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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 . |
34 | Together these programs replace rlogin and rsh programs, and | |
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. | |
49 | .Pp | |
50 | .Nm | |
4fe2af09 | 51 | works as follows. |
52 | Each host has a host-specific RSA key | |
53 | (normally 1024 bits) used to identify the host. | |
54 | Additionally, when | |
bf740959 | 55 | the daemon starts, it generates a server RSA key (normally 768 bits). |
56 | This key is normally regenerated every hour if it has been used, and | |
57 | is never stored on disk. | |
58 | .Pp | |
59 | Whenever a client connects the daemon, the daemon sends its host | |
4fe2af09 | 60 | and server public keys to the client. |
61 | The client compares the | |
bf740959 | 62 | host key against its own database to verify that it has not changed. |
4fe2af09 | 63 | The client then generates a 256 bit random number. |
64 | It encrypts this | |
bf740959 | 65 | random number using both the host key and the server key, and sends |
4fe2af09 | 66 | the encrypted number to the server. |
67 | Both sides then start to use this | |
bf740959 | 68 | random number as a session key which is used to encrypt all further |
4fe2af09 | 69 | communications in the session. |
70 | The rest of the session is encrypted | |
bf740959 | 71 | using a conventional cipher, currently Blowfish and 3DES, with 3DES |
7368a6c8 | 72 | being used by default. |
4fe2af09 | 73 | The client selects the encryption algorithm |
bf740959 | 74 | to use from those offered by the server. |
75 | .Pp | |
4fe2af09 | 76 | Next, the server and the client enter an authentication dialog. |
77 | The client tries to authenticate itself using | |
bf740959 | 78 | .Pa .rhosts |
79 | authentication, | |
80 | .Pa .rhosts | |
81 | authentication combined with RSA host | |
82 | authentication, RSA challenge-response authentication, or password | |
83 | based authentication. | |
84 | .Pp | |
85 | Rhosts authentication is normally disabled | |
86 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 87 | configuration file if desired. |
88 | System security is not improved unless | |
bf740959 | 89 | .Xr rshd 8 , |
90 | .Xr rlogind 8 , | |
91 | .Xr rexecd 8 , | |
92 | and | |
93 | .Xr rexd 8 | |
94 | are disabled (thus completely disabling | |
95 | .Xr rlogin 1 | |
96 | and | |
97 | .Xr rsh 1 | |
98 | into that machine). | |
99 | .Pp | |
100 | If the client successfully authenticates itself, a dialog for | |
4fe2af09 | 101 | preparing the session is entered. |
102 | At this time the client may request | |
bf740959 | 103 | things like allocating a pseudo-tty, forwarding X11 connections, |
104 | forwarding TCP/IP connections, or forwarding the authentication agent | |
105 | connection over the secure channel. | |
106 | .Pp | |
107 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 108 | The sides then enter session mode. |
109 | In this mode, either side may send | |
bf740959 | 110 | data at any time, and such data is forwarded to/from the shell or |
111 | command on the server side, and the user terminal in the client side. | |
112 | .Pp | |
113 | When the user program terminates and all forwarded X11 and other | |
114 | connections have been closed, the server sends command exit status to | |
115 | the client, and both sides exit. | |
116 | .Pp | |
117 | .Nm | |
118 | can be configured using command-line options or a configuration | |
4fe2af09 | 119 | file. |
120 | Command-line options override values specified in the | |
bf740959 | 121 | configuration file. |
122 | .Pp | |
9d6b7add | 123 | .Nm |
124 | rereads its configuration file when it receives a hangup signal, | |
125 | .Dv SIGHUP . | |
126 | .Pp | |
bf740959 | 127 | The options are as follows: |
128 | .Bl -tag -width Ds | |
129 | .It Fl b Ar bits | |
130 | Specifies the number of bits in the server key (default 768). | |
131 | .Pp | |
132 | .It Fl d | |
4fe2af09 | 133 | Debug mode. |
134 | The server sends verbose debug output to the system | |
135 | log, and does not put itself in the background. | |
136 | The server also will not fork and will only process one connection. | |
137 | This option is only intended for debugging for the server. | |
bf740959 | 138 | .It Fl f Ar configuration_file |
4fe2af09 | 139 | Specifies the name of the configuration file. |
140 | The default is | |
5f4fdfae | 141 | .Pa /etc/sshd_config . |
bf740959 | 142 | .Nm |
143 | refuses to start if there is no configuration file. | |
144 | .It Fl g Ar login_grace_time | |
145 | Gives the grace time for clients to authenticate themselves (default | |
4fe2af09 | 146 | 300 seconds). |
147 | If the client fails to authenticate the user within | |
148 | this many seconds, the server disconnects and exits. | |
149 | A value of zero indicates no limit. | |
bf740959 | 150 | .It Fl h Ar host_key_file |
151 | Specifies the file from which the host key is read (default | |
5f4fdfae | 152 | .Pa /etc/ssh_host_key ) . |
bf740959 | 153 | This option must be given if |
154 | .Nm | |
155 | is not run as root (as the normal | |
156 | host file is normally not readable by anyone but root). | |
157 | .It Fl i | |
158 | Specifies that | |
159 | .Nm | |
f54651ce | 160 | is being run from inetd. |
bf740959 | 161 | .Nm |
162 | is normally not run | |
163 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 164 | respond to the client, and this may take tens of seconds. |
165 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 166 | However, with small key sizes (e.g., 512) using |
bf740959 | 167 | .Nm |
168 | from inetd may | |
169 | be feasible. | |
170 | .It Fl k Ar key_gen_time | |
171 | Specifies how often the server key is regenerated (default 3600 | |
4fe2af09 | 172 | seconds, or one hour). |
173 | The motivation for regenerating the key fairly | |
bf740959 | 174 | often is that the key is not stored anywhere, and after about an hour, |
175 | it becomes impossible to recover the key for decrypting intercepted | |
176 | communications even if the machine is cracked into or physically | |
4fe2af09 | 177 | seized. |
178 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 179 | .It Fl p Ar port |
180 | Specifies the port on which the server listens for connections | |
181 | (default 22). | |
182 | .It Fl q | |
4fe2af09 | 183 | Quiet mode. |
184 | Nothing is sent to the system log. | |
185 | Normally the beginning, | |
bf740959 | 186 | authentication, and termination of each connection is logged. |
187 | .It Fl Q | |
188 | Do not print an error message if RSA support is missing. | |
5260325f | 189 | .It Fl V Ar client_protocol_id |
190 | SSH2 compatibility mode. | |
191 | When this options is specified | |
192 | .Nm | |
193 | assumes the client has sent the given version string | |
194 | and skips the | |
195 | Protocol Version Identification Exchange. | |
48e671d5 | 196 | .It Fl 4 |
197 | Forces | |
198 | .Nm | |
199 | to use IPv4 addresses only. | |
200 | .It Fl 6 | |
201 | Forces | |
202 | .Nm | |
203 | to use IPv6 addresses only. | |
bf740959 | 204 | .El |
205 | .Sh CONFIGURATION FILE | |
206 | .Nm | |
f54651ce | 207 | reads configuration data from |
5f4fdfae | 208 | .Pa /etc/sshd_config |
bf740959 | 209 | (or the file specified with |
210 | .Fl f | |
4fe2af09 | 211 | on the command line). |
212 | The file contains keyword-value pairs, one per line. | |
213 | Lines starting with | |
bf740959 | 214 | .Ql # |
215 | and empty lines are interpreted as comments. | |
216 | .Pp | |
217 | The following keywords are possible. | |
218 | .Bl -tag -width Ds | |
219 | .It Cm AFSTokenPassing | |
4fe2af09 | 220 | Specifies whether an AFS token may be forwarded to the server. |
221 | Default is | |
bf740959 | 222 | .Dq yes . |
223 | .It Cm AllowGroups | |
224 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 225 | by spaces. |
226 | If specified, login is allowed only for users whose primary | |
bf740959 | 227 | group matches one of the patterns. |
228 | .Ql \&* | |
229 | and | |
230 | .Ql ? | |
231 | can be used as | |
4fe2af09 | 232 | wildcards in the patterns. |
233 | Only group names are valid, a numerical group ID isn't recognized. | |
234 | By default login is allowed regardless of the primary group. | |
bf740959 | 235 | .Pp |
236 | .It Cm AllowUsers | |
237 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 238 | by spaces. |
239 | If specified, login is allowed only for users names that | |
bf740959 | 240 | match one of the patterns. |
241 | .Ql \&* | |
242 | and | |
243 | .Ql ? | |
244 | can be used as | |
4fe2af09 | 245 | wildcards in the patterns. |
246 | Only user names are valid, a numerical user ID isn't recognized. | |
247 | By default login is allowed regardless of the user name. | |
bf740959 | 248 | .Pp |
f54651ce | 249 | .It Cm Ciphers |
250 | Specifies the ciphers allowed for protocol version 2. | |
251 | Multiple ciphers must be comma-separated. | |
252 | The default is | |
253 | .Dq blowfish-cbc,3des-cbc,arcfour,cast128-cbc . | |
bf740959 | 254 | .It Cm CheckMail |
255 | Specifies whether | |
256 | .Nm | |
257 | should check for new mail for interactive logins. | |
258 | The default is | |
259 | .Dq no . | |
260 | .It Cm DenyGroups | |
261 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 262 | by spaces. |
263 | Users whose primary group matches one of the patterns | |
bf740959 | 264 | aren't allowed to log in. |
265 | .Ql \&* | |
266 | and | |
267 | .Ql ? | |
268 | can be used as | |
4fe2af09 | 269 | wildcards in the patterns. |
270 | Only group names are valid, a numerical group ID isn't recognized. | |
271 | By default login is allowed regardless of the primary group. | |
bf740959 | 272 | .Pp |
273 | .It Cm DenyUsers | |
274 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 275 | by spaces. |
276 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 277 | .Ql \&* |
278 | and | |
279 | .Ql ? | |
4fe2af09 | 280 | can be used as wildcards in the patterns. |
281 | Only user names are valid, a numerical user ID isn't recognized. | |
282 | By default login is allowed regardless of the user name. | |
bf740959 | 283 | .It Cm HostKey |
284 | Specifies the file containing the private host key (default | |
5f4fdfae | 285 | .Pa /etc/ssh_host_key ) . |
bf740959 | 286 | Note that |
287 | .Nm | |
288 | does not start if this file is group/world-accessible. | |
289 | .It Cm IgnoreRhosts | |
c8d54615 | 290 | Specifies that |
291 | .Pa .rhosts | |
f54651ce | 292 | and |
c8d54615 | 293 | .Pa .shosts |
294 | files will not be used in authentication. | |
bf740959 | 295 | .Pa /etc/hosts.equiv |
296 | and | |
f54651ce | 297 | .Pa /etc/shosts.equiv |
4fe2af09 | 298 | are still used. |
f54651ce | 299 | The default is |
c8d54615 | 300 | .Dq yes . |
b4748e2f | 301 | .It Cm IgnoreUserKnownHosts |
302 | Specifies whether | |
303 | .Nm | |
304 | should ignore the user's | |
305 | .Pa $HOME/.ssh/known_hosts | |
306 | during | |
307 | .Cm RhostsRSAAuthentication . | |
308 | The default is | |
309 | .Dq no . | |
bf740959 | 310 | .It Cm KeepAlive |
311 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 312 | other side. |
313 | If they are sent, death of the connection or crash of one | |
314 | of the machines will be properly noticed. | |
315 | However, this means that | |
bf740959 | 316 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 317 | find it annoying. |
318 | On the other hand, if keepalives are not send, | |
bf740959 | 319 | sessions may hang indefinitely on the server, leaving |
320 | .Dq ghost | |
321 | users and consuming server resources. | |
322 | .Pp | |
323 | The default is | |
324 | .Dq yes | |
325 | (to send keepalives), and the server will notice | |
4fe2af09 | 326 | if the network goes down or the client host reboots. |
327 | This avoids infinitely hanging sessions. | |
bf740959 | 328 | .Pp |
329 | To disable keepalives, the value should be set to | |
330 | .Dq no | |
331 | in both the server and the client configuration files. | |
332 | .It Cm KerberosAuthentication | |
4fe2af09 | 333 | Specifies whether Kerberos authentication is allowed. |
334 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 335 | .Cm PasswordAuthentication |
336 | is yes, the password provided by the user will be validated through | |
4fe2af09 | 337 | the Kerberos KDC. |
338 | Default is | |
bf740959 | 339 | .Dq yes . |
340 | .It Cm KerberosOrLocalPasswd | |
341 | If set then if password authentication through Kerberos fails then | |
342 | the password will be validated via any additional local mechanism | |
343 | such as | |
344 | .Pa /etc/passwd | |
4fe2af09 | 345 | or SecurID. |
346 | Default is | |
bf740959 | 347 | .Dq yes . |
348 | .It Cm KerberosTgtPassing | |
349 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 350 | Default is |
bf740959 | 351 | .Dq no , |
352 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
353 | .It Cm KerberosTicketCleanup | |
354 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 355 | file on logout. |
356 | Default is | |
bf740959 | 357 | .Dq yes . |
358 | .It Cm KeyRegenerationInterval | |
359 | The server key is automatically regenerated after this many seconds | |
4fe2af09 | 360 | (if it has been used). |
361 | The purpose of regeneration is to prevent | |
bf740959 | 362 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 363 | stealing the keys. |
364 | The key is never stored anywhere. | |
365 | If the value is 0, the key is never regenerated. | |
366 | The default is 3600 (seconds). | |
bf740959 | 367 | .It Cm ListenAddress |
368 | Specifies what local address | |
369 | .Nm | |
370 | should listen on. | |
371 | The default is to listen to all local addresses. | |
48e671d5 | 372 | Multiple options of this type are permitted. |
373 | Additionally, the | |
374 | .Cm Ports | |
375 | options must precede this option. | |
bf740959 | 376 | .It Cm LoginGraceTime |
377 | The server disconnects after this time if the user has not | |
4fe2af09 | 378 | successfully logged in. |
379 | If the value is 0, there is no time limit. | |
bf740959 | 380 | The default is 600 (seconds). |
6a17f9c2 | 381 | .It Cm LogLevel |
382 | Gives the verbosity level that is used when logging messages from | |
383 | .Nm sshd . | |
384 | The possible values are: | |
5260325f | 385 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
6a17f9c2 | 386 | The default is INFO. |
387 | Logging with level DEBUG violates the privacy of users | |
388 | and is not recommended. | |
bf740959 | 389 | .It Cm PasswordAuthentication |
390 | Specifies whether password authentication is allowed. | |
391 | The default is | |
392 | .Dq yes . | |
393 | .It Cm PermitEmptyPasswords | |
394 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 395 | server allows login to accounts with empty password strings. |
396 | The default is | |
c8d54615 | 397 | .Dq no . |
bf740959 | 398 | .It Cm PermitRootLogin |
399 | Specifies whether the root can log in using | |
400 | .Xr ssh 1 . | |
401 | The argument must be | |
402 | .Dq yes , | |
403 | .Dq without-password | |
404 | or | |
405 | .Dq no . | |
406 | The default is | |
407 | .Dq yes . | |
408 | If this options is set to | |
409 | .Dq without-password | |
410 | only password authentication is disabled for root. | |
411 | .Pp | |
412 | Root login with RSA authentication when the | |
413 | .Ar command | |
414 | option has been | |
415 | specified will be allowed regardless of the value of this setting | |
416 | (which may be useful for taking remote backups even if root login is | |
417 | normally not allowed). | |
418 | .It Cm Port | |
419 | Specifies the port number that | |
420 | .Nm | |
4fe2af09 | 421 | listens on. |
422 | The default is 22. | |
48e671d5 | 423 | Multiple options of this type are permitted. |
bf740959 | 424 | .It Cm PrintMotd |
425 | Specifies whether | |
426 | .Nm | |
f54651ce | 427 | should print |
bf740959 | 428 | .Pa /etc/motd |
4fe2af09 | 429 | when a user logs in interactively. |
430 | (On some systems it is also printed by the shell, | |
bf740959 | 431 | .Pa /etc/profile , |
4fe2af09 | 432 | or equivalent.) |
433 | The default is | |
bf740959 | 434 | .Dq yes . |
f54651ce | 435 | .It Cm Protocol |
436 | Specifies the protocol versions | |
437 | .Nm | |
438 | should support. | |
439 | The possible values are | |
440 | .Dq 1 | |
441 | and | |
442 | .Dq 2 . | |
443 | Multiple versions must be comma-separated. | |
444 | The default is | |
445 | .Dq 1 . | |
bf740959 | 446 | .It Cm RandomSeed |
4fe2af09 | 447 | Obsolete. |
448 | Random number generation uses other techniques. | |
bf740959 | 449 | .It Cm RhostsAuthentication |
450 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 451 | files is sufficient. |
452 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 453 | .Cm RhostsRSAAuthentication |
454 | should be used | |
455 | instead, because it performs RSA-based host authentication in addition | |
456 | to normal rhosts or /etc/hosts.equiv authentication. | |
457 | The default is | |
458 | .Dq no . | |
459 | .It Cm RhostsRSAAuthentication | |
460 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 461 | with successful RSA host authentication is allowed. |
462 | The default is | |
c8d54615 | 463 | .Dq no . |
bf740959 | 464 | .It Cm RSAAuthentication |
4fe2af09 | 465 | Specifies whether pure RSA authentication is allowed. |
466 | The default is | |
bf740959 | 467 | .Dq yes . |
468 | .It Cm ServerKeyBits | |
4fe2af09 | 469 | Defines the number of bits in the server key. |
470 | The minimum value is 512, and the default is 768. | |
bf740959 | 471 | .It Cm SkeyAuthentication |
472 | Specifies whether | |
f54651ce | 473 | .Xr skey 1 |
4fe2af09 | 474 | authentication is allowed. |
475 | The default is | |
bf740959 | 476 | .Dq yes . |
477 | Note that s/key authentication is enabled only if | |
478 | .Cm PasswordAuthentication | |
479 | is allowed, too. | |
480 | .It Cm StrictModes | |
481 | Specifies whether | |
482 | .Nm | |
483 | should check file modes and ownership of the | |
4fe2af09 | 484 | user's files and home directory before accepting login. |
485 | This is normally desirable because novices sometimes accidentally leave their | |
486 | directory or files world-writable. | |
487 | The default is | |
bf740959 | 488 | .Dq yes . |
489 | .It Cm SyslogFacility | |
490 | Gives the facility code that is used when logging messages from | |
491 | .Nm sshd . | |
492 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 493 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
494 | The default is AUTH. | |
bf740959 | 495 | .It Cm UseLogin |
496 | Specifies whether | |
497 | .Xr login 1 | |
4fe2af09 | 498 | is used. |
499 | The default is | |
bf740959 | 500 | .Dq no . |
bf740959 | 501 | .It Cm X11DisplayOffset |
502 | Specifies the first display number available for | |
503 | .Nm sshd Ns 's | |
4fe2af09 | 504 | X11 forwarding. |
505 | This prevents | |
bf740959 | 506 | .Nm |
507 | from interfering with real X11 servers. | |
c8d54615 | 508 | The default is 10. |
7b2ea3a1 | 509 | .It Cm X11Forwarding |
4fe2af09 | 510 | Specifies whether X11 forwarding is permitted. |
511 | The default is | |
c8d54615 | 512 | .Dq no . |
7b2ea3a1 | 513 | Note that disabling X11 forwarding does not improve security in any |
514 | way, as users can always install their own forwarders. | |
bf740959 | 515 | .El |
516 | .Sh LOGIN PROCESS | |
517 | When a user successfully logs in, | |
518 | .Nm | |
519 | does the following: | |
520 | .Bl -enum -offset indent | |
521 | .It | |
522 | If the login is on a tty, and no command has been specified, | |
f54651ce | 523 | prints last login time and |
bf740959 | 524 | .Pa /etc/motd |
525 | (unless prevented in the configuration file or by | |
526 | .Pa $HOME/.hushlogin ; | |
527 | see the | |
f54651ce | 528 | .Sx FILES |
bf740959 | 529 | section). |
530 | .It | |
531 | If the login is on a tty, records login time. | |
532 | .It | |
533 | Checks | |
534 | .Pa /etc/nologin ; | |
535 | if it exists, prints contents and quits | |
536 | (unless root). | |
537 | .It | |
538 | Changes to run with normal user privileges. | |
539 | .It | |
540 | Sets up basic environment. | |
541 | .It | |
542 | Reads | |
543 | .Pa $HOME/.ssh/environment | |
544 | if it exists. | |
545 | .It | |
546 | Changes to user's home directory. | |
547 | .It | |
548 | If | |
549 | .Pa $HOME/.ssh/rc | |
550 | exists, runs it; else if | |
5f4fdfae | 551 | .Pa /etc/sshrc |
bf740959 | 552 | exists, runs |
4fe2af09 | 553 | it; otherwise runs xauth. |
554 | The | |
bf740959 | 555 | .Dq rc |
556 | files are given the X11 | |
557 | authentication protocol and cookie in standard input. | |
558 | .It | |
559 | Runs user's shell or command. | |
560 | .El | |
561 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
f54651ce | 562 | The |
bf740959 | 563 | .Pa $HOME/.ssh/authorized_keys |
564 | file lists the RSA keys that are | |
4fe2af09 | 565 | permitted for RSA authentication. |
566 | Each line of the file contains one | |
bf740959 | 567 | key (empty lines and lines starting with a |
568 | .Ql # | |
569 | are ignored as | |
4fe2af09 | 570 | comments). |
571 | Each line consists of the following fields, separated by | |
572 | spaces: options, bits, exponent, modulus, comment. | |
573 | The options field | |
bf740959 | 574 | is optional; its presence is determined by whether the line starts |
575 | with a number or not (the option field never starts with a number). | |
576 | The bits, exponent, modulus and comment fields give the RSA key; the | |
577 | comment field is not used for anything (but may be convenient for the | |
578 | user to identify the key). | |
579 | .Pp | |
580 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 581 | (because of the size of the RSA key modulus). |
582 | You don't want to type them in; instead, copy the | |
bf740959 | 583 | .Pa identity.pub |
584 | file and edit it. | |
585 | .Pp | |
586 | The options (if present) consists of comma-separated option | |
4fe2af09 | 587 | specifications. |
588 | No spaces are permitted, except within double quotes. | |
bf740959 | 589 | The following option specifications are supported: |
590 | .Bl -tag -width Ds | |
591 | .It Cm from="pattern-list" | |
592 | Specifies that in addition to RSA authentication, the canonical name | |
593 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 594 | patterns |
595 | .Pf ( Ql * | |
596 | and | |
597 | .Ql ? | |
598 | serve as wildcards). | |
599 | The list may also contain | |
600 | patterns negated by prefixing them with | |
601 | .Ql ! ; | |
602 | if the canonical host name matches a negated pattern, the key is not accepted. | |
603 | The purpose | |
bf740959 | 604 | of this option is to optionally increase security: RSA authentication |
605 | by itself does not trust the network or name servers or anything (but | |
606 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 607 | permits an intruder to log in from anywhere in the world. |
608 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 609 | servers and/or routers would have to be compromised in addition to |
610 | just the key). | |
611 | .It Cm command="command" | |
612 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 613 | authentication. |
614 | The command supplied by the user (if any) is ignored. | |
bf740959 | 615 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 616 | otherwise it is run without a tty. |
617 | A quote may be included in the command by quoting it with a backslash. | |
618 | This option might be useful | |
619 | to restrict certain RSA keys to perform just a specific operation. | |
620 | An example might be a key that permits remote backups but nothing else. | |
621 | Notice that the client may specify TCP/IP and/or X11 | |
bf740959 | 622 | forwardings unless they are explicitly prohibited. |
623 | .It Cm environment="NAME=value" | |
624 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 625 | logging in using this key. |
626 | Environment variables set this way | |
627 | override other default environment values. | |
628 | Multiple options of this type are permitted. | |
bf740959 | 629 | .It Cm no-port-forwarding |
630 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 631 | Any port forward requests by the client will return an error. |
632 | This might be used, e.g., in connection with the | |
bf740959 | 633 | .Cm command |
634 | option. | |
635 | .It Cm no-X11-forwarding | |
636 | Forbids X11 forwarding when this key is used for authentication. | |
637 | Any X11 forward requests by the client will return an error. | |
638 | .It Cm no-agent-forwarding | |
639 | Forbids authentication agent forwarding when this key is used for | |
640 | authentication. | |
641 | .It Cm no-pty | |
642 | Prevents tty allocation (a request to allocate a pty will fail). | |
643 | .El | |
644 | .Ss Examples | |
645 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
646 | .Pp | |
647 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
648 | .Pp | |
649 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
650 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
f54651ce | 651 | The |
5f4fdfae | 652 | .Pa /etc/ssh_known_hosts |
f54651ce | 653 | and |
bf740959 | 654 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 655 | files contain host public keys for all known hosts. |
656 | The global file should | |
657 | be prepared by the administrator (optional), and the per-user file is | |
bf740959 | 658 | maintained automatically: whenever the user connects an unknown host |
4fe2af09 | 659 | its key is added to the per-user file. |
bf740959 | 660 | .Pp |
661 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 662 | bits, exponent, modulus, comment. |
663 | The fields are separated by spaces. | |
bf740959 | 664 | .Pp |
665 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
666 | wildcards); each pattern in turn is matched against the canonical host | |
667 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 668 | name (when authenticating a server). |
669 | A pattern may also be preceded by | |
bf740959 | 670 | .Ql ! |
671 | to indicate negation: if the host name matches a negated | |
672 | pattern, it is not accepted (by that line) even if it matched another | |
673 | pattern on the line. | |
674 | .Pp | |
675 | Bits, exponent, and modulus are taken directly from the host key; they | |
676 | can be obtained, e.g., from | |
5f4fdfae | 677 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 678 | The optional comment field continues to the end of the line, and is not used. |
679 | .Pp | |
680 | Lines starting with | |
681 | .Ql # | |
682 | and empty lines are ignored as comments. | |
683 | .Pp | |
684 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 685 | matching line has the proper key. |
686 | It is thus permissible (but not | |
bf740959 | 687 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 688 | names. |
689 | This will inevitably happen when short forms of host names | |
690 | from different domains are put in the file. | |
691 | It is possible | |
bf740959 | 692 | that the files contain conflicting information; authentication is |
693 | accepted if valid information can be found from either file. | |
694 | .Pp | |
695 | Note that the lines in these files are typically hundreds of characters | |
696 | long, and you definitely don't want to type in the host keys by hand. | |
697 | Rather, generate them by a script | |
f54651ce | 698 | or by taking |
5f4fdfae | 699 | .Pa /etc/ssh_host_key.pub |
bf740959 | 700 | and adding the host names at the front. |
701 | .Ss Examples | |
702 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
703 | .Sh FILES | |
704 | .Bl -tag -width Ds | |
5f4fdfae | 705 | .It Pa /etc/sshd_config |
bf740959 | 706 | Contains configuration data for |
707 | .Nm sshd . | |
708 | This file should be writable by root only, but it is recommended | |
709 | (though not necessary) that it be world-readable. | |
5f4fdfae | 710 | .It Pa /etc/ssh_host_key |
bf740959 | 711 | Contains the private part of the host key. |
712 | This file should only be owned by root, readable only by root, and not | |
713 | accessible to others. | |
714 | Note that | |
715 | .Nm | |
716 | does not start if this file is group/world-accessible. | |
5f4fdfae | 717 | .It Pa /etc/ssh_host_key.pub |
bf740959 | 718 | Contains the public part of the host key. |
719 | This file should be world-readable but writable only by | |
4fe2af09 | 720 | root. |
721 | Its contents should match the private part. | |
722 | This file is not | |
bf740959 | 723 | really used for anything; it is only provided for the convenience of |
724 | the user so its contents can be copied to known hosts files. | |
725 | These two files are created using | |
726 | .Xr ssh-keygen 1 . | |
5f4fdfae | 727 | .It Pa /var/run/sshd.pid |
bf740959 | 728 | Contains the process ID of the |
729 | .Nm | |
730 | listening for connections (if there are several daemons running | |
731 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 732 | started last). |
733 | The contents of this file are not sensitive; it can be world-readable. | |
bf740959 | 734 | .It Pa $HOME/.ssh/authorized_keys |
735 | Lists the RSA keys that can be used to log into the user's account. | |
736 | This file must be readable by root (which may on some machines imply | |
737 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 738 | volume). |
739 | It is recommended that it not be accessible by others. | |
740 | The format of this file is described above. | |
5f4fdfae | 741 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 742 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 743 | authentication to check the public key of the host. |
744 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 745 | The client uses the same files |
4fe2af09 | 746 | to verify that the remote host is the one we intended to connect. |
747 | These files should be writable only by root/the owner. | |
5f4fdfae | 748 | .Pa /etc/ssh_known_hosts |
bf740959 | 749 | should be world-readable, and |
750 | .Pa $HOME/.ssh/known_hosts | |
751 | can but need not be world-readable. | |
752 | .It Pa /etc/nologin | |
f54651ce | 753 | If this file exists, |
bf740959 | 754 | .Nm |
4fe2af09 | 755 | refuses to let anyone except root log in. |
756 | The contents of the file | |
bf740959 | 757 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 758 | refused. |
759 | The file should be world-readable. | |
bf740959 | 760 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
761 | If compiled with | |
762 | .Sy LIBWRAP | |
763 | support, tcp-wrappers access controls may be defined here as described in | |
764 | .Xr hosts_access 5 . | |
765 | .It Pa $HOME/.rhosts | |
766 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 767 | line. |
768 | The given user on the corresponding host is permitted to log in | |
769 | without password. | |
770 | The same file is used by rlogind and rshd. | |
bf740959 | 771 | The file must |
772 | be writable only by the user; it is recommended that it not be | |
773 | accessible by others. | |
774 | .Pp | |
4fe2af09 | 775 | If is also possible to use netgroups in the file. |
776 | Either host or user | |
bf740959 | 777 | name may be of the form +@groupname to specify all hosts or all users |
778 | in the group. | |
779 | .It Pa $HOME/.shosts | |
780 | For ssh, | |
781 | this file is exactly the same as for | |
782 | .Pa .rhosts . | |
783 | However, this file is | |
784 | not used by rlogin and rshd, so using this permits access using SSH only. | |
785 | .Pa /etc/hosts.equiv | |
786 | This file is used during | |
787 | .Pa .rhosts | |
4fe2af09 | 788 | authentication. |
789 | In the simplest form, this file contains host names, one per line. | |
790 | Users on | |
bf740959 | 791 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 792 | have the same user name on both machines. |
793 | The host name may also be | |
bf740959 | 794 | followed by a user name; such users are permitted to log in as |
795 | .Em any | |
4fe2af09 | 796 | user on this machine (except root). |
797 | Additionally, the syntax | |
bf740959 | 798 | .Dq +@group |
4fe2af09 | 799 | can be used to specify netgroups. |
800 | Negated entries start with | |
bf740959 | 801 | .Ql \&- . |
802 | .Pp | |
803 | If the client host/user is successfully matched in this file, login is | |
804 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 805 | same. |
806 | Additionally, successful RSA host authentication is normally required. | |
807 | This file must be writable only by root; it is recommended | |
bf740959 | 808 | that it be world-readable. |
809 | .Pp | |
810 | .Sy "Warning: It is almost never a good idea to use user names in" | |
811 | .Pa hosts.equiv . | |
812 | Beware that it really means that the named user(s) can log in as | |
813 | .Em anybody , | |
814 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 815 | binaries and directories. |
816 | Using a user name practically grants the user root access. | |
817 | The only valid use for user names that I can think | |
bf740959 | 818 | of is in negative entries. |
819 | .Pp | |
820 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 821 | .It Pa /etc/shosts.equiv |
bf740959 | 822 | This is processed exactly as |
823 | .Pa /etc/hosts.equiv . | |
824 | However, this file may be useful in environments that want to run both | |
825 | rsh/rlogin and ssh. | |
826 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 827 | This file is read into the environment at login (if it exists). |
828 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 829 | .Ql # ) , |
4fe2af09 | 830 | and assignment lines of the form name=value. |
831 | The file should be writable | |
bf740959 | 832 | only by the user; it need not be readable by anyone else. |
833 | .It Pa $HOME/.ssh/rc | |
834 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 835 | environment files but before starting the user's shell or command. |
836 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 837 | standard input (and |
838 | .Ev DISPLAY | |
4fe2af09 | 839 | in environment). |
840 | This must call | |
bf740959 | 841 | .Xr xauth 1 |
842 | in that case. | |
843 | .Pp | |
844 | The primary purpose of this file is to run any initialization routines | |
845 | which may be needed before the user's home directory becomes | |
846 | accessible; AFS is a particular example of such an environment. | |
847 | .Pp | |
848 | This file will probably contain some initialization code followed by | |
849 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
850 | $proto $cookie | xauth -q -; fi". | |
851 | .Pp | |
852 | If this file does not exist, | |
5f4fdfae | 853 | .Pa /etc/sshrc |
bf740959 | 854 | is run, and if that |
855 | does not exist either, xauth is used to store the cookie. | |
856 | .Pp | |
857 | This file should be writable only by the user, and need not be | |
858 | readable by anyone else. | |
5f4fdfae | 859 | .It Pa /etc/sshrc |
bf740959 | 860 | Like |
861 | .Pa $HOME/.ssh/rc . | |
862 | This can be used to specify | |
4fe2af09 | 863 | machine-specific login-time initializations globally. |
864 | This file should be writable only by root, and should be world-readable. | |
bf740959 | 865 | .Sh AUTHOR |
bf740959 | 866 | OpenSSH |
c8d54615 | 867 | is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, |
4fe2af09 | 868 | but with bugs removed and newer features re-added. |
869 | Rapidly after the | |
c8d54615 | 870 | 1.2.12 release, newer versions of the original ssh bore successively |
871 | more restrictive licenses, and thus demand for a free version was born. | |
872 | This version of OpenSSH | |
bf740959 | 873 | .Bl -bullet |
874 | .It | |
399d9d44 | 875 | has all components of a restrictive nature (i.e., patents, see |
bf740959 | 876 | .Xr ssl 8 ) |
877 | directly removed from the source code; any licensed or patented components | |
878 | are chosen from | |
879 | external libraries. | |
880 | .It | |
c8d54615 | 881 | has been updated to support ssh protocol 1.5, making it compatible with |
882 | all other ssh protocol 1 clients and servers. | |
bf740959 | 883 | .It |
f54651ce | 884 | contains added support for |
bf740959 | 885 | .Xr kerberos 8 |
886 | authentication and ticket passing. | |
887 | .It | |
888 | supports one-time password authentication with | |
889 | .Xr skey 1 . | |
890 | .El | |
891 | .Pp | |
892 | The libraries described in | |
893 | .Xr ssl 8 | |
894 | are required for proper operation. | |
895 | .Sh SEE ALSO | |
bf740959 | 896 | .Xr scp 1 , |
897 | .Xr ssh 1 , | |
898 | .Xr ssh-add 1 , | |
899 | .Xr ssh-agent 1 , | |
900 | .Xr ssh-keygen 1 , | |
7368a6c8 | 901 | .Xr ssl 8 , |
902 | .Xr rlogin 1 , | |
903 | .Xr rsh 1 |