Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.230
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.230 ! jmc 36: .\" $OpenBSD: ssh_config.5,v 1.229 2016/04/15 00:30:19 djm Exp $
! 37: .Dd $Mdocdate: April 15 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).
1.228 sobrado 265: The default is
266: .Dq any .
1.1 stevesk 267: .It Cm BatchMode
268: If set to
269: .Dq yes ,
270: passphrase/password querying will be disabled.
271: This option is useful in scripts and other batch jobs where no user
272: is present to supply the password.
273: The argument must be
274: .Dq yes
275: or
276: .Dq no .
277: The default is
278: .Dq no .
279: .It Cm BindAddress
1.60 dtucker 280: Use the specified address on the local machine as the source address of
1.61 jmc 281: the connection.
282: Only useful on systems with more than one address.
1.1 stevesk 283: Note that this option does not work if
284: .Cm UsePrivilegedPort
285: is set to
286: .Dq yes .
1.171 djm 287: .It Cm CanonicalDomains
1.172 jmc 288: When
1.173 djm 289: .Cm CanonicalizeHostname
1.171 djm 290: is enabled, this option specifies the list of domain suffixes in which to
291: search for the specified destination host.
1.173 djm 292: .It Cm CanonicalizeFallbackLocal
1.174 djm 293: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 294: The default,
1.184 djm 295: .Dq yes ,
1.172 jmc 296: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 297: search rules.
298: A value of
1.184 djm 299: .Dq no
1.171 djm 300: will cause
301: .Xr ssh 1
302: to fail instantly if
1.173 djm 303: .Cm CanonicalizeHostname
1.171 djm 304: is enabled and the target hostname cannot be found in any of the domains
305: specified by
306: .Cm CanonicalDomains .
1.173 djm 307: .It Cm CanonicalizeHostname
1.174 djm 308: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 309: The default,
310: .Dq no ,
1.171 djm 311: is not to perform any name rewriting and let the system resolver handle all
312: hostname lookups.
313: If set to
314: .Dq yes
315: then, for connections that do not use a
316: .Cm ProxyCommand ,
317: .Xr ssh 1
1.173 djm 318: will attempt to canonicalize the hostname specified on the command line
1.171 djm 319: using the
320: .Cm CanonicalDomains
321: suffixes and
1.173 djm 322: .Cm CanonicalizePermittedCNAMEs
1.171 djm 323: rules.
324: If
1.173 djm 325: .Cm CanonicalizeHostname
1.171 djm 326: is set to
327: .Dq always ,
1.174 djm 328: then canonicalization is applied to proxied connections too.
1.185 djm 329: .Pp
1.193 djm 330: If this option is enabled, then the configuration files are processed
331: again using the new target name to pick up any new configuration in matching
1.185 djm 332: .Cm Host
1.193 djm 333: and
334: .Cm Match
1.185 djm 335: stanzas.
1.173 djm 336: .It Cm CanonicalizeMaxDots
1.172 jmc 337: Specifies the maximum number of dot characters in a hostname before
1.174 djm 338: canonicalization is disabled.
1.172 jmc 339: The default,
340: .Dq 1 ,
341: allows a single dot (i.e. hostname.subdomain).
1.173 djm 342: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 343: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 344: canonicalizing hostnames.
1.171 djm 345: The rules consist of one or more arguments of
1.172 jmc 346: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 347: where
348: .Ar source_domain_list
1.174 djm 349: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 350: and
351: .Ar target_domain_list
1.172 jmc 352: is a pattern-list of domains that they may resolve to.
1.171 djm 353: .Pp
354: For example,
355: .Dq *.a.example.com:*.b.example.com,*.c.example.com
356: will allow hostnames matching
357: .Dq *.a.example.com
1.173 djm 358: to be canonicalized to names in the
1.171 djm 359: .Dq *.b.example.com
360: or
361: .Dq *.c.example.com
362: domains.
1.221 djm 363: .It Cm CertificateFile
364: Specifies a file from which the user's certificate is read.
365: A corresponding private key must be provided separately in order
366: to use this certificate either
367: from an
368: .Cm IdentityFile
369: directive or
370: .Fl i
371: flag to
372: .Xr ssh 1 ,
373: via
374: .Xr ssh-agent 1 ,
375: or via a
376: .Cm PKCS11Provider .
377: .Pp
378: The file name may use the tilde
379: syntax to refer to a user's home directory or one of the following
380: escape characters:
381: .Ql %d
382: (local user's home directory),
383: .Ql %u
384: (local user name),
385: .Ql %l
386: (local host name),
387: .Ql %h
388: (remote host name) or
389: .Ql %r
390: (remote user name).
391: .Pp
392: It is possible to have multiple certificate files specified in
393: configuration files; these certificates will be tried in sequence.
394: Multiple
395: .Cm CertificateFile
396: directives will add to the list of certificates used for
397: authentication.
1.1 stevesk 398: .It Cm ChallengeResponseAuthentication
1.82 jmc 399: Specifies whether to use challenge-response authentication.
1.1 stevesk 400: The argument to this keyword must be
401: .Dq yes
402: or
403: .Dq no .
404: The default is
405: .Dq yes .
406: .It Cm CheckHostIP
407: If this flag is set to
408: .Dq yes ,
1.84 jmc 409: .Xr ssh 1
410: will additionally check the host IP address in the
1.1 stevesk 411: .Pa known_hosts
412: file.
1.211 djm 413: This allows ssh to detect if a host key changed due to DNS spoofing
414: and will add addresses of destination hosts to
415: .Pa ~/.ssh/known_hosts
416: in the process, regardless of the setting of
417: .Cm StrictHostKeyChecking .
1.107 grunk 418: If the option is set to
1.1 stevesk 419: .Dq no ,
420: the check will not be executed.
421: The default is
422: .Dq yes .
423: .It Cm Cipher
424: Specifies the cipher to use for encrypting the session
425: in protocol version 1.
426: Currently,
427: .Dq blowfish ,
428: .Dq 3des ,
429: and
430: .Dq des
431: are supported.
432: .Ar des
433: is only supported in the
1.84 jmc 434: .Xr ssh 1
1.1 stevesk 435: client for interoperability with legacy protocol 1 implementations
436: that do not support the
437: .Ar 3des
1.7 jmc 438: cipher.
439: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 440: The default is
441: .Dq 3des .
442: .It Cm Ciphers
443: Specifies the ciphers allowed for protocol version 2
444: in order of preference.
445: Multiple ciphers must be comma-separated.
1.214 djm 446: If the specified value begins with a
447: .Sq +
448: character, then the specified ciphers will be appended to the default set
449: instead of replacing them.
450: .Pp
1.180 djm 451: The supported ciphers are:
452: .Pp
1.186 naddy 453: .Bl -item -compact -offset indent
454: .It
455: 3des-cbc
456: .It
457: aes128-cbc
458: .It
459: aes192-cbc
460: .It
461: aes256-cbc
462: .It
463: aes128-ctr
464: .It
465: aes192-ctr
466: .It
467: aes256-ctr
468: .It
469: aes128-gcm@openssh.com
470: .It
471: aes256-gcm@openssh.com
472: .It
473: arcfour
474: .It
475: arcfour128
476: .It
477: arcfour256
478: .It
479: blowfish-cbc
480: .It
481: cast128-cbc
482: .It
483: chacha20-poly1305@openssh.com
484: .El
1.180 djm 485: .Pp
1.84 jmc 486: The default is:
1.186 naddy 487: .Bd -literal -offset indent
1.215 jmc 488: chacha20-poly1305@openssh.com,
1.186 naddy 489: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 490: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.224 djm 491: aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc
1.1 stevesk 492: .Ed
1.180 djm 493: .Pp
494: The list of available ciphers may also be obtained using the
495: .Fl Q
496: option of
1.198 djm 497: .Xr ssh 1
498: with an argument of
499: .Dq cipher .
1.1 stevesk 500: .It Cm ClearAllForwardings
1.84 jmc 501: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 502: specified in the configuration files or on the command line be
1.7 jmc 503: cleared.
504: This option is primarily useful when used from the
1.84 jmc 505: .Xr ssh 1
1.1 stevesk 506: command line to clear port forwardings set in
507: configuration files, and is automatically set by
508: .Xr scp 1
509: and
510: .Xr sftp 1 .
511: The argument must be
512: .Dq yes
513: or
514: .Dq no .
515: The default is
516: .Dq no .
517: .It Cm Compression
518: Specifies whether to use compression.
519: The argument must be
520: .Dq yes
521: or
522: .Dq no .
523: The default is
524: .Dq no .
525: .It Cm CompressionLevel
526: Specifies the compression level to use if compression is enabled.
527: The argument must be an integer from 1 (fast) to 9 (slow, best).
528: The default level is 6, which is good for most applications.
529: The meaning of the values is the same as in
530: .Xr gzip 1 .
531: Note that this option applies to protocol version 1 only.
532: .It Cm ConnectionAttempts
533: Specifies the number of tries (one per second) to make before exiting.
534: The argument must be an integer.
535: This may be useful in scripts if the connection sometimes fails.
536: The default is 1.
1.9 djm 537: .It Cm ConnectTimeout
1.84 jmc 538: Specifies the timeout (in seconds) used when connecting to the
539: SSH server, instead of using the default system TCP timeout.
1.11 jmc 540: This value is used only when the target is down or really unreachable,
541: not when it refuses the connection.
1.36 djm 542: .It Cm ControlMaster
543: Enables the sharing of multiple sessions over a single network connection.
544: When set to
1.84 jmc 545: .Dq yes ,
546: .Xr ssh 1
1.36 djm 547: will listen for connections on a control socket specified using the
548: .Cm ControlPath
549: argument.
550: Additional sessions can connect to this socket using the same
551: .Cm ControlPath
552: with
553: .Cm ControlMaster
554: set to
555: .Dq no
1.38 jmc 556: (the default).
1.64 jmc 557: These sessions will try to reuse the master instance's network connection
1.63 djm 558: rather than initiating new ones, but will fall back to connecting normally
559: if the control socket does not exist, or is not listening.
560: .Pp
1.37 djm 561: Setting this to
562: .Dq ask
1.84 jmc 563: will cause ssh
1.206 jmc 564: to listen for control connections, but require confirmation using
565: .Xr ssh-askpass 1 .
1.51 jakob 566: If the
567: .Cm ControlPath
1.84 jmc 568: cannot be opened,
569: ssh will continue without connecting to a master instance.
1.58 djm 570: .Pp
571: X11 and
1.59 jmc 572: .Xr ssh-agent 1
1.58 djm 573: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 574: display and agent forwarded will be the one belonging to the master
1.59 jmc 575: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 576: .Pp
577: Two additional options allow for opportunistic multiplexing: try to use a
578: master connection but fall back to creating a new one if one does not already
579: exist.
580: These options are:
581: .Dq auto
582: and
583: .Dq autoask .
584: The latter requires confirmation like the
585: .Dq ask
586: option.
1.36 djm 587: .It Cm ControlPath
1.55 djm 588: Specify the path to the control socket used for connection sharing as described
589: in the
1.36 djm 590: .Cm ControlMaster
1.57 djm 591: section above or the string
592: .Dq none
593: to disable connection sharing.
1.55 djm 594: In the path,
1.147 djm 595: .Ql %L
596: will be substituted by the first component of the local host name,
1.77 djm 597: .Ql %l
1.147 djm 598: will be substituted by the local host name (including any domain name),
1.55 djm 599: .Ql %h
600: will be substituted by the target host name,
1.150 jmc 601: .Ql %n
602: will be substituted by the original target host name
603: specified on the command line,
1.55 djm 604: .Ql %p
1.175 djm 605: the destination port,
1.55 djm 606: .Ql %r
1.188 djm 607: by the remote login username,
1.147 djm 608: .Ql %u
1.218 djm 609: by the username and
610: .Ql %i
1.219 jmc 611: by the numeric user ID (uid) of the user running
612: .Xr ssh 1 ,
613: and
1.189 jmc 614: .Ql \&%C
1.188 djm 615: by a hash of the concatenation: %l%h%p%r.
1.56 djm 616: It is recommended that any
617: .Cm ControlPath
618: used for opportunistic connection sharing include
1.195 djm 619: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
620: that is not writable by other users.
1.56 djm 621: This ensures that shared connections are uniquely identified.
1.137 djm 622: .It Cm ControlPersist
623: When used in conjunction with
624: .Cm ControlMaster ,
625: specifies that the master connection should remain open
626: in the background (waiting for future client connections)
627: after the initial client connection has been closed.
628: If set to
629: .Dq no ,
630: then the master connection will not be placed into the background,
631: and will close as soon as the initial client connection is closed.
632: If set to
1.195 djm 633: .Dq yes
634: or
635: .Dq 0 ,
1.137 djm 636: then the master connection will remain in the background indefinitely
637: (until killed or closed via a mechanism such as the
638: .Xr ssh 1
639: .Dq Fl O No exit
640: option).
641: If set to a time in seconds, or a time in any of the formats documented in
642: .Xr sshd_config 5 ,
643: then the backgrounded master connection will automatically terminate
644: after it has remained idle (with no client connections) for the
645: specified time.
1.38 jmc 646: .It Cm DynamicForward
1.74 jmc 647: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 648: over the secure channel, and the application
649: protocol is then used to determine where to connect to from the
650: remote machine.
1.62 djm 651: .Pp
652: The argument must be
653: .Sm off
654: .Oo Ar bind_address : Oc Ar port .
655: .Sm on
1.138 djm 656: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 657: By default, the local port is bound in accordance with the
658: .Cm GatewayPorts
659: setting.
660: However, an explicit
661: .Ar bind_address
662: may be used to bind the connection to a specific address.
663: The
664: .Ar bind_address
665: of
666: .Dq localhost
667: indicates that the listening port be bound for local use only, while an
668: empty address or
669: .Sq *
670: indicates that the port should be available from all interfaces.
671: .Pp
1.38 jmc 672: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 673: .Xr ssh 1
1.38 jmc 674: will act as a SOCKS server.
675: Multiple forwardings may be specified, and
676: additional forwardings can be given on the command line.
677: Only the superuser can forward privileged ports.
1.14 markus 678: .It Cm EnableSSHKeysign
679: Setting this option to
680: .Dq yes
681: in the global client configuration file
682: .Pa /etc/ssh/ssh_config
683: enables the use of the helper program
684: .Xr ssh-keysign 8
685: during
686: .Cm HostbasedAuthentication .
687: The argument must be
688: .Dq yes
689: or
690: .Dq no .
691: The default is
692: .Dq no .
1.23 jmc 693: This option should be placed in the non-hostspecific section.
1.14 markus 694: See
695: .Xr ssh-keysign 8
696: for more information.
1.1 stevesk 697: .It Cm EscapeChar
698: Sets the escape character (default:
699: .Ql ~ ) .
700: The escape character can also
701: be set on the command line.
702: The argument should be a single character,
703: .Ql ^
704: followed by a letter, or
705: .Dq none
706: to disable the escape
707: character entirely (making the connection transparent for binary
708: data).
1.96 markus 709: .It Cm ExitOnForwardFailure
710: Specifies whether
711: .Xr ssh 1
712: should terminate the connection if it cannot set up all requested
1.216 djm 713: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 714: if either end is unable to bind and listen on a specified port).
1.216 djm 715: Note that
716: .Cm ExitOnForwardFailure
717: does not apply to connections made over port forwardings and will not,
718: for example, cause
719: .Xr ssh 1
720: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 721: The argument must be
722: .Dq yes
723: or
724: .Dq no .
725: The default is
726: .Dq no .
1.197 djm 727: .It Cm FingerprintHash
728: Specifies the hash algorithm used when displaying key fingerprints.
729: Valid options are:
730: .Dq md5
731: and
732: .Dq sha256 .
733: The default is
734: .Dq sha256 .
1.1 stevesk 735: .It Cm ForwardAgent
736: Specifies whether the connection to the authentication agent (if any)
737: will be forwarded to the remote machine.
738: The argument must be
739: .Dq yes
740: or
741: .Dq no .
742: The default is
743: .Dq no .
1.3 stevesk 744: .Pp
1.7 jmc 745: Agent forwarding should be enabled with caution.
746: Users with the ability to bypass file permissions on the remote host
747: (for the agent's Unix-domain socket)
748: can access the local agent through the forwarded connection.
749: An attacker cannot obtain key material from the agent,
1.3 stevesk 750: however they can perform operations on the keys that enable them to
751: authenticate using the identities loaded into the agent.
1.1 stevesk 752: .It Cm ForwardX11
753: Specifies whether X11 connections will be automatically redirected
754: over the secure channel and
755: .Ev DISPLAY
756: set.
757: The argument must be
758: .Dq yes
759: or
760: .Dq no .
761: The default is
762: .Dq no .
1.3 stevesk 763: .Pp
1.7 jmc 764: X11 forwarding should be enabled with caution.
765: Users with the ability to bypass file permissions on the remote host
1.22 markus 766: (for the user's X11 authorization database)
1.7 jmc 767: can access the local X11 display through the forwarded connection.
1.22 markus 768: An attacker may then be able to perform activities such as keystroke monitoring
769: if the
770: .Cm ForwardX11Trusted
771: option is also enabled.
1.134 djm 772: .It Cm ForwardX11Timeout
1.135 jmc 773: Specify a timeout for untrusted X11 forwarding
774: using the format described in the
1.166 jmc 775: TIME FORMATS section of
1.134 djm 776: .Xr sshd_config 5 .
777: X11 connections received by
778: .Xr ssh 1
779: after this time will be refused.
780: The default is to disable untrusted X11 forwarding after twenty minutes has
781: elapsed.
1.22 markus 782: .It Cm ForwardX11Trusted
1.34 jmc 783: If this option is set to
1.84 jmc 784: .Dq yes ,
785: remote X11 clients will have full access to the original X11 display.
1.42 djm 786: .Pp
1.22 markus 787: If this option is set to
1.84 jmc 788: .Dq no ,
789: remote X11 clients will be considered untrusted and prevented
1.22 markus 790: from stealing or tampering with data belonging to trusted X11
791: clients.
1.42 djm 792: Furthermore, the
793: .Xr xauth 1
794: token used for the session will be set to expire after 20 minutes.
795: Remote clients will be refused access after this time.
1.22 markus 796: .Pp
797: The default is
798: .Dq no .
799: .Pp
800: See the X11 SECURITY extension specification for full details on
801: the restrictions imposed on untrusted clients.
1.1 stevesk 802: .It Cm GatewayPorts
803: Specifies whether remote hosts are allowed to connect to local
804: forwarded ports.
805: By default,
1.84 jmc 806: .Xr ssh 1
1.7 jmc 807: binds local port forwardings to the loopback address.
808: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 809: .Cm GatewayPorts
1.84 jmc 810: can be used to specify that ssh
1.1 stevesk 811: should bind local port forwardings to the wildcard address,
812: thus allowing remote hosts to connect to forwarded ports.
813: The argument must be
814: .Dq yes
815: or
816: .Dq no .
817: The default is
818: .Dq no .
819: .It Cm GlobalKnownHostsFile
1.151 djm 820: Specifies one or more files to use for the global
821: host key database, separated by whitespace.
822: The default is
823: .Pa /etc/ssh/ssh_known_hosts ,
824: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 825: .It Cm GSSAPIAuthentication
1.27 markus 826: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 827: The default is
1.21 markus 828: .Dq no .
1.18 markus 829: .It Cm GSSAPIDelegateCredentials
830: Forward (delegate) credentials to the server.
831: The default is
832: .Dq no .
1.44 djm 833: .It Cm HashKnownHosts
834: Indicates that
1.84 jmc 835: .Xr ssh 1
1.44 djm 836: should hash host names and addresses when they are added to
1.50 djm 837: .Pa ~/.ssh/known_hosts .
1.44 djm 838: These hashed names may be used normally by
1.84 jmc 839: .Xr ssh 1
1.44 djm 840: and
1.84 jmc 841: .Xr sshd 8 ,
1.44 djm 842: but they do not reveal identifying information should the file's contents
843: be disclosed.
844: The default is
845: .Dq no .
1.97 jmc 846: Note that existing names and addresses in known hosts files
847: will not be converted automatically,
848: but may be manually hashed using
1.45 djm 849: .Xr ssh-keygen 1 .
1.1 stevesk 850: .It Cm HostbasedAuthentication
851: Specifies whether to try rhosts based authentication with public key
852: authentication.
853: The argument must be
854: .Dq yes
855: or
856: .Dq no .
857: The default is
858: .Dq no .
1.202 djm 859: .It Cm HostbasedKeyTypes
860: Specifies the key types that will be used for hostbased authentication
861: as a comma-separated pattern list.
1.214 djm 862: Alternately if the specified value begins with a
863: .Sq +
864: character, then the specified key types will be appended to the default set
865: instead of replacing them.
1.213 markus 866: The default for this option is:
867: .Bd -literal -offset 3n
868: ecdsa-sha2-nistp256-cert-v01@openssh.com,
869: ecdsa-sha2-nistp384-cert-v01@openssh.com,
870: ecdsa-sha2-nistp521-cert-v01@openssh.com,
871: ssh-ed25519-cert-v01@openssh.com,
872: ssh-rsa-cert-v01@openssh.com,
873: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.227 djm 874: ssh-ed25519,ssh-rsa
1.213 markus 875: .Ed
876: .Pp
1.202 djm 877: The
878: .Fl Q
879: option of
880: .Xr ssh 1
881: may be used to list supported key types.
1.1 stevesk 882: .It Cm HostKeyAlgorithms
1.226 jmc 883: Specifies the host key algorithms
1.1 stevesk 884: that the client wants to use in order of preference.
1.214 djm 885: Alternately if the specified value begins with a
886: .Sq +
887: character, then the specified key types will be appended to the default set
888: instead of replacing them.
1.1 stevesk 889: The default for this option is:
1.139 djm 890: .Bd -literal -offset 3n
891: ecdsa-sha2-nistp256-cert-v01@openssh.com,
892: ecdsa-sha2-nistp384-cert-v01@openssh.com,
893: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 894: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 895: ssh-rsa-cert-v01@openssh.com,
1.139 djm 896: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.227 djm 897: ssh-ed25519,ssh-rsa
1.139 djm 898: .Ed
1.145 djm 899: .Pp
900: If hostkeys are known for the destination host then this default is modified
901: to prefer their algorithms.
1.198 djm 902: .Pp
903: The list of available key types may also be obtained using the
904: .Fl Q
905: option of
906: .Xr ssh 1
907: with an argument of
908: .Dq key .
1.1 stevesk 909: .It Cm HostKeyAlias
910: Specifies an alias that should be used instead of the
911: real host name when looking up or saving the host key
912: in the host key database files.
1.84 jmc 913: This option is useful for tunneling SSH connections
1.1 stevesk 914: or for multiple servers running on a single host.
915: .It Cm HostName
916: Specifies the real host name to log into.
917: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 918: If the hostname contains the character sequence
919: .Ql %h ,
1.150 jmc 920: then this will be replaced with the host name specified on the command line
1.136 djm 921: (this is useful for manipulating unqualified names).
1.187 djm 922: The character sequence
923: .Ql %%
924: will be replaced by a single
925: .Ql %
926: character, which may be used when specifying IPv6 link-local addresses.
927: .Pp
1.84 jmc 928: The default is the name given on the command line.
1.1 stevesk 929: Numeric IP addresses are also permitted (both on the command line and in
930: .Cm HostName
931: specifications).
1.29 markus 932: .It Cm IdentitiesOnly
933: Specifies that
1.84 jmc 934: .Xr ssh 1
1.221 djm 935: should only use the authentication identity and certificate files explicitly
936: configured in the
1.31 jmc 937: .Nm
1.221 djm 938: files
939: or passed on the
940: .Xr ssh 1
941: command-line,
1.84 jmc 942: even if
943: .Xr ssh-agent 1
1.159 djm 944: or a
945: .Cm PKCS11Provider
1.29 markus 946: offers more identities.
947: The argument to this keyword must be
948: .Dq yes
949: or
950: .Dq no .
1.84 jmc 951: This option is intended for situations where ssh-agent
1.29 markus 952: offers many different identities.
953: The default is
954: .Dq no .
1.67 jmc 955: .It Cm IdentityFile
1.192 sobrado 956: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 957: identity is read.
1.67 jmc 958: The default is
959: .Pa ~/.ssh/identity
960: for protocol version 1, and
1.139 djm 961: .Pa ~/.ssh/id_dsa ,
1.183 naddy 962: .Pa ~/.ssh/id_ecdsa ,
963: .Pa ~/.ssh/id_ed25519
1.139 djm 964: and
1.67 jmc 965: .Pa ~/.ssh/id_rsa
966: for protocol version 2.
967: Additionally, any identities represented by the authentication agent
1.165 djm 968: will be used for authentication unless
969: .Cm IdentitiesOnly
970: is set.
1.221 djm 971: If no certificates have been explicitly specified by
972: .Cm CertificateFile ,
1.129 djm 973: .Xr ssh 1
974: will try to load certificate information from the filename obtained by
975: appending
976: .Pa -cert.pub
977: to the path of a specified
978: .Cm IdentityFile .
1.90 djm 979: .Pp
1.67 jmc 980: The file name may use the tilde
1.91 jmc 981: syntax to refer to a user's home directory or one of the following
1.90 djm 982: escape characters:
983: .Ql %d
984: (local user's home directory),
985: .Ql %u
986: (local user name),
987: .Ql %l
988: (local host name),
989: .Ql %h
990: (remote host name) or
1.92 djm 991: .Ql %r
1.90 djm 992: (remote user name).
993: .Pp
1.67 jmc 994: It is possible to have
995: multiple identity files specified in configuration files; all these
996: identities will be tried in sequence.
1.152 djm 997: Multiple
998: .Cm IdentityFile
999: directives will add to the list of identities tried (this behaviour
1000: differs from that of other configuration directives).
1.165 djm 1001: .Pp
1002: .Cm IdentityFile
1003: may be used in conjunction with
1004: .Cm IdentitiesOnly
1005: to select which identities in an agent are offered during authentication.
1.221 djm 1006: .Cm IdentityFile
1007: may also be used in conjunction with
1008: .Cm CertificateFile
1009: in order to provide any certificate also needed for authentication with
1010: the identity.
1.164 jmc 1011: .It Cm IgnoreUnknown
1012: Specifies a pattern-list of unknown options to be ignored if they are
1013: encountered in configuration parsing.
1014: This may be used to suppress errors if
1015: .Nm
1016: contains options that are unrecognised by
1017: .Xr ssh 1 .
1018: It is recommended that
1019: .Cm IgnoreUnknown
1020: be listed early in the configuration file as it will not be applied
1021: to unknown options that appear before it.
1.229 djm 1022: .It Cm Include
1023: Include the specified configuration file(s).
1.230 ! jmc 1024: Multiple pathnames may be specified and each pathname may contain
1.229 djm 1025: .Xr glob 3
1026: wildcards and, for user configurations, shell-like
1027: .Dq ~
1028: references to user home directories.
1029: Files without absolute paths are assumed to be in
1030: .Pa ~/.ssh
1.230 ! jmc 1031: if included in a user configuration file or
1.229 djm 1032: .Pa /etc/ssh
1033: if included from the system configuration file.
1034: .Cm Include
1035: directive may appear inside a
1036: .Cm Match
1037: or
1038: .Cm Host
1039: block
1040: to perform conditional inclusion.
1.143 djm 1041: .It Cm IPQoS
1042: Specifies the IPv4 type-of-service or DSCP class for connections.
1043: Accepted values are
1044: .Dq af11 ,
1045: .Dq af12 ,
1046: .Dq af13 ,
1.154 djm 1047: .Dq af21 ,
1.143 djm 1048: .Dq af22 ,
1049: .Dq af23 ,
1050: .Dq af31 ,
1051: .Dq af32 ,
1052: .Dq af33 ,
1053: .Dq af41 ,
1054: .Dq af42 ,
1055: .Dq af43 ,
1056: .Dq cs0 ,
1057: .Dq cs1 ,
1058: .Dq cs2 ,
1059: .Dq cs3 ,
1060: .Dq cs4 ,
1061: .Dq cs5 ,
1062: .Dq cs6 ,
1063: .Dq cs7 ,
1064: .Dq ef ,
1065: .Dq lowdelay ,
1066: .Dq throughput ,
1067: .Dq reliability ,
1068: or a numeric value.
1.146 djm 1069: This option may take one or two arguments, separated by whitespace.
1.143 djm 1070: If one argument is specified, it is used as the packet class unconditionally.
1071: If two values are specified, the first is automatically selected for
1072: interactive sessions and the second for non-interactive sessions.
1073: The default is
1074: .Dq lowdelay
1075: for interactive sessions and
1076: .Dq throughput
1077: for non-interactive sessions.
1.103 djm 1078: .It Cm KbdInteractiveAuthentication
1079: Specifies whether to use keyboard-interactive authentication.
1080: The argument to this keyword must be
1081: .Dq yes
1082: or
1083: .Dq no .
1084: The default is
1085: .Dq yes .
1.39 djm 1086: .It Cm KbdInteractiveDevices
1087: Specifies the list of methods to use in keyboard-interactive authentication.
1088: Multiple method names must be comma-separated.
1089: The default is to use the server specified list.
1.85 jmc 1090: The methods available vary depending on what the server supports.
1091: For an OpenSSH server,
1092: it may be zero or more of:
1093: .Dq bsdauth ,
1094: .Dq pam ,
1095: and
1096: .Dq skey .
1.140 djm 1097: .It Cm KexAlgorithms
1098: Specifies the available KEX (Key Exchange) algorithms.
1099: Multiple algorithms must be comma-separated.
1.214 djm 1100: Alternately if the specified value begins with a
1101: .Sq +
1102: character, then the specified methods will be appended to the default set
1103: instead of replacing them.
1.141 jmc 1104: The default is:
1105: .Bd -literal -offset indent
1.179 markus 1106: curve25519-sha256@libssh.org,
1.141 jmc 1107: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1108: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1109: diffie-hellman-group-exchange-sha1,
1.212 djm 1110: diffie-hellman-group14-sha1
1.141 jmc 1111: .Ed
1.198 djm 1112: .Pp
1113: The list of available key exchange algorithms may also be obtained using the
1114: .Fl Q
1115: option of
1116: .Xr ssh 1
1117: with an argument of
1118: .Dq kex .
1.65 reyk 1119: .It Cm LocalCommand
1120: Specifies a command to execute on the local machine after successfully
1121: connecting to the server.
1122: The command string extends to the end of the line, and is executed with
1.105 jmc 1123: the user's shell.
1.109 dtucker 1124: The following escape character substitutions will be performed:
1125: .Ql %d
1126: (local user's home directory),
1127: .Ql %h
1128: (remote host name),
1129: .Ql %l
1130: (local host name),
1131: .Ql %n
1132: (host name as provided on the command line),
1133: .Ql %p
1134: (remote port),
1135: .Ql %r
1136: (remote user name) or
1137: .Ql %u
1.188 djm 1138: (local user name) or
1.189 jmc 1139: .Ql \&%C
1.188 djm 1140: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1141: .Pp
1142: The command is run synchronously and does not have access to the
1143: session of the
1144: .Xr ssh 1
1145: that spawned it.
1146: It should not be used for interactive commands.
1147: .Pp
1.65 reyk 1148: This directive is ignored unless
1149: .Cm PermitLocalCommand
1150: has been enabled.
1.1 stevesk 1151: .It Cm LocalForward
1.74 jmc 1152: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1153: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1154: The first argument must be
1.43 djm 1155: .Sm off
1.49 jmc 1156: .Oo Ar bind_address : Oc Ar port
1.43 djm 1157: .Sm on
1.49 jmc 1158: and the second argument must be
1159: .Ar host : Ns Ar hostport .
1.138 djm 1160: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1161: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1162: given on the command line.
1.1 stevesk 1163: Only the superuser can forward privileged ports.
1.43 djm 1164: By default, the local port is bound in accordance with the
1165: .Cm GatewayPorts
1166: setting.
1167: However, an explicit
1168: .Ar bind_address
1169: may be used to bind the connection to a specific address.
1170: The
1171: .Ar bind_address
1172: of
1173: .Dq localhost
1.46 jmc 1174: indicates that the listening port be bound for local use only, while an
1175: empty address or
1176: .Sq *
1.43 djm 1177: indicates that the port should be available from all interfaces.
1.1 stevesk 1178: .It Cm LogLevel
1179: Gives the verbosity level that is used when logging messages from
1.84 jmc 1180: .Xr ssh 1 .
1.1 stevesk 1181: The possible values are:
1.84 jmc 1182: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1183: The default is INFO.
1184: DEBUG and DEBUG1 are equivalent.
1185: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1186: .It Cm MACs
1187: Specifies the MAC (message authentication code) algorithms
1188: in order of preference.
1.226 jmc 1189: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1190: Multiple algorithms must be comma-separated.
1.214 djm 1191: If the specified value begins with a
1192: .Sq +
1193: character, then the specified algorithms will be appended to the default set
1194: instead of replacing them.
1195: .Pp
1.160 markus 1196: The algorithms that contain
1197: .Dq -etm
1198: calculate the MAC after encryption (encrypt-then-mac).
1199: These are considered safer and their use recommended.
1.214 djm 1200: .Pp
1.84 jmc 1201: The default is:
1.101 jmc 1202: .Bd -literal -offset indent
1.160 markus 1203: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1204: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1205: hmac-sha1-etm@openssh.com,
1.186 naddy 1206: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1207: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1208: .Ed
1.198 djm 1209: .Pp
1210: The list of available MAC algorithms may also be obtained using the
1211: .Fl Q
1212: option of
1213: .Xr ssh 1
1214: with an argument of
1215: .Dq mac .
1.1 stevesk 1216: .It Cm NoHostAuthenticationForLocalhost
1217: This option can be used if the home directory is shared across machines.
1218: In this case localhost will refer to a different machine on each of
1219: the machines and the user will get many warnings about changed host keys.
1220: However, this option disables host authentication for localhost.
1221: The argument to this keyword must be
1222: .Dq yes
1223: or
1224: .Dq no .
1225: The default is to check the host key for localhost.
1226: .It Cm NumberOfPasswordPrompts
1227: Specifies the number of password prompts before giving up.
1228: The argument to this keyword must be an integer.
1.84 jmc 1229: The default is 3.
1.1 stevesk 1230: .It Cm PasswordAuthentication
1231: Specifies whether to use password authentication.
1232: The argument to this keyword must be
1233: .Dq yes
1234: or
1235: .Dq no .
1236: The default is
1237: .Dq yes .
1.65 reyk 1238: .It Cm PermitLocalCommand
1239: Allow local command execution via the
1240: .Ic LocalCommand
1241: option or using the
1.66 jmc 1242: .Ic !\& Ns Ar command
1.65 reyk 1243: escape sequence in
1244: .Xr ssh 1 .
1245: The argument must be
1246: .Dq yes
1247: or
1248: .Dq no .
1249: The default is
1250: .Dq no .
1.127 markus 1251: .It Cm PKCS11Provider
1252: Specifies which PKCS#11 provider to use.
1.144 jmc 1253: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1254: .Xr ssh 1
1.128 markus 1255: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1256: private RSA key.
1.67 jmc 1257: .It Cm Port
1258: Specifies the port number to connect on the remote host.
1.84 jmc 1259: The default is 22.
1.1 stevesk 1260: .It Cm PreferredAuthentications
1.226 jmc 1261: Specifies the order in which the client should try authentication methods.
1.48 jmc 1262: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1263: .Cm keyboard-interactive )
1.48 jmc 1264: over another method (e.g.\&
1.131 jmc 1265: .Cm password ) .
1266: The default is:
1267: .Bd -literal -offset indent
1268: gssapi-with-mic,hostbased,publickey,
1269: keyboard-interactive,password
1270: .Ed
1.1 stevesk 1271: .It Cm Protocol
1272: Specifies the protocol versions
1.84 jmc 1273: .Xr ssh 1
1.1 stevesk 1274: should support in order of preference.
1275: The possible values are
1.84 jmc 1276: .Sq 1
1.1 stevesk 1277: and
1.84 jmc 1278: .Sq 2 .
1.1 stevesk 1279: Multiple versions must be comma-separated.
1.120 markus 1280: When this option is set to
1.121 jmc 1281: .Dq 2,1
1.120 markus 1282: .Nm ssh
1283: will try version 2 and fall back to version 1
1284: if version 2 is not available.
1.1 stevesk 1285: The default is
1.121 jmc 1286: .Sq 2 .
1.225 djm 1287: Protocol 1 suffers from a number of cryptographic weaknesses and should
1288: not be used.
1289: It is only offered to support legacy devices.
1.1 stevesk 1290: .It Cm ProxyCommand
1291: Specifies the command to use to connect to the server.
1292: The command
1.190 djm 1293: string extends to the end of the line, and is executed
1294: using the user's shell
1295: .Ql exec
1296: directive to avoid a lingering shell process.
1297: .Pp
1.133 jmc 1298: In the command string, any occurrence of
1.1 stevesk 1299: .Ql %h
1300: will be substituted by the host name to
1.132 djm 1301: connect,
1.1 stevesk 1302: .Ql %p
1.133 jmc 1303: by the port, and
1304: .Ql %r
1.132 djm 1305: by the remote user name.
1.1 stevesk 1306: The command can be basically anything,
1307: and should read from its standard input and write to its standard output.
1308: It should eventually connect an
1309: .Xr sshd 8
1310: server running on some machine, or execute
1311: .Ic sshd -i
1312: somewhere.
1313: Host key management will be done using the
1314: HostName of the host being connected (defaulting to the name typed by
1315: the user).
1.7 jmc 1316: Setting the command to
1317: .Dq none
1.6 markus 1318: disables this option entirely.
1.1 stevesk 1319: Note that
1320: .Cm CheckHostIP
1321: is not available for connects with a proxy command.
1.52 djm 1322: .Pp
1323: This directive is useful in conjunction with
1324: .Xr nc 1
1325: and its proxy support.
1.53 jmc 1326: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1327: 192.0.2.0:
1328: .Bd -literal -offset 3n
1329: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1330: .Ed
1.167 djm 1331: .It Cm ProxyUseFdpass
1.168 jmc 1332: Specifies that
1.167 djm 1333: .Cm ProxyCommand
1334: will pass a connected file descriptor back to
1.168 jmc 1335: .Xr ssh 1
1.167 djm 1336: instead of continuing to execute and pass data.
1337: The default is
1338: .Dq no .
1.213 markus 1339: .It Cm PubkeyAcceptedKeyTypes
1340: Specifies the key types that will be used for public key authentication
1341: as a comma-separated pattern list.
1.214 djm 1342: Alternately if the specified value begins with a
1343: .Sq +
1344: character, then the key types after it will be appended to the default
1345: instead of replacing it.
1.213 markus 1346: The default for this option is:
1347: .Bd -literal -offset 3n
1348: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1349: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1350: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1351: ssh-ed25519-cert-v01@openssh.com,
1352: ssh-rsa-cert-v01@openssh.com,
1353: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.227 djm 1354: ssh-ed25519,ssh-rsa
1.213 markus 1355: .Ed
1356: .Pp
1357: The
1358: .Fl Q
1359: option of
1360: .Xr ssh 1
1361: may be used to list supported key types.
1.1 stevesk 1362: .It Cm PubkeyAuthentication
1363: Specifies whether to try public key authentication.
1364: The argument to this keyword must be
1365: .Dq yes
1366: or
1367: .Dq no .
1368: The default is
1369: .Dq yes .
1.75 dtucker 1370: .It Cm RekeyLimit
1371: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1372: session key is renegotiated, optionally followed a maximum amount of
1373: time that may pass before the session key is renegotiated.
1374: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1375: .Sq K ,
1376: .Sq M ,
1.75 dtucker 1377: or
1.76 jmc 1378: .Sq G
1.75 dtucker 1379: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1380: The default is between
1.84 jmc 1381: .Sq 1G
1.75 dtucker 1382: and
1.84 jmc 1383: .Sq 4G ,
1.75 dtucker 1384: depending on the cipher.
1.162 dtucker 1385: The optional second value is specified in seconds and may use any of the
1386: units documented in the
1.166 jmc 1387: TIME FORMATS section of
1.162 dtucker 1388: .Xr sshd_config 5 .
1389: The default value for
1390: .Cm RekeyLimit
1391: is
1392: .Dq default none ,
1393: which means that rekeying is performed after the cipher's default amount
1394: of data has been sent or received and no time based rekeying is done.
1.1 stevesk 1395: .It Cm RemoteForward
1.74 jmc 1396: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1397: the secure channel to the specified host and port from the local machine.
1.49 jmc 1398: The first argument must be
1.43 djm 1399: .Sm off
1.49 jmc 1400: .Oo Ar bind_address : Oc Ar port
1.43 djm 1401: .Sm on
1.49 jmc 1402: and the second argument must be
1403: .Ar host : Ns Ar hostport .
1.138 djm 1404: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1405: Multiple forwardings may be specified, and additional
1406: forwardings can be given on the command line.
1.113 stevesk 1407: Privileged ports can be forwarded only when
1408: logging in as root on the remote machine.
1.118 jmc 1409: .Pp
1.117 djm 1410: If the
1411: .Ar port
1412: argument is
1413: .Ql 0 ,
1414: the listen port will be dynamically allocated on the server and reported
1415: to the client at run time.
1.43 djm 1416: .Pp
1417: If the
1418: .Ar bind_address
1419: is not specified, the default is to only bind to loopback addresses.
1420: If the
1421: .Ar bind_address
1422: is
1423: .Ql *
1424: or an empty string, then the forwarding is requested to listen on all
1425: interfaces.
1426: Specifying a remote
1427: .Ar bind_address
1.46 jmc 1428: will only succeed if the server's
1429: .Cm GatewayPorts
1.43 djm 1430: option is enabled (see
1.46 jmc 1431: .Xr sshd_config 5 ) .
1.149 djm 1432: .It Cm RequestTTY
1433: Specifies whether to request a pseudo-tty for the session.
1434: The argument may be one of:
1435: .Dq no
1436: (never request a TTY),
1437: .Dq yes
1438: (always request a TTY when standard input is a TTY),
1439: .Dq force
1440: (always request a TTY) or
1441: .Dq auto
1442: (request a TTY when opening a login session).
1443: This option mirrors the
1444: .Fl t
1445: and
1446: .Fl T
1447: flags for
1448: .Xr ssh 1 .
1.196 djm 1449: .It Cm RevokedHostKeys
1450: Specifies revoked host public keys.
1451: Keys listed in this file will be refused for host authentication.
1452: Note that if this file does not exist or is not readable,
1453: then host authentication will be refused for all hosts.
1454: Keys may be specified as a text file, listing one public key per line, or as
1455: an OpenSSH Key Revocation List (KRL) as generated by
1456: .Xr ssh-keygen 1 .
1457: For more information on KRLs, see the KEY REVOCATION LISTS section in
1458: .Xr ssh-keygen 1 .
1.1 stevesk 1459: .It Cm RhostsRSAAuthentication
1460: Specifies whether to try rhosts based authentication with RSA host
1461: authentication.
1462: The argument must be
1463: .Dq yes
1464: or
1465: .Dq no .
1466: The default is
1467: .Dq no .
1468: This option applies to protocol version 1 only and requires
1.84 jmc 1469: .Xr ssh 1
1.1 stevesk 1470: to be setuid root.
1471: .It Cm RSAAuthentication
1472: Specifies whether to try RSA authentication.
1473: The argument to this keyword must be
1474: .Dq yes
1475: or
1476: .Dq no .
1477: RSA authentication will only be
1478: attempted if the identity file exists, or an authentication agent is
1479: running.
1480: The default is
1481: .Dq yes .
1482: Note that this option applies to protocol version 1 only.
1.32 djm 1483: .It Cm SendEnv
1484: Specifies what variables from the local
1485: .Xr environ 7
1486: should be sent to the server.
1.84 jmc 1487: The server must also support it, and the server must be configured to
1.33 djm 1488: accept these environment variables.
1.207 dtucker 1489: Note that the
1490: .Ev TERM
1.208 jmc 1491: environment variable is always sent whenever a
1.207 dtucker 1492: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1493: Refer to
1494: .Cm AcceptEnv
1495: in
1496: .Xr sshd_config 5
1497: for how to configure the server.
1.80 jmc 1498: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1499: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1500: across multiple
1501: .Cm SendEnv
1502: directives.
1503: The default is not to send any environment variables.
1.81 jmc 1504: .Pp
1505: See
1506: .Sx PATTERNS
1507: for more information on patterns.
1.28 markus 1508: .It Cm ServerAliveCountMax
1.73 jmc 1509: Sets the number of server alive messages (see below) which may be
1.28 markus 1510: sent without
1.84 jmc 1511: .Xr ssh 1
1.28 markus 1512: receiving any messages back from the server.
1513: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1514: ssh will disconnect from the server, terminating the session.
1.28 markus 1515: It is important to note that the use of server alive messages is very
1516: different from
1517: .Cm TCPKeepAlive
1518: (below).
1519: The server alive messages are sent through the encrypted channel
1520: and therefore will not be spoofable.
1521: The TCP keepalive option enabled by
1522: .Cm TCPKeepAlive
1523: is spoofable.
1524: The server alive mechanism is valuable when the client or
1525: server depend on knowing when a connection has become inactive.
1526: .Pp
1527: The default value is 3.
1528: If, for example,
1529: .Cm ServerAliveInterval
1.84 jmc 1530: (see below) is set to 15 and
1.28 markus 1531: .Cm ServerAliveCountMax
1.84 jmc 1532: is left at the default, if the server becomes unresponsive,
1533: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1534: .It Cm ServerAliveInterval
1535: Sets a timeout interval in seconds after which if no data has been received
1536: from the server,
1.84 jmc 1537: .Xr ssh 1
1.67 jmc 1538: will send a message through the encrypted
1539: channel to request a response from the server.
1540: The default
1541: is 0, indicating that these messages will not be sent to the server.
1.191 millert 1542: .It Cm StreamLocalBindMask
1543: Sets the octal file creation mode mask
1544: .Pq umask
1545: used when creating a Unix-domain socket file for local or remote
1546: port forwarding.
1547: This option is only used for port forwarding to a Unix-domain socket file.
1548: .Pp
1549: The default value is 0177, which creates a Unix-domain socket file that is
1550: readable and writable only by the owner.
1551: Note that not all operating systems honor the file mode on Unix-domain
1552: socket files.
1553: .It Cm StreamLocalBindUnlink
1554: Specifies whether to remove an existing Unix-domain socket file for local
1555: or remote port forwarding before creating a new one.
1556: If the socket file already exists and
1557: .Cm StreamLocalBindUnlink
1558: is not enabled,
1559: .Nm ssh
1560: will be unable to forward the port to the Unix-domain socket file.
1561: This option is only used for port forwarding to a Unix-domain socket file.
1562: .Pp
1563: The argument must be
1564: .Dq yes
1565: or
1566: .Dq no .
1567: The default is
1568: .Dq no .
1.1 stevesk 1569: .It Cm StrictHostKeyChecking
1570: If this flag is set to
1571: .Dq yes ,
1.84 jmc 1572: .Xr ssh 1
1.1 stevesk 1573: will never automatically add host keys to the
1.50 djm 1574: .Pa ~/.ssh/known_hosts
1.1 stevesk 1575: file, and refuses to connect to hosts whose host key has changed.
1576: This provides maximum protection against trojan horse attacks,
1.84 jmc 1577: though it can be annoying when the
1.1 stevesk 1578: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1579: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1580: frequently made.
1581: This option forces the user to manually
1582: add all new hosts.
1583: If this flag is set to
1584: .Dq no ,
1.84 jmc 1585: ssh will automatically add new host keys to the
1.1 stevesk 1586: user known hosts files.
1587: If this flag is set to
1588: .Dq ask ,
1589: new host keys
1590: will be added to the user known host files only after the user
1591: has confirmed that is what they really want to do, and
1.84 jmc 1592: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1593: The host keys of
1594: known hosts will be verified automatically in all cases.
1595: The argument must be
1596: .Dq yes ,
1.84 jmc 1597: .Dq no ,
1.1 stevesk 1598: or
1599: .Dq ask .
1600: The default is
1601: .Dq ask .
1.26 markus 1602: .It Cm TCPKeepAlive
1603: Specifies whether the system should send TCP keepalive messages to the
1604: other side.
1605: If they are sent, death of the connection or crash of one
1606: of the machines will be properly noticed.
1607: However, this means that
1608: connections will die if the route is down temporarily, and some people
1609: find it annoying.
1610: .Pp
1611: The default is
1612: .Dq yes
1613: (to send TCP keepalive messages), and the client will notice
1614: if the network goes down or the remote host dies.
1615: This is important in scripts, and many users want it too.
1616: .Pp
1617: To disable TCP keepalive messages, the value should be set to
1618: .Dq no .
1.65 reyk 1619: .It Cm Tunnel
1.95 stevesk 1620: Request
1.65 reyk 1621: .Xr tun 4
1.69 jmc 1622: device forwarding between the client and the server.
1.65 reyk 1623: The argument must be
1.68 reyk 1624: .Dq yes ,
1.95 stevesk 1625: .Dq point-to-point
1626: (layer 3),
1627: .Dq ethernet
1628: (layer 2),
1.65 reyk 1629: or
1630: .Dq no .
1.95 stevesk 1631: Specifying
1632: .Dq yes
1633: requests the default tunnel mode, which is
1634: .Dq point-to-point .
1.65 reyk 1635: The default is
1636: .Dq no .
1637: .It Cm TunnelDevice
1.95 stevesk 1638: Specifies the
1.65 reyk 1639: .Xr tun 4
1.95 stevesk 1640: devices to open on the client
1641: .Pq Ar local_tun
1642: and the server
1643: .Pq Ar remote_tun .
1644: .Pp
1645: The argument must be
1646: .Sm off
1647: .Ar local_tun Op : Ar remote_tun .
1648: .Sm on
1649: The devices may be specified by numerical ID or the keyword
1650: .Dq any ,
1651: which uses the next available tunnel device.
1652: If
1653: .Ar remote_tun
1654: is not specified, it defaults to
1655: .Dq any .
1656: The default is
1657: .Dq any:any .
1.201 djm 1658: .It Cm UpdateHostKeys
1.200 djm 1659: Specifies whether
1660: .Xr ssh 1
1661: should accept notifications of additional hostkeys from the server sent
1662: after authentication has completed and add them to
1663: .Cm UserKnownHostsFile .
1664: The argument must be
1.204 djm 1665: .Dq yes ,
1.203 djm 1666: .Dq no
1.204 djm 1667: (the default) or
1668: .Dq ask .
1.200 djm 1669: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1670: and supports graceful key rotation by allowing a server to send replacement
1671: public keys before old ones are removed.
1.200 djm 1672: Additional hostkeys are only accepted if the key used to authenticate the
1.220 sobrado 1673: host was already trusted or explicitly accepted by the user.
1.204 djm 1674: If
1675: .Cm UpdateHostKeys
1676: is set to
1677: .Dq ask ,
1678: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1679: Confirmation is currently incompatible with
1680: .Cm ControlPersist ,
1681: and will be disabled if it is enabled.
1.200 djm 1682: .Pp
1683: Presently, only
1684: .Xr sshd 8
1685: from OpenSSH 6.8 and greater support the
1686: .Dq hostkeys@openssh.com
1687: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1688: .It Cm UsePrivilegedPort
1689: Specifies whether to use a privileged port for outgoing connections.
1690: The argument must be
1691: .Dq yes
1692: or
1693: .Dq no .
1694: The default is
1695: .Dq no .
1696: If set to
1.84 jmc 1697: .Dq yes ,
1698: .Xr ssh 1
1.72 jmc 1699: must be setuid root.
1700: Note that this option must be set to
1701: .Dq yes
1702: for
1703: .Cm RhostsRSAAuthentication
1704: with older servers.
1.1 stevesk 1705: .It Cm User
1706: Specifies the user to log in as.
1707: This can be useful when a different user name is used on different machines.
1708: This saves the trouble of
1709: having to remember to give the user name on the command line.
1710: .It Cm UserKnownHostsFile
1.151 djm 1711: Specifies one or more files to use for the user
1712: host key database, separated by whitespace.
1713: The default is
1714: .Pa ~/.ssh/known_hosts ,
1715: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1716: .It Cm VerifyHostKeyDNS
1717: Specifies whether to verify the remote key using DNS and SSHFP resource
1718: records.
1.24 jakob 1719: If this option is set to
1720: .Dq yes ,
1.25 jmc 1721: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1722: from DNS.
1723: Insecure fingerprints will be handled as if this option was set to
1724: .Dq ask .
1725: If this option is set to
1726: .Dq ask ,
1727: information on fingerprint match will be displayed, but the user will still
1728: need to confirm new host keys according to the
1729: .Cm StrictHostKeyChecking
1730: option.
1731: The argument must be
1732: .Dq yes ,
1.84 jmc 1733: .Dq no ,
1.25 jmc 1734: or
1735: .Dq ask .
1.8 jakob 1736: The default is
1737: .Dq no .
1.84 jmc 1738: .Pp
1.166 jmc 1739: See also VERIFYING HOST KEYS in
1.84 jmc 1740: .Xr ssh 1 .
1.111 grunk 1741: .It Cm VisualHostKey
1742: If this flag is set to
1743: .Dq yes ,
1744: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1745: printed in addition to the fingerprint string at login and
1.114 stevesk 1746: for unknown host keys.
1.111 grunk 1747: If this flag is set to
1748: .Dq no ,
1.114 stevesk 1749: no fingerprint strings are printed at login and
1.197 djm 1750: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1751: The default is
1752: .Dq no .
1.1 stevesk 1753: .It Cm XAuthLocation
1.5 stevesk 1754: Specifies the full pathname of the
1.1 stevesk 1755: .Xr xauth 1
1756: program.
1757: The default is
1758: .Pa /usr/X11R6/bin/xauth .
1759: .El
1.86 jmc 1760: .Sh PATTERNS
1761: A
1762: .Em pattern
1763: consists of zero or more non-whitespace characters,
1764: .Sq *
1765: (a wildcard that matches zero or more characters),
1766: or
1767: .Sq ?\&
1768: (a wildcard that matches exactly one character).
1769: For example, to specify a set of declarations for any host in the
1770: .Dq .co.uk
1771: set of domains,
1772: the following pattern could be used:
1773: .Pp
1774: .Dl Host *.co.uk
1775: .Pp
1776: The following pattern
1777: would match any host in the 192.168.0.[0-9] network range:
1778: .Pp
1779: .Dl Host 192.168.0.?
1780: .Pp
1781: A
1782: .Em pattern-list
1783: is a comma-separated list of patterns.
1784: Patterns within pattern-lists may be negated
1785: by preceding them with an exclamation mark
1786: .Pq Sq !\& .
1787: For example,
1.174 djm 1788: to allow a key to be used from anywhere within an organization
1.86 jmc 1789: except from the
1790: .Dq dialup
1791: pool,
1792: the following entry (in authorized_keys) could be used:
1793: .Pp
1794: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1795: .Sh FILES
1796: .Bl -tag -width Ds
1.50 djm 1797: .It Pa ~/.ssh/config
1.1 stevesk 1798: This is the per-user configuration file.
1799: The format of this file is described above.
1.84 jmc 1800: This file is used by the SSH client.
1.30 djm 1801: Because of the potential for abuse, this file must have strict permissions:
1802: read/write for the user, and not accessible by others.
1.1 stevesk 1803: .It Pa /etc/ssh/ssh_config
1804: Systemwide configuration file.
1805: This file provides defaults for those
1806: values that are not specified in the user's configuration file, and
1807: for those users who do not have a configuration file.
1808: This file must be world-readable.
1809: .El
1.13 jmc 1810: .Sh SEE ALSO
1811: .Xr ssh 1
1.1 stevesk 1812: .Sh AUTHORS
1813: OpenSSH is a derivative of the original and free
1814: ssh 1.2.12 release by Tatu Ylonen.
1815: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1816: Theo de Raadt and Dug Song
1817: removed many bugs, re-added newer features and
1818: created OpenSSH.
1819: Markus Friedl contributed the support for SSH
1820: protocol versions 1.5 and 2.0.
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