Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.193
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,
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33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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1.193 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.192 2014/08/30 15:33:50 sobrado Exp $
! 37: .Dd $Mdocdate: August 30 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
! 157: .Cm canonical.
! 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
1.188 djm 540: at least %h, %p, and %r (or alternatively %C).
1.56 djm 541: This ensures that shared connections are uniquely identified.
1.137 djm 542: .It Cm ControlPersist
543: When used in conjunction with
544: .Cm ControlMaster ,
545: specifies that the master connection should remain open
546: in the background (waiting for future client connections)
547: after the initial client connection has been closed.
548: If set to
549: .Dq no ,
550: then the master connection will not be placed into the background,
551: and will close as soon as the initial client connection is closed.
552: If set to
553: .Dq yes ,
554: then the master connection will remain in the background indefinitely
555: (until killed or closed via a mechanism such as the
556: .Xr ssh 1
557: .Dq Fl O No exit
558: option).
559: If set to a time in seconds, or a time in any of the formats documented in
560: .Xr sshd_config 5 ,
561: then the backgrounded master connection will automatically terminate
562: after it has remained idle (with no client connections) for the
563: specified time.
1.38 jmc 564: .It Cm DynamicForward
1.74 jmc 565: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 566: over the secure channel, and the application
567: protocol is then used to determine where to connect to from the
568: remote machine.
1.62 djm 569: .Pp
570: The argument must be
571: .Sm off
572: .Oo Ar bind_address : Oc Ar port .
573: .Sm on
1.138 djm 574: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 575: By default, the local port is bound in accordance with the
576: .Cm GatewayPorts
577: setting.
578: However, an explicit
579: .Ar bind_address
580: may be used to bind the connection to a specific address.
581: The
582: .Ar bind_address
583: of
584: .Dq localhost
585: indicates that the listening port be bound for local use only, while an
586: empty address or
587: .Sq *
588: indicates that the port should be available from all interfaces.
589: .Pp
1.38 jmc 590: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 591: .Xr ssh 1
1.38 jmc 592: will act as a SOCKS server.
593: Multiple forwardings may be specified, and
594: additional forwardings can be given on the command line.
595: Only the superuser can forward privileged ports.
1.14 markus 596: .It Cm EnableSSHKeysign
597: Setting this option to
598: .Dq yes
599: in the global client configuration file
600: .Pa /etc/ssh/ssh_config
601: enables the use of the helper program
602: .Xr ssh-keysign 8
603: during
604: .Cm HostbasedAuthentication .
605: The argument must be
606: .Dq yes
607: or
608: .Dq no .
609: The default is
610: .Dq no .
1.23 jmc 611: This option should be placed in the non-hostspecific section.
1.14 markus 612: See
613: .Xr ssh-keysign 8
614: for more information.
1.1 stevesk 615: .It Cm EscapeChar
616: Sets the escape character (default:
617: .Ql ~ ) .
618: The escape character can also
619: be set on the command line.
620: The argument should be a single character,
621: .Ql ^
622: followed by a letter, or
623: .Dq none
624: to disable the escape
625: character entirely (making the connection transparent for binary
626: data).
1.96 markus 627: .It Cm ExitOnForwardFailure
628: Specifies whether
629: .Xr ssh 1
630: should terminate the connection if it cannot set up all requested
1.102 stevesk 631: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 632: The argument must be
633: .Dq yes
634: or
635: .Dq no .
636: The default is
637: .Dq no .
1.1 stevesk 638: .It Cm ForwardAgent
639: Specifies whether the connection to the authentication agent (if any)
640: will be forwarded to the remote machine.
641: The argument must be
642: .Dq yes
643: or
644: .Dq no .
645: The default is
646: .Dq no .
1.3 stevesk 647: .Pp
1.7 jmc 648: Agent forwarding should be enabled with caution.
649: Users with the ability to bypass file permissions on the remote host
650: (for the agent's Unix-domain socket)
651: can access the local agent through the forwarded connection.
652: An attacker cannot obtain key material from the agent,
1.3 stevesk 653: however they can perform operations on the keys that enable them to
654: authenticate using the identities loaded into the agent.
1.1 stevesk 655: .It Cm ForwardX11
656: Specifies whether X11 connections will be automatically redirected
657: over the secure channel and
658: .Ev DISPLAY
659: set.
660: The argument must be
661: .Dq yes
662: or
663: .Dq no .
664: The default is
665: .Dq no .
1.3 stevesk 666: .Pp
1.7 jmc 667: X11 forwarding should be enabled with caution.
668: Users with the ability to bypass file permissions on the remote host
1.22 markus 669: (for the user's X11 authorization database)
1.7 jmc 670: can access the local X11 display through the forwarded connection.
1.22 markus 671: An attacker may then be able to perform activities such as keystroke monitoring
672: if the
673: .Cm ForwardX11Trusted
674: option is also enabled.
1.134 djm 675: .It Cm ForwardX11Timeout
1.135 jmc 676: Specify a timeout for untrusted X11 forwarding
677: using the format described in the
1.166 jmc 678: TIME FORMATS section of
1.134 djm 679: .Xr sshd_config 5 .
680: X11 connections received by
681: .Xr ssh 1
682: after this time will be refused.
683: The default is to disable untrusted X11 forwarding after twenty minutes has
684: elapsed.
1.22 markus 685: .It Cm ForwardX11Trusted
1.34 jmc 686: If this option is set to
1.84 jmc 687: .Dq yes ,
688: remote X11 clients will have full access to the original X11 display.
1.42 djm 689: .Pp
1.22 markus 690: If this option is set to
1.84 jmc 691: .Dq no ,
692: remote X11 clients will be considered untrusted and prevented
1.22 markus 693: from stealing or tampering with data belonging to trusted X11
694: clients.
1.42 djm 695: Furthermore, the
696: .Xr xauth 1
697: token used for the session will be set to expire after 20 minutes.
698: Remote clients will be refused access after this time.
1.22 markus 699: .Pp
700: The default is
701: .Dq no .
702: .Pp
703: See the X11 SECURITY extension specification for full details on
704: the restrictions imposed on untrusted clients.
1.1 stevesk 705: .It Cm GatewayPorts
706: Specifies whether remote hosts are allowed to connect to local
707: forwarded ports.
708: By default,
1.84 jmc 709: .Xr ssh 1
1.7 jmc 710: binds local port forwardings to the loopback address.
711: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 712: .Cm GatewayPorts
1.84 jmc 713: can be used to specify that ssh
1.1 stevesk 714: should bind local port forwardings to the wildcard address,
715: thus allowing remote hosts to connect to forwarded ports.
716: The argument must be
717: .Dq yes
718: or
719: .Dq no .
720: The default is
721: .Dq no .
722: .It Cm GlobalKnownHostsFile
1.151 djm 723: Specifies one or more files to use for the global
724: host key database, separated by whitespace.
725: The default is
726: .Pa /etc/ssh/ssh_known_hosts ,
727: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 728: .It Cm GSSAPIAuthentication
1.27 markus 729: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 730: The default is
1.21 markus 731: .Dq no .
1.18 markus 732: Note that this option applies to protocol version 2 only.
733: .It Cm GSSAPIDelegateCredentials
734: Forward (delegate) credentials to the server.
735: The default is
736: .Dq no .
737: Note that this option applies to protocol version 2 only.
1.44 djm 738: .It Cm HashKnownHosts
739: Indicates that
1.84 jmc 740: .Xr ssh 1
1.44 djm 741: should hash host names and addresses when they are added to
1.50 djm 742: .Pa ~/.ssh/known_hosts .
1.44 djm 743: These hashed names may be used normally by
1.84 jmc 744: .Xr ssh 1
1.44 djm 745: and
1.84 jmc 746: .Xr sshd 8 ,
1.44 djm 747: but they do not reveal identifying information should the file's contents
748: be disclosed.
749: The default is
750: .Dq no .
1.97 jmc 751: Note that existing names and addresses in known hosts files
752: will not be converted automatically,
753: but may be manually hashed using
1.45 djm 754: .Xr ssh-keygen 1 .
1.1 stevesk 755: .It Cm HostbasedAuthentication
756: Specifies whether to try rhosts based authentication with public key
757: authentication.
758: The argument must be
759: .Dq yes
760: or
761: .Dq no .
762: The default is
763: .Dq no .
764: This option applies to protocol version 2 only and
765: is similar to
766: .Cm RhostsRSAAuthentication .
767: .It Cm HostKeyAlgorithms
768: Specifies the protocol version 2 host key algorithms
769: that the client wants to use in order of preference.
770: The default for this option is:
1.139 djm 771: .Bd -literal -offset 3n
772: ecdsa-sha2-nistp256-cert-v01@openssh.com,
773: ecdsa-sha2-nistp384-cert-v01@openssh.com,
774: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 775: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 776: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
777: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
778: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.182 markus 779: ssh-ed25519,ssh-rsa,ssh-dss
1.139 djm 780: .Ed
1.145 djm 781: .Pp
782: If hostkeys are known for the destination host then this default is modified
783: to prefer their algorithms.
1.1 stevesk 784: .It Cm HostKeyAlias
785: Specifies an alias that should be used instead of the
786: real host name when looking up or saving the host key
787: in the host key database files.
1.84 jmc 788: This option is useful for tunneling SSH connections
1.1 stevesk 789: or for multiple servers running on a single host.
790: .It Cm HostName
791: Specifies the real host name to log into.
792: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 793: If the hostname contains the character sequence
794: .Ql %h ,
1.150 jmc 795: then this will be replaced with the host name specified on the command line
1.136 djm 796: (this is useful for manipulating unqualified names).
1.187 djm 797: The character sequence
798: .Ql %%
799: will be replaced by a single
800: .Ql %
801: character, which may be used when specifying IPv6 link-local addresses.
802: .Pp
1.84 jmc 803: The default is the name given on the command line.
1.1 stevesk 804: Numeric IP addresses are also permitted (both on the command line and in
805: .Cm HostName
806: specifications).
1.29 markus 807: .It Cm IdentitiesOnly
808: Specifies that
1.84 jmc 809: .Xr ssh 1
1.29 markus 810: should only use the authentication identity files configured in the
1.31 jmc 811: .Nm
1.29 markus 812: files,
1.84 jmc 813: even if
814: .Xr ssh-agent 1
1.159 djm 815: or a
816: .Cm PKCS11Provider
1.29 markus 817: offers more identities.
818: The argument to this keyword must be
819: .Dq yes
820: or
821: .Dq no .
1.84 jmc 822: This option is intended for situations where ssh-agent
1.29 markus 823: offers many different identities.
824: The default is
825: .Dq no .
1.67 jmc 826: .It Cm IdentityFile
1.192 sobrado 827: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 828: identity is read.
1.67 jmc 829: The default is
830: .Pa ~/.ssh/identity
831: for protocol version 1, and
1.139 djm 832: .Pa ~/.ssh/id_dsa ,
1.183 naddy 833: .Pa ~/.ssh/id_ecdsa ,
834: .Pa ~/.ssh/id_ed25519
1.139 djm 835: and
1.67 jmc 836: .Pa ~/.ssh/id_rsa
837: for protocol version 2.
838: Additionally, any identities represented by the authentication agent
1.165 djm 839: will be used for authentication unless
840: .Cm IdentitiesOnly
841: is set.
1.129 djm 842: .Xr ssh 1
843: will try to load certificate information from the filename obtained by
844: appending
845: .Pa -cert.pub
846: to the path of a specified
847: .Cm IdentityFile .
1.90 djm 848: .Pp
1.67 jmc 849: The file name may use the tilde
1.91 jmc 850: syntax to refer to a user's home directory or one of the following
1.90 djm 851: escape characters:
852: .Ql %d
853: (local user's home directory),
854: .Ql %u
855: (local user name),
856: .Ql %l
857: (local host name),
858: .Ql %h
859: (remote host name) or
1.92 djm 860: .Ql %r
1.90 djm 861: (remote user name).
862: .Pp
1.67 jmc 863: It is possible to have
864: multiple identity files specified in configuration files; all these
865: identities will be tried in sequence.
1.152 djm 866: Multiple
867: .Cm IdentityFile
868: directives will add to the list of identities tried (this behaviour
869: differs from that of other configuration directives).
1.165 djm 870: .Pp
871: .Cm IdentityFile
872: may be used in conjunction with
873: .Cm IdentitiesOnly
874: to select which identities in an agent are offered during authentication.
1.164 jmc 875: .It Cm IgnoreUnknown
876: Specifies a pattern-list of unknown options to be ignored if they are
877: encountered in configuration parsing.
878: This may be used to suppress errors if
879: .Nm
880: contains options that are unrecognised by
881: .Xr ssh 1 .
882: It is recommended that
883: .Cm IgnoreUnknown
884: be listed early in the configuration file as it will not be applied
885: to unknown options that appear before it.
1.143 djm 886: .It Cm IPQoS
887: Specifies the IPv4 type-of-service or DSCP class for connections.
888: Accepted values are
889: .Dq af11 ,
890: .Dq af12 ,
891: .Dq af13 ,
1.154 djm 892: .Dq af21 ,
1.143 djm 893: .Dq af22 ,
894: .Dq af23 ,
895: .Dq af31 ,
896: .Dq af32 ,
897: .Dq af33 ,
898: .Dq af41 ,
899: .Dq af42 ,
900: .Dq af43 ,
901: .Dq cs0 ,
902: .Dq cs1 ,
903: .Dq cs2 ,
904: .Dq cs3 ,
905: .Dq cs4 ,
906: .Dq cs5 ,
907: .Dq cs6 ,
908: .Dq cs7 ,
909: .Dq ef ,
910: .Dq lowdelay ,
911: .Dq throughput ,
912: .Dq reliability ,
913: or a numeric value.
1.146 djm 914: This option may take one or two arguments, separated by whitespace.
1.143 djm 915: If one argument is specified, it is used as the packet class unconditionally.
916: If two values are specified, the first is automatically selected for
917: interactive sessions and the second for non-interactive sessions.
918: The default is
919: .Dq lowdelay
920: for interactive sessions and
921: .Dq throughput
922: for non-interactive sessions.
1.103 djm 923: .It Cm KbdInteractiveAuthentication
924: Specifies whether to use keyboard-interactive authentication.
925: The argument to this keyword must be
926: .Dq yes
927: or
928: .Dq no .
929: The default is
930: .Dq yes .
1.39 djm 931: .It Cm KbdInteractiveDevices
932: Specifies the list of methods to use in keyboard-interactive authentication.
933: Multiple method names must be comma-separated.
934: The default is to use the server specified list.
1.85 jmc 935: The methods available vary depending on what the server supports.
936: For an OpenSSH server,
937: it may be zero or more of:
938: .Dq bsdauth ,
939: .Dq pam ,
940: and
941: .Dq skey .
1.140 djm 942: .It Cm KexAlgorithms
943: Specifies the available KEX (Key Exchange) algorithms.
944: Multiple algorithms must be comma-separated.
1.141 jmc 945: The default is:
946: .Bd -literal -offset indent
1.179 markus 947: curve25519-sha256@libssh.org,
1.141 jmc 948: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
949: diffie-hellman-group-exchange-sha256,
1.186 naddy 950: diffie-hellman-group14-sha1,
1.141 jmc 951: diffie-hellman-group-exchange-sha1,
952: diffie-hellman-group1-sha1
953: .Ed
1.65 reyk 954: .It Cm LocalCommand
955: Specifies a command to execute on the local machine after successfully
956: connecting to the server.
957: The command string extends to the end of the line, and is executed with
1.105 jmc 958: the user's shell.
1.109 dtucker 959: The following escape character substitutions will be performed:
960: .Ql %d
961: (local user's home directory),
962: .Ql %h
963: (remote host name),
964: .Ql %l
965: (local host name),
966: .Ql %n
967: (host name as provided on the command line),
968: .Ql %p
969: (remote port),
970: .Ql %r
971: (remote user name) or
972: .Ql %u
1.188 djm 973: (local user name) or
1.189 jmc 974: .Ql \&%C
1.188 djm 975: by a hash of the concatenation: %l%h%p%r.
1.123 djm 976: .Pp
977: The command is run synchronously and does not have access to the
978: session of the
979: .Xr ssh 1
980: that spawned it.
981: It should not be used for interactive commands.
982: .Pp
1.65 reyk 983: This directive is ignored unless
984: .Cm PermitLocalCommand
985: has been enabled.
1.1 stevesk 986: .It Cm LocalForward
1.74 jmc 987: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 988: the secure channel to the specified host and port from the remote machine.
1.49 jmc 989: The first argument must be
1.43 djm 990: .Sm off
1.49 jmc 991: .Oo Ar bind_address : Oc Ar port
1.43 djm 992: .Sm on
1.49 jmc 993: and the second argument must be
994: .Ar host : Ns Ar hostport .
1.138 djm 995: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 996: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 997: given on the command line.
1.1 stevesk 998: Only the superuser can forward privileged ports.
1.43 djm 999: By default, the local port is bound in accordance with the
1000: .Cm GatewayPorts
1001: setting.
1002: However, an explicit
1003: .Ar bind_address
1004: may be used to bind the connection to a specific address.
1005: The
1006: .Ar bind_address
1007: of
1008: .Dq localhost
1.46 jmc 1009: indicates that the listening port be bound for local use only, while an
1010: empty address or
1011: .Sq *
1.43 djm 1012: indicates that the port should be available from all interfaces.
1.1 stevesk 1013: .It Cm LogLevel
1014: Gives the verbosity level that is used when logging messages from
1.84 jmc 1015: .Xr ssh 1 .
1.1 stevesk 1016: The possible values are:
1.84 jmc 1017: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1018: The default is INFO.
1019: DEBUG and DEBUG1 are equivalent.
1020: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1021: .It Cm MACs
1022: Specifies the MAC (message authentication code) algorithms
1023: in order of preference.
1024: The MAC algorithm is used in protocol version 2
1025: for data integrity protection.
1026: Multiple algorithms must be comma-separated.
1.160 markus 1027: The algorithms that contain
1028: .Dq -etm
1029: calculate the MAC after encryption (encrypt-then-mac).
1030: These are considered safer and their use recommended.
1.84 jmc 1031: The default is:
1.101 jmc 1032: .Bd -literal -offset indent
1.160 markus 1033: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1034: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1035: umac-64@openssh.com,umac-128@openssh.com,
1036: hmac-sha2-256,hmac-sha2-512,
1037: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1038: hmac-ripemd160-etm@openssh.com,
1039: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1040: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1041: hmac-sha1-96,hmac-md5-96
1.101 jmc 1042: .Ed
1.1 stevesk 1043: .It Cm NoHostAuthenticationForLocalhost
1044: This option can be used if the home directory is shared across machines.
1045: In this case localhost will refer to a different machine on each of
1046: the machines and the user will get many warnings about changed host keys.
1047: However, this option disables host authentication for localhost.
1048: The argument to this keyword must be
1049: .Dq yes
1050: or
1051: .Dq no .
1052: The default is to check the host key for localhost.
1053: .It Cm NumberOfPasswordPrompts
1054: Specifies the number of password prompts before giving up.
1055: The argument to this keyword must be an integer.
1.84 jmc 1056: The default is 3.
1.1 stevesk 1057: .It Cm PasswordAuthentication
1058: Specifies whether to use password authentication.
1059: The argument to this keyword must be
1060: .Dq yes
1061: or
1062: .Dq no .
1063: The default is
1064: .Dq yes .
1.65 reyk 1065: .It Cm PermitLocalCommand
1066: Allow local command execution via the
1067: .Ic LocalCommand
1068: option or using the
1.66 jmc 1069: .Ic !\& Ns Ar command
1.65 reyk 1070: escape sequence in
1071: .Xr ssh 1 .
1072: The argument must be
1073: .Dq yes
1074: or
1075: .Dq no .
1076: The default is
1077: .Dq no .
1.127 markus 1078: .It Cm PKCS11Provider
1079: Specifies which PKCS#11 provider to use.
1.144 jmc 1080: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1081: .Xr ssh 1
1.128 markus 1082: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1083: private RSA key.
1.67 jmc 1084: .It Cm Port
1085: Specifies the port number to connect on the remote host.
1.84 jmc 1086: The default is 22.
1.1 stevesk 1087: .It Cm PreferredAuthentications
1088: Specifies the order in which the client should try protocol 2
1.11 jmc 1089: authentication methods.
1.48 jmc 1090: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1091: .Cm keyboard-interactive )
1.48 jmc 1092: over another method (e.g.\&
1.131 jmc 1093: .Cm password ) .
1094: The default is:
1095: .Bd -literal -offset indent
1096: gssapi-with-mic,hostbased,publickey,
1097: keyboard-interactive,password
1098: .Ed
1.1 stevesk 1099: .It Cm Protocol
1100: Specifies the protocol versions
1.84 jmc 1101: .Xr ssh 1
1.1 stevesk 1102: should support in order of preference.
1103: The possible values are
1.84 jmc 1104: .Sq 1
1.1 stevesk 1105: and
1.84 jmc 1106: .Sq 2 .
1.1 stevesk 1107: Multiple versions must be comma-separated.
1.120 markus 1108: When this option is set to
1.121 jmc 1109: .Dq 2,1
1.120 markus 1110: .Nm ssh
1111: will try version 2 and fall back to version 1
1112: if version 2 is not available.
1.1 stevesk 1113: The default is
1.121 jmc 1114: .Sq 2 .
1.1 stevesk 1115: .It Cm ProxyCommand
1116: Specifies the command to use to connect to the server.
1117: The command
1.190 djm 1118: string extends to the end of the line, and is executed
1119: using the user's shell
1120: .Ql exec
1121: directive to avoid a lingering shell process.
1122: .Pp
1.133 jmc 1123: In the command string, any occurrence of
1.1 stevesk 1124: .Ql %h
1125: will be substituted by the host name to
1.132 djm 1126: connect,
1.1 stevesk 1127: .Ql %p
1.133 jmc 1128: by the port, and
1129: .Ql %r
1.132 djm 1130: by the remote user name.
1.1 stevesk 1131: The command can be basically anything,
1132: and should read from its standard input and write to its standard output.
1133: It should eventually connect an
1134: .Xr sshd 8
1135: server running on some machine, or execute
1136: .Ic sshd -i
1137: somewhere.
1138: Host key management will be done using the
1139: HostName of the host being connected (defaulting to the name typed by
1140: the user).
1.7 jmc 1141: Setting the command to
1142: .Dq none
1.6 markus 1143: disables this option entirely.
1.1 stevesk 1144: Note that
1145: .Cm CheckHostIP
1146: is not available for connects with a proxy command.
1.52 djm 1147: .Pp
1148: This directive is useful in conjunction with
1149: .Xr nc 1
1150: and its proxy support.
1.53 jmc 1151: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1152: 192.0.2.0:
1153: .Bd -literal -offset 3n
1154: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1155: .Ed
1.167 djm 1156: .It Cm ProxyUseFdpass
1.168 jmc 1157: Specifies that
1.167 djm 1158: .Cm ProxyCommand
1159: will pass a connected file descriptor back to
1.168 jmc 1160: .Xr ssh 1
1.167 djm 1161: instead of continuing to execute and pass data.
1162: The default is
1163: .Dq no .
1.1 stevesk 1164: .It Cm PubkeyAuthentication
1165: Specifies whether to try public key authentication.
1166: The argument to this keyword must be
1167: .Dq yes
1168: or
1169: .Dq no .
1170: The default is
1171: .Dq yes .
1172: This option applies to protocol version 2 only.
1.75 dtucker 1173: .It Cm RekeyLimit
1174: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1175: session key is renegotiated, optionally followed a maximum amount of
1176: time that may pass before the session key is renegotiated.
1177: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1178: .Sq K ,
1179: .Sq M ,
1.75 dtucker 1180: or
1.76 jmc 1181: .Sq G
1.75 dtucker 1182: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1183: The default is between
1.84 jmc 1184: .Sq 1G
1.75 dtucker 1185: and
1.84 jmc 1186: .Sq 4G ,
1.75 dtucker 1187: depending on the cipher.
1.162 dtucker 1188: The optional second value is specified in seconds and may use any of the
1189: units documented in the
1.166 jmc 1190: TIME FORMATS section of
1.162 dtucker 1191: .Xr sshd_config 5 .
1192: The default value for
1193: .Cm RekeyLimit
1194: is
1195: .Dq default none ,
1196: which means that rekeying is performed after the cipher's default amount
1197: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1198: This option applies to protocol version 2 only.
1.1 stevesk 1199: .It Cm RemoteForward
1.74 jmc 1200: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1201: the secure channel to the specified host and port from the local machine.
1.49 jmc 1202: The first argument must be
1.43 djm 1203: .Sm off
1.49 jmc 1204: .Oo Ar bind_address : Oc Ar port
1.43 djm 1205: .Sm on
1.49 jmc 1206: and the second argument must be
1207: .Ar host : Ns Ar hostport .
1.138 djm 1208: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1209: Multiple forwardings may be specified, and additional
1210: forwardings can be given on the command line.
1.113 stevesk 1211: Privileged ports can be forwarded only when
1212: logging in as root on the remote machine.
1.118 jmc 1213: .Pp
1.117 djm 1214: If the
1215: .Ar port
1216: argument is
1217: .Ql 0 ,
1218: the listen port will be dynamically allocated on the server and reported
1219: to the client at run time.
1.43 djm 1220: .Pp
1221: If the
1222: .Ar bind_address
1223: is not specified, the default is to only bind to loopback addresses.
1224: If the
1225: .Ar bind_address
1226: is
1227: .Ql *
1228: or an empty string, then the forwarding is requested to listen on all
1229: interfaces.
1230: Specifying a remote
1231: .Ar bind_address
1.46 jmc 1232: will only succeed if the server's
1233: .Cm GatewayPorts
1.43 djm 1234: option is enabled (see
1.46 jmc 1235: .Xr sshd_config 5 ) .
1.149 djm 1236: .It Cm RequestTTY
1237: Specifies whether to request a pseudo-tty for the session.
1238: The argument may be one of:
1239: .Dq no
1240: (never request a TTY),
1241: .Dq yes
1242: (always request a TTY when standard input is a TTY),
1243: .Dq force
1244: (always request a TTY) or
1245: .Dq auto
1246: (request a TTY when opening a login session).
1247: This option mirrors the
1248: .Fl t
1249: and
1250: .Fl T
1251: flags for
1252: .Xr ssh 1 .
1.1 stevesk 1253: .It Cm RhostsRSAAuthentication
1254: Specifies whether to try rhosts based authentication with RSA host
1255: authentication.
1256: The argument must be
1257: .Dq yes
1258: or
1259: .Dq no .
1260: The default is
1261: .Dq no .
1262: This option applies to protocol version 1 only and requires
1.84 jmc 1263: .Xr ssh 1
1.1 stevesk 1264: to be setuid root.
1265: .It Cm RSAAuthentication
1266: Specifies whether to try RSA authentication.
1267: The argument to this keyword must be
1268: .Dq yes
1269: or
1270: .Dq no .
1271: RSA authentication will only be
1272: attempted if the identity file exists, or an authentication agent is
1273: running.
1274: The default is
1275: .Dq yes .
1276: Note that this option applies to protocol version 1 only.
1.32 djm 1277: .It Cm SendEnv
1278: Specifies what variables from the local
1279: .Xr environ 7
1280: should be sent to the server.
1.84 jmc 1281: Note that environment passing is only supported for protocol 2.
1282: The server must also support it, and the server must be configured to
1.33 djm 1283: accept these environment variables.
1.32 djm 1284: Refer to
1285: .Cm AcceptEnv
1286: in
1287: .Xr sshd_config 5
1288: for how to configure the server.
1.80 jmc 1289: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1290: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1291: across multiple
1292: .Cm SendEnv
1293: directives.
1294: The default is not to send any environment variables.
1.81 jmc 1295: .Pp
1296: See
1297: .Sx PATTERNS
1298: for more information on patterns.
1.28 markus 1299: .It Cm ServerAliveCountMax
1.73 jmc 1300: Sets the number of server alive messages (see below) which may be
1.28 markus 1301: sent without
1.84 jmc 1302: .Xr ssh 1
1.28 markus 1303: receiving any messages back from the server.
1304: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1305: ssh will disconnect from the server, terminating the session.
1.28 markus 1306: It is important to note that the use of server alive messages is very
1307: different from
1308: .Cm TCPKeepAlive
1309: (below).
1310: The server alive messages are sent through the encrypted channel
1311: and therefore will not be spoofable.
1312: The TCP keepalive option enabled by
1313: .Cm TCPKeepAlive
1314: is spoofable.
1315: The server alive mechanism is valuable when the client or
1316: server depend on knowing when a connection has become inactive.
1317: .Pp
1318: The default value is 3.
1319: If, for example,
1320: .Cm ServerAliveInterval
1.84 jmc 1321: (see below) is set to 15 and
1.28 markus 1322: .Cm ServerAliveCountMax
1.84 jmc 1323: is left at the default, if the server becomes unresponsive,
1324: ssh will disconnect after approximately 45 seconds.
1.89 markus 1325: This option applies to protocol version 2 only.
1.67 jmc 1326: .It Cm ServerAliveInterval
1327: Sets a timeout interval in seconds after which if no data has been received
1328: from the server,
1.84 jmc 1329: .Xr ssh 1
1.67 jmc 1330: will send a message through the encrypted
1331: channel to request a response from the server.
1332: The default
1333: is 0, indicating that these messages will not be sent to the server.
1334: This option applies to protocol version 2 only.
1.191 millert 1335: .It Cm StreamLocalBindMask
1336: Sets the octal file creation mode mask
1337: .Pq umask
1338: used when creating a Unix-domain socket file for local or remote
1339: port forwarding.
1340: This option is only used for port forwarding to a Unix-domain socket file.
1341: .Pp
1342: The default value is 0177, which creates a Unix-domain socket file that is
1343: readable and writable only by the owner.
1344: Note that not all operating systems honor the file mode on Unix-domain
1345: socket files.
1346: .It Cm StreamLocalBindUnlink
1347: Specifies whether to remove an existing Unix-domain socket file for local
1348: or remote port forwarding before creating a new one.
1349: If the socket file already exists and
1350: .Cm StreamLocalBindUnlink
1351: is not enabled,
1352: .Nm ssh
1353: will be unable to forward the port to the Unix-domain socket file.
1354: This option is only used for port forwarding to a Unix-domain socket file.
1355: .Pp
1356: The argument must be
1357: .Dq yes
1358: or
1359: .Dq no .
1360: The default is
1361: .Dq no .
1.1 stevesk 1362: .It Cm StrictHostKeyChecking
1363: If this flag is set to
1364: .Dq yes ,
1.84 jmc 1365: .Xr ssh 1
1.1 stevesk 1366: will never automatically add host keys to the
1.50 djm 1367: .Pa ~/.ssh/known_hosts
1.1 stevesk 1368: file, and refuses to connect to hosts whose host key has changed.
1369: This provides maximum protection against trojan horse attacks,
1.84 jmc 1370: though it can be annoying when the
1.1 stevesk 1371: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1372: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1373: frequently made.
1374: This option forces the user to manually
1375: add all new hosts.
1376: If this flag is set to
1377: .Dq no ,
1.84 jmc 1378: ssh will automatically add new host keys to the
1.1 stevesk 1379: user known hosts files.
1380: If this flag is set to
1381: .Dq ask ,
1382: new host keys
1383: will be added to the user known host files only after the user
1384: has confirmed that is what they really want to do, and
1.84 jmc 1385: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1386: The host keys of
1387: known hosts will be verified automatically in all cases.
1388: The argument must be
1389: .Dq yes ,
1.84 jmc 1390: .Dq no ,
1.1 stevesk 1391: or
1392: .Dq ask .
1393: The default is
1394: .Dq ask .
1.26 markus 1395: .It Cm TCPKeepAlive
1396: Specifies whether the system should send TCP keepalive messages to the
1397: other side.
1398: If they are sent, death of the connection or crash of one
1399: of the machines will be properly noticed.
1400: However, this means that
1401: connections will die if the route is down temporarily, and some people
1402: find it annoying.
1403: .Pp
1404: The default is
1405: .Dq yes
1406: (to send TCP keepalive messages), and the client will notice
1407: if the network goes down or the remote host dies.
1408: This is important in scripts, and many users want it too.
1409: .Pp
1410: To disable TCP keepalive messages, the value should be set to
1411: .Dq no .
1.65 reyk 1412: .It Cm Tunnel
1.95 stevesk 1413: Request
1.65 reyk 1414: .Xr tun 4
1.69 jmc 1415: device forwarding between the client and the server.
1.65 reyk 1416: The argument must be
1.68 reyk 1417: .Dq yes ,
1.95 stevesk 1418: .Dq point-to-point
1419: (layer 3),
1420: .Dq ethernet
1421: (layer 2),
1.65 reyk 1422: or
1423: .Dq no .
1.95 stevesk 1424: Specifying
1425: .Dq yes
1426: requests the default tunnel mode, which is
1427: .Dq point-to-point .
1.65 reyk 1428: The default is
1429: .Dq no .
1430: .It Cm TunnelDevice
1.95 stevesk 1431: Specifies the
1.65 reyk 1432: .Xr tun 4
1.95 stevesk 1433: devices to open on the client
1434: .Pq Ar local_tun
1435: and the server
1436: .Pq Ar remote_tun .
1437: .Pp
1438: The argument must be
1439: .Sm off
1440: .Ar local_tun Op : Ar remote_tun .
1441: .Sm on
1442: The devices may be specified by numerical ID or the keyword
1443: .Dq any ,
1444: which uses the next available tunnel device.
1445: If
1446: .Ar remote_tun
1447: is not specified, it defaults to
1448: .Dq any .
1449: The default is
1450: .Dq any:any .
1.72 jmc 1451: .It Cm UsePrivilegedPort
1452: Specifies whether to use a privileged port for outgoing connections.
1453: The argument must be
1454: .Dq yes
1455: or
1456: .Dq no .
1457: The default is
1458: .Dq no .
1459: If set to
1.84 jmc 1460: .Dq yes ,
1461: .Xr ssh 1
1.72 jmc 1462: must be setuid root.
1463: Note that this option must be set to
1464: .Dq yes
1465: for
1466: .Cm RhostsRSAAuthentication
1467: with older servers.
1.1 stevesk 1468: .It Cm User
1469: Specifies the user to log in as.
1470: This can be useful when a different user name is used on different machines.
1471: This saves the trouble of
1472: having to remember to give the user name on the command line.
1473: .It Cm UserKnownHostsFile
1.151 djm 1474: Specifies one or more files to use for the user
1475: host key database, separated by whitespace.
1476: The default is
1477: .Pa ~/.ssh/known_hosts ,
1478: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1479: .It Cm VerifyHostKeyDNS
1480: Specifies whether to verify the remote key using DNS and SSHFP resource
1481: records.
1.24 jakob 1482: If this option is set to
1483: .Dq yes ,
1.25 jmc 1484: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1485: from DNS.
1486: Insecure fingerprints will be handled as if this option was set to
1487: .Dq ask .
1488: If this option is set to
1489: .Dq ask ,
1490: information on fingerprint match will be displayed, but the user will still
1491: need to confirm new host keys according to the
1492: .Cm StrictHostKeyChecking
1493: option.
1494: The argument must be
1495: .Dq yes ,
1.84 jmc 1496: .Dq no ,
1.25 jmc 1497: or
1498: .Dq ask .
1.8 jakob 1499: The default is
1500: .Dq no .
1.12 jakob 1501: Note that this option applies to protocol version 2 only.
1.84 jmc 1502: .Pp
1.166 jmc 1503: See also VERIFYING HOST KEYS in
1.84 jmc 1504: .Xr ssh 1 .
1.111 grunk 1505: .It Cm VisualHostKey
1506: If this flag is set to
1507: .Dq yes ,
1508: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1509: printed in addition to the hex fingerprint string at login and
1510: for unknown host keys.
1.111 grunk 1511: If this flag is set to
1512: .Dq no ,
1.114 stevesk 1513: no fingerprint strings are printed at login and
1514: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1515: The default is
1516: .Dq no .
1.1 stevesk 1517: .It Cm XAuthLocation
1.5 stevesk 1518: Specifies the full pathname of the
1.1 stevesk 1519: .Xr xauth 1
1520: program.
1521: The default is
1522: .Pa /usr/X11R6/bin/xauth .
1523: .El
1.86 jmc 1524: .Sh PATTERNS
1525: A
1526: .Em pattern
1527: consists of zero or more non-whitespace characters,
1528: .Sq *
1529: (a wildcard that matches zero or more characters),
1530: or
1531: .Sq ?\&
1532: (a wildcard that matches exactly one character).
1533: For example, to specify a set of declarations for any host in the
1534: .Dq .co.uk
1535: set of domains,
1536: the following pattern could be used:
1537: .Pp
1538: .Dl Host *.co.uk
1539: .Pp
1540: The following pattern
1541: would match any host in the 192.168.0.[0-9] network range:
1542: .Pp
1543: .Dl Host 192.168.0.?
1544: .Pp
1545: A
1546: .Em pattern-list
1547: is a comma-separated list of patterns.
1548: Patterns within pattern-lists may be negated
1549: by preceding them with an exclamation mark
1550: .Pq Sq !\& .
1551: For example,
1.174 djm 1552: to allow a key to be used from anywhere within an organization
1.86 jmc 1553: except from the
1554: .Dq dialup
1555: pool,
1556: the following entry (in authorized_keys) could be used:
1557: .Pp
1558: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1559: .Sh FILES
1560: .Bl -tag -width Ds
1.50 djm 1561: .It Pa ~/.ssh/config
1.1 stevesk 1562: This is the per-user configuration file.
1563: The format of this file is described above.
1.84 jmc 1564: This file is used by the SSH client.
1.30 djm 1565: Because of the potential for abuse, this file must have strict permissions:
1566: read/write for the user, and not accessible by others.
1.1 stevesk 1567: .It Pa /etc/ssh/ssh_config
1568: Systemwide configuration file.
1569: This file provides defaults for those
1570: values that are not specified in the user's configuration file, and
1571: for those users who do not have a configuration file.
1572: This file must be world-readable.
1573: .El
1.13 jmc 1574: .Sh SEE ALSO
1575: .Xr ssh 1
1.1 stevesk 1576: .Sh AUTHORS
1577: OpenSSH is a derivative of the original and free
1578: ssh 1.2.12 release by Tatu Ylonen.
1579: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1580: Theo de Raadt and Dug Song
1581: removed many bugs, re-added newer features and
1582: created OpenSSH.
1583: Markus Friedl contributed the support for SSH
1584: protocol versions 1.5 and 2.0.
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