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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
bf740959 | 3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
bf740959 | 4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
5 | .\" All rights reserved | |
6 | .\" | |
bcbf86ec | 7 | .\" As far as I am concerned, the code I have written for this software |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
f3c7c613 | 13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
bcbf86ec | 16 | .\" |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
bf740959 | 25 | .\" |
bcbf86ec | 26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
bf740959 | 36 | .\" |
713d61f7 | 37 | .\" $OpenBSD: sshd.8,v 1.154 2001/11/07 22:12:01 markus Exp $ |
bf740959 | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD 8 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd | |
da89cf4d | 43 | .Nd OpenSSH SSH daemon |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm sshd | |
2043936f | 46 | .Op Fl deiqtD46 |
bf740959 | 47 | .Op Fl b Ar bits |
48 | .Op Fl f Ar config_file | |
49 | .Op Fl g Ar login_grace_time | |
50 | .Op Fl h Ar host_key_file | |
51 | .Op Fl k Ar key_gen_time | |
52 | .Op Fl p Ar port | |
c345cf9d | 53 | .Op Fl u Ar len |
f54651ce | 54 | .Sh DESCRIPTION |
bf740959 | 55 | .Nm |
2c86906e | 56 | (SSH Daemon) is the daemon program for |
bf740959 | 57 | .Xr ssh 1 . |
3189621b | 58 | Together these programs replace rlogin and rsh, and |
bf740959 | 59 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 60 | over an insecure network. |
61 | The programs are intended to be as easy to | |
bf740959 | 62 | install and use as possible. |
63 | .Pp | |
64 | .Nm | |
4fe2af09 | 65 | is the daemon that listens for connections from clients. |
f54651ce | 66 | It is normally started at boot from |
bf740959 | 67 | .Pa /etc/rc . |
68 | It forks a new | |
4fe2af09 | 69 | daemon for each incoming connection. |
70 | The forked daemons handle | |
bf740959 | 71 | key exchange, encryption, authentication, command execution, |
72 | and data exchange. | |
1d1ffb87 | 73 | This implementation of |
74 | .Nm | |
75 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 76 | .Nm |
4fe2af09 | 77 | works as follows. |
1d1ffb87 | 78 | .Pp |
79 | .Ss SSH protocol version 1 | |
80 | .Pp | |
4fe2af09 | 81 | Each host has a host-specific RSA key |
82 | (normally 1024 bits) used to identify the host. | |
83 | Additionally, when | |
bf740959 | 84 | the daemon starts, it generates a server RSA key (normally 768 bits). |
85 | This key is normally regenerated every hour if it has been used, and | |
86 | is never stored on disk. | |
87 | .Pp | |
3189621b | 88 | Whenever a client connects the daemon responds with its public |
89 | host and server keys. | |
4fe2af09 | 90 | The client compares the |
1d1ffb87 | 91 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 92 | The client then generates a 256 bit random number. |
93 | It encrypts this | |
bf740959 | 94 | random number using both the host key and the server key, and sends |
4fe2af09 | 95 | the encrypted number to the server. |
3189621b | 96 | Both sides then use this |
bf740959 | 97 | random number as a session key which is used to encrypt all further |
4fe2af09 | 98 | communications in the session. |
99 | The rest of the session is encrypted | |
3189621b | 100 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 101 | being used by default. |
4fe2af09 | 102 | The client selects the encryption algorithm |
bf740959 | 103 | to use from those offered by the server. |
104 | .Pp | |
4fe2af09 | 105 | Next, the server and the client enter an authentication dialog. |
106 | The client tries to authenticate itself using | |
bf740959 | 107 | .Pa .rhosts |
108 | authentication, | |
109 | .Pa .rhosts | |
110 | authentication combined with RSA host | |
111 | authentication, RSA challenge-response authentication, or password | |
112 | based authentication. | |
113 | .Pp | |
114 | Rhosts authentication is normally disabled | |
115 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 116 | configuration file if desired. |
117 | System security is not improved unless | |
bf740959 | 118 | .Xr rshd 8 , |
119 | .Xr rlogind 8 , | |
bf740959 | 120 | and |
9495bfc5 | 121 | .Xr rexecd 8 |
bf740959 | 122 | are disabled (thus completely disabling |
123 | .Xr rlogin 1 | |
124 | and | |
125 | .Xr rsh 1 | |
3189621b | 126 | into the machine). |
bf740959 | 127 | .Pp |
1d1ffb87 | 128 | .Ss SSH protocol version 2 |
129 | .Pp | |
c345cf9d | 130 | Version 2 works similarly: |
7f19f8bb | 131 | Each host has a host-specific key (RSA or DSA) used to identify the host. |
1d1ffb87 | 132 | However, when the daemon starts, it does not generate a server key. |
133 | Forward security is provided through a Diffie-Hellman key agreement. | |
134 | This key agreement results in a shared session key. | |
da89cf4d | 135 | .Pp |
51c251f0 | 136 | The rest of the session is encrypted using a symmetric cipher, currently |
da89cf4d | 137 | 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. |
1d1ffb87 | 138 | The client selects the encryption algorithm |
139 | to use from those offered by the server. | |
140 | Additionally, session integrity is provided | |
d0c832f3 | 141 | through a cryptographic message authentication code |
1d1ffb87 | 142 | (hmac-sha1 or hmac-md5). |
143 | .Pp | |
144 | Protocol version 2 provides a public key based | |
da89cf4d | 145 | user (PubkeyAuthentication) or |
146 | client host (HostbasedAuthentication) authentication method, | |
147 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 148 | .Pp |
149 | .Ss Command execution and data forwarding | |
150 | .Pp | |
bf740959 | 151 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 152 | preparing the session is entered. |
153 | At this time the client may request | |
bf740959 | 154 | things like allocating a pseudo-tty, forwarding X11 connections, |
155 | forwarding TCP/IP connections, or forwarding the authentication agent | |
156 | connection over the secure channel. | |
157 | .Pp | |
158 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 159 | The sides then enter session mode. |
160 | In this mode, either side may send | |
bf740959 | 161 | data at any time, and such data is forwarded to/from the shell or |
162 | command on the server side, and the user terminal in the client side. | |
163 | .Pp | |
164 | When the user program terminates and all forwarded X11 and other | |
165 | connections have been closed, the server sends command exit status to | |
166 | the client, and both sides exit. | |
167 | .Pp | |
168 | .Nm | |
169 | can be configured using command-line options or a configuration | |
4fe2af09 | 170 | file. |
171 | Command-line options override values specified in the | |
bf740959 | 172 | configuration file. |
173 | .Pp | |
9d6b7add | 174 | .Nm |
175 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 176 | .Dv SIGHUP , |
a5a2da3b | 177 | by executing itself with the name it was started as, i.e., |
409edaba | 178 | .Pa /usr/sbin/sshd . |
9d6b7add | 179 | .Pp |
bf740959 | 180 | The options are as follows: |
181 | .Bl -tag -width Ds | |
182 | .It Fl b Ar bits | |
da89cf4d | 183 | Specifies the number of bits in the ephemeral protocol version 1 |
184 | server key (default 768). | |
bf740959 | 185 | .It Fl d |
4fe2af09 | 186 | Debug mode. |
187 | The server sends verbose debug output to the system | |
188 | log, and does not put itself in the background. | |
189 | The server also will not fork and will only process one connection. | |
190 | This option is only intended for debugging for the server. | |
da89cf4d | 191 | Multiple -d options increase the debugging level. |
94ec8c6b | 192 | Maximum is 3. |
da89cf4d | 193 | .It Fl e |
194 | When this option is specified, | |
195 | .Nm | |
196 | will send the output to the standard error instead of the system log. | |
bf740959 | 197 | .It Fl f Ar configuration_file |
4fe2af09 | 198 | Specifies the name of the configuration file. |
199 | The default is | |
5f4fdfae | 200 | .Pa /etc/sshd_config . |
bf740959 | 201 | .Nm |
202 | refuses to start if there is no configuration file. | |
203 | .It Fl g Ar login_grace_time | |
204 | Gives the grace time for clients to authenticate themselves (default | |
ee5e6612 | 205 | 600 seconds). |
4fe2af09 | 206 | If the client fails to authenticate the user within |
207 | this many seconds, the server disconnects and exits. | |
208 | A value of zero indicates no limit. | |
bf740959 | 209 | .It Fl h Ar host_key_file |
8abcdba4 | 210 | Specifies the file from which the host key is read (default |
5f4fdfae | 211 | .Pa /etc/ssh_host_key ) . |
bf740959 | 212 | This option must be given if |
213 | .Nm | |
214 | is not run as root (as the normal | |
215 | host file is normally not readable by anyone but root). | |
8abcdba4 | 216 | It is possible to have multiple host key files for |
da89cf4d | 217 | the different protocol versions and host key algorithms. |
bf740959 | 218 | .It Fl i |
219 | Specifies that | |
220 | .Nm | |
f54651ce | 221 | is being run from inetd. |
bf740959 | 222 | .Nm |
223 | is normally not run | |
224 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 225 | respond to the client, and this may take tens of seconds. |
226 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 227 | However, with small key sizes (e.g., 512) using |
bf740959 | 228 | .Nm |
229 | from inetd may | |
230 | be feasible. | |
231 | .It Fl k Ar key_gen_time | |
da89cf4d | 232 | Specifies how often the ephemeral protocol version 1 server key is |
233 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 234 | The motivation for regenerating the key fairly |
bf740959 | 235 | often is that the key is not stored anywhere, and after about an hour, |
236 | it becomes impossible to recover the key for decrypting intercepted | |
237 | communications even if the machine is cracked into or physically | |
4fe2af09 | 238 | seized. |
239 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 240 | .It Fl p Ar port |
241 | Specifies the port on which the server listens for connections | |
242 | (default 22). | |
243 | .It Fl q | |
4fe2af09 | 244 | Quiet mode. |
245 | Nothing is sent to the system log. | |
246 | Normally the beginning, | |
bf740959 | 247 | authentication, and termination of each connection is logged. |
f87f09aa | 248 | .It Fl t |
249 | Test mode. | |
250 | Only check the validity of the configuration file and sanity of the keys. | |
251 | This is useful for updating | |
252 | .Nm | |
253 | reliably as configuration options may change. | |
c345cf9d | 254 | .It Fl u Ar len |
255 | This option is used to specify the size of the field | |
256 | in the | |
257 | .Li utmp | |
258 | structure that holds the remote host name. | |
259 | If the resolved host name is longer than | |
260 | .Ar len , | |
261 | the dotted decimal value will be used instead. | |
262 | This allows hosts with very long host names that | |
263 | overflow this field to still be uniquely identified. | |
264 | Specifying | |
265 | .Fl u0 | |
266 | indicates that only dotted decimal addresses | |
267 | should be put into the | |
268 | .Pa utmp | |
269 | file. | |
e675b851 | 270 | .Fl u0 |
271 | is also be used to prevent | |
272 | .Nm | |
273 | from making DNS requests unless the authentication | |
274 | mechanism or configuration requires it. | |
275 | Authentication mechanisms that may require DNS include | |
276 | .Cm RhostsAuthentication , | |
277 | .Cm RhostsRSAAuthentication , | |
278 | .Cm HostbasedAuthentication | |
279 | and using a | |
280 | .Cm from="pattern-list" | |
281 | option in a key file. | |
8abcdba4 | 282 | .It Fl D |
283 | When this option is specified | |
284 | .Nm | |
285 | will not detach and does not become a daemon. | |
286 | This allows easy monitoring of | |
287 | .Nm sshd . | |
48e671d5 | 288 | .It Fl 4 |
289 | Forces | |
290 | .Nm | |
291 | to use IPv4 addresses only. | |
292 | .It Fl 6 | |
293 | Forces | |
294 | .Nm | |
295 | to use IPv6 addresses only. | |
bf740959 | 296 | .El |
297 | .Sh CONFIGURATION FILE | |
298 | .Nm | |
f54651ce | 299 | reads configuration data from |
5f4fdfae | 300 | .Pa /etc/sshd_config |
bf740959 | 301 | (or the file specified with |
302 | .Fl f | |
4fe2af09 | 303 | on the command line). |
54bf768d | 304 | The file contains keyword-argument pairs, one per line. |
4fe2af09 | 305 | Lines starting with |
bf740959 | 306 | .Ql # |
307 | and empty lines are interpreted as comments. | |
308 | .Pp | |
54bf768d | 309 | The possible |
310 | keywords and their meanings are as follows (note that | |
311 | keywords are case-insensitive and arguments are case-sensitive): | |
bf740959 | 312 | .Bl -tag -width Ds |
313 | .It Cm AFSTokenPassing | |
4fe2af09 | 314 | Specifies whether an AFS token may be forwarded to the server. |
315 | Default is | |
bf740959 | 316 | .Dq yes . |
317 | .It Cm AllowGroups | |
76896b4e | 318 | This keyword can be followed by a list of group names, separated |
4fe2af09 | 319 | by spaces. |
320 | If specified, login is allowed only for users whose primary | |
c6a69271 | 321 | group or supplementary group list matches one of the patterns. |
bf740959 | 322 | .Ql \&* |
323 | and | |
324 | .Ql ? | |
325 | can be used as | |
4fe2af09 | 326 | wildcards in the patterns. |
91789042 | 327 | Only group names are valid; a numerical group ID is not recognized. |
c6a69271 | 328 | By default login is allowed regardless of the group list. |
bf740959 | 329 | .Pp |
33de75a3 | 330 | .It Cm AllowTcpForwarding |
331 | Specifies whether TCP forwarding is permitted. | |
332 | The default is | |
333 | .Dq yes . | |
334 | Note that disabling TCP forwarding does not improve security unless | |
335 | users are also denied shell access, as they can always install their | |
336 | own forwarders. | |
337 | .Pp | |
bf740959 | 338 | .It Cm AllowUsers |
76896b4e | 339 | This keyword can be followed by a list of user names, separated |
4fe2af09 | 340 | by spaces. |
341 | If specified, login is allowed only for users names that | |
bf740959 | 342 | match one of the patterns. |
343 | .Ql \&* | |
344 | and | |
345 | .Ql ? | |
346 | can be used as | |
4fe2af09 | 347 | wildcards in the patterns. |
91789042 | 348 | Only user names are valid; a numerical user ID is not recognized. |
4fe2af09 | 349 | By default login is allowed regardless of the user name. |
80f8f24f | 350 | If the pattern takes the form USER@HOST then USER and HOST |
91789042 | 351 | are separately checked, restricting logins to particular |
80f8f24f | 352 | users from particular hosts. |
bf740959 | 353 | .Pp |
c8445989 | 354 | .It Cm AuthorizedKeysFile |
7f19f8bb | 355 | Specifies the file that contains the public keys that can be used |
356 | for user authentication. | |
c8445989 | 357 | .Cm AuthorizedKeysFile |
358 | may contain tokens of the form %T which are substituted during connection | |
ae897d7c | 359 | set-up. The following tokens are defined: %% is replaced by a literal '%', |
c8445989 | 360 | %h is replaced by the home directory of the user being authenticated and |
361 | %u is replaced by the username of that user. | |
362 | After expansion, | |
363 | .Cm AuthorizedKeysFile | |
f740438e | 364 | is taken to be an absolute path or one relative to the user's home |
c8445989 | 365 | directory. |
366 | The default is | |
367 | .Dq .ssh/authorized_keys | |
eea39c02 | 368 | .It Cm Banner |
369 | In some jurisdictions, sending a warning message before authentication | |
370 | may be relevant for getting legal protection. | |
371 | The contents of the specified file are sent to the remote user before | |
372 | authentication is allowed. | |
373 | This option is only available for protocol version 2. | |
374 | .Pp | |
657297ff | 375 | .It Cm ChallengeResponseAuthentication |
248bad82 | 376 | Specifies whether challenge response authentication is allowed. |
377 | All authentication styles from | |
378 | .Xr login.conf 5 | |
379 | are supported. | |
657297ff | 380 | The default is |
381 | .Dq yes . | |
a18395da | 382 | .It Cm Ciphers |
383 | Specifies the ciphers allowed for protocol version 2. | |
384 | Multiple ciphers must be comma-separated. | |
385 | The default is | |
386 | .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. | |
3ffc6336 | 387 | .It Cm ClientAliveInterval |
388 | Sets a timeout interval in seconds after which if no data has been received | |
3730bb22 | 389 | from the client, |
3ffc6336 | 390 | .Nm |
391 | will send a message through the encrypted | |
84fc17bf | 392 | channel to request a response from the client. |
393 | The default | |
3ffc6336 | 394 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 395 | This option applies to protocol version 2 only. |
3ffc6336 | 396 | .It Cm ClientAliveCountMax |
397 | Sets the number of client alive messages (see above) which may be | |
398 | sent without | |
399 | .Nm | |
400 | receiving any messages back from the client. If this threshold is | |
3730bb22 | 401 | reached while client alive messages are being sent, |
3ffc6336 | 402 | .Nm |
403 | will disconnect the client, terminating the session. It is important | |
3730bb22 | 404 | to note that the use of client alive messages is very different from |
713d61f7 | 405 | .Cm KeepAlive |
84fc17bf | 406 | (below). The client alive messages are sent through the |
3ffc6336 | 407 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 408 | option enabled by |
713d61f7 | 409 | .Cm KeepAlive |
91789042 | 410 | is spoofable. The client alive mechanism is valuable when the client or |
411 | server depend on knowing when a connection has become inactive. | |
84fc17bf | 412 | .Pp |
91789042 | 413 | The default value is 3. If |
84fc17bf | 414 | .Cm ClientAliveInterval |
91789042 | 415 | (above) is set to 15, and |
fc7eca52 | 416 | .Cm ClientAliveCountMax |
417 | is left at the default, unresponsive ssh clients | |
3730bb22 | 418 | will be disconnected after approximately 45 seconds. |
bf740959 | 419 | .It Cm DenyGroups |
420 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 421 | by spaces. |
c6a69271 | 422 | Users whose primary group or supplementary group list matches |
423 | one of the patterns aren't allowed to log in. | |
bf740959 | 424 | .Ql \&* |
425 | and | |
426 | .Ql ? | |
427 | can be used as | |
4fe2af09 | 428 | wildcards in the patterns. |
91789042 | 429 | Only group names are valid; a numerical group ID is not recognized. |
c6a69271 | 430 | By default login is allowed regardless of the group list. |
bf740959 | 431 | .Pp |
432 | .It Cm DenyUsers | |
433 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 434 | by spaces. |
435 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 436 | .Ql \&* |
437 | and | |
438 | .Ql ? | |
4fe2af09 | 439 | can be used as wildcards in the patterns. |
91789042 | 440 | Only user names are valid; a numerical user ID is not recognized. |
4fe2af09 | 441 | By default login is allowed regardless of the user name. |
1d1ffb87 | 442 | .It Cm GatewayPorts |
443 | Specifies whether remote hosts are allowed to connect to ports | |
444 | forwarded for the client. | |
70068acc | 445 | By default, |
446 | .Nm | |
447 | binds remote port forwardings to the loopback addresss. This | |
448 | prevents other remote hosts from connecting to forwarded ports. | |
449 | .Cm GatewayPorts | |
450 | can be used to specify that | |
451 | .Nm | |
452 | should bind remote port forwardings to the wildcard address, | |
453 | thus allowing remote hosts to connect to forwarded ports. | |
1d1ffb87 | 454 | The argument must be |
455 | .Dq yes | |
456 | or | |
457 | .Dq no . | |
458 | The default is | |
459 | .Dq no . | |
da89cf4d | 460 | .It Cm HostbasedAuthentication |
461 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
462 | with successful public key client host authentication is allowed | |
463 | (hostbased authentication). | |
464 | This option is similar to | |
465 | .Cm RhostsRSAAuthentication | |
466 | and applies to protocol version 2 only. | |
467 | The default is | |
468 | .Dq no . | |
bf740959 | 469 | .It Cm HostKey |
fa08c86b | 470 | Specifies the file containing the private host keys (default |
1d1ffb87 | 471 | .Pa /etc/ssh_host_key ) |
fa08c86b | 472 | used by SSH protocol versions 1 and 2. |
bf740959 | 473 | Note that |
474 | .Nm | |
42f11eb2 | 475 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 476 | It is possible to have multiple host key files. |
477 | .Dq rsa1 | |
478 | keys are used for version 1 and | |
479 | .Dq dsa | |
480 | or | |
481 | .Dq rsa | |
482 | are used for version 2 of the SSH protocol. | |
bf740959 | 483 | .It Cm IgnoreRhosts |
c8d54615 | 484 | Specifies that |
485 | .Pa .rhosts | |
f54651ce | 486 | and |
c8d54615 | 487 | .Pa .shosts |
da89cf4d | 488 | files will not be used in |
489 | .Cm RhostsAuthentication , | |
490 | .Cm RhostsRSAAuthentication | |
491 | or | |
492 | .Cm HostbasedAuthentication . | |
493 | .Pp | |
bf740959 | 494 | .Pa /etc/hosts.equiv |
495 | and | |
f54651ce | 496 | .Pa /etc/shosts.equiv |
4fe2af09 | 497 | are still used. |
f54651ce | 498 | The default is |
c8d54615 | 499 | .Dq yes . |
b4748e2f | 500 | .It Cm IgnoreUserKnownHosts |
501 | Specifies whether | |
502 | .Nm | |
503 | should ignore the user's | |
504 | .Pa $HOME/.ssh/known_hosts | |
505 | during | |
da89cf4d | 506 | .Cm RhostsRSAAuthentication |
507 | or | |
508 | .Cm HostbasedAuthentication . | |
b4748e2f | 509 | The default is |
510 | .Dq no . | |
bf740959 | 511 | .It Cm KeepAlive |
512 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 513 | other side. |
514 | If they are sent, death of the connection or crash of one | |
515 | of the machines will be properly noticed. | |
516 | However, this means that | |
bf740959 | 517 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 518 | find it annoying. |
d0c832f3 | 519 | On the other hand, if keepalives are not sent, |
bf740959 | 520 | sessions may hang indefinitely on the server, leaving |
521 | .Dq ghost | |
522 | users and consuming server resources. | |
523 | .Pp | |
524 | The default is | |
525 | .Dq yes | |
526 | (to send keepalives), and the server will notice | |
4fe2af09 | 527 | if the network goes down or the client host reboots. |
528 | This avoids infinitely hanging sessions. | |
bf740959 | 529 | .Pp |
530 | To disable keepalives, the value should be set to | |
531 | .Dq no | |
532 | in both the server and the client configuration files. | |
533 | .It Cm KerberosAuthentication | |
4fe2af09 | 534 | Specifies whether Kerberos authentication is allowed. |
535 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 536 | .Cm PasswordAuthentication |
537 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 538 | the Kerberos KDC. |
539 | To use this option, the server needs a | |
c345cf9d | 540 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 541 | Default is |
bf740959 | 542 | .Dq yes . |
543 | .It Cm KerberosOrLocalPasswd | |
544 | If set then if password authentication through Kerberos fails then | |
545 | the password will be validated via any additional local mechanism | |
546 | such as | |
188adeb2 | 547 | .Pa /etc/passwd . |
4fe2af09 | 548 | Default is |
bf740959 | 549 | .Dq yes . |
550 | .It Cm KerberosTgtPassing | |
551 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 552 | Default is |
bf740959 | 553 | .Dq no , |
554 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
555 | .It Cm KerberosTicketCleanup | |
556 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 557 | file on logout. |
558 | Default is | |
bf740959 | 559 | .Dq yes . |
560 | .It Cm KeyRegenerationInterval | |
da89cf4d | 561 | In protocol version 1, the ephemeral server key is automatically regenerated |
562 | after this many seconds (if it has been used). | |
4fe2af09 | 563 | The purpose of regeneration is to prevent |
bf740959 | 564 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 565 | stealing the keys. |
566 | The key is never stored anywhere. | |
567 | If the value is 0, the key is never regenerated. | |
568 | The default is 3600 (seconds). | |
bf740959 | 569 | .It Cm ListenAddress |
6e9944b8 | 570 | Specifies the local addresses |
da89cf4d | 571 | .Nm |
bf740959 | 572 | should listen on. |
6e9944b8 | 573 | The following forms may be used: |
574 | .Pp | |
575 | .Bl -item -offset indent -compact | |
576 | .It | |
577 | .Cm ListenAddress | |
b8a297f1 | 578 | .Sm off |
579 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
580 | .Sm on | |
6e9944b8 | 581 | .It |
582 | .Cm ListenAddress | |
b8a297f1 | 583 | .Sm off |
584 | .Ar host No | Ar IPv4_addr No : Ar port | |
585 | .Sm on | |
6e9944b8 | 586 | .It |
587 | .Cm ListenAddress | |
b8a297f1 | 588 | .Sm off |
589 | .Oo | |
590 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
591 | .Sm on | |
6e9944b8 | 592 | .El |
593 | .Pp | |
594 | If | |
b8a297f1 | 595 | .Ar port |
6e9944b8 | 596 | is not specified, |
da89cf4d | 597 | .Nm |
6e9944b8 | 598 | will listen on the address and all prior |
599 | .Cm Port | |
600 | options specified. The default is to listen on all local | |
601 | addresses. Multiple | |
602 | .Cm ListenAddress | |
603 | options are permitted. Additionally, any | |
604 | .Cm Port | |
605 | options must precede this option for non port qualified addresses. | |
bf740959 | 606 | .It Cm LoginGraceTime |
607 | The server disconnects after this time if the user has not | |
4fe2af09 | 608 | successfully logged in. |
609 | If the value is 0, there is no time limit. | |
bf740959 | 610 | The default is 600 (seconds). |
6a17f9c2 | 611 | .It Cm LogLevel |
612 | Gives the verbosity level that is used when logging messages from | |
613 | .Nm sshd . | |
614 | The possible values are: | |
59c97189 | 615 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
616 | The default is INFO. | |
6a17f9c2 | 617 | Logging with level DEBUG violates the privacy of users |
618 | and is not recommended. | |
b2552997 | 619 | .It Cm MACs |
620 | Specifies the available MAC (message authentication code) algorithms. | |
621 | The MAC algorithm is used in protocol version 2 | |
622 | for data integrity protection. | |
623 | Multiple algorithms must be comma-separated. | |
624 | The default is | |
47bf6266 | 625 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
089fbbd2 | 626 | .It Cm MaxStartups |
627 | Specifies the maximum number of concurrent unauthenticated connections to the | |
628 | .Nm | |
629 | daemon. | |
630 | Additional connections will be dropped until authentication succeeds or the | |
631 | .Cm LoginGraceTime | |
632 | expires for a connection. | |
633 | The default is 10. | |
c345cf9d | 634 | .Pp |
635 | Alternatively, random early drop can be enabled by specifying | |
636 | the three colon separated values | |
637 | .Dq start:rate:full | |
94ec8c6b | 638 | (e.g., "10:30:60"). |
c345cf9d | 639 | .Nm |
6f37606e | 640 | will refuse connection attempts with a probability of |
c345cf9d | 641 | .Dq rate/100 |
642 | (30%) | |
643 | if there are currently | |
644 | .Dq start | |
645 | (10) | |
646 | unauthenticated connections. | |
6f37606e | 647 | The probability increases linearly and all connection attempts |
c345cf9d | 648 | are refused if the number of unauthenticated connections reaches |
649 | .Dq full | |
650 | (60). | |
10f72868 | 651 | .It Cm PAMAuthenticationViaKbdInt |
652 | Specifies whether PAM challenge response authentication is allowed. This | |
653 | allows the use of most PAM challenge response authentication modules, but | |
654 | it will allow password authentication regardless of whether | |
655 | .Cm PasswordAuthentication | |
656 | is disabled. | |
657 | The default is | |
658 | .Dq no . | |
bf740959 | 659 | .It Cm PasswordAuthentication |
660 | Specifies whether password authentication is allowed. | |
661 | The default is | |
662 | .Dq yes . | |
663 | .It Cm PermitEmptyPasswords | |
664 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 665 | server allows login to accounts with empty password strings. |
666 | The default is | |
c8d54615 | 667 | .Dq no . |
bf740959 | 668 | .It Cm PermitRootLogin |
46f23b8d | 669 | Specifies whether root can login using |
bf740959 | 670 | .Xr ssh 1 . |
671 | The argument must be | |
672 | .Dq yes , | |
15853e93 | 673 | .Dq without-password , |
674 | .Dq forced-commands-only | |
bf740959 | 675 | or |
676 | .Dq no . | |
677 | The default is | |
678 | .Dq yes . | |
15853e93 | 679 | .Pp |
680 | If this option is set to | |
bf740959 | 681 | .Dq without-password |
15853e93 | 682 | password authentication is disabled for root. |
bf740959 | 683 | .Pp |
15853e93 | 684 | If this option is set to |
685 | .Dq forced-commands-only | |
686 | root login with public key authentication will be allowed, | |
687 | but only if the | |
bf740959 | 688 | .Ar command |
15853e93 | 689 | option has been specified |
bf740959 | 690 | (which may be useful for taking remote backups even if root login is |
15853e93 | 691 | normally not allowed). All other authentication methods are disabled |
692 | for root. | |
46f23b8d | 693 | .Pp |
694 | If this option is set to | |
695 | .Dq no | |
696 | root is not allowed to login. | |
0fbe8c74 | 697 | .It Cm PidFile |
698 | Specifies the file that contains the process identifier of the | |
699 | .Nm | |
700 | daemon. | |
701 | The default is | |
702 | .Pa /var/run/sshd.pid . | |
bf740959 | 703 | .It Cm Port |
704 | Specifies the port number that | |
705 | .Nm | |
4fe2af09 | 706 | listens on. |
707 | The default is 22. | |
48e671d5 | 708 | Multiple options of this type are permitted. |
da89cf4d | 709 | See also |
710 | .Cm ListenAddress . | |
4f4648f9 | 711 | .It Cm PrintLastLog |
712 | Specifies whether | |
713 | .Nm | |
714 | should print the date and time when the user last logged in. | |
715 | The default is | |
716 | .Dq yes . | |
bf740959 | 717 | .It Cm PrintMotd |
718 | Specifies whether | |
719 | .Nm | |
f54651ce | 720 | should print |
bf740959 | 721 | .Pa /etc/motd |
4fe2af09 | 722 | when a user logs in interactively. |
723 | (On some systems it is also printed by the shell, | |
bf740959 | 724 | .Pa /etc/profile , |
4fe2af09 | 725 | or equivalent.) |
726 | The default is | |
bf740959 | 727 | .Dq yes . |
f54651ce | 728 | .It Cm Protocol |
729 | Specifies the protocol versions | |
730 | .Nm | |
731 | should support. | |
732 | The possible values are | |
733 | .Dq 1 | |
734 | and | |
735 | .Dq 2 . | |
736 | Multiple versions must be comma-separated. | |
737 | The default is | |
6023325e | 738 | .Dq 2,1 . |
657297ff | 739 | .It Cm PubkeyAuthentication |
740 | Specifies whether public key authentication is allowed. | |
741 | The default is | |
742 | .Dq yes . | |
743 | Note that this option applies to protocol version 2 only. | |
61e96248 | 744 | .It Cm ReverseMappingCheck |
745 | Specifies whether | |
746 | .Nm | |
747 | should try to verify the remote host name and check that | |
748 | the resolved host name for the remote IP address maps back to the | |
749 | very same IP address. | |
750 | The default is | |
751 | .Dq no . | |
bf740959 | 752 | .It Cm RhostsAuthentication |
753 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 754 | files is sufficient. |
755 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 756 | .Cm RhostsRSAAuthentication |
757 | should be used | |
758 | instead, because it performs RSA-based host authentication in addition | |
759 | to normal rhosts or /etc/hosts.equiv authentication. | |
760 | The default is | |
761 | .Dq no . | |
da89cf4d | 762 | This option applies to protocol version 1 only. |
bf740959 | 763 | .It Cm RhostsRSAAuthentication |
764 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 765 | with successful RSA host authentication is allowed. |
766 | The default is | |
c8d54615 | 767 | .Dq no . |
da89cf4d | 768 | This option applies to protocol version 1 only. |
bf740959 | 769 | .It Cm RSAAuthentication |
4fe2af09 | 770 | Specifies whether pure RSA authentication is allowed. |
771 | The default is | |
bf740959 | 772 | .Dq yes . |
da89cf4d | 773 | This option applies to protocol version 1 only. |
bf740959 | 774 | .It Cm ServerKeyBits |
da89cf4d | 775 | Defines the number of bits in the ephemeral protocol version 1 server key. |
4fe2af09 | 776 | The minimum value is 512, and the default is 768. |
bf740959 | 777 | .It Cm StrictModes |
778 | Specifies whether | |
779 | .Nm | |
780 | should check file modes and ownership of the | |
4fe2af09 | 781 | user's files and home directory before accepting login. |
782 | This is normally desirable because novices sometimes accidentally leave their | |
783 | directory or files world-writable. | |
784 | The default is | |
bf740959 | 785 | .Dq yes . |
38c295d6 | 786 | .It Cm Subsystem |
94ec8c6b | 787 | Configures an external subsystem (e.g., file transfer daemon). |
788 | Arguments should be a subsystem name and a command to execute upon subsystem | |
789 | request. | |
b5e300c2 | 790 | The command |
791 | .Xr sftp-server 8 | |
792 | implements the | |
793 | .Dq sftp | |
794 | file transfer subsystem. | |
38c295d6 | 795 | By default no subsystems are defined. |
796 | Note that this option applies to protocol version 2 only. | |
bf740959 | 797 | .It Cm SyslogFacility |
798 | Gives the facility code that is used when logging messages from | |
799 | .Nm sshd . | |
800 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 801 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
802 | The default is AUTH. | |
bf740959 | 803 | .It Cm UseLogin |
804 | Specifies whether | |
805 | .Xr login 1 | |
fa649821 | 806 | is used for interactive login sessions. |
ff027d84 | 807 | The default is |
808 | .Dq no . | |
fa649821 | 809 | Note that |
810 | .Xr login 1 | |
c345cf9d | 811 | is never used for remote command execution. |
3730bb22 | 812 | Note also, that if this is enabled, |
813 | .Cm X11Forwarding | |
ff027d84 | 814 | will be disabled because |
815 | .Xr login 1 | |
816 | does not know how to handle | |
3730bb22 | 817 | .Xr xauth 1 |
ff027d84 | 818 | cookies. |
bf740959 | 819 | .It Cm X11DisplayOffset |
820 | Specifies the first display number available for | |
821 | .Nm sshd Ns 's | |
4fe2af09 | 822 | X11 forwarding. |
823 | This prevents | |
bf740959 | 824 | .Nm |
825 | from interfering with real X11 servers. | |
c8d54615 | 826 | The default is 10. |
7b2ea3a1 | 827 | .It Cm X11Forwarding |
4fe2af09 | 828 | Specifies whether X11 forwarding is permitted. |
829 | The default is | |
c8d54615 | 830 | .Dq no . |
7b2ea3a1 | 831 | Note that disabling X11 forwarding does not improve security in any |
832 | way, as users can always install their own forwarders. | |
3730bb22 | 833 | X11 forwarding is automatically disabled if |
834 | .Cm UseLogin | |
835 | is enabled. | |
fa649821 | 836 | .It Cm XAuthLocation |
837 | Specifies the location of the | |
838 | .Xr xauth 1 | |
839 | program. | |
840 | The default is | |
841 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 842 | .El |
e2b1fb42 | 843 | .Ss Time Formats |
844 | .Pp | |
845 | .Nm | |
846 | command-line arguments and configuration file options that specify time | |
847 | may be expressed using a sequence of the form: | |
848 | .Sm off | |
849 | .Ar time Oo Ar qualifier Oc , | |
850 | .Sm on | |
851 | where | |
852 | .Ar time | |
853 | is a positive integer value and | |
854 | .Ar qualifier | |
855 | is one of the following: | |
856 | .Pp | |
857 | .Bl -tag -width Ds -compact -offset indent | |
858 | .It Cm <none> | |
859 | seconds | |
860 | .It Cm s | Cm S | |
861 | seconds | |
862 | .It Cm m | Cm M | |
863 | minutes | |
864 | .It Cm h | Cm H | |
865 | hours | |
866 | .It Cm d | Cm D | |
867 | days | |
868 | .It Cm w | Cm W | |
869 | weeks | |
870 | .El | |
871 | .Pp | |
872 | Each member of the sequence is added together to calculate | |
873 | the total time value. | |
874 | .Pp | |
875 | Time format examples: | |
876 | .Pp | |
877 | .Bl -tag -width Ds -compact -offset indent | |
878 | .It 600 | |
879 | 600 seconds (10 minutes) | |
880 | .It 10m | |
881 | 10 minutes | |
882 | .It 1h30m | |
883 | 1 hour 30 minutes (90 minutes) | |
884 | .El | |
bf740959 | 885 | .Sh LOGIN PROCESS |
886 | When a user successfully logs in, | |
887 | .Nm | |
888 | does the following: | |
889 | .Bl -enum -offset indent | |
890 | .It | |
891 | If the login is on a tty, and no command has been specified, | |
f54651ce | 892 | prints last login time and |
bf740959 | 893 | .Pa /etc/motd |
894 | (unless prevented in the configuration file or by | |
895 | .Pa $HOME/.hushlogin ; | |
896 | see the | |
f54651ce | 897 | .Sx FILES |
bf740959 | 898 | section). |
899 | .It | |
900 | If the login is on a tty, records login time. | |
901 | .It | |
902 | Checks | |
903 | .Pa /etc/nologin ; | |
904 | if it exists, prints contents and quits | |
905 | (unless root). | |
906 | .It | |
907 | Changes to run with normal user privileges. | |
908 | .It | |
909 | Sets up basic environment. | |
910 | .It | |
911 | Reads | |
912 | .Pa $HOME/.ssh/environment | |
913 | if it exists. | |
914 | .It | |
915 | Changes to user's home directory. | |
916 | .It | |
917 | If | |
918 | .Pa $HOME/.ssh/rc | |
919 | exists, runs it; else if | |
5f4fdfae | 920 | .Pa /etc/sshrc |
bf740959 | 921 | exists, runs |
4fe2af09 | 922 | it; otherwise runs xauth. |
923 | The | |
bf740959 | 924 | .Dq rc |
925 | files are given the X11 | |
926 | authentication protocol and cookie in standard input. | |
927 | .It | |
928 | Runs user's shell or command. | |
929 | .El | |
930 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 931 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 932 | is the default file that lists the public keys that are |
933 | permitted for RSA authentication in protocol version 1 | |
934 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 935 | in protocol version 2. |
96a7b0cc | 936 | .Cm AuthorizedKeysFile |
c8445989 | 937 | may be used to specify an alternative file. |
8abcdba4 | 938 | .Pp |
4fe2af09 | 939 | Each line of the file contains one |
bf740959 | 940 | key (empty lines and lines starting with a |
941 | .Ql # | |
942 | are ignored as | |
4fe2af09 | 943 | comments). |
8abcdba4 | 944 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 945 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 946 | Each protocol version 2 public key consists of: |
947 | options, keytype, base64 encoded key, comment. | |
948 | The options fields | |
949 | are optional; its presence is determined by whether the line starts | |
bf740959 | 950 | with a number or not (the option field never starts with a number). |
8abcdba4 | 951 | The bits, exponent, modulus and comment fields give the RSA key for |
952 | protocol version 1; the | |
bf740959 | 953 | comment field is not used for anything (but may be convenient for the |
954 | user to identify the key). | |
8abcdba4 | 955 | For protocol version 2 the keytype is |
956 | .Dq ssh-dss | |
957 | or | |
958 | .Dq ssh-rsa . | |
bf740959 | 959 | .Pp |
960 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 961 | (because of the size of the RSA key modulus). |
962 | You don't want to type them in; instead, copy the | |
c0ecc314 | 963 | .Pa identity.pub , |
8abcdba4 | 964 | .Pa id_dsa.pub |
c0ecc314 | 965 | or the |
966 | .Pa id_rsa.pub | |
bf740959 | 967 | file and edit it. |
968 | .Pp | |
c345cf9d | 969 | The options (if present) consist of comma-separated option |
4fe2af09 | 970 | specifications. |
971 | No spaces are permitted, except within double quotes. | |
54bf768d | 972 | The following option specifications are supported (note |
973 | that option keywords are case-insensitive): | |
bf740959 | 974 | .Bl -tag -width Ds |
975 | .It Cm from="pattern-list" | |
976 | Specifies that in addition to RSA authentication, the canonical name | |
977 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 978 | patterns |
979 | .Pf ( Ql * | |
980 | and | |
981 | .Ql ? | |
982 | serve as wildcards). | |
983 | The list may also contain | |
984 | patterns negated by prefixing them with | |
985 | .Ql ! ; | |
986 | if the canonical host name matches a negated pattern, the key is not accepted. | |
987 | The purpose | |
bf740959 | 988 | of this option is to optionally increase security: RSA authentication |
989 | by itself does not trust the network or name servers or anything (but | |
990 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 991 | permits an intruder to log in from anywhere in the world. |
992 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 993 | servers and/or routers would have to be compromised in addition to |
994 | just the key). | |
995 | .It Cm command="command" | |
996 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 997 | authentication. |
998 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 999 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 1000 | otherwise it is run without a tty. |
91789042 | 1001 | If a 8-bit clean channel is required, |
1002 | one must not request a pty or should specify | |
61e96248 | 1003 | .Cm no-pty . |
4fe2af09 | 1004 | A quote may be included in the command by quoting it with a backslash. |
1005 | This option might be useful | |
1006 | to restrict certain RSA keys to perform just a specific operation. | |
1007 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 1008 | Note that the client may specify TCP/IP and/or X11 |
1009 | forwarding unless they are explicitly prohibited. | |
4cdbc654 | 1010 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 1011 | .It Cm environment="NAME=value" |
1012 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 1013 | logging in using this key. |
1014 | Environment variables set this way | |
1015 | override other default environment values. | |
1016 | Multiple options of this type are permitted. | |
bf740959 | 1017 | .It Cm no-port-forwarding |
1018 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 1019 | Any port forward requests by the client will return an error. |
1020 | This might be used, e.g., in connection with the | |
bf740959 | 1021 | .Cm command |
1022 | option. | |
1023 | .It Cm no-X11-forwarding | |
1024 | Forbids X11 forwarding when this key is used for authentication. | |
1025 | Any X11 forward requests by the client will return an error. | |
1026 | .It Cm no-agent-forwarding | |
1027 | Forbids authentication agent forwarding when this key is used for | |
1028 | authentication. | |
1029 | .It Cm no-pty | |
1030 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 1031 | .It Cm permitopen="host:port" |
3730bb22 | 1032 | Limit local |
dc504afd | 1033 | .Li ``ssh -L'' |
b2ae83b8 | 1034 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 1035 | port. |
1036 | IPv6 addresses can be specified with an alternative syntax: | |
1037 | .Ar host/port . | |
1038 | Multiple | |
dc504afd | 1039 | .Cm permitopen |
3730bb22 | 1040 | options may be applied separated by commas. No pattern matching is |
1041 | performed on the specified hostnames, they must be literal domains or | |
dc504afd | 1042 | addresses. |
bf740959 | 1043 | .El |
1044 | .Ss Examples | |
1045 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
1046 | .Pp | |
1047 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
1048 | .Pp | |
1049 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 1050 | .Pp |
1051 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 1052 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 1053 | The |
1d1ffb87 | 1054 | .Pa /etc/ssh_known_hosts , |
f54651ce | 1055 | and |
f49bc4f7 | 1056 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 1057 | files contain host public keys for all known hosts. |
1058 | The global file should | |
1059 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 1060 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 1061 | its key is added to the per-user file. |
bf740959 | 1062 | .Pp |
1063 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 1064 | bits, exponent, modulus, comment. |
1065 | The fields are separated by spaces. | |
bf740959 | 1066 | .Pp |
1067 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
1068 | wildcards); each pattern in turn is matched against the canonical host | |
1069 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 1070 | name (when authenticating a server). |
1071 | A pattern may also be preceded by | |
bf740959 | 1072 | .Ql ! |
1073 | to indicate negation: if the host name matches a negated | |
1074 | pattern, it is not accepted (by that line) even if it matched another | |
1075 | pattern on the line. | |
1076 | .Pp | |
1d1ffb87 | 1077 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 1078 | can be obtained, e.g., from |
5f4fdfae | 1079 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 1080 | The optional comment field continues to the end of the line, and is not used. |
1081 | .Pp | |
1082 | Lines starting with | |
1083 | .Ql # | |
1084 | and empty lines are ignored as comments. | |
1085 | .Pp | |
1086 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 1087 | matching line has the proper key. |
1088 | It is thus permissible (but not | |
bf740959 | 1089 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 1090 | names. |
1091 | This will inevitably happen when short forms of host names | |
1092 | from different domains are put in the file. | |
1093 | It is possible | |
bf740959 | 1094 | that the files contain conflicting information; authentication is |
1095 | accepted if valid information can be found from either file. | |
1096 | .Pp | |
1097 | Note that the lines in these files are typically hundreds of characters | |
1098 | long, and you definitely don't want to type in the host keys by hand. | |
1099 | Rather, generate them by a script | |
f54651ce | 1100 | or by taking |
5f4fdfae | 1101 | .Pa /etc/ssh_host_key.pub |
bf740959 | 1102 | and adding the host names at the front. |
1103 | .Ss Examples | |
da89cf4d | 1104 | .Bd -literal |
1105 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
1106 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
1107 | .Ed | |
bf740959 | 1108 | .Sh FILES |
1109 | .Bl -tag -width Ds | |
5f4fdfae | 1110 | .It Pa /etc/sshd_config |
bf740959 | 1111 | Contains configuration data for |
1112 | .Nm sshd . | |
1113 | This file should be writable by root only, but it is recommended | |
1114 | (though not necessary) that it be world-readable. | |
b8dc87d3 | 1115 | .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
da89cf4d | 1116 | These three files contain the private parts of the host keys. |
b8dc87d3 | 1117 | These files should only be owned by root, readable only by root, and not |
bf740959 | 1118 | accessible to others. |
1119 | Note that | |
1120 | .Nm | |
1121 | does not start if this file is group/world-accessible. | |
b8dc87d3 | 1122 | .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
da89cf4d | 1123 | These three files contain the public parts of the host keys. |
b8dc87d3 | 1124 | These files should be world-readable but writable only by |
4fe2af09 | 1125 | root. |
b8dc87d3 | 1126 | Their contents should match the respective private parts. |
1127 | These files are not | |
1128 | really used for anything; they are provided for the convenience of | |
1129 | the user so their contents can be copied to known hosts files. | |
1130 | These files are created using | |
bf740959 | 1131 | .Xr ssh-keygen 1 . |
e2432638 | 1132 | .It Pa /etc/moduli |
c523303b | 1133 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
5f4fdfae | 1134 | .It Pa /var/run/sshd.pid |
bf740959 | 1135 | Contains the process ID of the |
1136 | .Nm | |
1137 | listening for connections (if there are several daemons running | |
1138 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1139 | started last). |
c345cf9d | 1140 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1141 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 1142 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 1143 | This file must be readable by root (which may on some machines imply |
1144 | it being world-readable if the user's home directory resides on an NFS | |
1145 | volume). | |
1146 | It is recommended that it not be accessible by others. | |
1147 | The format of this file is described above. | |
1148 | Users will place the contents of their | |
96a7b0cc | 1149 | .Pa identity.pub , |
1d1ffb87 | 1150 | .Pa id_dsa.pub |
c0ecc314 | 1151 | and/or |
1152 | .Pa id_rsa.pub | |
1d1ffb87 | 1153 | files into this file, as described in |
1154 | .Xr ssh-keygen 1 . | |
5f4fdfae | 1155 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1156 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 1157 | authentication or protocol version 2 hostbased authentication |
1158 | to check the public key of the host. | |
4fe2af09 | 1159 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 1160 | The client uses the same files |
e91c60f2 | 1161 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1162 | These files should be writable only by root/the owner. |
5f4fdfae | 1163 | .Pa /etc/ssh_known_hosts |
bf740959 | 1164 | should be world-readable, and |
1165 | .Pa $HOME/.ssh/known_hosts | |
1166 | can but need not be world-readable. | |
1167 | .It Pa /etc/nologin | |
f54651ce | 1168 | If this file exists, |
bf740959 | 1169 | .Nm |
4fe2af09 | 1170 | refuses to let anyone except root log in. |
1171 | The contents of the file | |
bf740959 | 1172 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1173 | refused. |
1174 | The file should be world-readable. | |
bf740959 | 1175 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
5b263aae | 1176 | Access controls that should be enforced by tcp-wrappers are defined here. |
1177 | Further details are described in | |
bf740959 | 1178 | .Xr hosts_access 5 . |
1179 | .It Pa $HOME/.rhosts | |
1180 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1181 | line. |
1182 | The given user on the corresponding host is permitted to log in | |
1183 | without password. | |
1184 | The same file is used by rlogind and rshd. | |
bf740959 | 1185 | The file must |
1186 | be writable only by the user; it is recommended that it not be | |
1187 | accessible by others. | |
1188 | .Pp | |
4fe2af09 | 1189 | If is also possible to use netgroups in the file. |
1190 | Either host or user | |
bf740959 | 1191 | name may be of the form +@groupname to specify all hosts or all users |
1192 | in the group. | |
1193 | .It Pa $HOME/.shosts | |
1194 | For ssh, | |
1195 | this file is exactly the same as for | |
1196 | .Pa .rhosts . | |
1197 | However, this file is | |
1198 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1199 | .It Pa /etc/hosts.equiv |
bf740959 | 1200 | This file is used during |
1201 | .Pa .rhosts | |
4fe2af09 | 1202 | authentication. |
1203 | In the simplest form, this file contains host names, one per line. | |
1204 | Users on | |
bf740959 | 1205 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1206 | have the same user name on both machines. |
1207 | The host name may also be | |
bf740959 | 1208 | followed by a user name; such users are permitted to log in as |
1209 | .Em any | |
4fe2af09 | 1210 | user on this machine (except root). |
1211 | Additionally, the syntax | |
bf740959 | 1212 | .Dq +@group |
4fe2af09 | 1213 | can be used to specify netgroups. |
1214 | Negated entries start with | |
bf740959 | 1215 | .Ql \&- . |
1216 | .Pp | |
1217 | If the client host/user is successfully matched in this file, login is | |
1218 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1219 | same. |
1220 | Additionally, successful RSA host authentication is normally required. | |
1221 | This file must be writable only by root; it is recommended | |
bf740959 | 1222 | that it be world-readable. |
1223 | .Pp | |
1224 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1225 | .Pa hosts.equiv . | |
1226 | Beware that it really means that the named user(s) can log in as | |
1227 | .Em anybody , | |
1228 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1229 | binaries and directories. |
1230 | Using a user name practically grants the user root access. | |
1231 | The only valid use for user names that I can think | |
bf740959 | 1232 | of is in negative entries. |
1233 | .Pp | |
1234 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1235 | .It Pa /etc/shosts.equiv |
bf740959 | 1236 | This is processed exactly as |
1237 | .Pa /etc/hosts.equiv . | |
1238 | However, this file may be useful in environments that want to run both | |
1239 | rsh/rlogin and ssh. | |
1240 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1241 | This file is read into the environment at login (if it exists). |
1242 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1243 | .Ql # ) , |
4fe2af09 | 1244 | and assignment lines of the form name=value. |
1245 | The file should be writable | |
bf740959 | 1246 | only by the user; it need not be readable by anyone else. |
1247 | .It Pa $HOME/.ssh/rc | |
1248 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1249 | environment files but before starting the user's shell or command. |
1250 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 1251 | standard input (and |
1252 | .Ev DISPLAY | |
4fe2af09 | 1253 | in environment). |
1254 | This must call | |
bf740959 | 1255 | .Xr xauth 1 |
1256 | in that case. | |
1257 | .Pp | |
1258 | The primary purpose of this file is to run any initialization routines | |
1259 | which may be needed before the user's home directory becomes | |
1260 | accessible; AFS is a particular example of such an environment. | |
1261 | .Pp | |
1262 | This file will probably contain some initialization code followed by | |
da89cf4d | 1263 | something similar to: |
1264 | .Bd -literal | |
1265 | if read proto cookie; then | |
1266 | echo add $DISPLAY $proto $cookie | xauth -q - | |
1267 | fi | |
1268 | .Ed | |
bf740959 | 1269 | .Pp |
1270 | If this file does not exist, | |
5f4fdfae | 1271 | .Pa /etc/sshrc |
bf740959 | 1272 | is run, and if that |
1273 | does not exist either, xauth is used to store the cookie. | |
1274 | .Pp | |
1275 | This file should be writable only by the user, and need not be | |
1276 | readable by anyone else. | |
5f4fdfae | 1277 | .It Pa /etc/sshrc |
bf740959 | 1278 | Like |
1279 | .Pa $HOME/.ssh/rc . | |
1280 | This can be used to specify | |
4fe2af09 | 1281 | machine-specific login-time initializations globally. |
1282 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1283 | .El |
fa08c86b | 1284 | .Sh AUTHORS |
7f5c4295 | 1285 | OpenSSH is a derivative of the original and free |
1286 | ssh 1.2.12 release by Tatu Ylonen. | |
1287 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1288 | Theo de Raadt and Dug Song | |
1289 | removed many bugs, re-added newer features and | |
1290 | created OpenSSH. | |
1291 | Markus Friedl contributed the support for SSH | |
1292 | protocol versions 1.5 and 2.0. | |
bf740959 | 1293 | .Sh SEE ALSO |
bf740959 | 1294 | .Xr scp 1 , |
61e96248 | 1295 | .Xr sftp 1 , |
bf740959 | 1296 | .Xr ssh 1 , |
1297 | .Xr ssh-add 1 , | |
1298 | .Xr ssh-agent 1 , | |
1299 | .Xr ssh-keygen 1 , | |
248bad82 | 1300 | .Xr login.conf 5 , |
1301 | .Xr moduli 5 , | |
a5a2da3b | 1302 | .Xr sftp-server 8 |
2cad6cef | 1303 | .Rs |
1304 | .%A T. Ylonen | |
1305 | .%A T. Kivinen | |
1306 | .%A M. Saarinen | |
1307 | .%A T. Rinne | |
1308 | .%A S. Lehtinen | |
1309 | .%T "SSH Protocol Architecture" | |
1930af48 | 1310 | .%N draft-ietf-secsh-architecture-09.txt |
1311 | .%D July 2001 | |
2cad6cef | 1312 | .%O work in progress material |
1313 | .Re | |
da89cf4d | 1314 | .Rs |
1315 | .%A M. Friedl | |
1316 | .%A N. Provos | |
1317 | .%A W. A. Simpson | |
1318 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
7d747e89 | 1319 | .%N draft-ietf-secsh-dh-group-exchange-01.txt |
1320 | .%D April 2001 | |
da89cf4d | 1321 | .%O work in progress material |
1322 | .Re |