Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.225
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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1.225 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.224 2016/02/11 02:56:32 djm Exp $
! 37: .Dd $Mdocdate: February 11 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: Note that this option applies to protocol version 2 only.
828: .It Cm GSSAPIDelegateCredentials
829: Forward (delegate) credentials to the server.
830: The default is
831: .Dq no .
832: Note that this option applies to protocol version 2 only.
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 .
859: This option applies to protocol version 2 only and
860: is similar to
861: .Cm RhostsRSAAuthentication .
1.202 djm 862: .It Cm HostbasedKeyTypes
863: Specifies the key types that will be used for hostbased authentication
864: as a comma-separated pattern list.
1.214 djm 865: Alternately if the specified value begins with a
866: .Sq +
867: character, then the specified key types will be appended to the default set
868: instead of replacing them.
1.213 markus 869: The default for this option is:
870: .Bd -literal -offset 3n
871: ecdsa-sha2-nistp256-cert-v01@openssh.com,
872: ecdsa-sha2-nistp384-cert-v01@openssh.com,
873: ecdsa-sha2-nistp521-cert-v01@openssh.com,
874: ssh-ed25519-cert-v01@openssh.com,
875: ssh-rsa-cert-v01@openssh.com,
876: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 877: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 878: .Ed
879: .Pp
1.202 djm 880: The
881: .Fl Q
882: option of
883: .Xr ssh 1
884: may be used to list supported key types.
1.1 stevesk 885: .It Cm HostKeyAlgorithms
886: Specifies the protocol version 2 host key algorithms
887: that the client wants to use in order of preference.
1.214 djm 888: Alternately if the specified value begins with a
889: .Sq +
890: character, then the specified key types will be appended to the default set
891: instead of replacing them.
1.1 stevesk 892: The default for this option is:
1.139 djm 893: .Bd -literal -offset 3n
894: ecdsa-sha2-nistp256-cert-v01@openssh.com,
895: ecdsa-sha2-nistp384-cert-v01@openssh.com,
896: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 897: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 898: ssh-rsa-cert-v01@openssh.com,
1.139 djm 899: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 900: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.139 djm 901: .Ed
1.145 djm 902: .Pp
903: If hostkeys are known for the destination host then this default is modified
904: to prefer their algorithms.
1.198 djm 905: .Pp
906: The list of available key types may also be obtained using the
907: .Fl Q
908: option of
909: .Xr ssh 1
910: with an argument of
911: .Dq key .
1.1 stevesk 912: .It Cm HostKeyAlias
913: Specifies an alias that should be used instead of the
914: real host name when looking up or saving the host key
915: in the host key database files.
1.84 jmc 916: This option is useful for tunneling SSH connections
1.1 stevesk 917: or for multiple servers running on a single host.
918: .It Cm HostName
919: Specifies the real host name to log into.
920: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 921: If the hostname contains the character sequence
922: .Ql %h ,
1.150 jmc 923: then this will be replaced with the host name specified on the command line
1.136 djm 924: (this is useful for manipulating unqualified names).
1.187 djm 925: The character sequence
926: .Ql %%
927: will be replaced by a single
928: .Ql %
929: character, which may be used when specifying IPv6 link-local addresses.
930: .Pp
1.84 jmc 931: The default is the name given on the command line.
1.1 stevesk 932: Numeric IP addresses are also permitted (both on the command line and in
933: .Cm HostName
934: specifications).
1.29 markus 935: .It Cm IdentitiesOnly
936: Specifies that
1.84 jmc 937: .Xr ssh 1
1.221 djm 938: should only use the authentication identity and certificate files explicitly
939: configured in the
1.31 jmc 940: .Nm
1.221 djm 941: files
942: or passed on the
943: .Xr ssh 1
944: command-line,
1.84 jmc 945: even if
946: .Xr ssh-agent 1
1.159 djm 947: or a
948: .Cm PKCS11Provider
1.29 markus 949: offers more identities.
950: The argument to this keyword must be
951: .Dq yes
952: or
953: .Dq no .
1.84 jmc 954: This option is intended for situations where ssh-agent
1.29 markus 955: offers many different identities.
956: The default is
957: .Dq no .
1.67 jmc 958: .It Cm IdentityFile
1.192 sobrado 959: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 960: identity is read.
1.67 jmc 961: The default is
962: .Pa ~/.ssh/identity
963: for protocol version 1, and
1.139 djm 964: .Pa ~/.ssh/id_dsa ,
1.183 naddy 965: .Pa ~/.ssh/id_ecdsa ,
966: .Pa ~/.ssh/id_ed25519
1.139 djm 967: and
1.67 jmc 968: .Pa ~/.ssh/id_rsa
969: for protocol version 2.
970: Additionally, any identities represented by the authentication agent
1.165 djm 971: will be used for authentication unless
972: .Cm IdentitiesOnly
973: is set.
1.221 djm 974: If no certificates have been explicitly specified by
975: .Cm CertificateFile ,
1.129 djm 976: .Xr ssh 1
977: will try to load certificate information from the filename obtained by
978: appending
979: .Pa -cert.pub
980: to the path of a specified
981: .Cm IdentityFile .
1.90 djm 982: .Pp
1.67 jmc 983: The file name may use the tilde
1.91 jmc 984: syntax to refer to a user's home directory or one of the following
1.90 djm 985: escape characters:
986: .Ql %d
987: (local user's home directory),
988: .Ql %u
989: (local user name),
990: .Ql %l
991: (local host name),
992: .Ql %h
993: (remote host name) or
1.92 djm 994: .Ql %r
1.90 djm 995: (remote user name).
996: .Pp
1.67 jmc 997: It is possible to have
998: multiple identity files specified in configuration files; all these
999: identities will be tried in sequence.
1.152 djm 1000: Multiple
1001: .Cm IdentityFile
1002: directives will add to the list of identities tried (this behaviour
1003: differs from that of other configuration directives).
1.165 djm 1004: .Pp
1005: .Cm IdentityFile
1006: may be used in conjunction with
1007: .Cm IdentitiesOnly
1008: to select which identities in an agent are offered during authentication.
1.221 djm 1009: .Cm IdentityFile
1010: may also be used in conjunction with
1011: .Cm CertificateFile
1012: in order to provide any certificate also needed for authentication with
1013: the identity.
1.164 jmc 1014: .It Cm IgnoreUnknown
1015: Specifies a pattern-list of unknown options to be ignored if they are
1016: encountered in configuration parsing.
1017: This may be used to suppress errors if
1018: .Nm
1019: contains options that are unrecognised by
1020: .Xr ssh 1 .
1021: It is recommended that
1022: .Cm IgnoreUnknown
1023: be listed early in the configuration file as it will not be applied
1024: to unknown options that appear before it.
1.143 djm 1025: .It Cm IPQoS
1026: Specifies the IPv4 type-of-service or DSCP class for connections.
1027: Accepted values are
1028: .Dq af11 ,
1029: .Dq af12 ,
1030: .Dq af13 ,
1.154 djm 1031: .Dq af21 ,
1.143 djm 1032: .Dq af22 ,
1033: .Dq af23 ,
1034: .Dq af31 ,
1035: .Dq af32 ,
1036: .Dq af33 ,
1037: .Dq af41 ,
1038: .Dq af42 ,
1039: .Dq af43 ,
1040: .Dq cs0 ,
1041: .Dq cs1 ,
1042: .Dq cs2 ,
1043: .Dq cs3 ,
1044: .Dq cs4 ,
1045: .Dq cs5 ,
1046: .Dq cs6 ,
1047: .Dq cs7 ,
1048: .Dq ef ,
1049: .Dq lowdelay ,
1050: .Dq throughput ,
1051: .Dq reliability ,
1052: or a numeric value.
1.146 djm 1053: This option may take one or two arguments, separated by whitespace.
1.143 djm 1054: If one argument is specified, it is used as the packet class unconditionally.
1055: If two values are specified, the first is automatically selected for
1056: interactive sessions and the second for non-interactive sessions.
1057: The default is
1058: .Dq lowdelay
1059: for interactive sessions and
1060: .Dq throughput
1061: for non-interactive sessions.
1.103 djm 1062: .It Cm KbdInteractiveAuthentication
1063: Specifies whether to use keyboard-interactive authentication.
1064: The argument to this keyword must be
1065: .Dq yes
1066: or
1067: .Dq no .
1068: The default is
1069: .Dq yes .
1.39 djm 1070: .It Cm KbdInteractiveDevices
1071: Specifies the list of methods to use in keyboard-interactive authentication.
1072: Multiple method names must be comma-separated.
1073: The default is to use the server specified list.
1.85 jmc 1074: The methods available vary depending on what the server supports.
1075: For an OpenSSH server,
1076: it may be zero or more of:
1077: .Dq bsdauth ,
1078: .Dq pam ,
1079: and
1080: .Dq skey .
1.140 djm 1081: .It Cm KexAlgorithms
1082: Specifies the available KEX (Key Exchange) algorithms.
1083: Multiple algorithms must be comma-separated.
1.214 djm 1084: Alternately if the specified value begins with a
1085: .Sq +
1086: character, then the specified methods will be appended to the default set
1087: instead of replacing them.
1.141 jmc 1088: The default is:
1089: .Bd -literal -offset indent
1.179 markus 1090: curve25519-sha256@libssh.org,
1.141 jmc 1091: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1092: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1093: diffie-hellman-group-exchange-sha1,
1.212 djm 1094: diffie-hellman-group14-sha1
1.141 jmc 1095: .Ed
1.198 djm 1096: .Pp
1097: The list of available key exchange algorithms may also be obtained using the
1098: .Fl Q
1099: option of
1100: .Xr ssh 1
1101: with an argument of
1102: .Dq kex .
1.65 reyk 1103: .It Cm LocalCommand
1104: Specifies a command to execute on the local machine after successfully
1105: connecting to the server.
1106: The command string extends to the end of the line, and is executed with
1.105 jmc 1107: the user's shell.
1.109 dtucker 1108: The following escape character substitutions will be performed:
1109: .Ql %d
1110: (local user's home directory),
1111: .Ql %h
1112: (remote host name),
1113: .Ql %l
1114: (local host name),
1115: .Ql %n
1116: (host name as provided on the command line),
1117: .Ql %p
1118: (remote port),
1119: .Ql %r
1120: (remote user name) or
1121: .Ql %u
1.188 djm 1122: (local user name) or
1.189 jmc 1123: .Ql \&%C
1.188 djm 1124: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1125: .Pp
1126: The command is run synchronously and does not have access to the
1127: session of the
1128: .Xr ssh 1
1129: that spawned it.
1130: It should not be used for interactive commands.
1131: .Pp
1.65 reyk 1132: This directive is ignored unless
1133: .Cm PermitLocalCommand
1134: has been enabled.
1.1 stevesk 1135: .It Cm LocalForward
1.74 jmc 1136: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1137: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1138: The first argument must be
1.43 djm 1139: .Sm off
1.49 jmc 1140: .Oo Ar bind_address : Oc Ar port
1.43 djm 1141: .Sm on
1.49 jmc 1142: and the second argument must be
1143: .Ar host : Ns Ar hostport .
1.138 djm 1144: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1145: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1146: given on the command line.
1.1 stevesk 1147: Only the superuser can forward privileged ports.
1.43 djm 1148: By default, the local port is bound in accordance with the
1149: .Cm GatewayPorts
1150: setting.
1151: However, an explicit
1152: .Ar bind_address
1153: may be used to bind the connection to a specific address.
1154: The
1155: .Ar bind_address
1156: of
1157: .Dq localhost
1.46 jmc 1158: indicates that the listening port be bound for local use only, while an
1159: empty address or
1160: .Sq *
1.43 djm 1161: indicates that the port should be available from all interfaces.
1.1 stevesk 1162: .It Cm LogLevel
1163: Gives the verbosity level that is used when logging messages from
1.84 jmc 1164: .Xr ssh 1 .
1.1 stevesk 1165: The possible values are:
1.84 jmc 1166: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1167: The default is INFO.
1168: DEBUG and DEBUG1 are equivalent.
1169: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1170: .It Cm MACs
1171: Specifies the MAC (message authentication code) algorithms
1172: in order of preference.
1173: The MAC algorithm is used in protocol version 2
1174: for data integrity protection.
1175: Multiple algorithms must be comma-separated.
1.214 djm 1176: If the specified value begins with a
1177: .Sq +
1178: character, then the specified algorithms will be appended to the default set
1179: instead of replacing them.
1180: .Pp
1.160 markus 1181: The algorithms that contain
1182: .Dq -etm
1183: calculate the MAC after encryption (encrypt-then-mac).
1184: These are considered safer and their use recommended.
1.214 djm 1185: .Pp
1.84 jmc 1186: The default is:
1.101 jmc 1187: .Bd -literal -offset indent
1.160 markus 1188: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1189: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1190: hmac-sha1-etm@openssh.com,
1.186 naddy 1191: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1192: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1193: .Ed
1.198 djm 1194: .Pp
1195: The list of available MAC algorithms may also be obtained using the
1196: .Fl Q
1197: option of
1198: .Xr ssh 1
1199: with an argument of
1200: .Dq mac .
1.1 stevesk 1201: .It Cm NoHostAuthenticationForLocalhost
1202: This option can be used if the home directory is shared across machines.
1203: In this case localhost will refer to a different machine on each of
1204: the machines and the user will get many warnings about changed host keys.
1205: However, this option disables host authentication for localhost.
1206: The argument to this keyword must be
1207: .Dq yes
1208: or
1209: .Dq no .
1210: The default is to check the host key for localhost.
1211: .It Cm NumberOfPasswordPrompts
1212: Specifies the number of password prompts before giving up.
1213: The argument to this keyword must be an integer.
1.84 jmc 1214: The default is 3.
1.1 stevesk 1215: .It Cm PasswordAuthentication
1216: Specifies whether to use password authentication.
1217: The argument to this keyword must be
1218: .Dq yes
1219: or
1220: .Dq no .
1221: The default is
1222: .Dq yes .
1.65 reyk 1223: .It Cm PermitLocalCommand
1224: Allow local command execution via the
1225: .Ic LocalCommand
1226: option or using the
1.66 jmc 1227: .Ic !\& Ns Ar command
1.65 reyk 1228: escape sequence in
1229: .Xr ssh 1 .
1230: The argument must be
1231: .Dq yes
1232: or
1233: .Dq no .
1234: The default is
1235: .Dq no .
1.127 markus 1236: .It Cm PKCS11Provider
1237: Specifies which PKCS#11 provider to use.
1.144 jmc 1238: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1239: .Xr ssh 1
1.128 markus 1240: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1241: private RSA key.
1.67 jmc 1242: .It Cm Port
1243: Specifies the port number to connect on the remote host.
1.84 jmc 1244: The default is 22.
1.1 stevesk 1245: .It Cm PreferredAuthentications
1246: Specifies the order in which the client should try protocol 2
1.11 jmc 1247: authentication methods.
1.48 jmc 1248: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1249: .Cm keyboard-interactive )
1.48 jmc 1250: over another method (e.g.\&
1.131 jmc 1251: .Cm password ) .
1252: The default is:
1253: .Bd -literal -offset indent
1254: gssapi-with-mic,hostbased,publickey,
1255: keyboard-interactive,password
1256: .Ed
1.1 stevesk 1257: .It Cm Protocol
1258: Specifies the protocol versions
1.84 jmc 1259: .Xr ssh 1
1.1 stevesk 1260: should support in order of preference.
1261: The possible values are
1.84 jmc 1262: .Sq 1
1.1 stevesk 1263: and
1.84 jmc 1264: .Sq 2 .
1.1 stevesk 1265: Multiple versions must be comma-separated.
1.120 markus 1266: When this option is set to
1.121 jmc 1267: .Dq 2,1
1.120 markus 1268: .Nm ssh
1269: will try version 2 and fall back to version 1
1270: if version 2 is not available.
1.1 stevesk 1271: The default is
1.121 jmc 1272: .Sq 2 .
1.225 ! djm 1273: Protocol 1 suffers from a number of cryptographic weaknesses and should
! 1274: not be used.
! 1275: It is only offered to support legacy devices.
1.1 stevesk 1276: .It Cm ProxyCommand
1277: Specifies the command to use to connect to the server.
1278: The command
1.190 djm 1279: string extends to the end of the line, and is executed
1280: using the user's shell
1281: .Ql exec
1282: directive to avoid a lingering shell process.
1283: .Pp
1.133 jmc 1284: In the command string, any occurrence of
1.1 stevesk 1285: .Ql %h
1286: will be substituted by the host name to
1.132 djm 1287: connect,
1.1 stevesk 1288: .Ql %p
1.133 jmc 1289: by the port, and
1290: .Ql %r
1.132 djm 1291: by the remote user name.
1.1 stevesk 1292: The command can be basically anything,
1293: and should read from its standard input and write to its standard output.
1294: It should eventually connect an
1295: .Xr sshd 8
1296: server running on some machine, or execute
1297: .Ic sshd -i
1298: somewhere.
1299: Host key management will be done using the
1300: HostName of the host being connected (defaulting to the name typed by
1301: the user).
1.7 jmc 1302: Setting the command to
1303: .Dq none
1.6 markus 1304: disables this option entirely.
1.1 stevesk 1305: Note that
1306: .Cm CheckHostIP
1307: is not available for connects with a proxy command.
1.52 djm 1308: .Pp
1309: This directive is useful in conjunction with
1310: .Xr nc 1
1311: and its proxy support.
1.53 jmc 1312: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1313: 192.0.2.0:
1314: .Bd -literal -offset 3n
1315: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1316: .Ed
1.167 djm 1317: .It Cm ProxyUseFdpass
1.168 jmc 1318: Specifies that
1.167 djm 1319: .Cm ProxyCommand
1320: will pass a connected file descriptor back to
1.168 jmc 1321: .Xr ssh 1
1.167 djm 1322: instead of continuing to execute and pass data.
1323: The default is
1324: .Dq no .
1.213 markus 1325: .It Cm PubkeyAcceptedKeyTypes
1326: Specifies the key types that will be used for public key authentication
1327: as a comma-separated pattern list.
1.214 djm 1328: Alternately if the specified value begins with a
1329: .Sq +
1330: character, then the key types after it will be appended to the default
1331: instead of replacing it.
1.213 markus 1332: The default for this option is:
1333: .Bd -literal -offset 3n
1334: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1335: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1336: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1337: ssh-ed25519-cert-v01@openssh.com,
1338: ssh-rsa-cert-v01@openssh.com,
1339: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.224 djm 1340: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 1341: .Ed
1342: .Pp
1343: The
1344: .Fl Q
1345: option of
1346: .Xr ssh 1
1347: may be used to list supported key types.
1.1 stevesk 1348: .It Cm PubkeyAuthentication
1349: Specifies whether to try public key authentication.
1350: The argument to this keyword must be
1351: .Dq yes
1352: or
1353: .Dq no .
1354: The default is
1355: .Dq yes .
1356: This option applies to protocol version 2 only.
1.75 dtucker 1357: .It Cm RekeyLimit
1358: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1359: session key is renegotiated, optionally followed a maximum amount of
1360: time that may pass before the session key is renegotiated.
1361: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1362: .Sq K ,
1363: .Sq M ,
1.75 dtucker 1364: or
1.76 jmc 1365: .Sq G
1.75 dtucker 1366: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1367: The default is between
1.84 jmc 1368: .Sq 1G
1.75 dtucker 1369: and
1.84 jmc 1370: .Sq 4G ,
1.75 dtucker 1371: depending on the cipher.
1.162 dtucker 1372: The optional second value is specified in seconds and may use any of the
1373: units documented in the
1.166 jmc 1374: TIME FORMATS section of
1.162 dtucker 1375: .Xr sshd_config 5 .
1376: The default value for
1377: .Cm RekeyLimit
1378: is
1379: .Dq default none ,
1380: which means that rekeying is performed after the cipher's default amount
1381: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1382: This option applies to protocol version 2 only.
1.1 stevesk 1383: .It Cm RemoteForward
1.74 jmc 1384: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1385: the secure channel to the specified host and port from the local machine.
1.49 jmc 1386: The first argument must be
1.43 djm 1387: .Sm off
1.49 jmc 1388: .Oo Ar bind_address : Oc Ar port
1.43 djm 1389: .Sm on
1.49 jmc 1390: and the second argument must be
1391: .Ar host : Ns Ar hostport .
1.138 djm 1392: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1393: Multiple forwardings may be specified, and additional
1394: forwardings can be given on the command line.
1.113 stevesk 1395: Privileged ports can be forwarded only when
1396: logging in as root on the remote machine.
1.118 jmc 1397: .Pp
1.117 djm 1398: If the
1399: .Ar port
1400: argument is
1401: .Ql 0 ,
1402: the listen port will be dynamically allocated on the server and reported
1403: to the client at run time.
1.43 djm 1404: .Pp
1405: If the
1406: .Ar bind_address
1407: is not specified, the default is to only bind to loopback addresses.
1408: If the
1409: .Ar bind_address
1410: is
1411: .Ql *
1412: or an empty string, then the forwarding is requested to listen on all
1413: interfaces.
1414: Specifying a remote
1415: .Ar bind_address
1.46 jmc 1416: will only succeed if the server's
1417: .Cm GatewayPorts
1.43 djm 1418: option is enabled (see
1.46 jmc 1419: .Xr sshd_config 5 ) .
1.149 djm 1420: .It Cm RequestTTY
1421: Specifies whether to request a pseudo-tty for the session.
1422: The argument may be one of:
1423: .Dq no
1424: (never request a TTY),
1425: .Dq yes
1426: (always request a TTY when standard input is a TTY),
1427: .Dq force
1428: (always request a TTY) or
1429: .Dq auto
1430: (request a TTY when opening a login session).
1431: This option mirrors the
1432: .Fl t
1433: and
1434: .Fl T
1435: flags for
1436: .Xr ssh 1 .
1.196 djm 1437: .It Cm RevokedHostKeys
1438: Specifies revoked host public keys.
1439: Keys listed in this file will be refused for host authentication.
1440: Note that if this file does not exist or is not readable,
1441: then host authentication will be refused for all hosts.
1442: Keys may be specified as a text file, listing one public key per line, or as
1443: an OpenSSH Key Revocation List (KRL) as generated by
1444: .Xr ssh-keygen 1 .
1445: For more information on KRLs, see the KEY REVOCATION LISTS section in
1446: .Xr ssh-keygen 1 .
1.1 stevesk 1447: .It Cm RhostsRSAAuthentication
1448: Specifies whether to try rhosts based authentication with RSA host
1449: authentication.
1450: The argument must be
1451: .Dq yes
1452: or
1453: .Dq no .
1454: The default is
1455: .Dq no .
1456: This option applies to protocol version 1 only and requires
1.84 jmc 1457: .Xr ssh 1
1.1 stevesk 1458: to be setuid root.
1459: .It Cm RSAAuthentication
1460: Specifies whether to try RSA authentication.
1461: The argument to this keyword must be
1462: .Dq yes
1463: or
1464: .Dq no .
1465: RSA authentication will only be
1466: attempted if the identity file exists, or an authentication agent is
1467: running.
1468: The default is
1469: .Dq yes .
1470: Note that this option applies to protocol version 1 only.
1.32 djm 1471: .It Cm SendEnv
1472: Specifies what variables from the local
1473: .Xr environ 7
1474: should be sent to the server.
1.84 jmc 1475: Note that environment passing is only supported for protocol 2.
1476: The server must also support it, and the server must be configured to
1.33 djm 1477: accept these environment variables.
1.207 dtucker 1478: Note that the
1479: .Ev TERM
1.208 jmc 1480: environment variable is always sent whenever a
1.207 dtucker 1481: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1482: Refer to
1483: .Cm AcceptEnv
1484: in
1485: .Xr sshd_config 5
1486: for how to configure the server.
1.80 jmc 1487: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1488: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1489: across multiple
1490: .Cm SendEnv
1491: directives.
1492: The default is not to send any environment variables.
1.81 jmc 1493: .Pp
1494: See
1495: .Sx PATTERNS
1496: for more information on patterns.
1.28 markus 1497: .It Cm ServerAliveCountMax
1.73 jmc 1498: Sets the number of server alive messages (see below) which may be
1.28 markus 1499: sent without
1.84 jmc 1500: .Xr ssh 1
1.28 markus 1501: receiving any messages back from the server.
1502: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1503: ssh will disconnect from the server, terminating the session.
1.28 markus 1504: It is important to note that the use of server alive messages is very
1505: different from
1506: .Cm TCPKeepAlive
1507: (below).
1508: The server alive messages are sent through the encrypted channel
1509: and therefore will not be spoofable.
1510: The TCP keepalive option enabled by
1511: .Cm TCPKeepAlive
1512: is spoofable.
1513: The server alive mechanism is valuable when the client or
1514: server depend on knowing when a connection has become inactive.
1515: .Pp
1516: The default value is 3.
1517: If, for example,
1518: .Cm ServerAliveInterval
1.84 jmc 1519: (see below) is set to 15 and
1.28 markus 1520: .Cm ServerAliveCountMax
1.84 jmc 1521: is left at the default, if the server becomes unresponsive,
1522: ssh will disconnect after approximately 45 seconds.
1.89 markus 1523: This option applies to protocol version 2 only.
1.67 jmc 1524: .It Cm ServerAliveInterval
1525: Sets a timeout interval in seconds after which if no data has been received
1526: from the server,
1.84 jmc 1527: .Xr ssh 1
1.67 jmc 1528: will send a message through the encrypted
1529: channel to request a response from the server.
1530: The default
1531: is 0, indicating that these messages will not be sent to the server.
1532: This option applies to protocol version 2 only.
1.191 millert 1533: .It Cm StreamLocalBindMask
1534: Sets the octal file creation mode mask
1535: .Pq umask
1536: used when creating a Unix-domain socket file for local or remote
1537: port forwarding.
1538: This option is only used for port forwarding to a Unix-domain socket file.
1539: .Pp
1540: The default value is 0177, which creates a Unix-domain socket file that is
1541: readable and writable only by the owner.
1542: Note that not all operating systems honor the file mode on Unix-domain
1543: socket files.
1544: .It Cm StreamLocalBindUnlink
1545: Specifies whether to remove an existing Unix-domain socket file for local
1546: or remote port forwarding before creating a new one.
1547: If the socket file already exists and
1548: .Cm StreamLocalBindUnlink
1549: is not enabled,
1550: .Nm ssh
1551: will be unable to forward the port to the Unix-domain socket file.
1552: This option is only used for port forwarding to a Unix-domain socket file.
1553: .Pp
1554: The argument must be
1555: .Dq yes
1556: or
1557: .Dq no .
1558: The default is
1559: .Dq no .
1.1 stevesk 1560: .It Cm StrictHostKeyChecking
1561: If this flag is set to
1562: .Dq yes ,
1.84 jmc 1563: .Xr ssh 1
1.1 stevesk 1564: will never automatically add host keys to the
1.50 djm 1565: .Pa ~/.ssh/known_hosts
1.1 stevesk 1566: file, and refuses to connect to hosts whose host key has changed.
1567: This provides maximum protection against trojan horse attacks,
1.84 jmc 1568: though it can be annoying when the
1.1 stevesk 1569: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1570: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1571: frequently made.
1572: This option forces the user to manually
1573: add all new hosts.
1574: If this flag is set to
1575: .Dq no ,
1.84 jmc 1576: ssh will automatically add new host keys to the
1.1 stevesk 1577: user known hosts files.
1578: If this flag is set to
1579: .Dq ask ,
1580: new host keys
1581: will be added to the user known host files only after the user
1582: has confirmed that is what they really want to do, and
1.84 jmc 1583: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1584: The host keys of
1585: known hosts will be verified automatically in all cases.
1586: The argument must be
1587: .Dq yes ,
1.84 jmc 1588: .Dq no ,
1.1 stevesk 1589: or
1590: .Dq ask .
1591: The default is
1592: .Dq ask .
1.26 markus 1593: .It Cm TCPKeepAlive
1594: Specifies whether the system should send TCP keepalive messages to the
1595: other side.
1596: If they are sent, death of the connection or crash of one
1597: of the machines will be properly noticed.
1598: However, this means that
1599: connections will die if the route is down temporarily, and some people
1600: find it annoying.
1601: .Pp
1602: The default is
1603: .Dq yes
1604: (to send TCP keepalive messages), and the client will notice
1605: if the network goes down or the remote host dies.
1606: This is important in scripts, and many users want it too.
1607: .Pp
1608: To disable TCP keepalive messages, the value should be set to
1609: .Dq no .
1.65 reyk 1610: .It Cm Tunnel
1.95 stevesk 1611: Request
1.65 reyk 1612: .Xr tun 4
1.69 jmc 1613: device forwarding between the client and the server.
1.65 reyk 1614: The argument must be
1.68 reyk 1615: .Dq yes ,
1.95 stevesk 1616: .Dq point-to-point
1617: (layer 3),
1618: .Dq ethernet
1619: (layer 2),
1.65 reyk 1620: or
1621: .Dq no .
1.95 stevesk 1622: Specifying
1623: .Dq yes
1624: requests the default tunnel mode, which is
1625: .Dq point-to-point .
1.65 reyk 1626: The default is
1627: .Dq no .
1628: .It Cm TunnelDevice
1.95 stevesk 1629: Specifies the
1.65 reyk 1630: .Xr tun 4
1.95 stevesk 1631: devices to open on the client
1632: .Pq Ar local_tun
1633: and the server
1634: .Pq Ar remote_tun .
1635: .Pp
1636: The argument must be
1637: .Sm off
1638: .Ar local_tun Op : Ar remote_tun .
1639: .Sm on
1640: The devices may be specified by numerical ID or the keyword
1641: .Dq any ,
1642: which uses the next available tunnel device.
1643: If
1644: .Ar remote_tun
1645: is not specified, it defaults to
1646: .Dq any .
1647: The default is
1648: .Dq any:any .
1.201 djm 1649: .It Cm UpdateHostKeys
1.200 djm 1650: Specifies whether
1651: .Xr ssh 1
1652: should accept notifications of additional hostkeys from the server sent
1653: after authentication has completed and add them to
1654: .Cm UserKnownHostsFile .
1655: The argument must be
1.204 djm 1656: .Dq yes ,
1.203 djm 1657: .Dq no
1.204 djm 1658: (the default) or
1659: .Dq ask .
1.200 djm 1660: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1661: and supports graceful key rotation by allowing a server to send replacement
1662: public keys before old ones are removed.
1.200 djm 1663: Additional hostkeys are only accepted if the key used to authenticate the
1.220 sobrado 1664: host was already trusted or explicitly accepted by the user.
1.204 djm 1665: If
1666: .Cm UpdateHostKeys
1667: is set to
1668: .Dq ask ,
1669: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1670: Confirmation is currently incompatible with
1671: .Cm ControlPersist ,
1672: and will be disabled if it is enabled.
1.200 djm 1673: .Pp
1674: Presently, only
1675: .Xr sshd 8
1676: from OpenSSH 6.8 and greater support the
1677: .Dq hostkeys@openssh.com
1678: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1679: .It Cm UsePrivilegedPort
1680: Specifies whether to use a privileged port for outgoing connections.
1681: The argument must be
1682: .Dq yes
1683: or
1684: .Dq no .
1685: The default is
1686: .Dq no .
1687: If set to
1.84 jmc 1688: .Dq yes ,
1689: .Xr ssh 1
1.72 jmc 1690: must be setuid root.
1691: Note that this option must be set to
1692: .Dq yes
1693: for
1694: .Cm RhostsRSAAuthentication
1695: with older servers.
1.1 stevesk 1696: .It Cm User
1697: Specifies the user to log in as.
1698: This can be useful when a different user name is used on different machines.
1699: This saves the trouble of
1700: having to remember to give the user name on the command line.
1701: .It Cm UserKnownHostsFile
1.151 djm 1702: Specifies one or more files to use for the user
1703: host key database, separated by whitespace.
1704: The default is
1705: .Pa ~/.ssh/known_hosts ,
1706: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1707: .It Cm VerifyHostKeyDNS
1708: Specifies whether to verify the remote key using DNS and SSHFP resource
1709: records.
1.24 jakob 1710: If this option is set to
1711: .Dq yes ,
1.25 jmc 1712: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1713: from DNS.
1714: Insecure fingerprints will be handled as if this option was set to
1715: .Dq ask .
1716: If this option is set to
1717: .Dq ask ,
1718: information on fingerprint match will be displayed, but the user will still
1719: need to confirm new host keys according to the
1720: .Cm StrictHostKeyChecking
1721: option.
1722: The argument must be
1723: .Dq yes ,
1.84 jmc 1724: .Dq no ,
1.25 jmc 1725: or
1726: .Dq ask .
1.8 jakob 1727: The default is
1728: .Dq no .
1.12 jakob 1729: Note that this option applies to protocol version 2 only.
1.84 jmc 1730: .Pp
1.166 jmc 1731: See also VERIFYING HOST KEYS in
1.84 jmc 1732: .Xr ssh 1 .
1.111 grunk 1733: .It Cm VisualHostKey
1734: If this flag is set to
1735: .Dq yes ,
1736: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1737: printed in addition to the fingerprint string at login and
1.114 stevesk 1738: for unknown host keys.
1.111 grunk 1739: If this flag is set to
1740: .Dq no ,
1.114 stevesk 1741: no fingerprint strings are printed at login and
1.197 djm 1742: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1743: The default is
1744: .Dq no .
1.1 stevesk 1745: .It Cm XAuthLocation
1.5 stevesk 1746: Specifies the full pathname of the
1.1 stevesk 1747: .Xr xauth 1
1748: program.
1749: The default is
1750: .Pa /usr/X11R6/bin/xauth .
1751: .El
1.86 jmc 1752: .Sh PATTERNS
1753: A
1754: .Em pattern
1755: consists of zero or more non-whitespace characters,
1756: .Sq *
1757: (a wildcard that matches zero or more characters),
1758: or
1759: .Sq ?\&
1760: (a wildcard that matches exactly one character).
1761: For example, to specify a set of declarations for any host in the
1762: .Dq .co.uk
1763: set of domains,
1764: the following pattern could be used:
1765: .Pp
1766: .Dl Host *.co.uk
1767: .Pp
1768: The following pattern
1769: would match any host in the 192.168.0.[0-9] network range:
1770: .Pp
1771: .Dl Host 192.168.0.?
1772: .Pp
1773: A
1774: .Em pattern-list
1775: is a comma-separated list of patterns.
1776: Patterns within pattern-lists may be negated
1777: by preceding them with an exclamation mark
1778: .Pq Sq !\& .
1779: For example,
1.174 djm 1780: to allow a key to be used from anywhere within an organization
1.86 jmc 1781: except from the
1782: .Dq dialup
1783: pool,
1784: the following entry (in authorized_keys) could be used:
1785: .Pp
1786: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1787: .Sh FILES
1788: .Bl -tag -width Ds
1.50 djm 1789: .It Pa ~/.ssh/config
1.1 stevesk 1790: This is the per-user configuration file.
1791: The format of this file is described above.
1.84 jmc 1792: This file is used by the SSH client.
1.30 djm 1793: Because of the potential for abuse, this file must have strict permissions:
1794: read/write for the user, and not accessible by others.
1.1 stevesk 1795: .It Pa /etc/ssh/ssh_config
1796: Systemwide configuration file.
1797: This file provides defaults for those
1798: values that are not specified in the user's configuration file, and
1799: for those users who do not have a configuration file.
1800: This file must be world-readable.
1801: .El
1.13 jmc 1802: .Sh SEE ALSO
1803: .Xr ssh 1
1.1 stevesk 1804: .Sh AUTHORS
1805: OpenSSH is a derivative of the original and free
1806: ssh 1.2.12 release by Tatu Ylonen.
1807: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1808: Theo de Raadt and Dug Song
1809: removed many bugs, re-added newer features and
1810: created OpenSSH.
1811: Markus Friedl contributed the support for SSH
1812: protocol versions 1.5 and 2.0.