Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.226
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
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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1.226 ! jmc 36: .\" $OpenBSD: ssh_config.5,v 1.225 2016/02/16 05:11:04 djm Exp $
! 37: .Dd $Mdocdate: February 16 2016 $
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.
1.220 sobrado 142: Match conditions are specified using one or more criteria
1.178 dtucker 143: or the single token
144: .Cm all
1.193 djm 145: which always matches.
146: The available criteria keywords are:
147: .Cm canonical ,
1.176 djm 148: .Cm exec ,
1.169 djm 149: .Cm host ,
150: .Cm originalhost ,
151: .Cm user ,
152: and
153: .Cm localuser .
1.193 djm 154: The
155: .Cm all
156: criteria must appear alone or immediately after
1.194 jmc 157: .Cm canonical .
1.193 djm 158: Other criteria may be combined arbitrarily.
159: All criteria but
160: .Cm all
161: and
162: .Cm canonical
163: require an argument.
164: Criteria may be negated by prepending an exclamation mark
165: .Pq Sq !\& .
1.169 djm 166: .Pp
1.177 jmc 167: The
1.193 djm 168: .Cm canonical
1.210 dtucker 169: keyword matches only when the configuration file is being re-parsed
1.193 djm 170: after hostname canonicalization (see the
171: .Cm CanonicalizeHostname
172: option.)
173: This may be useful to specify conditions that work with canonical host
174: names only.
175: The
1.176 djm 176: .Cm exec
1.177 jmc 177: keyword executes the specified command under the user's shell.
1.169 djm 178: If the command returns a zero exit status then the condition is considered true.
179: Commands containing whitespace characters must be quoted.
1.175 djm 180: The following character sequences in the command will be expanded prior to
181: execution:
182: .Ql %L
183: will be substituted by the first component of the local host name,
184: .Ql %l
185: will be substituted by the local host name (including any domain name),
186: .Ql %h
187: will be substituted by the target host name,
188: .Ql %n
189: will be substituted by the original target host name
1.176 djm 190: specified on the command-line,
1.175 djm 191: .Ql %p
192: the destination port,
193: .Ql %r
194: by the remote login username, and
195: .Ql %u
196: by the username of the user running
197: .Xr ssh 1 .
1.169 djm 198: .Pp
199: The other keywords' criteria must be single entries or comma-separated
200: lists and may use the wildcard and negation operators described in the
201: .Sx PATTERNS
202: section.
203: The criteria for the
204: .Cm host
205: keyword are matched against the target hostname, after any substitution
206: by the
207: .Cm Hostname
1.193 djm 208: or
209: .Cm CanonicalizeHostname
210: options.
1.169 djm 211: The
212: .Cm originalhost
213: keyword matches against the hostname as it was specified on the command-line.
214: The
215: .Cm user
216: keyword matches against the target username on the remote host.
217: The
218: .Cm localuser
219: keyword matches against the name of the local user running
220: .Xr ssh 1
221: (this keyword may be useful in system-wide
222: .Nm
223: files).
1.222 jcs 224: .It Cm AddKeysToAgent
225: Specifies whether keys should be automatically added to a running
1.223 jmc 226: .Xr ssh-agent 1 .
1.222 jcs 227: If this option is set to
228: .Dq yes
229: and a key is loaded from a file, the key and its passphrase are added to
230: the agent with the default lifetime, as if by
231: .Xr ssh-add 1 .
232: If this option is set to
233: .Dq ask ,
234: .Nm ssh
235: will require confirmation using the
236: .Ev SSH_ASKPASS
237: program before adding a key (see
238: .Xr ssh-add 1
239: for details).
240: If this option is set to
241: .Dq confirm ,
242: each use of the key must be confirmed, as if the
243: .Fl c
244: option was specified to
245: .Xr ssh-add 1 .
246: If this option is set to
247: .Dq no ,
248: no keys are added to the agent.
249: The argument must be
250: .Dq yes ,
251: .Dq confirm ,
252: .Dq ask ,
253: or
254: .Dq no .
255: The default is
256: .Dq no .
1.10 djm 257: .It Cm AddressFamily
1.11 jmc 258: Specifies which address family to use when connecting.
259: Valid arguments are
1.10 djm 260: .Dq any ,
261: .Dq inet
1.84 jmc 262: (use IPv4 only), or
1.10 djm 263: .Dq inet6
1.40 jmc 264: (use IPv6 only).
1.1 stevesk 265: .It Cm BatchMode
266: If set to
267: .Dq yes ,
268: passphrase/password querying will be disabled.
269: This option is useful in scripts and other batch jobs where no user
270: is present to supply the password.
271: The argument must be
272: .Dq yes
273: or
274: .Dq no .
275: The default is
276: .Dq no .
277: .It Cm BindAddress
1.60 dtucker 278: Use the specified address on the local machine as the source address of
1.61 jmc 279: the connection.
280: Only useful on systems with more than one address.
1.1 stevesk 281: Note that this option does not work if
282: .Cm UsePrivilegedPort
283: is set to
284: .Dq yes .
1.171 djm 285: .It Cm CanonicalDomains
1.172 jmc 286: When
1.173 djm 287: .Cm CanonicalizeHostname
1.171 djm 288: is enabled, this option specifies the list of domain suffixes in which to
289: search for the specified destination host.
1.173 djm 290: .It Cm CanonicalizeFallbackLocal
1.174 djm 291: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 292: The default,
1.184 djm 293: .Dq yes ,
1.172 jmc 294: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 295: search rules.
296: A value of
1.184 djm 297: .Dq no
1.171 djm 298: will cause
299: .Xr ssh 1
300: to fail instantly if
1.173 djm 301: .Cm CanonicalizeHostname
1.171 djm 302: is enabled and the target hostname cannot be found in any of the domains
303: specified by
304: .Cm CanonicalDomains .
1.173 djm 305: .It Cm CanonicalizeHostname
1.174 djm 306: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 307: The default,
308: .Dq no ,
1.171 djm 309: is not to perform any name rewriting and let the system resolver handle all
310: hostname lookups.
311: If set to
312: .Dq yes
313: then, for connections that do not use a
314: .Cm ProxyCommand ,
315: .Xr ssh 1
1.173 djm 316: will attempt to canonicalize the hostname specified on the command line
1.171 djm 317: using the
318: .Cm CanonicalDomains
319: suffixes and
1.173 djm 320: .Cm CanonicalizePermittedCNAMEs
1.171 djm 321: rules.
322: If
1.173 djm 323: .Cm CanonicalizeHostname
1.171 djm 324: is set to
325: .Dq always ,
1.174 djm 326: then canonicalization is applied to proxied connections too.
1.185 djm 327: .Pp
1.193 djm 328: If this option is enabled, then the configuration files are processed
329: again using the new target name to pick up any new configuration in matching
1.185 djm 330: .Cm Host
1.193 djm 331: and
332: .Cm Match
1.185 djm 333: stanzas.
1.173 djm 334: .It Cm CanonicalizeMaxDots
1.172 jmc 335: Specifies the maximum number of dot characters in a hostname before
1.174 djm 336: canonicalization is disabled.
1.172 jmc 337: The default,
338: .Dq 1 ,
339: allows a single dot (i.e. hostname.subdomain).
1.173 djm 340: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 341: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 342: canonicalizing hostnames.
1.171 djm 343: The rules consist of one or more arguments of
1.172 jmc 344: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 345: where
346: .Ar source_domain_list
1.174 djm 347: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 348: and
349: .Ar target_domain_list
1.172 jmc 350: is a pattern-list of domains that they may resolve to.
1.171 djm 351: .Pp
352: For example,
353: .Dq *.a.example.com:*.b.example.com,*.c.example.com
354: will allow hostnames matching
355: .Dq *.a.example.com
1.173 djm 356: to be canonicalized to names in the
1.171 djm 357: .Dq *.b.example.com
358: or
359: .Dq *.c.example.com
360: domains.
1.221 djm 361: .It Cm CertificateFile
362: Specifies a file from which the user's certificate is read.
363: A corresponding private key must be provided separately in order
364: to use this certificate either
365: from an
366: .Cm IdentityFile
367: directive or
368: .Fl i
369: flag to
370: .Xr ssh 1 ,
371: via
372: .Xr ssh-agent 1 ,
373: or via a
374: .Cm PKCS11Provider .
375: .Pp
376: The file name may use the tilde
377: syntax to refer to a user's home directory or one of the following
378: escape characters:
379: .Ql %d
380: (local user's home directory),
381: .Ql %u
382: (local user name),
383: .Ql %l
384: (local host name),
385: .Ql %h
386: (remote host name) or
387: .Ql %r
388: (remote user name).
389: .Pp
390: It is possible to have multiple certificate files specified in
391: configuration files; these certificates will be tried in sequence.
392: Multiple
393: .Cm CertificateFile
394: directives will add to the list of certificates used for
395: authentication.
1.1 stevesk 396: .It Cm ChallengeResponseAuthentication
1.82 jmc 397: Specifies whether to use challenge-response authentication.
1.1 stevesk 398: The argument to this keyword must be
399: .Dq yes
400: or
401: .Dq no .
402: The default is
403: .Dq yes .
404: .It Cm CheckHostIP
405: If this flag is set to
406: .Dq yes ,
1.84 jmc 407: .Xr ssh 1
408: will additionally check the host IP address in the
1.1 stevesk 409: .Pa known_hosts
410: file.
1.211 djm 411: This allows ssh to detect if a host key changed due to DNS spoofing
412: and will add addresses of destination hosts to
413: .Pa ~/.ssh/known_hosts
414: in the process, regardless of the setting of
415: .Cm StrictHostKeyChecking .
1.107 grunk 416: If the option is set to
1.1 stevesk 417: .Dq no ,
418: the check will not be executed.
419: The default is
420: .Dq yes .
421: .It Cm Cipher
422: Specifies the cipher to use for encrypting the session
423: in protocol version 1.
424: Currently,
425: .Dq blowfish ,
426: .Dq 3des ,
427: and
428: .Dq des
429: are supported.
430: .Ar des
431: is only supported in the
1.84 jmc 432: .Xr ssh 1
1.1 stevesk 433: client for interoperability with legacy protocol 1 implementations
434: that do not support the
435: .Ar 3des
1.7 jmc 436: cipher.
437: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 438: The default is
439: .Dq 3des .
440: .It Cm Ciphers
441: Specifies the ciphers allowed for protocol version 2
442: in order of preference.
443: Multiple ciphers must be comma-separated.
1.214 djm 444: If the specified value begins with a
445: .Sq +
446: character, then the specified ciphers will be appended to the default set
447: instead of replacing them.
448: .Pp
1.180 djm 449: The supported ciphers are:
450: .Pp
1.186 naddy 451: .Bl -item -compact -offset indent
452: .It
453: 3des-cbc
454: .It
455: aes128-cbc
456: .It
457: aes192-cbc
458: .It
459: aes256-cbc
460: .It
461: aes128-ctr
462: .It
463: aes192-ctr
464: .It
465: aes256-ctr
466: .It
467: aes128-gcm@openssh.com
468: .It
469: aes256-gcm@openssh.com
470: .It
471: arcfour
472: .It
473: arcfour128
474: .It
475: arcfour256
476: .It
477: blowfish-cbc
478: .It
479: cast128-cbc
480: .It
481: chacha20-poly1305@openssh.com
482: .El
1.180 djm 483: .Pp
1.84 jmc 484: The default is:
1.186 naddy 485: .Bd -literal -offset indent
1.215 jmc 486: chacha20-poly1305@openssh.com,
1.186 naddy 487: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 488: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.224 djm 489: aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc
1.1 stevesk 490: .Ed
1.180 djm 491: .Pp
492: The list of available ciphers may also be obtained using the
493: .Fl Q
494: option of
1.198 djm 495: .Xr ssh 1
496: with an argument of
497: .Dq cipher .
1.1 stevesk 498: .It Cm ClearAllForwardings
1.84 jmc 499: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 500: specified in the configuration files or on the command line be
1.7 jmc 501: cleared.
502: This option is primarily useful when used from the
1.84 jmc 503: .Xr ssh 1
1.1 stevesk 504: command line to clear port forwardings set in
505: configuration files, and is automatically set by
506: .Xr scp 1
507: and
508: .Xr sftp 1 .
509: The argument must be
510: .Dq yes
511: or
512: .Dq no .
513: The default is
514: .Dq no .
515: .It Cm Compression
516: Specifies whether to use compression.
517: The argument must be
518: .Dq yes
519: or
520: .Dq no .
521: The default is
522: .Dq no .
523: .It Cm CompressionLevel
524: Specifies the compression level to use if compression is enabled.
525: The argument must be an integer from 1 (fast) to 9 (slow, best).
526: The default level is 6, which is good for most applications.
527: The meaning of the values is the same as in
528: .Xr gzip 1 .
529: Note that this option applies to protocol version 1 only.
530: .It Cm ConnectionAttempts
531: Specifies the number of tries (one per second) to make before exiting.
532: The argument must be an integer.
533: This may be useful in scripts if the connection sometimes fails.
534: The default is 1.
1.9 djm 535: .It Cm ConnectTimeout
1.84 jmc 536: Specifies the timeout (in seconds) used when connecting to the
537: SSH server, instead of using the default system TCP timeout.
1.11 jmc 538: This value is used only when the target is down or really unreachable,
539: not when it refuses the connection.
1.36 djm 540: .It Cm ControlMaster
541: Enables the sharing of multiple sessions over a single network connection.
542: When set to
1.84 jmc 543: .Dq yes ,
544: .Xr ssh 1
1.36 djm 545: will listen for connections on a control socket specified using the
546: .Cm ControlPath
547: argument.
548: Additional sessions can connect to this socket using the same
549: .Cm ControlPath
550: with
551: .Cm ControlMaster
552: set to
553: .Dq no
1.38 jmc 554: (the default).
1.64 jmc 555: These sessions will try to reuse the master instance's network connection
1.63 djm 556: rather than initiating new ones, but will fall back to connecting normally
557: if the control socket does not exist, or is not listening.
558: .Pp
1.37 djm 559: Setting this to
560: .Dq ask
1.84 jmc 561: will cause ssh
1.206 jmc 562: to listen for control connections, but require confirmation using
563: .Xr ssh-askpass 1 .
1.51 jakob 564: If the
565: .Cm ControlPath
1.84 jmc 566: cannot be opened,
567: ssh will continue without connecting to a master instance.
1.58 djm 568: .Pp
569: X11 and
1.59 jmc 570: .Xr ssh-agent 1
1.58 djm 571: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 572: display and agent forwarded will be the one belonging to the master
1.59 jmc 573: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 574: .Pp
575: Two additional options allow for opportunistic multiplexing: try to use a
576: master connection but fall back to creating a new one if one does not already
577: exist.
578: These options are:
579: .Dq auto
580: and
581: .Dq autoask .
582: The latter requires confirmation like the
583: .Dq ask
584: option.
1.36 djm 585: .It Cm ControlPath
1.55 djm 586: Specify the path to the control socket used for connection sharing as described
587: in the
1.36 djm 588: .Cm ControlMaster
1.57 djm 589: section above or the string
590: .Dq none
591: to disable connection sharing.
1.55 djm 592: In the path,
1.147 djm 593: .Ql %L
594: will be substituted by the first component of the local host name,
1.77 djm 595: .Ql %l
1.147 djm 596: will be substituted by the local host name (including any domain name),
1.55 djm 597: .Ql %h
598: will be substituted by the target host name,
1.150 jmc 599: .Ql %n
600: will be substituted by the original target host name
601: specified on the command line,
1.55 djm 602: .Ql %p
1.175 djm 603: the destination port,
1.55 djm 604: .Ql %r
1.188 djm 605: by the remote login username,
1.147 djm 606: .Ql %u
1.218 djm 607: by the username and
608: .Ql %i
1.219 jmc 609: by the numeric user ID (uid) of the user running
610: .Xr ssh 1 ,
611: and
1.189 jmc 612: .Ql \&%C
1.188 djm 613: by a hash of the concatenation: %l%h%p%r.
1.56 djm 614: It is recommended that any
615: .Cm ControlPath
616: used for opportunistic connection sharing include
1.195 djm 617: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
618: that is not writable by other users.
1.56 djm 619: This ensures that shared connections are uniquely identified.
1.137 djm 620: .It Cm ControlPersist
621: When used in conjunction with
622: .Cm ControlMaster ,
623: specifies that the master connection should remain open
624: in the background (waiting for future client connections)
625: after the initial client connection has been closed.
626: If set to
627: .Dq no ,
628: then the master connection will not be placed into the background,
629: and will close as soon as the initial client connection is closed.
630: If set to
1.195 djm 631: .Dq yes
632: or
633: .Dq 0 ,
1.137 djm 634: then the master connection will remain in the background indefinitely
635: (until killed or closed via a mechanism such as the
636: .Xr ssh 1
637: .Dq Fl O No exit
638: option).
639: If set to a time in seconds, or a time in any of the formats documented in
640: .Xr sshd_config 5 ,
641: then the backgrounded master connection will automatically terminate
642: after it has remained idle (with no client connections) for the
643: specified time.
1.38 jmc 644: .It Cm DynamicForward
1.74 jmc 645: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 646: over the secure channel, and the application
647: protocol is then used to determine where to connect to from the
648: remote machine.
1.62 djm 649: .Pp
650: The argument must be
651: .Sm off
652: .Oo Ar bind_address : Oc Ar port .
653: .Sm on
1.138 djm 654: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 655: By default, the local port is bound in accordance with the
656: .Cm GatewayPorts
657: setting.
658: However, an explicit
659: .Ar bind_address
660: may be used to bind the connection to a specific address.
661: The
662: .Ar bind_address
663: of
664: .Dq localhost
665: indicates that the listening port be bound for local use only, while an
666: empty address or
667: .Sq *
668: indicates that the port should be available from all interfaces.
669: .Pp
1.38 jmc 670: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 671: .Xr ssh 1
1.38 jmc 672: will act as a SOCKS server.
673: Multiple forwardings may be specified, and
674: additional forwardings can be given on the command line.
675: Only the superuser can forward privileged ports.
1.14 markus 676: .It Cm EnableSSHKeysign
677: Setting this option to
678: .Dq yes
679: in the global client configuration file
680: .Pa /etc/ssh/ssh_config
681: enables the use of the helper program
682: .Xr ssh-keysign 8
683: during
684: .Cm HostbasedAuthentication .
685: The argument must be
686: .Dq yes
687: or
688: .Dq no .
689: The default is
690: .Dq no .
1.23 jmc 691: This option should be placed in the non-hostspecific section.
1.14 markus 692: See
693: .Xr ssh-keysign 8
694: for more information.
1.1 stevesk 695: .It Cm EscapeChar
696: Sets the escape character (default:
697: .Ql ~ ) .
698: The escape character can also
699: be set on the command line.
700: The argument should be a single character,
701: .Ql ^
702: followed by a letter, or
703: .Dq none
704: to disable the escape
705: character entirely (making the connection transparent for binary
706: data).
1.96 markus 707: .It Cm ExitOnForwardFailure
708: Specifies whether
709: .Xr ssh 1
710: should terminate the connection if it cannot set up all requested
1.216 djm 711: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 712: if either end is unable to bind and listen on a specified port).
1.216 djm 713: Note that
714: .Cm ExitOnForwardFailure
715: does not apply to connections made over port forwardings and will not,
716: for example, cause
717: .Xr ssh 1
718: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 719: The argument must be
720: .Dq yes
721: or
722: .Dq no .
723: The default is
724: .Dq no .
1.197 djm 725: .It Cm FingerprintHash
726: Specifies the hash algorithm used when displaying key fingerprints.
727: Valid options are:
728: .Dq md5
729: and
730: .Dq sha256 .
731: The default is
732: .Dq sha256 .
1.1 stevesk 733: .It Cm ForwardAgent
734: Specifies whether the connection to the authentication agent (if any)
735: will be forwarded to the remote machine.
736: The argument must be
737: .Dq yes
738: or
739: .Dq no .
740: The default is
741: .Dq no .
1.3 stevesk 742: .Pp
1.7 jmc 743: Agent forwarding should be enabled with caution.
744: Users with the ability to bypass file permissions on the remote host
745: (for the agent's Unix-domain socket)
746: can access the local agent through the forwarded connection.
747: An attacker cannot obtain key material from the agent,
1.3 stevesk 748: however they can perform operations on the keys that enable them to
749: authenticate using the identities loaded into the agent.
1.1 stevesk 750: .It Cm ForwardX11
751: Specifies whether X11 connections will be automatically redirected
752: over the secure channel and
753: .Ev DISPLAY
754: set.
755: The argument must be
756: .Dq yes
757: or
758: .Dq no .
759: The default is
760: .Dq no .
1.3 stevesk 761: .Pp
1.7 jmc 762: X11 forwarding should be enabled with caution.
763: Users with the ability to bypass file permissions on the remote host
1.22 markus 764: (for the user's X11 authorization database)
1.7 jmc 765: can access the local X11 display through the forwarded connection.
1.22 markus 766: An attacker may then be able to perform activities such as keystroke monitoring
767: if the
768: .Cm ForwardX11Trusted
769: option is also enabled.
1.134 djm 770: .It Cm ForwardX11Timeout
1.135 jmc 771: Specify a timeout for untrusted X11 forwarding
772: using the format described in the
1.166 jmc 773: TIME FORMATS section of
1.134 djm 774: .Xr sshd_config 5 .
775: X11 connections received by
776: .Xr ssh 1
777: after this time will be refused.
778: The default is to disable untrusted X11 forwarding after twenty minutes has
779: elapsed.
1.22 markus 780: .It Cm ForwardX11Trusted
1.34 jmc 781: If this option is set to
1.84 jmc 782: .Dq yes ,
783: remote X11 clients will have full access to the original X11 display.
1.42 djm 784: .Pp
1.22 markus 785: If this option is set to
1.84 jmc 786: .Dq no ,
787: remote X11 clients will be considered untrusted and prevented
1.22 markus 788: from stealing or tampering with data belonging to trusted X11
789: clients.
1.42 djm 790: Furthermore, the
791: .Xr xauth 1
792: token used for the session will be set to expire after 20 minutes.
793: Remote clients will be refused access after this time.
1.22 markus 794: .Pp
795: The default is
796: .Dq no .
797: .Pp
798: See the X11 SECURITY extension specification for full details on
799: the restrictions imposed on untrusted clients.
1.1 stevesk 800: .It Cm GatewayPorts
801: Specifies whether remote hosts are allowed to connect to local
802: forwarded ports.
803: By default,
1.84 jmc 804: .Xr ssh 1
1.7 jmc 805: binds local port forwardings to the loopback address.
806: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 807: .Cm GatewayPorts
1.84 jmc 808: can be used to specify that ssh
1.1 stevesk 809: should bind local port forwardings to the wildcard address,
810: thus allowing remote hosts to connect to forwarded ports.
811: The argument must be
812: .Dq yes
813: or
814: .Dq no .
815: The default is
816: .Dq no .
817: .It Cm GlobalKnownHostsFile
1.151 djm 818: Specifies one or more files to use for the global
819: host key database, separated by whitespace.
820: The default is
821: .Pa /etc/ssh/ssh_known_hosts ,
822: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 823: .It Cm GSSAPIAuthentication
1.27 markus 824: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 825: The default is
1.21 markus 826: .Dq no .
1.18 markus 827: .It Cm GSSAPIDelegateCredentials
828: Forward (delegate) credentials to the server.
829: The default is
830: .Dq no .
1.44 djm 831: .It Cm HashKnownHosts
832: Indicates that
1.84 jmc 833: .Xr ssh 1
1.44 djm 834: should hash host names and addresses when they are added to
1.50 djm 835: .Pa ~/.ssh/known_hosts .
1.44 djm 836: These hashed names may be used normally by
1.84 jmc 837: .Xr ssh 1
1.44 djm 838: and
1.84 jmc 839: .Xr sshd 8 ,
1.44 djm 840: but they do not reveal identifying information should the file's contents
841: be disclosed.
842: The default is
843: .Dq no .
1.97 jmc 844: Note that existing names and addresses in known hosts files
845: will not be converted automatically,
846: but may be manually hashed using
1.45 djm 847: .Xr ssh-keygen 1 .
1.1 stevesk 848: .It Cm HostbasedAuthentication
849: Specifies whether to try rhosts based authentication with public key
850: authentication.
851: The argument must be
852: .Dq yes
853: or
854: .Dq no .
855: The default is
856: .Dq no .
1.202 djm 857: .It Cm HostbasedKeyTypes
858: Specifies the key types that will be used for hostbased authentication
859: as a comma-separated pattern list.
1.214 djm 860: Alternately if the specified value begins with a
861: .Sq +
862: character, then the specified key types will be appended to the default set
863: instead of replacing them.
1.213 markus 864: The default for this option is:
865: .Bd -literal -offset 3n
866: ecdsa-sha2-nistp256-cert-v01@openssh.com,
867: ecdsa-sha2-nistp384-cert-v01@openssh.com,
868: ecdsa-sha2-nistp521-cert-v01@openssh.com,
869: ssh-ed25519-cert-v01@openssh.com,
870: ssh-rsa-cert-v01@openssh.com,
871: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 872: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 873: .Ed
874: .Pp
1.202 djm 875: The
876: .Fl Q
877: option of
878: .Xr ssh 1
879: may be used to list supported key types.
1.1 stevesk 880: .It Cm HostKeyAlgorithms
1.226 ! jmc 881: Specifies the host key algorithms
1.1 stevesk 882: that the client wants to use in order of preference.
1.214 djm 883: Alternately if the specified value begins with a
884: .Sq +
885: character, then the specified key types will be appended to the default set
886: instead of replacing them.
1.1 stevesk 887: The default for this option is:
1.139 djm 888: .Bd -literal -offset 3n
889: ecdsa-sha2-nistp256-cert-v01@openssh.com,
890: ecdsa-sha2-nistp384-cert-v01@openssh.com,
891: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 892: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 893: ssh-rsa-cert-v01@openssh.com,
1.139 djm 894: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 895: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.139 djm 896: .Ed
1.145 djm 897: .Pp
898: If hostkeys are known for the destination host then this default is modified
899: to prefer their algorithms.
1.198 djm 900: .Pp
901: The list of available key types may also be obtained using the
902: .Fl Q
903: option of
904: .Xr ssh 1
905: with an argument of
906: .Dq key .
1.1 stevesk 907: .It Cm HostKeyAlias
908: Specifies an alias that should be used instead of the
909: real host name when looking up or saving the host key
910: in the host key database files.
1.84 jmc 911: This option is useful for tunneling SSH connections
1.1 stevesk 912: or for multiple servers running on a single host.
913: .It Cm HostName
914: Specifies the real host name to log into.
915: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 916: If the hostname contains the character sequence
917: .Ql %h ,
1.150 jmc 918: then this will be replaced with the host name specified on the command line
1.136 djm 919: (this is useful for manipulating unqualified names).
1.187 djm 920: The character sequence
921: .Ql %%
922: will be replaced by a single
923: .Ql %
924: character, which may be used when specifying IPv6 link-local addresses.
925: .Pp
1.84 jmc 926: The default is the name given on the command line.
1.1 stevesk 927: Numeric IP addresses are also permitted (both on the command line and in
928: .Cm HostName
929: specifications).
1.29 markus 930: .It Cm IdentitiesOnly
931: Specifies that
1.84 jmc 932: .Xr ssh 1
1.221 djm 933: should only use the authentication identity and certificate files explicitly
934: configured in the
1.31 jmc 935: .Nm
1.221 djm 936: files
937: or passed on the
938: .Xr ssh 1
939: command-line,
1.84 jmc 940: even if
941: .Xr ssh-agent 1
1.159 djm 942: or a
943: .Cm PKCS11Provider
1.29 markus 944: offers more identities.
945: The argument to this keyword must be
946: .Dq yes
947: or
948: .Dq no .
1.84 jmc 949: This option is intended for situations where ssh-agent
1.29 markus 950: offers many different identities.
951: The default is
952: .Dq no .
1.67 jmc 953: .It Cm IdentityFile
1.192 sobrado 954: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 955: identity is read.
1.67 jmc 956: The default is
957: .Pa ~/.ssh/identity
958: for protocol version 1, and
1.139 djm 959: .Pa ~/.ssh/id_dsa ,
1.183 naddy 960: .Pa ~/.ssh/id_ecdsa ,
961: .Pa ~/.ssh/id_ed25519
1.139 djm 962: and
1.67 jmc 963: .Pa ~/.ssh/id_rsa
964: for protocol version 2.
965: Additionally, any identities represented by the authentication agent
1.165 djm 966: will be used for authentication unless
967: .Cm IdentitiesOnly
968: is set.
1.221 djm 969: If no certificates have been explicitly specified by
970: .Cm CertificateFile ,
1.129 djm 971: .Xr ssh 1
972: will try to load certificate information from the filename obtained by
973: appending
974: .Pa -cert.pub
975: to the path of a specified
976: .Cm IdentityFile .
1.90 djm 977: .Pp
1.67 jmc 978: The file name may use the tilde
1.91 jmc 979: syntax to refer to a user's home directory or one of the following
1.90 djm 980: escape characters:
981: .Ql %d
982: (local user's home directory),
983: .Ql %u
984: (local user name),
985: .Ql %l
986: (local host name),
987: .Ql %h
988: (remote host name) or
1.92 djm 989: .Ql %r
1.90 djm 990: (remote user name).
991: .Pp
1.67 jmc 992: It is possible to have
993: multiple identity files specified in configuration files; all these
994: identities will be tried in sequence.
1.152 djm 995: Multiple
996: .Cm IdentityFile
997: directives will add to the list of identities tried (this behaviour
998: differs from that of other configuration directives).
1.165 djm 999: .Pp
1000: .Cm IdentityFile
1001: may be used in conjunction with
1002: .Cm IdentitiesOnly
1003: to select which identities in an agent are offered during authentication.
1.221 djm 1004: .Cm IdentityFile
1005: may also be used in conjunction with
1006: .Cm CertificateFile
1007: in order to provide any certificate also needed for authentication with
1008: the identity.
1.164 jmc 1009: .It Cm IgnoreUnknown
1010: Specifies a pattern-list of unknown options to be ignored if they are
1011: encountered in configuration parsing.
1012: This may be used to suppress errors if
1013: .Nm
1014: contains options that are unrecognised by
1015: .Xr ssh 1 .
1016: It is recommended that
1017: .Cm IgnoreUnknown
1018: be listed early in the configuration file as it will not be applied
1019: to unknown options that appear before it.
1.143 djm 1020: .It Cm IPQoS
1021: Specifies the IPv4 type-of-service or DSCP class for connections.
1022: Accepted values are
1023: .Dq af11 ,
1024: .Dq af12 ,
1025: .Dq af13 ,
1.154 djm 1026: .Dq af21 ,
1.143 djm 1027: .Dq af22 ,
1028: .Dq af23 ,
1029: .Dq af31 ,
1030: .Dq af32 ,
1031: .Dq af33 ,
1032: .Dq af41 ,
1033: .Dq af42 ,
1034: .Dq af43 ,
1035: .Dq cs0 ,
1036: .Dq cs1 ,
1037: .Dq cs2 ,
1038: .Dq cs3 ,
1039: .Dq cs4 ,
1040: .Dq cs5 ,
1041: .Dq cs6 ,
1042: .Dq cs7 ,
1043: .Dq ef ,
1044: .Dq lowdelay ,
1045: .Dq throughput ,
1046: .Dq reliability ,
1047: or a numeric value.
1.146 djm 1048: This option may take one or two arguments, separated by whitespace.
1.143 djm 1049: If one argument is specified, it is used as the packet class unconditionally.
1050: If two values are specified, the first is automatically selected for
1051: interactive sessions and the second for non-interactive sessions.
1052: The default is
1053: .Dq lowdelay
1054: for interactive sessions and
1055: .Dq throughput
1056: for non-interactive sessions.
1.103 djm 1057: .It Cm KbdInteractiveAuthentication
1058: Specifies whether to use keyboard-interactive authentication.
1059: The argument to this keyword must be
1060: .Dq yes
1061: or
1062: .Dq no .
1063: The default is
1064: .Dq yes .
1.39 djm 1065: .It Cm KbdInteractiveDevices
1066: Specifies the list of methods to use in keyboard-interactive authentication.
1067: Multiple method names must be comma-separated.
1068: The default is to use the server specified list.
1.85 jmc 1069: The methods available vary depending on what the server supports.
1070: For an OpenSSH server,
1071: it may be zero or more of:
1072: .Dq bsdauth ,
1073: .Dq pam ,
1074: and
1075: .Dq skey .
1.140 djm 1076: .It Cm KexAlgorithms
1077: Specifies the available KEX (Key Exchange) algorithms.
1078: Multiple algorithms must be comma-separated.
1.214 djm 1079: Alternately if the specified value begins with a
1080: .Sq +
1081: character, then the specified methods will be appended to the default set
1082: instead of replacing them.
1.141 jmc 1083: The default is:
1084: .Bd -literal -offset indent
1.179 markus 1085: curve25519-sha256@libssh.org,
1.141 jmc 1086: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1087: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1088: diffie-hellman-group-exchange-sha1,
1.212 djm 1089: diffie-hellman-group14-sha1
1.141 jmc 1090: .Ed
1.198 djm 1091: .Pp
1092: The list of available key exchange algorithms may also be obtained using the
1093: .Fl Q
1094: option of
1095: .Xr ssh 1
1096: with an argument of
1097: .Dq kex .
1.65 reyk 1098: .It Cm LocalCommand
1099: Specifies a command to execute on the local machine after successfully
1100: connecting to the server.
1101: The command string extends to the end of the line, and is executed with
1.105 jmc 1102: the user's shell.
1.109 dtucker 1103: The following escape character substitutions will be performed:
1104: .Ql %d
1105: (local user's home directory),
1106: .Ql %h
1107: (remote host name),
1108: .Ql %l
1109: (local host name),
1110: .Ql %n
1111: (host name as provided on the command line),
1112: .Ql %p
1113: (remote port),
1114: .Ql %r
1115: (remote user name) or
1116: .Ql %u
1.188 djm 1117: (local user name) or
1.189 jmc 1118: .Ql \&%C
1.188 djm 1119: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1120: .Pp
1121: The command is run synchronously and does not have access to the
1122: session of the
1123: .Xr ssh 1
1124: that spawned it.
1125: It should not be used for interactive commands.
1126: .Pp
1.65 reyk 1127: This directive is ignored unless
1128: .Cm PermitLocalCommand
1129: has been enabled.
1.1 stevesk 1130: .It Cm LocalForward
1.74 jmc 1131: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1132: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1133: The first argument must be
1.43 djm 1134: .Sm off
1.49 jmc 1135: .Oo Ar bind_address : Oc Ar port
1.43 djm 1136: .Sm on
1.49 jmc 1137: and the second argument must be
1138: .Ar host : Ns Ar hostport .
1.138 djm 1139: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1140: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1141: given on the command line.
1.1 stevesk 1142: Only the superuser can forward privileged ports.
1.43 djm 1143: By default, the local port is bound in accordance with the
1144: .Cm GatewayPorts
1145: setting.
1146: However, an explicit
1147: .Ar bind_address
1148: may be used to bind the connection to a specific address.
1149: The
1150: .Ar bind_address
1151: of
1152: .Dq localhost
1.46 jmc 1153: indicates that the listening port be bound for local use only, while an
1154: empty address or
1155: .Sq *
1.43 djm 1156: indicates that the port should be available from all interfaces.
1.1 stevesk 1157: .It Cm LogLevel
1158: Gives the verbosity level that is used when logging messages from
1.84 jmc 1159: .Xr ssh 1 .
1.1 stevesk 1160: The possible values are:
1.84 jmc 1161: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1162: The default is INFO.
1163: DEBUG and DEBUG1 are equivalent.
1164: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1165: .It Cm MACs
1166: Specifies the MAC (message authentication code) algorithms
1167: in order of preference.
1.226 ! jmc 1168: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1169: Multiple algorithms must be comma-separated.
1.214 djm 1170: If the specified value begins with a
1171: .Sq +
1172: character, then the specified algorithms will be appended to the default set
1173: instead of replacing them.
1174: .Pp
1.160 markus 1175: The algorithms that contain
1176: .Dq -etm
1177: calculate the MAC after encryption (encrypt-then-mac).
1178: These are considered safer and their use recommended.
1.214 djm 1179: .Pp
1.84 jmc 1180: The default is:
1.101 jmc 1181: .Bd -literal -offset indent
1.160 markus 1182: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1183: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1184: hmac-sha1-etm@openssh.com,
1.186 naddy 1185: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1186: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1187: .Ed
1.198 djm 1188: .Pp
1189: The list of available MAC algorithms may also be obtained using the
1190: .Fl Q
1191: option of
1192: .Xr ssh 1
1193: with an argument of
1194: .Dq mac .
1.1 stevesk 1195: .It Cm NoHostAuthenticationForLocalhost
1196: This option can be used if the home directory is shared across machines.
1197: In this case localhost will refer to a different machine on each of
1198: the machines and the user will get many warnings about changed host keys.
1199: However, this option disables host authentication for localhost.
1200: The argument to this keyword must be
1201: .Dq yes
1202: or
1203: .Dq no .
1204: The default is to check the host key for localhost.
1205: .It Cm NumberOfPasswordPrompts
1206: Specifies the number of password prompts before giving up.
1207: The argument to this keyword must be an integer.
1.84 jmc 1208: The default is 3.
1.1 stevesk 1209: .It Cm PasswordAuthentication
1210: Specifies whether to use password authentication.
1211: The argument to this keyword must be
1212: .Dq yes
1213: or
1214: .Dq no .
1215: The default is
1216: .Dq yes .
1.65 reyk 1217: .It Cm PermitLocalCommand
1218: Allow local command execution via the
1219: .Ic LocalCommand
1220: option or using the
1.66 jmc 1221: .Ic !\& Ns Ar command
1.65 reyk 1222: escape sequence in
1223: .Xr ssh 1 .
1224: The argument must be
1225: .Dq yes
1226: or
1227: .Dq no .
1228: The default is
1229: .Dq no .
1.127 markus 1230: .It Cm PKCS11Provider
1231: Specifies which PKCS#11 provider to use.
1.144 jmc 1232: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1233: .Xr ssh 1
1.128 markus 1234: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1235: private RSA key.
1.67 jmc 1236: .It Cm Port
1237: Specifies the port number to connect on the remote host.
1.84 jmc 1238: The default is 22.
1.1 stevesk 1239: .It Cm PreferredAuthentications
1.226 ! jmc 1240: Specifies the order in which the client should try authentication methods.
1.48 jmc 1241: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1242: .Cm keyboard-interactive )
1.48 jmc 1243: over another method (e.g.\&
1.131 jmc 1244: .Cm password ) .
1245: The default is:
1246: .Bd -literal -offset indent
1247: gssapi-with-mic,hostbased,publickey,
1248: keyboard-interactive,password
1249: .Ed
1.1 stevesk 1250: .It Cm Protocol
1251: Specifies the protocol versions
1.84 jmc 1252: .Xr ssh 1
1.1 stevesk 1253: should support in order of preference.
1254: The possible values are
1.84 jmc 1255: .Sq 1
1.1 stevesk 1256: and
1.84 jmc 1257: .Sq 2 .
1.1 stevesk 1258: Multiple versions must be comma-separated.
1.120 markus 1259: When this option is set to
1.121 jmc 1260: .Dq 2,1
1.120 markus 1261: .Nm ssh
1262: will try version 2 and fall back to version 1
1263: if version 2 is not available.
1.1 stevesk 1264: The default is
1.121 jmc 1265: .Sq 2 .
1.225 djm 1266: Protocol 1 suffers from a number of cryptographic weaknesses and should
1267: not be used.
1268: It is only offered to support legacy devices.
1.1 stevesk 1269: .It Cm ProxyCommand
1270: Specifies the command to use to connect to the server.
1271: The command
1.190 djm 1272: string extends to the end of the line, and is executed
1273: using the user's shell
1274: .Ql exec
1275: directive to avoid a lingering shell process.
1276: .Pp
1.133 jmc 1277: In the command string, any occurrence of
1.1 stevesk 1278: .Ql %h
1279: will be substituted by the host name to
1.132 djm 1280: connect,
1.1 stevesk 1281: .Ql %p
1.133 jmc 1282: by the port, and
1283: .Ql %r
1.132 djm 1284: by the remote user name.
1.1 stevesk 1285: The command can be basically anything,
1286: and should read from its standard input and write to its standard output.
1287: It should eventually connect an
1288: .Xr sshd 8
1289: server running on some machine, or execute
1290: .Ic sshd -i
1291: somewhere.
1292: Host key management will be done using the
1293: HostName of the host being connected (defaulting to the name typed by
1294: the user).
1.7 jmc 1295: Setting the command to
1296: .Dq none
1.6 markus 1297: disables this option entirely.
1.1 stevesk 1298: Note that
1299: .Cm CheckHostIP
1300: is not available for connects with a proxy command.
1.52 djm 1301: .Pp
1302: This directive is useful in conjunction with
1303: .Xr nc 1
1304: and its proxy support.
1.53 jmc 1305: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1306: 192.0.2.0:
1307: .Bd -literal -offset 3n
1308: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1309: .Ed
1.167 djm 1310: .It Cm ProxyUseFdpass
1.168 jmc 1311: Specifies that
1.167 djm 1312: .Cm ProxyCommand
1313: will pass a connected file descriptor back to
1.168 jmc 1314: .Xr ssh 1
1.167 djm 1315: instead of continuing to execute and pass data.
1316: The default is
1317: .Dq no .
1.213 markus 1318: .It Cm PubkeyAcceptedKeyTypes
1319: Specifies the key types that will be used for public key authentication
1320: as a comma-separated pattern list.
1.214 djm 1321: Alternately if the specified value begins with a
1322: .Sq +
1323: character, then the key types after it will be appended to the default
1324: instead of replacing it.
1.213 markus 1325: The default for this option is:
1326: .Bd -literal -offset 3n
1327: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1328: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1329: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1330: ssh-ed25519-cert-v01@openssh.com,
1331: ssh-rsa-cert-v01@openssh.com,
1332: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 1333: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 1334: .Ed
1335: .Pp
1336: The
1337: .Fl Q
1338: option of
1339: .Xr ssh 1
1340: may be used to list supported key types.
1.1 stevesk 1341: .It Cm PubkeyAuthentication
1342: Specifies whether to try public key authentication.
1343: The argument to this keyword must be
1344: .Dq yes
1345: or
1346: .Dq no .
1347: The default is
1348: .Dq yes .
1.75 dtucker 1349: .It Cm RekeyLimit
1350: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1351: session key is renegotiated, optionally followed a maximum amount of
1352: time that may pass before the session key is renegotiated.
1353: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1354: .Sq K ,
1355: .Sq M ,
1.75 dtucker 1356: or
1.76 jmc 1357: .Sq G
1.75 dtucker 1358: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1359: The default is between
1.84 jmc 1360: .Sq 1G
1.75 dtucker 1361: and
1.84 jmc 1362: .Sq 4G ,
1.75 dtucker 1363: depending on the cipher.
1.162 dtucker 1364: The optional second value is specified in seconds and may use any of the
1365: units documented in the
1.166 jmc 1366: TIME FORMATS section of
1.162 dtucker 1367: .Xr sshd_config 5 .
1368: The default value for
1369: .Cm RekeyLimit
1370: is
1371: .Dq default none ,
1372: which means that rekeying is performed after the cipher's default amount
1373: of data has been sent or received and no time based rekeying is done.
1.1 stevesk 1374: .It Cm RemoteForward
1.74 jmc 1375: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1376: the secure channel to the specified host and port from the local machine.
1.49 jmc 1377: The first argument must be
1.43 djm 1378: .Sm off
1.49 jmc 1379: .Oo Ar bind_address : Oc Ar port
1.43 djm 1380: .Sm on
1.49 jmc 1381: and the second argument must be
1382: .Ar host : Ns Ar hostport .
1.138 djm 1383: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1384: Multiple forwardings may be specified, and additional
1385: forwardings can be given on the command line.
1.113 stevesk 1386: Privileged ports can be forwarded only when
1387: logging in as root on the remote machine.
1.118 jmc 1388: .Pp
1.117 djm 1389: If the
1390: .Ar port
1391: argument is
1392: .Ql 0 ,
1393: the listen port will be dynamically allocated on the server and reported
1394: to the client at run time.
1.43 djm 1395: .Pp
1396: If the
1397: .Ar bind_address
1398: is not specified, the default is to only bind to loopback addresses.
1399: If the
1400: .Ar bind_address
1401: is
1402: .Ql *
1403: or an empty string, then the forwarding is requested to listen on all
1404: interfaces.
1405: Specifying a remote
1406: .Ar bind_address
1.46 jmc 1407: will only succeed if the server's
1408: .Cm GatewayPorts
1.43 djm 1409: option is enabled (see
1.46 jmc 1410: .Xr sshd_config 5 ) .
1.149 djm 1411: .It Cm RequestTTY
1412: Specifies whether to request a pseudo-tty for the session.
1413: The argument may be one of:
1414: .Dq no
1415: (never request a TTY),
1416: .Dq yes
1417: (always request a TTY when standard input is a TTY),
1418: .Dq force
1419: (always request a TTY) or
1420: .Dq auto
1421: (request a TTY when opening a login session).
1422: This option mirrors the
1423: .Fl t
1424: and
1425: .Fl T
1426: flags for
1427: .Xr ssh 1 .
1.196 djm 1428: .It Cm RevokedHostKeys
1429: Specifies revoked host public keys.
1430: Keys listed in this file will be refused for host authentication.
1431: Note that if this file does not exist or is not readable,
1432: then host authentication will be refused for all hosts.
1433: Keys may be specified as a text file, listing one public key per line, or as
1434: an OpenSSH Key Revocation List (KRL) as generated by
1435: .Xr ssh-keygen 1 .
1436: For more information on KRLs, see the KEY REVOCATION LISTS section in
1437: .Xr ssh-keygen 1 .
1.1 stevesk 1438: .It Cm RhostsRSAAuthentication
1439: Specifies whether to try rhosts based authentication with RSA host
1440: authentication.
1441: The argument must be
1442: .Dq yes
1443: or
1444: .Dq no .
1445: The default is
1446: .Dq no .
1447: This option applies to protocol version 1 only and requires
1.84 jmc 1448: .Xr ssh 1
1.1 stevesk 1449: to be setuid root.
1450: .It Cm RSAAuthentication
1451: Specifies whether to try RSA authentication.
1452: The argument to this keyword must be
1453: .Dq yes
1454: or
1455: .Dq no .
1456: RSA authentication will only be
1457: attempted if the identity file exists, or an authentication agent is
1458: running.
1459: The default is
1460: .Dq yes .
1461: Note that this option applies to protocol version 1 only.
1.32 djm 1462: .It Cm SendEnv
1463: Specifies what variables from the local
1464: .Xr environ 7
1465: should be sent to the server.
1.84 jmc 1466: The server must also support it, and the server must be configured to
1.33 djm 1467: accept these environment variables.
1.207 dtucker 1468: Note that the
1469: .Ev TERM
1.208 jmc 1470: environment variable is always sent whenever a
1.207 dtucker 1471: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1472: Refer to
1473: .Cm AcceptEnv
1474: in
1475: .Xr sshd_config 5
1476: for how to configure the server.
1.80 jmc 1477: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1478: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1479: across multiple
1480: .Cm SendEnv
1481: directives.
1482: The default is not to send any environment variables.
1.81 jmc 1483: .Pp
1484: See
1485: .Sx PATTERNS
1486: for more information on patterns.
1.28 markus 1487: .It Cm ServerAliveCountMax
1.73 jmc 1488: Sets the number of server alive messages (see below) which may be
1.28 markus 1489: sent without
1.84 jmc 1490: .Xr ssh 1
1.28 markus 1491: receiving any messages back from the server.
1492: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1493: ssh will disconnect from the server, terminating the session.
1.28 markus 1494: It is important to note that the use of server alive messages is very
1495: different from
1496: .Cm TCPKeepAlive
1497: (below).
1498: The server alive messages are sent through the encrypted channel
1499: and therefore will not be spoofable.
1500: The TCP keepalive option enabled by
1501: .Cm TCPKeepAlive
1502: is spoofable.
1503: The server alive mechanism is valuable when the client or
1504: server depend on knowing when a connection has become inactive.
1505: .Pp
1506: The default value is 3.
1507: If, for example,
1508: .Cm ServerAliveInterval
1.84 jmc 1509: (see below) is set to 15 and
1.28 markus 1510: .Cm ServerAliveCountMax
1.84 jmc 1511: is left at the default, if the server becomes unresponsive,
1512: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1513: .It Cm ServerAliveInterval
1514: Sets a timeout interval in seconds after which if no data has been received
1515: from the server,
1.84 jmc 1516: .Xr ssh 1
1.67 jmc 1517: will send a message through the encrypted
1518: channel to request a response from the server.
1519: The default
1520: is 0, indicating that these messages will not be sent to the server.
1.191 millert 1521: .It Cm StreamLocalBindMask
1522: Sets the octal file creation mode mask
1523: .Pq umask
1524: used when creating a Unix-domain socket file for local or remote
1525: port forwarding.
1526: This option is only used for port forwarding to a Unix-domain socket file.
1527: .Pp
1528: The default value is 0177, which creates a Unix-domain socket file that is
1529: readable and writable only by the owner.
1530: Note that not all operating systems honor the file mode on Unix-domain
1531: socket files.
1532: .It Cm StreamLocalBindUnlink
1533: Specifies whether to remove an existing Unix-domain socket file for local
1534: or remote port forwarding before creating a new one.
1535: If the socket file already exists and
1536: .Cm StreamLocalBindUnlink
1537: is not enabled,
1538: .Nm ssh
1539: will be unable to forward the port to the Unix-domain socket file.
1540: This option is only used for port forwarding to a Unix-domain socket file.
1541: .Pp
1542: The argument must be
1543: .Dq yes
1544: or
1545: .Dq no .
1546: The default is
1547: .Dq no .
1.1 stevesk 1548: .It Cm StrictHostKeyChecking
1549: If this flag is set to
1550: .Dq yes ,
1.84 jmc 1551: .Xr ssh 1
1.1 stevesk 1552: will never automatically add host keys to the
1.50 djm 1553: .Pa ~/.ssh/known_hosts
1.1 stevesk 1554: file, and refuses to connect to hosts whose host key has changed.
1555: This provides maximum protection against trojan horse attacks,
1.84 jmc 1556: though it can be annoying when the
1.1 stevesk 1557: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1558: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1559: frequently made.
1560: This option forces the user to manually
1561: add all new hosts.
1562: If this flag is set to
1563: .Dq no ,
1.84 jmc 1564: ssh will automatically add new host keys to the
1.1 stevesk 1565: user known hosts files.
1566: If this flag is set to
1567: .Dq ask ,
1568: new host keys
1569: will be added to the user known host files only after the user
1570: has confirmed that is what they really want to do, and
1.84 jmc 1571: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1572: The host keys of
1573: known hosts will be verified automatically in all cases.
1574: The argument must be
1575: .Dq yes ,
1.84 jmc 1576: .Dq no ,
1.1 stevesk 1577: or
1578: .Dq ask .
1579: The default is
1580: .Dq ask .
1.26 markus 1581: .It Cm TCPKeepAlive
1582: Specifies whether the system should send TCP keepalive messages to the
1583: other side.
1584: If they are sent, death of the connection or crash of one
1585: of the machines will be properly noticed.
1586: However, this means that
1587: connections will die if the route is down temporarily, and some people
1588: find it annoying.
1589: .Pp
1590: The default is
1591: .Dq yes
1592: (to send TCP keepalive messages), and the client will notice
1593: if the network goes down or the remote host dies.
1594: This is important in scripts, and many users want it too.
1595: .Pp
1596: To disable TCP keepalive messages, the value should be set to
1597: .Dq no .
1.65 reyk 1598: .It Cm Tunnel
1.95 stevesk 1599: Request
1.65 reyk 1600: .Xr tun 4
1.69 jmc 1601: device forwarding between the client and the server.
1.65 reyk 1602: The argument must be
1.68 reyk 1603: .Dq yes ,
1.95 stevesk 1604: .Dq point-to-point
1605: (layer 3),
1606: .Dq ethernet
1607: (layer 2),
1.65 reyk 1608: or
1609: .Dq no .
1.95 stevesk 1610: Specifying
1611: .Dq yes
1612: requests the default tunnel mode, which is
1613: .Dq point-to-point .
1.65 reyk 1614: The default is
1615: .Dq no .
1616: .It Cm TunnelDevice
1.95 stevesk 1617: Specifies the
1.65 reyk 1618: .Xr tun 4
1.95 stevesk 1619: devices to open on the client
1620: .Pq Ar local_tun
1621: and the server
1622: .Pq Ar remote_tun .
1623: .Pp
1624: The argument must be
1625: .Sm off
1626: .Ar local_tun Op : Ar remote_tun .
1627: .Sm on
1628: The devices may be specified by numerical ID or the keyword
1629: .Dq any ,
1630: which uses the next available tunnel device.
1631: If
1632: .Ar remote_tun
1633: is not specified, it defaults to
1634: .Dq any .
1635: The default is
1636: .Dq any:any .
1.201 djm 1637: .It Cm UpdateHostKeys
1.200 djm 1638: Specifies whether
1639: .Xr ssh 1
1640: should accept notifications of additional hostkeys from the server sent
1641: after authentication has completed and add them to
1642: .Cm UserKnownHostsFile .
1643: The argument must be
1.204 djm 1644: .Dq yes ,
1.203 djm 1645: .Dq no
1.204 djm 1646: (the default) or
1647: .Dq ask .
1.200 djm 1648: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1649: and supports graceful key rotation by allowing a server to send replacement
1650: public keys before old ones are removed.
1.200 djm 1651: Additional hostkeys are only accepted if the key used to authenticate the
1.220 sobrado 1652: host was already trusted or explicitly accepted by the user.
1.204 djm 1653: If
1654: .Cm UpdateHostKeys
1655: is set to
1656: .Dq ask ,
1657: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1658: Confirmation is currently incompatible with
1659: .Cm ControlPersist ,
1660: and will be disabled if it is enabled.
1.200 djm 1661: .Pp
1662: Presently, only
1663: .Xr sshd 8
1664: from OpenSSH 6.8 and greater support the
1665: .Dq hostkeys@openssh.com
1666: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1667: .It Cm UsePrivilegedPort
1668: Specifies whether to use a privileged port for outgoing connections.
1669: The argument must be
1670: .Dq yes
1671: or
1672: .Dq no .
1673: The default is
1674: .Dq no .
1675: If set to
1.84 jmc 1676: .Dq yes ,
1677: .Xr ssh 1
1.72 jmc 1678: must be setuid root.
1679: Note that this option must be set to
1680: .Dq yes
1681: for
1682: .Cm RhostsRSAAuthentication
1683: with older servers.
1.1 stevesk 1684: .It Cm User
1685: Specifies the user to log in as.
1686: This can be useful when a different user name is used on different machines.
1687: This saves the trouble of
1688: having to remember to give the user name on the command line.
1689: .It Cm UserKnownHostsFile
1.151 djm 1690: Specifies one or more files to use for the user
1691: host key database, separated by whitespace.
1692: The default is
1693: .Pa ~/.ssh/known_hosts ,
1694: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1695: .It Cm VerifyHostKeyDNS
1696: Specifies whether to verify the remote key using DNS and SSHFP resource
1697: records.
1.24 jakob 1698: If this option is set to
1699: .Dq yes ,
1.25 jmc 1700: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1701: from DNS.
1702: Insecure fingerprints will be handled as if this option was set to
1703: .Dq ask .
1704: If this option is set to
1705: .Dq ask ,
1706: information on fingerprint match will be displayed, but the user will still
1707: need to confirm new host keys according to the
1708: .Cm StrictHostKeyChecking
1709: option.
1710: The argument must be
1711: .Dq yes ,
1.84 jmc 1712: .Dq no ,
1.25 jmc 1713: or
1714: .Dq ask .
1.8 jakob 1715: The default is
1716: .Dq no .
1.84 jmc 1717: .Pp
1.166 jmc 1718: See also VERIFYING HOST KEYS in
1.84 jmc 1719: .Xr ssh 1 .
1.111 grunk 1720: .It Cm VisualHostKey
1721: If this flag is set to
1722: .Dq yes ,
1723: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1724: printed in addition to the fingerprint string at login and
1.114 stevesk 1725: for unknown host keys.
1.111 grunk 1726: If this flag is set to
1727: .Dq no ,
1.114 stevesk 1728: no fingerprint strings are printed at login and
1.197 djm 1729: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1730: The default is
1731: .Dq no .
1.1 stevesk 1732: .It Cm XAuthLocation
1.5 stevesk 1733: Specifies the full pathname of the
1.1 stevesk 1734: .Xr xauth 1
1735: program.
1736: The default is
1737: .Pa /usr/X11R6/bin/xauth .
1738: .El
1.86 jmc 1739: .Sh PATTERNS
1740: A
1741: .Em pattern
1742: consists of zero or more non-whitespace characters,
1743: .Sq *
1744: (a wildcard that matches zero or more characters),
1745: or
1746: .Sq ?\&
1747: (a wildcard that matches exactly one character).
1748: For example, to specify a set of declarations for any host in the
1749: .Dq .co.uk
1750: set of domains,
1751: the following pattern could be used:
1752: .Pp
1753: .Dl Host *.co.uk
1754: .Pp
1755: The following pattern
1756: would match any host in the 192.168.0.[0-9] network range:
1757: .Pp
1758: .Dl Host 192.168.0.?
1759: .Pp
1760: A
1761: .Em pattern-list
1762: is a comma-separated list of patterns.
1763: Patterns within pattern-lists may be negated
1764: by preceding them with an exclamation mark
1765: .Pq Sq !\& .
1766: For example,
1.174 djm 1767: to allow a key to be used from anywhere within an organization
1.86 jmc 1768: except from the
1769: .Dq dialup
1770: pool,
1771: the following entry (in authorized_keys) could be used:
1772: .Pp
1773: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1774: .Sh FILES
1775: .Bl -tag -width Ds
1.50 djm 1776: .It Pa ~/.ssh/config
1.1 stevesk 1777: This is the per-user configuration file.
1778: The format of this file is described above.
1.84 jmc 1779: This file is used by the SSH client.
1.30 djm 1780: Because of the potential for abuse, this file must have strict permissions:
1781: read/write for the user, and not accessible by others.
1.1 stevesk 1782: .It Pa /etc/ssh/ssh_config
1783: Systemwide configuration file.
1784: This file provides defaults for those
1785: values that are not specified in the user's configuration file, and
1786: for those users who do not have a configuration file.
1787: This file must be world-readable.
1788: .El
1.13 jmc 1789: .Sh SEE ALSO
1790: .Xr ssh 1
1.1 stevesk 1791: .Sh AUTHORS
1792: OpenSSH is a derivative of the original and free
1793: ssh 1.2.12 release by Tatu Ylonen.
1794: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1795: Theo de Raadt and Dug Song
1796: removed many bugs, re-added newer features and
1797: created OpenSSH.
1798: Markus Friedl contributed the support for SSH
1799: protocol versions 1.5 and 2.0.
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