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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
bf740959 | 3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
bf740959 | 4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
5 | .\" All rights reserved | |
6 | .\" | |
bcbf86ec | 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 | .\" | |
f3c7c613 | 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. | |
bcbf86ec | 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. | |
bf740959 | 25 | .\" |
bcbf86ec | 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. | |
bf740959 | 36 | .\" |
da0561eb | 37 | .\" $OpenBSD: ssh.1,v 1.166 2002/09/12 19:50:36 stevesk Exp $ |
bf740959 | 38 | .Dd September 25, 1999 |
39 | .Dt SSH 1 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh | |
2c86906e | 43 | .Nd OpenSSH SSH client (remote login program) |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm ssh | |
46 | .Op Fl l Ar login_name | |
c78e5800 | 47 | .Ar hostname | user@hostname |
bf740959 | 48 | .Op Ar command |
49 | .Pp | |
50 | .Nm ssh | |
e59404d1 | 51 | .Op Fl afgknqstvxACNTX1246 |
3435f5a6 | 52 | .Op Fl b Ar bind_address |
d0c832f3 | 53 | .Op Fl c Ar cipher_spec |
bf740959 | 54 | .Op Fl e Ar escape_char |
55 | .Op Fl i Ar identity_file | |
56 | .Op Fl l Ar login_name | |
b2552997 | 57 | .Op Fl m Ar mac_spec |
bf740959 | 58 | .Op Fl o Ar option |
59 | .Op Fl p Ar port | |
e591b98a | 60 | .Op Fl F Ar configfile |
bf740959 | 61 | .Oo Fl L Xo |
62 | .Sm off | |
bf740959 | 63 | .Ar port : |
7b2ea3a1 | 64 | .Ar host : |
bf740959 | 65 | .Ar hostport |
66 | .Sm on | |
67 | .Xc | |
68 | .Oc | |
69 | .Oo Fl R Xo | |
70 | .Sm off | |
bf740959 | 71 | .Ar port : |
7b2ea3a1 | 72 | .Ar host : |
bf740959 | 73 | .Ar hostport |
74 | .Sm on | |
75 | .Xc | |
76 | .Oc | |
ad3e169f | 77 | .Op Fl D Ar port |
c78e5800 | 78 | .Ar hostname | user@hostname |
bf740959 | 79 | .Op Ar command |
f54651ce | 80 | .Sh DESCRIPTION |
bf740959 | 81 | .Nm |
2c86906e | 82 | (SSH client) is a program for logging into a remote machine and for |
610cd5c6 | 83 | executing commands on a remote machine. |
84 | It is intended to replace | |
bf740959 | 85 | rlogin and rsh, and provide secure encrypted communications between |
610cd5c6 | 86 | two untrusted hosts over an insecure network. |
87 | X11 connections and | |
bf740959 | 88 | arbitrary TCP/IP ports can also be forwarded over the secure channel. |
89 | .Pp | |
90 | .Nm | |
f54651ce | 91 | connects and logs into the specified |
bf740959 | 92 | .Ar hostname . |
93 | The user must prove | |
1d1ffb87 | 94 | his/her identity to the remote machine using one of several methods |
95 | depending on the protocol version used: | |
96 | .Pp | |
97 | .Ss SSH protocol version 1 | |
bf740959 | 98 | .Pp |
99 | First, if the machine the user logs in from is listed in | |
100 | .Pa /etc/hosts.equiv | |
101 | or | |
5f4fdfae | 102 | .Pa /etc/shosts.equiv |
bf740959 | 103 | on the remote machine, and the user names are |
104 | the same on both sides, the user is immediately permitted to log in. | |
f54651ce | 105 | Second, if |
bf740959 | 106 | .Pa \&.rhosts |
107 | or | |
108 | .Pa \&.shosts | |
109 | exists in the user's home directory on the | |
110 | remote machine and contains a line containing the name of the client | |
111 | machine and the name of the user on that machine, the user is | |
610cd5c6 | 112 | permitted to log in. |
113 | This form of authentication alone is normally not | |
bf740959 | 114 | allowed by the server because it is not secure. |
115 | .Pp | |
da89cf4d | 116 | The second authentication method is the |
bf740959 | 117 | .Pa rhosts |
118 | or | |
119 | .Pa hosts.equiv | |
610cd5c6 | 120 | method combined with RSA-based host authentication. |
121 | It means that if the login would be permitted by | |
1d1ffb87 | 122 | .Pa $HOME/.rhosts , |
123 | .Pa $HOME/.shosts , | |
bf740959 | 124 | .Pa /etc/hosts.equiv , |
125 | or | |
5f4fdfae | 126 | .Pa /etc/shosts.equiv , |
bf740959 | 127 | and if additionally the server can verify the client's |
f54651ce | 128 | host key (see |
2a8a6488 | 129 | .Pa /etc/ssh/ssh_known_hosts |
5bbb5681 | 130 | and |
131 | .Pa $HOME/.ssh/known_hosts | |
bf740959 | 132 | in the |
133 | .Sx FILES | |
610cd5c6 | 134 | section), only then login is permitted. |
135 | This authentication method closes security holes due to IP | |
136 | spoofing, DNS spoofing and routing spoofing. | |
137 | [Note to the administrator: | |
bf740959 | 138 | .Pa /etc/hosts.equiv , |
1d1ffb87 | 139 | .Pa $HOME/.rhosts , |
bf740959 | 140 | and the rlogin/rsh protocol in general, are inherently insecure and should be |
141 | disabled if security is desired.] | |
142 | .Pp | |
f54651ce | 143 | As a third authentication method, |
bf740959 | 144 | .Nm |
145 | supports RSA based authentication. | |
146 | The scheme is based on public-key cryptography: there are cryptosystems | |
147 | where encryption and decryption are done using separate keys, and it | |
148 | is not possible to derive the decryption key from the encryption key. | |
610cd5c6 | 149 | RSA is one such system. |
f54651ce | 150 | The idea is that each user creates a public/private |
610cd5c6 | 151 | key pair for authentication purposes. |
152 | The server knows the public key, and only the user knows the private key. | |
f54651ce | 153 | The file |
bf740959 | 154 | .Pa $HOME/.ssh/authorized_keys |
155 | lists the public keys that are permitted for logging | |
610cd5c6 | 156 | in. |
157 | When the user logs in, the | |
bf740959 | 158 | .Nm |
159 | program tells the server which key pair it would like to use for | |
610cd5c6 | 160 | authentication. |
161 | The server checks if this key is permitted, and if | |
bf740959 | 162 | so, sends the user (actually the |
163 | .Nm | |
164 | program running on behalf of the user) a challenge, a random number, | |
610cd5c6 | 165 | encrypted by the user's public key. |
166 | The challenge can only be | |
167 | decrypted using the proper private key. | |
168 | The user's client then decrypts the | |
bf740959 | 169 | challenge using the private key, proving that he/she knows the private |
170 | key but without disclosing it to the server. | |
171 | .Pp | |
172 | .Nm | |
610cd5c6 | 173 | implements the RSA authentication protocol automatically. |
174 | The user creates his/her RSA key pair by running | |
bf740959 | 175 | .Xr ssh-keygen 1 . |
f54651ce | 176 | This stores the private key in |
1d1ffb87 | 177 | .Pa $HOME/.ssh/identity |
bf740959 | 178 | and the public key in |
1d1ffb87 | 179 | .Pa $HOME/.ssh/identity.pub |
610cd5c6 | 180 | in the user's home directory. |
181 | The user should then copy the | |
bf740959 | 182 | .Pa identity.pub |
f54651ce | 183 | to |
1d1ffb87 | 184 | .Pa $HOME/.ssh/authorized_keys |
f54651ce | 185 | in his/her home directory on the remote machine (the |
bf740959 | 186 | .Pa authorized_keys |
f54651ce | 187 | file corresponds to the conventional |
1d1ffb87 | 188 | .Pa $HOME/.rhosts |
bf740959 | 189 | file, and has one key |
610cd5c6 | 190 | per line, though the lines can be very long). |
191 | After this, the user can log in without giving the password. | |
192 | RSA authentication is much | |
bf740959 | 193 | more secure than rhosts authentication. |
194 | .Pp | |
195 | The most convenient way to use RSA authentication may be with an | |
610cd5c6 | 196 | authentication agent. |
197 | See | |
bf740959 | 198 | .Xr ssh-agent 1 |
199 | for more information. | |
200 | .Pp | |
f54651ce | 201 | If other authentication methods fail, |
bf740959 | 202 | .Nm |
610cd5c6 | 203 | prompts the user for a password. |
204 | The password is sent to the remote | |
bf740959 | 205 | host for checking; however, since all communications are encrypted, |
206 | the password cannot be seen by someone listening on the network. | |
207 | .Pp | |
1d1ffb87 | 208 | .Ss SSH protocol version 2 |
209 | .Pp | |
16210ef7 | 210 | When a user connects using protocol version 2 |
211 | similar authentication methods are available. | |
da89cf4d | 212 | Using the default values for |
213 | .Cm PreferredAuthentications , | |
29c440a0 | 214 | the client will try to authenticate first using the hostbased method; |
215 | if this method fails public key authentication is attempted, | |
216 | and finally if this method fails keyboard-interactive and | |
217 | password authentication are tried. | |
1d1ffb87 | 218 | .Pp |
219 | The public key method is similar to RSA authentication described | |
da89cf4d | 220 | in the previous section and allows the RSA or DSA algorithm to be used: |
c0ecc314 | 221 | The client uses his private key, |
1d1ffb87 | 222 | .Pa $HOME/.ssh/id_dsa |
c0ecc314 | 223 | or |
224 | .Pa $HOME/.ssh/id_rsa , | |
1d1ffb87 | 225 | to sign the session identifier and sends the result to the server. |
226 | The server checks whether the matching public key is listed in | |
96a7b0cc | 227 | .Pa $HOME/.ssh/authorized_keys |
1d1ffb87 | 228 | and grants access if both the key is found and the signature is correct. |
229 | The session identifier is derived from a shared Diffie-Hellman value | |
230 | and is only known to the client and the server. | |
231 | .Pp | |
232 | If public key authentication fails or is not available a password | |
233 | can be sent encrypted to the remote host for proving the user's identity. | |
da89cf4d | 234 | .Pp |
235 | Additionally, | |
236 | .Nm | |
237 | supports hostbased or challenge response authentication. | |
1d1ffb87 | 238 | .Pp |
239 | Protocol 2 provides additional mechanisms for confidentiality | |
d0c832f3 | 240 | (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) |
f2ba0775 | 241 | and integrity (hmac-md5, hmac-sha1). |
1d1ffb87 | 242 | Note that protocol 1 lacks a strong mechanism for ensuring the |
243 | integrity of the connection. | |
244 | .Pp | |
245 | .Ss Login session and remote execution | |
246 | .Pp | |
bf740959 | 247 | When the user's identity has been accepted by the server, the server |
248 | either executes the given command, or logs into the machine and gives | |
610cd5c6 | 249 | the user a normal shell on the remote machine. |
250 | All communication with | |
bf740959 | 251 | the remote command or shell will be automatically encrypted. |
252 | .Pp | |
253 | If a pseudo-terminal has been allocated (normal login session), the | |
df841692 | 254 | user may use the escape characters noted below. |
bf740959 | 255 | .Pp |
256 | If no pseudo tty has been allocated, the | |
257 | session is transparent and can be used to reliably transfer binary | |
610cd5c6 | 258 | data. |
259 | On most systems, setting the escape character to | |
bf740959 | 260 | .Dq none |
261 | will also make the session transparent even if a tty is used. | |
262 | .Pp | |
ce9c0b75 | 263 | The session terminates when the command or shell on the remote |
e91c60f2 | 264 | machine exits and all X11 and TCP/IP connections have been closed. |
bf740959 | 265 | The exit status of the remote program is returned as the exit status |
266 | of | |
267 | .Nm ssh . | |
268 | .Pp | |
df841692 | 269 | .Ss Escape Characters |
270 | .Pp | |
271 | When a pseudo terminal has been requested, ssh supports a number of functions | |
3730bb22 | 272 | through the use of an escape character. |
df841692 | 273 | .Pp |
274 | A single tilde character can be sent as | |
275 | .Ic ~~ | |
5bef3c35 | 276 | or by following the tilde by a character other than those described below. |
df841692 | 277 | The escape character must always follow a newline to be interpreted as |
278 | special. | |
279 | The escape character can be changed in configuration files using the | |
280 | .Cm EscapeChar | |
3730bb22 | 281 | configuration directive or on the command line by the |
df841692 | 282 | .Fl e |
283 | option. | |
284 | .Pp | |
285 | The supported escapes (assuming the default | |
286 | .Ql ~ ) | |
287 | are: | |
288 | .Bl -tag -width Ds | |
289 | .It Cm ~. | |
290 | Disconnect | |
291 | .It Cm ~^Z | |
292 | Background ssh | |
293 | .It Cm ~# | |
294 | List forwarded connections | |
295 | .It Cm ~& | |
296 | Background ssh at logout when waiting for forwarded connection / X11 sessions | |
03cf595c | 297 | to terminate |
df841692 | 298 | .It Cm ~? |
299 | Display a list of escape characters | |
c53c54c2 | 300 | .It Cm ~C |
301 | Open command line (only useful for adding port forwardings using the | |
302 | .Fl L | |
303 | and | |
304 | .Fl R | |
305 | options) | |
df841692 | 306 | .It Cm ~R |
67b964a1 | 307 | Request rekeying of the connection (only useful for SSH protocol version 2 |
308 | and if the peer supports it) | |
df841692 | 309 | .El |
310 | .Pp | |
1d1ffb87 | 311 | .Ss X11 and TCP forwarding |
312 | .Pp | |
6efa3d14 | 313 | If the |
314 | .Cm ForwardX11 | |
315 | variable is set to | |
316 | .Dq yes | |
317 | (or, see the description of the | |
318 | .Fl X | |
319 | and | |
320 | .Fl x | |
321 | options described later) | |
322 | and the user is using X11 (the | |
bf740959 | 323 | .Ev DISPLAY |
324 | environment variable is set), the connection to the X11 display is | |
325 | automatically forwarded to the remote side in such a way that any X11 | |
326 | programs started from the shell (or command) will go through the | |
327 | encrypted channel, and the connection to the real X server will be made | |
610cd5c6 | 328 | from the local machine. |
329 | The user should not manually set | |
bf740959 | 330 | .Ev DISPLAY . |
331 | Forwarding of X11 connections can be | |
332 | configured on the command line or in configuration files. | |
333 | .Pp | |
334 | The | |
f54651ce | 335 | .Ev DISPLAY |
bf740959 | 336 | value set by |
337 | .Nm | |
338 | will point to the server machine, but with a display number greater | |
610cd5c6 | 339 | than zero. |
340 | This is normal, and happens because | |
bf740959 | 341 | .Nm |
342 | creates a | |
343 | .Dq proxy | |
344 | X server on the server machine for forwarding the | |
345 | connections over the encrypted channel. | |
346 | .Pp | |
347 | .Nm | |
348 | will also automatically set up Xauthority data on the server machine. | |
349 | For this purpose, it will generate a random authorization cookie, | |
350 | store it in Xauthority on the server, and verify that any forwarded | |
351 | connections carry this cookie and replace it by the real cookie when | |
610cd5c6 | 352 | the connection is opened. |
353 | The real authentication cookie is never | |
bf740959 | 354 | sent to the server machine (and no cookies are sent in the plain). |
355 | .Pp | |
24794905 | 356 | If the |
357 | .Cm ForwardAgent | |
358 | variable is set to | |
359 | .Dq yes | |
360 | (or, see the description of the | |
361 | .Fl A | |
362 | and | |
363 | .Fl a | |
364 | options described later) and | |
365 | the user is using an authentication agent, the connection to the agent | |
366 | is automatically forwarded to the remote side. | |
bf740959 | 367 | .Pp |
368 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
491f5f7b | 369 | be specified either on the command line or in a configuration file. |
610cd5c6 | 370 | One possible application of TCP/IP forwarding is a secure connection to an |
e91c60f2 | 371 | electronic purse; another is going through firewalls. |
bf740959 | 372 | .Pp |
1d1ffb87 | 373 | .Ss Server authentication |
374 | .Pp | |
bf740959 | 375 | .Nm |
1d1ffb87 | 376 | automatically maintains and checks a database containing |
610cd5c6 | 377 | identifications for all hosts it has ever been used with. |
f49bc4f7 | 378 | Host keys are stored in |
1d1ffb87 | 379 | .Pa $HOME/.ssh/known_hosts |
610cd5c6 | 380 | in the user's home directory. |
f49bc4f7 | 381 | Additionally, the file |
2a8a6488 | 382 | .Pa /etc/ssh/ssh_known_hosts |
f49bc4f7 | 383 | is automatically checked for known hosts. |
610cd5c6 | 384 | Any new hosts are automatically added to the user's file. |
385 | If a host's identification | |
bf740959 | 386 | ever changes, |
387 | .Nm | |
388 | warns about this and disables password authentication to prevent a | |
610cd5c6 | 389 | trojan horse from getting the user's password. |
390 | Another purpose of | |
bf740959 | 391 | this mechanism is to prevent man-in-the-middle attacks which could |
610cd5c6 | 392 | otherwise be used to circumvent the encryption. |
393 | The | |
bf740959 | 394 | .Cm StrictHostKeyChecking |
588df31a | 395 | option can be used to prevent logins to machines whose |
bf740959 | 396 | host key is not known or has changed. |
fa08c86b | 397 | .Pp |
398 | The options are as follows: | |
bf740959 | 399 | .Bl -tag -width Ds |
400 | .It Fl a | |
4fe2af09 | 401 | Disables forwarding of the authentication agent connection. |
71276795 | 402 | .It Fl A |
403 | Enables forwarding of the authentication agent connection. | |
404 | This can also be specified on a per-host basis in a configuration file. | |
07d688d5 | 405 | .Pp |
406 | Agent forwarding should be enabled with caution. Users with the | |
407 | ability to bypass file permissions on the remote host (for the agent's | |
408 | Unix-domain socket) can access the local agent through the forwarded | |
409 | connection. An attacker cannot obtain key material from the agent, | |
410 | however they can perform operations on the keys that enable them to | |
411 | authenticate using the identities loaded into the agent. | |
3435f5a6 | 412 | .It Fl b Ar bind_address |
413 | Specify the interface to transmit from on machines with multiple | |
414 | interfaces or aliased addresses. | |
29999e54 | 415 | .It Fl c Ar blowfish|3des|des |
f54651ce | 416 | Selects the cipher to use for encrypting the session. |
bf740959 | 417 | .Ar 3des |
610cd5c6 | 418 | is used by default. |
f54651ce | 419 | It is believed to be secure. |
bf740959 | 420 | .Ar 3des |
421 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
bf740959 | 422 | .Ar blowfish |
423 | is a fast block cipher, it appears very secure and is much faster than | |
610cd5c6 | 424 | .Ar 3des . |
29999e54 | 425 | .Ar des |
426 | is only supported in the | |
427 | .Nm | |
428 | client for interoperability with legacy protocol 1 implementations | |
429 | that do not support the | |
430 | .Ar 3des | |
431 | cipher. Its use is strongly discouraged due to cryptographic | |
432 | weaknesses. | |
9afadca8 | 433 | .It Fl c Ar cipher_spec |
d0c832f3 | 434 | Additionally, for protocol version 2 a comma-separated list of ciphers can |
94ec8c6b | 435 | be specified in order of preference. |
9afadca8 | 436 | See |
437 | .Cm Ciphers | |
438 | for more information. | |
bf740959 | 439 | .It Fl e Ar ch|^ch|none |
440 | Sets the escape character for sessions with a pty (default: | |
441 | .Ql ~ ) . | |
610cd5c6 | 442 | The escape character is only recognized at the beginning of a line. |
443 | The escape character followed by a dot | |
bf740959 | 444 | .Pq Ql \&. |
445 | closes the connection, followed | |
446 | by control-Z suspends the connection, and followed by itself sends the | |
610cd5c6 | 447 | escape character once. |
448 | Setting the character to | |
bf740959 | 449 | .Dq none |
450 | disables any escapes and makes the session fully transparent. | |
451 | .It Fl f | |
452 | Requests | |
453 | .Nm | |
610cd5c6 | 454 | to go to background just before command execution. |
455 | This is useful if | |
bf740959 | 456 | .Nm |
457 | is going to ask for passwords or passphrases, but the user | |
610cd5c6 | 458 | wants it in the background. |
f54651ce | 459 | This implies |
bf740959 | 460 | .Fl n . |
461 | The recommended way to start X11 programs at a remote site is with | |
462 | something like | |
463 | .Ic ssh -f host xterm . | |
7b2ea3a1 | 464 | .It Fl g |
465 | Allows remote hosts to connect to local forwarded ports. | |
bf740959 | 466 | .It Fl i Ar identity_file |
cf5a07a8 | 467 | Selects a file from which the identity (private key) for |
fa08c86b | 468 | RSA or DSA authentication is read. |
cf5a07a8 | 469 | The default is |
1d1ffb87 | 470 | .Pa $HOME/.ssh/identity |
cf5a07a8 | 471 | for protocol version 1, and |
472 | .Pa $HOME/.ssh/id_rsa | |
473 | and | |
474 | .Pa $HOME/.ssh/id_dsa | |
475 | for protocol version 2. | |
610cd5c6 | 476 | Identity files may also be specified on |
477 | a per-host basis in the configuration file. | |
478 | It is possible to have multiple | |
bf740959 | 479 | .Fl i |
480 | options (and multiple identities specified in | |
481 | configuration files). | |
eea098a3 | 482 | .It Fl I Ar smartcard_device |
483 | Specifies which smartcard device to use. The argument is | |
484 | the device | |
485 | .Nm | |
486 | should use to communicate with a smartcard used for storing the user's | |
487 | private RSA key. | |
bf740959 | 488 | .It Fl k |
4fe2af09 | 489 | Disables forwarding of Kerberos tickets and AFS tokens. |
490 | This may also be specified on a per-host basis in the configuration file. | |
bf740959 | 491 | .It Fl l Ar login_name |
610cd5c6 | 492 | Specifies the user to log in as on the remote machine. |
493 | This also may be specified on a per-host basis in the configuration file. | |
b2552997 | 494 | .It Fl m Ar mac_spec |
495 | Additionally, for protocol version 2 a comma-separated list of MAC | |
496 | (message authentication code) algorithms can | |
497 | be specified in order of preference. | |
498 | See the | |
499 | .Cm MACs | |
500 | keyword for more information. | |
bf740959 | 501 | .It Fl n |
502 | Redirects stdin from | |
503 | .Pa /dev/null | |
504 | (actually, prevents reading from stdin). | |
505 | This must be used when | |
506 | .Nm | |
610cd5c6 | 507 | is run in the background. |
508 | A common trick is to use this to run X11 programs on a remote machine. | |
509 | For example, | |
bf740959 | 510 | .Ic ssh -n shadows.cs.hut.fi emacs & |
511 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
512 | connection will be automatically forwarded over an encrypted channel. | |
513 | The | |
514 | .Nm | |
515 | program will be put in the background. | |
516 | (This does not work if | |
517 | .Nm | |
518 | needs to ask for a password or passphrase; see also the | |
519 | .Fl f | |
520 | option.) | |
0e73cc53 | 521 | .It Fl N |
522 | Do not execute a remote command. | |
91789042 | 523 | This is useful for just forwarding ports |
0e73cc53 | 524 | (protocol version 2 only). |
bf740959 | 525 | .It Fl o Ar option |
38539909 | 526 | Can be used to give options in the format used in the configuration file. |
bf740959 | 527 | This is useful for specifying options for which there is no separate |
610cd5c6 | 528 | command-line flag. |
bf740959 | 529 | .It Fl p Ar port |
610cd5c6 | 530 | Port to connect to on the remote host. |
531 | This can be specified on a | |
bf740959 | 532 | per-host basis in the configuration file. |
bf740959 | 533 | .It Fl q |
610cd5c6 | 534 | Quiet mode. |
535 | Causes all warning and diagnostic messages to be suppressed. | |
ae810de7 | 536 | .It Fl s |
3730bb22 | 537 | May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use |
538 | of SSH as a secure transport for other applications (eg. sftp). The | |
ae810de7 | 539 | subsystem is specified as the remote command. |
bf740959 | 540 | .It Fl t |
610cd5c6 | 541 | Force pseudo-tty allocation. |
4fe2af09 | 542 | This can be used to execute arbitrary |
610cd5c6 | 543 | screen-based programs on a remote machine, which can be very useful, |
544 | e.g., when implementing menu services. | |
8abcdba4 | 545 | Multiple |
546 | .Fl t | |
547 | options force tty allocation, even if | |
548 | .Nm | |
549 | has no local tty. | |
0e73cc53 | 550 | .It Fl T |
0b6fbf03 | 551 | Disable pseudo-tty allocation. |
bf740959 | 552 | .It Fl v |
610cd5c6 | 553 | Verbose mode. |
554 | Causes | |
bf740959 | 555 | .Nm |
610cd5c6 | 556 | to print debugging messages about its progress. |
557 | This is helpful in | |
bf740959 | 558 | debugging connection, authentication, and configuration problems. |
8abcdba4 | 559 | Multiple |
560 | .Fl v | |
561 | options increases the verbosity. | |
94ec8c6b | 562 | Maximum is 3. |
bf740959 | 563 | .It Fl x |
610cd5c6 | 564 | Disables X11 forwarding. |
bf740959 | 565 | .It Fl X |
566 | Enables X11 forwarding. | |
71276795 | 567 | This can also be specified on a per-host basis in a configuration file. |
07d688d5 | 568 | .Pp |
569 | X11 forwarding should be enabled with caution. Users with the ability | |
570 | to bypass file permissions on the remote host (for the user's X | |
571 | authorization database) can access the local X11 display through the | |
572 | forwarded connection. An attacker may then be able to perform | |
573 | activities such as keystroke monitoring. | |
bf740959 | 574 | .It Fl C |
575 | Requests compression of all data (including stdin, stdout, stderr, and | |
610cd5c6 | 576 | data for forwarded X11 and TCP/IP connections). |
577 | The compression algorithm is the same used by | |
7b2ea3a1 | 578 | .Xr gzip 1 , |
579 | and the | |
bf740959 | 580 | .Dq level |
581 | can be controlled by the | |
582 | .Cm CompressionLevel | |
588df31a | 583 | option. |
610cd5c6 | 584 | Compression is desirable on modem lines and other |
bf740959 | 585 | slow connections, but will only slow down things on fast networks. |
586 | The default value can be set on a host-by-host basis in the | |
587 | configuration files; see the | |
0aea6c59 | 588 | .Cm Compression |
588df31a | 589 | option. |
e591b98a | 590 | .It Fl F Ar configfile |
591 | Specifies an alternative per-user configuration file. | |
592 | If a configuration file is given on the command line, | |
593 | the system-wide configuration file | |
2a8a6488 | 594 | .Pq Pa /etc/ssh/ssh_config |
e591b98a | 595 | will be ignored. |
596 | The default for the per-user configuration file is | |
597 | .Pa $HOME/.ssh/config . | |
bf740959 | 598 | .It Fl L Ar port:host:hostport |
599 | Specifies that the given port on the local (client) host is to be | |
610cd5c6 | 600 | forwarded to the given host and port on the remote side. |
601 | This works by allocating a socket to listen to | |
bf740959 | 602 | .Ar port |
603 | on the local side, and whenever a connection is made to this port, the | |
604 | connection is forwarded over the secure channel, and a connection is | |
605 | made to | |
48e671d5 | 606 | .Ar host |
607 | port | |
608 | .Ar hostport | |
610cd5c6 | 609 | from the remote machine. |
610 | Port forwardings can also be specified in the configuration file. | |
611 | Only root can forward privileged ports. | |
48e671d5 | 612 | IPv6 addresses can be specified with an alternative syntax: |
613 | .Ar port/host/hostport | |
bf740959 | 614 | .It Fl R Ar port:host:hostport |
615 | Specifies that the given port on the remote (server) host is to be | |
610cd5c6 | 616 | forwarded to the given host and port on the local side. |
617 | This works by allocating a socket to listen to | |
bf740959 | 618 | .Ar port |
619 | on the remote side, and whenever a connection is made to this port, the | |
620 | connection is forwarded over the secure channel, and a connection is | |
621 | made to | |
48e671d5 | 622 | .Ar host |
623 | port | |
624 | .Ar hostport | |
610cd5c6 | 625 | from the local machine. |
626 | Port forwardings can also be specified in the configuration file. | |
627 | Privileged ports can be forwarded only when | |
bf740959 | 628 | logging in as root on the remote machine. |
da89cf4d | 629 | IPv6 addresses can be specified with an alternative syntax: |
630 | .Ar port/host/hostport | |
4c780c2a | 631 | .It Fl D Ar port |
632 | Specifies a local | |
633 | .Dq dynamic | |
634 | application-level port forwarding. | |
635 | This works by allocating a socket to listen to | |
636 | .Ar port | |
637 | on the local side, and whenever a connection is made to this port, the | |
638 | connection is forwarded over the secure channel, and the application | |
639 | protocol is then used to determine where to connect to from the | |
640 | remote machine. Currently the SOCKS4 protocol is supported, and | |
641 | .Nm | |
642 | will act as a SOCKS4 server. | |
643 | Only root can forward privileged ports. | |
644 | Dynamic port forwardings can also be specified in the configuration file. | |
9afadca8 | 645 | .It Fl 1 |
646 | Forces | |
647 | .Nm | |
648 | to try protocol version 1 only. | |
6ae2364d | 649 | .It Fl 2 |
650 | Forces | |
651 | .Nm | |
1d1ffb87 | 652 | to try protocol version 2 only. |
48e671d5 | 653 | .It Fl 4 |
654 | Forces | |
655 | .Nm | |
656 | to use IPv4 addresses only. | |
657 | .It Fl 6 | |
658 | Forces | |
659 | .Nm | |
660 | to use IPv6 addresses only. | |
bf740959 | 661 | .El |
662 | .Sh CONFIGURATION FILES | |
663 | .Nm | |
588df31a | 664 | may additionally obtain configuration data from |
665 | a per-user configuration file and a system-wide configuration file. | |
666 | The file format and configuration options are described in | |
667 | .Xr ssh_config 5 . | |
bf740959 | 668 | .Sh ENVIRONMENT |
669 | .Nm | |
670 | will normally set the following environment variables: | |
671 | .Bl -tag -width Ds | |
672 | .It Ev DISPLAY | |
673 | The | |
674 | .Ev DISPLAY | |
610cd5c6 | 675 | variable indicates the location of the X11 server. |
f54651ce | 676 | It is automatically set by |
bf740959 | 677 | .Nm |
678 | to point to a value of the form | |
679 | .Dq hostname:n | |
680 | where hostname indicates | |
610cd5c6 | 681 | the host where the shell runs, and n is an integer >= 1. |
682 | .Nm | |
683 | uses this special value to forward X11 connections over the secure | |
684 | channel. | |
da89cf4d | 685 | The user should normally not set |
686 | .Ev DISPLAY | |
687 | explicitly, as that | |
bf740959 | 688 | will render the X11 connection insecure (and will require the user to |
689 | manually copy any required authorization cookies). | |
690 | .It Ev HOME | |
691 | Set to the path of the user's home directory. | |
692 | .It Ev LOGNAME | |
693 | Synonym for | |
694 | .Ev USER ; | |
695 | set for compatibility with systems that use this variable. | |
696 | .It Ev MAIL | |
ae897d7c | 697 | Set to the path of the user's mailbox. |
610cd5c6 | 698 | .It Ev PATH |
bf740959 | 699 | Set to the default |
700 | .Ev PATH , | |
701 | as specified when compiling | |
702 | .Nm ssh . | |
3474b2b4 | 703 | .It Ev SSH_ASKPASS |
704 | If | |
705 | .Nm | |
706 | needs a passphrase, it will read the passphrase from the current | |
707 | terminal if it was run from a terminal. | |
708 | If | |
709 | .Nm | |
710 | does not have a terminal associated with it but | |
711 | .Ev DISPLAY | |
712 | and | |
713 | .Ev SSH_ASKPASS | |
714 | are set, it will execute the program specified by | |
715 | .Ev SSH_ASKPASS | |
716 | and open an X11 window to read the passphrase. | |
717 | This is particularly useful when calling | |
718 | .Nm | |
719 | from a | |
720 | .Pa .Xsession | |
721 | or related script. | |
722 | (Note that on some machines it | |
723 | may be necessary to redirect the input from | |
724 | .Pa /dev/null | |
725 | to make this work.) | |
bf740959 | 726 | .It Ev SSH_AUTH_SOCK |
ae897d7c | 727 | Identifies the path of a unix-domain socket used to communicate with the |
bf740959 | 728 | agent. |
da0561eb | 729 | .It Ev SSH_CONNECTION |
730 | Identifies the client and server ends of the connection. | |
610cd5c6 | 731 | The variable contains |
da0561eb | 732 | four space-separated values: client ip-address, client port number, |
733 | server ip-address and server port number. | |
8abcdba4 | 734 | .It Ev SSH_ORIGINAL_COMMAND |
735 | The variable contains the original command line if a forced command | |
736 | is executed. | |
737 | It can be used to extract the original arguments. | |
bf740959 | 738 | .It Ev SSH_TTY |
739 | This is set to the name of the tty (path to the device) associated | |
610cd5c6 | 740 | with the current shell or command. |
741 | If the current session has no tty, | |
bf740959 | 742 | this variable is not set. |
743 | .It Ev TZ | |
744 | The timezone variable is set to indicate the present timezone if it | |
c345cf9d | 745 | was set when the daemon was started (i.e., the daemon passes the value |
bf740959 | 746 | on to new connections). |
747 | .It Ev USER | |
748 | Set to the name of the user logging in. | |
749 | .El | |
750 | .Pp | |
f54651ce | 751 | Additionally, |
bf740959 | 752 | .Nm |
f54651ce | 753 | reads |
754 | .Pa $HOME/.ssh/environment , | |
bf740959 | 755 | and adds lines of the format |
756 | .Dq VARNAME=value | |
6a342527 | 757 | to the environment if the file exists and if users are allowed to |
758 | change their environment. | |
35453849 | 759 | See the |
6a342527 | 760 | .Cm PermitUserEnvironment |
35453849 | 761 | option in |
6a342527 | 762 | .Xr sshd_config 5 . |
bf740959 | 763 | .Sh FILES |
c8d54615 | 764 | .Bl -tag -width Ds |
f49bc4f7 | 765 | .It Pa $HOME/.ssh/known_hosts |
ae897d7c | 766 | Records host keys for all hosts the user has logged into that are not |
bf740959 | 767 | in |
2a8a6488 | 768 | .Pa /etc/ssh/ssh_known_hosts . |
bf740959 | 769 | See |
770 | .Xr sshd 8 . | |
c0ecc314 | 771 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa |
772 | Contains the authentication identity of the user. | |
773 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
1d1ffb87 | 774 | These files |
775 | contain sensitive data and should be readable by the user but not | |
bf740959 | 776 | accessible by others (read/write/execute). |
777 | Note that | |
778 | .Nm | |
1d1ffb87 | 779 | ignores a private key file if it is accessible by others. |
bf740959 | 780 | It is possible to specify a passphrase when |
781 | generating the key; the passphrase will be used to encrypt the | |
782 | sensitive part of this file using 3DES. | |
c0ecc314 | 783 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub |
bf740959 | 784 | Contains the public key for authentication (public part of the |
610cd5c6 | 785 | identity file in human-readable form). |
1d1ffb87 | 786 | The contents of the |
787 | .Pa $HOME/.ssh/identity.pub | |
788 | file should be added to | |
bf740959 | 789 | .Pa $HOME/.ssh/authorized_keys |
790 | on all machines | |
91789042 | 791 | where the user wishes to log in using protocol version 1 RSA authentication. |
1d1ffb87 | 792 | The contents of the |
793 | .Pa $HOME/.ssh/id_dsa.pub | |
c0ecc314 | 794 | and |
795 | .Pa $HOME/.ssh/id_rsa.pub | |
1d1ffb87 | 796 | file should be added to |
96a7b0cc | 797 | .Pa $HOME/.ssh/authorized_keys |
1d1ffb87 | 798 | on all machines |
91789042 | 799 | where the user wishes to log in using protocol version 2 DSA/RSA authentication. |
1d1ffb87 | 800 | These files are not |
610cd5c6 | 801 | sensitive and can (but need not) be readable by anyone. |
1d1ffb87 | 802 | These files are |
c44559d2 | 803 | never used automatically and are not necessary; they are only provided for |
bf740959 | 804 | the convenience of the user. |
805 | .It Pa $HOME/.ssh/config | |
610cd5c6 | 806 | This is the per-user configuration file. |
588df31a | 807 | The file format and configuration options are described in |
808 | .Xr ssh_config 5 . | |
bf740959 | 809 | .It Pa $HOME/.ssh/authorized_keys |
96a7b0cc | 810 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. |
610cd5c6 | 811 | The format of this file is described in the |
bf740959 | 812 | .Xr sshd 8 |
610cd5c6 | 813 | manual page. |
f49bc4f7 | 814 | In the simplest form the format is the same as the .pub |
815 | identity files. | |
1d1ffb87 | 816 | This file is not highly sensitive, but the recommended |
817 | permissions are read/write for the user, and not accessible by others. | |
2a8a6488 | 818 | .It Pa /etc/ssh/ssh_known_hosts |
610cd5c6 | 819 | Systemwide list of known host keys. |
f49bc4f7 | 820 | This file should be prepared by the |
bf740959 | 821 | system administrator to contain the public host keys of all machines in the |
610cd5c6 | 822 | organization. |
823 | This file should be world-readable. | |
824 | This file contains | |
bf740959 | 825 | public keys, one per line, in the following format (fields separated |
f49bc4f7 | 826 | by spaces): system name, public key and optional comment field. |
610cd5c6 | 827 | When different names are used |
bf740959 | 828 | for the same machine, all such names should be listed, separated by |
610cd5c6 | 829 | commas. |
830 | The format is described on the | |
bf740959 | 831 | .Xr sshd 8 |
832 | manual page. | |
833 | .Pp | |
834 | The canonical system name (as returned by name servers) is used by | |
835 | .Xr sshd 8 | |
836 | to verify the client host when logging in; other names are needed because | |
837 | .Nm | |
838 | does not convert the user-supplied name to a canonical name before | |
839 | checking the key, because someone with access to the name servers | |
840 | would then be able to fool host authentication. | |
2a8a6488 | 841 | .It Pa /etc/ssh/ssh_config |
610cd5c6 | 842 | Systemwide configuration file. |
588df31a | 843 | The file format and configuration options are described in |
844 | .Xr ssh_config 5 . | |
2a8a6488 | 845 | .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key |
dd58cb5e | 846 | These three files contain the private parts of the host keys |
847 | and are used for | |
848 | .Cm RhostsRSAAuthentication | |
849 | and | |
850 | .Cm HostbasedAuthentication . | |
d83cbdc3 | 851 | If the protocol version 1 |
852 | .Cm RhostsRSAAuthentication | |
7203d6bb | 853 | method is used, |
d83cbdc3 | 854 | .Nm |
855 | must be setuid root, since the host key is readable only by root. | |
856 | For protocol version 2, | |
857 | .Nm | |
858 | uses | |
859 | .Xr ssh-keysign 8 | |
860 | to access the host keys for | |
861 | .Cm HostbasedAuthentication . | |
862 | This eliminates the requirement that | |
863 | .Nm | |
864 | be setuid root when that authentication method is used. | |
865 | By default | |
dd58cb5e | 866 | .Nm |
d83cbdc3 | 867 | is not setuid root. |
bf740959 | 868 | .It Pa $HOME/.rhosts |
869 | This file is used in | |
870 | .Pa \&.rhosts | |
871 | authentication to list the | |
610cd5c6 | 872 | host/user pairs that are permitted to log in. |
873 | (Note that this file is | |
bf740959 | 874 | also used by rlogin and rsh, which makes using this file insecure.) |
875 | Each line of the file contains a host name (in the canonical form | |
876 | returned by name servers), and then a user name on that host, | |
610cd5c6 | 877 | separated by a space. |
e91c60f2 | 878 | On some machines this file may need to be |
bf740959 | 879 | world-readable if the user's home directory is on a NFS partition, |
880 | because | |
881 | .Xr sshd 8 | |
610cd5c6 | 882 | reads it as root. |
883 | Additionally, this file must be owned by the user, | |
884 | and must not have write permissions for anyone else. | |
885 | The recommended | |
bf740959 | 886 | permission for most machines is read/write for the user, and not |
887 | accessible by others. | |
888 | .Pp | |
889 | Note that by default | |
890 | .Xr sshd 8 | |
891 | will be installed so that it requires successful RSA host | |
610cd5c6 | 892 | authentication before permitting \s+2.\s0rhosts authentication. |
91789042 | 893 | If the server machine does not have the client's host key in |
2a8a6488 | 894 | .Pa /etc/ssh/ssh_known_hosts , |
91789042 | 895 | it can be stored in |
bf740959 | 896 | .Pa $HOME/.ssh/known_hosts . |
897 | The easiest way to do this is to | |
898 | connect back to the client from the server machine using ssh; this | |
1d1ffb87 | 899 | will automatically add the host key to |
bf740959 | 900 | .Pa $HOME/.ssh/known_hosts . |
901 | .It Pa $HOME/.shosts | |
902 | This file is used exactly the same way as | |
903 | .Pa \&.rhosts . | |
904 | The purpose for | |
905 | having this file is to be able to use rhosts authentication with | |
906 | .Nm | |
907 | without permitting login with | |
7e276482 | 908 | .Nm rlogin |
bf740959 | 909 | or |
910 | .Xr rsh 1 . | |
911 | .It Pa /etc/hosts.equiv | |
912 | This file is used during | |
610cd5c6 | 913 | .Pa \&.rhosts authentication. |
914 | It contains | |
bf740959 | 915 | canonical hosts names, one per line (the full format is described on |
916 | the | |
917 | .Xr sshd 8 | |
610cd5c6 | 918 | manual page). |
919 | If the client host is found in this file, login is | |
bf740959 | 920 | automatically permitted provided client and server user names are the |
610cd5c6 | 921 | same. |
922 | Additionally, successful RSA host authentication is normally | |
923 | required. | |
924 | This file should only be writable by root. | |
5f4fdfae | 925 | .It Pa /etc/shosts.equiv |
f54651ce | 926 | This file is processed exactly as |
bf740959 | 927 | .Pa /etc/hosts.equiv . |
928 | This file may be useful to permit logins using | |
929 | .Nm | |
930 | but not using rsh/rlogin. | |
2a8a6488 | 931 | .It Pa /etc/ssh/sshrc |
bf740959 | 932 | Commands in this file are executed by |
933 | .Nm | |
934 | when the user logs in just before the user's shell (or command) is started. | |
935 | See the | |
936 | .Xr sshd 8 | |
937 | manual page for more information. | |
938 | .It Pa $HOME/.ssh/rc | |
939 | Commands in this file are executed by | |
940 | .Nm | |
941 | when the user logs in just before the user's shell (or command) is | |
942 | started. | |
f54651ce | 943 | See the |
bf740959 | 944 | .Xr sshd 8 |
945 | manual page for more information. | |
83b7f649 | 946 | .It Pa $HOME/.ssh/environment |
947 | Contains additional definitions for environment variables, see section | |
948 | .Sx ENVIRONMENT | |
949 | above. | |
b5e300c2 | 950 | .El |
16210ef7 | 951 | .Sh DIAGNOSTICS |
952 | .Nm | |
953 | exits with the exit status of the remote command or with 255 | |
954 | if an error occurred. | |
fa08c86b | 955 | .Sh AUTHORS |
5fb622e4 | 956 | OpenSSH is a derivative of the original and free |
957 | ssh 1.2.12 release by Tatu Ylonen. | |
958 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
959 | Theo de Raadt and Dug Song | |
960 | removed many bugs, re-added newer features and | |
961 | created OpenSSH. | |
962 | Markus Friedl contributed the support for SSH | |
963 | protocol versions 1.5 and 2.0. | |
bf740959 | 964 | .Sh SEE ALSO |
bf740959 | 965 | .Xr rsh 1 , |
966 | .Xr scp 1 , | |
61e96248 | 967 | .Xr sftp 1 , |
bf740959 | 968 | .Xr ssh-add 1 , |
969 | .Xr ssh-agent 1 , | |
970 | .Xr ssh-keygen 1 , | |
971 | .Xr telnet 1 , | |
1ae02182 | 972 | .Xr ssh_config 5 , |
b2843ec6 | 973 | .Xr ssh-keysign 8 , |
9afadca8 | 974 | .Xr sshd 8 |
2cad6cef | 975 | .Rs |
976 | .%A T. Ylonen | |
977 | .%A T. Kivinen | |
978 | .%A M. Saarinen | |
979 | .%A T. Rinne | |
980 | .%A S. Lehtinen | |
981 | .%T "SSH Protocol Architecture" | |
17f5e68a | 982 | .%N draft-ietf-secsh-architecture-12.txt |
983 | .%D January 2002 | |
2cad6cef | 984 | .%O work in progress material |
985 | .Re |