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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 | .\" |
ce773142 | 37 | .\" $OpenBSD: ssh.1,v 1.124 2001/07/25 14:35:18 markus 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 | |
47 | .Op Ar hostname | user@hostname | |
48 | .Op Ar command | |
49 | .Pp | |
50 | .Nm ssh | |
9afadca8 | 51 | .Op Fl afgknqstvxACNPTX1246 |
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 | |
60 | .Oo Fl L Xo | |
61 | .Sm off | |
bf740959 | 62 | .Ar port : |
7b2ea3a1 | 63 | .Ar host : |
bf740959 | 64 | .Ar hostport |
65 | .Sm on | |
66 | .Xc | |
67 | .Oc | |
68 | .Oo Fl R Xo | |
69 | .Sm off | |
bf740959 | 70 | .Ar port : |
7b2ea3a1 | 71 | .Ar host : |
bf740959 | 72 | .Ar hostport |
73 | .Sm on | |
74 | .Xc | |
75 | .Oc | |
76 | .Op Ar hostname | user@hostname | |
77 | .Op Ar command | |
f54651ce | 78 | .Sh DESCRIPTION |
bf740959 | 79 | .Nm |
2c86906e | 80 | (SSH client) is a program for logging into a remote machine and for |
610cd5c6 | 81 | executing commands on a remote machine. |
82 | It is intended to replace | |
bf740959 | 83 | rlogin and rsh, and provide secure encrypted communications between |
610cd5c6 | 84 | two untrusted hosts over an insecure network. |
85 | X11 connections and | |
bf740959 | 86 | arbitrary TCP/IP ports can also be forwarded over the secure channel. |
87 | .Pp | |
88 | .Nm | |
f54651ce | 89 | connects and logs into the specified |
bf740959 | 90 | .Ar hostname . |
91 | The user must prove | |
1d1ffb87 | 92 | his/her identity to the remote machine using one of several methods |
93 | depending on the protocol version used: | |
94 | .Pp | |
95 | .Ss SSH protocol version 1 | |
bf740959 | 96 | .Pp |
97 | First, if the machine the user logs in from is listed in | |
98 | .Pa /etc/hosts.equiv | |
99 | or | |
5f4fdfae | 100 | .Pa /etc/shosts.equiv |
bf740959 | 101 | on the remote machine, and the user names are |
102 | the same on both sides, the user is immediately permitted to log in. | |
f54651ce | 103 | Second, if |
bf740959 | 104 | .Pa \&.rhosts |
105 | or | |
106 | .Pa \&.shosts | |
107 | exists in the user's home directory on the | |
108 | remote machine and contains a line containing the name of the client | |
109 | machine and the name of the user on that machine, the user is | |
610cd5c6 | 110 | permitted to log in. |
111 | This form of authentication alone is normally not | |
bf740959 | 112 | allowed by the server because it is not secure. |
113 | .Pp | |
da89cf4d | 114 | The second authentication method is the |
bf740959 | 115 | .Pa rhosts |
116 | or | |
117 | .Pa hosts.equiv | |
610cd5c6 | 118 | method combined with RSA-based host authentication. |
119 | It means that if the login would be permitted by | |
1d1ffb87 | 120 | .Pa $HOME/.rhosts , |
121 | .Pa $HOME/.shosts , | |
bf740959 | 122 | .Pa /etc/hosts.equiv , |
123 | or | |
5f4fdfae | 124 | .Pa /etc/shosts.equiv , |
bf740959 | 125 | and if additionally the server can verify the client's |
f54651ce | 126 | host key (see |
5f4fdfae | 127 | .Pa /etc/ssh_known_hosts |
5bbb5681 | 128 | and |
129 | .Pa $HOME/.ssh/known_hosts | |
bf740959 | 130 | in the |
131 | .Sx FILES | |
610cd5c6 | 132 | section), only then login is permitted. |
133 | This authentication method closes security holes due to IP | |
134 | spoofing, DNS spoofing and routing spoofing. | |
135 | [Note to the administrator: | |
bf740959 | 136 | .Pa /etc/hosts.equiv , |
1d1ffb87 | 137 | .Pa $HOME/.rhosts , |
bf740959 | 138 | and the rlogin/rsh protocol in general, are inherently insecure and should be |
139 | disabled if security is desired.] | |
140 | .Pp | |
f54651ce | 141 | As a third authentication method, |
bf740959 | 142 | .Nm |
143 | supports RSA based authentication. | |
144 | The scheme is based on public-key cryptography: there are cryptosystems | |
145 | where encryption and decryption are done using separate keys, and it | |
146 | is not possible to derive the decryption key from the encryption key. | |
610cd5c6 | 147 | RSA is one such system. |
f54651ce | 148 | The idea is that each user creates a public/private |
610cd5c6 | 149 | key pair for authentication purposes. |
150 | The server knows the public key, and only the user knows the private key. | |
f54651ce | 151 | The file |
bf740959 | 152 | .Pa $HOME/.ssh/authorized_keys |
153 | lists the public keys that are permitted for logging | |
610cd5c6 | 154 | in. |
155 | When the user logs in, the | |
bf740959 | 156 | .Nm |
157 | program tells the server which key pair it would like to use for | |
610cd5c6 | 158 | authentication. |
159 | The server checks if this key is permitted, and if | |
bf740959 | 160 | so, sends the user (actually the |
161 | .Nm | |
162 | program running on behalf of the user) a challenge, a random number, | |
610cd5c6 | 163 | encrypted by the user's public key. |
164 | The challenge can only be | |
165 | decrypted using the proper private key. | |
166 | The user's client then decrypts the | |
bf740959 | 167 | challenge using the private key, proving that he/she knows the private |
168 | key but without disclosing it to the server. | |
169 | .Pp | |
170 | .Nm | |
610cd5c6 | 171 | implements the RSA authentication protocol automatically. |
172 | The user creates his/her RSA key pair by running | |
bf740959 | 173 | .Xr ssh-keygen 1 . |
f54651ce | 174 | This stores the private key in |
1d1ffb87 | 175 | .Pa $HOME/.ssh/identity |
bf740959 | 176 | and the public key in |
1d1ffb87 | 177 | .Pa $HOME/.ssh/identity.pub |
610cd5c6 | 178 | in the user's home directory. |
179 | The user should then copy the | |
bf740959 | 180 | .Pa identity.pub |
f54651ce | 181 | to |
1d1ffb87 | 182 | .Pa $HOME/.ssh/authorized_keys |
f54651ce | 183 | in his/her home directory on the remote machine (the |
bf740959 | 184 | .Pa authorized_keys |
f54651ce | 185 | file corresponds to the conventional |
1d1ffb87 | 186 | .Pa $HOME/.rhosts |
bf740959 | 187 | file, and has one key |
610cd5c6 | 188 | per line, though the lines can be very long). |
189 | After this, the user can log in without giving the password. | |
190 | RSA authentication is much | |
bf740959 | 191 | more secure than rhosts authentication. |
192 | .Pp | |
193 | The most convenient way to use RSA authentication may be with an | |
610cd5c6 | 194 | authentication agent. |
195 | See | |
bf740959 | 196 | .Xr ssh-agent 1 |
197 | for more information. | |
198 | .Pp | |
f54651ce | 199 | If other authentication methods fail, |
bf740959 | 200 | .Nm |
610cd5c6 | 201 | prompts the user for a password. |
202 | The password is sent to the remote | |
bf740959 | 203 | host for checking; however, since all communications are encrypted, |
204 | the password cannot be seen by someone listening on the network. | |
205 | .Pp | |
1d1ffb87 | 206 | .Ss SSH protocol version 2 |
207 | .Pp | |
208 | When a user connects using the protocol version 2 | |
da89cf4d | 209 | different authentication methods are available. |
210 | Using the default values for | |
211 | .Cm PreferredAuthentications , | |
29c440a0 | 212 | the client will try to authenticate first using the hostbased method; |
213 | if this method fails public key authentication is attempted, | |
214 | and finally if this method fails keyboard-interactive and | |
215 | password authentication are tried. | |
1d1ffb87 | 216 | .Pp |
217 | The public key method is similar to RSA authentication described | |
da89cf4d | 218 | in the previous section and allows the RSA or DSA algorithm to be used: |
c0ecc314 | 219 | The client uses his private key, |
1d1ffb87 | 220 | .Pa $HOME/.ssh/id_dsa |
c0ecc314 | 221 | or |
222 | .Pa $HOME/.ssh/id_rsa , | |
1d1ffb87 | 223 | to sign the session identifier and sends the result to the server. |
224 | The server checks whether the matching public key is listed in | |
96a7b0cc | 225 | .Pa $HOME/.ssh/authorized_keys |
1d1ffb87 | 226 | and grants access if both the key is found and the signature is correct. |
227 | The session identifier is derived from a shared Diffie-Hellman value | |
228 | and is only known to the client and the server. | |
229 | .Pp | |
230 | If public key authentication fails or is not available a password | |
231 | can be sent encrypted to the remote host for proving the user's identity. | |
da89cf4d | 232 | .Pp |
233 | Additionally, | |
234 | .Nm | |
235 | supports hostbased or challenge response authentication. | |
1d1ffb87 | 236 | .Pp |
237 | Protocol 2 provides additional mechanisms for confidentiality | |
d0c832f3 | 238 | (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) |
f2ba0775 | 239 | and integrity (hmac-md5, hmac-sha1). |
1d1ffb87 | 240 | Note that protocol 1 lacks a strong mechanism for ensuring the |
241 | integrity of the connection. | |
242 | .Pp | |
243 | .Ss Login session and remote execution | |
244 | .Pp | |
bf740959 | 245 | When the user's identity has been accepted by the server, the server |
246 | either executes the given command, or logs into the machine and gives | |
610cd5c6 | 247 | the user a normal shell on the remote machine. |
248 | All communication with | |
bf740959 | 249 | the remote command or shell will be automatically encrypted. |
250 | .Pp | |
251 | If a pseudo-terminal has been allocated (normal login session), the | |
df841692 | 252 | user may use the escape characters noted below. |
bf740959 | 253 | .Pp |
254 | If no pseudo tty has been allocated, the | |
255 | session is transparent and can be used to reliably transfer binary | |
610cd5c6 | 256 | data. |
257 | On most systems, setting the escape character to | |
bf740959 | 258 | .Dq none |
259 | will also make the session transparent even if a tty is used. | |
260 | .Pp | |
ce9c0b75 | 261 | The session terminates when the command or shell on the remote |
e91c60f2 | 262 | machine exits and all X11 and TCP/IP connections have been closed. |
bf740959 | 263 | The exit status of the remote program is returned as the exit status |
264 | of | |
265 | .Nm ssh . | |
266 | .Pp | |
df841692 | 267 | .Ss Escape Characters |
268 | .Pp | |
269 | When a pseudo terminal has been requested, ssh supports a number of functions | |
3730bb22 | 270 | through the use of an escape character. |
df841692 | 271 | .Pp |
272 | A single tilde character can be sent as | |
273 | .Ic ~~ | |
5bef3c35 | 274 | or by following the tilde by a character other than those described below. |
df841692 | 275 | The escape character must always follow a newline to be interpreted as |
276 | special. | |
277 | The escape character can be changed in configuration files using the | |
278 | .Cm EscapeChar | |
3730bb22 | 279 | configuration directive or on the command line by the |
df841692 | 280 | .Fl e |
281 | option. | |
282 | .Pp | |
283 | The supported escapes (assuming the default | |
284 | .Ql ~ ) | |
285 | are: | |
286 | .Bl -tag -width Ds | |
287 | .It Cm ~. | |
288 | Disconnect | |
289 | .It Cm ~^Z | |
290 | Background ssh | |
291 | .It Cm ~# | |
292 | List forwarded connections | |
293 | .It Cm ~& | |
294 | Background ssh at logout when waiting for forwarded connection / X11 sessions | |
da89cf4d | 295 | to terminate (protocol version 1 only) |
df841692 | 296 | .It Cm ~? |
297 | Display a list of escape characters | |
298 | .It Cm ~R | |
67b964a1 | 299 | Request rekeying of the connection (only useful for SSH protocol version 2 |
300 | and if the peer supports it) | |
df841692 | 301 | .El |
302 | .Pp | |
1d1ffb87 | 303 | .Ss X11 and TCP forwarding |
304 | .Pp | |
6efa3d14 | 305 | If the |
306 | .Cm ForwardX11 | |
307 | variable is set to | |
308 | .Dq yes | |
309 | (or, see the description of the | |
310 | .Fl X | |
311 | and | |
312 | .Fl x | |
313 | options described later) | |
314 | and the user is using X11 (the | |
bf740959 | 315 | .Ev DISPLAY |
316 | environment variable is set), the connection to the X11 display is | |
317 | automatically forwarded to the remote side in such a way that any X11 | |
318 | programs started from the shell (or command) will go through the | |
319 | encrypted channel, and the connection to the real X server will be made | |
610cd5c6 | 320 | from the local machine. |
321 | The user should not manually set | |
bf740959 | 322 | .Ev DISPLAY . |
323 | Forwarding of X11 connections can be | |
324 | configured on the command line or in configuration files. | |
325 | .Pp | |
326 | The | |
f54651ce | 327 | .Ev DISPLAY |
bf740959 | 328 | value set by |
329 | .Nm | |
330 | will point to the server machine, but with a display number greater | |
610cd5c6 | 331 | than zero. |
332 | This is normal, and happens because | |
bf740959 | 333 | .Nm |
334 | creates a | |
335 | .Dq proxy | |
336 | X server on the server machine for forwarding the | |
337 | connections over the encrypted channel. | |
338 | .Pp | |
339 | .Nm | |
340 | will also automatically set up Xauthority data on the server machine. | |
341 | For this purpose, it will generate a random authorization cookie, | |
342 | store it in Xauthority on the server, and verify that any forwarded | |
343 | connections carry this cookie and replace it by the real cookie when | |
610cd5c6 | 344 | the connection is opened. |
345 | The real authentication cookie is never | |
bf740959 | 346 | sent to the server machine (and no cookies are sent in the plain). |
347 | .Pp | |
348 | If the user is using an authentication agent, the connection to the agent | |
349 | is automatically forwarded to the remote side unless disabled on | |
491f5f7b | 350 | the command line or in a configuration file. |
bf740959 | 351 | .Pp |
352 | Forwarding of arbitrary TCP/IP connections over the secure channel can | |
491f5f7b | 353 | be specified either on the command line or in a configuration file. |
610cd5c6 | 354 | One possible application of TCP/IP forwarding is a secure connection to an |
e91c60f2 | 355 | electronic purse; another is going through firewalls. |
bf740959 | 356 | .Pp |
1d1ffb87 | 357 | .Ss Server authentication |
358 | .Pp | |
bf740959 | 359 | .Nm |
1d1ffb87 | 360 | automatically maintains and checks a database containing |
610cd5c6 | 361 | identifications for all hosts it has ever been used with. |
f49bc4f7 | 362 | Host keys are stored in |
1d1ffb87 | 363 | .Pa $HOME/.ssh/known_hosts |
610cd5c6 | 364 | in the user's home directory. |
f49bc4f7 | 365 | Additionally, the file |
5f4fdfae | 366 | .Pa /etc/ssh_known_hosts |
f49bc4f7 | 367 | is automatically checked for known hosts. |
610cd5c6 | 368 | Any new hosts are automatically added to the user's file. |
369 | If a host's identification | |
bf740959 | 370 | ever changes, |
371 | .Nm | |
372 | warns about this and disables password authentication to prevent a | |
610cd5c6 | 373 | trojan horse from getting the user's password. |
374 | Another purpose of | |
bf740959 | 375 | this mechanism is to prevent man-in-the-middle attacks which could |
610cd5c6 | 376 | otherwise be used to circumvent the encryption. |
377 | The | |
bf740959 | 378 | .Cm StrictHostKeyChecking |
379 | option (see below) can be used to prevent logins to machines whose | |
380 | host key is not known or has changed. | |
fa08c86b | 381 | .Pp |
382 | The options are as follows: | |
bf740959 | 383 | .Bl -tag -width Ds |
384 | .It Fl a | |
4fe2af09 | 385 | Disables forwarding of the authentication agent connection. |
71276795 | 386 | .It Fl A |
387 | Enables forwarding of the authentication agent connection. | |
388 | This can also be specified on a per-host basis in a configuration file. | |
3435f5a6 | 389 | .It Fl b Ar bind_address |
390 | Specify the interface to transmit from on machines with multiple | |
391 | interfaces or aliased addresses. | |
bf740959 | 392 | .It Fl c Ar blowfish|3des |
f54651ce | 393 | Selects the cipher to use for encrypting the session. |
bf740959 | 394 | .Ar 3des |
610cd5c6 | 395 | is used by default. |
f54651ce | 396 | It is believed to be secure. |
bf740959 | 397 | .Ar 3des |
398 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
399 | It is presumably more secure than the | |
400 | .Ar des | |
a22aff1f | 401 | cipher which is no longer fully supported in |
d0c832f3 | 402 | .Nm ssh . |
bf740959 | 403 | .Ar blowfish |
404 | is a fast block cipher, it appears very secure and is much faster than | |
610cd5c6 | 405 | .Ar 3des . |
9afadca8 | 406 | .It Fl c Ar cipher_spec |
d0c832f3 | 407 | Additionally, for protocol version 2 a comma-separated list of ciphers can |
94ec8c6b | 408 | be specified in order of preference. |
9afadca8 | 409 | See |
410 | .Cm Ciphers | |
411 | for more information. | |
bf740959 | 412 | .It Fl e Ar ch|^ch|none |
413 | Sets the escape character for sessions with a pty (default: | |
414 | .Ql ~ ) . | |
610cd5c6 | 415 | The escape character is only recognized at the beginning of a line. |
416 | The escape character followed by a dot | |
bf740959 | 417 | .Pq Ql \&. |
418 | closes the connection, followed | |
419 | by control-Z suspends the connection, and followed by itself sends the | |
610cd5c6 | 420 | escape character once. |
421 | Setting the character to | |
bf740959 | 422 | .Dq none |
423 | disables any escapes and makes the session fully transparent. | |
424 | .It Fl f | |
425 | Requests | |
426 | .Nm | |
610cd5c6 | 427 | to go to background just before command execution. |
428 | This is useful if | |
bf740959 | 429 | .Nm |
430 | is going to ask for passwords or passphrases, but the user | |
610cd5c6 | 431 | wants it in the background. |
f54651ce | 432 | This implies |
bf740959 | 433 | .Fl n . |
434 | The recommended way to start X11 programs at a remote site is with | |
435 | something like | |
436 | .Ic ssh -f host xterm . | |
7b2ea3a1 | 437 | .It Fl g |
438 | Allows remote hosts to connect to local forwarded ports. | |
bf740959 | 439 | .It Fl i Ar identity_file |
f54651ce | 440 | Selects the file from which the identity (private key) for |
fa08c86b | 441 | RSA or DSA authentication is read. |
f54651ce | 442 | Default is |
1d1ffb87 | 443 | .Pa $HOME/.ssh/identity |
610cd5c6 | 444 | in the user's home directory. |
445 | Identity files may also be specified on | |
446 | a per-host basis in the configuration file. | |
447 | It is possible to have multiple | |
bf740959 | 448 | .Fl i |
449 | options (and multiple identities specified in | |
450 | configuration files). | |
bf740959 | 451 | .It Fl k |
4fe2af09 | 452 | Disables forwarding of Kerberos tickets and AFS tokens. |
453 | This may also be specified on a per-host basis in the configuration file. | |
bf740959 | 454 | .It Fl l Ar login_name |
610cd5c6 | 455 | Specifies the user to log in as on the remote machine. |
456 | This also may be specified on a per-host basis in the configuration file. | |
b2552997 | 457 | .It Fl m Ar mac_spec |
458 | Additionally, for protocol version 2 a comma-separated list of MAC | |
459 | (message authentication code) algorithms can | |
460 | be specified in order of preference. | |
461 | See the | |
462 | .Cm MACs | |
463 | keyword for more information. | |
bf740959 | 464 | .It Fl n |
465 | Redirects stdin from | |
466 | .Pa /dev/null | |
467 | (actually, prevents reading from stdin). | |
468 | This must be used when | |
469 | .Nm | |
610cd5c6 | 470 | is run in the background. |
471 | A common trick is to use this to run X11 programs on a remote machine. | |
472 | For example, | |
bf740959 | 473 | .Ic ssh -n shadows.cs.hut.fi emacs & |
474 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
475 | connection will be automatically forwarded over an encrypted channel. | |
476 | The | |
477 | .Nm | |
478 | program will be put in the background. | |
479 | (This does not work if | |
480 | .Nm | |
481 | needs to ask for a password or passphrase; see also the | |
482 | .Fl f | |
483 | option.) | |
0e73cc53 | 484 | .It Fl N |
485 | Do not execute a remote command. | |
0b6fbf03 | 486 | This is useful if you just want to forward ports |
0e73cc53 | 487 | (protocol version 2 only). |
bf740959 | 488 | .It Fl o Ar option |
489 | Can be used to give options in the format used in the config file. | |
490 | This is useful for specifying options for which there is no separate | |
610cd5c6 | 491 | command-line flag. |
492 | The option has the same format as a line in the configuration file. | |
bf740959 | 493 | .It Fl p Ar port |
610cd5c6 | 494 | Port to connect to on the remote host. |
495 | This can be specified on a | |
bf740959 | 496 | per-host basis in the configuration file. |
497 | .It Fl P | |
498 | Use a non-privileged port for outgoing connections. | |
499 | This can be used if your firewall does | |
500 | not permit connections from privileged ports. | |
95f1eccc | 501 | Note that this option turns off |
bf740959 | 502 | .Cm RhostsAuthentication |
503 | and | |
6ffc9c88 | 504 | .Cm RhostsRSAAuthentication |
505 | for older servers. | |
bf740959 | 506 | .It Fl q |
610cd5c6 | 507 | Quiet mode. |
508 | Causes all warning and diagnostic messages to be suppressed. | |
509 | Only fatal errors are displayed. | |
ae810de7 | 510 | .It Fl s |
3730bb22 | 511 | 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 |
512 | of SSH as a secure transport for other applications (eg. sftp). The | |
ae810de7 | 513 | subsystem is specified as the remote command. |
bf740959 | 514 | .It Fl t |
610cd5c6 | 515 | Force pseudo-tty allocation. |
4fe2af09 | 516 | This can be used to execute arbitrary |
610cd5c6 | 517 | screen-based programs on a remote machine, which can be very useful, |
518 | e.g., when implementing menu services. | |
8abcdba4 | 519 | Multiple |
520 | .Fl t | |
521 | options force tty allocation, even if | |
522 | .Nm | |
523 | has no local tty. | |
0e73cc53 | 524 | .It Fl T |
0b6fbf03 | 525 | Disable pseudo-tty allocation. |
bf740959 | 526 | .It Fl v |
610cd5c6 | 527 | Verbose mode. |
528 | Causes | |
bf740959 | 529 | .Nm |
610cd5c6 | 530 | to print debugging messages about its progress. |
531 | This is helpful in | |
bf740959 | 532 | debugging connection, authentication, and configuration problems. |
8abcdba4 | 533 | Multiple |
534 | .Fl v | |
535 | options increases the verbosity. | |
94ec8c6b | 536 | Maximum is 3. |
bf740959 | 537 | .It Fl x |
610cd5c6 | 538 | Disables X11 forwarding. |
bf740959 | 539 | .It Fl X |
540 | Enables X11 forwarding. | |
71276795 | 541 | This can also be specified on a per-host basis in a configuration file. |
bf740959 | 542 | .It Fl C |
543 | Requests compression of all data (including stdin, stdout, stderr, and | |
610cd5c6 | 544 | data for forwarded X11 and TCP/IP connections). |
545 | The compression algorithm is the same used by | |
7b2ea3a1 | 546 | .Xr gzip 1 , |
547 | and the | |
bf740959 | 548 | .Dq level |
549 | can be controlled by the | |
550 | .Cm CompressionLevel | |
610cd5c6 | 551 | option (see below). |
552 | Compression is desirable on modem lines and other | |
bf740959 | 553 | slow connections, but will only slow down things on fast networks. |
554 | The default value can be set on a host-by-host basis in the | |
555 | configuration files; see the | |
0aea6c59 | 556 | .Cm Compression |
bf740959 | 557 | option below. |
558 | .It Fl L Ar port:host:hostport | |
559 | Specifies that the given port on the local (client) host is to be | |
610cd5c6 | 560 | forwarded to the given host and port on the remote side. |
561 | This works by allocating a socket to listen to | |
bf740959 | 562 | .Ar port |
563 | on the local side, and whenever a connection is made to this port, the | |
564 | connection is forwarded over the secure channel, and a connection is | |
565 | made to | |
48e671d5 | 566 | .Ar host |
567 | port | |
568 | .Ar hostport | |
610cd5c6 | 569 | from the remote machine. |
570 | Port forwardings can also be specified in the configuration file. | |
571 | Only root can forward privileged ports. | |
48e671d5 | 572 | IPv6 addresses can be specified with an alternative syntax: |
573 | .Ar port/host/hostport | |
bf740959 | 574 | .It Fl R Ar port:host:hostport |
575 | Specifies that the given port on the remote (server) host is to be | |
610cd5c6 | 576 | forwarded to the given host and port on the local side. |
577 | This works by allocating a socket to listen to | |
bf740959 | 578 | .Ar port |
579 | on the remote side, and whenever a connection is made to this port, the | |
580 | connection is forwarded over the secure channel, and a connection is | |
581 | made to | |
48e671d5 | 582 | .Ar host |
583 | port | |
584 | .Ar hostport | |
610cd5c6 | 585 | from the local machine. |
586 | Port forwardings can also be specified in the configuration file. | |
587 | Privileged ports can be forwarded only when | |
bf740959 | 588 | logging in as root on the remote machine. |
da89cf4d | 589 | IPv6 addresses can be specified with an alternative syntax: |
590 | .Ar port/host/hostport | |
9afadca8 | 591 | .It Fl 1 |
592 | Forces | |
593 | .Nm | |
594 | to try protocol version 1 only. | |
6ae2364d | 595 | .It Fl 2 |
596 | Forces | |
597 | .Nm | |
1d1ffb87 | 598 | to try protocol version 2 only. |
48e671d5 | 599 | .It Fl 4 |
600 | Forces | |
601 | .Nm | |
602 | to use IPv4 addresses only. | |
603 | .It Fl 6 | |
604 | Forces | |
605 | .Nm | |
606 | to use IPv6 addresses only. | |
bf740959 | 607 | .El |
608 | .Sh CONFIGURATION FILES | |
609 | .Nm | |
610 | obtains configuration data from the following sources (in this order): | |
611 | command line options, user's configuration file | |
612 | .Pq Pa $HOME/.ssh/config , | |
613 | and system-wide configuration file | |
5f4fdfae | 614 | .Pq Pa /etc/ssh_config . |
bf740959 | 615 | For each parameter, the first obtained value |
610cd5c6 | 616 | will be used. |
617 | The configuration files contain sections bracketed by | |
618 | .Dq Host | |
619 | specifications, and that section is only applied for hosts that | |
620 | match one of the patterns given in the specification. | |
621 | The matched host name is the one given on the command line. | |
bf740959 | 622 | .Pp |
623 | Since the first obtained value for each parameter is used, more | |
624 | host-specific declarations should be given near the beginning of the | |
625 | file, and general defaults at the end. | |
626 | .Pp | |
627 | The configuration file has the following format: | |
628 | .Pp | |
629 | Empty lines and lines starting with | |
630 | .Ql # | |
631 | are comments. | |
632 | .Pp | |
633 | Otherwise a line is of the format | |
634 | .Dq keyword arguments . | |
635 | The possible | |
636 | keywords and their meanings are as follows (note that the | |
637 | configuration files are case-sensitive): | |
638 | .Bl -tag -width Ds | |
639 | .It Cm Host | |
640 | Restricts the following declarations (up to the next | |
641 | .Cm Host | |
642 | keyword) to be only for those hosts that match one of the patterns | |
643 | given after the keyword. | |
644 | .Ql \&* | |
645 | and | |
646 | .Ql ? | |
647 | can be used as wildcards in the | |
610cd5c6 | 648 | patterns. |
649 | A single | |
bf740959 | 650 | .Ql \&* |
651 | as a pattern can be used to provide global | |
610cd5c6 | 652 | defaults for all hosts. |
653 | The host is the | |
bf740959 | 654 | .Ar hostname |
655 | argument given on the command line (i.e., the name is not converted to | |
656 | a canonicalized host name before matching). | |
657 | .It Cm AFSTokenPassing | |
4fe2af09 | 658 | Specifies whether to pass AFS tokens to remote host. |
659 | The argument to this keyword must be | |
bf740959 | 660 | .Dq yes |
661 | or | |
662 | .Dq no . | |
da89cf4d | 663 | This option applies to protocol version 1 only. |
bf740959 | 664 | .It Cm BatchMode |
665 | If set to | |
666 | .Dq yes , | |
610cd5c6 | 667 | passphrase/password querying will be disabled. |
668 | This option is useful in scripts and other batch jobs where you have no | |
669 | user to supply the password. | |
670 | The argument must be | |
bf740959 | 671 | .Dq yes |
672 | or | |
673 | .Dq no . | |
01ce749f | 674 | The default is |
675 | .Dq no . | |
3435f5a6 | 676 | .It Cm BindAddress |
677 | Specify the interface to transmit from on machines with multiple | |
678 | interfaces or aliased addresses. | |
679 | Note that this option does not work if | |
680 | .Cm UsePrivilegedPort | |
681 | is set to | |
682 | .Dq yes . | |
7b2ea3a1 | 683 | .It Cm CheckHostIP |
684 | If this flag is set to | |
685 | .Dq yes , | |
01ce749f | 686 | ssh will additionally check the host IP address in the |
7b2ea3a1 | 687 | .Pa known_hosts |
4fe2af09 | 688 | file. |
689 | This allows ssh to detect if a host key changed due to DNS spoofing. | |
7b2ea3a1 | 690 | If the option is set to |
691 | .Dq no , | |
692 | the check will not be executed. | |
01ce749f | 693 | The default is |
694 | .Dq yes . | |
bf740959 | 695 | .It Cm Cipher |
33de75a3 | 696 | Specifies the cipher to use for encrypting the session |
a22aff1f | 697 | in protocol version 1. |
610cd5c6 | 698 | Currently, |
3730bb22 | 699 | .Dq blowfish |
bf740959 | 700 | and |
701 | .Dq 3des | |
610cd5c6 | 702 | are supported. |
703 | The default is | |
bf740959 | 704 | .Dq 3des . |
f54651ce | 705 | .It Cm Ciphers |
706 | Specifies the ciphers allowed for protocol version 2 | |
707 | in order of preference. | |
708 | Multiple ciphers must be comma-separated. | |
709 | The default is | |
9afadca8 | 710 | .Pp |
711 | .Bd -literal | |
f2ba0775 | 712 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, |
da89cf4d | 713 | aes192-cbc,aes256-cbc'' |
9afadca8 | 714 | .Ed |
bf740959 | 715 | .It Cm Compression |
610cd5c6 | 716 | Specifies whether to use compression. |
717 | The argument must be | |
bf740959 | 718 | .Dq yes |
719 | or | |
720 | .Dq no . | |
01ce749f | 721 | The default is |
722 | .Dq no . | |
bf740959 | 723 | .It Cm CompressionLevel |
01ce749f | 724 | Specifies the compression level to use if compression is enabled. |
610cd5c6 | 725 | The argument must be an integer from 1 (fast) to 9 (slow, best). |
726 | The default level is 6, which is good for most applications. | |
727 | The meaning of the values is the same as in | |
7b2ea3a1 | 728 | .Xr gzip 1 . |
da89cf4d | 729 | Note that this option applies to protocol version 1 only. |
bf740959 | 730 | .It Cm ConnectionAttempts |
731 | Specifies the number of tries (one per second) to make before falling | |
610cd5c6 | 732 | back to rsh or exiting. |
733 | The argument must be an integer. | |
734 | This may be useful in scripts if the connection sometimes fails. | |
ce773142 | 735 | The default is 1. |
bf740959 | 736 | .It Cm EscapeChar |
737 | Sets the escape character (default: | |
738 | .Ql ~ ) . | |
739 | The escape character can also | |
610cd5c6 | 740 | be set on the command line. |
741 | The argument should be a single character, | |
bf740959 | 742 | .Ql ^ |
743 | followed by a letter, or | |
744 | .Dq none | |
745 | to disable the escape | |
746 | character entirely (making the connection transparent for binary | |
747 | data). | |
f54651ce | 748 | .It Cm FallBackToRsh |
bf740959 | 749 | Specifies that if connecting via |
750 | .Nm | |
751 | fails due to a connection refused error (there is no | |
752 | .Xr sshd 8 | |
f54651ce | 753 | listening on the remote host), |
bf740959 | 754 | .Xr rsh 1 |
755 | should automatically be used instead (after a suitable warning about | |
610cd5c6 | 756 | the session being unencrypted). |
757 | The argument must be | |
bf740959 | 758 | .Dq yes |
759 | or | |
760 | .Dq no . | |
01ce749f | 761 | The default is |
762 | .Dq no . | |
bf740959 | 763 | .It Cm ForwardAgent |
764 | Specifies whether the connection to the authentication agent (if any) | |
610cd5c6 | 765 | will be forwarded to the remote machine. |
766 | The argument must be | |
bf740959 | 767 | .Dq yes |
768 | or | |
769 | .Dq no . | |
71276795 | 770 | The default is |
771 | .Dq no . | |
bf740959 | 772 | .It Cm ForwardX11 |
773 | Specifies whether X11 connections will be automatically redirected | |
f54651ce | 774 | over the secure channel and |
bf740959 | 775 | .Ev DISPLAY |
610cd5c6 | 776 | set. |
f54651ce | 777 | The argument must be |
bf740959 | 778 | .Dq yes |
779 | or | |
780 | .Dq no . | |
c8d54615 | 781 | The default is |
782 | .Dq no . | |
bf740959 | 783 | .It Cm GatewayPorts |
784 | Specifies whether remote hosts are allowed to connect to local | |
785 | forwarded ports. | |
786 | The argument must be | |
787 | .Dq yes | |
788 | or | |
789 | .Dq no . | |
790 | The default is | |
791 | .Dq no . | |
792 | .It Cm GlobalKnownHostsFile | |
f49bc4f7 | 793 | Specifies a file to use for the global |
a5df12e9 | 794 | host key database instead of |
5f4fdfae | 795 | .Pa /etc/ssh_known_hosts . |
da89cf4d | 796 | .It Cm HostbasedAuthentication |
797 | Specifies whether to try rhosts based authentication with public key | |
798 | authentication. | |
799 | The argument must be | |
800 | .Dq yes | |
801 | or | |
802 | .Dq no . | |
803 | The default is | |
3398dda9 | 804 | .Dq no . |
da89cf4d | 805 | This option applies to protocol version 2 only and |
806 | is similar to | |
807 | .Cm RhostsRSAAuthentication . | |
e961a8f9 | 808 | .It Cm HostKeyAlgorithms |
98143cfc | 809 | Specifies the protocol version 2 host key algorithms |
e961a8f9 | 810 | that the client wants to use in order of preference. |
811 | The default for this option is: | |
812 | .Dq ssh-rsa,ssh-dss | |
da89cf4d | 813 | .It Cm HostKeyAlias |
814 | Specifies an alias that should be used instead of the | |
815 | real host name when looking up or saving the host key | |
816 | in the host key database files. | |
817 | This option is useful for tunneling ssh connections | |
818 | or if you have multiple servers running on a single host. | |
bf740959 | 819 | .It Cm HostName |
610cd5c6 | 820 | Specifies the real host name to log into. |
821 | This can be used to specify nicknames or abbreviations for hosts. | |
822 | Default is the name given on the command line. | |
823 | Numeric IP addresses are also permitted (both on the command line and in | |
bf740959 | 824 | .Cm HostName |
825 | specifications). | |
826 | .It Cm IdentityFile | |
da89cf4d | 827 | Specifies the file from which the user's RSA or DSA authentication identity |
bf740959 | 828 | is read (default |
1d1ffb87 | 829 | .Pa $HOME/.ssh/identity |
bf740959 | 830 | in the user's home directory). |
831 | Additionally, any identities represented by the authentication agent | |
610cd5c6 | 832 | will be used for authentication. |
833 | The file name may use the tilde | |
834 | syntax to refer to a user's home directory. | |
835 | It is possible to have | |
bf740959 | 836 | multiple identity files specified in configuration files; all these |
837 | identities will be tried in sequence. | |
838 | .It Cm KeepAlive | |
839 | Specifies whether the system should send keepalive messages to the | |
610cd5c6 | 840 | other side. |
841 | If they are sent, death of the connection or crash of one | |
842 | of the machines will be properly noticed. | |
843 | However, this means that | |
bf740959 | 844 | connections will die if the route is down temporarily, and some people |
4fe2af09 | 845 | find it annoying. |
bf740959 | 846 | .Pp |
847 | The default is | |
848 | .Dq yes | |
849 | (to send keepalives), and the client will notice | |
610cd5c6 | 850 | if the network goes down or the remote host dies. |
851 | This is important in scripts, and many users want it too. | |
bf740959 | 852 | .Pp |
853 | To disable keepalives, the value should be set to | |
854 | .Dq no | |
855 | in both the server and the client configuration files. | |
856 | .It Cm KerberosAuthentication | |
4fe2af09 | 857 | Specifies whether Kerberos authentication will be used. |
858 | The argument to this keyword must be | |
bf740959 | 859 | .Dq yes |
860 | or | |
861 | .Dq no . | |
862 | .It Cm KerberosTgtPassing | |
4fe2af09 | 863 | Specifies whether a Kerberos TGT will be forwarded to the server. |
864 | This will only work if the Kerberos server is actually an AFS kaserver. | |
865 | The argument to this keyword must be | |
bf740959 | 866 | .Dq yes |
867 | or | |
868 | .Dq no . | |
869 | .It Cm LocalForward | |
870 | Specifies that a TCP/IP port on the local machine be forwarded over | |
610cd5c6 | 871 | the secure channel to given host:port from the remote machine. |
872 | The first argument must be a port number, and the second must be | |
873 | host:port. | |
874 | Multiple forwardings may be specified, and additional | |
875 | forwardings can be given on the command line. | |
876 | Only the superuser can forward privileged ports. | |
6a17f9c2 | 877 | .It Cm LogLevel |
878 | Gives the verbosity level that is used when logging messages from | |
879 | .Nm ssh . | |
880 | The possible values are: | |
59c97189 | 881 | QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
882 | The default is INFO. | |
b2552997 | 883 | .It Cm MACs |
3730bb22 | 884 | Specifies the MAC (message authentication code) algorithms |
b2552997 | 885 | in order of preference. |
886 | The MAC algorithm is used in protocol version 2 | |
887 | for data integrity protection. | |
888 | Multiple algorithms must be comma-separated. | |
889 | The default is | |
47bf6266 | 890 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
bf740959 | 891 | .It Cm NumberOfPasswordPrompts |
4fe2af09 | 892 | Specifies the number of password prompts before giving up. |
893 | The argument to this keyword must be an integer. | |
894 | Default is 3. | |
7b2ea3a1 | 895 | .It Cm PasswordAuthentication |
610cd5c6 | 896 | Specifies whether to use password authentication. |
897 | The argument to this keyword must be | |
7b2ea3a1 | 898 | .Dq yes |
899 | or | |
900 | .Dq no . | |
01ce749f | 901 | The default is |
902 | .Dq yes . | |
bf740959 | 903 | .It Cm Port |
610cd5c6 | 904 | Specifies the port number to connect on the remote host. |
905 | Default is 22. | |
babd91d4 | 906 | .It Cm PreferredAuthentications |
3730bb22 | 907 | Specifies the order in which the client should try protocol 2 |
908 | authentication methods. This allows a client to prefer one method (e.g. | |
babd91d4 | 909 | .Cm keyboard-interactive ) |
910 | over another method (e.g. | |
911 | .Cm password ) | |
912 | The default for this option is: | |
29c440a0 | 913 | .Dq hostbased,publickey,keyboard-interactive,password |
f54651ce | 914 | .It Cm Protocol |
915 | Specifies the protocol versions | |
916 | .Nm | |
917 | should support in order of preference. | |
918 | The possible values are | |
919 | .Dq 1 | |
920 | and | |
921 | .Dq 2 . | |
922 | Multiple versions must be comma-separated. | |
923 | The default is | |
b4a19d21 | 924 | .Dq 2,1 . |
1d1ffb87 | 925 | This means that |
926 | .Nm | |
b4a19d21 | 927 | tries version 2 and falls back to version 1 |
928 | if version 2 is not available. | |
bf740959 | 929 | .It Cm ProxyCommand |
610cd5c6 | 930 | Specifies the command to use to connect to the server. |
931 | The command | |
932 | string extends to the end of the line, and is executed with | |
933 | .Pa /bin/sh . | |
934 | In the command string, | |
935 | .Ql %h | |
936 | will be substituted by the host name to | |
937 | connect and | |
938 | .Ql %p | |
939 | by the port. | |
940 | The command can be basically anything, | |
941 | and should read from its standard input and write to its standard output. | |
942 | It should eventually connect an | |
bf740959 | 943 | .Xr sshd 8 |
944 | server running on some machine, or execute | |
945 | .Ic sshd -i | |
610cd5c6 | 946 | somewhere. |
947 | Host key management will be done using the | |
bf740959 | 948 | HostName of the host being connected (defaulting to the name typed by |
949 | the user). | |
57112b5a | 950 | Note that |
951 | .Cm CheckHostIP | |
952 | is not available for connects with a proxy command. | |
bf740959 | 953 | .Pp |
da89cf4d | 954 | .It Cm PubkeyAuthentication |
955 | Specifies whether to try public key authentication. | |
956 | The argument to this keyword must be | |
957 | .Dq yes | |
958 | or | |
959 | .Dq no . | |
960 | The default is | |
961 | .Dq yes . | |
962 | This option applies to protocol version 2 only. | |
bf740959 | 963 | .It Cm RemoteForward |
964 | Specifies that a TCP/IP port on the remote machine be forwarded over | |
610cd5c6 | 965 | the secure channel to given host:port from the local machine. |
966 | The first argument must be a port number, and the second must be | |
967 | host:port. | |
968 | Multiple forwardings may be specified, and additional | |
969 | forwardings can be given on the command line. | |
970 | Only the superuser can forward privileged ports. | |
bf740959 | 971 | .It Cm RhostsAuthentication |
610cd5c6 | 972 | Specifies whether to try rhosts based authentication. |
973 | Note that this | |
bf740959 | 974 | declaration only affects the client side and has no effect whatsoever |
610cd5c6 | 975 | on security. |
976 | Disabling rhosts authentication may reduce | |
bf740959 | 977 | authentication time on slow connections when rhosts authentication is |
610cd5c6 | 978 | not used. |
979 | Most servers do not permit RhostsAuthentication because it | |
3730bb22 | 980 | is not secure (see |
a5a2da3b | 981 | .Cm RhostsRSAAuthentication ) . |
610cd5c6 | 982 | The argument to this keyword must be |
bf740959 | 983 | .Dq yes |
984 | or | |
985 | .Dq no . | |
01ce749f | 986 | The default is |
987 | .Dq yes . | |
da89cf4d | 988 | This option applies to protocol version 1 only. |
bf740959 | 989 | .It Cm RhostsRSAAuthentication |
990 | Specifies whether to try rhosts based authentication with RSA host | |
610cd5c6 | 991 | authentication. |
610cd5c6 | 992 | The argument must be |
bf740959 | 993 | .Dq yes |
994 | or | |
995 | .Dq no . | |
01ce749f | 996 | The default is |
997 | .Dq yes . | |
da89cf4d | 998 | This option applies to protocol version 1 only. |
bf740959 | 999 | .It Cm RSAAuthentication |
610cd5c6 | 1000 | Specifies whether to try RSA authentication. |
1001 | The argument to this keyword must be | |
bf740959 | 1002 | .Dq yes |
1003 | or | |
1004 | .Dq no . | |
1005 | RSA authentication will only be | |
1006 | attempted if the identity file exists, or an authentication agent is | |
1007 | running. | |
01ce749f | 1008 | The default is |
1009 | .Dq yes . | |
1d1ffb87 | 1010 | Note that this option applies to protocol version 1 only. |
61e96248 | 1011 | .It Cm ChallengeResponseAuthentication |
1012 | Specifies whether to use challenge response authentication. | |
610cd5c6 | 1013 | The argument to this keyword must be |
5260325f | 1014 | .Dq yes |
1015 | or | |
1016 | .Dq no . | |
1017 | The default is | |
10a2cbef | 1018 | .Dq yes . |
bf740959 | 1019 | .It Cm StrictHostKeyChecking |
1020 | If this flag is set to | |
f54651ce | 1021 | .Dq yes , |
bf740959 | 1022 | .Nm |
a877488a | 1023 | will never automatically add host keys to the |
bf740959 | 1024 | .Pa $HOME/.ssh/known_hosts |
f49bc4f7 | 1025 | file, and refuses to connect to hosts whose host key has changed. |
610cd5c6 | 1026 | This provides maximum protection against trojan horse attacks. |
1027 | However, it can be somewhat annoying if you don't have good | |
5f4fdfae | 1028 | .Pa /etc/ssh_known_hosts |
bf740959 | 1029 | files installed and frequently |
a877488a | 1030 | connect to new hosts. |
1031 | This option forces the user to manually | |
1032 | add all new hosts. | |
1033 | If this flag is set to | |
1034 | .Dq no , | |
1035 | .Nm | |
1036 | will automatically add new host keys to the | |
1037 | user known hosts files. | |
1038 | If this flag is set to | |
1039 | .Dq ask , | |
1040 | new host keys | |
1041 | will be added to the user known host files only after the user | |
1042 | has confirmed that is what they really want to do, and | |
1043 | .Nm | |
1044 | will refuse to connect to hosts whose host key has changed. | |
610cd5c6 | 1045 | The host keys of |
a877488a | 1046 | known hosts will be verified automatically in all cases. |
610cd5c6 | 1047 | The argument must be |
a877488a | 1048 | .Dq yes , |
1049 | .Dq no | |
bf740959 | 1050 | or |
a877488a | 1051 | .Dq ask . |
1052 | The default is | |
1053 | .Dq ask . | |
bf740959 | 1054 | .It Cm UsePrivilegedPort |
1055 | Specifies whether to use a privileged port for outgoing connections. | |
1056 | The argument must be | |
1057 | .Dq yes | |
1058 | or | |
1059 | .Dq no . | |
1060 | The default is | |
d5ebca2b | 1061 | .Dq no . |
da89cf4d | 1062 | Note that you need to set this option to |
1063 | .Dq yes | |
1064 | if you want to use | |
bf740959 | 1065 | .Cm RhostsAuthentication |
1066 | and | |
6ffc9c88 | 1067 | .Cm RhostsRSAAuthentication |
da89cf4d | 1068 | with older servers. |
7b2ea3a1 | 1069 | .It Cm User |
610cd5c6 | 1070 | Specifies the user to log in as. |
1071 | This can be useful if you have a different user name on different machines. | |
1072 | This saves the trouble of | |
7b2ea3a1 | 1073 | having to remember to give the user name on the command line. |
1074 | .It Cm UserKnownHostsFile | |
f49bc4f7 | 1075 | Specifies a file to use for the user |
a5df12e9 | 1076 | host key database instead of |
7b2ea3a1 | 1077 | .Pa $HOME/.ssh/known_hosts . |
bf740959 | 1078 | .It Cm UseRsh |
610cd5c6 | 1079 | Specifies that rlogin/rsh should be used for this host. |
1080 | It is possible that the host does not at all support the | |
bf740959 | 1081 | .Nm |
610cd5c6 | 1082 | protocol. |
1083 | This causes | |
bf740959 | 1084 | .Nm |
610cd5c6 | 1085 | to immediately execute |
bf740959 | 1086 | .Xr rsh 1 . |
1087 | All other options (except | |
1088 | .Cm HostName ) | |
610cd5c6 | 1089 | are ignored if this has been specified. |
1090 | The argument must be | |
bf740959 | 1091 | .Dq yes |
1092 | or | |
1093 | .Dq no . | |
fa649821 | 1094 | .It Cm XAuthLocation |
1095 | Specifies the location of the | |
1096 | .Xr xauth 1 | |
1097 | program. | |
1098 | The default is | |
1099 | .Pa /usr/X11R6/bin/xauth . | |
b5e300c2 | 1100 | .El |
bf740959 | 1101 | .Sh ENVIRONMENT |
1102 | .Nm | |
1103 | will normally set the following environment variables: | |
1104 | .Bl -tag -width Ds | |
1105 | .It Ev DISPLAY | |
1106 | The | |
1107 | .Ev DISPLAY | |
610cd5c6 | 1108 | variable indicates the location of the X11 server. |
f54651ce | 1109 | It is automatically set by |
bf740959 | 1110 | .Nm |
1111 | to point to a value of the form | |
1112 | .Dq hostname:n | |
1113 | where hostname indicates | |
610cd5c6 | 1114 | the host where the shell runs, and n is an integer >= 1. |
1115 | .Nm | |
1116 | uses this special value to forward X11 connections over the secure | |
1117 | channel. | |
da89cf4d | 1118 | The user should normally not set |
1119 | .Ev DISPLAY | |
1120 | explicitly, as that | |
bf740959 | 1121 | will render the X11 connection insecure (and will require the user to |
1122 | manually copy any required authorization cookies). | |
1123 | .It Ev HOME | |
1124 | Set to the path of the user's home directory. | |
1125 | .It Ev LOGNAME | |
1126 | Synonym for | |
1127 | .Ev USER ; | |
1128 | set for compatibility with systems that use this variable. | |
1129 | .It Ev MAIL | |
1130 | Set to point the user's mailbox. | |
610cd5c6 | 1131 | .It Ev PATH |
bf740959 | 1132 | Set to the default |
1133 | .Ev PATH , | |
1134 | as specified when compiling | |
1135 | .Nm ssh . | |
3474b2b4 | 1136 | .It Ev SSH_ASKPASS |
1137 | If | |
1138 | .Nm | |
1139 | needs a passphrase, it will read the passphrase from the current | |
1140 | terminal if it was run from a terminal. | |
1141 | If | |
1142 | .Nm | |
1143 | does not have a terminal associated with it but | |
1144 | .Ev DISPLAY | |
1145 | and | |
1146 | .Ev SSH_ASKPASS | |
1147 | are set, it will execute the program specified by | |
1148 | .Ev SSH_ASKPASS | |
1149 | and open an X11 window to read the passphrase. | |
1150 | This is particularly useful when calling | |
1151 | .Nm | |
1152 | from a | |
1153 | .Pa .Xsession | |
1154 | or related script. | |
1155 | (Note that on some machines it | |
1156 | may be necessary to redirect the input from | |
1157 | .Pa /dev/null | |
1158 | to make this work.) | |
bf740959 | 1159 | .It Ev SSH_AUTH_SOCK |
1160 | indicates the path of a unix-domain socket used to communicate with the | |
1161 | agent. | |
1162 | .It Ev SSH_CLIENT | |
610cd5c6 | 1163 | Identifies the client end of the connection. |
1164 | The variable contains | |
bf740959 | 1165 | three space-separated values: client ip-address, client port number, |
1166 | and server port number. | |
8abcdba4 | 1167 | .It Ev SSH_ORIGINAL_COMMAND |
1168 | The variable contains the original command line if a forced command | |
1169 | is executed. | |
1170 | It can be used to extract the original arguments. | |
bf740959 | 1171 | .It Ev SSH_TTY |
1172 | This is set to the name of the tty (path to the device) associated | |
610cd5c6 | 1173 | with the current shell or command. |
1174 | If the current session has no tty, | |
bf740959 | 1175 | this variable is not set. |
1176 | .It Ev TZ | |
1177 | The timezone variable is set to indicate the present timezone if it | |
c345cf9d | 1178 | was set when the daemon was started (i.e., the daemon passes the value |
bf740959 | 1179 | on to new connections). |
1180 | .It Ev USER | |
1181 | Set to the name of the user logging in. | |
1182 | .El | |
1183 | .Pp | |
f54651ce | 1184 | Additionally, |
bf740959 | 1185 | .Nm |
f54651ce | 1186 | reads |
1187 | .Pa $HOME/.ssh/environment , | |
bf740959 | 1188 | and adds lines of the format |
1189 | .Dq VARNAME=value | |
1190 | to the environment. | |
1191 | .Sh FILES | |
c8d54615 | 1192 | .Bl -tag -width Ds |
f49bc4f7 | 1193 | .It Pa $HOME/.ssh/known_hosts |
bf740959 | 1194 | Records host keys for all hosts the user has logged into (that are not |
1195 | in | |
f49bc4f7 | 1196 | .Pa /etc/ssh_known_hosts . |
bf740959 | 1197 | See |
1198 | .Xr sshd 8 . | |
c0ecc314 | 1199 | .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa |
1200 | Contains the authentication identity of the user. | |
1201 | They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | |
1d1ffb87 | 1202 | These files |
1203 | contain sensitive data and should be readable by the user but not | |
bf740959 | 1204 | accessible by others (read/write/execute). |
1205 | Note that | |
1206 | .Nm | |
1d1ffb87 | 1207 | ignores a private key file if it is accessible by others. |
bf740959 | 1208 | It is possible to specify a passphrase when |
1209 | generating the key; the passphrase will be used to encrypt the | |
1210 | sensitive part of this file using 3DES. | |
c0ecc314 | 1211 | .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub |
bf740959 | 1212 | Contains the public key for authentication (public part of the |
610cd5c6 | 1213 | identity file in human-readable form). |
1d1ffb87 | 1214 | The contents of the |
1215 | .Pa $HOME/.ssh/identity.pub | |
1216 | file should be added to | |
bf740959 | 1217 | .Pa $HOME/.ssh/authorized_keys |
1218 | on all machines | |
c0ecc314 | 1219 | where you wish to log in using protocol version 1 RSA authentication. |
1d1ffb87 | 1220 | The contents of the |
1221 | .Pa $HOME/.ssh/id_dsa.pub | |
c0ecc314 | 1222 | and |
1223 | .Pa $HOME/.ssh/id_rsa.pub | |
1d1ffb87 | 1224 | file should be added to |
96a7b0cc | 1225 | .Pa $HOME/.ssh/authorized_keys |
1d1ffb87 | 1226 | on all machines |
c0ecc314 | 1227 | where you wish to log in using protocol version 2 DSA/RSA authentication. |
1d1ffb87 | 1228 | These files are not |
610cd5c6 | 1229 | sensitive and can (but need not) be readable by anyone. |
1d1ffb87 | 1230 | These files are |
c44559d2 | 1231 | never used automatically and are not necessary; they are only provided for |
bf740959 | 1232 | the convenience of the user. |
1233 | .It Pa $HOME/.ssh/config | |
610cd5c6 | 1234 | This is the per-user configuration file. |
1235 | The format of this file is described above. | |
1236 | This file is used by the | |
bf740959 | 1237 | .Nm |
610cd5c6 | 1238 | client. |
1239 | This file does not usually contain any sensitive information, | |
bf740959 | 1240 | but the recommended permissions are read/write for the user, and not |
1241 | accessible by others. | |
1242 | .It Pa $HOME/.ssh/authorized_keys | |
96a7b0cc | 1243 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. |
610cd5c6 | 1244 | The format of this file is described in the |
bf740959 | 1245 | .Xr sshd 8 |
610cd5c6 | 1246 | manual page. |
f49bc4f7 | 1247 | In the simplest form the format is the same as the .pub |
1248 | identity files. | |
1d1ffb87 | 1249 | This file is not highly sensitive, but the recommended |
1250 | permissions are read/write for the user, and not accessible by others. | |
f49bc4f7 | 1251 | .It Pa /etc/ssh_known_hosts |
610cd5c6 | 1252 | Systemwide list of known host keys. |
f49bc4f7 | 1253 | This file should be prepared by the |
bf740959 | 1254 | system administrator to contain the public host keys of all machines in the |
610cd5c6 | 1255 | organization. |
1256 | This file should be world-readable. | |
1257 | This file contains | |
bf740959 | 1258 | public keys, one per line, in the following format (fields separated |
f49bc4f7 | 1259 | by spaces): system name, public key and optional comment field. |
610cd5c6 | 1260 | When different names are used |
bf740959 | 1261 | for the same machine, all such names should be listed, separated by |
610cd5c6 | 1262 | commas. |
1263 | The format is described on the | |
bf740959 | 1264 | .Xr sshd 8 |
1265 | manual page. | |
1266 | .Pp | |
1267 | The canonical system name (as returned by name servers) is used by | |
1268 | .Xr sshd 8 | |
1269 | to verify the client host when logging in; other names are needed because | |
1270 | .Nm | |
1271 | does not convert the user-supplied name to a canonical name before | |
1272 | checking the key, because someone with access to the name servers | |
1273 | would then be able to fool host authentication. | |
5f4fdfae | 1274 | .It Pa /etc/ssh_config |
610cd5c6 | 1275 | Systemwide configuration file. |
1276 | This file provides defaults for those | |
bf740959 | 1277 | values that are not specified in the user's configuration file, and |
610cd5c6 | 1278 | for those users who do not have a configuration file. |
1279 | This file must be world-readable. | |
bf740959 | 1280 | .It Pa $HOME/.rhosts |
1281 | This file is used in | |
1282 | .Pa \&.rhosts | |
1283 | authentication to list the | |
610cd5c6 | 1284 | host/user pairs that are permitted to log in. |
1285 | (Note that this file is | |
bf740959 | 1286 | also used by rlogin and rsh, which makes using this file insecure.) |
1287 | Each line of the file contains a host name (in the canonical form | |
1288 | returned by name servers), and then a user name on that host, | |
610cd5c6 | 1289 | separated by a space. |
e91c60f2 | 1290 | On some machines this file may need to be |
bf740959 | 1291 | world-readable if the user's home directory is on a NFS partition, |
1292 | because | |
1293 | .Xr sshd 8 | |
610cd5c6 | 1294 | reads it as root. |
1295 | Additionally, this file must be owned by the user, | |
1296 | and must not have write permissions for anyone else. | |
1297 | The recommended | |
bf740959 | 1298 | permission for most machines is read/write for the user, and not |
1299 | accessible by others. | |
1300 | .Pp | |
1301 | Note that by default | |
1302 | .Xr sshd 8 | |
1303 | will be installed so that it requires successful RSA host | |
610cd5c6 | 1304 | authentication before permitting \s+2.\s0rhosts authentication. |
1305 | If your server machine does not have the client's host key in | |
5f4fdfae | 1306 | .Pa /etc/ssh_known_hosts , |
bf740959 | 1307 | you can store it in |
1308 | .Pa $HOME/.ssh/known_hosts . | |
1309 | The easiest way to do this is to | |
1310 | connect back to the client from the server machine using ssh; this | |
1d1ffb87 | 1311 | will automatically add the host key to |
bf740959 | 1312 | .Pa $HOME/.ssh/known_hosts . |
1313 | .It Pa $HOME/.shosts | |
1314 | This file is used exactly the same way as | |
1315 | .Pa \&.rhosts . | |
1316 | The purpose for | |
1317 | having this file is to be able to use rhosts authentication with | |
1318 | .Nm | |
1319 | without permitting login with | |
1320 | .Xr rlogin 1 | |
1321 | or | |
1322 | .Xr rsh 1 . | |
1323 | .It Pa /etc/hosts.equiv | |
1324 | This file is used during | |
610cd5c6 | 1325 | .Pa \&.rhosts authentication. |
1326 | It contains | |
bf740959 | 1327 | canonical hosts names, one per line (the full format is described on |
1328 | the | |
1329 | .Xr sshd 8 | |
610cd5c6 | 1330 | manual page). |
1331 | If the client host is found in this file, login is | |
bf740959 | 1332 | automatically permitted provided client and server user names are the |
610cd5c6 | 1333 | same. |
1334 | Additionally, successful RSA host authentication is normally | |
1335 | required. | |
1336 | This file should only be writable by root. | |
5f4fdfae | 1337 | .It Pa /etc/shosts.equiv |
f54651ce | 1338 | This file is processed exactly as |
bf740959 | 1339 | .Pa /etc/hosts.equiv . |
1340 | This file may be useful to permit logins using | |
1341 | .Nm | |
1342 | but not using rsh/rlogin. | |
5f4fdfae | 1343 | .It Pa /etc/sshrc |
bf740959 | 1344 | Commands in this file are executed by |
1345 | .Nm | |
1346 | when the user logs in just before the user's shell (or command) is started. | |
1347 | See the | |
1348 | .Xr sshd 8 | |
1349 | manual page for more information. | |
1350 | .It Pa $HOME/.ssh/rc | |
1351 | Commands in this file are executed by | |
1352 | .Nm | |
1353 | when the user logs in just before the user's shell (or command) is | |
1354 | started. | |
f54651ce | 1355 | See the |
bf740959 | 1356 | .Xr sshd 8 |
1357 | manual page for more information. | |
83b7f649 | 1358 | .It Pa $HOME/.ssh/environment |
1359 | Contains additional definitions for environment variables, see section | |
1360 | .Sx ENVIRONMENT | |
1361 | above. | |
b5e300c2 | 1362 | .El |
fa08c86b | 1363 | .Sh AUTHORS |
5fb622e4 | 1364 | OpenSSH is a derivative of the original and free |
1365 | ssh 1.2.12 release by Tatu Ylonen. | |
1366 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1367 | Theo de Raadt and Dug Song | |
1368 | removed many bugs, re-added newer features and | |
1369 | created OpenSSH. | |
1370 | Markus Friedl contributed the support for SSH | |
1371 | protocol versions 1.5 and 2.0. | |
bf740959 | 1372 | .Sh SEE ALSO |
1373 | .Xr rlogin 1 , | |
1374 | .Xr rsh 1 , | |
1375 | .Xr scp 1 , | |
61e96248 | 1376 | .Xr sftp 1 , |
bf740959 | 1377 | .Xr ssh-add 1 , |
1378 | .Xr ssh-agent 1 , | |
1379 | .Xr ssh-keygen 1 , | |
1380 | .Xr telnet 1 , | |
9afadca8 | 1381 | .Xr sshd 8 |
2cad6cef | 1382 | .Rs |
1383 | .%A T. Ylonen | |
1384 | .%A T. Kivinen | |
1385 | .%A M. Saarinen | |
1386 | .%A T. Rinne | |
1387 | .%A S. Lehtinen | |
1388 | .%T "SSH Protocol Architecture" | |
1389 | .%N draft-ietf-secsh-architecture-07.txt | |
1390 | .%D January 2001 | |
1391 | .%O work in progress material | |
1392 | .Re |