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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 | .\" |
98143cfc | 37 | .\" $OpenBSD: sshd.8,v 1.121 2001/05/04 14:21:56 stevesk Exp $ |
bf740959 | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
da89cf4d | 43 | .Nd OpenSSH SSH daemon |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm sshd | |
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 , |
179 | by executing itself with the name it was started as, ie. | |
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 |
eea39c02 | 334 | .It Cm Banner |
335 | In some jurisdictions, sending a warning message before authentication | |
336 | may be relevant for getting legal protection. | |
337 | The contents of the specified file are sent to the remote user before | |
338 | authentication is allowed. | |
339 | This option is only available for protocol version 2. | |
340 | .Pp | |
657297ff | 341 | .It Cm ChallengeResponseAuthentication |
342 | Specifies whether | |
343 | challenge response | |
344 | authentication is allowed. | |
345 | Currently there is only support for | |
346 | .Xr skey 1 | |
347 | authentication. | |
348 | The default is | |
349 | .Dq yes . | |
f54651ce | 350 | .It Cm Ciphers |
351 | Specifies the ciphers allowed for protocol version 2. | |
352 | Multiple ciphers must be comma-separated. | |
353 | The default is | |
7a6c39a3 | 354 | .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. |
bf740959 | 355 | .It Cm CheckMail |
356 | Specifies whether | |
357 | .Nm | |
358 | should check for new mail for interactive logins. | |
359 | The default is | |
360 | .Dq no . | |
3ffc6336 | 361 | .It Cm ClientAliveInterval |
362 | Sets a timeout interval in seconds after which if no data has been received | |
363 | from the client, | |
364 | .Nm | |
365 | will send a message through the encrypted | |
84fc17bf | 366 | channel to request a response from the client. |
367 | The default | |
3ffc6336 | 368 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 369 | This option applies to protocol version 2 only. |
3ffc6336 | 370 | .It Cm ClientAliveCountMax |
371 | Sets the number of client alive messages (see above) which may be | |
372 | sent without | |
373 | .Nm | |
374 | receiving any messages back from the client. If this threshold is | |
375 | reached while client alive messages are being sent, | |
376 | .Nm | |
377 | will disconnect the client, terminating the session. It is important | |
378 | to note that the use of client alive messages is very different from | |
84fc17bf | 379 | .Cm Keepalive |
380 | (below). The client alive messages are sent through the | |
3ffc6336 | 381 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 382 | option enabled by |
383 | .Cm Keepalive | |
384 | is spoofable. You want to use the client | |
3ffc6336 | 385 | alive mechanism when you are basing something important on |
386 | clients having an active connection to the server. | |
84fc17bf | 387 | .Pp |
388 | The default value is 3. If you set | |
389 | .Cm ClientAliveInterval | |
3ffc6336 | 390 | (above) to 15, and leave this value at the default, unresponsive ssh clients |
391 | will be disconnected after approximately 45 seconds. | |
bf740959 | 392 | .It Cm DenyGroups |
393 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 394 | by spaces. |
c6a69271 | 395 | Users whose primary group or supplementary group list matches |
396 | one of the patterns aren't allowed to log in. | |
bf740959 | 397 | .Ql \&* |
398 | and | |
399 | .Ql ? | |
400 | can be used as | |
4fe2af09 | 401 | wildcards in the patterns. |
c345cf9d | 402 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 403 | By default login is allowed regardless of the group list. |
bf740959 | 404 | .Pp |
405 | .It Cm DenyUsers | |
406 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 407 | by spaces. |
408 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 409 | .Ql \&* |
410 | and | |
411 | .Ql ? | |
4fe2af09 | 412 | can be used as wildcards in the patterns. |
c345cf9d | 413 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 414 | By default login is allowed regardless of the user name. |
1d1ffb87 | 415 | .It Cm GatewayPorts |
416 | Specifies whether remote hosts are allowed to connect to ports | |
417 | forwarded for the client. | |
418 | The argument must be | |
419 | .Dq yes | |
420 | or | |
421 | .Dq no . | |
422 | The default is | |
423 | .Dq no . | |
da89cf4d | 424 | .It Cm HostbasedAuthentication |
425 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
426 | with successful public key client host authentication is allowed | |
427 | (hostbased authentication). | |
428 | This option is similar to | |
429 | .Cm RhostsRSAAuthentication | |
430 | and applies to protocol version 2 only. | |
431 | The default is | |
432 | .Dq no . | |
bf740959 | 433 | .It Cm HostKey |
fa08c86b | 434 | Specifies the file containing the private host keys (default |
1d1ffb87 | 435 | .Pa /etc/ssh_host_key ) |
fa08c86b | 436 | used by SSH protocol versions 1 and 2. |
bf740959 | 437 | Note that |
438 | .Nm | |
42f11eb2 | 439 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 440 | It is possible to have multiple host key files. |
441 | .Dq rsa1 | |
442 | keys are used for version 1 and | |
443 | .Dq dsa | |
444 | or | |
445 | .Dq rsa | |
446 | are used for version 2 of the SSH protocol. | |
bf740959 | 447 | .It Cm IgnoreRhosts |
c8d54615 | 448 | Specifies that |
449 | .Pa .rhosts | |
f54651ce | 450 | and |
c8d54615 | 451 | .Pa .shosts |
da89cf4d | 452 | files will not be used in |
453 | .Cm RhostsAuthentication , | |
454 | .Cm RhostsRSAAuthentication | |
455 | or | |
456 | .Cm HostbasedAuthentication . | |
457 | .Pp | |
bf740959 | 458 | .Pa /etc/hosts.equiv |
459 | and | |
f54651ce | 460 | .Pa /etc/shosts.equiv |
4fe2af09 | 461 | are still used. |
f54651ce | 462 | The default is |
c8d54615 | 463 | .Dq yes . |
b4748e2f | 464 | .It Cm IgnoreUserKnownHosts |
465 | Specifies whether | |
466 | .Nm | |
467 | should ignore the user's | |
468 | .Pa $HOME/.ssh/known_hosts | |
469 | during | |
da89cf4d | 470 | .Cm RhostsRSAAuthentication |
471 | or | |
472 | .Cm HostbasedAuthentication . | |
b4748e2f | 473 | The default is |
474 | .Dq no . | |
bf740959 | 475 | .It Cm KeepAlive |
476 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 477 | other side. |
478 | If they are sent, death of the connection or crash of one | |
479 | of the machines will be properly noticed. | |
480 | However, this means that | |
bf740959 | 481 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 482 | find it annoying. |
d0c832f3 | 483 | On the other hand, if keepalives are not sent, |
bf740959 | 484 | sessions may hang indefinitely on the server, leaving |
485 | .Dq ghost | |
486 | users and consuming server resources. | |
487 | .Pp | |
488 | The default is | |
489 | .Dq yes | |
490 | (to send keepalives), and the server will notice | |
4fe2af09 | 491 | if the network goes down or the client host reboots. |
492 | This avoids infinitely hanging sessions. | |
bf740959 | 493 | .Pp |
494 | To disable keepalives, the value should be set to | |
495 | .Dq no | |
496 | in both the server and the client configuration files. | |
497 | .It Cm KerberosAuthentication | |
4fe2af09 | 498 | Specifies whether Kerberos authentication is allowed. |
499 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 500 | .Cm PasswordAuthentication |
501 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 502 | the Kerberos KDC. |
503 | To use this option, the server needs a | |
c345cf9d | 504 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 505 | Default is |
bf740959 | 506 | .Dq yes . |
507 | .It Cm KerberosOrLocalPasswd | |
508 | If set then if password authentication through Kerberos fails then | |
509 | the password will be validated via any additional local mechanism | |
510 | such as | |
188adeb2 | 511 | .Pa /etc/passwd . |
4fe2af09 | 512 | Default is |
bf740959 | 513 | .Dq yes . |
514 | .It Cm KerberosTgtPassing | |
515 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 516 | Default is |
bf740959 | 517 | .Dq no , |
518 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
519 | .It Cm KerberosTicketCleanup | |
520 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 521 | file on logout. |
522 | Default is | |
bf740959 | 523 | .Dq yes . |
524 | .It Cm KeyRegenerationInterval | |
da89cf4d | 525 | In protocol version 1, the ephemeral server key is automatically regenerated |
526 | after this many seconds (if it has been used). | |
4fe2af09 | 527 | The purpose of regeneration is to prevent |
bf740959 | 528 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 529 | stealing the keys. |
530 | The key is never stored anywhere. | |
531 | If the value is 0, the key is never regenerated. | |
532 | The default is 3600 (seconds). | |
bf740959 | 533 | .It Cm ListenAddress |
6e9944b8 | 534 | Specifies the local addresses |
da89cf4d | 535 | .Nm |
bf740959 | 536 | should listen on. |
6e9944b8 | 537 | The following forms may be used: |
538 | .Pp | |
539 | .Bl -item -offset indent -compact | |
540 | .It | |
541 | .Cm ListenAddress | |
b8a297f1 | 542 | .Sm off |
543 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
544 | .Sm on | |
6e9944b8 | 545 | .It |
546 | .Cm ListenAddress | |
b8a297f1 | 547 | .Sm off |
548 | .Ar host No | Ar IPv4_addr No : Ar port | |
549 | .Sm on | |
6e9944b8 | 550 | .It |
551 | .Cm ListenAddress | |
b8a297f1 | 552 | .Sm off |
553 | .Oo | |
554 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
555 | .Sm on | |
6e9944b8 | 556 | .El |
557 | .Pp | |
558 | If | |
b8a297f1 | 559 | .Ar port |
6e9944b8 | 560 | is not specified, |
da89cf4d | 561 | .Nm |
6e9944b8 | 562 | will listen on the address and all prior |
563 | .Cm Port | |
564 | options specified. The default is to listen on all local | |
565 | addresses. Multiple | |
566 | .Cm ListenAddress | |
567 | options are permitted. Additionally, any | |
568 | .Cm Port | |
569 | options must precede this option for non port qualified addresses. | |
bf740959 | 570 | .It Cm LoginGraceTime |
571 | The server disconnects after this time if the user has not | |
4fe2af09 | 572 | successfully logged in. |
573 | If the value is 0, there is no time limit. | |
bf740959 | 574 | The default is 600 (seconds). |
6a17f9c2 | 575 | .It Cm LogLevel |
576 | Gives the verbosity level that is used when logging messages from | |
577 | .Nm sshd . | |
578 | The possible values are: | |
59c97189 | 579 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
580 | The default is INFO. | |
6a17f9c2 | 581 | Logging with level DEBUG violates the privacy of users |
582 | and is not recommended. | |
b2552997 | 583 | .It Cm MACs |
584 | Specifies the available MAC (message authentication code) algorithms. | |
585 | The MAC algorithm is used in protocol version 2 | |
586 | for data integrity protection. | |
587 | Multiple algorithms must be comma-separated. | |
588 | The default is | |
589 | .Pp | |
590 | .Bd -literal | |
7a6c39a3 | 591 | ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com, |
b2552997 | 592 | hmac-sha1-96,hmac-md5-96'' |
593 | .Ed | |
089fbbd2 | 594 | .It Cm MaxStartups |
595 | Specifies the maximum number of concurrent unauthenticated connections to the | |
596 | .Nm | |
597 | daemon. | |
598 | Additional connections will be dropped until authentication succeeds or the | |
599 | .Cm LoginGraceTime | |
600 | expires for a connection. | |
601 | The default is 10. | |
c345cf9d | 602 | .Pp |
603 | Alternatively, random early drop can be enabled by specifying | |
604 | the three colon separated values | |
605 | .Dq start:rate:full | |
94ec8c6b | 606 | (e.g., "10:30:60"). |
c345cf9d | 607 | .Nm |
6f37606e | 608 | will refuse connection attempts with a probability of |
c345cf9d | 609 | .Dq rate/100 |
610 | (30%) | |
611 | if there are currently | |
612 | .Dq start | |
613 | (10) | |
614 | unauthenticated connections. | |
6f37606e | 615 | The probability increases linearly and all connection attempts |
c345cf9d | 616 | are refused if the number of unauthenticated connections reaches |
617 | .Dq full | |
618 | (60). | |
10f72868 | 619 | .It Cm PAMAuthenticationViaKbdInt |
620 | Specifies whether PAM challenge response authentication is allowed. This | |
621 | allows the use of most PAM challenge response authentication modules, but | |
622 | it will allow password authentication regardless of whether | |
623 | .Cm PasswordAuthentication | |
624 | is disabled. | |
625 | The default is | |
626 | .Dq no . | |
bf740959 | 627 | .It Cm PasswordAuthentication |
628 | Specifies whether password authentication is allowed. | |
629 | The default is | |
630 | .Dq yes . | |
631 | .It Cm PermitEmptyPasswords | |
632 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 633 | server allows login to accounts with empty password strings. |
634 | The default is | |
c8d54615 | 635 | .Dq no . |
bf740959 | 636 | .It Cm PermitRootLogin |
46f23b8d | 637 | Specifies whether root can login using |
bf740959 | 638 | .Xr ssh 1 . |
639 | The argument must be | |
640 | .Dq yes , | |
15853e93 | 641 | .Dq without-password , |
642 | .Dq forced-commands-only | |
bf740959 | 643 | or |
644 | .Dq no . | |
645 | The default is | |
646 | .Dq yes . | |
15853e93 | 647 | .Pp |
648 | If this option is set to | |
bf740959 | 649 | .Dq without-password |
15853e93 | 650 | password authentication is disabled for root. |
bf740959 | 651 | .Pp |
15853e93 | 652 | If this option is set to |
653 | .Dq forced-commands-only | |
654 | root login with public key authentication will be allowed, | |
655 | but only if the | |
bf740959 | 656 | .Ar command |
15853e93 | 657 | option has been specified |
bf740959 | 658 | (which may be useful for taking remote backups even if root login is |
15853e93 | 659 | normally not allowed). All other authentication methods are disabled |
660 | for root. | |
46f23b8d | 661 | .Pp |
662 | If this option is set to | |
663 | .Dq no | |
664 | root is not allowed to login. | |
0fbe8c74 | 665 | .It Cm PidFile |
666 | Specifies the file that contains the process identifier of the | |
667 | .Nm | |
668 | daemon. | |
669 | The default is | |
670 | .Pa /var/run/sshd.pid . | |
bf740959 | 671 | .It Cm Port |
672 | Specifies the port number that | |
673 | .Nm | |
4fe2af09 | 674 | listens on. |
675 | The default is 22. | |
48e671d5 | 676 | Multiple options of this type are permitted. |
da89cf4d | 677 | See also |
678 | .Cm ListenAddress . | |
4f4648f9 | 679 | .It Cm PrintLastLog |
680 | Specifies whether | |
681 | .Nm | |
682 | should print the date and time when the user last logged in. | |
683 | The default is | |
684 | .Dq yes . | |
bf740959 | 685 | .It Cm PrintMotd |
686 | Specifies whether | |
687 | .Nm | |
f54651ce | 688 | should print |
bf740959 | 689 | .Pa /etc/motd |
4fe2af09 | 690 | when a user logs in interactively. |
691 | (On some systems it is also printed by the shell, | |
bf740959 | 692 | .Pa /etc/profile , |
4fe2af09 | 693 | or equivalent.) |
694 | The default is | |
bf740959 | 695 | .Dq yes . |
f54651ce | 696 | .It Cm Protocol |
697 | Specifies the protocol versions | |
698 | .Nm | |
699 | should support. | |
700 | The possible values are | |
701 | .Dq 1 | |
702 | and | |
703 | .Dq 2 . | |
704 | Multiple versions must be comma-separated. | |
705 | The default is | |
6023325e | 706 | .Dq 2,1 . |
657297ff | 707 | .It Cm PubkeyAuthentication |
708 | Specifies whether public key authentication is allowed. | |
709 | The default is | |
710 | .Dq yes . | |
711 | Note that this option applies to protocol version 2 only. | |
61e96248 | 712 | .It Cm ReverseMappingCheck |
713 | Specifies whether | |
714 | .Nm | |
715 | should try to verify the remote host name and check that | |
716 | the resolved host name for the remote IP address maps back to the | |
717 | very same IP address. | |
718 | The default is | |
719 | .Dq no . | |
bf740959 | 720 | .It Cm RhostsAuthentication |
721 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 722 | files is sufficient. |
723 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 724 | .Cm RhostsRSAAuthentication |
725 | should be used | |
726 | instead, because it performs RSA-based host authentication in addition | |
727 | to normal rhosts or /etc/hosts.equiv authentication. | |
728 | The default is | |
729 | .Dq no . | |
da89cf4d | 730 | This option applies to protocol version 1 only. |
bf740959 | 731 | .It Cm RhostsRSAAuthentication |
732 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 733 | with successful RSA host authentication is allowed. |
734 | The default is | |
c8d54615 | 735 | .Dq no . |
da89cf4d | 736 | This option applies to protocol version 1 only. |
bf740959 | 737 | .It Cm RSAAuthentication |
4fe2af09 | 738 | Specifies whether pure RSA authentication is allowed. |
739 | The default is | |
bf740959 | 740 | .Dq yes . |
da89cf4d | 741 | This option applies to protocol version 1 only. |
bf740959 | 742 | .It Cm ServerKeyBits |
da89cf4d | 743 | Defines the number of bits in the ephemeral protocol version 1 server key. |
4fe2af09 | 744 | The minimum value is 512, and the default is 768. |
bf740959 | 745 | .It Cm StrictModes |
746 | Specifies whether | |
747 | .Nm | |
748 | should check file modes and ownership of the | |
4fe2af09 | 749 | user's files and home directory before accepting login. |
750 | This is normally desirable because novices sometimes accidentally leave their | |
751 | directory or files world-writable. | |
752 | The default is | |
bf740959 | 753 | .Dq yes . |
38c295d6 | 754 | .It Cm Subsystem |
94ec8c6b | 755 | Configures an external subsystem (e.g., file transfer daemon). |
756 | Arguments should be a subsystem name and a command to execute upon subsystem | |
757 | request. | |
b5e300c2 | 758 | The command |
759 | .Xr sftp-server 8 | |
760 | implements the | |
761 | .Dq sftp | |
762 | file transfer subsystem. | |
38c295d6 | 763 | By default no subsystems are defined. |
764 | Note that this option applies to protocol version 2 only. | |
bf740959 | 765 | .It Cm SyslogFacility |
766 | Gives the facility code that is used when logging messages from | |
767 | .Nm sshd . | |
768 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 769 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
770 | The default is AUTH. | |
bf740959 | 771 | .It Cm UseLogin |
772 | Specifies whether | |
773 | .Xr login 1 | |
fa649821 | 774 | is used for interactive login sessions. |
775 | Note that | |
776 | .Xr login 1 | |
c345cf9d | 777 | is never used for remote command execution. |
4fe2af09 | 778 | The default is |
bf740959 | 779 | .Dq no . |
bf740959 | 780 | .It Cm X11DisplayOffset |
781 | Specifies the first display number available for | |
782 | .Nm sshd Ns 's | |
4fe2af09 | 783 | X11 forwarding. |
784 | This prevents | |
bf740959 | 785 | .Nm |
786 | from interfering with real X11 servers. | |
c8d54615 | 787 | The default is 10. |
7b2ea3a1 | 788 | .It Cm X11Forwarding |
4fe2af09 | 789 | Specifies whether X11 forwarding is permitted. |
790 | The default is | |
c8d54615 | 791 | .Dq no . |
7b2ea3a1 | 792 | Note that disabling X11 forwarding does not improve security in any |
793 | way, as users can always install their own forwarders. | |
fa649821 | 794 | .It Cm XAuthLocation |
795 | Specifies the location of the | |
796 | .Xr xauth 1 | |
797 | program. | |
798 | The default is | |
799 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 800 | .El |
801 | .Sh LOGIN PROCESS | |
802 | When a user successfully logs in, | |
803 | .Nm | |
804 | does the following: | |
805 | .Bl -enum -offset indent | |
806 | .It | |
807 | If the login is on a tty, and no command has been specified, | |
f54651ce | 808 | prints last login time and |
bf740959 | 809 | .Pa /etc/motd |
810 | (unless prevented in the configuration file or by | |
811 | .Pa $HOME/.hushlogin ; | |
812 | see the | |
f54651ce | 813 | .Sx FILES |
bf740959 | 814 | section). |
815 | .It | |
816 | If the login is on a tty, records login time. | |
817 | .It | |
818 | Checks | |
819 | .Pa /etc/nologin ; | |
820 | if it exists, prints contents and quits | |
821 | (unless root). | |
822 | .It | |
823 | Changes to run with normal user privileges. | |
824 | .It | |
825 | Sets up basic environment. | |
826 | .It | |
827 | Reads | |
828 | .Pa $HOME/.ssh/environment | |
829 | if it exists. | |
830 | .It | |
831 | Changes to user's home directory. | |
832 | .It | |
833 | If | |
834 | .Pa $HOME/.ssh/rc | |
835 | exists, runs it; else if | |
5f4fdfae | 836 | .Pa /etc/sshrc |
bf740959 | 837 | exists, runs |
4fe2af09 | 838 | it; otherwise runs xauth. |
839 | The | |
bf740959 | 840 | .Dq rc |
841 | files are given the X11 | |
842 | authentication protocol and cookie in standard input. | |
843 | .It | |
844 | Runs user's shell or command. | |
845 | .El | |
846 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
f54651ce | 847 | The |
bf740959 | 848 | .Pa $HOME/.ssh/authorized_keys |
849 | file lists the RSA keys that are | |
98143cfc | 850 | permitted for RSA authentication in protocol version 1. |
d0c832f3 | 851 | Similarly, the |
1d1ffb87 | 852 | .Pa $HOME/.ssh/authorized_keys2 |
8abcdba4 | 853 | file lists the DSA and RSA keys that are |
854 | permitted for public key authentication (PubkeyAuthentication) | |
da89cf4d | 855 | in protocol version 2. |
8abcdba4 | 856 | .Pp |
4fe2af09 | 857 | Each line of the file contains one |
bf740959 | 858 | key (empty lines and lines starting with a |
859 | .Ql # | |
860 | are ignored as | |
4fe2af09 | 861 | comments). |
8abcdba4 | 862 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 863 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 864 | Each protocol version 2 public key consists of: |
865 | options, keytype, base64 encoded key, comment. | |
866 | The options fields | |
867 | are optional; its presence is determined by whether the line starts | |
bf740959 | 868 | with a number or not (the option field never starts with a number). |
8abcdba4 | 869 | The bits, exponent, modulus and comment fields give the RSA key for |
870 | protocol version 1; the | |
bf740959 | 871 | comment field is not used for anything (but may be convenient for the |
872 | user to identify the key). | |
8abcdba4 | 873 | For protocol version 2 the keytype is |
874 | .Dq ssh-dss | |
875 | or | |
876 | .Dq ssh-rsa . | |
bf740959 | 877 | .Pp |
878 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 879 | (because of the size of the RSA key modulus). |
880 | You don't want to type them in; instead, copy the | |
c0ecc314 | 881 | .Pa identity.pub , |
8abcdba4 | 882 | .Pa id_dsa.pub |
c0ecc314 | 883 | or the |
884 | .Pa id_rsa.pub | |
bf740959 | 885 | file and edit it. |
886 | .Pp | |
c345cf9d | 887 | The options (if present) consist of comma-separated option |
4fe2af09 | 888 | specifications. |
889 | No spaces are permitted, except within double quotes. | |
bf740959 | 890 | The following option specifications are supported: |
891 | .Bl -tag -width Ds | |
892 | .It Cm from="pattern-list" | |
893 | Specifies that in addition to RSA authentication, the canonical name | |
894 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 895 | patterns |
896 | .Pf ( Ql * | |
897 | and | |
898 | .Ql ? | |
899 | serve as wildcards). | |
900 | The list may also contain | |
901 | patterns negated by prefixing them with | |
902 | .Ql ! ; | |
903 | if the canonical host name matches a negated pattern, the key is not accepted. | |
904 | The purpose | |
bf740959 | 905 | of this option is to optionally increase security: RSA authentication |
906 | by itself does not trust the network or name servers or anything (but | |
907 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 908 | permits an intruder to log in from anywhere in the world. |
909 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 910 | servers and/or routers would have to be compromised in addition to |
911 | just the key). | |
912 | .It Cm command="command" | |
913 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 914 | authentication. |
915 | The command supplied by the user (if any) is ignored. | |
bf740959 | 916 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 917 | otherwise it is run without a tty. |
61e96248 | 918 | Note that if you want a 8-bit clean channel, |
919 | you must not request a pty or should specify | |
920 | .Cm no-pty . | |
4fe2af09 | 921 | A quote may be included in the command by quoting it with a backslash. |
922 | This option might be useful | |
923 | to restrict certain RSA keys to perform just a specific operation. | |
924 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 925 | Note that the client may specify TCP/IP and/or X11 |
926 | forwarding unless they are explicitly prohibited. | |
bf740959 | 927 | .It Cm environment="NAME=value" |
928 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 929 | logging in using this key. |
930 | Environment variables set this way | |
931 | override other default environment values. | |
932 | Multiple options of this type are permitted. | |
bf740959 | 933 | .It Cm no-port-forwarding |
934 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 935 | Any port forward requests by the client will return an error. |
936 | This might be used, e.g., in connection with the | |
bf740959 | 937 | .Cm command |
938 | option. | |
939 | .It Cm no-X11-forwarding | |
940 | Forbids X11 forwarding when this key is used for authentication. | |
941 | Any X11 forward requests by the client will return an error. | |
942 | .It Cm no-agent-forwarding | |
943 | Forbids authentication agent forwarding when this key is used for | |
944 | authentication. | |
945 | .It Cm no-pty | |
946 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 947 | .It Cm permitopen="host:port" |
948 | Limit local | |
949 | .Li ``ssh -L'' | |
b2ae83b8 | 950 | port forwarding such that it may only connect to the specified host and |
dc504afd | 951 | port. Multiple |
952 | .Cm permitopen | |
b2ae83b8 | 953 | options may be applied separated by commas. No pattern matching is |
dc504afd | 954 | performed on the specified hostnames, they must be literal domains or |
955 | addresses. | |
bf740959 | 956 | .El |
957 | .Ss Examples | |
958 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
959 | .Pp | |
960 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
961 | .Pp | |
962 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 963 | .Pp |
964 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 965 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 966 | The |
1d1ffb87 | 967 | .Pa /etc/ssh_known_hosts , |
968 | .Pa /etc/ssh_known_hosts2 , | |
969 | .Pa $HOME/.ssh/known_hosts , | |
f54651ce | 970 | and |
1d1ffb87 | 971 | .Pa $HOME/.ssh/known_hosts2 |
4fe2af09 | 972 | files contain host public keys for all known hosts. |
973 | The global file should | |
974 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 975 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 976 | its key is added to the per-user file. |
bf740959 | 977 | .Pp |
978 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 979 | bits, exponent, modulus, comment. |
980 | The fields are separated by spaces. | |
bf740959 | 981 | .Pp |
982 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
983 | wildcards); each pattern in turn is matched against the canonical host | |
984 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 985 | name (when authenticating a server). |
986 | A pattern may also be preceded by | |
bf740959 | 987 | .Ql ! |
988 | to indicate negation: if the host name matches a negated | |
989 | pattern, it is not accepted (by that line) even if it matched another | |
990 | pattern on the line. | |
991 | .Pp | |
1d1ffb87 | 992 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 993 | can be obtained, e.g., from |
5f4fdfae | 994 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 995 | The optional comment field continues to the end of the line, and is not used. |
996 | .Pp | |
997 | Lines starting with | |
998 | .Ql # | |
999 | and empty lines are ignored as comments. | |
1000 | .Pp | |
1001 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 1002 | matching line has the proper key. |
1003 | It is thus permissible (but not | |
bf740959 | 1004 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 1005 | names. |
1006 | This will inevitably happen when short forms of host names | |
1007 | from different domains are put in the file. | |
1008 | It is possible | |
bf740959 | 1009 | that the files contain conflicting information; authentication is |
1010 | accepted if valid information can be found from either file. | |
1011 | .Pp | |
1012 | Note that the lines in these files are typically hundreds of characters | |
1013 | long, and you definitely don't want to type in the host keys by hand. | |
1014 | Rather, generate them by a script | |
f54651ce | 1015 | or by taking |
5f4fdfae | 1016 | .Pa /etc/ssh_host_key.pub |
bf740959 | 1017 | and adding the host names at the front. |
1018 | .Ss Examples | |
da89cf4d | 1019 | .Bd -literal |
1020 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
1021 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
1022 | .Ed | |
bf740959 | 1023 | .Sh FILES |
1024 | .Bl -tag -width Ds | |
5f4fdfae | 1025 | .It Pa /etc/sshd_config |
bf740959 | 1026 | Contains configuration data for |
1027 | .Nm sshd . | |
1028 | This file should be writable by root only, but it is recommended | |
1029 | (though not necessary) that it be world-readable. | |
b8dc87d3 | 1030 | .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
da89cf4d | 1031 | These three files contain the private parts of the host keys. |
b8dc87d3 | 1032 | These files should only be owned by root, readable only by root, and not |
bf740959 | 1033 | accessible to others. |
1034 | Note that | |
1035 | .Nm | |
1036 | does not start if this file is group/world-accessible. | |
b8dc87d3 | 1037 | .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
da89cf4d | 1038 | These three files contain the public parts of the host keys. |
b8dc87d3 | 1039 | These files should be world-readable but writable only by |
4fe2af09 | 1040 | root. |
b8dc87d3 | 1041 | Their contents should match the respective private parts. |
1042 | These files are not | |
1043 | really used for anything; they are provided for the convenience of | |
1044 | the user so their contents can be copied to known hosts files. | |
1045 | These files are created using | |
bf740959 | 1046 | .Xr ssh-keygen 1 . |
c523303b | 1047 | .It Pa /etc/primes |
1048 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
5f4fdfae | 1049 | .It Pa /var/run/sshd.pid |
bf740959 | 1050 | Contains the process ID of the |
1051 | .Nm | |
1052 | listening for connections (if there are several daemons running | |
1053 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1054 | started last). |
c345cf9d | 1055 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1056 | .It Pa $HOME/.ssh/authorized_keys |
1057 | Lists the RSA keys that can be used to log into the user's account. | |
1058 | This file must be readable by root (which may on some machines imply | |
1059 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 1060 | volume). |
1061 | It is recommended that it not be accessible by others. | |
1062 | The format of this file is described above. | |
1d1ffb87 | 1063 | Users will place the contents of their |
1064 | .Pa identity.pub | |
1065 | files into this file, as described in | |
1066 | .Xr ssh-keygen 1 . | |
1067 | .It Pa $HOME/.ssh/authorized_keys2 | |
da89cf4d | 1068 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 1069 | This file must be readable by root (which may on some machines imply |
1070 | it being world-readable if the user's home directory resides on an NFS | |
1071 | volume). | |
1072 | It is recommended that it not be accessible by others. | |
1073 | The format of this file is described above. | |
1074 | Users will place the contents of their | |
1075 | .Pa id_dsa.pub | |
c0ecc314 | 1076 | and/or |
1077 | .Pa id_rsa.pub | |
1d1ffb87 | 1078 | files into this file, as described in |
1079 | .Xr ssh-keygen 1 . | |
5f4fdfae | 1080 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1081 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 1082 | authentication to check the public key of the host. |
1083 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 1084 | The client uses the same files |
e91c60f2 | 1085 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1086 | These files should be writable only by root/the owner. |
5f4fdfae | 1087 | .Pa /etc/ssh_known_hosts |
bf740959 | 1088 | should be world-readable, and |
1089 | .Pa $HOME/.ssh/known_hosts | |
1090 | can but need not be world-readable. | |
da89cf4d | 1091 | .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2" |
1092 | These files are consulted when using protocol version 2 hostbased | |
1093 | authentication to check the public key of the host. | |
1094 | The key must be listed in one of these files to be accepted. | |
1095 | The client uses the same files | |
1096 | to verify that it is connecting to the correct remote host. | |
1097 | These files should be writable only by root/the owner. | |
1098 | .Pa /etc/ssh_known_hosts2 | |
1099 | should be world-readable, and | |
1100 | .Pa $HOME/.ssh/known_hosts2 | |
1101 | can but need not be world-readable. | |
bf740959 | 1102 | .It Pa /etc/nologin |
f54651ce | 1103 | If this file exists, |
bf740959 | 1104 | .Nm |
4fe2af09 | 1105 | refuses to let anyone except root log in. |
1106 | The contents of the file | |
bf740959 | 1107 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1108 | refused. |
1109 | The file should be world-readable. | |
bf740959 | 1110 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
1111 | If compiled with | |
1112 | .Sy LIBWRAP | |
1113 | support, tcp-wrappers access controls may be defined here as described in | |
1114 | .Xr hosts_access 5 . | |
1115 | .It Pa $HOME/.rhosts | |
1116 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1117 | line. |
1118 | The given user on the corresponding host is permitted to log in | |
1119 | without password. | |
1120 | The same file is used by rlogind and rshd. | |
bf740959 | 1121 | The file must |
1122 | be writable only by the user; it is recommended that it not be | |
1123 | accessible by others. | |
1124 | .Pp | |
4fe2af09 | 1125 | If is also possible to use netgroups in the file. |
1126 | Either host or user | |
bf740959 | 1127 | name may be of the form +@groupname to specify all hosts or all users |
1128 | in the group. | |
1129 | .It Pa $HOME/.shosts | |
1130 | For ssh, | |
1131 | this file is exactly the same as for | |
1132 | .Pa .rhosts . | |
1133 | However, this file is | |
1134 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1135 | .It Pa /etc/hosts.equiv |
bf740959 | 1136 | This file is used during |
1137 | .Pa .rhosts | |
4fe2af09 | 1138 | authentication. |
1139 | In the simplest form, this file contains host names, one per line. | |
1140 | Users on | |
bf740959 | 1141 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1142 | have the same user name on both machines. |
1143 | The host name may also be | |
bf740959 | 1144 | followed by a user name; such users are permitted to log in as |
1145 | .Em any | |
4fe2af09 | 1146 | user on this machine (except root). |
1147 | Additionally, the syntax | |
bf740959 | 1148 | .Dq +@group |
4fe2af09 | 1149 | can be used to specify netgroups. |
1150 | Negated entries start with | |
bf740959 | 1151 | .Ql \&- . |
1152 | .Pp | |
1153 | If the client host/user is successfully matched in this file, login is | |
1154 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1155 | same. |
1156 | Additionally, successful RSA host authentication is normally required. | |
1157 | This file must be writable only by root; it is recommended | |
bf740959 | 1158 | that it be world-readable. |
1159 | .Pp | |
1160 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1161 | .Pa hosts.equiv . | |
1162 | Beware that it really means that the named user(s) can log in as | |
1163 | .Em anybody , | |
1164 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1165 | binaries and directories. |
1166 | Using a user name practically grants the user root access. | |
1167 | The only valid use for user names that I can think | |
bf740959 | 1168 | of is in negative entries. |
1169 | .Pp | |
1170 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1171 | .It Pa /etc/shosts.equiv |
bf740959 | 1172 | This is processed exactly as |
1173 | .Pa /etc/hosts.equiv . | |
1174 | However, this file may be useful in environments that want to run both | |
1175 | rsh/rlogin and ssh. | |
1176 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1177 | This file is read into the environment at login (if it exists). |
1178 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1179 | .Ql # ) , |
4fe2af09 | 1180 | and assignment lines of the form name=value. |
1181 | The file should be writable | |
bf740959 | 1182 | only by the user; it need not be readable by anyone else. |
1183 | .It Pa $HOME/.ssh/rc | |
1184 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1185 | environment files but before starting the user's shell or command. |
1186 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 1187 | standard input (and |
1188 | .Ev DISPLAY | |
4fe2af09 | 1189 | in environment). |
1190 | This must call | |
bf740959 | 1191 | .Xr xauth 1 |
1192 | in that case. | |
1193 | .Pp | |
1194 | The primary purpose of this file is to run any initialization routines | |
1195 | which may be needed before the user's home directory becomes | |
1196 | accessible; AFS is a particular example of such an environment. | |
1197 | .Pp | |
1198 | This file will probably contain some initialization code followed by | |
da89cf4d | 1199 | something similar to: |
1200 | .Bd -literal | |
1201 | if read proto cookie; then | |
1202 | echo add $DISPLAY $proto $cookie | xauth -q - | |
1203 | fi | |
1204 | .Ed | |
bf740959 | 1205 | .Pp |
1206 | If this file does not exist, | |
5f4fdfae | 1207 | .Pa /etc/sshrc |
bf740959 | 1208 | is run, and if that |
1209 | does not exist either, xauth is used to store the cookie. | |
1210 | .Pp | |
1211 | This file should be writable only by the user, and need not be | |
1212 | readable by anyone else. | |
5f4fdfae | 1213 | .It Pa /etc/sshrc |
bf740959 | 1214 | Like |
1215 | .Pa $HOME/.ssh/rc . | |
1216 | This can be used to specify | |
4fe2af09 | 1217 | machine-specific login-time initializations globally. |
1218 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1219 | .El |
fa08c86b | 1220 | .Sh AUTHORS |
7f5c4295 | 1221 | OpenSSH is a derivative of the original and free |
1222 | ssh 1.2.12 release by Tatu Ylonen. | |
1223 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1224 | Theo de Raadt and Dug Song | |
1225 | removed many bugs, re-added newer features and | |
1226 | created OpenSSH. | |
1227 | Markus Friedl contributed the support for SSH | |
1228 | protocol versions 1.5 and 2.0. | |
bf740959 | 1229 | .Sh SEE ALSO |
bf740959 | 1230 | .Xr scp 1 , |
61e96248 | 1231 | .Xr sftp 1 , |
b5e300c2 | 1232 | .Xr sftp-server 8 , |
bf740959 | 1233 | .Xr ssh 1 , |
1234 | .Xr ssh-add 1 , | |
1235 | .Xr ssh-agent 1 , | |
1236 | .Xr ssh-keygen 1 , | |
7368a6c8 | 1237 | .Xr rlogin 1 , |
1238 | .Xr rsh 1 | |
2cad6cef | 1239 | .Rs |
1240 | .%A T. Ylonen | |
1241 | .%A T. Kivinen | |
1242 | .%A M. Saarinen | |
1243 | .%A T. Rinne | |
1244 | .%A S. Lehtinen | |
1245 | .%T "SSH Protocol Architecture" | |
1246 | .%N draft-ietf-secsh-architecture-07.txt | |
1247 | .%D January 2001 | |
1248 | .%O work in progress material | |
1249 | .Re | |
da89cf4d | 1250 | .Rs |
1251 | .%A M. Friedl | |
1252 | .%A N. Provos | |
1253 | .%A W. A. Simpson | |
1254 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
1255 | .%N draft-ietf-secsh-dh-group-exchange-00.txt | |
1256 | .%D January 2001 | |
1257 | .%O work in progress material | |
1258 | .Re |