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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 | |
22 | .Op Fl diq | |
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 | |
29 | .Sh DESCRIPTION | |
30 | .Nm | |
31 | (Secure Shell Daemon) is the daemon program for | |
32 | .Xr ssh 1 . | |
33 | Together these programs replace rlogin and rsh programs, and | |
34 | provide secure encrypted communications between two untrusted hosts | |
35 | over an insecure network. The programs are intended to be as easy to | |
36 | install and use as possible. | |
37 | .Pp | |
38 | .Nm | |
39 | is the daemon that listens for connections from clients. It is | |
40 | normally started at boot from | |
41 | .Pa /etc/rc . | |
42 | It forks a new | |
43 | daemon for each incoming connection. The forked daemons handle | |
44 | key exchange, encryption, authentication, command execution, | |
45 | and data exchange. | |
46 | .Pp | |
47 | .Nm | |
48 | works as follows. Each host has a host-specific RSA key | |
49 | (normally 1024 bits) used to identify the host. Additionally, when | |
50 | the daemon starts, it generates a server RSA key (normally 768 bits). | |
51 | This key is normally regenerated every hour if it has been used, and | |
52 | is never stored on disk. | |
53 | .Pp | |
54 | Whenever a client connects the daemon, the daemon sends its host | |
55 | and server public keys to the client. The client compares the | |
56 | host key against its own database to verify that it has not changed. | |
57 | The client then generates a 256 bit random number. It encrypts this | |
58 | random number using both the host key and the server key, and sends | |
59 | the encrypted number to the server. Both sides then start to use this | |
60 | random number as a session key which is used to encrypt all further | |
61 | communications in the session. The rest of the session is encrypted | |
62 | using a conventional cipher, currently Blowfish and 3DES, with 3DES | |
63 | being is used by default. The client selects the encryption algorithm | |
64 | to use from those offered by the server. | |
65 | .Pp | |
66 | Next, the server and the client enter an authentication dialog. The | |
67 | client tries to authenticate itself using | |
68 | .Pa .rhosts | |
69 | authentication, | |
70 | .Pa .rhosts | |
71 | authentication combined with RSA host | |
72 | authentication, RSA challenge-response authentication, or password | |
73 | based authentication. | |
74 | .Pp | |
75 | Rhosts authentication is normally disabled | |
76 | because it is fundamentally insecure, but can be enabled in the server | |
77 | configuration file if desired. System security is not improved unless | |
78 | .Xr rshd 8 , | |
79 | .Xr rlogind 8 , | |
80 | .Xr rexecd 8 , | |
81 | and | |
82 | .Xr rexd 8 | |
83 | are disabled (thus completely disabling | |
84 | .Xr rlogin 1 | |
85 | and | |
86 | .Xr rsh 1 | |
87 | into that machine). | |
88 | .Pp | |
89 | If the client successfully authenticates itself, a dialog for | |
90 | preparing the session is entered. At this time the client may request | |
91 | things like allocating a pseudo-tty, forwarding X11 connections, | |
92 | forwarding TCP/IP connections, or forwarding the authentication agent | |
93 | connection over the secure channel. | |
94 | .Pp | |
95 | Finally, the client either requests a shell or execution of a command. | |
96 | The sides then enter session mode. In this mode, either side may send | |
97 | data at any time, and such data is forwarded to/from the shell or | |
98 | command on the server side, and the user terminal in the client side. | |
99 | .Pp | |
100 | When the user program terminates and all forwarded X11 and other | |
101 | connections have been closed, the server sends command exit status to | |
102 | the client, and both sides exit. | |
103 | .Pp | |
104 | .Nm | |
105 | can be configured using command-line options or a configuration | |
106 | file. Command-line options override values specified in the | |
107 | configuration file. | |
108 | .Pp | |
109 | The options are as follows: | |
110 | .Bl -tag -width Ds | |
111 | .It Fl b Ar bits | |
112 | Specifies the number of bits in the server key (default 768). | |
113 | .Pp | |
114 | .It Fl d | |
115 | Debug mode. The server sends verbose debug output to the system | |
116 | log, and does not put itself in the background. The server also will | |
117 | not fork and will only process one connection. This option is only | |
118 | intended for debugging for the server. | |
119 | .It Fl f Ar configuration_file | |
120 | Specifies the name of the configuration file. The default is | |
916a2f43 | 121 | .Pa /etc/ssh/sshd_config . |
bf740959 | 122 | .Nm |
123 | refuses to start if there is no configuration file. | |
124 | .It Fl g Ar login_grace_time | |
125 | Gives the grace time for clients to authenticate themselves (default | |
126 | 300 seconds). If the client fails to authenticate the user within | |
127 | this many seconds, the server disconnects and exits. A value of zero | |
128 | indicates no limit. | |
129 | .It Fl h Ar host_key_file | |
130 | Specifies the file from which the host key is read (default | |
916a2f43 | 131 | .Pa /etc/ssh/ssh_host_key ) . |
bf740959 | 132 | This option must be given if |
133 | .Nm | |
134 | is not run as root (as the normal | |
135 | host file is normally not readable by anyone but root). | |
136 | .It Fl i | |
137 | Specifies that | |
138 | .Nm | |
139 | is being run from inetd. | |
140 | .Nm | |
141 | is normally not run | |
142 | from inetd because it needs to generate the server key before it can | |
143 | respond to the client, and this may take tens of seconds. Clients | |
144 | would have to wait too long if the key was regenerated every time. | |
145 | However, with small key sizes (e.g. 512) using | |
146 | .Nm | |
147 | from inetd may | |
148 | be feasible. | |
149 | .It Fl k Ar key_gen_time | |
150 | Specifies how often the server key is regenerated (default 3600 | |
151 | seconds, or one hour). The motivation for regenerating the key fairly | |
152 | often is that the key is not stored anywhere, and after about an hour, | |
153 | it becomes impossible to recover the key for decrypting intercepted | |
154 | communications even if the machine is cracked into or physically | |
155 | seized. A value of zero indicates that the key will never be regenerated. | |
156 | .It Fl p Ar port | |
157 | Specifies the port on which the server listens for connections | |
158 | (default 22). | |
159 | .It Fl q | |
160 | Quiet mode. Nothing is sent to the system log. Normally the beginning, | |
161 | authentication, and termination of each connection is logged. | |
162 | .It Fl Q | |
163 | Do not print an error message if RSA support is missing. | |
164 | .El | |
165 | .Sh CONFIGURATION FILE | |
166 | .Nm | |
167 | reads configuration data from | |
916a2f43 | 168 | .Pa /etc/ssh/sshd_config |
bf740959 | 169 | (or the file specified with |
170 | .Fl f | |
171 | on the command line). The file | |
172 | contains keyword-value pairs, one per line. Lines starting with | |
173 | .Ql # | |
174 | and empty lines are interpreted as comments. | |
175 | .Pp | |
176 | The following keywords are possible. | |
177 | .Bl -tag -width Ds | |
178 | .It Cm AFSTokenPassing | |
179 | Specifies whether an AFS token may be forwarded to the server. Default is | |
180 | .Dq yes . | |
181 | .It Cm AllowGroups | |
182 | This keyword can be followed by a number of group names, separated | |
183 | by spaces. If specified, login is allowed only for users whose primary | |
184 | group matches one of the patterns. | |
185 | .Ql \&* | |
186 | and | |
187 | .Ql ? | |
188 | can be used as | |
189 | wildcards in the patterns. Only group names are valid, a numerical group | |
190 | id isn't recognized. By default login is allowed regardless of | |
191 | the primary group. | |
192 | .Pp | |
193 | .It Cm AllowUsers | |
194 | This keyword can be followed by a number of user names, separated | |
195 | by spaces. If specified, login is allowed only for users names that | |
196 | match one of the patterns. | |
197 | .Ql \&* | |
198 | and | |
199 | .Ql ? | |
200 | can be used as | |
201 | wildcards in the patterns. Only user names are valid, a numerical user | |
202 | id isn't recognized. By default login is allowed regardless of | |
203 | the user name. | |
204 | .Pp | |
205 | .It Cm CheckMail | |
206 | Specifies whether | |
207 | .Nm | |
208 | should check for new mail for interactive logins. | |
209 | The default is | |
210 | .Dq no . | |
211 | .It Cm DenyGroups | |
212 | This keyword can be followed by a number of group names, separated | |
213 | by spaces. Users whose primary group matches one of the patterns | |
214 | aren't allowed to log in. | |
215 | .Ql \&* | |
216 | and | |
217 | .Ql ? | |
218 | can be used as | |
219 | wildcards in the patterns. Only group names are valid, a numerical group | |
220 | id isn't recognized. By default login is allowed regardless of | |
221 | the primary group. | |
222 | .Pp | |
223 | .It Cm DenyUsers | |
224 | This keyword can be followed by a number of user names, separated | |
225 | by spaces. Login is allowed disallowed for user names that match | |
226 | one of the patterns. | |
227 | .Ql \&* | |
228 | and | |
229 | .Ql ? | |
230 | can be used as | |
231 | wildcards in the patterns. Only user names are valid, a numerical user | |
232 | id isn't recognized. By default login is allowed regardless of | |
233 | the user name. | |
bf740959 | 234 | .It Cm HostKey |
235 | Specifies the file containing the private host key (default | |
916a2f43 | 236 | .Pa /etc/ssh/ssh_host_key ) . |
bf740959 | 237 | Note that |
238 | .Nm | |
239 | does not start if this file is group/world-accessible. | |
240 | .It Cm IgnoreRhosts | |
241 | Specifies that rhosts and shosts files will not be used in | |
242 | authentication. | |
243 | .Pa /etc/hosts.equiv | |
244 | and | |
916a2f43 | 245 | .Pa /etc/ssh/shosts.equiv |
bf740959 | 246 | are still used. The default is |
247 | .Dq no . | |
248 | .It Cm KeepAlive | |
249 | Specifies whether the system should send keepalive messages to the | |
250 | other side. If they are sent, death of the connection or crash of one | |
251 | of the machines will be properly noticed. However, this means that | |
252 | connections will die if the route is down temporarily, and some people | |
253 | find it annoying. On the other hand, if keepalives are not send, | |
254 | sessions may hang indefinitely on the server, leaving | |
255 | .Dq ghost | |
256 | users and consuming server resources. | |
257 | .Pp | |
258 | The default is | |
259 | .Dq yes | |
260 | (to send keepalives), and the server will notice | |
261 | if the network goes down or the client host reboots. This avoids | |
262 | infinitely hanging sessions. | |
263 | .Pp | |
264 | To disable keepalives, the value should be set to | |
265 | .Dq no | |
266 | in both the server and the client configuration files. | |
267 | .It Cm KerberosAuthentication | |
268 | Specifies whether Kerberos authentication is allowed. This can | |
269 | be in the form of a Kerberos ticket, or if | |
270 | .Cm PasswordAuthentication | |
271 | is yes, the password provided by the user will be validated through | |
272 | the Kerberos KDC. Default is | |
273 | .Dq yes . | |
274 | .It Cm KerberosOrLocalPasswd | |
275 | If set then if password authentication through Kerberos fails then | |
276 | the password will be validated via any additional local mechanism | |
277 | such as | |
278 | .Pa /etc/passwd | |
279 | or SecurID. Default is | |
280 | .Dq yes . | |
281 | .It Cm KerberosTgtPassing | |
282 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
283 | Default is | |
284 | .Dq no , | |
285 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
286 | .It Cm KerberosTicketCleanup | |
287 | Specifies whether to automatically destroy the user's ticket cache | |
288 | file on logout. Default is | |
289 | .Dq yes . | |
290 | .It Cm KeyRegenerationInterval | |
291 | The server key is automatically regenerated after this many seconds | |
292 | (if it has been used). The purpose of regeneration is to prevent | |
293 | decrypting captured sessions by later breaking into the machine and | |
294 | stealing the keys. The key is never stored anywhere. If the value is | |
295 | 0, the key is never regenerated. The default is 3600 | |
296 | (seconds). | |
297 | .It Cm ListenAddress | |
298 | Specifies what local address | |
299 | .Nm | |
300 | should listen on. | |
301 | The default is to listen to all local addresses. | |
302 | .It Cm LoginGraceTime | |
303 | The server disconnects after this time if the user has not | |
304 | successfully logged in. If the value is 0, there is no time limit. | |
305 | The default is 600 (seconds). | |
6a17f9c2 | 306 | .It Cm LogLevel |
307 | Gives the verbosity level that is used when logging messages from | |
308 | .Nm sshd . | |
309 | The possible values are: | |
310 | QUIET, FATAL, ERROR, INFO, CHAT and DEBUG. | |
311 | The default is INFO. | |
312 | Logging with level DEBUG violates the privacy of users | |
313 | and is not recommended. | |
bf740959 | 314 | .It Cm PasswordAuthentication |
315 | Specifies whether password authentication is allowed. | |
316 | The default is | |
317 | .Dq yes . | |
318 | .It Cm PermitEmptyPasswords | |
319 | When password authentication is allowed, it specifies whether the | |
320 | server allows login to accounts with empty password strings. The default | |
321 | is | |
322 | .Dq yes . | |
323 | .It Cm PermitRootLogin | |
324 | Specifies whether the root can log in using | |
325 | .Xr ssh 1 . | |
326 | The argument must be | |
327 | .Dq yes , | |
328 | .Dq without-password | |
329 | or | |
330 | .Dq no . | |
331 | The default is | |
332 | .Dq yes . | |
333 | If this options is set to | |
334 | .Dq without-password | |
335 | only password authentication is disabled for root. | |
336 | .Pp | |
337 | Root login with RSA authentication when the | |
338 | .Ar command | |
339 | option has been | |
340 | specified will be allowed regardless of the value of this setting | |
341 | (which may be useful for taking remote backups even if root login is | |
342 | normally not allowed). | |
343 | .It Cm Port | |
344 | Specifies the port number that | |
345 | .Nm | |
346 | listens on. The default is 22. | |
347 | .It Cm PrintMotd | |
348 | Specifies whether | |
349 | .Nm | |
350 | should print | |
351 | .Pa /etc/motd | |
352 | when a user logs in interactively. (On some systems it is also | |
353 | printed by the shell, | |
354 | .Pa /etc/profile , | |
355 | or equivalent.) The default is | |
356 | .Dq yes . | |
bf740959 | 357 | .It Cm RandomSeed |
358 | Obsolete. Random number generation uses other techniques. | |
359 | .It Cm RhostsAuthentication | |
360 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
361 | files is sufficient. Normally, this method should not be permitted | |
362 | because it is insecure. | |
363 | .Cm RhostsRSAAuthentication | |
364 | should be used | |
365 | instead, because it performs RSA-based host authentication in addition | |
366 | to normal rhosts or /etc/hosts.equiv authentication. | |
367 | The default is | |
368 | .Dq no . | |
369 | .It Cm RhostsRSAAuthentication | |
370 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
371 | with successful RSA host authentication is allowed. The default is | |
372 | .Dq yes . | |
373 | .It Cm RSAAuthentication | |
374 | Specifies whether pure RSA authentication is allowed. The default is | |
375 | .Dq yes . | |
376 | .It Cm ServerKeyBits | |
377 | Defines the number of bits in the server key. The minimum value is | |
378 | 512, and the default is 768. | |
379 | .It Cm SkeyAuthentication | |
380 | Specifies whether | |
381 | .Xr skey 1 | |
382 | authentication is allowed. The default is | |
383 | .Dq yes . | |
384 | Note that s/key authentication is enabled only if | |
385 | .Cm PasswordAuthentication | |
386 | is allowed, too. | |
387 | .It Cm StrictModes | |
388 | Specifies whether | |
389 | .Nm | |
390 | should check file modes and ownership of the | |
391 | user's files and home directory before accepting login. This | |
392 | is normally desirable because novices sometimes accidentally leave their | |
393 | directory or files world-writable. The default is | |
394 | .Dq yes . | |
395 | .It Cm SyslogFacility | |
396 | Gives the facility code that is used when logging messages from | |
397 | .Nm sshd . | |
398 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
399 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. | |
400 | .It Cm UseLogin | |
401 | Specifies whether | |
402 | .Xr login 1 | |
403 | is used. The default is | |
404 | .Dq no . | |
405 | .It Cm X11Forwarding | |
406 | Specifies whether X11 forwarding is permitted. The default is | |
407 | .Dq yes . | |
408 | Note that disabling X11 forwarding does not improve security in any | |
409 | way, as users can always install their own forwarders. | |
410 | .It Cm X11DisplayOffset | |
411 | Specifies the first display number available for | |
412 | .Nm sshd Ns 's | |
413 | X11 forwarding. This prevents | |
414 | .Nm | |
415 | from interfering with real X11 servers. | |
416 | .El | |
417 | .Sh LOGIN PROCESS | |
418 | When a user successfully logs in, | |
419 | .Nm | |
420 | does the following: | |
421 | .Bl -enum -offset indent | |
422 | .It | |
423 | If the login is on a tty, and no command has been specified, | |
424 | prints last login time and | |
425 | .Pa /etc/motd | |
426 | (unless prevented in the configuration file or by | |
427 | .Pa $HOME/.hushlogin ; | |
428 | see the | |
429 | .Sx FILES | |
430 | section). | |
431 | .It | |
432 | If the login is on a tty, records login time. | |
433 | .It | |
434 | Checks | |
435 | .Pa /etc/nologin ; | |
436 | if it exists, prints contents and quits | |
437 | (unless root). | |
438 | .It | |
439 | Changes to run with normal user privileges. | |
440 | .It | |
441 | Sets up basic environment. | |
442 | .It | |
443 | Reads | |
444 | .Pa $HOME/.ssh/environment | |
445 | if it exists. | |
446 | .It | |
447 | Changes to user's home directory. | |
448 | .It | |
449 | If | |
450 | .Pa $HOME/.ssh/rc | |
451 | exists, runs it; else if | |
916a2f43 | 452 | .Pa /etc/ssh/sshrc |
bf740959 | 453 | exists, runs |
454 | it; otherwise runs xauth. The | |
455 | .Dq rc | |
456 | files are given the X11 | |
457 | authentication protocol and cookie in standard input. | |
458 | .It | |
459 | Runs user's shell or command. | |
460 | .El | |
461 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
462 | The | |
463 | .Pa $HOME/.ssh/authorized_keys | |
464 | file lists the RSA keys that are | |
465 | permitted for RSA authentication. Each line of the file contains one | |
466 | key (empty lines and lines starting with a | |
467 | .Ql # | |
468 | are ignored as | |
469 | comments). Each line consists of the following fields, separated by | |
470 | spaces: options, bits, exponent, modulus, comment. The options field | |
471 | is optional; its presence is determined by whether the line starts | |
472 | with a number or not (the option field never starts with a number). | |
473 | The bits, exponent, modulus and comment fields give the RSA key; the | |
474 | comment field is not used for anything (but may be convenient for the | |
475 | user to identify the key). | |
476 | .Pp | |
477 | Note that lines in this file are usually several hundred bytes long | |
478 | (because of the size of the RSA key modulus). You don't want to type | |
479 | them in; instead, copy the | |
480 | .Pa identity.pub | |
481 | file and edit it. | |
482 | .Pp | |
483 | The options (if present) consists of comma-separated option | |
484 | specifications. No spaces are permitted, except within double quotes. | |
485 | The following option specifications are supported: | |
486 | .Bl -tag -width Ds | |
487 | .It Cm from="pattern-list" | |
488 | Specifies that in addition to RSA authentication, the canonical name | |
489 | of the remote host must be present in the comma-separated list of | |
490 | patterns ('*' and '?' serve as wildcards). The list may also contain | |
491 | patterns negated by prefixing them with '!'; if the canonical host | |
492 | name matches a negated pattern, the key is not accepted. The purpose | |
493 | of this option is to optionally increase security: RSA authentication | |
494 | by itself does not trust the network or name servers or anything (but | |
495 | the key); however, if somebody somehow steals the key, the key | |
496 | permits an intruder to log in from anywhere in the world. This | |
497 | additional option makes using a stolen key more difficult (name | |
498 | servers and/or routers would have to be compromised in addition to | |
499 | just the key). | |
500 | .It Cm command="command" | |
501 | Specifies that the command is executed whenever this key is used for | |
502 | authentication. The command supplied by the user (if any) is ignored. | |
503 | The command is run on a pty if the connection requests a pty; | |
504 | otherwise it is run without a tty. A quote may be included in the | |
505 | command by quoting it with a backslash. This option might be useful | |
506 | to restrict certain RSA keys to perform just a specific operation. An | |
507 | example might be a key that permits remote backups but nothing | |
508 | else. Notice that the client may specify TCP/IP and/or X11 | |
509 | forwardings unless they are explicitly prohibited. | |
510 | .It Cm environment="NAME=value" | |
511 | Specifies that the string is to be added to the environment when | |
512 | logging in using this key. Environment variables set this way | |
513 | override other default environment values. Multiple options of this | |
514 | type are permitted. | |
515 | .It Cm no-port-forwarding | |
516 | Forbids TCP/IP forwarding when this key is used for authentication. | |
517 | Any port forward requests by the client will return an error. This | |
518 | might be used, e.g., in connection with the | |
519 | .Cm command | |
520 | option. | |
521 | .It Cm no-X11-forwarding | |
522 | Forbids X11 forwarding when this key is used for authentication. | |
523 | Any X11 forward requests by the client will return an error. | |
524 | .It Cm no-agent-forwarding | |
525 | Forbids authentication agent forwarding when this key is used for | |
526 | authentication. | |
527 | .It Cm no-pty | |
528 | Prevents tty allocation (a request to allocate a pty will fail). | |
529 | .El | |
530 | .Ss Examples | |
531 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
532 | .Pp | |
533 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
534 | .Pp | |
535 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
536 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
537 | The | |
916a2f43 | 538 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 539 | and |
540 | .Pa $HOME/.ssh/known_hosts | |
541 | files contain host public keys for all known hosts. The global file should | |
542 | be prepared by the admistrator (optional), and the per-user file is | |
543 | maintained automatically: whenever the user connects an unknown host | |
544 | its key is added to the per-user file. | |
545 | .Pp | |
546 | Each line in these files contains the following fields: hostnames, | |
547 | bits, exponent, modulus, comment. The fields are separated by spaces. | |
548 | .Pp | |
549 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
550 | wildcards); each pattern in turn is matched against the canonical host | |
551 | name (when authenticating a client) or against the user-supplied | |
552 | name (when authenticating a server). A pattern may also be preceded | |
553 | by | |
554 | .Ql ! | |
555 | to indicate negation: if the host name matches a negated | |
556 | pattern, it is not accepted (by that line) even if it matched another | |
557 | pattern on the line. | |
558 | .Pp | |
559 | Bits, exponent, and modulus are taken directly from the host key; they | |
560 | can be obtained, e.g., from | |
916a2f43 | 561 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 562 | The optional comment field continues to the end of the line, and is not used. |
563 | .Pp | |
564 | Lines starting with | |
565 | .Ql # | |
566 | and empty lines are ignored as comments. | |
567 | .Pp | |
568 | When performing host authentication, authentication is accepted if any | |
569 | matching line has the proper key. It is thus permissible (but not | |
570 | recommended) to have several lines or different host keys for the same | |
571 | names. This will inevitably happen when short forms of host names | |
572 | from different domains are put in the file. It is possible | |
573 | that the files contain conflicting information; authentication is | |
574 | accepted if valid information can be found from either file. | |
575 | .Pp | |
576 | Note that the lines in these files are typically hundreds of characters | |
577 | long, and you definitely don't want to type in the host keys by hand. | |
578 | Rather, generate them by a script | |
579 | or by taking | |
916a2f43 | 580 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 581 | and adding the host names at the front. |
582 | .Ss Examples | |
583 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
584 | .Sh FILES | |
585 | .Bl -tag -width Ds | |
916a2f43 | 586 | .It Pa /etc/ssh/sshd_config |
bf740959 | 587 | Contains configuration data for |
588 | .Nm sshd . | |
589 | This file should be writable by root only, but it is recommended | |
590 | (though not necessary) that it be world-readable. | |
916a2f43 | 591 | .It Pa /etc/ssh/ssh_host_key |
bf740959 | 592 | Contains the private part of the host key. |
593 | This file should only be owned by root, readable only by root, and not | |
594 | accessible to others. | |
595 | Note that | |
596 | .Nm | |
597 | does not start if this file is group/world-accessible. | |
916a2f43 | 598 | .It Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 599 | Contains the public part of the host key. |
600 | This file should be world-readable but writable only by | |
601 | root. Its contents should match the private part. This file is not | |
602 | really used for anything; it is only provided for the convenience of | |
603 | the user so its contents can be copied to known hosts files. | |
604 | These two files are created using | |
605 | .Xr ssh-keygen 1 . | |
606 | .It Pa /var/run/sshd.pid | |
607 | Contains the process ID of the | |
608 | .Nm | |
609 | listening for connections (if there are several daemons running | |
610 | concurrently for different ports, this contains the pid of the one | |
611 | started last). The contents of this file are not sensitive; it can be | |
612 | world-readable. | |
613 | .It Pa $HOME/.ssh/authorized_keys | |
614 | Lists the RSA keys that can be used to log into the user's account. | |
615 | This file must be readable by root (which may on some machines imply | |
616 | it being world-readable if the user's home directory resides on an NFS | |
617 | volume). It is recommended that it not be accessible by others. The | |
618 | format of this file is described above. | |
6a17f9c2 | 619 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
620 | These files are consulted when using rhosts with RSA host | |
bf740959 | 621 | authentication to check the public key of the host. The key must be |
5bbb5681 | 622 | listed in one of these files to be accepted. |
623 | The client uses the same files | |
bf740959 | 624 | to verify that the remote host is the one we intended to |
625 | connect. These files should be writable only by root/the owner. | |
916a2f43 | 626 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 627 | should be world-readable, and |
628 | .Pa $HOME/.ssh/known_hosts | |
629 | can but need not be world-readable. | |
630 | .It Pa /etc/nologin | |
631 | If this file exists, | |
632 | .Nm | |
633 | refuses to let anyone except root log in. The contents of the file | |
634 | are displayed to anyone trying to log in, and non-root connections are | |
635 | refused. The file should be world-readable. | |
636 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
637 | If compiled with | |
638 | .Sy LIBWRAP | |
639 | support, tcp-wrappers access controls may be defined here as described in | |
640 | .Xr hosts_access 5 . | |
641 | .It Pa $HOME/.rhosts | |
642 | This file contains host-username pairs, separated by a space, one per | |
643 | line. The given user on the corresponding host is permitted to log in | |
644 | without password. The same file is used by rlogind and rshd. | |
645 | The file must | |
646 | be writable only by the user; it is recommended that it not be | |
647 | accessible by others. | |
648 | .Pp | |
649 | If is also possible to use netgroups in the file. Either host or user | |
650 | name may be of the form +@groupname to specify all hosts or all users | |
651 | in the group. | |
652 | .It Pa $HOME/.shosts | |
653 | For ssh, | |
654 | this file is exactly the same as for | |
655 | .Pa .rhosts . | |
656 | However, this file is | |
657 | not used by rlogin and rshd, so using this permits access using SSH only. | |
658 | .Pa /etc/hosts.equiv | |
659 | This file is used during | |
660 | .Pa .rhosts | |
661 | authentication. In the | |
662 | simplest form, this file contains host names, one per line. Users on | |
663 | those hosts are permitted to log in without a password, provided they | |
664 | have the same user name on both machines. The host name may also be | |
665 | followed by a user name; such users are permitted to log in as | |
666 | .Em any | |
667 | user on this machine (except root). Additionally, the syntax | |
668 | .Dq +@group | |
669 | can be used to specify netgroups. Negated entries start with | |
670 | .Ql \&- . | |
671 | .Pp | |
672 | If the client host/user is successfully matched in this file, login is | |
673 | automatically permitted provided the client and server user names are the | |
674 | same. Additionally, successful RSA host authentication is normally | |
675 | required. This file must be writable only by root; it is recommended | |
676 | that it be world-readable. | |
677 | .Pp | |
678 | .Sy "Warning: It is almost never a good idea to use user names in" | |
679 | .Pa hosts.equiv . | |
680 | Beware that it really means that the named user(s) can log in as | |
681 | .Em anybody , | |
682 | which includes bin, daemon, adm, and other accounts that own critical | |
683 | binaries and directories. Using a user name practically grants the | |
684 | user root access. The only valid use for user names that I can think | |
685 | of is in negative entries. | |
686 | .Pp | |
687 | Note that this warning also applies to rsh/rlogin. | |
916a2f43 | 688 | .It Pa /etc/ssh/shosts.equiv |
bf740959 | 689 | This is processed exactly as |
690 | .Pa /etc/hosts.equiv . | |
691 | However, this file may be useful in environments that want to run both | |
692 | rsh/rlogin and ssh. | |
693 | .It Pa $HOME/.ssh/environment | |
694 | This file is read into the environment at login (if it exists). It | |
695 | can only contain empty lines, comment lines (that start with | |
696 | .Ql # ) , | |
697 | and assignment lines of the form name=value. The file should be writable | |
698 | only by the user; it need not be readable by anyone else. | |
699 | .It Pa $HOME/.ssh/rc | |
700 | If this file exists, it is run with /bin/sh after reading the | |
701 | environment files but before starting the user's shell or command. If | |
702 | X11 spoofing is in use, this will receive the "proto cookie" pair in | |
703 | standard input (and | |
704 | .Ev DISPLAY | |
705 | in environment). This must call | |
706 | .Xr xauth 1 | |
707 | in that case. | |
708 | .Pp | |
709 | The primary purpose of this file is to run any initialization routines | |
710 | which may be needed before the user's home directory becomes | |
711 | accessible; AFS is a particular example of such an environment. | |
712 | .Pp | |
713 | This file will probably contain some initialization code followed by | |
714 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
715 | $proto $cookie | xauth -q -; fi". | |
716 | .Pp | |
717 | If this file does not exist, | |
916a2f43 | 718 | .Pa /etc/ssh/sshrc |
bf740959 | 719 | is run, and if that |
720 | does not exist either, xauth is used to store the cookie. | |
721 | .Pp | |
722 | This file should be writable only by the user, and need not be | |
723 | readable by anyone else. | |
916a2f43 | 724 | .It Pa /etc/ssh/sshrc |
bf740959 | 725 | Like |
726 | .Pa $HOME/.ssh/rc . | |
727 | This can be used to specify | |
728 | machine-specific login-time initializations globally. This file | |
729 | should be writable only by root, and should be world-readable. | |
730 | .Sh AUTHOR | |
731 | Tatu Ylonen <ylo@cs.hut.fi> | |
732 | .Pp | |
733 | Information about new releases, mailing lists, and other related | |
734 | issues can be found from the SSH WWW home page: | |
735 | .Pp | |
736 | .Dl http://www.cs.hut.fi/ssh. | |
737 | .Pp | |
738 | OpenSSH | |
739 | is a derivative of the original (free) ssh 1.2.12 release, but with bugs | |
740 | removed and newer features re-added. Rapidly after the 1.2.12 release, | |
741 | newer versions bore successively more restrictive licenses. This version | |
742 | of OpenSSH | |
743 | .Bl -bullet | |
744 | .It | |
745 | has all components of a restrictive nature (ie. patents, see | |
746 | .Xr ssl 8 ) | |
747 | directly removed from the source code; any licensed or patented components | |
748 | are chosen from | |
749 | external libraries. | |
750 | .It | |
751 | has been updated to support ssh protocol 1.5. | |
752 | .It | |
753 | contains added support for | |
754 | .Xr kerberos 8 | |
755 | authentication and ticket passing. | |
756 | .It | |
757 | supports one-time password authentication with | |
758 | .Xr skey 1 . | |
759 | .El | |
760 | .Pp | |
761 | The libraries described in | |
762 | .Xr ssl 8 | |
763 | are required for proper operation. | |
764 | .Sh SEE ALSO | |
765 | .Xr rlogin 1 , | |
766 | .Xr rsh 1 , | |
767 | .Xr scp 1 , | |
768 | .Xr ssh 1 , | |
769 | .Xr ssh-add 1 , | |
770 | .Xr ssh-agent 1 , | |
771 | .Xr ssh-keygen 1 , | |
772 | .Xr ssl 8 |