Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.371
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
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1.371 ! naddy 36: .\" $OpenBSD: ssh_config.5,v 1.370 2022/03/31 17:27:27 naddy Exp $
! 37: .Dd $Mdocdate: March 31 2022 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
1.310 jmc 42: .Nd OpenSSH client configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.84 jmc 44: .Xr ssh 1
1.1 stevesk 45: obtains configuration data from the following sources in
46: the following order:
1.79 jmc 47: .Pp
1.2 stevesk 48: .Bl -enum -offset indent -compact
49: .It
50: command-line options
51: .It
52: user's configuration file
1.50 djm 53: .Pq Pa ~/.ssh/config
1.2 stevesk 54: .It
55: system-wide configuration file
56: .Pq Pa /etc/ssh/ssh_config
57: .El
1.1 stevesk 58: .Pp
59: For each parameter, the first obtained value
60: will be used.
1.41 jmc 61: The configuration files contain sections separated by
1.240 jmc 62: .Cm Host
1.1 stevesk 63: specifications, and that section is only applied for hosts that
64: match one of the patterns given in the specification.
1.193 djm 65: The matched host name is usually the one given on the command line
66: (see the
67: .Cm CanonicalizeHostname
1.240 jmc 68: option for exceptions).
1.1 stevesk 69: .Pp
70: Since the first obtained value for each parameter is used, more
71: host-specific declarations should be given near the beginning of the
72: file, and general defaults at the end.
1.80 jmc 73: .Pp
1.240 jmc 74: The file contains keyword-argument pairs, one per line.
75: Lines starting with
1.1 stevesk 76: .Ql #
1.240 jmc 77: and empty lines are interpreted as comments.
78: Arguments may optionally be enclosed in double quotes
79: .Pq \&"
80: in order to represent arguments containing spaces.
1.1 stevesk 81: Configuration options may be separated by whitespace or
82: optional whitespace and exactly one
83: .Ql = ;
84: the latter format is useful to avoid the need to quote whitespace
85: when specifying configuration options using the
86: .Nm ssh ,
1.87 jmc 87: .Nm scp ,
1.1 stevesk 88: and
89: .Nm sftp
90: .Fl o
91: option.
92: .Pp
93: The possible
94: keywords and their meanings are as follows (note that
95: keywords are case-insensitive and arguments are case-sensitive):
96: .Bl -tag -width Ds
97: .It Cm Host
98: Restricts the following declarations (up to the next
99: .Cm Host
1.169 djm 100: or
101: .Cm Match
1.1 stevesk 102: keyword) to be only for those hosts that match one of the patterns
103: given after the keyword.
1.112 krw 104: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 105: A single
1.83 jmc 106: .Ql *
1.1 stevesk 107: as a pattern can be used to provide global
108: defaults for all hosts.
1.193 djm 109: The host is usually the
1.1 stevesk 110: .Ar hostname
1.193 djm 111: argument given on the command line
112: (see the
113: .Cm CanonicalizeHostname
1.240 jmc 114: keyword for exceptions).
1.148 djm 115: .Pp
116: A pattern entry may be negated by prefixing it with an exclamation mark
117: .Pq Sq !\& .
118: If a negated entry is matched, then the
119: .Cm Host
120: entry is ignored, regardless of whether any other patterns on the line
121: match.
122: Negated matches are therefore useful to provide exceptions for wildcard
123: matches.
1.81 jmc 124: .Pp
125: See
126: .Sx PATTERNS
127: for more information on patterns.
1.170 jmc 128: .It Cm Match
1.169 djm 129: Restricts the following declarations (up to the next
130: .Cm Host
131: or
132: .Cm Match
133: keyword) to be used only when the conditions following the
134: .Cm Match
135: keyword are satisfied.
1.220 sobrado 136: Match conditions are specified using one or more criteria
1.178 dtucker 137: or the single token
138: .Cm all
1.193 djm 139: which always matches.
140: The available criteria keywords are:
141: .Cm canonical ,
1.287 djm 142: .Cm final ,
1.176 djm 143: .Cm exec ,
1.169 djm 144: .Cm host ,
145: .Cm originalhost ,
146: .Cm user ,
147: and
148: .Cm localuser .
1.193 djm 149: The
150: .Cm all
151: criteria must appear alone or immediately after
1.287 djm 152: .Cm canonical
153: or
154: .Cm final .
1.193 djm 155: Other criteria may be combined arbitrarily.
156: All criteria but
1.288 jmc 157: .Cm all ,
158: .Cm canonical ,
1.193 djm 159: and
1.287 djm 160: .Cm final
1.193 djm 161: require an argument.
162: Criteria may be negated by prepending an exclamation mark
163: .Pq Sq !\& .
1.169 djm 164: .Pp
1.177 jmc 165: The
1.193 djm 166: .Cm canonical
1.210 dtucker 167: keyword matches only when the configuration file is being re-parsed
1.193 djm 168: after hostname canonicalization (see the
169: .Cm CanonicalizeHostname
1.288 jmc 170: option).
1.193 djm 171: This may be useful to specify conditions that work with canonical host
172: names only.
1.287 djm 173: .Pp
174: The
175: .Cm final
176: keyword requests that the configuration be re-parsed (regardless of whether
177: .Cm CanonicalizeHostname
178: is enabled), and matches only during this final pass.
179: If
180: .Cm CanonicalizeHostname
181: is enabled, then
182: .Cm canonical
183: and
184: .Cm final
185: match during the same pass.
186: .Pp
1.193 djm 187: The
1.176 djm 188: .Cm exec
1.177 jmc 189: keyword executes the specified command under the user's shell.
1.169 djm 190: If the command returns a zero exit status then the condition is considered true.
191: Commands containing whitespace characters must be quoted.
1.239 jmc 192: Arguments to
193: .Cm exec
194: accept the tokens described in the
195: .Sx TOKENS
196: section.
1.169 djm 197: .Pp
198: The other keywords' criteria must be single entries or comma-separated
199: lists and may use the wildcard and negation operators described in the
200: .Sx PATTERNS
201: section.
202: The criteria for the
203: .Cm host
204: keyword are matched against the target hostname, after any substitution
205: by the
1.295 jmc 206: .Cm Hostname
1.193 djm 207: or
208: .Cm CanonicalizeHostname
209: options.
1.169 djm 210: The
211: .Cm originalhost
212: keyword matches against the hostname as it was specified on the command-line.
213: The
214: .Cm user
215: keyword matches against the target username on the remote host.
216: The
217: .Cm localuser
218: keyword matches against the name of the local user running
219: .Xr ssh 1
220: (this keyword may be useful in system-wide
221: .Nm
222: files).
1.222 jcs 223: .It Cm AddKeysToAgent
224: Specifies whether keys should be automatically added to a running
1.223 jmc 225: .Xr ssh-agent 1 .
1.222 jcs 226: If this option is set to
1.240 jmc 227: .Cm yes
1.222 jcs 228: and a key is loaded from a file, the key and its passphrase are added to
229: the agent with the default lifetime, as if by
230: .Xr ssh-add 1 .
231: If this option is set to
1.240 jmc 232: .Cm ask ,
233: .Xr ssh 1
1.222 jcs 234: will require confirmation using the
235: .Ev SSH_ASKPASS
236: program before adding a key (see
237: .Xr ssh-add 1
238: for details).
239: If this option is set to
1.240 jmc 240: .Cm confirm ,
1.222 jcs 241: each use of the key must be confirmed, as if the
242: .Fl c
243: option was specified to
244: .Xr ssh-add 1 .
245: If this option is set to
1.240 jmc 246: .Cm no ,
1.222 jcs 247: no keys are added to the agent.
1.332 djm 248: Alternately, this option may be specified as a time interval
249: using the format described in the
250: .Sx TIME FORMATS
251: section of
252: .Xr sshd_config 5
253: to specify the key's lifetime in
254: .Xr ssh-agent 1 ,
255: after which it will automatically be removed.
1.222 jcs 256: The argument must be
1.332 djm 257: .Cm no
258: (the default),
1.240 jmc 259: .Cm yes ,
1.332 djm 260: .Cm confirm
261: (optionally followed by a time interval),
262: .Cm ask
263: or a time interval.
1.10 djm 264: .It Cm AddressFamily
1.11 jmc 265: Specifies which address family to use when connecting.
266: Valid arguments are
1.240 jmc 267: .Cm any
268: (the default),
269: .Cm inet
1.84 jmc 270: (use IPv4 only), or
1.240 jmc 271: .Cm inet6
1.40 jmc 272: (use IPv6 only).
1.1 stevesk 273: .It Cm BatchMode
274: If set to
1.240 jmc 275: .Cm yes ,
1.318 djm 276: user interaction such as password prompts and host key confirmation requests
277: will be disabled.
1.1 stevesk 278: This option is useful in scripts and other batch jobs where no user
1.318 djm 279: is present to interact with
280: .Xr ssh 1 .
1.1 stevesk 281: The argument must be
1.240 jmc 282: .Cm yes
1.1 stevesk 283: or
1.240 jmc 284: .Cm no
285: (the default).
1.268 jmc 286: .It Cm BindAddress
287: Use the specified address on the local machine as the source address of
288: the connection.
289: Only useful on systems with more than one address.
290: .It Cm BindInterface
291: Use the address of the specified interface on the local machine as the
292: source address of the connection.
1.171 djm 293: .It Cm CanonicalDomains
1.172 jmc 294: When
1.173 djm 295: .Cm CanonicalizeHostname
1.171 djm 296: is enabled, this option specifies the list of domain suffixes in which to
297: search for the specified destination host.
1.173 djm 298: .It Cm CanonicalizeFallbackLocal
1.174 djm 299: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 300: The default,
1.240 jmc 301: .Cm yes ,
1.172 jmc 302: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 303: search rules.
304: A value of
1.240 jmc 305: .Cm no
1.171 djm 306: will cause
307: .Xr ssh 1
308: to fail instantly if
1.173 djm 309: .Cm CanonicalizeHostname
1.171 djm 310: is enabled and the target hostname cannot be found in any of the domains
311: specified by
312: .Cm CanonicalDomains .
1.173 djm 313: .It Cm CanonicalizeHostname
1.174 djm 314: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 315: The default,
1.240 jmc 316: .Cm no ,
1.171 djm 317: is not to perform any name rewriting and let the system resolver handle all
318: hostname lookups.
319: If set to
1.240 jmc 320: .Cm yes
1.171 djm 321: then, for connections that do not use a
1.284 djm 322: .Cm ProxyCommand
323: or
324: .Cm ProxyJump ,
1.171 djm 325: .Xr ssh 1
1.173 djm 326: will attempt to canonicalize the hostname specified on the command line
1.171 djm 327: using the
328: .Cm CanonicalDomains
329: suffixes and
1.173 djm 330: .Cm CanonicalizePermittedCNAMEs
1.171 djm 331: rules.
332: If
1.173 djm 333: .Cm CanonicalizeHostname
1.171 djm 334: is set to
1.240 jmc 335: .Cm always ,
1.174 djm 336: then canonicalization is applied to proxied connections too.
1.185 djm 337: .Pp
1.193 djm 338: If this option is enabled, then the configuration files are processed
339: again using the new target name to pick up any new configuration in matching
1.185 djm 340: .Cm Host
1.193 djm 341: and
342: .Cm Match
1.185 djm 343: stanzas.
1.361 dtucker 344: A value of
345: .Cm none
346: disables the use of a
347: .Cm ProxyJump
348: host.
1.173 djm 349: .It Cm CanonicalizeMaxDots
1.172 jmc 350: Specifies the maximum number of dot characters in a hostname before
1.174 djm 351: canonicalization is disabled.
1.240 jmc 352: The default, 1,
1.172 jmc 353: allows a single dot (i.e. hostname.subdomain).
1.173 djm 354: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 355: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 356: canonicalizing hostnames.
1.171 djm 357: The rules consist of one or more arguments of
1.172 jmc 358: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 359: where
360: .Ar source_domain_list
1.174 djm 361: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 362: and
363: .Ar target_domain_list
1.172 jmc 364: is a pattern-list of domains that they may resolve to.
1.171 djm 365: .Pp
366: For example,
1.240 jmc 367: .Qq *.a.example.com:*.b.example.com,*.c.example.com
1.171 djm 368: will allow hostnames matching
1.240 jmc 369: .Qq *.a.example.com
1.173 djm 370: to be canonicalized to names in the
1.240 jmc 371: .Qq *.b.example.com
1.171 djm 372: or
1.240 jmc 373: .Qq *.c.example.com
1.171 djm 374: domains.
1.365 djm 375: .Pp
376: A single argument of
377: .Qq none
378: causes no CNAMEs to be considered for canonicalization.
379: This is the default behaviour.
1.283 jmc 380: .It Cm CASignatureAlgorithms
381: Specifies which algorithms are allowed for signing of certificates
382: by certificate authorities (CAs).
383: The default is:
384: .Bd -literal -offset indent
1.362 djm 385: ssh-ed25519,ecdsa-sha2-nistp256,
386: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
387: sk-ssh-ed25519@openssh.com,
388: sk-ecdsa-sha2-nistp256@openssh.com,
1.351 djm 389: rsa-sha2-512,rsa-sha2-256
1.283 jmc 390: .Ed
1.362 djm 391: .Pp
392: If the specified list begins with a
393: .Sq +
394: character, then the specified algorithms will be appended to the default set
395: instead of replacing them.
396: If the specified list begins with a
397: .Sq -
398: character, then the specified algorithms (including wildcards) will be removed
399: from the default set instead of replacing them.
1.283 jmc 400: .Pp
401: .Xr ssh 1
402: will not accept host certificates signed using algorithms other than those
403: specified.
1.221 djm 404: .It Cm CertificateFile
405: Specifies a file from which the user's certificate is read.
406: A corresponding private key must be provided separately in order
407: to use this certificate either
408: from an
409: .Cm IdentityFile
410: directive or
411: .Fl i
412: flag to
413: .Xr ssh 1 ,
414: via
415: .Xr ssh-agent 1 ,
416: or via a
1.305 naddy 417: .Cm PKCS11Provider
418: or
419: .Cm SecurityKeyProvider .
1.221 djm 420: .Pp
1.239 jmc 421: Arguments to
422: .Cm CertificateFile
1.326 dtucker 423: may use the tilde syntax to refer to a user's home directory,
424: the tokens described in the
1.239 jmc 425: .Sx TOKENS
1.326 dtucker 426: section and environment variables as described in the
427: .Sx ENVIRONMENT VARIABLES
1.239 jmc 428: section.
1.221 djm 429: .Pp
430: It is possible to have multiple certificate files specified in
431: configuration files; these certificates will be tried in sequence.
432: Multiple
433: .Cm CertificateFile
434: directives will add to the list of certificates used for
435: authentication.
1.1 stevesk 436: .It Cm CheckHostIP
1.240 jmc 437: If set to
1.370 naddy 438: .Cm yes ,
1.84 jmc 439: .Xr ssh 1
440: will additionally check the host IP address in the
1.1 stevesk 441: .Pa known_hosts
442: file.
1.240 jmc 443: This allows it to detect if a host key changed due to DNS spoofing
1.211 djm 444: and will add addresses of destination hosts to
445: .Pa ~/.ssh/known_hosts
446: in the process, regardless of the setting of
447: .Cm StrictHostKeyChecking .
1.107 grunk 448: If the option is set to
1.341 djm 449: .Cm no
450: (the default),
1.1 stevesk 451: the check will not be executed.
452: .It Cm Ciphers
1.245 djm 453: Specifies the ciphers allowed and their order of preference.
1.1 stevesk 454: Multiple ciphers must be comma-separated.
1.299 kn 455: If the specified list begins with a
1.214 djm 456: .Sq +
457: character, then the specified ciphers will be appended to the default set
458: instead of replacing them.
1.299 kn 459: If the specified list begins with a
1.241 djm 460: .Sq -
461: character, then the specified ciphers (including wildcards) will be removed
462: from the default set instead of replacing them.
1.301 naddy 463: If the specified list begins with a
464: .Sq ^
465: character, then the specified ciphers will be placed at the head of the
466: default set.
1.214 djm 467: .Pp
1.180 djm 468: The supported ciphers are:
1.240 jmc 469: .Bd -literal -offset indent
1.186 naddy 470: 3des-cbc
471: aes128-cbc
472: aes192-cbc
473: aes256-cbc
474: aes128-ctr
475: aes192-ctr
476: aes256-ctr
477: aes128-gcm@openssh.com
478: aes256-gcm@openssh.com
479: chacha20-poly1305@openssh.com
1.240 jmc 480: .Ed
1.180 djm 481: .Pp
1.84 jmc 482: The default is:
1.186 naddy 483: .Bd -literal -offset indent
1.215 jmc 484: chacha20-poly1305@openssh.com,
1.186 naddy 485: aes128-ctr,aes192-ctr,aes256-ctr,
1.270 djm 486: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 487: .Ed
1.180 djm 488: .Pp
1.240 jmc 489: The list of available ciphers may also be obtained using
490: .Qq ssh -Q cipher .
1.1 stevesk 491: .It Cm ClearAllForwardings
1.84 jmc 492: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 493: specified in the configuration files or on the command line be
1.7 jmc 494: cleared.
495: This option is primarily useful when used from the
1.84 jmc 496: .Xr ssh 1
1.1 stevesk 497: command line to clear port forwardings set in
498: configuration files, and is automatically set by
499: .Xr scp 1
500: and
501: .Xr sftp 1 .
502: The argument must be
1.240 jmc 503: .Cm yes
1.1 stevesk 504: or
1.240 jmc 505: .Cm no
506: (the default).
1.1 stevesk 507: .It Cm Compression
508: Specifies whether to use compression.
509: The argument must be
1.240 jmc 510: .Cm yes
1.1 stevesk 511: or
1.240 jmc 512: .Cm no
513: (the default).
1.247 naddy 514: .It Cm ConnectionAttempts
515: Specifies the number of tries (one per second) to make before exiting.
516: The argument must be an integer.
517: This may be useful in scripts if the connection sometimes fails.
518: The default is 1.
1.9 djm 519: .It Cm ConnectTimeout
1.84 jmc 520: Specifies the timeout (in seconds) used when connecting to the
521: SSH server, instead of using the default system TCP timeout.
1.302 djm 522: This timeout is applied both to establishing the connection and to performing
523: the initial SSH protocol handshake and key exchange.
1.36 djm 524: .It Cm ControlMaster
525: Enables the sharing of multiple sessions over a single network connection.
526: When set to
1.240 jmc 527: .Cm yes ,
1.84 jmc 528: .Xr ssh 1
1.36 djm 529: will listen for connections on a control socket specified using the
530: .Cm ControlPath
531: argument.
532: Additional sessions can connect to this socket using the same
533: .Cm ControlPath
534: with
535: .Cm ControlMaster
536: set to
1.240 jmc 537: .Cm no
1.38 jmc 538: (the default).
1.64 jmc 539: These sessions will try to reuse the master instance's network connection
1.63 djm 540: rather than initiating new ones, but will fall back to connecting normally
541: if the control socket does not exist, or is not listening.
542: .Pp
1.37 djm 543: Setting this to
1.240 jmc 544: .Cm ask
545: will cause
546: .Xr ssh 1
1.206 jmc 547: to listen for control connections, but require confirmation using
548: .Xr ssh-askpass 1 .
1.51 jakob 549: If the
550: .Cm ControlPath
1.84 jmc 551: cannot be opened,
1.240 jmc 552: .Xr ssh 1
553: will continue without connecting to a master instance.
1.58 djm 554: .Pp
555: X11 and
1.59 jmc 556: .Xr ssh-agent 1
1.58 djm 557: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 558: display and agent forwarded will be the one belonging to the master
1.59 jmc 559: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 560: .Pp
561: Two additional options allow for opportunistic multiplexing: try to use a
562: master connection but fall back to creating a new one if one does not already
563: exist.
564: These options are:
1.240 jmc 565: .Cm auto
1.56 djm 566: and
1.240 jmc 567: .Cm autoask .
1.56 djm 568: The latter requires confirmation like the
1.240 jmc 569: .Cm ask
1.56 djm 570: option.
1.36 djm 571: .It Cm ControlPath
1.55 djm 572: Specify the path to the control socket used for connection sharing as described
573: in the
1.36 djm 574: .Cm ControlMaster
1.57 djm 575: section above or the string
1.240 jmc 576: .Cm none
1.57 djm 577: to disable connection sharing.
1.239 jmc 578: Arguments to
579: .Cm ControlPath
1.326 dtucker 580: may use the tilde syntax to refer to a user's home directory,
581: the tokens described in the
1.239 jmc 582: .Sx TOKENS
1.326 dtucker 583: section and environment variables as described in the
584: .Sx ENVIRONMENT VARIABLES
1.239 jmc 585: section.
1.56 djm 586: It is recommended that any
587: .Cm ControlPath
588: used for opportunistic connection sharing include
1.195 djm 589: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
590: that is not writable by other users.
1.56 djm 591: This ensures that shared connections are uniquely identified.
1.137 djm 592: .It Cm ControlPersist
593: When used in conjunction with
594: .Cm ControlMaster ,
595: specifies that the master connection should remain open
596: in the background (waiting for future client connections)
597: after the initial client connection has been closed.
598: If set to
1.314 naddy 599: .Cm no
600: (the default),
1.137 djm 601: then the master connection will not be placed into the background,
602: and will close as soon as the initial client connection is closed.
603: If set to
1.240 jmc 604: .Cm yes
605: or 0,
1.137 djm 606: then the master connection will remain in the background indefinitely
607: (until killed or closed via a mechanism such as the
1.240 jmc 608: .Qq ssh -O exit ) .
1.137 djm 609: If set to a time in seconds, or a time in any of the formats documented in
610: .Xr sshd_config 5 ,
611: then the backgrounded master connection will automatically terminate
612: after it has remained idle (with no client connections) for the
613: specified time.
1.38 jmc 614: .It Cm DynamicForward
1.74 jmc 615: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 616: over the secure channel, and the application
617: protocol is then used to determine where to connect to from the
618: remote machine.
1.62 djm 619: .Pp
620: The argument must be
621: .Sm off
622: .Oo Ar bind_address : Oc Ar port .
623: .Sm on
1.138 djm 624: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 625: By default, the local port is bound in accordance with the
626: .Cm GatewayPorts
627: setting.
628: However, an explicit
629: .Ar bind_address
630: may be used to bind the connection to a specific address.
631: The
632: .Ar bind_address
633: of
1.240 jmc 634: .Cm localhost
1.62 djm 635: indicates that the listening port be bound for local use only, while an
636: empty address or
637: .Sq *
638: indicates that the port should be available from all interfaces.
639: .Pp
1.38 jmc 640: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 641: .Xr ssh 1
1.38 jmc 642: will act as a SOCKS server.
643: Multiple forwardings may be specified, and
644: additional forwardings can be given on the command line.
645: Only the superuser can forward privileged ports.
1.14 markus 646: .It Cm EnableSSHKeysign
647: Setting this option to
1.240 jmc 648: .Cm yes
1.14 markus 649: in the global client configuration file
650: .Pa /etc/ssh/ssh_config
651: enables the use of the helper program
652: .Xr ssh-keysign 8
653: during
654: .Cm HostbasedAuthentication .
655: The argument must be
1.240 jmc 656: .Cm yes
1.14 markus 657: or
1.240 jmc 658: .Cm no
659: (the default).
1.23 jmc 660: This option should be placed in the non-hostspecific section.
1.14 markus 661: See
662: .Xr ssh-keysign 8
663: for more information.
1.1 stevesk 664: .It Cm EscapeChar
665: Sets the escape character (default:
666: .Ql ~ ) .
667: The escape character can also
668: be set on the command line.
669: The argument should be a single character,
670: .Ql ^
671: followed by a letter, or
1.240 jmc 672: .Cm none
1.1 stevesk 673: to disable the escape
674: character entirely (making the connection transparent for binary
675: data).
1.96 markus 676: .It Cm ExitOnForwardFailure
677: Specifies whether
678: .Xr ssh 1
679: should terminate the connection if it cannot set up all requested
1.216 djm 680: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 681: if either end is unable to bind and listen on a specified port).
1.216 djm 682: Note that
683: .Cm ExitOnForwardFailure
684: does not apply to connections made over port forwardings and will not,
685: for example, cause
686: .Xr ssh 1
687: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 688: The argument must be
1.240 jmc 689: .Cm yes
1.96 markus 690: or
1.240 jmc 691: .Cm no
692: (the default).
1.197 djm 693: .It Cm FingerprintHash
694: Specifies the hash algorithm used when displaying key fingerprints.
695: Valid options are:
1.240 jmc 696: .Cm md5
1.197 djm 697: and
1.240 jmc 698: .Cm sha256
1.359 djm 699: (the default).
700: .It Cm ForkAfterAuthentication
701: Requests
702: .Nm ssh
703: to go to background just before command execution.
704: This is useful if
705: .Nm ssh
706: is going to ask for passwords or passphrases, but the user
707: wants it in the background.
708: This implies the
709: .Cm StdinNull
710: configuration option being set to
711: .Dq yes .
712: The recommended way to start X11 programs at a remote site is with
713: something like
714: .Ic ssh -f host xterm ,
715: which is the same as
716: .Ic ssh host xterm
717: if the
718: .Cm ForkAfterAuthentication
719: configuration option is set to
720: .Dq yes .
721: .Pp
722: If the
723: .Cm ExitOnForwardFailure
724: configuration option is set to
725: .Dq yes ,
726: then a client started with the
727: .Cm ForkAfterAuthentication
728: configuration option being set to
729: .Dq yes
730: will wait for all remote port forwards to be successfully established
731: before placing itself in the background.
732: The argument to this keyword must be
733: .Cm yes
734: (same as the
735: .Fl f
736: option) or
737: .Cm no
1.240 jmc 738: (the default).
1.1 stevesk 739: .It Cm ForwardAgent
740: Specifies whether the connection to the authentication agent (if any)
741: will be forwarded to the remote machine.
1.312 djm 742: The argument may be
743: .Cm yes ,
1.240 jmc 744: .Cm no
1.312 djm 745: (the default),
746: an explicit path to an agent socket or the name of an environment variable
747: (beginning with
748: .Sq $ )
749: in which to find the path.
1.3 stevesk 750: .Pp
1.7 jmc 751: Agent forwarding should be enabled with caution.
752: Users with the ability to bypass file permissions on the remote host
753: (for the agent's Unix-domain socket)
754: can access the local agent through the forwarded connection.
755: An attacker cannot obtain key material from the agent,
1.3 stevesk 756: however they can perform operations on the keys that enable them to
757: authenticate using the identities loaded into the agent.
1.1 stevesk 758: .It Cm ForwardX11
759: Specifies whether X11 connections will be automatically redirected
760: over the secure channel and
761: .Ev DISPLAY
762: set.
763: The argument must be
1.240 jmc 764: .Cm yes
1.1 stevesk 765: or
1.240 jmc 766: .Cm no
767: (the default).
1.3 stevesk 768: .Pp
1.7 jmc 769: X11 forwarding should be enabled with caution.
770: Users with the ability to bypass file permissions on the remote host
1.22 markus 771: (for the user's X11 authorization database)
1.7 jmc 772: can access the local X11 display through the forwarded connection.
1.22 markus 773: An attacker may then be able to perform activities such as keystroke monitoring
774: if the
775: .Cm ForwardX11Trusted
776: option is also enabled.
1.134 djm 777: .It Cm ForwardX11Timeout
1.135 jmc 778: Specify a timeout for untrusted X11 forwarding
779: using the format described in the
1.240 jmc 780: .Sx TIME FORMATS
781: section of
1.134 djm 782: .Xr sshd_config 5 .
783: X11 connections received by
784: .Xr ssh 1
785: after this time will be refused.
1.285 djm 786: Setting
787: .Cm ForwardX11Timeout
788: to zero will disable the timeout and permit X11 forwarding for the life
789: of the connection.
1.134 djm 790: The default is to disable untrusted X11 forwarding after twenty minutes has
791: elapsed.
1.22 markus 792: .It Cm ForwardX11Trusted
1.34 jmc 793: If this option is set to
1.240 jmc 794: .Cm yes ,
1.84 jmc 795: remote X11 clients will have full access to the original X11 display.
1.42 djm 796: .Pp
1.22 markus 797: If this option is set to
1.240 jmc 798: .Cm no
799: (the default),
1.84 jmc 800: remote X11 clients will be considered untrusted and prevented
1.22 markus 801: from stealing or tampering with data belonging to trusted X11
802: clients.
1.42 djm 803: Furthermore, the
804: .Xr xauth 1
805: token used for the session will be set to expire after 20 minutes.
806: Remote clients will be refused access after this time.
1.22 markus 807: .Pp
808: See the X11 SECURITY extension specification for full details on
809: the restrictions imposed on untrusted clients.
1.1 stevesk 810: .It Cm GatewayPorts
811: Specifies whether remote hosts are allowed to connect to local
812: forwarded ports.
813: By default,
1.84 jmc 814: .Xr ssh 1
1.7 jmc 815: binds local port forwardings to the loopback address.
816: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 817: .Cm GatewayPorts
1.84 jmc 818: can be used to specify that ssh
1.1 stevesk 819: should bind local port forwardings to the wildcard address,
820: thus allowing remote hosts to connect to forwarded ports.
821: The argument must be
1.240 jmc 822: .Cm yes
1.1 stevesk 823: or
1.240 jmc 824: .Cm no
825: (the default).
1.1 stevesk 826: .It Cm GlobalKnownHostsFile
1.151 djm 827: Specifies one or more files to use for the global
828: host key database, separated by whitespace.
829: The default is
830: .Pa /etc/ssh/ssh_known_hosts ,
831: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 832: .It Cm GSSAPIAuthentication
1.27 markus 833: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 834: The default is
1.240 jmc 835: .Cm no .
1.18 markus 836: .It Cm GSSAPIDelegateCredentials
837: Forward (delegate) credentials to the server.
838: The default is
1.240 jmc 839: .Cm no .
1.44 djm 840: .It Cm HashKnownHosts
841: Indicates that
1.84 jmc 842: .Xr ssh 1
1.44 djm 843: should hash host names and addresses when they are added to
1.50 djm 844: .Pa ~/.ssh/known_hosts .
1.44 djm 845: These hashed names may be used normally by
1.84 jmc 846: .Xr ssh 1
1.44 djm 847: and
1.84 jmc 848: .Xr sshd 8 ,
1.316 djm 849: but they do not visually reveal identifying information if the
850: file's contents are disclosed.
1.44 djm 851: The default is
1.240 jmc 852: .Cm no .
1.97 jmc 853: Note that existing names and addresses in known hosts files
854: will not be converted automatically,
855: but may be manually hashed using
1.45 djm 856: .Xr ssh-keygen 1 .
1.344 dtucker 857: .It Cm HostbasedAcceptedAlgorithms
1.348 djm 858: Specifies the signature algorithms that will be used for hostbased
859: authentication as a comma-separated list of patterns.
1.300 naddy 860: Alternately if the specified list begins with a
1.214 djm 861: .Sq +
1.348 djm 862: character, then the specified signature algorithms will be appended
863: to the default set instead of replacing them.
1.300 naddy 864: If the specified list begins with a
1.241 djm 865: .Sq -
1.348 djm 866: character, then the specified signature algorithms (including wildcards)
867: will be removed from the default set instead of replacing them.
1.301 naddy 868: If the specified list begins with a
869: .Sq ^
1.348 djm 870: character, then the specified signature algorithms will be placed
871: at the head of the default set.
1.213 markus 872: The default for this option is:
873: .Bd -literal -offset 3n
1.333 djm 874: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 875: ecdsa-sha2-nistp256-cert-v01@openssh.com,
876: ecdsa-sha2-nistp384-cert-v01@openssh.com,
877: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 878: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 879: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 880: rsa-sha2-512-cert-v01@openssh.com,
881: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 882: ssh-ed25519,
1.213 markus 883: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 884: sk-ssh-ed25519@openssh.com,
1.311 naddy 885: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 886: rsa-sha2-512,rsa-sha2-256
1.213 markus 887: .Ed
888: .Pp
1.202 djm 889: The
890: .Fl Q
891: option of
892: .Xr ssh 1
1.348 djm 893: may be used to list supported signature algorithms.
1.344 dtucker 894: This was formerly named HostbasedKeyTypes.
1.345 naddy 895: .It Cm HostbasedAuthentication
896: Specifies whether to try rhosts based authentication with public key
897: authentication.
898: The argument must be
899: .Cm yes
900: or
901: .Cm no
902: (the default).
1.1 stevesk 903: .It Cm HostKeyAlgorithms
1.348 djm 904: Specifies the host key signature algorithms
1.1 stevesk 905: that the client wants to use in order of preference.
1.300 naddy 906: Alternately if the specified list begins with a
1.214 djm 907: .Sq +
1.348 djm 908: character, then the specified signature algorithms will be appended to
909: the default set instead of replacing them.
1.300 naddy 910: If the specified list begins with a
1.241 djm 911: .Sq -
1.348 djm 912: character, then the specified signature algorithms (including wildcards)
913: will be removed from the default set instead of replacing them.
1.301 naddy 914: If the specified list begins with a
915: .Sq ^
1.348 djm 916: character, then the specified signature algorithms will be placed
917: at the head of the default set.
1.1 stevesk 918: The default for this option is:
1.139 djm 919: .Bd -literal -offset 3n
1.333 djm 920: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 921: ecdsa-sha2-nistp256-cert-v01@openssh.com,
922: ecdsa-sha2-nistp384-cert-v01@openssh.com,
923: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 924: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 925: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 926: rsa-sha2-512-cert-v01@openssh.com,
927: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 928: ssh-ed25519,
1.139 djm 929: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 930: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 931: sk-ssh-ed25519@openssh.com,
1.366 kn 932: rsa-sha2-512,rsa-sha2-256
1.139 djm 933: .Ed
1.145 djm 934: .Pp
935: If hostkeys are known for the destination host then this default is modified
936: to prefer their algorithms.
1.198 djm 937: .Pp
1.348 djm 938: The list of available signature algorithms may also be obtained using
1.322 dtucker 939: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 940: .It Cm HostKeyAlias
941: Specifies an alias that should be used instead of the
942: real host name when looking up or saving the host key
1.251 djm 943: in the host key database files and when validating host certificates.
1.84 jmc 944: This option is useful for tunneling SSH connections
1.1 stevesk 945: or for multiple servers running on a single host.
1.295 jmc 946: .It Cm Hostname
1.1 stevesk 947: Specifies the real host name to log into.
948: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 949: Arguments to
1.295 jmc 950: .Cm Hostname
1.239 jmc 951: accept the tokens described in the
952: .Sx TOKENS
953: section.
1.1 stevesk 954: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 955: .Cm Hostname
1.1 stevesk 956: specifications).
1.239 jmc 957: The default is the name given on the command line.
1.29 markus 958: .It Cm IdentitiesOnly
959: Specifies that
1.84 jmc 960: .Xr ssh 1
1.304 djm 961: should only use the configured authentication identity and certificate files
962: (either the default files, or those explicitly configured in the
1.31 jmc 963: .Nm
1.221 djm 964: files
965: or passed on the
966: .Xr ssh 1
1.304 djm 967: command-line),
1.84 jmc 968: even if
969: .Xr ssh-agent 1
1.159 djm 970: or a
971: .Cm PKCS11Provider
1.305 naddy 972: or
973: .Cm SecurityKeyProvider
1.29 markus 974: offers more identities.
975: The argument to this keyword must be
1.240 jmc 976: .Cm yes
1.29 markus 977: or
1.240 jmc 978: .Cm no
979: (the default).
1.84 jmc 980: This option is intended for situations where ssh-agent
1.29 markus 981: offers many different identities.
1.231 markus 982: .It Cm IdentityAgent
983: Specifies the
984: .Ux Ns -domain
985: socket used to communicate with the authentication agent.
986: .Pp
987: This option overrides the
1.240 jmc 988: .Ev SSH_AUTH_SOCK
1.231 markus 989: environment variable and can be used to select a specific agent.
990: Setting the socket name to
1.240 jmc 991: .Cm none
1.231 markus 992: disables the use of an authentication agent.
1.232 markus 993: If the string
1.240 jmc 994: .Qq SSH_AUTH_SOCK
1.232 markus 995: is specified, the location of the socket will be read from the
996: .Ev SSH_AUTH_SOCK
997: environment variable.
1.286 djm 998: Otherwise if the specified value begins with a
999: .Sq $
1000: character, then it will be treated as an environment variable containing
1001: the location of the socket.
1.231 markus 1002: .Pp
1.239 jmc 1003: Arguments to
1004: .Cm IdentityAgent
1.326 dtucker 1005: may use the tilde syntax to refer to a user's home directory,
1006: the tokens described in the
1.239 jmc 1007: .Sx TOKENS
1.326 dtucker 1008: section and environment variables as described in the
1009: .Sx ENVIRONMENT VARIABLES
1.239 jmc 1010: section.
1.67 jmc 1011: .It Cm IdentityFile
1.313 naddy 1012: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1013: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.67 jmc 1014: The default is
1.368 dtucker 1015: .Pa ~/.ssh/id_rsa ,
1.183 naddy 1016: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 1017: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 1018: .Pa ~/.ssh/id_ed25519 ,
1019: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 1020: and
1.368 dtucker 1021: .Pa ~/.ssh/id_dsa .
1.67 jmc 1022: Additionally, any identities represented by the authentication agent
1.165 djm 1023: will be used for authentication unless
1024: .Cm IdentitiesOnly
1025: is set.
1.221 djm 1026: If no certificates have been explicitly specified by
1027: .Cm CertificateFile ,
1.129 djm 1028: .Xr ssh 1
1029: will try to load certificate information from the filename obtained by
1030: appending
1031: .Pa -cert.pub
1032: to the path of a specified
1033: .Cm IdentityFile .
1.90 djm 1034: .Pp
1.239 jmc 1035: Arguments to
1036: .Cm IdentityFile
1037: may use the tilde syntax to refer to a user's home directory
1038: or the tokens described in the
1039: .Sx TOKENS
1040: section.
1.90 djm 1041: .Pp
1.67 jmc 1042: It is possible to have
1043: multiple identity files specified in configuration files; all these
1044: identities will be tried in sequence.
1.152 djm 1045: Multiple
1046: .Cm IdentityFile
1047: directives will add to the list of identities tried (this behaviour
1048: differs from that of other configuration directives).
1.165 djm 1049: .Pp
1050: .Cm IdentityFile
1051: may be used in conjunction with
1052: .Cm IdentitiesOnly
1053: to select which identities in an agent are offered during authentication.
1.221 djm 1054: .Cm IdentityFile
1055: may also be used in conjunction with
1056: .Cm CertificateFile
1057: in order to provide any certificate also needed for authentication with
1058: the identity.
1.164 jmc 1059: .It Cm IgnoreUnknown
1060: Specifies a pattern-list of unknown options to be ignored if they are
1061: encountered in configuration parsing.
1062: This may be used to suppress errors if
1063: .Nm
1064: contains options that are unrecognised by
1065: .Xr ssh 1 .
1066: It is recommended that
1067: .Cm IgnoreUnknown
1068: be listed early in the configuration file as it will not be applied
1069: to unknown options that appear before it.
1.229 djm 1070: .It Cm Include
1071: Include the specified configuration file(s).
1.230 jmc 1072: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1073: .Xr glob 7
1.229 djm 1074: wildcards and, for user configurations, shell-like
1.240 jmc 1075: .Sq ~
1.229 djm 1076: references to user home directories.
1.327 djm 1077: Wildcards will be expanded and processed in lexical order.
1.229 djm 1078: Files without absolute paths are assumed to be in
1079: .Pa ~/.ssh
1.230 jmc 1080: if included in a user configuration file or
1.229 djm 1081: .Pa /etc/ssh
1082: if included from the system configuration file.
1083: .Cm Include
1084: directive may appear inside a
1085: .Cm Match
1086: or
1087: .Cm Host
1088: block
1089: to perform conditional inclusion.
1.143 djm 1090: .It Cm IPQoS
1091: Specifies the IPv4 type-of-service or DSCP class for connections.
1092: Accepted values are
1.240 jmc 1093: .Cm af11 ,
1094: .Cm af12 ,
1095: .Cm af13 ,
1096: .Cm af21 ,
1097: .Cm af22 ,
1098: .Cm af23 ,
1099: .Cm af31 ,
1100: .Cm af32 ,
1101: .Cm af33 ,
1102: .Cm af41 ,
1103: .Cm af42 ,
1104: .Cm af43 ,
1105: .Cm cs0 ,
1106: .Cm cs1 ,
1107: .Cm cs2 ,
1108: .Cm cs3 ,
1109: .Cm cs4 ,
1110: .Cm cs5 ,
1111: .Cm cs6 ,
1112: .Cm cs7 ,
1113: .Cm ef ,
1.319 djm 1114: .Cm le ,
1.240 jmc 1115: .Cm lowdelay ,
1116: .Cm throughput ,
1117: .Cm reliability ,
1.253 djm 1118: a numeric value, or
1119: .Cm none
1120: to use the operating system default.
1.146 djm 1121: This option may take one or two arguments, separated by whitespace.
1.143 djm 1122: If one argument is specified, it is used as the packet class unconditionally.
1123: If two values are specified, the first is automatically selected for
1124: interactive sessions and the second for non-interactive sessions.
1125: The default is
1.269 job 1126: .Cm af21
1.272 jmc 1127: (Low-Latency Data)
1.143 djm 1128: for interactive sessions and
1.269 job 1129: .Cm cs1
1.272 jmc 1130: (Lower Effort)
1.143 djm 1131: for non-interactive sessions.
1.103 djm 1132: .It Cm KbdInteractiveAuthentication
1133: Specifies whether to use keyboard-interactive authentication.
1134: The argument to this keyword must be
1.240 jmc 1135: .Cm yes
1136: (the default)
1.103 djm 1137: or
1.240 jmc 1138: .Cm no .
1.355 dtucker 1139: .Cm ChallengeResponseAuthentication
1140: is a deprecated alias for this.
1.39 djm 1141: .It Cm KbdInteractiveDevices
1142: Specifies the list of methods to use in keyboard-interactive authentication.
1143: Multiple method names must be comma-separated.
1144: The default is to use the server specified list.
1.85 jmc 1145: The methods available vary depending on what the server supports.
1146: For an OpenSSH server,
1147: it may be zero or more of:
1.240 jmc 1148: .Cm bsdauth ,
1149: .Cm pam ,
1.85 jmc 1150: and
1.240 jmc 1151: .Cm skey .
1.140 djm 1152: .It Cm KexAlgorithms
1153: Specifies the available KEX (Key Exchange) algorithms.
1154: Multiple algorithms must be comma-separated.
1.299 kn 1155: If the specified list begins with a
1.214 djm 1156: .Sq +
1.363 dtucker 1157: character, then the specified algorithms will be appended to the default set
1.214 djm 1158: instead of replacing them.
1.299 kn 1159: If the specified list begins with a
1.241 djm 1160: .Sq -
1.363 dtucker 1161: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1162: from the default set instead of replacing them.
1.301 naddy 1163: If the specified list begins with a
1164: .Sq ^
1.363 dtucker 1165: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1166: default set.
1.141 jmc 1167: The default is:
1168: .Bd -literal -offset indent
1.371 ! naddy 1169: sntrup761x25519-sha512@openssh.com,
1.238 djm 1170: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1171: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1172: diffie-hellman-group-exchange-sha256,
1.266 djm 1173: diffie-hellman-group16-sha512,
1174: diffie-hellman-group18-sha512,
1.317 tedu 1175: diffie-hellman-group14-sha256
1.141 jmc 1176: .Ed
1.198 djm 1177: .Pp
1.240 jmc 1178: The list of available key exchange algorithms may also be obtained using
1179: .Qq ssh -Q kex .
1.339 djm 1180: .It Cm KnownHostsCommand
1.340 jmc 1181: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1182: those listed in
1183: .Cm UserKnownHostsFile
1184: and
1185: .Cm GlobalKnownHostsFile .
1186: This command is executed after the files have been read.
1.340 jmc 1187: It may write host key lines to standard output in identical format to the
1.339 djm 1188: usual files (described in the
1189: .Sx VERIFYING HOST KEYS
1190: section in
1191: .Xr ssh 1 ) .
1192: Arguments to
1193: .Cm KnownHostsCommand
1194: accept the tokens described in the
1195: .Sx TOKENS
1196: section.
1.340 jmc 1197: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1198: the preference list of host key algorithms to use, again to obtain the
1199: host key for the requested host name and, if
1200: .Cm CheckHostIP
1201: is enabled, one more time to obtain the host key matching the server's
1202: address.
1203: If the command exits abnormally or returns a non-zero exit status then the
1204: connection is terminated.
1.65 reyk 1205: .It Cm LocalCommand
1206: Specifies a command to execute on the local machine after successfully
1207: connecting to the server.
1208: The command string extends to the end of the line, and is executed with
1.105 jmc 1209: the user's shell.
1.239 jmc 1210: Arguments to
1211: .Cm LocalCommand
1212: accept the tokens described in the
1213: .Sx TOKENS
1214: section.
1.123 djm 1215: .Pp
1216: The command is run synchronously and does not have access to the
1217: session of the
1218: .Xr ssh 1
1219: that spawned it.
1220: It should not be used for interactive commands.
1221: .Pp
1.65 reyk 1222: This directive is ignored unless
1223: .Cm PermitLocalCommand
1224: has been enabled.
1.1 stevesk 1225: .It Cm LocalForward
1.74 jmc 1226: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1227: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1228: The first argument specifies the listener and may be
1.43 djm 1229: .Sm off
1.49 jmc 1230: .Oo Ar bind_address : Oc Ar port
1.43 djm 1231: .Sm on
1.324 dtucker 1232: or a Unix domain socket path.
1233: The second argument is the destination and may be
1234: .Ar host : Ns Ar hostport
1235: or a Unix domain socket path if the remote host supports it.
1236: .Pp
1.138 djm 1237: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1238: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1239: given on the command line.
1.1 stevesk 1240: Only the superuser can forward privileged ports.
1.43 djm 1241: By default, the local port is bound in accordance with the
1242: .Cm GatewayPorts
1243: setting.
1244: However, an explicit
1245: .Ar bind_address
1246: may be used to bind the connection to a specific address.
1247: The
1248: .Ar bind_address
1249: of
1.240 jmc 1250: .Cm localhost
1.46 jmc 1251: indicates that the listening port be bound for local use only, while an
1252: empty address or
1253: .Sq *
1.43 djm 1254: indicates that the port should be available from all interfaces.
1.326 dtucker 1255: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1256: .Sx TOKENS
1.326 dtucker 1257: section and environment variables as described in the
1258: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1259: section.
1.1 stevesk 1260: .It Cm LogLevel
1261: Gives the verbosity level that is used when logging messages from
1.84 jmc 1262: .Xr ssh 1 .
1.1 stevesk 1263: The possible values are:
1.84 jmc 1264: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1265: The default is INFO.
1266: DEBUG and DEBUG1 are equivalent.
1267: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1268: .It Cm LogVerbose
1269: Specify one or more overrides to LogLevel.
1270: An override consists of a pattern lists that matches the source file, function
1271: and line number to force detailed logging for.
1272: For example, an override pattern of:
1273: .Bd -literal -offset indent
1274: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1275: .Ed
1276: .Pp
1277: would enable detailed logging for line 1000 of
1.338 jmc 1278: .Pa kex.c ,
1.337 djm 1279: everything in the
1280: .Fn kex_exchange_identification
1281: function, and all code in the
1282: .Pa packet.c
1283: file.
1284: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1285: .It Cm MACs
1286: Specifies the MAC (message authentication code) algorithms
1287: in order of preference.
1.226 jmc 1288: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1289: Multiple algorithms must be comma-separated.
1.299 kn 1290: If the specified list begins with a
1.214 djm 1291: .Sq +
1292: character, then the specified algorithms will be appended to the default set
1293: instead of replacing them.
1.299 kn 1294: If the specified list begins with a
1.241 djm 1295: .Sq -
1296: character, then the specified algorithms (including wildcards) will be removed
1297: from the default set instead of replacing them.
1.301 naddy 1298: If the specified list begins with a
1299: .Sq ^
1300: character, then the specified algorithms will be placed at the head of the
1301: default set.
1.214 djm 1302: .Pp
1.160 markus 1303: The algorithms that contain
1.240 jmc 1304: .Qq -etm
1.160 markus 1305: calculate the MAC after encryption (encrypt-then-mac).
1306: These are considered safer and their use recommended.
1.214 djm 1307: .Pp
1.84 jmc 1308: The default is:
1.101 jmc 1309: .Bd -literal -offset indent
1.160 markus 1310: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1311: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1312: hmac-sha1-etm@openssh.com,
1.186 naddy 1313: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1314: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1315: .Ed
1.198 djm 1316: .Pp
1.240 jmc 1317: The list of available MAC algorithms may also be obtained using
1318: .Qq ssh -Q mac .
1.1 stevesk 1319: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1320: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1321: The argument to this keyword must be
1.240 jmc 1322: .Cm yes
1.1 stevesk 1323: or
1.242 jmc 1324: .Cm no
1.240 jmc 1325: (the default).
1.1 stevesk 1326: .It Cm NumberOfPasswordPrompts
1327: Specifies the number of password prompts before giving up.
1328: The argument to this keyword must be an integer.
1.84 jmc 1329: The default is 3.
1.1 stevesk 1330: .It Cm PasswordAuthentication
1331: Specifies whether to use password authentication.
1332: The argument to this keyword must be
1.240 jmc 1333: .Cm yes
1334: (the default)
1.1 stevesk 1335: or
1.240 jmc 1336: .Cm no .
1.65 reyk 1337: .It Cm PermitLocalCommand
1338: Allow local command execution via the
1339: .Ic LocalCommand
1340: option or using the
1.66 jmc 1341: .Ic !\& Ns Ar command
1.65 reyk 1342: escape sequence in
1343: .Xr ssh 1 .
1344: The argument must be
1.240 jmc 1345: .Cm yes
1.65 reyk 1346: or
1.240 jmc 1347: .Cm no
1348: (the default).
1.347 markus 1349: .It Cm PermitRemoteOpen
1350: Specifies the destinations to which remote TCP port forwarding is permitted when
1351: .Cm RemoteForward
1352: is used as a SOCKS proxy.
1353: The forwarding specification must be one of the following forms:
1354: .Pp
1355: .Bl -item -offset indent -compact
1356: .It
1357: .Cm PermitRemoteOpen
1358: .Sm off
1359: .Ar host : port
1360: .Sm on
1361: .It
1362: .Cm PermitRemoteOpen
1363: .Sm off
1364: .Ar IPv4_addr : port
1365: .Sm on
1366: .It
1367: .Cm PermitRemoteOpen
1368: .Sm off
1369: .Ar \&[ IPv6_addr \&] : port
1370: .Sm on
1371: .El
1372: .Pp
1373: Multiple forwards may be specified by separating them with whitespace.
1374: An argument of
1375: .Cm any
1376: can be used to remove all restrictions and permit any forwarding requests.
1377: An argument of
1378: .Cm none
1379: can be used to prohibit all forwarding requests.
1380: The wildcard
1381: .Sq *
1382: can be used for host or port to allow all hosts or ports respectively.
1383: Otherwise, no pattern matching or address lookups are performed on supplied
1384: names.
1.127 markus 1385: .It Cm PKCS11Provider
1.292 djm 1386: Specifies which PKCS#11 provider to use or
1387: .Cm none
1388: to indicate that no provider should be used (the default).
1389: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1390: .Xr ssh 1
1.292 djm 1391: should use to communicate with a PKCS#11 token providing keys for user
1392: authentication.
1.67 jmc 1393: .It Cm Port
1394: Specifies the port number to connect on the remote host.
1.84 jmc 1395: The default is 22.
1.1 stevesk 1396: .It Cm PreferredAuthentications
1.226 jmc 1397: Specifies the order in which the client should try authentication methods.
1.48 jmc 1398: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1399: .Cm keyboard-interactive )
1.48 jmc 1400: over another method (e.g.\&
1.131 jmc 1401: .Cm password ) .
1402: The default is:
1403: .Bd -literal -offset indent
1404: gssapi-with-mic,hostbased,publickey,
1405: keyboard-interactive,password
1406: .Ed
1.1 stevesk 1407: .It Cm ProxyCommand
1408: Specifies the command to use to connect to the server.
1409: The command
1.190 djm 1410: string extends to the end of the line, and is executed
1411: using the user's shell
1412: .Ql exec
1413: directive to avoid a lingering shell process.
1414: .Pp
1.239 jmc 1415: Arguments to
1416: .Cm ProxyCommand
1417: accept the tokens described in the
1418: .Sx TOKENS
1419: section.
1.1 stevesk 1420: The command can be basically anything,
1421: and should read from its standard input and write to its standard output.
1422: It should eventually connect an
1423: .Xr sshd 8
1424: server running on some machine, or execute
1425: .Ic sshd -i
1426: somewhere.
1427: Host key management will be done using the
1.296 jmc 1428: .Cm Hostname
1429: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1430: Setting the command to
1.240 jmc 1431: .Cm none
1.6 markus 1432: disables this option entirely.
1.1 stevesk 1433: Note that
1434: .Cm CheckHostIP
1435: is not available for connects with a proxy command.
1.52 djm 1436: .Pp
1437: This directive is useful in conjunction with
1438: .Xr nc 1
1439: and its proxy support.
1.53 jmc 1440: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1441: 192.0.2.0:
1442: .Bd -literal -offset 3n
1443: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1444: .Ed
1.233 djm 1445: .It Cm ProxyJump
1.260 millert 1446: Specifies one or more jump proxies as either
1.233 djm 1447: .Xo
1448: .Sm off
1.234 jmc 1449: .Op Ar user No @
1.233 djm 1450: .Ar host
1.234 jmc 1451: .Op : Ns Ar port
1.233 djm 1452: .Sm on
1.260 millert 1453: or an ssh URI
1.233 djm 1454: .Xc .
1.235 djm 1455: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1456: sequentially.
1.233 djm 1457: Setting this option will cause
1458: .Xr ssh 1
1459: to connect to the target host by first making a
1460: .Xr ssh 1
1461: connection to the specified
1462: .Cm ProxyJump
1463: host and then establishing a
1.234 jmc 1464: TCP forwarding to the ultimate target from there.
1.346 dlg 1465: Setting the host to
1466: .Cm none
1467: disables this option entirely.
1.233 djm 1468: .Pp
1469: Note that this option will compete with the
1470: .Cm ProxyCommand
1471: option - whichever is specified first will prevent later instances of the
1472: other from taking effect.
1.289 djm 1473: .Pp
1474: Note also that the configuration for the destination host (either supplied
1475: via the command-line or the configuration file) is not generally applied
1476: to jump hosts.
1477: .Pa ~/.ssh/config
1478: should be used if specific configuration is required for jump hosts.
1.167 djm 1479: .It Cm ProxyUseFdpass
1.168 jmc 1480: Specifies that
1.167 djm 1481: .Cm ProxyCommand
1482: will pass a connected file descriptor back to
1.168 jmc 1483: .Xr ssh 1
1.167 djm 1484: instead of continuing to execute and pass data.
1485: The default is
1.240 jmc 1486: .Cm no .
1.343 dtucker 1487: .It Cm PubkeyAcceptedAlgorithms
1488: Specifies the signature algorithms that will be used for public key
1489: authentication as a comma-separated list of patterns.
1.299 kn 1490: If the specified list begins with a
1.214 djm 1491: .Sq +
1.343 dtucker 1492: character, then the algorithms after it will be appended to the default
1.214 djm 1493: instead of replacing it.
1.299 kn 1494: If the specified list begins with a
1.241 djm 1495: .Sq -
1.343 dtucker 1496: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1497: from the default set instead of replacing them.
1.301 naddy 1498: If the specified list begins with a
1499: .Sq ^
1.343 dtucker 1500: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1501: default set.
1.213 markus 1502: The default for this option is:
1503: .Bd -literal -offset 3n
1.333 djm 1504: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1505: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1506: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1507: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1508: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1509: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1510: rsa-sha2-512-cert-v01@openssh.com,
1511: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 1512: ssh-ed25519,
1.311 naddy 1513: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1514: sk-ssh-ed25519@openssh.com,
1.305 naddy 1515: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 1516: rsa-sha2-512,rsa-sha2-256
1.213 markus 1517: .Ed
1518: .Pp
1.348 djm 1519: The list of available signature algorithms may also be obtained using
1.343 dtucker 1520: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1521: .It Cm PubkeyAuthentication
1522: Specifies whether to try public key authentication.
1523: The argument to this keyword must be
1.240 jmc 1524: .Cm yes
1.369 djm 1525: (the default),
1526: .Cm no ,
1527: .Cm unbound
1.1 stevesk 1528: or
1.369 djm 1529: .Cm host-bound .
1530: The final two options enable public key authentication while respectively
1531: disabling or enabling the OpenSSH host-bound authentication protocol
1532: extension required for restricted
1533: .Xr ssh-agent 1
1534: forwarding.
1.75 dtucker 1535: .It Cm RekeyLimit
1536: Specifies the maximum amount of data that may be transmitted before the
1.342 rob 1537: session key is renegotiated, optionally followed by a maximum amount of
1.162 dtucker 1538: time that may pass before the session key is renegotiated.
1539: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1540: .Sq K ,
1541: .Sq M ,
1.75 dtucker 1542: or
1.76 jmc 1543: .Sq G
1.75 dtucker 1544: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1545: The default is between
1.84 jmc 1546: .Sq 1G
1.75 dtucker 1547: and
1.84 jmc 1548: .Sq 4G ,
1.75 dtucker 1549: depending on the cipher.
1.162 dtucker 1550: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1551: units documented in the TIME FORMATS section of
1.162 dtucker 1552: .Xr sshd_config 5 .
1553: The default value for
1554: .Cm RekeyLimit
1555: is
1.240 jmc 1556: .Cm default none ,
1.162 dtucker 1557: which means that rekeying is performed after the cipher's default amount
1558: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1559: .It Cm RemoteCommand
1560: Specifies a command to execute on the remote machine after successfully
1561: connecting to the server.
1562: The command string extends to the end of the line, and is executed with
1563: the user's shell.
1.250 jmc 1564: Arguments to
1565: .Cm RemoteCommand
1566: accept the tokens described in the
1567: .Sx TOKENS
1568: section.
1.1 stevesk 1569: .It Cm RemoteForward
1.74 jmc 1570: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1571: the secure channel.
1.273 djm 1572: The remote port may either be forwarded to a specified host and port
1.256 markus 1573: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1574: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1575: The first argument is the listening specification and may be
1.43 djm 1576: .Sm off
1.49 jmc 1577: .Oo Ar bind_address : Oc Ar port
1.43 djm 1578: .Sm on
1.324 dtucker 1579: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1580: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1581: .Ar host : Ns Ar hostport
1582: or a Unix domain socket path,
1.256 markus 1583: otherwise if no destination argument is specified then the remote forwarding
1584: will be established as a SOCKS proxy.
1.370 naddy 1585: When acting as a SOCKS proxy, the destination of the connection can be
1.347 markus 1586: restricted by
1587: .Cm PermitRemoteOpen .
1.256 markus 1588: .Pp
1.138 djm 1589: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1590: Multiple forwardings may be specified, and additional
1591: forwardings can be given on the command line.
1.113 stevesk 1592: Privileged ports can be forwarded only when
1593: logging in as root on the remote machine.
1.326 dtucker 1594: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1595: .Sx TOKENS
1.326 dtucker 1596: section and environment variables as described in the
1597: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1598: section.
1.118 jmc 1599: .Pp
1.117 djm 1600: If the
1601: .Ar port
1.240 jmc 1602: argument is 0,
1.117 djm 1603: the listen port will be dynamically allocated on the server and reported
1604: to the client at run time.
1.43 djm 1605: .Pp
1606: If the
1607: .Ar bind_address
1608: is not specified, the default is to only bind to loopback addresses.
1609: If the
1610: .Ar bind_address
1611: is
1612: .Ql *
1613: or an empty string, then the forwarding is requested to listen on all
1614: interfaces.
1615: Specifying a remote
1616: .Ar bind_address
1.46 jmc 1617: will only succeed if the server's
1618: .Cm GatewayPorts
1.43 djm 1619: option is enabled (see
1.46 jmc 1620: .Xr sshd_config 5 ) .
1.149 djm 1621: .It Cm RequestTTY
1622: Specifies whether to request a pseudo-tty for the session.
1623: The argument may be one of:
1.240 jmc 1624: .Cm no
1.149 djm 1625: (never request a TTY),
1.240 jmc 1626: .Cm yes
1.149 djm 1627: (always request a TTY when standard input is a TTY),
1.240 jmc 1628: .Cm force
1.149 djm 1629: (always request a TTY) or
1.240 jmc 1630: .Cm auto
1.149 djm 1631: (request a TTY when opening a login session).
1632: This option mirrors the
1633: .Fl t
1634: and
1635: .Fl T
1636: flags for
1637: .Xr ssh 1 .
1.196 djm 1638: .It Cm RevokedHostKeys
1639: Specifies revoked host public keys.
1640: Keys listed in this file will be refused for host authentication.
1641: Note that if this file does not exist or is not readable,
1642: then host authentication will be refused for all hosts.
1643: Keys may be specified as a text file, listing one public key per line, or as
1644: an OpenSSH Key Revocation List (KRL) as generated by
1645: .Xr ssh-keygen 1 .
1646: For more information on KRLs, see the KEY REVOCATION LISTS section in
1647: .Xr ssh-keygen 1 .
1.305 naddy 1648: .It Cm SecurityKeyProvider
1.313 naddy 1649: Specifies a path to a library that will be used when loading any
1650: FIDO authenticator-hosted keys, overriding the default of using
1651: the built-in USB HID support.
1.309 naddy 1652: .Pp
1653: If the specified value begins with a
1654: .Sq $
1655: character, then it will be treated as an environment variable containing
1656: the path to the library.
1.32 djm 1657: .It Cm SendEnv
1658: Specifies what variables from the local
1659: .Xr environ 7
1660: should be sent to the server.
1.84 jmc 1661: The server must also support it, and the server must be configured to
1.33 djm 1662: accept these environment variables.
1.207 dtucker 1663: Note that the
1664: .Ev TERM
1.208 jmc 1665: environment variable is always sent whenever a
1.207 dtucker 1666: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1667: Refer to
1668: .Cm AcceptEnv
1669: in
1670: .Xr sshd_config 5
1671: for how to configure the server.
1.80 jmc 1672: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1673: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1674: across multiple
1675: .Cm SendEnv
1676: directives.
1.81 jmc 1677: .Pp
1678: See
1679: .Sx PATTERNS
1680: for more information on patterns.
1.271 djm 1681: .Pp
1.272 jmc 1682: It is possible to clear previously set
1.271 djm 1683: .Cm SendEnv
1684: variable names by prefixing patterns with
1685: .Pa - .
1686: The default is not to send any environment variables.
1.28 markus 1687: .It Cm ServerAliveCountMax
1.73 jmc 1688: Sets the number of server alive messages (see below) which may be
1.28 markus 1689: sent without
1.84 jmc 1690: .Xr ssh 1
1.28 markus 1691: receiving any messages back from the server.
1692: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1693: ssh will disconnect from the server, terminating the session.
1.28 markus 1694: It is important to note that the use of server alive messages is very
1695: different from
1696: .Cm TCPKeepAlive
1697: (below).
1698: The server alive messages are sent through the encrypted channel
1699: and therefore will not be spoofable.
1700: The TCP keepalive option enabled by
1701: .Cm TCPKeepAlive
1702: is spoofable.
1703: The server alive mechanism is valuable when the client or
1.298 dtucker 1704: server depend on knowing when a connection has become unresponsive.
1.28 markus 1705: .Pp
1706: The default value is 3.
1707: If, for example,
1708: .Cm ServerAliveInterval
1.84 jmc 1709: (see below) is set to 15 and
1.28 markus 1710: .Cm ServerAliveCountMax
1.84 jmc 1711: is left at the default, if the server becomes unresponsive,
1712: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1713: .It Cm ServerAliveInterval
1714: Sets a timeout interval in seconds after which if no data has been received
1715: from the server,
1.84 jmc 1716: .Xr ssh 1
1.67 jmc 1717: will send a message through the encrypted
1718: channel to request a response from the server.
1719: The default
1720: is 0, indicating that these messages will not be sent to the server.
1.357 jmc 1721: .It Cm SessionType
1722: May be used to either request invocation of a subsystem on the remote system,
1723: or to prevent the execution of a remote command at all.
1724: The latter is useful for just forwarding ports.
1725: The argument to this keyword must be
1726: .Cm none
1727: (same as the
1728: .Fl N
1729: option),
1730: .Cm subsystem
1731: (same as the
1732: .Fl s
1733: option) or
1734: .Cm default
1735: (shell or command execution).
1.277 jmc 1736: .It Cm SetEnv
1737: Directly specify one or more environment variables and their contents to
1738: be sent to the server.
1739: Similarly to
1740: .Cm SendEnv ,
1.354 djm 1741: with the exception of the
1742: .Ev TERM
1743: variable, the server must be prepared to accept the environment variable.
1.358 djm 1744: .It Cm StdinNull
1745: Redirects stdin from
1746: .Pa /dev/null
1747: (actually, prevents reading from stdin).
1748: Either this or the equivalent
1749: .Fl n
1750: option must be used when
1751: .Nm ssh
1752: is run in the background.
1753: The argument to this keyword must be
1754: .Cm yes
1755: (same as the
1756: .Fl n
1757: option) or
1758: .Cm no
1759: (the default).
1.191 millert 1760: .It Cm StreamLocalBindMask
1761: Sets the octal file creation mode mask
1762: .Pq umask
1763: used when creating a Unix-domain socket file for local or remote
1764: port forwarding.
1765: This option is only used for port forwarding to a Unix-domain socket file.
1766: .Pp
1767: The default value is 0177, which creates a Unix-domain socket file that is
1768: readable and writable only by the owner.
1769: Note that not all operating systems honor the file mode on Unix-domain
1770: socket files.
1771: .It Cm StreamLocalBindUnlink
1772: Specifies whether to remove an existing Unix-domain socket file for local
1773: or remote port forwarding before creating a new one.
1774: If the socket file already exists and
1775: .Cm StreamLocalBindUnlink
1776: is not enabled,
1777: .Nm ssh
1778: will be unable to forward the port to the Unix-domain socket file.
1779: This option is only used for port forwarding to a Unix-domain socket file.
1780: .Pp
1781: The argument must be
1.240 jmc 1782: .Cm yes
1.191 millert 1783: or
1.240 jmc 1784: .Cm no
1785: (the default).
1.1 stevesk 1786: .It Cm StrictHostKeyChecking
1787: If this flag is set to
1.240 jmc 1788: .Cm yes ,
1.84 jmc 1789: .Xr ssh 1
1.1 stevesk 1790: will never automatically add host keys to the
1.50 djm 1791: .Pa ~/.ssh/known_hosts
1.1 stevesk 1792: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1793: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1794: though it can be annoying when the
1.1 stevesk 1795: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1796: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1797: frequently made.
1798: This option forces the user to manually
1799: add all new hosts.
1.255 jmc 1800: .Pp
1.1 stevesk 1801: If this flag is set to
1.364 dtucker 1802: .Cm accept-new
1.360 jmc 1803: then ssh will automatically add new host keys to the user's
1804: .Pa known_hosts
1805: file, but will not permit connections to hosts with
1.254 djm 1806: changed host keys.
1807: If this flag is set to
1.364 dtucker 1808: .Cm no
1.254 djm 1809: or
1.364 dtucker 1810: .Cm off ,
1.255 jmc 1811: ssh will automatically add new host keys to the user known hosts files
1812: and allow connections to hosts with changed hostkeys to proceed,
1813: subject to some restrictions.
1.1 stevesk 1814: If this flag is set to
1.240 jmc 1815: .Cm ask
1816: (the default),
1.1 stevesk 1817: new host keys
1818: will be added to the user known host files only after the user
1819: has confirmed that is what they really want to do, and
1.84 jmc 1820: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1821: The host keys of
1822: known hosts will be verified automatically in all cases.
1.244 jmc 1823: .It Cm SyslogFacility
1824: Gives the facility code that is used when logging messages from
1825: .Xr ssh 1 .
1826: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1827: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1828: The default is USER.
1.26 markus 1829: .It Cm TCPKeepAlive
1830: Specifies whether the system should send TCP keepalive messages to the
1831: other side.
1832: If they are sent, death of the connection or crash of one
1833: of the machines will be properly noticed.
1834: However, this means that
1835: connections will die if the route is down temporarily, and some people
1836: find it annoying.
1837: .Pp
1838: The default is
1.240 jmc 1839: .Cm yes
1.26 markus 1840: (to send TCP keepalive messages), and the client will notice
1841: if the network goes down or the remote host dies.
1842: This is important in scripts, and many users want it too.
1843: .Pp
1844: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1845: .Cm no .
1.265 djm 1846: See also
1847: .Cm ServerAliveInterval
1848: for protocol-level keepalives.
1.65 reyk 1849: .It Cm Tunnel
1.95 stevesk 1850: Request
1.65 reyk 1851: .Xr tun 4
1.69 jmc 1852: device forwarding between the client and the server.
1.65 reyk 1853: The argument must be
1.240 jmc 1854: .Cm yes ,
1855: .Cm point-to-point
1.95 stevesk 1856: (layer 3),
1.240 jmc 1857: .Cm ethernet
1.95 stevesk 1858: (layer 2),
1.65 reyk 1859: or
1.240 jmc 1860: .Cm no
1861: (the default).
1.95 stevesk 1862: Specifying
1.240 jmc 1863: .Cm yes
1.95 stevesk 1864: requests the default tunnel mode, which is
1.240 jmc 1865: .Cm point-to-point .
1.65 reyk 1866: .It Cm TunnelDevice
1.95 stevesk 1867: Specifies the
1.65 reyk 1868: .Xr tun 4
1.95 stevesk 1869: devices to open on the client
1870: .Pq Ar local_tun
1871: and the server
1872: .Pq Ar remote_tun .
1873: .Pp
1874: The argument must be
1875: .Sm off
1876: .Ar local_tun Op : Ar remote_tun .
1877: .Sm on
1878: The devices may be specified by numerical ID or the keyword
1.240 jmc 1879: .Cm any ,
1.95 stevesk 1880: which uses the next available tunnel device.
1881: If
1882: .Ar remote_tun
1883: is not specified, it defaults to
1.240 jmc 1884: .Cm any .
1.95 stevesk 1885: The default is
1.240 jmc 1886: .Cm any:any .
1.201 djm 1887: .It Cm UpdateHostKeys
1.200 djm 1888: Specifies whether
1889: .Xr ssh 1
1890: should accept notifications of additional hostkeys from the server sent
1891: after authentication has completed and add them to
1892: .Cm UserKnownHostsFile .
1893: The argument must be
1.240 jmc 1894: .Cm yes ,
1895: .Cm no
1.320 djm 1896: or
1.240 jmc 1897: .Cm ask .
1.320 djm 1898: This option allows learning alternate hostkeys for a server
1.201 djm 1899: and supports graceful key rotation by allowing a server to send replacement
1900: public keys before old ones are removed.
1.336 djm 1901: .Pp
1.200 djm 1902: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1903: host was already trusted or explicitly accepted by the user, the host was
1904: authenticated via
1905: .Cm UserKnownHostsFile
1906: (i.e. not
1907: .Cm GlobalKnownHostsFile )
1908: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1909: .Pp
1910: .Cm UpdateHostKeys
1.321 jmc 1911: is enabled by default if the user has not overridden the default
1.320 djm 1912: .Cm UserKnownHostsFile
1.335 djm 1913: setting and has not enabled
1914: .Cm VerifyHostKeyDNS ,
1915: otherwise
1.320 djm 1916: .Cm UpdateHostKeys
1917: will be set to
1.334 djm 1918: .Cm no .
1.320 djm 1919: .Pp
1.204 djm 1920: If
1921: .Cm UpdateHostKeys
1922: is set to
1.240 jmc 1923: .Cm ask ,
1.204 djm 1924: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1925: Confirmation is currently incompatible with
1926: .Cm ControlPersist ,
1927: and will be disabled if it is enabled.
1.200 djm 1928: .Pp
1929: Presently, only
1930: .Xr sshd 8
1931: from OpenSSH 6.8 and greater support the
1.240 jmc 1932: .Qq hostkeys@openssh.com
1.200 djm 1933: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1934: .It Cm User
1935: Specifies the user to log in as.
1936: This can be useful when a different user name is used on different machines.
1937: This saves the trouble of
1938: having to remember to give the user name on the command line.
1939: .It Cm UserKnownHostsFile
1.151 djm 1940: Specifies one or more files to use for the user
1941: host key database, separated by whitespace.
1.329 dtucker 1942: Each filename may use tilde notation to refer to the user's home directory,
1943: the tokens described in the
1944: .Sx TOKENS
1945: section and environment variables as described in the
1946: .Sx ENVIRONMENT VARIABLES
1947: section.
1.151 djm 1948: The default is
1949: .Pa ~/.ssh/known_hosts ,
1950: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1951: .It Cm VerifyHostKeyDNS
1952: Specifies whether to verify the remote key using DNS and SSHFP resource
1953: records.
1.24 jakob 1954: If this option is set to
1.240 jmc 1955: .Cm yes ,
1.25 jmc 1956: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1957: from DNS.
1958: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 1959: .Cm ask .
1.24 jakob 1960: If this option is set to
1.240 jmc 1961: .Cm ask ,
1.24 jakob 1962: information on fingerprint match will be displayed, but the user will still
1963: need to confirm new host keys according to the
1964: .Cm StrictHostKeyChecking
1965: option.
1.8 jakob 1966: The default is
1.240 jmc 1967: .Cm no .
1.84 jmc 1968: .Pp
1.240 jmc 1969: See also
1970: .Sx VERIFYING HOST KEYS
1971: in
1.84 jmc 1972: .Xr ssh 1 .
1.111 grunk 1973: .It Cm VisualHostKey
1974: If this flag is set to
1.240 jmc 1975: .Cm yes ,
1.111 grunk 1976: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1977: printed in addition to the fingerprint string at login and
1.114 stevesk 1978: for unknown host keys.
1.111 grunk 1979: If this flag is set to
1.240 jmc 1980: .Cm no
1981: (the default),
1.114 stevesk 1982: no fingerprint strings are printed at login and
1.197 djm 1983: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 1984: .It Cm XAuthLocation
1.5 stevesk 1985: Specifies the full pathname of the
1.1 stevesk 1986: .Xr xauth 1
1987: program.
1988: The default is
1989: .Pa /usr/X11R6/bin/xauth .
1990: .El
1.86 jmc 1991: .Sh PATTERNS
1992: A
1993: .Em pattern
1994: consists of zero or more non-whitespace characters,
1995: .Sq *
1996: (a wildcard that matches zero or more characters),
1997: or
1998: .Sq ?\&
1999: (a wildcard that matches exactly one character).
2000: For example, to specify a set of declarations for any host in the
1.240 jmc 2001: .Qq .co.uk
1.86 jmc 2002: set of domains,
2003: the following pattern could be used:
2004: .Pp
2005: .Dl Host *.co.uk
2006: .Pp
2007: The following pattern
2008: would match any host in the 192.168.0.[0-9] network range:
2009: .Pp
2010: .Dl Host 192.168.0.?
2011: .Pp
2012: A
2013: .Em pattern-list
2014: is a comma-separated list of patterns.
2015: Patterns within pattern-lists may be negated
2016: by preceding them with an exclamation mark
2017: .Pq Sq !\& .
2018: For example,
1.174 djm 2019: to allow a key to be used from anywhere within an organization
1.86 jmc 2020: except from the
1.240 jmc 2021: .Qq dialup
1.86 jmc 2022: pool,
2023: the following entry (in authorized_keys) could be used:
2024: .Pp
2025: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 2026: .Pp
2027: Note that a negated match will never produce a positive result by itself.
2028: For example, attempting to match
2029: .Qq host3
2030: against the following pattern-list will fail:
2031: .Pp
2032: .Dl from=\&"!host1,!host2\&"
2033: .Pp
2034: The solution here is to include a term that will yield a positive match,
2035: such as a wildcard:
2036: .Pp
2037: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 2038: .Sh TOKENS
2039: Arguments to some keywords can make use of tokens,
2040: which are expanded at runtime:
2041: .Pp
2042: .Bl -tag -width XXXX -offset indent -compact
2043: .It %%
2044: A literal
2045: .Sq % .
2046: .It \&%C
1.257 jmc 2047: Hash of %l%h%p%r.
1.239 jmc 2048: .It %d
2049: Local user's home directory.
1.339 djm 2050: .It %f
2051: The fingerprint of the server's host key.
2052: .It %H
2053: The
2054: .Pa known_hosts
2055: hostname or address that is being searched for.
1.239 jmc 2056: .It %h
2057: The remote hostname.
1.340 jmc 2058: .It \%%I
1.339 djm 2059: A string describing the reason for a
2060: .Cm KnownHostsCommand
1.340 jmc 2061: execution: either
2062: .Cm ADDRESS
1.339 djm 2063: when looking up a host by address (only when
2064: .Cm CheckHostIP
2065: is enabled),
1.340 jmc 2066: .Cm HOSTNAME
2067: when searching by hostname, or
2068: .Cm ORDER
1.339 djm 2069: when preparing the host key algorithm preference list to use for the
2070: destination host.
1.239 jmc 2071: .It %i
2072: The local user ID.
1.339 djm 2073: .It %K
2074: The base64 encoded host key.
1.330 dtucker 2075: .It %k
1.350 jsg 2076: The host key alias if specified, otherwise the original remote hostname given
1.330 dtucker 2077: on the command line.
1.239 jmc 2078: .It %L
2079: The local hostname.
2080: .It %l
2081: The local hostname, including the domain name.
2082: .It %n
2083: The original remote hostname, as given on the command line.
2084: .It %p
2085: The remote port.
2086: .It %r
2087: The remote username.
1.261 djm 2088: .It \&%T
2089: The local
2090: .Xr tun 4
2091: or
2092: .Xr tap 4
2093: network interface assigned if
1.262 jmc 2094: tunnel forwarding was requested, or
2095: .Qq NONE
1.261 djm 2096: otherwise.
1.339 djm 2097: .It %t
2098: The type of the server host key, e.g.
1.360 jmc 2099: .Cm ssh-ed25519 .
1.239 jmc 2100: .It %u
2101: The local username.
2102: .El
2103: .Pp
1.323 dtucker 2104: .Cm CertificateFile ,
2105: .Cm ControlPath ,
2106: .Cm IdentityAgent ,
2107: .Cm IdentityFile ,
1.339 djm 2108: .Cm KnownHostsCommand ,
1.325 jmc 2109: .Cm LocalForward ,
1.324 dtucker 2110: .Cm Match exec ,
2111: .Cm RemoteCommand ,
1.329 dtucker 2112: .Cm RemoteForward ,
1.323 dtucker 2113: and
1.331 jmc 2114: .Cm UserKnownHostsFile
1.349 dtucker 2115: accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2116: .Pp
1.339 djm 2117: .Cm KnownHostsCommand
2118: additionally accepts the tokens %f, %H, %I, %K and %t.
2119: .Pp
1.295 jmc 2120: .Cm Hostname
1.239 jmc 2121: accepts the tokens %% and %h.
2122: .Pp
2123: .Cm LocalCommand
1.323 dtucker 2124: accepts all tokens.
1.239 jmc 2125: .Pp
2126: .Cm ProxyCommand
1.303 djm 2127: accepts the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2128: .Sh ENVIRONMENT VARIABLES
2129: Arguments to some keywords can be expanded at runtime from environment
2130: variables on the client by enclosing them in
2131: .Ic ${} ,
2132: for example
2133: .Ic ${HOME}/.ssh
2134: would refer to the user's .ssh directory.
2135: If a specified environment variable does not exist then an error will be
2136: returned and the setting for that keyword will be ignored.
2137: .Pp
2138: The keywords
2139: .Cm CertificateFile ,
2140: .Cm ControlPath ,
1.329 dtucker 2141: .Cm IdentityAgent ,
1.352 jmc 2142: .Cm IdentityFile ,
1.339 djm 2143: .Cm KnownHostsCommand ,
1.326 dtucker 2144: and
1.329 dtucker 2145: .Cm UserKnownHostsFile
1.326 dtucker 2146: support environment variables.
2147: The keywords
2148: .Cm LocalForward
2149: and
2150: .Cm RemoteForward
2151: support environment variables only for Unix domain socket paths.
1.1 stevesk 2152: .Sh FILES
2153: .Bl -tag -width Ds
1.50 djm 2154: .It Pa ~/.ssh/config
1.1 stevesk 2155: This is the per-user configuration file.
2156: The format of this file is described above.
1.84 jmc 2157: This file is used by the SSH client.
1.30 djm 2158: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2159: read/write for the user, and not writable by others.
1.1 stevesk 2160: .It Pa /etc/ssh/ssh_config
2161: Systemwide configuration file.
2162: This file provides defaults for those
2163: values that are not specified in the user's configuration file, and
2164: for those users who do not have a configuration file.
2165: This file must be world-readable.
2166: .El
1.13 jmc 2167: .Sh SEE ALSO
2168: .Xr ssh 1
1.1 stevesk 2169: .Sh AUTHORS
1.240 jmc 2170: .An -nosplit
1.1 stevesk 2171: OpenSSH is a derivative of the original and free
1.240 jmc 2172: ssh 1.2.12 release by
2173: .An Tatu Ylonen .
2174: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2175: .An Niels Provos , Theo de Raadt
2176: and
2177: .An Dug Song
1.1 stevesk 2178: removed many bugs, re-added newer features and
2179: created OpenSSH.
1.240 jmc 2180: .An Markus Friedl
2181: contributed the support for SSH protocol versions 1.5 and 2.0.
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