Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.364
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.364 ! dtucker 36: .\" $OpenBSD: ssh_config.5,v 1.363 2021/09/03 05:25:50 dtucker Exp $
! 37: .Dd $Mdocdate: September 3 2021 $
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.
1.283 jmc 375: .It Cm CASignatureAlgorithms
376: Specifies which algorithms are allowed for signing of certificates
377: by certificate authorities (CAs).
378: The default is:
379: .Bd -literal -offset indent
1.362 djm 380: ssh-ed25519,ecdsa-sha2-nistp256,
381: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
382: sk-ssh-ed25519@openssh.com,
383: sk-ecdsa-sha2-nistp256@openssh.com,
1.351 djm 384: rsa-sha2-512,rsa-sha2-256
1.283 jmc 385: .Ed
1.362 djm 386: .Pp
387: If the specified list begins with a
388: .Sq +
389: character, then the specified algorithms will be appended to the default set
390: instead of replacing them.
391: If the specified list begins with a
392: .Sq -
393: character, then the specified algorithms (including wildcards) will be removed
394: from the default set instead of replacing them.
1.283 jmc 395: .Pp
396: .Xr ssh 1
397: will not accept host certificates signed using algorithms other than those
398: specified.
1.221 djm 399: .It Cm CertificateFile
400: Specifies a file from which the user's certificate is read.
401: A corresponding private key must be provided separately in order
402: to use this certificate either
403: from an
404: .Cm IdentityFile
405: directive or
406: .Fl i
407: flag to
408: .Xr ssh 1 ,
409: via
410: .Xr ssh-agent 1 ,
411: or via a
1.305 naddy 412: .Cm PKCS11Provider
413: or
414: .Cm SecurityKeyProvider .
1.221 djm 415: .Pp
1.239 jmc 416: Arguments to
417: .Cm CertificateFile
1.326 dtucker 418: may use the tilde syntax to refer to a user's home directory,
419: the tokens described in the
1.239 jmc 420: .Sx TOKENS
1.326 dtucker 421: section and environment variables as described in the
422: .Sx ENVIRONMENT VARIABLES
1.239 jmc 423: section.
1.221 djm 424: .Pp
425: It is possible to have multiple certificate files specified in
426: configuration files; these certificates will be tried in sequence.
427: Multiple
428: .Cm CertificateFile
429: directives will add to the list of certificates used for
430: authentication.
1.1 stevesk 431: .It Cm CheckHostIP
1.240 jmc 432: If set to
433: .Cm yes
1.84 jmc 434: .Xr ssh 1
435: will additionally check the host IP address in the
1.1 stevesk 436: .Pa known_hosts
437: file.
1.240 jmc 438: This allows it to detect if a host key changed due to DNS spoofing
1.211 djm 439: and will add addresses of destination hosts to
440: .Pa ~/.ssh/known_hosts
441: in the process, regardless of the setting of
442: .Cm StrictHostKeyChecking .
1.107 grunk 443: If the option is set to
1.341 djm 444: .Cm no
445: (the default),
1.1 stevesk 446: the check will not be executed.
447: .It Cm Ciphers
1.245 djm 448: Specifies the ciphers allowed and their order of preference.
1.1 stevesk 449: Multiple ciphers must be comma-separated.
1.299 kn 450: If the specified list begins with a
1.214 djm 451: .Sq +
452: character, then the specified ciphers will be appended to the default set
453: instead of replacing them.
1.299 kn 454: If the specified list begins with a
1.241 djm 455: .Sq -
456: character, then the specified ciphers (including wildcards) will be removed
457: from the default set instead of replacing them.
1.301 naddy 458: If the specified list begins with a
459: .Sq ^
460: character, then the specified ciphers will be placed at the head of the
461: default set.
1.214 djm 462: .Pp
1.180 djm 463: The supported ciphers are:
1.240 jmc 464: .Bd -literal -offset indent
1.186 naddy 465: 3des-cbc
466: aes128-cbc
467: aes192-cbc
468: aes256-cbc
469: aes128-ctr
470: aes192-ctr
471: aes256-ctr
472: aes128-gcm@openssh.com
473: aes256-gcm@openssh.com
474: chacha20-poly1305@openssh.com
1.240 jmc 475: .Ed
1.180 djm 476: .Pp
1.84 jmc 477: The default is:
1.186 naddy 478: .Bd -literal -offset indent
1.215 jmc 479: chacha20-poly1305@openssh.com,
1.186 naddy 480: aes128-ctr,aes192-ctr,aes256-ctr,
1.270 djm 481: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 482: .Ed
1.180 djm 483: .Pp
1.240 jmc 484: The list of available ciphers may also be obtained using
485: .Qq ssh -Q cipher .
1.1 stevesk 486: .It Cm ClearAllForwardings
1.84 jmc 487: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 488: specified in the configuration files or on the command line be
1.7 jmc 489: cleared.
490: This option is primarily useful when used from the
1.84 jmc 491: .Xr ssh 1
1.1 stevesk 492: command line to clear port forwardings set in
493: configuration files, and is automatically set by
494: .Xr scp 1
495: and
496: .Xr sftp 1 .
497: The argument must be
1.240 jmc 498: .Cm yes
1.1 stevesk 499: or
1.240 jmc 500: .Cm no
501: (the default).
1.1 stevesk 502: .It Cm Compression
503: Specifies whether to use compression.
504: The argument must be
1.240 jmc 505: .Cm yes
1.1 stevesk 506: or
1.240 jmc 507: .Cm no
508: (the default).
1.247 naddy 509: .It Cm ConnectionAttempts
510: Specifies the number of tries (one per second) to make before exiting.
511: The argument must be an integer.
512: This may be useful in scripts if the connection sometimes fails.
513: The default is 1.
1.9 djm 514: .It Cm ConnectTimeout
1.84 jmc 515: Specifies the timeout (in seconds) used when connecting to the
516: SSH server, instead of using the default system TCP timeout.
1.302 djm 517: This timeout is applied both to establishing the connection and to performing
518: the initial SSH protocol handshake and key exchange.
1.36 djm 519: .It Cm ControlMaster
520: Enables the sharing of multiple sessions over a single network connection.
521: When set to
1.240 jmc 522: .Cm yes ,
1.84 jmc 523: .Xr ssh 1
1.36 djm 524: will listen for connections on a control socket specified using the
525: .Cm ControlPath
526: argument.
527: Additional sessions can connect to this socket using the same
528: .Cm ControlPath
529: with
530: .Cm ControlMaster
531: set to
1.240 jmc 532: .Cm no
1.38 jmc 533: (the default).
1.64 jmc 534: These sessions will try to reuse the master instance's network connection
1.63 djm 535: rather than initiating new ones, but will fall back to connecting normally
536: if the control socket does not exist, or is not listening.
537: .Pp
1.37 djm 538: Setting this to
1.240 jmc 539: .Cm ask
540: will cause
541: .Xr ssh 1
1.206 jmc 542: to listen for control connections, but require confirmation using
543: .Xr ssh-askpass 1 .
1.51 jakob 544: If the
545: .Cm ControlPath
1.84 jmc 546: cannot be opened,
1.240 jmc 547: .Xr ssh 1
548: will continue without connecting to a master instance.
1.58 djm 549: .Pp
550: X11 and
1.59 jmc 551: .Xr ssh-agent 1
1.58 djm 552: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 553: display and agent forwarded will be the one belonging to the master
1.59 jmc 554: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 555: .Pp
556: Two additional options allow for opportunistic multiplexing: try to use a
557: master connection but fall back to creating a new one if one does not already
558: exist.
559: These options are:
1.240 jmc 560: .Cm auto
1.56 djm 561: and
1.240 jmc 562: .Cm autoask .
1.56 djm 563: The latter requires confirmation like the
1.240 jmc 564: .Cm ask
1.56 djm 565: option.
1.36 djm 566: .It Cm ControlPath
1.55 djm 567: Specify the path to the control socket used for connection sharing as described
568: in the
1.36 djm 569: .Cm ControlMaster
1.57 djm 570: section above or the string
1.240 jmc 571: .Cm none
1.57 djm 572: to disable connection sharing.
1.239 jmc 573: Arguments to
574: .Cm ControlPath
1.326 dtucker 575: may use the tilde syntax to refer to a user's home directory,
576: the tokens described in the
1.239 jmc 577: .Sx TOKENS
1.326 dtucker 578: section and environment variables as described in the
579: .Sx ENVIRONMENT VARIABLES
1.239 jmc 580: section.
1.56 djm 581: It is recommended that any
582: .Cm ControlPath
583: used for opportunistic connection sharing include
1.195 djm 584: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
585: that is not writable by other users.
1.56 djm 586: This ensures that shared connections are uniquely identified.
1.137 djm 587: .It Cm ControlPersist
588: When used in conjunction with
589: .Cm ControlMaster ,
590: specifies that the master connection should remain open
591: in the background (waiting for future client connections)
592: after the initial client connection has been closed.
593: If set to
1.314 naddy 594: .Cm no
595: (the default),
1.137 djm 596: then the master connection will not be placed into the background,
597: and will close as soon as the initial client connection is closed.
598: If set to
1.240 jmc 599: .Cm yes
600: or 0,
1.137 djm 601: then the master connection will remain in the background indefinitely
602: (until killed or closed via a mechanism such as the
1.240 jmc 603: .Qq ssh -O exit ) .
1.137 djm 604: If set to a time in seconds, or a time in any of the formats documented in
605: .Xr sshd_config 5 ,
606: then the backgrounded master connection will automatically terminate
607: after it has remained idle (with no client connections) for the
608: specified time.
1.38 jmc 609: .It Cm DynamicForward
1.74 jmc 610: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 611: over the secure channel, and the application
612: protocol is then used to determine where to connect to from the
613: remote machine.
1.62 djm 614: .Pp
615: The argument must be
616: .Sm off
617: .Oo Ar bind_address : Oc Ar port .
618: .Sm on
1.138 djm 619: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 620: By default, the local port is bound in accordance with the
621: .Cm GatewayPorts
622: setting.
623: However, an explicit
624: .Ar bind_address
625: may be used to bind the connection to a specific address.
626: The
627: .Ar bind_address
628: of
1.240 jmc 629: .Cm localhost
1.62 djm 630: indicates that the listening port be bound for local use only, while an
631: empty address or
632: .Sq *
633: indicates that the port should be available from all interfaces.
634: .Pp
1.38 jmc 635: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 636: .Xr ssh 1
1.38 jmc 637: will act as a SOCKS server.
638: Multiple forwardings may be specified, and
639: additional forwardings can be given on the command line.
640: Only the superuser can forward privileged ports.
1.14 markus 641: .It Cm EnableSSHKeysign
642: Setting this option to
1.240 jmc 643: .Cm yes
1.14 markus 644: in the global client configuration file
645: .Pa /etc/ssh/ssh_config
646: enables the use of the helper program
647: .Xr ssh-keysign 8
648: during
649: .Cm HostbasedAuthentication .
650: The argument must be
1.240 jmc 651: .Cm yes
1.14 markus 652: or
1.240 jmc 653: .Cm no
654: (the default).
1.23 jmc 655: This option should be placed in the non-hostspecific section.
1.14 markus 656: See
657: .Xr ssh-keysign 8
658: for more information.
1.1 stevesk 659: .It Cm EscapeChar
660: Sets the escape character (default:
661: .Ql ~ ) .
662: The escape character can also
663: be set on the command line.
664: The argument should be a single character,
665: .Ql ^
666: followed by a letter, or
1.240 jmc 667: .Cm none
1.1 stevesk 668: to disable the escape
669: character entirely (making the connection transparent for binary
670: data).
1.96 markus 671: .It Cm ExitOnForwardFailure
672: Specifies whether
673: .Xr ssh 1
674: should terminate the connection if it cannot set up all requested
1.216 djm 675: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 676: if either end is unable to bind and listen on a specified port).
1.216 djm 677: Note that
678: .Cm ExitOnForwardFailure
679: does not apply to connections made over port forwardings and will not,
680: for example, cause
681: .Xr ssh 1
682: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 683: The argument must be
1.240 jmc 684: .Cm yes
1.96 markus 685: or
1.240 jmc 686: .Cm no
687: (the default).
1.197 djm 688: .It Cm FingerprintHash
689: Specifies the hash algorithm used when displaying key fingerprints.
690: Valid options are:
1.240 jmc 691: .Cm md5
1.197 djm 692: and
1.240 jmc 693: .Cm sha256
1.359 djm 694: (the default).
695: .It Cm ForkAfterAuthentication
696: Requests
697: .Nm ssh
698: to go to background just before command execution.
699: This is useful if
700: .Nm ssh
701: is going to ask for passwords or passphrases, but the user
702: wants it in the background.
703: This implies the
704: .Cm StdinNull
705: configuration option being set to
706: .Dq yes .
707: The recommended way to start X11 programs at a remote site is with
708: something like
709: .Ic ssh -f host xterm ,
710: which is the same as
711: .Ic ssh host xterm
712: if the
713: .Cm ForkAfterAuthentication
714: configuration option is set to
715: .Dq yes .
716: .Pp
717: If the
718: .Cm ExitOnForwardFailure
719: configuration option is set to
720: .Dq yes ,
721: then a client started with the
722: .Cm ForkAfterAuthentication
723: configuration option being set to
724: .Dq yes
725: will wait for all remote port forwards to be successfully established
726: before placing itself in the background.
727: The argument to this keyword must be
728: .Cm yes
729: (same as the
730: .Fl f
731: option) or
732: .Cm no
1.240 jmc 733: (the default).
1.1 stevesk 734: .It Cm ForwardAgent
735: Specifies whether the connection to the authentication agent (if any)
736: will be forwarded to the remote machine.
1.312 djm 737: The argument may be
738: .Cm yes ,
1.240 jmc 739: .Cm no
1.312 djm 740: (the default),
741: an explicit path to an agent socket or the name of an environment variable
742: (beginning with
743: .Sq $ )
744: in which to find the path.
1.3 stevesk 745: .Pp
1.7 jmc 746: Agent forwarding should be enabled with caution.
747: Users with the ability to bypass file permissions on the remote host
748: (for the agent's Unix-domain socket)
749: can access the local agent through the forwarded connection.
750: An attacker cannot obtain key material from the agent,
1.3 stevesk 751: however they can perform operations on the keys that enable them to
752: authenticate using the identities loaded into the agent.
1.1 stevesk 753: .It Cm ForwardX11
754: Specifies whether X11 connections will be automatically redirected
755: over the secure channel and
756: .Ev DISPLAY
757: set.
758: The argument must be
1.240 jmc 759: .Cm yes
1.1 stevesk 760: or
1.240 jmc 761: .Cm no
762: (the default).
1.3 stevesk 763: .Pp
1.7 jmc 764: X11 forwarding should be enabled with caution.
765: Users with the ability to bypass file permissions on the remote host
1.22 markus 766: (for the user's X11 authorization database)
1.7 jmc 767: can access the local X11 display through the forwarded connection.
1.22 markus 768: An attacker may then be able to perform activities such as keystroke monitoring
769: if the
770: .Cm ForwardX11Trusted
771: option is also enabled.
1.134 djm 772: .It Cm ForwardX11Timeout
1.135 jmc 773: Specify a timeout for untrusted X11 forwarding
774: using the format described in the
1.240 jmc 775: .Sx TIME FORMATS
776: section of
1.134 djm 777: .Xr sshd_config 5 .
778: X11 connections received by
779: .Xr ssh 1
780: after this time will be refused.
1.285 djm 781: Setting
782: .Cm ForwardX11Timeout
783: to zero will disable the timeout and permit X11 forwarding for the life
784: of the connection.
1.134 djm 785: The default is to disable untrusted X11 forwarding after twenty minutes has
786: elapsed.
1.22 markus 787: .It Cm ForwardX11Trusted
1.34 jmc 788: If this option is set to
1.240 jmc 789: .Cm yes ,
1.84 jmc 790: remote X11 clients will have full access to the original X11 display.
1.42 djm 791: .Pp
1.22 markus 792: If this option is set to
1.240 jmc 793: .Cm no
794: (the default),
1.84 jmc 795: remote X11 clients will be considered untrusted and prevented
1.22 markus 796: from stealing or tampering with data belonging to trusted X11
797: clients.
1.42 djm 798: Furthermore, the
799: .Xr xauth 1
800: token used for the session will be set to expire after 20 minutes.
801: Remote clients will be refused access after this time.
1.22 markus 802: .Pp
803: See the X11 SECURITY extension specification for full details on
804: the restrictions imposed on untrusted clients.
1.1 stevesk 805: .It Cm GatewayPorts
806: Specifies whether remote hosts are allowed to connect to local
807: forwarded ports.
808: By default,
1.84 jmc 809: .Xr ssh 1
1.7 jmc 810: binds local port forwardings to the loopback address.
811: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 812: .Cm GatewayPorts
1.84 jmc 813: can be used to specify that ssh
1.1 stevesk 814: should bind local port forwardings to the wildcard address,
815: thus allowing remote hosts to connect to forwarded ports.
816: The argument must be
1.240 jmc 817: .Cm yes
1.1 stevesk 818: or
1.240 jmc 819: .Cm no
820: (the default).
1.1 stevesk 821: .It Cm GlobalKnownHostsFile
1.151 djm 822: Specifies one or more files to use for the global
823: host key database, separated by whitespace.
824: The default is
825: .Pa /etc/ssh/ssh_known_hosts ,
826: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 827: .It Cm GSSAPIAuthentication
1.27 markus 828: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 829: The default is
1.240 jmc 830: .Cm no .
1.18 markus 831: .It Cm GSSAPIDelegateCredentials
832: Forward (delegate) credentials to the server.
833: The default is
1.240 jmc 834: .Cm no .
1.44 djm 835: .It Cm HashKnownHosts
836: Indicates that
1.84 jmc 837: .Xr ssh 1
1.44 djm 838: should hash host names and addresses when they are added to
1.50 djm 839: .Pa ~/.ssh/known_hosts .
1.44 djm 840: These hashed names may be used normally by
1.84 jmc 841: .Xr ssh 1
1.44 djm 842: and
1.84 jmc 843: .Xr sshd 8 ,
1.316 djm 844: but they do not visually reveal identifying information if the
845: file's contents are disclosed.
1.44 djm 846: The default is
1.240 jmc 847: .Cm no .
1.97 jmc 848: Note that existing names and addresses in known hosts files
849: will not be converted automatically,
850: but may be manually hashed using
1.45 djm 851: .Xr ssh-keygen 1 .
1.344 dtucker 852: .It Cm HostbasedAcceptedAlgorithms
1.348 djm 853: Specifies the signature algorithms that will be used for hostbased
854: authentication as a comma-separated list of patterns.
1.300 naddy 855: Alternately if the specified list begins with a
1.214 djm 856: .Sq +
1.348 djm 857: character, then the specified signature algorithms will be appended
858: to the default set instead of replacing them.
1.300 naddy 859: If the specified list begins with a
1.241 djm 860: .Sq -
1.348 djm 861: character, then the specified signature algorithms (including wildcards)
862: will be removed from the default set instead of replacing them.
1.301 naddy 863: If the specified list begins with a
864: .Sq ^
1.348 djm 865: character, then the specified signature algorithms will be placed
866: at the head of the default set.
1.213 markus 867: The default for this option is:
868: .Bd -literal -offset 3n
1.333 djm 869: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 870: ecdsa-sha2-nistp256-cert-v01@openssh.com,
871: ecdsa-sha2-nistp384-cert-v01@openssh.com,
872: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 873: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 874: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 875: rsa-sha2-512-cert-v01@openssh.com,
876: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 877: ssh-rsa-cert-v01@openssh.com,
1.333 djm 878: ssh-ed25519,
1.213 markus 879: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 880: sk-ssh-ed25519@openssh.com,
1.311 naddy 881: sk-ecdsa-sha2-nistp256@openssh.com,
882: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 883: .Ed
884: .Pp
1.202 djm 885: The
886: .Fl Q
887: option of
888: .Xr ssh 1
1.348 djm 889: may be used to list supported signature algorithms.
1.344 dtucker 890: This was formerly named HostbasedKeyTypes.
1.345 naddy 891: .It Cm HostbasedAuthentication
892: Specifies whether to try rhosts based authentication with public key
893: authentication.
894: The argument must be
895: .Cm yes
896: or
897: .Cm no
898: (the default).
1.1 stevesk 899: .It Cm HostKeyAlgorithms
1.348 djm 900: Specifies the host key signature algorithms
1.1 stevesk 901: that the client wants to use in order of preference.
1.300 naddy 902: Alternately if the specified list begins with a
1.214 djm 903: .Sq +
1.348 djm 904: character, then the specified signature algorithms will be appended to
905: the default set instead of replacing them.
1.300 naddy 906: If the specified list begins with a
1.241 djm 907: .Sq -
1.348 djm 908: character, then the specified signature algorithms (including wildcards)
909: will be removed from the default set instead of replacing them.
1.301 naddy 910: If the specified list begins with a
911: .Sq ^
1.348 djm 912: character, then the specified signature algorithms will be placed
913: at the head of the default set.
1.1 stevesk 914: The default for this option is:
1.139 djm 915: .Bd -literal -offset 3n
1.333 djm 916: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 917: ecdsa-sha2-nistp256-cert-v01@openssh.com,
918: ecdsa-sha2-nistp384-cert-v01@openssh.com,
919: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 920: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 921: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 922: rsa-sha2-512-cert-v01@openssh.com,
923: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 924: ssh-rsa-cert-v01@openssh.com,
1.333 djm 925: ssh-ed25519,
1.139 djm 926: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 927: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 928: sk-ssh-ed25519@openssh.com,
1.311 naddy 929: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.139 djm 930: .Ed
1.145 djm 931: .Pp
932: If hostkeys are known for the destination host then this default is modified
933: to prefer their algorithms.
1.198 djm 934: .Pp
1.348 djm 935: The list of available signature algorithms may also be obtained using
1.322 dtucker 936: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 937: .It Cm HostKeyAlias
938: Specifies an alias that should be used instead of the
939: real host name when looking up or saving the host key
1.251 djm 940: in the host key database files and when validating host certificates.
1.84 jmc 941: This option is useful for tunneling SSH connections
1.1 stevesk 942: or for multiple servers running on a single host.
1.295 jmc 943: .It Cm Hostname
1.1 stevesk 944: Specifies the real host name to log into.
945: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 946: Arguments to
1.295 jmc 947: .Cm Hostname
1.239 jmc 948: accept the tokens described in the
949: .Sx TOKENS
950: section.
1.1 stevesk 951: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 952: .Cm Hostname
1.1 stevesk 953: specifications).
1.239 jmc 954: The default is the name given on the command line.
1.29 markus 955: .It Cm IdentitiesOnly
956: Specifies that
1.84 jmc 957: .Xr ssh 1
1.304 djm 958: should only use the configured authentication identity and certificate files
959: (either the default files, or those explicitly configured in the
1.31 jmc 960: .Nm
1.221 djm 961: files
962: or passed on the
963: .Xr ssh 1
1.304 djm 964: command-line),
1.84 jmc 965: even if
966: .Xr ssh-agent 1
1.159 djm 967: or a
968: .Cm PKCS11Provider
1.305 naddy 969: or
970: .Cm SecurityKeyProvider
1.29 markus 971: offers more identities.
972: The argument to this keyword must be
1.240 jmc 973: .Cm yes
1.29 markus 974: or
1.240 jmc 975: .Cm no
976: (the default).
1.84 jmc 977: This option is intended for situations where ssh-agent
1.29 markus 978: offers many different identities.
1.231 markus 979: .It Cm IdentityAgent
980: Specifies the
981: .Ux Ns -domain
982: socket used to communicate with the authentication agent.
983: .Pp
984: This option overrides the
1.240 jmc 985: .Ev SSH_AUTH_SOCK
1.231 markus 986: environment variable and can be used to select a specific agent.
987: Setting the socket name to
1.240 jmc 988: .Cm none
1.231 markus 989: disables the use of an authentication agent.
1.232 markus 990: If the string
1.240 jmc 991: .Qq SSH_AUTH_SOCK
1.232 markus 992: is specified, the location of the socket will be read from the
993: .Ev SSH_AUTH_SOCK
994: environment variable.
1.286 djm 995: Otherwise if the specified value begins with a
996: .Sq $
997: character, then it will be treated as an environment variable containing
998: the location of the socket.
1.231 markus 999: .Pp
1.239 jmc 1000: Arguments to
1001: .Cm IdentityAgent
1.326 dtucker 1002: may use the tilde syntax to refer to a user's home directory,
1003: the tokens described in the
1.239 jmc 1004: .Sx TOKENS
1.326 dtucker 1005: section and environment variables as described in the
1006: .Sx ENVIRONMENT VARIABLES
1.239 jmc 1007: section.
1.67 jmc 1008: .It Cm IdentityFile
1.313 naddy 1009: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1010: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.67 jmc 1011: The default is
1.139 djm 1012: .Pa ~/.ssh/id_dsa ,
1.183 naddy 1013: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 1014: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 1015: .Pa ~/.ssh/id_ed25519 ,
1016: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 1017: and
1.245 djm 1018: .Pa ~/.ssh/id_rsa .
1.67 jmc 1019: Additionally, any identities represented by the authentication agent
1.165 djm 1020: will be used for authentication unless
1021: .Cm IdentitiesOnly
1022: is set.
1.221 djm 1023: If no certificates have been explicitly specified by
1024: .Cm CertificateFile ,
1.129 djm 1025: .Xr ssh 1
1026: will try to load certificate information from the filename obtained by
1027: appending
1028: .Pa -cert.pub
1029: to the path of a specified
1030: .Cm IdentityFile .
1.90 djm 1031: .Pp
1.239 jmc 1032: Arguments to
1033: .Cm IdentityFile
1034: may use the tilde syntax to refer to a user's home directory
1035: or the tokens described in the
1036: .Sx TOKENS
1037: section.
1.90 djm 1038: .Pp
1.67 jmc 1039: It is possible to have
1040: multiple identity files specified in configuration files; all these
1041: identities will be tried in sequence.
1.152 djm 1042: Multiple
1043: .Cm IdentityFile
1044: directives will add to the list of identities tried (this behaviour
1045: differs from that of other configuration directives).
1.165 djm 1046: .Pp
1047: .Cm IdentityFile
1048: may be used in conjunction with
1049: .Cm IdentitiesOnly
1050: to select which identities in an agent are offered during authentication.
1.221 djm 1051: .Cm IdentityFile
1052: may also be used in conjunction with
1053: .Cm CertificateFile
1054: in order to provide any certificate also needed for authentication with
1055: the identity.
1.164 jmc 1056: .It Cm IgnoreUnknown
1057: Specifies a pattern-list of unknown options to be ignored if they are
1058: encountered in configuration parsing.
1059: This may be used to suppress errors if
1060: .Nm
1061: contains options that are unrecognised by
1062: .Xr ssh 1 .
1063: It is recommended that
1064: .Cm IgnoreUnknown
1065: be listed early in the configuration file as it will not be applied
1066: to unknown options that appear before it.
1.229 djm 1067: .It Cm Include
1068: Include the specified configuration file(s).
1.230 jmc 1069: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1070: .Xr glob 7
1.229 djm 1071: wildcards and, for user configurations, shell-like
1.240 jmc 1072: .Sq ~
1.229 djm 1073: references to user home directories.
1.327 djm 1074: Wildcards will be expanded and processed in lexical order.
1.229 djm 1075: Files without absolute paths are assumed to be in
1076: .Pa ~/.ssh
1.230 jmc 1077: if included in a user configuration file or
1.229 djm 1078: .Pa /etc/ssh
1079: if included from the system configuration file.
1080: .Cm Include
1081: directive may appear inside a
1082: .Cm Match
1083: or
1084: .Cm Host
1085: block
1086: to perform conditional inclusion.
1.143 djm 1087: .It Cm IPQoS
1088: Specifies the IPv4 type-of-service or DSCP class for connections.
1089: Accepted values are
1.240 jmc 1090: .Cm af11 ,
1091: .Cm af12 ,
1092: .Cm af13 ,
1093: .Cm af21 ,
1094: .Cm af22 ,
1095: .Cm af23 ,
1096: .Cm af31 ,
1097: .Cm af32 ,
1098: .Cm af33 ,
1099: .Cm af41 ,
1100: .Cm af42 ,
1101: .Cm af43 ,
1102: .Cm cs0 ,
1103: .Cm cs1 ,
1104: .Cm cs2 ,
1105: .Cm cs3 ,
1106: .Cm cs4 ,
1107: .Cm cs5 ,
1108: .Cm cs6 ,
1109: .Cm cs7 ,
1110: .Cm ef ,
1.319 djm 1111: .Cm le ,
1.240 jmc 1112: .Cm lowdelay ,
1113: .Cm throughput ,
1114: .Cm reliability ,
1.253 djm 1115: a numeric value, or
1116: .Cm none
1117: to use the operating system default.
1.146 djm 1118: This option may take one or two arguments, separated by whitespace.
1.143 djm 1119: If one argument is specified, it is used as the packet class unconditionally.
1120: If two values are specified, the first is automatically selected for
1121: interactive sessions and the second for non-interactive sessions.
1122: The default is
1.269 job 1123: .Cm af21
1.272 jmc 1124: (Low-Latency Data)
1.143 djm 1125: for interactive sessions and
1.269 job 1126: .Cm cs1
1.272 jmc 1127: (Lower Effort)
1.143 djm 1128: for non-interactive sessions.
1.103 djm 1129: .It Cm KbdInteractiveAuthentication
1130: Specifies whether to use keyboard-interactive authentication.
1131: The argument to this keyword must be
1.240 jmc 1132: .Cm yes
1133: (the default)
1.103 djm 1134: or
1.240 jmc 1135: .Cm no .
1.355 dtucker 1136: .Cm ChallengeResponseAuthentication
1137: is a deprecated alias for this.
1.39 djm 1138: .It Cm KbdInteractiveDevices
1139: Specifies the list of methods to use in keyboard-interactive authentication.
1140: Multiple method names must be comma-separated.
1141: The default is to use the server specified list.
1.85 jmc 1142: The methods available vary depending on what the server supports.
1143: For an OpenSSH server,
1144: it may be zero or more of:
1.240 jmc 1145: .Cm bsdauth ,
1146: .Cm pam ,
1.85 jmc 1147: and
1.240 jmc 1148: .Cm skey .
1.140 djm 1149: .It Cm KexAlgorithms
1150: Specifies the available KEX (Key Exchange) algorithms.
1151: Multiple algorithms must be comma-separated.
1.299 kn 1152: If the specified list begins with a
1.214 djm 1153: .Sq +
1.363 dtucker 1154: character, then the specified algorithms will be appended to the default set
1.214 djm 1155: instead of replacing them.
1.299 kn 1156: If the specified list begins with a
1.241 djm 1157: .Sq -
1.363 dtucker 1158: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1159: from the default set instead of replacing them.
1.301 naddy 1160: If the specified list begins with a
1161: .Sq ^
1.363 dtucker 1162: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1163: default set.
1.141 jmc 1164: The default is:
1165: .Bd -literal -offset indent
1.238 djm 1166: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1167: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1168: diffie-hellman-group-exchange-sha256,
1.266 djm 1169: diffie-hellman-group16-sha512,
1170: diffie-hellman-group18-sha512,
1.317 tedu 1171: diffie-hellman-group14-sha256
1.141 jmc 1172: .Ed
1.198 djm 1173: .Pp
1.240 jmc 1174: The list of available key exchange algorithms may also be obtained using
1175: .Qq ssh -Q kex .
1.339 djm 1176: .It Cm KnownHostsCommand
1.340 jmc 1177: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1178: those listed in
1179: .Cm UserKnownHostsFile
1180: and
1181: .Cm GlobalKnownHostsFile .
1182: This command is executed after the files have been read.
1.340 jmc 1183: It may write host key lines to standard output in identical format to the
1.339 djm 1184: usual files (described in the
1185: .Sx VERIFYING HOST KEYS
1186: section in
1187: .Xr ssh 1 ) .
1188: Arguments to
1189: .Cm KnownHostsCommand
1190: accept the tokens described in the
1191: .Sx TOKENS
1192: section.
1.340 jmc 1193: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1194: the preference list of host key algorithms to use, again to obtain the
1195: host key for the requested host name and, if
1196: .Cm CheckHostIP
1197: is enabled, one more time to obtain the host key matching the server's
1198: address.
1199: If the command exits abnormally or returns a non-zero exit status then the
1200: connection is terminated.
1.65 reyk 1201: .It Cm LocalCommand
1202: Specifies a command to execute on the local machine after successfully
1203: connecting to the server.
1204: The command string extends to the end of the line, and is executed with
1.105 jmc 1205: the user's shell.
1.239 jmc 1206: Arguments to
1207: .Cm LocalCommand
1208: accept the tokens described in the
1209: .Sx TOKENS
1210: section.
1.123 djm 1211: .Pp
1212: The command is run synchronously and does not have access to the
1213: session of the
1214: .Xr ssh 1
1215: that spawned it.
1216: It should not be used for interactive commands.
1217: .Pp
1.65 reyk 1218: This directive is ignored unless
1219: .Cm PermitLocalCommand
1220: has been enabled.
1.1 stevesk 1221: .It Cm LocalForward
1.74 jmc 1222: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1223: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1224: The first argument specifies the listener and may be
1.43 djm 1225: .Sm off
1.49 jmc 1226: .Oo Ar bind_address : Oc Ar port
1.43 djm 1227: .Sm on
1.324 dtucker 1228: or a Unix domain socket path.
1229: The second argument is the destination and may be
1230: .Ar host : Ns Ar hostport
1231: or a Unix domain socket path if the remote host supports it.
1232: .Pp
1.138 djm 1233: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1234: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1235: given on the command line.
1.1 stevesk 1236: Only the superuser can forward privileged ports.
1.43 djm 1237: By default, the local port is bound in accordance with the
1238: .Cm GatewayPorts
1239: setting.
1240: However, an explicit
1241: .Ar bind_address
1242: may be used to bind the connection to a specific address.
1243: The
1244: .Ar bind_address
1245: of
1.240 jmc 1246: .Cm localhost
1.46 jmc 1247: indicates that the listening port be bound for local use only, while an
1248: empty address or
1249: .Sq *
1.43 djm 1250: indicates that the port should be available from all interfaces.
1.326 dtucker 1251: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1252: .Sx TOKENS
1.326 dtucker 1253: section and environment variables as described in the
1254: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1255: section.
1.1 stevesk 1256: .It Cm LogLevel
1257: Gives the verbosity level that is used when logging messages from
1.84 jmc 1258: .Xr ssh 1 .
1.1 stevesk 1259: The possible values are:
1.84 jmc 1260: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1261: The default is INFO.
1262: DEBUG and DEBUG1 are equivalent.
1263: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1264: .It Cm LogVerbose
1265: Specify one or more overrides to LogLevel.
1266: An override consists of a pattern lists that matches the source file, function
1267: and line number to force detailed logging for.
1268: For example, an override pattern of:
1269: .Bd -literal -offset indent
1270: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1271: .Ed
1272: .Pp
1273: would enable detailed logging for line 1000 of
1.338 jmc 1274: .Pa kex.c ,
1.337 djm 1275: everything in the
1276: .Fn kex_exchange_identification
1277: function, and all code in the
1278: .Pa packet.c
1279: file.
1280: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1281: .It Cm MACs
1282: Specifies the MAC (message authentication code) algorithms
1283: in order of preference.
1.226 jmc 1284: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1285: Multiple algorithms must be comma-separated.
1.299 kn 1286: If the specified list begins with a
1.214 djm 1287: .Sq +
1288: character, then the specified algorithms will be appended to the default set
1289: instead of replacing them.
1.299 kn 1290: If the specified list begins with a
1.241 djm 1291: .Sq -
1292: character, then the specified algorithms (including wildcards) will be removed
1293: from the default set instead of replacing them.
1.301 naddy 1294: If the specified list begins with a
1295: .Sq ^
1296: character, then the specified algorithms will be placed at the head of the
1297: default set.
1.214 djm 1298: .Pp
1.160 markus 1299: The algorithms that contain
1.240 jmc 1300: .Qq -etm
1.160 markus 1301: calculate the MAC after encryption (encrypt-then-mac).
1302: These are considered safer and their use recommended.
1.214 djm 1303: .Pp
1.84 jmc 1304: The default is:
1.101 jmc 1305: .Bd -literal -offset indent
1.160 markus 1306: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1307: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1308: hmac-sha1-etm@openssh.com,
1.186 naddy 1309: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1310: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1311: .Ed
1.198 djm 1312: .Pp
1.240 jmc 1313: The list of available MAC algorithms may also be obtained using
1314: .Qq ssh -Q mac .
1.1 stevesk 1315: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1316: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1317: The argument to this keyword must be
1.240 jmc 1318: .Cm yes
1.1 stevesk 1319: or
1.242 jmc 1320: .Cm no
1.240 jmc 1321: (the default).
1.1 stevesk 1322: .It Cm NumberOfPasswordPrompts
1323: Specifies the number of password prompts before giving up.
1324: The argument to this keyword must be an integer.
1.84 jmc 1325: The default is 3.
1.1 stevesk 1326: .It Cm PasswordAuthentication
1327: Specifies whether to use password authentication.
1328: The argument to this keyword must be
1.240 jmc 1329: .Cm yes
1330: (the default)
1.1 stevesk 1331: or
1.240 jmc 1332: .Cm no .
1.65 reyk 1333: .It Cm PermitLocalCommand
1334: Allow local command execution via the
1335: .Ic LocalCommand
1336: option or using the
1.66 jmc 1337: .Ic !\& Ns Ar command
1.65 reyk 1338: escape sequence in
1339: .Xr ssh 1 .
1340: The argument must be
1.240 jmc 1341: .Cm yes
1.65 reyk 1342: or
1.240 jmc 1343: .Cm no
1344: (the default).
1.347 markus 1345: .It Cm PermitRemoteOpen
1346: Specifies the destinations to which remote TCP port forwarding is permitted when
1347: .Cm RemoteForward
1348: is used as a SOCKS proxy.
1349: The forwarding specification must be one of the following forms:
1350: .Pp
1351: .Bl -item -offset indent -compact
1352: .It
1353: .Cm PermitRemoteOpen
1354: .Sm off
1355: .Ar host : port
1356: .Sm on
1357: .It
1358: .Cm PermitRemoteOpen
1359: .Sm off
1360: .Ar IPv4_addr : port
1361: .Sm on
1362: .It
1363: .Cm PermitRemoteOpen
1364: .Sm off
1365: .Ar \&[ IPv6_addr \&] : port
1366: .Sm on
1367: .El
1368: .Pp
1369: Multiple forwards may be specified by separating them with whitespace.
1370: An argument of
1371: .Cm any
1372: can be used to remove all restrictions and permit any forwarding requests.
1373: An argument of
1374: .Cm none
1375: can be used to prohibit all forwarding requests.
1376: The wildcard
1377: .Sq *
1378: can be used for host or port to allow all hosts or ports respectively.
1379: Otherwise, no pattern matching or address lookups are performed on supplied
1380: names.
1.127 markus 1381: .It Cm PKCS11Provider
1.292 djm 1382: Specifies which PKCS#11 provider to use or
1383: .Cm none
1384: to indicate that no provider should be used (the default).
1385: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1386: .Xr ssh 1
1.292 djm 1387: should use to communicate with a PKCS#11 token providing keys for user
1388: authentication.
1.67 jmc 1389: .It Cm Port
1390: Specifies the port number to connect on the remote host.
1.84 jmc 1391: The default is 22.
1.1 stevesk 1392: .It Cm PreferredAuthentications
1.226 jmc 1393: Specifies the order in which the client should try authentication methods.
1.48 jmc 1394: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1395: .Cm keyboard-interactive )
1.48 jmc 1396: over another method (e.g.\&
1.131 jmc 1397: .Cm password ) .
1398: The default is:
1399: .Bd -literal -offset indent
1400: gssapi-with-mic,hostbased,publickey,
1401: keyboard-interactive,password
1402: .Ed
1.1 stevesk 1403: .It Cm ProxyCommand
1404: Specifies the command to use to connect to the server.
1405: The command
1.190 djm 1406: string extends to the end of the line, and is executed
1407: using the user's shell
1408: .Ql exec
1409: directive to avoid a lingering shell process.
1410: .Pp
1.239 jmc 1411: Arguments to
1412: .Cm ProxyCommand
1413: accept the tokens described in the
1414: .Sx TOKENS
1415: section.
1.1 stevesk 1416: The command can be basically anything,
1417: and should read from its standard input and write to its standard output.
1418: It should eventually connect an
1419: .Xr sshd 8
1420: server running on some machine, or execute
1421: .Ic sshd -i
1422: somewhere.
1423: Host key management will be done using the
1.296 jmc 1424: .Cm Hostname
1425: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1426: Setting the command to
1.240 jmc 1427: .Cm none
1.6 markus 1428: disables this option entirely.
1.1 stevesk 1429: Note that
1430: .Cm CheckHostIP
1431: is not available for connects with a proxy command.
1.52 djm 1432: .Pp
1433: This directive is useful in conjunction with
1434: .Xr nc 1
1435: and its proxy support.
1.53 jmc 1436: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1437: 192.0.2.0:
1438: .Bd -literal -offset 3n
1439: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1440: .Ed
1.233 djm 1441: .It Cm ProxyJump
1.260 millert 1442: Specifies one or more jump proxies as either
1.233 djm 1443: .Xo
1444: .Sm off
1.234 jmc 1445: .Op Ar user No @
1.233 djm 1446: .Ar host
1.234 jmc 1447: .Op : Ns Ar port
1.233 djm 1448: .Sm on
1.260 millert 1449: or an ssh URI
1.233 djm 1450: .Xc .
1.235 djm 1451: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1452: sequentially.
1.233 djm 1453: Setting this option will cause
1454: .Xr ssh 1
1455: to connect to the target host by first making a
1456: .Xr ssh 1
1457: connection to the specified
1458: .Cm ProxyJump
1459: host and then establishing a
1.234 jmc 1460: TCP forwarding to the ultimate target from there.
1.346 dlg 1461: Setting the host to
1462: .Cm none
1463: disables this option entirely.
1.233 djm 1464: .Pp
1465: Note that this option will compete with the
1466: .Cm ProxyCommand
1467: option - whichever is specified first will prevent later instances of the
1468: other from taking effect.
1.289 djm 1469: .Pp
1470: Note also that the configuration for the destination host (either supplied
1471: via the command-line or the configuration file) is not generally applied
1472: to jump hosts.
1473: .Pa ~/.ssh/config
1474: should be used if specific configuration is required for jump hosts.
1.167 djm 1475: .It Cm ProxyUseFdpass
1.168 jmc 1476: Specifies that
1.167 djm 1477: .Cm ProxyCommand
1478: will pass a connected file descriptor back to
1.168 jmc 1479: .Xr ssh 1
1.167 djm 1480: instead of continuing to execute and pass data.
1481: The default is
1.240 jmc 1482: .Cm no .
1.343 dtucker 1483: .It Cm PubkeyAcceptedAlgorithms
1484: Specifies the signature algorithms that will be used for public key
1485: authentication as a comma-separated list of patterns.
1.299 kn 1486: If the specified list begins with a
1.214 djm 1487: .Sq +
1.343 dtucker 1488: character, then the algorithms after it will be appended to the default
1.214 djm 1489: instead of replacing it.
1.299 kn 1490: If the specified list begins with a
1.241 djm 1491: .Sq -
1.343 dtucker 1492: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1493: from the default set instead of replacing them.
1.301 naddy 1494: If the specified list begins with a
1495: .Sq ^
1.343 dtucker 1496: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1497: default set.
1.213 markus 1498: The default for this option is:
1499: .Bd -literal -offset 3n
1.333 djm 1500: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1501: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1502: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1503: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1504: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1505: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1506: rsa-sha2-512-cert-v01@openssh.com,
1507: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 1508: ssh-rsa-cert-v01@openssh.com,
1.333 djm 1509: ssh-ed25519,
1.311 naddy 1510: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1511: sk-ssh-ed25519@openssh.com,
1.305 naddy 1512: sk-ecdsa-sha2-nistp256@openssh.com,
1.311 naddy 1513: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 1514: .Ed
1515: .Pp
1.348 djm 1516: The list of available signature algorithms may also be obtained using
1.343 dtucker 1517: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1518: .It Cm PubkeyAuthentication
1519: Specifies whether to try public key authentication.
1520: The argument to this keyword must be
1.240 jmc 1521: .Cm yes
1522: (the default)
1.1 stevesk 1523: or
1.240 jmc 1524: .Cm no .
1.75 dtucker 1525: .It Cm RekeyLimit
1526: Specifies the maximum amount of data that may be transmitted before the
1.342 rob 1527: session key is renegotiated, optionally followed by a maximum amount of
1.162 dtucker 1528: time that may pass before the session key is renegotiated.
1529: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1530: .Sq K ,
1531: .Sq M ,
1.75 dtucker 1532: or
1.76 jmc 1533: .Sq G
1.75 dtucker 1534: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1535: The default is between
1.84 jmc 1536: .Sq 1G
1.75 dtucker 1537: and
1.84 jmc 1538: .Sq 4G ,
1.75 dtucker 1539: depending on the cipher.
1.162 dtucker 1540: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1541: units documented in the TIME FORMATS section of
1.162 dtucker 1542: .Xr sshd_config 5 .
1543: The default value for
1544: .Cm RekeyLimit
1545: is
1.240 jmc 1546: .Cm default none ,
1.162 dtucker 1547: which means that rekeying is performed after the cipher's default amount
1548: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1549: .It Cm RemoteCommand
1550: Specifies a command to execute on the remote machine after successfully
1551: connecting to the server.
1552: The command string extends to the end of the line, and is executed with
1553: the user's shell.
1.250 jmc 1554: Arguments to
1555: .Cm RemoteCommand
1556: accept the tokens described in the
1557: .Sx TOKENS
1558: section.
1.1 stevesk 1559: .It Cm RemoteForward
1.74 jmc 1560: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1561: the secure channel.
1.273 djm 1562: The remote port may either be forwarded to a specified host and port
1.256 markus 1563: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1564: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1565: The first argument is the listening specification and may be
1.43 djm 1566: .Sm off
1.49 jmc 1567: .Oo Ar bind_address : Oc Ar port
1.43 djm 1568: .Sm on
1.324 dtucker 1569: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1570: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1571: .Ar host : Ns Ar hostport
1572: or a Unix domain socket path,
1.256 markus 1573: otherwise if no destination argument is specified then the remote forwarding
1574: will be established as a SOCKS proxy.
1.347 markus 1575: When acting as a SOCKS proxy the destination of the connection can be
1576: restricted by
1577: .Cm PermitRemoteOpen .
1.256 markus 1578: .Pp
1.138 djm 1579: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1580: Multiple forwardings may be specified, and additional
1581: forwardings can be given on the command line.
1.113 stevesk 1582: Privileged ports can be forwarded only when
1583: logging in as root on the remote machine.
1.326 dtucker 1584: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1585: .Sx TOKENS
1.326 dtucker 1586: section and environment variables as described in the
1587: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1588: section.
1.118 jmc 1589: .Pp
1.117 djm 1590: If the
1591: .Ar port
1.240 jmc 1592: argument is 0,
1.117 djm 1593: the listen port will be dynamically allocated on the server and reported
1594: to the client at run time.
1.43 djm 1595: .Pp
1596: If the
1597: .Ar bind_address
1598: is not specified, the default is to only bind to loopback addresses.
1599: If the
1600: .Ar bind_address
1601: is
1602: .Ql *
1603: or an empty string, then the forwarding is requested to listen on all
1604: interfaces.
1605: Specifying a remote
1606: .Ar bind_address
1.46 jmc 1607: will only succeed if the server's
1608: .Cm GatewayPorts
1.43 djm 1609: option is enabled (see
1.46 jmc 1610: .Xr sshd_config 5 ) .
1.149 djm 1611: .It Cm RequestTTY
1612: Specifies whether to request a pseudo-tty for the session.
1613: The argument may be one of:
1.240 jmc 1614: .Cm no
1.149 djm 1615: (never request a TTY),
1.240 jmc 1616: .Cm yes
1.149 djm 1617: (always request a TTY when standard input is a TTY),
1.240 jmc 1618: .Cm force
1.149 djm 1619: (always request a TTY) or
1.240 jmc 1620: .Cm auto
1.149 djm 1621: (request a TTY when opening a login session).
1622: This option mirrors the
1623: .Fl t
1624: and
1625: .Fl T
1626: flags for
1627: .Xr ssh 1 .
1.196 djm 1628: .It Cm RevokedHostKeys
1629: Specifies revoked host public keys.
1630: Keys listed in this file will be refused for host authentication.
1631: Note that if this file does not exist or is not readable,
1632: then host authentication will be refused for all hosts.
1633: Keys may be specified as a text file, listing one public key per line, or as
1634: an OpenSSH Key Revocation List (KRL) as generated by
1635: .Xr ssh-keygen 1 .
1636: For more information on KRLs, see the KEY REVOCATION LISTS section in
1637: .Xr ssh-keygen 1 .
1.305 naddy 1638: .It Cm SecurityKeyProvider
1.313 naddy 1639: Specifies a path to a library that will be used when loading any
1640: FIDO authenticator-hosted keys, overriding the default of using
1641: the built-in USB HID support.
1.309 naddy 1642: .Pp
1643: If the specified value begins with a
1644: .Sq $
1645: character, then it will be treated as an environment variable containing
1646: the path to the library.
1.32 djm 1647: .It Cm SendEnv
1648: Specifies what variables from the local
1649: .Xr environ 7
1650: should be sent to the server.
1.84 jmc 1651: The server must also support it, and the server must be configured to
1.33 djm 1652: accept these environment variables.
1.207 dtucker 1653: Note that the
1654: .Ev TERM
1.208 jmc 1655: environment variable is always sent whenever a
1.207 dtucker 1656: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1657: Refer to
1658: .Cm AcceptEnv
1659: in
1660: .Xr sshd_config 5
1661: for how to configure the server.
1.80 jmc 1662: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1663: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1664: across multiple
1665: .Cm SendEnv
1666: directives.
1.81 jmc 1667: .Pp
1668: See
1669: .Sx PATTERNS
1670: for more information on patterns.
1.271 djm 1671: .Pp
1.272 jmc 1672: It is possible to clear previously set
1.271 djm 1673: .Cm SendEnv
1674: variable names by prefixing patterns with
1675: .Pa - .
1676: The default is not to send any environment variables.
1.28 markus 1677: .It Cm ServerAliveCountMax
1.73 jmc 1678: Sets the number of server alive messages (see below) which may be
1.28 markus 1679: sent without
1.84 jmc 1680: .Xr ssh 1
1.28 markus 1681: receiving any messages back from the server.
1682: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1683: ssh will disconnect from the server, terminating the session.
1.28 markus 1684: It is important to note that the use of server alive messages is very
1685: different from
1686: .Cm TCPKeepAlive
1687: (below).
1688: The server alive messages are sent through the encrypted channel
1689: and therefore will not be spoofable.
1690: The TCP keepalive option enabled by
1691: .Cm TCPKeepAlive
1692: is spoofable.
1693: The server alive mechanism is valuable when the client or
1.298 dtucker 1694: server depend on knowing when a connection has become unresponsive.
1.28 markus 1695: .Pp
1696: The default value is 3.
1697: If, for example,
1698: .Cm ServerAliveInterval
1.84 jmc 1699: (see below) is set to 15 and
1.28 markus 1700: .Cm ServerAliveCountMax
1.84 jmc 1701: is left at the default, if the server becomes unresponsive,
1702: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1703: .It Cm ServerAliveInterval
1704: Sets a timeout interval in seconds after which if no data has been received
1705: from the server,
1.84 jmc 1706: .Xr ssh 1
1.67 jmc 1707: will send a message through the encrypted
1708: channel to request a response from the server.
1709: The default
1710: is 0, indicating that these messages will not be sent to the server.
1.357 jmc 1711: .It Cm SessionType
1712: May be used to either request invocation of a subsystem on the remote system,
1713: or to prevent the execution of a remote command at all.
1714: The latter is useful for just forwarding ports.
1715: The argument to this keyword must be
1716: .Cm none
1717: (same as the
1718: .Fl N
1719: option),
1720: .Cm subsystem
1721: (same as the
1722: .Fl s
1723: option) or
1724: .Cm default
1725: (shell or command execution).
1.277 jmc 1726: .It Cm SetEnv
1727: Directly specify one or more environment variables and their contents to
1728: be sent to the server.
1729: Similarly to
1730: .Cm SendEnv ,
1.354 djm 1731: with the exception of the
1732: .Ev TERM
1733: variable, the server must be prepared to accept the environment variable.
1.358 djm 1734: .It Cm StdinNull
1735: Redirects stdin from
1736: .Pa /dev/null
1737: (actually, prevents reading from stdin).
1738: Either this or the equivalent
1739: .Fl n
1740: option must be used when
1741: .Nm ssh
1742: is run in the background.
1743: The argument to this keyword must be
1744: .Cm yes
1745: (same as the
1746: .Fl n
1747: option) or
1748: .Cm no
1749: (the default).
1.191 millert 1750: .It Cm StreamLocalBindMask
1751: Sets the octal file creation mode mask
1752: .Pq umask
1753: used when creating a Unix-domain socket file for local or remote
1754: port forwarding.
1755: This option is only used for port forwarding to a Unix-domain socket file.
1756: .Pp
1757: The default value is 0177, which creates a Unix-domain socket file that is
1758: readable and writable only by the owner.
1759: Note that not all operating systems honor the file mode on Unix-domain
1760: socket files.
1761: .It Cm StreamLocalBindUnlink
1762: Specifies whether to remove an existing Unix-domain socket file for local
1763: or remote port forwarding before creating a new one.
1764: If the socket file already exists and
1765: .Cm StreamLocalBindUnlink
1766: is not enabled,
1767: .Nm ssh
1768: will be unable to forward the port to the Unix-domain socket file.
1769: This option is only used for port forwarding to a Unix-domain socket file.
1770: .Pp
1771: The argument must be
1.240 jmc 1772: .Cm yes
1.191 millert 1773: or
1.240 jmc 1774: .Cm no
1775: (the default).
1.1 stevesk 1776: .It Cm StrictHostKeyChecking
1777: If this flag is set to
1.240 jmc 1778: .Cm yes ,
1.84 jmc 1779: .Xr ssh 1
1.1 stevesk 1780: will never automatically add host keys to the
1.50 djm 1781: .Pa ~/.ssh/known_hosts
1.1 stevesk 1782: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1783: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1784: though it can be annoying when the
1.1 stevesk 1785: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1786: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1787: frequently made.
1788: This option forces the user to manually
1789: add all new hosts.
1.255 jmc 1790: .Pp
1.1 stevesk 1791: If this flag is set to
1.364 ! dtucker 1792: .Cm accept-new
1.360 jmc 1793: then ssh will automatically add new host keys to the user's
1794: .Pa known_hosts
1795: file, but will not permit connections to hosts with
1.254 djm 1796: changed host keys.
1797: If this flag is set to
1.364 ! dtucker 1798: .Cm no
1.254 djm 1799: or
1.364 ! dtucker 1800: .Cm off ,
1.255 jmc 1801: ssh will automatically add new host keys to the user known hosts files
1802: and allow connections to hosts with changed hostkeys to proceed,
1803: subject to some restrictions.
1.1 stevesk 1804: If this flag is set to
1.240 jmc 1805: .Cm ask
1806: (the default),
1.1 stevesk 1807: new host keys
1808: will be added to the user known host files only after the user
1809: has confirmed that is what they really want to do, and
1.84 jmc 1810: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1811: The host keys of
1812: known hosts will be verified automatically in all cases.
1.244 jmc 1813: .It Cm SyslogFacility
1814: Gives the facility code that is used when logging messages from
1815: .Xr ssh 1 .
1816: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1817: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1818: The default is USER.
1.26 markus 1819: .It Cm TCPKeepAlive
1820: Specifies whether the system should send TCP keepalive messages to the
1821: other side.
1822: If they are sent, death of the connection or crash of one
1823: of the machines will be properly noticed.
1824: However, this means that
1825: connections will die if the route is down temporarily, and some people
1826: find it annoying.
1827: .Pp
1828: The default is
1.240 jmc 1829: .Cm yes
1.26 markus 1830: (to send TCP keepalive messages), and the client will notice
1831: if the network goes down or the remote host dies.
1832: This is important in scripts, and many users want it too.
1833: .Pp
1834: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1835: .Cm no .
1.265 djm 1836: See also
1837: .Cm ServerAliveInterval
1838: for protocol-level keepalives.
1.65 reyk 1839: .It Cm Tunnel
1.95 stevesk 1840: Request
1.65 reyk 1841: .Xr tun 4
1.69 jmc 1842: device forwarding between the client and the server.
1.65 reyk 1843: The argument must be
1.240 jmc 1844: .Cm yes ,
1845: .Cm point-to-point
1.95 stevesk 1846: (layer 3),
1.240 jmc 1847: .Cm ethernet
1.95 stevesk 1848: (layer 2),
1.65 reyk 1849: or
1.240 jmc 1850: .Cm no
1851: (the default).
1.95 stevesk 1852: Specifying
1.240 jmc 1853: .Cm yes
1.95 stevesk 1854: requests the default tunnel mode, which is
1.240 jmc 1855: .Cm point-to-point .
1.65 reyk 1856: .It Cm TunnelDevice
1.95 stevesk 1857: Specifies the
1.65 reyk 1858: .Xr tun 4
1.95 stevesk 1859: devices to open on the client
1860: .Pq Ar local_tun
1861: and the server
1862: .Pq Ar remote_tun .
1863: .Pp
1864: The argument must be
1865: .Sm off
1866: .Ar local_tun Op : Ar remote_tun .
1867: .Sm on
1868: The devices may be specified by numerical ID or the keyword
1.240 jmc 1869: .Cm any ,
1.95 stevesk 1870: which uses the next available tunnel device.
1871: If
1872: .Ar remote_tun
1873: is not specified, it defaults to
1.240 jmc 1874: .Cm any .
1.95 stevesk 1875: The default is
1.240 jmc 1876: .Cm any:any .
1.201 djm 1877: .It Cm UpdateHostKeys
1.200 djm 1878: Specifies whether
1879: .Xr ssh 1
1880: should accept notifications of additional hostkeys from the server sent
1881: after authentication has completed and add them to
1882: .Cm UserKnownHostsFile .
1883: The argument must be
1.240 jmc 1884: .Cm yes ,
1885: .Cm no
1.320 djm 1886: or
1.240 jmc 1887: .Cm ask .
1.320 djm 1888: This option allows learning alternate hostkeys for a server
1.201 djm 1889: and supports graceful key rotation by allowing a server to send replacement
1890: public keys before old ones are removed.
1.336 djm 1891: .Pp
1.200 djm 1892: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1893: host was already trusted or explicitly accepted by the user, the host was
1894: authenticated via
1895: .Cm UserKnownHostsFile
1896: (i.e. not
1897: .Cm GlobalKnownHostsFile )
1898: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1899: .Pp
1900: .Cm UpdateHostKeys
1.321 jmc 1901: is enabled by default if the user has not overridden the default
1.320 djm 1902: .Cm UserKnownHostsFile
1.335 djm 1903: setting and has not enabled
1904: .Cm VerifyHostKeyDNS ,
1905: otherwise
1.320 djm 1906: .Cm UpdateHostKeys
1907: will be set to
1.334 djm 1908: .Cm no .
1.320 djm 1909: .Pp
1.204 djm 1910: If
1911: .Cm UpdateHostKeys
1912: is set to
1.240 jmc 1913: .Cm ask ,
1.204 djm 1914: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1915: Confirmation is currently incompatible with
1916: .Cm ControlPersist ,
1917: and will be disabled if it is enabled.
1.200 djm 1918: .Pp
1919: Presently, only
1920: .Xr sshd 8
1921: from OpenSSH 6.8 and greater support the
1.240 jmc 1922: .Qq hostkeys@openssh.com
1.200 djm 1923: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1924: .It Cm User
1925: Specifies the user to log in as.
1926: This can be useful when a different user name is used on different machines.
1927: This saves the trouble of
1928: having to remember to give the user name on the command line.
1929: .It Cm UserKnownHostsFile
1.151 djm 1930: Specifies one or more files to use for the user
1931: host key database, separated by whitespace.
1.329 dtucker 1932: Each filename may use tilde notation to refer to the user's home directory,
1933: the tokens described in the
1934: .Sx TOKENS
1935: section and environment variables as described in the
1936: .Sx ENVIRONMENT VARIABLES
1937: section.
1.151 djm 1938: The default is
1939: .Pa ~/.ssh/known_hosts ,
1940: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1941: .It Cm VerifyHostKeyDNS
1942: Specifies whether to verify the remote key using DNS and SSHFP resource
1943: records.
1.24 jakob 1944: If this option is set to
1.240 jmc 1945: .Cm yes ,
1.25 jmc 1946: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1947: from DNS.
1948: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 1949: .Cm ask .
1.24 jakob 1950: If this option is set to
1.240 jmc 1951: .Cm ask ,
1.24 jakob 1952: information on fingerprint match will be displayed, but the user will still
1953: need to confirm new host keys according to the
1954: .Cm StrictHostKeyChecking
1955: option.
1.8 jakob 1956: The default is
1.240 jmc 1957: .Cm no .
1.84 jmc 1958: .Pp
1.240 jmc 1959: See also
1960: .Sx VERIFYING HOST KEYS
1961: in
1.84 jmc 1962: .Xr ssh 1 .
1.111 grunk 1963: .It Cm VisualHostKey
1964: If this flag is set to
1.240 jmc 1965: .Cm yes ,
1.111 grunk 1966: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1967: printed in addition to the fingerprint string at login and
1.114 stevesk 1968: for unknown host keys.
1.111 grunk 1969: If this flag is set to
1.240 jmc 1970: .Cm no
1971: (the default),
1.114 stevesk 1972: no fingerprint strings are printed at login and
1.197 djm 1973: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 1974: .It Cm XAuthLocation
1.5 stevesk 1975: Specifies the full pathname of the
1.1 stevesk 1976: .Xr xauth 1
1977: program.
1978: The default is
1979: .Pa /usr/X11R6/bin/xauth .
1980: .El
1.86 jmc 1981: .Sh PATTERNS
1982: A
1983: .Em pattern
1984: consists of zero or more non-whitespace characters,
1985: .Sq *
1986: (a wildcard that matches zero or more characters),
1987: or
1988: .Sq ?\&
1989: (a wildcard that matches exactly one character).
1990: For example, to specify a set of declarations for any host in the
1.240 jmc 1991: .Qq .co.uk
1.86 jmc 1992: set of domains,
1993: the following pattern could be used:
1994: .Pp
1995: .Dl Host *.co.uk
1996: .Pp
1997: The following pattern
1998: would match any host in the 192.168.0.[0-9] network range:
1999: .Pp
2000: .Dl Host 192.168.0.?
2001: .Pp
2002: A
2003: .Em pattern-list
2004: is a comma-separated list of patterns.
2005: Patterns within pattern-lists may be negated
2006: by preceding them with an exclamation mark
2007: .Pq Sq !\& .
2008: For example,
1.174 djm 2009: to allow a key to be used from anywhere within an organization
1.86 jmc 2010: except from the
1.240 jmc 2011: .Qq dialup
1.86 jmc 2012: pool,
2013: the following entry (in authorized_keys) could be used:
2014: .Pp
2015: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 2016: .Pp
2017: Note that a negated match will never produce a positive result by itself.
2018: For example, attempting to match
2019: .Qq host3
2020: against the following pattern-list will fail:
2021: .Pp
2022: .Dl from=\&"!host1,!host2\&"
2023: .Pp
2024: The solution here is to include a term that will yield a positive match,
2025: such as a wildcard:
2026: .Pp
2027: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 2028: .Sh TOKENS
2029: Arguments to some keywords can make use of tokens,
2030: which are expanded at runtime:
2031: .Pp
2032: .Bl -tag -width XXXX -offset indent -compact
2033: .It %%
2034: A literal
2035: .Sq % .
2036: .It \&%C
1.257 jmc 2037: Hash of %l%h%p%r.
1.239 jmc 2038: .It %d
2039: Local user's home directory.
1.339 djm 2040: .It %f
2041: The fingerprint of the server's host key.
2042: .It %H
2043: The
2044: .Pa known_hosts
2045: hostname or address that is being searched for.
1.239 jmc 2046: .It %h
2047: The remote hostname.
1.340 jmc 2048: .It \%%I
1.339 djm 2049: A string describing the reason for a
2050: .Cm KnownHostsCommand
1.340 jmc 2051: execution: either
2052: .Cm ADDRESS
1.339 djm 2053: when looking up a host by address (only when
2054: .Cm CheckHostIP
2055: is enabled),
1.340 jmc 2056: .Cm HOSTNAME
2057: when searching by hostname, or
2058: .Cm ORDER
1.339 djm 2059: when preparing the host key algorithm preference list to use for the
2060: destination host.
1.239 jmc 2061: .It %i
2062: The local user ID.
1.339 djm 2063: .It %K
2064: The base64 encoded host key.
1.330 dtucker 2065: .It %k
1.350 jsg 2066: The host key alias if specified, otherwise the original remote hostname given
1.330 dtucker 2067: on the command line.
1.239 jmc 2068: .It %L
2069: The local hostname.
2070: .It %l
2071: The local hostname, including the domain name.
2072: .It %n
2073: The original remote hostname, as given on the command line.
2074: .It %p
2075: The remote port.
2076: .It %r
2077: The remote username.
1.261 djm 2078: .It \&%T
2079: The local
2080: .Xr tun 4
2081: or
2082: .Xr tap 4
2083: network interface assigned if
1.262 jmc 2084: tunnel forwarding was requested, or
2085: .Qq NONE
1.261 djm 2086: otherwise.
1.339 djm 2087: .It %t
2088: The type of the server host key, e.g.
1.360 jmc 2089: .Cm ssh-ed25519 .
1.239 jmc 2090: .It %u
2091: The local username.
2092: .El
2093: .Pp
1.323 dtucker 2094: .Cm CertificateFile ,
2095: .Cm ControlPath ,
2096: .Cm IdentityAgent ,
2097: .Cm IdentityFile ,
1.339 djm 2098: .Cm KnownHostsCommand ,
1.325 jmc 2099: .Cm LocalForward ,
1.324 dtucker 2100: .Cm Match exec ,
2101: .Cm RemoteCommand ,
1.329 dtucker 2102: .Cm RemoteForward ,
1.323 dtucker 2103: and
1.331 jmc 2104: .Cm UserKnownHostsFile
1.349 dtucker 2105: accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2106: .Pp
1.339 djm 2107: .Cm KnownHostsCommand
2108: additionally accepts the tokens %f, %H, %I, %K and %t.
2109: .Pp
1.295 jmc 2110: .Cm Hostname
1.239 jmc 2111: accepts the tokens %% and %h.
2112: .Pp
2113: .Cm LocalCommand
1.323 dtucker 2114: accepts all tokens.
1.239 jmc 2115: .Pp
2116: .Cm ProxyCommand
1.303 djm 2117: accepts the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2118: .Sh ENVIRONMENT VARIABLES
2119: Arguments to some keywords can be expanded at runtime from environment
2120: variables on the client by enclosing them in
2121: .Ic ${} ,
2122: for example
2123: .Ic ${HOME}/.ssh
2124: would refer to the user's .ssh directory.
2125: If a specified environment variable does not exist then an error will be
2126: returned and the setting for that keyword will be ignored.
2127: .Pp
2128: The keywords
2129: .Cm CertificateFile ,
2130: .Cm ControlPath ,
1.329 dtucker 2131: .Cm IdentityAgent ,
1.352 jmc 2132: .Cm IdentityFile ,
1.339 djm 2133: .Cm KnownHostsCommand ,
1.326 dtucker 2134: and
1.329 dtucker 2135: .Cm UserKnownHostsFile
1.326 dtucker 2136: support environment variables.
2137: The keywords
2138: .Cm LocalForward
2139: and
2140: .Cm RemoteForward
2141: support environment variables only for Unix domain socket paths.
1.1 stevesk 2142: .Sh FILES
2143: .Bl -tag -width Ds
1.50 djm 2144: .It Pa ~/.ssh/config
1.1 stevesk 2145: This is the per-user configuration file.
2146: The format of this file is described above.
1.84 jmc 2147: This file is used by the SSH client.
1.30 djm 2148: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2149: read/write for the user, and not writable by others.
1.1 stevesk 2150: .It Pa /etc/ssh/ssh_config
2151: Systemwide configuration file.
2152: This file provides defaults for those
2153: values that are not specified in the user's configuration file, and
2154: for those users who do not have a configuration file.
2155: This file must be world-readable.
2156: .El
1.13 jmc 2157: .Sh SEE ALSO
2158: .Xr ssh 1
1.1 stevesk 2159: .Sh AUTHORS
1.240 jmc 2160: .An -nosplit
1.1 stevesk 2161: OpenSSH is a derivative of the original and free
1.240 jmc 2162: ssh 1.2.12 release by
2163: .An Tatu Ylonen .
2164: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2165: .An Niels Provos , Theo de Raadt
2166: and
2167: .An Dug Song
1.1 stevesk 2168: removed many bugs, re-added newer features and
2169: created OpenSSH.
1.240 jmc 2170: .An Markus Friedl
2171: contributed the support for SSH protocol versions 1.5 and 2.0.