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1 | .\" -*- nroff -*- | ||
2 | .\" | ||
3 | .\" ssh.1.in | ||
4 | .\" | ||
5 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | ||
6 | .\" | ||
7 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | ||
8 | .\" All rights reserved | ||
9 | .\" | ||
10 | .\" Created: Sat Apr 22 21:55:14 1995 ylo | ||
11 | .\" | ||
12 | .\" $Id: openssh.1,v 1.1 1999/10/28 04:03:14 damien Exp $ | ||
13 | .\" | ||
14 | .Dd September 25, 1999 | ||
15 | .Dt SSH 1 | ||
16 | .Os | ||
17 | .Sh NAME | ||
18 | .Nm ssh | ||
19 | .Nd OpenSSH secure shell client (remote login program) | ||
20 | .Sh SYNOPSIS | ||
21 | .Nm ssh | ||
22 | .Op Fl l Ar login_name | ||
23 | .Op Ar hostname | user@hostname | ||
24 | .Op Ar command | ||
25 | .Pp | ||
26 | .Nm ssh | ||
27 | .Op Fl afgknqtvxCPX | ||
28 | .Op Fl c Ar blowfish | 3des | ||
29 | .Op Fl e Ar escape_char | ||
30 | .Op Fl i Ar identity_file | ||
31 | .Op Fl l Ar login_name | ||
32 | .Op Fl o Ar option | ||
33 | .Op Fl p Ar port | ||
34 | .Oo Fl L Xo | ||
35 | .Sm off | ||
36 | .Ar host : | ||
37 | .Ar port : | ||
38 | .Ar hostport | ||
39 | .Sm on | ||
40 | .Xc | ||
41 | .Oc | ||
42 | .Oo Fl R Xo | ||
43 | .Sm off | ||
44 | .Ar host : | ||
45 | .Ar port : | ||
46 | .Ar hostport | ||
47 | .Sm on | ||
48 | .Xc | ||
49 | .Oc | ||
50 | .Op Ar hostname | user@hostname | ||
51 | .Op Ar command | ||
52 | .Sh DESCRIPTION | ||
53 | .Nm | ||
54 | (Secure Shell) is a program for logging into a remote machine and for | ||
55 | executing commands on a remote machine. It is intended to replace | ||
56 | rlogin and rsh, and provide secure encrypted communications between | ||
57 | two untrusted hosts over an insecure network. X11 connections and | ||
58 | arbitrary TCP/IP ports can also be forwarded over the secure channel. | ||
59 | .Pp | ||
60 | .Nm | ||
61 | connects and logs into the specified | ||
62 | .Ar hostname . | ||
63 | The user must prove | ||
64 | his/her identity to the remote machine using one of several methods. | ||
65 | .Pp | ||
66 | First, if the machine the user logs in from is listed in | ||
67 | .Pa /etc/hosts.equiv | ||
68 | or | ||
69 | .Pa /etc/openssh/shosts.equiv | ||
70 | on the remote machine, and the user names are | ||
71 | the same on both sides, the user is immediately permitted to log in. | ||
72 | Second, if | ||
73 | .Pa \&.rhosts | ||
74 | or | ||
75 | .Pa \&.shosts | ||
76 | exists in the user's home directory on the | ||
77 | remote machine and contains a line containing the name of the client | ||
78 | machine and the name of the user on that machine, the user is | ||
79 | permitted to log in. This form of authentication alone is normally not | ||
80 | allowed by the server because it is not secure. | ||
81 | .Pp | ||
82 | The second (and primary) authentication method is the | ||
83 | .Pa rhosts | ||
84 | or | ||
85 | .Pa hosts.equiv | ||
86 | method combined with RSA-based host authentication. It | ||
87 | means that if the login would be permitted by | ||
88 | .Pa \&.rhosts , | ||
89 | .Pa \&.shosts , | ||
90 | .Pa /etc/hosts.equiv , | ||
91 | or | ||
92 | .Pa /etc/openssh/shosts.equiv , | ||
93 | and if additionally the server can verify the client's | ||
94 | host key (see | ||
95 | .Pa /etc/openssh/ssh_known_hosts | ||
96 | in the | ||
97 | .Sx FILES | ||
98 | section), only then login is | ||
99 | permitted. This authentication method closes security holes due to IP | ||
100 | spoofing, DNS spoofing and routing spoofing. [Note to the | ||
101 | administrator: | ||
102 | .Pa /etc/hosts.equiv , | ||
103 | .Pa \&.rhosts , | ||
104 | and the rlogin/rsh protocol in general, are inherently insecure and should be | ||
105 | disabled if security is desired.] | ||
106 | .Pp | ||
107 | As a third authentication method, | ||
108 | .Nm | ||
109 | supports RSA based authentication. | ||
110 | The scheme is based on public-key cryptography: there are cryptosystems | ||
111 | where encryption and decryption are done using separate keys, and it | ||
112 | is not possible to derive the decryption key from the encryption key. | ||
113 | RSA is one such system. The idea is that each user creates a public/private | ||
114 | key pair for authentication purposes. The | ||
115 | server knows the public key, and only the user knows the private key. | ||
116 | The file | ||
117 | .Pa $HOME/.ssh/authorized_keys | ||
118 | lists the public keys that are permitted for logging | ||
119 | in. When the user logs in, the | ||
120 | .Nm | ||
121 | program tells the server which key pair it would like to use for | ||
122 | authentication. The server checks if this key is permitted, and if | ||
123 | so, sends the user (actually the | ||
124 | .Nm | ||
125 | program running on behalf of the user) a challenge, a random number, | ||
126 | encrypted by the user's public key. The challenge can only be | ||
127 | decrypted using the proper private key. The user's client then decrypts the | ||
128 | challenge using the private key, proving that he/she knows the private | ||
129 | key but without disclosing it to the server. | ||
130 | .Pp | ||
131 | .Nm | ||
132 | implements the RSA authentication protocol automatically. The user | ||
133 | creates his/her RSA key pair by running | ||
134 | .Xr ssh-keygen 1 . | ||
135 | This stores the private key in | ||
136 | .Pa \&.ssh/identity | ||
137 | and the public key in | ||
138 | .Pa \&.ssh/identity.pub | ||
139 | in the user's home directory. The user should then | ||
140 | copy the | ||
141 | .Pa identity.pub | ||
142 | to | ||
143 | .Pa \&.ssh/authorized_keys | ||
144 | in his/her home directory on the remote machine (the | ||
145 | .Pa authorized_keys | ||
146 | file corresponds to the conventional | ||
147 | .Pa \&.rhosts | ||
148 | file, and has one key | ||
149 | per line, though the lines can be very long). After this, the user | ||
150 | can log in without giving the password. RSA authentication is much | ||
151 | more secure than rhosts authentication. | ||
152 | .Pp | ||
153 | The most convenient way to use RSA authentication may be with an | ||
154 | authentication agent. See | ||
155 | .Xr ssh-agent 1 | ||
156 | for more information. | ||
157 | .Pp | ||
158 | If other authentication methods fail, | ||
159 | .Nm | ||
160 | prompts the user for a password. The password is sent to the remote | ||
161 | host for checking; however, since all communications are encrypted, | ||
162 | the password cannot be seen by someone listening on the network. | ||
163 | .Pp | ||
164 | When the user's identity has been accepted by the server, the server | ||
165 | either executes the given command, or logs into the machine and gives | ||
166 | the user a normal shell on the remote machine. All communication with | ||
167 | the remote command or shell will be automatically encrypted. | ||
168 | .Pp | ||
169 | If a pseudo-terminal has been allocated (normal login session), the | ||
170 | user can disconnect with | ||
171 | .Ic ~. , | ||
172 | and suspend | ||
173 | .Nm | ||
174 | with | ||
175 | .Ic ~^Z . | ||
176 | All forwarded connections can be listed with | ||
177 | .Ic ~# | ||
178 | and if | ||
179 | the session blocks waiting for forwarded X11 or TCP/IP | ||
180 | connections to terminate, it can be backgrounded with | ||
181 | .Ic ~& | ||
182 | (this should not be used while the user shell is active, as it can cause the | ||
183 | shell to hang). All available escapes can be listed with | ||
184 | .Ic ~? . | ||
185 | .Pp | ||
186 | A single tilde character can be sent as | ||
187 | .Ic ~~ | ||
188 | (or by following the tilde by a character other than those described above). | ||
189 | The escape character must always follow a newline to be interpreted as | ||
190 | special. The escape character can be changed in configuration files | ||
191 | or on the command line. | ||
192 | .Pp | ||
193 | If no pseudo tty has been allocated, the | ||
194 | session is transparent and can be used to reliably transfer binary | ||
195 | data. On most systems, setting the escape character to | ||
196 | .Dq none | ||
197 | will also make the session transparent even if a tty is used. | ||
198 | .Pp | ||
199 | The session terminates when the command or shell in on the remote | ||
200 | machine exists and all X11 and TCP/IP connections have been closed. | ||
201 | The exit status of the remote program is returned as the exit status | ||
202 | of | ||
203 | .Nm ssh . | ||
204 | .Pp | ||
205 | If the user is using X11 (the | ||
206 | .Ev DISPLAY | ||
207 | environment variable is set), the connection to the X11 display is | ||
208 | automatically forwarded to the remote side in such a way that any X11 | ||
209 | programs started from the shell (or command) will go through the | ||
210 | encrypted channel, and the connection to the real X server will be made | ||
211 | from the local machine. The user should not manually set | ||
212 | .Ev DISPLAY . | ||
213 | Forwarding of X11 connections can be | ||
214 | configured on the command line or in configuration files. | ||
215 | .Pp | ||
216 | The | ||
217 | .Ev DISPLAY | ||
218 | value set by | ||
219 | .Nm | ||
220 | will point to the server machine, but with a display number greater | ||
221 | than zero. This is normal, and happens because | ||
222 | .Nm | ||
223 | creates a | ||
224 | .Dq proxy | ||
225 | X server on the server machine for forwarding the | ||
226 | connections over the encrypted channel. | ||
227 | .Pp | ||
228 | .Nm | ||
229 | will also automatically set up Xauthority data on the server machine. | ||
230 | For this purpose, it will generate a random authorization cookie, | ||
231 | store it in Xauthority on the server, and verify that any forwarded | ||
232 | connections carry this cookie and replace it by the real cookie when | ||
233 | the connection is opened. The real authentication cookie is never | ||
234 | sent to the server machine (and no cookies are sent in the plain). | ||
235 | .Pp | ||
236 | If the user is using an authentication agent, the connection to the agent | ||
237 | is automatically forwarded to the remote side unless disabled on | ||
238 | command line or in a configuration file. | ||
239 | .Pp | ||
240 | Forwarding of arbitrary TCP/IP connections over the secure channel can | ||
241 | be specified either on command line or in a configuration file. One | ||
242 | possible application of TCP/IP forwarding is a secure connection to an | ||
243 | electronic purse; another is going trough firewalls. | ||
244 | .Pp | ||
245 | .Nm | ||
246 | automatically maintains and checks a database containing RSA-based | ||
247 | identifications for all hosts it has ever been used with. The | ||
248 | database is stored in | ||
249 | .Pa \&.ssh/known_hosts | ||
250 | in the user's home directory. Additionally, the file | ||
251 | .Pa /etc/openssh/ssh_known_hosts | ||
252 | is automatically checked for known hosts. Any new hosts are | ||
253 | automatically added to the user's file. If a host's identification | ||
254 | ever changes, | ||
255 | .Nm | ||
256 | warns about this and disables password authentication to prevent a | ||
257 | trojan horse from getting the user's password. Another purpose of | ||
258 | this mechanism is to prevent man-in-the-middle attacks which could | ||
259 | otherwise be used to circumvent the encryption. The | ||
260 | .Cm StrictHostKeyChecking | ||
261 | option (see below) can be used to prevent logins to machines whose | ||
262 | host key is not known or has changed. | ||
263 | .Sh OPTIONS | ||
264 | .Bl -tag -width Ds | ||
265 | .It Fl a | ||
266 | Disables forwarding of the authentication agent connection. This may | ||
267 | also be specified on a per-host basis in the configuration file. | ||
268 | .It Fl c Ar blowfish|3des | ||
269 | Selects the cipher to use for encrypting the session. | ||
270 | .Ar 3des | ||
271 | is used by default. It is believed to be secure. | ||
272 | .Ar 3des | ||
273 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | ||
274 | It is presumably more secure than the | ||
275 | .Ar des | ||
276 | cipher which is no longer supported in ssh. | ||
277 | .Ar blowfish | ||
278 | is a fast block cipher, it appears very secure and is much faster than | ||
279 | .Ar 3des . | ||
280 | .It Fl e Ar ch|^ch|none | ||
281 | Sets the escape character for sessions with a pty (default: | ||
282 | .Ql ~ ) . | ||
283 | The escape character is only recognized at the beginning of a line. The | ||
284 | escape character followed by a dot | ||
285 | .Pq Ql \&. | ||
286 | closes the connection, followed | ||
287 | by control-Z suspends the connection, and followed by itself sends the | ||
288 | escape character once. Setting the character to | ||
289 | .Dq none | ||
290 | disables any escapes and makes the session fully transparent. | ||
291 | .It Fl f | ||
292 | Requests | ||
293 | .Nm | ||
294 | to go to background after authentication. This is useful | ||
295 | if | ||
296 | .Nm | ||
297 | is going to ask for passwords or passphrases, but the user | ||
298 | wants it in the background. This implies | ||
299 | .Fl n . | ||
300 | The recommended way to start X11 programs at a remote site is with | ||
301 | something like | ||
302 | .Ic ssh -f host xterm . | ||
303 | .It Fl i Ar identity_file | ||
304 | Selects the file from which the identity (private key) for | ||
305 | RSA authentication is read. Default is | ||
306 | .Pa \&.ssh/identity | ||
307 | in the user's home directory. Identity files may also be specified on | ||
308 | a per-host basis in the configuration file. It is possible to have | ||
309 | multiple | ||
310 | .Fl i | ||
311 | options (and multiple identities specified in | ||
312 | configuration files). | ||
313 | .It Fl g | ||
314 | Allows remote hosts to connect to local forwarded ports. | ||
315 | .It Fl k | ||
316 | Disables forwarding of Kerberos tickets and AFS tokens. This may | ||
317 | also be specified on a per-host basis in the configuration file. | ||
318 | .It Fl l Ar login_name | ||
319 | Specifies the user to log in as on the remote machine. This may also | ||
320 | be specified on a per-host basis in the configuration file. | ||
321 | .It Fl n | ||
322 | Redirects stdin from | ||
323 | .Pa /dev/null | ||
324 | (actually, prevents reading from stdin). | ||
325 | This must be used when | ||
326 | .Nm | ||
327 | is run in the background. A common trick is to use this to run X11 | ||
328 | programs in a remote machine. For example, | ||
329 | .Ic ssh -n shadows.cs.hut.fi emacs & | ||
330 | will start an emacs on shadows.cs.hut.fi, and the X11 | ||
331 | connection will be automatically forwarded over an encrypted channel. | ||
332 | The | ||
333 | .Nm | ||
334 | program will be put in the background. | ||
335 | (This does not work if | ||
336 | .Nm | ||
337 | needs to ask for a password or passphrase; see also the | ||
338 | .Fl f | ||
339 | option.) | ||
340 | .It Fl o Ar option | ||
341 | Can be used to give options in the format used in the config file. | ||
342 | This is useful for specifying options for which there is no separate | ||
343 | command-line flag. The option has the same format as a line in the | ||
344 | configuration file. | ||
345 | .It Fl p Ar port | ||
346 | Port to connect to on the remote host. This can be specified on a | ||
347 | per-host basis in the configuration file. | ||
348 | .It Fl P | ||
349 | Use a non-privileged port for outgoing connections. | ||
350 | This can be used if your firewall does | ||
351 | not permit connections from privileged ports. | ||
352 | Note that this option turns of | ||
353 | .Cm RhostsAuthentication | ||
354 | and | ||
355 | .Cm RhostsRSAAuthentication . | ||
356 | .It Fl q | ||
357 | Quiet mode. Causes all warning and diagnostic messages to be | ||
358 | suppressed. Only fatal errors are displayed. | ||
359 | .It Fl t | ||
360 | Force pseudo-tty allocation. This can be used to execute arbitary | ||
361 | screen-based programs on a remote machine, which can be very useful | ||
362 | e.g. when implementing menu services. | ||
363 | .It Fl v | ||
364 | Verbose mode. Causes | ||
365 | .Nm | ||
366 | to print debugging messages about its progress. This is helpful in | ||
367 | debugging connection, authentication, and configuration problems. | ||
368 | The verbose mode is also used to display | ||
369 | .Xr skey 1 | ||
370 | challenges, if the user entered "s/key" as password. | ||
371 | .It Fl x | ||
372 | Disables X11 forwarding. This can also be specified on a per-host | ||
373 | basis in a configuration file. | ||
374 | .It Fl X | ||
375 | Enables X11 forwarding. | ||
376 | .It Fl C | ||
377 | Requests compression of all data (including stdin, stdout, stderr, and | ||
378 | data for forwarded X11 and TCP/IP connections). The compression | ||
379 | algorithm is the same used by gzip, and the | ||
380 | .Dq level | ||
381 | can be controlled by the | ||
382 | .Cm CompressionLevel | ||
383 | option (see below). Compression is desirable on modem lines and other | ||
384 | slow connections, but will only slow down things on fast networks. | ||
385 | The default value can be set on a host-by-host basis in the | ||
386 | configuration files; see the | ||
387 | .Cm Compress | ||
388 | option below. | ||
389 | .It Fl L Ar port:host:hostport | ||
390 | Specifies that the given port on the local (client) host is to be | ||
391 | forwarded to the given host and port on the remote side. This works | ||
392 | by allocating a socket to listen to | ||
393 | .Ar port | ||
394 | on the local side, and whenever a connection is made to this port, the | ||
395 | connection is forwarded over the secure channel, and a connection is | ||
396 | made to | ||
397 | .Ar host:hostport | ||
398 | from the remote machine. Port forwardings can also be specified in the | ||
399 | configuration file. Only root can forward privileged ports. | ||
400 | .It Fl R Ar port:host:hostport | ||
401 | Specifies that the given port on the remote (server) host is to be | ||
402 | forwarded to the given host and port on the local side. This works | ||
403 | by allocating a socket to listen to | ||
404 | .Ar port | ||
405 | on the remote side, and whenever a connection is made to this port, the | ||
406 | connection is forwarded over the secure channel, and a connection is | ||
407 | made to | ||
408 | .Ar host:hostport | ||
409 | from the local machine. Port forwardings can also be specified in the | ||
410 | configuration file. Privileged ports can be forwarded only when | ||
411 | logging in as root on the remote machine. | ||
412 | .El | ||
413 | .Sh CONFIGURATION FILES | ||
414 | .Nm | ||
415 | obtains configuration data from the following sources (in this order): | ||
416 | command line options, user's configuration file | ||
417 | .Pq Pa $HOME/.ssh/config , | ||
418 | and system-wide configuration file | ||
419 | .Pq Pa /etc/openssh/ssh_config . | ||
420 | For each parameter, the first obtained value | ||
421 | will be used. The configuration files contain sections bracketed by | ||
422 | "Host" specifications, and that section is only applied for hosts that | ||
423 | match one of the patterns given in the specification. The matched | ||
424 | host name is the one given on the command line. | ||
425 | .Pp | ||
426 | Since the first obtained value for each parameter is used, more | ||
427 | host-specific declarations should be given near the beginning of the | ||
428 | file, and general defaults at the end. | ||
429 | .Pp | ||
430 | The configuration file has the following format: | ||
431 | .Pp | ||
432 | Empty lines and lines starting with | ||
433 | .Ql # | ||
434 | are comments. | ||
435 | .Pp | ||
436 | Otherwise a line is of the format | ||
437 | .Dq keyword arguments . | ||
438 | The possible | ||
439 | keywords and their meanings are as follows (note that the | ||
440 | configuration files are case-sensitive): | ||
441 | .Bl -tag -width Ds | ||
442 | .It Cm Host | ||
443 | Restricts the following declarations (up to the next | ||
444 | .Cm Host | ||
445 | keyword) to be only for those hosts that match one of the patterns | ||
446 | given after the keyword. | ||
447 | .Ql \&* | ||
448 | and | ||
449 | .Ql ? | ||
450 | can be used as wildcards in the | ||
451 | patterns. A single | ||
452 | .Ql \&* | ||
453 | as a pattern can be used to provide global | ||
454 | defaults for all hosts. The host is the | ||
455 | .Ar hostname | ||
456 | argument given on the command line (i.e., the name is not converted to | ||
457 | a canonicalized host name before matching). | ||
458 | .It Cm AFSTokenPassing | ||
459 | Specifies whether to pass AFS tokens to remote host. The argument to | ||
460 | this keyword must be | ||
461 | .Dq yes | ||
462 | or | ||
463 | .Dq no . | ||
464 | .It Cm BatchMode | ||
465 | If set to | ||
466 | .Dq yes , | ||
467 | passphrase/password querying will be disabled. This | ||
468 | option is useful in scripts and other batch jobs where you have no | ||
469 | user to supply the password. The argument must be | ||
470 | .Dq yes | ||
471 | or | ||
472 | .Dq no . | ||
473 | .It Cm Cipher | ||
474 | Specifies the cipher to use for encrypting the session. Currently, | ||
475 | .Dq blowfish , | ||
476 | and | ||
477 | .Dq 3des | ||
478 | are supported. The default is | ||
479 | .Dq 3des . | ||
480 | .It Cm Compression | ||
481 | Specifies whether to use compression. The argument must be | ||
482 | .Dq yes | ||
483 | or | ||
484 | .Dq no . | ||
485 | .It Cm CompressionLevel | ||
486 | Specifies the compression level to use if compression is enable. The | ||
487 | argument must be an integer from 1 (fast) to 9 (slow, best). The | ||
488 | default level is 6, which is good for most applications. The meaning | ||
489 | of the values is the same as in GNU GZIP. | ||
490 | .It Cm ConnectionAttempts | ||
491 | Specifies the number of tries (one per second) to make before falling | ||
492 | back to rsh or exiting. The argument must be an integer. This may be | ||
493 | useful in scripts if the connection sometimes fails. | ||
494 | .It Cm EscapeChar | ||
495 | Sets the escape character (default: | ||
496 | .Ql ~ ) . | ||
497 | The escape character can also | ||
498 | be set on the command line. The argument should be a single | ||
499 | character, | ||
500 | .Ql ^ | ||
501 | followed by a letter, or | ||
502 | .Dq none | ||
503 | to disable the escape | ||
504 | character entirely (making the connection transparent for binary | ||
505 | data). | ||
506 | .It Cm FallBackToRsh | ||
507 | Specifies that if connecting via | ||
508 | .Nm | ||
509 | fails due to a connection refused error (there is no | ||
510 | .Xr sshd 8 | ||
511 | listening on the remote host), | ||
512 | .Xr rsh 1 | ||
513 | should automatically be used instead (after a suitable warning about | ||
514 | the session being unencrypted). The argument must be | ||
515 | .Dq yes | ||
516 | or | ||
517 | .Dq no . | ||
518 | .It Cm ForwardAgent | ||
519 | Specifies whether the connection to the authentication agent (if any) | ||
520 | will be forwarded to the remote machine. The argument must be | ||
521 | .Dq yes | ||
522 | or | ||
523 | .Dq no . | ||
524 | .It Cm ForwardX11 | ||
525 | Specifies whether X11 connections will be automatically redirected | ||
526 | over the secure channel and | ||
527 | .Ev DISPLAY | ||
528 | set. The argument must be | ||
529 | .Dq yes | ||
530 | or | ||
531 | .Dq no . | ||
532 | .It Cm GatewayPorts | ||
533 | Specifies whether remote hosts are allowed to connect to local | ||
534 | forwarded ports. | ||
535 | The argument must be | ||
536 | .Dq yes | ||
537 | or | ||
538 | .Dq no . | ||
539 | The default is | ||
540 | .Dq no . | ||
541 | .It Cm GlobalKnownHostsFile | ||
542 | Specifies a file to use instead of | ||
543 | .Pa /etc/openssh/ssh_known_hosts . | ||
544 | .It Cm HostName | ||
545 | Specifies the real host name to log into. This can be used to specify | ||
546 | nicnames or abbreviations for hosts. Default is the name given on the | ||
547 | command line. Numeric IP addresses are also permitted (both on the | ||
548 | command line and in | ||
549 | .Cm HostName | ||
550 | specifications). | ||
551 | .It Cm IdentityFile | ||
552 | Specifies the file from which the user's RSA authentication identity | ||
553 | is read (default | ||
554 | .Pa .ssh/identity | ||
555 | in the user's home directory). | ||
556 | Additionally, any identities represented by the authentication agent | ||
557 | will be used for authentication. The file name may use the tilde | ||
558 | syntax to refer to a user's home directory. It is possible to have | ||
559 | multiple identity files specified in configuration files; all these | ||
560 | identities will be tried in sequence. | ||
561 | .It Cm KeepAlive | ||
562 | Specifies whether the system should send keepalive messages to the | ||
563 | other side. If they are sent, death of the connection or crash of one | ||
564 | of the machines will be properly noticed. However, this means that | ||
565 | connections will die if the route is down temporarily, and some people | ||
566 | find it annoying. | ||
567 | .Pp | ||
568 | The default is | ||
569 | .Dq yes | ||
570 | (to send keepalives), and the client will notice | ||
571 | if the network goes down or the remote host dies. This is important | ||
572 | in scripts, and many users want it too. | ||
573 | .Pp | ||
574 | To disable keepalives, the value should be set to | ||
575 | .Dq no | ||
576 | in both the server and the client configuration files. | ||
577 | .It Cm KerberosAuthentication | ||
578 | Specifies whether Kerberos authentication will be used. The argument to | ||
579 | this keyword must be | ||
580 | .Dq yes | ||
581 | or | ||
582 | .Dq no . | ||
583 | .It Cm KerberosTgtPassing | ||
584 | Specifies whether a Kerberos TGT will be forwarded to the server. This | ||
585 | will only work if the Kerberos server is actually an AFS kaserver. The | ||
586 | argument to this keyword must be | ||
587 | .Dq yes | ||
588 | or | ||
589 | .Dq no . | ||
590 | .It Cm LocalForward | ||
591 | Specifies that a TCP/IP port on the local machine be forwarded over | ||
592 | the secure channel to given host:port from the remote machine. The | ||
593 | first argument must be a port number, and the second must be | ||
594 | host:port. Multiple forwardings may be specified, and additional | ||
595 | forwardings can be given on the command line. Only the root can | ||
596 | forward privileged ports. | ||
597 | .It Cm PasswordAuthentication | ||
598 | Specifies whether to use password authentication. The argument to | ||
599 | this keyword must be | ||
600 | .Dq yes | ||
601 | or | ||
602 | .Dq no . | ||
603 | .It Cm NumberOfPasswordPrompts | ||
604 | Specifies the number of password prompts before giving up. The | ||
605 | argument to this keyword must be an integer. Default is 3. | ||
606 | .It Cm Port | ||
607 | Specifies the port number to connect on the remote host. Default is | ||
608 | 22. | ||
609 | .It Cm ProxyCommand | ||
610 | Specifies the command to use to connect to the server. The command | ||
611 | string extends to the end of the line, and is executed with /bin/sh. | ||
612 | In the command string, %h will be substituted by the host name to | ||
613 | connect and %p by the port. The command can be basically anything, | ||
614 | and should read from its stdin and write to its stdout. It should | ||
615 | eventually connect an | ||
616 | .Xr sshd 8 | ||
617 | server running on some machine, or execute | ||
618 | .Ic sshd -i | ||
619 | somewhere. Host key management will be done using the | ||
620 | HostName of the host being connected (defaulting to the name typed by | ||
621 | the user). | ||
622 | .Pp | ||
623 | .It Cm RemoteForward | ||
624 | Specifies that a TCP/IP port on the remote machine be forwarded over | ||
625 | the secure channel to given host:port from the local machine. The | ||
626 | first argument must be a port number, and the second must be | ||
627 | host:port. Multiple forwardings may be specified, and additional | ||
628 | forwardings can be given on the command line. Only the root can | ||
629 | forward privileged ports. | ||
630 | .It Cm RhostsAuthentication | ||
631 | Specifies whether to try rhosts based authentication. Note that this | ||
632 | declaration only affects the client side and has no effect whatsoever | ||
633 | on security. Disabling rhosts authentication may reduce | ||
634 | authentication time on slow connections when rhosts authentication is | ||
635 | not used. Most servers do not permit RhostsAuthentication because it | ||
636 | is not secure (see RhostsRSAAuthentication). The argument to this | ||
637 | keyword must be | ||
638 | .Dq yes | ||
639 | or | ||
640 | .Dq no . | ||
641 | .It Cm RhostsRSAAuthentication | ||
642 | Specifies whether to try rhosts based authentication with RSA host | ||
643 | authentication. This is the primary authentication method for most | ||
644 | sites. The argument must be | ||
645 | .Dq yes | ||
646 | or | ||
647 | .Dq no . | ||
648 | .It Cm RSAAuthentication | ||
649 | Specifies whether to try RSA authentication. The argument to this | ||
650 | keyword must be | ||
651 | .Dq yes | ||
652 | or | ||
653 | .Dq no . | ||
654 | RSA authentication will only be | ||
655 | attempted if the identity file exists, or an authentication agent is | ||
656 | running. | ||
657 | .It Cm CheckHostIP | ||
658 | If this flag is set to | ||
659 | .Dq yes , | ||
660 | ssh will additionally check the host ip address in the | ||
661 | .Pa known_hosts | ||
662 | file. This allows ssh to detect if a host key changed due to DNS spoofing. | ||
663 | If the option is set to | ||
664 | .Dq no , | ||
665 | the check will not be executed. | ||
666 | .It Cm StrictHostKeyChecking | ||
667 | If this flag is set to | ||
668 | .Dq yes , | ||
669 | .Nm | ||
670 | ssh will never automatically add host keys to the | ||
671 | .Pa $HOME/.ssh/known_hosts | ||
672 | file, and refuses to connect hosts whose host key has changed. This | ||
673 | provides maximum protection against trojan horse attacks. However, it | ||
674 | can be somewhat annoying if you don't have good | ||
675 | .Pa /etc/openssh/ssh_known_hosts | ||
676 | files installed and frequently | ||
677 | connect new hosts. Basically this option forces the user to manually | ||
678 | add any new hosts. Normally this option is disabled, and new hosts | ||
679 | will automatically be added to the known host files. The host keys of | ||
680 | known hosts will be verified automatically in either case. The | ||
681 | argument must be | ||
682 | .Dq yes | ||
683 | or | ||
684 | .Dq no . | ||
685 | .It Cm User | ||
686 | Specifies the user to log in as. This can be useful if you have a | ||
687 | different user name in different machines. This saves the trouble of | ||
688 | having to remember to give the user name on the command line. | ||
689 | .It Cm UserKnownHostsFile | ||
690 | Specifies a file to use instead of | ||
691 | .Pa $HOME/.ssh/known_hosts . | ||
692 | .It Cm UsePrivilegedPort | ||
693 | Specifies whether to use a privileged port for outgoing connections. | ||
694 | The argument must be | ||
695 | .Dq yes | ||
696 | or | ||
697 | .Dq no . | ||
698 | The default is | ||
699 | .Dq yes . | ||
700 | Note that setting this option to | ||
701 | .Dq no | ||
702 | turns of | ||
703 | .Cm RhostsAuthentication | ||
704 | and | ||
705 | .Cm RhostsRSAAuthentication . | ||
706 | .It Cm UseRsh | ||
707 | Specifies that rlogin/rsh should be used for this host. It is | ||
708 | possible that the host does not at all support the | ||
709 | .Nm | ||
710 | protocol. This causes | ||
711 | .Nm | ||
712 | to immediately exec | ||
713 | .Xr rsh 1 . | ||
714 | All other options (except | ||
715 | .Cm HostName ) | ||
716 | are ignored if this has been specified. The argument must be | ||
717 | .Dq yes | ||
718 | or | ||
719 | .Dq no . | ||
720 | .Sh ENVIRONMENT | ||
721 | .Nm | ||
722 | will normally set the following environment variables: | ||
723 | .Bl -tag -width Ds | ||
724 | .It Ev DISPLAY | ||
725 | The | ||
726 | .Ev DISPLAY | ||
727 | variable indicates the location of the X11 server. It is | ||
728 | automatically set by | ||
729 | .Nm | ||
730 | to point to a value of the form | ||
731 | .Dq hostname:n | ||
732 | where hostname indicates | ||
733 | the host where the shell runs, and n is an integer >= 1. Ssh uses | ||
734 | this special value to forward X11 connections over the secure | ||
735 | channel. The user should normally not set DISPLAY explicitly, as that | ||
736 | will render the X11 connection insecure (and will require the user to | ||
737 | manually copy any required authorization cookies). | ||
738 | .It Ev HOME | ||
739 | Set to the path of the user's home directory. | ||
740 | .It Ev LOGNAME | ||
741 | Synonym for | ||
742 | .Ev USER ; | ||
743 | set for compatibility with systems that use this variable. | ||
744 | .It Ev MAIL | ||
745 | Set to point the user's mailbox. | ||
746 | .It Ev PATH | ||
747 | Set to the default | ||
748 | .Ev PATH , | ||
749 | as specified when compiling | ||
750 | .Nm ssh . | ||
751 | .It Ev SSH_AUTH_SOCK | ||
752 | indicates the path of a unix-domain socket used to communicate with the | ||
753 | agent. | ||
754 | .It Ev SSH_CLIENT | ||
755 | Identifies the client end of the connection. The variable contains | ||
756 | three space-separated values: client ip-address, client port number, | ||
757 | and server port number. | ||
758 | .It Ev SSH_TTY | ||
759 | This is set to the name of the tty (path to the device) associated | ||
760 | with the current shell or command. If the current session has no tty, | ||
761 | this variable is not set. | ||
762 | .It Ev TZ | ||
763 | The timezone variable is set to indicate the present timezone if it | ||
764 | was set when the daemon was started (e.i., the daemon passes the value | ||
765 | on to new connections). | ||
766 | .It Ev USER | ||
767 | Set to the name of the user logging in. | ||
768 | .El | ||
769 | .Pp | ||
770 | Additionally, | ||
771 | .Nm | ||
772 | reads | ||
773 | .Pa $HOME/.ssh/environment , | ||
774 | and adds lines of the format | ||
775 | .Dq VARNAME=value | ||
776 | to the environment. | ||
777 | .Sh FILES | ||
778 | .Bl -tag -width $HOME/.ssh/known_hosts | ||
779 | .It Pa $HOME/.ssh/known_hosts | ||
780 | Records host keys for all hosts the user has logged into (that are not | ||
781 | in | ||
782 | .Pa /etc/openssh/ssh_known_hosts ) . | ||
783 | See | ||
784 | .Xr sshd 8 . | ||
785 | .It Pa $HOME/.ssh/random_seed | ||
786 | Used for seeding the random number generator. This file contains | ||
787 | sensitive data and should read/write for the user and not accessible | ||
788 | for others. This file is created the first time the program is run | ||
789 | and updated automatically. The user should never need to read or | ||
790 | modify this file. | ||
791 | .It Pa $HOME/.ssh/identity | ||
792 | Contains the RSA authentication identity of the user. This file | ||
793 | contains sensitive data and should be readable by the user but not | ||
794 | accessible by others (read/write/execute). | ||
795 | Note that | ||
796 | .Nm | ||
797 | ignores this file if it is accessible by others. | ||
798 | It is possible to specify a passphrase when | ||
799 | generating the key; the passphrase will be used to encrypt the | ||
800 | sensitive part of this file using 3DES. | ||
801 | .It Pa $HOME/.ssh/identity.pub | ||
802 | Contains the public key for authentication (public part of the | ||
803 | identity file in human-readable form). The contents of this file | ||
804 | should be added to | ||
805 | .Pa $HOME/.ssh/authorized_keys | ||
806 | on all machines | ||
807 | where you wish to log in using RSA authentication. This file is not | ||
808 | sensitive and can (but need not) be readable by anyone. This file is | ||
809 | never used automatically and is not necessary; it is only provided for | ||
810 | the convenience of the user. | ||
811 | .It Pa $HOME/.ssh/config | ||
812 | This is the per-user configuration file. The format of this file is | ||
813 | described above. This file is used by the | ||
814 | .Nm | ||
815 | client. This file does not usually contain any sensitive information, | ||
816 | but the recommended permissions are read/write for the user, and not | ||
817 | accessible by others. | ||
818 | .It Pa $HOME/.ssh/authorized_keys | ||
819 | Lists the RSA keys that can be used for logging in as this user. The | ||
820 | format of this file is described in the | ||
821 | .Xr sshd 8 | ||
822 | manual page. In the simplest form the format is the same as the .pub | ||
823 | identity files (that is, each line contains the number of bits in | ||
824 | modulus, public exponent, modulus, and comment fields, separated by | ||
825 | spaces). This file is not highly sensitive, but the recommended | ||
826 | permissions are read/write for the user, and not accessible by others. | ||
827 | .It Pa /etc/openssh/ssh_known_hosts | ||
828 | Systemwide list of known host keys. This file should be prepared by the | ||
829 | system administrator to contain the public host keys of all machines in the | ||
830 | organization. This file should be world-readable. This file contains | ||
831 | public keys, one per line, in the following format (fields separated | ||
832 | by spaces): system name, number of bits in modulus, public exponent, | ||
833 | modulus, and optional comment field. When different names are used | ||
834 | for the same machine, all such names should be listed, separated by | ||
835 | commas. The format is described on the | ||
836 | .Xr sshd 8 | ||
837 | manual page. | ||
838 | .Pp | ||
839 | The canonical system name (as returned by name servers) is used by | ||
840 | .Xr sshd 8 | ||
841 | to verify the client host when logging in; other names are needed because | ||
842 | .Nm | ||
843 | does not convert the user-supplied name to a canonical name before | ||
844 | checking the key, because someone with access to the name servers | ||
845 | would then be able to fool host authentication. | ||
846 | .It Pa /etc/openssh/ssh_config | ||
847 | Systemwide configuration file. This file provides defaults for those | ||
848 | values that are not specified in the user's configuration file, and | ||
849 | for those users who do not have a configuration file. This file must | ||
850 | be world-readable. | ||
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. (Note that this file is | ||
856 | also used by rlogin and rsh, which makes using this file insecure.) | ||
857 | Each line of the file contains a host name (in the canonical form | ||
858 | returned by name servers), and then a user name on that host, | ||
859 | separated by a space. One some machines this file may need to be | ||
860 | world-readable if the user's home directory is on a NFS partition, | ||
861 | because | ||
862 | .Xr sshd 8 | ||
863 | reads it as root. Additionally, this file must be owned by the user, | ||
864 | and must not have write permissions for anyone else. The recommended | ||
865 | permission for most machines is read/write for the user, and not | ||
866 | accessible by others. | ||
867 | .Pp | ||
868 | Note that by default | ||
869 | .Xr sshd 8 | ||
870 | will be installed so that it requires successful RSA host | ||
871 | authentication before permitting \s+2.\s0rhosts authentication. If your | ||
872 | server machine does not have the client's host key in | ||
873 | .Pa /etc/openssh/ssh_known_hosts , | ||
874 | you can store it in | ||
875 | .Pa $HOME/.ssh/known_hosts . | ||
876 | The easiest way to do this is to | ||
877 | connect back to the client from the server machine using ssh; this | ||
878 | will automatically add the host key inxi | ||
879 | .Pa $HOME/.ssh/known_hosts . | ||
880 | .It Pa $HOME/.shosts | ||
881 | This file is used exactly the same way as | ||
882 | .Pa \&.rhosts . | ||
883 | The purpose for | ||
884 | having this file is to be able to use rhosts authentication with | ||
885 | .Nm | ||
886 | without permitting login with | ||
887 | .Xr rlogin 1 | ||
888 | or | ||
889 | .Xr rsh 1 . | ||
890 | .It Pa /etc/hosts.equiv | ||
891 | This file is used during | ||
892 | .Pa \&.rhosts authentication. It contains | ||
893 | canonical hosts names, one per line (the full format is described on | ||
894 | the | ||
895 | .Xr sshd 8 | ||
896 | manual page). If the client host is found in this file, login is | ||
897 | automatically permitted provided client and server user names are the | ||
898 | same. Additionally, successful RSA host authentication is normally | ||
899 | required. This file should only be writable by root. | ||
900 | .It Pa /etc/openssh/shosts.equiv | ||
901 | This file is processed exactly as | ||
902 | .Pa /etc/hosts.equiv . | ||
903 | This file may be useful to permit logins using | ||
904 | .Nm | ||
905 | but not using rsh/rlogin. | ||
906 | .It Pa /etc/openssh/sshrc | ||
907 | Commands in this file are executed by | ||
908 | .Nm | ||
909 | when the user logs in just before the user's shell (or command) is started. | ||
910 | See the | ||
911 | .Xr sshd 8 | ||
912 | manual page for more information. | ||
913 | .It Pa $HOME/.ssh/rc | ||
914 | Commands in this file are executed by | ||
915 | .Nm | ||
916 | when the user logs in just before the user's shell (or command) is | ||
917 | started. | ||
918 | See the | ||
919 | .Xr sshd 8 | ||
920 | manual page for more information. | ||
921 | .It Pa libcrypto.so.X.1 | ||
922 | A version of this library which includes support for the RSA algorithm | ||
923 | is required for proper operation. | ||
924 | .Sh AUTHOR | ||
925 | Tatu Ylonen <ylo@cs.hut.fi> | ||
926 | .Pp | ||
927 | Issues can be found from the SSH WWW home page: | ||
928 | .Pp | ||
929 | .Dl http://www.cs.hut.fi/ssh | ||
930 | .Pp | ||
931 | OpenSSH | ||
932 | is a derivative of the original (free) ssh 1.2.12 release, but with bugs | ||
933 | removed and newer features re-added. Rapidly after the 1.2.12 release, | ||
934 | newer versions bore successively more restrictive licenses. This version | ||
935 | of OpenSSH | ||
936 | .Bl -bullet | ||
937 | .It | ||
938 | has all components of a restrictive nature (ie. patents, see | ||
939 | .Xr ssl 8 ) | ||
940 | directly removed from the source code; any licensed or patented components | ||
941 | are chosen from | ||
942 | external libraries. | ||
943 | .It | ||
944 | has been updated to support ssh protocol 1.5. | ||
945 | .It | ||
946 | contains added support for | ||
947 | .Xr kerberos 8 | ||
948 | authentication and ticket passing. | ||
949 | .It | ||
950 | supports one-time password authentication with | ||
951 | .Xr skey 1 . | ||
952 | .El | ||
953 | .Pp | ||
954 | The libraries described in | ||
955 | .Xr ssl 8 | ||
956 | are required for proper operation. | ||
957 | .Sh SEE ALSO | ||
958 | .Xr rlogin 1 , | ||
959 | .Xr rsh 1 , | ||
960 | .Xr scp 1 , | ||
961 | .Xr ssh-add 1 , | ||
962 | .Xr ssh-agent 1 , | ||
963 | .Xr ssh-keygen 1 , | ||
964 | .Xr telnet 1 , | ||
965 | .Xr sshd 8 , | ||
966 | .Xr ssl 8 | ||