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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 | |
c345cf9d | 29 | .Op Fl u Ar len |
5260325f | 30 | .Op Fl V Ar client_protocol_id |
f54651ce | 31 | .Sh DESCRIPTION |
bf740959 | 32 | .Nm |
f54651ce | 33 | (Secure Shell Daemon) is the daemon program for |
bf740959 | 34 | .Xr ssh 1 . |
3189621b | 35 | Together these programs replace rlogin and rsh, and |
bf740959 | 36 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 37 | over an insecure network. |
38 | The programs are intended to be as easy to | |
bf740959 | 39 | install and use as possible. |
40 | .Pp | |
41 | .Nm | |
4fe2af09 | 42 | is the daemon that listens for connections from clients. |
f54651ce | 43 | It is normally started at boot from |
bf740959 | 44 | .Pa /etc/rc . |
45 | It forks a new | |
4fe2af09 | 46 | daemon for each incoming connection. |
47 | The forked daemons handle | |
bf740959 | 48 | key exchange, encryption, authentication, command execution, |
49 | and data exchange. | |
1d1ffb87 | 50 | This implementation of |
51 | .Nm | |
52 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 53 | .Nm |
4fe2af09 | 54 | works as follows. |
1d1ffb87 | 55 | .Pp |
56 | .Ss SSH protocol version 1 | |
57 | .Pp | |
4fe2af09 | 58 | Each host has a host-specific RSA key |
59 | (normally 1024 bits) used to identify the host. | |
60 | Additionally, when | |
bf740959 | 61 | the daemon starts, it generates a server RSA key (normally 768 bits). |
62 | This key is normally regenerated every hour if it has been used, and | |
63 | is never stored on disk. | |
64 | .Pp | |
3189621b | 65 | Whenever a client connects the daemon responds with its public |
66 | host and server keys. | |
4fe2af09 | 67 | The client compares the |
1d1ffb87 | 68 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 69 | The client then generates a 256 bit random number. |
70 | It encrypts this | |
bf740959 | 71 | random number using both the host key and the server key, and sends |
4fe2af09 | 72 | the encrypted number to the server. |
3189621b | 73 | Both sides then use this |
bf740959 | 74 | random number as a session key which is used to encrypt all further |
4fe2af09 | 75 | communications in the session. |
76 | The rest of the session is encrypted | |
3189621b | 77 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 78 | being used by default. |
4fe2af09 | 79 | The client selects the encryption algorithm |
bf740959 | 80 | to use from those offered by the server. |
81 | .Pp | |
4fe2af09 | 82 | Next, the server and the client enter an authentication dialog. |
83 | The client tries to authenticate itself using | |
bf740959 | 84 | .Pa .rhosts |
85 | authentication, | |
86 | .Pa .rhosts | |
87 | authentication combined with RSA host | |
88 | authentication, RSA challenge-response authentication, or password | |
89 | based authentication. | |
90 | .Pp | |
91 | Rhosts authentication is normally disabled | |
92 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 93 | configuration file if desired. |
94 | System security is not improved unless | |
bf740959 | 95 | .Xr rshd 8 , |
96 | .Xr rlogind 8 , | |
97 | .Xr rexecd 8 , | |
98 | and | |
99 | .Xr rexd 8 | |
100 | are disabled (thus completely disabling | |
101 | .Xr rlogin 1 | |
102 | and | |
103 | .Xr rsh 1 | |
3189621b | 104 | into the machine). |
bf740959 | 105 | .Pp |
1d1ffb87 | 106 | .Ss SSH protocol version 2 |
107 | .Pp | |
c345cf9d | 108 | Version 2 works similarly: |
1d1ffb87 | 109 | Each host has a host-specific DSA key used to identify the host. |
110 | However, when the daemon starts, it does not generate a server key. | |
111 | Forward security is provided through a Diffie-Hellman key agreement. | |
112 | This key agreement results in a shared session key. | |
113 | The rest of the session is encrypted | |
114 | using a symmetric cipher, currently | |
115 | Blowfish, 3DES or CAST128 in CBC mode or Arcfour. | |
116 | The client selects the encryption algorithm | |
117 | to use from those offered by the server. | |
118 | Additionally, session integrity is provided | |
d0c832f3 | 119 | through a cryptographic message authentication code |
1d1ffb87 | 120 | (hmac-sha1 or hmac-md5). |
121 | .Pp | |
122 | Protocol version 2 provides a public key based | |
123 | user authentication method (DSAAuthentication) | |
124 | and conventional password authentication. | |
125 | .Pp | |
126 | .Ss Command execution and data forwarding | |
127 | .Pp | |
bf740959 | 128 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 129 | preparing the session is entered. |
130 | At this time the client may request | |
bf740959 | 131 | things like allocating a pseudo-tty, forwarding X11 connections, |
132 | forwarding TCP/IP connections, or forwarding the authentication agent | |
133 | connection over the secure channel. | |
134 | .Pp | |
135 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 136 | The sides then enter session mode. |
137 | In this mode, either side may send | |
bf740959 | 138 | data at any time, and such data is forwarded to/from the shell or |
139 | command on the server side, and the user terminal in the client side. | |
140 | .Pp | |
141 | When the user program terminates and all forwarded X11 and other | |
142 | connections have been closed, the server sends command exit status to | |
143 | the client, and both sides exit. | |
144 | .Pp | |
145 | .Nm | |
146 | can be configured using command-line options or a configuration | |
4fe2af09 | 147 | file. |
148 | Command-line options override values specified in the | |
bf740959 | 149 | configuration file. |
150 | .Pp | |
9d6b7add | 151 | .Nm |
152 | rereads its configuration file when it receives a hangup signal, | |
153 | .Dv SIGHUP . | |
154 | .Pp | |
bf740959 | 155 | The options are as follows: |
156 | .Bl -tag -width Ds | |
157 | .It Fl b Ar bits | |
158 | Specifies the number of bits in the server key (default 768). | |
159 | .Pp | |
160 | .It Fl d | |
4fe2af09 | 161 | Debug mode. |
162 | The server sends verbose debug output to the system | |
163 | log, and does not put itself in the background. | |
164 | The server also will not fork and will only process one connection. | |
165 | This option is only intended for debugging for the server. | |
bf740959 | 166 | .It Fl f Ar configuration_file |
4fe2af09 | 167 | Specifies the name of the configuration file. |
168 | The default is | |
5f4fdfae | 169 | .Pa /etc/sshd_config . |
bf740959 | 170 | .Nm |
171 | refuses to start if there is no configuration file. | |
172 | .It Fl g Ar login_grace_time | |
173 | Gives the grace time for clients to authenticate themselves (default | |
4fe2af09 | 174 | 300 seconds). |
175 | If the client fails to authenticate the user within | |
176 | this many seconds, the server disconnects and exits. | |
177 | A value of zero indicates no limit. | |
bf740959 | 178 | .It Fl h Ar host_key_file |
1d1ffb87 | 179 | Specifies the file from which the RSA host key is read (default |
5f4fdfae | 180 | .Pa /etc/ssh_host_key ) . |
bf740959 | 181 | This option must be given if |
182 | .Nm | |
183 | is not run as root (as the normal | |
184 | host file is normally not readable by anyone but root). | |
185 | .It Fl i | |
186 | Specifies that | |
187 | .Nm | |
f54651ce | 188 | is being run from inetd. |
bf740959 | 189 | .Nm |
190 | is normally not run | |
191 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 192 | respond to the client, and this may take tens of seconds. |
193 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 194 | However, with small key sizes (e.g., 512) using |
bf740959 | 195 | .Nm |
196 | from inetd may | |
197 | be feasible. | |
198 | .It Fl k Ar key_gen_time | |
199 | Specifies how often the server key is regenerated (default 3600 | |
4fe2af09 | 200 | seconds, or one hour). |
201 | The motivation for regenerating the key fairly | |
bf740959 | 202 | often is that the key is not stored anywhere, and after about an hour, |
203 | it becomes impossible to recover the key for decrypting intercepted | |
204 | communications even if the machine is cracked into or physically | |
4fe2af09 | 205 | seized. |
206 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 207 | .It Fl p Ar port |
208 | Specifies the port on which the server listens for connections | |
209 | (default 22). | |
210 | .It Fl q | |
4fe2af09 | 211 | Quiet mode. |
212 | Nothing is sent to the system log. | |
213 | Normally the beginning, | |
bf740959 | 214 | authentication, and termination of each connection is logged. |
c345cf9d | 215 | .It Fl u Ar len |
216 | This option is used to specify the size of the field | |
217 | in the | |
218 | .Li utmp | |
219 | structure that holds the remote host name. | |
220 | If the resolved host name is longer than | |
221 | .Ar len , | |
222 | the dotted decimal value will be used instead. | |
223 | This allows hosts with very long host names that | |
224 | overflow this field to still be uniquely identified. | |
225 | Specifying | |
226 | .Fl u0 | |
227 | indicates that only dotted decimal addresses | |
228 | should be put into the | |
229 | .Pa utmp | |
230 | file. | |
bf740959 | 231 | .It Fl Q |
232 | Do not print an error message if RSA support is missing. | |
5260325f | 233 | .It Fl V Ar client_protocol_id |
234 | SSH2 compatibility mode. | |
3189621b | 235 | When this option is specified |
5260325f | 236 | .Nm |
3189621b | 237 | assumes the client has sent the supplied version string |
5260325f | 238 | and skips the |
239 | Protocol Version Identification Exchange. | |
48e671d5 | 240 | .It Fl 4 |
241 | Forces | |
242 | .Nm | |
243 | to use IPv4 addresses only. | |
244 | .It Fl 6 | |
245 | Forces | |
246 | .Nm | |
247 | to use IPv6 addresses only. | |
bf740959 | 248 | .El |
249 | .Sh CONFIGURATION FILE | |
250 | .Nm | |
f54651ce | 251 | reads configuration data from |
5f4fdfae | 252 | .Pa /etc/sshd_config |
bf740959 | 253 | (or the file specified with |
254 | .Fl f | |
4fe2af09 | 255 | on the command line). |
256 | The file contains keyword-value pairs, one per line. | |
257 | Lines starting with | |
bf740959 | 258 | .Ql # |
259 | and empty lines are interpreted as comments. | |
260 | .Pp | |
261 | The following keywords are possible. | |
262 | .Bl -tag -width Ds | |
263 | .It Cm AFSTokenPassing | |
4fe2af09 | 264 | Specifies whether an AFS token may be forwarded to the server. |
265 | Default is | |
bf740959 | 266 | .Dq yes . |
267 | .It Cm AllowGroups | |
268 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 269 | by spaces. |
270 | If specified, login is allowed only for users whose primary | |
bf740959 | 271 | group matches one of the patterns. |
272 | .Ql \&* | |
273 | and | |
274 | .Ql ? | |
275 | can be used as | |
4fe2af09 | 276 | wildcards in the patterns. |
c345cf9d | 277 | Only group names are valid; a numerical group ID isn't recognized. |
4fe2af09 | 278 | By default login is allowed regardless of the primary group. |
bf740959 | 279 | .Pp |
280 | .It Cm AllowUsers | |
281 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 282 | by spaces. |
283 | If specified, login is allowed only for users names that | |
bf740959 | 284 | match one of the patterns. |
285 | .Ql \&* | |
286 | and | |
287 | .Ql ? | |
288 | can be used as | |
4fe2af09 | 289 | wildcards in the patterns. |
c345cf9d | 290 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 291 | By default login is allowed regardless of the user name. |
bf740959 | 292 | .Pp |
f54651ce | 293 | .It Cm Ciphers |
294 | Specifies the ciphers allowed for protocol version 2. | |
295 | Multiple ciphers must be comma-separated. | |
296 | The default is | |
d0c832f3 | 297 | .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc . |
bf740959 | 298 | .It Cm CheckMail |
299 | Specifies whether | |
300 | .Nm | |
301 | should check for new mail for interactive logins. | |
302 | The default is | |
303 | .Dq no . | |
304 | .It Cm DenyGroups | |
305 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 306 | by spaces. |
307 | Users whose primary group matches one of the patterns | |
bf740959 | 308 | aren't allowed to log in. |
309 | .Ql \&* | |
310 | and | |
311 | .Ql ? | |
312 | can be used as | |
4fe2af09 | 313 | wildcards in the patterns. |
c345cf9d | 314 | Only group names are valid; a numerical group ID isn't recognized. |
4fe2af09 | 315 | By default login is allowed regardless of the primary group. |
bf740959 | 316 | .Pp |
317 | .It Cm DenyUsers | |
318 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 319 | by spaces. |
320 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 321 | .Ql \&* |
322 | and | |
323 | .Ql ? | |
4fe2af09 | 324 | can be used as wildcards in the patterns. |
c345cf9d | 325 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 326 | By default login is allowed regardless of the user name. |
1d1ffb87 | 327 | .It Cm DSAAuthentication |
328 | Specifies whether DSA authentication is allowed. | |
329 | The default is | |
330 | .Dq yes . | |
331 | Note that this option applies to protocol version 2 only. | |
332 | .It Cm GatewayPorts | |
333 | Specifies whether remote hosts are allowed to connect to ports | |
334 | forwarded for the client. | |
335 | The argument must be | |
336 | .Dq yes | |
337 | or | |
338 | .Dq no . | |
339 | The default is | |
340 | .Dq no . | |
c345cf9d | 341 | .It Cm HostDSAKey |
1d1ffb87 | 342 | Specifies the file containing the private DSA host key (default |
343 | .Pa /etc/ssh_host_dsa_key ) | |
344 | used by SSH protocol 2.0. | |
345 | Note that | |
346 | .Nm | |
d0c832f3 | 347 | disables protocol 2.0 if this file is group/world-accessible. |
bf740959 | 348 | .It Cm HostKey |
1d1ffb87 | 349 | Specifies the file containing the private RSA host key (default |
350 | .Pa /etc/ssh_host_key ) | |
351 | used by SSH protocols 1.3 and 1.5. | |
bf740959 | 352 | Note that |
353 | .Nm | |
d0c832f3 | 354 | disables protocols 1.3 and 1.5 if this file is group/world-accessible. |
bf740959 | 355 | .It Cm IgnoreRhosts |
c8d54615 | 356 | Specifies that |
357 | .Pa .rhosts | |
f54651ce | 358 | and |
c8d54615 | 359 | .Pa .shosts |
360 | files will not be used in authentication. | |
bf740959 | 361 | .Pa /etc/hosts.equiv |
362 | and | |
f54651ce | 363 | .Pa /etc/shosts.equiv |
4fe2af09 | 364 | are still used. |
f54651ce | 365 | The default is |
c8d54615 | 366 | .Dq yes . |
b4748e2f | 367 | .It Cm IgnoreUserKnownHosts |
368 | Specifies whether | |
369 | .Nm | |
370 | should ignore the user's | |
371 | .Pa $HOME/.ssh/known_hosts | |
372 | during | |
373 | .Cm RhostsRSAAuthentication . | |
374 | The default is | |
375 | .Dq no . | |
bf740959 | 376 | .It Cm KeepAlive |
377 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 378 | other side. |
379 | If they are sent, death of the connection or crash of one | |
380 | of the machines will be properly noticed. | |
381 | However, this means that | |
bf740959 | 382 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 383 | find it annoying. |
d0c832f3 | 384 | On the other hand, if keepalives are not sent, |
bf740959 | 385 | sessions may hang indefinitely on the server, leaving |
386 | .Dq ghost | |
387 | users and consuming server resources. | |
388 | .Pp | |
389 | The default is | |
390 | .Dq yes | |
391 | (to send keepalives), and the server will notice | |
4fe2af09 | 392 | if the network goes down or the client host reboots. |
393 | This avoids infinitely hanging sessions. | |
bf740959 | 394 | .Pp |
395 | To disable keepalives, the value should be set to | |
396 | .Dq no | |
397 | in both the server and the client configuration files. | |
398 | .It Cm KerberosAuthentication | |
4fe2af09 | 399 | Specifies whether Kerberos authentication is allowed. |
400 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 401 | .Cm PasswordAuthentication |
402 | is yes, the password provided by the user will be validated through | |
c345cf9d | 403 | the Kerberos KDC. To use this option, the server needs a |
404 | Kerberos servtab which allows the verification of the KDC's identity. | |
4fe2af09 | 405 | Default is |
bf740959 | 406 | .Dq yes . |
407 | .It Cm KerberosOrLocalPasswd | |
408 | If set then if password authentication through Kerberos fails then | |
409 | the password will be validated via any additional local mechanism | |
410 | such as | |
411 | .Pa /etc/passwd | |
4fe2af09 | 412 | or SecurID. |
413 | Default is | |
bf740959 | 414 | .Dq yes . |
415 | .It Cm KerberosTgtPassing | |
416 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 417 | Default is |
bf740959 | 418 | .Dq no , |
419 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
420 | .It Cm KerberosTicketCleanup | |
421 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 422 | file on logout. |
423 | Default is | |
bf740959 | 424 | .Dq yes . |
425 | .It Cm KeyRegenerationInterval | |
426 | The server key is automatically regenerated after this many seconds | |
4fe2af09 | 427 | (if it has been used). |
428 | The purpose of regeneration is to prevent | |
bf740959 | 429 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 430 | stealing the keys. |
431 | The key is never stored anywhere. | |
432 | If the value is 0, the key is never regenerated. | |
433 | The default is 3600 (seconds). | |
bf740959 | 434 | .It Cm ListenAddress |
435 | Specifies what local address | |
436 | .Nm | |
437 | should listen on. | |
438 | The default is to listen to all local addresses. | |
48e671d5 | 439 | Multiple options of this type are permitted. |
440 | Additionally, the | |
441 | .Cm Ports | |
442 | options must precede this option. | |
bf740959 | 443 | .It Cm LoginGraceTime |
444 | The server disconnects after this time if the user has not | |
4fe2af09 | 445 | successfully logged in. |
446 | If the value is 0, there is no time limit. | |
bf740959 | 447 | The default is 600 (seconds). |
6a17f9c2 | 448 | .It Cm LogLevel |
449 | Gives the verbosity level that is used when logging messages from | |
450 | .Nm sshd . | |
451 | The possible values are: | |
5260325f | 452 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
6a17f9c2 | 453 | The default is INFO. |
454 | Logging with level DEBUG violates the privacy of users | |
455 | and is not recommended. | |
089fbbd2 | 456 | .It Cm MaxStartups |
457 | Specifies the maximum number of concurrent unauthenticated connections to the | |
458 | .Nm | |
459 | daemon. | |
460 | Additional connections will be dropped until authentication succeeds or the | |
461 | .Cm LoginGraceTime | |
462 | expires for a connection. | |
463 | The default is 10. | |
c345cf9d | 464 | .Pp |
465 | Alternatively, random early drop can be enabled by specifying | |
466 | the three colon separated values | |
467 | .Dq start:rate:full | |
468 | (e.g. "10:30:60"). | |
469 | .Nm | |
470 | will refuse connection attempts with a probabillity of | |
471 | .Dq rate/100 | |
472 | (30%) | |
473 | if there are currently | |
474 | .Dq start | |
475 | (10) | |
476 | unauthenticated connections. | |
477 | The probabillity increases linearly and all connection attempts | |
478 | are refused if the number of unauthenticated connections reaches | |
479 | .Dq full | |
480 | (60). | |
bf740959 | 481 | .It Cm PasswordAuthentication |
482 | Specifies whether password authentication is allowed. | |
483 | The default is | |
484 | .Dq yes . | |
c345cf9d | 485 | Note that this option applies to both protocol versions 1 and 2. |
bf740959 | 486 | .It Cm PermitEmptyPasswords |
487 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 488 | server allows login to accounts with empty password strings. |
489 | The default is | |
c8d54615 | 490 | .Dq no . |
bf740959 | 491 | .It Cm PermitRootLogin |
492 | Specifies whether the root can log in using | |
493 | .Xr ssh 1 . | |
494 | The argument must be | |
495 | .Dq yes , | |
496 | .Dq without-password | |
497 | or | |
498 | .Dq no . | |
499 | The default is | |
500 | .Dq yes . | |
501 | If this options is set to | |
502 | .Dq without-password | |
503 | only password authentication is disabled for root. | |
504 | .Pp | |
505 | Root login with RSA authentication when the | |
506 | .Ar command | |
507 | option has been | |
508 | specified will be allowed regardless of the value of this setting | |
509 | (which may be useful for taking remote backups even if root login is | |
510 | normally not allowed). | |
0fbe8c74 | 511 | .It Cm PidFile |
512 | Specifies the file that contains the process identifier of the | |
513 | .Nm | |
514 | daemon. | |
515 | The default is | |
516 | .Pa /var/run/sshd.pid . | |
bf740959 | 517 | .It Cm Port |
518 | Specifies the port number that | |
519 | .Nm | |
4fe2af09 | 520 | listens on. |
521 | The default is 22. | |
48e671d5 | 522 | Multiple options of this type are permitted. |
bf740959 | 523 | .It Cm PrintMotd |
524 | Specifies whether | |
525 | .Nm | |
f54651ce | 526 | should print |
bf740959 | 527 | .Pa /etc/motd |
4fe2af09 | 528 | when a user logs in interactively. |
529 | (On some systems it is also printed by the shell, | |
bf740959 | 530 | .Pa /etc/profile , |
4fe2af09 | 531 | or equivalent.) |
532 | The default is | |
bf740959 | 533 | .Dq yes . |
f54651ce | 534 | .It Cm Protocol |
535 | Specifies the protocol versions | |
536 | .Nm | |
537 | should support. | |
538 | The possible values are | |
539 | .Dq 1 | |
540 | and | |
541 | .Dq 2 . | |
542 | Multiple versions must be comma-separated. | |
543 | The default is | |
544 | .Dq 1 . | |
bf740959 | 545 | .It Cm RandomSeed |
4fe2af09 | 546 | Obsolete. |
547 | Random number generation uses other techniques. | |
bf740959 | 548 | .It Cm RhostsAuthentication |
549 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 550 | files is sufficient. |
551 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 552 | .Cm RhostsRSAAuthentication |
553 | should be used | |
554 | instead, because it performs RSA-based host authentication in addition | |
555 | to normal rhosts or /etc/hosts.equiv authentication. | |
556 | The default is | |
557 | .Dq no . | |
558 | .It Cm RhostsRSAAuthentication | |
559 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 560 | with successful RSA host authentication is allowed. |
561 | The default is | |
c8d54615 | 562 | .Dq no . |
bf740959 | 563 | .It Cm RSAAuthentication |
4fe2af09 | 564 | Specifies whether pure RSA authentication is allowed. |
565 | The default is | |
bf740959 | 566 | .Dq yes . |
1d1ffb87 | 567 | Note that this option applies to protocol version 1 only. |
bf740959 | 568 | .It Cm ServerKeyBits |
4fe2af09 | 569 | Defines the number of bits in the server key. |
570 | The minimum value is 512, and the default is 768. | |
bf740959 | 571 | .It Cm SkeyAuthentication |
572 | Specifies whether | |
f54651ce | 573 | .Xr skey 1 |
4fe2af09 | 574 | authentication is allowed. |
575 | The default is | |
bf740959 | 576 | .Dq yes . |
577 | Note that s/key authentication is enabled only if | |
578 | .Cm PasswordAuthentication | |
579 | is allowed, too. | |
580 | .It Cm StrictModes | |
581 | Specifies whether | |
582 | .Nm | |
583 | should check file modes and ownership of the | |
4fe2af09 | 584 | user's files and home directory before accepting login. |
585 | This is normally desirable because novices sometimes accidentally leave their | |
586 | directory or files world-writable. | |
587 | The default is | |
bf740959 | 588 | .Dq yes . |
38c295d6 | 589 | .It Cm Subsystem |
590 | Configures an external subsystem (e.g. file transfer daemon). | |
591 | Arguments should be a subsystem name and a command to execute upon subsystem request. | |
592 | By default no subsystems are defined. | |
593 | Note that this option applies to protocol version 2 only. | |
bf740959 | 594 | .It Cm SyslogFacility |
595 | Gives the facility code that is used when logging messages from | |
596 | .Nm sshd . | |
597 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 598 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
599 | The default is AUTH. | |
bf740959 | 600 | .It Cm UseLogin |
601 | Specifies whether | |
602 | .Xr login 1 | |
fa649821 | 603 | is used for interactive login sessions. |
604 | Note that | |
605 | .Xr login 1 | |
c345cf9d | 606 | is never used for remote command execution. |
4fe2af09 | 607 | The default is |
bf740959 | 608 | .Dq no . |
bf740959 | 609 | .It Cm X11DisplayOffset |
610 | Specifies the first display number available for | |
611 | .Nm sshd Ns 's | |
4fe2af09 | 612 | X11 forwarding. |
613 | This prevents | |
bf740959 | 614 | .Nm |
615 | from interfering with real X11 servers. | |
c8d54615 | 616 | The default is 10. |
7b2ea3a1 | 617 | .It Cm X11Forwarding |
4fe2af09 | 618 | Specifies whether X11 forwarding is permitted. |
619 | The default is | |
c8d54615 | 620 | .Dq no . |
7b2ea3a1 | 621 | Note that disabling X11 forwarding does not improve security in any |
622 | way, as users can always install their own forwarders. | |
fa649821 | 623 | .It Cm XAuthLocation |
624 | Specifies the location of the | |
625 | .Xr xauth 1 | |
626 | program. | |
627 | The default is | |
628 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 629 | .El |
630 | .Sh LOGIN PROCESS | |
631 | When a user successfully logs in, | |
632 | .Nm | |
633 | does the following: | |
634 | .Bl -enum -offset indent | |
635 | .It | |
636 | If the login is on a tty, and no command has been specified, | |
f54651ce | 637 | prints last login time and |
bf740959 | 638 | .Pa /etc/motd |
639 | (unless prevented in the configuration file or by | |
640 | .Pa $HOME/.hushlogin ; | |
641 | see the | |
f54651ce | 642 | .Sx FILES |
bf740959 | 643 | section). |
644 | .It | |
645 | If the login is on a tty, records login time. | |
646 | .It | |
647 | Checks | |
648 | .Pa /etc/nologin ; | |
649 | if it exists, prints contents and quits | |
650 | (unless root). | |
651 | .It | |
652 | Changes to run with normal user privileges. | |
653 | .It | |
654 | Sets up basic environment. | |
655 | .It | |
656 | Reads | |
657 | .Pa $HOME/.ssh/environment | |
658 | if it exists. | |
659 | .It | |
660 | Changes to user's home directory. | |
661 | .It | |
662 | If | |
663 | .Pa $HOME/.ssh/rc | |
664 | exists, runs it; else if | |
5f4fdfae | 665 | .Pa /etc/sshrc |
bf740959 | 666 | exists, runs |
4fe2af09 | 667 | it; otherwise runs xauth. |
668 | The | |
bf740959 | 669 | .Dq rc |
670 | files are given the X11 | |
671 | authentication protocol and cookie in standard input. | |
672 | .It | |
673 | Runs user's shell or command. | |
674 | .El | |
675 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
f54651ce | 676 | The |
bf740959 | 677 | .Pa $HOME/.ssh/authorized_keys |
678 | file lists the RSA keys that are | |
1d1ffb87 | 679 | permitted for RSA authentication in SSH protocols 1.3 and 1.5 |
d0c832f3 | 680 | Similarly, the |
1d1ffb87 | 681 | .Pa $HOME/.ssh/authorized_keys2 |
682 | file lists the DSA keys that are | |
683 | permitted for DSA authentication in SSH protocol 2.0. | |
4fe2af09 | 684 | Each line of the file contains one |
bf740959 | 685 | key (empty lines and lines starting with a |
686 | .Ql # | |
687 | are ignored as | |
4fe2af09 | 688 | comments). |
689 | Each line consists of the following fields, separated by | |
690 | spaces: options, bits, exponent, modulus, comment. | |
691 | The options field | |
bf740959 | 692 | is optional; its presence is determined by whether the line starts |
693 | with a number or not (the option field never starts with a number). | |
694 | The bits, exponent, modulus and comment fields give the RSA key; the | |
695 | comment field is not used for anything (but may be convenient for the | |
696 | user to identify the key). | |
697 | .Pp | |
698 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 699 | (because of the size of the RSA key modulus). |
700 | You don't want to type them in; instead, copy the | |
bf740959 | 701 | .Pa identity.pub |
702 | file and edit it. | |
703 | .Pp | |
c345cf9d | 704 | The options (if present) consist of comma-separated option |
4fe2af09 | 705 | specifications. |
706 | No spaces are permitted, except within double quotes. | |
bf740959 | 707 | The following option specifications are supported: |
708 | .Bl -tag -width Ds | |
709 | .It Cm from="pattern-list" | |
710 | Specifies that in addition to RSA authentication, the canonical name | |
711 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 712 | patterns |
713 | .Pf ( Ql * | |
714 | and | |
715 | .Ql ? | |
716 | serve as wildcards). | |
717 | The list may also contain | |
718 | patterns negated by prefixing them with | |
719 | .Ql ! ; | |
720 | if the canonical host name matches a negated pattern, the key is not accepted. | |
721 | The purpose | |
bf740959 | 722 | of this option is to optionally increase security: RSA authentication |
723 | by itself does not trust the network or name servers or anything (but | |
724 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 725 | permits an intruder to log in from anywhere in the world. |
726 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 727 | servers and/or routers would have to be compromised in addition to |
728 | just the key). | |
729 | .It Cm command="command" | |
730 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 731 | authentication. |
732 | The command supplied by the user (if any) is ignored. | |
bf740959 | 733 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 734 | otherwise it is run without a tty. |
735 | A quote may be included in the command by quoting it with a backslash. | |
736 | This option might be useful | |
737 | to restrict certain RSA keys to perform just a specific operation. | |
738 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 739 | Note that the client may specify TCP/IP and/or X11 |
740 | forwarding unless they are explicitly prohibited. | |
bf740959 | 741 | .It Cm environment="NAME=value" |
742 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 743 | logging in using this key. |
744 | Environment variables set this way | |
745 | override other default environment values. | |
746 | Multiple options of this type are permitted. | |
bf740959 | 747 | .It Cm no-port-forwarding |
748 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 749 | Any port forward requests by the client will return an error. |
750 | This might be used, e.g., in connection with the | |
bf740959 | 751 | .Cm command |
752 | option. | |
753 | .It Cm no-X11-forwarding | |
754 | Forbids X11 forwarding when this key is used for authentication. | |
755 | Any X11 forward requests by the client will return an error. | |
756 | .It Cm no-agent-forwarding | |
757 | Forbids authentication agent forwarding when this key is used for | |
758 | authentication. | |
759 | .It Cm no-pty | |
760 | Prevents tty allocation (a request to allocate a pty will fail). | |
761 | .El | |
762 | .Ss Examples | |
763 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
764 | .Pp | |
765 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
766 | .Pp | |
767 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
768 | .Sh SSH_KNOWN_HOSTS FILE FORMAT | |
f54651ce | 769 | The |
1d1ffb87 | 770 | .Pa /etc/ssh_known_hosts , |
771 | .Pa /etc/ssh_known_hosts2 , | |
772 | .Pa $HOME/.ssh/known_hosts , | |
f54651ce | 773 | and |
1d1ffb87 | 774 | .Pa $HOME/.ssh/known_hosts2 |
4fe2af09 | 775 | files contain host public keys for all known hosts. |
776 | The global file should | |
777 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 778 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 779 | its key is added to the per-user file. |
bf740959 | 780 | .Pp |
781 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 782 | bits, exponent, modulus, comment. |
783 | The fields are separated by spaces. | |
bf740959 | 784 | .Pp |
785 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
786 | wildcards); each pattern in turn is matched against the canonical host | |
787 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 788 | name (when authenticating a server). |
789 | A pattern may also be preceded by | |
bf740959 | 790 | .Ql ! |
791 | to indicate negation: if the host name matches a negated | |
792 | pattern, it is not accepted (by that line) even if it matched another | |
793 | pattern on the line. | |
794 | .Pp | |
1d1ffb87 | 795 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 796 | can be obtained, e.g., from |
5f4fdfae | 797 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 798 | The optional comment field continues to the end of the line, and is not used. |
799 | .Pp | |
800 | Lines starting with | |
801 | .Ql # | |
802 | and empty lines are ignored as comments. | |
803 | .Pp | |
804 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 805 | matching line has the proper key. |
806 | It is thus permissible (but not | |
bf740959 | 807 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 808 | names. |
809 | This will inevitably happen when short forms of host names | |
810 | from different domains are put in the file. | |
811 | It is possible | |
bf740959 | 812 | that the files contain conflicting information; authentication is |
813 | accepted if valid information can be found from either file. | |
814 | .Pp | |
815 | Note that the lines in these files are typically hundreds of characters | |
816 | long, and you definitely don't want to type in the host keys by hand. | |
817 | Rather, generate them by a script | |
f54651ce | 818 | or by taking |
5f4fdfae | 819 | .Pa /etc/ssh_host_key.pub |
bf740959 | 820 | and adding the host names at the front. |
821 | .Ss Examples | |
822 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
823 | .Sh FILES | |
824 | .Bl -tag -width Ds | |
5f4fdfae | 825 | .It Pa /etc/sshd_config |
bf740959 | 826 | Contains configuration data for |
827 | .Nm sshd . | |
828 | This file should be writable by root only, but it is recommended | |
829 | (though not necessary) that it be world-readable. | |
5f4fdfae | 830 | .It Pa /etc/ssh_host_key |
bf740959 | 831 | Contains the private part of the host key. |
832 | This file should only be owned by root, readable only by root, and not | |
833 | accessible to others. | |
834 | Note that | |
835 | .Nm | |
836 | does not start if this file is group/world-accessible. | |
5f4fdfae | 837 | .It Pa /etc/ssh_host_key.pub |
bf740959 | 838 | Contains the public part of the host key. |
839 | This file should be world-readable but writable only by | |
4fe2af09 | 840 | root. |
841 | Its contents should match the private part. | |
842 | This file is not | |
bf740959 | 843 | really used for anything; it is only provided for the convenience of |
844 | the user so its contents can be copied to known hosts files. | |
845 | These two files are created using | |
846 | .Xr ssh-keygen 1 . | |
5f4fdfae | 847 | .It Pa /var/run/sshd.pid |
bf740959 | 848 | Contains the process ID of the |
849 | .Nm | |
850 | listening for connections (if there are several daemons running | |
851 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 852 | started last). |
c345cf9d | 853 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 854 | .It Pa $HOME/.ssh/authorized_keys |
855 | Lists the RSA keys that can be used to log into the user's account. | |
856 | This file must be readable by root (which may on some machines imply | |
857 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 858 | volume). |
859 | It is recommended that it not be accessible by others. | |
860 | The format of this file is described above. | |
1d1ffb87 | 861 | Users will place the contents of their |
862 | .Pa identity.pub | |
863 | files into this file, as described in | |
864 | .Xr ssh-keygen 1 . | |
865 | .It Pa $HOME/.ssh/authorized_keys2 | |
866 | Lists the DSA keys that can be used to log into the user's account. | |
867 | This file must be readable by root (which may on some machines imply | |
868 | it being world-readable if the user's home directory resides on an NFS | |
869 | volume). | |
870 | It is recommended that it not be accessible by others. | |
871 | The format of this file is described above. | |
872 | Users will place the contents of their | |
873 | .Pa id_dsa.pub | |
874 | files into this file, as described in | |
875 | .Xr ssh-keygen 1 . | |
5f4fdfae | 876 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 877 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 878 | authentication to check the public key of the host. |
879 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 880 | The client uses the same files |
c345cf9d | 881 | to verify that the remote host is the one it intended to connect. |
4fe2af09 | 882 | These files should be writable only by root/the owner. |
5f4fdfae | 883 | .Pa /etc/ssh_known_hosts |
bf740959 | 884 | should be world-readable, and |
885 | .Pa $HOME/.ssh/known_hosts | |
886 | can but need not be world-readable. | |
887 | .It Pa /etc/nologin | |
f54651ce | 888 | If this file exists, |
bf740959 | 889 | .Nm |
4fe2af09 | 890 | refuses to let anyone except root log in. |
891 | The contents of the file | |
bf740959 | 892 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 893 | refused. |
894 | The file should be world-readable. | |
bf740959 | 895 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
896 | If compiled with | |
897 | .Sy LIBWRAP | |
898 | support, tcp-wrappers access controls may be defined here as described in | |
899 | .Xr hosts_access 5 . | |
900 | .It Pa $HOME/.rhosts | |
901 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 902 | line. |
903 | The given user on the corresponding host is permitted to log in | |
904 | without password. | |
905 | The same file is used by rlogind and rshd. | |
bf740959 | 906 | The file must |
907 | be writable only by the user; it is recommended that it not be | |
908 | accessible by others. | |
909 | .Pp | |
4fe2af09 | 910 | If is also possible to use netgroups in the file. |
911 | Either host or user | |
bf740959 | 912 | name may be of the form +@groupname to specify all hosts or all users |
913 | in the group. | |
914 | .It Pa $HOME/.shosts | |
915 | For ssh, | |
916 | this file is exactly the same as for | |
917 | .Pa .rhosts . | |
918 | However, this file is | |
919 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 920 | .It Pa /etc/hosts.equiv |
bf740959 | 921 | This file is used during |
922 | .Pa .rhosts | |
4fe2af09 | 923 | authentication. |
924 | In the simplest form, this file contains host names, one per line. | |
925 | Users on | |
bf740959 | 926 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 927 | have the same user name on both machines. |
928 | The host name may also be | |
bf740959 | 929 | followed by a user name; such users are permitted to log in as |
930 | .Em any | |
4fe2af09 | 931 | user on this machine (except root). |
932 | Additionally, the syntax | |
bf740959 | 933 | .Dq +@group |
4fe2af09 | 934 | can be used to specify netgroups. |
935 | Negated entries start with | |
bf740959 | 936 | .Ql \&- . |
937 | .Pp | |
938 | If the client host/user is successfully matched in this file, login is | |
939 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 940 | same. |
941 | Additionally, successful RSA host authentication is normally required. | |
942 | This file must be writable only by root; it is recommended | |
bf740959 | 943 | that it be world-readable. |
944 | .Pp | |
945 | .Sy "Warning: It is almost never a good idea to use user names in" | |
946 | .Pa hosts.equiv . | |
947 | Beware that it really means that the named user(s) can log in as | |
948 | .Em anybody , | |
949 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 950 | binaries and directories. |
951 | Using a user name practically grants the user root access. | |
952 | The only valid use for user names that I can think | |
bf740959 | 953 | of is in negative entries. |
954 | .Pp | |
955 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 956 | .It Pa /etc/shosts.equiv |
bf740959 | 957 | This is processed exactly as |
958 | .Pa /etc/hosts.equiv . | |
959 | However, this file may be useful in environments that want to run both | |
960 | rsh/rlogin and ssh. | |
961 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 962 | This file is read into the environment at login (if it exists). |
963 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 964 | .Ql # ) , |
4fe2af09 | 965 | and assignment lines of the form name=value. |
966 | The file should be writable | |
bf740959 | 967 | only by the user; it need not be readable by anyone else. |
968 | .It Pa $HOME/.ssh/rc | |
969 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 970 | environment files but before starting the user's shell or command. |
971 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 972 | standard input (and |
973 | .Ev DISPLAY | |
4fe2af09 | 974 | in environment). |
975 | This must call | |
bf740959 | 976 | .Xr xauth 1 |
977 | in that case. | |
978 | .Pp | |
979 | The primary purpose of this file is to run any initialization routines | |
980 | which may be needed before the user's home directory becomes | |
981 | accessible; AFS is a particular example of such an environment. | |
982 | .Pp | |
983 | This file will probably contain some initialization code followed by | |
984 | something similar to: "if read proto cookie; then echo add $DISPLAY | |
985 | $proto $cookie | xauth -q -; fi". | |
986 | .Pp | |
987 | If this file does not exist, | |
5f4fdfae | 988 | .Pa /etc/sshrc |
bf740959 | 989 | is run, and if that |
990 | does not exist either, xauth is used to store the cookie. | |
991 | .Pp | |
992 | This file should be writable only by the user, and need not be | |
993 | readable by anyone else. | |
5f4fdfae | 994 | .It Pa /etc/sshrc |
bf740959 | 995 | Like |
996 | .Pa $HOME/.ssh/rc . | |
997 | This can be used to specify | |
4fe2af09 | 998 | machine-specific login-time initializations globally. |
999 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1000 | .El |
bf740959 | 1001 | .Sh AUTHOR |
bf740959 | 1002 | OpenSSH |
c8d54615 | 1003 | is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen, |
4fe2af09 | 1004 | but with bugs removed and newer features re-added. |
1005 | Rapidly after the | |
c8d54615 | 1006 | 1.2.12 release, newer versions of the original ssh bore successively |
1007 | more restrictive licenses, and thus demand for a free version was born. | |
1d1ffb87 | 1008 | .Pp |
c8d54615 | 1009 | This version of OpenSSH |
bf740959 | 1010 | .Bl -bullet |
1011 | .It | |
371ecff9 | 1012 | has all components of a restrictive nature (i.e., patents) |
bf740959 | 1013 | directly removed from the source code; any licensed or patented components |
1014 | are chosen from | |
1015 | external libraries. | |
1016 | .It | |
1d1ffb87 | 1017 | has been updated to support SSH protocol 1.5 and 2, making it compatible with |
1018 | all other SSH clients and servers. | |
bf740959 | 1019 | .It |
f54651ce | 1020 | contains added support for |
bf740959 | 1021 | .Xr kerberos 8 |
1022 | authentication and ticket passing. | |
1023 | .It | |
1024 | supports one-time password authentication with | |
1025 | .Xr skey 1 . | |
1026 | .El | |
1d1ffb87 | 1027 | .Pp |
1028 | OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, | |
1029 | Niels Provos, Theo de Raadt, and Dug Song. | |
1030 | .Pp | |
1031 | The support for SSH protocol 2 was written by Markus Friedl. | |
bf740959 | 1032 | .Sh SEE ALSO |
bf740959 | 1033 | .Xr scp 1 , |
1034 | .Xr ssh 1 , | |
1035 | .Xr ssh-add 1 , | |
1036 | .Xr ssh-agent 1 , | |
1037 | .Xr ssh-keygen 1 , | |
7368a6c8 | 1038 | .Xr rlogin 1 , |
1039 | .Xr rsh 1 |