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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" sshd.8.in | |
4 | .\" | |
5 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
6 | .\" | |
7 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
8 | .\" All rights reserved | |
9 | .\" | |
10 | .\" Created: Sat Apr 22 21:55:14 1995 ylo | |
11 | .\" | |
12 | .\" $Id$ | |
13 | .\" | |
14 | .Dd September 25, 1999 | |
15 | .Dt SSHD 8 | |
16 | .Os | |
17 | .Sh NAME | |
18 | .Nm sshd | |
19 | .Nd secure shell daemon | |
20 | .Sh SYNOPSIS | |
21 | .Nm sshd | |
48e671d5 | 22 | .Op Fl diqQ46 |
bf740959 | 23 | .Op Fl b Ar bits |
24 | .Op Fl f Ar config_file | |
25 | .Op Fl g Ar login_grace_time | |
26 | .Op Fl h Ar host_key_file | |
27 | .Op Fl k Ar key_gen_time | |
28 | .Op Fl p Ar port | |
5260325f | 29 | .Op Fl V Ar client_protocol_id |
f54651ce | 30 | .Sh DESCRIPTION |
bf740959 | 31 | .Nm |
f54651ce | 32 | (Secure Shell Daemon) is the daemon program for |
bf740959 | 33 | .Xr ssh 1 . |
3189621b | 34 | Together these programs replace rlogin and rsh, and |
bf740959 | 35 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 36 | over an insecure network. |
37 | The programs are intended to be as easy to | |
bf740959 | 38 | install and use as possible. |
39 | .Pp | |
40 | .Nm | |
4fe2af09 | 41 | is the daemon that listens for connections from clients. |
f54651ce | 42 | It is normally started at boot from |
bf740959 | 43 | .Pa /etc/rc . |
44 | It forks a new | |
4fe2af09 | 45 | daemon for each incoming connection. |
46 | The forked daemons handle | |
bf740959 | 47 | key exchange, encryption, authentication, command execution, |
48 | and data exchange. | |
1d1ffb87 | 49 | This implementation of |
50 | .Nm | |
51 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 52 | .Nm |
4fe2af09 | 53 | works as follows. |
1d1ffb87 | 54 | .Pp |
55 | .Ss SSH protocol version 1 | |
56 | .Pp | |
4fe2af09 | 57 | Each host has a host-specific RSA key |
58 | (normally 1024 bits) used to identify the host. | |
59 | Additionally, when | |
bf740959 | 60 | the daemon starts, it generates a server RSA key (normally 768 bits). |
61 | This key is normally regenerated every hour if it has been used, and | |
62 | is never stored on disk. | |
63 | .Pp | |
3189621b | 64 | Whenever a client connects the daemon responds with its public |
65 | host and server keys. | |
4fe2af09 | 66 | The client compares the |
1d1ffb87 | 67 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 68 | The client then generates a 256 bit random number. |
69 | It encrypts this | |
bf740959 | 70 | random number using both the host key and the server key, and sends |
4fe2af09 | 71 | the encrypted number to the server. |
3189621b | 72 | Both sides then use this |
bf740959 | 73 | random number as a session key which is used to encrypt all further |
4fe2af09 | 74 | communications in the session. |
75 | The rest of the session is encrypted | |
3189621b | 76 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 77 | being used by default. |
4fe2af09 | 78 | The client selects the encryption algorithm |
bf740959 | 79 | to use from those offered by the server. |
80 | .Pp | |
4fe2af09 | 81 | Next, the server and the client enter an authentication dialog. |
82 | The client tries to authenticate itself using | |
bf740959 | 83 | .Pa .rhosts |
84 | authentication, | |
85 | .Pa .rhosts | |
86 | authentication combined with RSA host | |
87 | authentication, RSA challenge-response authentication, or password | |
88 | based authentication. | |
89 | .Pp | |
90 | Rhosts authentication is normally disabled | |
91 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 92 | configuration file if desired. |
93 | System security is not improved unless | |
bf740959 | 94 | .Xr rshd 8 , |
95 | .Xr rlogind 8 , | |
96 | .Xr rexecd 8 , | |
97 | and | |
98 | .Xr rexd 8 | |
99 | are disabled (thus completely disabling | |
100 | .Xr rlogin 1 | |
101 | and | |
102 | .Xr rsh 1 | |
3189621b | 103 | into the machine). |
bf740959 | 104 | .Pp |
1d1ffb87 | 105 | .Ss SSH protocol version 2 |
106 | .Pp | |
107 | Version 2 works similar: | |
108 | Each host has a host-specific DSA key used to identify the host. | |
109 | However, when the daemon starts, it does not generate a server key. | |
110 | Forward security is provided through a Diffie-Hellman key agreement. | |
111 | This key agreement results in a shared session key. | |
112 | The rest of the session is encrypted | |
113 | using a symmetric cipher, currently | |
114 | Blowfish, 3DES or CAST128 in CBC mode or Arcfour. | |
115 | The client selects the encryption algorithm | |
116 | to use from those offered by the server. | |
117 | Additionally, session integrity is provided | |
d0c832f3 | 118 | through a cryptographic message authentication code |
1d1ffb87 | 119 | (hmac-sha1 or hmac-md5). |
120 | .Pp | |
121 | Protocol version 2 provides a public key based | |
122 | user authentication method (DSAAuthentication) | |
123 | and conventional password authentication. | |
124 | .Pp | |
125 | .Ss Command execution and data forwarding | |
126 | .Pp | |
bf740959 | 127 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 128 | preparing the session is entered. |
129 | At this time the client may request | |
bf740959 | 130 | things like allocating a pseudo-tty, forwarding X11 connections, |
131 | forwarding TCP/IP connections, or forwarding the authentication agent | |
132 | connection over the secure channel. | |
133 | .Pp | |
134 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 135 | The sides then enter session mode. |
136 | In this mode, either side may send | |
bf740959 | 137 | data at any time, and such data is forwarded to/from the shell or |
138 | command on the server side, and the user terminal in the client side. | |
139 | .Pp | |
140 | When the user program terminates and all forwarded X11 and other | |
141 | connections have been closed, the server sends command exit status to | |
142 | the client, and both sides exit. | |
143 | .Pp | |
144 | .Nm | |
145 | can be configured using command-line options or a configuration | |
4fe2af09 | 146 | file. |
147 | Command-line options override values specified in the | |
bf740959 | 148 | configuration file. |
149 | .Pp | |
9d6b7add | 150 | .Nm |
151 | rereads its configuration file when it receives a hangup signal, | |
152 | .Dv SIGHUP . | |
153 | .Pp | |
bf740959 | 154 | The options are as follows: |
155 | .Bl -tag -width Ds | |
156 | .It Fl b Ar bits | |
157 | Specifies the number of bits in the server key (default 768). | |
158 | .Pp | |
159 | .It Fl d | |
4fe2af09 | 160 | Debug mode. |
161 | The server sends verbose debug output to the system | |
162 | log, and does not put itself in the background. | |
163 | The server also will not fork and will only process one connection. | |
164 | This option is only intended for debugging for the server. | |
bf740959 | 165 | .It Fl f Ar configuration_file |
4fe2af09 | 166 | Specifies the name of the configuration file. |
167 | The default is | |
5f4fdfae | 168 | .Pa /etc/sshd_config . |
bf740959 | 169 | .Nm |
170 | refuses to start if there is no configuration file. | |
171 | .It Fl g Ar login_grace_time | |
172 | Gives the grace time for clients to authenticate themselves (default | |
4fe2af09 | 173 | 300 seconds). |
174 | If the client fails to authenticate the user within | |
175 | this many seconds, the server disconnects and exits. | |
176 | A value of zero indicates no limit. | |
bf740959 | 177 | .It Fl h Ar host_key_file |
1d1ffb87 | 178 | Specifies the file from which the RSA host key is read (default |
5f4fdfae | 179 | .Pa /etc/ssh_host_key ) . |
bf740959 | 180 | This option must be given if |
181 | .Nm | |
182 | is not run as root (as the normal | |
183 | host file is normally not readable by anyone but root). | |
184 | .It Fl i | |
185 | Specifies that | |
186 | .Nm | |
f54651ce | 187 | is being run from inetd. |
bf740959 | 188 | .Nm |
189 | is normally not run | |
190 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 191 | respond to the client, and this may take tens of seconds. |
192 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 193 | However, with small key sizes (e.g., 512) using |
bf740959 | 194 | .Nm |
195 | from inetd may | |
196 | be feasible. | |
197 | .It Fl k Ar key_gen_time | |
198 | Specifies how often the server key is regenerated (default 3600 | |
4fe2af09 | 199 | seconds, or one hour). |
200 | The motivation for regenerating the key fairly | |
bf740959 | 201 | often is that the key is not stored anywhere, and after about an hour, |
202 | it becomes impossible to recover the key for decrypting intercepted | |
203 | communications even if the machine is cracked into or physically | |
4fe2af09 | 204 | seized. |
205 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 206 | .It Fl p Ar port |
207 | Specifies the port on which the server listens for connections | |
208 | (default 22). | |
209 | .It Fl q | |
4fe2af09 | 210 | Quiet mode. |
211 | Nothing is sent to the system log. | |
212 | Normally the beginning, | |
bf740959 | 213 | authentication, and termination of each connection is logged. |
214 | .It Fl Q | |
215 | Do not print an error message if RSA support is missing. | |
5260325f | 216 | .It Fl V Ar client_protocol_id |
217 | SSH2 compatibility mode. | |
3189621b | 218 | When this option is specified |
5260325f | 219 | .Nm |
3189621b | 220 | assumes the client has sent the supplied version string |
5260325f | 221 | and skips the |
222 | Protocol Version Identification Exchange. | |
48e671d5 | 223 | .It Fl 4 |
224 | Forces | |
225 | .Nm | |
226 | to use IPv4 addresses only. | |
227 | .It Fl 6 | |
228 | Forces | |
229 | .Nm | |
230 | to use IPv6 addresses only. | |
bf740959 | 231 | .El |
232 | .Sh CONFIGURATION FILE | |
233 | .Nm | |
f54651ce | 234 | reads configuration data from |
5f4fdfae | 235 | .Pa /etc/sshd_config |
bf740959 | 236 | (or the file specified with |
237 | .Fl f | |
4fe2af09 | 238 | on the command line). |
239 | The file contains keyword-value pairs, one per line. | |
240 | Lines starting with | |
bf740959 | 241 | .Ql # |
242 | and empty lines are interpreted as comments. | |
243 | .Pp | |
244 | The following keywords are possible. | |
245 | .Bl -tag -width Ds | |
246 | .It Cm AFSTokenPassing | |
4fe2af09 | 247 | Specifies whether an AFS token may be forwarded to the server. |
248 | Default is | |
bf740959 | 249 | .Dq yes . |
250 | .It Cm AllowGroups | |
251 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 252 | by spaces. |
253 | If specified, login is allowed only for users whose primary | |
bf740959 | 254 | group matches one of the patterns. |
255 | .Ql \&* | |
256 | and | |
257 | .Ql ? | |
258 | can be used as | |
4fe2af09 | 259 | wildcards in the patterns. |
260 | Only group names are valid, a numerical group ID isn't recognized. | |
261 | By default login is allowed regardless of the primary group. | |
bf740959 | 262 | .Pp |
263 | .It Cm AllowUsers | |
264 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 265 | by spaces. |
266 | If specified, login is allowed only for users names that | |
bf740959 | 267 | match one of the patterns. |
268 | .Ql \&* | |
269 | and | |
270 | .Ql ? | |
271 | can be used as | |
4fe2af09 | 272 | wildcards in the patterns. |
273 | Only user names are valid, a numerical user ID isn't recognized. | |
274 | By default login is allowed regardless of the user name. | |
bf740959 | 275 | .Pp |
f54651ce | 276 | .It Cm Ciphers |
277 | Specifies the ciphers allowed for protocol version 2. | |
278 | Multiple ciphers must be comma-separated. | |
279 | The default is | |
d0c832f3 | 280 | .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc . |
bf740959 | 281 | .It Cm CheckMail |
282 | Specifies whether | |
283 | .Nm | |
284 | should check for new mail for interactive logins. | |
285 | The default is | |
286 | .Dq no . | |
287 | .It Cm DenyGroups | |
288 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 289 | by spaces. |
290 | Users whose primary group matches one of the patterns | |
bf740959 | 291 | aren't allowed to log in. |
292 | .Ql \&* | |
293 | and | |
294 | .Ql ? | |
295 | can be used as | |
4fe2af09 | 296 | wildcards in the patterns. |
297 | Only group names are valid, a numerical group ID isn't recognized. | |
298 | By default login is allowed regardless of the primary group. | |
bf740959 | 299 | .Pp |
300 | .It Cm DenyUsers | |
301 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 302 | by spaces. |
303 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 304 | .Ql \&* |
305 | and | |
306 | .Ql ? | |
4fe2af09 | 307 | can be used as wildcards in the patterns. |
308 | Only user names are valid, a numerical user ID isn't recognized. | |
309 | By default login is allowed regardless of the user name. | |
1d1ffb87 | 310 | .It Cm DSAAuthentication |
311 | Specifies whether DSA authentication is allowed. | |
312 | The default is | |
313 | .Dq yes . | |
314 | Note that this option applies to protocol version 2 only. | |
315 | .It Cm GatewayPorts | |
316 | Specifies whether remote hosts are allowed to connect to ports | |
317 | forwarded for the client. | |
318 | The argument must be | |
319 | .Dq yes | |
320 | or | |
321 | .Dq no . | |
322 | The default is | |
323 | .Dq no . | |
324 | .It Cm HostDsaKey | |
325 | Specifies the file containing the private DSA host key (default | |
326 | .Pa /etc/ssh_host_dsa_key ) | |
327 | used by SSH protocol 2.0. | |
328 | Note that | |
329 | .Nm | |
d0c832f3 | 330 | disables protocol 2.0 if this file is group/world-accessible. |
bf740959 | 331 | .It Cm HostKey |
1d1ffb87 | 332 | Specifies the file containing the private RSA host key (default |
333 | .Pa /etc/ssh_host_key ) | |
334 | used by SSH protocols 1.3 and 1.5. | |
bf740959 | 335 | Note that |
336 | .Nm | |
d0c832f3 | 337 | disables protocols 1.3 and 1.5 if this file is group/world-accessible. |
bf740959 | 338 | .It Cm IgnoreRhosts |
c8d54615 | 339 | Specifies that |
340 | .Pa .rhosts | |
f54651ce | 341 | and |
c8d54615 | 342 | .Pa .shosts |
343 | files will not be used in authentication. | |
bf740959 | 344 | .Pa /etc/hosts.equiv |
345 | and | |
f54651ce | 346 | .Pa /etc/shosts.equiv |
4fe2af09 | 347 | are still used. |
f54651ce | 348 | The default is |
c8d54615 | 349 | .Dq yes . |
b4748e2f | 350 | .It Cm IgnoreUserKnownHosts |
351 | Specifies whether | |
352 | .Nm | |
353 | should ignore the user's | |
354 | .Pa $HOME/.ssh/known_hosts | |
355 | during | |
356 | .Cm RhostsRSAAuthentication . | |
357 | The default is | |
358 | .Dq no . | |
bf740959 | 359 | .It Cm KeepAlive |
360 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 361 | other side. |
362 | If they are sent, death of the connection or crash of one | |
363 | of the machines will be properly noticed. | |
364 | However, this means that | |
bf740959 | 365 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 366 | find it annoying. |
d0c832f3 | 367 | On the other hand, if keepalives are not sent, |
bf740959 | 368 | sessions may hang indefinitely on the server, leaving |
369 | .Dq ghost | |
370 | users and consuming server resources. | |
371 | .Pp | |
372 | The default is | |
373 | .Dq yes | |
374 | (to send keepalives), and the server will notice | |
4fe2af09 | 375 | if the network goes down or the client host reboots. |
376 | This avoids infinitely hanging sessions. | |
bf740959 | 377 | .Pp |
378 | To disable keepalives, the value should be set to | |
379 | .Dq no | |
380 | in both the server and the client configuration files. | |
381 | .It Cm KerberosAuthentication | |
4fe2af09 | 382 | Specifies whether Kerberos authentication is allowed. |
383 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 384 | .Cm PasswordAuthentication |
385 | is yes, the password provided by the user will be validated through | |
4fe2af09 | 386 | the Kerberos KDC. |
387 | Default is | |
bf740959 | 388 | .Dq yes . |
389 | .It Cm KerberosOrLocalPasswd | |
390 | If set then if password authentication through Kerberos fails then | |
391 | the password will be validated via any additional local mechanism | |
392 | such as | |
393 | .Pa /etc/passwd | |
4fe2af09 | 394 | or SecurID. |
395 | Default is | |
bf740959 | 396 | .Dq yes . |
397 | .It Cm KerberosTgtPassing | |
398 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 399 | Default is |
bf740959 | 400 | .Dq no , |
401 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
402 | .It Cm KerberosTicketCleanup | |
403 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 404 | file on logout. |
405 | Default is | |
bf740959 | 406 | .Dq yes . |
407 | .It Cm KeyRegenerationInterval | |
408 | The server key is automatically regenerated after this many seconds | |
4fe2af09 | 409 | (if it has been used). |
410 | The purpose of regeneration is to prevent | |
bf740959 | 411 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 412 | stealing the keys. |
413 | The key is never stored anywhere. | |
414 | If the value is 0, the key is never regenerated. | |
415 | The default is 3600 (seconds). | |
bf740959 | 416 | .It Cm ListenAddress |
417 | Specifies what local address | |
418 | .Nm | |
419 | should listen on. | |
420 | The default is to listen to all local addresses. | |
48e671d5 | 421 | Multiple options of this type are permitted. |
422 | Additionally, the | |
423 | .Cm Ports | |
424 | options must precede this option. | |
bf740959 | 425 | .It Cm LoginGraceTime |
426 | The server disconnects after this time if the user has not | |
4fe2af09 | 427 | successfully logged in. |
428 | If the value is 0, there is no time limit. | |
bf740959 | 429 | The default is 600 (seconds). |
6a17f9c2 | 430 | .It Cm LogLevel |
431 | Gives the verbosity level that is used when logging messages from | |
432 | .Nm sshd . | |
433 | The possible values are: | |
5260325f | 434 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
6a17f9c2 | 435 | The default is INFO. |
436 | Logging with level DEBUG violates the privacy of users | |
437 | and is not recommended. | |
089fbbd2 | 438 | .It Cm MaxStartups |
439 | Specifies the maximum number of concurrent unauthenticated connections to the | |
440 | .Nm | |
441 | daemon. | |
442 | Additional connections will be dropped until authentication succeeds or the | |
443 | .Cm LoginGraceTime | |
444 | expires for a connection. | |
445 | The default is 10. | |
bf740959 | 446 | .It Cm PasswordAuthentication |
447 | Specifies whether password authentication is allowed. | |
448 | The default is | |
449 | .Dq yes . | |
1d1ffb87 | 450 | Note that this option applies to both protocol version 1 and 2. |
bf740959 | 451 | .It Cm PermitEmptyPasswords |
452 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 453 | server allows login to accounts with empty password strings. |
454 | The default is | |
c8d54615 | 455 | .Dq no . |
bf740959 | 456 | .It Cm PermitRootLogin |
457 | Specifies whether the root can log in using | |
458 | .Xr ssh 1 . | |
459 | The argument must be | |
460 | .Dq yes , | |
461 | .Dq without-password | |
462 | or | |
463 | .Dq no . | |
464 | The default is | |
465 | .Dq yes . | |
466 | If this options is set to | |
467 | .Dq without-password | |
468 | only password authentication is disabled for root. | |
469 | .Pp | |
470 | Root login with RSA authentication when the | |
471 | .Ar command | |
472 | option has been | |
473 | specified will be allowed regardless of the value of this setting | |
474 | (which may be useful for taking remote backups even if root login is | |
475 | normally not allowed). | |
0fbe8c74 | 476 | .It Cm PidFile |
477 | Specifies the file that contains the process identifier of the | |
478 | .Nm | |
479 | daemon. | |
480 | The default is | |
481 | .Pa /var/run/sshd.pid . | |
bf740959 | 482 | .It Cm Port |
483 | Specifies the port number that | |
484 | .Nm | |
4fe2af09 | 485 | listens on. |
486 | The default is 22. | |
48e671d5 | 487 | Multiple options of this type are permitted. |
bf740959 | 488 | .It Cm PrintMotd |
489 | Specifies whether | |
490 | .Nm | |
f54651ce | 491 | should print |
bf740959 | 492 | .Pa /etc/motd |
4fe2af09 | 493 | when a user logs in interactively. |
494 | (On some systems it is also printed by the shell, | |
bf740959 | 495 | .Pa /etc/profile , |
4fe2af09 | 496 | or equivalent.) |
497 | The default is | |
bf740959 | 498 | .Dq yes . |
f54651ce | 499 | .It Cm Protocol |
500 | Specifies the protocol versions | |
501 | .Nm | |
502 | should support. | |
503 | The possible values are | |
504 | .Dq 1 | |
505 | and | |
506 | .Dq 2 . | |
507 | Multiple versions must be comma-separated. | |
508 | The default is | |
509 | .Dq 1 . | |
bf740959 | 510 | .It Cm RandomSeed |
4fe2af09 | 511 | Obsolete. |
512 | Random number generation uses other techniques. | |
bf740959 | 513 | .It Cm RhostsAuthentication |
514 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 515 | files is sufficient. |
516 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 517 | .Cm RhostsRSAAuthentication |
518 | should be used | |
519 | instead, because it performs RSA-based host authentication in addition | |
520 | to normal rhosts or /etc/hosts.equiv authentication. | |
521 | The default is | |
522 | .Dq no . | |
523 | .It Cm RhostsRSAAuthentication | |
524 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 525 | with successful RSA host authentication is allowed. |
526 | The default is | |
c8d54615 | 527 | .Dq no . |
bf740959 | 528 | .It Cm RSAAuthentication |
4fe2af09 | 529 | Specifies whether pure RSA authentication is allowed. |
530 | The default is | |
bf740959 | 531 | .Dq yes . |
1d1ffb87 | 532 | Note that this option applies to protocol version 1 only. |
bf740959 | 533 | .It Cm ServerKeyBits |
4fe2af09 | 534 | Defines the number of bits in the server key. |
535 | The minimum value is 512, and the default is 768. | |
bf740959 | 536 | .It Cm SkeyAuthentication |
537 | Specifies whether | |
f54651ce | 538 | .Xr skey 1 |
4fe2af09 | 539 | authentication is allowed. |
540 | The default is | |
bf740959 | 541 | .Dq yes . |
542 | Note that s/key authentication is enabled only if | |
543 | .Cm PasswordAuthentication | |
544 | is allowed, too. | |
545 | .It Cm StrictModes | |
546 | Specifies whether | |
547 | .Nm | |
548 | should check file modes and ownership of the | |
4fe2af09 | 549 | user's files and home directory before accepting login. |
550 | This is normally desirable because novices sometimes accidentally leave their | |
551 | directory or files world-writable. | |
552 | The default is | |
bf740959 | 553 | .Dq yes . |
38c295d6 | 554 | .It Cm Subsystem |
555 | Configures an external subsystem (e.g. file transfer daemon). | |
556 | Arguments should be a subsystem name and a command to execute upon subsystem request. | |
557 | By default no subsystems are defined. | |
558 | Note that this option applies to protocol version 2 only. | |
bf740959 | 559 | .It Cm SyslogFacility |
560 | Gives the facility code that is used when logging messages from | |
561 | .Nm sshd . | |
562 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 563 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
564 | The default is AUTH. | |
bf740959 | 565 | .It Cm UseLogin |
566 | Specifies whether | |
567 | .Xr login 1 | |
fa649821 | 568 | is used for interactive login sessions. |
569 | Note that | |
570 | .Xr login 1 | |
571 | is not never for remote command execution. | |
4fe2af09 | 572 | The default is |
bf740959 | 573 | .Dq no . |
bf740959 | 574 | .It Cm X11DisplayOffset |
575 | Specifies the first display number available for | |
576 | .Nm sshd Ns 's | |
4fe2af09 | 577 | X11 forwarding. |
578 | This prevents | |
bf740959 | 579 | .Nm |
580 | from interfering with real X11 servers. | |
c8d54615 | 581 | The default is 10. |
7b2ea3a1 | 582 | .It Cm X11Forwarding |
4fe2af09 | 583 | Specifies whether X11 forwarding is permitted. |
584 | The default is | |
c8d54615 | 585 | .Dq no . |
7b2ea3a1 | 586 | Note that disabling X11 forwarding does not improve security in any |
587 | way, as users can always install their own forwarders. | |
fa649821 | 588 | .It Cm XAuthLocation |
589 | Specifies the location of the | |
590 | .Xr xauth 1 | |
591 | program. | |
592 | The default is | |
593 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 594 | .El |
595 | .Sh LOGIN PROCESS | |
596 | When a user successfully logs in, | |
597 | .Nm | |
598 | does the following: | |
599 | .Bl -enum -offset indent | |
600 | .It | |
601 | If the login is on a tty, and no command has been specified, | |
f54651ce | 602 | prints last login time and |
bf740959 | 603 | .Pa /etc/motd |
604 | (unless prevented in the configuration file or by | |
605 | .Pa $HOME/.hushlogin ; | |
606 | see the | |
f54651ce | 607 | .Sx FILES |
bf740959 | 608 | section). |
609 | .It | |
610 | If the login is on a tty, records login time. | |
611 | .It | |
612 | Checks | |
613 | .Pa /etc/nologin ; | |
614 | if it exists, prints contents and quits | |
615 | (unless root). | |
616 | .It | |
617 | Changes to run with normal user privileges. | |
618 | .It | |
619 | Sets up basic environment. | |
620 | .It | |
621 | Reads | |
622 | .Pa $HOME/.ssh/environment | |
623 | if it exists. | |
624 | .It | |
625 | Changes to user's home directory. | |
626 | .It | |
627 | If | |
628 | .Pa $HOME/.ssh/rc | |
629 | exists, runs it; else if | |
5f4fdfae | 630 | .Pa /etc/sshrc |
bf740959 | 631 | exists, runs |
4fe2af09 | 632 | it; otherwise runs xauth. |
633 | The | |
bf740959 | 634 | .Dq rc |
635 | files are given the X11 | |
636 | authentication protocol and cookie in standard input. | |
637 | .It | |
638 | Runs user's shell or command. | |
639 | .El | |
640 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
f54651ce | 641 | The |
bf740959 | 642 | .Pa $HOME/.ssh/authorized_keys |
643 | file lists the RSA keys that are | |
1d1ffb87 | 644 | permitted for RSA authentication in SSH protocols 1.3 and 1.5 |
d0c832f3 | 645 | Similarly, the |
1d1ffb87 | 646 | .Pa $HOME/.ssh/authorized_keys2 |
647 | file lists the DSA keys that are | |
648 | permitted for DSA authentication in SSH protocol 2.0. | |
4fe2af09 | 649 | Each line of the file contains one |
bf740959 | 650 | key (empty lines and lines starting with a |
651 | .Ql # | |
652 | are ignored as | |
4fe2af09 | 653 | comments). |
654 | Each line consists of the following fields, separated by | |
655 | spaces: options, bits, exponent, modulus, comment. | |
656 | The options field | |
bf740959 | 657 | is optional; its presence is determined by whether the line starts |
658 | with a number or not (the option field never starts with a number). | |
659 | The bits, exponent, modulus and comment fields give the RSA key; the | |
660 | comment field is not used for anything (but may be convenient for the | |
661 | user to identify the key). | |
662 | .Pp | |
663 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 664 | (because of the size of the RSA key modulus). |
665 | You don't want to type them in; instead, copy the | |
bf740959 | 666 | .Pa identity.pub |
667 | file and edit it. | |
668 | .Pp | |
669 | The options (if present) consists of comma-separated option | |
4fe2af09 | 670 | specifications. |
671 | No spaces are permitted, except within double quotes. | |
bf740959 | 672 | The following option specifications are supported: |
673 | .Bl -tag -width Ds | |
674 | .It Cm from="pattern-list" | |
675 | Specifies that in addition to RSA authentication, the canonical name | |
676 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 677 | patterns |
678 | .Pf ( Ql * | |
679 | and | |
680 | .Ql ? | |
681 | serve as wildcards). | |
682 | The list may also contain | |
683 | patterns negated by prefixing them with | |
684 | .Ql ! ; | |
685 | if the canonical host name matches a negated pattern, the key is not accepted. | |
686 | The purpose | |
bf740959 | 687 | of this option is to optionally increase security: RSA authentication |
688 | by itself does not trust the network or name servers or anything (but | |
689 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 690 | permits an intruder to log in from anywhere in the world. |
691 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 692 | servers and/or routers would have to be compromised in addition to |
693 | just the key). | |
694 | .It Cm command="command" | |
695 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 696 | authentication. |
697 | The command supplied by the user (if any) is ignored. | |
bf740959 | 698 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 699 | otherwise it is run without a tty. |
700 | A quote may be included in the command by quoting it with a backslash. | |
701 | This option might be useful | |
702 | to restrict certain RSA keys to perform just a specific operation. | |
703 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 704 | Note that the client may specify TCP/IP and/or X11 |
705 | forwarding unless they are explicitly prohibited. | |
bf740959 | 706 | .It Cm environment="NAME=value" |
707 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 708 | logging in using this key. |
709 | Environment variables set this way | |
710 | override other default environment values. | |
711 | Multiple options of this type are permitted. | |
bf740959 | 712 | .It Cm no-port-forwarding |
713 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 714 | Any port forward requests by the client will return an error. |
715 | This might be used, e.g., in connection with the | |
bf740959 | 716 | .Cm command |
717 | option. | |
718 | .It Cm no-X11-forwarding | |
719 | Forbids X11 forwarding when this key is used for authentication. | |
720 | Any X11 forward requests by the client will return an error. | |
721 | .It Cm no-agent-forwarding | |
722 | Forbids authentication agent forwarding when this key is used for | |
723 | authentication. | |
724 | .It Cm no-pty | |
725 | Prevents tty allocation (a request to allocate a pty will fail). | |
726 | .El | |
727 | .Ss Examples | |
728 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
729 | .Pp | |
730 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
731 | .Pp | |
732 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
733 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
f54651ce | 734 | The |
1d1ffb87 | 735 | .Pa /etc/ssh_known_hosts , |
736 | .Pa /etc/ssh_known_hosts2 , | |
737 | .Pa $HOME/.ssh/known_hosts , | |
f54651ce | 738 | and |
1d1ffb87 | 739 | .Pa $HOME/.ssh/known_hosts2 |
4fe2af09 | 740 | files contain host public keys for all known hosts. |
741 | The global file should | |
742 | be prepared by the administrator (optional), and the per-user file is | |
bf740959 | 743 | maintained automatically: whenever the user connects an unknown host |
4fe2af09 | 744 | its key is added to the per-user file. |
bf740959 | 745 | .Pp |
746 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 747 | bits, exponent, modulus, comment. |
748 | The fields are separated by spaces. | |
bf740959 | 749 | .Pp |
750 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
751 | wildcards); each pattern in turn is matched against the canonical host | |
752 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 753 | name (when authenticating a server). |
754 | A pattern may also be preceded by | |
bf740959 | 755 | .Ql ! |
756 | to indicate negation: if the host name matches a negated | |
757 | pattern, it is not accepted (by that line) even if it matched another | |
758 | pattern on the line. | |
759 | .Pp | |
1d1ffb87 | 760 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 761 | can be obtained, e.g., from |
5f4fdfae | 762 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 763 | The optional comment field continues to the end of the line, and is not used. |
764 | .Pp | |
765 | Lines starting with | |
766 | .Ql # | |
767 | and empty lines are ignored as comments. | |
768 | .Pp | |
769 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 770 | matching line has the proper key. |
771 | It is thus permissible (but not | |
bf740959 | 772 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 773 | names. |
774 | This will inevitably happen when short forms of host names | |
775 | from different domains are put in the file. | |
776 | It is possible | |
bf740959 | 777 | that the files contain conflicting information; authentication is |
778 | accepted if valid information can be found from either file. | |
779 | .Pp | |
780 | Note that the lines in these files are typically hundreds of characters | |
781 | long, and you definitely don't want to type in the host keys by hand. | |
782 | Rather, generate them by a script | |
f54651ce | 783 | or by taking |
5f4fdfae | 784 | .Pa /etc/ssh_host_key.pub |
bf740959 | 785 | and adding the host names at the front. |
786 | .Ss Examples | |
787 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
788 | .Sh FILES | |
789 | .Bl -tag -width Ds | |
5f4fdfae | 790 | .It Pa /etc/sshd_config |
bf740959 | 791 | Contains configuration data for |
792 | .Nm sshd . | |
793 | This file should be writable by root only, but it is recommended | |
794 | (though not necessary) that it be world-readable. | |
5f4fdfae | 795 | .It Pa /etc/ssh_host_key |
bf740959 | 796 | Contains the private part of the host key. |
797 | This file should only be owned by root, readable only by root, and not | |
798 | accessible to others. | |
799 | Note that | |
800 | .Nm | |
801 | does not start if this file is group/world-accessible. | |
5f4fdfae | 802 | .It Pa /etc/ssh_host_key.pub |
bf740959 | 803 | Contains the public part of the host key. |
804 | This file should be world-readable but writable only by | |
4fe2af09 | 805 | root. |
806 | Its contents should match the private part. | |
807 | This file is not | |
bf740959 | 808 | really used for anything; it is only provided for the convenience of |
809 | the user so its contents can be copied to known hosts files. | |
810 | These two files are created using | |
811 | .Xr ssh-keygen 1 . | |
5f4fdfae | 812 | .It Pa /var/run/sshd.pid |
bf740959 | 813 | Contains the process ID of the |
814 | .Nm | |
815 | listening for connections (if there are several daemons running | |
816 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 817 | started last). |
818 | The contents of this file are not sensitive; it can be world-readable. | |
bf740959 | 819 | .It Pa $HOME/.ssh/authorized_keys |
820 | Lists the RSA keys that can be used to log into the user's account. | |
821 | This file must be readable by root (which may on some machines imply | |
822 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 823 | volume). |
824 | It is recommended that it not be accessible by others. | |
825 | The format of this file is described above. | |
1d1ffb87 | 826 | Users will place the contents of their |
827 | .Pa identity.pub | |
828 | files into this file, as described in | |
829 | .Xr ssh-keygen 1 . | |
830 | .It Pa $HOME/.ssh/authorized_keys2 | |
831 | Lists the DSA keys that can be used to log into the user's account. | |
832 | This file must be readable by root (which may on some machines imply | |
833 | it being world-readable if the user's home directory resides on an NFS | |
834 | volume). | |
835 | It is recommended that it not be accessible by others. | |
836 | The format of this file is described above. | |
837 | Users will place the contents of their | |
838 | .Pa id_dsa.pub | |
839 | files into this file, as described in | |
840 | .Xr ssh-keygen 1 . | |
5f4fdfae | 841 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 842 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 843 | authentication to check the public key of the host. |
844 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 845 | The client uses the same files |
4fe2af09 | 846 | to verify that the remote host is the one we intended to connect. |
847 | These files should be writable only by root/the owner. | |
5f4fdfae | 848 | .Pa /etc/ssh_known_hosts |
bf740959 | 849 | should be world-readable, and |
850 | .Pa $HOME/.ssh/known_hosts | |
851 | can but need not be world-readable. | |
852 | .It Pa /etc/nologin | |
f54651ce | 853 | If this file exists, |
bf740959 | 854 | .Nm |
4fe2af09 | 855 | refuses to let anyone except root log in. |
856 | The contents of the file | |
bf740959 | 857 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 858 | refused. |
859 | The file should be world-readable. | |
bf740959 | 860 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
861 | If compiled with | |
862 | .Sy LIBWRAP | |
863 | support, tcp-wrappers access controls may be defined here as described in | |
864 | .Xr hosts_access 5 . | |
865 | .It Pa $HOME/.rhosts | |
866 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 867 | line. |
868 | The given user on the corresponding host is permitted to log in | |
869 | without password. | |
870 | The same file is used by rlogind and rshd. | |
bf740959 | 871 | The file must |
872 | be writable only by the user; it is recommended that it not be | |
873 | accessible by others. | |
874 | .Pp | |
4fe2af09 | 875 | If is also possible to use netgroups in the file. |
876 | Either host or user | |
bf740959 | 877 | name may be of the form +@groupname to specify all hosts or all users |
878 | in the group. | |
879 | .It Pa $HOME/.shosts | |
880 | For ssh, | |
881 | this file is exactly the same as for | |
882 | .Pa .rhosts . | |
883 | However, this file is | |
884 | not used by rlogin and rshd, so using this permits access using SSH only. | |
885 | .Pa /etc/hosts.equiv | |
886 | This file is used during | |
887 | .Pa .rhosts | |
4fe2af09 | 888 | authentication. |
889 | In the simplest form, this file contains host names, one per line. | |
890 | Users on | |
bf740959 | 891 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 892 | have the same user name on both machines. |
893 | The host name may also be | |
bf740959 | 894 | followed by a user name; such users are permitted to log in as |
895 | .Em any | |
4fe2af09 | 896 | user on this machine (except root). |
897 | Additionally, the syntax | |
bf740959 | 898 | .Dq +@group |
4fe2af09 | 899 | can be used to specify netgroups. |
900 | Negated entries start with | |
bf740959 | 901 | .Ql \&- . |
902 | .Pp | |
903 | If the client host/user is successfully matched in this file, login is | |
904 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 905 | same. |
906 | Additionally, successful RSA host authentication is normally required. | |
907 | This file must be writable only by root; it is recommended | |
bf740959 | 908 | that it be world-readable. |
909 | .Pp | |
910 | .Sy "Warning: It is almost never a good idea to use user names in" | |
911 | .Pa hosts.equiv . | |
912 | Beware that it really means that the named user(s) can log in as | |
913 | .Em anybody , | |
914 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 915 | binaries and directories. |
916 | Using a user name practically grants the user root access. | |
917 | The only valid use for user names that I can think | |
bf740959 | 918 | of is in negative entries. |
919 | .Pp | |
920 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 921 | .It Pa /etc/shosts.equiv |
bf740959 | 922 | This is processed exactly as |
923 | .Pa /etc/hosts.equiv . | |
924 | However, this file may be useful in environments that want to run both | |
925 | rsh/rlogin and ssh. | |
926 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 927 | This file is read into the environment at login (if it exists). |
928 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 929 | .Ql # ) , |
4fe2af09 | 930 | and assignment lines of the form name=value. |
931 | The file should be writable | |
bf740959 | 932 | only by the user; it need not be readable by anyone else. |
933 | .It Pa $HOME/.ssh/rc | |
934 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 935 | environment files but before starting the user's shell or command. |
936 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 937 | standard input (and |
938 | .Ev DISPLAY | |
4fe2af09 | 939 | in environment). |
940 | This must call | |
bf740959 | 941 | .Xr xauth 1 |
942 | in that case. | |
943 | .Pp | |
944 | The primary purpose of this file is to run any initialization routines | |
945 | which may be needed before the user's home directory becomes | |
946 | accessible; AFS is a particular example of such an environment. | |
947 | .Pp | |
948 | This file will probably contain some initialization code followed by | |
949 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
950 | $proto $cookie | xauth -q -; fi". | |
951 | .Pp | |
952 | If this file does not exist, | |
5f4fdfae | 953 | .Pa /etc/sshrc |
bf740959 | 954 | is run, and if that |
955 | does not exist either, xauth is used to store the cookie. | |
956 | .Pp | |
957 | This file should be writable only by the user, and need not be | |
958 | readable by anyone else. | |
5f4fdfae | 959 | .It Pa /etc/sshrc |
bf740959 | 960 | Like |
961 | .Pa $HOME/.ssh/rc . | |
962 | This can be used to specify | |
4fe2af09 | 963 | machine-specific login-time initializations globally. |
964 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 965 | .El |
bf740959 | 966 | .Sh AUTHOR |
bf740959 | 967 | OpenSSH |
c8d54615 | 968 | is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, |
4fe2af09 | 969 | but with bugs removed and newer features re-added. |
970 | Rapidly after the | |
c8d54615 | 971 | 1.2.12 release, newer versions of the original ssh bore successively |
972 | more restrictive licenses, and thus demand for a free version was born. | |
1d1ffb87 | 973 | .Pp |
c8d54615 | 974 | This version of OpenSSH |
bf740959 | 975 | .Bl -bullet |
976 | .It | |
371ecff9 | 977 | has all components of a restrictive nature (i.e., patents) |
bf740959 | 978 | directly removed from the source code; any licensed or patented components |
979 | are chosen from | |
980 | external libraries. | |
981 | .It | |
1d1ffb87 | 982 | has been updated to support SSH protocol 1.5 and 2, making it compatible with |
983 | all other SSH clients and servers. | |
bf740959 | 984 | .It |
f54651ce | 985 | contains added support for |
bf740959 | 986 | .Xr kerberos 8 |
987 | authentication and ticket passing. | |
988 | .It | |
989 | supports one-time password authentication with | |
990 | .Xr skey 1 . | |
991 | .El | |
1d1ffb87 | 992 | .Pp |
993 | OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, | |
994 | Niels Provos, Theo de Raadt, and Dug Song. | |
995 | .Pp | |
996 | The support for SSH protocol 2 was written by Markus Friedl. | |
bf740959 | 997 | .Sh SEE ALSO |
bf740959 | 998 | .Xr scp 1 , |
999 | .Xr ssh 1 , | |
1000 | .Xr ssh-add 1 , | |
1001 | .Xr ssh-agent 1 , | |
1002 | .Xr ssh-keygen 1 , | |
7368a6c8 | 1003 | .Xr rlogin 1 , |
1004 | .Xr rsh 1 |