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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 | .\" |
a5a2da3b | 37 | .\" $OpenBSD: sshd.8,v 1.128 2001/06/22 10:17:51 mpech 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 | |
ff14faf1 | 46 | .Op Fl deiqD46 |
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 |
5260325f | 54 | .Op Fl V Ar client_protocol_id |
f54651ce | 55 | .Sh DESCRIPTION |
bf740959 | 56 | .Nm |
2c86906e | 57 | (SSH Daemon) is the daemon program for |
bf740959 | 58 | .Xr ssh 1 . |
3189621b | 59 | Together these programs replace rlogin and rsh, and |
bf740959 | 60 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 61 | over an insecure network. |
62 | The programs are intended to be as easy to | |
bf740959 | 63 | install and use as possible. |
64 | .Pp | |
65 | .Nm | |
4fe2af09 | 66 | is the daemon that listens for connections from clients. |
f54651ce | 67 | It is normally started at boot from |
bf740959 | 68 | .Pa /etc/rc . |
69 | It forks a new | |
4fe2af09 | 70 | daemon for each incoming connection. |
71 | The forked daemons handle | |
bf740959 | 72 | key exchange, encryption, authentication, command execution, |
73 | and data exchange. | |
1d1ffb87 | 74 | This implementation of |
75 | .Nm | |
76 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 77 | .Nm |
4fe2af09 | 78 | works as follows. |
1d1ffb87 | 79 | .Pp |
80 | .Ss SSH protocol version 1 | |
81 | .Pp | |
4fe2af09 | 82 | Each host has a host-specific RSA key |
83 | (normally 1024 bits) used to identify the host. | |
84 | Additionally, when | |
bf740959 | 85 | the daemon starts, it generates a server RSA key (normally 768 bits). |
86 | This key is normally regenerated every hour if it has been used, and | |
87 | is never stored on disk. | |
88 | .Pp | |
3189621b | 89 | Whenever a client connects the daemon responds with its public |
90 | host and server keys. | |
4fe2af09 | 91 | The client compares the |
1d1ffb87 | 92 | RSA host key against its own database to verify that it has not changed. |
4fe2af09 | 93 | The client then generates a 256 bit random number. |
94 | It encrypts this | |
bf740959 | 95 | random number using both the host key and the server key, and sends |
4fe2af09 | 96 | the encrypted number to the server. |
3189621b | 97 | Both sides then use this |
bf740959 | 98 | random number as a session key which is used to encrypt all further |
4fe2af09 | 99 | communications in the session. |
100 | The rest of the session is encrypted | |
3189621b | 101 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
7368a6c8 | 102 | being used by default. |
4fe2af09 | 103 | The client selects the encryption algorithm |
bf740959 | 104 | to use from those offered by the server. |
105 | .Pp | |
4fe2af09 | 106 | Next, the server and the client enter an authentication dialog. |
107 | The client tries to authenticate itself using | |
bf740959 | 108 | .Pa .rhosts |
109 | authentication, | |
110 | .Pa .rhosts | |
111 | authentication combined with RSA host | |
112 | authentication, RSA challenge-response authentication, or password | |
113 | based authentication. | |
114 | .Pp | |
115 | Rhosts authentication is normally disabled | |
116 | because it is fundamentally insecure, but can be enabled in the server | |
4fe2af09 | 117 | configuration file if desired. |
118 | System security is not improved unless | |
bf740959 | 119 | .Xr rshd 8 , |
120 | .Xr rlogind 8 , | |
121 | .Xr rexecd 8 , | |
122 | and | |
123 | .Xr rexd 8 | |
124 | are disabled (thus completely disabling | |
125 | .Xr rlogin 1 | |
126 | and | |
127 | .Xr rsh 1 | |
3189621b | 128 | into the machine). |
bf740959 | 129 | .Pp |
1d1ffb87 | 130 | .Ss SSH protocol version 2 |
131 | .Pp | |
c345cf9d | 132 | Version 2 works similarly: |
1d1ffb87 | 133 | Each host has a host-specific DSA key used to identify the host. |
134 | However, when the daemon starts, it does not generate a server key. | |
135 | Forward security is provided through a Diffie-Hellman key agreement. | |
136 | This key agreement results in a shared session key. | |
da89cf4d | 137 | .Pp |
51c251f0 | 138 | The rest of the session is encrypted using a symmetric cipher, currently |
da89cf4d | 139 | 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. |
1d1ffb87 | 140 | The client selects the encryption algorithm |
141 | to use from those offered by the server. | |
142 | Additionally, session integrity is provided | |
d0c832f3 | 143 | through a cryptographic message authentication code |
1d1ffb87 | 144 | (hmac-sha1 or hmac-md5). |
145 | .Pp | |
146 | Protocol version 2 provides a public key based | |
da89cf4d | 147 | user (PubkeyAuthentication) or |
148 | client host (HostbasedAuthentication) authentication method, | |
149 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 150 | .Pp |
151 | .Ss Command execution and data forwarding | |
152 | .Pp | |
bf740959 | 153 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 154 | preparing the session is entered. |
155 | At this time the client may request | |
bf740959 | 156 | things like allocating a pseudo-tty, forwarding X11 connections, |
157 | forwarding TCP/IP connections, or forwarding the authentication agent | |
158 | connection over the secure channel. | |
159 | .Pp | |
160 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 161 | The sides then enter session mode. |
162 | In this mode, either side may send | |
bf740959 | 163 | data at any time, and such data is forwarded to/from the shell or |
164 | command on the server side, and the user terminal in the client side. | |
165 | .Pp | |
166 | When the user program terminates and all forwarded X11 and other | |
167 | connections have been closed, the server sends command exit status to | |
168 | the client, and both sides exit. | |
169 | .Pp | |
170 | .Nm | |
171 | can be configured using command-line options or a configuration | |
4fe2af09 | 172 | file. |
173 | Command-line options override values specified in the | |
bf740959 | 174 | configuration file. |
175 | .Pp | |
9d6b7add | 176 | .Nm |
177 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 178 | .Dv SIGHUP , |
a5a2da3b | 179 | by executing itself with the name it was started as, i.e., |
409edaba | 180 | .Pa /usr/sbin/sshd . |
9d6b7add | 181 | .Pp |
bf740959 | 182 | The options are as follows: |
183 | .Bl -tag -width Ds | |
184 | .It Fl b Ar bits | |
da89cf4d | 185 | Specifies the number of bits in the ephemeral protocol version 1 |
186 | server key (default 768). | |
bf740959 | 187 | .Pp |
188 | .It Fl d | |
4fe2af09 | 189 | Debug mode. |
190 | The server sends verbose debug output to the system | |
191 | log, and does not put itself in the background. | |
192 | The server also will not fork and will only process one connection. | |
193 | This option is only intended for debugging for the server. | |
da89cf4d | 194 | Multiple -d options increase the debugging level. |
94ec8c6b | 195 | Maximum is 3. |
da89cf4d | 196 | .It Fl e |
197 | When this option is specified, | |
198 | .Nm | |
199 | will send the output to the standard error instead of the system log. | |
bf740959 | 200 | .It Fl f Ar configuration_file |
4fe2af09 | 201 | Specifies the name of the configuration file. |
202 | The default is | |
5f4fdfae | 203 | .Pa /etc/sshd_config . |
bf740959 | 204 | .Nm |
205 | refuses to start if there is no configuration file. | |
206 | .It Fl g Ar login_grace_time | |
207 | Gives the grace time for clients to authenticate themselves (default | |
ee5e6612 | 208 | 600 seconds). |
4fe2af09 | 209 | If the client fails to authenticate the user within |
210 | this many seconds, the server disconnects and exits. | |
211 | A value of zero indicates no limit. | |
bf740959 | 212 | .It Fl h Ar host_key_file |
8abcdba4 | 213 | Specifies the file from which the host key is read (default |
5f4fdfae | 214 | .Pa /etc/ssh_host_key ) . |
bf740959 | 215 | This option must be given if |
216 | .Nm | |
217 | is not run as root (as the normal | |
218 | host file is normally not readable by anyone but root). | |
8abcdba4 | 219 | It is possible to have multiple host key files for |
da89cf4d | 220 | the different protocol versions and host key algorithms. |
bf740959 | 221 | .It Fl i |
222 | Specifies that | |
223 | .Nm | |
f54651ce | 224 | is being run from inetd. |
bf740959 | 225 | .Nm |
226 | is normally not run | |
227 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 228 | respond to the client, and this may take tens of seconds. |
229 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 230 | However, with small key sizes (e.g., 512) using |
bf740959 | 231 | .Nm |
232 | from inetd may | |
233 | be feasible. | |
234 | .It Fl k Ar key_gen_time | |
da89cf4d | 235 | Specifies how often the ephemeral protocol version 1 server key is |
236 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 237 | The motivation for regenerating the key fairly |
bf740959 | 238 | often is that the key is not stored anywhere, and after about an hour, |
239 | it becomes impossible to recover the key for decrypting intercepted | |
240 | communications even if the machine is cracked into or physically | |
4fe2af09 | 241 | seized. |
242 | A value of zero indicates that the key will never be regenerated. | |
bf740959 | 243 | .It Fl p Ar port |
244 | Specifies the port on which the server listens for connections | |
245 | (default 22). | |
246 | .It Fl q | |
4fe2af09 | 247 | Quiet mode. |
248 | Nothing is sent to the system log. | |
249 | Normally the beginning, | |
bf740959 | 250 | authentication, and termination of each connection is logged. |
c345cf9d | 251 | .It Fl u Ar len |
252 | This option is used to specify the size of the field | |
253 | in the | |
254 | .Li utmp | |
255 | structure that holds the remote host name. | |
256 | If the resolved host name is longer than | |
257 | .Ar len , | |
258 | the dotted decimal value will be used instead. | |
259 | This allows hosts with very long host names that | |
260 | overflow this field to still be uniquely identified. | |
261 | Specifying | |
262 | .Fl u0 | |
263 | indicates that only dotted decimal addresses | |
264 | should be put into the | |
265 | .Pa utmp | |
266 | file. | |
8abcdba4 | 267 | .It Fl D |
268 | When this option is specified | |
269 | .Nm | |
270 | will not detach and does not become a daemon. | |
271 | This allows easy monitoring of | |
272 | .Nm sshd . | |
48e671d5 | 273 | .It Fl 4 |
274 | Forces | |
275 | .Nm | |
276 | to use IPv4 addresses only. | |
277 | .It Fl 6 | |
278 | Forces | |
279 | .Nm | |
280 | to use IPv6 addresses only. | |
bf740959 | 281 | .El |
282 | .Sh CONFIGURATION FILE | |
283 | .Nm | |
f54651ce | 284 | reads configuration data from |
5f4fdfae | 285 | .Pa /etc/sshd_config |
bf740959 | 286 | (or the file specified with |
287 | .Fl f | |
4fe2af09 | 288 | on the command line). |
289 | The file contains keyword-value pairs, one per line. | |
290 | Lines starting with | |
bf740959 | 291 | .Ql # |
292 | and empty lines are interpreted as comments. | |
293 | .Pp | |
294 | The following keywords are possible. | |
295 | .Bl -tag -width Ds | |
296 | .It Cm AFSTokenPassing | |
4fe2af09 | 297 | Specifies whether an AFS token may be forwarded to the server. |
298 | Default is | |
bf740959 | 299 | .Dq yes . |
300 | .It Cm AllowGroups | |
76896b4e | 301 | This keyword can be followed by a list of group names, separated |
4fe2af09 | 302 | by spaces. |
303 | If specified, login is allowed only for users whose primary | |
c6a69271 | 304 | group or supplementary group list matches one of the patterns. |
bf740959 | 305 | .Ql \&* |
306 | and | |
307 | .Ql ? | |
308 | can be used as | |
4fe2af09 | 309 | wildcards in the patterns. |
c345cf9d | 310 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 311 | By default login is allowed regardless of the group list. |
bf740959 | 312 | .Pp |
33de75a3 | 313 | .It Cm AllowTcpForwarding |
314 | Specifies whether TCP forwarding is permitted. | |
315 | The default is | |
316 | .Dq yes . | |
317 | Note that disabling TCP forwarding does not improve security unless | |
318 | users are also denied shell access, as they can always install their | |
319 | own forwarders. | |
320 | .Pp | |
bf740959 | 321 | .It Cm AllowUsers |
76896b4e | 322 | This keyword can be followed by a list of user names, separated |
4fe2af09 | 323 | by spaces. |
324 | If specified, login is allowed only for users names that | |
bf740959 | 325 | match one of the patterns. |
326 | .Ql \&* | |
327 | and | |
328 | .Ql ? | |
329 | can be used as | |
4fe2af09 | 330 | wildcards in the patterns. |
c345cf9d | 331 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 332 | By default login is allowed regardless of the user name. |
bf740959 | 333 | .Pp |
c8445989 | 334 | .It Cm AuthorizedKeysFile |
335 | Specifies the file that contains the public RSA keys that can be used | |
336 | for RSA authentication in protocol version 1. | |
337 | .Cm AuthorizedKeysFile | |
338 | may contain tokens of the form %T which are substituted during connection | |
339 | set-up. The following tokens are defined; %% is replaces by a literal '%', | |
340 | %h is replaced by the home directory of the user being authenticated and | |
341 | %u is replaced by the username of that user. | |
342 | After expansion, | |
343 | .Cm AuthorizedKeysFile | |
f740438e | 344 | is taken to be an absolute path or one relative to the user's home |
c8445989 | 345 | directory. |
346 | The default is | |
347 | .Dq .ssh/authorized_keys | |
348 | .It Cm AuthorizedKeysFile2 | |
349 | Specifies the file that contains the public keys that can be used | |
350 | for public key authentication in protocol version 2. | |
351 | .Cm AuthorizedKeysFile2 | |
352 | may contain tokens of the form %T which are substituted during connection | |
353 | set-up. The following tokens are defined; %% is replaces by a literal '%', | |
354 | %h is replaced by the home directory of the user being authenticated and | |
355 | %u is replaced by the username of that user. | |
356 | After expansion, | |
357 | .Cm AuthorizedKeysFile2 | |
f740438e | 358 | is taken to be an absolute path or one relative to the user's home |
c8445989 | 359 | directory. |
360 | The default is | |
361 | .Dq .ssh/authorized_keys2 | |
eea39c02 | 362 | .It Cm Banner |
363 | In some jurisdictions, sending a warning message before authentication | |
364 | may be relevant for getting legal protection. | |
365 | The contents of the specified file are sent to the remote user before | |
366 | authentication is allowed. | |
367 | This option is only available for protocol version 2. | |
368 | .Pp | |
657297ff | 369 | .It Cm ChallengeResponseAuthentication |
370 | Specifies whether | |
371 | challenge response | |
372 | authentication is allowed. | |
373 | Currently there is only support for | |
374 | .Xr skey 1 | |
375 | authentication. | |
376 | The default is | |
377 | .Dq yes . | |
bf740959 | 378 | .It Cm CheckMail |
379 | Specifies whether | |
380 | .Nm | |
381 | should check for new mail for interactive logins. | |
382 | The default is | |
383 | .Dq no . | |
a18395da | 384 | .It Cm Ciphers |
385 | Specifies the ciphers allowed for protocol version 2. | |
386 | Multiple ciphers must be comma-separated. | |
387 | The default is | |
388 | .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. | |
3ffc6336 | 389 | .It Cm ClientAliveInterval |
390 | Sets a timeout interval in seconds after which if no data has been received | |
391 | from the client, | |
392 | .Nm | |
393 | will send a message through the encrypted | |
84fc17bf | 394 | channel to request a response from the client. |
395 | The default | |
3ffc6336 | 396 | is 0, indicating that these messages will not be sent to the client. |
84fc17bf | 397 | This option applies to protocol version 2 only. |
3ffc6336 | 398 | .It Cm ClientAliveCountMax |
399 | Sets the number of client alive messages (see above) which may be | |
400 | sent without | |
401 | .Nm | |
402 | receiving any messages back from the client. If this threshold is | |
403 | reached while client alive messages are being sent, | |
404 | .Nm | |
405 | will disconnect the client, terminating the session. It is important | |
406 | to note that the use of client alive messages is very different from | |
84fc17bf | 407 | .Cm Keepalive |
408 | (below). The client alive messages are sent through the | |
3ffc6336 | 409 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
84fc17bf | 410 | option enabled by |
411 | .Cm Keepalive | |
412 | is spoofable. You want to use the client | |
3ffc6336 | 413 | alive mechanism when you are basing something important on |
414 | clients having an active connection to the server. | |
84fc17bf | 415 | .Pp |
416 | The default value is 3. If you set | |
417 | .Cm ClientAliveInterval | |
3ffc6336 | 418 | (above) to 15, and leave this value at the default, unresponsive ssh clients |
419 | will be disconnected after approximately 45 seconds. | |
bf740959 | 420 | .It Cm DenyGroups |
421 | This keyword can be followed by a number of group names, separated | |
4fe2af09 | 422 | by spaces. |
c6a69271 | 423 | Users whose primary group or supplementary group list matches |
424 | one of the patterns aren't allowed to log in. | |
bf740959 | 425 | .Ql \&* |
426 | and | |
427 | .Ql ? | |
428 | can be used as | |
4fe2af09 | 429 | wildcards in the patterns. |
c345cf9d | 430 | Only group names are valid; a numerical group ID isn't recognized. |
c6a69271 | 431 | By default login is allowed regardless of the group list. |
bf740959 | 432 | .Pp |
433 | .It Cm DenyUsers | |
434 | This keyword can be followed by a number of user names, separated | |
4fe2af09 | 435 | by spaces. |
436 | Login is disallowed for user names that match one of the patterns. | |
bf740959 | 437 | .Ql \&* |
438 | and | |
439 | .Ql ? | |
4fe2af09 | 440 | can be used as wildcards in the patterns. |
c345cf9d | 441 | Only user names are valid; a numerical user ID isn't recognized. |
4fe2af09 | 442 | By default login is allowed regardless of the user name. |
1d1ffb87 | 443 | .It Cm GatewayPorts |
444 | Specifies whether remote hosts are allowed to connect to ports | |
445 | forwarded for the client. | |
446 | The argument must be | |
447 | .Dq yes | |
448 | or | |
449 | .Dq no . | |
450 | The default is | |
451 | .Dq no . | |
da89cf4d | 452 | .It Cm HostbasedAuthentication |
453 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
454 | with successful public key client host authentication is allowed | |
455 | (hostbased authentication). | |
456 | This option is similar to | |
457 | .Cm RhostsRSAAuthentication | |
458 | and applies to protocol version 2 only. | |
459 | The default is | |
460 | .Dq no . | |
bf740959 | 461 | .It Cm HostKey |
fa08c86b | 462 | Specifies the file containing the private host keys (default |
1d1ffb87 | 463 | .Pa /etc/ssh_host_key ) |
fa08c86b | 464 | used by SSH protocol versions 1 and 2. |
bf740959 | 465 | Note that |
466 | .Nm | |
42f11eb2 | 467 | will refuse to use a file if it is group/world-accessible. |
fa08c86b | 468 | It is possible to have multiple host key files. |
469 | .Dq rsa1 | |
470 | keys are used for version 1 and | |
471 | .Dq dsa | |
472 | or | |
473 | .Dq rsa | |
474 | are used for version 2 of the SSH protocol. | |
bf740959 | 475 | .It Cm IgnoreRhosts |
c8d54615 | 476 | Specifies that |
477 | .Pa .rhosts | |
f54651ce | 478 | and |
c8d54615 | 479 | .Pa .shosts |
da89cf4d | 480 | files will not be used in |
481 | .Cm RhostsAuthentication , | |
482 | .Cm RhostsRSAAuthentication | |
483 | or | |
484 | .Cm HostbasedAuthentication . | |
485 | .Pp | |
bf740959 | 486 | .Pa /etc/hosts.equiv |
487 | and | |
f54651ce | 488 | .Pa /etc/shosts.equiv |
4fe2af09 | 489 | are still used. |
f54651ce | 490 | The default is |
c8d54615 | 491 | .Dq yes . |
b4748e2f | 492 | .It Cm IgnoreUserKnownHosts |
493 | Specifies whether | |
494 | .Nm | |
495 | should ignore the user's | |
496 | .Pa $HOME/.ssh/known_hosts | |
497 | during | |
da89cf4d | 498 | .Cm RhostsRSAAuthentication |
499 | or | |
500 | .Cm HostbasedAuthentication . | |
b4748e2f | 501 | The default is |
502 | .Dq no . | |
bf740959 | 503 | .It Cm KeepAlive |
504 | Specifies whether the system should send keepalive messages to the | |
4fe2af09 | 505 | other side. |
506 | If they are sent, death of the connection or crash of one | |
507 | of the machines will be properly noticed. | |
508 | However, this means that | |
bf740959 | 509 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 510 | find it annoying. |
d0c832f3 | 511 | On the other hand, if keepalives are not sent, |
bf740959 | 512 | sessions may hang indefinitely on the server, leaving |
513 | .Dq ghost | |
514 | users and consuming server resources. | |
515 | .Pp | |
516 | The default is | |
517 | .Dq yes | |
518 | (to send keepalives), and the server will notice | |
4fe2af09 | 519 | if the network goes down or the client host reboots. |
520 | This avoids infinitely hanging sessions. | |
bf740959 | 521 | .Pp |
522 | To disable keepalives, the value should be set to | |
523 | .Dq no | |
524 | in both the server and the client configuration files. | |
525 | .It Cm KerberosAuthentication | |
4fe2af09 | 526 | Specifies whether Kerberos authentication is allowed. |
527 | This can be in the form of a Kerberos ticket, or if | |
bf740959 | 528 | .Cm PasswordAuthentication |
529 | is yes, the password provided by the user will be validated through | |
94ec8c6b | 530 | the Kerberos KDC. |
531 | To use this option, the server needs a | |
c345cf9d | 532 | Kerberos servtab which allows the verification of the KDC's identity. |
4fe2af09 | 533 | Default is |
bf740959 | 534 | .Dq yes . |
535 | .It Cm KerberosOrLocalPasswd | |
536 | If set then if password authentication through Kerberos fails then | |
537 | the password will be validated via any additional local mechanism | |
538 | such as | |
188adeb2 | 539 | .Pa /etc/passwd . |
4fe2af09 | 540 | Default is |
bf740959 | 541 | .Dq yes . |
542 | .It Cm KerberosTgtPassing | |
543 | Specifies whether a Kerberos TGT may be forwarded to the server. | |
f54651ce | 544 | Default is |
bf740959 | 545 | .Dq no , |
546 | as this only works when the Kerberos KDC is actually an AFS kaserver. | |
547 | .It Cm KerberosTicketCleanup | |
548 | Specifies whether to automatically destroy the user's ticket cache | |
4fe2af09 | 549 | file on logout. |
550 | Default is | |
bf740959 | 551 | .Dq yes . |
552 | .It Cm KeyRegenerationInterval | |
da89cf4d | 553 | In protocol version 1, the ephemeral server key is automatically regenerated |
554 | after this many seconds (if it has been used). | |
4fe2af09 | 555 | The purpose of regeneration is to prevent |
bf740959 | 556 | decrypting captured sessions by later breaking into the machine and |
4fe2af09 | 557 | stealing the keys. |
558 | The key is never stored anywhere. | |
559 | If the value is 0, the key is never regenerated. | |
560 | The default is 3600 (seconds). | |
bf740959 | 561 | .It Cm ListenAddress |
6e9944b8 | 562 | Specifies the local addresses |
da89cf4d | 563 | .Nm |
bf740959 | 564 | should listen on. |
6e9944b8 | 565 | The following forms may be used: |
566 | .Pp | |
567 | .Bl -item -offset indent -compact | |
568 | .It | |
569 | .Cm ListenAddress | |
b8a297f1 | 570 | .Sm off |
571 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
572 | .Sm on | |
6e9944b8 | 573 | .It |
574 | .Cm ListenAddress | |
b8a297f1 | 575 | .Sm off |
576 | .Ar host No | Ar IPv4_addr No : Ar port | |
577 | .Sm on | |
6e9944b8 | 578 | .It |
579 | .Cm ListenAddress | |
b8a297f1 | 580 | .Sm off |
581 | .Oo | |
582 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
583 | .Sm on | |
6e9944b8 | 584 | .El |
585 | .Pp | |
586 | If | |
b8a297f1 | 587 | .Ar port |
6e9944b8 | 588 | is not specified, |
da89cf4d | 589 | .Nm |
6e9944b8 | 590 | will listen on the address and all prior |
591 | .Cm Port | |
592 | options specified. The default is to listen on all local | |
593 | addresses. Multiple | |
594 | .Cm ListenAddress | |
595 | options are permitted. Additionally, any | |
596 | .Cm Port | |
597 | options must precede this option for non port qualified addresses. | |
bf740959 | 598 | .It Cm LoginGraceTime |
599 | The server disconnects after this time if the user has not | |
4fe2af09 | 600 | successfully logged in. |
601 | If the value is 0, there is no time limit. | |
bf740959 | 602 | The default is 600 (seconds). |
6a17f9c2 | 603 | .It Cm LogLevel |
604 | Gives the verbosity level that is used when logging messages from | |
605 | .Nm sshd . | |
606 | The possible values are: | |
59c97189 | 607 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
608 | The default is INFO. | |
6a17f9c2 | 609 | Logging with level DEBUG violates the privacy of users |
610 | and is not recommended. | |
b2552997 | 611 | .It Cm MACs |
612 | Specifies the available MAC (message authentication code) algorithms. | |
613 | The MAC algorithm is used in protocol version 2 | |
614 | for data integrity protection. | |
615 | Multiple algorithms must be comma-separated. | |
616 | The default is | |
47bf6266 | 617 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
089fbbd2 | 618 | .It Cm MaxStartups |
619 | Specifies the maximum number of concurrent unauthenticated connections to the | |
620 | .Nm | |
621 | daemon. | |
622 | Additional connections will be dropped until authentication succeeds or the | |
623 | .Cm LoginGraceTime | |
624 | expires for a connection. | |
625 | The default is 10. | |
c345cf9d | 626 | .Pp |
627 | Alternatively, random early drop can be enabled by specifying | |
628 | the three colon separated values | |
629 | .Dq start:rate:full | |
94ec8c6b | 630 | (e.g., "10:30:60"). |
c345cf9d | 631 | .Nm |
6f37606e | 632 | will refuse connection attempts with a probability of |
c345cf9d | 633 | .Dq rate/100 |
634 | (30%) | |
635 | if there are currently | |
636 | .Dq start | |
637 | (10) | |
638 | unauthenticated connections. | |
6f37606e | 639 | The probability increases linearly and all connection attempts |
c345cf9d | 640 | are refused if the number of unauthenticated connections reaches |
641 | .Dq full | |
642 | (60). | |
10f72868 | 643 | .It Cm PAMAuthenticationViaKbdInt |
644 | Specifies whether PAM challenge response authentication is allowed. This | |
645 | allows the use of most PAM challenge response authentication modules, but | |
646 | it will allow password authentication regardless of whether | |
647 | .Cm PasswordAuthentication | |
648 | is disabled. | |
649 | The default is | |
650 | .Dq no . | |
bf740959 | 651 | .It Cm PasswordAuthentication |
652 | Specifies whether password authentication is allowed. | |
653 | The default is | |
654 | .Dq yes . | |
655 | .It Cm PermitEmptyPasswords | |
656 | When password authentication is allowed, it specifies whether the | |
4fe2af09 | 657 | server allows login to accounts with empty password strings. |
658 | The default is | |
c8d54615 | 659 | .Dq no . |
bf740959 | 660 | .It Cm PermitRootLogin |
46f23b8d | 661 | Specifies whether root can login using |
bf740959 | 662 | .Xr ssh 1 . |
663 | The argument must be | |
664 | .Dq yes , | |
15853e93 | 665 | .Dq without-password , |
666 | .Dq forced-commands-only | |
bf740959 | 667 | or |
668 | .Dq no . | |
669 | The default is | |
670 | .Dq yes . | |
15853e93 | 671 | .Pp |
672 | If this option is set to | |
bf740959 | 673 | .Dq without-password |
15853e93 | 674 | password authentication is disabled for root. |
bf740959 | 675 | .Pp |
15853e93 | 676 | If this option is set to |
677 | .Dq forced-commands-only | |
678 | root login with public key authentication will be allowed, | |
679 | but only if the | |
bf740959 | 680 | .Ar command |
15853e93 | 681 | option has been specified |
bf740959 | 682 | (which may be useful for taking remote backups even if root login is |
15853e93 | 683 | normally not allowed). All other authentication methods are disabled |
684 | for root. | |
46f23b8d | 685 | .Pp |
686 | If this option is set to | |
687 | .Dq no | |
688 | root is not allowed to login. | |
0fbe8c74 | 689 | .It Cm PidFile |
690 | Specifies the file that contains the process identifier of the | |
691 | .Nm | |
692 | daemon. | |
693 | The default is | |
694 | .Pa /var/run/sshd.pid . | |
bf740959 | 695 | .It Cm Port |
696 | Specifies the port number that | |
697 | .Nm | |
4fe2af09 | 698 | listens on. |
699 | The default is 22. | |
48e671d5 | 700 | Multiple options of this type are permitted. |
da89cf4d | 701 | See also |
702 | .Cm ListenAddress . | |
4f4648f9 | 703 | .It Cm PrintLastLog |
704 | Specifies whether | |
705 | .Nm | |
706 | should print the date and time when the user last logged in. | |
707 | The default is | |
708 | .Dq yes . | |
bf740959 | 709 | .It Cm PrintMotd |
710 | Specifies whether | |
711 | .Nm | |
f54651ce | 712 | should print |
bf740959 | 713 | .Pa /etc/motd |
4fe2af09 | 714 | when a user logs in interactively. |
715 | (On some systems it is also printed by the shell, | |
bf740959 | 716 | .Pa /etc/profile , |
4fe2af09 | 717 | or equivalent.) |
718 | The default is | |
bf740959 | 719 | .Dq yes . |
f54651ce | 720 | .It Cm Protocol |
721 | Specifies the protocol versions | |
722 | .Nm | |
723 | should support. | |
724 | The possible values are | |
725 | .Dq 1 | |
726 | and | |
727 | .Dq 2 . | |
728 | Multiple versions must be comma-separated. | |
729 | The default is | |
6023325e | 730 | .Dq 2,1 . |
657297ff | 731 | .It Cm PubkeyAuthentication |
732 | Specifies whether public key authentication is allowed. | |
733 | The default is | |
734 | .Dq yes . | |
735 | Note that this option applies to protocol version 2 only. | |
61e96248 | 736 | .It Cm ReverseMappingCheck |
737 | Specifies whether | |
738 | .Nm | |
739 | should try to verify the remote host name and check that | |
740 | the resolved host name for the remote IP address maps back to the | |
741 | very same IP address. | |
742 | The default is | |
743 | .Dq no . | |
bf740959 | 744 | .It Cm RhostsAuthentication |
745 | Specifies whether authentication using rhosts or /etc/hosts.equiv | |
4fe2af09 | 746 | files is sufficient. |
747 | Normally, this method should not be permitted because it is insecure. | |
bf740959 | 748 | .Cm RhostsRSAAuthentication |
749 | should be used | |
750 | instead, because it performs RSA-based host authentication in addition | |
751 | to normal rhosts or /etc/hosts.equiv authentication. | |
752 | The default is | |
753 | .Dq no . | |
da89cf4d | 754 | This option applies to protocol version 1 only. |
bf740959 | 755 | .It Cm RhostsRSAAuthentication |
756 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
4fe2af09 | 757 | with successful RSA host authentication is allowed. |
758 | The default is | |
c8d54615 | 759 | .Dq no . |
da89cf4d | 760 | This option applies to protocol version 1 only. |
bf740959 | 761 | .It Cm RSAAuthentication |
4fe2af09 | 762 | Specifies whether pure RSA authentication is allowed. |
763 | The default is | |
bf740959 | 764 | .Dq yes . |
da89cf4d | 765 | This option applies to protocol version 1 only. |
bf740959 | 766 | .It Cm ServerKeyBits |
da89cf4d | 767 | Defines the number of bits in the ephemeral protocol version 1 server key. |
4fe2af09 | 768 | The minimum value is 512, and the default is 768. |
bf740959 | 769 | .It Cm StrictModes |
770 | Specifies whether | |
771 | .Nm | |
772 | should check file modes and ownership of the | |
4fe2af09 | 773 | user's files and home directory before accepting login. |
774 | This is normally desirable because novices sometimes accidentally leave their | |
775 | directory or files world-writable. | |
776 | The default is | |
bf740959 | 777 | .Dq yes . |
38c295d6 | 778 | .It Cm Subsystem |
94ec8c6b | 779 | Configures an external subsystem (e.g., file transfer daemon). |
780 | Arguments should be a subsystem name and a command to execute upon subsystem | |
781 | request. | |
b5e300c2 | 782 | The command |
783 | .Xr sftp-server 8 | |
784 | implements the | |
785 | .Dq sftp | |
786 | file transfer subsystem. | |
38c295d6 | 787 | By default no subsystems are defined. |
788 | Note that this option applies to protocol version 2 only. | |
bf740959 | 789 | .It Cm SyslogFacility |
790 | Gives the facility code that is used when logging messages from | |
791 | .Nm sshd . | |
792 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
4fe2af09 | 793 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
794 | The default is AUTH. | |
bf740959 | 795 | .It Cm UseLogin |
796 | Specifies whether | |
797 | .Xr login 1 | |
fa649821 | 798 | is used for interactive login sessions. |
ff027d84 | 799 | The default is |
800 | .Dq no . | |
fa649821 | 801 | Note that |
802 | .Xr login 1 | |
c345cf9d | 803 | is never used for remote command execution. |
ff027d84 | 804 | Note also, that if this is enabled, |
805 | .Cm X11Forwarding | |
806 | will be disabled because | |
807 | .Xr login 1 | |
808 | does not know how to handle | |
809 | .Xr xauth 1 | |
810 | cookies. | |
bf740959 | 811 | .It Cm X11DisplayOffset |
812 | Specifies the first display number available for | |
813 | .Nm sshd Ns 's | |
4fe2af09 | 814 | X11 forwarding. |
815 | This prevents | |
bf740959 | 816 | .Nm |
817 | from interfering with real X11 servers. | |
c8d54615 | 818 | The default is 10. |
7b2ea3a1 | 819 | .It Cm X11Forwarding |
4fe2af09 | 820 | Specifies whether X11 forwarding is permitted. |
821 | The default is | |
c8d54615 | 822 | .Dq no . |
7b2ea3a1 | 823 | Note that disabling X11 forwarding does not improve security in any |
824 | way, as users can always install their own forwarders. | |
ff027d84 | 825 | X11 forwarding is automatically disabled if |
826 | .Cm UseLogin | |
827 | is enabled. | |
fa649821 | 828 | .It Cm XAuthLocation |
829 | Specifies the location of the | |
830 | .Xr xauth 1 | |
831 | program. | |
832 | The default is | |
833 | .Pa /usr/X11R6/bin/xauth . | |
bf740959 | 834 | .El |
e2b1fb42 | 835 | .Ss Time Formats |
836 | .Pp | |
837 | .Nm | |
838 | command-line arguments and configuration file options that specify time | |
839 | may be expressed using a sequence of the form: | |
840 | .Sm off | |
841 | .Ar time Oo Ar qualifier Oc , | |
842 | .Sm on | |
843 | where | |
844 | .Ar time | |
845 | is a positive integer value and | |
846 | .Ar qualifier | |
847 | is one of the following: | |
848 | .Pp | |
849 | .Bl -tag -width Ds -compact -offset indent | |
850 | .It Cm <none> | |
851 | seconds | |
852 | .It Cm s | Cm S | |
853 | seconds | |
854 | .It Cm m | Cm M | |
855 | minutes | |
856 | .It Cm h | Cm H | |
857 | hours | |
858 | .It Cm d | Cm D | |
859 | days | |
860 | .It Cm w | Cm W | |
861 | weeks | |
862 | .El | |
863 | .Pp | |
864 | Each member of the sequence is added together to calculate | |
865 | the total time value. | |
866 | .Pp | |
867 | Time format examples: | |
868 | .Pp | |
869 | .Bl -tag -width Ds -compact -offset indent | |
870 | .It 600 | |
871 | 600 seconds (10 minutes) | |
872 | .It 10m | |
873 | 10 minutes | |
874 | .It 1h30m | |
875 | 1 hour 30 minutes (90 minutes) | |
876 | .El | |
bf740959 | 877 | .Sh LOGIN PROCESS |
878 | When a user successfully logs in, | |
879 | .Nm | |
880 | does the following: | |
881 | .Bl -enum -offset indent | |
882 | .It | |
883 | If the login is on a tty, and no command has been specified, | |
f54651ce | 884 | prints last login time and |
bf740959 | 885 | .Pa /etc/motd |
886 | (unless prevented in the configuration file or by | |
887 | .Pa $HOME/.hushlogin ; | |
888 | see the | |
f54651ce | 889 | .Sx FILES |
bf740959 | 890 | section). |
891 | .It | |
892 | If the login is on a tty, records login time. | |
893 | .It | |
894 | Checks | |
895 | .Pa /etc/nologin ; | |
896 | if it exists, prints contents and quits | |
897 | (unless root). | |
898 | .It | |
899 | Changes to run with normal user privileges. | |
900 | .It | |
901 | Sets up basic environment. | |
902 | .It | |
903 | Reads | |
904 | .Pa $HOME/.ssh/environment | |
905 | if it exists. | |
906 | .It | |
907 | Changes to user's home directory. | |
908 | .It | |
909 | If | |
910 | .Pa $HOME/.ssh/rc | |
911 | exists, runs it; else if | |
5f4fdfae | 912 | .Pa /etc/sshrc |
bf740959 | 913 | exists, runs |
4fe2af09 | 914 | it; otherwise runs xauth. |
915 | The | |
bf740959 | 916 | .Dq rc |
917 | files are given the X11 | |
918 | authentication protocol and cookie in standard input. | |
919 | .It | |
920 | Runs user's shell or command. | |
921 | .El | |
922 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 923 | .Pa $HOME/.ssh/authorized_keys |
c8445989 | 924 | is the default file that lists the RSA keys that are |
98143cfc | 925 | permitted for RSA authentication in protocol version 1. |
c8445989 | 926 | .Cm AuthorizedKeysFile |
927 | may be used to specify an alternative file. | |
928 | Similarly, | |
1d1ffb87 | 929 | .Pa $HOME/.ssh/authorized_keys2 |
c8445989 | 930 | is the default file that lists the DSA and RSA keys that are |
8abcdba4 | 931 | permitted for public key authentication (PubkeyAuthentication) |
da89cf4d | 932 | in protocol version 2. |
c8445989 | 933 | .Cm AuthorizedKeysFile2 |
934 | may be used to specify an alternative file. | |
8abcdba4 | 935 | .Pp |
4fe2af09 | 936 | Each line of the file contains one |
bf740959 | 937 | key (empty lines and lines starting with a |
938 | .Ql # | |
939 | are ignored as | |
4fe2af09 | 940 | comments). |
8abcdba4 | 941 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 942 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 943 | Each protocol version 2 public key consists of: |
944 | options, keytype, base64 encoded key, comment. | |
945 | The options fields | |
946 | are optional; its presence is determined by whether the line starts | |
bf740959 | 947 | with a number or not (the option field never starts with a number). |
8abcdba4 | 948 | The bits, exponent, modulus and comment fields give the RSA key for |
949 | protocol version 1; the | |
bf740959 | 950 | comment field is not used for anything (but may be convenient for the |
951 | user to identify the key). | |
8abcdba4 | 952 | For protocol version 2 the keytype is |
953 | .Dq ssh-dss | |
954 | or | |
955 | .Dq ssh-rsa . | |
bf740959 | 956 | .Pp |
957 | Note that lines in this file are usually several hundred bytes long | |
4fe2af09 | 958 | (because of the size of the RSA key modulus). |
959 | You don't want to type them in; instead, copy the | |
c0ecc314 | 960 | .Pa identity.pub , |
8abcdba4 | 961 | .Pa id_dsa.pub |
c0ecc314 | 962 | or the |
963 | .Pa id_rsa.pub | |
bf740959 | 964 | file and edit it. |
965 | .Pp | |
c345cf9d | 966 | The options (if present) consist of comma-separated option |
4fe2af09 | 967 | specifications. |
968 | No spaces are permitted, except within double quotes. | |
bf740959 | 969 | The following option specifications are supported: |
970 | .Bl -tag -width Ds | |
971 | .It Cm from="pattern-list" | |
972 | Specifies that in addition to RSA authentication, the canonical name | |
973 | of the remote host must be present in the comma-separated list of | |
4fe2af09 | 974 | patterns |
975 | .Pf ( Ql * | |
976 | and | |
977 | .Ql ? | |
978 | serve as wildcards). | |
979 | The list may also contain | |
980 | patterns negated by prefixing them with | |
981 | .Ql ! ; | |
982 | if the canonical host name matches a negated pattern, the key is not accepted. | |
983 | The purpose | |
bf740959 | 984 | of this option is to optionally increase security: RSA authentication |
985 | by itself does not trust the network or name servers or anything (but | |
986 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 987 | permits an intruder to log in from anywhere in the world. |
988 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 989 | servers and/or routers would have to be compromised in addition to |
990 | just the key). | |
991 | .It Cm command="command" | |
992 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 993 | authentication. |
994 | The command supplied by the user (if any) is ignored. | |
bf740959 | 995 | The command is run on a pty if the connection requests a pty; |
4fe2af09 | 996 | otherwise it is run without a tty. |
61e96248 | 997 | Note that if you want a 8-bit clean channel, |
998 | you must not request a pty or should specify | |
999 | .Cm no-pty . | |
4fe2af09 | 1000 | A quote may be included in the command by quoting it with a backslash. |
1001 | This option might be useful | |
1002 | to restrict certain RSA keys to perform just a specific operation. | |
1003 | An example might be a key that permits remote backups but nothing else. | |
d0c832f3 | 1004 | Note that the client may specify TCP/IP and/or X11 |
1005 | forwarding unless they are explicitly prohibited. | |
bf740959 | 1006 | .It Cm environment="NAME=value" |
1007 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 1008 | logging in using this key. |
1009 | Environment variables set this way | |
1010 | override other default environment values. | |
1011 | Multiple options of this type are permitted. | |
bf740959 | 1012 | .It Cm no-port-forwarding |
1013 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 1014 | Any port forward requests by the client will return an error. |
1015 | This might be used, e.g., in connection with the | |
bf740959 | 1016 | .Cm command |
1017 | option. | |
1018 | .It Cm no-X11-forwarding | |
1019 | Forbids X11 forwarding when this key is used for authentication. | |
1020 | Any X11 forward requests by the client will return an error. | |
1021 | .It Cm no-agent-forwarding | |
1022 | Forbids authentication agent forwarding when this key is used for | |
1023 | authentication. | |
1024 | .It Cm no-pty | |
1025 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 1026 | .It Cm permitopen="host:port" |
1027 | Limit local | |
1028 | .Li ``ssh -L'' | |
b2ae83b8 | 1029 | port forwarding such that it may only connect to the specified host and |
dc504afd | 1030 | port. Multiple |
1031 | .Cm permitopen | |
b2ae83b8 | 1032 | options may be applied separated by commas. No pattern matching is |
dc504afd | 1033 | performed on the specified hostnames, they must be literal domains or |
1034 | addresses. | |
bf740959 | 1035 | .El |
1036 | .Ss Examples | |
1037 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar | |
1038 | .Pp | |
1039 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula | |
1040 | .Pp | |
1041 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi | |
dc504afd | 1042 | .Pp |
1043 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 | |
bf740959 | 1044 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 1045 | The |
1d1ffb87 | 1046 | .Pa /etc/ssh_known_hosts , |
1047 | .Pa /etc/ssh_known_hosts2 , | |
1048 | .Pa $HOME/.ssh/known_hosts , | |
f54651ce | 1049 | and |
1d1ffb87 | 1050 | .Pa $HOME/.ssh/known_hosts2 |
4fe2af09 | 1051 | files contain host public keys for all known hosts. |
1052 | The global file should | |
1053 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 1054 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 1055 | its key is added to the per-user file. |
bf740959 | 1056 | .Pp |
1057 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 1058 | bits, exponent, modulus, comment. |
1059 | The fields are separated by spaces. | |
bf740959 | 1060 | .Pp |
1061 | Hostnames is a comma-separated list of patterns ('*' and '?' act as | |
1062 | wildcards); each pattern in turn is matched against the canonical host | |
1063 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 1064 | name (when authenticating a server). |
1065 | A pattern may also be preceded by | |
bf740959 | 1066 | .Ql ! |
1067 | to indicate negation: if the host name matches a negated | |
1068 | pattern, it is not accepted (by that line) even if it matched another | |
1069 | pattern on the line. | |
1070 | .Pp | |
1d1ffb87 | 1071 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 1072 | can be obtained, e.g., from |
5f4fdfae | 1073 | .Pa /etc/ssh_host_key.pub . |
bf740959 | 1074 | The optional comment field continues to the end of the line, and is not used. |
1075 | .Pp | |
1076 | Lines starting with | |
1077 | .Ql # | |
1078 | and empty lines are ignored as comments. | |
1079 | .Pp | |
1080 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 1081 | matching line has the proper key. |
1082 | It is thus permissible (but not | |
bf740959 | 1083 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 1084 | names. |
1085 | This will inevitably happen when short forms of host names | |
1086 | from different domains are put in the file. | |
1087 | It is possible | |
bf740959 | 1088 | that the files contain conflicting information; authentication is |
1089 | accepted if valid information can be found from either file. | |
1090 | .Pp | |
1091 | Note that the lines in these files are typically hundreds of characters | |
1092 | long, and you definitely don't want to type in the host keys by hand. | |
1093 | Rather, generate them by a script | |
f54651ce | 1094 | or by taking |
5f4fdfae | 1095 | .Pa /etc/ssh_host_key.pub |
bf740959 | 1096 | and adding the host names at the front. |
1097 | .Ss Examples | |
da89cf4d | 1098 | .Bd -literal |
1099 | closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi | |
1100 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= | |
1101 | .Ed | |
bf740959 | 1102 | .Sh FILES |
1103 | .Bl -tag -width Ds | |
5f4fdfae | 1104 | .It Pa /etc/sshd_config |
bf740959 | 1105 | Contains configuration data for |
1106 | .Nm sshd . | |
1107 | This file should be writable by root only, but it is recommended | |
1108 | (though not necessary) that it be world-readable. | |
b8dc87d3 | 1109 | .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key |
da89cf4d | 1110 | These three files contain the private parts of the host keys. |
b8dc87d3 | 1111 | These files should only be owned by root, readable only by root, and not |
bf740959 | 1112 | accessible to others. |
1113 | Note that | |
1114 | .Nm | |
1115 | does not start if this file is group/world-accessible. | |
b8dc87d3 | 1116 | .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub |
da89cf4d | 1117 | These three files contain the public parts of the host keys. |
b8dc87d3 | 1118 | These files should be world-readable but writable only by |
4fe2af09 | 1119 | root. |
b8dc87d3 | 1120 | Their contents should match the respective private parts. |
1121 | These files are not | |
1122 | really used for anything; they are provided for the convenience of | |
1123 | the user so their contents can be copied to known hosts files. | |
1124 | These files are created using | |
bf740959 | 1125 | .Xr ssh-keygen 1 . |
c523303b | 1126 | .It Pa /etc/primes |
1127 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". | |
5f4fdfae | 1128 | .It Pa /var/run/sshd.pid |
bf740959 | 1129 | Contains the process ID of the |
1130 | .Nm | |
1131 | listening for connections (if there are several daemons running | |
1132 | concurrently for different ports, this contains the pid of the one | |
4fe2af09 | 1133 | started last). |
c345cf9d | 1134 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 1135 | .It Pa $HOME/.ssh/authorized_keys |
1136 | Lists the RSA keys that can be used to log into the user's account. | |
1137 | This file must be readable by root (which may on some machines imply | |
1138 | it being world-readable if the user's home directory resides on an NFS | |
4fe2af09 | 1139 | volume). |
1140 | It is recommended that it not be accessible by others. | |
1141 | The format of this file is described above. | |
1d1ffb87 | 1142 | Users will place the contents of their |
1143 | .Pa identity.pub | |
1144 | files into this file, as described in | |
1145 | .Xr ssh-keygen 1 . | |
1146 | .It Pa $HOME/.ssh/authorized_keys2 | |
da89cf4d | 1147 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 1148 | This file must be readable by root (which may on some machines imply |
1149 | it being world-readable if the user's home directory resides on an NFS | |
1150 | volume). | |
1151 | It is recommended that it not be accessible by others. | |
1152 | The format of this file is described above. | |
1153 | Users will place the contents of their | |
1154 | .Pa id_dsa.pub | |
c0ecc314 | 1155 | and/or |
1156 | .Pa id_rsa.pub | |
1d1ffb87 | 1157 | files into this file, as described in |
1158 | .Xr ssh-keygen 1 . | |
5f4fdfae | 1159 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
6a17f9c2 | 1160 | These files are consulted when using rhosts with RSA host |
4fe2af09 | 1161 | authentication to check the public key of the host. |
1162 | The key must be listed in one of these files to be accepted. | |
5bbb5681 | 1163 | The client uses the same files |
e91c60f2 | 1164 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 1165 | These files should be writable only by root/the owner. |
5f4fdfae | 1166 | .Pa /etc/ssh_known_hosts |
bf740959 | 1167 | should be world-readable, and |
1168 | .Pa $HOME/.ssh/known_hosts | |
1169 | can but need not be world-readable. | |
da89cf4d | 1170 | .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2" |
1171 | These files are consulted when using protocol version 2 hostbased | |
1172 | authentication to check the public key of the host. | |
1173 | The key must be listed in one of these files to be accepted. | |
1174 | The client uses the same files | |
1175 | to verify that it is connecting to the correct remote host. | |
1176 | These files should be writable only by root/the owner. | |
1177 | .Pa /etc/ssh_known_hosts2 | |
1178 | should be world-readable, and | |
1179 | .Pa $HOME/.ssh/known_hosts2 | |
1180 | can but need not be world-readable. | |
bf740959 | 1181 | .It Pa /etc/nologin |
f54651ce | 1182 | If this file exists, |
bf740959 | 1183 | .Nm |
4fe2af09 | 1184 | refuses to let anyone except root log in. |
1185 | The contents of the file | |
bf740959 | 1186 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 1187 | refused. |
1188 | The file should be world-readable. | |
bf740959 | 1189 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
1190 | If compiled with | |
1191 | .Sy LIBWRAP | |
1192 | support, tcp-wrappers access controls may be defined here as described in | |
1193 | .Xr hosts_access 5 . | |
1194 | .It Pa $HOME/.rhosts | |
1195 | This file contains host-username pairs, separated by a space, one per | |
4fe2af09 | 1196 | line. |
1197 | The given user on the corresponding host is permitted to log in | |
1198 | without password. | |
1199 | The same file is used by rlogind and rshd. | |
bf740959 | 1200 | The file must |
1201 | be writable only by the user; it is recommended that it not be | |
1202 | accessible by others. | |
1203 | .Pp | |
4fe2af09 | 1204 | If is also possible to use netgroups in the file. |
1205 | Either host or user | |
bf740959 | 1206 | name may be of the form +@groupname to specify all hosts or all users |
1207 | in the group. | |
1208 | .It Pa $HOME/.shosts | |
1209 | For ssh, | |
1210 | this file is exactly the same as for | |
1211 | .Pa .rhosts . | |
1212 | However, this file is | |
1213 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 1214 | .It Pa /etc/hosts.equiv |
bf740959 | 1215 | This file is used during |
1216 | .Pa .rhosts | |
4fe2af09 | 1217 | authentication. |
1218 | In the simplest form, this file contains host names, one per line. | |
1219 | Users on | |
bf740959 | 1220 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 1221 | have the same user name on both machines. |
1222 | The host name may also be | |
bf740959 | 1223 | followed by a user name; such users are permitted to log in as |
1224 | .Em any | |
4fe2af09 | 1225 | user on this machine (except root). |
1226 | Additionally, the syntax | |
bf740959 | 1227 | .Dq +@group |
4fe2af09 | 1228 | can be used to specify netgroups. |
1229 | Negated entries start with | |
bf740959 | 1230 | .Ql \&- . |
1231 | .Pp | |
1232 | If the client host/user is successfully matched in this file, login is | |
1233 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 1234 | same. |
1235 | Additionally, successful RSA host authentication is normally required. | |
1236 | This file must be writable only by root; it is recommended | |
bf740959 | 1237 | that it be world-readable. |
1238 | .Pp | |
1239 | .Sy "Warning: It is almost never a good idea to use user names in" | |
1240 | .Pa hosts.equiv . | |
1241 | Beware that it really means that the named user(s) can log in as | |
1242 | .Em anybody , | |
1243 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 1244 | binaries and directories. |
1245 | Using a user name practically grants the user root access. | |
1246 | The only valid use for user names that I can think | |
bf740959 | 1247 | of is in negative entries. |
1248 | .Pp | |
1249 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 1250 | .It Pa /etc/shosts.equiv |
bf740959 | 1251 | This is processed exactly as |
1252 | .Pa /etc/hosts.equiv . | |
1253 | However, this file may be useful in environments that want to run both | |
1254 | rsh/rlogin and ssh. | |
1255 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 1256 | This file is read into the environment at login (if it exists). |
1257 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 1258 | .Ql # ) , |
4fe2af09 | 1259 | and assignment lines of the form name=value. |
1260 | The file should be writable | |
bf740959 | 1261 | only by the user; it need not be readable by anyone else. |
1262 | .It Pa $HOME/.ssh/rc | |
1263 | If this file exists, it is run with /bin/sh after reading the | |
4fe2af09 | 1264 | environment files but before starting the user's shell or command. |
1265 | If X11 spoofing is in use, this will receive the "proto cookie" pair in | |
bf740959 | 1266 | standard input (and |
1267 | .Ev DISPLAY | |
4fe2af09 | 1268 | in environment). |
1269 | This must call | |
bf740959 | 1270 | .Xr xauth 1 |
1271 | in that case. | |
1272 | .Pp | |
1273 | The primary purpose of this file is to run any initialization routines | |
1274 | which may be needed before the user's home directory becomes | |
1275 | accessible; AFS is a particular example of such an environment. | |
1276 | .Pp | |
1277 | This file will probably contain some initialization code followed by | |
da89cf4d | 1278 | something similar to: |
1279 | .Bd -literal | |
1280 | if read proto cookie; then | |
1281 | echo add $DISPLAY $proto $cookie | xauth -q - | |
1282 | fi | |
1283 | .Ed | |
bf740959 | 1284 | .Pp |
1285 | If this file does not exist, | |
5f4fdfae | 1286 | .Pa /etc/sshrc |
bf740959 | 1287 | is run, and if that |
1288 | does not exist either, xauth is used to store the cookie. | |
1289 | .Pp | |
1290 | This file should be writable only by the user, and need not be | |
1291 | readable by anyone else. | |
5f4fdfae | 1292 | .It Pa /etc/sshrc |
bf740959 | 1293 | Like |
1294 | .Pa $HOME/.ssh/rc . | |
1295 | This can be used to specify | |
4fe2af09 | 1296 | machine-specific login-time initializations globally. |
1297 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 1298 | .El |
fa08c86b | 1299 | .Sh AUTHORS |
7f5c4295 | 1300 | OpenSSH is a derivative of the original and free |
1301 | ssh 1.2.12 release by Tatu Ylonen. | |
1302 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1303 | Theo de Raadt and Dug Song | |
1304 | removed many bugs, re-added newer features and | |
1305 | created OpenSSH. | |
1306 | Markus Friedl contributed the support for SSH | |
1307 | protocol versions 1.5 and 2.0. | |
bf740959 | 1308 | .Sh SEE ALSO |
a5a2da3b | 1309 | .Xr rlogin 1 , |
1310 | .Xr rsh 1 , | |
bf740959 | 1311 | .Xr scp 1 , |
61e96248 | 1312 | .Xr sftp 1 , |
bf740959 | 1313 | .Xr ssh 1 , |
1314 | .Xr ssh-add 1 , | |
1315 | .Xr ssh-agent 1 , | |
1316 | .Xr ssh-keygen 1 , | |
a5a2da3b | 1317 | .Xr sftp-server 8 |
2cad6cef | 1318 | .Rs |
1319 | .%A T. Ylonen | |
1320 | .%A T. Kivinen | |
1321 | .%A M. Saarinen | |
1322 | .%A T. Rinne | |
1323 | .%A S. Lehtinen | |
1324 | .%T "SSH Protocol Architecture" | |
1325 | .%N draft-ietf-secsh-architecture-07.txt | |
1326 | .%D January 2001 | |
1327 | .%O work in progress material | |
1328 | .Re | |
da89cf4d | 1329 | .Rs |
1330 | .%A M. Friedl | |
1331 | .%A N. Provos | |
1332 | .%A W. A. Simpson | |
1333 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
1334 | .%N draft-ietf-secsh-dh-group-exchange-00.txt | |
1335 | .%D January 2001 | |
1336 | .%O work in progress material | |
1337 | .Re |