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8efc0c15 | 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 | |
5aecb327 | 121 | .Pa /etc/openssh/sshd_config . |
8efc0c15 | 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 | |
5aecb327 | 131 | .Pa /etc/openssh/ssh_host_key ) . |
8efc0c15 | 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 | |
5aecb327 | 168 | .Pa /etc/openssh/sshd_config |
8efc0c15 | 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. | |
234 | .Pp | |
235 | .It Cm FascistLogging | |
236 | Specifies whether to use verbose logging. Verbose logging violates | |
237 | the privacy of users and is not recommended. The argument must be | |
238 | .Dq yes | |
239 | or | |
240 | .Dq no . | |
241 | The default is | |
242 | .Dq no . | |
243 | .It Cm HostKey | |
244 | Specifies the file containing the private host key (default | |
5aecb327 | 245 | .Pa /etc/openssh/ssh_host_key ) . |
8efc0c15 | 246 | Note that |
247 | .Nm | |
248 | does not start if this file is group/world-accessible. | |
249 | .It Cm IgnoreRhosts | |
250 | Specifies that rhosts and shosts files will not be used in | |
251 | authentication. | |
252 | .Pa /etc/hosts.equiv | |
253 | and | |
5aecb327 | 254 | .Pa /etc/openssh/shosts.equiv |
8efc0c15 | 255 | are still used. The default is |
256 | .Dq no . | |
257 | .It Cm KeepAlive | |
258 | Specifies whether the system should send keepalive messages to the | |
259 | other side. If they are sent, death of the connection or crash of one | |
260 | of the machines will be properly noticed. However, this means that | |
261 | connections will die if the route is down temporarily, and some people | |
262 | find it annoying. On the other hand, if keepalives are not send, | |
263 | sessions may hang indefinitely on the server, leaving | |
264 | .Dq ghost | |
265 | users and consuming server resources. | |
266 | .Pp | |
267 | The default is | |
268 | .Dq yes | |
269 | (to send keepalives), and the server will notice | |
270 | if the network goes down or the client host reboots. This avoids | |
271 | infinitely hanging sessions. | |
272 | .Pp | |
273 | To disable keepalives, the value should be set to | |
274 | .Dq no | |
275 | in both the server and the client configuration files. | |
276 | .It Cm KerberosAuthentication | |
277 | Specifies whether Kerberos authentication is allowed. This can | |
278 | be in the form of a Kerberos ticket, or if | |
279 | .Cm PasswordAuthentication | |
280 | is yes, the password provided by the user will be validated through | |
281 | the Kerberos KDC. Default is | |
282 | .Dq yes . | |
283 | .It Cm KerberosOrLocalPasswd | |
284 | If set then if password authentication through Kerberos fails then | |
285 | the password will be validated via any additional local mechanism | |
286 | such as | |
287 | .Pa /etc/passwd | |
288 | or SecurID. Default is | |
289 | .Dq yes . | |
290 | .It Cm KerberosTgtPassing | |
291 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
292 | Default is | |
293 | .Dq no , | |
294 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
295 | .It Cm KerberosTicketCleanup | |
296 | Specifies whether to automatically destroy the user's ticket cache | |
297 | file on logout. Default is | |
298 | .Dq yes . | |
299 | .It Cm KeyRegenerationInterval | |
300 | The server key is automatically regenerated after this many seconds | |
301 | (if it has been used). The purpose of regeneration is to prevent | |
302 | decrypting captured sessions by later breaking into the machine and | |
303 | stealing the keys. The key is never stored anywhere. If the value is | |
304 | 0, the key is never regenerated. The default is 3600 | |
305 | (seconds). | |
306 | .It Cm ListenAddress | |
307 | Specifies what local address | |
308 | .Nm | |
309 | should listen on. | |
310 | The default is to listen to all local addresses. | |
311 | .It Cm LoginGraceTime | |
312 | The server disconnects after this time if the user has not | |
313 | successfully logged in. If the value is 0, there is no time limit. | |
314 | The default is 600 (seconds). | |
315 | .It Cm PasswordAuthentication | |
316 | Specifies whether password authentication is allowed. | |
317 | The default is | |
318 | .Dq yes . | |
319 | .It Cm PermitEmptyPasswords | |
320 | When password authentication is allowed, it specifies whether the | |
321 | server allows login to accounts with empty password strings. The default | |
322 | is | |
323 | .Dq yes . | |
324 | .It Cm PermitRootLogin | |
325 | Specifies whether the root can log in using | |
326 | .Xr ssh 1 . | |
327 | The argument must be | |
328 | .Dq yes , | |
329 | .Dq without-password | |
330 | or | |
331 | .Dq no . | |
332 | The default is | |
333 | .Dq yes . | |
334 | If this options is set to | |
335 | .Dq without-password | |
336 | only password authentication is disabled for root. | |
337 | .Pp | |
338 | Root login with RSA authentication when the | |
339 | .Ar command | |
340 | option has been | |
341 | specified will be allowed regardless of the value of this setting | |
342 | (which may be useful for taking remote backups even if root login is | |
343 | normally not allowed). | |
344 | .It Cm Port | |
345 | Specifies the port number that | |
346 | .Nm | |
347 | listens on. The default is 22. | |
348 | .It Cm PrintMotd | |
349 | Specifies whether | |
350 | .Nm | |
351 | should print | |
352 | .Pa /etc/motd | |
353 | when a user logs in interactively. (On some systems it is also | |
354 | printed by the shell, | |
355 | .Pa /etc/profile , | |
356 | or equivalent.) The default is | |
357 | .Dq yes . | |
358 | .It Cm QuietMode | |
359 | Specifies whether the system runs in quiet mode. In quiet mode, | |
360 | nothing is logged in the system log, except fatal errors. The default | |
361 | is | |
362 | .Dq no . | |
363 | .It Cm RandomSeed | |
364 | Obsolete. Random number generation uses other techniques. | |
365 | .It Cm RhostsAuthentication | |
366 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
367 | files is sufficient. Normally, this method should not be permitted | |
368 | because it is insecure. | |
369 | .Cm RhostsRSAAuthentication | |
370 | should be used | |
371 | instead, because it performs RSA-based host authentication in addition | |
372 | to normal rhosts or /etc/hosts.equiv authentication. | |
373 | The default is | |
374 | .Dq no . | |
375 | .It Cm RhostsRSAAuthentication | |
376 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
377 | with successful RSA host authentication is allowed. The default is | |
378 | .Dq yes . | |
379 | .It Cm RSAAuthentication | |
380 | Specifies whether pure RSA authentication is allowed. The default is | |
381 | .Dq yes . | |
382 | .It Cm ServerKeyBits | |
383 | Defines the number of bits in the server key. The minimum value is | |
384 | 512, and the default is 768. | |
385 | .It Cm SkeyAuthentication | |
386 | Specifies whether | |
387 | .Xr skey 1 | |
388 | authentication is allowed. The default is | |
389 | .Dq yes . | |
390 | Note that s/key authentication is enabled only if | |
391 | .Cm PasswordAuthentication | |
392 | is allowed, too. | |
393 | .It Cm StrictModes | |
394 | Specifies whether | |
395 | .Nm | |
396 | should check file modes and ownership of the | |
397 | user's files and home directory before accepting login. This | |
398 | is normally desirable because novices sometimes accidentally leave their | |
399 | directory or files world-writable. The default is | |
400 | .Dq yes . | |
401 | .It Cm SyslogFacility | |
402 | Gives the facility code that is used when logging messages from | |
403 | .Nm sshd . | |
404 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
405 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. | |
406 | .It Cm UseLogin | |
407 | Specifies whether | |
408 | .Xr login 1 | |
409 | is used. The default is | |
410 | .Dq no . | |
411 | .It Cm X11Forwarding | |
412 | Specifies whether X11 forwarding is permitted. The default is | |
413 | .Dq yes . | |
414 | Note that disabling X11 forwarding does not improve security in any | |
415 | way, as users can always install their own forwarders. | |
416 | .It Cm X11DisplayOffset | |
417 | Specifies the first display number available for | |
418 | .Nm sshd Ns 's | |
419 | X11 forwarding. This prevents | |
420 | .Nm | |
421 | from interfering with real X11 servers. | |
422 | .El | |
423 | .Sh LOGIN PROCESS | |
424 | When a user successfully logs in, | |
425 | .Nm | |
426 | does the following: | |
427 | .Bl -enum -offset indent | |
428 | .It | |
429 | If the login is on a tty, and no command has been specified, | |
430 | prints last login time and | |
431 | .Pa /etc/motd | |
432 | (unless prevented in the configuration file or by | |
433 | .Pa $HOME/.hushlogin ; | |
434 | see the | |
435 | .Sx FILES | |
436 | section). | |
437 | .It | |
438 | If the login is on a tty, records login time. | |
439 | .It | |
440 | Checks | |
441 | .Pa /etc/nologin ; | |
442 | if it exists, prints contents and quits | |
443 | (unless root). | |
444 | .It | |
445 | Changes to run with normal user privileges. | |
446 | .It | |
447 | Sets up basic environment. | |
448 | .It | |
449 | Reads | |
450 | .Pa $HOME/.ssh/environment | |
451 | if it exists. | |
452 | .It | |
453 | Changes to user's home directory. | |
454 | .It | |
455 | If | |
456 | .Pa $HOME/.ssh/rc | |
457 | exists, runs it; else if | |
5aecb327 | 458 | .Pa /etc/openssh/sshrc |
8efc0c15 | 459 | exists, runs |
460 | it; otherwise runs xauth. The | |
461 | .Dq rc | |
462 | files are given the X11 | |
463 | authentication protocol and cookie in standard input. | |
464 | .It | |
465 | Runs user's shell or command. | |
466 | .El | |
467 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
468 | The | |
469 | .Pa $HOME/.ssh/authorized_keys | |
470 | file lists the RSA keys that are | |
471 | permitted for RSA authentication. Each line of the file contains one | |
472 | key (empty lines and lines starting with a | |
473 | .Ql # | |
474 | are ignored as | |
475 | comments). Each line consists of the following fields, separated by | |
476 | spaces: options, bits, exponent, modulus, comment. The options field | |
477 | is optional; its presence is determined by whether the line starts | |
478 | with a number or not (the option field never starts with a number). | |
479 | The bits, exponent, modulus and comment fields give the RSA key; the | |
480 | comment field is not used for anything (but may be convenient for the | |
481 | user to identify the key). | |
482 | .Pp | |
483 | Note that lines in this file are usually several hundred bytes long | |
484 | (because of the size of the RSA key modulus). You don't want to type | |
485 | them in; instead, copy the | |
486 | .Pa identity.pub | |
487 | file and edit it. | |
488 | .Pp | |
489 | The options (if present) consists of comma-separated option | |
490 | specifications. No spaces are permitted, except within double quotes. | |
491 | The following option specifications are supported: | |
492 | .Bl -tag -width Ds | |
493 | .It Cm from="pattern-list" | |
494 | Specifies that in addition to RSA authentication, the canonical name | |
495 | of the remote host must be present in the comma-separated list of | |
496 | patterns ('*' and '?' serve as wildcards). The list may also contain | |
497 | patterns negated by prefixing them with '!'; if the canonical host | |
498 | name matches a negated pattern, the key is not accepted. The purpose | |
499 | of this option is to optionally increase security: RSA authentication | |
500 | by itself does not trust the network or name servers or anything (but | |
501 | the key); however, if somebody somehow steals the key, the key | |
502 | permits an intruder to log in from anywhere in the world. This | |
503 | additional option makes using a stolen key more difficult (name | |
504 | servers and/or routers would have to be compromised in addition to | |
505 | just the key). | |
506 | .It Cm command="command" | |
507 | Specifies that the command is executed whenever this key is used for | |
508 | authentication. The command supplied by the user (if any) is ignored. | |
509 | The command is run on a pty if the connection requests a pty; | |
510 | otherwise it is run without a tty. A quote may be included in the | |
511 | command by quoting it with a backslash. This option might be useful | |
512 | to restrict certain RSA keys to perform just a specific operation. An | |
513 | example might be a key that permits remote backups but nothing | |
514 | else. Notice that the client may specify TCP/IP and/or X11 | |
515 | forwardings unless they are explicitly prohibited. | |
516 | .It Cm environment="NAME=value" | |
517 | Specifies that the string is to be added to the environment when | |
518 | logging in using this key. Environment variables set this way | |
519 | override other default environment values. Multiple options of this | |
520 | type are permitted. | |
521 | .It Cm no-port-forwarding | |
522 | Forbids TCP/IP forwarding when this key is used for authentication. | |
523 | Any port forward requests by the client will return an error. This | |
524 | might be used, e.g., in connection with the | |
525 | .Cm command | |
526 | option. | |
527 | .It Cm no-X11-forwarding | |
528 | Forbids X11 forwarding when this key is used for authentication. | |
529 | Any X11 forward requests by the client will return an error. | |
530 | .It Cm no-agent-forwarding | |
531 | Forbids authentication agent forwarding when this key is used for | |
532 | authentication. | |
533 | .It Cm no-pty | |
534 | Prevents tty allocation (a request to allocate a pty will fail). | |
535 | .El | |
536 | .Ss Examples | |
537 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
538 | .Pp | |
539 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
540 | .Pp | |
541 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
542 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
543 | The | |
5aecb327 | 544 | .Pa /etc/openssh/ssh_known_hosts |
8efc0c15 | 545 | and |
546 | .Pa $HOME/.ssh/known_hosts | |
547 | files contain host public keys for all known hosts. The global file should | |
548 | be prepared by the admistrator (optional), and the per-user file is | |
549 | maintained automatically: whenever the user connects an unknown host | |
550 | its key is added to the per-user file. | |
551 | .Pp | |
552 | Each line in these files contains the following fields: hostnames, | |
553 | bits, exponent, modulus, comment. The fields are separated by spaces. | |
554 | .Pp | |
555 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
556 | wildcards); each pattern in turn is matched against the canonical host | |
557 | name (when authenticating a client) or against the user-supplied | |
558 | name (when authenticating a server). A pattern may also be preceded | |
559 | by | |
560 | .Ql ! | |
561 | to indicate negation: if the host name matches a negated | |
562 | pattern, it is not accepted (by that line) even if it matched another | |
563 | pattern on the line. | |
564 | .Pp | |
565 | Bits, exponent, and modulus are taken directly from the host key; they | |
566 | can be obtained, e.g., from | |
5aecb327 | 567 | .Pa /etc/openssh/ssh_host_key.pub . |
8efc0c15 | 568 | The optional comment field continues to the end of the line, and is not used. |
569 | .Pp | |
570 | Lines starting with | |
571 | .Ql # | |
572 | and empty lines are ignored as comments. | |
573 | .Pp | |
574 | When performing host authentication, authentication is accepted if any | |
575 | matching line has the proper key. It is thus permissible (but not | |
576 | recommended) to have several lines or different host keys for the same | |
577 | names. This will inevitably happen when short forms of host names | |
578 | from different domains are put in the file. It is possible | |
579 | that the files contain conflicting information; authentication is | |
580 | accepted if valid information can be found from either file. | |
581 | .Pp | |
582 | Note that the lines in these files are typically hundreds of characters | |
583 | long, and you definitely don't want to type in the host keys by hand. | |
584 | Rather, generate them by a script | |
585 | or by taking | |
5aecb327 | 586 | .Pa /etc/openssh/ssh_host_key.pub |
8efc0c15 | 587 | and adding the host names at the front. |
588 | .Ss Examples | |
589 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
590 | .Sh FILES | |
591 | .Bl -tag -width Ds | |
5aecb327 | 592 | .It Pa /etc/openssh/sshd_config |
8efc0c15 | 593 | Contains configuration data for |
594 | .Nm sshd . | |
595 | This file should be writable by root only, but it is recommended | |
596 | (though not necessary) that it be world-readable. | |
5aecb327 | 597 | .It Pa /etc/openssh/ssh_host_key |
8efc0c15 | 598 | Contains the private part of the host key. |
599 | This file should only be owned by root, readable only by root, and not | |
600 | accessible to others. | |
601 | Note that | |
602 | .Nm | |
603 | does not start if this file is group/world-accessible. | |
5aecb327 | 604 | .It Pa /etc/openssh/ssh_host_key.pub |
8efc0c15 | 605 | Contains the public part of the host key. |
606 | This file should be world-readable but writable only by | |
607 | root. Its contents should match the private part. This file is not | |
608 | really used for anything; it is only provided for the convenience of | |
609 | the user so its contents can be copied to known hosts files. | |
610 | These two files are created using | |
611 | .Xr ssh-keygen 1 . | |
612 | .It Pa /var/run/sshd.pid | |
613 | Contains the process ID of the | |
614 | .Nm | |
615 | listening for connections (if there are several daemons running | |
616 | concurrently for different ports, this contains the pid of the one | |
617 | started last). The contents of this file are not sensitive; it can be | |
618 | world-readable. | |
619 | .It Pa $HOME/.ssh/authorized_keys | |
620 | Lists the RSA keys that can be used to log into the user's account. | |
621 | This file must be readable by root (which may on some machines imply | |
622 | it being world-readable if the user's home directory resides on an NFS | |
623 | volume). It is recommended that it not be accessible by others. The | |
624 | format of this file is described above. | |
5aecb327 | 625 | .It Pa /etc/openssh/ssh_known_hosts |
8efc0c15 | 626 | This file is consulted when using rhosts with RSA host |
627 | authentication to check the public key of the host. The key must be | |
628 | listed in this file to be accepted. | |
629 | .It Pa $HOME/.ssh/known_hosts | |
630 | The client uses this file | |
631 | and | |
5aecb327 | 632 | .Pa /etc/openssh/ssh_known_hosts |
8efc0c15 | 633 | to verify that the remote host is the one we intended to |
634 | connect. These files should be writable only by root/the owner. | |
5aecb327 | 635 | .Pa /etc/openssh/ssh_known_hosts |
8efc0c15 | 636 | should be world-readable, and |
637 | .Pa $HOME/.ssh/known_hosts | |
638 | can but need not be world-readable. | |
639 | .It Pa /etc/nologin | |
640 | If this file exists, | |
641 | .Nm | |
642 | refuses to let anyone except root log in. The contents of the file | |
643 | are displayed to anyone trying to log in, and non-root connections are | |
644 | refused. The file should be world-readable. | |
645 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
646 | If compiled with | |
647 | .Sy LIBWRAP | |
648 | support, tcp-wrappers access controls may be defined here as described in | |
649 | .Xr hosts_access 5 . | |
650 | .It Pa $HOME/.rhosts | |
651 | This file contains host-username pairs, separated by a space, one per | |
652 | line. The given user on the corresponding host is permitted to log in | |
653 | without password. The same file is used by rlogind and rshd. | |
654 | The file must | |
655 | be writable only by the user; it is recommended that it not be | |
656 | accessible by others. | |
657 | .Pp | |
658 | If is also possible to use netgroups in the file. Either host or user | |
659 | name may be of the form +@groupname to specify all hosts or all users | |
660 | in the group. | |
661 | .It Pa $HOME/.shosts | |
662 | For ssh, | |
663 | this file is exactly the same as for | |
664 | .Pa .rhosts . | |
665 | However, this file is | |
666 | not used by rlogin and rshd, so using this permits access using SSH only. | |
667 | .Pa /etc/hosts.equiv | |
668 | This file is used during | |
669 | .Pa .rhosts | |
670 | authentication. In the | |
671 | simplest form, this file contains host names, one per line. Users on | |
672 | those hosts are permitted to log in without a password, provided they | |
673 | have the same user name on both machines. The host name may also be | |
674 | followed by a user name; such users are permitted to log in as | |
675 | .Em any | |
676 | user on this machine (except root). Additionally, the syntax | |
677 | .Dq +@group | |
678 | can be used to specify netgroups. Negated entries start with | |
679 | .Ql \&- . | |
680 | .Pp | |
681 | If the client host/user is successfully matched in this file, login is | |
682 | automatically permitted provided the client and server user names are the | |
683 | same. Additionally, successful RSA host authentication is normally | |
684 | required. This file must be writable only by root; it is recommended | |
685 | that it be world-readable. | |
686 | .Pp | |
687 | .Sy "Warning: It is almost never a good idea to use user names in" | |
688 | .Pa hosts.equiv . | |
689 | Beware that it really means that the named user(s) can log in as | |
690 | .Em anybody , | |
691 | which includes bin, daemon, adm, and other accounts that own critical | |
692 | binaries and directories. Using a user name practically grants the | |
693 | user root access. The only valid use for user names that I can think | |
694 | of is in negative entries. | |
695 | .Pp | |
696 | Note that this warning also applies to rsh/rlogin. | |
5aecb327 | 697 | .It Pa /etc/openssh/shosts.equiv |
8efc0c15 | 698 | This is processed exactly as |
699 | .Pa /etc/hosts.equiv . | |
700 | However, this file may be useful in environments that want to run both | |
701 | rsh/rlogin and ssh. | |
702 | .It Pa $HOME/.ssh/environment | |
703 | This file is read into the environment at login (if it exists). It | |
704 | can only contain empty lines, comment lines (that start with | |
705 | .Ql # ) , | |
706 | and assignment lines of the form name=value. The file should be writable | |
707 | only by the user; it need not be readable by anyone else. | |
708 | .It Pa $HOME/.ssh/rc | |
709 | If this file exists, it is run with /bin/sh after reading the | |
710 | environment files but before starting the user's shell or command. If | |
711 | X11 spoofing is in use, this will receive the "proto cookie" pair in | |
712 | standard input (and | |
713 | .Ev DISPLAY | |
714 | in environment). This must call | |
715 | .Xr xauth 1 | |
716 | in that case. | |
717 | .Pp | |
718 | The primary purpose of this file is to run any initialization routines | |
719 | which may be needed before the user's home directory becomes | |
720 | accessible; AFS is a particular example of such an environment. | |
721 | .Pp | |
722 | This file will probably contain some initialization code followed by | |
723 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
724 | $proto $cookie | xauth -q -; fi". | |
725 | .Pp | |
726 | If this file does not exist, | |
5aecb327 | 727 | .Pa /etc/openssh/sshrc |
8efc0c15 | 728 | is run, and if that |
729 | does not exist either, xauth is used to store the cookie. | |
730 | .Pp | |
731 | This file should be writable only by the user, and need not be | |
732 | readable by anyone else. | |
5aecb327 | 733 | .It Pa /etc/openssh/sshrc |
8efc0c15 | 734 | Like |
735 | .Pa $HOME/.ssh/rc . | |
736 | This can be used to specify | |
737 | machine-specific login-time initializations globally. This file | |
738 | should be writable only by root, and should be world-readable. | |
739 | .Sh AUTHOR | |
740 | Tatu Ylonen <ylo@cs.hut.fi> | |
741 | .Pp | |
742 | Information about new releases, mailing lists, and other related | |
743 | issues can be found from the SSH WWW home page: | |
744 | .Pp | |
745 | .Dl http://www.cs.hut.fi/ssh. | |
746 | .Pp | |
747 | OpenSSH | |
748 | is a derivative of the original (free) ssh 1.2.12 release, but with bugs | |
749 | removed and newer features re-added. Rapidly after the 1.2.12 release, | |
750 | newer versions bore successively more restrictive licenses. This version | |
751 | of OpenSSH | |
752 | .Bl -bullet | |
753 | .It | |
754 | has all components of a restrictive nature (ie. patents, see | |
755 | .Xr ssl 8 ) | |
756 | directly removed from the source code; any licensed or patented components | |
757 | are chosen from | |
758 | external libraries. | |
759 | .It | |
760 | has been updated to support ssh protocol 1.5. | |
761 | .It | |
762 | contains added support for | |
763 | .Xr kerberos 8 | |
764 | authentication and ticket passing. | |
765 | .It | |
766 | supports one-time password authentication with | |
767 | .Xr skey 1 . | |
768 | .El | |
769 | .Pp | |
770 | The libraries described in | |
771 | .Xr ssl 8 | |
772 | are required for proper operation. | |
773 | .Sh SEE ALSO | |
774 | .Xr rlogin 1 , | |
775 | .Xr rsh 1 , | |
776 | .Xr scp 1 , | |
777 | .Xr ssh 1 , | |
778 | .Xr ssh-add 1 , | |
779 | .Xr ssh-agent 1 , | |
780 | .Xr ssh-keygen 1 , | |
781 | .Xr ssl 8 |