]>
Commit | Line | Data |
---|---|---|
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 | .\" | |
37 | .\" $OpenBSD: ssh.1,v 1.177 2003/10/08 08:27:36 jmc Exp $ | |
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 1246AaCfgkNnqsTtVvXx | |
47 | .Op Fl b Ar bind_address | |
48 | .Op Fl c Ar cipher_spec | |
49 | .Op Fl D Ar port | |
50 | .Op Fl e Ar escape_char | |
51 | .Op Fl F Ar configfile | |
52 | .Op Fl i Ar identity_file | |
53 | .Bk -words | |
54 | .Oo Fl L Xo | |
55 | .Sm off | |
56 | .Ar port : | |
57 | .Ar host : | |
58 | .Ar hostport | |
59 | .Sm on | |
60 | .Xc | |
61 | .Oc | |
62 | .Ek | |
63 | .Op Fl l Ar login_name | |
64 | .Op Fl m Ar mac_spec | |
65 | .Op Fl o Ar option | |
66 | .Bk -words | |
67 | .Op Fl p Ar port | |
68 | .Ek | |
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 | .Oo Ar user Ns @ Oc Ns Ar hostname | |
78 | .Op Ar command | |
79 | .Sh DESCRIPTION | |
80 | .Nm | |
81 | (SSH client) is a program for logging into a remote machine and for | |
82 | executing commands on a remote machine. | |
83 | It is intended to replace rlogin and rsh, | |
84 | and provide secure encrypted communications between | |
85 | two untrusted hosts over an insecure network. | |
86 | X11 connections and arbitrary TCP/IP ports | |
87 | can also be forwarded over the secure channel. | |
88 | .Pp | |
89 | .Nm | |
90 | connects and logs into the specified | |
91 | .Ar hostname | |
92 | (with optional | |
93 | .Ar user | |
94 | name). | |
95 | The user must prove | |
96 | his/her identity to the remote machine using one of several methods | |
97 | depending on the protocol version used. | |
98 | .Pp | |
99 | If | |
100 | .Ar command | |
101 | is specified, | |
102 | .Ar command | |
103 | is executed on the remote host instead of a login shell. | |
104 | .Ss SSH protocol version 1 | |
105 | First, if the machine the user logs in from is listed in | |
106 | .Pa /etc/hosts.equiv | |
107 | or | |
108 | .Pa /etc/shosts.equiv | |
109 | on the remote machine, and the user names are | |
110 | the same on both sides, the user is immediately permitted to log in. | |
111 | Second, if | |
112 | .Pa .rhosts | |
113 | or | |
114 | .Pa .shosts | |
115 | exists in the user's home directory on the | |
116 | remote machine and contains a line containing the name of the client | |
117 | machine and the name of the user on that machine, the user is | |
118 | permitted to log in. | |
119 | This form of authentication alone is normally not | |
120 | allowed by the server because it is not secure. | |
121 | .Pp | |
122 | The second authentication method is the | |
123 | .Em rhosts | |
124 | or | |
125 | .Em hosts.equiv | |
126 | method combined with RSA-based host authentication. | |
127 | It means that if the login would be permitted by | |
128 | .Pa $HOME/.rhosts , | |
129 | .Pa $HOME/.shosts , | |
130 | .Pa /etc/hosts.equiv , | |
131 | or | |
132 | .Pa /etc/shosts.equiv , | |
133 | and if additionally the server can verify the client's | |
134 | host key (see | |
135 | .Pa /etc/ssh/ssh_known_hosts | |
136 | and | |
137 | .Pa $HOME/.ssh/known_hosts | |
138 | in the | |
139 | .Sx FILES | |
140 | section), only then is login permitted. | |
141 | This authentication method closes security holes due to IP | |
142 | spoofing, DNS spoofing and routing spoofing. | |
143 | [Note to the administrator: | |
144 | .Pa /etc/hosts.equiv , | |
145 | .Pa $HOME/.rhosts , | |
146 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
147 | disabled if security is desired.] | |
148 | .Pp | |
149 | As a third authentication method, | |
150 | .Nm | |
151 | supports RSA based authentication. | |
152 | The scheme is based on public-key cryptography: there are cryptosystems | |
153 | where encryption and decryption are done using separate keys, and it | |
154 | is not possible to derive the decryption key from the encryption key. | |
155 | RSA is one such system. | |
156 | The idea is that each user creates a public/private | |
157 | key pair for authentication purposes. | |
158 | The server knows the public key, and only the user knows the private key. | |
159 | .Pp | |
160 | The file | |
161 | .Pa $HOME/.ssh/authorized_keys | |
162 | lists the public keys that are permitted for logging in. | |
163 | When the user logs in, the | |
164 | .Nm | |
165 | program tells the server which key pair it would like to use for | |
166 | authentication. | |
167 | The server checks if this key is permitted, and if so, | |
168 | sends the user (actually the | |
169 | .Nm | |
170 | program running on behalf of the user) a challenge, a random number, | |
171 | encrypted by the user's public key. | |
172 | The challenge can only be decrypted using the proper private key. | |
173 | The user's client then decrypts the challenge using the private key, | |
174 | proving that he/she knows the private key | |
175 | but without disclosing it to the server. | |
176 | .Pp | |
177 | .Nm | |
178 | implements the RSA authentication protocol automatically. | |
179 | The user creates his/her RSA key pair by running | |
180 | .Xr ssh-keygen 1 . | |
181 | This stores the private key in | |
182 | .Pa $HOME/.ssh/identity | |
183 | and stores the public key in | |
184 | .Pa $HOME/.ssh/identity.pub | |
185 | in the user's home directory. | |
186 | The user should then copy the | |
187 | .Pa identity.pub | |
188 | to | |
189 | .Pa $HOME/.ssh/authorized_keys | |
190 | in his/her home directory on the remote machine (the | |
191 | .Pa authorized_keys | |
192 | file corresponds to the conventional | |
193 | .Pa $HOME/.rhosts | |
194 | file, and has one key | |
195 | per line, though the lines can be very long). | |
196 | After this, the user can log in without giving the password. | |
197 | RSA authentication is much more secure than | |
198 | .Em rhosts | |
199 | authentication. | |
200 | .Pp | |
201 | The most convenient way to use RSA authentication may be with an | |
202 | authentication agent. | |
203 | See | |
204 | .Xr ssh-agent 1 | |
205 | for more information. | |
206 | .Pp | |
207 | If other authentication methods fail, | |
208 | .Nm | |
209 | prompts the user for a password. | |
210 | The password is sent to the remote | |
211 | host for checking; however, since all communications are encrypted, | |
212 | the password cannot be seen by someone listening on the network. | |
213 | .Ss SSH protocol version 2 | |
214 | When a user connects using protocol version 2, | |
215 | similar authentication methods are available. | |
216 | Using the default values for | |
217 | .Cm PreferredAuthentications , | |
218 | the client will try to authenticate first using the hostbased method; | |
219 | if this method fails, public key authentication is attempted, | |
220 | and finally if this method fails, keyboard-interactive and | |
221 | password authentication are tried. | |
222 | .Pp | |
223 | The public key method is similar to RSA authentication described | |
224 | in the previous section and allows the RSA or DSA algorithm to be used: | |
225 | The client uses his private key, | |
226 | .Pa $HOME/.ssh/id_dsa | |
227 | or | |
228 | .Pa $HOME/.ssh/id_rsa , | |
229 | to sign the session identifier and sends the result to the server. | |
230 | The server checks whether the matching public key is listed in | |
231 | .Pa $HOME/.ssh/authorized_keys | |
232 | and grants access if both the key is found and the signature is correct. | |
233 | The session identifier is derived from a shared Diffie-Hellman value | |
234 | and is only known to the client and the server. | |
235 | .Pp | |
236 | If public key authentication fails or is not available, a password | |
237 | can be sent encrypted to the remote host to prove the user's identity. | |
238 | .Pp | |
239 | Additionally, | |
240 | .Nm | |
241 | supports hostbased or challenge response authentication. | |
242 | .Pp | |
243 | Protocol 2 provides additional mechanisms for confidentiality | |
244 | (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) | |
245 | and integrity (hmac-md5, hmac-sha1). | |
246 | Note that protocol 1 lacks a strong mechanism for ensuring the | |
247 | integrity of the connection. | |
248 | .Ss Login session and remote execution | |
249 | When the user's identity has been accepted by the server, the server | |
250 | either executes the given command, or logs into the machine and gives | |
251 | the user a normal shell on the remote machine. | |
252 | All communication with | |
253 | the remote command or shell will be automatically encrypted. | |
254 | .Pp | |
255 | If a pseudo-terminal has been allocated (normal login session), the | |
256 | user may use the escape characters noted below. | |
257 | .Pp | |
258 | If no pseudo-tty has been allocated, | |
259 | the session is transparent and can be used to reliably transfer binary data. | |
260 | On most systems, setting the escape character to | |
261 | .Dq none | |
262 | will also make the session transparent even if a tty is used. | |
263 | .Pp | |
264 | The session terminates when the command or shell on the remote | |
265 | machine exits and all X11 and TCP/IP connections have been closed. | |
266 | The exit status of the remote program is returned as the exit status of | |
267 | .Nm ssh . | |
268 | .Ss Escape Characters | |
269 | When a pseudo-terminal has been requested, | |
270 | .Nm | |
271 | supports a number of functions through the use of an escape character. | |
272 | .Pp | |
273 | A single tilde character can be sent as | |
274 | .Ic ~~ | |
275 | or by following the tilde by a character other than those described below. | |
276 | The escape character must always follow a newline to be interpreted as | |
277 | special. | |
278 | The escape character can be changed in configuration files using the | |
279 | .Cm EscapeChar | |
280 | configuration directive or on the command line by the | |
281 | .Fl e | |
282 | option. | |
283 | .Pp | |
284 | The supported escapes (assuming the default | |
285 | .Ql ~ ) | |
286 | are: | |
287 | .Bl -tag -width Ds | |
288 | .It Cm ~. | |
289 | Disconnect. | |
290 | .It Cm ~^Z | |
291 | Background | |
292 | .Nm ssh . | |
293 | .It Cm ~# | |
294 | List forwarded connections. | |
295 | .It Cm ~& | |
296 | Background | |
297 | .Nm | |
298 | at logout when waiting for forwarded connection / X11 sessions to terminate. | |
299 | .It Cm ~? | |
300 | Display a list of escape characters. | |
301 | .It Cm ~B | |
302 | Send a BREAK to the remote system | |
303 | (only useful for SSH protocol version 2 and if the peer supports it). | |
304 | .It Cm ~C | |
305 | Open command line (only useful for adding port forwardings using the | |
306 | .Fl L | |
307 | and | |
308 | .Fl R | |
309 | options). | |
310 | .It Cm ~R | |
311 | Request rekeying of the connection | |
312 | (only useful for SSH protocol version 2 and if the peer supports it). | |
313 | .El | |
314 | .Ss X11 and TCP forwarding | |
315 | If the | |
316 | .Cm ForwardX11 | |
317 | variable is set to | |
318 | .Dq yes | |
319 | (or see the description of the | |
320 | .Fl X | |
321 | and | |
322 | .Fl x | |
323 | options described later) | |
324 | and the user is using X11 (the | |
325 | .Ev DISPLAY | |
326 | environment variable is set), the connection to the X11 display is | |
327 | automatically forwarded to the remote side in such a way that any X11 | |
328 | programs started from the shell (or command) will go through the | |
329 | encrypted channel, and the connection to the real X server will be made | |
330 | from the local machine. | |
331 | The user should not manually set | |
332 | .Ev DISPLAY . | |
333 | Forwarding of X11 connections can be | |
334 | configured on the command line or in configuration files. | |
335 | .Pp | |
336 | The | |
337 | .Ev DISPLAY | |
338 | value set by | |
339 | .Nm | |
340 | will point to the server machine, but with a display number greater than zero. | |
341 | This is normal, and happens because | |
342 | .Nm | |
343 | creates a | |
344 | .Dq proxy | |
345 | X server on the server machine for forwarding the | |
346 | connections over the encrypted channel. | |
347 | .Pp | |
348 | .Nm | |
349 | will also automatically set up Xauthority data on the server machine. | |
350 | For this purpose, it will generate a random authorization cookie, | |
351 | store it in Xauthority on the server, and verify that any forwarded | |
352 | connections carry this cookie and replace it by the real cookie when | |
353 | the connection is opened. | |
354 | The real authentication cookie is never | |
355 | sent to the server machine (and no cookies are sent in the plain). | |
356 | .Pp | |
357 | If the | |
358 | .Cm ForwardAgent | |
359 | variable is set to | |
360 | .Dq yes | |
361 | (or see the description of the | |
362 | .Fl A | |
363 | and | |
364 | .Fl a | |
365 | options described later) and | |
366 | the user is using an authentication agent, the connection to the agent | |
367 | is automatically forwarded to the remote side. | |
368 | .Pp | |
369 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
370 | be specified either on the command line or in a configuration file. | |
371 | One possible application of TCP/IP forwarding is a secure connection to an | |
372 | electronic purse; another is going through firewalls. | |
373 | .Ss Server authentication | |
374 | .Nm | |
375 | automatically maintains and checks a database containing | |
376 | identifications for all hosts it has ever been used with. | |
377 | Host keys are stored in | |
378 | .Pa $HOME/.ssh/known_hosts | |
379 | in the user's home directory. | |
380 | Additionally, the file | |
381 | .Pa /etc/ssh/ssh_known_hosts | |
382 | is automatically checked for known hosts. | |
383 | Any new hosts are automatically added to the user's file. | |
384 | If a host's identification ever changes, | |
385 | .Nm | |
386 | warns about this and disables password authentication to prevent a | |
387 | trojan horse from getting the user's password. | |
388 | Another purpose of this mechanism is to prevent man-in-the-middle attacks | |
389 | which could otherwise be used to circumvent the encryption. | |
390 | The | |
391 | .Cm StrictHostKeyChecking | |
392 | option can be used to prevent logins to machines whose | |
393 | host key is not known or has changed. | |
394 | .Pp | |
395 | The options are as follows: | |
396 | .Bl -tag -width Ds | |
397 | .It Fl 1 | |
398 | Forces | |
399 | .Nm | |
400 | to try protocol version 1 only. | |
401 | .It Fl 2 | |
402 | Forces | |
403 | .Nm | |
404 | to try protocol version 2 only. | |
405 | .It Fl 4 | |
406 | Forces | |
407 | .Nm | |
408 | to use IPv4 addresses only. | |
409 | .It Fl 6 | |
410 | Forces | |
411 | .Nm | |
412 | to use IPv6 addresses only. | |
413 | .It Fl A | |
414 | Enables forwarding of the authentication agent connection. | |
415 | This can also be specified on a per-host basis in a configuration file. | |
416 | .Pp | |
417 | Agent forwarding should be enabled with caution. | |
418 | Users with the ability to bypass file permissions on the remote host | |
419 | (for the agent's Unix-domain socket) | |
420 | can access the local agent through the forwarded connection. | |
421 | An attacker cannot obtain key material from the agent, | |
422 | however they can perform operations on the keys that enable them to | |
423 | authenticate using the identities loaded into the agent. | |
424 | .It Fl a | |
425 | Disables forwarding of the authentication agent connection. | |
426 | .It Fl b Ar bind_address | |
427 | Specify the interface to transmit from on machines with multiple | |
428 | interfaces or aliased addresses. | |
429 | .It Fl C | |
430 | Requests compression of all data (including stdin, stdout, stderr, and | |
431 | data for forwarded X11 and TCP/IP connections). | |
432 | The compression algorithm is the same used by | |
433 | .Xr gzip 1 , | |
434 | and the | |
435 | .Dq level | |
436 | can be controlled by the | |
437 | .Cm CompressionLevel | |
438 | option for protocol version 1. | |
439 | Compression is desirable on modem lines and other | |
440 | slow connections, but will only slow down things on fast networks. | |
441 | The default value can be set on a host-by-host basis in the | |
442 | configuration files; see the | |
443 | .Cm Compression | |
444 | option. | |
445 | .It Fl c Ar blowfish | 3des | des | |
446 | Selects the cipher to use for encrypting the session. | |
447 | .Ar 3des | |
448 | is used by default. | |
449 | It is believed to be secure. | |
450 | .Ar 3des | |
451 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
452 | .Ar blowfish | |
453 | is a fast block cipher; it appears very secure and is much faster than | |
454 | .Ar 3des . | |
455 | .Ar des | |
456 | is only supported in the | |
457 | .Nm | |
458 | client for interoperability with legacy protocol 1 implementations | |
459 | that do not support the | |
460 | .Ar 3des | |
461 | cipher. | |
462 | Its use is strongly discouraged due to cryptographic weaknesses. | |
463 | .It Fl c Ar cipher_spec | |
464 | Additionally, for protocol version 2 a comma-separated list of ciphers can | |
465 | be specified in order of preference. | |
466 | See | |
467 | .Cm Ciphers | |
468 | for more information. | |
469 | .It Fl D Ar port | |
470 | Specifies a local | |
471 | .Dq dynamic | |
472 | application-level port forwarding. | |
473 | This works by allocating a socket to listen to | |
474 | .Ar port | |
475 | on the local side, and whenever a connection is made to this port, the | |
476 | connection is forwarded over the secure channel, and the application | |
477 | protocol is then used to determine where to connect to from the | |
478 | remote machine. | |
479 | Currently the SOCKS4 and SOCKS5 protocols are supported, and | |
480 | .Nm | |
481 | will act as a SOCKS server. | |
482 | Only root can forward privileged ports. | |
483 | Dynamic port forwardings can also be specified in the configuration file. | |
484 | .It Fl e Ar ch | ^ch | none | |
485 | Sets the escape character for sessions with a pty (default: | |
486 | .Ql ~ ) . | |
487 | The escape character is only recognized at the beginning of a line. | |
488 | The escape character followed by a dot | |
489 | .Pq Ql \&. | |
490 | closes the connection; | |
491 | followed by control-Z suspends the connection; | |
492 | and followed by itself sends the escape character once. | |
493 | Setting the character to | |
494 | .Dq none | |
495 | disables any escapes and makes the session fully transparent. | |
496 | .It Fl F Ar configfile | |
497 | Specifies an alternative per-user configuration file. | |
498 | If a configuration file is given on the command line, | |
499 | the system-wide configuration file | |
500 | .Pq Pa /etc/ssh/ssh_config | |
501 | will be ignored. | |
502 | The default for the per-user configuration file is | |
503 | .Pa $HOME/.ssh/config . | |
504 | .It Fl f | |
505 | Requests | |
506 | .Nm | |
507 | to go to background just before command execution. | |
508 | This is useful if | |
509 | .Nm | |
510 | is going to ask for passwords or passphrases, but the user | |
511 | wants it in the background. | |
512 | This implies | |
513 | .Fl n . | |
514 | The recommended way to start X11 programs at a remote site is with | |
515 | something like | |
516 | .Ic ssh -f host xterm . | |
517 | .It Fl g | |
518 | Allows remote hosts to connect to local forwarded ports. | |
519 | .It Fl I Ar smartcard_device | |
520 | Specifies which smartcard device to use. | |
521 | The argument is the device | |
522 | .Nm | |
523 | should use to communicate with a smartcard used for storing the user's | |
524 | private RSA key. | |
525 | .It Fl i Ar identity_file | |
526 | Selects a file from which the identity (private key) for | |
527 | RSA or DSA authentication is read. | |
528 | The default is | |
529 | .Pa $HOME/.ssh/identity | |
530 | for protocol version 1, and | |
531 | .Pa $HOME/.ssh/id_rsa | |
532 | and | |
533 | .Pa $HOME/.ssh/id_dsa | |
534 | for protocol version 2. | |
535 | Identity files may also be specified on | |
536 | a per-host basis in the configuration file. | |
537 | It is possible to have multiple | |
538 | .Fl i | |
539 | options (and multiple identities specified in | |
540 | configuration files). | |
541 | .It Fl k | |
542 | Disables forwarding of Kerberos tickets. | |
543 | This may also be specified on a per-host basis in the configuration file. | |
544 | .It Fl L Xo | |
545 | .Sm off | |
546 | .Ar port : host : hostport | |
547 | .Sm on | |
548 | .Xc | |
549 | Specifies that the given port on the local (client) host is to be | |
550 | forwarded to the given host and port on the remote side. | |
551 | This works by allocating a socket to listen to | |
552 | .Ar port | |
553 | on the local side, and whenever a connection is made to this port, the | |
554 | connection is forwarded over the secure channel, and a connection is | |
555 | made to | |
556 | .Ar host | |
557 | port | |
558 | .Ar hostport | |
559 | from the remote machine. | |
560 | Port forwardings can also be specified in the configuration file. | |
561 | Only root can forward privileged ports. | |
562 | IPv6 addresses can be specified with an alternative syntax: | |
563 | .Sm off | |
564 | .Xo | |
565 | .Ar port No / Ar host No / | |
566 | .Ar hostport . | |
567 | .Xc | |
568 | .Sm on | |
569 | .It Fl l Ar login_name | |
570 | Specifies the user to log in as on the remote machine. | |
571 | This also may be specified on a per-host basis in the configuration file. | |
572 | .It Fl m Ar mac_spec | |
573 | Additionally, for protocol version 2 a comma-separated list of MAC | |
574 | (message authentication code) algorithms can | |
575 | be specified in order of preference. | |
576 | See the | |
577 | .Cm MACs | |
578 | keyword for more information. | |
579 | .It Fl N | |
580 | Do not execute a remote command. | |
581 | This is useful for just forwarding ports | |
582 | (protocol version 2 only). | |
583 | .It Fl n | |
584 | Redirects stdin from | |
585 | .Pa /dev/null | |
586 | (actually, prevents reading from stdin). | |
587 | This must be used when | |
588 | .Nm | |
589 | is run in the background. | |
590 | A common trick is to use this to run X11 programs on a remote machine. | |
591 | For example, | |
592 | .Ic ssh -n shadows.cs.hut.fi emacs & | |
593 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
594 | connection will be automatically forwarded over an encrypted channel. | |
595 | The | |
596 | .Nm | |
597 | program will be put in the background. | |
598 | (This does not work if | |
599 | .Nm | |
600 | needs to ask for a password or passphrase; see also the | |
601 | .Fl f | |
602 | option.) | |
603 | .It Fl o Ar option | |
604 | Can be used to give options in the format used in the configuration file. | |
605 | This is useful for specifying options for which there is no separate | |
606 | command-line flag. | |
607 | For full details of the options listed below, and their possible values, see | |
608 | .Xr ssh_config 5 . | |
609 | .Pp | |
610 | .Bl -tag -width Ds -offset indent -compact | |
611 | .It AddressFamily | |
612 | .It BatchMode | |
613 | .It BindAddress | |
614 | .It ChallengeResponseAuthentication | |
615 | .It CheckHostIP | |
616 | .It Cipher | |
617 | .It Ciphers | |
618 | .It ClearAllForwardings | |
619 | .It Compression | |
620 | .It CompressionLevel | |
621 | .It ConnectionAttempts | |
622 | .It ConnectionTimeout | |
623 | .It DynamicForward | |
624 | .It EscapeChar | |
625 | .It ForwardAgent | |
626 | .It ForwardX11 | |
627 | .It GatewayPorts | |
628 | .It GlobalKnownHostsFile | |
629 | .It GSSAPIAuthentication | |
630 | .It GSSAPIDelegateCredentials | |
631 | .It Host | |
632 | .It HostbasedAuthentication | |
633 | .It HostKeyAlgorithms | |
634 | .It HostKeyAlias | |
635 | .It HostName | |
636 | .It IdentityFile | |
637 | .It KeepAlive | |
638 | .It LocalForward | |
639 | .It LogLevel | |
640 | .It MACs | |
641 | .It NoHostAuthenticationForLocalhost | |
642 | .It NumberOfPasswordPrompts | |
643 | .It PasswordAuthentication | |
644 | .It Port | |
645 | .It PreferredAuthentications | |
646 | .It Protocol | |
647 | .It ProxyCommand | |
648 | .It PubkeyAuthentication | |
649 | .It RemoteForward | |
650 | .It RhostsRSAAuthentication | |
651 | .It RSAAuthentication | |
652 | .It SmartcardDevice | |
653 | .It StrictHostKeyChecking | |
654 | .It UsePrivilegedPort | |
655 | .It User | |
656 | .It UserKnownHostsFile | |
657 | .It VerifyHostKeyDNS | |
658 | .It XAuthLocation | |
659 | .El | |
660 | .It Fl p Ar port | |
661 | Port to connect to on the remote host. | |
662 | This can be specified on a | |
663 | per-host basis in the configuration file. | |
664 | .It Fl q | |
665 | Quiet mode. | |
666 | Causes all warning and diagnostic messages to be suppressed. | |
667 | .It Fl R Xo | |
668 | .Sm off | |
669 | .Ar port : host : hostport | |
670 | .Sm on | |
671 | .Xc | |
672 | Specifies that the given port on the remote (server) host is to be | |
673 | forwarded to the given host and port on the local side. | |
674 | This works by allocating a socket to listen to | |
675 | .Ar port | |
676 | on the remote side, and whenever a connection is made to this port, the | |
677 | connection is forwarded over the secure channel, and a connection is | |
678 | made to | |
679 | .Ar host | |
680 | port | |
681 | .Ar hostport | |
682 | from the local machine. | |
683 | Port forwardings can also be specified in the configuration file. | |
684 | Privileged ports can be forwarded only when | |
685 | logging in as root on the remote machine. | |
686 | IPv6 addresses can be specified with an alternative syntax: | |
687 | .Sm off | |
688 | .Xo | |
689 | .Ar port No / Ar host No / | |
690 | .Ar hostport . | |
691 | .Xc | |
692 | .Sm on | |
693 | .It Fl s | |
694 | May be used to request invocation of a subsystem on the remote system. | |
695 | Subsystems are a feature of the SSH2 protocol which facilitate the use | |
696 | of SSH as a secure transport for other applications (eg.\& | |
697 | .Xr sftp 1 ) . | |
698 | The subsystem is specified as the remote command. | |
699 | .It Fl T | |
700 | Disable pseudo-tty allocation. | |
701 | .It Fl t | |
702 | Force pseudo-tty allocation. | |
703 | This can be used to execute arbitrary | |
704 | screen-based programs on a remote machine, which can be very useful, | |
705 | e.g., when implementing menu services. | |
706 | Multiple | |
707 | .Fl t | |
708 | options force tty allocation, even if | |
709 | .Nm | |
710 | has no local tty. | |
711 | .It Fl V | |
712 | Display the version number and exit. | |
713 | .It Fl v | |
714 | Verbose mode. | |
715 | Causes | |
716 | .Nm | |
717 | to print debugging messages about its progress. | |
718 | This is helpful in | |
719 | debugging connection, authentication, and configuration problems. | |
720 | Multiple | |
721 | .Fl v | |
722 | options increase the verbosity. | |
723 | The maximum is 3. | |
724 | .It Fl X | |
725 | Enables X11 forwarding. | |
726 | This can also be specified on a per-host basis in a configuration file. | |
727 | .Pp | |
728 | X11 forwarding should be enabled with caution. | |
729 | Users with the ability to bypass file permissions on the remote host | |
730 | (for the user's X authorization database) | |
731 | can access the local X11 display through the forwarded connection. | |
732 | An attacker may then be able to perform activities such as keystroke monitoring. | |
733 | .It Fl x | |
734 | Disables X11 forwarding. | |
735 | .El | |
736 | .Sh CONFIGURATION FILES | |
737 | .Nm | |
738 | may additionally obtain configuration data from | |
739 | a per-user configuration file and a system-wide configuration file. | |
740 | The file format and configuration options are described in | |
741 | .Xr ssh_config 5 . | |
742 | .Sh ENVIRONMENT | |
743 | .Nm | |
744 | will normally set the following environment variables: | |
745 | .Bl -tag -width LOGNAME | |
746 | .It Ev DISPLAY | |
747 | The | |
748 | .Ev DISPLAY | |
749 | variable indicates the location of the X11 server. | |
750 | It is automatically set by | |
751 | .Nm | |
752 | to point to a value of the form | |
753 | .Dq hostname:n | |
754 | where hostname indicates | |
755 | the host where the shell runs, and n is an integer \*(Ge 1. | |
756 | .Nm | |
757 | uses this special value to forward X11 connections over the secure | |
758 | channel. | |
759 | The user should normally not set | |
760 | .Ev DISPLAY | |
761 | explicitly, as that | |
762 | will render the X11 connection insecure (and will require the user to | |
763 | manually copy any required authorization cookies). | |
764 | .It Ev HOME | |
765 | Set to the path of the user's home directory. | |
766 | .It Ev LOGNAME | |
767 | Synonym for | |
768 | .Ev USER ; | |
769 | set for compatibility with systems that use this variable. | |
770 | .It Ev MAIL | |
771 | Set to the path of the user's mailbox. | |
772 | .It Ev PATH | |
773 | Set to the default | |
774 | .Ev PATH , | |
775 | as specified when compiling | |
776 | .Nm ssh . | |
777 | .It Ev SSH_ASKPASS | |
778 | If | |
779 | .Nm | |
780 | needs a passphrase, it will read the passphrase from the current | |
781 | terminal if it was run from a terminal. | |
782 | If | |
783 | .Nm | |
784 | does not have a terminal associated with it but | |
785 | .Ev DISPLAY | |
786 | and | |
787 | .Ev SSH_ASKPASS | |
788 | are set, it will execute the program specified by | |
789 | .Ev SSH_ASKPASS | |
790 | and open an X11 window to read the passphrase. | |
791 | This is particularly useful when calling | |
792 | .Nm | |
793 | from a | |
794 | .Pa .Xsession | |
795 | or related script. | |
796 | (Note that on some machines it | |
797 | may be necessary to redirect the input from | |
798 | .Pa /dev/null | |
799 | to make this work.) | |
800 | .It Ev SSH_AUTH_SOCK | |
801 | Identifies the path of a unix-domain socket used to communicate with the | |
802 | agent. | |
803 | .It Ev SSH_CONNECTION | |
804 | Identifies the client and server ends of the connection. | |
805 | The variable contains | |
806 | four space-separated values: client ip-address, client port number, | |
807 | server ip-address and server port number. | |
808 | .It Ev SSH_ORIGINAL_COMMAND | |
809 | The variable contains the original command line if a forced command | |
810 | is executed. | |
811 | It can be used to extract the original arguments. | |
812 | .It Ev SSH_TTY | |
813 | This is set to the name of the tty (path to the device) associated | |
814 | with the current shell or command. | |
815 | If the current session has no tty, | |
816 | this variable is not set. | |
817 | .It Ev TZ | |
818 | The timezone variable is set to indicate the present timezone if it | |
819 | was set when the daemon was started (i.e., the daemon passes the value | |
820 | on to new connections). | |
821 | .It Ev USER | |
822 | Set to the name of the user logging in. | |
823 | .El | |
824 | .Pp | |
825 | Additionally, | |
826 | .Nm | |
827 | reads | |
828 | .Pa $HOME/.ssh/environment , | |
829 | and adds lines of the format | |
830 | .Dq VARNAME=value | |
831 | to the environment if the file exists and if users are allowed to | |
832 | change their environment. | |
833 | For more information, see the | |
834 | .Cm PermitUserEnvironment | |
835 | option in | |
836 | .Xr sshd_config 5 . | |
837 | .Sh FILES | |
838 | .Bl -tag -width Ds | |
839 | .It Pa $HOME/.ssh/known_hosts | |
840 | Records host keys for all hosts the user has logged into that are not | |
841 | in | |
842 | .Pa /etc/ssh/ssh_known_hosts . | |
843 | See | |
844 | .Xr sshd 8 . | |
845 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | |
846 | Contains the authentication identity of the user. | |
847 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
848 | These files | |
849 | contain sensitive data and should be readable by the user but not | |
850 | accessible by others (read/write/execute). | |
851 | Note that | |
852 | .Nm | |
853 | ignores a private key file if it is accessible by others. | |
854 | It is possible to specify a passphrase when | |
855 | generating the key; the passphrase will be used to encrypt the | |
856 | sensitive part of this file using 3DES. | |
857 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | |
858 | Contains the public key for authentication (public part of the | |
859 | identity file in human-readable form). | |
860 | The contents of the | |
861 | .Pa $HOME/.ssh/identity.pub | |
862 | file should be added to the file | |
863 | .Pa $HOME/.ssh/authorized_keys | |
864 | on all machines | |
865 | where the user wishes to log in using protocol version 1 RSA authentication. | |
866 | The contents of the | |
867 | .Pa $HOME/.ssh/id_dsa.pub | |
868 | and | |
869 | .Pa $HOME/.ssh/id_rsa.pub | |
870 | file should be added to | |
871 | .Pa $HOME/.ssh/authorized_keys | |
872 | on all machines | |
873 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. | |
874 | These files are not | |
875 | sensitive and can (but need not) be readable by anyone. | |
876 | These files are | |
877 | never used automatically and are not necessary; they are only provided for | |
878 | the convenience of the user. | |
879 | .It Pa $HOME/.ssh/config | |
880 | This is the per-user configuration file. | |
881 | The file format and configuration options are described in | |
882 | .Xr ssh_config 5 . | |
883 | .It Pa $HOME/.ssh/authorized_keys | |
884 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
885 | The format of this file is described in the | |
886 | .Xr sshd 8 | |
887 | manual page. | |
888 | In the simplest form the format is the same as the | |
889 | .Pa .pub | |
890 | identity files. | |
891 | This file is not highly sensitive, but the recommended | |
892 | permissions are read/write for the user, and not accessible by others. | |
893 | .It Pa /etc/ssh/ssh_known_hosts | |
894 | Systemwide list of known host keys. | |
895 | This file should be prepared by the | |
896 | system administrator to contain the public host keys of all machines in the | |
897 | organization. | |
898 | This file should be world-readable. | |
899 | This file contains | |
900 | public keys, one per line, in the following format (fields separated | |
901 | by spaces): system name, public key and optional comment field. | |
902 | When different names are used | |
903 | for the same machine, all such names should be listed, separated by | |
904 | commas. | |
905 | The format is described in the | |
906 | .Xr sshd 8 | |
907 | manual page. | |
908 | .Pp | |
909 | The canonical system name (as returned by name servers) is used by | |
910 | .Xr sshd 8 | |
911 | to verify the client host when logging in; other names are needed because | |
912 | .Nm | |
913 | does not convert the user-supplied name to a canonical name before | |
914 | checking the key, because someone with access to the name servers | |
915 | would then be able to fool host authentication. | |
916 | .It Pa /etc/ssh/ssh_config | |
917 | Systemwide configuration file. | |
918 | The file format and configuration options are described in | |
919 | .Xr ssh_config 5 . | |
920 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key | |
921 | These three files contain the private parts of the host keys | |
922 | and are used for | |
923 | .Cm RhostsRSAAuthentication | |
924 | and | |
925 | .Cm HostbasedAuthentication . | |
926 | If the protocol version 1 | |
927 | .Cm RhostsRSAAuthentication | |
928 | method is used, | |
929 | .Nm | |
930 | must be setuid root, since the host key is readable only by root. | |
931 | For protocol version 2, | |
932 | .Nm | |
933 | uses | |
934 | .Xr ssh-keysign 8 | |
935 | to access the host keys for | |
936 | .Cm HostbasedAuthentication . | |
937 | This eliminates the requirement that | |
938 | .Nm | |
939 | be setuid root when that authentication method is used. | |
940 | By default | |
941 | .Nm | |
942 | is not setuid root. | |
943 | .It Pa $HOME/.rhosts | |
944 | This file is used in | |
945 | .Em rhosts | |
946 | authentication to list the | |
947 | host/user pairs that are permitted to log in. | |
948 | (Note that this file is | |
949 | also used by rlogin and rsh, which makes using this file insecure.) | |
950 | Each line of the file contains a host name (in the canonical form | |
951 | returned by name servers), and then a user name on that host, | |
952 | separated by a space. | |
953 | On some machines this file may need to be | |
954 | world-readable if the user's home directory is on a NFS partition, | |
955 | because | |
956 | .Xr sshd 8 | |
957 | reads it as root. | |
958 | Additionally, this file must be owned by the user, | |
959 | and must not have write permissions for anyone else. | |
960 | The recommended | |
961 | permission for most machines is read/write for the user, and not | |
962 | accessible by others. | |
963 | .Pp | |
964 | Note that by default | |
965 | .Xr sshd 8 | |
966 | will be installed so that it requires successful RSA host | |
967 | authentication before permitting | |
968 | .Em rhosts | |
969 | authentication. | |
970 | If the server machine does not have the client's host key in | |
971 | .Pa /etc/ssh/ssh_known_hosts , | |
972 | it can be stored in | |
973 | .Pa $HOME/.ssh/known_hosts . | |
974 | The easiest way to do this is to | |
975 | connect back to the client from the server machine using ssh; this | |
976 | will automatically add the host key to | |
977 | .Pa $HOME/.ssh/known_hosts . | |
978 | .It Pa $HOME/.shosts | |
979 | This file is used exactly the same way as | |
980 | .Pa .rhosts . | |
981 | The purpose for | |
982 | having this file is to be able to use rhosts authentication with | |
983 | .Nm | |
984 | without permitting login with | |
985 | .Xr rlogin | |
986 | or | |
987 | .Xr rsh 1 . | |
988 | .It Pa /etc/hosts.equiv | |
989 | This file is used during | |
990 | .Em rhosts | |
991 | authentication. | |
992 | It contains | |
993 | canonical hosts names, one per line (the full format is described in the | |
994 | .Xr sshd 8 | |
995 | manual page). | |
996 | If the client host is found in this file, login is | |
997 | automatically permitted provided client and server user names are the | |
998 | same. | |
999 | Additionally, successful RSA host authentication is normally | |
1000 | required. | |
1001 | This file should only be writable by root. | |
1002 | .It Pa /etc/shosts.equiv | |
1003 | This file is processed exactly as | |
1004 | .Pa /etc/hosts.equiv . | |
1005 | This file may be useful to permit logins using | |
1006 | .Nm | |
1007 | but not using rsh/rlogin. | |
1008 | .It Pa /etc/ssh/sshrc | |
1009 | Commands in this file are executed by | |
1010 | .Nm | |
1011 | when the user logs in just before the user's shell (or command) is started. | |
1012 | See the | |
1013 | .Xr sshd 8 | |
1014 | manual page for more information. | |
1015 | .It Pa $HOME/.ssh/rc | |
1016 | Commands in this file are executed by | |
1017 | .Nm | |
1018 | when the user logs in just before the user's shell (or command) is | |
1019 | started. | |
1020 | See the | |
1021 | .Xr sshd 8 | |
1022 | manual page for more information. | |
1023 | .It Pa $HOME/.ssh/environment | |
1024 | Contains additional definitions for environment variables, see section | |
1025 | .Sx ENVIRONMENT | |
1026 | above. | |
1027 | .El | |
1028 | .Sh DIAGNOSTICS | |
1029 | .Nm | |
1030 | exits with the exit status of the remote command or with 255 | |
1031 | if an error occurred. | |
1032 | .Sh SEE ALSO | |
1033 | .Xr gzip 1 , | |
1034 | .Xr rsh 1 , | |
1035 | .Xr scp 1 , | |
1036 | .Xr sftp 1 , | |
1037 | .Xr ssh-add 1 , | |
1038 | .Xr ssh-agent 1 , | |
1039 | .Xr ssh-keygen 1 , | |
1040 | .Xr telnet 1 , | |
1041 | .Xr hosts.equiv 5 , | |
1042 | .Xr ssh_config 5 , | |
1043 | .Xr ssh-keysign 8 , | |
1044 | .Xr sshd 8 | |
1045 | .Rs | |
1046 | .%A T. Ylonen | |
1047 | .%A T. Kivinen | |
1048 | .%A M. Saarinen | |
1049 | .%A T. Rinne | |
1050 | .%A S. Lehtinen | |
1051 | .%T "SSH Protocol Architecture" | |
1052 | .%N draft-ietf-secsh-architecture-12.txt | |
1053 | .%D January 2002 | |
1054 | .%O work in progress material | |
1055 | .Re | |
1056 | .Sh AUTHORS | |
1057 | OpenSSH is a derivative of the original and free | |
1058 | ssh 1.2.12 release by Tatu Ylonen. | |
1059 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1060 | Theo de Raadt and Dug Song | |
1061 | removed many bugs, re-added newer features and | |
1062 | created OpenSSH. | |
1063 | Markus Friedl contributed the support for SSH | |
1064 | protocol versions 1.5 and 2.0. |