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8 .TH SSH_CONFIG 4 "Jan 17, 2013"
9 .SH NAME
10 ssh_config \- ssh configuration file
11 .SH SYNOPSIS
12 .LP
13 .nf
14 \fB/etc/ssh/ssh_config\fR
15 .fi
17 .LP
18 .nf
19 \fB$HOME/.ssh/config\fR
20 .fi
22 .SH DESCRIPTION
23 .sp
24 .LP
25 The first \fBssh_config\fR path, above, provides the system-wide defaults for
26 \fBssh\fR(1). The second version is user-specific defaults for \fBssh\fR.
27 .sp
28 .LP
29 \fBssh\fR obtains configuration data from the following sources, in this order:
30 command line options, user's configuration file (\fB$HOME/.ssh/config\fR), and
31 system-wide configuration file (\fB/etc/ssh/ssh_config\fR). For each parameter,
32 the first obtained value is used. The configuration files contain sections
33 bracketed by \fBHost\fR specifications, and that section is applied only for
34 hosts that match one of the patterns given in the specification. The matched
35 host name is the one given on the command line.
36 .sp
37 .LP
38 Since the first obtained value for each parameter is used, host-specific
39 declarations should be given near the beginning of the file, and general
40 defaults at the end.
41 .sp
42 .LP
43 The configuration file has the following format and syntax:
44 .RS +4
45 .TP
46 .ie t \(bu
47 .el o
48 Empty lines and lines starting with \fB#\fR are comments.
49 .RE
50 .RS +4
51 .TP
52 .ie t \(bu
53 .el o
54 Non-commented lines are of the form:
55 .sp
56 .in +2
57 .nf
58 \fIkeyword\fR \fIarguments\fR
59 .fi
60 .in -2
61 .sp
63 .RE
64 .RS +4
65 .TP
66 .ie t \(bu
67 .el o
68 Configuration options can be separated by white space or optional whitespace
69 and exactly one equal sign. The latter format allows you to avoid the need to
70 quote white space when specifying configuration options using the \fB-o\fR
71 option to \fBssh\fR, \fBscp\fR, and \fBsftp\fR.
72 .RE
73 .sp
74 .LP
75 The possible keywords and their meanings are listed in the following
76 list.Keywords are case-insensitive and arguments are case-sensitive.
77 .sp
78 .ne 2
79 .na
80 \fB\fBBatchMode\fR\fR
81 .ad
82 .sp .6
83 .RS 4n
84 The argument must be \fByes\fR or \fBno\fR. If set to \fByes\fR,
85 passphrase/password querying is disabled. This option is useful in scripts and
86 other batch jobs where you have no user to supply the password.
87 .RE
89 .sp
90 .ne 2
91 .na
92 \fB\fBBindAddress\fR\fR
93 .ad
94 .sp .6
95 .RS 4n
96 Specify the interface to transmit from on machines with multiple interfaces or
97 aliased addresses. This option does not work if \fBUsePrivilegedPort\fR is set
98 to \fByes\fR.
99 .RE
101 .sp
102 .ne 2
103 .na
104 \fB\fBCheckHostIP\fR\fR
105 .ad
106 .sp .6
107 .RS 4n
108 If this flag is set to \fByes\fR, \fBssh\fR additionally checks the host IP
109 address in the \fBknown_hosts\fR file. This allows \fBssh\fR to detect if a
110 host key changed due to DNS spoofing. If the option is set to \fBno\fR, the
111 check is not executed.
112 .RE
114 .sp
115 .ne 2
116 .na
117 \fB\fBCipher\fR\fR
118 .ad
119 .sp .6
120 .RS 4n
121 Specifies the cipher to use for encrypting the session in protocol version 1.
122 Only a single cipher can be specified. Currently, \fBblowfish, 3des,\fR and
123 \fBdes\fR are supported. \fB3des\fR (triple-\fBdes\fR) is an
124 encrypt-decrypt-encrypt triple with three different keys. It is believed to be
125 secure. \fBblowfish\fR is a fast block cipher. It appears very secure and is
126 much faster than \fB3des\fR. \fBdes\fR is only supported in the \fBssh\fR
127 client for interoperability with legacy protocol 1 implementations that do not
128 support the \fB3des\fR cipher. Its use is strongly discouraged due to
129 cryptographic weaknesses. The default is \fB3des\fR.
130 .RE
132 .sp
133 .ne 2
134 .na
135 \fB\fBCiphers\fR\fR
136 .ad
137 .sp .6
138 .RS 4n
139 Specifies the ciphers allowed for protocol version 2 in order of preference.
140 Multiple ciphers must be comma separated.
141 .sp
142 The default cipher list contains all supported ciphers in this order:
143 .sp
144 .in +2
145 .nf
146 aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, aes128-cbc,
147 aes192-cbc, aes256-cbc, arcfour, 3des-cbc,blowfish-cbc
148 .fi
149 .in -2
150 .sp
152 While CBC modes are not considered as secure as other modes in connection with
153 the SSH protocol 2 they are present at the back of the default client cipher
154 list for backward compatibility with SSH servers that do not support other
155 cipher modes.
156 .RE
158 .sp
159 .ne 2
160 .na
161 \fB\fBClearAllForwardings\fR\fR
162 .ad
163 .sp .6
164 .RS 4n
165 Specifies that all local, remote, and dynamic port forwardings specified in the
166 configuration files or on the command line be cleared. This option is primarily
167 useful when used from the \fBssh\fR command line to clear port forwardings set
168 in configuration files and is automatically set by \fBscp\fR(1) and
169 \fBsftp\fR(1). The argument must be \fByes\fR or \fBno\fR. The default is
170 \fBno\fR.
171 .RE
173 .sp
174 .ne 2
175 .na
176 \fB\fBCompression\fR\fR
177 .ad
178 .sp .6
179 .RS 4n
180 Specifies whether to use compression. The argument must be \fByes\fR or
181 \fBno\fR. Defaults to \fBno\fR.
182 .RE
184 .sp
185 .ne 2
186 .na
187 \fB\fBCompressionLevel\fR\fR
188 .ad
189 .sp .6
190 .RS 4n
191 Specifies the compression level to use if compression is enabled. The argument
192 must be an integer from 1 (fast) to 9 (slow, best). The default level is 6,
193 which is good for most applications. This option applies to protocol version 1
194 only.
195 .RE
197 .sp
198 .ne 2
199 .na
200 \fB\fBConnectionAttempts\fR\fR
201 .ad
202 .sp .6
203 .RS 4n
204 Specifies the number of tries (one per second) to make before falling back to
205 \fBrsh\fR or exiting. The argument must be an integer. This can be useful in
206 scripts if the connection sometimes fails. The default is 1.
207 .RE
209 .sp
210 .ne 2
211 .na
212 \fB\fBConnectTimeout\fR\fR
213 .ad
214 .sp .6
215 .RS 4n
216 Specifies the timeout (in seconds) used when connecting to the \fBssh\fR
217 server, instead of using the default system TCP timeout. This value is used
218 only when the target is down or truly unreachable, not when it refuses the
219 connection.
220 .RE
222 .sp
223 .ne 2
224 .na
225 \fB\fBDisableBanner\fR\fR
226 .ad
227 .sp .6
228 .RS 4n
229 If set to \fByes\fR, disables the display of the banner message. If set to
230 \fBin-exec-mode\fR, disables the display of banner message when in remote
231 command mode only.
232 .sp
233 The default value is \fBno\fR, which means that the banner is displayed unless
234 the log level is \fBQUIET\fR, \fBFATAL\fR, or \fBERROR\fR. See also the
235 \fBBanner\fR option in \fBsshd_config\fR(4). This option applies to protocol
236 version 2 only.
237 .RE
239 .sp
240 .ne 2
241 .na
242 \fB\fBDynamicForward\fR\fR
243 .ad
244 .sp .6
245 .RS 4n
246 Specifies that a TCP/IP port on the local machine be forwarded over the secure
247 channel. The application protocol is then used to determine where to connect to
248 from the remote machine.
249 .sp
250 The argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR. IPv6
251 addresses can be specified by enclosing addresses in square brackets or by
252 using an alternative syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR. By
253 default, the local port is bound in accordance with the \fBGatewayPorts\fR
254 setting. However, an explicit \fIbind_address\fR can be used to bind the
255 connection to a specific address. The \fIbind_address\fR of \fBlocalhost\fR
256 indicates that the listening port be bound for local use only, while an empty
257 address or \fB*\fR indicates that the port should be available from all
258 interfaces.
259 .sp
260 Currently the \fBSOCKS4\fR and \fBSOCKS5\fR protocols are supported, and
261 \fBssh\fR acts as a \fBSOCKS\fR server. Multiple forwardings can be specified
262 and additional forwardings can be specified on the command line. Only a user
263 with enough privileges can forward privileged ports.
264 .RE
266 .sp
267 .ne 2
268 .na
269 \fB\fBEscapeChar\fR\fR
270 .ad
271 .sp .6
272 .RS 4n
273 Sets the escape character. The default is tilde (\fB~\fR). The escape character
274 can also be set on the command line. The argument should be a single character,
275 \fB^\fR, followed by a letter, or \fBnone\fR to disable the escape character
276 entirely (making the connection transparent for binary data).
277 .RE
279 .sp
280 .ne 2
281 .na
282 \fB\fBFallBackToRsh\fR\fR
283 .ad
284 .sp .6
285 .RS 4n
286 Specifies that if connecting with \fBssh\fR fails due to a connection refused
287 error (there is no \fBsshd\fR(1M) listening on the remote host), \fBrsh\fR(1)
288 should automatically be used instead (after a suitable warning about the
289 session being unencrypted). The argument must be \fByes\fR or \fBno\fR.
290 .RE
292 .sp
293 .ne 2
294 .na
295 \fB\fBForwardAgent\fR\fR
296 .ad
297 .sp .6
298 .RS 4n
299 Specifies whether the connection to the authentication agent (if any) is
300 forwarded to the remote machine. The argument must be \fByes\fR or \fBno\fR.
301 The default is \fBno\fR.
302 .sp
303 Agent forwarding should be enabled with caution. Users with the ability to
304 bypass file permissions on the remote host (for the agent's Unix-domain socket)
305 can access the local agent through the forwarded connection. An attacker cannot
306 obtain key material from the agent, however he can perform operations on the
307 keys that enable him to authenticate using the identities loaded into the
308 agent.
309 .RE
311 .sp
312 .ne 2
313 .na
314 \fB\fBForwardX11\fR\fR
315 .ad
316 .sp .6
317 .RS 4n
318 Specifies whether X11 connections are automatically redirected over the secure
319 channel and \fBDISPLAY\fR set. The argument must be \fByes\fR or \fBno\fR. The
320 default is \fBno\fR.
321 .sp
322 X11 forwarding should be enabled with caution. Users with the ability to bypass
323 file permissions on the remote host (for the user's X authorization database)
324 can access the local \fBX11\fR display through the forwarded connection. An
325 attacker might then be able to perform activities such as keystroke monitoring.
326 See the \fBForwardX11Trusted\fR option for more information how to prevent
327 this.
328 .RE
330 .sp
331 .ne 2
332 .na
333 \fB\fBForwardX11Trusted\fR\fR
334 .ad
335 .sp .6
336 .RS 4n
337 If this option is set to \fByes\fR, remote X11 clients have full access to the
338 original X11 display. This option is set to \fByes\fR by default.
339 .sp
340 If this option is set to \fBno\fR, remote X11 clients are considered untrusted
341 and prevented from stealing or tampering with data belonging to trusted X11
342 clients. Furthermore, the \fBxauth\fR(1) token used for the session is set to
343 expire after 20 minutes. Remote clients are refused access after this time.
344 .sp
345 See the X11 SECURITY extension specification for full details on the
346 restrictions imposed on untrusted clients.
347 .RE
349 .sp
350 .ne 2
351 .na
352 \fB\fBGatewayPorts\fR\fR
353 .ad
354 .sp .6
355 .RS 4n
356 Specifies whether remote hosts are allowed to connect to local forwarded ports.
357 By default, \fBssh\fR binds local port forwardings to the loopback address.
358 This prevents other remote hosts from connecting to forwarded ports.
359 \fBGatewayPorts\fR can be used to specify that \fBssh\fR should bind local port
360 forwardings to the wildcard address, thus allowing remote hosts to connect to
361 forwarded ports. The argument must be \fByes\fR or \fBno\fR. The default is
362 \fBno\fR.
363 .RE
365 .sp
366 .ne 2
367 .na
368 \fB\fBGlobalKnownHostsFile\fR\fR
369 .ad
370 .sp .6
371 .RS 4n
372 Specifies a file to use instead of \fB/etc/ssh/ssh_known_hosts\fR.
373 .RE
375 .sp
376 .ne 2
377 .na
378 \fB\fBGSSAPIAuthentication\fR\fR
379 .ad
380 .sp .6
381 .RS 4n
382 Enables/disables GSS-API user authentication. The default is \fByes\fR.
383 .RE
385 .sp
386 .ne 2
387 .na
388 \fB\fBGSSAPIDelegateCredentials\fR\fR
389 .ad
390 .sp .6
391 .RS 4n
392 Enables/disables GSS-API credential forwarding. The default is \fBno\fR.
393 .RE
395 .sp
396 .ne 2
397 .na
398 \fB\fBGSSAPIKeyExchange\fR\fR
399 .ad
400 .sp .6
401 .RS 4n
402 Enables/disables GSS-API-authenticated key exchanges. The default is \fByes\fR.
403 .sp
404 This option is intended primarily to allow users to disable the use of GSS-API
405 key exchange for SSHv2 when it would otherwise be selected and then fail (due
406 to server misconfiguration, for example). SSHv2 key exchange failure always
407 results in disconnection.
408 .sp
409 This option also enables the use of the GSS-API to authenticate the user to the
410 server after the key exchange. GSS-API key exchange can succeed but the
411 subsequent authentication using the GSS-API fail if the server does not
412 authorize the user's GSS principal name to the target user account.
413 .RE
415 .sp
416 .ne 2
417 .na
418 \fB\fBHashKnownHosts\fR\fR
419 .ad
420 .sp .6
421 .RS 4n
422 Indicates that \fBssh\fR(1), should hash host names and addresses when they are
423 added to \fB~/.ssh/known_hosts\fR. These hashed names can be used normally by
424 \fBssh\fR(1) and \fBsshd\fR(1M), but they do not reveal identifying information
425 should the file's contents be disclosed. The default is \fBno\fR. Existing
426 names and addresses in known hosts files are not be converted automatically,
427 but can be manually hashed using \fBssh-keygen\fR(1).
428 .RE
430 .sp
431 .ne 2
432 .na
433 \fB\fBHost\fR\fR
434 .ad
435 .sp .6
436 .RS 4n
437 Restricts the following declarations (up to the next \fBHost\fR keyword) to be
438 only for those hosts that match one of the patterns given after the keyword. An
439 asterisk (\fB*\fR) and a question mark (\fB?\fR) can be used as wildcards in
440 the patterns. A single asterisk as a pattern can be used to provide global
441 defaults for all hosts. The host is the host name argument given on the command
442 line (that is, the name is not converted to a canonicalized host name before
443 matching).
444 .RE
446 .sp
447 .ne 2
448 .na
449 \fB\fBHostbasedAuthentication\fR\fR
450 .ad
451 .sp .6
452 .RS 4n
453 Specifies whether to try \fBrhosts\fR-based authentication with public key
454 authentication. The argument must be \fByes\fR or \fBno\fR. The default is
455 \fBno\fR. This option applies to protocol version 2 only and is similar to
456 \fBRhostsRSAAuthentication\fR.
457 .RE
459 .sp
460 .ne 2
461 .na
462 \fB\fBHostKeyAlgorithms\fR\fR
463 .ad
464 .sp .6
465 .RS 4n
466 Specifies the protocol version 2 host key algorithms that the client wants to
467 use in order of preference. The default for this option is:
468 \fBssh-rsa,ssh-dss\fR.
469 .RE
471 .sp
472 .ne 2
473 .na
474 \fB\fBHostKeyAlias\fR\fR
475 .ad
476 .sp .6
477 .RS 4n
478 Specifies an alias that should be used instead of the real host name when
479 looking up or saving the host key in the host key database files. This option
480 is useful for tunneling \fBssh\fR connections or for multiple servers running
481 on a single host.
482 .RE
484 .sp
485 .ne 2
486 .na
487 \fB\fBHostName\fR\fR
488 .ad
489 .sp .6
490 .RS 4n
491 Specifies the real host name to log into. This can be used to specify nicknames
492 or abbreviations for hosts. Default is the name given on the command line.
493 Numeric IP addresses are also permitted (both on the command line and in
494 \fBHostName\fR specifications).
495 .RE
497 .sp
498 .ne 2
499 .na
500 \fB\fBIdentityFile\fR\fR
501 .ad
502 .sp .6
503 .RS 4n
504 Specifies a file from which the user's RSA or DSA authentication identity is
505 read. The default is \fB$HOME/.ssh/identity\fR for protocol version 1 and
506 \fB$HOME/.ssh/id_rsa\fR and \fB$HOME/.ssh/id_dsa\fR for protocol version 2.
507 Additionally, any identities represented by the authentication agent is used
508 for authentication. The file name can use the tilde syntax to refer to a user's
509 home directory. It is possible to have multiple identity files specified in
510 configuration files; all these identities is tried in sequence.
511 .RE
513 .sp
514 .ne 2
515 .na
516 \fB\fBIgnoreIfUnknown\fR\fR
517 .ad
518 .sp .6
519 .RS 4n
520 Specifies a comma-separated list of \fBssh_config\fR parameters, which, if
521 unknown to \fBssh\fR(1), are to be ignored by \fBssh\fR.
522 .sp
523 This parameter is primarily intended to be used in the per-user
524 \fBssh_config\fR, \fB~/.ssh/config\fR. While this parameter can also be used in
525 the system wide \fB/etc/ssh/ssh_config\fR file, it is generally useless as the
526 capabilities of the \fBssh\fR(1) client on that host should match that file.
527 .RE
529 .sp
530 .ne 2
531 .na
532 \fB\fBTCPKeepAlive\fR\fR
533 .ad
534 .sp .6
535 .RS 4n
536 Specifies whether the system should send TCP keepalive messages to the other
537 side. If they are sent, death of the connection or crash of one of the machines
538 is properly noticed. However, this means that connections die if the route is
539 down temporarily, which can be a source of annoyance.
540 .sp
541 The default is \fByes\fR (to send keepalives), which means the client notices
542 if the network goes down or the remote host dies. This is important in scripts,
543 and many users want it too. To disable keepalives, the value should be set to
544 \fBno\fR in both the server and the client configuration files.
545 .RE
547 .sp
548 .ne 2
549 .na
550 \fB\fBLocalForward\fR\fR
551 .ad
552 .sp .6
553 .RS 4n
554 Specifies that a TCP/IP port on the local machine be forwarded over the secure
555 channel to a given \fIhost\fR:\fIport\fR from the remote machine. The first
556 argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the second
557 must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be specified by
558 enclosing addresses in square brackets or by using an alternative syntax:
559 \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and \fIhost\fR\fB/\fR\fIport\fR.
560 Multiple forwardings can be specified and additional forwardings can be given
561 on the command line. Only a user with enough privileges can forward privileged
562 ports. By default, the local port is bound in accordance with the
563 \fBGatewayPorts\fR setting. However, an explicit \fIbind_address\fR can be used
564 to bind the connection to a specific address. The \fIbind_address\fR of
565 \fIlocalhost\fR indicates that the listening port be bound for local use only,
566 while an empty address or \fB*\fR indicates that the port should be available
567 from all interfaces.
568 .RE
570 .sp
571 .ne 2
572 .na
573 \fB\fBLogLevel\fR\fR
574 .ad
575 .sp .6
576 .RS 4n
577 Gives the verbosity level that is used when logging messages from \fBssh\fR.
578 The possible values are: \fBFATAL\fR, \fBERROR\fR, \fBQUIET\fR, \fBINFO\fR,
579 \fBVERBOSE\fR, \fBDEBUG\fR, \fBDEBUG1\fR, \fBDEBUG2\fR, and \fBDEBUG3\fR. The
580 default is \fBINFO\fR. \fBDEBUG\fR and \fBDEBUG1\fR are equivalent.
581 \fBDEBUG2\fR and \fBDEBUG3\fR each specify higher levels of verbose output.
582 .RE
584 .sp
585 .ne 2
586 .na
587 \fB\fBMACs\fR\fR
588 .ad
589 .sp .6
590 .RS 4n
591 Specifies the MAC (message authentication code) algorithms in order of
592 preference. The MAC algorithm is used in protocol version 2 for data integrity
593 protection. Multiple algorithms must be comma-separated. The default is
594 \fBhmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96\fR.
595 .RE
597 .sp
598 .ne 2
599 .na
600 \fB\fBNoHostAuthenticationForLocalhost\fR\fR
601 .ad
602 .sp .6
603 .RS 4n
604 This option can be used if the home directory is shared across machines. In
605 this case \fBlocalhost\fR refers to a different machine on each of the machines
606 and the user gets many warnings about changed host keys. However, this option
607 disables host authentication for \fBlocalhost\fR. The argument to this keyword
608 must be \fByes\fR or \fBno\fR. The default is to check the host key for
609 \fBlocalhost\fR.
610 .RE
612 .sp
613 .ne 2
614 .na
615 \fB\fBNumberOfPasswordPrompts\fR\fR
616 .ad
617 .sp .6
618 .RS 4n
619 Specifies the number of attempts before giving up for password and
620 keyboard-interactive methods. Attempts for each method are counted separately.
621 The argument to this keyword must be an integer. The default is 3.
622 .RE
624 .sp
625 .ne 2
626 .na
627 \fB\fBPasswordAuthentication\fR\fR
628 .ad
629 .sp .6
630 .RS 4n
631 Specifies whether to use password authentication. The argument to this keyword
632 must be \fByes\fR or \fBno\fR. This option applies to both protocol versions 1
633 and 2. The default is \fByes\fR.
634 .RE
636 .sp
637 .ne 2
638 .na
639 \fB\fBPort\fR\fR
640 .ad
641 .sp .6
642 .RS 4n
643 Specifies the port number to connect on the remote host. The default is 22.
644 .RE
646 .sp
647 .ne 2
648 .na
649 \fB\fBPreferredAuthentications\fR\fR
650 .ad
651 .sp .6
652 .RS 4n
653 Specifies the order in which the client should try protocol 2 authentication
654 methods. This allows a client to prefer one method (for example,
655 \fBkeyboard-interactive\fR) over another method (for example, \fBpassword\fR).
656 The default for this option is:
657 \fBhostbased,publickey,keyboard-interactive,password\fR.
658 .RE
660 .sp
661 .ne 2
662 .na
663 \fB\fBProtocol\fR\fR
664 .ad
665 .sp .6
666 .RS 4n
667 Specifies the protocol versions \fBssh\fR should support in order of
668 preference. The possible values are \fB1\fR and \fB2\fR. Multiple versions must
669 be comma-separated. The default is \fB2,1\fR. This means that \fBssh\fR tries
670 version 2 and falls back to version 1 if version 2 is not available.
671 .RE
673 .sp
674 .ne 2
675 .na
676 \fB\fBProxyCommand\fR\fR
677 .ad
678 .sp .6
679 .RS 4n
680 Specifies the command to use to connect to the server. The command string
681 extends to the end of the line, and is executed with \fB/bin/sh\fR. In the
682 command string, \fB%h\fR is substituted by the host name to connect and
683 \fB%p\fR by the port. The string can be any valid command, and should read from
684 its standard input and write to its standard output. It should eventually
685 connect an \fBsshd\fR(1M) server running on some machine, or execute \fBsshd\fR
686 \fB-i\fR somewhere. Host key management is done using the \fBHostName\fR of the
687 host being connected (defaulting to the name typed by the user).
688 \fBCheckHostIP\fR is not available for connects with a proxy command.
689 .RE
691 .sp
692 .ne 2
693 .na
694 \fB\fBPubkeyAuthentication\fR\fR
695 .ad
696 .sp .6
697 .RS 4n
698 Specifies whether to try public key authentication. The argument to this
699 keyword must be \fByes\fR or \fBno\fR. The default is \fByes\fR. This option
700 applies to protocol version 2 only.
701 .RE
703 .sp
704 .ne 2
705 .na
706 \fB\fBRekeyLimit\fR\fR
707 .ad
708 .sp .6
709 .RS 4n
710 Specifies the maximum amount of data that can be transmitted before the session
711 key is renegotiated. The argument is the number of bytes, with an optional
712 suffix of \fBK\fR, \fBM\fR, or \fBG\fR to indicate Kilobytes, Megabytes, or
713 Gigabytes, respectively. The default is between \fB1G\fR and \fB4G\fR,
714 depending on the cipher. This option applies to protocol version 2 only.
715 .RE
717 .sp
718 .ne 2
719 .na
720 \fB\fBRemoteForward\fR\fR
721 .ad
722 .sp .6
723 .RS 4n
724 Specifies that a TCP/IP port on the remote machine be forwarded over the secure
725 channel to a given \fB\fIhost\fR:\fIport\fR\fR from the local machine. The
726 first argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the
727 second argument must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be
728 specified by enclosing addresses in square brackets or by using an alternative
729 syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and
730 \fIhost\fR\fB/\fR\fIport\fR. You can specify multiple forwardings and give
731 additional forwardings on the command line. Only a user with enough privileges
732 can forward privileged ports.
733 .sp
734 If the \fIbind_address\fR is not specified, the default is to only bind to
735 loopback addresses. If the \fIbind_address\fR is \fB*\fR or an empty string,
736 then the forwarding is requested to listen on all interfaces. Specifying a
737 remote \fIbind_address\fR only succeeds if the server's \fBGatewayPorts\fR
738 option is enabled. See \fBsshd_config\fR(4).
739 .RE
741 .sp
742 .ne 2
743 .na
744 \fB\fBRhostsAuthentication\fR\fR
745 .ad
746 .sp .6
747 .RS 4n
748 Specifies whether to try \fBrhosts\fR-based authentication. This declaration
749 affects only the client side and has no effect whatsoever on security.
750 Disabling \fBrhosts\fR authentication can reduce authentication time on slow
751 connections when \fBrhosts\fR authentication is not used. Most servers do not
752 permit \fBRhostsAuthentication\fR because it is not secure (see
753 \fBRhostsRSAAuthentication\fR). The argument to this keyword must be \fByes\fR
754 or \fBno\fR. This option applies only to the protocol version 1 and requires
755 that \fBssh\fR be \fBsetuid\fR root and that \fBUsePrivilegedPort\fR be set to
756 \fByes\fR.
757 .RE
759 .sp
760 .ne 2
761 .na
762 \fB\fBRhostsRSAAuthentication\fR\fR
763 .ad
764 .sp .6
765 .RS 4n
766 Specifies whether to try \fBrhosts\fR-based authentication with RSA host
767 authentication. This is the primary authentication method for most sites. The
768 argument must be \fByes\fR or \fBno\fR. This option applies only to the
769 protocol version 1 and requires that \fBssh\fR be \fBsetuid\fR root and that
770 \fBUsePrivilegedPort\fR be set to \fByes\fR.
771 .RE
773 .sp
774 .ne 2
775 .na
776 \fB\fBServerAliveCountMax\fR\fR
777 .ad
778 .sp .6
779 .RS 4n
780 Sets the number of server alive messages which can be sent without \fBssh\fR(1)
781 receiving messages back from the server. If this threshold is reached while
782 server alive messages are being sent, \fBssh\fR disconnects from the server,
783 terminating the session. The use of server alive messages differs from
784 \fBTCPKeepAlive\fR. Server alive messages are sent through the encrypted
785 channel and are not spoofable. The TCP keep alive option enabled by
786 \fBTCPKeepAlive\fR is spoofable. The server alive mechanism is valuable when
787 the client or server depend on knowing when a connection has become inactive.
788 .sp
789 The default value is 3. If, for example, \fBServerAliveInterval\fR is set to 15
790 and \fBServerAliveCountMax\fR is left at the default, \fBssh\fR disconnects in
791 45-60 seconds if the server becomes unresponsive. This option applies to
792 protocol version 2 only.
793 .RE
795 .sp
796 .ne 2
797 .na
798 \fB\fBServerAliveInterval\fR\fR
799 .ad
800 .sp .6
801 .RS 4n
802 Sets a timeout interval in seconds after which if no data has been received
803 from the server, \fBssh\fR(1) sends a message through the encrypted channel to
804 request a response from the server. The default is 0, indicating that these
805 messages are not sent to the server. This option applies to protocol version 2
806 only.
807 .RE
809 .sp
810 .ne 2
811 .na
812 \fB\fBStrictHostKeyChecking\fR\fR
813 .ad
814 .sp .6
815 .RS 4n
816 If this flag is set to \fByes\fR, \fBssh\fR never automatically adds host keys
817 to the \fB$HOME/.ssh/known_hosts\fR file, and refuses to connect hosts whose
818 host key has changed. This provides maximum protection against trojan horse
819 attacks. However, it can be a source of inconvenience if you do not have good
820 \fB/etc/ssh/ssh_known_hosts\fR files installed and frequently connect new
821 hosts. This option forces the user to manually add any new hosts. Normally this
822 option is disabled, and new hosts are automatically added to the known host
823 files. The host keys of known hosts are verified automatically in either case.
824 The argument must be \fByes\fR or \fBno\fR or \fBask\fR. The default is
825 \fBask\fR.
826 .RE
828 .sp
829 .ne 2
830 .na
831 \fB\fBUseOpenSSLEngine\fR\fR
832 .ad
833 .sp .6
834 .RS 4n
835 Specifies whether \fBssh\fR should use the OpenSSL PKCS#11 engine for
836 offloading cryptographic operations to the Cryptographic Framework.
837 Cryptographic operations are accelerated according to the available installed
838 plug-ins. When no suitable plug-ins are present this option does not have an
839 effect. The default is \fByes\fR.
840 .RE
842 .sp
843 .ne 2
844 .na
845 \fB\fBUsePrivilegedPort\fR\fR
846 .ad
847 .sp .6
848 .RS 4n
849 Specifies whether to use a privileged port for outgoing connections. The
850 argument must be \fByes\fR or \fBno\fR. The default is \fByes\fR. Setting this
851 option to \fBno\fR turns off \fBRhostsAuthentication\fR and
852 \fBRhostsRSAAuthentication\fR. If set to \fByes\fR \fBssh\fR must be
853 \fBsetuid\fR root. Defaults to \fBno\fR.
854 .RE
856 .sp
857 .ne 2
858 .na
859 \fB\fBUser\fR\fR
860 .ad
861 .sp .6
862 .RS 4n
863 Specifies the user to log in as. This can be useful if you have different user
864 names on different machines. This saves you the trouble of having to remember
865 to enter the user name on the command line.
866 .RE
868 .sp
869 .ne 2
870 .na
871 \fB\fBUserKnownHostsFile\fR\fR
872 .ad
873 .sp .6
874 .RS 4n
875 Specifies a file to use instead of \fB$HOME/.ssh/known_hosts\fR.
876 .RE
878 .sp
879 .ne 2
880 .na
881 \fB\fBUseRsh\fR\fR
882 .ad
883 .sp .6
884 .RS 4n
885 Specifies that \fBrlogin\fR or \fBrsh\fR should be used for this host. It is
886 possible that the host does not support the \fBssh\fR protocol. This causes
887 \fBssh\fR to immediately execute \fBrsh\fR(1). All other options (except
888 \fBHostName\fR) are ignored if this has been specified. The argument must be
889 \fByes\fR or \fBno\fR.
890 .RE
892 .sp
893 .ne 2
894 .na
895 \fB\fBXAuthLocation\fR\fR
896 .ad
897 .sp .6
898 .RS 4n
899 Specifies the location of the \fBxauth\fR(1) program. The default is
900 \fB/usr/openwin/bin/xauth\fR.
901 .RE
903 .SH SEE ALSO
904 .sp
905 .LP
906 \fBrsh\fR(1), \fBssh\fR(1), \fBssh-http-proxy-connect\fR(1),
907 \fBssh-keygen\fR(1), \fBssh-socks5-proxy-connect\fR(1), \fBsshd\fR(1M),
908 \fBsshd_config\fR(4), \fBkerberos\fR(5)
909 .sp
910 .LP
911 \fIRFC 4252\fR