Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.209
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.209 ! dtucker 36: .\" $OpenBSD: ssh_config.5,v 1.208 2015/04/28 13:47:38 jmc Exp $
1.208 jmc 37: .Dd $Mdocdate: April 28 2015 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
42: .Nd OpenSSH SSH client configuration files
43: .Sh SYNOPSIS
1.98 jmc 44: .Nm ~/.ssh/config
45: .Nm /etc/ssh/ssh_config
1.1 stevesk 46: .Sh DESCRIPTION
1.84 jmc 47: .Xr ssh 1
1.1 stevesk 48: obtains configuration data from the following sources in
49: the following order:
1.79 jmc 50: .Pp
1.2 stevesk 51: .Bl -enum -offset indent -compact
52: .It
53: command-line options
54: .It
55: user's configuration file
1.50 djm 56: .Pq Pa ~/.ssh/config
1.2 stevesk 57: .It
58: system-wide configuration file
59: .Pq Pa /etc/ssh/ssh_config
60: .El
1.1 stevesk 61: .Pp
62: For each parameter, the first obtained value
63: will be used.
1.41 jmc 64: The configuration files contain sections separated by
1.1 stevesk 65: .Dq Host
66: specifications, and that section is only applied for hosts that
67: match one of the patterns given in the specification.
1.193 djm 68: The matched host name is usually the one given on the command line
69: (see the
70: .Cm CanonicalizeHostname
71: option for exceptions.)
1.1 stevesk 72: .Pp
73: Since the first obtained value for each parameter is used, more
74: host-specific declarations should be given near the beginning of the
75: file, and general defaults at the end.
1.80 jmc 76: .Pp
1.1 stevesk 77: The configuration file has the following format:
78: .Pp
79: Empty lines and lines starting with
80: .Ql #
81: are comments.
82: Otherwise a line is of the format
83: .Dq keyword arguments .
84: Configuration options may be separated by whitespace or
85: optional whitespace and exactly one
86: .Ql = ;
87: the latter format is useful to avoid the need to quote whitespace
88: when specifying configuration options using the
89: .Nm ssh ,
1.87 jmc 90: .Nm scp ,
1.1 stevesk 91: and
92: .Nm sftp
93: .Fl o
94: option.
1.88 dtucker 95: Arguments may optionally be enclosed in double quotes
96: .Pq \&"
97: in order to represent arguments containing spaces.
1.1 stevesk 98: .Pp
99: The possible
100: keywords and their meanings are as follows (note that
101: keywords are case-insensitive and arguments are case-sensitive):
102: .Bl -tag -width Ds
103: .It Cm Host
104: Restricts the following declarations (up to the next
105: .Cm Host
1.169 djm 106: or
107: .Cm Match
1.1 stevesk 108: keyword) to be only for those hosts that match one of the patterns
109: given after the keyword.
1.112 krw 110: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 111: A single
1.83 jmc 112: .Ql *
1.1 stevesk 113: as a pattern can be used to provide global
114: defaults for all hosts.
1.193 djm 115: The host is usually the
1.1 stevesk 116: .Ar hostname
1.193 djm 117: argument given on the command line
118: (see the
119: .Cm CanonicalizeHostname
120: option for exceptions.)
1.148 djm 121: .Pp
122: A pattern entry may be negated by prefixing it with an exclamation mark
123: .Pq Sq !\& .
124: If a negated entry is matched, then the
125: .Cm Host
126: entry is ignored, regardless of whether any other patterns on the line
127: match.
128: Negated matches are therefore useful to provide exceptions for wildcard
129: matches.
1.81 jmc 130: .Pp
131: See
132: .Sx PATTERNS
133: for more information on patterns.
1.170 jmc 134: .It Cm Match
1.169 djm 135: Restricts the following declarations (up to the next
136: .Cm Host
137: or
138: .Cm Match
139: keyword) to be used only when the conditions following the
140: .Cm Match
141: keyword are satisfied.
1.193 djm 142: Match conditions are specified using one or more critera
1.178 dtucker 143: or the single token
144: .Cm all
1.193 djm 145: which always matches.
146: The available criteria keywords are:
147: .Cm canonical ,
1.176 djm 148: .Cm exec ,
1.169 djm 149: .Cm host ,
150: .Cm originalhost ,
151: .Cm user ,
152: and
153: .Cm localuser .
1.193 djm 154: The
155: .Cm all
156: criteria must appear alone or immediately after
1.194 jmc 157: .Cm canonical .
1.193 djm 158: Other criteria may be combined arbitrarily.
159: All criteria but
160: .Cm all
161: and
162: .Cm canonical
163: require an argument.
164: Criteria may be negated by prepending an exclamation mark
165: .Pq Sq !\& .
1.169 djm 166: .Pp
1.177 jmc 167: The
1.193 djm 168: .Cm canonical
169: keywork matches only when the configuration file is being re-parsed
170: after hostname canonicalization (see the
171: .Cm CanonicalizeHostname
172: option.)
173: This may be useful to specify conditions that work with canonical host
174: names only.
175: The
1.176 djm 176: .Cm exec
1.177 jmc 177: keyword executes the specified command under the user's shell.
1.169 djm 178: If the command returns a zero exit status then the condition is considered true.
179: Commands containing whitespace characters must be quoted.
1.175 djm 180: The following character sequences in the command will be expanded prior to
181: execution:
182: .Ql %L
183: will be substituted by the first component of the local host name,
184: .Ql %l
185: will be substituted by the local host name (including any domain name),
186: .Ql %h
187: will be substituted by the target host name,
188: .Ql %n
189: will be substituted by the original target host name
1.176 djm 190: specified on the command-line,
1.175 djm 191: .Ql %p
192: the destination port,
193: .Ql %r
194: by the remote login username, and
195: .Ql %u
196: by the username of the user running
197: .Xr ssh 1 .
1.169 djm 198: .Pp
199: The other keywords' criteria must be single entries or comma-separated
200: lists and may use the wildcard and negation operators described in the
201: .Sx PATTERNS
202: section.
203: The criteria for the
204: .Cm host
205: keyword are matched against the target hostname, after any substitution
206: by the
207: .Cm Hostname
1.193 djm 208: or
209: .Cm CanonicalizeHostname
210: options.
1.169 djm 211: The
212: .Cm originalhost
213: keyword matches against the hostname as it was specified on the command-line.
214: The
215: .Cm user
216: keyword matches against the target username on the remote host.
217: The
218: .Cm localuser
219: keyword matches against the name of the local user running
220: .Xr ssh 1
221: (this keyword may be useful in system-wide
222: .Nm
223: files).
1.10 djm 224: .It Cm AddressFamily
1.11 jmc 225: Specifies which address family to use when connecting.
226: Valid arguments are
1.10 djm 227: .Dq any ,
228: .Dq inet
1.84 jmc 229: (use IPv4 only), or
1.10 djm 230: .Dq inet6
1.40 jmc 231: (use IPv6 only).
1.1 stevesk 232: .It Cm BatchMode
233: If set to
234: .Dq yes ,
235: passphrase/password querying will be disabled.
236: This option is useful in scripts and other batch jobs where no user
237: is present to supply the password.
238: The argument must be
239: .Dq yes
240: or
241: .Dq no .
242: The default is
243: .Dq no .
244: .It Cm BindAddress
1.60 dtucker 245: Use the specified address on the local machine as the source address of
1.61 jmc 246: the connection.
247: Only useful on systems with more than one address.
1.1 stevesk 248: Note that this option does not work if
249: .Cm UsePrivilegedPort
250: is set to
251: .Dq yes .
1.171 djm 252: .It Cm CanonicalDomains
1.172 jmc 253: When
1.173 djm 254: .Cm CanonicalizeHostname
1.171 djm 255: is enabled, this option specifies the list of domain suffixes in which to
256: search for the specified destination host.
1.173 djm 257: .It Cm CanonicalizeFallbackLocal
1.174 djm 258: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 259: The default,
1.184 djm 260: .Dq yes ,
1.172 jmc 261: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 262: search rules.
263: A value of
1.184 djm 264: .Dq no
1.171 djm 265: will cause
266: .Xr ssh 1
267: to fail instantly if
1.173 djm 268: .Cm CanonicalizeHostname
1.171 djm 269: is enabled and the target hostname cannot be found in any of the domains
270: specified by
271: .Cm CanonicalDomains .
1.173 djm 272: .It Cm CanonicalizeHostname
1.174 djm 273: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 274: The default,
275: .Dq no ,
1.171 djm 276: is not to perform any name rewriting and let the system resolver handle all
277: hostname lookups.
278: If set to
279: .Dq yes
280: then, for connections that do not use a
281: .Cm ProxyCommand ,
282: .Xr ssh 1
1.173 djm 283: will attempt to canonicalize the hostname specified on the command line
1.171 djm 284: using the
285: .Cm CanonicalDomains
286: suffixes and
1.173 djm 287: .Cm CanonicalizePermittedCNAMEs
1.171 djm 288: rules.
289: If
1.173 djm 290: .Cm CanonicalizeHostname
1.171 djm 291: is set to
292: .Dq always ,
1.174 djm 293: then canonicalization is applied to proxied connections too.
1.185 djm 294: .Pp
1.193 djm 295: If this option is enabled, then the configuration files are processed
296: again using the new target name to pick up any new configuration in matching
1.185 djm 297: .Cm Host
1.193 djm 298: and
299: .Cm Match
1.185 djm 300: stanzas.
1.173 djm 301: .It Cm CanonicalizeMaxDots
1.172 jmc 302: Specifies the maximum number of dot characters in a hostname before
1.174 djm 303: canonicalization is disabled.
1.172 jmc 304: The default,
305: .Dq 1 ,
306: allows a single dot (i.e. hostname.subdomain).
1.173 djm 307: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 308: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 309: canonicalizing hostnames.
1.171 djm 310: The rules consist of one or more arguments of
1.172 jmc 311: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 312: where
313: .Ar source_domain_list
1.174 djm 314: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 315: and
316: .Ar target_domain_list
1.172 jmc 317: is a pattern-list of domains that they may resolve to.
1.171 djm 318: .Pp
319: For example,
320: .Dq *.a.example.com:*.b.example.com,*.c.example.com
321: will allow hostnames matching
322: .Dq *.a.example.com
1.173 djm 323: to be canonicalized to names in the
1.171 djm 324: .Dq *.b.example.com
325: or
326: .Dq *.c.example.com
327: domains.
1.1 stevesk 328: .It Cm ChallengeResponseAuthentication
1.82 jmc 329: Specifies whether to use challenge-response authentication.
1.1 stevesk 330: The argument to this keyword must be
331: .Dq yes
332: or
333: .Dq no .
334: The default is
335: .Dq yes .
336: .It Cm CheckHostIP
337: If this flag is set to
338: .Dq yes ,
1.84 jmc 339: .Xr ssh 1
340: will additionally check the host IP address in the
1.1 stevesk 341: .Pa known_hosts
342: file.
343: This allows ssh to detect if a host key changed due to DNS spoofing.
1.107 grunk 344: If the option is set to
1.1 stevesk 345: .Dq no ,
346: the check will not be executed.
347: The default is
348: .Dq yes .
349: .It Cm Cipher
350: Specifies the cipher to use for encrypting the session
351: in protocol version 1.
352: Currently,
353: .Dq blowfish ,
354: .Dq 3des ,
355: and
356: .Dq des
357: are supported.
358: .Ar des
359: is only supported in the
1.84 jmc 360: .Xr ssh 1
1.1 stevesk 361: client for interoperability with legacy protocol 1 implementations
362: that do not support the
363: .Ar 3des
1.7 jmc 364: cipher.
365: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 366: The default is
367: .Dq 3des .
368: .It Cm Ciphers
369: Specifies the ciphers allowed for protocol version 2
370: in order of preference.
371: Multiple ciphers must be comma-separated.
1.180 djm 372: The supported ciphers are:
373: .Pp
1.186 naddy 374: .Bl -item -compact -offset indent
375: .It
376: 3des-cbc
377: .It
378: aes128-cbc
379: .It
380: aes192-cbc
381: .It
382: aes256-cbc
383: .It
384: aes128-ctr
385: .It
386: aes192-ctr
387: .It
388: aes256-ctr
389: .It
390: aes128-gcm@openssh.com
391: .It
392: aes256-gcm@openssh.com
393: .It
394: arcfour
395: .It
396: arcfour128
397: .It
398: arcfour256
399: .It
400: blowfish-cbc
401: .It
402: cast128-cbc
403: .It
404: chacha20-poly1305@openssh.com
405: .El
1.180 djm 406: .Pp
1.84 jmc 407: The default is:
1.186 naddy 408: .Bd -literal -offset indent
409: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 410: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.180 djm 411: chacha20-poly1305@openssh.com,
1.186 naddy 412: arcfour256,arcfour128,
413: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
414: aes192-cbc,aes256-cbc,arcfour
1.1 stevesk 415: .Ed
1.180 djm 416: .Pp
417: The list of available ciphers may also be obtained using the
418: .Fl Q
419: option of
1.198 djm 420: .Xr ssh 1
421: with an argument of
422: .Dq cipher .
1.1 stevesk 423: .It Cm ClearAllForwardings
1.84 jmc 424: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 425: specified in the configuration files or on the command line be
1.7 jmc 426: cleared.
427: This option is primarily useful when used from the
1.84 jmc 428: .Xr ssh 1
1.1 stevesk 429: command line to clear port forwardings set in
430: configuration files, and is automatically set by
431: .Xr scp 1
432: and
433: .Xr sftp 1 .
434: The argument must be
435: .Dq yes
436: or
437: .Dq no .
438: The default is
439: .Dq no .
440: .It Cm Compression
441: Specifies whether to use compression.
442: The argument must be
443: .Dq yes
444: or
445: .Dq no .
446: The default is
447: .Dq no .
448: .It Cm CompressionLevel
449: Specifies the compression level to use if compression is enabled.
450: The argument must be an integer from 1 (fast) to 9 (slow, best).
451: The default level is 6, which is good for most applications.
452: The meaning of the values is the same as in
453: .Xr gzip 1 .
454: Note that this option applies to protocol version 1 only.
455: .It Cm ConnectionAttempts
456: Specifies the number of tries (one per second) to make before exiting.
457: The argument must be an integer.
458: This may be useful in scripts if the connection sometimes fails.
459: The default is 1.
1.9 djm 460: .It Cm ConnectTimeout
1.84 jmc 461: Specifies the timeout (in seconds) used when connecting to the
462: SSH server, instead of using the default system TCP timeout.
1.11 jmc 463: This value is used only when the target is down or really unreachable,
464: not when it refuses the connection.
1.36 djm 465: .It Cm ControlMaster
466: Enables the sharing of multiple sessions over a single network connection.
467: When set to
1.84 jmc 468: .Dq yes ,
469: .Xr ssh 1
1.36 djm 470: will listen for connections on a control socket specified using the
471: .Cm ControlPath
472: argument.
473: Additional sessions can connect to this socket using the same
474: .Cm ControlPath
475: with
476: .Cm ControlMaster
477: set to
478: .Dq no
1.38 jmc 479: (the default).
1.64 jmc 480: These sessions will try to reuse the master instance's network connection
1.63 djm 481: rather than initiating new ones, but will fall back to connecting normally
482: if the control socket does not exist, or is not listening.
483: .Pp
1.37 djm 484: Setting this to
485: .Dq ask
1.84 jmc 486: will cause ssh
1.206 jmc 487: to listen for control connections, but require confirmation using
488: .Xr ssh-askpass 1 .
1.51 jakob 489: If the
490: .Cm ControlPath
1.84 jmc 491: cannot be opened,
492: ssh will continue without connecting to a master instance.
1.58 djm 493: .Pp
494: X11 and
1.59 jmc 495: .Xr ssh-agent 1
1.58 djm 496: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 497: display and agent forwarded will be the one belonging to the master
1.59 jmc 498: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 499: .Pp
500: Two additional options allow for opportunistic multiplexing: try to use a
501: master connection but fall back to creating a new one if one does not already
502: exist.
503: These options are:
504: .Dq auto
505: and
506: .Dq autoask .
507: The latter requires confirmation like the
508: .Dq ask
509: option.
1.36 djm 510: .It Cm ControlPath
1.55 djm 511: Specify the path to the control socket used for connection sharing as described
512: in the
1.36 djm 513: .Cm ControlMaster
1.57 djm 514: section above or the string
515: .Dq none
516: to disable connection sharing.
1.55 djm 517: In the path,
1.147 djm 518: .Ql %L
519: will be substituted by the first component of the local host name,
1.77 djm 520: .Ql %l
1.147 djm 521: will be substituted by the local host name (including any domain name),
1.55 djm 522: .Ql %h
523: will be substituted by the target host name,
1.150 jmc 524: .Ql %n
525: will be substituted by the original target host name
526: specified on the command line,
1.55 djm 527: .Ql %p
1.175 djm 528: the destination port,
1.55 djm 529: .Ql %r
1.188 djm 530: by the remote login username,
1.147 djm 531: .Ql %u
532: by the username of the user running
1.188 djm 533: .Xr ssh 1 , and
1.189 jmc 534: .Ql \&%C
1.188 djm 535: by a hash of the concatenation: %l%h%p%r.
1.56 djm 536: It is recommended that any
537: .Cm ControlPath
538: used for opportunistic connection sharing include
1.195 djm 539: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
540: that is not writable by other users.
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
1.195 djm 553: .Dq yes
554: or
555: .Dq 0 ,
1.137 djm 556: then the master connection will remain in the background indefinitely
557: (until killed or closed via a mechanism such as the
558: .Xr ssh 1
559: .Dq Fl O No exit
560: option).
561: If set to a time in seconds, or a time in any of the formats documented in
562: .Xr sshd_config 5 ,
563: then the backgrounded master connection will automatically terminate
564: after it has remained idle (with no client connections) for the
565: specified time.
1.38 jmc 566: .It Cm DynamicForward
1.74 jmc 567: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 568: over the secure channel, and the application
569: protocol is then used to determine where to connect to from the
570: remote machine.
1.62 djm 571: .Pp
572: The argument must be
573: .Sm off
574: .Oo Ar bind_address : Oc Ar port .
575: .Sm on
1.138 djm 576: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 577: By default, the local port is bound in accordance with the
578: .Cm GatewayPorts
579: setting.
580: However, an explicit
581: .Ar bind_address
582: may be used to bind the connection to a specific address.
583: The
584: .Ar bind_address
585: of
586: .Dq localhost
587: indicates that the listening port be bound for local use only, while an
588: empty address or
589: .Sq *
590: indicates that the port should be available from all interfaces.
591: .Pp
1.38 jmc 592: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 593: .Xr ssh 1
1.38 jmc 594: will act as a SOCKS server.
595: Multiple forwardings may be specified, and
596: additional forwardings can be given on the command line.
597: Only the superuser can forward privileged ports.
1.14 markus 598: .It Cm EnableSSHKeysign
599: Setting this option to
600: .Dq yes
601: in the global client configuration file
602: .Pa /etc/ssh/ssh_config
603: enables the use of the helper program
604: .Xr ssh-keysign 8
605: during
606: .Cm HostbasedAuthentication .
607: The argument must be
608: .Dq yes
609: or
610: .Dq no .
611: The default is
612: .Dq no .
1.23 jmc 613: This option should be placed in the non-hostspecific section.
1.14 markus 614: See
615: .Xr ssh-keysign 8
616: for more information.
1.1 stevesk 617: .It Cm EscapeChar
618: Sets the escape character (default:
619: .Ql ~ ) .
620: The escape character can also
621: be set on the command line.
622: The argument should be a single character,
623: .Ql ^
624: followed by a letter, or
625: .Dq none
626: to disable the escape
627: character entirely (making the connection transparent for binary
628: data).
1.96 markus 629: .It Cm ExitOnForwardFailure
630: Specifies whether
631: .Xr ssh 1
632: should terminate the connection if it cannot set up all requested
1.102 stevesk 633: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 634: The argument must be
635: .Dq yes
636: or
637: .Dq no .
638: The default is
639: .Dq no .
1.197 djm 640: .It Cm FingerprintHash
641: Specifies the hash algorithm used when displaying key fingerprints.
642: Valid options are:
643: .Dq md5
644: and
645: .Dq sha256 .
646: The default is
647: .Dq sha256 .
1.1 stevesk 648: .It Cm ForwardAgent
649: Specifies whether the connection to the authentication agent (if any)
650: will be forwarded to the remote machine.
651: The argument must be
652: .Dq yes
653: or
654: .Dq no .
655: The default is
656: .Dq no .
1.3 stevesk 657: .Pp
1.7 jmc 658: Agent forwarding should be enabled with caution.
659: Users with the ability to bypass file permissions on the remote host
660: (for the agent's Unix-domain socket)
661: can access the local agent through the forwarded connection.
662: An attacker cannot obtain key material from the agent,
1.3 stevesk 663: however they can perform operations on the keys that enable them to
664: authenticate using the identities loaded into the agent.
1.1 stevesk 665: .It Cm ForwardX11
666: Specifies whether X11 connections will be automatically redirected
667: over the secure channel and
668: .Ev DISPLAY
669: set.
670: The argument must be
671: .Dq yes
672: or
673: .Dq no .
674: The default is
675: .Dq no .
1.3 stevesk 676: .Pp
1.7 jmc 677: X11 forwarding should be enabled with caution.
678: Users with the ability to bypass file permissions on the remote host
1.22 markus 679: (for the user's X11 authorization database)
1.7 jmc 680: can access the local X11 display through the forwarded connection.
1.22 markus 681: An attacker may then be able to perform activities such as keystroke monitoring
682: if the
683: .Cm ForwardX11Trusted
684: option is also enabled.
1.134 djm 685: .It Cm ForwardX11Timeout
1.135 jmc 686: Specify a timeout for untrusted X11 forwarding
687: using the format described in the
1.166 jmc 688: TIME FORMATS section of
1.134 djm 689: .Xr sshd_config 5 .
690: X11 connections received by
691: .Xr ssh 1
692: after this time will be refused.
693: The default is to disable untrusted X11 forwarding after twenty minutes has
694: elapsed.
1.22 markus 695: .It Cm ForwardX11Trusted
1.34 jmc 696: If this option is set to
1.84 jmc 697: .Dq yes ,
698: remote X11 clients will have full access to the original X11 display.
1.42 djm 699: .Pp
1.22 markus 700: If this option is set to
1.84 jmc 701: .Dq no ,
702: remote X11 clients will be considered untrusted and prevented
1.22 markus 703: from stealing or tampering with data belonging to trusted X11
704: clients.
1.42 djm 705: Furthermore, the
706: .Xr xauth 1
707: token used for the session will be set to expire after 20 minutes.
708: Remote clients will be refused access after this time.
1.22 markus 709: .Pp
710: The default is
711: .Dq no .
712: .Pp
713: See the X11 SECURITY extension specification for full details on
714: the restrictions imposed on untrusted clients.
1.1 stevesk 715: .It Cm GatewayPorts
716: Specifies whether remote hosts are allowed to connect to local
717: forwarded ports.
718: By default,
1.84 jmc 719: .Xr ssh 1
1.7 jmc 720: binds local port forwardings to the loopback address.
721: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 722: .Cm GatewayPorts
1.84 jmc 723: can be used to specify that ssh
1.1 stevesk 724: should bind local port forwardings to the wildcard address,
725: thus allowing remote hosts to connect to forwarded ports.
726: The argument must be
727: .Dq yes
728: or
729: .Dq no .
730: The default is
731: .Dq no .
732: .It Cm GlobalKnownHostsFile
1.151 djm 733: Specifies one or more files to use for the global
734: host key database, separated by whitespace.
735: The default is
736: .Pa /etc/ssh/ssh_known_hosts ,
737: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 738: .It Cm GSSAPIAuthentication
1.27 markus 739: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 740: The default is
1.21 markus 741: .Dq no .
1.18 markus 742: Note that this option applies to protocol version 2 only.
743: .It Cm GSSAPIDelegateCredentials
744: Forward (delegate) credentials to the server.
745: The default is
746: .Dq no .
747: Note that this option applies to protocol version 2 only.
1.44 djm 748: .It Cm HashKnownHosts
749: Indicates that
1.84 jmc 750: .Xr ssh 1
1.44 djm 751: should hash host names and addresses when they are added to
1.50 djm 752: .Pa ~/.ssh/known_hosts .
1.44 djm 753: These hashed names may be used normally by
1.84 jmc 754: .Xr ssh 1
1.44 djm 755: and
1.84 jmc 756: .Xr sshd 8 ,
1.44 djm 757: but they do not reveal identifying information should the file's contents
758: be disclosed.
759: The default is
760: .Dq no .
1.97 jmc 761: Note that existing names and addresses in known hosts files
762: will not be converted automatically,
763: but may be manually hashed using
1.45 djm 764: .Xr ssh-keygen 1 .
1.1 stevesk 765: .It Cm HostbasedAuthentication
766: Specifies whether to try rhosts based authentication with public key
767: authentication.
768: The argument must be
769: .Dq yes
770: or
771: .Dq no .
772: The default is
773: .Dq no .
774: This option applies to protocol version 2 only and
775: is similar to
776: .Cm RhostsRSAAuthentication .
1.202 djm 777: .It Cm HostbasedKeyTypes
778: Specifies the key types that will be used for hostbased authentication
779: as a comma-separated pattern list.
780: The default
781: .Dq *
782: will allow all key types.
783: The
784: .Fl Q
785: option of
786: .Xr ssh 1
787: may be used to list supported key types.
1.1 stevesk 788: .It Cm HostKeyAlgorithms
789: Specifies the protocol version 2 host key algorithms
790: that the client wants to use in order of preference.
791: The default for this option is:
1.139 djm 792: .Bd -literal -offset 3n
793: ecdsa-sha2-nistp256-cert-v01@openssh.com,
794: ecdsa-sha2-nistp384-cert-v01@openssh.com,
795: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 796: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 797: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
798: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
799: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.182 markus 800: ssh-ed25519,ssh-rsa,ssh-dss
1.139 djm 801: .Ed
1.145 djm 802: .Pp
803: If hostkeys are known for the destination host then this default is modified
804: to prefer their algorithms.
1.198 djm 805: .Pp
806: The list of available key types may also be obtained using the
807: .Fl Q
808: option of
809: .Xr ssh 1
810: with an argument of
811: .Dq key .
1.1 stevesk 812: .It Cm HostKeyAlias
813: Specifies an alias that should be used instead of the
814: real host name when looking up or saving the host key
815: in the host key database files.
1.84 jmc 816: This option is useful for tunneling SSH connections
1.1 stevesk 817: or for multiple servers running on a single host.
818: .It Cm HostName
819: Specifies the real host name to log into.
820: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 821: If the hostname contains the character sequence
822: .Ql %h ,
1.150 jmc 823: then this will be replaced with the host name specified on the command line
1.136 djm 824: (this is useful for manipulating unqualified names).
1.187 djm 825: The character sequence
826: .Ql %%
827: will be replaced by a single
828: .Ql %
829: character, which may be used when specifying IPv6 link-local addresses.
830: .Pp
1.84 jmc 831: The default is the name given on the command line.
1.1 stevesk 832: Numeric IP addresses are also permitted (both on the command line and in
833: .Cm HostName
834: specifications).
1.29 markus 835: .It Cm IdentitiesOnly
836: Specifies that
1.84 jmc 837: .Xr ssh 1
1.29 markus 838: should only use the authentication identity files configured in the
1.31 jmc 839: .Nm
1.29 markus 840: files,
1.84 jmc 841: even if
842: .Xr ssh-agent 1
1.159 djm 843: or a
844: .Cm PKCS11Provider
1.29 markus 845: offers more identities.
846: The argument to this keyword must be
847: .Dq yes
848: or
849: .Dq no .
1.84 jmc 850: This option is intended for situations where ssh-agent
1.29 markus 851: offers many different identities.
852: The default is
853: .Dq no .
1.67 jmc 854: .It Cm IdentityFile
1.192 sobrado 855: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 856: identity is read.
1.67 jmc 857: The default is
858: .Pa ~/.ssh/identity
859: for protocol version 1, and
1.139 djm 860: .Pa ~/.ssh/id_dsa ,
1.183 naddy 861: .Pa ~/.ssh/id_ecdsa ,
862: .Pa ~/.ssh/id_ed25519
1.139 djm 863: and
1.67 jmc 864: .Pa ~/.ssh/id_rsa
865: for protocol version 2.
866: Additionally, any identities represented by the authentication agent
1.165 djm 867: will be used for authentication unless
868: .Cm IdentitiesOnly
869: is set.
1.129 djm 870: .Xr ssh 1
871: will try to load certificate information from the filename obtained by
872: appending
873: .Pa -cert.pub
874: to the path of a specified
875: .Cm IdentityFile .
1.90 djm 876: .Pp
1.67 jmc 877: The file name may use the tilde
1.91 jmc 878: syntax to refer to a user's home directory or one of the following
1.90 djm 879: escape characters:
880: .Ql %d
881: (local user's home directory),
882: .Ql %u
883: (local user name),
884: .Ql %l
885: (local host name),
886: .Ql %h
887: (remote host name) or
1.92 djm 888: .Ql %r
1.90 djm 889: (remote user name).
890: .Pp
1.67 jmc 891: It is possible to have
892: multiple identity files specified in configuration files; all these
893: identities will be tried in sequence.
1.152 djm 894: Multiple
895: .Cm IdentityFile
896: directives will add to the list of identities tried (this behaviour
897: differs from that of other configuration directives).
1.165 djm 898: .Pp
899: .Cm IdentityFile
900: may be used in conjunction with
901: .Cm IdentitiesOnly
902: to select which identities in an agent are offered during authentication.
1.164 jmc 903: .It Cm IgnoreUnknown
904: Specifies a pattern-list of unknown options to be ignored if they are
905: encountered in configuration parsing.
906: This may be used to suppress errors if
907: .Nm
908: contains options that are unrecognised by
909: .Xr ssh 1 .
910: It is recommended that
911: .Cm IgnoreUnknown
912: be listed early in the configuration file as it will not be applied
913: to unknown options that appear before it.
1.143 djm 914: .It Cm IPQoS
915: Specifies the IPv4 type-of-service or DSCP class for connections.
916: Accepted values are
917: .Dq af11 ,
918: .Dq af12 ,
919: .Dq af13 ,
1.154 djm 920: .Dq af21 ,
1.143 djm 921: .Dq af22 ,
922: .Dq af23 ,
923: .Dq af31 ,
924: .Dq af32 ,
925: .Dq af33 ,
926: .Dq af41 ,
927: .Dq af42 ,
928: .Dq af43 ,
929: .Dq cs0 ,
930: .Dq cs1 ,
931: .Dq cs2 ,
932: .Dq cs3 ,
933: .Dq cs4 ,
934: .Dq cs5 ,
935: .Dq cs6 ,
936: .Dq cs7 ,
937: .Dq ef ,
938: .Dq lowdelay ,
939: .Dq throughput ,
940: .Dq reliability ,
941: or a numeric value.
1.146 djm 942: This option may take one or two arguments, separated by whitespace.
1.143 djm 943: If one argument is specified, it is used as the packet class unconditionally.
944: If two values are specified, the first is automatically selected for
945: interactive sessions and the second for non-interactive sessions.
946: The default is
947: .Dq lowdelay
948: for interactive sessions and
949: .Dq throughput
950: for non-interactive sessions.
1.103 djm 951: .It Cm KbdInteractiveAuthentication
952: Specifies whether to use keyboard-interactive authentication.
953: The argument to this keyword must be
954: .Dq yes
955: or
956: .Dq no .
957: The default is
958: .Dq yes .
1.39 djm 959: .It Cm KbdInteractiveDevices
960: Specifies the list of methods to use in keyboard-interactive authentication.
961: Multiple method names must be comma-separated.
962: The default is to use the server specified list.
1.85 jmc 963: The methods available vary depending on what the server supports.
964: For an OpenSSH server,
965: it may be zero or more of:
966: .Dq bsdauth ,
967: .Dq pam ,
968: and
969: .Dq skey .
1.140 djm 970: .It Cm KexAlgorithms
971: Specifies the available KEX (Key Exchange) algorithms.
972: Multiple algorithms must be comma-separated.
1.141 jmc 973: The default is:
974: .Bd -literal -offset indent
1.179 markus 975: curve25519-sha256@libssh.org,
1.141 jmc 976: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
977: diffie-hellman-group-exchange-sha256,
1.209 ! dtucker 978: diffie-hellman-group-exchange-sha1,
1.186 naddy 979: diffie-hellman-group14-sha1,
1.141 jmc 980: diffie-hellman-group1-sha1
981: .Ed
1.198 djm 982: .Pp
983: The list of available key exchange algorithms may also be obtained using the
984: .Fl Q
985: option of
986: .Xr ssh 1
987: with an argument of
988: .Dq kex .
1.65 reyk 989: .It Cm LocalCommand
990: Specifies a command to execute on the local machine after successfully
991: connecting to the server.
992: The command string extends to the end of the line, and is executed with
1.105 jmc 993: the user's shell.
1.109 dtucker 994: The following escape character substitutions will be performed:
995: .Ql %d
996: (local user's home directory),
997: .Ql %h
998: (remote host name),
999: .Ql %l
1000: (local host name),
1001: .Ql %n
1002: (host name as provided on the command line),
1003: .Ql %p
1004: (remote port),
1005: .Ql %r
1006: (remote user name) or
1007: .Ql %u
1.188 djm 1008: (local user name) or
1.189 jmc 1009: .Ql \&%C
1.188 djm 1010: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1011: .Pp
1012: The command is run synchronously and does not have access to the
1013: session of the
1014: .Xr ssh 1
1015: that spawned it.
1016: It should not be used for interactive commands.
1017: .Pp
1.65 reyk 1018: This directive is ignored unless
1019: .Cm PermitLocalCommand
1020: has been enabled.
1.1 stevesk 1021: .It Cm LocalForward
1.74 jmc 1022: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1023: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1024: The first argument must be
1.43 djm 1025: .Sm off
1.49 jmc 1026: .Oo Ar bind_address : Oc Ar port
1.43 djm 1027: .Sm on
1.49 jmc 1028: and the second argument must be
1029: .Ar host : Ns Ar hostport .
1.138 djm 1030: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1031: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1032: given on the command line.
1.1 stevesk 1033: Only the superuser can forward privileged ports.
1.43 djm 1034: By default, the local port is bound in accordance with the
1035: .Cm GatewayPorts
1036: setting.
1037: However, an explicit
1038: .Ar bind_address
1039: may be used to bind the connection to a specific address.
1040: The
1041: .Ar bind_address
1042: of
1043: .Dq localhost
1.46 jmc 1044: indicates that the listening port be bound for local use only, while an
1045: empty address or
1046: .Sq *
1.43 djm 1047: indicates that the port should be available from all interfaces.
1.1 stevesk 1048: .It Cm LogLevel
1049: Gives the verbosity level that is used when logging messages from
1.84 jmc 1050: .Xr ssh 1 .
1.1 stevesk 1051: The possible values are:
1.84 jmc 1052: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1053: The default is INFO.
1054: DEBUG and DEBUG1 are equivalent.
1055: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1056: .It Cm MACs
1057: Specifies the MAC (message authentication code) algorithms
1058: in order of preference.
1059: The MAC algorithm is used in protocol version 2
1060: for data integrity protection.
1061: Multiple algorithms must be comma-separated.
1.160 markus 1062: The algorithms that contain
1063: .Dq -etm
1064: calculate the MAC after encryption (encrypt-then-mac).
1065: These are considered safer and their use recommended.
1.84 jmc 1066: The default is:
1.101 jmc 1067: .Bd -literal -offset indent
1.160 markus 1068: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1069: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1070: umac-64@openssh.com,umac-128@openssh.com,
1071: hmac-sha2-256,hmac-sha2-512,
1072: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1073: hmac-ripemd160-etm@openssh.com,
1074: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1075: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1076: hmac-sha1-96,hmac-md5-96
1.101 jmc 1077: .Ed
1.198 djm 1078: .Pp
1079: The list of available MAC algorithms may also be obtained using the
1080: .Fl Q
1081: option of
1082: .Xr ssh 1
1083: with an argument of
1084: .Dq mac .
1.1 stevesk 1085: .It Cm NoHostAuthenticationForLocalhost
1086: This option can be used if the home directory is shared across machines.
1087: In this case localhost will refer to a different machine on each of
1088: the machines and the user will get many warnings about changed host keys.
1089: However, this option disables host authentication for localhost.
1090: The argument to this keyword must be
1091: .Dq yes
1092: or
1093: .Dq no .
1094: The default is to check the host key for localhost.
1095: .It Cm NumberOfPasswordPrompts
1096: Specifies the number of password prompts before giving up.
1097: The argument to this keyword must be an integer.
1.84 jmc 1098: The default is 3.
1.1 stevesk 1099: .It Cm PasswordAuthentication
1100: Specifies whether to use password authentication.
1101: The argument to this keyword must be
1102: .Dq yes
1103: or
1104: .Dq no .
1105: The default is
1106: .Dq yes .
1.65 reyk 1107: .It Cm PermitLocalCommand
1108: Allow local command execution via the
1109: .Ic LocalCommand
1110: option or using the
1.66 jmc 1111: .Ic !\& Ns Ar command
1.65 reyk 1112: escape sequence in
1113: .Xr ssh 1 .
1114: The argument must be
1115: .Dq yes
1116: or
1117: .Dq no .
1118: The default is
1119: .Dq no .
1.127 markus 1120: .It Cm PKCS11Provider
1121: Specifies which PKCS#11 provider to use.
1.144 jmc 1122: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1123: .Xr ssh 1
1.128 markus 1124: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1125: private RSA key.
1.67 jmc 1126: .It Cm Port
1127: Specifies the port number to connect on the remote host.
1.84 jmc 1128: The default is 22.
1.1 stevesk 1129: .It Cm PreferredAuthentications
1130: Specifies the order in which the client should try protocol 2
1.11 jmc 1131: authentication methods.
1.48 jmc 1132: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1133: .Cm keyboard-interactive )
1.48 jmc 1134: over another method (e.g.\&
1.131 jmc 1135: .Cm password ) .
1136: The default is:
1137: .Bd -literal -offset indent
1138: gssapi-with-mic,hostbased,publickey,
1139: keyboard-interactive,password
1140: .Ed
1.1 stevesk 1141: .It Cm Protocol
1142: Specifies the protocol versions
1.84 jmc 1143: .Xr ssh 1
1.1 stevesk 1144: should support in order of preference.
1145: The possible values are
1.84 jmc 1146: .Sq 1
1.1 stevesk 1147: and
1.84 jmc 1148: .Sq 2 .
1.1 stevesk 1149: Multiple versions must be comma-separated.
1.120 markus 1150: When this option is set to
1.121 jmc 1151: .Dq 2,1
1.120 markus 1152: .Nm ssh
1153: will try version 2 and fall back to version 1
1154: if version 2 is not available.
1.1 stevesk 1155: The default is
1.121 jmc 1156: .Sq 2 .
1.1 stevesk 1157: .It Cm ProxyCommand
1158: Specifies the command to use to connect to the server.
1159: The command
1.190 djm 1160: string extends to the end of the line, and is executed
1161: using the user's shell
1162: .Ql exec
1163: directive to avoid a lingering shell process.
1164: .Pp
1.133 jmc 1165: In the command string, any occurrence of
1.1 stevesk 1166: .Ql %h
1167: will be substituted by the host name to
1.132 djm 1168: connect,
1.1 stevesk 1169: .Ql %p
1.133 jmc 1170: by the port, and
1171: .Ql %r
1.132 djm 1172: by the remote user name.
1.1 stevesk 1173: The command can be basically anything,
1174: and should read from its standard input and write to its standard output.
1175: It should eventually connect an
1176: .Xr sshd 8
1177: server running on some machine, or execute
1178: .Ic sshd -i
1179: somewhere.
1180: Host key management will be done using the
1181: HostName of the host being connected (defaulting to the name typed by
1182: the user).
1.7 jmc 1183: Setting the command to
1184: .Dq none
1.6 markus 1185: disables this option entirely.
1.1 stevesk 1186: Note that
1187: .Cm CheckHostIP
1188: is not available for connects with a proxy command.
1.52 djm 1189: .Pp
1190: This directive is useful in conjunction with
1191: .Xr nc 1
1192: and its proxy support.
1.53 jmc 1193: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1194: 192.0.2.0:
1195: .Bd -literal -offset 3n
1196: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1197: .Ed
1.167 djm 1198: .It Cm ProxyUseFdpass
1.168 jmc 1199: Specifies that
1.167 djm 1200: .Cm ProxyCommand
1201: will pass a connected file descriptor back to
1.168 jmc 1202: .Xr ssh 1
1.167 djm 1203: instead of continuing to execute and pass data.
1204: The default is
1205: .Dq no .
1.1 stevesk 1206: .It Cm PubkeyAuthentication
1207: Specifies whether to try public key authentication.
1208: The argument to this keyword must be
1209: .Dq yes
1210: or
1211: .Dq no .
1212: The default is
1213: .Dq yes .
1214: This option applies to protocol version 2 only.
1.75 dtucker 1215: .It Cm RekeyLimit
1216: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1217: session key is renegotiated, optionally followed a maximum amount of
1218: time that may pass before the session key is renegotiated.
1219: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1220: .Sq K ,
1221: .Sq M ,
1.75 dtucker 1222: or
1.76 jmc 1223: .Sq G
1.75 dtucker 1224: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1225: The default is between
1.84 jmc 1226: .Sq 1G
1.75 dtucker 1227: and
1.84 jmc 1228: .Sq 4G ,
1.75 dtucker 1229: depending on the cipher.
1.162 dtucker 1230: The optional second value is specified in seconds and may use any of the
1231: units documented in the
1.166 jmc 1232: TIME FORMATS section of
1.162 dtucker 1233: .Xr sshd_config 5 .
1234: The default value for
1235: .Cm RekeyLimit
1236: is
1237: .Dq default none ,
1238: which means that rekeying is performed after the cipher's default amount
1239: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1240: This option applies to protocol version 2 only.
1.1 stevesk 1241: .It Cm RemoteForward
1.74 jmc 1242: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1243: the secure channel to the specified host and port from the local machine.
1.49 jmc 1244: The first argument must be
1.43 djm 1245: .Sm off
1.49 jmc 1246: .Oo Ar bind_address : Oc Ar port
1.43 djm 1247: .Sm on
1.49 jmc 1248: and the second argument must be
1249: .Ar host : Ns Ar hostport .
1.138 djm 1250: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1251: Multiple forwardings may be specified, and additional
1252: forwardings can be given on the command line.
1.113 stevesk 1253: Privileged ports can be forwarded only when
1254: logging in as root on the remote machine.
1.118 jmc 1255: .Pp
1.117 djm 1256: If the
1257: .Ar port
1258: argument is
1259: .Ql 0 ,
1260: the listen port will be dynamically allocated on the server and reported
1261: to the client at run time.
1.43 djm 1262: .Pp
1263: If the
1264: .Ar bind_address
1265: is not specified, the default is to only bind to loopback addresses.
1266: If the
1267: .Ar bind_address
1268: is
1269: .Ql *
1270: or an empty string, then the forwarding is requested to listen on all
1271: interfaces.
1272: Specifying a remote
1273: .Ar bind_address
1.46 jmc 1274: will only succeed if the server's
1275: .Cm GatewayPorts
1.43 djm 1276: option is enabled (see
1.46 jmc 1277: .Xr sshd_config 5 ) .
1.149 djm 1278: .It Cm RequestTTY
1279: Specifies whether to request a pseudo-tty for the session.
1280: The argument may be one of:
1281: .Dq no
1282: (never request a TTY),
1283: .Dq yes
1284: (always request a TTY when standard input is a TTY),
1285: .Dq force
1286: (always request a TTY) or
1287: .Dq auto
1288: (request a TTY when opening a login session).
1289: This option mirrors the
1290: .Fl t
1291: and
1292: .Fl T
1293: flags for
1294: .Xr ssh 1 .
1.196 djm 1295: .It Cm RevokedHostKeys
1296: Specifies revoked host public keys.
1297: Keys listed in this file will be refused for host authentication.
1298: Note that if this file does not exist or is not readable,
1299: then host authentication will be refused for all hosts.
1300: Keys may be specified as a text file, listing one public key per line, or as
1301: an OpenSSH Key Revocation List (KRL) as generated by
1302: .Xr ssh-keygen 1 .
1303: For more information on KRLs, see the KEY REVOCATION LISTS section in
1304: .Xr ssh-keygen 1 .
1.1 stevesk 1305: .It Cm RhostsRSAAuthentication
1306: Specifies whether to try rhosts based authentication with RSA host
1307: authentication.
1308: The argument must be
1309: .Dq yes
1310: or
1311: .Dq no .
1312: The default is
1313: .Dq no .
1314: This option applies to protocol version 1 only and requires
1.84 jmc 1315: .Xr ssh 1
1.1 stevesk 1316: to be setuid root.
1317: .It Cm RSAAuthentication
1318: Specifies whether to try RSA authentication.
1319: The argument to this keyword must be
1320: .Dq yes
1321: or
1322: .Dq no .
1323: RSA authentication will only be
1324: attempted if the identity file exists, or an authentication agent is
1325: running.
1326: The default is
1327: .Dq yes .
1328: Note that this option applies to protocol version 1 only.
1.32 djm 1329: .It Cm SendEnv
1330: Specifies what variables from the local
1331: .Xr environ 7
1332: should be sent to the server.
1.84 jmc 1333: Note that environment passing is only supported for protocol 2.
1334: The server must also support it, and the server must be configured to
1.33 djm 1335: accept these environment variables.
1.207 dtucker 1336: Note that the
1337: .Ev TERM
1.208 jmc 1338: environment variable is always sent whenever a
1.207 dtucker 1339: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1340: Refer to
1341: .Cm AcceptEnv
1342: in
1343: .Xr sshd_config 5
1344: for how to configure the server.
1.80 jmc 1345: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1346: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1347: across multiple
1348: .Cm SendEnv
1349: directives.
1350: The default is not to send any environment variables.
1.81 jmc 1351: .Pp
1352: See
1353: .Sx PATTERNS
1354: for more information on patterns.
1.28 markus 1355: .It Cm ServerAliveCountMax
1.73 jmc 1356: Sets the number of server alive messages (see below) which may be
1.28 markus 1357: sent without
1.84 jmc 1358: .Xr ssh 1
1.28 markus 1359: receiving any messages back from the server.
1360: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1361: ssh will disconnect from the server, terminating the session.
1.28 markus 1362: It is important to note that the use of server alive messages is very
1363: different from
1364: .Cm TCPKeepAlive
1365: (below).
1366: The server alive messages are sent through the encrypted channel
1367: and therefore will not be spoofable.
1368: The TCP keepalive option enabled by
1369: .Cm TCPKeepAlive
1370: is spoofable.
1371: The server alive mechanism is valuable when the client or
1372: server depend on knowing when a connection has become inactive.
1373: .Pp
1374: The default value is 3.
1375: If, for example,
1376: .Cm ServerAliveInterval
1.84 jmc 1377: (see below) is set to 15 and
1.28 markus 1378: .Cm ServerAliveCountMax
1.84 jmc 1379: is left at the default, if the server becomes unresponsive,
1380: ssh will disconnect after approximately 45 seconds.
1.89 markus 1381: This option applies to protocol version 2 only.
1.67 jmc 1382: .It Cm ServerAliveInterval
1383: Sets a timeout interval in seconds after which if no data has been received
1384: from the server,
1.84 jmc 1385: .Xr ssh 1
1.67 jmc 1386: will send a message through the encrypted
1387: channel to request a response from the server.
1388: The default
1389: is 0, indicating that these messages will not be sent to the server.
1390: This option applies to protocol version 2 only.
1.191 millert 1391: .It Cm StreamLocalBindMask
1392: Sets the octal file creation mode mask
1393: .Pq umask
1394: used when creating a Unix-domain socket file for local or remote
1395: port forwarding.
1396: This option is only used for port forwarding to a Unix-domain socket file.
1397: .Pp
1398: The default value is 0177, which creates a Unix-domain socket file that is
1399: readable and writable only by the owner.
1400: Note that not all operating systems honor the file mode on Unix-domain
1401: socket files.
1402: .It Cm StreamLocalBindUnlink
1403: Specifies whether to remove an existing Unix-domain socket file for local
1404: or remote port forwarding before creating a new one.
1405: If the socket file already exists and
1406: .Cm StreamLocalBindUnlink
1407: is not enabled,
1408: .Nm ssh
1409: will be unable to forward the port to the Unix-domain socket file.
1410: This option is only used for port forwarding to a Unix-domain socket file.
1411: .Pp
1412: The argument must be
1413: .Dq yes
1414: or
1415: .Dq no .
1416: The default is
1417: .Dq no .
1.1 stevesk 1418: .It Cm StrictHostKeyChecking
1419: If this flag is set to
1420: .Dq yes ,
1.84 jmc 1421: .Xr ssh 1
1.1 stevesk 1422: will never automatically add host keys to the
1.50 djm 1423: .Pa ~/.ssh/known_hosts
1.1 stevesk 1424: file, and refuses to connect to hosts whose host key has changed.
1425: This provides maximum protection against trojan horse attacks,
1.84 jmc 1426: though it can be annoying when the
1.1 stevesk 1427: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1428: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1429: frequently made.
1430: This option forces the user to manually
1431: add all new hosts.
1432: If this flag is set to
1433: .Dq no ,
1.84 jmc 1434: ssh will automatically add new host keys to the
1.1 stevesk 1435: user known hosts files.
1436: If this flag is set to
1437: .Dq ask ,
1438: new host keys
1439: will be added to the user known host files only after the user
1440: has confirmed that is what they really want to do, and
1.84 jmc 1441: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1442: The host keys of
1443: known hosts will be verified automatically in all cases.
1444: The argument must be
1445: .Dq yes ,
1.84 jmc 1446: .Dq no ,
1.1 stevesk 1447: or
1448: .Dq ask .
1449: The default is
1450: .Dq ask .
1.26 markus 1451: .It Cm TCPKeepAlive
1452: Specifies whether the system should send TCP keepalive messages to the
1453: other side.
1454: If they are sent, death of the connection or crash of one
1455: of the machines will be properly noticed.
1456: However, this means that
1457: connections will die if the route is down temporarily, and some people
1458: find it annoying.
1459: .Pp
1460: The default is
1461: .Dq yes
1462: (to send TCP keepalive messages), and the client will notice
1463: if the network goes down or the remote host dies.
1464: This is important in scripts, and many users want it too.
1465: .Pp
1466: To disable TCP keepalive messages, the value should be set to
1467: .Dq no .
1.65 reyk 1468: .It Cm Tunnel
1.95 stevesk 1469: Request
1.65 reyk 1470: .Xr tun 4
1.69 jmc 1471: device forwarding between the client and the server.
1.65 reyk 1472: The argument must be
1.68 reyk 1473: .Dq yes ,
1.95 stevesk 1474: .Dq point-to-point
1475: (layer 3),
1476: .Dq ethernet
1477: (layer 2),
1.65 reyk 1478: or
1479: .Dq no .
1.95 stevesk 1480: Specifying
1481: .Dq yes
1482: requests the default tunnel mode, which is
1483: .Dq point-to-point .
1.65 reyk 1484: The default is
1485: .Dq no .
1486: .It Cm TunnelDevice
1.95 stevesk 1487: Specifies the
1.65 reyk 1488: .Xr tun 4
1.95 stevesk 1489: devices to open on the client
1490: .Pq Ar local_tun
1491: and the server
1492: .Pq Ar remote_tun .
1493: .Pp
1494: The argument must be
1495: .Sm off
1496: .Ar local_tun Op : Ar remote_tun .
1497: .Sm on
1498: The devices may be specified by numerical ID or the keyword
1499: .Dq any ,
1500: which uses the next available tunnel device.
1501: If
1502: .Ar remote_tun
1503: is not specified, it defaults to
1504: .Dq any .
1505: The default is
1506: .Dq any:any .
1.201 djm 1507: .It Cm UpdateHostKeys
1.200 djm 1508: Specifies whether
1509: .Xr ssh 1
1510: should accept notifications of additional hostkeys from the server sent
1511: after authentication has completed and add them to
1512: .Cm UserKnownHostsFile .
1513: The argument must be
1.204 djm 1514: .Dq yes ,
1.203 djm 1515: .Dq no
1.204 djm 1516: (the default) or
1517: .Dq ask .
1.200 djm 1518: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1519: and supports graceful key rotation by allowing a server to send replacement
1520: public keys before old ones are removed.
1.200 djm 1521: Additional hostkeys are only accepted if the key used to authenticate the
1522: host was already trusted or explicity accepted by the user.
1.204 djm 1523: If
1524: .Cm UpdateHostKeys
1525: is set to
1526: .Dq ask ,
1527: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1528: Confirmation is currently incompatible with
1529: .Cm ControlPersist ,
1530: and will be disabled if it is enabled.
1.200 djm 1531: .Pp
1532: Presently, only
1533: .Xr sshd 8
1534: from OpenSSH 6.8 and greater support the
1535: .Dq hostkeys@openssh.com
1536: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1537: .It Cm UsePrivilegedPort
1538: Specifies whether to use a privileged port for outgoing connections.
1539: The argument must be
1540: .Dq yes
1541: or
1542: .Dq no .
1543: The default is
1544: .Dq no .
1545: If set to
1.84 jmc 1546: .Dq yes ,
1547: .Xr ssh 1
1.72 jmc 1548: must be setuid root.
1549: Note that this option must be set to
1550: .Dq yes
1551: for
1552: .Cm RhostsRSAAuthentication
1553: with older servers.
1.1 stevesk 1554: .It Cm User
1555: Specifies the user to log in as.
1556: This can be useful when a different user name is used on different machines.
1557: This saves the trouble of
1558: having to remember to give the user name on the command line.
1559: .It Cm UserKnownHostsFile
1.151 djm 1560: Specifies one or more files to use for the user
1561: host key database, separated by whitespace.
1562: The default is
1563: .Pa ~/.ssh/known_hosts ,
1564: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1565: .It Cm VerifyHostKeyDNS
1566: Specifies whether to verify the remote key using DNS and SSHFP resource
1567: records.
1.24 jakob 1568: If this option is set to
1569: .Dq yes ,
1.25 jmc 1570: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1571: from DNS.
1572: Insecure fingerprints will be handled as if this option was set to
1573: .Dq ask .
1574: If this option is set to
1575: .Dq ask ,
1576: information on fingerprint match will be displayed, but the user will still
1577: need to confirm new host keys according to the
1578: .Cm StrictHostKeyChecking
1579: option.
1580: The argument must be
1581: .Dq yes ,
1.84 jmc 1582: .Dq no ,
1.25 jmc 1583: or
1584: .Dq ask .
1.8 jakob 1585: The default is
1586: .Dq no .
1.12 jakob 1587: Note that this option applies to protocol version 2 only.
1.84 jmc 1588: .Pp
1.166 jmc 1589: See also VERIFYING HOST KEYS in
1.84 jmc 1590: .Xr ssh 1 .
1.111 grunk 1591: .It Cm VisualHostKey
1592: If this flag is set to
1593: .Dq yes ,
1594: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1595: printed in addition to the fingerprint string at login and
1.114 stevesk 1596: for unknown host keys.
1.111 grunk 1597: If this flag is set to
1598: .Dq no ,
1.114 stevesk 1599: no fingerprint strings are printed at login and
1.197 djm 1600: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1601: The default is
1602: .Dq no .
1.1 stevesk 1603: .It Cm XAuthLocation
1.5 stevesk 1604: Specifies the full pathname of the
1.1 stevesk 1605: .Xr xauth 1
1606: program.
1607: The default is
1608: .Pa /usr/X11R6/bin/xauth .
1609: .El
1.86 jmc 1610: .Sh PATTERNS
1611: A
1612: .Em pattern
1613: consists of zero or more non-whitespace characters,
1614: .Sq *
1615: (a wildcard that matches zero or more characters),
1616: or
1617: .Sq ?\&
1618: (a wildcard that matches exactly one character).
1619: For example, to specify a set of declarations for any host in the
1620: .Dq .co.uk
1621: set of domains,
1622: the following pattern could be used:
1623: .Pp
1624: .Dl Host *.co.uk
1625: .Pp
1626: The following pattern
1627: would match any host in the 192.168.0.[0-9] network range:
1628: .Pp
1629: .Dl Host 192.168.0.?
1630: .Pp
1631: A
1632: .Em pattern-list
1633: is a comma-separated list of patterns.
1634: Patterns within pattern-lists may be negated
1635: by preceding them with an exclamation mark
1636: .Pq Sq !\& .
1637: For example,
1.174 djm 1638: to allow a key to be used from anywhere within an organization
1.86 jmc 1639: except from the
1640: .Dq dialup
1641: pool,
1642: the following entry (in authorized_keys) could be used:
1643: .Pp
1644: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1645: .Sh FILES
1646: .Bl -tag -width Ds
1.50 djm 1647: .It Pa ~/.ssh/config
1.1 stevesk 1648: This is the per-user configuration file.
1649: The format of this file is described above.
1.84 jmc 1650: This file is used by the SSH client.
1.30 djm 1651: Because of the potential for abuse, this file must have strict permissions:
1652: read/write for the user, and not accessible by others.
1.1 stevesk 1653: .It Pa /etc/ssh/ssh_config
1654: Systemwide configuration file.
1655: This file provides defaults for those
1656: values that are not specified in the user's configuration file, and
1657: for those users who do not have a configuration file.
1658: This file must be world-readable.
1659: .El
1.13 jmc 1660: .Sh SEE ALSO
1661: .Xr ssh 1
1.1 stevesk 1662: .Sh AUTHORS
1663: OpenSSH is a derivative of the original and free
1664: ssh 1.2.12 release by Tatu Ylonen.
1665: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1666: Theo de Raadt and Dug Song
1667: removed many bugs, re-added newer features and
1668: created OpenSSH.
1669: Markus Friedl contributed the support for SSH
1670: protocol versions 1.5 and 2.0.
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