Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.211
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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1.211 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.210 2015/05/28 05:09:45 dtucker Exp $
! 37: .Dd $Mdocdate: May 28 2015 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
42: .Nd OpenSSH SSH client configuration files
43: .Sh SYNOPSIS
1.98 jmc 44: .Nm ~/.ssh/config
45: .Nm /etc/ssh/ssh_config
1.1 stevesk 46: .Sh DESCRIPTION
1.84 jmc 47: .Xr ssh 1
1.1 stevesk 48: obtains configuration data from the following sources in
49: the following order:
1.79 jmc 50: .Pp
1.2 stevesk 51: .Bl -enum -offset indent -compact
52: .It
53: command-line options
54: .It
55: user's configuration file
1.50 djm 56: .Pq Pa ~/.ssh/config
1.2 stevesk 57: .It
58: system-wide configuration file
59: .Pq Pa /etc/ssh/ssh_config
60: .El
1.1 stevesk 61: .Pp
62: For each parameter, the first obtained value
63: will be used.
1.41 jmc 64: The configuration files contain sections separated by
1.1 stevesk 65: .Dq Host
66: specifications, and that section is only applied for hosts that
67: match one of the patterns given in the specification.
1.193 djm 68: The matched host name is usually the one given on the command line
69: (see the
70: .Cm CanonicalizeHostname
71: option for exceptions.)
1.1 stevesk 72: .Pp
73: Since the first obtained value for each parameter is used, more
74: host-specific declarations should be given near the beginning of the
75: file, and general defaults at the end.
1.80 jmc 76: .Pp
1.1 stevesk 77: The configuration file has the following format:
78: .Pp
79: Empty lines and lines starting with
80: .Ql #
81: are comments.
82: Otherwise a line is of the format
83: .Dq keyword arguments .
84: Configuration options may be separated by whitespace or
85: optional whitespace and exactly one
86: .Ql = ;
87: the latter format is useful to avoid the need to quote whitespace
88: when specifying configuration options using the
89: .Nm ssh ,
1.87 jmc 90: .Nm scp ,
1.1 stevesk 91: and
92: .Nm sftp
93: .Fl o
94: option.
1.88 dtucker 95: Arguments may optionally be enclosed in double quotes
96: .Pq \&"
97: in order to represent arguments containing spaces.
1.1 stevesk 98: .Pp
99: The possible
100: keywords and their meanings are as follows (note that
101: keywords are case-insensitive and arguments are case-sensitive):
102: .Bl -tag -width Ds
103: .It Cm Host
104: Restricts the following declarations (up to the next
105: .Cm Host
1.169 djm 106: or
107: .Cm Match
1.1 stevesk 108: keyword) to be only for those hosts that match one of the patterns
109: given after the keyword.
1.112 krw 110: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 111: A single
1.83 jmc 112: .Ql *
1.1 stevesk 113: as a pattern can be used to provide global
114: defaults for all hosts.
1.193 djm 115: The host is usually the
1.1 stevesk 116: .Ar hostname
1.193 djm 117: argument given on the command line
118: (see the
119: .Cm CanonicalizeHostname
120: option for exceptions.)
1.148 djm 121: .Pp
122: A pattern entry may be negated by prefixing it with an exclamation mark
123: .Pq Sq !\& .
124: If a negated entry is matched, then the
125: .Cm Host
126: entry is ignored, regardless of whether any other patterns on the line
127: match.
128: Negated matches are therefore useful to provide exceptions for wildcard
129: matches.
1.81 jmc 130: .Pp
131: See
132: .Sx PATTERNS
133: for more information on patterns.
1.170 jmc 134: .It Cm Match
1.169 djm 135: Restricts the following declarations (up to the next
136: .Cm Host
137: or
138: .Cm Match
139: keyword) to be used only when the conditions following the
140: .Cm Match
141: keyword are satisfied.
1.193 djm 142: Match conditions are specified using one or more critera
1.178 dtucker 143: or the single token
144: .Cm all
1.193 djm 145: which always matches.
146: The available criteria keywords are:
147: .Cm canonical ,
1.176 djm 148: .Cm exec ,
1.169 djm 149: .Cm host ,
150: .Cm originalhost ,
151: .Cm user ,
152: and
153: .Cm localuser .
1.193 djm 154: The
155: .Cm all
156: criteria must appear alone or immediately after
1.194 jmc 157: .Cm canonical .
1.193 djm 158: Other criteria may be combined arbitrarily.
159: All criteria but
160: .Cm all
161: and
162: .Cm canonical
163: require an argument.
164: Criteria may be negated by prepending an exclamation mark
165: .Pq Sq !\& .
1.169 djm 166: .Pp
1.177 jmc 167: The
1.193 djm 168: .Cm canonical
1.210 dtucker 169: keyword matches only when the configuration file is being re-parsed
1.193 djm 170: after hostname canonicalization (see the
171: .Cm CanonicalizeHostname
172: option.)
173: This may be useful to specify conditions that work with canonical host
174: names only.
175: The
1.176 djm 176: .Cm exec
1.177 jmc 177: keyword executes the specified command under the user's shell.
1.169 djm 178: If the command returns a zero exit status then the condition is considered true.
179: Commands containing whitespace characters must be quoted.
1.175 djm 180: The following character sequences in the command will be expanded prior to
181: execution:
182: .Ql %L
183: will be substituted by the first component of the local host name,
184: .Ql %l
185: will be substituted by the local host name (including any domain name),
186: .Ql %h
187: will be substituted by the target host name,
188: .Ql %n
189: will be substituted by the original target host name
1.176 djm 190: specified on the command-line,
1.175 djm 191: .Ql %p
192: the destination port,
193: .Ql %r
194: by the remote login username, and
195: .Ql %u
196: by the username of the user running
197: .Xr ssh 1 .
1.169 djm 198: .Pp
199: The other keywords' criteria must be single entries or comma-separated
200: lists and may use the wildcard and negation operators described in the
201: .Sx PATTERNS
202: section.
203: The criteria for the
204: .Cm host
205: keyword are matched against the target hostname, after any substitution
206: by the
207: .Cm Hostname
1.193 djm 208: or
209: .Cm CanonicalizeHostname
210: options.
1.169 djm 211: The
212: .Cm originalhost
213: keyword matches against the hostname as it was specified on the command-line.
214: The
215: .Cm user
216: keyword matches against the target username on the remote host.
217: The
218: .Cm localuser
219: keyword matches against the name of the local user running
220: .Xr ssh 1
221: (this keyword may be useful in system-wide
222: .Nm
223: files).
1.10 djm 224: .It Cm AddressFamily
1.11 jmc 225: Specifies which address family to use when connecting.
226: Valid arguments are
1.10 djm 227: .Dq any ,
228: .Dq inet
1.84 jmc 229: (use IPv4 only), or
1.10 djm 230: .Dq inet6
1.40 jmc 231: (use IPv6 only).
1.1 stevesk 232: .It Cm BatchMode
233: If set to
234: .Dq yes ,
235: passphrase/password querying will be disabled.
236: This option is useful in scripts and other batch jobs where no user
237: is present to supply the password.
238: The argument must be
239: .Dq yes
240: or
241: .Dq no .
242: The default is
243: .Dq no .
244: .It Cm BindAddress
1.60 dtucker 245: Use the specified address on the local machine as the source address of
1.61 jmc 246: the connection.
247: Only useful on systems with more than one address.
1.1 stevesk 248: Note that this option does not work if
249: .Cm UsePrivilegedPort
250: is set to
251: .Dq yes .
1.171 djm 252: .It Cm CanonicalDomains
1.172 jmc 253: When
1.173 djm 254: .Cm CanonicalizeHostname
1.171 djm 255: is enabled, this option specifies the list of domain suffixes in which to
256: search for the specified destination host.
1.173 djm 257: .It Cm CanonicalizeFallbackLocal
1.174 djm 258: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 259: The default,
1.184 djm 260: .Dq yes ,
1.172 jmc 261: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 262: search rules.
263: A value of
1.184 djm 264: .Dq no
1.171 djm 265: will cause
266: .Xr ssh 1
267: to fail instantly if
1.173 djm 268: .Cm CanonicalizeHostname
1.171 djm 269: is enabled and the target hostname cannot be found in any of the domains
270: specified by
271: .Cm CanonicalDomains .
1.173 djm 272: .It Cm CanonicalizeHostname
1.174 djm 273: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 274: The default,
275: .Dq no ,
1.171 djm 276: is not to perform any name rewriting and let the system resolver handle all
277: hostname lookups.
278: If set to
279: .Dq yes
280: then, for connections that do not use a
281: .Cm ProxyCommand ,
282: .Xr ssh 1
1.173 djm 283: will attempt to canonicalize the hostname specified on the command line
1.171 djm 284: using the
285: .Cm CanonicalDomains
286: suffixes and
1.173 djm 287: .Cm CanonicalizePermittedCNAMEs
1.171 djm 288: rules.
289: If
1.173 djm 290: .Cm CanonicalizeHostname
1.171 djm 291: is set to
292: .Dq always ,
1.174 djm 293: then canonicalization is applied to proxied connections too.
1.185 djm 294: .Pp
1.193 djm 295: If this option is enabled, then the configuration files are processed
296: again using the new target name to pick up any new configuration in matching
1.185 djm 297: .Cm Host
1.193 djm 298: and
299: .Cm Match
1.185 djm 300: stanzas.
1.173 djm 301: .It Cm CanonicalizeMaxDots
1.172 jmc 302: Specifies the maximum number of dot characters in a hostname before
1.174 djm 303: canonicalization is disabled.
1.172 jmc 304: The default,
305: .Dq 1 ,
306: allows a single dot (i.e. hostname.subdomain).
1.173 djm 307: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 308: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 309: canonicalizing hostnames.
1.171 djm 310: The rules consist of one or more arguments of
1.172 jmc 311: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 312: where
313: .Ar source_domain_list
1.174 djm 314: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 315: and
316: .Ar target_domain_list
1.172 jmc 317: is a pattern-list of domains that they may resolve to.
1.171 djm 318: .Pp
319: For example,
320: .Dq *.a.example.com:*.b.example.com,*.c.example.com
321: will allow hostnames matching
322: .Dq *.a.example.com
1.173 djm 323: to be canonicalized to names in the
1.171 djm 324: .Dq *.b.example.com
325: or
326: .Dq *.c.example.com
327: domains.
1.1 stevesk 328: .It Cm ChallengeResponseAuthentication
1.82 jmc 329: Specifies whether to use challenge-response authentication.
1.1 stevesk 330: The argument to this keyword must be
331: .Dq yes
332: or
333: .Dq no .
334: The default is
335: .Dq yes .
336: .It Cm CheckHostIP
337: If this flag is set to
338: .Dq yes ,
1.84 jmc 339: .Xr ssh 1
340: will additionally check the host IP address in the
1.1 stevesk 341: .Pa known_hosts
342: file.
1.211 ! djm 343: This allows ssh to detect if a host key changed due to DNS spoofing
! 344: and will add addresses of destination hosts to
! 345: .Pa ~/.ssh/known_hosts
! 346: in the process, regardless of the setting of
! 347: .Cm StrictHostKeyChecking .
1.107 grunk 348: If the option is set to
1.1 stevesk 349: .Dq no ,
350: the check will not be executed.
351: The default is
352: .Dq yes .
353: .It Cm Cipher
354: Specifies the cipher to use for encrypting the session
355: in protocol version 1.
356: Currently,
357: .Dq blowfish ,
358: .Dq 3des ,
359: and
360: .Dq des
361: are supported.
362: .Ar des
363: is only supported in the
1.84 jmc 364: .Xr ssh 1
1.1 stevesk 365: client for interoperability with legacy protocol 1 implementations
366: that do not support the
367: .Ar 3des
1.7 jmc 368: cipher.
369: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 370: The default is
371: .Dq 3des .
372: .It Cm Ciphers
373: Specifies the ciphers allowed for protocol version 2
374: in order of preference.
375: Multiple ciphers must be comma-separated.
1.180 djm 376: The supported ciphers are:
377: .Pp
1.186 naddy 378: .Bl -item -compact -offset indent
379: .It
380: 3des-cbc
381: .It
382: aes128-cbc
383: .It
384: aes192-cbc
385: .It
386: aes256-cbc
387: .It
388: aes128-ctr
389: .It
390: aes192-ctr
391: .It
392: aes256-ctr
393: .It
394: aes128-gcm@openssh.com
395: .It
396: aes256-gcm@openssh.com
397: .It
398: arcfour
399: .It
400: arcfour128
401: .It
402: arcfour256
403: .It
404: blowfish-cbc
405: .It
406: cast128-cbc
407: .It
408: chacha20-poly1305@openssh.com
409: .El
1.180 djm 410: .Pp
1.84 jmc 411: The default is:
1.186 naddy 412: .Bd -literal -offset indent
413: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 414: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.180 djm 415: chacha20-poly1305@openssh.com,
1.186 naddy 416: arcfour256,arcfour128,
417: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
418: aes192-cbc,aes256-cbc,arcfour
1.1 stevesk 419: .Ed
1.180 djm 420: .Pp
421: The list of available ciphers may also be obtained using the
422: .Fl Q
423: option of
1.198 djm 424: .Xr ssh 1
425: with an argument of
426: .Dq cipher .
1.1 stevesk 427: .It Cm ClearAllForwardings
1.84 jmc 428: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 429: specified in the configuration files or on the command line be
1.7 jmc 430: cleared.
431: This option is primarily useful when used from the
1.84 jmc 432: .Xr ssh 1
1.1 stevesk 433: command line to clear port forwardings set in
434: configuration files, and is automatically set by
435: .Xr scp 1
436: and
437: .Xr sftp 1 .
438: The argument must be
439: .Dq yes
440: or
441: .Dq no .
442: The default is
443: .Dq no .
444: .It Cm Compression
445: Specifies whether to use compression.
446: The argument must be
447: .Dq yes
448: or
449: .Dq no .
450: The default is
451: .Dq no .
452: .It Cm CompressionLevel
453: Specifies the compression level to use if compression is enabled.
454: The argument must be an integer from 1 (fast) to 9 (slow, best).
455: The default level is 6, which is good for most applications.
456: The meaning of the values is the same as in
457: .Xr gzip 1 .
458: Note that this option applies to protocol version 1 only.
459: .It Cm ConnectionAttempts
460: Specifies the number of tries (one per second) to make before exiting.
461: The argument must be an integer.
462: This may be useful in scripts if the connection sometimes fails.
463: The default is 1.
1.9 djm 464: .It Cm ConnectTimeout
1.84 jmc 465: Specifies the timeout (in seconds) used when connecting to the
466: SSH server, instead of using the default system TCP timeout.
1.11 jmc 467: This value is used only when the target is down or really unreachable,
468: not when it refuses the connection.
1.36 djm 469: .It Cm ControlMaster
470: Enables the sharing of multiple sessions over a single network connection.
471: When set to
1.84 jmc 472: .Dq yes ,
473: .Xr ssh 1
1.36 djm 474: will listen for connections on a control socket specified using the
475: .Cm ControlPath
476: argument.
477: Additional sessions can connect to this socket using the same
478: .Cm ControlPath
479: with
480: .Cm ControlMaster
481: set to
482: .Dq no
1.38 jmc 483: (the default).
1.64 jmc 484: These sessions will try to reuse the master instance's network connection
1.63 djm 485: rather than initiating new ones, but will fall back to connecting normally
486: if the control socket does not exist, or is not listening.
487: .Pp
1.37 djm 488: Setting this to
489: .Dq ask
1.84 jmc 490: will cause ssh
1.206 jmc 491: to listen for control connections, but require confirmation using
492: .Xr ssh-askpass 1 .
1.51 jakob 493: If the
494: .Cm ControlPath
1.84 jmc 495: cannot be opened,
496: ssh will continue without connecting to a master instance.
1.58 djm 497: .Pp
498: X11 and
1.59 jmc 499: .Xr ssh-agent 1
1.58 djm 500: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 501: display and agent forwarded will be the one belonging to the master
1.59 jmc 502: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 503: .Pp
504: Two additional options allow for opportunistic multiplexing: try to use a
505: master connection but fall back to creating a new one if one does not already
506: exist.
507: These options are:
508: .Dq auto
509: and
510: .Dq autoask .
511: The latter requires confirmation like the
512: .Dq ask
513: option.
1.36 djm 514: .It Cm ControlPath
1.55 djm 515: Specify the path to the control socket used for connection sharing as described
516: in the
1.36 djm 517: .Cm ControlMaster
1.57 djm 518: section above or the string
519: .Dq none
520: to disable connection sharing.
1.55 djm 521: In the path,
1.147 djm 522: .Ql %L
523: will be substituted by the first component of the local host name,
1.77 djm 524: .Ql %l
1.147 djm 525: will be substituted by the local host name (including any domain name),
1.55 djm 526: .Ql %h
527: will be substituted by the target host name,
1.150 jmc 528: .Ql %n
529: will be substituted by the original target host name
530: specified on the command line,
1.55 djm 531: .Ql %p
1.175 djm 532: the destination port,
1.55 djm 533: .Ql %r
1.188 djm 534: by the remote login username,
1.147 djm 535: .Ql %u
536: by the username of the user running
1.188 djm 537: .Xr ssh 1 , and
1.189 jmc 538: .Ql \&%C
1.188 djm 539: by a hash of the concatenation: %l%h%p%r.
1.56 djm 540: It is recommended that any
541: .Cm ControlPath
542: used for opportunistic connection sharing include
1.195 djm 543: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
544: that is not writable by other users.
1.56 djm 545: This ensures that shared connections are uniquely identified.
1.137 djm 546: .It Cm ControlPersist
547: When used in conjunction with
548: .Cm ControlMaster ,
549: specifies that the master connection should remain open
550: in the background (waiting for future client connections)
551: after the initial client connection has been closed.
552: If set to
553: .Dq no ,
554: then the master connection will not be placed into the background,
555: and will close as soon as the initial client connection is closed.
556: If set to
1.195 djm 557: .Dq yes
558: or
559: .Dq 0 ,
1.137 djm 560: then the master connection will remain in the background indefinitely
561: (until killed or closed via a mechanism such as the
562: .Xr ssh 1
563: .Dq Fl O No exit
564: option).
565: If set to a time in seconds, or a time in any of the formats documented in
566: .Xr sshd_config 5 ,
567: then the backgrounded master connection will automatically terminate
568: after it has remained idle (with no client connections) for the
569: specified time.
1.38 jmc 570: .It Cm DynamicForward
1.74 jmc 571: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 572: over the secure channel, and the application
573: protocol is then used to determine where to connect to from the
574: remote machine.
1.62 djm 575: .Pp
576: The argument must be
577: .Sm off
578: .Oo Ar bind_address : Oc Ar port .
579: .Sm on
1.138 djm 580: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 581: By default, the local port is bound in accordance with the
582: .Cm GatewayPorts
583: setting.
584: However, an explicit
585: .Ar bind_address
586: may be used to bind the connection to a specific address.
587: The
588: .Ar bind_address
589: of
590: .Dq localhost
591: indicates that the listening port be bound for local use only, while an
592: empty address or
593: .Sq *
594: indicates that the port should be available from all interfaces.
595: .Pp
1.38 jmc 596: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 597: .Xr ssh 1
1.38 jmc 598: will act as a SOCKS server.
599: Multiple forwardings may be specified, and
600: additional forwardings can be given on the command line.
601: Only the superuser can forward privileged ports.
1.14 markus 602: .It Cm EnableSSHKeysign
603: Setting this option to
604: .Dq yes
605: in the global client configuration file
606: .Pa /etc/ssh/ssh_config
607: enables the use of the helper program
608: .Xr ssh-keysign 8
609: during
610: .Cm HostbasedAuthentication .
611: The argument must be
612: .Dq yes
613: or
614: .Dq no .
615: The default is
616: .Dq no .
1.23 jmc 617: This option should be placed in the non-hostspecific section.
1.14 markus 618: See
619: .Xr ssh-keysign 8
620: for more information.
1.1 stevesk 621: .It Cm EscapeChar
622: Sets the escape character (default:
623: .Ql ~ ) .
624: The escape character can also
625: be set on the command line.
626: The argument should be a single character,
627: .Ql ^
628: followed by a letter, or
629: .Dq none
630: to disable the escape
631: character entirely (making the connection transparent for binary
632: data).
1.96 markus 633: .It Cm ExitOnForwardFailure
634: Specifies whether
635: .Xr ssh 1
636: should terminate the connection if it cannot set up all requested
1.102 stevesk 637: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 638: The argument must be
639: .Dq yes
640: or
641: .Dq no .
642: The default is
643: .Dq no .
1.197 djm 644: .It Cm FingerprintHash
645: Specifies the hash algorithm used when displaying key fingerprints.
646: Valid options are:
647: .Dq md5
648: and
649: .Dq sha256 .
650: The default is
651: .Dq sha256 .
1.1 stevesk 652: .It Cm ForwardAgent
653: Specifies whether the connection to the authentication agent (if any)
654: will be forwarded to the remote machine.
655: The argument must be
656: .Dq yes
657: or
658: .Dq no .
659: The default is
660: .Dq no .
1.3 stevesk 661: .Pp
1.7 jmc 662: Agent forwarding should be enabled with caution.
663: Users with the ability to bypass file permissions on the remote host
664: (for the agent's Unix-domain socket)
665: can access the local agent through the forwarded connection.
666: An attacker cannot obtain key material from the agent,
1.3 stevesk 667: however they can perform operations on the keys that enable them to
668: authenticate using the identities loaded into the agent.
1.1 stevesk 669: .It Cm ForwardX11
670: Specifies whether X11 connections will be automatically redirected
671: over the secure channel and
672: .Ev DISPLAY
673: set.
674: The argument must be
675: .Dq yes
676: or
677: .Dq no .
678: The default is
679: .Dq no .
1.3 stevesk 680: .Pp
1.7 jmc 681: X11 forwarding should be enabled with caution.
682: Users with the ability to bypass file permissions on the remote host
1.22 markus 683: (for the user's X11 authorization database)
1.7 jmc 684: can access the local X11 display through the forwarded connection.
1.22 markus 685: An attacker may then be able to perform activities such as keystroke monitoring
686: if the
687: .Cm ForwardX11Trusted
688: option is also enabled.
1.134 djm 689: .It Cm ForwardX11Timeout
1.135 jmc 690: Specify a timeout for untrusted X11 forwarding
691: using the format described in the
1.166 jmc 692: TIME FORMATS section of
1.134 djm 693: .Xr sshd_config 5 .
694: X11 connections received by
695: .Xr ssh 1
696: after this time will be refused.
697: The default is to disable untrusted X11 forwarding after twenty minutes has
698: elapsed.
1.22 markus 699: .It Cm ForwardX11Trusted
1.34 jmc 700: If this option is set to
1.84 jmc 701: .Dq yes ,
702: remote X11 clients will have full access to the original X11 display.
1.42 djm 703: .Pp
1.22 markus 704: If this option is set to
1.84 jmc 705: .Dq no ,
706: remote X11 clients will be considered untrusted and prevented
1.22 markus 707: from stealing or tampering with data belonging to trusted X11
708: clients.
1.42 djm 709: Furthermore, the
710: .Xr xauth 1
711: token used for the session will be set to expire after 20 minutes.
712: Remote clients will be refused access after this time.
1.22 markus 713: .Pp
714: The default is
715: .Dq no .
716: .Pp
717: See the X11 SECURITY extension specification for full details on
718: the restrictions imposed on untrusted clients.
1.1 stevesk 719: .It Cm GatewayPorts
720: Specifies whether remote hosts are allowed to connect to local
721: forwarded ports.
722: By default,
1.84 jmc 723: .Xr ssh 1
1.7 jmc 724: binds local port forwardings to the loopback address.
725: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 726: .Cm GatewayPorts
1.84 jmc 727: can be used to specify that ssh
1.1 stevesk 728: should bind local port forwardings to the wildcard address,
729: thus allowing remote hosts to connect to forwarded ports.
730: The argument must be
731: .Dq yes
732: or
733: .Dq no .
734: The default is
735: .Dq no .
736: .It Cm GlobalKnownHostsFile
1.151 djm 737: Specifies one or more files to use for the global
738: host key database, separated by whitespace.
739: The default is
740: .Pa /etc/ssh/ssh_known_hosts ,
741: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 742: .It Cm GSSAPIAuthentication
1.27 markus 743: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 744: The default is
1.21 markus 745: .Dq no .
1.18 markus 746: Note that this option applies to protocol version 2 only.
747: .It Cm GSSAPIDelegateCredentials
748: Forward (delegate) credentials to the server.
749: The default is
750: .Dq no .
751: Note that this option applies to protocol version 2 only.
1.44 djm 752: .It Cm HashKnownHosts
753: Indicates that
1.84 jmc 754: .Xr ssh 1
1.44 djm 755: should hash host names and addresses when they are added to
1.50 djm 756: .Pa ~/.ssh/known_hosts .
1.44 djm 757: These hashed names may be used normally by
1.84 jmc 758: .Xr ssh 1
1.44 djm 759: and
1.84 jmc 760: .Xr sshd 8 ,
1.44 djm 761: but they do not reveal identifying information should the file's contents
762: be disclosed.
763: The default is
764: .Dq no .
1.97 jmc 765: Note that existing names and addresses in known hosts files
766: will not be converted automatically,
767: but may be manually hashed using
1.45 djm 768: .Xr ssh-keygen 1 .
1.1 stevesk 769: .It Cm HostbasedAuthentication
770: Specifies whether to try rhosts based authentication with public key
771: authentication.
772: The argument must be
773: .Dq yes
774: or
775: .Dq no .
776: The default is
777: .Dq no .
778: This option applies to protocol version 2 only and
779: is similar to
780: .Cm RhostsRSAAuthentication .
1.202 djm 781: .It Cm HostbasedKeyTypes
782: Specifies the key types that will be used for hostbased authentication
783: as a comma-separated pattern list.
784: The default
785: .Dq *
786: will allow all key types.
787: The
788: .Fl Q
789: option of
790: .Xr ssh 1
791: may be used to list supported key types.
1.1 stevesk 792: .It Cm HostKeyAlgorithms
793: Specifies the protocol version 2 host key algorithms
794: that the client wants to use in order of preference.
795: The default for this option is:
1.139 djm 796: .Bd -literal -offset 3n
797: ecdsa-sha2-nistp256-cert-v01@openssh.com,
798: ecdsa-sha2-nistp384-cert-v01@openssh.com,
799: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 800: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 801: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
802: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
803: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.182 markus 804: ssh-ed25519,ssh-rsa,ssh-dss
1.139 djm 805: .Ed
1.145 djm 806: .Pp
807: If hostkeys are known for the destination host then this default is modified
808: to prefer their algorithms.
1.198 djm 809: .Pp
810: The list of available key types may also be obtained using the
811: .Fl Q
812: option of
813: .Xr ssh 1
814: with an argument of
815: .Dq key .
1.1 stevesk 816: .It Cm HostKeyAlias
817: Specifies an alias that should be used instead of the
818: real host name when looking up or saving the host key
819: in the host key database files.
1.84 jmc 820: This option is useful for tunneling SSH connections
1.1 stevesk 821: or for multiple servers running on a single host.
822: .It Cm HostName
823: Specifies the real host name to log into.
824: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 825: If the hostname contains the character sequence
826: .Ql %h ,
1.150 jmc 827: then this will be replaced with the host name specified on the command line
1.136 djm 828: (this is useful for manipulating unqualified names).
1.187 djm 829: The character sequence
830: .Ql %%
831: will be replaced by a single
832: .Ql %
833: character, which may be used when specifying IPv6 link-local addresses.
834: .Pp
1.84 jmc 835: The default is the name given on the command line.
1.1 stevesk 836: Numeric IP addresses are also permitted (both on the command line and in
837: .Cm HostName
838: specifications).
1.29 markus 839: .It Cm IdentitiesOnly
840: Specifies that
1.84 jmc 841: .Xr ssh 1
1.29 markus 842: should only use the authentication identity files configured in the
1.31 jmc 843: .Nm
1.29 markus 844: files,
1.84 jmc 845: even if
846: .Xr ssh-agent 1
1.159 djm 847: or a
848: .Cm PKCS11Provider
1.29 markus 849: offers more identities.
850: The argument to this keyword must be
851: .Dq yes
852: or
853: .Dq no .
1.84 jmc 854: This option is intended for situations where ssh-agent
1.29 markus 855: offers many different identities.
856: The default is
857: .Dq no .
1.67 jmc 858: .It Cm IdentityFile
1.192 sobrado 859: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 860: identity is read.
1.67 jmc 861: The default is
862: .Pa ~/.ssh/identity
863: for protocol version 1, and
1.139 djm 864: .Pa ~/.ssh/id_dsa ,
1.183 naddy 865: .Pa ~/.ssh/id_ecdsa ,
866: .Pa ~/.ssh/id_ed25519
1.139 djm 867: and
1.67 jmc 868: .Pa ~/.ssh/id_rsa
869: for protocol version 2.
870: Additionally, any identities represented by the authentication agent
1.165 djm 871: will be used for authentication unless
872: .Cm IdentitiesOnly
873: is set.
1.129 djm 874: .Xr ssh 1
875: will try to load certificate information from the filename obtained by
876: appending
877: .Pa -cert.pub
878: to the path of a specified
879: .Cm IdentityFile .
1.90 djm 880: .Pp
1.67 jmc 881: The file name may use the tilde
1.91 jmc 882: syntax to refer to a user's home directory or one of the following
1.90 djm 883: escape characters:
884: .Ql %d
885: (local user's home directory),
886: .Ql %u
887: (local user name),
888: .Ql %l
889: (local host name),
890: .Ql %h
891: (remote host name) or
1.92 djm 892: .Ql %r
1.90 djm 893: (remote user name).
894: .Pp
1.67 jmc 895: It is possible to have
896: multiple identity files specified in configuration files; all these
897: identities will be tried in sequence.
1.152 djm 898: Multiple
899: .Cm IdentityFile
900: directives will add to the list of identities tried (this behaviour
901: differs from that of other configuration directives).
1.165 djm 902: .Pp
903: .Cm IdentityFile
904: may be used in conjunction with
905: .Cm IdentitiesOnly
906: to select which identities in an agent are offered during authentication.
1.164 jmc 907: .It Cm IgnoreUnknown
908: Specifies a pattern-list of unknown options to be ignored if they are
909: encountered in configuration parsing.
910: This may be used to suppress errors if
911: .Nm
912: contains options that are unrecognised by
913: .Xr ssh 1 .
914: It is recommended that
915: .Cm IgnoreUnknown
916: be listed early in the configuration file as it will not be applied
917: to unknown options that appear before it.
1.143 djm 918: .It Cm IPQoS
919: Specifies the IPv4 type-of-service or DSCP class for connections.
920: Accepted values are
921: .Dq af11 ,
922: .Dq af12 ,
923: .Dq af13 ,
1.154 djm 924: .Dq af21 ,
1.143 djm 925: .Dq af22 ,
926: .Dq af23 ,
927: .Dq af31 ,
928: .Dq af32 ,
929: .Dq af33 ,
930: .Dq af41 ,
931: .Dq af42 ,
932: .Dq af43 ,
933: .Dq cs0 ,
934: .Dq cs1 ,
935: .Dq cs2 ,
936: .Dq cs3 ,
937: .Dq cs4 ,
938: .Dq cs5 ,
939: .Dq cs6 ,
940: .Dq cs7 ,
941: .Dq ef ,
942: .Dq lowdelay ,
943: .Dq throughput ,
944: .Dq reliability ,
945: or a numeric value.
1.146 djm 946: This option may take one or two arguments, separated by whitespace.
1.143 djm 947: If one argument is specified, it is used as the packet class unconditionally.
948: If two values are specified, the first is automatically selected for
949: interactive sessions and the second for non-interactive sessions.
950: The default is
951: .Dq lowdelay
952: for interactive sessions and
953: .Dq throughput
954: for non-interactive sessions.
1.103 djm 955: .It Cm KbdInteractiveAuthentication
956: Specifies whether to use keyboard-interactive authentication.
957: The argument to this keyword must be
958: .Dq yes
959: or
960: .Dq no .
961: The default is
962: .Dq yes .
1.39 djm 963: .It Cm KbdInteractiveDevices
964: Specifies the list of methods to use in keyboard-interactive authentication.
965: Multiple method names must be comma-separated.
966: The default is to use the server specified list.
1.85 jmc 967: The methods available vary depending on what the server supports.
968: For an OpenSSH server,
969: it may be zero or more of:
970: .Dq bsdauth ,
971: .Dq pam ,
972: and
973: .Dq skey .
1.140 djm 974: .It Cm KexAlgorithms
975: Specifies the available KEX (Key Exchange) algorithms.
976: Multiple algorithms must be comma-separated.
1.141 jmc 977: The default is:
978: .Bd -literal -offset indent
1.179 markus 979: curve25519-sha256@libssh.org,
1.141 jmc 980: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
981: diffie-hellman-group-exchange-sha256,
1.209 dtucker 982: diffie-hellman-group-exchange-sha1,
1.186 naddy 983: diffie-hellman-group14-sha1,
1.141 jmc 984: diffie-hellman-group1-sha1
985: .Ed
1.198 djm 986: .Pp
987: The list of available key exchange algorithms may also be obtained using the
988: .Fl Q
989: option of
990: .Xr ssh 1
991: with an argument of
992: .Dq kex .
1.65 reyk 993: .It Cm LocalCommand
994: Specifies a command to execute on the local machine after successfully
995: connecting to the server.
996: The command string extends to the end of the line, and is executed with
1.105 jmc 997: the user's shell.
1.109 dtucker 998: The following escape character substitutions will be performed:
999: .Ql %d
1000: (local user's home directory),
1001: .Ql %h
1002: (remote host name),
1003: .Ql %l
1004: (local host name),
1005: .Ql %n
1006: (host name as provided on the command line),
1007: .Ql %p
1008: (remote port),
1009: .Ql %r
1010: (remote user name) or
1011: .Ql %u
1.188 djm 1012: (local user name) or
1.189 jmc 1013: .Ql \&%C
1.188 djm 1014: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1015: .Pp
1016: The command is run synchronously and does not have access to the
1017: session of the
1018: .Xr ssh 1
1019: that spawned it.
1020: It should not be used for interactive commands.
1021: .Pp
1.65 reyk 1022: This directive is ignored unless
1023: .Cm PermitLocalCommand
1024: has been enabled.
1.1 stevesk 1025: .It Cm LocalForward
1.74 jmc 1026: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1027: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1028: The first argument must be
1.43 djm 1029: .Sm off
1.49 jmc 1030: .Oo Ar bind_address : Oc Ar port
1.43 djm 1031: .Sm on
1.49 jmc 1032: and the second argument must be
1033: .Ar host : Ns Ar hostport .
1.138 djm 1034: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1035: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1036: given on the command line.
1.1 stevesk 1037: Only the superuser can forward privileged ports.
1.43 djm 1038: By default, the local port is bound in accordance with the
1039: .Cm GatewayPorts
1040: setting.
1041: However, an explicit
1042: .Ar bind_address
1043: may be used to bind the connection to a specific address.
1044: The
1045: .Ar bind_address
1046: of
1047: .Dq localhost
1.46 jmc 1048: indicates that the listening port be bound for local use only, while an
1049: empty address or
1050: .Sq *
1.43 djm 1051: indicates that the port should be available from all interfaces.
1.1 stevesk 1052: .It Cm LogLevel
1053: Gives the verbosity level that is used when logging messages from
1.84 jmc 1054: .Xr ssh 1 .
1.1 stevesk 1055: The possible values are:
1.84 jmc 1056: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1057: The default is INFO.
1058: DEBUG and DEBUG1 are equivalent.
1059: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1060: .It Cm MACs
1061: Specifies the MAC (message authentication code) algorithms
1062: in order of preference.
1063: The MAC algorithm is used in protocol version 2
1064: for data integrity protection.
1065: Multiple algorithms must be comma-separated.
1.160 markus 1066: The algorithms that contain
1067: .Dq -etm
1068: calculate the MAC after encryption (encrypt-then-mac).
1069: These are considered safer and their use recommended.
1.84 jmc 1070: The default is:
1.101 jmc 1071: .Bd -literal -offset indent
1.160 markus 1072: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1073: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1074: umac-64@openssh.com,umac-128@openssh.com,
1075: hmac-sha2-256,hmac-sha2-512,
1076: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1077: hmac-ripemd160-etm@openssh.com,
1078: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1079: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1080: hmac-sha1-96,hmac-md5-96
1.101 jmc 1081: .Ed
1.198 djm 1082: .Pp
1083: The list of available MAC algorithms may also be obtained using the
1084: .Fl Q
1085: option of
1086: .Xr ssh 1
1087: with an argument of
1088: .Dq mac .
1.1 stevesk 1089: .It Cm NoHostAuthenticationForLocalhost
1090: This option can be used if the home directory is shared across machines.
1091: In this case localhost will refer to a different machine on each of
1092: the machines and the user will get many warnings about changed host keys.
1093: However, this option disables host authentication for localhost.
1094: The argument to this keyword must be
1095: .Dq yes
1096: or
1097: .Dq no .
1098: The default is to check the host key for localhost.
1099: .It Cm NumberOfPasswordPrompts
1100: Specifies the number of password prompts before giving up.
1101: The argument to this keyword must be an integer.
1.84 jmc 1102: The default is 3.
1.1 stevesk 1103: .It Cm PasswordAuthentication
1104: Specifies whether to use password authentication.
1105: The argument to this keyword must be
1106: .Dq yes
1107: or
1108: .Dq no .
1109: The default is
1110: .Dq yes .
1.65 reyk 1111: .It Cm PermitLocalCommand
1112: Allow local command execution via the
1113: .Ic LocalCommand
1114: option or using the
1.66 jmc 1115: .Ic !\& Ns Ar command
1.65 reyk 1116: escape sequence in
1117: .Xr ssh 1 .
1118: The argument must be
1119: .Dq yes
1120: or
1121: .Dq no .
1122: The default is
1123: .Dq no .
1.127 markus 1124: .It Cm PKCS11Provider
1125: Specifies which PKCS#11 provider to use.
1.144 jmc 1126: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1127: .Xr ssh 1
1.128 markus 1128: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1129: private RSA key.
1.67 jmc 1130: .It Cm Port
1131: Specifies the port number to connect on the remote host.
1.84 jmc 1132: The default is 22.
1.1 stevesk 1133: .It Cm PreferredAuthentications
1134: Specifies the order in which the client should try protocol 2
1.11 jmc 1135: authentication methods.
1.48 jmc 1136: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1137: .Cm keyboard-interactive )
1.48 jmc 1138: over another method (e.g.\&
1.131 jmc 1139: .Cm password ) .
1140: The default is:
1141: .Bd -literal -offset indent
1142: gssapi-with-mic,hostbased,publickey,
1143: keyboard-interactive,password
1144: .Ed
1.1 stevesk 1145: .It Cm Protocol
1146: Specifies the protocol versions
1.84 jmc 1147: .Xr ssh 1
1.1 stevesk 1148: should support in order of preference.
1149: The possible values are
1.84 jmc 1150: .Sq 1
1.1 stevesk 1151: and
1.84 jmc 1152: .Sq 2 .
1.1 stevesk 1153: Multiple versions must be comma-separated.
1.120 markus 1154: When this option is set to
1.121 jmc 1155: .Dq 2,1
1.120 markus 1156: .Nm ssh
1157: will try version 2 and fall back to version 1
1158: if version 2 is not available.
1.1 stevesk 1159: The default is
1.121 jmc 1160: .Sq 2 .
1.1 stevesk 1161: .It Cm ProxyCommand
1162: Specifies the command to use to connect to the server.
1163: The command
1.190 djm 1164: string extends to the end of the line, and is executed
1165: using the user's shell
1166: .Ql exec
1167: directive to avoid a lingering shell process.
1168: .Pp
1.133 jmc 1169: In the command string, any occurrence of
1.1 stevesk 1170: .Ql %h
1171: will be substituted by the host name to
1.132 djm 1172: connect,
1.1 stevesk 1173: .Ql %p
1.133 jmc 1174: by the port, and
1175: .Ql %r
1.132 djm 1176: by the remote user name.
1.1 stevesk 1177: The command can be basically anything,
1178: and should read from its standard input and write to its standard output.
1179: It should eventually connect an
1180: .Xr sshd 8
1181: server running on some machine, or execute
1182: .Ic sshd -i
1183: somewhere.
1184: Host key management will be done using the
1185: HostName of the host being connected (defaulting to the name typed by
1186: the user).
1.7 jmc 1187: Setting the command to
1188: .Dq none
1.6 markus 1189: disables this option entirely.
1.1 stevesk 1190: Note that
1191: .Cm CheckHostIP
1192: is not available for connects with a proxy command.
1.52 djm 1193: .Pp
1194: This directive is useful in conjunction with
1195: .Xr nc 1
1196: and its proxy support.
1.53 jmc 1197: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1198: 192.0.2.0:
1199: .Bd -literal -offset 3n
1200: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1201: .Ed
1.167 djm 1202: .It Cm ProxyUseFdpass
1.168 jmc 1203: Specifies that
1.167 djm 1204: .Cm ProxyCommand
1205: will pass a connected file descriptor back to
1.168 jmc 1206: .Xr ssh 1
1.167 djm 1207: instead of continuing to execute and pass data.
1208: The default is
1209: .Dq no .
1.1 stevesk 1210: .It Cm PubkeyAuthentication
1211: Specifies whether to try public key authentication.
1212: The argument to this keyword must be
1213: .Dq yes
1214: or
1215: .Dq no .
1216: The default is
1217: .Dq yes .
1218: This option applies to protocol version 2 only.
1.75 dtucker 1219: .It Cm RekeyLimit
1220: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1221: session key is renegotiated, optionally followed a maximum amount of
1222: time that may pass before the session key is renegotiated.
1223: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1224: .Sq K ,
1225: .Sq M ,
1.75 dtucker 1226: or
1.76 jmc 1227: .Sq G
1.75 dtucker 1228: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1229: The default is between
1.84 jmc 1230: .Sq 1G
1.75 dtucker 1231: and
1.84 jmc 1232: .Sq 4G ,
1.75 dtucker 1233: depending on the cipher.
1.162 dtucker 1234: The optional second value is specified in seconds and may use any of the
1235: units documented in the
1.166 jmc 1236: TIME FORMATS section of
1.162 dtucker 1237: .Xr sshd_config 5 .
1238: The default value for
1239: .Cm RekeyLimit
1240: is
1241: .Dq default none ,
1242: which means that rekeying is performed after the cipher's default amount
1243: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1244: This option applies to protocol version 2 only.
1.1 stevesk 1245: .It Cm RemoteForward
1.74 jmc 1246: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1247: the secure channel to the specified host and port from the local machine.
1.49 jmc 1248: The first argument must be
1.43 djm 1249: .Sm off
1.49 jmc 1250: .Oo Ar bind_address : Oc Ar port
1.43 djm 1251: .Sm on
1.49 jmc 1252: and the second argument must be
1253: .Ar host : Ns Ar hostport .
1.138 djm 1254: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1255: Multiple forwardings may be specified, and additional
1256: forwardings can be given on the command line.
1.113 stevesk 1257: Privileged ports can be forwarded only when
1258: logging in as root on the remote machine.
1.118 jmc 1259: .Pp
1.117 djm 1260: If the
1261: .Ar port
1262: argument is
1263: .Ql 0 ,
1264: the listen port will be dynamically allocated on the server and reported
1265: to the client at run time.
1.43 djm 1266: .Pp
1267: If the
1268: .Ar bind_address
1269: is not specified, the default is to only bind to loopback addresses.
1270: If the
1271: .Ar bind_address
1272: is
1273: .Ql *
1274: or an empty string, then the forwarding is requested to listen on all
1275: interfaces.
1276: Specifying a remote
1277: .Ar bind_address
1.46 jmc 1278: will only succeed if the server's
1279: .Cm GatewayPorts
1.43 djm 1280: option is enabled (see
1.46 jmc 1281: .Xr sshd_config 5 ) .
1.149 djm 1282: .It Cm RequestTTY
1283: Specifies whether to request a pseudo-tty for the session.
1284: The argument may be one of:
1285: .Dq no
1286: (never request a TTY),
1287: .Dq yes
1288: (always request a TTY when standard input is a TTY),
1289: .Dq force
1290: (always request a TTY) or
1291: .Dq auto
1292: (request a TTY when opening a login session).
1293: This option mirrors the
1294: .Fl t
1295: and
1296: .Fl T
1297: flags for
1298: .Xr ssh 1 .
1.196 djm 1299: .It Cm RevokedHostKeys
1300: Specifies revoked host public keys.
1301: Keys listed in this file will be refused for host authentication.
1302: Note that if this file does not exist or is not readable,
1303: then host authentication will be refused for all hosts.
1304: Keys may be specified as a text file, listing one public key per line, or as
1305: an OpenSSH Key Revocation List (KRL) as generated by
1306: .Xr ssh-keygen 1 .
1307: For more information on KRLs, see the KEY REVOCATION LISTS section in
1308: .Xr ssh-keygen 1 .
1.1 stevesk 1309: .It Cm RhostsRSAAuthentication
1310: Specifies whether to try rhosts based authentication with RSA host
1311: authentication.
1312: The argument must be
1313: .Dq yes
1314: or
1315: .Dq no .
1316: The default is
1317: .Dq no .
1318: This option applies to protocol version 1 only and requires
1.84 jmc 1319: .Xr ssh 1
1.1 stevesk 1320: to be setuid root.
1321: .It Cm RSAAuthentication
1322: Specifies whether to try RSA authentication.
1323: The argument to this keyword must be
1324: .Dq yes
1325: or
1326: .Dq no .
1327: RSA authentication will only be
1328: attempted if the identity file exists, or an authentication agent is
1329: running.
1330: The default is
1331: .Dq yes .
1332: Note that this option applies to protocol version 1 only.
1.32 djm 1333: .It Cm SendEnv
1334: Specifies what variables from the local
1335: .Xr environ 7
1336: should be sent to the server.
1.84 jmc 1337: Note that environment passing is only supported for protocol 2.
1338: The server must also support it, and the server must be configured to
1.33 djm 1339: accept these environment variables.
1.207 dtucker 1340: Note that the
1341: .Ev TERM
1.208 jmc 1342: environment variable is always sent whenever a
1.207 dtucker 1343: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1344: Refer to
1345: .Cm AcceptEnv
1346: in
1347: .Xr sshd_config 5
1348: for how to configure the server.
1.80 jmc 1349: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1350: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1351: across multiple
1352: .Cm SendEnv
1353: directives.
1354: The default is not to send any environment variables.
1.81 jmc 1355: .Pp
1356: See
1357: .Sx PATTERNS
1358: for more information on patterns.
1.28 markus 1359: .It Cm ServerAliveCountMax
1.73 jmc 1360: Sets the number of server alive messages (see below) which may be
1.28 markus 1361: sent without
1.84 jmc 1362: .Xr ssh 1
1.28 markus 1363: receiving any messages back from the server.
1364: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1365: ssh will disconnect from the server, terminating the session.
1.28 markus 1366: It is important to note that the use of server alive messages is very
1367: different from
1368: .Cm TCPKeepAlive
1369: (below).
1370: The server alive messages are sent through the encrypted channel
1371: and therefore will not be spoofable.
1372: The TCP keepalive option enabled by
1373: .Cm TCPKeepAlive
1374: is spoofable.
1375: The server alive mechanism is valuable when the client or
1376: server depend on knowing when a connection has become inactive.
1377: .Pp
1378: The default value is 3.
1379: If, for example,
1380: .Cm ServerAliveInterval
1.84 jmc 1381: (see below) is set to 15 and
1.28 markus 1382: .Cm ServerAliveCountMax
1.84 jmc 1383: is left at the default, if the server becomes unresponsive,
1384: ssh will disconnect after approximately 45 seconds.
1.89 markus 1385: This option applies to protocol version 2 only.
1.67 jmc 1386: .It Cm ServerAliveInterval
1387: Sets a timeout interval in seconds after which if no data has been received
1388: from the server,
1.84 jmc 1389: .Xr ssh 1
1.67 jmc 1390: will send a message through the encrypted
1391: channel to request a response from the server.
1392: The default
1393: is 0, indicating that these messages will not be sent to the server.
1394: This option applies to protocol version 2 only.
1.191 millert 1395: .It Cm StreamLocalBindMask
1396: Sets the octal file creation mode mask
1397: .Pq umask
1398: used when creating a Unix-domain socket file for local or remote
1399: port forwarding.
1400: This option is only used for port forwarding to a Unix-domain socket file.
1401: .Pp
1402: The default value is 0177, which creates a Unix-domain socket file that is
1403: readable and writable only by the owner.
1404: Note that not all operating systems honor the file mode on Unix-domain
1405: socket files.
1406: .It Cm StreamLocalBindUnlink
1407: Specifies whether to remove an existing Unix-domain socket file for local
1408: or remote port forwarding before creating a new one.
1409: If the socket file already exists and
1410: .Cm StreamLocalBindUnlink
1411: is not enabled,
1412: .Nm ssh
1413: will be unable to forward the port to the Unix-domain socket file.
1414: This option is only used for port forwarding to a Unix-domain socket file.
1415: .Pp
1416: The argument must be
1417: .Dq yes
1418: or
1419: .Dq no .
1420: The default is
1421: .Dq no .
1.1 stevesk 1422: .It Cm StrictHostKeyChecking
1423: If this flag is set to
1424: .Dq yes ,
1.84 jmc 1425: .Xr ssh 1
1.1 stevesk 1426: will never automatically add host keys to the
1.50 djm 1427: .Pa ~/.ssh/known_hosts
1.1 stevesk 1428: file, and refuses to connect to hosts whose host key has changed.
1429: This provides maximum protection against trojan horse attacks,
1.84 jmc 1430: though it can be annoying when the
1.1 stevesk 1431: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1432: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1433: frequently made.
1434: This option forces the user to manually
1435: add all new hosts.
1436: If this flag is set to
1437: .Dq no ,
1.84 jmc 1438: ssh will automatically add new host keys to the
1.1 stevesk 1439: user known hosts files.
1440: If this flag is set to
1441: .Dq ask ,
1442: new host keys
1443: will be added to the user known host files only after the user
1444: has confirmed that is what they really want to do, and
1.84 jmc 1445: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1446: The host keys of
1447: known hosts will be verified automatically in all cases.
1448: The argument must be
1449: .Dq yes ,
1.84 jmc 1450: .Dq no ,
1.1 stevesk 1451: or
1452: .Dq ask .
1453: The default is
1454: .Dq ask .
1.26 markus 1455: .It Cm TCPKeepAlive
1456: Specifies whether the system should send TCP keepalive messages to the
1457: other side.
1458: If they are sent, death of the connection or crash of one
1459: of the machines will be properly noticed.
1460: However, this means that
1461: connections will die if the route is down temporarily, and some people
1462: find it annoying.
1463: .Pp
1464: The default is
1465: .Dq yes
1466: (to send TCP keepalive messages), and the client will notice
1467: if the network goes down or the remote host dies.
1468: This is important in scripts, and many users want it too.
1469: .Pp
1470: To disable TCP keepalive messages, the value should be set to
1471: .Dq no .
1.65 reyk 1472: .It Cm Tunnel
1.95 stevesk 1473: Request
1.65 reyk 1474: .Xr tun 4
1.69 jmc 1475: device forwarding between the client and the server.
1.65 reyk 1476: The argument must be
1.68 reyk 1477: .Dq yes ,
1.95 stevesk 1478: .Dq point-to-point
1479: (layer 3),
1480: .Dq ethernet
1481: (layer 2),
1.65 reyk 1482: or
1483: .Dq no .
1.95 stevesk 1484: Specifying
1485: .Dq yes
1486: requests the default tunnel mode, which is
1487: .Dq point-to-point .
1.65 reyk 1488: The default is
1489: .Dq no .
1490: .It Cm TunnelDevice
1.95 stevesk 1491: Specifies the
1.65 reyk 1492: .Xr tun 4
1.95 stevesk 1493: devices to open on the client
1494: .Pq Ar local_tun
1495: and the server
1496: .Pq Ar remote_tun .
1497: .Pp
1498: The argument must be
1499: .Sm off
1500: .Ar local_tun Op : Ar remote_tun .
1501: .Sm on
1502: The devices may be specified by numerical ID or the keyword
1503: .Dq any ,
1504: which uses the next available tunnel device.
1505: If
1506: .Ar remote_tun
1507: is not specified, it defaults to
1508: .Dq any .
1509: The default is
1510: .Dq any:any .
1.201 djm 1511: .It Cm UpdateHostKeys
1.200 djm 1512: Specifies whether
1513: .Xr ssh 1
1514: should accept notifications of additional hostkeys from the server sent
1515: after authentication has completed and add them to
1516: .Cm UserKnownHostsFile .
1517: The argument must be
1.204 djm 1518: .Dq yes ,
1.203 djm 1519: .Dq no
1.204 djm 1520: (the default) or
1521: .Dq ask .
1.200 djm 1522: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1523: and supports graceful key rotation by allowing a server to send replacement
1524: public keys before old ones are removed.
1.200 djm 1525: Additional hostkeys are only accepted if the key used to authenticate the
1526: host was already trusted or explicity accepted by the user.
1.204 djm 1527: If
1528: .Cm UpdateHostKeys
1529: is set to
1530: .Dq ask ,
1531: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1532: Confirmation is currently incompatible with
1533: .Cm ControlPersist ,
1534: and will be disabled if it is enabled.
1.200 djm 1535: .Pp
1536: Presently, only
1537: .Xr sshd 8
1538: from OpenSSH 6.8 and greater support the
1539: .Dq hostkeys@openssh.com
1540: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1541: .It Cm UsePrivilegedPort
1542: Specifies whether to use a privileged port for outgoing connections.
1543: The argument must be
1544: .Dq yes
1545: or
1546: .Dq no .
1547: The default is
1548: .Dq no .
1549: If set to
1.84 jmc 1550: .Dq yes ,
1551: .Xr ssh 1
1.72 jmc 1552: must be setuid root.
1553: Note that this option must be set to
1554: .Dq yes
1555: for
1556: .Cm RhostsRSAAuthentication
1557: with older servers.
1.1 stevesk 1558: .It Cm User
1559: Specifies the user to log in as.
1560: This can be useful when a different user name is used on different machines.
1561: This saves the trouble of
1562: having to remember to give the user name on the command line.
1563: .It Cm UserKnownHostsFile
1.151 djm 1564: Specifies one or more files to use for the user
1565: host key database, separated by whitespace.
1566: The default is
1567: .Pa ~/.ssh/known_hosts ,
1568: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1569: .It Cm VerifyHostKeyDNS
1570: Specifies whether to verify the remote key using DNS and SSHFP resource
1571: records.
1.24 jakob 1572: If this option is set to
1573: .Dq yes ,
1.25 jmc 1574: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1575: from DNS.
1576: Insecure fingerprints will be handled as if this option was set to
1577: .Dq ask .
1578: If this option is set to
1579: .Dq ask ,
1580: information on fingerprint match will be displayed, but the user will still
1581: need to confirm new host keys according to the
1582: .Cm StrictHostKeyChecking
1583: option.
1584: The argument must be
1585: .Dq yes ,
1.84 jmc 1586: .Dq no ,
1.25 jmc 1587: or
1588: .Dq ask .
1.8 jakob 1589: The default is
1590: .Dq no .
1.12 jakob 1591: Note that this option applies to protocol version 2 only.
1.84 jmc 1592: .Pp
1.166 jmc 1593: See also VERIFYING HOST KEYS in
1.84 jmc 1594: .Xr ssh 1 .
1.111 grunk 1595: .It Cm VisualHostKey
1596: If this flag is set to
1597: .Dq yes ,
1598: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1599: printed in addition to the fingerprint string at login and
1.114 stevesk 1600: for unknown host keys.
1.111 grunk 1601: If this flag is set to
1602: .Dq no ,
1.114 stevesk 1603: no fingerprint strings are printed at login and
1.197 djm 1604: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1605: The default is
1606: .Dq no .
1.1 stevesk 1607: .It Cm XAuthLocation
1.5 stevesk 1608: Specifies the full pathname of the
1.1 stevesk 1609: .Xr xauth 1
1610: program.
1611: The default is
1612: .Pa /usr/X11R6/bin/xauth .
1613: .El
1.86 jmc 1614: .Sh PATTERNS
1615: A
1616: .Em pattern
1617: consists of zero or more non-whitespace characters,
1618: .Sq *
1619: (a wildcard that matches zero or more characters),
1620: or
1621: .Sq ?\&
1622: (a wildcard that matches exactly one character).
1623: For example, to specify a set of declarations for any host in the
1624: .Dq .co.uk
1625: set of domains,
1626: the following pattern could be used:
1627: .Pp
1628: .Dl Host *.co.uk
1629: .Pp
1630: The following pattern
1631: would match any host in the 192.168.0.[0-9] network range:
1632: .Pp
1633: .Dl Host 192.168.0.?
1634: .Pp
1635: A
1636: .Em pattern-list
1637: is a comma-separated list of patterns.
1638: Patterns within pattern-lists may be negated
1639: by preceding them with an exclamation mark
1640: .Pq Sq !\& .
1641: For example,
1.174 djm 1642: to allow a key to be used from anywhere within an organization
1.86 jmc 1643: except from the
1644: .Dq dialup
1645: pool,
1646: the following entry (in authorized_keys) could be used:
1647: .Pp
1648: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1649: .Sh FILES
1650: .Bl -tag -width Ds
1.50 djm 1651: .It Pa ~/.ssh/config
1.1 stevesk 1652: This is the per-user configuration file.
1653: The format of this file is described above.
1.84 jmc 1654: This file is used by the SSH client.
1.30 djm 1655: Because of the potential for abuse, this file must have strict permissions:
1656: read/write for the user, and not accessible by others.
1.1 stevesk 1657: .It Pa /etc/ssh/ssh_config
1658: Systemwide configuration file.
1659: This file provides defaults for those
1660: values that are not specified in the user's configuration file, and
1661: for those users who do not have a configuration file.
1662: This file must be world-readable.
1663: .El
1.13 jmc 1664: .Sh SEE ALSO
1665: .Xr ssh 1
1.1 stevesk 1666: .Sh AUTHORS
1667: OpenSSH is a derivative of the original and free
1668: ssh 1.2.12 release by Tatu Ylonen.
1669: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1670: Theo de Raadt and Dug Song
1671: removed many bugs, re-added newer features and
1672: created OpenSSH.
1673: Markus Friedl contributed the support for SSH
1674: protocol versions 1.5 and 2.0.
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