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3c0ef626 | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
9cb1827b | 37 | .\" $OpenBSD: ssh.1,v 1.175 2003/07/22 13:35:22 markus Exp $ |
3c0ef626 | 38 | .Dd September 25, 1999 |
39 | .Dt SSH 1 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh | |
43 | .Nd OpenSSH SSH client (remote login program) | |
44 | .Sh SYNOPSIS | |
45 | .Nm ssh | |
9cb1827b | 46 | .Op Fl l Ar login_name |
47 | .Ar hostname | user@hostname | |
48 | .Op Ar command | |
49 | .Pp | |
50 | .Nm ssh | |
51 | .Bk -words | |
52 | .Op Fl afgknqstvxACNTVX1246 | |
3c0ef626 | 53 | .Op Fl b Ar bind_address |
54 | .Op Fl c Ar cipher_spec | |
55 | .Op Fl e Ar escape_char | |
12408a1b | 56 | .Op Fl i Ar identity_file |
9cb1827b | 57 | .Op Fl l Ar login_name |
58 | .Op Fl m Ar mac_spec | |
59 | .Op Fl o Ar option | |
60 | .Op Fl p Ar port | |
61 | .Op Fl F Ar configfile | |
3c0ef626 | 62 | .Oo Fl L Xo |
63 | .Sm off | |
64 | .Ar port : | |
65 | .Ar host : | |
66 | .Ar hostport | |
67 | .Sm on | |
68 | .Xc | |
69 | .Oc | |
6a9b3198 | 70 | .Ek |
71 | .Bk -words | |
3c0ef626 | 72 | .Oo Fl R Xo |
73 | .Sm off | |
74 | .Ar port : | |
75 | .Ar host : | |
76 | .Ar hostport | |
77 | .Sm on | |
78 | .Xc | |
79 | .Oc | |
9cb1827b | 80 | .Op Fl D Ar port |
81 | .Ar hostname | user@hostname | |
3c0ef626 | 82 | .Op Ar command |
9cb1827b | 83 | .Ek |
3c0ef626 | 84 | .Sh DESCRIPTION |
85 | .Nm | |
86 | (SSH client) is a program for logging into a remote machine and for | |
87 | executing commands on a remote machine. | |
9cb1827b | 88 | It is intended to replace |
89 | rlogin and rsh, and provide secure encrypted communications between | |
3c0ef626 | 90 | two untrusted hosts over an insecure network. |
9cb1827b | 91 | X11 connections and |
92 | arbitrary TCP/IP ports can also be forwarded over the secure channel. | |
3c0ef626 | 93 | .Pp |
94 | .Nm | |
95 | connects and logs into the specified | |
9cb1827b | 96 | .Ar hostname . |
3c0ef626 | 97 | The user must prove |
98 | his/her identity to the remote machine using one of several methods | |
9cb1827b | 99 | depending on the protocol version used: |
3c0ef626 | 100 | .Pp |
101 | .Ss SSH protocol version 1 | |
9cb1827b | 102 | .Pp |
3c0ef626 | 103 | First, if the machine the user logs in from is listed in |
104 | .Pa /etc/hosts.equiv | |
105 | or | |
106 | .Pa /etc/shosts.equiv | |
107 | on the remote machine, and the user names are | |
108 | the same on both sides, the user is immediately permitted to log in. | |
109 | Second, if | |
9cb1827b | 110 | .Pa \&.rhosts |
3c0ef626 | 111 | or |
9cb1827b | 112 | .Pa \&.shosts |
3c0ef626 | 113 | exists in the user's home directory on the |
114 | remote machine and contains a line containing the name of the client | |
115 | machine and the name of the user on that machine, the user is | |
116 | permitted to log in. | |
117 | This form of authentication alone is normally not | |
118 | allowed by the server because it is not secure. | |
119 | .Pp | |
120 | The second authentication method is the | |
9cb1827b | 121 | .Pa rhosts |
3c0ef626 | 122 | or |
9cb1827b | 123 | .Pa hosts.equiv |
3c0ef626 | 124 | method combined with RSA-based host authentication. |
125 | It means that if the login would be permitted by | |
126 | .Pa $HOME/.rhosts , | |
127 | .Pa $HOME/.shosts , | |
128 | .Pa /etc/hosts.equiv , | |
129 | or | |
130 | .Pa /etc/shosts.equiv , | |
131 | and if additionally the server can verify the client's | |
132 | host key (see | |
e9a17296 | 133 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 134 | and |
135 | .Pa $HOME/.ssh/known_hosts | |
136 | in the | |
137 | .Sx FILES | |
9cb1827b | 138 | section), only then login is permitted. |
3c0ef626 | 139 | This authentication method closes security holes due to IP |
140 | spoofing, DNS spoofing and routing spoofing. | |
141 | [Note to the administrator: | |
142 | .Pa /etc/hosts.equiv , | |
143 | .Pa $HOME/.rhosts , | |
144 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
145 | disabled if security is desired.] | |
146 | .Pp | |
147 | As a third authentication method, | |
148 | .Nm | |
149 | supports RSA based authentication. | |
150 | The scheme is based on public-key cryptography: there are cryptosystems | |
151 | where encryption and decryption are done using separate keys, and it | |
152 | is not possible to derive the decryption key from the encryption key. | |
153 | RSA is one such system. | |
154 | The idea is that each user creates a public/private | |
155 | key pair for authentication purposes. | |
156 | The server knows the public key, and only the user knows the private key. | |
157 | The file | |
158 | .Pa $HOME/.ssh/authorized_keys | |
9cb1827b | 159 | lists the public keys that are permitted for logging |
160 | in. | |
3c0ef626 | 161 | When the user logs in, the |
162 | .Nm | |
163 | program tells the server which key pair it would like to use for | |
164 | authentication. | |
9cb1827b | 165 | The server checks if this key is permitted, and if |
166 | so, sends the user (actually the | |
3c0ef626 | 167 | .Nm |
168 | program running on behalf of the user) a challenge, a random number, | |
169 | encrypted by the user's public key. | |
9cb1827b | 170 | The challenge can only be |
171 | decrypted using the proper private key. | |
172 | The user's client then decrypts the | |
173 | challenge using the private key, proving that he/she knows the private | |
174 | key but without disclosing it to the server. | |
3c0ef626 | 175 | .Pp |
176 | .Nm | |
177 | implements the RSA authentication protocol automatically. | |
178 | The user creates his/her RSA key pair by running | |
179 | .Xr ssh-keygen 1 . | |
180 | This stores the private key in | |
181 | .Pa $HOME/.ssh/identity | |
9cb1827b | 182 | and the public key in |
3c0ef626 | 183 | .Pa $HOME/.ssh/identity.pub |
184 | in the user's home directory. | |
185 | The user should then copy the | |
186 | .Pa identity.pub | |
187 | to | |
188 | .Pa $HOME/.ssh/authorized_keys | |
189 | in his/her home directory on the remote machine (the | |
190 | .Pa authorized_keys | |
191 | file corresponds to the conventional | |
192 | .Pa $HOME/.rhosts | |
193 | file, and has one key | |
194 | per line, though the lines can be very long). | |
195 | After this, the user can log in without giving the password. | |
9cb1827b | 196 | RSA authentication is much |
197 | more secure than rhosts authentication. | |
3c0ef626 | 198 | .Pp |
199 | The most convenient way to use RSA authentication may be with an | |
200 | authentication agent. | |
201 | See | |
202 | .Xr ssh-agent 1 | |
203 | for more information. | |
204 | .Pp | |
205 | If other authentication methods fail, | |
206 | .Nm | |
207 | prompts the user for a password. | |
208 | The password is sent to the remote | |
209 | host for checking; however, since all communications are encrypted, | |
210 | the password cannot be seen by someone listening on the network. | |
9cb1827b | 211 | .Pp |
3c0ef626 | 212 | .Ss SSH protocol version 2 |
9cb1827b | 213 | .Pp |
214 | When a user connects using protocol version 2 | |
e9a17296 | 215 | similar authentication methods are available. |
3c0ef626 | 216 | Using the default values for |
217 | .Cm PreferredAuthentications , | |
218 | the client will try to authenticate first using the hostbased method; | |
9cb1827b | 219 | if this method fails public key authentication is attempted, |
220 | and finally if this method fails keyboard-interactive and | |
3c0ef626 | 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 | |
9cb1827b | 236 | If public key authentication fails or is not available a password |
237 | can be sent encrypted to the remote host for proving the user's identity. | |
3c0ef626 | 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. | |
9cb1827b | 248 | .Pp |
3c0ef626 | 249 | .Ss Login session and remote execution |
9cb1827b | 250 | .Pp |
3c0ef626 | 251 | When the user's identity has been accepted by the server, the server |
252 | either executes the given command, or logs into the machine and gives | |
253 | the user a normal shell on the remote machine. | |
254 | All communication with | |
255 | the remote command or shell will be automatically encrypted. | |
256 | .Pp | |
257 | If a pseudo-terminal has been allocated (normal login session), the | |
258 | user may use the escape characters noted below. | |
259 | .Pp | |
9cb1827b | 260 | If no pseudo tty has been allocated, the |
261 | session is transparent and can be used to reliably transfer binary | |
262 | data. | |
3c0ef626 | 263 | On most systems, setting the escape character to |
264 | .Dq none | |
265 | will also make the session transparent even if a tty is used. | |
266 | .Pp | |
267 | The session terminates when the command or shell on the remote | |
268 | machine exits and all X11 and TCP/IP connections have been closed. | |
9cb1827b | 269 | The exit status of the remote program is returned as the exit status |
270 | of | |
3c0ef626 | 271 | .Nm ssh . |
9cb1827b | 272 | .Pp |
3c0ef626 | 273 | .Ss Escape Characters |
9cb1827b | 274 | .Pp |
275 | When a pseudo terminal has been requested, ssh supports a number of functions | |
276 | through the use of an escape character. | |
3c0ef626 | 277 | .Pp |
278 | A single tilde character can be sent as | |
279 | .Ic ~~ | |
280 | or by following the tilde by a character other than those described below. | |
281 | The escape character must always follow a newline to be interpreted as | |
282 | special. | |
283 | The escape character can be changed in configuration files using the | |
284 | .Cm EscapeChar | |
285 | configuration directive or on the command line by the | |
286 | .Fl e | |
287 | option. | |
288 | .Pp | |
289 | The supported escapes (assuming the default | |
290 | .Ql ~ ) | |
291 | are: | |
292 | .Bl -tag -width Ds | |
293 | .It Cm ~. | |
9cb1827b | 294 | Disconnect |
3c0ef626 | 295 | .It Cm ~^Z |
9cb1827b | 296 | Background ssh |
3c0ef626 | 297 | .It Cm ~# |
9cb1827b | 298 | List forwarded connections |
3c0ef626 | 299 | .It Cm ~& |
9cb1827b | 300 | Background ssh at logout when waiting for forwarded connection / X11 sessions |
301 | to terminate | |
3c0ef626 | 302 | .It Cm ~? |
9cb1827b | 303 | Display a list of escape characters |
0fff78ff | 304 | .It Cm ~B |
9cb1827b | 305 | Send a BREAK to the remote system (only useful for SSH protocol version 2 |
306 | and if the peer supports it) | |
700318f3 | 307 | .It Cm ~C |
308 | Open command line (only useful for adding port forwardings using the | |
309 | .Fl L | |
310 | and | |
311 | .Fl R | |
9cb1827b | 312 | options) |
3c0ef626 | 313 | .It Cm ~R |
9cb1827b | 314 | Request rekeying of the connection (only useful for SSH protocol version 2 |
315 | and if the peer supports it) | |
3c0ef626 | 316 | .El |
9cb1827b | 317 | .Pp |
3c0ef626 | 318 | .Ss X11 and TCP forwarding |
9cb1827b | 319 | .Pp |
3c0ef626 | 320 | If the |
321 | .Cm ForwardX11 | |
322 | variable is set to | |
323 | .Dq yes | |
9cb1827b | 324 | (or, see the description of the |
3c0ef626 | 325 | .Fl X |
326 | and | |
327 | .Fl x | |
328 | options described later) | |
329 | and the user is using X11 (the | |
330 | .Ev DISPLAY | |
331 | environment variable is set), the connection to the X11 display is | |
332 | automatically forwarded to the remote side in such a way that any X11 | |
333 | programs started from the shell (or command) will go through the | |
334 | encrypted channel, and the connection to the real X server will be made | |
335 | from the local machine. | |
336 | The user should not manually set | |
337 | .Ev DISPLAY . | |
338 | Forwarding of X11 connections can be | |
339 | configured on the command line or in configuration files. | |
340 | .Pp | |
341 | The | |
342 | .Ev DISPLAY | |
343 | value set by | |
344 | .Nm | |
9cb1827b | 345 | will point to the server machine, but with a display number greater |
346 | than zero. | |
3c0ef626 | 347 | This is normal, and happens because |
348 | .Nm | |
349 | creates a | |
350 | .Dq proxy | |
351 | X server on the server machine for forwarding the | |
352 | connections over the encrypted channel. | |
353 | .Pp | |
354 | .Nm | |
355 | will also automatically set up Xauthority data on the server machine. | |
356 | For this purpose, it will generate a random authorization cookie, | |
357 | store it in Xauthority on the server, and verify that any forwarded | |
358 | connections carry this cookie and replace it by the real cookie when | |
359 | the connection is opened. | |
360 | The real authentication cookie is never | |
361 | sent to the server machine (and no cookies are sent in the plain). | |
362 | .Pp | |
41b2f314 | 363 | If the |
364 | .Cm ForwardAgent | |
365 | variable is set to | |
366 | .Dq yes | |
9cb1827b | 367 | (or, see the description of the |
41b2f314 | 368 | .Fl A |
369 | and | |
370 | .Fl a | |
6a9b3198 | 371 | options described later) and |
41b2f314 | 372 | the user is using an authentication agent, the connection to the agent |
373 | is automatically forwarded to the remote side. | |
3c0ef626 | 374 | .Pp |
375 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
376 | be specified either on the command line or in a configuration file. | |
377 | One possible application of TCP/IP forwarding is a secure connection to an | |
378 | electronic purse; another is going through firewalls. | |
9cb1827b | 379 | .Pp |
3c0ef626 | 380 | .Ss Server authentication |
9cb1827b | 381 | .Pp |
3c0ef626 | 382 | .Nm |
383 | automatically maintains and checks a database containing | |
384 | identifications for all hosts it has ever been used with. | |
385 | Host keys are stored in | |
386 | .Pa $HOME/.ssh/known_hosts | |
387 | in the user's home directory. | |
388 | Additionally, the file | |
e9a17296 | 389 | .Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 390 | is automatically checked for known hosts. |
391 | Any new hosts are automatically added to the user's file. | |
9cb1827b | 392 | If a host's identification |
393 | ever changes, | |
3c0ef626 | 394 | .Nm |
395 | warns about this and disables password authentication to prevent a | |
396 | trojan horse from getting the user's password. | |
9cb1827b | 397 | Another purpose of |
398 | this mechanism is to prevent man-in-the-middle attacks which could | |
399 | otherwise be used to circumvent the encryption. | |
3c0ef626 | 400 | The |
401 | .Cm StrictHostKeyChecking | |
f5799ae1 | 402 | option can be used to prevent logins to machines whose |
3c0ef626 | 403 | host key is not known or has changed. |
404 | .Pp | |
405 | The options are as follows: | |
406 | .Bl -tag -width Ds | |
9cb1827b | 407 | .It Fl a |
408 | Disables forwarding of the authentication agent connection. | |
3c0ef626 | 409 | .It Fl A |
410 | Enables forwarding of the authentication agent connection. | |
411 | This can also be specified on a per-host basis in a configuration file. | |
41b2f314 | 412 | .Pp |
6a9b3198 | 413 | Agent forwarding should be enabled with caution. |
414 | Users with the ability to bypass file permissions on the remote host | |
415 | (for the agent's Unix-domain socket) | |
416 | can access the local agent through the forwarded connection. | |
417 | An attacker cannot obtain key material from the agent, | |
41b2f314 | 418 | however they can perform operations on the keys that enable them to |
419 | authenticate using the identities loaded into the agent. | |
3c0ef626 | 420 | .It Fl b Ar bind_address |
421 | Specify the interface to transmit from on machines with multiple | |
422 | interfaces or aliased addresses. | |
9cb1827b | 423 | .It Fl c Ar blowfish|3des|des |
3c0ef626 | 424 | Selects the cipher to use for encrypting the session. |
425 | .Ar 3des | |
426 | is used by default. | |
427 | It is believed to be secure. | |
428 | .Ar 3des | |
429 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
430 | .Ar blowfish | |
9cb1827b | 431 | is a fast block cipher, it appears very secure and is much faster than |
3c0ef626 | 432 | .Ar 3des . |
433 | .Ar des | |
434 | is only supported in the | |
435 | .Nm | |
436 | client for interoperability with legacy protocol 1 implementations | |
437 | that do not support the | |
438 | .Ar 3des | |
6a9b3198 | 439 | cipher. |
440 | Its use is strongly discouraged due to cryptographic weaknesses. | |
3c0ef626 | 441 | .It Fl c Ar cipher_spec |
442 | Additionally, for protocol version 2 a comma-separated list of ciphers can | |
443 | be specified in order of preference. | |
444 | See | |
445 | .Cm Ciphers | |
446 | for more information. | |
9cb1827b | 447 | .It Fl e Ar ch|^ch|none |
3c0ef626 | 448 | Sets the escape character for sessions with a pty (default: |
449 | .Ql ~ ) . | |
450 | The escape character is only recognized at the beginning of a line. | |
451 | The escape character followed by a dot | |
452 | .Pq Ql \&. | |
9cb1827b | 453 | closes the connection, followed |
454 | by control-Z suspends the connection, and followed by itself sends the | |
455 | escape character once. | |
3c0ef626 | 456 | Setting the character to |
457 | .Dq none | |
458 | disables any escapes and makes the session fully transparent. | |
459 | .It Fl f | |
460 | Requests | |
461 | .Nm | |
462 | to go to background just before command execution. | |
463 | This is useful if | |
464 | .Nm | |
465 | is going to ask for passwords or passphrases, but the user | |
466 | wants it in the background. | |
467 | This implies | |
468 | .Fl n . | |
469 | The recommended way to start X11 programs at a remote site is with | |
470 | something like | |
471 | .Ic ssh -f host xterm . | |
472 | .It Fl g | |
473 | Allows remote hosts to connect to local forwarded ports. | |
474 | .It Fl i Ar identity_file | |
e9a17296 | 475 | Selects a file from which the identity (private key) for |
3c0ef626 | 476 | RSA or DSA authentication is read. |
e9a17296 | 477 | The default is |
3c0ef626 | 478 | .Pa $HOME/.ssh/identity |
e9a17296 | 479 | for protocol version 1, and |
480 | .Pa $HOME/.ssh/id_rsa | |
481 | and | |
482 | .Pa $HOME/.ssh/id_dsa | |
483 | for protocol version 2. | |
3c0ef626 | 484 | Identity files may also be specified on |
485 | a per-host basis in the configuration file. | |
486 | It is possible to have multiple | |
487 | .Fl i | |
488 | options (and multiple identities specified in | |
489 | configuration files). | |
9cb1827b | 490 | .It Fl I Ar smartcard_device |
491 | Specifies which smartcard device to use. | |
492 | The argument is the device | |
493 | .Nm | |
494 | should use to communicate with a smartcard used for storing the user's | |
495 | private RSA key. | |
3c0ef626 | 496 | .It Fl k |
9cb1827b | 497 | Disables forwarding of Kerberos tickets. |
498 | This may also be specified on a per-host basis in the configuration file. | |
3c0ef626 | 499 | .It Fl l Ar login_name |
500 | Specifies the user to log in as on the remote machine. | |
501 | This also may be specified on a per-host basis in the configuration file. | |
502 | .It Fl m Ar mac_spec | |
503 | Additionally, for protocol version 2 a comma-separated list of MAC | |
504 | (message authentication code) algorithms can | |
505 | be specified in order of preference. | |
506 | See the | |
507 | .Cm MACs | |
508 | keyword for more information. | |
509 | .It Fl n | |
510 | Redirects stdin from | |
511 | .Pa /dev/null | |
512 | (actually, prevents reading from stdin). | |
513 | This must be used when | |
514 | .Nm | |
515 | is run in the background. | |
516 | A common trick is to use this to run X11 programs on a remote machine. | |
517 | For example, | |
518 | .Ic ssh -n shadows.cs.hut.fi emacs & | |
519 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
520 | connection will be automatically forwarded over an encrypted channel. | |
521 | The | |
522 | .Nm | |
523 | program will be put in the background. | |
524 | (This does not work if | |
525 | .Nm | |
526 | needs to ask for a password or passphrase; see also the | |
527 | .Fl f | |
528 | option.) | |
9cb1827b | 529 | .It Fl N |
530 | Do not execute a remote command. | |
531 | This is useful for just forwarding ports | |
532 | (protocol version 2 only). | |
3c0ef626 | 533 | .It Fl o Ar option |
534 | Can be used to give options in the format used in the configuration file. | |
535 | This is useful for specifying options for which there is no separate | |
536 | command-line flag. | |
537 | .It Fl p Ar port | |
538 | Port to connect to on the remote host. | |
539 | This can be specified on a | |
540 | per-host basis in the configuration file. | |
3c0ef626 | 541 | .It Fl q |
542 | Quiet mode. | |
543 | Causes all warning and diagnostic messages to be suppressed. | |
3c0ef626 | 544 | .It Fl s |
0fff78ff | 545 | May be used to request invocation of a subsystem on the remote system. |
546 | Subsystems are a feature of the SSH2 protocol which facilitate the use | |
9cb1827b | 547 | of SSH as a secure transport for other applications (eg. sftp). |
0fff78ff | 548 | The subsystem is specified as the remote command. |
3c0ef626 | 549 | .It Fl t |
550 | Force pseudo-tty allocation. | |
551 | This can be used to execute arbitrary | |
552 | screen-based programs on a remote machine, which can be very useful, | |
553 | e.g., when implementing menu services. | |
554 | Multiple | |
555 | .Fl t | |
556 | options force tty allocation, even if | |
557 | .Nm | |
558 | has no local tty. | |
9cb1827b | 559 | .It Fl T |
560 | Disable pseudo-tty allocation. | |
3c0ef626 | 561 | .It Fl v |
562 | Verbose mode. | |
563 | Causes | |
564 | .Nm | |
565 | to print debugging messages about its progress. | |
566 | This is helpful in | |
567 | debugging connection, authentication, and configuration problems. | |
568 | Multiple | |
569 | .Fl v | |
0fff78ff | 570 | options increase the verbosity. |
571 | The maximum is 3. | |
9cb1827b | 572 | .It Fl V |
573 | Display the version number and exit. | |
574 | .It Fl x | |
575 | Disables X11 forwarding. | |
3c0ef626 | 576 | .It Fl X |
577 | Enables X11 forwarding. | |
578 | This can also be specified on a per-host basis in a configuration file. | |
41b2f314 | 579 | .Pp |
6a9b3198 | 580 | X11 forwarding should be enabled with caution. |
581 | Users with the ability to bypass file permissions on the remote host | |
582 | (for the user's X authorization database) | |
583 | can access the local X11 display through the forwarded connection. | |
584 | An attacker may then be able to perform activities such as keystroke monitoring. | |
9cb1827b | 585 | .It Fl C |
586 | Requests compression of all data (including stdin, stdout, stderr, and | |
587 | data for forwarded X11 and TCP/IP connections). | |
588 | The compression algorithm is the same used by | |
589 | .Xr gzip 1 , | |
590 | and the | |
591 | .Dq level | |
592 | can be controlled by the | |
593 | .Cm CompressionLevel | |
594 | option for protocol version 1. | |
595 | Compression is desirable on modem lines and other | |
596 | slow connections, but will only slow down things on fast networks. | |
597 | The default value can be set on a host-by-host basis in the | |
598 | configuration files; see the | |
599 | .Cm Compression | |
600 | option. | |
601 | .It Fl F Ar configfile | |
602 | Specifies an alternative per-user configuration file. | |
603 | If a configuration file is given on the command line, | |
604 | the system-wide configuration file | |
605 | .Pq Pa /etc/ssh/ssh_config | |
606 | will be ignored. | |
607 | The default for the per-user configuration file is | |
608 | .Pa $HOME/.ssh/config . | |
609 | .It Fl L Ar port:host:hostport | |
610 | Specifies that the given port on the local (client) host is to be | |
611 | forwarded to the given host and port on the remote side. | |
612 | This works by allocating a socket to listen to | |
613 | .Ar port | |
614 | on the local side, and whenever a connection is made to this port, the | |
615 | connection is forwarded over the secure channel, and a connection is | |
616 | made to | |
617 | .Ar host | |
618 | port | |
619 | .Ar hostport | |
620 | from the remote machine. | |
621 | Port forwardings can also be specified in the configuration file. | |
622 | Only root can forward privileged ports. | |
623 | IPv6 addresses can be specified with an alternative syntax: | |
624 | .Ar port/host/hostport | |
625 | .It Fl R Ar port:host:hostport | |
626 | Specifies that the given port on the remote (server) host is to be | |
627 | forwarded to the given host and port on the local side. | |
628 | This works by allocating a socket to listen to | |
629 | .Ar port | |
630 | on the remote side, and whenever a connection is made to this port, the | |
631 | connection is forwarded over the secure channel, and a connection is | |
632 | made to | |
633 | .Ar host | |
634 | port | |
635 | .Ar hostport | |
636 | from the local machine. | |
637 | Port forwardings can also be specified in the configuration file. | |
638 | Privileged ports can be forwarded only when | |
639 | logging in as root on the remote machine. | |
640 | IPv6 addresses can be specified with an alternative syntax: | |
641 | .Ar port/host/hostport | |
642 | .It Fl D Ar port | |
643 | Specifies a local | |
644 | .Dq dynamic | |
645 | application-level port forwarding. | |
646 | This works by allocating a socket to listen to | |
647 | .Ar port | |
648 | on the local side, and whenever a connection is made to this port, the | |
649 | connection is forwarded over the secure channel, and the application | |
650 | protocol is then used to determine where to connect to from the | |
651 | remote machine. | |
652 | Currently the SOCKS4 and SOCKS5 protocols are supported, and | |
653 | .Nm | |
654 | will act as a SOCKS server. | |
655 | Only root can forward privileged ports. | |
656 | Dynamic port forwardings can also be specified in the configuration file. | |
657 | .It Fl 1 | |
658 | Forces | |
659 | .Nm | |
660 | to try protocol version 1 only. | |
661 | .It Fl 2 | |
662 | Forces | |
663 | .Nm | |
664 | to try protocol version 2 only. | |
665 | .It Fl 4 | |
666 | Forces | |
667 | .Nm | |
668 | to use IPv4 addresses only. | |
669 | .It Fl 6 | |
670 | Forces | |
671 | .Nm | |
672 | to use IPv6 addresses only. | |
3c0ef626 | 673 | .El |
674 | .Sh CONFIGURATION FILES | |
675 | .Nm | |
f5799ae1 | 676 | may additionally obtain configuration data from |
677 | a per-user configuration file and a system-wide configuration file. | |
678 | The file format and configuration options are described in | |
679 | .Xr ssh_config 5 . | |
3c0ef626 | 680 | .Sh ENVIRONMENT |
681 | .Nm | |
682 | will normally set the following environment variables: | |
9cb1827b | 683 | .Bl -tag -width Ds |
3c0ef626 | 684 | .It Ev DISPLAY |
685 | The | |
686 | .Ev DISPLAY | |
687 | variable indicates the location of the X11 server. | |
688 | It is automatically set by | |
689 | .Nm | |
690 | to point to a value of the form | |
691 | .Dq hostname:n | |
692 | where hostname indicates | |
9cb1827b | 693 | the host where the shell runs, and n is an integer >= 1. |
3c0ef626 | 694 | .Nm |
695 | uses this special value to forward X11 connections over the secure | |
696 | channel. | |
697 | The user should normally not set | |
698 | .Ev DISPLAY | |
699 | explicitly, as that | |
700 | will render the X11 connection insecure (and will require the user to | |
701 | manually copy any required authorization cookies). | |
702 | .It Ev HOME | |
703 | Set to the path of the user's home directory. | |
704 | .It Ev LOGNAME | |
705 | Synonym for | |
706 | .Ev USER ; | |
707 | set for compatibility with systems that use this variable. | |
708 | .It Ev MAIL | |
709 | Set to the path of the user's mailbox. | |
710 | .It Ev PATH | |
711 | Set to the default | |
712 | .Ev PATH , | |
713 | as specified when compiling | |
714 | .Nm ssh . | |
715 | .It Ev SSH_ASKPASS | |
716 | If | |
717 | .Nm | |
718 | needs a passphrase, it will read the passphrase from the current | |
719 | terminal if it was run from a terminal. | |
720 | If | |
721 | .Nm | |
722 | does not have a terminal associated with it but | |
723 | .Ev DISPLAY | |
724 | and | |
725 | .Ev SSH_ASKPASS | |
726 | are set, it will execute the program specified by | |
727 | .Ev SSH_ASKPASS | |
728 | and open an X11 window to read the passphrase. | |
729 | This is particularly useful when calling | |
730 | .Nm | |
731 | from a | |
732 | .Pa .Xsession | |
733 | or related script. | |
734 | (Note that on some machines it | |
735 | may be necessary to redirect the input from | |
736 | .Pa /dev/null | |
737 | to make this work.) | |
738 | .It Ev SSH_AUTH_SOCK | |
739 | Identifies the path of a unix-domain socket used to communicate with the | |
740 | agent. | |
41b2f314 | 741 | .It Ev SSH_CONNECTION |
742 | Identifies the client and server ends of the connection. | |
3c0ef626 | 743 | The variable contains |
41b2f314 | 744 | four space-separated values: client ip-address, client port number, |
745 | server ip-address and server port number. | |
3c0ef626 | 746 | .It Ev SSH_ORIGINAL_COMMAND |
747 | The variable contains the original command line if a forced command | |
748 | is executed. | |
749 | It can be used to extract the original arguments. | |
750 | .It Ev SSH_TTY | |
751 | This is set to the name of the tty (path to the device) associated | |
752 | with the current shell or command. | |
753 | If the current session has no tty, | |
754 | this variable is not set. | |
755 | .It Ev TZ | |
756 | The timezone variable is set to indicate the present timezone if it | |
757 | was set when the daemon was started (i.e., the daemon passes the value | |
758 | on to new connections). | |
759 | .It Ev USER | |
760 | Set to the name of the user logging in. | |
761 | .El | |
762 | .Pp | |
763 | Additionally, | |
764 | .Nm | |
765 | reads | |
766 | .Pa $HOME/.ssh/environment , | |
767 | and adds lines of the format | |
768 | .Dq VARNAME=value | |
41b2f314 | 769 | to the environment if the file exists and if users are allowed to |
770 | change their environment. | |
9cb1827b | 771 | See the |
41b2f314 | 772 | .Cm PermitUserEnvironment |
773 | option in | |
774 | .Xr sshd_config 5 . | |
3c0ef626 | 775 | .Sh FILES |
776 | .Bl -tag -width Ds | |
777 | .It Pa $HOME/.ssh/known_hosts | |
778 | Records host keys for all hosts the user has logged into that are not | |
779 | in | |
e9a17296 | 780 | .Pa /etc/ssh/ssh_known_hosts . |
3c0ef626 | 781 | See |
782 | .Xr sshd 8 . | |
783 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | |
784 | Contains the authentication identity of the user. | |
785 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
786 | These files | |
787 | contain sensitive data and should be readable by the user but not | |
788 | accessible by others (read/write/execute). | |
789 | Note that | |
790 | .Nm | |
791 | ignores a private key file if it is accessible by others. | |
792 | It is possible to specify a passphrase when | |
793 | generating the key; the passphrase will be used to encrypt the | |
794 | sensitive part of this file using 3DES. | |
795 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | |
796 | Contains the public key for authentication (public part of the | |
797 | identity file in human-readable form). | |
798 | The contents of the | |
799 | .Pa $HOME/.ssh/identity.pub | |
9cb1827b | 800 | file should be added to |
3c0ef626 | 801 | .Pa $HOME/.ssh/authorized_keys |
802 | on all machines | |
803 | where the user wishes to log in using protocol version 1 RSA authentication. | |
804 | The contents of the | |
805 | .Pa $HOME/.ssh/id_dsa.pub | |
806 | and | |
807 | .Pa $HOME/.ssh/id_rsa.pub | |
808 | file should be added to | |
809 | .Pa $HOME/.ssh/authorized_keys | |
810 | on all machines | |
811 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. | |
812 | These files are not | |
813 | sensitive and can (but need not) be readable by anyone. | |
814 | These files are | |
815 | never used automatically and are not necessary; they are only provided for | |
816 | the convenience of the user. | |
817 | .It Pa $HOME/.ssh/config | |
818 | This is the per-user configuration file. | |
f5799ae1 | 819 | The file format and configuration options are described in |
820 | .Xr ssh_config 5 . | |
3c0ef626 | 821 | .It Pa $HOME/.ssh/authorized_keys |
822 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
823 | The format of this file is described in the | |
824 | .Xr sshd 8 | |
825 | manual page. | |
9cb1827b | 826 | In the simplest form the format is the same as the .pub |
3c0ef626 | 827 | identity files. |
828 | This file is not highly sensitive, but the recommended | |
829 | permissions are read/write for the user, and not accessible by others. | |
e9a17296 | 830 | .It Pa /etc/ssh/ssh_known_hosts |
3c0ef626 | 831 | Systemwide list of known host keys. |
832 | This file should be prepared by the | |
833 | system administrator to contain the public host keys of all machines in the | |
834 | organization. | |
835 | This file should be world-readable. | |
836 | This file contains | |
837 | public keys, one per line, in the following format (fields separated | |
838 | by spaces): system name, public key and optional comment field. | |
839 | When different names are used | |
840 | for the same machine, all such names should be listed, separated by | |
841 | commas. | |
9cb1827b | 842 | The format is described on the |
3c0ef626 | 843 | .Xr sshd 8 |
844 | manual page. | |
845 | .Pp | |
846 | The canonical system name (as returned by name servers) is used by | |
847 | .Xr sshd 8 | |
848 | to verify the client host when logging in; other names are needed because | |
849 | .Nm | |
850 | does not convert the user-supplied name to a canonical name before | |
851 | checking the key, because someone with access to the name servers | |
852 | would then be able to fool host authentication. | |
e9a17296 | 853 | .It Pa /etc/ssh/ssh_config |
3c0ef626 | 854 | Systemwide configuration file. |
f5799ae1 | 855 | The file format and configuration options are described in |
856 | .Xr ssh_config 5 . | |
e9a17296 | 857 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
3c0ef626 | 858 | These three files contain the private parts of the host keys |
859 | and are used for | |
860 | .Cm RhostsRSAAuthentication | |
861 | and | |
862 | .Cm HostbasedAuthentication . | |
f5799ae1 | 863 | If the protocol version 1 |
864 | .Cm RhostsRSAAuthentication | |
865 | method is used, | |
866 | .Nm | |
867 | must be setuid root, since the host key is readable only by root. | |
868 | For protocol version 2, | |
869 | .Nm | |
870 | uses | |
871 | .Xr ssh-keysign 8 | |
872 | to access the host keys for | |
873 | .Cm HostbasedAuthentication . | |
874 | This eliminates the requirement that | |
875 | .Nm | |
876 | be setuid root when that authentication method is used. | |
877 | By default | |
3c0ef626 | 878 | .Nm |
f5799ae1 | 879 | is not setuid root. |
3c0ef626 | 880 | .It Pa $HOME/.rhosts |
881 | This file is used in | |
9cb1827b | 882 | .Pa \&.rhosts |
3c0ef626 | 883 | authentication to list the |
884 | host/user pairs that are permitted to log in. | |
885 | (Note that this file is | |
886 | also used by rlogin and rsh, which makes using this file insecure.) | |
887 | Each line of the file contains a host name (in the canonical form | |
888 | returned by name servers), and then a user name on that host, | |
889 | separated by a space. | |
890 | On some machines this file may need to be | |
891 | world-readable if the user's home directory is on a NFS partition, | |
892 | because | |
893 | .Xr sshd 8 | |
894 | reads it as root. | |
895 | Additionally, this file must be owned by the user, | |
896 | and must not have write permissions for anyone else. | |
897 | The recommended | |
898 | permission for most machines is read/write for the user, and not | |
899 | accessible by others. | |
900 | .Pp | |
901 | Note that by default | |
902 | .Xr sshd 8 | |
903 | will be installed so that it requires successful RSA host | |
9cb1827b | 904 | authentication before permitting \s+2.\s0rhosts authentication. |
3c0ef626 | 905 | If the server machine does not have the client's host key in |
e9a17296 | 906 | .Pa /etc/ssh/ssh_known_hosts , |
3c0ef626 | 907 | it can be stored in |
908 | .Pa $HOME/.ssh/known_hosts . | |
909 | The easiest way to do this is to | |
910 | connect back to the client from the server machine using ssh; this | |
911 | will automatically add the host key to | |
912 | .Pa $HOME/.ssh/known_hosts . | |
913 | .It Pa $HOME/.shosts | |
914 | This file is used exactly the same way as | |
9cb1827b | 915 | .Pa \&.rhosts . |
3c0ef626 | 916 | The purpose for |
917 | having this file is to be able to use rhosts authentication with | |
918 | .Nm | |
919 | without permitting login with | |
9cb1827b | 920 | .Nm rlogin |
3c0ef626 | 921 | or |
922 | .Xr rsh 1 . | |
923 | .It Pa /etc/hosts.equiv | |
924 | This file is used during | |
9cb1827b | 925 | .Pa \&.rhosts |
0fff78ff | 926 | authentication. |
3c0ef626 | 927 | It contains |
9cb1827b | 928 | canonical hosts names, one per line (the full format is described on |
929 | the | |
3c0ef626 | 930 | .Xr sshd 8 |
931 | manual page). | |
932 | If the client host is found in this file, login is | |
933 | automatically permitted provided client and server user names are the | |
934 | same. | |
935 | Additionally, successful RSA host authentication is normally | |
936 | required. | |
937 | This file should only be writable by root. | |
938 | .It Pa /etc/shosts.equiv | |
939 | This file is processed exactly as | |
940 | .Pa /etc/hosts.equiv . | |
941 | This file may be useful to permit logins using | |
942 | .Nm | |
943 | but not using rsh/rlogin. | |
e9a17296 | 944 | .It Pa /etc/ssh/sshrc |
3c0ef626 | 945 | Commands in this file are executed by |
946 | .Nm | |
947 | when the user logs in just before the user's shell (or command) is started. | |
948 | See the | |
949 | .Xr sshd 8 | |
950 | manual page for more information. | |
951 | .It Pa $HOME/.ssh/rc | |
952 | Commands in this file are executed by | |
953 | .Nm | |
954 | when the user logs in just before the user's shell (or command) is | |
955 | started. | |
956 | See the | |
957 | .Xr sshd 8 | |
958 | manual page for more information. | |
959 | .It Pa $HOME/.ssh/environment | |
960 | Contains additional definitions for environment variables, see section | |
961 | .Sx ENVIRONMENT | |
962 | above. | |
963 | .El | |
e9a17296 | 964 | .Sh DIAGNOSTICS |
965 | .Nm | |
966 | exits with the exit status of the remote command or with 255 | |
967 | if an error occurred. | |
3c0ef626 | 968 | .Sh SEE ALSO |
3c0ef626 | 969 | .Xr rsh 1 , |
970 | .Xr scp 1 , | |
971 | .Xr sftp 1 , | |
972 | .Xr ssh-add 1 , | |
973 | .Xr ssh-agent 1 , | |
974 | .Xr ssh-keygen 1 , | |
975 | .Xr telnet 1 , | |
680cee3b | 976 | .Xr ssh_config 5 , |
977 | .Xr ssh-keysign 8 , | |
3c0ef626 | 978 | .Xr sshd 8 |
979 | .Rs | |
980 | .%A T. Ylonen | |
981 | .%A T. Kivinen | |
982 | .%A M. Saarinen | |
983 | .%A T. Rinne | |
984 | .%A S. Lehtinen | |
985 | .%T "SSH Protocol Architecture" | |
700318f3 | 986 | .%N draft-ietf-secsh-architecture-12.txt |
987 | .%D January 2002 | |
3c0ef626 | 988 | .%O work in progress material |
989 | .Re | |
0fff78ff | 990 | .Sh AUTHORS |
991 | OpenSSH is a derivative of the original and free | |
992 | ssh 1.2.12 release by Tatu Ylonen. | |
993 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
994 | Theo de Raadt and Dug Song | |
995 | removed many bugs, re-added newer features and | |
996 | created OpenSSH. | |
997 | Markus Friedl contributed the support for SSH | |
998 | protocol versions 1.5 and 2.0. |