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1 | .\" -*- nroff -*- | |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
37 | .\" $OpenBSD: sshd.8,v 1.73 2000/11/22 15:38:30 provos Exp $ | |
38 | .Dd September 25, 1999 | |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
43 | .Nd secure shell daemon | |
44 | .Sh SYNOPSIS | |
45 | .Nm sshd | |
46 | .Op Fl diqQ46 | |
47 | .Op Fl b Ar bits | |
48 | .Op Fl f Ar config_file | |
49 | .Op Fl g Ar login_grace_time | |
50 | .Op Fl h Ar host_key_file | |
51 | .Op Fl k Ar key_gen_time | |
52 | .Op Fl p Ar port | |
53 | .Op Fl u Ar len | |
54 | .Op Fl V Ar client_protocol_id | |
55 | .Sh DESCRIPTION | |
56 | .Nm | |
57 | (Secure Shell Daemon) is the daemon program for | |
58 | .Xr ssh 1 . | |
59 | Together these programs replace rlogin and rsh, and | |
60 | provide secure encrypted communications between two untrusted hosts | |
61 | over an insecure network. | |
62 | The programs are intended to be as easy to | |
63 | install and use as possible. | |
64 | .Pp | |
65 | .Nm | |
66 | is the daemon that listens for connections from clients. | |
67 | It is normally started at boot from | |
68 | .Pa /etc/rc . | |
69 | It forks a new | |
70 | daemon for each incoming connection. | |
71 | The forked daemons handle | |
72 | key exchange, encryption, authentication, command execution, | |
73 | and data exchange. | |
74 | This implementation of | |
75 | .Nm | |
76 | supports both SSH protocol version 1 and 2 simultaneously. | |
77 | .Nm | |
78 | works as follows. | |
79 | .Pp | |
80 | .Ss SSH protocol version 1 | |
81 | .Pp | |
82 | Each host has a host-specific RSA key | |
83 | (normally 1024 bits) used to identify the host. | |
84 | Additionally, when | |
85 | the daemon starts, it generates a server RSA key (normally 768 bits). | |
86 | This key is normally regenerated every hour if it has been used, and | |
87 | is never stored on disk. | |
88 | .Pp | |
89 | Whenever a client connects the daemon responds with its public | |
90 | host and server keys. | |
91 | The client compares the | |
92 | RSA host key against its own database to verify that it has not changed. | |
93 | The client then generates a 256 bit random number. | |
94 | It encrypts this | |
95 | random number using both the host key and the server key, and sends | |
96 | the encrypted number to the server. | |
97 | Both sides then use this | |
98 | random number as a session key which is used to encrypt all further | |
99 | communications in the session. | |
100 | The rest of the session is encrypted | |
101 | using a conventional cipher, currently Blowfish or 3DES, with 3DES | |
102 | being used by default. | |
103 | The client selects the encryption algorithm | |
104 | to use from those offered by the server. | |
105 | .Pp | |
106 | Next, the server and the client enter an authentication dialog. | |
107 | The client tries to authenticate itself using | |
108 | .Pa .rhosts | |
109 | authentication, | |
110 | .Pa .rhosts | |
111 | authentication combined with RSA host | |
112 | authentication, RSA challenge-response authentication, or password | |
113 | based authentication. | |
114 | .Pp | |
115 | Rhosts authentication is normally disabled | |
116 | because it is fundamentally insecure, but can be enabled in the server | |
117 | configuration file if desired. | |
118 | System security is not improved unless | |
119 | .Xr rshd 8 , | |
120 | .Xr rlogind 8 , | |
121 | .Xr rexecd 8 , | |
122 | and | |
123 | .Xr rexd 8 | |
124 | are disabled (thus completely disabling | |
125 | .Xr rlogin 1 | |
126 | and | |
127 | .Xr rsh 1 | |
128 | into the machine). | |
129 | .Pp | |
130 | .Ss SSH protocol version 2 | |
131 | .Pp | |
132 | Version 2 works similarly: | |
133 | Each host has a host-specific DSA key used to identify the host. | |
134 | However, when the daemon starts, it does not generate a server key. | |
135 | Forward security is provided through a Diffie-Hellman key agreement. | |
136 | This key agreement results in a shared session key. | |
137 | The rest of the session is encrypted | |
138 | using a symmetric cipher, currently | |
139 | Blowfish, 3DES or CAST128 in CBC mode or Arcfour. | |
140 | The client selects the encryption algorithm | |
141 | to use from those offered by the server. | |
142 | Additionally, session integrity is provided | |
143 | through a cryptographic message authentication code | |
144 | (hmac-sha1 or hmac-md5). | |
145 | .Pp | |
146 | Protocol version 2 provides a public key based | |
147 | user authentication method (PubkeyAuthentication) | |
148 | and conventional password authentication. | |
149 | .Pp | |
150 | .Ss Command execution and data forwarding | |
151 | .Pp | |
152 | If the client successfully authenticates itself, a dialog for | |
153 | preparing the session is entered. | |
154 | At this time the client may request | |
155 | things like allocating a pseudo-tty, forwarding X11 connections, | |
156 | forwarding TCP/IP connections, or forwarding the authentication agent | |
157 | connection over the secure channel. | |
158 | .Pp | |
159 | Finally, the client either requests a shell or execution of a command. | |
160 | The sides then enter session mode. | |
161 | In this mode, either side may send | |
162 | data at any time, and such data is forwarded to/from the shell or | |
163 | command on the server side, and the user terminal in the client side. | |
164 | .Pp | |
165 | When the user program terminates and all forwarded X11 and other | |
166 | connections have been closed, the server sends command exit status to | |
167 | the client, and both sides exit. | |
168 | .Pp | |
169 | .Nm | |
170 | can be configured using command-line options or a configuration | |
171 | file. | |
172 | Command-line options override values specified in the | |
173 | configuration file. | |
174 | .Pp | |
175 | .Nm | |
176 | rereads its configuration file when it receives a hangup signal, | |
177 | .Dv SIGHUP . | |
178 | .Pp | |
179 | The options are as follows: | |
180 | .Bl -tag -width Ds | |
181 | .It Fl b Ar bits | |
182 | Specifies the number of bits in the server key (default 768). | |
183 | .Pp | |
184 | .It Fl d | |
185 | Debug mode. | |
186 | The server sends verbose debug output to the system | |
187 | log, and does not put itself in the background. | |
188 | The server also will not fork and will only process one connection. | |
189 | This option is only intended for debugging for the server. | |
190 | Multiple -d options increases the debugging level. | |
191 | Maximum is 3. | |
192 | .It Fl f Ar configuration_file | |
193 | Specifies the name of the configuration file. | |
194 | The default is | |
195 | .Pa /etc/sshd_config . | |
196 | .Nm | |
197 | refuses to start if there is no configuration file. | |
198 | .It Fl g Ar login_grace_time | |
199 | Gives the grace time for clients to authenticate themselves (default | |
200 | 300 seconds). | |
201 | If the client fails to authenticate the user within | |
202 | this many seconds, the server disconnects and exits. | |
203 | A value of zero indicates no limit. | |
204 | .It Fl h Ar host_key_file | |
205 | Specifies the file from which the RSA host key is read (default | |
206 | .Pa /etc/ssh_host_key ) . | |
207 | This option must be given if | |
208 | .Nm | |
209 | is not run as root (as the normal | |
210 | host file is normally not readable by anyone but root). | |
211 | .It Fl i | |
212 | Specifies that | |
213 | .Nm | |
214 | is being run from inetd. | |
215 | .Nm | |
216 | is normally not run | |
217 | from inetd because it needs to generate the server key before it can | |
218 | respond to the client, and this may take tens of seconds. | |
219 | Clients would have to wait too long if the key was regenerated every time. | |
220 | However, with small key sizes (e.g., 512) using | |
221 | .Nm | |
222 | from inetd may | |
223 | be feasible. | |
224 | .It Fl k Ar key_gen_time | |
225 | Specifies how often the server key is regenerated (default 3600 | |
226 | seconds, or one hour). | |
227 | The motivation for regenerating the key fairly | |
228 | often is that the key is not stored anywhere, and after about an hour, | |
229 | it becomes impossible to recover the key for decrypting intercepted | |
230 | communications even if the machine is cracked into or physically | |
231 | seized. | |
232 | A value of zero indicates that the key will never be regenerated. | |
233 | .It Fl p Ar port | |
234 | Specifies the port on which the server listens for connections | |
235 | (default 22). | |
236 | .It Fl q | |
237 | Quiet mode. | |
238 | Nothing is sent to the system log. | |
239 | Normally the beginning, | |
240 | authentication, and termination of each connection is logged. | |
241 | .It Fl u Ar len | |
242 | This option is used to specify the size of the field | |
243 | in the | |
244 | .Li utmp | |
245 | structure that holds the remote host name. | |
246 | If the resolved host name is longer than | |
247 | .Ar len , | |
248 | the dotted decimal value will be used instead. | |
249 | This allows hosts with very long host names that | |
250 | overflow this field to still be uniquely identified. | |
251 | Specifying | |
252 | .Fl u0 | |
253 | indicates that only dotted decimal addresses | |
254 | should be put into the | |
255 | .Pa utmp | |
256 | file. | |
257 | .It Fl Q | |
258 | Do not print an error message if RSA support is missing. | |
259 | .It Fl V Ar client_protocol_id | |
260 | SSH-2 compatibility mode. | |
261 | When this option is specified | |
262 | .Nm | |
263 | assumes the client has sent the supplied version string | |
264 | and skips the | |
265 | Protocol Version Identification Exchange. | |
266 | This option is not intended to be called directly. | |
267 | .It Fl 4 | |
268 | Forces | |
269 | .Nm | |
270 | to use IPv4 addresses only. | |
271 | .It Fl 6 | |
272 | Forces | |
273 | .Nm | |
274 | to use IPv6 addresses only. | |
275 | .El | |
276 | .Sh CONFIGURATION FILE | |
277 | .Nm | |
278 | reads configuration data from | |
279 | .Pa /etc/sshd_config | |
280 | (or the file specified with | |
281 | .Fl f | |
282 | on the command line). | |
283 | The file contains keyword-value pairs, one per line. | |
284 | Lines starting with | |
285 | .Ql # | |
286 | and empty lines are interpreted as comments. | |
287 | .Pp | |
288 | The following keywords are possible. | |
289 | .Bl -tag -width Ds | |
290 | .It Cm AFSTokenPassing | |
291 | Specifies whether an AFS token may be forwarded to the server. | |
292 | Default is | |
293 | .Dq yes . | |
294 | .It Cm AllowGroups | |
295 | This keyword can be followed by a number of group names, separated | |
296 | by spaces. | |
297 | If specified, login is allowed only for users whose primary | |
298 | group matches one of the patterns. | |
299 | .Ql \&* | |
300 | and | |
301 | .Ql ? | |
302 | can be used as | |
303 | wildcards in the patterns. | |
304 | Only group names are valid; a numerical group ID isn't recognized. | |
305 | By default login is allowed regardless of the primary group. | |
306 | .Pp | |
307 | .It Cm AllowTcpForwarding | |
308 | Specifies whether TCP forwarding is permitted. | |
309 | The default is | |
310 | .Dq yes . | |
311 | Note that disabling TCP forwarding does not improve security unless | |
312 | users are also denied shell access, as they can always install their | |
313 | own forwarders. | |
314 | .Pp | |
315 | .It Cm AllowUsers | |
316 | This keyword can be followed by a number of user names, separated | |
317 | by spaces. | |
318 | If specified, login is allowed only for users names that | |
319 | match one of the patterns. | |
320 | .Ql \&* | |
321 | and | |
322 | .Ql ? | |
323 | can be used as | |
324 | wildcards in the patterns. | |
325 | Only user names are valid; a numerical user ID isn't recognized. | |
326 | By default login is allowed regardless of the user name. | |
327 | .Pp | |
328 | .It Cm Ciphers | |
329 | Specifies the ciphers allowed for protocol version 2. | |
330 | Multiple ciphers must be comma-separated. | |
331 | The default is | |
332 | .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc . | |
333 | .It Cm CheckMail | |
334 | Specifies whether | |
335 | .Nm | |
336 | should check for new mail for interactive logins. | |
337 | The default is | |
338 | .Dq no . | |
339 | .It Cm DenyGroups | |
340 | This keyword can be followed by a number of group names, separated | |
341 | by spaces. | |
342 | Users whose primary group matches one of the patterns | |
343 | aren't allowed to log in. | |
344 | .Ql \&* | |
345 | and | |
346 | .Ql ? | |
347 | can be used as | |
348 | wildcards in the patterns. | |
349 | Only group names are valid; a numerical group ID isn't recognized. | |
350 | By default login is allowed regardless of the primary group. | |
351 | .Pp | |
352 | .It Cm DenyUsers | |
353 | This keyword can be followed by a number of user names, separated | |
354 | by spaces. | |
355 | Login is disallowed for user names that match one of the patterns. | |
356 | .Ql \&* | |
357 | and | |
358 | .Ql ? | |
359 | can be used as wildcards in the patterns. | |
360 | Only user names are valid; a numerical user ID isn't recognized. | |
361 | By default login is allowed regardless of the user name. | |
362 | .It Cm PubkeyAuthentication | |
363 | Specifies whether public key authentication is allowed. | |
364 | The default is | |
365 | .Dq yes . | |
366 | Note that this option applies to protocol version 2 only. | |
367 | .It Cm GatewayPorts | |
368 | Specifies whether remote hosts are allowed to connect to ports | |
369 | forwarded for the client. | |
370 | The argument must be | |
371 | .Dq yes | |
372 | or | |
373 | .Dq no . | |
374 | The default is | |
375 | .Dq no . | |
376 | .It Cm HostKey | |
377 | Specifies the file containing the private host keys (default | |
378 | .Pa /etc/ssh_host_key ) | |
379 | used by SSH protocol versions 1 and 2. | |
380 | Note that | |
381 | .Nm | |
382 | if this file is group/world-accessible. | |
383 | It is possible to have multiple host key files. | |
384 | .Dq rsa1 | |
385 | keys are used for version 1 and | |
386 | .Dq dsa | |
387 | or | |
388 | .Dq rsa | |
389 | are used for version 2 of the SSH protocol. | |
390 | .It Cm IgnoreRhosts | |
391 | Specifies that | |
392 | .Pa .rhosts | |
393 | and | |
394 | .Pa .shosts | |
395 | files will not be used in authentication. | |
396 | .Pa /etc/hosts.equiv | |
397 | and | |
398 | .Pa /etc/shosts.equiv | |
399 | are still used. | |
400 | The default is | |
401 | .Dq yes . | |
402 | .It Cm IgnoreUserKnownHosts | |
403 | Specifies whether | |
404 | .Nm | |
405 | should ignore the user's | |
406 | .Pa $HOME/.ssh/known_hosts | |
407 | during | |
408 | .Cm RhostsRSAAuthentication . | |
409 | The default is | |
410 | .Dq no . | |
411 | .It Cm KeepAlive | |
412 | Specifies whether the system should send keepalive messages to the | |
413 | other side. | |
414 | If they are sent, death of the connection or crash of one | |
415 | of the machines will be properly noticed. | |
416 | However, this means that | |
417 | connections will die if the route is down temporarily, and some people | |
418 | find it annoying. | |
419 | On the other hand, if keepalives are not sent, | |
420 | sessions may hang indefinitely on the server, leaving | |
421 | .Dq ghost | |
422 | users and consuming server resources. | |
423 | .Pp | |
424 | The default is | |
425 | .Dq yes | |
426 | (to send keepalives), and the server will notice | |
427 | if the network goes down or the client host reboots. | |
428 | This avoids infinitely hanging sessions. | |
429 | .Pp | |
430 | To disable keepalives, the value should be set to | |
431 | .Dq no | |
432 | in both the server and the client configuration files. | |
433 | .It Cm KerberosAuthentication | |
434 | Specifies whether Kerberos authentication is allowed. | |
435 | This can be in the form of a Kerberos ticket, or if | |
436 | .Cm PasswordAuthentication | |
437 | is yes, the password provided by the user will be validated through | |
438 | the Kerberos KDC. | |
439 | To use this option, the server needs a | |
440 | Kerberos servtab which allows the verification of the KDC's identity. | |
441 | Default is | |
442 | .Dq yes . | |
443 | .It Cm KerberosOrLocalPasswd | |
444 | If set then if password authentication through Kerberos fails then | |
445 | the password will be validated via any additional local mechanism | |
446 | such as | |
447 | .Pa /etc/passwd . | |
448 | Default is | |
449 | .Dq yes . | |
450 | .It Cm KerberosTgtPassing | |
451 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
452 | Default is | |
453 | .Dq no , | |
454 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
455 | .It Cm KerberosTicketCleanup | |
456 | Specifies whether to automatically destroy the user's ticket cache | |
457 | file on logout. | |
458 | Default is | |
459 | .Dq yes . | |
460 | .It Cm KeyRegenerationInterval | |
461 | The server key is automatically regenerated after this many seconds | |
462 | (if it has been used). | |
463 | The purpose of regeneration is to prevent | |
464 | decrypting captured sessions by later breaking into the machine and | |
465 | stealing the keys. | |
466 | The key is never stored anywhere. | |
467 | If the value is 0, the key is never regenerated. | |
468 | The default is 3600 (seconds). | |
469 | .It Cm ListenAddress | |
470 | Specifies what local address | |
471 | .Nm | |
472 | should listen on. | |
473 | The default is to listen to all local addresses. | |
474 | Multiple options of this type are permitted. | |
475 | Additionally, the | |
476 | .Cm Ports | |
477 | options must precede this option. | |
478 | .It Cm LoginGraceTime | |
479 | The server disconnects after this time if the user has not | |
480 | successfully logged in. | |
481 | If the value is 0, there is no time limit. | |
482 | The default is 600 (seconds). | |
483 | .It Cm LogLevel | |
484 | Gives the verbosity level that is used when logging messages from | |
485 | .Nm sshd . | |
486 | The possible values are: | |
487 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. | |
488 | The default is INFO. | |
489 | Logging with level DEBUG violates the privacy of users | |
490 | and is not recommended. | |
491 | .It Cm MaxStartups | |
492 | Specifies the maximum number of concurrent unauthenticated connections to the | |
493 | .Nm | |
494 | daemon. | |
495 | Additional connections will be dropped until authentication succeeds or the | |
496 | .Cm LoginGraceTime | |
497 | expires for a connection. | |
498 | The default is 10. | |
499 | .Pp | |
500 | Alternatively, random early drop can be enabled by specifying | |
501 | the three colon separated values | |
502 | .Dq start:rate:full | |
503 | (e.g., "10:30:60"). | |
504 | .Nm | |
505 | will refuse connection attempts with a probabillity of | |
506 | .Dq rate/100 | |
507 | (30%) | |
508 | if there are currently | |
509 | .Dq start | |
510 | (10) | |
511 | unauthenticated connections. | |
512 | The probabillity increases linearly and all connection attempts | |
513 | are refused if the number of unauthenticated connections reaches | |
514 | .Dq full | |
515 | (60). | |
516 | .It Cm PasswordAuthentication | |
517 | Specifies whether password authentication is allowed. | |
518 | The default is | |
519 | .Dq yes . | |
520 | Note that this option applies to both protocol versions 1 and 2. | |
521 | .It Cm PermitEmptyPasswords | |
522 | When password authentication is allowed, it specifies whether the | |
523 | server allows login to accounts with empty password strings. | |
524 | The default is | |
525 | .Dq no . | |
526 | .It Cm PermitRootLogin | |
527 | Specifies whether the root can log in using | |
528 | .Xr ssh 1 . | |
529 | The argument must be | |
530 | .Dq yes , | |
531 | .Dq without-password | |
532 | or | |
533 | .Dq no . | |
534 | The default is | |
535 | .Dq yes . | |
536 | If this options is set to | |
537 | .Dq without-password | |
538 | only password authentication is disabled for root. | |
539 | .Pp | |
540 | Root login with RSA authentication when the | |
541 | .Ar command | |
542 | option has been | |
543 | specified will be allowed regardless of the value of this setting | |
544 | (which may be useful for taking remote backups even if root login is | |
545 | normally not allowed). | |
546 | .It Cm PidFile | |
547 | Specifies the file that contains the process identifier of the | |
548 | .Nm | |
549 | daemon. | |
550 | The default is | |
551 | .Pa /var/run/sshd.pid . | |
552 | .It Cm Port | |
553 | Specifies the port number that | |
554 | .Nm | |
555 | listens on. | |
556 | The default is 22. | |
557 | Multiple options of this type are permitted. | |
558 | .It Cm PrintMotd | |
559 | Specifies whether | |
560 | .Nm | |
561 | should print | |
562 | .Pa /etc/motd | |
563 | when a user logs in interactively. | |
564 | (On some systems it is also printed by the shell, | |
565 | .Pa /etc/profile , | |
566 | or equivalent.) | |
567 | The default is | |
568 | .Dq yes . | |
569 | .It Cm Protocol | |
570 | Specifies the protocol versions | |
571 | .Nm | |
572 | should support. | |
573 | The possible values are | |
574 | .Dq 1 | |
575 | and | |
576 | .Dq 2 . | |
577 | Multiple versions must be comma-separated. | |
578 | The default is | |
579 | .Dq 1 . | |
580 | .It Cm RandomSeed | |
581 | Obsolete. | |
582 | Random number generation uses other techniques. | |
583 | .It Cm RhostsAuthentication | |
584 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
585 | files is sufficient. | |
586 | Normally, this method should not be permitted because it is insecure. | |
587 | .Cm RhostsRSAAuthentication | |
588 | should be used | |
589 | instead, because it performs RSA-based host authentication in addition | |
590 | to normal rhosts or /etc/hosts.equiv authentication. | |
591 | The default is | |
592 | .Dq no . | |
593 | .It Cm RhostsRSAAuthentication | |
594 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
595 | with successful RSA host authentication is allowed. | |
596 | The default is | |
597 | .Dq no . | |
598 | .It Cm RSAAuthentication | |
599 | Specifies whether pure RSA authentication is allowed. | |
600 | The default is | |
601 | .Dq yes . | |
602 | Note that this option applies to protocol version 1 only. | |
603 | .It Cm ServerKeyBits | |
604 | Defines the number of bits in the server key. | |
605 | The minimum value is 512, and the default is 768. | |
606 | .It Cm SkeyAuthentication | |
607 | Specifies whether | |
608 | .Xr skey 1 | |
609 | authentication is allowed. | |
610 | The default is | |
611 | .Dq yes . | |
612 | Note that s/key authentication is enabled only if | |
613 | .Cm PasswordAuthentication | |
614 | is allowed, too. | |
615 | .It Cm StrictModes | |
616 | Specifies whether | |
617 | .Nm | |
618 | should check file modes and ownership of the | |
619 | user's files and home directory before accepting login. | |
620 | This is normally desirable because novices sometimes accidentally leave their | |
621 | directory or files world-writable. | |
622 | The default is | |
623 | .Dq yes . | |
624 | .It Cm Subsystem | |
625 | Configures an external subsystem (e.g., file transfer daemon). | |
626 | Arguments should be a subsystem name and a command to execute upon subsystem | |
627 | request. | |
628 | The command | |
629 | .Xr sftp-server 8 | |
630 | implements the | |
631 | .Dq sftp | |
632 | file transfer subsystem. | |
633 | By default no subsystems are defined. | |
634 | Note that this option applies to protocol version 2 only. | |
635 | .It Cm SyslogFacility | |
636 | Gives the facility code that is used when logging messages from | |
637 | .Nm sshd . | |
638 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
639 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
640 | The default is AUTH. | |
641 | .It Cm UseLogin | |
642 | Specifies whether | |
643 | .Xr login 1 | |
644 | is used for interactive login sessions. | |
645 | Note that | |
646 | .Xr login 1 | |
647 | is never used for remote command execution. | |
648 | The default is | |
649 | .Dq no . | |
650 | .It Cm X11DisplayOffset | |
651 | Specifies the first display number available for | |
652 | .Nm sshd Ns 's | |
653 | X11 forwarding. | |
654 | This prevents | |
655 | .Nm | |
656 | from interfering with real X11 servers. | |
657 | The default is 10. | |
658 | .It Cm X11Forwarding | |
659 | Specifies whether X11 forwarding is permitted. | |
660 | The default is | |
661 | .Dq no . | |
662 | Note that disabling X11 forwarding does not improve security in any | |
663 | way, as users can always install their own forwarders. | |
664 | .It Cm XAuthLocation | |
665 | Specifies the location of the | |
666 | .Xr xauth 1 | |
667 | program. | |
668 | The default is | |
669 | .Pa /usr/X11R6/bin/xauth . | |
670 | .El | |
671 | .Sh LOGIN PROCESS | |
672 | When a user successfully logs in, | |
673 | .Nm | |
674 | does the following: | |
675 | .Bl -enum -offset indent | |
676 | .It | |
677 | If the login is on a tty, and no command has been specified, | |
678 | prints last login time and | |
679 | .Pa /etc/motd | |
680 | (unless prevented in the configuration file or by | |
681 | .Pa $HOME/.hushlogin ; | |
682 | see the | |
683 | .Sx FILES | |
684 | section). | |
685 | .It | |
686 | If the login is on a tty, records login time. | |
687 | .It | |
688 | Checks | |
689 | .Pa /etc/nologin ; | |
690 | if it exists, prints contents and quits | |
691 | (unless root). | |
692 | .It | |
693 | Changes to run with normal user privileges. | |
694 | .It | |
695 | Sets up basic environment. | |
696 | .It | |
697 | Reads | |
698 | .Pa $HOME/.ssh/environment | |
699 | if it exists. | |
700 | .It | |
701 | Changes to user's home directory. | |
702 | .It | |
703 | If | |
704 | .Pa $HOME/.ssh/rc | |
705 | exists, runs it; else if | |
706 | .Pa /etc/sshrc | |
707 | exists, runs | |
708 | it; otherwise runs xauth. | |
709 | The | |
710 | .Dq rc | |
711 | files are given the X11 | |
712 | authentication protocol and cookie in standard input. | |
713 | .It | |
714 | Runs user's shell or command. | |
715 | .El | |
716 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
717 | The | |
718 | .Pa $HOME/.ssh/authorized_keys | |
719 | file lists the RSA keys that are | |
720 | permitted for RSA authentication in SSH protocols 1.3 and 1.5 | |
721 | Similarly, the | |
722 | .Pa $HOME/.ssh/authorized_keys2 | |
723 | file lists the DSA keys that are | |
724 | permitted for DSA authentication in SSH protocol 2.0. | |
725 | Each line of the file contains one | |
726 | key (empty lines and lines starting with a | |
727 | .Ql # | |
728 | are ignored as | |
729 | comments). | |
730 | Each line consists of the following fields, separated by | |
731 | spaces: options, bits, exponent, modulus, comment. | |
732 | The options field | |
733 | is optional; its presence is determined by whether the line starts | |
734 | with a number or not (the option field never starts with a number). | |
735 | The bits, exponent, modulus and comment fields give the RSA key; the | |
736 | comment field is not used for anything (but may be convenient for the | |
737 | user to identify the key). | |
738 | .Pp | |
739 | Note that lines in this file are usually several hundred bytes long | |
740 | (because of the size of the RSA key modulus). | |
741 | You don't want to type them in; instead, copy the | |
742 | .Pa identity.pub | |
743 | file and edit it. | |
744 | .Pp | |
745 | The options (if present) consist of comma-separated option | |
746 | specifications. | |
747 | No spaces are permitted, except within double quotes. | |
748 | The following option specifications are supported: | |
749 | .Bl -tag -width Ds | |
750 | .It Cm from="pattern-list" | |
751 | Specifies that in addition to RSA authentication, the canonical name | |
752 | of the remote host must be present in the comma-separated list of | |
753 | patterns | |
754 | .Pf ( Ql * | |
755 | and | |
756 | .Ql ? | |
757 | serve as wildcards). | |
758 | The list may also contain | |
759 | patterns negated by prefixing them with | |
760 | .Ql ! ; | |
761 | if the canonical host name matches a negated pattern, the key is not accepted. | |
762 | The purpose | |
763 | of this option is to optionally increase security: RSA authentication | |
764 | by itself does not trust the network or name servers or anything (but | |
765 | the key); however, if somebody somehow steals the key, the key | |
766 | permits an intruder to log in from anywhere in the world. | |
767 | This additional option makes using a stolen key more difficult (name | |
768 | servers and/or routers would have to be compromised in addition to | |
769 | just the key). | |
770 | .It Cm command="command" | |
771 | Specifies that the command is executed whenever this key is used for | |
772 | authentication. | |
773 | The command supplied by the user (if any) is ignored. | |
774 | The command is run on a pty if the connection requests a pty; | |
775 | otherwise it is run without a tty. | |
776 | A quote may be included in the command by quoting it with a backslash. | |
777 | This option might be useful | |
778 | to restrict certain RSA keys to perform just a specific operation. | |
779 | An example might be a key that permits remote backups but nothing else. | |
780 | Note that the client may specify TCP/IP and/or X11 | |
781 | forwarding unless they are explicitly prohibited. | |
782 | .It Cm environment="NAME=value" | |
783 | Specifies that the string is to be added to the environment when | |
784 | logging in using this key. | |
785 | Environment variables set this way | |
786 | override other default environment values. | |
787 | Multiple options of this type are permitted. | |
788 | .It Cm no-port-forwarding | |
789 | Forbids TCP/IP forwarding when this key is used for authentication. | |
790 | Any port forward requests by the client will return an error. | |
791 | This might be used, e.g., in connection with the | |
792 | .Cm command | |
793 | option. | |
794 | .It Cm no-X11-forwarding | |
795 | Forbids X11 forwarding when this key is used for authentication. | |
796 | Any X11 forward requests by the client will return an error. | |
797 | .It Cm no-agent-forwarding | |
798 | Forbids authentication agent forwarding when this key is used for | |
799 | authentication. | |
800 | .It Cm no-pty | |
801 | Prevents tty allocation (a request to allocate a pty will fail). | |
802 | .El | |
803 | .Ss Examples | |
804 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
805 | .Pp | |
806 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
807 | .Pp | |
808 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
809 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
810 | The | |
811 | .Pa /etc/ssh_known_hosts , | |
812 | .Pa /etc/ssh_known_hosts2 , | |
813 | .Pa $HOME/.ssh/known_hosts , | |
814 | and | |
815 | .Pa $HOME/.ssh/known_hosts2 | |
816 | files contain host public keys for all known hosts. | |
817 | The global file should | |
818 | be prepared by the administrator (optional), and the per-user file is | |
819 | maintained automatically: whenever the user connects from an unknown host | |
820 | its key is added to the per-user file. | |
821 | .Pp | |
822 | Each line in these files contains the following fields: hostnames, | |
823 | bits, exponent, modulus, comment. | |
824 | The fields are separated by spaces. | |
825 | .Pp | |
826 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
827 | wildcards); each pattern in turn is matched against the canonical host | |
828 | name (when authenticating a client) or against the user-supplied | |
829 | name (when authenticating a server). | |
830 | A pattern may also be preceded by | |
831 | .Ql ! | |
832 | to indicate negation: if the host name matches a negated | |
833 | pattern, it is not accepted (by that line) even if it matched another | |
834 | pattern on the line. | |
835 | .Pp | |
836 | Bits, exponent, and modulus are taken directly from the RSA host key; they | |
837 | can be obtained, e.g., from | |
838 | .Pa /etc/ssh_host_key.pub . | |
839 | The optional comment field continues to the end of the line, and is not used. | |
840 | .Pp | |
841 | Lines starting with | |
842 | .Ql # | |
843 | and empty lines are ignored as comments. | |
844 | .Pp | |
845 | When performing host authentication, authentication is accepted if any | |
846 | matching line has the proper key. | |
847 | It is thus permissible (but not | |
848 | recommended) to have several lines or different host keys for the same | |
849 | names. | |
850 | This will inevitably happen when short forms of host names | |
851 | from different domains are put in the file. | |
852 | It is possible | |
853 | that the files contain conflicting information; authentication is | |
854 | accepted if valid information can be found from either file. | |
855 | .Pp | |
856 | Note that the lines in these files are typically hundreds of characters | |
857 | long, and you definitely don't want to type in the host keys by hand. | |
858 | Rather, generate them by a script | |
859 | or by taking | |
860 | .Pa /etc/ssh_host_key.pub | |
861 | and adding the host names at the front. | |
862 | .Ss Examples | |
863 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
864 | .Sh FILES | |
865 | .Bl -tag -width Ds | |
866 | .It Pa /etc/sshd_config | |
867 | Contains configuration data for | |
868 | .Nm sshd . | |
869 | This file should be writable by root only, but it is recommended | |
870 | (though not necessary) that it be world-readable. | |
871 | .It Pa /etc/ssh_host_key | |
872 | Contains the private part of the host key. | |
873 | This file should only be owned by root, readable only by root, and not | |
874 | accessible to others. | |
875 | Note that | |
876 | .Nm | |
877 | does not start if this file is group/world-accessible. | |
878 | .It Pa /etc/ssh_host_key.pub | |
879 | Contains the public part of the host key. | |
880 | This file should be world-readable but writable only by | |
881 | root. | |
882 | Its contents should match the private part. | |
883 | This file is not | |
884 | really used for anything; it is only provided for the convenience of | |
885 | the user so its contents can be copied to known hosts files. | |
886 | These two files are created using | |
887 | .Xr ssh-keygen 1 . | |
888 | .It Pa /etc/primes | |
889 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
890 | .It Pa /var/run/sshd.pid | |
891 | Contains the process ID of the | |
892 | .Nm | |
893 | listening for connections (if there are several daemons running | |
894 | concurrently for different ports, this contains the pid of the one | |
895 | started last). | |
896 | The content of this file is not sensitive; it can be world-readable. | |
897 | .It Pa $HOME/.ssh/authorized_keys | |
898 | Lists the RSA keys that can be used to log into the user's account. | |
899 | This file must be readable by root (which may on some machines imply | |
900 | it being world-readable if the user's home directory resides on an NFS | |
901 | volume). | |
902 | It is recommended that it not be accessible by others. | |
903 | The format of this file is described above. | |
904 | Users will place the contents of their | |
905 | .Pa identity.pub | |
906 | files into this file, as described in | |
907 | .Xr ssh-keygen 1 . | |
908 | .It Pa $HOME/.ssh/authorized_keys2 | |
909 | Lists the DSA keys that can be used to log into the user's account. | |
910 | This file must be readable by root (which may on some machines imply | |
911 | it being world-readable if the user's home directory resides on an NFS | |
912 | volume). | |
913 | It is recommended that it not be accessible by others. | |
914 | The format of this file is described above. | |
915 | Users will place the contents of their | |
916 | .Pa id_dsa.pub | |
917 | files into this file, as described in | |
918 | .Xr ssh-keygen 1 . | |
919 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" | |
920 | These files are consulted when using rhosts with RSA host | |
921 | authentication to check the public key of the host. | |
922 | The key must be listed in one of these files to be accepted. | |
923 | The client uses the same files | |
924 | to verify that the remote host is the one it intended to connect. | |
925 | These files should be writable only by root/the owner. | |
926 | .Pa /etc/ssh_known_hosts | |
927 | should be world-readable, and | |
928 | .Pa $HOME/.ssh/known_hosts | |
929 | can but need not be world-readable. | |
930 | .It Pa /etc/nologin | |
931 | If this file exists, | |
932 | .Nm | |
933 | refuses to let anyone except root log in. | |
934 | The contents of the file | |
935 | are displayed to anyone trying to log in, and non-root connections are | |
936 | refused. | |
937 | The file should be world-readable. | |
938 | .It Pa /etc/hosts.allow, /etc/hosts.deny | |
939 | If compiled with | |
940 | .Sy LIBWRAP | |
941 | support, tcp-wrappers access controls may be defined here as described in | |
942 | .Xr hosts_access 5 . | |
943 | .It Pa $HOME/.rhosts | |
944 | This file contains host-username pairs, separated by a space, one per | |
945 | line. | |
946 | The given user on the corresponding host is permitted to log in | |
947 | without password. | |
948 | The same file is used by rlogind and rshd. | |
949 | The file must | |
950 | be writable only by the user; it is recommended that it not be | |
951 | accessible by others. | |
952 | .Pp | |
953 | If is also possible to use netgroups in the file. | |
954 | Either host or user | |
955 | name may be of the form +@groupname to specify all hosts or all users | |
956 | in the group. | |
957 | .It Pa $HOME/.shosts | |
958 | For ssh, | |
959 | this file is exactly the same as for | |
960 | .Pa .rhosts . | |
961 | However, this file is | |
962 | not used by rlogin and rshd, so using this permits access using SSH only. | |
963 | .It Pa /etc/hosts.equiv | |
964 | This file is used during | |
965 | .Pa .rhosts | |
966 | authentication. | |
967 | In the simplest form, this file contains host names, one per line. | |
968 | Users on | |
969 | those hosts are permitted to log in without a password, provided they | |
970 | have the same user name on both machines. | |
971 | The host name may also be | |
972 | followed by a user name; such users are permitted to log in as | |
973 | .Em any | |
974 | user on this machine (except root). | |
975 | Additionally, the syntax | |
976 | .Dq +@group | |
977 | can be used to specify netgroups. | |
978 | Negated entries start with | |
979 | .Ql \&- . | |
980 | .Pp | |
981 | If the client host/user is successfully matched in this file, login is | |
982 | automatically permitted provided the client and server user names are the | |
983 | same. | |
984 | Additionally, successful RSA host authentication is normally required. | |
985 | This file must be writable only by root; it is recommended | |
986 | that it be world-readable. | |
987 | .Pp | |
988 | .Sy "Warning: It is almost never a good idea to use user names in" | |
989 | .Pa hosts.equiv . | |
990 | Beware that it really means that the named user(s) can log in as | |
991 | .Em anybody , | |
992 | which includes bin, daemon, adm, and other accounts that own critical | |
993 | binaries and directories. | |
994 | Using a user name practically grants the user root access. | |
995 | The only valid use for user names that I can think | |
996 | of is in negative entries. | |
997 | .Pp | |
998 | Note that this warning also applies to rsh/rlogin. | |
999 | .It Pa /etc/shosts.equiv | |
1000 | This is processed exactly as | |
1001 | .Pa /etc/hosts.equiv . | |
1002 | However, this file may be useful in environments that want to run both | |
1003 | rsh/rlogin and ssh. | |
1004 | .It Pa $HOME/.ssh/environment | |
1005 | This file is read into the environment at login (if it exists). | |
1006 | It can only contain empty lines, comment lines (that start with | |
1007 | .Ql # ) , | |
1008 | and assignment lines of the form name=value. | |
1009 | The file should be writable | |
1010 | only by the user; it need not be readable by anyone else. | |
1011 | .It Pa $HOME/.ssh/rc | |
1012 | If this file exists, it is run with /bin/sh after reading the | |
1013 | environment files but before starting the user's shell or command. | |
1014 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
1015 | standard input (and | |
1016 | .Ev DISPLAY | |
1017 | in environment). | |
1018 | This must call | |
1019 | .Xr xauth 1 | |
1020 | in that case. | |
1021 | .Pp | |
1022 | The primary purpose of this file is to run any initialization routines | |
1023 | which may be needed before the user's home directory becomes | |
1024 | accessible; AFS is a particular example of such an environment. | |
1025 | .Pp | |
1026 | This file will probably contain some initialization code followed by | |
1027 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
1028 | $proto $cookie | xauth -q -; fi". | |
1029 | .Pp | |
1030 | If this file does not exist, | |
1031 | .Pa /etc/sshrc | |
1032 | is run, and if that | |
1033 | does not exist either, xauth is used to store the cookie. | |
1034 | .Pp | |
1035 | This file should be writable only by the user, and need not be | |
1036 | readable by anyone else. | |
1037 | .It Pa /etc/sshrc | |
1038 | Like | |
1039 | .Pa $HOME/.ssh/rc . | |
1040 | This can be used to specify | |
1041 | machine-specific login-time initializations globally. | |
1042 | This file should be writable only by root, and should be world-readable. | |
1043 | .El | |
1044 | .Sh AUTHORS | |
1045 | OpenSSH | |
1046 | is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, | |
1047 | but with bugs removed and newer features re-added. | |
1048 | Rapidly after the | |
1049 | 1.2.12 release, newer versions of the original ssh bore successively | |
1050 | more restrictive licenses, and thus demand for a free version was born. | |
1051 | .Pp | |
1052 | This version of OpenSSH | |
1053 | .Bl -bullet | |
1054 | .It | |
1055 | has all components of a restrictive nature (i.e., patents, see | |
1056 | .Xr crypto 3 ) | |
1057 | directly removed from the source code; any licensed or patented components | |
1058 | are chosen from | |
1059 | external libraries. | |
1060 | .It | |
1061 | has been updated to support SSH protocol 1.5 and 2, making it compatible with | |
1062 | all other SSH clients and servers. | |
1063 | .It | |
1064 | contains added support for | |
1065 | .Xr kerberos 8 | |
1066 | authentication and ticket passing. | |
1067 | .It | |
1068 | supports one-time password authentication with | |
1069 | .Xr skey 1 . | |
1070 | .El | |
1071 | .Pp | |
1072 | OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, | |
1073 | Niels Provos, Theo de Raadt, and Dug Song. | |
1074 | .Pp | |
1075 | The support for SSH protocol 2 was written by Markus Friedl. | |
1076 | .Sh SEE ALSO | |
1077 | .Xr scp 1 , | |
1078 | .Xr sftp-server 8 , | |
1079 | .Xr ssh 1 , | |
1080 | .Xr ssh-add 1 , | |
1081 | .Xr ssh-agent 1 , | |
1082 | .Xr ssh-keygen 1 , | |
1083 | .Xr crypto 3 , | |
1084 | .Xr rlogin 1 , | |
1085 | .Xr rsh 1 |