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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" sshd.8.in | |
4 | .\" | |
5 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
6 | .\" | |
7 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
8 | .\" All rights reserved | |
9 | .\" | |
10 | .\" Created: Sat Apr 22 21:55:14 1995 ylo | |
11 | .\" | |
12 | .\" $Id$ | |
13 | .\" | |
14 | .Dd September 25, 1999 | |
15 | .Dt SSHD 8 | |
16 | .Os | |
17 | .Sh NAME | |
18 | .Nm sshd | |
19 | .Nd secure shell daemon | |
20 | .Sh SYNOPSIS | |
21 | .Nm sshd | |
48e671d5 | 22 | .Op Fl diqQ46 |
bf740959 | 23 | .Op Fl b Ar bits |
24 | .Op Fl f Ar config_file | |
25 | .Op Fl g Ar login_grace_time | |
26 | .Op Fl h Ar host_key_file | |
27 | .Op Fl k Ar key_gen_time | |
28 | .Op Fl p Ar port | |
5260325f | 29 | .Op Fl V Ar client_protocol_id |
bf740959 | 30 | .Sh DESCRIPTION |
31 | .Nm | |
32 | (Secure Shell Daemon) is the daemon program for | |
33 | .Xr ssh 1 . | |
34 | Together these programs replace rlogin and rsh programs, and | |
35 | provide secure encrypted communications between two untrusted hosts | |
4fe2af09 | 36 | over an insecure network. |
37 | The programs are intended to be as easy to | |
bf740959 | 38 | install and use as possible. |
39 | .Pp | |
40 | .Nm | |
4fe2af09 | 41 | is the daemon that listens for connections from clients. |
42 | It is normally started at boot from | |
bf740959 | 43 | .Pa /etc/rc . |
44 | It forks a new | |
4fe2af09 | 45 | daemon for each incoming connection. |
46 | The forked daemons handle | |
bf740959 | 47 | key exchange, encryption, authentication, command execution, |
48 | and data exchange. | |
49 | .Pp | |
50 | .Nm | |
4fe2af09 | 51 | works as follows. |
52 | Each host has a host-specific RSA key | |
53 | (normally 1024 bits) used to identify the host. | |
54 | Additionally, when | |
bf740959 | 55 | the daemon starts, it generates a server RSA key (normally 768 bits). |
56 | This key is normally regenerated every hour if it has been used, and | |
57 | is never stored on disk. | |
58 | .Pp | |
59 | Whenever a client connects the daemon, the daemon sends its host | |
4fe2af09 | 60 | and server public keys to the client. |
61 | The client compares the | |
bf740959 | 62 | host key against its own database to verify that it has not changed. |
4fe2af09 | 63 | The client then generates a 256 bit random number. |
64 | It encrypts this | |
bf740959 | 65 | random number using both the host key and the server key, and sends |
4fe2af09 | 66 | the encrypted number to the server. |
67 | Both sides then start to use this | |
bf740959 | 68 | random number as a session key which is used to encrypt all further |
4fe2af09 | 69 | communications in the session. |
70 | The rest of the session is encrypted | |
bf740959 | 71 | using a conventional cipher, currently Blowfish and 3DES, with 3DES |
7368a6c8 | 72 | being used by default. |
4fe2af09 | 73 | The client selects the encryption algorithm |
bf740959 | 74 | to use from those offered by the server. |
75 | .Pp | |
4fe2af09 | 76 | Next, the server and the client enter an authentication dialog. |
77 | The client tries to authenticate itself using | |
bf740959 | 78 | .Pa .rhosts |
79 | authentication, | |
80 | .Pa .rhosts | |
81 | authentication combined with RSA host | |
82 | authentication, RSA challenge-response authentication, or password | |
83 | based authentication. | |
84 | .Pp | |
85 | Rhosts authentication is normally disabled | |
86 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 87 | configuration file if desired. |
88 | System security is not improved unless | |
bf740959 | 89 | .Xr rshd 8 , |
90 | .Xr rlogind 8 , | |
91 | .Xr rexecd 8 , | |
92 | and | |
93 | .Xr rexd 8 | |
94 | are disabled (thus completely disabling | |
95 | .Xr rlogin 1 | |
96 | and | |
97 | .Xr rsh 1 | |
98 | into that machine). | |
99 | .Pp | |
100 | If the client successfully authenticates itself, a dialog for | |
4fe2af09 | 101 | preparing the session is entered. |
102 | At this time the client may request | |
bf740959 | 103 | things like allocating a pseudo-tty, forwarding X11 connections, |
104 | forwarding TCP/IP connections, or forwarding the authentication agent | |
105 | connection over the secure channel. | |
106 | .Pp | |
107 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 108 | The sides then enter session mode. |
109 | In this mode, either side may send | |
bf740959 | 110 | data at any time, and such data is forwarded to/from the shell or |
111 | command on the server side, and the user terminal in the client side. | |
112 | .Pp | |
113 | When the user program terminates and all forwarded X11 and other | |
114 | connections have been closed, the server sends command exit status to | |
115 | the client, and both sides exit. | |
116 | .Pp | |
117 | .Nm | |
118 | can be configured using command-line options or a configuration | |
4fe2af09 | 119 | file. |
120 | Command-line options override values specified in the | |
bf740959 | 121 | configuration file. |
122 | .Pp | |
9d6b7add | 123 | .Nm |
124 | rereads its configuration file when it receives a hangup signal, | |
125 | .Dv SIGHUP . | |
126 | .Pp | |
bf740959 | 127 | The options are as follows: |
128 | .Bl -tag -width Ds | |
129 | .It Fl b Ar bits | |
130 | Specifies the number of bits in the server key (default 768). | |
131 | .Pp | |
132 | .It Fl d | |
4fe2af09 | 133 | Debug mode. |
134 | The server sends verbose debug output to the system | |
135 | log, and does not put itself in the background. | |
136 | The server also will not fork and will only process one connection. | |
137 | This option is only intended for debugging for the server. | |
bf740959 | 138 | .It Fl f Ar configuration_file |
4fe2af09 | 139 | Specifies the name of the configuration file. |
140 | The default is | |
5f4fdfae | 141 | .Pa /etc/sshd_config . |
bf740959 | 142 | .Nm |
143 | refuses to start if there is no configuration file. | |
144 | .It Fl g Ar login_grace_time | |
145 | Gives the grace time for clients to authenticate themselves (default | |
4fe2af09 | 146 | 300 seconds). |
147 | If the client fails to authenticate the user within | |
148 | this many seconds, the server disconnects and exits. | |
149 | A value of zero indicates no limit. | |
bf740959 | 150 | .It Fl h Ar host_key_file |
151 | Specifies the file from which the host key is read (default | |
5f4fdfae | 152 | .Pa /etc/ssh_host_key ) . |
bf740959 | 153 | This option must be given if |
154 | .Nm | |
155 | is not run as root (as the normal | |
156 | host file is normally not readable by anyone but root). | |
157 | .It Fl i | |
158 | Specifies that | |
159 | .Nm | |
160 | is being run from inetd. | |
161 | .Nm | |
162 | is normally not run | |
163 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 164 | respond to the client, and this may take tens of seconds. |
165 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 166 | However, with small key sizes (e.g., 512) using |
bf740959 | 167 | .Nm |
168 | from inetd may | |
169 | be feasible. | |
170 | .It Fl k Ar key_gen_time | |
171 | Specifies how often the server key is regenerated (default 3600 | |
4fe2af09 | 172 | seconds, or one hour). |
173 | The motivation for regenerating the key fairly | |
bf740959 | 174 | often is that the key is not stored anywhere, and after about an hour, |
175 | it becomes impossible to recover the key for decrypting intercepted | |
176 | communications even if the machine is cracked into or physically | |
4fe2af09 | 177 | seized. |
178 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 179 | .It Fl p Ar port |
180 | Specifies the port on which the server listens for connections | |
181 | (default 22). | |
182 | .It Fl q | |
4fe2af09 | 183 | Quiet mode. |
184 | Nothing is sent to the system log. | |
185 | Normally the beginning, | |
bf740959 | 186 | authentication, and termination of each connection is logged. |
187 | .It Fl Q | |
188 | Do not print an error message if RSA support is missing. | |
5260325f | 189 | .It Fl V Ar client_protocol_id |
190 | SSH2 compatibility mode. | |
191 | When this options is specified | |
192 | .Nm | |
193 | assumes the client has sent the given version string | |
194 | and skips the | |
195 | Protocol Version Identification Exchange. | |
48e671d5 | 196 | .It Fl 4 |
197 | Forces | |
198 | .Nm | |
199 | to use IPv4 addresses only. | |
200 | .It Fl 6 | |
201 | Forces | |
202 | .Nm | |
203 | to use IPv6 addresses only. | |
bf740959 | 204 | .El |
205 | .Sh CONFIGURATION FILE | |
206 | .Nm | |
207 | reads configuration data from | |
5f4fdfae | 208 | .Pa /etc/sshd_config |
bf740959 | 209 | (or the file specified with |
210 | .Fl f | |
4fe2af09 | 211 | on the command line). |
212 | The file contains keyword-value pairs, one per line. | |
213 | Lines starting with | |
bf740959 | 214 | .Ql # |
215 | and empty lines are interpreted as comments. | |
216 | .Pp | |
217 | The following keywords are possible. | |
218 | .Bl -tag -width Ds | |
219 | .It Cm AFSTokenPassing | |
4fe2af09 | 220 | Specifies whether an AFS token may be forwarded to the server. |
221 | Default is | |
bf740959 | 222 | .Dq yes . |
223 | .It Cm AllowGroups | |
224 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 225 | by spaces. |
226 | If specified, login is allowed only for users whose primary | |
bf740959 | 227 | group matches one of the patterns. |
228 | .Ql \&* | |
229 | and | |
230 | .Ql ? | |
231 | can be used as | |
4fe2af09 | 232 | wildcards in the patterns. |
233 | Only group names are valid, a numerical group ID isn't recognized. | |
234 | By default login is allowed regardless of the primary group. | |
bf740959 | 235 | .Pp |
236 | .It Cm AllowUsers | |
237 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 238 | by spaces. |
239 | If specified, login is allowed only for users names that | |
bf740959 | 240 | match one of the patterns. |
241 | .Ql \&* | |
242 | and | |
243 | .Ql ? | |
244 | can be used as | |
4fe2af09 | 245 | wildcards in the patterns. |
246 | Only user names are valid, a numerical user ID isn't recognized. | |
247 | By default login is allowed regardless of the user name. | |
bf740959 | 248 | .Pp |
249 | .It Cm CheckMail | |
250 | Specifies whether | |
251 | .Nm | |
252 | should check for new mail for interactive logins. | |
253 | The default is | |
254 | .Dq no . | |
255 | .It Cm DenyGroups | |
256 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 257 | by spaces. |
258 | Users whose primary group matches one of the patterns | |
bf740959 | 259 | aren't allowed to log in. |
260 | .Ql \&* | |
261 | and | |
262 | .Ql ? | |
263 | can be used as | |
4fe2af09 | 264 | wildcards in the patterns. |
265 | Only group names are valid, a numerical group ID isn't recognized. | |
266 | By default login is allowed regardless of the primary group. | |
bf740959 | 267 | .Pp |
268 | .It Cm DenyUsers | |
269 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 270 | by spaces. |
271 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 272 | .Ql \&* |
273 | and | |
274 | .Ql ? | |
4fe2af09 | 275 | can be used as wildcards in the patterns. |
276 | Only user names are valid, a numerical user ID isn't recognized. | |
277 | By default login is allowed regardless of the user name. | |
bf740959 | 278 | .It Cm HostKey |
279 | Specifies the file containing the private host key (default | |
5f4fdfae | 280 | .Pa /etc/ssh_host_key ) . |
bf740959 | 281 | Note that |
282 | .Nm | |
283 | does not start if this file is group/world-accessible. | |
284 | .It Cm IgnoreRhosts | |
c8d54615 | 285 | Specifies that |
286 | .Pa .rhosts | |
287 | and | |
288 | .Pa .shosts | |
289 | files will not be used in authentication. | |
bf740959 | 290 | .Pa /etc/hosts.equiv |
291 | and | |
5f4fdfae | 292 | .Pa /etc/shosts.equiv |
4fe2af09 | 293 | are still used. |
294 | The default is | |
c8d54615 | 295 | .Dq yes . |
b4748e2f | 296 | .It Cm IgnoreUserKnownHosts |
297 | Specifies whether | |
298 | .Nm | |
299 | should ignore the user's | |
300 | .Pa $HOME/.ssh/known_hosts | |
301 | during | |
302 | .Cm RhostsRSAAuthentication . | |
303 | The default is | |
304 | .Dq no . | |
bf740959 | 305 | .It Cm KeepAlive |
306 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 307 | other side. |
308 | If they are sent, death of the connection or crash of one | |
309 | of the machines will be properly noticed. | |
310 | However, this means that | |
bf740959 | 311 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 312 | find it annoying. |
313 | On the other hand, if keepalives are not send, | |
bf740959 | 314 | sessions may hang indefinitely on the server, leaving |
315 | .Dq ghost | |
316 | users and consuming server resources. | |
317 | .Pp | |
318 | The default is | |
319 | .Dq yes | |
320 | (to send keepalives), and the server will notice | |
4fe2af09 | 321 | if the network goes down or the client host reboots. |
322 | This avoids infinitely hanging sessions. | |
bf740959 | 323 | .Pp |
324 | To disable keepalives, the value should be set to | |
325 | .Dq no | |
326 | in both the server and the client configuration files. | |
327 | .It Cm KerberosAuthentication | |
4fe2af09 | 328 | Specifies whether Kerberos authentication is allowed. |
329 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 330 | .Cm PasswordAuthentication |
331 | is yes, the password provided by the user will be validated through | |
4fe2af09 | 332 | the Kerberos KDC. |
333 | Default is | |
bf740959 | 334 | .Dq yes . |
335 | .It Cm KerberosOrLocalPasswd | |
336 | If set then if password authentication through Kerberos fails then | |
337 | the password will be validated via any additional local mechanism | |
338 | such as | |
339 | .Pa /etc/passwd | |
4fe2af09 | 340 | or SecurID. |
341 | Default is | |
bf740959 | 342 | .Dq yes . |
343 | .It Cm KerberosTgtPassing | |
344 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
345 | Default is | |
346 | .Dq no , | |
347 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
348 | .It Cm KerberosTicketCleanup | |
349 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 350 | file on logout. |
351 | Default is | |
bf740959 | 352 | .Dq yes . |
353 | .It Cm KeyRegenerationInterval | |
354 | The server key is automatically regenerated after this many seconds | |
4fe2af09 | 355 | (if it has been used). |
356 | The purpose of regeneration is to prevent | |
bf740959 | 357 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 358 | stealing the keys. |
359 | The key is never stored anywhere. | |
360 | If the value is 0, the key is never regenerated. | |
361 | The default is 3600 (seconds). | |
bf740959 | 362 | .It Cm ListenAddress |
363 | Specifies what local address | |
364 | .Nm | |
365 | should listen on. | |
366 | The default is to listen to all local addresses. | |
48e671d5 | 367 | Multiple options of this type are permitted. |
368 | Additionally, the | |
369 | .Cm Ports | |
370 | options must precede this option. | |
bf740959 | 371 | .It Cm LoginGraceTime |
372 | The server disconnects after this time if the user has not | |
4fe2af09 | 373 | successfully logged in. |
374 | If the value is 0, there is no time limit. | |
bf740959 | 375 | The default is 600 (seconds). |
6a17f9c2 | 376 | .It Cm LogLevel |
377 | Gives the verbosity level that is used when logging messages from | |
378 | .Nm sshd . | |
379 | The possible values are: | |
5260325f | 380 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
6a17f9c2 | 381 | The default is INFO. |
382 | Logging with level DEBUG violates the privacy of users | |
383 | and is not recommended. | |
bf740959 | 384 | .It Cm PasswordAuthentication |
385 | Specifies whether password authentication is allowed. | |
386 | The default is | |
387 | .Dq yes . | |
388 | .It Cm PermitEmptyPasswords | |
389 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 390 | server allows login to accounts with empty password strings. |
391 | The default is | |
c8d54615 | 392 | .Dq no . |
bf740959 | 393 | .It Cm PermitRootLogin |
394 | Specifies whether the root can log in using | |
395 | .Xr ssh 1 . | |
396 | The argument must be | |
397 | .Dq yes , | |
398 | .Dq without-password | |
399 | or | |
400 | .Dq no . | |
401 | The default is | |
402 | .Dq yes . | |
403 | If this options is set to | |
404 | .Dq without-password | |
405 | only password authentication is disabled for root. | |
406 | .Pp | |
407 | Root login with RSA authentication when the | |
408 | .Ar command | |
409 | option has been | |
410 | specified will be allowed regardless of the value of this setting | |
411 | (which may be useful for taking remote backups even if root login is | |
412 | normally not allowed). | |
413 | .It Cm Port | |
414 | Specifies the port number that | |
415 | .Nm | |
4fe2af09 | 416 | listens on. |
417 | The default is 22. | |
48e671d5 | 418 | Multiple options of this type are permitted. |
bf740959 | 419 | .It Cm PrintMotd |
420 | Specifies whether | |
421 | .Nm | |
422 | should print | |
423 | .Pa /etc/motd | |
4fe2af09 | 424 | when a user logs in interactively. |
425 | (On some systems it is also printed by the shell, | |
bf740959 | 426 | .Pa /etc/profile , |
4fe2af09 | 427 | or equivalent.) |
428 | The default is | |
bf740959 | 429 | .Dq yes . |
bf740959 | 430 | .It Cm RandomSeed |
4fe2af09 | 431 | Obsolete. |
432 | Random number generation uses other techniques. | |
bf740959 | 433 | .It Cm RhostsAuthentication |
434 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 435 | files is sufficient. |
436 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 437 | .Cm RhostsRSAAuthentication |
438 | should be used | |
439 | instead, because it performs RSA-based host authentication in addition | |
440 | to normal rhosts or /etc/hosts.equiv authentication. | |
441 | The default is | |
442 | .Dq no . | |
443 | .It Cm RhostsRSAAuthentication | |
444 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 445 | with successful RSA host authentication is allowed. |
446 | The default is | |
c8d54615 | 447 | .Dq no . |
bf740959 | 448 | .It Cm RSAAuthentication |
4fe2af09 | 449 | Specifies whether pure RSA authentication is allowed. |
450 | The default is | |
bf740959 | 451 | .Dq yes . |
452 | .It Cm ServerKeyBits | |
4fe2af09 | 453 | Defines the number of bits in the server key. |
454 | The minimum value is 512, and the default is 768. | |
bf740959 | 455 | .It Cm SkeyAuthentication |
456 | Specifies whether | |
457 | .Xr skey 1 | |
4fe2af09 | 458 | authentication is allowed. |
459 | The default is | |
bf740959 | 460 | .Dq yes . |
461 | Note that s/key authentication is enabled only if | |
462 | .Cm PasswordAuthentication | |
463 | is allowed, too. | |
464 | .It Cm StrictModes | |
465 | Specifies whether | |
466 | .Nm | |
467 | should check file modes and ownership of the | |
4fe2af09 | 468 | user's files and home directory before accepting login. |
469 | This is normally desirable because novices sometimes accidentally leave their | |
470 | directory or files world-writable. | |
471 | The default is | |
bf740959 | 472 | .Dq yes . |
473 | .It Cm SyslogFacility | |
474 | Gives the facility code that is used when logging messages from | |
475 | .Nm sshd . | |
476 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 477 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
478 | The default is AUTH. | |
bf740959 | 479 | .It Cm UseLogin |
480 | Specifies whether | |
481 | .Xr login 1 | |
4fe2af09 | 482 | is used. |
483 | The default is | |
bf740959 | 484 | .Dq no . |
bf740959 | 485 | .It Cm X11DisplayOffset |
486 | Specifies the first display number available for | |
487 | .Nm sshd Ns 's | |
4fe2af09 | 488 | X11 forwarding. |
489 | This prevents | |
bf740959 | 490 | .Nm |
491 | from interfering with real X11 servers. | |
c8d54615 | 492 | The default is 10. |
7b2ea3a1 | 493 | .It Cm X11Forwarding |
4fe2af09 | 494 | Specifies whether X11 forwarding is permitted. |
495 | The default is | |
c8d54615 | 496 | .Dq no . |
7b2ea3a1 | 497 | Note that disabling X11 forwarding does not improve security in any |
498 | way, as users can always install their own forwarders. | |
bf740959 | 499 | .El |
500 | .Sh LOGIN PROCESS | |
501 | When a user successfully logs in, | |
502 | .Nm | |
503 | does the following: | |
504 | .Bl -enum -offset indent | |
505 | .It | |
506 | If the login is on a tty, and no command has been specified, | |
507 | prints last login time and | |
508 | .Pa /etc/motd | |
509 | (unless prevented in the configuration file or by | |
510 | .Pa $HOME/.hushlogin ; | |
511 | see the | |
512 | .Sx FILES | |
513 | section). | |
514 | .It | |
515 | If the login is on a tty, records login time. | |
516 | .It | |
517 | Checks | |
518 | .Pa /etc/nologin ; | |
519 | if it exists, prints contents and quits | |
520 | (unless root). | |
521 | .It | |
522 | Changes to run with normal user privileges. | |
523 | .It | |
524 | Sets up basic environment. | |
525 | .It | |
526 | Reads | |
527 | .Pa $HOME/.ssh/environment | |
528 | if it exists. | |
529 | .It | |
530 | Changes to user's home directory. | |
531 | .It | |
532 | If | |
533 | .Pa $HOME/.ssh/rc | |
534 | exists, runs it; else if | |
5f4fdfae | 535 | .Pa /etc/sshrc |
bf740959 | 536 | exists, runs |
4fe2af09 | 537 | it; otherwise runs xauth. |
538 | The | |
bf740959 | 539 | .Dq rc |
540 | files are given the X11 | |
541 | authentication protocol and cookie in standard input. | |
542 | .It | |
543 | Runs user's shell or command. | |
544 | .El | |
545 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
546 | The | |
547 | .Pa $HOME/.ssh/authorized_keys | |
548 | file lists the RSA keys that are | |
4fe2af09 | 549 | permitted for RSA authentication. |
550 | Each line of the file contains one | |
bf740959 | 551 | key (empty lines and lines starting with a |
552 | .Ql # | |
553 | are ignored as | |
4fe2af09 | 554 | comments). |
555 | Each line consists of the following fields, separated by | |
556 | spaces: options, bits, exponent, modulus, comment. | |
557 | The options field | |
bf740959 | 558 | is optional; its presence is determined by whether the line starts |
559 | with a number or not (the option field never starts with a number). | |
560 | The bits, exponent, modulus and comment fields give the RSA key; the | |
561 | comment field is not used for anything (but may be convenient for the | |
562 | user to identify the key). | |
563 | .Pp | |
564 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 565 | (because of the size of the RSA key modulus). |
566 | You don't want to type them in; instead, copy the | |
bf740959 | 567 | .Pa identity.pub |
568 | file and edit it. | |
569 | .Pp | |
570 | The options (if present) consists of comma-separated option | |
4fe2af09 | 571 | specifications. |
572 | No spaces are permitted, except within double quotes. | |
bf740959 | 573 | The following option specifications are supported: |
574 | .Bl -tag -width Ds | |
575 | .It Cm from="pattern-list" | |
576 | Specifies that in addition to RSA authentication, the canonical name | |
577 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 578 | patterns |
579 | .Pf ( Ql * | |
580 | and | |
581 | .Ql ? | |
582 | serve as wildcards). | |
583 | The list may also contain | |
584 | patterns negated by prefixing them with | |
585 | .Ql ! ; | |
586 | if the canonical host name matches a negated pattern, the key is not accepted. | |
587 | The purpose | |
bf740959 | 588 | of this option is to optionally increase security: RSA authentication |
589 | by itself does not trust the network or name servers or anything (but | |
590 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 591 | permits an intruder to log in from anywhere in the world. |
592 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 593 | servers and/or routers would have to be compromised in addition to |
594 | just the key). | |
595 | .It Cm command="command" | |
596 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 597 | authentication. |
598 | The command supplied by the user (if any) is ignored. | |
bf740959 | 599 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 600 | otherwise it is run without a tty. |
601 | A quote may be included in the command by quoting it with a backslash. | |
602 | This option might be useful | |
603 | to restrict certain RSA keys to perform just a specific operation. | |
604 | An example might be a key that permits remote backups but nothing else. | |
605 | Notice that the client may specify TCP/IP and/or X11 | |
bf740959 | 606 | forwardings unless they are explicitly prohibited. |
607 | .It Cm environment="NAME=value" | |
608 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 609 | logging in using this key. |
610 | Environment variables set this way | |
611 | override other default environment values. | |
612 | Multiple options of this type are permitted. | |
bf740959 | 613 | .It Cm no-port-forwarding |
614 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 615 | Any port forward requests by the client will return an error. |
616 | This might be used, e.g., in connection with the | |
bf740959 | 617 | .Cm command |
618 | option. | |
619 | .It Cm no-X11-forwarding | |
620 | Forbids X11 forwarding when this key is used for authentication. | |
621 | Any X11 forward requests by the client will return an error. | |
622 | .It Cm no-agent-forwarding | |
623 | Forbids authentication agent forwarding when this key is used for | |
624 | authentication. | |
625 | .It Cm no-pty | |
626 | Prevents tty allocation (a request to allocate a pty will fail). | |
627 | .El | |
628 | .Ss Examples | |
629 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
630 | .Pp | |
631 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
632 | .Pp | |
633 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
634 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
635 | The | |
5f4fdfae | 636 | .Pa /etc/ssh_known_hosts |
bf740959 | 637 | and |
638 | .Pa $HOME/.ssh/known_hosts | |
4fe2af09 | 639 | files contain host public keys for all known hosts. |
640 | The global file should | |
641 | be prepared by the administrator (optional), and the per-user file is | |
bf740959 | 642 | maintained automatically: whenever the user connects an unknown host |
4fe2af09 | 643 | its key is added to the per-user file. |
bf740959 | 644 | .Pp |
645 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 646 | bits, exponent, modulus, comment. |
647 | The fields are separated by spaces. | |
bf740959 | 648 | .Pp |
649 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
650 | wildcards); each pattern in turn is matched against the canonical host | |
651 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 652 | name (when authenticating a server). |
653 | A pattern may also be preceded by | |
bf740959 | 654 | .Ql ! |
655 | to indicate negation: if the host name matches a negated | |
656 | pattern, it is not accepted (by that line) even if it matched another | |
657 | pattern on the line. | |
658 | .Pp | |
659 | Bits, exponent, and modulus are taken directly from the host key; they | |
660 | can be obtained, e.g., from | |
5f4fdfae | 661 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 662 | The optional comment field continues to the end of the line, and is not used. |
663 | .Pp | |
664 | Lines starting with | |
665 | .Ql # | |
666 | and empty lines are ignored as comments. | |
667 | .Pp | |
668 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 669 | matching line has the proper key. |
670 | It is thus permissible (but not | |
bf740959 | 671 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 672 | names. |
673 | This will inevitably happen when short forms of host names | |
674 | from different domains are put in the file. | |
675 | It is possible | |
bf740959 | 676 | that the files contain conflicting information; authentication is |
677 | accepted if valid information can be found from either file. | |
678 | .Pp | |
679 | Note that the lines in these files are typically hundreds of characters | |
680 | long, and you definitely don't want to type in the host keys by hand. | |
681 | Rather, generate them by a script | |
682 | or by taking | |
5f4fdfae | 683 | .Pa /etc/ssh_host_key.pub |
bf740959 | 684 | and adding the host names at the front. |
685 | .Ss Examples | |
686 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
687 | .Sh FILES | |
688 | .Bl -tag -width Ds | |
5f4fdfae | 689 | .It Pa /etc/sshd_config |
bf740959 | 690 | Contains configuration data for |
691 | .Nm sshd . | |
692 | This file should be writable by root only, but it is recommended | |
693 | (though not necessary) that it be world-readable. | |
5f4fdfae | 694 | .It Pa /etc/ssh_host_key |
bf740959 | 695 | Contains the private part of the host key. |
696 | This file should only be owned by root, readable only by root, and not | |
697 | accessible to others. | |
698 | Note that | |
699 | .Nm | |
700 | does not start if this file is group/world-accessible. | |
5f4fdfae | 701 | .It Pa /etc/ssh_host_key.pub |
bf740959 | 702 | Contains the public part of the host key. |
703 | This file should be world-readable but writable only by | |
4fe2af09 | 704 | root. |
705 | Its contents should match the private part. | |
706 | This file is not | |
bf740959 | 707 | really used for anything; it is only provided for the convenience of |
708 | the user so its contents can be copied to known hosts files. | |
709 | These two files are created using | |
710 | .Xr ssh-keygen 1 . | |
5f4fdfae | 711 | .It Pa /var/run/sshd.pid |
bf740959 | 712 | Contains the process ID of the |
713 | .Nm | |
714 | listening for connections (if there are several daemons running | |
715 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 716 | started last). |
717 | The contents of this file are not sensitive; it can be world-readable. | |
bf740959 | 718 | .It Pa $HOME/.ssh/authorized_keys |
719 | Lists the RSA keys that can be used to log into the user's account. | |
720 | This file must be readable by root (which may on some machines imply | |
721 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 722 | volume). |
723 | It is recommended that it not be accessible by others. | |
724 | The format of this file is described above. | |
5f4fdfae | 725 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 726 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 727 | authentication to check the public key of the host. |
728 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 729 | The client uses the same files |
4fe2af09 | 730 | to verify that the remote host is the one we intended to connect. |
731 | These files should be writable only by root/the owner. | |
5f4fdfae | 732 | .Pa /etc/ssh_known_hosts |
bf740959 | 733 | should be world-readable, and |
734 | .Pa $HOME/.ssh/known_hosts | |
735 | can but need not be world-readable. | |
736 | .It Pa /etc/nologin | |
737 | If this file exists, | |
738 | .Nm | |
4fe2af09 | 739 | refuses to let anyone except root log in. |
740 | The contents of the file | |
bf740959 | 741 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 742 | refused. |
743 | The file should be world-readable. | |
bf740959 | 744 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
745 | If compiled with | |
746 | .Sy LIBWRAP | |
747 | support, tcp-wrappers access controls may be defined here as described in | |
748 | .Xr hosts_access 5 . | |
749 | .It Pa $HOME/.rhosts | |
750 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 751 | line. |
752 | The given user on the corresponding host is permitted to log in | |
753 | without password. | |
754 | The same file is used by rlogind and rshd. | |
bf740959 | 755 | The file must |
756 | be writable only by the user; it is recommended that it not be | |
757 | accessible by others. | |
758 | .Pp | |
4fe2af09 | 759 | If is also possible to use netgroups in the file. |
760 | Either host or user | |
bf740959 | 761 | name may be of the form +@groupname to specify all hosts or all users |
762 | in the group. | |
763 | .It Pa $HOME/.shosts | |
764 | For ssh, | |
765 | this file is exactly the same as for | |
766 | .Pa .rhosts . | |
767 | However, this file is | |
768 | not used by rlogin and rshd, so using this permits access using SSH only. | |
769 | .Pa /etc/hosts.equiv | |
770 | This file is used during | |
771 | .Pa .rhosts | |
4fe2af09 | 772 | authentication. |
773 | In the simplest form, this file contains host names, one per line. | |
774 | Users on | |
bf740959 | 775 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 776 | have the same user name on both machines. |
777 | The host name may also be | |
bf740959 | 778 | followed by a user name; such users are permitted to log in as |
779 | .Em any | |
4fe2af09 | 780 | user on this machine (except root). |
781 | Additionally, the syntax | |
bf740959 | 782 | .Dq +@group |
4fe2af09 | 783 | can be used to specify netgroups. |
784 | Negated entries start with | |
bf740959 | 785 | .Ql \&- . |
786 | .Pp | |
787 | If the client host/user is successfully matched in this file, login is | |
788 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 789 | same. |
790 | Additionally, successful RSA host authentication is normally required. | |
791 | This file must be writable only by root; it is recommended | |
bf740959 | 792 | that it be world-readable. |
793 | .Pp | |
794 | .Sy "Warning: It is almost never a good idea to use user names in" | |
795 | .Pa hosts.equiv . | |
796 | Beware that it really means that the named user(s) can log in as | |
797 | .Em anybody , | |
798 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 799 | binaries and directories. |
800 | Using a user name practically grants the user root access. | |
801 | The only valid use for user names that I can think | |
bf740959 | 802 | of is in negative entries. |
803 | .Pp | |
804 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 805 | .It Pa /etc/shosts.equiv |
bf740959 | 806 | This is processed exactly as |
807 | .Pa /etc/hosts.equiv . | |
808 | However, this file may be useful in environments that want to run both | |
809 | rsh/rlogin and ssh. | |
810 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 811 | This file is read into the environment at login (if it exists). |
812 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 813 | .Ql # ) , |
4fe2af09 | 814 | and assignment lines of the form name=value. |
815 | The file should be writable | |
bf740959 | 816 | only by the user; it need not be readable by anyone else. |
817 | .It Pa $HOME/.ssh/rc | |
818 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 819 | environment files but before starting the user's shell or command. |
820 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 821 | standard input (and |
822 | .Ev DISPLAY | |
4fe2af09 | 823 | in environment). |
824 | This must call | |
bf740959 | 825 | .Xr xauth 1 |
826 | in that case. | |
827 | .Pp | |
828 | The primary purpose of this file is to run any initialization routines | |
829 | which may be needed before the user's home directory becomes | |
830 | accessible; AFS is a particular example of such an environment. | |
831 | .Pp | |
832 | This file will probably contain some initialization code followed by | |
833 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
834 | $proto $cookie | xauth -q -; fi". | |
835 | .Pp | |
836 | If this file does not exist, | |
5f4fdfae | 837 | .Pa /etc/sshrc |
bf740959 | 838 | is run, and if that |
839 | does not exist either, xauth is used to store the cookie. | |
840 | .Pp | |
841 | This file should be writable only by the user, and need not be | |
842 | readable by anyone else. | |
5f4fdfae | 843 | .It Pa /etc/sshrc |
bf740959 | 844 | Like |
845 | .Pa $HOME/.ssh/rc . | |
846 | This can be used to specify | |
4fe2af09 | 847 | machine-specific login-time initializations globally. |
848 | This file should be writable only by root, and should be world-readable. | |
bf740959 | 849 | .Sh AUTHOR |
bf740959 | 850 | OpenSSH |
c8d54615 | 851 | is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, |
4fe2af09 | 852 | but with bugs removed and newer features re-added. |
853 | Rapidly after the | |
c8d54615 | 854 | 1.2.12 release, newer versions of the original ssh bore successively |
855 | more restrictive licenses, and thus demand for a free version was born. | |
856 | This version of OpenSSH | |
bf740959 | 857 | .Bl -bullet |
858 | .It | |
399d9d44 | 859 | has all components of a restrictive nature (i.e., patents, see |
bf740959 | 860 | .Xr ssl 8 ) |
861 | directly removed from the source code; any licensed or patented components | |
862 | are chosen from | |
863 | external libraries. | |
864 | .It | |
c8d54615 | 865 | has been updated to support ssh protocol 1.5, making it compatible with |
866 | all other ssh protocol 1 clients and servers. | |
bf740959 | 867 | .It |
868 | contains added support for | |
869 | .Xr kerberos 8 | |
870 | authentication and ticket passing. | |
871 | .It | |
872 | supports one-time password authentication with | |
873 | .Xr skey 1 . | |
874 | .El | |
875 | .Pp | |
876 | The libraries described in | |
877 | .Xr ssl 8 | |
878 | are required for proper operation. | |
879 | .Sh SEE ALSO | |
bf740959 | 880 | .Xr scp 1 , |
881 | .Xr ssh 1 , | |
882 | .Xr ssh-add 1 , | |
883 | .Xr ssh-agent 1 , | |
884 | .Xr ssh-keygen 1 , | |
7368a6c8 | 885 | .Xr ssl 8 , |
886 | .Xr rlogin 1 , | |
887 | .Xr rsh 1 |