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7 .TH SSH_CONFIG 4 "Apr 20, 2009"
8 .SH NAME
9 ssh_config \- ssh configuration file
10 .SH SYNOPSIS
11 .LP
12 .nf
13 \fB/etc/ssh/ssh_config\fR
14 .fi
16 .LP
17 .nf
18 \fB$HOME/.ssh/config\fR
19 .fi
21 .SH DESCRIPTION
22 .sp
23 .LP
24 The first \fBssh_config\fR path, above, provides the system-wide defaults for
25 \fBssh\fR(1). The second version is user-specific defaults for \fBssh\fR.
26 .sp
27 .LP
28 \fBssh\fR obtains configuration data from the following sources, in this order:
29 command line options, user's configuration file (\fB$HOME/.ssh/config\fR), and
30 system-wide configuration file (\fB/etc/ssh/ssh_config\fR). For each parameter,
31 the first obtained value is used. The configuration files contain sections
32 bracketed by \fBHost\fR specifications, and that section is applied only for
33 hosts that match one of the patterns given in the specification. The matched
34 host name is the one given on the command line.
35 .sp
36 .LP
37 Since the first obtained value for each parameter is used, host-specific
38 declarations should be given near the beginning of the file, and general
39 defaults at the end.
40 .sp
41 .LP
42 The configuration file has the following format and syntax:
43 .RS +4
44 .TP
45 .ie t \(bu
46 .el o
47 Empty lines and lines starting with \fB#\fR are comments.
48 .RE
49 .RS +4
50 .TP
51 .ie t \(bu
52 .el o
53 Non-commented lines are of the form:
54 .sp
55 .in +2
56 .nf
57 \fIkeyword\fR \fIarguments\fR
58 .fi
59 .in -2
60 .sp
62 .RE
63 .RS +4
64 .TP
65 .ie t \(bu
66 .el o
67 Configuration options can be separated by white space or optional whitespace
68 and exactly one equal sign. The latter format allows you to avoid the need to
69 quote white space when specifying configuration options using the \fB-o\fR
70 option to \fBssh\fR, \fBscp\fR, and \fBsftp\fR.
71 .RE
72 .sp
73 .LP
74 The possible keywords and their meanings are listed in the following
75 list.Keywords are case-insensitive and arguments are case-sensitive.
76 .sp
77 .ne 2
78 .na
79 \fB\fBBatchMode\fR\fR
80 .ad
81 .sp .6
82 .RS 4n
83 The argument must be \fByes\fR or \fBno\fR. If set to \fByes\fR,
84 passphrase/password querying is disabled. This option is useful in scripts and
85 other batch jobs where you have no user to supply the password.
86 .RE
88 .sp
89 .ne 2
90 .na
91 \fB\fBBindAddress\fR\fR
92 .ad
93 .sp .6
94 .RS 4n
95 Specify the interface to transmit from on machines with multiple interfaces or
96 aliased addresses. This option does not work if \fBUsePrivilegedPort\fR is set
97 to \fByes\fR.
98 .RE
100 .sp
101 .ne 2
102 .na
103 \fB\fBCheckHostIP\fR\fR
104 .ad
105 .sp .6
106 .RS 4n
107 If this flag is set to \fByes\fR, \fBssh\fR additionally checks the host IP
108 address in the \fBknown_hosts\fR file. This allows \fBssh\fR to detect if a
109 host key changed due to DNS spoofing. If the option is set to \fBno\fR, the
110 check is not executed.
111 .RE
113 .sp
114 .ne 2
115 .na
116 \fB\fBCipher\fR\fR
117 .ad
118 .sp .6
119 .RS 4n
120 Specifies the cipher to use for encrypting the session in protocol version 1.
121 Only a single cipher can be specified. Currently, \fBblowfish, 3des,\fR and
122 \fBdes\fR are supported. \fB3des\fR (triple-\fBdes\fR) is an
123 encrypt-decrypt-encrypt triple with three different keys. It is believed to be
124 secure. \fBblowfish\fR is a fast block cipher. It appears very secure and is
125 much faster than \fB3des\fR. \fBdes\fR is only supported in the \fBssh\fR
126 client for interoperability with legacy protocol 1 implementations that do not
127 support the \fB3des\fR cipher. Its use is strongly discouraged due to
128 cryptographic weaknesses. The default is \fB3des\fR.
129 .RE
131 .sp
132 .ne 2
133 .na
134 \fB\fBCiphers\fR\fR
135 .ad
136 .sp .6
137 .RS 4n
138 Specifies the ciphers allowed for protocol version 2 in order of preference.
139 Multiple ciphers must be comma separated.
140 .sp
141 The default cipher list contains all supported ciphers in this order:
142 .sp
143 .in +2
144 .nf
145 aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, aes128-cbc,
146 aes192-cbc, aes256-cbc, arcfour, 3des-cbc,blowfish-cbc
147 .fi
148 .in -2
149 .sp
151 While CBC modes are not considered as secure as other modes in connection with
152 the SSH protocol 2 they are present at the back of the default client cipher
153 list for backward compatibility with SSH servers that do not support other
154 cipher modes.
155 .RE
157 .sp
158 .ne 2
159 .na
160 \fB\fBClearAllForwardings\fR\fR
161 .ad
162 .sp .6
163 .RS 4n
164 Specifies that all local, remote, and dynamic port forwardings specified in the
165 configuration files or on the command line be cleared. This option is primarily
166 useful when used from the \fBssh\fR command line to clear port forwardings set
167 in configuration files and is automatically set by \fBscp\fR(1) and
168 \fBsftp\fR(1). The argument must be \fByes\fR or \fBno\fR. The default is
169 \fBno\fR.
170 .RE
172 .sp
173 .ne 2
174 .na
175 \fB\fBCompression\fR\fR
176 .ad
177 .sp .6
178 .RS 4n
179 Specifies whether to use compression. The argument must be \fByes\fR or
180 \fBno\fR. Defaults to \fBno\fR.
181 .RE
183 .sp
184 .ne 2
185 .na
186 \fB\fBCompressionLevel\fR\fR
187 .ad
188 .sp .6
189 .RS 4n
190 Specifies the compression level to use if compression is enabled. The argument
191 must be an integer from 1 (fast) to 9 (slow, best). The default level is 6,
192 which is good for most applications. This option applies to protocol version 1
193 only.
194 .RE
196 .sp
197 .ne 2
198 .na
199 \fB\fBConnectionAttempts\fR\fR
200 .ad
201 .sp .6
202 .RS 4n
203 Specifies the number of tries (one per second) to make before falling back to
204 \fBrsh\fR or exiting. The argument must be an integer. This can be useful in
205 scripts if the connection sometimes fails. The default is 1.
206 .RE
208 .sp
209 .ne 2
210 .na
211 \fB\fBConnectTimeout\fR\fR
212 .ad
213 .sp .6
214 .RS 4n
215 Specifies the timeout (in seconds) used when connecting to the \fBssh\fR
216 server, instead of using the default system TCP timeout. This value is used
217 only when the target is down or truly unreachable, not when it refuses the
218 connection.
219 .RE
221 .sp
222 .ne 2
223 .na
224 \fB\fBDisableBanner\fR\fR
225 .ad
226 .sp .6
227 .RS 4n
228 If set to \fByes\fR, disables the display of the banner message. If set to
229 \fBin-exec-mode\fR, disables the display of banner message when in remote
230 command mode only.
231 .sp
232 The default value is \fBno\fR, which means that the banner is displayed unless
233 the log level is \fBQUIET\fR, \fBFATAL\fR, or \fBERROR\fR. See also the
234 \fBBanner\fR option in \fBsshd_config\fR(4). This option applies to protocol
235 version 2 only.
236 .RE
238 .sp
239 .ne 2
240 .na
241 \fB\fBDynamicForward\fR\fR
242 .ad
243 .sp .6
244 .RS 4n
245 Specifies that a TCP/IP port on the local machine be forwarded over the secure
246 channel. The application protocol is then used to determine where to connect to
247 from the remote machine.
248 .sp
249 The argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR. IPv6
250 addresses can be specified by enclosing addresses in square brackets or by
251 using an alternative syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR. By
252 default, the local port is bound in accordance with the \fBGatewayPorts\fR
253 setting. However, an explicit \fIbind_address\fR can be used to bind the
254 connection to a specific address. The \fIbind_address\fR of \fBlocalhost\fR
255 indicates that the listening port be bound for local use only, while an empty
256 address or \fB*\fR indicates that the port should be available from all
257 interfaces.
258 .sp
259 Currently the \fBSOCKS4\fR and \fBSOCKS5\fR protocols are supported, and
260 \fBssh\fR acts as a \fBSOCKS\fR server. Multiple forwardings can be specified
261 and additional forwardings can be specified on the command line. Only a user
262 with enough privileges can forward privileged ports.
263 .RE
265 .sp
266 .ne 2
267 .na
268 \fB\fBEscapeChar\fR\fR
269 .ad
270 .sp .6
271 .RS 4n
272 Sets the escape character. The default is tilde (\fB~\fR). The escape character
273 can also be set on the command line. The argument should be a single character,
274 \fB^\fR, followed by a letter, or \fBnone\fR to disable the escape character
275 entirely (making the connection transparent for binary data).
276 .RE
278 .sp
279 .ne 2
280 .na
281 \fB\fBFallBackToRsh\fR\fR
282 .ad
283 .sp .6
284 .RS 4n
285 Specifies that if connecting with \fBssh\fR fails due to a connection refused
286 error (there is no \fBsshd\fR(1M) listening on the remote host), \fBrsh\fR(1)
287 should automatically be used instead (after a suitable warning about the
288 session being unencrypted). The argument must be \fByes\fR or \fBno\fR.
289 .RE
291 .sp
292 .ne 2
293 .na
294 \fB\fBForwardAgent\fR\fR
295 .ad
296 .sp .6
297 .RS 4n
298 Specifies whether the connection to the authentication agent (if any) is
299 forwarded to the remote machine. The argument must be \fByes\fR or \fBno\fR.
300 The default is \fBno\fR.
301 .sp
302 Agent forwarding should be enabled with caution. Users with the ability to
303 bypass file permissions on the remote host (for the agent's Unix-domain socket)
304 can access the local agent through the forwarded connection. An attacker cannot
305 obtain key material from the agent, however he can perform operations on the
306 keys that enable him to authenticate using the identities loaded into the
307 agent.
308 .RE
310 .sp
311 .ne 2
312 .na
313 \fB\fBForwardX11\fR\fR
314 .ad
315 .sp .6
316 .RS 4n
317 Specifies whether X11 connections are automatically redirected over the secure
318 channel and \fBDISPLAY\fR set. The argument must be \fByes\fR or \fBno\fR. The
319 default is \fBno\fR.
320 .sp
321 X11 forwarding should be enabled with caution. Users with the ability to bypass
322 file permissions on the remote host (for the user's X authorization database)
323 can access the local \fBX11\fR display through the forwarded connection. An
324 attacker might then be able to perform activities such as keystroke monitoring.
325 See the \fBForwardX11Trusted\fR option for more information how to prevent
326 this.
327 .RE
329 .sp
330 .ne 2
331 .na
332 \fB\fBForwardX11Trusted\fR\fR
333 .ad
334 .sp .6
335 .RS 4n
336 If this option is set to \fByes\fR, remote X11 clients have full access to the
337 original X11 display. This option is set to \fByes\fR by default.
338 .sp
339 If this option is set to \fBno\fR, remote X11 clients are considered untrusted
340 and prevented from stealing or tampering with data belonging to trusted X11
341 clients. Furthermore, the \fBxauth\fR(1) token used for the session is set to
342 expire after 20 minutes. Remote clients are refused access after this time.
343 .sp
344 See the X11 SECURITY extension specification for full details on the
345 restrictions imposed on untrusted clients.
346 .RE
348 .sp
349 .ne 2
350 .na
351 \fB\fBGatewayPorts\fR\fR
352 .ad
353 .sp .6
354 .RS 4n
355 Specifies whether remote hosts are allowed to connect to local forwarded ports.
356 By default, \fBssh\fR binds local port forwardings to the loopback address.
357 This prevents other remote hosts from connecting to forwarded ports.
358 \fBGatewayPorts\fR can be used to specify that \fBssh\fR should bind local port
359 forwardings to the wildcard address, thus allowing remote hosts to connect to
360 forwarded ports. The argument must be \fByes\fR or \fBno\fR. The default is
361 \fBno\fR.
362 .RE
364 .sp
365 .ne 2
366 .na
367 \fB\fBGlobalKnownHostsFile\fR\fR
368 .ad
369 .sp .6
370 .RS 4n
371 Specifies a file to use instead of \fB/etc/ssh/ssh_known_hosts\fR.
372 .RE
374 .sp
375 .ne 2
376 .na
377 \fB\fBGSSAPIAuthentication\fR\fR
378 .ad
379 .sp .6
380 .RS 4n
381 Enables/disables GSS-API user authentication. The default is \fByes\fR.
382 .RE
384 .sp
385 .ne 2
386 .na
387 \fB\fBGSSAPIDelegateCredentials\fR\fR
388 .ad
389 .sp .6
390 .RS 4n
391 Enables/disables GSS-API credential forwarding. The default is \fBno\fR.
392 .RE
394 .sp
395 .ne 2
396 .na
397 \fB\fBGSSAPIKeyExchange\fR\fR
398 .ad
399 .sp .6
400 .RS 4n
401 Enables/disables GSS-API-authenticated key exchanges. The default is \fByes\fR.
402 .sp
403 This option is intended primarily to allow users to disable the use of GSS-API
404 key exchange for SSHv2 when it would otherwise be selected and then fail (due
405 to server misconfiguration, for example). SSHv2 key exchange failure always
406 results in disconnection.
407 .sp
408 This option also enables the use of the GSS-API to authenticate the user to the
409 server after the key exchange. GSS-API key exchange can succeed but the
410 subsequent authentication using the GSS-API fail if the server does not
411 authorize the user's GSS principal name to the target user account.
412 .RE
414 .sp
415 .ne 2
416 .na
417 \fB\fBHashKnownHosts\fR\fR
418 .ad
419 .sp .6
420 .RS 4n
421 Indicates that \fBssh\fR(1), should hash host names and addresses when they are
422 added to \fB~/.ssh/known_hosts\fR. These hashed names can be used normally by
423 \fBssh\fR(1) and \fBsshd\fR(1M), but they do not reveal identifying information
424 should the file's contents be disclosed. The default is \fBno\fR. Existing
425 names and addresses in known hosts files are not be converted automatically,
426 but can be manually hashed using \fBssh-keygen\fR(1).
427 .RE
429 .sp
430 .ne 2
431 .na
432 \fB\fBHost\fR\fR
433 .ad
434 .sp .6
435 .RS 4n
436 Restricts the following declarations (up to the next \fBHost\fR keyword) to be
437 only for those hosts that match one of the patterns given after the keyword. An
438 asterisk (\fB*\fR) and a question mark (\fB?\fR) can be used as wildcards in
439 the patterns. A single asterisk as a pattern can be used to provide global
440 defaults for all hosts. The host is the host name argument given on the command
441 line (that is, the name is not converted to a canonicalized host name before
442 matching).
443 .RE
445 .sp
446 .ne 2
447 .na
448 \fB\fBHostbasedAuthentication\fR\fR
449 .ad
450 .sp .6
451 .RS 4n
452 Specifies whether to try \fBrhosts\fR-based authentication with public key
453 authentication. The argument must be \fByes\fR or \fBno\fR. The default is
454 \fBno\fR. This option applies to protocol version 2 only and is similar to
455 \fBRhostsRSAAuthentication\fR.
456 .RE
458 .sp
459 .ne 2
460 .na
461 \fB\fBHostKeyAlgorithms\fR\fR
462 .ad
463 .sp .6
464 .RS 4n
465 Specifies the protocol version 2 host key algorithms that the client wants to
466 use in order of preference. The default for this option is:
467 \fBssh-rsa,ssh-dss\fR.
468 .RE
470 .sp
471 .ne 2
472 .na
473 \fB\fBHostKeyAlias\fR\fR
474 .ad
475 .sp .6
476 .RS 4n
477 Specifies an alias that should be used instead of the real host name when
478 looking up or saving the host key in the host key database files. This option
479 is useful for tunneling \fBssh\fR connections or for multiple servers running
480 on a single host.
481 .RE
483 .sp
484 .ne 2
485 .na
486 \fB\fBHostName\fR\fR
487 .ad
488 .sp .6
489 .RS 4n
490 Specifies the real host name to log into. This can be used to specify nicknames
491 or abbreviations for hosts. Default is the name given on the command line.
492 Numeric IP addresses are also permitted (both on the command line and in
493 \fBHostName\fR specifications).
494 .RE
496 .sp
497 .ne 2
498 .na
499 \fB\fBIdentityFile\fR\fR
500 .ad
501 .sp .6
502 .RS 4n
503 Specifies a file from which the user's RSA or DSA authentication identity is
504 read. The default is \fB$HOME/.ssh/identity\fR for protocol version 1 and
505 \fB$HOME/.ssh/id_rsa\fR and \fB$HOME/.ssh/id_dsa\fR for protocol version 2.
506 Additionally, any identities represented by the authentication agent is used
507 for authentication. The file name can use the tilde syntax to refer to a user's
508 home directory. It is possible to have multiple identity files specified in
509 configuration files; all these identities is tried in sequence.
510 .RE
512 .sp
513 .ne 2
514 .na
515 \fB\fBIgnoreIfUnknown\fR\fR
516 .ad
517 .sp .6
518 .RS 4n
519 Specifies a comma-separated list of \fBssh_config\fR parameters, which, if
520 unknown to \fBssh\fR(1), are to be ignored by \fBssh\fR.
521 .sp
522 This parameter is primarily intended to be used in the per-user
523 \fBssh_config\fR, \fB~/.ssh/config\fR. While this parameter can also be used in
524 the system wide \fB/etc/ssh/ssh_config\fR file, it is generally useless as the
525 capabilities of the \fBssh\fR(1) client on that host should match that file.
526 .RE
528 .sp
529 .ne 2
530 .na
531 \fB\fBKeepAlive\fR\fR
532 .ad
533 .sp .6
534 .RS 4n
535 Specifies whether the system should send TCP keepalive messages to the other
536 side. If they are sent, death of the connection or crash of one of the machines
537 is properly noticed. However, this means that connections die if the route is
538 down temporarily, which can be a source of annoyance.
539 .sp
540 The default is \fByes\fR (to send keepalives), which means the client notices
541 if the network goes down or the remote host dies. This is important in scripts,
542 and many users want it too. To disable keepalives, the value should be set to
543 \fBno\fR in both the server and the client configuration files.
544 .RE
546 .sp
547 .ne 2
548 .na
549 \fB\fBLocalForward\fR\fR
550 .ad
551 .sp .6
552 .RS 4n
553 Specifies that a TCP/IP port on the local machine be forwarded over the secure
554 channel to a given \fIhost\fR:\fIport\fR from the remote machine. The first
555 argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the second
556 must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be specified by
557 enclosing addresses in square brackets or by using an alternative syntax:
558 \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and \fIhost\fR\fB/\fR\fIport\fR.
559 Multiple forwardings can be specified and additional forwardings can be given
560 on the command line. Only a user with enough privileges can forward privileged
561 ports. By default, the local port is bound in accordance with the
562 \fBGatewayPorts\fR setting. However, an explicit \fIbind_address\fR can be used
563 to bind the connection to a specific address. The \fIbind_address\fR of
564 \fIlocalhost\fR indicates that the listening port be bound for local use only,
565 while an empty address or \fB*\fR indicates that the port should be available
566 from all interfaces.
567 .RE
569 .sp
570 .ne 2
571 .na
572 \fB\fBLogLevel\fR\fR
573 .ad
574 .sp .6
575 .RS 4n
576 Gives the verbosity level that is used when logging messages from \fBssh\fR.
577 The possible values are: \fBFATAL\fR, \fBERROR\fR, \fBQUIET\fR, \fBINFO\fR,
578 \fBVERBOSE\fR, \fBDEBUG\fR, \fBDEBUG1\fR, \fBDEBUG2\fR, and \fBDEBUG3\fR. The
579 default is \fBINFO\fR. \fBDEBUG\fR and \fBDEBUG1\fR are equivalent.
580 \fBDEBUG2\fR and \fBDEBUG3\fR each specify higher levels of verbose output.
581 .RE
583 .sp
584 .ne 2
585 .na
586 \fB\fBMACs\fR\fR
587 .ad
588 .sp .6
589 .RS 4n
590 Specifies the MAC (message authentication code) algorithms in order of
591 preference. The MAC algorithm is used in protocol version 2 for data integrity
592 protection. Multiple algorithms must be comma-separated. The default is
593 \fBhmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96\fR.
594 .RE
596 .sp
597 .ne 2
598 .na
599 \fB\fBNoHostAuthenticationForLocalhost\fR\fR
600 .ad
601 .sp .6
602 .RS 4n
603 This option can be used if the home directory is shared across machines. In
604 this case \fBlocalhost\fR refers to a different machine on each of the machines
605 and the user gets many warnings about changed host keys. However, this option
606 disables host authentication for \fBlocalhost\fR. The argument to this keyword
607 must be \fByes\fR or \fBno\fR. The default is to check the host key for
608 \fBlocalhost\fR.
609 .RE
611 .sp
612 .ne 2
613 .na
614 \fB\fBNumberOfPasswordPrompts\fR\fR
615 .ad
616 .sp .6
617 .RS 4n
618 Specifies the number of attempts before giving up for password and
619 keyboard-interactive methods. Attempts for each method are counted separately.
620 The argument to this keyword must be an integer. The default is 3.
621 .RE
623 .sp
624 .ne 2
625 .na
626 \fB\fBPasswordAuthentication\fR\fR
627 .ad
628 .sp .6
629 .RS 4n
630 Specifies whether to use password authentication. The argument to this keyword
631 must be \fByes\fR or \fBno\fR. This option applies to both protocol versions 1
632 and 2. The default is \fByes\fR.
633 .RE
635 .sp
636 .ne 2
637 .na
638 \fB\fBPort\fR\fR
639 .ad
640 .sp .6
641 .RS 4n
642 Specifies the port number to connect on the remote host. The default is 22.
643 .RE
645 .sp
646 .ne 2
647 .na
648 \fB\fBPreferredAuthentications\fR\fR
649 .ad
650 .sp .6
651 .RS 4n
652 Specifies the order in which the client should try protocol 2 authentication
653 methods. This allows a client to prefer one method (for example,
654 \fBkeyboard-interactive\fR) over another method (for example, \fBpassword\fR).
655 The default for this option is:
656 \fBhostbased,publickey,keyboard-interactive,password\fR.
657 .RE
659 .sp
660 .ne 2
661 .na
662 \fB\fBProtocol\fR\fR
663 .ad
664 .sp .6
665 .RS 4n
666 Specifies the protocol versions \fBssh\fR should support in order of
667 preference. The possible values are \fB1\fR and \fB2\fR. Multiple versions must
668 be comma-separated. The default is \fB2,1\fR. This means that \fBssh\fR tries
669 version 2 and falls back to version 1 if version 2 is not available.
670 .RE
672 .sp
673 .ne 2
674 .na
675 \fB\fBProxyCommand\fR\fR
676 .ad
677 .sp .6
678 .RS 4n
679 Specifies the command to use to connect to the server. The command string
680 extends to the end of the line, and is executed with \fB/bin/sh\fR. In the
681 command string, \fB%h\fR is substituted by the host name to connect and
682 \fB%p\fR by the port. The string can be any valid command, and should read from
683 its standard input and write to its standard output. It should eventually
684 connect an \fBsshd\fR(1M) server running on some machine, or execute \fBsshd\fR
685 \fB-i\fR somewhere. Host key management is done using the \fBHostName\fR of the
686 host being connected (defaulting to the name typed by the user).
687 \fBCheckHostIP\fR is not available for connects with a proxy command.
688 .RE
690 .sp
691 .ne 2
692 .na
693 \fB\fBPubkeyAuthentication\fR\fR
694 .ad
695 .sp .6
696 .RS 4n
697 Specifies whether to try public key authentication. The argument to this
698 keyword must be \fByes\fR or \fBno\fR. The default is \fByes\fR. This option
699 applies to protocol version 2 only.
700 .RE
702 .sp
703 .ne 2
704 .na
705 \fB\fBRekeyLimit\fR\fR
706 .ad
707 .sp .6
708 .RS 4n
709 Specifies the maximum amount of data that can be transmitted before the session
710 key is renegotiated. The argument is the number of bytes, with an optional
711 suffix of \fBK\fR, \fBM\fR, or \fBG\fR to indicate Kilobytes, Megabytes, or
712 Gigabytes, respectively. The default is between \fB1G\fR and \fB4G\fR,
713 depending on the cipher. This option applies to protocol version 2 only.
714 .RE
716 .sp
717 .ne 2
718 .na
719 \fB\fBRemoteForward\fR\fR
720 .ad
721 .sp .6
722 .RS 4n
723 Specifies that a TCP/IP port on the remote machine be forwarded over the secure
724 channel to a given \fB\fIhost\fR:\fIport\fR\fR from the local machine. The
725 first argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the
726 second argument must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be
727 specified by enclosing addresses in square brackets or by using an alternative
728 syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and
729 \fIhost\fR\fB/\fR\fIport\fR. You can specify multiple forwardings and give
730 additional forwardings on the command line. Only a user with enough privileges
731 can forward privileged ports.
732 .sp
733 If the \fIbind_address\fR is not specified, the default is to only bind to
734 loopback addresses. If the \fIbind_address\fR is \fB*\fR or an empty string,
735 then the forwarding is requested to listen on all interfaces. Specifying a
736 remote \fIbind_address\fR only succeeds if the server's \fBGatewayPorts\fR
737 option is enabled. See \fBsshd_config\fR(4).
738 .RE
740 .sp
741 .ne 2
742 .na
743 \fB\fBRhostsAuthentication\fR\fR
744 .ad
745 .sp .6
746 .RS 4n
747 Specifies whether to try \fBrhosts\fR-based authentication. This declaration
748 affects only the client side and has no effect whatsoever on security.
749 Disabling \fBrhosts\fR authentication can reduce authentication time on slow
750 connections when \fBrhosts\fR authentication is not used. Most servers do not
751 permit \fBRhostsAuthentication\fR because it is not secure (see
752 \fBRhostsRSAAuthentication\fR). The argument to this keyword must be \fByes\fR
753 or \fBno\fR. This option applies only to the protocol version 1 and requires
754 that \fBssh\fR be \fBsetuid\fR root and that \fBUsePrivilegedPort\fR be set to
755 \fByes\fR.
756 .RE
758 .sp
759 .ne 2
760 .na
761 \fB\fBRhostsRSAAuthentication\fR\fR
762 .ad
763 .sp .6
764 .RS 4n
765 Specifies whether to try \fBrhosts\fR-based authentication with RSA host
766 authentication. This is the primary authentication method for most sites. The
767 argument must be \fByes\fR or \fBno\fR. This option applies only to the
768 protocol version 1 and requires that \fBssh\fR be \fBsetuid\fR root and that
769 \fBUsePrivilegedPort\fR be set to \fByes\fR.
770 .RE
772 .sp
773 .ne 2
774 .na
775 \fB\fBServerAliveCountMax\fR\fR
776 .ad
777 .sp .6
778 .RS 4n
779 Sets the number of server alive messages which can be sent without \fBssh\fR(1)
780 receiving messages back from the server. If this threshold is reached while
781 server alive messages are being sent, \fBssh\fR disconnects from the server,
782 terminating the session. The use of server alive messages differs from
783 \fBTCPKeepAlive\fR. Server alive messages are sent through the encrypted
784 channel and are not spoofable. The TCP keep alive option enabled by
785 \fBTCPKeepAlive\fR is spoofable. The server alive mechanism is valuable when
786 the client or server depend on knowing when a connection has become inactive.
787 .sp
788 The default value is 3. If, for example, \fBServerAliveInterval\fR is set to 15
789 and \fBServerAliveCountMax\fR is left at the default, \fBssh\fR disconnects in
790 45-60 seconds if the server becomes unresponsive. This option applies to
791 protocol version 2 only.
792 .RE
794 .sp
795 .ne 2
796 .na
797 \fB\fBServerAliveInterval\fR\fR
798 .ad
799 .sp .6
800 .RS 4n
801 Sets a timeout interval in seconds after which if no data has been received
802 from the server, \fBssh\fR(1) sends a message through the encrypted channel to
803 request a response from the server. The default is 0, indicating that these
804 messages are not sent to the server. This option applies to protocol version 2
805 only.
806 .RE
808 .sp
809 .ne 2
810 .na
811 \fB\fBStrictHostKeyChecking\fR\fR
812 .ad
813 .sp .6
814 .RS 4n
815 If this flag is set to \fByes\fR, \fBssh\fR never automatically adds host keys
816 to the \fB$HOME/.ssh/known_hosts\fR file, and refuses to connect hosts whose
817 host key has changed. This provides maximum protection against trojan horse
818 attacks. However, it can be a source of inconvenience if you do not have good
819 \fB/etc/ssh/ssh_known_hosts\fR files installed and frequently connect new
820 hosts. This option forces the user to manually add any new hosts. Normally this
821 option is disabled, and new hosts are automatically added to the known host
822 files. The host keys of known hosts are verified automatically in either case.
823 The argument must be \fByes\fR or \fBno\fR or \fBask\fR. The default is
824 \fBask\fR.
825 .RE
827 .sp
828 .ne 2
829 .na
830 \fB\fBUseOpenSSLEngine\fR\fR
831 .ad
832 .sp .6
833 .RS 4n
834 Specifies whether \fBssh\fR should use the OpenSSL PKCS#11 engine for
835 offloading cryptographic operations to the Cryptographic Framework.
836 Cryptographic operations are accelerated according to the available installed
837 plug-ins. When no suitable plug-ins are present this option does not have an
838 effect. The default is \fByes\fR.
839 .RE
841 .sp
842 .ne 2
843 .na
844 \fB\fBUsePrivilegedPort\fR\fR
845 .ad
846 .sp .6
847 .RS 4n
848 Specifies whether to use a privileged port for outgoing connections. The
849 argument must be \fByes\fR or \fBno\fR. The default is \fByes\fR. Setting this
850 option to \fBno\fR turns off \fBRhostsAuthentication\fR and
851 \fBRhostsRSAAuthentication\fR. If set to \fByes\fR \fBssh\fR must be
852 \fBsetuid\fR root. Defaults to \fBno\fR.
853 .RE
855 .sp
856 .ne 2
857 .na
858 \fB\fBUser\fR\fR
859 .ad
860 .sp .6
861 .RS 4n
862 Specifies the user to log in as. This can be useful if you have different user
863 names on different machines. This saves you the trouble of having to remember
864 to enter the user name on the command line.
865 .RE
867 .sp
868 .ne 2
869 .na
870 \fB\fBUserKnownHostsFile\fR\fR
871 .ad
872 .sp .6
873 .RS 4n
874 Specifies a file to use instead of \fB$HOME/.ssh/known_hosts\fR.
875 .RE
877 .sp
878 .ne 2
879 .na
880 \fB\fBUseRsh\fR\fR
881 .ad
882 .sp .6
883 .RS 4n
884 Specifies that \fBrlogin\fR or \fBrsh\fR should be used for this host. It is
885 possible that the host does not support the \fBssh\fR protocol. This causes
886 \fBssh\fR to immediately execute \fBrsh\fR(1). All other options (except
887 \fBHostName\fR) are ignored if this has been specified. The argument must be
888 \fByes\fR or \fBno\fR.
889 .RE
891 .sp
892 .ne 2
893 .na
894 \fB\fBXAuthLocation\fR\fR
895 .ad
896 .sp .6
897 .RS 4n
898 Specifies the location of the \fBxauth\fR(1) program. The default is
899 \fB/usr/openwin/bin/xauth\fR.
900 .RE
902 .SH SEE ALSO
903 .sp
904 .LP
905 \fBrsh\fR(1), \fBssh\fR(1), \fBssh-http-proxy-connect\fR(1),
906 \fBssh-keygen\fR(1), \fBssh-socks5-proxy-connect\fR(1), \fBsshd\fR(1M),
907 \fBsshd_config\fR(4), \fBkerberos\fR(5)
908 .sp
909 .LP
910 \fIRFC 4252\fR