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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 1.196 2004/08/30 21:22:49 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 1246AaCfgkMNnqsTtVvXxY | |
47 | .Op Fl b Ar bind_address | |
48 | .Op Fl c Ar cipher_spec | |
49 | .Bk -words | |
50 | .Op Fl D Ar port | |
51 | .Op Fl e Ar escape_char | |
52 | .Op Fl F Ar configfile | |
53 | .Op Fl i Ar identity_file | |
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 | .Op Fl S Ar ctl | |
78 | .Oo Ar user Ns @ Oc Ns Ar 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 rlogin and rsh, | |
85 | and provide secure encrypted communications between | |
86 | two untrusted hosts over an insecure network. | |
87 | X11 connections and arbitrary TCP/IP ports | |
88 | can also be forwarded over the secure channel. | |
89 | .Pp | |
90 | .Nm | |
91 | connects and logs into the specified | |
92 | .Ar hostname | |
93 | (with optional | |
94 | .Ar user | |
95 | name). | |
96 | The user must prove | |
97 | his/her identity to the remote machine using one of several methods | |
98 | depending on the protocol version used. | |
99 | .Pp | |
100 | If | |
101 | .Ar command | |
102 | is specified, | |
103 | .Ar command | |
104 | is executed on the remote host instead of a login shell. | |
105 | .Ss SSH protocol version 1 | |
106 | The first authentication method is the | |
107 | .Em rhosts | |
108 | or | |
109 | .Em hosts.equiv | |
110 | method combined with RSA-based host authentication. | |
111 | If the machine the user logs in from is listed in | |
112 | .Pa /etc/hosts.equiv | |
113 | or | |
114 | .Pa /etc/shosts.equiv | |
115 | on the remote machine, and the user names are | |
116 | the same on both sides, or if the files | |
117 | .Pa $HOME/.rhosts | |
118 | or | |
119 | .Pa $HOME/.shosts | |
120 | exist in the user's home directory on the | |
121 | remote machine and contain a line containing the name of the client | |
122 | machine and the name of the user on that machine, the user is | |
123 | considered for log in. | |
124 | Additionally, if the server can verify the client's | |
125 | host key (see | |
126 | .Pa /etc/ssh/ssh_known_hosts | |
127 | and | |
128 | .Pa $HOME/.ssh/known_hosts | |
129 | in the | |
130 | .Sx FILES | |
131 | section), only then is login permitted. | |
132 | This authentication method closes security holes due to IP | |
133 | spoofing, DNS spoofing and routing spoofing. | |
134 | [Note to the administrator: | |
135 | .Pa /etc/hosts.equiv , | |
136 | .Pa $HOME/.rhosts , | |
137 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
138 | disabled if security is desired.] | |
139 | .Pp | |
140 | As a second authentication method, | |
141 | .Nm | |
142 | supports RSA based authentication. | |
143 | The scheme is based on public-key cryptography: there are cryptosystems | |
144 | where encryption and decryption are done using separate keys, and it | |
145 | is not possible to derive the decryption key from the encryption key. | |
146 | RSA is one such system. | |
147 | The idea is that each user creates a public/private | |
148 | key pair for authentication purposes. | |
149 | The server knows the public key, and only the user knows the private key. | |
150 | .Pp | |
151 | The file | |
152 | .Pa $HOME/.ssh/authorized_keys | |
153 | lists the public keys that are permitted for logging in. | |
154 | When the user logs in, the | |
155 | .Nm | |
156 | program tells the server which key pair it would like to use for | |
157 | authentication. | |
158 | The server checks if this key is permitted, and if so, | |
159 | sends the user (actually the | |
160 | .Nm | |
161 | program running on behalf of the user) a challenge, a random number, | |
162 | encrypted by the user's public key. | |
163 | The challenge can only be decrypted using the proper private key. | |
164 | The user's client then decrypts the challenge using the private key, | |
165 | proving that he/she knows the private key | |
166 | but without disclosing it to the server. | |
167 | .Pp | |
168 | .Nm | |
169 | implements the RSA authentication protocol automatically. | |
170 | The user creates his/her RSA key pair by running | |
171 | .Xr ssh-keygen 1 . | |
172 | This stores the private key in | |
173 | .Pa $HOME/.ssh/identity | |
174 | and stores the public key in | |
175 | .Pa $HOME/.ssh/identity.pub | |
176 | in the user's home directory. | |
177 | The user should then copy the | |
178 | .Pa identity.pub | |
179 | to | |
180 | .Pa $HOME/.ssh/authorized_keys | |
181 | in his/her home directory on the remote machine (the | |
182 | .Pa authorized_keys | |
183 | file corresponds to the conventional | |
184 | .Pa $HOME/.rhosts | |
185 | file, and has one key | |
186 | per line, though the lines can be very long). | |
187 | After this, the user can log in without giving the password. | |
188 | .Pp | |
189 | The most convenient way to use RSA authentication may be with an | |
190 | authentication agent. | |
191 | See | |
192 | .Xr ssh-agent 1 | |
193 | for more information. | |
194 | .Pp | |
195 | If other authentication methods fail, | |
196 | .Nm | |
197 | prompts the user for a password. | |
198 | The password is sent to the remote | |
199 | host for checking; however, since all communications are encrypted, | |
200 | the password cannot be seen by someone listening on the network. | |
201 | .Ss SSH protocol version 2 | |
202 | When a user connects using protocol version 2, | |
203 | similar authentication methods are available. | |
204 | Using the default values for | |
205 | .Cm PreferredAuthentications , | |
206 | the client will try to authenticate first using the hostbased method; | |
207 | if this method fails, public key authentication is attempted, | |
208 | and finally if this method fails, keyboard-interactive and | |
209 | password authentication are tried. | |
210 | .Pp | |
211 | The public key method is similar to RSA authentication described | |
212 | in the previous section and allows the RSA or DSA algorithm to be used: | |
213 | The client uses his private key, | |
214 | .Pa $HOME/.ssh/id_dsa | |
215 | or | |
216 | .Pa $HOME/.ssh/id_rsa , | |
217 | to sign the session identifier and sends the result to the server. | |
218 | The server checks whether the matching public key is listed in | |
219 | .Pa $HOME/.ssh/authorized_keys | |
220 | and grants access if both the key is found and the signature is correct. | |
221 | The session identifier is derived from a shared Diffie-Hellman value | |
222 | and is only known to the client and the server. | |
223 | .Pp | |
224 | If public key authentication fails or is not available, a password | |
225 | can be sent encrypted to the remote host to prove the user's identity. | |
226 | .Pp | |
227 | Additionally, | |
228 | .Nm | |
229 | supports hostbased or challenge response authentication. | |
230 | .Pp | |
231 | Protocol 2 provides additional mechanisms for confidentiality | |
232 | (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) | |
233 | and integrity (hmac-md5, hmac-sha1, hmac-ripemd160). | |
234 | Note that protocol 1 lacks a strong mechanism for ensuring the | |
235 | integrity of the connection. | |
236 | .Ss Login session and remote execution | |
237 | When the user's identity has been accepted by the server, the server | |
238 | either executes the given command, or logs into the machine and gives | |
239 | the user a normal shell on the remote machine. | |
240 | All communication with | |
241 | the remote command or shell will be automatically encrypted. | |
242 | .Pp | |
243 | If a pseudo-terminal has been allocated (normal login session), the | |
244 | user may use the escape characters noted below. | |
245 | .Pp | |
246 | If no pseudo-tty has been allocated, | |
247 | the session is transparent and can be used to reliably transfer binary data. | |
248 | On most systems, setting the escape character to | |
249 | .Dq none | |
250 | will also make the session transparent even if a tty is used. | |
251 | .Pp | |
252 | The session terminates when the command or shell on the remote | |
253 | machine exits and all X11 and TCP/IP connections have been closed. | |
254 | The exit status of the remote program is returned as the exit status of | |
255 | .Nm ssh . | |
256 | .Ss Escape Characters | |
257 | When a pseudo-terminal has been requested, | |
258 | .Nm | |
259 | supports a number of functions through the use of an escape character. | |
260 | .Pp | |
261 | A single tilde character can be sent as | |
262 | .Ic ~~ | |
263 | or by following the tilde by a character other than those described below. | |
264 | The escape character must always follow a newline to be interpreted as | |
265 | special. | |
266 | The escape character can be changed in configuration files using the | |
267 | .Cm EscapeChar | |
268 | configuration directive or on the command line by the | |
269 | .Fl e | |
270 | option. | |
271 | .Pp | |
272 | The supported escapes (assuming the default | |
273 | .Ql ~ ) | |
274 | are: | |
275 | .Bl -tag -width Ds | |
276 | .It Cm ~. | |
277 | Disconnect. | |
278 | .It Cm ~^Z | |
279 | Background | |
280 | .Nm ssh . | |
281 | .It Cm ~# | |
282 | List forwarded connections. | |
283 | .It Cm ~& | |
284 | Background | |
285 | .Nm | |
286 | at logout when waiting for forwarded connection / X11 sessions to terminate. | |
287 | .It Cm ~? | |
288 | Display a list of escape characters. | |
289 | .It Cm ~B | |
290 | Send a BREAK to the remote system | |
291 | (only useful for SSH protocol version 2 and if the peer supports it). | |
292 | .It Cm ~C | |
293 | Open command line. | |
294 | Currently this allows the addition of port forwardings using the | |
295 | .Fl L | |
296 | and | |
297 | .Fl R | |
298 | options (see below). | |
299 | It also allows the cancellation of existing remote port-forwardings | |
300 | using | |
301 | .Fl KR Ar hostport . | |
302 | Basic help is available, using the | |
303 | .Fl h | |
304 | option. | |
305 | .It Cm ~R | |
306 | Request rekeying of the connection | |
307 | (only useful for SSH protocol version 2 and if the peer supports it). | |
308 | .El | |
309 | .Ss X11 and TCP forwarding | |
310 | If the | |
311 | .Cm ForwardX11 | |
312 | variable is set to | |
313 | .Dq yes | |
314 | (or see the description of the | |
315 | .Fl X | |
316 | and | |
317 | .Fl x | |
318 | options described later) | |
319 | and the user is using X11 (the | |
320 | .Ev DISPLAY | |
321 | environment variable is set), the connection to the X11 display is | |
322 | automatically forwarded to the remote side in such a way that any X11 | |
323 | programs started from the shell (or command) will go through the | |
324 | encrypted channel, and the connection to the real X server will be made | |
325 | from the local machine. | |
326 | The user should not manually set | |
327 | .Ev DISPLAY . | |
328 | Forwarding of X11 connections can be | |
329 | configured on the command line or in configuration files. | |
330 | .Pp | |
331 | The | |
332 | .Ev DISPLAY | |
333 | value set by | |
334 | .Nm | |
335 | will point to the server machine, but with a display number greater than zero. | |
336 | This is normal, and happens because | |
337 | .Nm | |
338 | creates a | |
339 | .Dq proxy | |
340 | X server on the server machine for forwarding the | |
341 | connections over the encrypted channel. | |
342 | .Pp | |
343 | .Nm | |
344 | will also automatically set up Xauthority data on the server machine. | |
345 | For this purpose, it will generate a random authorization cookie, | |
346 | store it in Xauthority on the server, and verify that any forwarded | |
347 | connections carry this cookie and replace it by the real cookie when | |
348 | the connection is opened. | |
349 | The real authentication cookie is never | |
350 | sent to the server machine (and no cookies are sent in the plain). | |
351 | .Pp | |
352 | If the | |
353 | .Cm ForwardAgent | |
354 | variable is set to | |
355 | .Dq yes | |
356 | (or see the description of the | |
357 | .Fl A | |
358 | and | |
359 | .Fl a | |
360 | options described later) and | |
361 | the user is using an authentication agent, the connection to the agent | |
362 | is automatically forwarded to the remote side. | |
363 | .Pp | |
364 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
365 | be specified either on the command line or in a configuration file. | |
366 | One possible application of TCP/IP forwarding is a secure connection to an | |
367 | electronic purse; another is going through firewalls. | |
368 | .Ss Server authentication | |
369 | .Nm | |
370 | automatically maintains and checks a database containing | |
371 | identifications for all hosts it has ever been used with. | |
372 | Host keys are stored in | |
373 | .Pa $HOME/.ssh/known_hosts | |
374 | in the user's home directory. | |
375 | Additionally, the file | |
376 | .Pa /etc/ssh/ssh_known_hosts | |
377 | is automatically checked for known hosts. | |
378 | Any new hosts are automatically added to the user's file. | |
379 | If a host's identification ever changes, | |
380 | .Nm | |
381 | warns about this and disables password authentication to prevent a | |
382 | trojan horse from getting the user's password. | |
383 | Another purpose of this mechanism is to prevent man-in-the-middle attacks | |
384 | which could otherwise be used to circumvent the encryption. | |
385 | The | |
386 | .Cm StrictHostKeyChecking | |
387 | option can be used to prevent logins to machines whose | |
388 | host key is not known or has changed. | |
389 | .Pp | |
390 | .Nm | |
391 | can be configured to verify host identification using fingerprint resource | |
392 | records (SSHFP) published in DNS. | |
393 | The | |
394 | .Cm VerifyHostKeyDNS | |
395 | option can be used to control how DNS lookups are performed. | |
396 | SSHFP resource records can be generated using | |
397 | .Xr ssh-keygen 1 . | |
398 | .Pp | |
399 | The options are as follows: | |
400 | .Bl -tag -width Ds | |
401 | .It Fl 1 | |
402 | Forces | |
403 | .Nm | |
404 | to try protocol version 1 only. | |
405 | .It Fl 2 | |
406 | Forces | |
407 | .Nm | |
408 | to try protocol version 2 only. | |
409 | .It Fl 4 | |
410 | Forces | |
411 | .Nm | |
412 | to use IPv4 addresses only. | |
413 | .It Fl 6 | |
414 | Forces | |
415 | .Nm | |
416 | to use IPv6 addresses only. | |
417 | .It Fl A | |
418 | Enables forwarding of the authentication agent connection. | |
419 | This can also be specified on a per-host basis in a configuration file. | |
420 | .Pp | |
421 | Agent forwarding should be enabled with caution. | |
422 | Users with the ability to bypass file permissions on the remote host | |
423 | (for the agent's Unix-domain socket) | |
424 | can access the local agent through the forwarded connection. | |
425 | An attacker cannot obtain key material from the agent, | |
426 | however they can perform operations on the keys that enable them to | |
427 | authenticate using the identities loaded into the agent. | |
428 | .It Fl a | |
429 | Disables forwarding of the authentication agent connection. | |
430 | .It Fl b Ar bind_address | |
431 | Specify the interface to transmit from on machines with multiple | |
432 | interfaces or aliased addresses. | |
433 | .It Fl C | |
434 | Requests compression of all data (including stdin, stdout, stderr, and | |
435 | data for forwarded X11 and TCP/IP connections). | |
436 | The compression algorithm is the same used by | |
437 | .Xr gzip 1 , | |
438 | and the | |
439 | .Dq level | |
440 | can be controlled by the | |
441 | .Cm CompressionLevel | |
442 | option for protocol version 1. | |
443 | Compression is desirable on modem lines and other | |
444 | slow connections, but will only slow down things on fast networks. | |
445 | The default value can be set on a host-by-host basis in the | |
446 | configuration files; see the | |
447 | .Cm Compression | |
448 | option. | |
449 | .It Fl c Ar cipher_spec | |
450 | Selects the cipher specification for encrypting the session. | |
451 | .Pp | |
452 | Protocol version 1 allows specification of a single cipher. | |
453 | The suported values are | |
454 | .Dq 3des , | |
455 | .Dq blowfish | |
456 | and | |
457 | .Dq des . | |
458 | .Ar 3des | |
459 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
460 | It is believed to be secure. | |
461 | .Ar blowfish | |
462 | is a fast block cipher; it appears very secure and is much faster than | |
463 | .Ar 3des . | |
464 | .Ar des | |
465 | is only supported in the | |
466 | .Nm | |
467 | client for interoperability with legacy protocol 1 implementations | |
468 | that do not support the | |
469 | .Ar 3des | |
470 | cipher. | |
471 | Its use is strongly discouraged due to cryptographic weaknesses. | |
472 | The default is | |
473 | .Dq 3des . | |
474 | .Pp | |
475 | For protocol version 2 | |
476 | .Ar cipher_spec | |
477 | is a comma-separated list of ciphers | |
478 | listed in order of preference. | |
479 | The supported ciphers are | |
480 | .Dq 3des-cbc , | |
481 | .Dq aes128-cbc , | |
482 | .Dq aes192-cbc , | |
483 | .Dq aes256-cbc , | |
484 | .Dq aes128-ctr , | |
485 | .Dq aes192-ctr , | |
486 | .Dq aes256-ctr , | |
487 | .Dq arcfour , | |
488 | .Dq blowfish-cbc , | |
489 | and | |
490 | .Dq cast128-cbc . | |
491 | The default is | |
492 | .Bd -literal | |
493 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
494 | aes192-cbc,aes256-cbc'' | |
495 | .Ed | |
496 | .It Fl D Ar port | |
497 | Specifies a local | |
498 | .Dq dynamic | |
499 | application-level port forwarding. | |
500 | This works by allocating a socket to listen to | |
501 | .Ar port | |
502 | on the local side, and whenever a connection is made to this port, the | |
503 | connection is forwarded over the secure channel, and the application | |
504 | protocol is then used to determine where to connect to from the | |
505 | remote machine. | |
506 | Currently the SOCKS4 and SOCKS5 protocols are supported, and | |
507 | .Nm | |
508 | will act as a SOCKS server. | |
509 | Only root can forward privileged ports. | |
510 | Dynamic port forwardings can also be specified in the configuration file. | |
511 | .It Fl e Ar ch | ^ch | none | |
512 | Sets the escape character for sessions with a pty (default: | |
513 | .Ql ~ ) . | |
514 | The escape character is only recognized at the beginning of a line. | |
515 | The escape character followed by a dot | |
516 | .Pq Ql \&. | |
517 | closes the connection; | |
518 | followed by control-Z suspends the connection; | |
519 | and followed by itself sends the escape character once. | |
520 | Setting the character to | |
521 | .Dq none | |
522 | disables any escapes and makes the session fully transparent. | |
523 | .It Fl F Ar configfile | |
524 | Specifies an alternative per-user configuration file. | |
525 | If a configuration file is given on the command line, | |
526 | the system-wide configuration file | |
527 | .Pq Pa /etc/ssh/ssh_config | |
528 | will be ignored. | |
529 | The default for the per-user configuration file is | |
530 | .Pa $HOME/.ssh/config . | |
531 | .It Fl f | |
532 | Requests | |
533 | .Nm | |
534 | to go to background just before command execution. | |
535 | This is useful if | |
536 | .Nm | |
537 | is going to ask for passwords or passphrases, but the user | |
538 | wants it in the background. | |
539 | This implies | |
540 | .Fl n . | |
541 | The recommended way to start X11 programs at a remote site is with | |
542 | something like | |
543 | .Ic ssh -f host xterm . | |
544 | .It Fl g | |
545 | Allows remote hosts to connect to local forwarded ports. | |
546 | .It Fl I Ar smartcard_device | |
547 | Specifies which smartcard device to use. | |
548 | The argument is the device | |
549 | .Nm | |
550 | should use to communicate with a smartcard used for storing the user's | |
551 | private RSA key. | |
552 | .It Fl i Ar identity_file | |
553 | Selects a file from which the identity (private key) for | |
554 | RSA or DSA authentication is read. | |
555 | The default is | |
556 | .Pa $HOME/.ssh/identity | |
557 | for protocol version 1, and | |
558 | .Pa $HOME/.ssh/id_rsa | |
559 | and | |
560 | .Pa $HOME/.ssh/id_dsa | |
561 | for protocol version 2. | |
562 | Identity files may also be specified on | |
563 | a per-host basis in the configuration file. | |
564 | It is possible to have multiple | |
565 | .Fl i | |
566 | options (and multiple identities specified in | |
567 | configuration files). | |
568 | .It Fl k | |
569 | Disables forwarding (delegation) of GSSAPI credentials to the server. | |
570 | .It Fl L Xo | |
571 | .Sm off | |
572 | .Ar port : host : hostport | |
573 | .Sm on | |
574 | .Xc | |
575 | Specifies that the given port on the local (client) host is to be | |
576 | forwarded to the given host and port on the remote side. | |
577 | This works by allocating a socket to listen to | |
578 | .Ar port | |
579 | on the local side, and whenever a connection is made to this port, the | |
580 | connection is forwarded over the secure channel, and a connection is | |
581 | made to | |
582 | .Ar host | |
583 | port | |
584 | .Ar hostport | |
585 | from the remote machine. | |
586 | Port forwardings can also be specified in the configuration file. | |
587 | Only root can forward privileged ports. | |
588 | IPv6 addresses can be specified with an alternative syntax: | |
589 | .Sm off | |
590 | .Xo | |
591 | .Ar port No / Ar host No / | |
592 | .Ar hostport . | |
593 | .Xc | |
594 | .Sm on | |
595 | .It Fl l Ar login_name | |
596 | Specifies the user to log in as on the remote machine. | |
597 | This also may be specified on a per-host basis in the configuration file. | |
598 | .It Fl M | |
599 | Places the | |
600 | .Nm | |
601 | client into | |
602 | .Dq master | |
603 | mode for connection sharing. | |
604 | Refer to the description of | |
605 | .Cm ControlMaster | |
606 | in | |
607 | .Xr ssh_config 5 | |
608 | for details. | |
609 | .It Fl m Ar mac_spec | |
610 | Additionally, for protocol version 2 a comma-separated list of MAC | |
611 | (message authentication code) algorithms can | |
612 | be specified in order of preference. | |
613 | See the | |
614 | .Cm MACs | |
615 | keyword for more information. | |
616 | .It Fl N | |
617 | Do not execute a remote command. | |
618 | This is useful for just forwarding ports | |
619 | (protocol version 2 only). | |
620 | .It Fl n | |
621 | Redirects stdin from | |
622 | .Pa /dev/null | |
623 | (actually, prevents reading from stdin). | |
624 | This must be used when | |
625 | .Nm | |
626 | is run in the background. | |
627 | A common trick is to use this to run X11 programs on a remote machine. | |
628 | For example, | |
629 | .Ic ssh -n shadows.cs.hut.fi emacs & | |
630 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
631 | connection will be automatically forwarded over an encrypted channel. | |
632 | The | |
633 | .Nm | |
634 | program will be put in the background. | |
635 | (This does not work if | |
636 | .Nm | |
637 | needs to ask for a password or passphrase; see also the | |
638 | .Fl f | |
639 | option.) | |
640 | .It Fl o Ar option | |
641 | Can be used to give options in the format used in the configuration file. | |
642 | This is useful for specifying options for which there is no separate | |
643 | command-line flag. | |
644 | For full details of the options listed below, and their possible values, see | |
645 | .Xr ssh_config 5 . | |
646 | .Pp | |
647 | .Bl -tag -width Ds -offset indent -compact | |
648 | .It AddressFamily | |
649 | .It BatchMode | |
650 | .It BindAddress | |
651 | .It ChallengeResponseAuthentication | |
652 | .It CheckHostIP | |
653 | .It Cipher | |
654 | .It Ciphers | |
655 | .It ClearAllForwardings | |
656 | .It Compression | |
657 | .It CompressionLevel | |
658 | .It ConnectionAttempts | |
659 | .It ConnectTimeout | |
660 | .It ControlMaster | |
661 | .It ControlPath | |
662 | .It DynamicForward | |
663 | .It EscapeChar | |
664 | .It ForwardAgent | |
665 | .It ForwardX11 | |
666 | .It ForwardX11Trusted | |
667 | .It GatewayPorts | |
668 | .It GlobalKnownHostsFile | |
669 | .It GSSAPIAuthentication | |
670 | .It GSSAPIDelegateCredentials | |
671 | .It Host | |
672 | .It HostbasedAuthentication | |
673 | .It HostKeyAlgorithms | |
674 | .It HostKeyAlias | |
675 | .It HostName | |
676 | .It IdentityFile | |
677 | .It IdentitiesOnly | |
678 | .It LocalForward | |
679 | .It LogLevel | |
680 | .It MACs | |
681 | .It NoHostAuthenticationForLocalhost | |
682 | .It NumberOfPasswordPrompts | |
683 | .It PasswordAuthentication | |
684 | .It Port | |
685 | .It PreferredAuthentications | |
686 | .It Protocol | |
687 | .It ProxyCommand | |
688 | .It PubkeyAuthentication | |
689 | .It RemoteForward | |
690 | .It RhostsRSAAuthentication | |
691 | .It RSAAuthentication | |
692 | .It SendEnv | |
693 | .It ServerAliveInterval | |
694 | .It ServerAliveCountMax | |
695 | .It SmartcardDevice | |
696 | .It StrictHostKeyChecking | |
697 | .It TCPKeepAlive | |
698 | .It UsePrivilegedPort | |
699 | .It User | |
700 | .It UserKnownHostsFile | |
701 | .It VerifyHostKeyDNS | |
702 | .It XAuthLocation | |
703 | .El | |
704 | .It Fl p Ar port | |
705 | Port to connect to on the remote host. | |
706 | This can be specified on a | |
707 | per-host basis in the configuration file. | |
708 | .It Fl q | |
709 | Quiet mode. | |
710 | Causes all warning and diagnostic messages to be suppressed. | |
711 | .It Fl R Xo | |
712 | .Sm off | |
713 | .Ar port : host : hostport | |
714 | .Sm on | |
715 | .Xc | |
716 | Specifies that the given port on the remote (server) host is to be | |
717 | forwarded to the given host and port on the local side. | |
718 | This works by allocating a socket to listen to | |
719 | .Ar port | |
720 | on the remote side, and whenever a connection is made to this port, the | |
721 | connection is forwarded over the secure channel, and a connection is | |
722 | made to | |
723 | .Ar host | |
724 | port | |
725 | .Ar hostport | |
726 | from the local machine. | |
727 | Port forwardings can also be specified in the configuration file. | |
728 | Privileged ports can be forwarded only when | |
729 | logging in as root on the remote machine. | |
730 | IPv6 addresses can be specified with an alternative syntax: | |
731 | .Sm off | |
732 | .Xo | |
733 | .Ar port No / Ar host No / | |
734 | .Ar hostport . | |
735 | .Xc | |
736 | .Sm on | |
737 | .It Fl S Ar ctl | |
738 | Specifies the location of a control socket for connection sharing. | |
739 | Refer to the description of | |
740 | .Cm ControlPath | |
741 | and | |
742 | .Cm ControlMaster | |
743 | in | |
744 | .Xr ssh_config 5 | |
745 | for details. | |
746 | .It Fl s | |
747 | May be used to request invocation of a subsystem on the remote system. | |
748 | Subsystems are a feature of the SSH2 protocol which facilitate the use | |
749 | of SSH as a secure transport for other applications (eg.\& | |
750 | .Xr sftp 1 ) . | |
751 | The subsystem is specified as the remote command. | |
752 | .It Fl T | |
753 | Disable pseudo-tty allocation. | |
754 | .It Fl t | |
755 | Force pseudo-tty allocation. | |
756 | This can be used to execute arbitrary | |
757 | screen-based programs on a remote machine, which can be very useful, | |
758 | e.g., when implementing menu services. | |
759 | Multiple | |
760 | .Fl t | |
761 | options force tty allocation, even if | |
762 | .Nm | |
763 | has no local tty. | |
764 | .It Fl V | |
765 | Display the version number and exit. | |
766 | .It Fl v | |
767 | Verbose mode. | |
768 | Causes | |
769 | .Nm | |
770 | to print debugging messages about its progress. | |
771 | This is helpful in | |
772 | debugging connection, authentication, and configuration problems. | |
773 | Multiple | |
774 | .Fl v | |
775 | options increase the verbosity. | |
776 | The maximum is 3. | |
777 | .It Fl X | |
778 | Enables X11 forwarding. | |
779 | This can also be specified on a per-host basis in a configuration file. | |
780 | .Pp | |
781 | X11 forwarding should be enabled with caution. | |
782 | Users with the ability to bypass file permissions on the remote host | |
783 | (for the user's X authorization database) | |
784 | can access the local X11 display through the forwarded connection. | |
785 | An attacker may then be able to perform activities such as keystroke monitoring. | |
786 | .It Fl x | |
787 | Disables X11 forwarding. | |
788 | .It Fl Y | |
789 | Enables trusted X11 forwarding. | |
790 | .El | |
791 | .Sh CONFIGURATION FILES | |
792 | .Nm | |
793 | may additionally obtain configuration data from | |
794 | a per-user configuration file and a system-wide configuration file. | |
795 | The file format and configuration options are described in | |
796 | .Xr ssh_config 5 . | |
797 | .Sh ENVIRONMENT | |
798 | .Nm | |
799 | will normally set the following environment variables: | |
800 | .Bl -tag -width LOGNAME | |
801 | .It Ev DISPLAY | |
802 | The | |
803 | .Ev DISPLAY | |
804 | variable indicates the location of the X11 server. | |
805 | It is automatically set by | |
806 | .Nm | |
807 | to point to a value of the form | |
808 | .Dq hostname:n | |
809 | where hostname indicates | |
810 | the host where the shell runs, and n is an integer \*(Ge 1. | |
811 | .Nm | |
812 | uses this special value to forward X11 connections over the secure | |
813 | channel. | |
814 | The user should normally not set | |
815 | .Ev DISPLAY | |
816 | explicitly, as that | |
817 | will render the X11 connection insecure (and will require the user to | |
818 | manually copy any required authorization cookies). | |
819 | .It Ev HOME | |
820 | Set to the path of the user's home directory. | |
821 | .It Ev LOGNAME | |
822 | Synonym for | |
823 | .Ev USER ; | |
824 | set for compatibility with systems that use this variable. | |
825 | .It Ev MAIL | |
826 | Set to the path of the user's mailbox. | |
827 | .It Ev PATH | |
828 | Set to the default | |
829 | .Ev PATH , | |
830 | as specified when compiling | |
831 | .Nm ssh . | |
832 | .It Ev SSH_ASKPASS | |
833 | If | |
834 | .Nm | |
835 | needs a passphrase, it will read the passphrase from the current | |
836 | terminal if it was run from a terminal. | |
837 | If | |
838 | .Nm | |
839 | does not have a terminal associated with it but | |
840 | .Ev DISPLAY | |
841 | and | |
842 | .Ev SSH_ASKPASS | |
843 | are set, it will execute the program specified by | |
844 | .Ev SSH_ASKPASS | |
845 | and open an X11 window to read the passphrase. | |
846 | This is particularly useful when calling | |
847 | .Nm | |
848 | from a | |
849 | .Pa .xsession | |
850 | or related script. | |
851 | (Note that on some machines it | |
852 | may be necessary to redirect the input from | |
853 | .Pa /dev/null | |
854 | to make this work.) | |
855 | .It Ev SSH_AUTH_SOCK | |
856 | Identifies the path of a unix-domain socket used to communicate with the | |
857 | agent. | |
858 | .It Ev SSH_CONNECTION | |
859 | Identifies the client and server ends of the connection. | |
860 | The variable contains | |
861 | four space-separated values: client ip-address, client port number, | |
862 | server ip-address and server port number. | |
863 | .It Ev SSH_ORIGINAL_COMMAND | |
864 | The variable contains the original command line if a forced command | |
865 | is executed. | |
866 | It can be used to extract the original arguments. | |
867 | .It Ev SSH_TTY | |
868 | This is set to the name of the tty (path to the device) associated | |
869 | with the current shell or command. | |
870 | If the current session has no tty, | |
871 | this variable is not set. | |
872 | .It Ev TZ | |
873 | The timezone variable is set to indicate the present timezone if it | |
874 | was set when the daemon was started (i.e., the daemon passes the value | |
875 | on to new connections). | |
876 | .It Ev USER | |
877 | Set to the name of the user logging in. | |
878 | .El | |
879 | .Pp | |
880 | Additionally, | |
881 | .Nm | |
882 | reads | |
883 | .Pa $HOME/.ssh/environment , | |
884 | and adds lines of the format | |
885 | .Dq VARNAME=value | |
886 | to the environment if the file exists and if users are allowed to | |
887 | change their environment. | |
888 | For more information, see the | |
889 | .Cm PermitUserEnvironment | |
890 | option in | |
891 | .Xr sshd_config 5 . | |
892 | .Sh FILES | |
893 | .Bl -tag -width Ds | |
894 | .It Pa $HOME/.ssh/known_hosts | |
895 | Records host keys for all hosts the user has logged into that are not | |
896 | in | |
897 | .Pa /etc/ssh/ssh_known_hosts . | |
898 | See | |
899 | .Xr sshd 8 . | |
900 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | |
901 | Contains the authentication identity of the user. | |
902 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
903 | These files | |
904 | contain sensitive data and should be readable by the user but not | |
905 | accessible by others (read/write/execute). | |
906 | Note that | |
907 | .Nm | |
908 | ignores a private key file if it is accessible by others. | |
909 | It is possible to specify a passphrase when | |
910 | generating the key; the passphrase will be used to encrypt the | |
911 | sensitive part of this file using 3DES. | |
912 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | |
913 | Contains the public key for authentication (public part of the | |
914 | identity file in human-readable form). | |
915 | The contents of the | |
916 | .Pa $HOME/.ssh/identity.pub | |
917 | file should be added to the file | |
918 | .Pa $HOME/.ssh/authorized_keys | |
919 | on all machines | |
920 | where the user wishes to log in using protocol version 1 RSA authentication. | |
921 | The contents of the | |
922 | .Pa $HOME/.ssh/id_dsa.pub | |
923 | and | |
924 | .Pa $HOME/.ssh/id_rsa.pub | |
925 | file should be added to | |
926 | .Pa $HOME/.ssh/authorized_keys | |
927 | on all machines | |
928 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. | |
929 | These files are not | |
930 | sensitive and can (but need not) be readable by anyone. | |
931 | These files are | |
932 | never used automatically and are not necessary; they are only provided for | |
933 | the convenience of the user. | |
934 | .It Pa $HOME/.ssh/config | |
935 | This is the per-user configuration file. | |
936 | The file format and configuration options are described in | |
937 | .Xr ssh_config 5 . | |
938 | Because of the potential for abuse, this file must have strict permissions: | |
939 | read/write for the user, and not accessible by others. | |
940 | .It Pa $HOME/.ssh/authorized_keys | |
941 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
942 | The format of this file is described in the | |
943 | .Xr sshd 8 | |
944 | manual page. | |
945 | In the simplest form the format is the same as the | |
946 | .Pa .pub | |
947 | identity files. | |
948 | This file is not highly sensitive, but the recommended | |
949 | permissions are read/write for the user, and not accessible by others. | |
950 | .It Pa /etc/ssh/ssh_known_hosts | |
951 | Systemwide list of known host keys. | |
952 | This file should be prepared by the | |
953 | system administrator to contain the public host keys of all machines in the | |
954 | organization. | |
955 | This file should be world-readable. | |
956 | This file contains | |
957 | public keys, one per line, in the following format (fields separated | |
958 | by spaces): system name, public key and optional comment field. | |
959 | When different names are used | |
960 | for the same machine, all such names should be listed, separated by | |
961 | commas. | |
962 | The format is described in the | |
963 | .Xr sshd 8 | |
964 | manual page. | |
965 | .Pp | |
966 | The canonical system name (as returned by name servers) is used by | |
967 | .Xr sshd 8 | |
968 | to verify the client host when logging in; other names are needed because | |
969 | .Nm | |
970 | does not convert the user-supplied name to a canonical name before | |
971 | checking the key, because someone with access to the name servers | |
972 | would then be able to fool host authentication. | |
973 | .It Pa /etc/ssh/ssh_config | |
974 | Systemwide configuration file. | |
975 | The file format and configuration options are described in | |
976 | .Xr ssh_config 5 . | |
977 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key | |
978 | These three files contain the private parts of the host keys | |
979 | and are used for | |
980 | .Cm RhostsRSAAuthentication | |
981 | and | |
982 | .Cm HostbasedAuthentication . | |
983 | If the protocol version 1 | |
984 | .Cm RhostsRSAAuthentication | |
985 | method is used, | |
986 | .Nm | |
987 | must be setuid root, since the host key is readable only by root. | |
988 | For protocol version 2, | |
989 | .Nm | |
990 | uses | |
991 | .Xr ssh-keysign 8 | |
992 | to access the host keys for | |
993 | .Cm HostbasedAuthentication . | |
994 | This eliminates the requirement that | |
995 | .Nm | |
996 | be setuid root when that authentication method is used. | |
997 | By default | |
998 | .Nm | |
999 | is not setuid root. | |
1000 | .It Pa $HOME/.rhosts | |
1001 | This file is used in | |
1002 | .Cm RhostsRSAAuthentication | |
1003 | and | |
1004 | .Cm HostbasedAuthentication | |
1005 | authentication to list the | |
1006 | host/user pairs that are permitted to log in. | |
1007 | (Note that this file is | |
1008 | also used by rlogin and rsh, which makes using this file insecure.) | |
1009 | Each line of the file contains a host name (in the canonical form | |
1010 | returned by name servers), and then a user name on that host, | |
1011 | separated by a space. | |
1012 | On some machines this file may need to be | |
1013 | world-readable if the user's home directory is on a NFS partition, | |
1014 | because | |
1015 | .Xr sshd 8 | |
1016 | reads it as root. | |
1017 | Additionally, this file must be owned by the user, | |
1018 | and must not have write permissions for anyone else. | |
1019 | The recommended | |
1020 | permission for most machines is read/write for the user, and not | |
1021 | accessible by others. | |
1022 | .Pp | |
1023 | Note that | |
1024 | .Xr sshd 8 | |
1025 | allows authentication only in combination with client host key | |
1026 | authentication before permitting log in. | |
1027 | If the server machine does not have the client's host key in | |
1028 | .Pa /etc/ssh/ssh_known_hosts , | |
1029 | it can be stored in | |
1030 | .Pa $HOME/.ssh/known_hosts . | |
1031 | The easiest way to do this is to | |
1032 | connect back to the client from the server machine using ssh; this | |
1033 | will automatically add the host key to | |
1034 | .Pa $HOME/.ssh/known_hosts . | |
1035 | .It Pa $HOME/.shosts | |
1036 | This file is used exactly the same way as | |
1037 | .Pa .rhosts . | |
1038 | The purpose for | |
1039 | having this file is to be able to use | |
1040 | .Cm RhostsRSAAuthentication | |
1041 | and | |
1042 | .Cm HostbasedAuthentication | |
1043 | authentication without permitting login with | |
1044 | .Xr rlogin | |
1045 | or | |
1046 | .Xr rsh 1 . | |
1047 | .It Pa /etc/hosts.equiv | |
1048 | This file is used during | |
1049 | .Cm RhostsRSAAuthentication | |
1050 | and | |
1051 | .Cm HostbasedAuthentication | |
1052 | authentication. | |
1053 | It contains | |
1054 | canonical hosts names, one per line (the full format is described in the | |
1055 | .Xr sshd 8 | |
1056 | manual page). | |
1057 | If the client host is found in this file, login is | |
1058 | automatically permitted provided client and server user names are the | |
1059 | same. | |
1060 | Additionally, successful client host key authentication is required. | |
1061 | This file should only be writable by root. | |
1062 | .It Pa /etc/shosts.equiv | |
1063 | This file is processed exactly as | |
1064 | .Pa /etc/hosts.equiv . | |
1065 | This file may be useful to permit logins using | |
1066 | .Nm | |
1067 | but not using rsh/rlogin. | |
1068 | .It Pa /etc/ssh/sshrc | |
1069 | Commands in this file are executed by | |
1070 | .Nm | |
1071 | when the user logs in just before the user's shell (or command) is started. | |
1072 | See the | |
1073 | .Xr sshd 8 | |
1074 | manual page for more information. | |
1075 | .It Pa $HOME/.ssh/rc | |
1076 | Commands in this file are executed by | |
1077 | .Nm | |
1078 | when the user logs in just before the user's shell (or command) is | |
1079 | started. | |
1080 | See the | |
1081 | .Xr sshd 8 | |
1082 | manual page for more information. | |
1083 | .It Pa $HOME/.ssh/environment | |
1084 | Contains additional definitions for environment variables, see section | |
1085 | .Sx ENVIRONMENT | |
1086 | above. | |
1087 | .El | |
1088 | .Sh DIAGNOSTICS | |
1089 | .Nm | |
1090 | exits with the exit status of the remote command or with 255 | |
1091 | if an error occurred. | |
1092 | .Sh SEE ALSO | |
1093 | .Xr gzip 1 , | |
1094 | .Xr rsh 1 , | |
1095 | .Xr scp 1 , | |
1096 | .Xr sftp 1 , | |
1097 | .Xr ssh-add 1 , | |
1098 | .Xr ssh-agent 1 , | |
1099 | .Xr ssh-keygen 1 , | |
1100 | .Xr telnet 1 , | |
1101 | .Xr hosts.equiv 5 , | |
1102 | .Xr ssh_config 5 , | |
1103 | .Xr ssh-keysign 8 , | |
1104 | .Xr sshd 8 | |
1105 | .Rs | |
1106 | .%A T. Ylonen | |
1107 | .%A T. Kivinen | |
1108 | .%A M. Saarinen | |
1109 | .%A T. Rinne | |
1110 | .%A S. Lehtinen | |
1111 | .%T "SSH Protocol Architecture" | |
1112 | .%N draft-ietf-secsh-architecture-12.txt | |
1113 | .%D January 2002 | |
1114 | .%O work in progress material | |
1115 | .Re | |
1116 | .Sh AUTHORS | |
1117 | OpenSSH is a derivative of the original and free | |
1118 | ssh 1.2.12 release by Tatu Ylonen. | |
1119 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1120 | Theo de Raadt and Dug Song | |
1121 | removed many bugs, re-added newer features and | |
1122 | created OpenSSH. | |
1123 | Markus Friedl contributed the support for SSH | |
1124 | protocol versions 1.5 and 2.0. |