
1d1ffb87 | 1123 | .Pa id_dsa.pub |
c0ecc314 | 1124 | and/or |
1125 | .Pa id_rsa.pub | |
1d1ffb87 | 1126 | files into this file, as described in |
1127 | .Xr ssh-keygen 1 . | |
5f4fdfae | 1128 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1129 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 1130 | authentication or protocol version 2 hostbased authentication |
1131 | to check the public key of the host. | |
4fe2af09 | 1132 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 1133 | The client uses the same files |
e91c60f2 | 1134 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1135 | These files should be writable only by root/the owner. |
5f4fdfae | 1136 | .Pa /etc/ssh_known_hosts |
bf740959 | 1137 | should be world-readable, and |
1138 | .Pa $HOME/.ssh/known_hosts | |
1139 | can but need not be world-readable. | |
1140 | .It Pa /etc/nologin | |
f54651ce | 1141 | If this file exists, |
bf740959 | 1142 | .Nm |
4fe2af09 | 1143 | refuses to let anyone except root log in. |
1144 | The contents of the file | |
bf740959 | 1145 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1146 | refused. |
1147 | The file should be world-readable. | |
bf740959 | 1148 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
1149 | If compiled with | |
1150 | .Sy LIBWRAP | |
1151 | support, tcp-wrappers access controls may be defined here as described in | |
1152 | .Xr hosts_access 5 . | |
1153 | .It Pa $HOME/.rhosts | |
1154 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1155 | line. |
1156 | The given user on the corresponding host is permitted to log in | |
1157 | without password. | |
1158 | The same file is used by rlogind and rshd. | |
bf740959 | 1159 | The file must |
1160 | be writable only by the user; it is recommended that it not be | |
1161 | accessible by others. | |
1162 | .Pp | |
4fe2af09 | 1163 | If is also possible to use netgroups in the file. |
1164 | Either host or user | |
bf740959 | 1165 | name may be of the form +@groupname to specify all hosts or all users |
1166 | in the group. | |
1167 | .It Pa $HOME/.shosts | |
1168 | For ssh, | |
1169 | this file is exactly the same as for | |
1170 | .Pa .rhosts . | |
1171 | However, this file is | |
1172 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1173 | .It Pa /etc/hosts.equiv |
bf740959 | 1174 | This file is used during |
1175 | .Pa .rhosts | |
4fe2af09 | 1176 | authentication. |
1177 | In the simplest form, this file contains host names, one per line. | |
1178 | Users on | |
bf740959 | 1179 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1180 | have the same user name on both machines. |
1181 | The host name may also be | |
bf740959 | 1182 | followed by a user name; such users are permitted to log in as |
1183 | .Em any | |
4fe2af09 | 1184 | user on this machine (except root). |
1185 | Additionally, the syntax | |
bf740959 | 1186 | .Dq +@group |
4fe2af09 | 1187 | can be used to specify netgroups. |
1188 | Negated entries start with | |
bf740959 | 1189 | .Ql \&- . |
1190 | .Pp | |
1191 | If the client host/user is successfully matched in this file, login is | |
1192 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1193 | same. |
1194 | Additionally, successful RSA host authentication is normally required. | |
1195 | This file must be writable only by root; it is recommended | |
bf740959 | 1196 | that it be world-readable. |
1197 | .Pp | |
1198 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1199 | .Pa hosts.equiv . | |
1200 | Beware that it really means that the named user(s) can log in as | |
1201 | .Em anybody , | |
1202 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1203 | binaries and directories. |
1204 | Using a user name practically grants the user root access. | |
1205 | The only valid use for user names that I can think | |
bf740959 | 1206 | of is in negative entries. |
1207 | .Pp | |
1208 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1209 | .It Pa /etc/shosts.equiv |
bf740959 | 1210 | This is processed exactly as |
1211 | .Pa /etc/hosts.equiv . | |
1212 | However, this file may be useful in environments that want to run both | |
1213 | rsh/rlogin and ssh. | |
1214 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1215 | This file is read into the environment at login (if it exists). |
1216 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1217 | .Ql # ) , |
4fe2af09 | 1218 | and assignment lines of the form name=value. |
1219 | The file should be writable | |
bf740959 | 1220 | only by the user; it need not be readable by anyone else. |
1221 | .It Pa $HOME/.ssh/rc | |
1222 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1223 | environment files but before starting the user's shell or command. |
1224 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 1225 | standard input (and |
1226 | .Ev DISPLAY | |
4fe2af09 | 1227 | in environment). |
1228 | This must call | |
bf740959 | 1229 | .Xr xauth 1 |
1230 | in that case. | |
1231 | .Pp | |
1232 | The primary purpose of this file is to run any initialization routines | |
1233 | which may be needed before the user's home directory becomes | |
1234 | accessible; AFS is a particular example of such an environment. | |
1235 | .Pp | |
1236 | This file will probably contain some initialization code followed by | |
da89cf4d | 1237 | something similar to: |
1238 | .Bd -literal | |
1239 | if read proto cookie; then | |
1240 | echo add $DISPLAY $proto $cookie | xauth -q - | |
1241 | fi | |
1242 | .Ed | |
bf740959 | 1243 | .Pp |
1244 | If this file does not exist, | |
5f4fdfae | 1245 | .Pa /etc/sshrc |
bf740959 | 1246 | is run, and if that |
1247 | does not exist either, xauth is used to store the cookie. | |
1248 | .Pp | |
1249 | This file should be writable only by the user, and need not be | |
1250 | readable by anyone else. | |
5f4fdfae | 1251 | .It Pa /etc/sshrc |
bf740959 | 1252 | Like |
1253 | .Pa $HOME/.ssh/rc . | |
1254 | This can be used to specify | |
4fe2af09 | 1255 | machine-specific login-time initializations globally. |
1256 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1257 | .El |
fa08c86b | 1258 | .Sh AUTHORS |
7f5c4295 | 1259 | OpenSSH is a derivative of the original and free |
1260 | ssh 1.2.12 release by Tatu Ylonen. | |
1261 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1262 | Theo de Raadt and Dug Song | |
1263 | removed many bugs, re-added newer features and | |
1264 | created OpenSSH. | |
1265 | Markus Friedl contributed the support for SSH | |
1266 | protocol versions 1.5 and 2.0. | |
bf740959 | 1267 | .Sh SEE ALSO |
e2432638 | 1268 | .Xr moduli 5 , |
a5a2da3b | 1269 | .Xr rlogin 1 , |
1270 | .Xr rsh 1 , | |
bf740959 | 1271 | .Xr scp 1 , |
61e96248 | 1272 | .Xr sftp 1 , |
bf740959 | 1273 | .Xr ssh 1 , |
1274 | .Xr ssh-add 1 , | |
1275 | .Xr ssh-agent 1 , | |
1276 | .Xr ssh-keygen 1 , | |
a5a2da3b | 1277 | .Xr sftp-server 8 |
2cad6cef | 1278 | .Rs |
1279 | .%A T. Ylonen | |
1280 | .%A T. Kivinen | |
1281 | .%A M. Saarinen | |
1282 | .%A T. Rinne | |
1283 | .%A S. Lehtinen | |
1284 | .%T "SSH Protocol Architecture" | |
1285 | .%N draft-ietf-secsh-architecture-07.txt | |
1286 | .%D January 2001 | |
1287 | .%O work in progress material | |
1288 | .Re | |
da89cf4d | 1289 | .Rs |
1290 | .%A M. Friedl | |
1291 | .%A N. Provos | |
1292 | .%A W. A. Simpson | |
1293 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
7d747e89 | 1294 | .%N draft-ietf-secsh-dh-group-exchange-01.txt |
1295 | .%D April 2001 | |
da89cf4d | 1296 | .%O work in progress material |
1297 | .Re |