Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.196
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.
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1.196 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.195 2014/11/10 22:25:49 djm Exp $
! 37: .Dd $Mdocdate: November 10 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
420: .Xr ssh 1 .
1.1 stevesk 421: .It Cm ClearAllForwardings
1.84 jmc 422: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 423: specified in the configuration files or on the command line be
1.7 jmc 424: cleared.
425: This option is primarily useful when used from the
1.84 jmc 426: .Xr ssh 1
1.1 stevesk 427: command line to clear port forwardings set in
428: configuration files, and is automatically set by
429: .Xr scp 1
430: and
431: .Xr sftp 1 .
432: The argument must be
433: .Dq yes
434: or
435: .Dq no .
436: The default is
437: .Dq no .
438: .It Cm Compression
439: Specifies whether to use compression.
440: The argument must be
441: .Dq yes
442: or
443: .Dq no .
444: The default is
445: .Dq no .
446: .It Cm CompressionLevel
447: Specifies the compression level to use if compression is enabled.
448: The argument must be an integer from 1 (fast) to 9 (slow, best).
449: The default level is 6, which is good for most applications.
450: The meaning of the values is the same as in
451: .Xr gzip 1 .
452: Note that this option applies to protocol version 1 only.
453: .It Cm ConnectionAttempts
454: Specifies the number of tries (one per second) to make before exiting.
455: The argument must be an integer.
456: This may be useful in scripts if the connection sometimes fails.
457: The default is 1.
1.9 djm 458: .It Cm ConnectTimeout
1.84 jmc 459: Specifies the timeout (in seconds) used when connecting to the
460: SSH server, instead of using the default system TCP timeout.
1.11 jmc 461: This value is used only when the target is down or really unreachable,
462: not when it refuses the connection.
1.36 djm 463: .It Cm ControlMaster
464: Enables the sharing of multiple sessions over a single network connection.
465: When set to
1.84 jmc 466: .Dq yes ,
467: .Xr ssh 1
1.36 djm 468: will listen for connections on a control socket specified using the
469: .Cm ControlPath
470: argument.
471: Additional sessions can connect to this socket using the same
472: .Cm ControlPath
473: with
474: .Cm ControlMaster
475: set to
476: .Dq no
1.38 jmc 477: (the default).
1.64 jmc 478: These sessions will try to reuse the master instance's network connection
1.63 djm 479: rather than initiating new ones, but will fall back to connecting normally
480: if the control socket does not exist, or is not listening.
481: .Pp
1.37 djm 482: Setting this to
483: .Dq ask
1.84 jmc 484: will cause ssh
1.37 djm 485: to listen for control connections, but require confirmation using the
486: .Ev SSH_ASKPASS
487: program before they are accepted (see
488: .Xr ssh-add 1
1.38 jmc 489: for details).
1.51 jakob 490: If the
491: .Cm ControlPath
1.84 jmc 492: cannot be opened,
493: ssh will continue without connecting to a master instance.
1.58 djm 494: .Pp
495: X11 and
1.59 jmc 496: .Xr ssh-agent 1
1.58 djm 497: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 498: display and agent forwarded will be the one belonging to the master
1.59 jmc 499: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 500: .Pp
501: Two additional options allow for opportunistic multiplexing: try to use a
502: master connection but fall back to creating a new one if one does not already
503: exist.
504: These options are:
505: .Dq auto
506: and
507: .Dq autoask .
508: The latter requires confirmation like the
509: .Dq ask
510: option.
1.36 djm 511: .It Cm ControlPath
1.55 djm 512: Specify the path to the control socket used for connection sharing as described
513: in the
1.36 djm 514: .Cm ControlMaster
1.57 djm 515: section above or the string
516: .Dq none
517: to disable connection sharing.
1.55 djm 518: In the path,
1.147 djm 519: .Ql %L
520: will be substituted by the first component of the local host name,
1.77 djm 521: .Ql %l
1.147 djm 522: will be substituted by the local host name (including any domain name),
1.55 djm 523: .Ql %h
524: will be substituted by the target host name,
1.150 jmc 525: .Ql %n
526: will be substituted by the original target host name
527: specified on the command line,
1.55 djm 528: .Ql %p
1.175 djm 529: the destination port,
1.55 djm 530: .Ql %r
1.188 djm 531: by the remote login username,
1.147 djm 532: .Ql %u
533: by the username of the user running
1.188 djm 534: .Xr ssh 1 , and
1.189 jmc 535: .Ql \&%C
1.188 djm 536: by a hash of the concatenation: %l%h%p%r.
1.56 djm 537: It is recommended that any
538: .Cm ControlPath
539: used for opportunistic connection sharing include
1.195 djm 540: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
541: that is not writable by other users.
1.56 djm 542: This ensures that shared connections are uniquely identified.
1.137 djm 543: .It Cm ControlPersist
544: When used in conjunction with
545: .Cm ControlMaster ,
546: specifies that the master connection should remain open
547: in the background (waiting for future client connections)
548: after the initial client connection has been closed.
549: If set to
550: .Dq no ,
551: then the master connection will not be placed into the background,
552: and will close as soon as the initial client connection is closed.
553: If set to
1.195 djm 554: .Dq yes
555: or
556: .Dq 0 ,
1.137 djm 557: then the master connection will remain in the background indefinitely
558: (until killed or closed via a mechanism such as the
559: .Xr ssh 1
560: .Dq Fl O No exit
561: option).
562: If set to a time in seconds, or a time in any of the formats documented in
563: .Xr sshd_config 5 ,
564: then the backgrounded master connection will automatically terminate
565: after it has remained idle (with no client connections) for the
566: specified time.
1.38 jmc 567: .It Cm DynamicForward
1.74 jmc 568: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 569: over the secure channel, and the application
570: protocol is then used to determine where to connect to from the
571: remote machine.
1.62 djm 572: .Pp
573: The argument must be
574: .Sm off
575: .Oo Ar bind_address : Oc Ar port .
576: .Sm on
1.138 djm 577: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 578: By default, the local port is bound in accordance with the
579: .Cm GatewayPorts
580: setting.
581: However, an explicit
582: .Ar bind_address
583: may be used to bind the connection to a specific address.
584: The
585: .Ar bind_address
586: of
587: .Dq localhost
588: indicates that the listening port be bound for local use only, while an
589: empty address or
590: .Sq *
591: indicates that the port should be available from all interfaces.
592: .Pp
1.38 jmc 593: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 594: .Xr ssh 1
1.38 jmc 595: will act as a SOCKS server.
596: Multiple forwardings may be specified, and
597: additional forwardings can be given on the command line.
598: Only the superuser can forward privileged ports.
1.14 markus 599: .It Cm EnableSSHKeysign
600: Setting this option to
601: .Dq yes
602: in the global client configuration file
603: .Pa /etc/ssh/ssh_config
604: enables the use of the helper program
605: .Xr ssh-keysign 8
606: during
607: .Cm HostbasedAuthentication .
608: The argument must be
609: .Dq yes
610: or
611: .Dq no .
612: The default is
613: .Dq no .
1.23 jmc 614: This option should be placed in the non-hostspecific section.
1.14 markus 615: See
616: .Xr ssh-keysign 8
617: for more information.
1.1 stevesk 618: .It Cm EscapeChar
619: Sets the escape character (default:
620: .Ql ~ ) .
621: The escape character can also
622: be set on the command line.
623: The argument should be a single character,
624: .Ql ^
625: followed by a letter, or
626: .Dq none
627: to disable the escape
628: character entirely (making the connection transparent for binary
629: data).
1.96 markus 630: .It Cm ExitOnForwardFailure
631: Specifies whether
632: .Xr ssh 1
633: should terminate the connection if it cannot set up all requested
1.102 stevesk 634: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 635: The argument must be
636: .Dq yes
637: or
638: .Dq no .
639: The default is
640: .Dq no .
1.1 stevesk 641: .It Cm ForwardAgent
642: Specifies whether the connection to the authentication agent (if any)
643: will be forwarded to the remote machine.
644: The argument must be
645: .Dq yes
646: or
647: .Dq no .
648: The default is
649: .Dq no .
1.3 stevesk 650: .Pp
1.7 jmc 651: Agent forwarding should be enabled with caution.
652: Users with the ability to bypass file permissions on the remote host
653: (for the agent's Unix-domain socket)
654: can access the local agent through the forwarded connection.
655: An attacker cannot obtain key material from the agent,
1.3 stevesk 656: however they can perform operations on the keys that enable them to
657: authenticate using the identities loaded into the agent.
1.1 stevesk 658: .It Cm ForwardX11
659: Specifies whether X11 connections will be automatically redirected
660: over the secure channel and
661: .Ev DISPLAY
662: set.
663: The argument must be
664: .Dq yes
665: or
666: .Dq no .
667: The default is
668: .Dq no .
1.3 stevesk 669: .Pp
1.7 jmc 670: X11 forwarding should be enabled with caution.
671: Users with the ability to bypass file permissions on the remote host
1.22 markus 672: (for the user's X11 authorization database)
1.7 jmc 673: can access the local X11 display through the forwarded connection.
1.22 markus 674: An attacker may then be able to perform activities such as keystroke monitoring
675: if the
676: .Cm ForwardX11Trusted
677: option is also enabled.
1.134 djm 678: .It Cm ForwardX11Timeout
1.135 jmc 679: Specify a timeout for untrusted X11 forwarding
680: using the format described in the
1.166 jmc 681: TIME FORMATS section of
1.134 djm 682: .Xr sshd_config 5 .
683: X11 connections received by
684: .Xr ssh 1
685: after this time will be refused.
686: The default is to disable untrusted X11 forwarding after twenty minutes has
687: elapsed.
1.22 markus 688: .It Cm ForwardX11Trusted
1.34 jmc 689: If this option is set to
1.84 jmc 690: .Dq yes ,
691: remote X11 clients will have full access to the original X11 display.
1.42 djm 692: .Pp
1.22 markus 693: If this option is set to
1.84 jmc 694: .Dq no ,
695: remote X11 clients will be considered untrusted and prevented
1.22 markus 696: from stealing or tampering with data belonging to trusted X11
697: clients.
1.42 djm 698: Furthermore, the
699: .Xr xauth 1
700: token used for the session will be set to expire after 20 minutes.
701: Remote clients will be refused access after this time.
1.22 markus 702: .Pp
703: The default is
704: .Dq no .
705: .Pp
706: See the X11 SECURITY extension specification for full details on
707: the restrictions imposed on untrusted clients.
1.1 stevesk 708: .It Cm GatewayPorts
709: Specifies whether remote hosts are allowed to connect to local
710: forwarded ports.
711: By default,
1.84 jmc 712: .Xr ssh 1
1.7 jmc 713: binds local port forwardings to the loopback address.
714: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 715: .Cm GatewayPorts
1.84 jmc 716: can be used to specify that ssh
1.1 stevesk 717: should bind local port forwardings to the wildcard address,
718: thus allowing remote hosts to connect to forwarded ports.
719: The argument must be
720: .Dq yes
721: or
722: .Dq no .
723: The default is
724: .Dq no .
725: .It Cm GlobalKnownHostsFile
1.151 djm 726: Specifies one or more files to use for the global
727: host key database, separated by whitespace.
728: The default is
729: .Pa /etc/ssh/ssh_known_hosts ,
730: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 731: .It Cm GSSAPIAuthentication
1.27 markus 732: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 733: The default is
1.21 markus 734: .Dq no .
1.18 markus 735: Note that this option applies to protocol version 2 only.
736: .It Cm GSSAPIDelegateCredentials
737: Forward (delegate) credentials to the server.
738: The default is
739: .Dq no .
740: Note that this option applies to protocol version 2 only.
1.44 djm 741: .It Cm HashKnownHosts
742: Indicates that
1.84 jmc 743: .Xr ssh 1
1.44 djm 744: should hash host names and addresses when they are added to
1.50 djm 745: .Pa ~/.ssh/known_hosts .
1.44 djm 746: These hashed names may be used normally by
1.84 jmc 747: .Xr ssh 1
1.44 djm 748: and
1.84 jmc 749: .Xr sshd 8 ,
1.44 djm 750: but they do not reveal identifying information should the file's contents
751: be disclosed.
752: The default is
753: .Dq no .
1.97 jmc 754: Note that existing names and addresses in known hosts files
755: will not be converted automatically,
756: but may be manually hashed using
1.45 djm 757: .Xr ssh-keygen 1 .
1.1 stevesk 758: .It Cm HostbasedAuthentication
759: Specifies whether to try rhosts based authentication with public key
760: authentication.
761: The argument must be
762: .Dq yes
763: or
764: .Dq no .
765: The default is
766: .Dq no .
767: This option applies to protocol version 2 only and
768: is similar to
769: .Cm RhostsRSAAuthentication .
770: .It Cm HostKeyAlgorithms
771: Specifies the protocol version 2 host key algorithms
772: that the client wants to use in order of preference.
773: The default for this option is:
1.139 djm 774: .Bd -literal -offset 3n
775: ecdsa-sha2-nistp256-cert-v01@openssh.com,
776: ecdsa-sha2-nistp384-cert-v01@openssh.com,
777: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 778: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 779: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
780: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
781: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.182 markus 782: ssh-ed25519,ssh-rsa,ssh-dss
1.139 djm 783: .Ed
1.145 djm 784: .Pp
785: If hostkeys are known for the destination host then this default is modified
786: to prefer their algorithms.
1.1 stevesk 787: .It Cm HostKeyAlias
788: Specifies an alias that should be used instead of the
789: real host name when looking up or saving the host key
790: in the host key database files.
1.84 jmc 791: This option is useful for tunneling SSH connections
1.1 stevesk 792: or for multiple servers running on a single host.
793: .It Cm HostName
794: Specifies the real host name to log into.
795: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 796: If the hostname contains the character sequence
797: .Ql %h ,
1.150 jmc 798: then this will be replaced with the host name specified on the command line
1.136 djm 799: (this is useful for manipulating unqualified names).
1.187 djm 800: The character sequence
801: .Ql %%
802: will be replaced by a single
803: .Ql %
804: character, which may be used when specifying IPv6 link-local addresses.
805: .Pp
1.84 jmc 806: The default is the name given on the command line.
1.1 stevesk 807: Numeric IP addresses are also permitted (both on the command line and in
808: .Cm HostName
809: specifications).
1.29 markus 810: .It Cm IdentitiesOnly
811: Specifies that
1.84 jmc 812: .Xr ssh 1
1.29 markus 813: should only use the authentication identity files configured in the
1.31 jmc 814: .Nm
1.29 markus 815: files,
1.84 jmc 816: even if
817: .Xr ssh-agent 1
1.159 djm 818: or a
819: .Cm PKCS11Provider
1.29 markus 820: offers more identities.
821: The argument to this keyword must be
822: .Dq yes
823: or
824: .Dq no .
1.84 jmc 825: This option is intended for situations where ssh-agent
1.29 markus 826: offers many different identities.
827: The default is
828: .Dq no .
1.67 jmc 829: .It Cm IdentityFile
1.192 sobrado 830: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 831: identity is read.
1.67 jmc 832: The default is
833: .Pa ~/.ssh/identity
834: for protocol version 1, and
1.139 djm 835: .Pa ~/.ssh/id_dsa ,
1.183 naddy 836: .Pa ~/.ssh/id_ecdsa ,
837: .Pa ~/.ssh/id_ed25519
1.139 djm 838: and
1.67 jmc 839: .Pa ~/.ssh/id_rsa
840: for protocol version 2.
841: Additionally, any identities represented by the authentication agent
1.165 djm 842: will be used for authentication unless
843: .Cm IdentitiesOnly
844: is set.
1.129 djm 845: .Xr ssh 1
846: will try to load certificate information from the filename obtained by
847: appending
848: .Pa -cert.pub
849: to the path of a specified
850: .Cm IdentityFile .
1.90 djm 851: .Pp
1.67 jmc 852: The file name may use the tilde
1.91 jmc 853: syntax to refer to a user's home directory or one of the following
1.90 djm 854: escape characters:
855: .Ql %d
856: (local user's home directory),
857: .Ql %u
858: (local user name),
859: .Ql %l
860: (local host name),
861: .Ql %h
862: (remote host name) or
1.92 djm 863: .Ql %r
1.90 djm 864: (remote user name).
865: .Pp
1.67 jmc 866: It is possible to have
867: multiple identity files specified in configuration files; all these
868: identities will be tried in sequence.
1.152 djm 869: Multiple
870: .Cm IdentityFile
871: directives will add to the list of identities tried (this behaviour
872: differs from that of other configuration directives).
1.165 djm 873: .Pp
874: .Cm IdentityFile
875: may be used in conjunction with
876: .Cm IdentitiesOnly
877: to select which identities in an agent are offered during authentication.
1.164 jmc 878: .It Cm IgnoreUnknown
879: Specifies a pattern-list of unknown options to be ignored if they are
880: encountered in configuration parsing.
881: This may be used to suppress errors if
882: .Nm
883: contains options that are unrecognised by
884: .Xr ssh 1 .
885: It is recommended that
886: .Cm IgnoreUnknown
887: be listed early in the configuration file as it will not be applied
888: to unknown options that appear before it.
1.143 djm 889: .It Cm IPQoS
890: Specifies the IPv4 type-of-service or DSCP class for connections.
891: Accepted values are
892: .Dq af11 ,
893: .Dq af12 ,
894: .Dq af13 ,
1.154 djm 895: .Dq af21 ,
1.143 djm 896: .Dq af22 ,
897: .Dq af23 ,
898: .Dq af31 ,
899: .Dq af32 ,
900: .Dq af33 ,
901: .Dq af41 ,
902: .Dq af42 ,
903: .Dq af43 ,
904: .Dq cs0 ,
905: .Dq cs1 ,
906: .Dq cs2 ,
907: .Dq cs3 ,
908: .Dq cs4 ,
909: .Dq cs5 ,
910: .Dq cs6 ,
911: .Dq cs7 ,
912: .Dq ef ,
913: .Dq lowdelay ,
914: .Dq throughput ,
915: .Dq reliability ,
916: or a numeric value.
1.146 djm 917: This option may take one or two arguments, separated by whitespace.
1.143 djm 918: If one argument is specified, it is used as the packet class unconditionally.
919: If two values are specified, the first is automatically selected for
920: interactive sessions and the second for non-interactive sessions.
921: The default is
922: .Dq lowdelay
923: for interactive sessions and
924: .Dq throughput
925: for non-interactive sessions.
1.103 djm 926: .It Cm KbdInteractiveAuthentication
927: Specifies whether to use keyboard-interactive authentication.
928: The argument to this keyword must be
929: .Dq yes
930: or
931: .Dq no .
932: The default is
933: .Dq yes .
1.39 djm 934: .It Cm KbdInteractiveDevices
935: Specifies the list of methods to use in keyboard-interactive authentication.
936: Multiple method names must be comma-separated.
937: The default is to use the server specified list.
1.85 jmc 938: The methods available vary depending on what the server supports.
939: For an OpenSSH server,
940: it may be zero or more of:
941: .Dq bsdauth ,
942: .Dq pam ,
943: and
944: .Dq skey .
1.140 djm 945: .It Cm KexAlgorithms
946: Specifies the available KEX (Key Exchange) algorithms.
947: Multiple algorithms must be comma-separated.
1.141 jmc 948: The default is:
949: .Bd -literal -offset indent
1.179 markus 950: curve25519-sha256@libssh.org,
1.141 jmc 951: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
952: diffie-hellman-group-exchange-sha256,
1.186 naddy 953: diffie-hellman-group14-sha1,
1.141 jmc 954: diffie-hellman-group-exchange-sha1,
955: diffie-hellman-group1-sha1
956: .Ed
1.65 reyk 957: .It Cm LocalCommand
958: Specifies a command to execute on the local machine after successfully
959: connecting to the server.
960: The command string extends to the end of the line, and is executed with
1.105 jmc 961: the user's shell.
1.109 dtucker 962: The following escape character substitutions will be performed:
963: .Ql %d
964: (local user's home directory),
965: .Ql %h
966: (remote host name),
967: .Ql %l
968: (local host name),
969: .Ql %n
970: (host name as provided on the command line),
971: .Ql %p
972: (remote port),
973: .Ql %r
974: (remote user name) or
975: .Ql %u
1.188 djm 976: (local user name) or
1.189 jmc 977: .Ql \&%C
1.188 djm 978: by a hash of the concatenation: %l%h%p%r.
1.123 djm 979: .Pp
980: The command is run synchronously and does not have access to the
981: session of the
982: .Xr ssh 1
983: that spawned it.
984: It should not be used for interactive commands.
985: .Pp
1.65 reyk 986: This directive is ignored unless
987: .Cm PermitLocalCommand
988: has been enabled.
1.1 stevesk 989: .It Cm LocalForward
1.74 jmc 990: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 991: the secure channel to the specified host and port from the remote machine.
1.49 jmc 992: The first argument must be
1.43 djm 993: .Sm off
1.49 jmc 994: .Oo Ar bind_address : Oc Ar port
1.43 djm 995: .Sm on
1.49 jmc 996: and the second argument must be
997: .Ar host : Ns Ar hostport .
1.138 djm 998: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 999: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1000: given on the command line.
1.1 stevesk 1001: Only the superuser can forward privileged ports.
1.43 djm 1002: By default, the local port is bound in accordance with the
1003: .Cm GatewayPorts
1004: setting.
1005: However, an explicit
1006: .Ar bind_address
1007: may be used to bind the connection to a specific address.
1008: The
1009: .Ar bind_address
1010: of
1011: .Dq localhost
1.46 jmc 1012: indicates that the listening port be bound for local use only, while an
1013: empty address or
1014: .Sq *
1.43 djm 1015: indicates that the port should be available from all interfaces.
1.1 stevesk 1016: .It Cm LogLevel
1017: Gives the verbosity level that is used when logging messages from
1.84 jmc 1018: .Xr ssh 1 .
1.1 stevesk 1019: The possible values are:
1.84 jmc 1020: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1021: The default is INFO.
1022: DEBUG and DEBUG1 are equivalent.
1023: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1024: .It Cm MACs
1025: Specifies the MAC (message authentication code) algorithms
1026: in order of preference.
1027: The MAC algorithm is used in protocol version 2
1028: for data integrity protection.
1029: Multiple algorithms must be comma-separated.
1.160 markus 1030: The algorithms that contain
1031: .Dq -etm
1032: calculate the MAC after encryption (encrypt-then-mac).
1033: These are considered safer and their use recommended.
1.84 jmc 1034: The default is:
1.101 jmc 1035: .Bd -literal -offset indent
1.160 markus 1036: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1037: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1038: umac-64@openssh.com,umac-128@openssh.com,
1039: hmac-sha2-256,hmac-sha2-512,
1040: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1041: hmac-ripemd160-etm@openssh.com,
1042: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1043: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1044: hmac-sha1-96,hmac-md5-96
1.101 jmc 1045: .Ed
1.1 stevesk 1046: .It Cm NoHostAuthenticationForLocalhost
1047: This option can be used if the home directory is shared across machines.
1048: In this case localhost will refer to a different machine on each of
1049: the machines and the user will get many warnings about changed host keys.
1050: However, this option disables host authentication for localhost.
1051: The argument to this keyword must be
1052: .Dq yes
1053: or
1054: .Dq no .
1055: The default is to check the host key for localhost.
1056: .It Cm NumberOfPasswordPrompts
1057: Specifies the number of password prompts before giving up.
1058: The argument to this keyword must be an integer.
1.84 jmc 1059: The default is 3.
1.1 stevesk 1060: .It Cm PasswordAuthentication
1061: Specifies whether to use password authentication.
1062: The argument to this keyword must be
1063: .Dq yes
1064: or
1065: .Dq no .
1066: The default is
1067: .Dq yes .
1.65 reyk 1068: .It Cm PermitLocalCommand
1069: Allow local command execution via the
1070: .Ic LocalCommand
1071: option or using the
1.66 jmc 1072: .Ic !\& Ns Ar command
1.65 reyk 1073: escape sequence in
1074: .Xr ssh 1 .
1075: The argument must be
1076: .Dq yes
1077: or
1078: .Dq no .
1079: The default is
1080: .Dq no .
1.127 markus 1081: .It Cm PKCS11Provider
1082: Specifies which PKCS#11 provider to use.
1.144 jmc 1083: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1084: .Xr ssh 1
1.128 markus 1085: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1086: private RSA key.
1.67 jmc 1087: .It Cm Port
1088: Specifies the port number to connect on the remote host.
1.84 jmc 1089: The default is 22.
1.1 stevesk 1090: .It Cm PreferredAuthentications
1091: Specifies the order in which the client should try protocol 2
1.11 jmc 1092: authentication methods.
1.48 jmc 1093: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1094: .Cm keyboard-interactive )
1.48 jmc 1095: over another method (e.g.\&
1.131 jmc 1096: .Cm password ) .
1097: The default is:
1098: .Bd -literal -offset indent
1099: gssapi-with-mic,hostbased,publickey,
1100: keyboard-interactive,password
1101: .Ed
1.1 stevesk 1102: .It Cm Protocol
1103: Specifies the protocol versions
1.84 jmc 1104: .Xr ssh 1
1.1 stevesk 1105: should support in order of preference.
1106: The possible values are
1.84 jmc 1107: .Sq 1
1.1 stevesk 1108: and
1.84 jmc 1109: .Sq 2 .
1.1 stevesk 1110: Multiple versions must be comma-separated.
1.120 markus 1111: When this option is set to
1.121 jmc 1112: .Dq 2,1
1.120 markus 1113: .Nm ssh
1114: will try version 2 and fall back to version 1
1115: if version 2 is not available.
1.1 stevesk 1116: The default is
1.121 jmc 1117: .Sq 2 .
1.1 stevesk 1118: .It Cm ProxyCommand
1119: Specifies the command to use to connect to the server.
1120: The command
1.190 djm 1121: string extends to the end of the line, and is executed
1122: using the user's shell
1123: .Ql exec
1124: directive to avoid a lingering shell process.
1125: .Pp
1.133 jmc 1126: In the command string, any occurrence of
1.1 stevesk 1127: .Ql %h
1128: will be substituted by the host name to
1.132 djm 1129: connect,
1.1 stevesk 1130: .Ql %p
1.133 jmc 1131: by the port, and
1132: .Ql %r
1.132 djm 1133: by the remote user name.
1.1 stevesk 1134: The command can be basically anything,
1135: and should read from its standard input and write to its standard output.
1136: It should eventually connect an
1137: .Xr sshd 8
1138: server running on some machine, or execute
1139: .Ic sshd -i
1140: somewhere.
1141: Host key management will be done using the
1142: HostName of the host being connected (defaulting to the name typed by
1143: the user).
1.7 jmc 1144: Setting the command to
1145: .Dq none
1.6 markus 1146: disables this option entirely.
1.1 stevesk 1147: Note that
1148: .Cm CheckHostIP
1149: is not available for connects with a proxy command.
1.52 djm 1150: .Pp
1151: This directive is useful in conjunction with
1152: .Xr nc 1
1153: and its proxy support.
1.53 jmc 1154: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1155: 192.0.2.0:
1156: .Bd -literal -offset 3n
1157: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1158: .Ed
1.167 djm 1159: .It Cm ProxyUseFdpass
1.168 jmc 1160: Specifies that
1.167 djm 1161: .Cm ProxyCommand
1162: will pass a connected file descriptor back to
1.168 jmc 1163: .Xr ssh 1
1.167 djm 1164: instead of continuing to execute and pass data.
1165: The default is
1166: .Dq no .
1.1 stevesk 1167: .It Cm PubkeyAuthentication
1168: Specifies whether to try public key authentication.
1169: The argument to this keyword must be
1170: .Dq yes
1171: or
1172: .Dq no .
1173: The default is
1174: .Dq yes .
1175: This option applies to protocol version 2 only.
1.75 dtucker 1176: .It Cm RekeyLimit
1177: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1178: session key is renegotiated, optionally followed a maximum amount of
1179: time that may pass before the session key is renegotiated.
1180: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1181: .Sq K ,
1182: .Sq M ,
1.75 dtucker 1183: or
1.76 jmc 1184: .Sq G
1.75 dtucker 1185: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1186: The default is between
1.84 jmc 1187: .Sq 1G
1.75 dtucker 1188: and
1.84 jmc 1189: .Sq 4G ,
1.75 dtucker 1190: depending on the cipher.
1.162 dtucker 1191: The optional second value is specified in seconds and may use any of the
1192: units documented in the
1.166 jmc 1193: TIME FORMATS section of
1.162 dtucker 1194: .Xr sshd_config 5 .
1195: The default value for
1196: .Cm RekeyLimit
1197: is
1198: .Dq default none ,
1199: which means that rekeying is performed after the cipher's default amount
1200: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1201: This option applies to protocol version 2 only.
1.1 stevesk 1202: .It Cm RemoteForward
1.74 jmc 1203: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1204: the secure channel to the specified host and port from the local machine.
1.49 jmc 1205: The first argument must be
1.43 djm 1206: .Sm off
1.49 jmc 1207: .Oo Ar bind_address : Oc Ar port
1.43 djm 1208: .Sm on
1.49 jmc 1209: and the second argument must be
1210: .Ar host : Ns Ar hostport .
1.138 djm 1211: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1212: Multiple forwardings may be specified, and additional
1213: forwardings can be given on the command line.
1.113 stevesk 1214: Privileged ports can be forwarded only when
1215: logging in as root on the remote machine.
1.118 jmc 1216: .Pp
1.117 djm 1217: If the
1218: .Ar port
1219: argument is
1220: .Ql 0 ,
1221: the listen port will be dynamically allocated on the server and reported
1222: to the client at run time.
1.43 djm 1223: .Pp
1224: If the
1225: .Ar bind_address
1226: is not specified, the default is to only bind to loopback addresses.
1227: If the
1228: .Ar bind_address
1229: is
1230: .Ql *
1231: or an empty string, then the forwarding is requested to listen on all
1232: interfaces.
1233: Specifying a remote
1234: .Ar bind_address
1.46 jmc 1235: will only succeed if the server's
1236: .Cm GatewayPorts
1.43 djm 1237: option is enabled (see
1.46 jmc 1238: .Xr sshd_config 5 ) .
1.149 djm 1239: .It Cm RequestTTY
1240: Specifies whether to request a pseudo-tty for the session.
1241: The argument may be one of:
1242: .Dq no
1243: (never request a TTY),
1244: .Dq yes
1245: (always request a TTY when standard input is a TTY),
1246: .Dq force
1247: (always request a TTY) or
1248: .Dq auto
1249: (request a TTY when opening a login session).
1250: This option mirrors the
1251: .Fl t
1252: and
1253: .Fl T
1254: flags for
1255: .Xr ssh 1 .
1.196 ! djm 1256: .It Cm RevokedHostKeys
! 1257: Specifies revoked host public keys.
! 1258: Keys listed in this file will be refused for host authentication.
! 1259: Note that if this file does not exist or is not readable,
! 1260: then host authentication will be refused for all hosts.
! 1261: Keys may be specified as a text file, listing one public key per line, or as
! 1262: an OpenSSH Key Revocation List (KRL) as generated by
! 1263: .Xr ssh-keygen 1 .
! 1264: For more information on KRLs, see the KEY REVOCATION LISTS section in
! 1265: .Xr ssh-keygen 1 .
1.1 stevesk 1266: .It Cm RhostsRSAAuthentication
1267: Specifies whether to try rhosts based authentication with RSA host
1268: authentication.
1269: The argument must be
1270: .Dq yes
1271: or
1272: .Dq no .
1273: The default is
1274: .Dq no .
1275: This option applies to protocol version 1 only and requires
1.84 jmc 1276: .Xr ssh 1
1.1 stevesk 1277: to be setuid root.
1278: .It Cm RSAAuthentication
1279: Specifies whether to try RSA authentication.
1280: The argument to this keyword must be
1281: .Dq yes
1282: or
1283: .Dq no .
1284: RSA authentication will only be
1285: attempted if the identity file exists, or an authentication agent is
1286: running.
1287: The default is
1288: .Dq yes .
1289: Note that this option applies to protocol version 1 only.
1.32 djm 1290: .It Cm SendEnv
1291: Specifies what variables from the local
1292: .Xr environ 7
1293: should be sent to the server.
1.84 jmc 1294: Note that environment passing is only supported for protocol 2.
1295: The server must also support it, and the server must be configured to
1.33 djm 1296: accept these environment variables.
1.32 djm 1297: Refer to
1298: .Cm AcceptEnv
1299: in
1300: .Xr sshd_config 5
1301: for how to configure the server.
1.80 jmc 1302: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1303: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1304: across multiple
1305: .Cm SendEnv
1306: directives.
1307: The default is not to send any environment variables.
1.81 jmc 1308: .Pp
1309: See
1310: .Sx PATTERNS
1311: for more information on patterns.
1.28 markus 1312: .It Cm ServerAliveCountMax
1.73 jmc 1313: Sets the number of server alive messages (see below) which may be
1.28 markus 1314: sent without
1.84 jmc 1315: .Xr ssh 1
1.28 markus 1316: receiving any messages back from the server.
1317: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1318: ssh will disconnect from the server, terminating the session.
1.28 markus 1319: It is important to note that the use of server alive messages is very
1320: different from
1321: .Cm TCPKeepAlive
1322: (below).
1323: The server alive messages are sent through the encrypted channel
1324: and therefore will not be spoofable.
1325: The TCP keepalive option enabled by
1326: .Cm TCPKeepAlive
1327: is spoofable.
1328: The server alive mechanism is valuable when the client or
1329: server depend on knowing when a connection has become inactive.
1330: .Pp
1331: The default value is 3.
1332: If, for example,
1333: .Cm ServerAliveInterval
1.84 jmc 1334: (see below) is set to 15 and
1.28 markus 1335: .Cm ServerAliveCountMax
1.84 jmc 1336: is left at the default, if the server becomes unresponsive,
1337: ssh will disconnect after approximately 45 seconds.
1.89 markus 1338: This option applies to protocol version 2 only.
1.67 jmc 1339: .It Cm ServerAliveInterval
1340: Sets a timeout interval in seconds after which if no data has been received
1341: from the server,
1.84 jmc 1342: .Xr ssh 1
1.67 jmc 1343: will send a message through the encrypted
1344: channel to request a response from the server.
1345: The default
1346: is 0, indicating that these messages will not be sent to the server.
1347: This option applies to protocol version 2 only.
1.191 millert 1348: .It Cm StreamLocalBindMask
1349: Sets the octal file creation mode mask
1350: .Pq umask
1351: used when creating a Unix-domain socket file for local or remote
1352: port forwarding.
1353: This option is only used for port forwarding to a Unix-domain socket file.
1354: .Pp
1355: The default value is 0177, which creates a Unix-domain socket file that is
1356: readable and writable only by the owner.
1357: Note that not all operating systems honor the file mode on Unix-domain
1358: socket files.
1359: .It Cm StreamLocalBindUnlink
1360: Specifies whether to remove an existing Unix-domain socket file for local
1361: or remote port forwarding before creating a new one.
1362: If the socket file already exists and
1363: .Cm StreamLocalBindUnlink
1364: is not enabled,
1365: .Nm ssh
1366: will be unable to forward the port to the Unix-domain socket file.
1367: This option is only used for port forwarding to a Unix-domain socket file.
1368: .Pp
1369: The argument must be
1370: .Dq yes
1371: or
1372: .Dq no .
1373: The default is
1374: .Dq no .
1.1 stevesk 1375: .It Cm StrictHostKeyChecking
1376: If this flag is set to
1377: .Dq yes ,
1.84 jmc 1378: .Xr ssh 1
1.1 stevesk 1379: will never automatically add host keys to the
1.50 djm 1380: .Pa ~/.ssh/known_hosts
1.1 stevesk 1381: file, and refuses to connect to hosts whose host key has changed.
1382: This provides maximum protection against trojan horse attacks,
1.84 jmc 1383: though it can be annoying when the
1.1 stevesk 1384: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1385: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1386: frequently made.
1387: This option forces the user to manually
1388: add all new hosts.
1389: If this flag is set to
1390: .Dq no ,
1.84 jmc 1391: ssh will automatically add new host keys to the
1.1 stevesk 1392: user known hosts files.
1393: If this flag is set to
1394: .Dq ask ,
1395: new host keys
1396: will be added to the user known host files only after the user
1397: has confirmed that is what they really want to do, and
1.84 jmc 1398: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1399: The host keys of
1400: known hosts will be verified automatically in all cases.
1401: The argument must be
1402: .Dq yes ,
1.84 jmc 1403: .Dq no ,
1.1 stevesk 1404: or
1405: .Dq ask .
1406: The default is
1407: .Dq ask .
1.26 markus 1408: .It Cm TCPKeepAlive
1409: Specifies whether the system should send TCP keepalive messages to the
1410: other side.
1411: If they are sent, death of the connection or crash of one
1412: of the machines will be properly noticed.
1413: However, this means that
1414: connections will die if the route is down temporarily, and some people
1415: find it annoying.
1416: .Pp
1417: The default is
1418: .Dq yes
1419: (to send TCP keepalive messages), and the client will notice
1420: if the network goes down or the remote host dies.
1421: This is important in scripts, and many users want it too.
1422: .Pp
1423: To disable TCP keepalive messages, the value should be set to
1424: .Dq no .
1.65 reyk 1425: .It Cm Tunnel
1.95 stevesk 1426: Request
1.65 reyk 1427: .Xr tun 4
1.69 jmc 1428: device forwarding between the client and the server.
1.65 reyk 1429: The argument must be
1.68 reyk 1430: .Dq yes ,
1.95 stevesk 1431: .Dq point-to-point
1432: (layer 3),
1433: .Dq ethernet
1434: (layer 2),
1.65 reyk 1435: or
1436: .Dq no .
1.95 stevesk 1437: Specifying
1438: .Dq yes
1439: requests the default tunnel mode, which is
1440: .Dq point-to-point .
1.65 reyk 1441: The default is
1442: .Dq no .
1443: .It Cm TunnelDevice
1.95 stevesk 1444: Specifies the
1.65 reyk 1445: .Xr tun 4
1.95 stevesk 1446: devices to open on the client
1447: .Pq Ar local_tun
1448: and the server
1449: .Pq Ar remote_tun .
1450: .Pp
1451: The argument must be
1452: .Sm off
1453: .Ar local_tun Op : Ar remote_tun .
1454: .Sm on
1455: The devices may be specified by numerical ID or the keyword
1456: .Dq any ,
1457: which uses the next available tunnel device.
1458: If
1459: .Ar remote_tun
1460: is not specified, it defaults to
1461: .Dq any .
1462: The default is
1463: .Dq any:any .
1.72 jmc 1464: .It Cm UsePrivilegedPort
1465: Specifies whether to use a privileged port for outgoing connections.
1466: The argument must be
1467: .Dq yes
1468: or
1469: .Dq no .
1470: The default is
1471: .Dq no .
1472: If set to
1.84 jmc 1473: .Dq yes ,
1474: .Xr ssh 1
1.72 jmc 1475: must be setuid root.
1476: Note that this option must be set to
1477: .Dq yes
1478: for
1479: .Cm RhostsRSAAuthentication
1480: with older servers.
1.1 stevesk 1481: .It Cm User
1482: Specifies the user to log in as.
1483: This can be useful when a different user name is used on different machines.
1484: This saves the trouble of
1485: having to remember to give the user name on the command line.
1486: .It Cm UserKnownHostsFile
1.151 djm 1487: Specifies one or more files to use for the user
1488: host key database, separated by whitespace.
1489: The default is
1490: .Pa ~/.ssh/known_hosts ,
1491: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1492: .It Cm VerifyHostKeyDNS
1493: Specifies whether to verify the remote key using DNS and SSHFP resource
1494: records.
1.24 jakob 1495: If this option is set to
1496: .Dq yes ,
1.25 jmc 1497: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1498: from DNS.
1499: Insecure fingerprints will be handled as if this option was set to
1500: .Dq ask .
1501: If this option is set to
1502: .Dq ask ,
1503: information on fingerprint match will be displayed, but the user will still
1504: need to confirm new host keys according to the
1505: .Cm StrictHostKeyChecking
1506: option.
1507: The argument must be
1508: .Dq yes ,
1.84 jmc 1509: .Dq no ,
1.25 jmc 1510: or
1511: .Dq ask .
1.8 jakob 1512: The default is
1513: .Dq no .
1.12 jakob 1514: Note that this option applies to protocol version 2 only.
1.84 jmc 1515: .Pp
1.166 jmc 1516: See also VERIFYING HOST KEYS in
1.84 jmc 1517: .Xr ssh 1 .
1.111 grunk 1518: .It Cm VisualHostKey
1519: If this flag is set to
1520: .Dq yes ,
1521: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1522: printed in addition to the hex fingerprint string at login and
1523: for unknown host keys.
1.111 grunk 1524: If this flag is set to
1525: .Dq no ,
1.114 stevesk 1526: no fingerprint strings are printed at login and
1527: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1528: The default is
1529: .Dq no .
1.1 stevesk 1530: .It Cm XAuthLocation
1.5 stevesk 1531: Specifies the full pathname of the
1.1 stevesk 1532: .Xr xauth 1
1533: program.
1534: The default is
1535: .Pa /usr/X11R6/bin/xauth .
1536: .El
1.86 jmc 1537: .Sh PATTERNS
1538: A
1539: .Em pattern
1540: consists of zero or more non-whitespace characters,
1541: .Sq *
1542: (a wildcard that matches zero or more characters),
1543: or
1544: .Sq ?\&
1545: (a wildcard that matches exactly one character).
1546: For example, to specify a set of declarations for any host in the
1547: .Dq .co.uk
1548: set of domains,
1549: the following pattern could be used:
1550: .Pp
1551: .Dl Host *.co.uk
1552: .Pp
1553: The following pattern
1554: would match any host in the 192.168.0.[0-9] network range:
1555: .Pp
1556: .Dl Host 192.168.0.?
1557: .Pp
1558: A
1559: .Em pattern-list
1560: is a comma-separated list of patterns.
1561: Patterns within pattern-lists may be negated
1562: by preceding them with an exclamation mark
1563: .Pq Sq !\& .
1564: For example,
1.174 djm 1565: to allow a key to be used from anywhere within an organization
1.86 jmc 1566: except from the
1567: .Dq dialup
1568: pool,
1569: the following entry (in authorized_keys) could be used:
1570: .Pp
1571: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1572: .Sh FILES
1573: .Bl -tag -width Ds
1.50 djm 1574: .It Pa ~/.ssh/config
1.1 stevesk 1575: This is the per-user configuration file.
1576: The format of this file is described above.
1.84 jmc 1577: This file is used by the SSH client.
1.30 djm 1578: Because of the potential for abuse, this file must have strict permissions:
1579: read/write for the user, and not accessible by others.
1.1 stevesk 1580: .It Pa /etc/ssh/ssh_config
1581: Systemwide configuration file.
1582: This file provides defaults for those
1583: values that are not specified in the user's configuration file, and
1584: for those users who do not have a configuration file.
1585: This file must be world-readable.
1586: .El
1.13 jmc 1587: .Sh SEE ALSO
1588: .Xr ssh 1
1.1 stevesk 1589: .Sh AUTHORS
1590: OpenSSH is a derivative of the original and free
1591: ssh 1.2.12 release by Tatu Ylonen.
1592: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1593: Theo de Raadt and Dug Song
1594: removed many bugs, re-added newer features and
1595: created OpenSSH.
1596: Markus Friedl contributed the support for SSH
1597: protocol versions 1.5 and 2.0.
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