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3c0ef626 | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
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 | .\" | |
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. | |
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. | |
25 | .\" | |
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. | |
36 | .\" | |
db32a221 | 37 | .\" $OpenBSD: ssh.1,v 1.151 2002/05/06 23:34:33 millert Exp $ |
3c0ef626 | 38 | .Dd September 25, 1999 |
39 | .Dt SSH 1 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh | |
43 | .Nd OpenSSH SSH client (remote login program) | |
44 | .Sh SYNOPSIS | |
45 | .Nm ssh | |
46 | .Op Fl l Ar login_name | |
47 | .Ar hostname | user@hostname | |
48 | .Op Ar command | |
49 | .Pp | |
50 | .Nm ssh | |
51 | .Op Fl afgknqstvxACNPTX1246 | |
52 | .Op Fl b Ar bind_address | |
53 | .Op Fl c Ar cipher_spec | |
54 | .Op Fl e Ar escape_char | |
55 | .Op Fl i Ar identity_file | |
56 | .Op Fl l Ar login_name | |
57 | .Op Fl m Ar mac_spec | |
58 | .Op Fl o Ar option | |
59 | .Op Fl p Ar port | |
60 | .Op Fl F Ar configfile | |
61 | .Oo Fl L Xo | |
62 | .Sm off | |
63 | .Ar port : | |
64 | .Ar host : | |
65 | .Ar hostport | |
66 | .Sm on | |
67 | .Xc | |
68 | .Oc | |
69 | .Oo Fl R Xo | |
70 | .Sm off | |
71 | .Ar port : | |
72 | .Ar host : | |
73 | .Ar hostport | |
74 | .Sm on | |
75 | .Xc | |
76 | .Oc | |
77 | .Op Fl D Ar port | |
78 | .Ar hostname | user@hostname | |
79 | .Op Ar command | |
80 | .Sh DESCRIPTION | |
81 | .Nm | |
82 | (SSH client) is a program for logging into a remote machine and for | |
83 | executing commands on a remote machine. | |
84 | It is intended to replace | |
85 | rlogin and rsh, and provide secure encrypted communications between | |
86 | two untrusted hosts over an insecure network. | |
87 | X11 connections and | |
88 | arbitrary TCP/IP ports can also be forwarded over the secure channel. | |
89 | .Pp | |
90 | .Nm | |
91 | connects and logs into the specified | |
92 | .Ar hostname . | |
93 | The user must prove | |
94 | his/her identity to the remote machine using one of several methods | |
95 | depending on the protocol version used: | |
96 | .Pp | |
97 | .Ss SSH protocol version 1 | |
98 | .Pp | |
99 | First, if the machine the user logs in from is listed in | |
100 | .Pa /etc/hosts.equiv | |
101 | or | |
102 | .Pa /etc/shosts.equiv | |
103 | on the remote machine, and the user names are | |
104 | the same on both sides, the user is immediately permitted to log in. | |
105 | Second, if | |
106 | .Pa \&.rhosts | |
107 | or | |
108 | .Pa \&.shosts | |
109 | exists in the user's home directory on the | |
110 | remote machine and contains a line containing the name of the client | |
111 | machine and the name of the user on that machine, the user is | |
112 | permitted to log in. | |
113 | This form of authentication alone is normally not | |
114 | allowed by the server because it is not secure. | |
115 | .Pp | |
116 | The second authentication method is the | |
117 | .Pa rhosts | |
118 | or | |
119 | .Pa hosts.equiv | |
120 | method combined with RSA-based host authentication. | |
121 | It means that if the login would be permitted by | |
122 | .Pa $HOME/.rhosts , | |
123 | .Pa $HOME/.shosts , | |
124 | .Pa /etc/hosts.equiv , | |
125 | or | |
126 | .Pa /etc/shosts.equiv , | |
127 | and if additionally the server can verify the client's | |
128 | host key (see | |
db32a221 | 129 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 130 | and |
131 | .Pa $HOME/.ssh/known_hosts | |
132 | in the | |
133 | .Sx FILES | |
134 | section), only then login is permitted. | |
135 | This authentication method closes security holes due to IP | |
136 | spoofing, DNS spoofing and routing spoofing. | |
137 | [Note to the administrator: | |
138 | .Pa /etc/hosts.equiv , | |
139 | .Pa $HOME/.rhosts , | |
140 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
141 | disabled if security is desired.] | |
142 | .Pp | |
143 | As a third authentication method, | |
144 | .Nm | |
145 | supports RSA based authentication. | |
146 | The scheme is based on public-key cryptography: there are cryptosystems | |
147 | where encryption and decryption are done using separate keys, and it | |
148 | is not possible to derive the decryption key from the encryption key. | |
149 | RSA is one such system. | |
150 | The idea is that each user creates a public/private | |
151 | key pair for authentication purposes. | |
152 | The server knows the public key, and only the user knows the private key. | |
153 | The file | |
154 | .Pa $HOME/.ssh/authorized_keys | |
155 | lists the public keys that are permitted for logging | |
156 | in. | |
157 | When the user logs in, the | |
158 | .Nm | |
159 | program tells the server which key pair it would like to use for | |
160 | authentication. | |
161 | The server checks if this key is permitted, and if | |
162 | so, sends the user (actually the | |
163 | .Nm | |
164 | program running on behalf of the user) a challenge, a random number, | |
165 | encrypted by the user's public key. | |
166 | The challenge can only be | |
167 | decrypted using the proper private key. | |
168 | The user's client then decrypts the | |
169 | challenge using the private key, proving that he/she knows the private | |
170 | key but without disclosing it to the server. | |
171 | .Pp | |
172 | .Nm | |
173 | implements the RSA authentication protocol automatically. | |
174 | The user creates his/her RSA key pair by running | |
175 | .Xr ssh-keygen 1 . | |
176 | This stores the private key in | |
177 | .Pa $HOME/.ssh/identity | |
178 | and the public key in | |
179 | .Pa $HOME/.ssh/identity.pub | |
180 | in the user's home directory. | |
181 | The user should then copy the | |
182 | .Pa identity.pub | |
183 | to | |
184 | .Pa $HOME/.ssh/authorized_keys | |
185 | in his/her home directory on the remote machine (the | |
186 | .Pa authorized_keys | |
187 | file corresponds to the conventional | |
188 | .Pa $HOME/.rhosts | |
189 | file, and has one key | |
190 | per line, though the lines can be very long). | |
191 | After this, the user can log in without giving the password. | |
192 | RSA authentication is much | |
193 | more secure than rhosts authentication. | |
194 | .Pp | |
195 | The most convenient way to use RSA authentication may be with an | |
196 | authentication agent. | |
197 | See | |
198 | .Xr ssh-agent 1 | |
199 | for more information. | |
200 | .Pp | |
201 | If other authentication methods fail, | |
202 | .Nm | |
203 | prompts the user for a password. | |
204 | The password is sent to the remote | |
205 | host for checking; however, since all communications are encrypted, | |
206 | the password cannot be seen by someone listening on the network. | |
207 | .Pp | |
208 | .Ss SSH protocol version 2 | |
209 | .Pp | |
db32a221 | 210 | When a user connects using protocol version 2 |
211 | similar authentication methods are available. | |
3c0ef626 | 212 | Using the default values for |
213 | .Cm PreferredAuthentications , | |
214 | the client will try to authenticate first using the hostbased method; | |
215 | if this method fails public key authentication is attempted, | |
216 | and finally if this method fails keyboard-interactive and | |
217 | password authentication are tried. | |
218 | .Pp | |
219 | The public key method is similar to RSA authentication described | |
220 | in the previous section and allows the RSA or DSA algorithm to be used: | |
221 | The client uses his private key, | |
222 | .Pa $HOME/.ssh/id_dsa | |
223 | or | |
224 | .Pa $HOME/.ssh/id_rsa , | |
225 | to sign the session identifier and sends the result to the server. | |
226 | The server checks whether the matching public key is listed in | |
227 | .Pa $HOME/.ssh/authorized_keys | |
228 | and grants access if both the key is found and the signature is correct. | |
229 | The session identifier is derived from a shared Diffie-Hellman value | |
230 | and is only known to the client and the server. | |
231 | .Pp | |
232 | If public key authentication fails or is not available a password | |
233 | can be sent encrypted to the remote host for proving the user's identity. | |
234 | .Pp | |
235 | Additionally, | |
236 | .Nm | |
237 | supports hostbased or challenge response authentication. | |
238 | .Pp | |
239 | Protocol 2 provides additional mechanisms for confidentiality | |
240 | (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) | |
241 | and integrity (hmac-md5, hmac-sha1). | |
242 | Note that protocol 1 lacks a strong mechanism for ensuring the | |
243 | integrity of the connection. | |
244 | .Pp | |
245 | .Ss Login session and remote execution | |
246 | .Pp | |
247 | When the user's identity has been accepted by the server, the server | |
248 | either executes the given command, or logs into the machine and gives | |
249 | the user a normal shell on the remote machine. | |
250 | All communication with | |
251 | the remote command or shell will be automatically encrypted. | |
252 | .Pp | |
253 | If a pseudo-terminal has been allocated (normal login session), the | |
254 | user may use the escape characters noted below. | |
255 | .Pp | |
256 | If no pseudo tty has been allocated, the | |
257 | session is transparent and can be used to reliably transfer binary | |
258 | data. | |
259 | On most systems, setting the escape character to | |
260 | .Dq none | |
261 | will also make the session transparent even if a tty is used. | |
262 | .Pp | |
263 | The session terminates when the command or shell on the remote | |
264 | machine exits and all X11 and TCP/IP connections have been closed. | |
265 | The exit status of the remote program is returned as the exit status | |
266 | of | |
267 | .Nm ssh . | |
268 | .Pp | |
269 | .Ss Escape Characters | |
270 | .Pp | |
271 | When a pseudo terminal has been requested, ssh supports a number of functions | |
272 | through the use of an escape character. | |
273 | .Pp | |
274 | A single tilde character can be sent as | |
275 | .Ic ~~ | |
276 | or by following the tilde by a character other than those described below. | |
277 | The escape character must always follow a newline to be interpreted as | |
278 | special. | |
279 | The escape character can be changed in configuration files using the | |
280 | .Cm EscapeChar | |
281 | configuration directive or on the command line by the | |
282 | .Fl e | |
283 | option. | |
284 | .Pp | |
285 | The supported escapes (assuming the default | |
286 | .Ql ~ ) | |
287 | are: | |
288 | .Bl -tag -width Ds | |
289 | .It Cm ~. | |
290 | Disconnect | |
291 | .It Cm ~^Z | |
292 | Background ssh | |
293 | .It Cm ~# | |
294 | List forwarded connections | |
295 | .It Cm ~& | |
296 | Background ssh at logout when waiting for forwarded connection / X11 sessions | |
297 | to terminate | |
298 | .It Cm ~? | |
299 | Display a list of escape characters | |
db32a221 | 300 | .It Cm ~C |
301 | Open command line (only useful for adding port forwardings using the | |
302 | .Fl L | |
303 | and | |
304 | .Fl R | |
305 | options) | |
3c0ef626 | 306 | .It Cm ~R |
307 | Request rekeying of the connection (only useful for SSH protocol version 2 | |
308 | and if the peer supports it) | |
309 | .El | |
310 | .Pp | |
311 | .Ss X11 and TCP forwarding | |
312 | .Pp | |
313 | If the | |
314 | .Cm ForwardX11 | |
315 | variable is set to | |
316 | .Dq yes | |
317 | (or, see the description of the | |
318 | .Fl X | |
319 | and | |
320 | .Fl x | |
321 | options described later) | |
322 | and the user is using X11 (the | |
323 | .Ev DISPLAY | |
324 | environment variable is set), the connection to the X11 display is | |
325 | automatically forwarded to the remote side in such a way that any X11 | |
326 | programs started from the shell (or command) will go through the | |
327 | encrypted channel, and the connection to the real X server will be made | |
328 | from the local machine. | |
329 | The user should not manually set | |
330 | .Ev DISPLAY . | |
331 | Forwarding of X11 connections can be | |
332 | configured on the command line or in configuration files. | |
333 | .Pp | |
334 | The | |
335 | .Ev DISPLAY | |
336 | value set by | |
337 | .Nm | |
338 | will point to the server machine, but with a display number greater | |
339 | than zero. | |
340 | This is normal, and happens because | |
341 | .Nm | |
342 | creates a | |
343 | .Dq proxy | |
344 | X server on the server machine for forwarding the | |
345 | connections over the encrypted channel. | |
346 | .Pp | |
347 | .Nm | |
348 | will also automatically set up Xauthority data on the server machine. | |
349 | For this purpose, it will generate a random authorization cookie, | |
350 | store it in Xauthority on the server, and verify that any forwarded | |
351 | connections carry this cookie and replace it by the real cookie when | |
352 | the connection is opened. | |
353 | The real authentication cookie is never | |
354 | sent to the server machine (and no cookies are sent in the plain). | |
355 | .Pp | |
356 | If the user is using an authentication agent, the connection to the agent | |
357 | is automatically forwarded to the remote side unless disabled on | |
358 | the command line or in a configuration file. | |
359 | .Pp | |
360 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
361 | be specified either on the command line or in a configuration file. | |
362 | One possible application of TCP/IP forwarding is a secure connection to an | |
363 | electronic purse; another is going through firewalls. | |
364 | .Pp | |
365 | .Ss Server authentication | |
366 | .Pp | |
367 | .Nm | |
368 | automatically maintains and checks a database containing | |
369 | identifications for all hosts it has ever been used with. | |
370 | Host keys are stored in | |
371 | .Pa $HOME/.ssh/known_hosts | |
372 | in the user's home directory. | |
373 | Additionally, the file | |
db32a221 | 374 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 375 | is automatically checked for known hosts. |
376 | Any new hosts are automatically added to the user's file. | |
377 | If a host's identification | |
378 | ever changes, | |
379 | .Nm | |
380 | warns about this and disables password authentication to prevent a | |
381 | trojan horse from getting the user's password. | |
382 | Another purpose of | |
383 | this mechanism is to prevent man-in-the-middle attacks which could | |
384 | otherwise be used to circumvent the encryption. | |
385 | The | |
386 | .Cm StrictHostKeyChecking | |
387 | option (see below) can be used to prevent logins to machines whose | |
388 | host key is not known or has changed. | |
389 | .Pp | |
390 | The options are as follows: | |
391 | .Bl -tag -width Ds | |
392 | .It Fl a | |
393 | Disables forwarding of the authentication agent connection. | |
394 | .It Fl A | |
395 | Enables forwarding of the authentication agent connection. | |
396 | This can also be specified on a per-host basis in a configuration file. | |
397 | .It Fl b Ar bind_address | |
398 | Specify the interface to transmit from on machines with multiple | |
399 | interfaces or aliased addresses. | |
400 | .It Fl c Ar blowfish|3des|des | |
401 | Selects the cipher to use for encrypting the session. | |
402 | .Ar 3des | |
403 | is used by default. | |
404 | It is believed to be secure. | |
405 | .Ar 3des | |
406 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
407 | .Ar blowfish | |
408 | is a fast block cipher, it appears very secure and is much faster than | |
409 | .Ar 3des . | |
410 | .Ar des | |
411 | is only supported in the | |
412 | .Nm | |
413 | client for interoperability with legacy protocol 1 implementations | |
414 | that do not support the | |
415 | .Ar 3des | |
416 | cipher. Its use is strongly discouraged due to cryptographic | |
417 | weaknesses. | |
418 | .It Fl c Ar cipher_spec | |
419 | Additionally, for protocol version 2 a comma-separated list of ciphers can | |
420 | be specified in order of preference. | |
421 | See | |
422 | .Cm Ciphers | |
423 | for more information. | |
424 | .It Fl e Ar ch|^ch|none | |
425 | Sets the escape character for sessions with a pty (default: | |
426 | .Ql ~ ) . | |
427 | The escape character is only recognized at the beginning of a line. | |
428 | The escape character followed by a dot | |
429 | .Pq Ql \&. | |
430 | closes the connection, followed | |
431 | by control-Z suspends the connection, and followed by itself sends the | |
432 | escape character once. | |
433 | Setting the character to | |
434 | .Dq none | |
435 | disables any escapes and makes the session fully transparent. | |
436 | .It Fl f | |
437 | Requests | |
438 | .Nm | |
439 | to go to background just before command execution. | |
440 | This is useful if | |
441 | .Nm | |
442 | is going to ask for passwords or passphrases, but the user | |
443 | wants it in the background. | |
444 | This implies | |
445 | .Fl n . | |
446 | The recommended way to start X11 programs at a remote site is with | |
447 | something like | |
448 | .Ic ssh -f host xterm . | |
449 | .It Fl g | |
450 | Allows remote hosts to connect to local forwarded ports. | |
451 | .It Fl i Ar identity_file | |
db32a221 | 452 | Selects a file from which the identity (private key) for |
3c0ef626 | 453 | RSA or DSA authentication is read. |
db32a221 | 454 | The default is |
3c0ef626 | 455 | .Pa $HOME/.ssh/identity |
db32a221 | 456 | for protocol version 1, and |
457 | .Pa $HOME/.ssh/id_rsa | |
458 | and | |
459 | .Pa $HOME/.ssh/id_dsa | |
460 | for protocol version 2. | |
3c0ef626 | 461 | Identity files may also be specified on |
462 | a per-host basis in the configuration file. | |
463 | It is possible to have multiple | |
464 | .Fl i | |
465 | options (and multiple identities specified in | |
466 | configuration files). | |
467 | .It Fl I Ar smartcard_device | |
468 | Specifies which smartcard device to use. The argument is | |
469 | the device | |
470 | .Nm | |
471 | should use to communicate with a smartcard used for storing the user's | |
472 | private RSA key. | |
473 | .It Fl k | |
474 | Disables forwarding of Kerberos tickets and AFS tokens. | |
475 | This may also be specified on a per-host basis in the configuration file. | |
476 | .It Fl l Ar login_name | |
477 | Specifies the user to log in as on the remote machine. | |
478 | This also may be specified on a per-host basis in the configuration file. | |
479 | .It Fl m Ar mac_spec | |
480 | Additionally, for protocol version 2 a comma-separated list of MAC | |
481 | (message authentication code) algorithms can | |
482 | be specified in order of preference. | |
483 | See the | |
484 | .Cm MACs | |
485 | keyword for more information. | |
486 | .It Fl n | |
487 | Redirects stdin from | |
488 | .Pa /dev/null | |
489 | (actually, prevents reading from stdin). | |
490 | This must be used when | |
491 | .Nm | |
492 | is run in the background. | |
493 | A common trick is to use this to run X11 programs on a remote machine. | |
494 | For example, | |
495 | .Ic ssh -n shadows.cs.hut.fi emacs & | |
496 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
497 | connection will be automatically forwarded over an encrypted channel. | |
498 | The | |
499 | .Nm | |
500 | program will be put in the background. | |
501 | (This does not work if | |
502 | .Nm | |
503 | needs to ask for a password or passphrase; see also the | |
504 | .Fl f | |
505 | option.) | |
506 | .It Fl N | |
507 | Do not execute a remote command. | |
508 | This is useful for just forwarding ports | |
509 | (protocol version 2 only). | |
510 | .It Fl o Ar option | |
511 | Can be used to give options in the format used in the configuration file. | |
512 | This is useful for specifying options for which there is no separate | |
513 | command-line flag. | |
514 | .It Fl p Ar port | |
515 | Port to connect to on the remote host. | |
516 | This can be specified on a | |
517 | per-host basis in the configuration file. | |
518 | .It Fl P | |
519 | Use a non-privileged port for outgoing connections. | |
520 | This can be used if a firewall does | |
521 | not permit connections from privileged ports. | |
522 | Note that this option turns off | |
523 | .Cm RhostsAuthentication | |
524 | and | |
525 | .Cm RhostsRSAAuthentication | |
526 | for older servers. | |
527 | .It Fl q | |
528 | Quiet mode. | |
529 | Causes all warning and diagnostic messages to be suppressed. | |
3c0ef626 | 530 | .It Fl s |
531 | May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use | |
532 | of SSH as a secure transport for other applications (eg. sftp). The | |
533 | subsystem is specified as the remote command. | |
534 | .It Fl t | |
535 | Force pseudo-tty allocation. | |
536 | This can be used to execute arbitrary | |
537 | screen-based programs on a remote machine, which can be very useful, | |
538 | e.g., when implementing menu services. | |
539 | Multiple | |
540 | .Fl t | |
541 | options force tty allocation, even if | |
542 | .Nm | |
543 | has no local tty. | |
544 | .It Fl T | |
545 | Disable pseudo-tty allocation. | |
546 | .It Fl v | |
547 | Verbose mode. | |
548 | Causes | |
549 | .Nm | |
550 | to print debugging messages about its progress. | |
551 | This is helpful in | |
552 | debugging connection, authentication, and configuration problems. | |
553 | Multiple | |
554 | .Fl v | |
555 | options increases the verbosity. | |
556 | Maximum is 3. | |
557 | .It Fl x | |
558 | Disables X11 forwarding. | |
559 | .It Fl X | |
560 | Enables X11 forwarding. | |
561 | This can also be specified on a per-host basis in a configuration file. | |
562 | .It Fl C | |
563 | Requests compression of all data (including stdin, stdout, stderr, and | |
564 | data for forwarded X11 and TCP/IP connections). | |
565 | The compression algorithm is the same used by | |
566 | .Xr gzip 1 , | |
567 | and the | |
568 | .Dq level | |
569 | can be controlled by the | |
570 | .Cm CompressionLevel | |
571 | option (see below). | |
572 | Compression is desirable on modem lines and other | |
573 | slow connections, but will only slow down things on fast networks. | |
574 | The default value can be set on a host-by-host basis in the | |
575 | configuration files; see the | |
576 | .Cm Compression | |
577 | option below. | |
578 | .It Fl F Ar configfile | |
579 | Specifies an alternative per-user configuration file. | |
580 | If a configuration file is given on the command line, | |
581 | the system-wide configuration file | |
db32a221 | 582 | .Pq Pa /etc/ssh/ssh_config |
3c0ef626 | 583 | will be ignored. |
584 | The default for the per-user configuration file is | |
585 | .Pa $HOME/.ssh/config . | |
586 | .It Fl L Ar port:host:hostport | |
587 | Specifies that the given port on the local (client) host is to be | |
588 | forwarded to the given host and port on the remote side. | |
589 | This works by allocating a socket to listen to | |
590 | .Ar port | |
591 | on the local side, and whenever a connection is made to this port, the | |
592 | connection is forwarded over the secure channel, and a connection is | |
593 | made to | |
594 | .Ar host | |
595 | port | |
596 | .Ar hostport | |
597 | from the remote machine. | |
598 | Port forwardings can also be specified in the configuration file. | |
599 | Only root can forward privileged ports. | |
600 | IPv6 addresses can be specified with an alternative syntax: | |
601 | .Ar port/host/hostport | |
602 | .It Fl R Ar port:host:hostport | |
603 | Specifies that the given port on the remote (server) host is to be | |
604 | forwarded to the given host and port on the local side. | |
605 | This works by allocating a socket to listen to | |
606 | .Ar port | |
607 | on the remote side, and whenever a connection is made to this port, the | |
608 | connection is forwarded over the secure channel, and a connection is | |
609 | made to | |
610 | .Ar host | |
611 | port | |
612 | .Ar hostport | |
613 | from the local machine. | |
614 | Port forwardings can also be specified in the configuration file. | |
615 | Privileged ports can be forwarded only when | |
616 | logging in as root on the remote machine. | |
617 | IPv6 addresses can be specified with an alternative syntax: | |
618 | .Ar port/host/hostport | |
619 | .It Fl D Ar port | |
620 | Specifies a local | |
621 | .Dq dynamic | |
622 | application-level port forwarding. | |
623 | This works by allocating a socket to listen to | |
624 | .Ar port | |
625 | on the local side, and whenever a connection is made to this port, the | |
626 | connection is forwarded over the secure channel, and the application | |
627 | protocol is then used to determine where to connect to from the | |
628 | remote machine. Currently the SOCKS4 protocol is supported, and | |
629 | .Nm | |
630 | will act as a SOCKS4 server. | |
631 | Only root can forward privileged ports. | |
632 | Dynamic port forwardings can also be specified in the configuration file. | |
633 | .It Fl 1 | |
634 | Forces | |
635 | .Nm | |
636 | to try protocol version 1 only. | |
637 | .It Fl 2 | |
638 | Forces | |
639 | .Nm | |
640 | to try protocol version 2 only. | |
641 | .It Fl 4 | |
642 | Forces | |
643 | .Nm | |
644 | to use IPv4 addresses only. | |
645 | .It Fl 6 | |
646 | Forces | |
647 | .Nm | |
648 | to use IPv6 addresses only. | |
649 | .El | |
650 | .Sh CONFIGURATION FILES | |
651 | .Nm | |
652 | obtains configuration data from the following sources in | |
653 | the following order: | |
db32a221 | 654 | command line options, |
655 | feature-specific user configuration file(s) (see below), | |
656 | user's configuration file | |
3c0ef626 | 657 | .Pq Pa $HOME/.ssh/config , |
658 | and system-wide configuration file | |
db32a221 | 659 | .Pq Pa /etc/ssh/ssh_config . |
660 | For compatibility with other | |
661 | .Nm | |
662 | versions, the following feature-specific user configuration files | |
663 | will be processed after the command line options but before the user's | |
664 | main configuration file, so options that other | |
665 | .Nm | |
666 | versions may not support don't need to go in the main configuration file: | |
667 | .Bl -tag -width Ds | |
668 | .It Pa $HOME/.ssh/config.gssapi | |
669 | Read if GSSAPI authentication is supported. This is a good place for | |
670 | the GssapiAuthentication and GssapiDelegateCredentials options. | |
671 | .It Pa $HOME/.ssh/config.krb | |
672 | Read if Kerberos authentication is supported. This is a good place | |
673 | for the KerberosAuthentication and KerberosTgtPassing options. | |
674 | .It Pa $HOME/.ssh/config.afs | |
675 | Read if AFS token passing is supported. This is a good place for the | |
676 | AfsTokenPassing option. | |
677 | .El | |
678 | .Pp | |
3c0ef626 | 679 | For each parameter, the first obtained value |
680 | will be used. | |
681 | The configuration files contain sections bracketed by | |
682 | .Dq Host | |
683 | specifications, and that section is only applied for hosts that | |
684 | match one of the patterns given in the specification. | |
685 | The matched host name is the one given on the command line. | |
686 | .Pp | |
687 | Since the first obtained value for each parameter is used, more | |
688 | host-specific declarations should be given near the beginning of the | |
689 | file, and general defaults at the end. | |
690 | .Pp | |
691 | The configuration file has the following format: | |
692 | .Pp | |
693 | Empty lines and lines starting with | |
694 | .Ql # | |
695 | are comments. | |
696 | .Pp | |
697 | Otherwise a line is of the format | |
698 | .Dq keyword arguments . | |
699 | Configuration options may be separated by whitespace or | |
700 | optional whitespace and exactly one | |
701 | .Ql = ; | |
702 | the latter format is useful to avoid the need to quote whitespace | |
703 | when specifying configuration options using the | |
704 | .Nm ssh , | |
705 | .Nm scp | |
706 | and | |
707 | .Nm sftp | |
708 | .Fl o | |
709 | option. | |
710 | .Pp | |
711 | The possible | |
712 | keywords and their meanings are as follows (note that | |
713 | keywords are case-insensitive and arguments are case-sensitive): | |
714 | .Bl -tag -width Ds | |
715 | .It Cm Host | |
716 | Restricts the following declarations (up to the next | |
717 | .Cm Host | |
718 | keyword) to be only for those hosts that match one of the patterns | |
719 | given after the keyword. | |
720 | .Ql \&* | |
721 | and | |
722 | .Ql ? | |
723 | can be used as wildcards in the | |
724 | patterns. | |
725 | A single | |
726 | .Ql \&* | |
727 | as a pattern can be used to provide global | |
728 | defaults for all hosts. | |
729 | The host is the | |
730 | .Ar hostname | |
731 | argument given on the command line (i.e., the name is not converted to | |
732 | a canonicalized host name before matching). | |
733 | .It Cm AFSTokenPassing | |
734 | Specifies whether to pass AFS tokens to remote host. | |
735 | The argument to this keyword must be | |
736 | .Dq yes | |
737 | or | |
738 | .Dq no . | |
739 | This option applies to protocol version 1 only. | |
740 | .It Cm BatchMode | |
741 | If set to | |
742 | .Dq yes , | |
743 | passphrase/password querying will be disabled. | |
744 | This option is useful in scripts and other batch jobs where no user | |
745 | is present to supply the password. | |
746 | The argument must be | |
747 | .Dq yes | |
748 | or | |
749 | .Dq no . | |
750 | The default is | |
751 | .Dq no . | |
752 | .It Cm BindAddress | |
753 | Specify the interface to transmit from on machines with multiple | |
754 | interfaces or aliased addresses. | |
755 | Note that this option does not work if | |
756 | .Cm UsePrivilegedPort | |
757 | is set to | |
758 | .Dq yes . | |
759 | .It Cm CheckHostIP | |
760 | If this flag is set to | |
761 | .Dq yes , | |
762 | ssh will additionally check the host IP address in the | |
763 | .Pa known_hosts | |
764 | file. | |
765 | This allows ssh to detect if a host key changed due to DNS spoofing. | |
766 | If the option is set to | |
767 | .Dq no , | |
768 | the check will not be executed. | |
769 | The default is | |
770 | .Dq yes . | |
771 | .It Cm Cipher | |
772 | Specifies the cipher to use for encrypting the session | |
773 | in protocol version 1. | |
774 | Currently, | |
775 | .Dq blowfish , | |
776 | .Dq 3des , | |
777 | and | |
778 | .Dq des | |
779 | are supported. | |
780 | .Ar des | |
781 | is only supported in the | |
782 | .Nm | |
783 | client for interoperability with legacy protocol 1 implementations | |
784 | that do not support the | |
785 | .Ar 3des | |
786 | cipher. Its use is strongly discouraged due to cryptographic | |
787 | weaknesses. | |
788 | The default is | |
789 | .Dq 3des . | |
790 | .It Cm Ciphers | |
791 | Specifies the ciphers allowed for protocol version 2 | |
792 | in order of preference. | |
793 | Multiple ciphers must be comma-separated. | |
794 | The default is | |
795 | .Pp | |
796 | .Bd -literal | |
797 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
798 | aes192-cbc,aes256-cbc'' | |
799 | .Ed | |
800 | .It Cm ClearAllForwardings | |
801 | Specifies that all local, remote and dynamic port forwardings | |
802 | specified in the configuration files or on the command line be | |
803 | cleared. This option is primarily useful when used from the | |
804 | .Nm | |
805 | command line to clear port forwardings set in | |
806 | configuration files, and is automatically set by | |
807 | .Xr scp 1 | |
808 | and | |
809 | .Xr sftp 1 . | |
810 | The argument must be | |
811 | .Dq yes | |
812 | or | |
813 | .Dq no . | |
814 | The default is | |
815 | .Dq no . | |
816 | .It Cm Compression | |
817 | Specifies whether to use compression. | |
818 | The argument must be | |
819 | .Dq yes | |
820 | or | |
821 | .Dq no . | |
822 | The default is | |
823 | .Dq no . | |
824 | .It Cm CompressionLevel | |
825 | Specifies the compression level to use if compression is enabled. | |
826 | The argument must be an integer from 1 (fast) to 9 (slow, best). | |
827 | The default level is 6, which is good for most applications. | |
828 | The meaning of the values is the same as in | |
829 | .Xr gzip 1 . | |
830 | Note that this option applies to protocol version 1 only. | |
831 | .It Cm ConnectionAttempts | |
832 | Specifies the number of tries (one per second) to make before falling | |
833 | back to rsh or exiting. | |
834 | The argument must be an integer. | |
835 | This may be useful in scripts if the connection sometimes fails. | |
836 | The default is 1. | |
837 | .It Cm DynamicForward | |
838 | Specifies that a TCP/IP port on the local machine be forwarded | |
839 | over the secure channel, and the application | |
840 | protocol is then used to determine where to connect to from the | |
841 | remote machine. The argument must be a port number. | |
842 | Currently the SOCKS4 protocol is supported, and | |
843 | .Nm | |
844 | will act as a SOCKS4 server. | |
845 | Multiple forwardings may be specified, and | |
846 | additional forwardings can be given on the command line. Only | |
847 | the superuser can forward privileged ports. | |
848 | .It Cm EscapeChar | |
849 | Sets the escape character (default: | |
850 | .Ql ~ ) . | |
851 | The escape character can also | |
852 | be set on the command line. | |
853 | The argument should be a single character, | |
854 | .Ql ^ | |
855 | followed by a letter, or | |
856 | .Dq none | |
857 | to disable the escape | |
858 | character entirely (making the connection transparent for binary | |
859 | data). | |
860 | .It Cm FallBackToRsh | |
861 | Specifies that if connecting via | |
862 | .Nm | |
863 | fails due to a connection refused error (there is no | |
864 | .Xr sshd 8 | |
865 | listening on the remote host), | |
866 | .Xr rsh 1 | |
867 | should automatically be used instead (after a suitable warning about | |
868 | the session being unencrypted). | |
869 | The argument must be | |
870 | .Dq yes | |
871 | or | |
872 | .Dq no . | |
873 | The default is | |
874 | .Dq no . | |
875 | .It Cm ForwardAgent | |
876 | Specifies whether the connection to the authentication agent (if any) | |
877 | will be forwarded to the remote machine. | |
878 | The argument must be | |
879 | .Dq yes | |
880 | or | |
881 | .Dq no . | |
882 | The default is | |
883 | .Dq no . | |
884 | .It Cm ForwardX11 | |
885 | Specifies whether X11 connections will be automatically redirected | |
886 | over the secure channel and | |
887 | .Ev DISPLAY | |
888 | set. | |
889 | The argument must be | |
890 | .Dq yes | |
891 | or | |
892 | .Dq no . | |
893 | The default is | |
894 | .Dq no . | |
895 | .It Cm GatewayPorts | |
896 | Specifies whether remote hosts are allowed to connect to local | |
897 | forwarded ports. | |
898 | By default, | |
899 | .Nm | |
900 | binds local port forwardings to the loopback addresss. This | |
901 | prevents other remote hosts from connecting to forwarded ports. | |
902 | .Cm GatewayPorts | |
903 | can be used to specify that | |
904 | .Nm | |
905 | should bind local port forwardings to the wildcard address, | |
906 | thus allowing remote hosts to connect to forwarded ports. | |
907 | The argument must be | |
908 | .Dq yes | |
909 | or | |
910 | .Dq no . | |
911 | The default is | |
912 | .Dq no . | |
913 | .It Cm GlobalKnownHostsFile | |
914 | Specifies a file to use for the global | |
915 | host key database instead of | |
db32a221 | 916 | .Pa /etc/ssh/ssh_known_hosts . |
917 | .It Cm GssapiAuthentication | |
918 | Specifies whether authentication based on GSSAPI may be used, either using | |
919 | the result of a successful key exchange, or using GSSAPI user | |
920 | authentication. | |
921 | The default is | |
922 | .Dq yes . | |
923 | .It Cm GssapiDelegateCredentials | |
924 | Specifies whether GSSAPI credentials will be delegated (forwarded) to | |
925 | the server. | |
926 | The default is | |
927 | .Dq yes . | |
3c0ef626 | 928 | .It Cm HostbasedAuthentication |
929 | Specifies whether to try rhosts based authentication with public key | |
930 | authentication. | |
931 | The argument must be | |
932 | .Dq yes | |
933 | or | |
934 | .Dq no . | |
935 | The default is | |
936 | .Dq no . | |
937 | This option applies to protocol version 2 only and | |
938 | is similar to | |
939 | .Cm RhostsRSAAuthentication . | |
940 | .It Cm HostKeyAlgorithms | |
941 | Specifies the protocol version 2 host key algorithms | |
942 | that the client wants to use in order of preference. | |
943 | The default for this option is: | |
db32a221 | 944 | .Dq ssh-rsa,ssh-dss . |
3c0ef626 | 945 | .It Cm HostKeyAlias |
946 | Specifies an alias that should be used instead of the | |
947 | real host name when looking up or saving the host key | |
948 | in the host key database files. | |
949 | This option is useful for tunneling ssh connections | |
950 | or for multiple servers running on a single host. | |
951 | .It Cm HostName | |
952 | Specifies the real host name to log into. | |
953 | This can be used to specify nicknames or abbreviations for hosts. | |
954 | Default is the name given on the command line. | |
955 | Numeric IP addresses are also permitted (both on the command line and in | |
956 | .Cm HostName | |
957 | specifications). | |
958 | .It Cm IdentityFile | |
db32a221 | 959 | Specifies a file from which the user's RSA or DSA authentication identity |
960 | is read. The default is | |
3c0ef626 | 961 | .Pa $HOME/.ssh/identity |
db32a221 | 962 | for protocol version 1, and |
963 | .Pa $HOME/.ssh/id_rsa | |
964 | and | |
965 | .Pa $HOME/.ssh/id_dsa | |
966 | for protocol version 2. | |
3c0ef626 | 967 | Additionally, any identities represented by the authentication agent |
968 | will be used for authentication. | |
969 | The file name may use the tilde | |
970 | syntax to refer to a user's home directory. | |
971 | It is possible to have | |
972 | multiple identity files specified in configuration files; all these | |
973 | identities will be tried in sequence. | |
974 | .It Cm KeepAlive | |
db32a221 | 975 | Specifies whether the system should send TCP keepalive messages to the |
3c0ef626 | 976 | other side. |
977 | If they are sent, death of the connection or crash of one | |
978 | of the machines will be properly noticed. | |
979 | However, this means that | |
980 | connections will die if the route is down temporarily, and some people | |
981 | find it annoying. | |
982 | .Pp | |
983 | The default is | |
984 | .Dq yes | |
985 | (to send keepalives), and the client will notice | |
986 | if the network goes down or the remote host dies. | |
987 | This is important in scripts, and many users want it too. | |
988 | .Pp | |
989 | To disable keepalives, the value should be set to | |
db32a221 | 990 | .Dq no . |
3c0ef626 | 991 | .It Cm KerberosAuthentication |
992 | Specifies whether Kerberos authentication will be used. | |
993 | The argument to this keyword must be | |
994 | .Dq yes | |
995 | or | |
996 | .Dq no . | |
997 | .It Cm KerberosTgtPassing | |
998 | Specifies whether a Kerberos TGT will be forwarded to the server. | |
999 | This will only work if the Kerberos server is actually an AFS kaserver. | |
1000 | The argument to this keyword must be | |
1001 | .Dq yes | |
1002 | or | |
1003 | .Dq no . | |
1004 | .It Cm LocalForward | |
1005 | Specifies that a TCP/IP port on the local machine be forwarded over | |
1006 | the secure channel to the specified host and port from the remote machine. | |
1007 | The first argument must be a port number, and the second must be | |
1008 | .Ar host:port . | |
1009 | IPv6 addresses can be specified with an alternative syntax: | |
1010 | .Ar host/port . | |
1011 | Multiple forwardings may be specified, and additional | |
1012 | forwardings can be given on the command line. | |
1013 | Only the superuser can forward privileged ports. | |
1014 | .It Cm LogLevel | |
1015 | Gives the verbosity level that is used when logging messages from | |
1016 | .Nm ssh . | |
1017 | The possible values are: | |
db32a221 | 1018 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. |
1019 | The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 | |
1020 | and DEBUG3 each specify higher levels of verbose output. | |
3c0ef626 | 1021 | .It Cm MACs |
1022 | Specifies the MAC (message authentication code) algorithms | |
1023 | in order of preference. | |
1024 | The MAC algorithm is used in protocol version 2 | |
1025 | for data integrity protection. | |
1026 | Multiple algorithms must be comma-separated. | |
1027 | The default is | |
1028 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
1029 | .It Cm NoHostAuthenticationForLocalhost | |
1030 | This option can be used if the home directory is shared across machines. | |
1031 | In this case localhost will refer to a different machine on each of | |
1032 | the machines and the user will get many warnings about changed host keys. | |
1033 | However, this option disables host authentication for localhost. | |
1034 | The argument to this keyword must be | |
1035 | .Dq yes | |
1036 | or | |
1037 | .Dq no . | |
1038 | The default is to check the host key for localhost. | |
1039 | .It Cm NumberOfPasswordPrompts | |
1040 | Specifies the number of password prompts before giving up. | |
1041 | The argument to this keyword must be an integer. | |
1042 | Default is 3. | |
1043 | .It Cm PasswordAuthentication | |
1044 | Specifies whether to use password authentication. | |
1045 | The argument to this keyword must be | |
1046 | .Dq yes | |
1047 | or | |
1048 | .Dq no . | |
1049 | The default is | |
1050 | .Dq yes . | |
1051 | .It Cm Port | |
1052 | Specifies the port number to connect on the remote host. | |
1053 | Default is 22. | |
1054 | .It Cm PreferredAuthentications | |
1055 | Specifies the order in which the client should try protocol 2 | |
1056 | authentication methods. This allows a client to prefer one method (e.g. | |
1057 | .Cm keyboard-interactive ) | |
1058 | over another method (e.g. | |
1059 | .Cm password ) | |
1060 | The default for this option is: | |
db32a221 | 1061 | .Dq hostbased,publickey,keyboard-interactive,password . |
3c0ef626 | 1062 | .It Cm Protocol |
1063 | Specifies the protocol versions | |
1064 | .Nm | |
1065 | should support in order of preference. | |
1066 | The possible values are | |
1067 | .Dq 1 | |
1068 | and | |
1069 | .Dq 2 . | |
1070 | Multiple versions must be comma-separated. | |
1071 | The default is | |
1072 | .Dq 2,1 . | |
1073 | This means that | |
1074 | .Nm | |
1075 | tries version 2 and falls back to version 1 | |
1076 | if version 2 is not available. | |
1077 | .It Cm ProxyCommand | |
1078 | Specifies the command to use to connect to the server. | |
1079 | The command | |
1080 | string extends to the end of the line, and is executed with | |
1081 | .Pa /bin/sh . | |
1082 | In the command string, | |
1083 | .Ql %h | |
1084 | will be substituted by the host name to | |
1085 | connect and | |
1086 | .Ql %p | |
1087 | by the port. | |
1088 | The command can be basically anything, | |
1089 | and should read from its standard input and write to its standard output. | |
1090 | It should eventually connect an | |
1091 | .Xr sshd 8 | |
1092 | server running on some machine, or execute | |
1093 | .Ic sshd -i | |
1094 | somewhere. | |
1095 | Host key management will be done using the | |
1096 | HostName of the host being connected (defaulting to the name typed by | |
1097 | the user). | |
1098 | Note that | |
1099 | .Cm CheckHostIP | |
1100 | is not available for connects with a proxy command. | |
1101 | .Pp | |
1102 | .It Cm PubkeyAuthentication | |
1103 | Specifies whether to try public key authentication. | |
1104 | The argument to this keyword must be | |
1105 | .Dq yes | |
1106 | or | |
1107 | .Dq no . | |
1108 | The default is | |
1109 | .Dq yes . | |
1110 | This option applies to protocol version 2 only. | |
1111 | .It Cm RemoteForward | |
1112 | Specifies that a TCP/IP port on the remote machine be forwarded over | |
1113 | the secure channel to the specified host and port from the local machine. | |
1114 | The first argument must be a port number, and the second must be | |
1115 | .Ar host:port . | |
1116 | IPv6 addresses can be specified with an alternative syntax: | |
1117 | .Ar host/port . | |
1118 | Multiple forwardings may be specified, and additional | |
1119 | forwardings can be given on the command line. | |
1120 | Only the superuser can forward privileged ports. | |
1121 | .It Cm RhostsAuthentication | |
1122 | Specifies whether to try rhosts based authentication. | |
1123 | Note that this | |
1124 | declaration only affects the client side and has no effect whatsoever | |
1125 | on security. | |
1126 | Disabling rhosts authentication may reduce | |
1127 | authentication time on slow connections when rhosts authentication is | |
1128 | not used. | |
1129 | Most servers do not permit RhostsAuthentication because it | |
1130 | is not secure (see | |
1131 | .Cm RhostsRSAAuthentication ) . | |
1132 | The argument to this keyword must be | |
1133 | .Dq yes | |
1134 | or | |
1135 | .Dq no . | |
1136 | The default is | |
1137 | .Dq yes . | |
1138 | This option applies to protocol version 1 only. | |
1139 | .It Cm RhostsRSAAuthentication | |
1140 | Specifies whether to try rhosts based authentication with RSA host | |
1141 | authentication. | |
1142 | The argument must be | |
1143 | .Dq yes | |
1144 | or | |
1145 | .Dq no . | |
1146 | The default is | |
1147 | .Dq yes . | |
1148 | This option applies to protocol version 1 only. | |
1149 | .It Cm RSAAuthentication | |
1150 | Specifies whether to try RSA authentication. | |
1151 | The argument to this keyword must be | |
1152 | .Dq yes | |
1153 | or | |
1154 | .Dq no . | |
1155 | RSA authentication will only be | |
1156 | attempted if the identity file exists, or an authentication agent is | |
1157 | running. | |
1158 | The default is | |
1159 | .Dq yes . | |
1160 | Note that this option applies to protocol version 1 only. | |
1161 | .It Cm ChallengeResponseAuthentication | |
1162 | Specifies whether to use challenge response authentication. | |
1163 | The argument to this keyword must be | |
1164 | .Dq yes | |
1165 | or | |
1166 | .Dq no . | |
1167 | The default is | |
1168 | .Dq yes . | |
1169 | .It Cm SmartcardDevice | |
1170 | Specifies which smartcard device to use. The argument to this keyword is | |
1171 | the device | |
1172 | .Nm | |
1173 | should use to communicate with a smartcard used for storing the user's | |
1174 | private RSA key. By default, no device is specified and smartcard support | |
1175 | is not activated. | |
1176 | .It Cm StrictHostKeyChecking | |
1177 | If this flag is set to | |
1178 | .Dq yes , | |
1179 | .Nm | |
1180 | will never automatically add host keys to the | |
1181 | .Pa $HOME/.ssh/known_hosts | |
1182 | file, and refuses to connect to hosts whose host key has changed. | |
1183 | This provides maximum protection against trojan horse attacks, | |
1184 | however, can be annoying when the | |
db32a221 | 1185 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 1186 | file is poorly maintained, or connections to new hosts are |
1187 | frequently made. | |
1188 | This option forces the user to manually | |
1189 | add all new hosts. | |
1190 | If this flag is set to | |
1191 | .Dq no , | |
1192 | .Nm | |
1193 | will automatically add new host keys to the | |
1194 | user known hosts files. | |
1195 | If this flag is set to | |
1196 | .Dq ask , | |
1197 | new host keys | |
1198 | will be added to the user known host files only after the user | |
1199 | has confirmed that is what they really want to do, and | |
1200 | .Nm | |
1201 | will refuse to connect to hosts whose host key has changed. | |
1202 | The host keys of | |
1203 | known hosts will be verified automatically in all cases. | |
1204 | The argument must be | |
1205 | .Dq yes , | |
1206 | .Dq no | |
1207 | or | |
1208 | .Dq ask . | |
1209 | The default is | |
1210 | .Dq ask . | |
1211 | .It Cm UsePrivilegedPort | |
1212 | Specifies whether to use a privileged port for outgoing connections. | |
1213 | The argument must be | |
1214 | .Dq yes | |
1215 | or | |
1216 | .Dq no . | |
1217 | The default is | |
1218 | .Dq no . | |
1219 | Note that this option must be set to | |
1220 | .Dq yes | |
1221 | if | |
1222 | .Cm RhostsAuthentication | |
1223 | and | |
1224 | .Cm RhostsRSAAuthentication | |
1225 | authentications are needed with older servers. | |
1226 | .It Cm User | |
1227 | Specifies the user to log in as. | |
1228 | This can be useful when a different user name is used on different machines. | |
1229 | This saves the trouble of | |
1230 | having to remember to give the user name on the command line. | |
1231 | .It Cm UserKnownHostsFile | |
1232 | Specifies a file to use for the user | |
1233 | host key database instead of | |
1234 | .Pa $HOME/.ssh/known_hosts . | |
1235 | .It Cm UseRsh | |
1236 | Specifies that rlogin/rsh should be used for this host. | |
1237 | It is possible that the host does not at all support the | |
1238 | .Nm | |
1239 | protocol. | |
1240 | This causes | |
1241 | .Nm | |
1242 | to immediately execute | |
1243 | .Xr rsh 1 . | |
1244 | All other options (except | |
1245 | .Cm HostName ) | |
1246 | are ignored if this has been specified. | |
1247 | The argument must be | |
1248 | .Dq yes | |
1249 | or | |
1250 | .Dq no . | |
1251 | .It Cm XAuthLocation | |
1252 | Specifies the location of the | |
1253 | .Xr xauth 1 | |
1254 | program. | |
1255 | The default is | |
1256 | .Pa /usr/X11R6/bin/xauth . | |
1257 | .El | |
1258 | .Sh ENVIRONMENT | |
1259 | .Nm | |
1260 | will normally set the following environment variables: | |
1261 | .Bl -tag -width Ds | |
1262 | .It Ev DISPLAY | |
1263 | The | |
1264 | .Ev DISPLAY | |
1265 | variable indicates the location of the X11 server. | |
1266 | It is automatically set by | |
1267 | .Nm | |
1268 | to point to a value of the form | |
1269 | .Dq hostname:n | |
1270 | where hostname indicates | |
1271 | the host where the shell runs, and n is an integer >= 1. | |
1272 | .Nm | |
1273 | uses this special value to forward X11 connections over the secure | |
1274 | channel. | |
1275 | The user should normally not set | |
1276 | .Ev DISPLAY | |
1277 | explicitly, as that | |
1278 | will render the X11 connection insecure (and will require the user to | |
1279 | manually copy any required authorization cookies). | |
1280 | .It Ev HOME | |
1281 | Set to the path of the user's home directory. | |
1282 | .It Ev LOGNAME | |
1283 | Synonym for | |
1284 | .Ev USER ; | |
1285 | set for compatibility with systems that use this variable. | |
1286 | .It Ev MAIL | |
1287 | Set to the path of the user's mailbox. | |
1288 | .It Ev PATH | |
1289 | Set to the default | |
1290 | .Ev PATH , | |
1291 | as specified when compiling | |
1292 | .Nm ssh . | |
1293 | .It Ev SSH_ASKPASS | |
1294 | If | |
1295 | .Nm | |
1296 | needs a passphrase, it will read the passphrase from the current | |
1297 | terminal if it was run from a terminal. | |
1298 | If | |
1299 | .Nm | |
1300 | does not have a terminal associated with it but | |
1301 | .Ev DISPLAY | |
1302 | and | |
1303 | .Ev SSH_ASKPASS | |
1304 | are set, it will execute the program specified by | |
1305 | .Ev SSH_ASKPASS | |
1306 | and open an X11 window to read the passphrase. | |
1307 | This is particularly useful when calling | |
1308 | .Nm | |
1309 | from a | |
1310 | .Pa .Xsession | |
1311 | or related script. | |
1312 | (Note that on some machines it | |
1313 | may be necessary to redirect the input from | |
1314 | .Pa /dev/null | |
1315 | to make this work.) | |
1316 | .It Ev SSH_AUTH_SOCK | |
1317 | Identifies the path of a unix-domain socket used to communicate with the | |
1318 | agent. | |
1319 | .It Ev SSH_CLIENT | |
1320 | Identifies the client end of the connection. | |
1321 | The variable contains | |
1322 | three space-separated values: client ip-address, client port number, | |
1323 | and server port number. | |
1324 | .It Ev SSH_ORIGINAL_COMMAND | |
1325 | The variable contains the original command line if a forced command | |
1326 | is executed. | |
1327 | It can be used to extract the original arguments. | |
1328 | .It Ev SSH_TTY | |
1329 | This is set to the name of the tty (path to the device) associated | |
1330 | with the current shell or command. | |
1331 | If the current session has no tty, | |
1332 | this variable is not set. | |
1333 | .It Ev TZ | |
1334 | The timezone variable is set to indicate the present timezone if it | |
1335 | was set when the daemon was started (i.e., the daemon passes the value | |
1336 | on to new connections). | |
1337 | .It Ev USER | |
1338 | Set to the name of the user logging in. | |
1339 | .El | |
1340 | .Pp | |
1341 | Additionally, | |
1342 | .Nm | |
1343 | reads | |
1344 | .Pa $HOME/.ssh/environment , | |
1345 | and adds lines of the format | |
1346 | .Dq VARNAME=value | |
1347 | to the environment. | |
1348 | .Sh FILES | |
1349 | .Bl -tag -width Ds | |
1350 | .It Pa $HOME/.ssh/known_hosts | |
1351 | Records host keys for all hosts the user has logged into that are not | |
1352 | in | |
db32a221 | 1353 | .Pa /etc/ssh/ssh_known_hosts . |
3c0ef626 | 1354 | See |
1355 | .Xr sshd 8 . | |
1356 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | |
1357 | Contains the authentication identity of the user. | |
1358 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
1359 | These files | |
1360 | contain sensitive data and should be readable by the user but not | |
1361 | accessible by others (read/write/execute). | |
1362 | Note that | |
1363 | .Nm | |
1364 | ignores a private key file if it is accessible by others. | |
1365 | It is possible to specify a passphrase when | |
1366 | generating the key; the passphrase will be used to encrypt the | |
1367 | sensitive part of this file using 3DES. | |
1368 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | |
1369 | Contains the public key for authentication (public part of the | |
1370 | identity file in human-readable form). | |
1371 | The contents of the | |
1372 | .Pa $HOME/.ssh/identity.pub | |
1373 | file should be added to | |
1374 | .Pa $HOME/.ssh/authorized_keys | |
1375 | on all machines | |
1376 | where the user wishes to log in using protocol version 1 RSA authentication. | |
1377 | The contents of the | |
1378 | .Pa $HOME/.ssh/id_dsa.pub | |
1379 | and | |
1380 | .Pa $HOME/.ssh/id_rsa.pub | |
1381 | file should be added to | |
1382 | .Pa $HOME/.ssh/authorized_keys | |
1383 | on all machines | |
1384 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. | |
1385 | These files are not | |
1386 | sensitive and can (but need not) be readable by anyone. | |
1387 | These files are | |
1388 | never used automatically and are not necessary; they are only provided for | |
1389 | the convenience of the user. | |
1390 | .It Pa $HOME/.ssh/config | |
1391 | This is the per-user configuration file. | |
1392 | The format of this file is described above. | |
1393 | This file is used by the | |
1394 | .Nm | |
1395 | client. | |
1396 | This file does not usually contain any sensitive information, | |
1397 | but the recommended permissions are read/write for the user, and not | |
1398 | accessible by others. | |
1399 | .It Pa $HOME/.ssh/authorized_keys | |
1400 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
1401 | The format of this file is described in the | |
1402 | .Xr sshd 8 | |
1403 | manual page. | |
1404 | In the simplest form the format is the same as the .pub | |
1405 | identity files. | |
1406 | This file is not highly sensitive, but the recommended | |
1407 | permissions are read/write for the user, and not accessible by others. | |
db32a221 | 1408 | .It Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 1409 | Systemwide list of known host keys. |
1410 | This file should be prepared by the | |
1411 | system administrator to contain the public host keys of all machines in the | |
1412 | organization. | |
1413 | This file should be world-readable. | |
1414 | This file contains | |
1415 | public keys, one per line, in the following format (fields separated | |
1416 | by spaces): system name, public key and optional comment field. | |
1417 | When different names are used | |
1418 | for the same machine, all such names should be listed, separated by | |
1419 | commas. | |
1420 | The format is described on the | |
1421 | .Xr sshd 8 | |
1422 | manual page. | |
1423 | .Pp | |
1424 | The canonical system name (as returned by name servers) is used by | |
1425 | .Xr sshd 8 | |
1426 | to verify the client host when logging in; other names are needed because | |
1427 | .Nm | |
1428 | does not convert the user-supplied name to a canonical name before | |
1429 | checking the key, because someone with access to the name servers | |
1430 | would then be able to fool host authentication. | |
db32a221 | 1431 | .It Pa /etc/ssh/ssh_config |
3c0ef626 | 1432 | Systemwide configuration file. |
1433 | This file provides defaults for those | |
1434 | values that are not specified in the user's configuration file, and | |
1435 | for those users who do not have a configuration file. | |
1436 | This file must be world-readable. | |
db32a221 | 1437 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
3c0ef626 | 1438 | These three files contain the private parts of the host keys |
1439 | and are used for | |
1440 | .Cm RhostsRSAAuthentication | |
1441 | and | |
1442 | .Cm HostbasedAuthentication . | |
1443 | Since they are readable only by root | |
1444 | .Nm | |
1445 | must be setuid root if these authentication methods are desired. | |
1446 | .It Pa $HOME/.rhosts | |
1447 | This file is used in | |
1448 | .Pa \&.rhosts | |
1449 | authentication to list the | |
1450 | host/user pairs that are permitted to log in. | |
1451 | (Note that this file is | |
1452 | also used by rlogin and rsh, which makes using this file insecure.) | |
1453 | Each line of the file contains a host name (in the canonical form | |
1454 | returned by name servers), and then a user name on that host, | |
1455 | separated by a space. | |
1456 | On some machines this file may need to be | |
1457 | world-readable if the user's home directory is on a NFS partition, | |
1458 | because | |
1459 | .Xr sshd 8 | |
1460 | reads it as root. | |
1461 | Additionally, this file must be owned by the user, | |
1462 | and must not have write permissions for anyone else. | |
1463 | The recommended | |
1464 | permission for most machines is read/write for the user, and not | |
1465 | accessible by others. | |
1466 | .Pp | |
1467 | Note that by default | |
1468 | .Xr sshd 8 | |
1469 | will be installed so that it requires successful RSA host | |
1470 | authentication before permitting \s+2.\s0rhosts authentication. | |
1471 | If the server machine does not have the client's host key in | |
db32a221 | 1472 | .Pa /etc/ssh/ssh_known_hosts , |
3c0ef626 | 1473 | it can be stored in |
1474 | .Pa $HOME/.ssh/known_hosts . | |
1475 | The easiest way to do this is to | |
1476 | connect back to the client from the server machine using ssh; this | |
1477 | will automatically add the host key to | |
1478 | .Pa $HOME/.ssh/known_hosts . | |
1479 | .It Pa $HOME/.shosts | |
1480 | This file is used exactly the same way as | |
1481 | .Pa \&.rhosts . | |
1482 | The purpose for | |
1483 | having this file is to be able to use rhosts authentication with | |
1484 | .Nm | |
1485 | without permitting login with | |
db32a221 | 1486 | .Nm rlogin |
3c0ef626 | 1487 | or |
1488 | .Xr rsh 1 . | |
1489 | .It Pa /etc/hosts.equiv | |
1490 | This file is used during | |
1491 | .Pa \&.rhosts authentication. | |
1492 | It contains | |
1493 | canonical hosts names, one per line (the full format is described on | |
1494 | the | |
1495 | .Xr sshd 8 | |
1496 | manual page). | |
1497 | If the client host is found in this file, login is | |
1498 | automatically permitted provided client and server user names are the | |
1499 | same. | |
1500 | Additionally, successful RSA host authentication is normally | |
1501 | required. | |
1502 | This file should only be writable by root. | |
1503 | .It Pa /etc/shosts.equiv | |
1504 | This file is processed exactly as | |
1505 | .Pa /etc/hosts.equiv . | |
1506 | This file may be useful to permit logins using | |
1507 | .Nm | |
1508 | but not using rsh/rlogin. | |
db32a221 | 1509 | .It Pa /etc/ssh/sshrc |
3c0ef626 | 1510 | Commands in this file are executed by |
1511 | .Nm | |
1512 | when the user logs in just before the user's shell (or command) is started. | |
1513 | See the | |
1514 | .Xr sshd 8 | |
1515 | manual page for more information. | |
1516 | .It Pa $HOME/.ssh/rc | |
1517 | Commands in this file are executed by | |
1518 | .Nm | |
1519 | when the user logs in just before the user's shell (or command) is | |
1520 | started. | |
1521 | See the | |
1522 | .Xr sshd 8 | |
1523 | manual page for more information. | |
1524 | .It Pa $HOME/.ssh/environment | |
1525 | Contains additional definitions for environment variables, see section | |
1526 | .Sx ENVIRONMENT | |
1527 | above. | |
1528 | .El | |
db32a221 | 1529 | .Sh DIAGNOSTICS |
1530 | .Nm | |
1531 | exits with the exit status of the remote command or with 255 | |
1532 | if an error occurred. | |
3c0ef626 | 1533 | .Sh AUTHORS |
1534 | OpenSSH is a derivative of the original and free | |
1535 | ssh 1.2.12 release by Tatu Ylonen. | |
1536 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1537 | Theo de Raadt and Dug Song | |
1538 | removed many bugs, re-added newer features and | |
1539 | created OpenSSH. | |
1540 | Markus Friedl contributed the support for SSH | |
1541 | protocol versions 1.5 and 2.0. | |
1542 | .Sh SEE ALSO | |
3c0ef626 | 1543 | .Xr rsh 1 , |
1544 | .Xr scp 1 , | |
1545 | .Xr sftp 1 , | |
1546 | .Xr ssh-add 1 , | |
1547 | .Xr ssh-agent 1 , | |
1548 | .Xr ssh-keygen 1 , | |
1549 | .Xr telnet 1 , | |
1550 | .Xr sshd 8 | |
1551 | .Rs | |
1552 | .%A T. Ylonen | |
1553 | .%A T. Kivinen | |
1554 | .%A M. Saarinen | |
1555 | .%A T. Rinne | |
1556 | .%A S. Lehtinen | |
1557 | .%T "SSH Protocol Architecture" | |
db32a221 | 1558 | .%N draft-ietf-secsh-architecture-12.txt |
1559 | .%D January 2002 | |
3c0ef626 | 1560 | .%O work in progress material |
1561 | .Re |