Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.200
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
18: .\" are met:
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
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
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.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35: .\"
1.200 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.199 2014/12/22 09:24:59 jmc Exp $
1.199 jmc 37: .Dd $Mdocdate: December 22 2014 $
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
169: keywork matches only when the configuration file is being re-parsed
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.
343: This allows ssh to detect if a host key changed due to DNS spoofing.
1.107 grunk 344: If the option is set to
1.1 stevesk 345: .Dq no ,
346: the check will not be executed.
347: The default is
348: .Dq yes .
349: .It Cm Cipher
350: Specifies the cipher to use for encrypting the session
351: in protocol version 1.
352: Currently,
353: .Dq blowfish ,
354: .Dq 3des ,
355: and
356: .Dq des
357: are supported.
358: .Ar des
359: is only supported in the
1.84 jmc 360: .Xr ssh 1
1.1 stevesk 361: client for interoperability with legacy protocol 1 implementations
362: that do not support the
363: .Ar 3des
1.7 jmc 364: cipher.
365: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 366: The default is
367: .Dq 3des .
368: .It Cm Ciphers
369: Specifies the ciphers allowed for protocol version 2
370: in order of preference.
371: Multiple ciphers must be comma-separated.
1.180 djm 372: The supported ciphers are:
373: .Pp
1.186 naddy 374: .Bl -item -compact -offset indent
375: .It
376: 3des-cbc
377: .It
378: aes128-cbc
379: .It
380: aes192-cbc
381: .It
382: aes256-cbc
383: .It
384: aes128-ctr
385: .It
386: aes192-ctr
387: .It
388: aes256-ctr
389: .It
390: aes128-gcm@openssh.com
391: .It
392: aes256-gcm@openssh.com
393: .It
394: arcfour
395: .It
396: arcfour128
397: .It
398: arcfour256
399: .It
400: blowfish-cbc
401: .It
402: cast128-cbc
403: .It
404: chacha20-poly1305@openssh.com
405: .El
1.180 djm 406: .Pp
1.84 jmc 407: The default is:
1.186 naddy 408: .Bd -literal -offset indent
409: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 410: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.180 djm 411: chacha20-poly1305@openssh.com,
1.186 naddy 412: arcfour256,arcfour128,
413: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
414: aes192-cbc,aes256-cbc,arcfour
1.1 stevesk 415: .Ed
1.180 djm 416: .Pp
417: The list of available ciphers may also be obtained using the
418: .Fl Q
419: option of
1.198 djm 420: .Xr ssh 1
421: with an argument of
422: .Dq cipher .
1.1 stevesk 423: .It Cm ClearAllForwardings
1.84 jmc 424: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 425: specified in the configuration files or on the command line be
1.7 jmc 426: cleared.
427: This option is primarily useful when used from the
1.84 jmc 428: .Xr ssh 1
1.1 stevesk 429: command line to clear port forwardings set in
430: configuration files, and is automatically set by
431: .Xr scp 1
432: and
433: .Xr sftp 1 .
434: The argument must be
435: .Dq yes
436: or
437: .Dq no .
438: The default is
439: .Dq no .
440: .It Cm Compression
441: Specifies whether to use compression.
442: The argument must be
443: .Dq yes
444: or
445: .Dq no .
446: The default is
447: .Dq no .
448: .It Cm CompressionLevel
449: Specifies the compression level to use if compression is enabled.
450: The argument must be an integer from 1 (fast) to 9 (slow, best).
451: The default level is 6, which is good for most applications.
452: The meaning of the values is the same as in
453: .Xr gzip 1 .
454: Note that this option applies to protocol version 1 only.
455: .It Cm ConnectionAttempts
456: Specifies the number of tries (one per second) to make before exiting.
457: The argument must be an integer.
458: This may be useful in scripts if the connection sometimes fails.
459: The default is 1.
1.9 djm 460: .It Cm ConnectTimeout
1.84 jmc 461: Specifies the timeout (in seconds) used when connecting to the
462: SSH server, instead of using the default system TCP timeout.
1.11 jmc 463: This value is used only when the target is down or really unreachable,
464: not when it refuses the connection.
1.36 djm 465: .It Cm ControlMaster
466: Enables the sharing of multiple sessions over a single network connection.
467: When set to
1.84 jmc 468: .Dq yes ,
469: .Xr ssh 1
1.36 djm 470: will listen for connections on a control socket specified using the
471: .Cm ControlPath
472: argument.
473: Additional sessions can connect to this socket using the same
474: .Cm ControlPath
475: with
476: .Cm ControlMaster
477: set to
478: .Dq no
1.38 jmc 479: (the default).
1.64 jmc 480: These sessions will try to reuse the master instance's network connection
1.63 djm 481: rather than initiating new ones, but will fall back to connecting normally
482: if the control socket does not exist, or is not listening.
483: .Pp
1.37 djm 484: Setting this to
485: .Dq ask
1.84 jmc 486: will cause ssh
1.37 djm 487: to listen for control connections, but require confirmation using the
488: .Ev SSH_ASKPASS
489: program before they are accepted (see
490: .Xr ssh-add 1
1.38 jmc 491: for details).
1.51 jakob 492: If the
493: .Cm ControlPath
1.84 jmc 494: cannot be opened,
495: ssh will continue without connecting to a master instance.
1.58 djm 496: .Pp
497: X11 and
1.59 jmc 498: .Xr ssh-agent 1
1.58 djm 499: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 500: display and agent forwarded will be the one belonging to the master
1.59 jmc 501: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 502: .Pp
503: Two additional options allow for opportunistic multiplexing: try to use a
504: master connection but fall back to creating a new one if one does not already
505: exist.
506: These options are:
507: .Dq auto
508: and
509: .Dq autoask .
510: The latter requires confirmation like the
511: .Dq ask
512: option.
1.36 djm 513: .It Cm ControlPath
1.55 djm 514: Specify the path to the control socket used for connection sharing as described
515: in the
1.36 djm 516: .Cm ControlMaster
1.57 djm 517: section above or the string
518: .Dq none
519: to disable connection sharing.
1.55 djm 520: In the path,
1.147 djm 521: .Ql %L
522: will be substituted by the first component of the local host name,
1.77 djm 523: .Ql %l
1.147 djm 524: will be substituted by the local host name (including any domain name),
1.55 djm 525: .Ql %h
526: will be substituted by the target host name,
1.150 jmc 527: .Ql %n
528: will be substituted by the original target host name
529: specified on the command line,
1.55 djm 530: .Ql %p
1.175 djm 531: the destination port,
1.55 djm 532: .Ql %r
1.188 djm 533: by the remote login username,
1.147 djm 534: .Ql %u
535: by the username of the user running
1.188 djm 536: .Xr ssh 1 , and
1.189 jmc 537: .Ql \&%C
1.188 djm 538: by a hash of the concatenation: %l%h%p%r.
1.56 djm 539: It is recommended that any
540: .Cm ControlPath
541: used for opportunistic connection sharing include
1.195 djm 542: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
543: that is not writable by other users.
1.56 djm 544: This ensures that shared connections are uniquely identified.
1.137 djm 545: .It Cm ControlPersist
546: When used in conjunction with
547: .Cm ControlMaster ,
548: specifies that the master connection should remain open
549: in the background (waiting for future client connections)
550: after the initial client connection has been closed.
551: If set to
552: .Dq no ,
553: then the master connection will not be placed into the background,
554: and will close as soon as the initial client connection is closed.
555: If set to
1.195 djm 556: .Dq yes
557: or
558: .Dq 0 ,
1.137 djm 559: then the master connection will remain in the background indefinitely
560: (until killed or closed via a mechanism such as the
561: .Xr ssh 1
562: .Dq Fl O No exit
563: option).
564: If set to a time in seconds, or a time in any of the formats documented in
565: .Xr sshd_config 5 ,
566: then the backgrounded master connection will automatically terminate
567: after it has remained idle (with no client connections) for the
568: specified time.
1.38 jmc 569: .It Cm DynamicForward
1.74 jmc 570: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 571: over the secure channel, and the application
572: protocol is then used to determine where to connect to from the
573: remote machine.
1.62 djm 574: .Pp
575: The argument must be
576: .Sm off
577: .Oo Ar bind_address : Oc Ar port .
578: .Sm on
1.138 djm 579: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 580: By default, the local port is bound in accordance with the
581: .Cm GatewayPorts
582: setting.
583: However, an explicit
584: .Ar bind_address
585: may be used to bind the connection to a specific address.
586: The
587: .Ar bind_address
588: of
589: .Dq localhost
590: indicates that the listening port be bound for local use only, while an
591: empty address or
592: .Sq *
593: indicates that the port should be available from all interfaces.
594: .Pp
1.38 jmc 595: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 596: .Xr ssh 1
1.38 jmc 597: will act as a SOCKS server.
598: Multiple forwardings may be specified, and
599: additional forwardings can be given on the command line.
600: Only the superuser can forward privileged ports.
1.14 markus 601: .It Cm EnableSSHKeysign
602: Setting this option to
603: .Dq yes
604: in the global client configuration file
605: .Pa /etc/ssh/ssh_config
606: enables the use of the helper program
607: .Xr ssh-keysign 8
608: during
609: .Cm HostbasedAuthentication .
610: The argument must be
611: .Dq yes
612: or
613: .Dq no .
614: The default is
615: .Dq no .
1.23 jmc 616: This option should be placed in the non-hostspecific section.
1.14 markus 617: See
618: .Xr ssh-keysign 8
619: for more information.
1.1 stevesk 620: .It Cm EscapeChar
621: Sets the escape character (default:
622: .Ql ~ ) .
623: The escape character can also
624: be set on the command line.
625: The argument should be a single character,
626: .Ql ^
627: followed by a letter, or
628: .Dq none
629: to disable the escape
630: character entirely (making the connection transparent for binary
631: data).
1.96 markus 632: .It Cm ExitOnForwardFailure
633: Specifies whether
634: .Xr ssh 1
635: should terminate the connection if it cannot set up all requested
1.102 stevesk 636: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 637: The argument must be
638: .Dq yes
639: or
640: .Dq no .
641: The default is
642: .Dq no .
1.197 djm 643: .It Cm FingerprintHash
644: Specifies the hash algorithm used when displaying key fingerprints.
645: Valid options are:
646: .Dq md5
647: and
648: .Dq sha256 .
649: The default is
650: .Dq sha256 .
1.1 stevesk 651: .It Cm ForwardAgent
652: Specifies whether the connection to the authentication agent (if any)
653: will be forwarded to the remote machine.
654: The argument must be
655: .Dq yes
656: or
657: .Dq no .
658: The default is
659: .Dq no .
1.3 stevesk 660: .Pp
1.7 jmc 661: Agent forwarding should be enabled with caution.
662: Users with the ability to bypass file permissions on the remote host
663: (for the agent's Unix-domain socket)
664: can access the local agent through the forwarded connection.
665: An attacker cannot obtain key material from the agent,
1.3 stevesk 666: however they can perform operations on the keys that enable them to
667: authenticate using the identities loaded into the agent.
1.1 stevesk 668: .It Cm ForwardX11
669: Specifies whether X11 connections will be automatically redirected
670: over the secure channel and
671: .Ev DISPLAY
672: set.
673: The argument must be
674: .Dq yes
675: or
676: .Dq no .
677: The default is
678: .Dq no .
1.3 stevesk 679: .Pp
1.7 jmc 680: X11 forwarding should be enabled with caution.
681: Users with the ability to bypass file permissions on the remote host
1.22 markus 682: (for the user's X11 authorization database)
1.7 jmc 683: can access the local X11 display through the forwarded connection.
1.22 markus 684: An attacker may then be able to perform activities such as keystroke monitoring
685: if the
686: .Cm ForwardX11Trusted
687: option is also enabled.
1.134 djm 688: .It Cm ForwardX11Timeout
1.135 jmc 689: Specify a timeout for untrusted X11 forwarding
690: using the format described in the
1.166 jmc 691: TIME FORMATS section of
1.134 djm 692: .Xr sshd_config 5 .
693: X11 connections received by
694: .Xr ssh 1
695: after this time will be refused.
696: The default is to disable untrusted X11 forwarding after twenty minutes has
697: elapsed.
1.22 markus 698: .It Cm ForwardX11Trusted
1.34 jmc 699: If this option is set to
1.84 jmc 700: .Dq yes ,
701: remote X11 clients will have full access to the original X11 display.
1.42 djm 702: .Pp
1.22 markus 703: If this option is set to
1.84 jmc 704: .Dq no ,
705: remote X11 clients will be considered untrusted and prevented
1.22 markus 706: from stealing or tampering with data belonging to trusted X11
707: clients.
1.42 djm 708: Furthermore, the
709: .Xr xauth 1
710: token used for the session will be set to expire after 20 minutes.
711: Remote clients will be refused access after this time.
1.22 markus 712: .Pp
713: The default is
714: .Dq no .
715: .Pp
716: See the X11 SECURITY extension specification for full details on
717: the restrictions imposed on untrusted clients.
1.1 stevesk 718: .It Cm GatewayPorts
719: Specifies whether remote hosts are allowed to connect to local
720: forwarded ports.
721: By default,
1.84 jmc 722: .Xr ssh 1
1.7 jmc 723: binds local port forwardings to the loopback address.
724: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 725: .Cm GatewayPorts
1.84 jmc 726: can be used to specify that ssh
1.1 stevesk 727: should bind local port forwardings to the wildcard address,
728: thus allowing remote hosts to connect to forwarded ports.
729: The argument must be
730: .Dq yes
731: or
732: .Dq no .
733: The default is
734: .Dq no .
735: .It Cm GlobalKnownHostsFile
1.151 djm 736: Specifies one or more files to use for the global
737: host key database, separated by whitespace.
738: The default is
739: .Pa /etc/ssh/ssh_known_hosts ,
740: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 741: .It Cm GSSAPIAuthentication
1.27 markus 742: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 743: The default is
1.21 markus 744: .Dq no .
1.18 markus 745: Note that this option applies to protocol version 2 only.
746: .It Cm GSSAPIDelegateCredentials
747: Forward (delegate) credentials to the server.
748: The default is
749: .Dq no .
750: Note that this option applies to protocol version 2 only.
1.44 djm 751: .It Cm HashKnownHosts
752: Indicates that
1.84 jmc 753: .Xr ssh 1
1.44 djm 754: should hash host names and addresses when they are added to
1.50 djm 755: .Pa ~/.ssh/known_hosts .
1.44 djm 756: These hashed names may be used normally by
1.84 jmc 757: .Xr ssh 1
1.44 djm 758: and
1.84 jmc 759: .Xr sshd 8 ,
1.44 djm 760: but they do not reveal identifying information should the file's contents
761: be disclosed.
762: The default is
763: .Dq no .
1.97 jmc 764: Note that existing names and addresses in known hosts files
765: will not be converted automatically,
766: but may be manually hashed using
1.45 djm 767: .Xr ssh-keygen 1 .
1.1 stevesk 768: .It Cm HostbasedAuthentication
769: Specifies whether to try rhosts based authentication with public key
770: authentication.
771: The argument must be
772: .Dq yes
773: or
774: .Dq no .
775: The default is
776: .Dq no .
777: This option applies to protocol version 2 only and
778: is similar to
779: .Cm RhostsRSAAuthentication .
780: .It Cm HostKeyAlgorithms
781: Specifies the protocol version 2 host key algorithms
782: that the client wants to use in order of preference.
783: The default for this option is:
1.139 djm 784: .Bd -literal -offset 3n
785: ecdsa-sha2-nistp256-cert-v01@openssh.com,
786: ecdsa-sha2-nistp384-cert-v01@openssh.com,
787: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 788: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 789: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
790: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
791: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.182 markus 792: ssh-ed25519,ssh-rsa,ssh-dss
1.139 djm 793: .Ed
1.145 djm 794: .Pp
795: If hostkeys are known for the destination host then this default is modified
796: to prefer their algorithms.
1.198 djm 797: .Pp
798: The list of available key types may also be obtained using the
799: .Fl Q
800: option of
801: .Xr ssh 1
802: with an argument of
803: .Dq key .
1.1 stevesk 804: .It Cm HostKeyAlias
805: Specifies an alias that should be used instead of the
806: real host name when looking up or saving the host key
807: in the host key database files.
1.84 jmc 808: This option is useful for tunneling SSH connections
1.1 stevesk 809: or for multiple servers running on a single host.
810: .It Cm HostName
811: Specifies the real host name to log into.
812: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 813: If the hostname contains the character sequence
814: .Ql %h ,
1.150 jmc 815: then this will be replaced with the host name specified on the command line
1.136 djm 816: (this is useful for manipulating unqualified names).
1.187 djm 817: The character sequence
818: .Ql %%
819: will be replaced by a single
820: .Ql %
821: character, which may be used when specifying IPv6 link-local addresses.
822: .Pp
1.84 jmc 823: The default is the name given on the command line.
1.1 stevesk 824: Numeric IP addresses are also permitted (both on the command line and in
825: .Cm HostName
826: specifications).
1.29 markus 827: .It Cm IdentitiesOnly
828: Specifies that
1.84 jmc 829: .Xr ssh 1
1.29 markus 830: should only use the authentication identity files configured in the
1.31 jmc 831: .Nm
1.29 markus 832: files,
1.84 jmc 833: even if
834: .Xr ssh-agent 1
1.159 djm 835: or a
836: .Cm PKCS11Provider
1.29 markus 837: offers more identities.
838: The argument to this keyword must be
839: .Dq yes
840: or
841: .Dq no .
1.84 jmc 842: This option is intended for situations where ssh-agent
1.29 markus 843: offers many different identities.
844: The default is
845: .Dq no .
1.67 jmc 846: .It Cm IdentityFile
1.192 sobrado 847: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 848: identity is read.
1.67 jmc 849: The default is
850: .Pa ~/.ssh/identity
851: for protocol version 1, and
1.139 djm 852: .Pa ~/.ssh/id_dsa ,
1.183 naddy 853: .Pa ~/.ssh/id_ecdsa ,
854: .Pa ~/.ssh/id_ed25519
1.139 djm 855: and
1.67 jmc 856: .Pa ~/.ssh/id_rsa
857: for protocol version 2.
858: Additionally, any identities represented by the authentication agent
1.165 djm 859: will be used for authentication unless
860: .Cm IdentitiesOnly
861: is set.
1.129 djm 862: .Xr ssh 1
863: will try to load certificate information from the filename obtained by
864: appending
865: .Pa -cert.pub
866: to the path of a specified
867: .Cm IdentityFile .
1.90 djm 868: .Pp
1.67 jmc 869: The file name may use the tilde
1.91 jmc 870: syntax to refer to a user's home directory or one of the following
1.90 djm 871: escape characters:
872: .Ql %d
873: (local user's home directory),
874: .Ql %u
875: (local user name),
876: .Ql %l
877: (local host name),
878: .Ql %h
879: (remote host name) or
1.92 djm 880: .Ql %r
1.90 djm 881: (remote user name).
882: .Pp
1.67 jmc 883: It is possible to have
884: multiple identity files specified in configuration files; all these
885: identities will be tried in sequence.
1.152 djm 886: Multiple
887: .Cm IdentityFile
888: directives will add to the list of identities tried (this behaviour
889: differs from that of other configuration directives).
1.165 djm 890: .Pp
891: .Cm IdentityFile
892: may be used in conjunction with
893: .Cm IdentitiesOnly
894: to select which identities in an agent are offered during authentication.
1.164 jmc 895: .It Cm IgnoreUnknown
896: Specifies a pattern-list of unknown options to be ignored if they are
897: encountered in configuration parsing.
898: This may be used to suppress errors if
899: .Nm
900: contains options that are unrecognised by
901: .Xr ssh 1 .
902: It is recommended that
903: .Cm IgnoreUnknown
904: be listed early in the configuration file as it will not be applied
905: to unknown options that appear before it.
1.143 djm 906: .It Cm IPQoS
907: Specifies the IPv4 type-of-service or DSCP class for connections.
908: Accepted values are
909: .Dq af11 ,
910: .Dq af12 ,
911: .Dq af13 ,
1.154 djm 912: .Dq af21 ,
1.143 djm 913: .Dq af22 ,
914: .Dq af23 ,
915: .Dq af31 ,
916: .Dq af32 ,
917: .Dq af33 ,
918: .Dq af41 ,
919: .Dq af42 ,
920: .Dq af43 ,
921: .Dq cs0 ,
922: .Dq cs1 ,
923: .Dq cs2 ,
924: .Dq cs3 ,
925: .Dq cs4 ,
926: .Dq cs5 ,
927: .Dq cs6 ,
928: .Dq cs7 ,
929: .Dq ef ,
930: .Dq lowdelay ,
931: .Dq throughput ,
932: .Dq reliability ,
933: or a numeric value.
1.146 djm 934: This option may take one or two arguments, separated by whitespace.
1.143 djm 935: If one argument is specified, it is used as the packet class unconditionally.
936: If two values are specified, the first is automatically selected for
937: interactive sessions and the second for non-interactive sessions.
938: The default is
939: .Dq lowdelay
940: for interactive sessions and
941: .Dq throughput
942: for non-interactive sessions.
1.103 djm 943: .It Cm KbdInteractiveAuthentication
944: Specifies whether to use keyboard-interactive authentication.
945: The argument to this keyword must be
946: .Dq yes
947: or
948: .Dq no .
949: The default is
950: .Dq yes .
1.39 djm 951: .It Cm KbdInteractiveDevices
952: Specifies the list of methods to use in keyboard-interactive authentication.
953: Multiple method names must be comma-separated.
954: The default is to use the server specified list.
1.85 jmc 955: The methods available vary depending on what the server supports.
956: For an OpenSSH server,
957: it may be zero or more of:
958: .Dq bsdauth ,
959: .Dq pam ,
960: and
961: .Dq skey .
1.140 djm 962: .It Cm KexAlgorithms
963: Specifies the available KEX (Key Exchange) algorithms.
964: Multiple algorithms must be comma-separated.
1.141 jmc 965: The default is:
966: .Bd -literal -offset indent
1.179 markus 967: curve25519-sha256@libssh.org,
1.141 jmc 968: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
969: diffie-hellman-group-exchange-sha256,
1.186 naddy 970: diffie-hellman-group14-sha1,
1.141 jmc 971: diffie-hellman-group-exchange-sha1,
972: diffie-hellman-group1-sha1
973: .Ed
1.198 djm 974: .Pp
975: The list of available key exchange algorithms may also be obtained using the
976: .Fl Q
977: option of
978: .Xr ssh 1
979: with an argument of
980: .Dq kex .
1.65 reyk 981: .It Cm LocalCommand
982: Specifies a command to execute on the local machine after successfully
983: connecting to the server.
984: The command string extends to the end of the line, and is executed with
1.105 jmc 985: the user's shell.
1.109 dtucker 986: The following escape character substitutions will be performed:
987: .Ql %d
988: (local user's home directory),
989: .Ql %h
990: (remote host name),
991: .Ql %l
992: (local host name),
993: .Ql %n
994: (host name as provided on the command line),
995: .Ql %p
996: (remote port),
997: .Ql %r
998: (remote user name) or
999: .Ql %u
1.188 djm 1000: (local user name) or
1.189 jmc 1001: .Ql \&%C
1.188 djm 1002: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1003: .Pp
1004: The command is run synchronously and does not have access to the
1005: session of the
1006: .Xr ssh 1
1007: that spawned it.
1008: It should not be used for interactive commands.
1009: .Pp
1.65 reyk 1010: This directive is ignored unless
1011: .Cm PermitLocalCommand
1012: has been enabled.
1.1 stevesk 1013: .It Cm LocalForward
1.74 jmc 1014: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1015: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1016: The first argument must be
1.43 djm 1017: .Sm off
1.49 jmc 1018: .Oo Ar bind_address : Oc Ar port
1.43 djm 1019: .Sm on
1.49 jmc 1020: and the second argument must be
1021: .Ar host : Ns Ar hostport .
1.138 djm 1022: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1023: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1024: given on the command line.
1.1 stevesk 1025: Only the superuser can forward privileged ports.
1.43 djm 1026: By default, the local port is bound in accordance with the
1027: .Cm GatewayPorts
1028: setting.
1029: However, an explicit
1030: .Ar bind_address
1031: may be used to bind the connection to a specific address.
1032: The
1033: .Ar bind_address
1034: of
1035: .Dq localhost
1.46 jmc 1036: indicates that the listening port be bound for local use only, while an
1037: empty address or
1038: .Sq *
1.43 djm 1039: indicates that the port should be available from all interfaces.
1.1 stevesk 1040: .It Cm LogLevel
1041: Gives the verbosity level that is used when logging messages from
1.84 jmc 1042: .Xr ssh 1 .
1.1 stevesk 1043: The possible values are:
1.84 jmc 1044: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1045: The default is INFO.
1046: DEBUG and DEBUG1 are equivalent.
1047: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1048: .It Cm MACs
1049: Specifies the MAC (message authentication code) algorithms
1050: in order of preference.
1051: The MAC algorithm is used in protocol version 2
1052: for data integrity protection.
1053: Multiple algorithms must be comma-separated.
1.160 markus 1054: The algorithms that contain
1055: .Dq -etm
1056: calculate the MAC after encryption (encrypt-then-mac).
1057: These are considered safer and their use recommended.
1.84 jmc 1058: The default is:
1.101 jmc 1059: .Bd -literal -offset indent
1.160 markus 1060: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1061: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1062: umac-64@openssh.com,umac-128@openssh.com,
1063: hmac-sha2-256,hmac-sha2-512,
1064: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1065: hmac-ripemd160-etm@openssh.com,
1066: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1067: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1068: hmac-sha1-96,hmac-md5-96
1.101 jmc 1069: .Ed
1.198 djm 1070: .Pp
1071: The list of available MAC algorithms may also be obtained using the
1072: .Fl Q
1073: option of
1074: .Xr ssh 1
1075: with an argument of
1076: .Dq mac .
1.1 stevesk 1077: .It Cm NoHostAuthenticationForLocalhost
1078: This option can be used if the home directory is shared across machines.
1079: In this case localhost will refer to a different machine on each of
1080: the machines and the user will get many warnings about changed host keys.
1081: However, this option disables host authentication for localhost.
1082: The argument to this keyword must be
1083: .Dq yes
1084: or
1085: .Dq no .
1086: The default is to check the host key for localhost.
1087: .It Cm NumberOfPasswordPrompts
1088: Specifies the number of password prompts before giving up.
1089: The argument to this keyword must be an integer.
1.84 jmc 1090: The default is 3.
1.1 stevesk 1091: .It Cm PasswordAuthentication
1092: Specifies whether to use password authentication.
1093: The argument to this keyword must be
1094: .Dq yes
1095: or
1096: .Dq no .
1097: The default is
1098: .Dq yes .
1.65 reyk 1099: .It Cm PermitLocalCommand
1100: Allow local command execution via the
1101: .Ic LocalCommand
1102: option or using the
1.66 jmc 1103: .Ic !\& Ns Ar command
1.65 reyk 1104: escape sequence in
1105: .Xr ssh 1 .
1106: The argument must be
1107: .Dq yes
1108: or
1109: .Dq no .
1110: The default is
1111: .Dq no .
1.127 markus 1112: .It Cm PKCS11Provider
1113: Specifies which PKCS#11 provider to use.
1.144 jmc 1114: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1115: .Xr ssh 1
1.128 markus 1116: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1117: private RSA key.
1.67 jmc 1118: .It Cm Port
1119: Specifies the port number to connect on the remote host.
1.84 jmc 1120: The default is 22.
1.1 stevesk 1121: .It Cm PreferredAuthentications
1122: Specifies the order in which the client should try protocol 2
1.11 jmc 1123: authentication methods.
1.48 jmc 1124: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1125: .Cm keyboard-interactive )
1.48 jmc 1126: over another method (e.g.\&
1.131 jmc 1127: .Cm password ) .
1128: The default is:
1129: .Bd -literal -offset indent
1130: gssapi-with-mic,hostbased,publickey,
1131: keyboard-interactive,password
1132: .Ed
1.1 stevesk 1133: .It Cm Protocol
1134: Specifies the protocol versions
1.84 jmc 1135: .Xr ssh 1
1.1 stevesk 1136: should support in order of preference.
1137: The possible values are
1.84 jmc 1138: .Sq 1
1.1 stevesk 1139: and
1.84 jmc 1140: .Sq 2 .
1.1 stevesk 1141: Multiple versions must be comma-separated.
1.120 markus 1142: When this option is set to
1.121 jmc 1143: .Dq 2,1
1.120 markus 1144: .Nm ssh
1145: will try version 2 and fall back to version 1
1146: if version 2 is not available.
1.1 stevesk 1147: The default is
1.121 jmc 1148: .Sq 2 .
1.1 stevesk 1149: .It Cm ProxyCommand
1150: Specifies the command to use to connect to the server.
1151: The command
1.190 djm 1152: string extends to the end of the line, and is executed
1153: using the user's shell
1154: .Ql exec
1155: directive to avoid a lingering shell process.
1156: .Pp
1.133 jmc 1157: In the command string, any occurrence of
1.1 stevesk 1158: .Ql %h
1159: will be substituted by the host name to
1.132 djm 1160: connect,
1.1 stevesk 1161: .Ql %p
1.133 jmc 1162: by the port, and
1163: .Ql %r
1.132 djm 1164: by the remote user name.
1.1 stevesk 1165: The command can be basically anything,
1166: and should read from its standard input and write to its standard output.
1167: It should eventually connect an
1168: .Xr sshd 8
1169: server running on some machine, or execute
1170: .Ic sshd -i
1171: somewhere.
1172: Host key management will be done using the
1173: HostName of the host being connected (defaulting to the name typed by
1174: the user).
1.7 jmc 1175: Setting the command to
1176: .Dq none
1.6 markus 1177: disables this option entirely.
1.1 stevesk 1178: Note that
1179: .Cm CheckHostIP
1180: is not available for connects with a proxy command.
1.52 djm 1181: .Pp
1182: This directive is useful in conjunction with
1183: .Xr nc 1
1184: and its proxy support.
1.53 jmc 1185: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1186: 192.0.2.0:
1187: .Bd -literal -offset 3n
1188: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1189: .Ed
1.167 djm 1190: .It Cm ProxyUseFdpass
1.168 jmc 1191: Specifies that
1.167 djm 1192: .Cm ProxyCommand
1193: will pass a connected file descriptor back to
1.168 jmc 1194: .Xr ssh 1
1.167 djm 1195: instead of continuing to execute and pass data.
1196: The default is
1197: .Dq no .
1.1 stevesk 1198: .It Cm PubkeyAuthentication
1199: Specifies whether to try public key authentication.
1200: The argument to this keyword must be
1201: .Dq yes
1202: or
1203: .Dq no .
1204: The default is
1205: .Dq yes .
1206: This option applies to protocol version 2 only.
1.75 dtucker 1207: .It Cm RekeyLimit
1208: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1209: session key is renegotiated, optionally followed a maximum amount of
1210: time that may pass before the session key is renegotiated.
1211: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1212: .Sq K ,
1213: .Sq M ,
1.75 dtucker 1214: or
1.76 jmc 1215: .Sq G
1.75 dtucker 1216: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1217: The default is between
1.84 jmc 1218: .Sq 1G
1.75 dtucker 1219: and
1.84 jmc 1220: .Sq 4G ,
1.75 dtucker 1221: depending on the cipher.
1.162 dtucker 1222: The optional second value is specified in seconds and may use any of the
1223: units documented in the
1.166 jmc 1224: TIME FORMATS section of
1.162 dtucker 1225: .Xr sshd_config 5 .
1226: The default value for
1227: .Cm RekeyLimit
1228: is
1229: .Dq default none ,
1230: which means that rekeying is performed after the cipher's default amount
1231: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1232: This option applies to protocol version 2 only.
1.1 stevesk 1233: .It Cm RemoteForward
1.74 jmc 1234: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1235: the secure channel to the specified host and port from the local machine.
1.49 jmc 1236: The first argument must be
1.43 djm 1237: .Sm off
1.49 jmc 1238: .Oo Ar bind_address : Oc Ar port
1.43 djm 1239: .Sm on
1.49 jmc 1240: and the second argument must be
1241: .Ar host : Ns Ar hostport .
1.138 djm 1242: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1243: Multiple forwardings may be specified, and additional
1244: forwardings can be given on the command line.
1.113 stevesk 1245: Privileged ports can be forwarded only when
1246: logging in as root on the remote machine.
1.118 jmc 1247: .Pp
1.117 djm 1248: If the
1249: .Ar port
1250: argument is
1251: .Ql 0 ,
1252: the listen port will be dynamically allocated on the server and reported
1253: to the client at run time.
1.43 djm 1254: .Pp
1255: If the
1256: .Ar bind_address
1257: is not specified, the default is to only bind to loopback addresses.
1258: If the
1259: .Ar bind_address
1260: is
1261: .Ql *
1262: or an empty string, then the forwarding is requested to listen on all
1263: interfaces.
1264: Specifying a remote
1265: .Ar bind_address
1.46 jmc 1266: will only succeed if the server's
1267: .Cm GatewayPorts
1.43 djm 1268: option is enabled (see
1.46 jmc 1269: .Xr sshd_config 5 ) .
1.149 djm 1270: .It Cm RequestTTY
1271: Specifies whether to request a pseudo-tty for the session.
1272: The argument may be one of:
1273: .Dq no
1274: (never request a TTY),
1275: .Dq yes
1276: (always request a TTY when standard input is a TTY),
1277: .Dq force
1278: (always request a TTY) or
1279: .Dq auto
1280: (request a TTY when opening a login session).
1281: This option mirrors the
1282: .Fl t
1283: and
1284: .Fl T
1285: flags for
1286: .Xr ssh 1 .
1.196 djm 1287: .It Cm RevokedHostKeys
1288: Specifies revoked host public keys.
1289: Keys listed in this file will be refused for host authentication.
1290: Note that if this file does not exist or is not readable,
1291: then host authentication will be refused for all hosts.
1292: Keys may be specified as a text file, listing one public key per line, or as
1293: an OpenSSH Key Revocation List (KRL) as generated by
1294: .Xr ssh-keygen 1 .
1295: For more information on KRLs, see the KEY REVOCATION LISTS section in
1296: .Xr ssh-keygen 1 .
1.1 stevesk 1297: .It Cm RhostsRSAAuthentication
1298: Specifies whether to try rhosts based authentication with RSA host
1299: authentication.
1300: The argument must be
1301: .Dq yes
1302: or
1303: .Dq no .
1304: The default is
1305: .Dq no .
1306: This option applies to protocol version 1 only and requires
1.84 jmc 1307: .Xr ssh 1
1.1 stevesk 1308: to be setuid root.
1309: .It Cm RSAAuthentication
1310: Specifies whether to try RSA authentication.
1311: The argument to this keyword must be
1312: .Dq yes
1313: or
1314: .Dq no .
1315: RSA authentication will only be
1316: attempted if the identity file exists, or an authentication agent is
1317: running.
1318: The default is
1319: .Dq yes .
1320: Note that this option applies to protocol version 1 only.
1.32 djm 1321: .It Cm SendEnv
1322: Specifies what variables from the local
1323: .Xr environ 7
1324: should be sent to the server.
1.84 jmc 1325: Note that environment passing is only supported for protocol 2.
1326: The server must also support it, and the server must be configured to
1.33 djm 1327: accept these environment variables.
1.32 djm 1328: Refer to
1329: .Cm AcceptEnv
1330: in
1331: .Xr sshd_config 5
1332: for how to configure the server.
1.80 jmc 1333: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1334: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1335: across multiple
1336: .Cm SendEnv
1337: directives.
1338: The default is not to send any environment variables.
1.81 jmc 1339: .Pp
1340: See
1341: .Sx PATTERNS
1342: for more information on patterns.
1.28 markus 1343: .It Cm ServerAliveCountMax
1.73 jmc 1344: Sets the number of server alive messages (see below) which may be
1.28 markus 1345: sent without
1.84 jmc 1346: .Xr ssh 1
1.28 markus 1347: receiving any messages back from the server.
1348: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1349: ssh will disconnect from the server, terminating the session.
1.28 markus 1350: It is important to note that the use of server alive messages is very
1351: different from
1352: .Cm TCPKeepAlive
1353: (below).
1354: The server alive messages are sent through the encrypted channel
1355: and therefore will not be spoofable.
1356: The TCP keepalive option enabled by
1357: .Cm TCPKeepAlive
1358: is spoofable.
1359: The server alive mechanism is valuable when the client or
1360: server depend on knowing when a connection has become inactive.
1361: .Pp
1362: The default value is 3.
1363: If, for example,
1364: .Cm ServerAliveInterval
1.84 jmc 1365: (see below) is set to 15 and
1.28 markus 1366: .Cm ServerAliveCountMax
1.84 jmc 1367: is left at the default, if the server becomes unresponsive,
1368: ssh will disconnect after approximately 45 seconds.
1.89 markus 1369: This option applies to protocol version 2 only.
1.67 jmc 1370: .It Cm ServerAliveInterval
1371: Sets a timeout interval in seconds after which if no data has been received
1372: from the server,
1.84 jmc 1373: .Xr ssh 1
1.67 jmc 1374: will send a message through the encrypted
1375: channel to request a response from the server.
1376: The default
1377: is 0, indicating that these messages will not be sent to the server.
1378: This option applies to protocol version 2 only.
1.191 millert 1379: .It Cm StreamLocalBindMask
1380: Sets the octal file creation mode mask
1381: .Pq umask
1382: used when creating a Unix-domain socket file for local or remote
1383: port forwarding.
1384: This option is only used for port forwarding to a Unix-domain socket file.
1385: .Pp
1386: The default value is 0177, which creates a Unix-domain socket file that is
1387: readable and writable only by the owner.
1388: Note that not all operating systems honor the file mode on Unix-domain
1389: socket files.
1390: .It Cm StreamLocalBindUnlink
1391: Specifies whether to remove an existing Unix-domain socket file for local
1392: or remote port forwarding before creating a new one.
1393: If the socket file already exists and
1394: .Cm StreamLocalBindUnlink
1395: is not enabled,
1396: .Nm ssh
1397: will be unable to forward the port to the Unix-domain socket file.
1398: This option is only used for port forwarding to a Unix-domain socket file.
1399: .Pp
1400: The argument must be
1401: .Dq yes
1402: or
1403: .Dq no .
1404: The default is
1405: .Dq no .
1.1 stevesk 1406: .It Cm StrictHostKeyChecking
1407: If this flag is set to
1408: .Dq yes ,
1.84 jmc 1409: .Xr ssh 1
1.1 stevesk 1410: will never automatically add host keys to the
1.50 djm 1411: .Pa ~/.ssh/known_hosts
1.1 stevesk 1412: file, and refuses to connect to hosts whose host key has changed.
1413: This provides maximum protection against trojan horse attacks,
1.84 jmc 1414: though it can be annoying when the
1.1 stevesk 1415: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1416: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1417: frequently made.
1418: This option forces the user to manually
1419: add all new hosts.
1420: If this flag is set to
1421: .Dq no ,
1.84 jmc 1422: ssh will automatically add new host keys to the
1.1 stevesk 1423: user known hosts files.
1424: If this flag is set to
1425: .Dq ask ,
1426: new host keys
1427: will be added to the user known host files only after the user
1428: has confirmed that is what they really want to do, and
1.84 jmc 1429: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1430: The host keys of
1431: known hosts will be verified automatically in all cases.
1432: The argument must be
1433: .Dq yes ,
1.84 jmc 1434: .Dq no ,
1.1 stevesk 1435: or
1436: .Dq ask .
1437: The default is
1438: .Dq ask .
1.26 markus 1439: .It Cm TCPKeepAlive
1440: Specifies whether the system should send TCP keepalive messages to the
1441: other side.
1442: If they are sent, death of the connection or crash of one
1443: of the machines will be properly noticed.
1444: However, this means that
1445: connections will die if the route is down temporarily, and some people
1446: find it annoying.
1447: .Pp
1448: The default is
1449: .Dq yes
1450: (to send TCP keepalive messages), and the client will notice
1451: if the network goes down or the remote host dies.
1452: This is important in scripts, and many users want it too.
1453: .Pp
1454: To disable TCP keepalive messages, the value should be set to
1455: .Dq no .
1.65 reyk 1456: .It Cm Tunnel
1.95 stevesk 1457: Request
1.65 reyk 1458: .Xr tun 4
1.69 jmc 1459: device forwarding between the client and the server.
1.65 reyk 1460: The argument must be
1.68 reyk 1461: .Dq yes ,
1.95 stevesk 1462: .Dq point-to-point
1463: (layer 3),
1464: .Dq ethernet
1465: (layer 2),
1.65 reyk 1466: or
1467: .Dq no .
1.95 stevesk 1468: Specifying
1469: .Dq yes
1470: requests the default tunnel mode, which is
1471: .Dq point-to-point .
1.65 reyk 1472: The default is
1473: .Dq no .
1474: .It Cm TunnelDevice
1.95 stevesk 1475: Specifies the
1.65 reyk 1476: .Xr tun 4
1.95 stevesk 1477: devices to open on the client
1478: .Pq Ar local_tun
1479: and the server
1480: .Pq Ar remote_tun .
1481: .Pp
1482: The argument must be
1483: .Sm off
1484: .Ar local_tun Op : Ar remote_tun .
1485: .Sm on
1486: The devices may be specified by numerical ID or the keyword
1487: .Dq any ,
1488: which uses the next available tunnel device.
1489: If
1490: .Ar remote_tun
1491: is not specified, it defaults to
1492: .Dq any .
1493: The default is
1494: .Dq any:any .
1.200 ! djm 1495: .It Cm UpdateHostkeys
! 1496: Specifies whether
! 1497: .Xr ssh 1
! 1498: should accept notifications of additional hostkeys from the server sent
! 1499: after authentication has completed and add them to
! 1500: .Cm UserKnownHostsFile .
! 1501: The argument must be
! 1502: .Dq yes
! 1503: (the default)
! 1504: or
! 1505: .Dq no .
! 1506: Enabling this option allows learning alternate hostkeys for a server
! 1507: and supports graceful key rotation by allowing a server to public replacement
! 1508: keys before old ones are removed.
! 1509: Additional hostkeys are only accepted if the key used to authenticate the
! 1510: host was already trusted or explicity accepted by the user.
! 1511: .Pp
! 1512: Presently, only
! 1513: .Xr sshd 8
! 1514: from OpenSSH 6.8 and greater support the
! 1515: .Dq hostkeys@openssh.com
! 1516: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1517: .It Cm UsePrivilegedPort
1518: Specifies whether to use a privileged port for outgoing connections.
1519: The argument must be
1520: .Dq yes
1521: or
1522: .Dq no .
1523: The default is
1524: .Dq no .
1525: If set to
1.84 jmc 1526: .Dq yes ,
1527: .Xr ssh 1
1.72 jmc 1528: must be setuid root.
1529: Note that this option must be set to
1530: .Dq yes
1531: for
1532: .Cm RhostsRSAAuthentication
1533: with older servers.
1.1 stevesk 1534: .It Cm User
1535: Specifies the user to log in as.
1536: This can be useful when a different user name is used on different machines.
1537: This saves the trouble of
1538: having to remember to give the user name on the command line.
1539: .It Cm UserKnownHostsFile
1.151 djm 1540: Specifies one or more files to use for the user
1541: host key database, separated by whitespace.
1542: The default is
1543: .Pa ~/.ssh/known_hosts ,
1544: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1545: .It Cm VerifyHostKeyDNS
1546: Specifies whether to verify the remote key using DNS and SSHFP resource
1547: records.
1.24 jakob 1548: If this option is set to
1549: .Dq yes ,
1.25 jmc 1550: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1551: from DNS.
1552: Insecure fingerprints will be handled as if this option was set to
1553: .Dq ask .
1554: If this option is set to
1555: .Dq ask ,
1556: information on fingerprint match will be displayed, but the user will still
1557: need to confirm new host keys according to the
1558: .Cm StrictHostKeyChecking
1559: option.
1560: The argument must be
1561: .Dq yes ,
1.84 jmc 1562: .Dq no ,
1.25 jmc 1563: or
1564: .Dq ask .
1.8 jakob 1565: The default is
1566: .Dq no .
1.12 jakob 1567: Note that this option applies to protocol version 2 only.
1.84 jmc 1568: .Pp
1.166 jmc 1569: See also VERIFYING HOST KEYS in
1.84 jmc 1570: .Xr ssh 1 .
1.111 grunk 1571: .It Cm VisualHostKey
1572: If this flag is set to
1573: .Dq yes ,
1574: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1575: printed in addition to the fingerprint string at login and
1.114 stevesk 1576: for unknown host keys.
1.111 grunk 1577: If this flag is set to
1578: .Dq no ,
1.114 stevesk 1579: no fingerprint strings are printed at login and
1.197 djm 1580: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1581: The default is
1582: .Dq no .
1.1 stevesk 1583: .It Cm XAuthLocation
1.5 stevesk 1584: Specifies the full pathname of the
1.1 stevesk 1585: .Xr xauth 1
1586: program.
1587: The default is
1588: .Pa /usr/X11R6/bin/xauth .
1589: .El
1.86 jmc 1590: .Sh PATTERNS
1591: A
1592: .Em pattern
1593: consists of zero or more non-whitespace characters,
1594: .Sq *
1595: (a wildcard that matches zero or more characters),
1596: or
1597: .Sq ?\&
1598: (a wildcard that matches exactly one character).
1599: For example, to specify a set of declarations for any host in the
1600: .Dq .co.uk
1601: set of domains,
1602: the following pattern could be used:
1603: .Pp
1604: .Dl Host *.co.uk
1605: .Pp
1606: The following pattern
1607: would match any host in the 192.168.0.[0-9] network range:
1608: .Pp
1609: .Dl Host 192.168.0.?
1610: .Pp
1611: A
1612: .Em pattern-list
1613: is a comma-separated list of patterns.
1614: Patterns within pattern-lists may be negated
1615: by preceding them with an exclamation mark
1616: .Pq Sq !\& .
1617: For example,
1.174 djm 1618: to allow a key to be used from anywhere within an organization
1.86 jmc 1619: except from the
1620: .Dq dialup
1621: pool,
1622: the following entry (in authorized_keys) could be used:
1623: .Pp
1624: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1625: .Sh FILES
1626: .Bl -tag -width Ds
1.50 djm 1627: .It Pa ~/.ssh/config
1.1 stevesk 1628: This is the per-user configuration file.
1629: The format of this file is described above.
1.84 jmc 1630: This file is used by the SSH client.
1.30 djm 1631: Because of the potential for abuse, this file must have strict permissions:
1632: read/write for the user, and not accessible by others.
1.1 stevesk 1633: .It Pa /etc/ssh/ssh_config
1634: Systemwide configuration file.
1635: This file provides defaults for those
1636: values that are not specified in the user's configuration file, and
1637: for those users who do not have a configuration file.
1638: This file must be world-readable.
1639: .El
1.13 jmc 1640: .Sh SEE ALSO
1641: .Xr ssh 1
1.1 stevesk 1642: .Sh AUTHORS
1643: OpenSSH is a derivative of the original and free
1644: ssh 1.2.12 release by Tatu Ylonen.
1645: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1646: Theo de Raadt and Dug Song
1647: removed many bugs, re-added newer features and
1648: created OpenSSH.
1649: Markus Friedl contributed the support for SSH
1650: protocol versions 1.5 and 2.0.
![[BACK]](/icons/back.gif){kind=icon}
Return to ssh_config.5 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh