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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 | .\" | |
44a053a3 | 37 | .\" $OpenBSD: ssh.1,v 1.158 2002/06/20 19:56:07 stevesk 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 | |
44a053a3 | 387 | option can be used to prevent logins to machines whose |
3c0ef626 | 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 | |
44a053a3 | 571 | option. |
3c0ef626 | 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 | |
44a053a3 | 577 | option. |
3c0ef626 | 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 | |
44a053a3 | 652 | may additionally obtain configuration data from |
653 | a per-user configuration file and a system-wide configuration file. | |
654 | The file format and configuration options are described in | |
655 | .Xr ssh_config 5 . | |
3c0ef626 | 656 | .Sh ENVIRONMENT |
657 | .Nm | |
658 | will normally set the following environment variables: | |
659 | .Bl -tag -width Ds | |
660 | .It Ev DISPLAY | |
661 | The | |
662 | .Ev DISPLAY | |
663 | variable indicates the location of the X11 server. | |
664 | It is automatically set by | |
665 | .Nm | |
666 | to point to a value of the form | |
667 | .Dq hostname:n | |
668 | where hostname indicates | |
669 | the host where the shell runs, and n is an integer >= 1. | |
670 | .Nm | |
671 | uses this special value to forward X11 connections over the secure | |
672 | channel. | |
673 | The user should normally not set | |
674 | .Ev DISPLAY | |
675 | explicitly, as that | |
676 | will render the X11 connection insecure (and will require the user to | |
677 | manually copy any required authorization cookies). | |
678 | .It Ev HOME | |
679 | Set to the path of the user's home directory. | |
680 | .It Ev LOGNAME | |
681 | Synonym for | |
682 | .Ev USER ; | |
683 | set for compatibility with systems that use this variable. | |
684 | .It Ev MAIL | |
685 | Set to the path of the user's mailbox. | |
686 | .It Ev PATH | |
687 | Set to the default | |
688 | .Ev PATH , | |
689 | as specified when compiling | |
690 | .Nm ssh . | |
691 | .It Ev SSH_ASKPASS | |
692 | If | |
693 | .Nm | |
694 | needs a passphrase, it will read the passphrase from the current | |
695 | terminal if it was run from a terminal. | |
696 | If | |
697 | .Nm | |
698 | does not have a terminal associated with it but | |
699 | .Ev DISPLAY | |
700 | and | |
701 | .Ev SSH_ASKPASS | |
702 | are set, it will execute the program specified by | |
703 | .Ev SSH_ASKPASS | |
704 | and open an X11 window to read the passphrase. | |
705 | This is particularly useful when calling | |
706 | .Nm | |
707 | from a | |
708 | .Pa .Xsession | |
709 | or related script. | |
710 | (Note that on some machines it | |
711 | may be necessary to redirect the input from | |
712 | .Pa /dev/null | |
713 | to make this work.) | |
714 | .It Ev SSH_AUTH_SOCK | |
715 | Identifies the path of a unix-domain socket used to communicate with the | |
716 | agent. | |
717 | .It Ev SSH_CLIENT | |
718 | Identifies the client end of the connection. | |
719 | The variable contains | |
720 | three space-separated values: client ip-address, client port number, | |
721 | and server port number. | |
722 | .It Ev SSH_ORIGINAL_COMMAND | |
723 | The variable contains the original command line if a forced command | |
724 | is executed. | |
725 | It can be used to extract the original arguments. | |
726 | .It Ev SSH_TTY | |
727 | This is set to the name of the tty (path to the device) associated | |
728 | with the current shell or command. | |
729 | If the current session has no tty, | |
730 | this variable is not set. | |
731 | .It Ev TZ | |
732 | The timezone variable is set to indicate the present timezone if it | |
733 | was set when the daemon was started (i.e., the daemon passes the value | |
734 | on to new connections). | |
735 | .It Ev USER | |
736 | Set to the name of the user logging in. | |
737 | .El | |
738 | .Pp | |
739 | Additionally, | |
740 | .Nm | |
741 | reads | |
742 | .Pa $HOME/.ssh/environment , | |
743 | and adds lines of the format | |
744 | .Dq VARNAME=value | |
745 | to the environment. | |
746 | .Sh FILES | |
747 | .Bl -tag -width Ds | |
748 | .It Pa $HOME/.ssh/known_hosts | |
749 | Records host keys for all hosts the user has logged into that are not | |
750 | in | |
db32a221 | 751 | .Pa /etc/ssh/ssh_known_hosts . |
3c0ef626 | 752 | See |
753 | .Xr sshd 8 . | |
754 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | |
755 | Contains the authentication identity of the user. | |
756 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
757 | These files | |
758 | contain sensitive data and should be readable by the user but not | |
759 | accessible by others (read/write/execute). | |
760 | Note that | |
761 | .Nm | |
762 | ignores a private key file if it is accessible by others. | |
763 | It is possible to specify a passphrase when | |
764 | generating the key; the passphrase will be used to encrypt the | |
765 | sensitive part of this file using 3DES. | |
766 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | |
767 | Contains the public key for authentication (public part of the | |
768 | identity file in human-readable form). | |
769 | The contents of the | |
770 | .Pa $HOME/.ssh/identity.pub | |
771 | file should be added to | |
772 | .Pa $HOME/.ssh/authorized_keys | |
773 | on all machines | |
774 | where the user wishes to log in using protocol version 1 RSA authentication. | |
775 | The contents of the | |
776 | .Pa $HOME/.ssh/id_dsa.pub | |
777 | and | |
778 | .Pa $HOME/.ssh/id_rsa.pub | |
779 | file should be added to | |
780 | .Pa $HOME/.ssh/authorized_keys | |
781 | on all machines | |
782 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. | |
783 | These files are not | |
784 | sensitive and can (but need not) be readable by anyone. | |
785 | These files are | |
786 | never used automatically and are not necessary; they are only provided for | |
787 | the convenience of the user. | |
788 | .It Pa $HOME/.ssh/config | |
789 | This is the per-user configuration file. | |
44a053a3 | 790 | The file format and configuration options are described in |
791 | .Xr ssh_config 5 . | |
3c0ef626 | 792 | .It Pa $HOME/.ssh/authorized_keys |
793 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
794 | The format of this file is described in the | |
795 | .Xr sshd 8 | |
796 | manual page. | |
797 | In the simplest form the format is the same as the .pub | |
798 | identity files. | |
799 | This file is not highly sensitive, but the recommended | |
800 | permissions are read/write for the user, and not accessible by others. | |
db32a221 | 801 | .It Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 802 | Systemwide list of known host keys. |
803 | This file should be prepared by the | |
804 | system administrator to contain the public host keys of all machines in the | |
805 | organization. | |
806 | This file should be world-readable. | |
807 | This file contains | |
808 | public keys, one per line, in the following format (fields separated | |
809 | by spaces): system name, public key and optional comment field. | |
810 | When different names are used | |
811 | for the same machine, all such names should be listed, separated by | |
812 | commas. | |
813 | The format is described on the | |
814 | .Xr sshd 8 | |
815 | manual page. | |
816 | .Pp | |
817 | The canonical system name (as returned by name servers) is used by | |
818 | .Xr sshd 8 | |
819 | to verify the client host when logging in; other names are needed because | |
820 | .Nm | |
821 | does not convert the user-supplied name to a canonical name before | |
822 | checking the key, because someone with access to the name servers | |
823 | would then be able to fool host authentication. | |
db32a221 | 824 | .It Pa /etc/ssh/ssh_config |
3c0ef626 | 825 | Systemwide configuration file. |
44a053a3 | 826 | The file format and configuration options are described in |
827 | .Xr ssh_config 5 . | |
db32a221 | 828 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
3c0ef626 | 829 | These three files contain the private parts of the host keys |
830 | and are used for | |
831 | .Cm RhostsRSAAuthentication | |
832 | and | |
833 | .Cm HostbasedAuthentication . | |
44a053a3 | 834 | If the protocol version 1 |
835 | .Cm RhostsRSAAuthentication | |
836 | method is used, | |
837 | .Nm | |
838 | must be setuid root, since the host key is readable only by root. | |
839 | For protocol version 2, | |
840 | .Nm | |
841 | uses | |
842 | .Xr ssh-keysign 8 | |
843 | to access the host keys for | |
844 | .Cm HostbasedAuthentication . | |
845 | This eliminates the requirement that | |
846 | .Nm | |
847 | be setuid root when that authentication method is used. | |
848 | By default | |
3c0ef626 | 849 | .Nm |
44a053a3 | 850 | is not setuid root. |
3c0ef626 | 851 | .It Pa $HOME/.rhosts |
852 | This file is used in | |
853 | .Pa \&.rhosts | |
854 | authentication to list the | |
855 | host/user pairs that are permitted to log in. | |
856 | (Note that this file is | |
857 | also used by rlogin and rsh, which makes using this file insecure.) | |
858 | Each line of the file contains a host name (in the canonical form | |
859 | returned by name servers), and then a user name on that host, | |
860 | separated by a space. | |
861 | On some machines this file may need to be | |
862 | world-readable if the user's home directory is on a NFS partition, | |
863 | because | |
864 | .Xr sshd 8 | |
865 | reads it as root. | |
866 | Additionally, this file must be owned by the user, | |
867 | and must not have write permissions for anyone else. | |
868 | The recommended | |
869 | permission for most machines is read/write for the user, and not | |
870 | accessible by others. | |
871 | .Pp | |
872 | Note that by default | |
873 | .Xr sshd 8 | |
874 | will be installed so that it requires successful RSA host | |
875 | authentication before permitting \s+2.\s0rhosts authentication. | |
876 | If the server machine does not have the client's host key in | |
db32a221 | 877 | .Pa /etc/ssh/ssh_known_hosts , |
3c0ef626 | 878 | it can be stored in |
879 | .Pa $HOME/.ssh/known_hosts . | |
880 | The easiest way to do this is to | |
881 | connect back to the client from the server machine using ssh; this | |
882 | will automatically add the host key to | |
883 | .Pa $HOME/.ssh/known_hosts . | |
884 | .It Pa $HOME/.shosts | |
885 | This file is used exactly the same way as | |
886 | .Pa \&.rhosts . | |
887 | The purpose for | |
888 | having this file is to be able to use rhosts authentication with | |
889 | .Nm | |
890 | without permitting login with | |
db32a221 | 891 | .Nm rlogin |
3c0ef626 | 892 | or |
893 | .Xr rsh 1 . | |
894 | .It Pa /etc/hosts.equiv | |
895 | This file is used during | |
896 | .Pa \&.rhosts authentication. | |
897 | It contains | |
898 | canonical hosts names, one per line (the full format is described on | |
899 | the | |
900 | .Xr sshd 8 | |
901 | manual page). | |
902 | If the client host is found in this file, login is | |
903 | automatically permitted provided client and server user names are the | |
904 | same. | |
905 | Additionally, successful RSA host authentication is normally | |
906 | required. | |
907 | This file should only be writable by root. | |
908 | .It Pa /etc/shosts.equiv | |
909 | This file is processed exactly as | |
910 | .Pa /etc/hosts.equiv . | |
911 | This file may be useful to permit logins using | |
912 | .Nm | |
913 | but not using rsh/rlogin. | |
db32a221 | 914 | .It Pa /etc/ssh/sshrc |
3c0ef626 | 915 | Commands in this file are executed by |
916 | .Nm | |
917 | when the user logs in just before the user's shell (or command) is started. | |
918 | See the | |
919 | .Xr sshd 8 | |
920 | manual page for more information. | |
921 | .It Pa $HOME/.ssh/rc | |
922 | Commands in this file are executed by | |
923 | .Nm | |
924 | when the user logs in just before the user's shell (or command) is | |
925 | started. | |
926 | See the | |
927 | .Xr sshd 8 | |
928 | manual page for more information. | |
929 | .It Pa $HOME/.ssh/environment | |
930 | Contains additional definitions for environment variables, see section | |
931 | .Sx ENVIRONMENT | |
932 | above. | |
933 | .El | |
db32a221 | 934 | .Sh DIAGNOSTICS |
935 | .Nm | |
936 | exits with the exit status of the remote command or with 255 | |
937 | if an error occurred. | |
3c0ef626 | 938 | .Sh AUTHORS |
939 | OpenSSH is a derivative of the original and free | |
940 | ssh 1.2.12 release by Tatu Ylonen. | |
941 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
942 | Theo de Raadt and Dug Song | |
943 | removed many bugs, re-added newer features and | |
944 | created OpenSSH. | |
945 | Markus Friedl contributed the support for SSH | |
946 | protocol versions 1.5 and 2.0. | |
947 | .Sh SEE ALSO | |
3c0ef626 | 948 | .Xr rsh 1 , |
949 | .Xr scp 1 , | |
950 | .Xr sftp 1 , | |
951 | .Xr ssh-add 1 , | |
952 | .Xr ssh-agent 1 , | |
953 | .Xr ssh-keygen 1 , | |
954 | .Xr telnet 1 , | |
44a053a3 | 955 | .Xr ssh_config 4 , |
956 | .Xr ssh-keysign 8, | |
3c0ef626 | 957 | .Xr sshd 8 |
958 | .Rs | |
959 | .%A T. Ylonen | |
960 | .%A T. Kivinen | |
961 | .%A M. Saarinen | |
962 | .%A T. Rinne | |
963 | .%A S. Lehtinen | |
964 | .%T "SSH Protocol Architecture" | |
db32a221 | 965 | .%N draft-ietf-secsh-architecture-12.txt |
966 | .%D January 2002 | |
3c0ef626 | 967 | .%O work in progress material |
968 | .Re |