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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 | .\" |
c79ae9fd | 37 | .\" $OpenBSD: sshd.8,v 1.206 2005/03/01 14:59:49 jmc 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 | |
6691e41b | 46 | .Bk -words |
433e60ac | 47 | .Op Fl 46Ddeiqt |
bf740959 | 48 | .Op Fl b Ar bits |
49 | .Op Fl f Ar config_file | |
50 | .Op Fl g Ar login_grace_time | |
51 | .Op Fl h Ar host_key_file | |
52 | .Op Fl k Ar key_gen_time | |
2717fa0f | 53 | .Op Fl o Ar option |
bf740959 | 54 | .Op Fl p Ar port |
c345cf9d | 55 | .Op Fl u Ar len |
6691e41b | 56 | .Ek |
f54651ce | 57 | .Sh DESCRIPTION |
bf740959 | 58 | .Nm |
2c86906e | 59 | (SSH Daemon) is the daemon program for |
bf740959 | 60 | .Xr ssh 1 . |
3189621b | 61 | Together these programs replace rlogin and rsh, and |
bf740959 | 62 | provide secure encrypted communications between two untrusted hosts |
4fe2af09 | 63 | over an insecure network. |
64 | The programs are intended to be as easy to | |
bf740959 | 65 | install and use as possible. |
66 | .Pp | |
67 | .Nm | |
4fe2af09 | 68 | is the daemon that listens for connections from clients. |
f54651ce | 69 | It is normally started at boot from |
bf740959 | 70 | .Pa /etc/rc . |
71 | It forks a new | |
4fe2af09 | 72 | daemon for each incoming connection. |
73 | The forked daemons handle | |
bf740959 | 74 | key exchange, encryption, authentication, command execution, |
75 | and data exchange. | |
1d1ffb87 | 76 | This implementation of |
77 | .Nm | |
78 | supports both SSH protocol version 1 and 2 simultaneously. | |
bf740959 | 79 | .Nm |
6691e41b | 80 | works as follows: |
1d1ffb87 | 81 | .Ss SSH protocol version 1 |
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 | |
6691e41b | 89 | Whenever a client connects, the daemon responds with its public |
3189621b | 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. |
433e60ac | 93 | The client then generates a 256-bit random number. |
4fe2af09 | 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 | |
433e60ac | 108 | .Em .rhosts |
bf740959 | 109 | authentication combined with RSA host |
110 | authentication, RSA challenge-response authentication, or password | |
111 | based authentication. | |
112 | .Pp | |
3e6e3da0 | 113 | Regardless of the authentication type, the account is checked to |
114 | ensure that it is accessible. An account is not accessible if it is | |
115 | locked, listed in | |
116 | .Cm DenyUsers | |
117 | or its group is listed in | |
118 | .Cm DenyGroups | |
119 | \&. The definition of a locked account is system dependant. Some platforms | |
120 | have their own account database (eg AIX) and some modify the passwd field ( | |
121 | .Ql \&*LK\&* | |
122 | on Solaris, | |
123 | .Ql \&* | |
124 | on HP-UX, containing | |
125 | .Ql Nologin | |
126 | on Tru64 and a leading | |
127 | .Ql \&!! | |
128 | on Linux). If there is a requirement to disable password authentication | |
129 | for the account while allowing still public-key, then the passwd field | |
130 | should be set to something other than these values (eg | |
131 | .Ql NP | |
132 | or | |
133 | .Ql \&*NP\&* | |
134 | ). | |
135 | .Pp | |
7e276482 | 136 | .Nm rshd , |
137 | .Nm rlogind , | |
bf740959 | 138 | and |
6691e41b | 139 | .Nm rexecd |
bf740959 | 140 | are disabled (thus completely disabling |
7e276482 | 141 | .Xr rlogin |
bf740959 | 142 | and |
7e276482 | 143 | .Xr rsh |
3189621b | 144 | into the machine). |
1d1ffb87 | 145 | .Ss SSH protocol version 2 |
c345cf9d | 146 | Version 2 works similarly: |
7f19f8bb | 147 | Each host has a host-specific key (RSA or DSA) used to identify the host. |
1d1ffb87 | 148 | However, when the daemon starts, it does not generate a server key. |
149 | Forward security is provided through a Diffie-Hellman key agreement. | |
150 | This key agreement results in a shared session key. | |
da89cf4d | 151 | .Pp |
51c251f0 | 152 | The rest of the session is encrypted using a symmetric cipher, currently |
433e60ac | 153 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. |
1d1ffb87 | 154 | The client selects the encryption algorithm |
155 | to use from those offered by the server. | |
156 | Additionally, session integrity is provided | |
d0c832f3 | 157 | through a cryptographic message authentication code |
1d1ffb87 | 158 | (hmac-sha1 or hmac-md5). |
159 | .Pp | |
160 | Protocol version 2 provides a public key based | |
da89cf4d | 161 | user (PubkeyAuthentication) or |
162 | client host (HostbasedAuthentication) authentication method, | |
163 | conventional password authentication and challenge response based methods. | |
1d1ffb87 | 164 | .Ss Command execution and data forwarding |
bf740959 | 165 | If the client successfully authenticates itself, a dialog for |
4fe2af09 | 166 | preparing the session is entered. |
167 | At this time the client may request | |
bf740959 | 168 | things like allocating a pseudo-tty, forwarding X11 connections, |
169 | forwarding TCP/IP connections, or forwarding the authentication agent | |
170 | connection over the secure channel. | |
171 | .Pp | |
172 | Finally, the client either requests a shell or execution of a command. | |
4fe2af09 | 173 | The sides then enter session mode. |
174 | In this mode, either side may send | |
bf740959 | 175 | data at any time, and such data is forwarded to/from the shell or |
176 | command on the server side, and the user terminal in the client side. | |
177 | .Pp | |
178 | When the user program terminates and all forwarded X11 and other | |
179 | connections have been closed, the server sends command exit status to | |
180 | the client, and both sides exit. | |
181 | .Pp | |
182 | .Nm | |
433e60ac | 183 | can be configured using command-line options or a configuration file |
184 | (by default | |
185 | .Xr sshd_config 5 ) . | |
4fe2af09 | 186 | Command-line options override values specified in the |
bf740959 | 187 | configuration file. |
188 | .Pp | |
9d6b7add | 189 | .Nm |
190 | rereads its configuration file when it receives a hangup signal, | |
409edaba | 191 | .Dv SIGHUP , |
7b7385da | 192 | by executing itself with the name and options it was started with, e.g., |
409edaba | 193 | .Pa /usr/sbin/sshd . |
9d6b7add | 194 | .Pp |
bf740959 | 195 | The options are as follows: |
196 | .Bl -tag -width Ds | |
433e60ac | 197 | .It Fl 4 |
198 | Forces | |
199 | .Nm | |
200 | to use IPv4 addresses only. | |
201 | .It Fl 6 | |
202 | Forces | |
203 | .Nm | |
204 | to use IPv6 addresses only. | |
bf740959 | 205 | .It Fl b Ar bits |
da89cf4d | 206 | Specifies the number of bits in the ephemeral protocol version 1 |
207 | server key (default 768). | |
433e60ac | 208 | .It Fl D |
209 | When this option is specified, | |
210 | .Nm | |
211 | will not detach and does not become a daemon. | |
212 | This allows easy monitoring of | |
213 | .Nm sshd . | |
bf740959 | 214 | .It Fl d |
4fe2af09 | 215 | Debug mode. |
216 | The server sends verbose debug output to the system | |
217 | log, and does not put itself in the background. | |
218 | The server also will not fork and will only process one connection. | |
219 | This option is only intended for debugging for the server. | |
6691e41b | 220 | Multiple |
221 | .Fl d | |
222 | options increase the debugging level. | |
94ec8c6b | 223 | Maximum is 3. |
da89cf4d | 224 | .It Fl e |
225 | When this option is specified, | |
226 | .Nm | |
227 | will send the output to the standard error instead of the system log. | |
bf740959 | 228 | .It Fl f Ar configuration_file |
4fe2af09 | 229 | Specifies the name of the configuration file. |
230 | The default is | |
2a8a6488 | 231 | .Pa /etc/ssh/sshd_config . |
bf740959 | 232 | .Nm |
233 | refuses to start if there is no configuration file. | |
234 | .It Fl g Ar login_grace_time | |
235 | Gives the grace time for clients to authenticate themselves (default | |
3445ca02 | 236 | 120 seconds). |
4fe2af09 | 237 | If the client fails to authenticate the user within |
238 | this many seconds, the server disconnects and exits. | |
239 | A value of zero indicates no limit. | |
bf740959 | 240 | .It Fl h Ar host_key_file |
0f84fe37 | 241 | Specifies a file from which a host key is read. |
bf740959 | 242 | This option must be given if |
243 | .Nm | |
244 | is not run as root (as the normal | |
0f84fe37 | 245 | host key files are normally not readable by anyone but root). |
246 | The default is | |
2a8a6488 | 247 | .Pa /etc/ssh/ssh_host_key |
0f84fe37 | 248 | for protocol version 1, and |
2a8a6488 | 249 | .Pa /etc/ssh/ssh_host_rsa_key |
0f84fe37 | 250 | and |
2a8a6488 | 251 | .Pa /etc/ssh/ssh_host_dsa_key |
0f84fe37 | 252 | for protocol version 2. |
8abcdba4 | 253 | It is possible to have multiple host key files for |
da89cf4d | 254 | the different protocol versions and host key algorithms. |
bf740959 | 255 | .It Fl i |
256 | Specifies that | |
257 | .Nm | |
6691e41b | 258 | is being run from |
259 | .Xr inetd 8 . | |
bf740959 | 260 | .Nm |
261 | is normally not run | |
262 | from inetd because it needs to generate the server key before it can | |
4fe2af09 | 263 | respond to the client, and this may take tens of seconds. |
264 | Clients would have to wait too long if the key was regenerated every time. | |
610cd5c6 | 265 | However, with small key sizes (e.g., 512) using |
bf740959 | 266 | .Nm |
267 | from inetd may | |
268 | be feasible. | |
269 | .It Fl k Ar key_gen_time | |
da89cf4d | 270 | Specifies how often the ephemeral protocol version 1 server key is |
271 | regenerated (default 3600 seconds, or one hour). | |
4fe2af09 | 272 | The motivation for regenerating the key fairly |
433e60ac | 273 | often is that the key is not stored anywhere, and after about an hour |
bf740959 | 274 | it becomes impossible to recover the key for decrypting intercepted |
275 | communications even if the machine is cracked into or physically | |
4fe2af09 | 276 | seized. |
277 | A value of zero indicates that the key will never be regenerated. | |
2717fa0f | 278 | .It Fl o Ar option |
279 | Can be used to give options in the format used in the configuration file. | |
280 | This is useful for specifying options for which there is no separate | |
281 | command-line flag. | |
433e60ac | 282 | For full details of the options, and their values, see |
283 | .Xr sshd_config 5 . | |
bf740959 | 284 | .It Fl p Ar port |
285 | Specifies the port on which the server listens for connections | |
286 | (default 22). | |
135113a3 | 287 | Multiple port options are permitted. |
288 | Ports specified in the configuration file are ignored when a | |
289 | command-line port is specified. | |
bf740959 | 290 | .It Fl q |
4fe2af09 | 291 | Quiet mode. |
292 | Nothing is sent to the system log. | |
293 | Normally the beginning, | |
bf740959 | 294 | authentication, and termination of each connection is logged. |
f87f09aa | 295 | .It Fl t |
296 | Test mode. | |
297 | Only check the validity of the configuration file and sanity of the keys. | |
184eed6a | 298 | This is useful for updating |
f87f09aa | 299 | .Nm |
300 | reliably as configuration options may change. | |
c345cf9d | 301 | .It Fl u Ar len |
302 | This option is used to specify the size of the field | |
303 | in the | |
304 | .Li utmp | |
305 | structure that holds the remote host name. | |
306 | If the resolved host name is longer than | |
307 | .Ar len , | |
308 | the dotted decimal value will be used instead. | |
309 | This allows hosts with very long host names that | |
310 | overflow this field to still be uniquely identified. | |
311 | Specifying | |
312 | .Fl u0 | |
313 | indicates that only dotted decimal addresses | |
314 | should be put into the | |
315 | .Pa utmp | |
316 | file. | |
e675b851 | 317 | .Fl u0 |
6691e41b | 318 | may also be used to prevent |
e675b851 | 319 | .Nm |
320 | from making DNS requests unless the authentication | |
321 | mechanism or configuration requires it. | |
322 | Authentication mechanisms that may require DNS include | |
e675b851 | 323 | .Cm RhostsRSAAuthentication , |
324 | .Cm HostbasedAuthentication | |
325 | and using a | |
326 | .Cm from="pattern-list" | |
327 | option in a key file. | |
f464aad8 | 328 | Configuration options that require DNS include using a |
329 | USER@HOST pattern in | |
330 | .Cm AllowUsers | |
331 | or | |
332 | .Cm DenyUsers . | |
bf740959 | 333 | .El |
334 | .Sh CONFIGURATION FILE | |
335 | .Nm | |
f54651ce | 336 | reads configuration data from |
2a8a6488 | 337 | .Pa /etc/ssh/sshd_config |
bf740959 | 338 | (or the file specified with |
339 | .Fl f | |
4fe2af09 | 340 | on the command line). |
588df31a | 341 | The file format and configuration options are described in |
342 | .Xr sshd_config 5 . | |
bf740959 | 343 | .Sh LOGIN PROCESS |
344 | When a user successfully logs in, | |
345 | .Nm | |
346 | does the following: | |
347 | .Bl -enum -offset indent | |
348 | .It | |
349 | If the login is on a tty, and no command has been specified, | |
f54651ce | 350 | prints last login time and |
bf740959 | 351 | .Pa /etc/motd |
352 | (unless prevented in the configuration file or by | |
353 | .Pa $HOME/.hushlogin ; | |
354 | see the | |
f54651ce | 355 | .Sx FILES |
bf740959 | 356 | section). |
357 | .It | |
358 | If the login is on a tty, records login time. | |
359 | .It | |
360 | Checks | |
361 | .Pa /etc/nologin ; | |
362 | if it exists, prints contents and quits | |
363 | (unless root). | |
364 | .It | |
365 | Changes to run with normal user privileges. | |
366 | .It | |
367 | Sets up basic environment. | |
368 | .It | |
433e60ac | 369 | Reads the file |
370 | .Pa $HOME/.ssh/environment , | |
371 | if it exists, and users are allowed to change their environment. | |
35453849 | 372 | See the |
6a342527 | 373 | .Cm PermitUserEnvironment |
35453849 | 374 | option in |
6a342527 | 375 | .Xr sshd_config 5 . |
bf740959 | 376 | .It |
377 | Changes to user's home directory. | |
378 | .It | |
379 | If | |
380 | .Pa $HOME/.ssh/rc | |
381 | exists, runs it; else if | |
af98ced9 | 382 | .Pa /etc/ssh/sshrc |
bf740959 | 383 | exists, runs |
4fe2af09 | 384 | it; otherwise runs xauth. |
385 | The | |
bf740959 | 386 | .Dq rc |
387 | files are given the X11 | |
388 | authentication protocol and cookie in standard input. | |
389 | .It | |
390 | Runs user's shell or command. | |
391 | .El | |
392 | .Sh AUTHORIZED_KEYS FILE FORMAT | |
bf740959 | 393 | .Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 394 | is the default file that lists the public keys that are |
395 | permitted for RSA authentication in protocol version 1 | |
396 | and for public key authentication (PubkeyAuthentication) | |
da89cf4d | 397 | in protocol version 2. |
96a7b0cc | 398 | .Cm AuthorizedKeysFile |
c8445989 | 399 | may be used to specify an alternative file. |
8abcdba4 | 400 | .Pp |
4fe2af09 | 401 | Each line of the file contains one |
bf740959 | 402 | key (empty lines and lines starting with a |
403 | .Ql # | |
404 | are ignored as | |
4fe2af09 | 405 | comments). |
8abcdba4 | 406 | Each RSA public key consists of the following fields, separated by |
4fe2af09 | 407 | spaces: options, bits, exponent, modulus, comment. |
8abcdba4 | 408 | Each protocol version 2 public key consists of: |
409 | options, keytype, base64 encoded key, comment. | |
755c4339 | 410 | The options field |
411 | is optional; its presence is determined by whether the line starts | |
412 | with a number or not (the options field never starts with a number). | |
8abcdba4 | 413 | The bits, exponent, modulus and comment fields give the RSA key for |
414 | protocol version 1; the | |
bf740959 | 415 | comment field is not used for anything (but may be convenient for the |
416 | user to identify the key). | |
8abcdba4 | 417 | For protocol version 2 the keytype is |
418 | .Dq ssh-dss | |
419 | or | |
420 | .Dq ssh-rsa . | |
bf740959 | 421 | .Pp |
422 | Note that lines in this file are usually several hundred bytes long | |
ea067773 | 423 | (because of the size of the public key encoding) up to a limit of |
424 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA | |
425 | keys up to 16 kilobits. | |
4fe2af09 | 426 | You don't want to type them in; instead, copy the |
c0ecc314 | 427 | .Pa identity.pub , |
8abcdba4 | 428 | .Pa id_dsa.pub |
c0ecc314 | 429 | or the |
430 | .Pa id_rsa.pub | |
bf740959 | 431 | file and edit it. |
432 | .Pp | |
3bc822df | 433 | .Nm |
434 | enforces a minimum RSA key modulus size for protocol 1 | |
435 | and protocol 2 keys of 768 bits. | |
436 | .Pp | |
c345cf9d | 437 | The options (if present) consist of comma-separated option |
4fe2af09 | 438 | specifications. |
439 | No spaces are permitted, except within double quotes. | |
54bf768d | 440 | The following option specifications are supported (note |
441 | that option keywords are case-insensitive): | |
bf740959 | 442 | .Bl -tag -width Ds |
443 | .It Cm from="pattern-list" | |
755c4339 | 444 | Specifies that in addition to public key authentication, the canonical name |
bf740959 | 445 | of the remote host must be present in the comma-separated list of |
4fe2af09 | 446 | patterns |
09dc8896 | 447 | .Pf ( Ql \&* |
4fe2af09 | 448 | and |
9a26a6e2 | 449 | .Ql \&? |
4fe2af09 | 450 | serve as wildcards). |
451 | The list may also contain | |
452 | patterns negated by prefixing them with | |
9a26a6e2 | 453 | .Ql \&! ; |
4fe2af09 | 454 | if the canonical host name matches a negated pattern, the key is not accepted. |
455 | The purpose | |
755c4339 | 456 | of this option is to optionally increase security: public key authentication |
bf740959 | 457 | by itself does not trust the network or name servers or anything (but |
458 | the key); however, if somebody somehow steals the key, the key | |
4fe2af09 | 459 | permits an intruder to log in from anywhere in the world. |
460 | This additional option makes using a stolen key more difficult (name | |
bf740959 | 461 | servers and/or routers would have to be compromised in addition to |
462 | just the key). | |
463 | .It Cm command="command" | |
464 | Specifies that the command is executed whenever this key is used for | |
4fe2af09 | 465 | authentication. |
466 | The command supplied by the user (if any) is ignored. | |
9658ecbc | 467 | The command is run on a pty if the client requests a pty; |
4fe2af09 | 468 | otherwise it is run without a tty. |
6691e41b | 469 | If an 8-bit clean channel is required, |
91789042 | 470 | one must not request a pty or should specify |
61e96248 | 471 | .Cm no-pty . |
4fe2af09 | 472 | A quote may be included in the command by quoting it with a backslash. |
473 | This option might be useful | |
755c4339 | 474 | to restrict certain public keys to perform just a specific operation. |
4fe2af09 | 475 | An example might be a key that permits remote backups but nothing else. |
d0c832f3 | 476 | Note that the client may specify TCP/IP and/or X11 |
477 | forwarding unless they are explicitly prohibited. | |
4cdbc654 | 478 | Note that this option applies to shell, command or subsystem execution. |
bf740959 | 479 | .It Cm environment="NAME=value" |
480 | Specifies that the string is to be added to the environment when | |
4fe2af09 | 481 | logging in using this key. |
482 | Environment variables set this way | |
483 | override other default environment values. | |
484 | Multiple options of this type are permitted. | |
35453849 | 485 | Environment processing is disabled by default and is |
486 | controlled via the | |
487 | .Cm PermitUserEnvironment | |
488 | option. | |
2548961d | 489 | This option is automatically disabled if |
490 | .Cm UseLogin | |
491 | is enabled. | |
bf740959 | 492 | .It Cm no-port-forwarding |
493 | Forbids TCP/IP forwarding when this key is used for authentication. | |
4fe2af09 | 494 | Any port forward requests by the client will return an error. |
495 | This might be used, e.g., in connection with the | |
bf740959 | 496 | .Cm command |
497 | option. | |
498 | .It Cm no-X11-forwarding | |
499 | Forbids X11 forwarding when this key is used for authentication. | |
500 | Any X11 forward requests by the client will return an error. | |
501 | .It Cm no-agent-forwarding | |
502 | Forbids authentication agent forwarding when this key is used for | |
503 | authentication. | |
504 | .It Cm no-pty | |
505 | Prevents tty allocation (a request to allocate a pty will fail). | |
dc504afd | 506 | .It Cm permitopen="host:port" |
3730bb22 | 507 | Limit local |
dc504afd | 508 | .Li ``ssh -L'' |
b2ae83b8 | 509 | port forwarding such that it may only connect to the specified host and |
ed787d14 | 510 | port. |
511 | IPv6 addresses can be specified with an alternative syntax: | |
433e60ac | 512 | .Ar host Ns / Ns Ar port . |
ed787d14 | 513 | Multiple |
dc504afd | 514 | .Cm permitopen |
3cbc677d | 515 | options may be applied separated by commas. |
516 | No pattern matching is performed on the specified hostnames, | |
517 | they must be literal domains or addresses. | |
bf740959 | 518 | .El |
519 | .Ss Examples | |
433e60ac | 520 | 1024 33 12121...312314325 ylo@foo.bar |
bf740959 | 521 | .Pp |
433e60ac | 522 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula |
bf740959 | 523 | .Pp |
433e60ac | 524 | command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi |
dc504afd | 525 | .Pp |
433e60ac | 526 | permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323 |
bf740959 | 527 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
f54651ce | 528 | The |
6691e41b | 529 | .Pa /etc/ssh/ssh_known_hosts |
f54651ce | 530 | and |
f49bc4f7 | 531 | .Pa $HOME/.ssh/known_hosts |
4fe2af09 | 532 | files contain host public keys for all known hosts. |
533 | The global file should | |
534 | be prepared by the administrator (optional), and the per-user file is | |
c345cf9d | 535 | maintained automatically: whenever the user connects from an unknown host |
4fe2af09 | 536 | its key is added to the per-user file. |
bf740959 | 537 | .Pp |
538 | Each line in these files contains the following fields: hostnames, | |
4fe2af09 | 539 | bits, exponent, modulus, comment. |
540 | The fields are separated by spaces. | |
bf740959 | 541 | .Pp |
09dc8896 | 542 | Hostnames is a comma-separated list of patterns |
3cbc677d | 543 | .Pf ( Ql \&* |
544 | and | |
545 | .Ql \&? | |
9a26a6e2 | 546 | act as |
bf740959 | 547 | wildcards); each pattern in turn is matched against the canonical host |
548 | name (when authenticating a client) or against the user-supplied | |
4fe2af09 | 549 | name (when authenticating a server). |
550 | A pattern may also be preceded by | |
9a26a6e2 | 551 | .Ql \&! |
bf740959 | 552 | to indicate negation: if the host name matches a negated |
553 | pattern, it is not accepted (by that line) even if it matched another | |
554 | pattern on the line. | |
555 | .Pp | |
5c63c2ab | 556 | Alternately, hostnames may be stored in a hashed form which hides host names |
c79ae9fd | 557 | and addresses should the file's contents be disclosed. |
558 | Hashed hostnames start with a | |
559 | .Ql | | |
5c63c2ab | 560 | character. |
561 | Only one hashed hostname may appear on a single line and none of the above | |
562 | negation or wildcard operators may be applied. | |
563 | .Pp | |
1d1ffb87 | 564 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
bf740959 | 565 | can be obtained, e.g., from |
2a8a6488 | 566 | .Pa /etc/ssh/ssh_host_key.pub . |
bf740959 | 567 | The optional comment field continues to the end of the line, and is not used. |
568 | .Pp | |
569 | Lines starting with | |
570 | .Ql # | |
571 | and empty lines are ignored as comments. | |
572 | .Pp | |
573 | When performing host authentication, authentication is accepted if any | |
4fe2af09 | 574 | matching line has the proper key. |
575 | It is thus permissible (but not | |
bf740959 | 576 | recommended) to have several lines or different host keys for the same |
4fe2af09 | 577 | names. |
578 | This will inevitably happen when short forms of host names | |
579 | from different domains are put in the file. | |
580 | It is possible | |
bf740959 | 581 | that the files contain conflicting information; authentication is |
582 | accepted if valid information can be found from either file. | |
583 | .Pp | |
584 | Note that the lines in these files are typically hundreds of characters | |
585 | long, and you definitely don't want to type in the host keys by hand. | |
586 | Rather, generate them by a script | |
f54651ce | 587 | or by taking |
2a8a6488 | 588 | .Pa /etc/ssh/ssh_host_key.pub |
bf740959 | 589 | and adding the host names at the front. |
590 | .Ss Examples | |
da89cf4d | 591 | .Bd -literal |
433e60ac | 592 | closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi |
da89cf4d | 593 | cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= |
594 | .Ed | |
5c63c2ab | 595 | .Bd -literal |
596 | # A hashed hostname | |
597 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | |
598 | AAAA1234.....= | |
599 | .Ed | |
bf740959 | 600 | .Sh FILES |
601 | .Bl -tag -width Ds | |
2a8a6488 | 602 | .It Pa /etc/ssh/sshd_config |
bf740959 | 603 | Contains configuration data for |
604 | .Nm sshd . | |
588df31a | 605 | The file format and configuration options are described in |
606 | .Xr sshd_config 5 . | |
2a8a6488 | 607 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
da89cf4d | 608 | These three files contain the private parts of the host keys. |
b8dc87d3 | 609 | These files should only be owned by root, readable only by root, and not |
bf740959 | 610 | accessible to others. |
611 | Note that | |
612 | .Nm | |
613 | does not start if this file is group/world-accessible. | |
2a8a6488 | 614 | .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub |
da89cf4d | 615 | These three files contain the public parts of the host keys. |
b8dc87d3 | 616 | These files should be world-readable but writable only by |
4fe2af09 | 617 | root. |
b8dc87d3 | 618 | Their contents should match the respective private parts. |
619 | These files are not | |
620 | really used for anything; they are provided for the convenience of | |
621 | the user so their contents can be copied to known hosts files. | |
622 | These files are created using | |
bf740959 | 623 | .Xr ssh-keygen 1 . |
e2432638 | 624 | .It Pa /etc/moduli |
c523303b | 625 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
f1dcc34e | 626 | The file format is described in |
627 | .Xr moduli 5 . | |
75653d3e | 628 | .It Pa /var/empty |
629 | .Xr chroot 2 | |
630 | directory used by | |
631 | .Nm | |
632 | during privilege separation in the pre-authentication phase. | |
633 | The directory should not contain any files and must be owned by root | |
634 | and not group or world-writable. | |
5f4fdfae | 635 | .It Pa /var/run/sshd.pid |
bf740959 | 636 | Contains the process ID of the |
637 | .Nm | |
638 | listening for connections (if there are several daemons running | |
baa08b92 | 639 | concurrently for different ports, this contains the process ID of the one |
4fe2af09 | 640 | started last). |
c345cf9d | 641 | The content of this file is not sensitive; it can be world-readable. |
bf740959 | 642 | .It Pa $HOME/.ssh/authorized_keys |
da89cf4d | 643 | Lists the public keys (RSA or DSA) that can be used to log into the user's account. |
1d1ffb87 | 644 | This file must be readable by root (which may on some machines imply |
645 | it being world-readable if the user's home directory resides on an NFS | |
646 | volume). | |
647 | It is recommended that it not be accessible by others. | |
648 | The format of this file is described above. | |
649 | Users will place the contents of their | |
96a7b0cc | 650 | .Pa identity.pub , |
1d1ffb87 | 651 | .Pa id_dsa.pub |
c0ecc314 | 652 | and/or |
653 | .Pa id_rsa.pub | |
1d1ffb87 | 654 | files into this file, as described in |
655 | .Xr ssh-keygen 1 . | |
433e60ac | 656 | .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts" |
6a17f9c2 | 657 | These files are consulted when using rhosts with RSA host |
f49bc4f7 | 658 | authentication or protocol version 2 hostbased authentication |
659 | to check the public key of the host. | |
4fe2af09 | 660 | The key must be listed in one of these files to be accepted. |
5bbb5681 | 661 | The client uses the same files |
e91c60f2 | 662 | to verify that it is connecting to the correct remote host. |
4fe2af09 | 663 | These files should be writable only by root/the owner. |
2a8a6488 | 664 | .Pa /etc/ssh/ssh_known_hosts |
bf740959 | 665 | should be world-readable, and |
666 | .Pa $HOME/.ssh/known_hosts | |
6691e41b | 667 | can, but need not be, world-readable. |
a333272d | 668 | .It Pa /etc/motd |
669 | See | |
670 | .Xr motd 5 . | |
671 | .It Pa $HOME/.hushlogin | |
672 | This file is used to suppress printing the last login time and | |
673 | .Pa /etc/motd , | |
674 | if | |
675 | .Cm PrintLastLog | |
676 | and | |
677 | .Cm PrintMotd , | |
678 | respectively, | |
679 | are enabled. | |
680 | It does not suppress printing of the banner specified by | |
681 | .Cm Banner . | |
bf740959 | 682 | .It Pa /etc/nologin |
f54651ce | 683 | If this file exists, |
bf740959 | 684 | .Nm |
4fe2af09 | 685 | refuses to let anyone except root log in. |
686 | The contents of the file | |
bf740959 | 687 | are displayed to anyone trying to log in, and non-root connections are |
4fe2af09 | 688 | refused. |
689 | The file should be world-readable. | |
bf740959 | 690 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
5b263aae | 691 | Access controls that should be enforced by tcp-wrappers are defined here. |
692 | Further details are described in | |
bf740959 | 693 | .Xr hosts_access 5 . |
694 | .It Pa $HOME/.rhosts | |
6f5abc1e | 695 | This file is used during |
696 | .Cm RhostsRSAAuthentication | |
697 | and | |
698 | .Cm HostbasedAuthentication | |
699 | and contains host-username pairs, separated by a space, one per | |
4fe2af09 | 700 | line. |
701 | The given user on the corresponding host is permitted to log in | |
6691e41b | 702 | without a password. |
4fe2af09 | 703 | The same file is used by rlogind and rshd. |
bf740959 | 704 | The file must |
705 | be writable only by the user; it is recommended that it not be | |
706 | accessible by others. | |
707 | .Pp | |
433e60ac | 708 | It is also possible to use netgroups in the file. |
4fe2af09 | 709 | Either host or user |
bf740959 | 710 | name may be of the form +@groupname to specify all hosts or all users |
711 | in the group. | |
712 | .It Pa $HOME/.shosts | |
713 | For ssh, | |
714 | this file is exactly the same as for | |
715 | .Pa .rhosts . | |
716 | However, this file is | |
717 | not used by rlogin and rshd, so using this permits access using SSH only. | |
c345cf9d | 718 | .It Pa /etc/hosts.equiv |
bf740959 | 719 | This file is used during |
6f5abc1e | 720 | .Cm RhostsRSAAuthentication |
721 | and | |
722 | .Cm HostbasedAuthentication | |
4fe2af09 | 723 | authentication. |
724 | In the simplest form, this file contains host names, one per line. | |
725 | Users on | |
bf740959 | 726 | those hosts are permitted to log in without a password, provided they |
4fe2af09 | 727 | have the same user name on both machines. |
728 | The host name may also be | |
bf740959 | 729 | followed by a user name; such users are permitted to log in as |
730 | .Em any | |
4fe2af09 | 731 | user on this machine (except root). |
732 | Additionally, the syntax | |
bf740959 | 733 | .Dq +@group |
4fe2af09 | 734 | can be used to specify netgroups. |
735 | Negated entries start with | |
bf740959 | 736 | .Ql \&- . |
737 | .Pp | |
738 | If the client host/user is successfully matched in this file, login is | |
739 | automatically permitted provided the client and server user names are the | |
4fe2af09 | 740 | same. |
6f5abc1e | 741 | Additionally, successful client host key authentication is required. |
4fe2af09 | 742 | This file must be writable only by root; it is recommended |
bf740959 | 743 | that it be world-readable. |
744 | .Pp | |
745 | .Sy "Warning: It is almost never a good idea to use user names in" | |
746 | .Pa hosts.equiv . | |
747 | Beware that it really means that the named user(s) can log in as | |
748 | .Em anybody , | |
749 | which includes bin, daemon, adm, and other accounts that own critical | |
4fe2af09 | 750 | binaries and directories. |
751 | Using a user name practically grants the user root access. | |
752 | The only valid use for user names that I can think | |
bf740959 | 753 | of is in negative entries. |
754 | .Pp | |
755 | Note that this warning also applies to rsh/rlogin. | |
5f4fdfae | 756 | .It Pa /etc/shosts.equiv |
bf740959 | 757 | This is processed exactly as |
758 | .Pa /etc/hosts.equiv . | |
759 | However, this file may be useful in environments that want to run both | |
760 | rsh/rlogin and ssh. | |
761 | .It Pa $HOME/.ssh/environment | |
4fe2af09 | 762 | This file is read into the environment at login (if it exists). |
763 | It can only contain empty lines, comment lines (that start with | |
bf740959 | 764 | .Ql # ) , |
4fe2af09 | 765 | and assignment lines of the form name=value. |
766 | The file should be writable | |
bf740959 | 767 | only by the user; it need not be readable by anyone else. |
35453849 | 768 | Environment processing is disabled by default and is |
769 | controlled via the | |
770 | .Cm PermitUserEnvironment | |
771 | option. | |
bf740959 | 772 | .It Pa $HOME/.ssh/rc |
6691e41b | 773 | If this file exists, it is run with |
774 | .Pa /bin/sh | |
775 | after reading the | |
4fe2af09 | 776 | environment files but before starting the user's shell or command. |
d4c6ddff | 777 | It must not produce any output on stdout; stderr must be used |
778 | instead. | |
779 | If X11 forwarding is in use, it will receive the "proto cookie" pair in | |
780 | its standard input (and | |
bf740959 | 781 | .Ev DISPLAY |
d4c6ddff | 782 | in its environment). |
783 | The script must call | |
bf740959 | 784 | .Xr xauth 1 |
d4c6ddff | 785 | because |
786 | .Nm | |
787 | will not run xauth automatically to add X11 cookies. | |
bf740959 | 788 | .Pp |
789 | The primary purpose of this file is to run any initialization routines | |
790 | which may be needed before the user's home directory becomes | |
791 | accessible; AFS is a particular example of such an environment. | |
792 | .Pp | |
793 | This file will probably contain some initialization code followed by | |
da89cf4d | 794 | something similar to: |
795 | .Bd -literal | |
d4c6ddff | 796 | if read proto cookie && [ -n "$DISPLAY" ]; then |
797 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | |
798 | # X11UseLocalhost=yes | |
cb11b555 | 799 | echo add unix:`echo $DISPLAY | |
d4c6ddff | 800 | cut -c11-` $proto $cookie |
801 | else | |
802 | # X11UseLocalhost=no | |
cb11b555 | 803 | echo add $DISPLAY $proto $cookie |
804 | fi | xauth -q - | |
d4c6ddff | 805 | fi |
da89cf4d | 806 | .Ed |
bf740959 | 807 | .Pp |
808 | If this file does not exist, | |
2a8a6488 | 809 | .Pa /etc/ssh/sshrc |
bf740959 | 810 | is run, and if that |
d4c6ddff | 811 | does not exist either, xauth is used to add the cookie. |
bf740959 | 812 | .Pp |
813 | This file should be writable only by the user, and need not be | |
814 | readable by anyone else. | |
2a8a6488 | 815 | .It Pa /etc/ssh/sshrc |
bf740959 | 816 | Like |
817 | .Pa $HOME/.ssh/rc . | |
818 | This can be used to specify | |
4fe2af09 | 819 | machine-specific login-time initializations globally. |
820 | This file should be writable only by root, and should be world-readable. | |
089fbbd2 | 821 | .El |
bf740959 | 822 | .Sh SEE ALSO |
bf740959 | 823 | .Xr scp 1 , |
61e96248 | 824 | .Xr sftp 1 , |
bf740959 | 825 | .Xr ssh 1 , |
826 | .Xr ssh-add 1 , | |
827 | .Xr ssh-agent 1 , | |
828 | .Xr ssh-keygen 1 , | |
433e60ac | 829 | .Xr chroot 2 , |
830 | .Xr hosts_access 5 , | |
248bad82 | 831 | .Xr login.conf 5 , |
832 | .Xr moduli 5 , | |
588df31a | 833 | .Xr sshd_config 5 , |
433e60ac | 834 | .Xr inetd 8 , |
a5a2da3b | 835 | .Xr sftp-server 8 |
2cad6cef | 836 | .Rs |
837 | .%A T. Ylonen | |
838 | .%A T. Kivinen | |
839 | .%A M. Saarinen | |
840 | .%A T. Rinne | |
841 | .%A S. Lehtinen | |
842 | .%T "SSH Protocol Architecture" | |
17f5e68a | 843 | .%N draft-ietf-secsh-architecture-12.txt |
844 | .%D January 2002 | |
2cad6cef | 845 | .%O work in progress material |
846 | .Re | |
da89cf4d | 847 | .Rs |
848 | .%A M. Friedl | |
849 | .%A N. Provos | |
850 | .%A W. A. Simpson | |
851 | .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" | |
17f5e68a | 852 | .%N draft-ietf-secsh-dh-group-exchange-02.txt |
853 | .%D January 2002 | |
da89cf4d | 854 | .%O work in progress material |
855 | .Re | |
be193d89 | 856 | .Sh AUTHORS |
857 | OpenSSH is a derivative of the original and free | |
858 | ssh 1.2.12 release by Tatu Ylonen. | |
859 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
860 | Theo de Raadt and Dug Song | |
861 | removed many bugs, re-added newer features and | |
862 | created OpenSSH. | |
863 | Markus Friedl contributed the support for SSH | |
864 | protocol versions 1.5 and 2.0. | |
865 | Niels Provos and Markus Friedl contributed support | |
866 | for privilege separation. |