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