Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.378
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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1.378 ! jmc 36: .\" $OpenBSD: ssh_config.5,v 1.377 2023/01/13 03:16:29 dtucker Exp $
! 37: .Dd $Mdocdate: January 13 2023 $
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.
1.376 djm 646: .It Cm EnableEscapeCommandline
647: Enables the command line option in the
648: .Cm EscapeChar
649: menu for interactive sessions (default
650: .Ql ~C ) .
651: By default, the command line is disabled.
1.14 markus 652: .It Cm EnableSSHKeysign
653: Setting this option to
1.240 jmc 654: .Cm yes
1.14 markus 655: in the global client configuration file
656: .Pa /etc/ssh/ssh_config
657: enables the use of the helper program
658: .Xr ssh-keysign 8
659: during
660: .Cm HostbasedAuthentication .
661: The argument must be
1.240 jmc 662: .Cm yes
1.14 markus 663: or
1.240 jmc 664: .Cm no
665: (the default).
1.23 jmc 666: This option should be placed in the non-hostspecific section.
1.14 markus 667: See
668: .Xr ssh-keysign 8
669: for more information.
1.1 stevesk 670: .It Cm EscapeChar
671: Sets the escape character (default:
672: .Ql ~ ) .
673: The escape character can also
674: be set on the command line.
675: The argument should be a single character,
676: .Ql ^
677: followed by a letter, or
1.240 jmc 678: .Cm none
1.1 stevesk 679: to disable the escape
680: character entirely (making the connection transparent for binary
681: data).
1.96 markus 682: .It Cm ExitOnForwardFailure
683: Specifies whether
684: .Xr ssh 1
685: should terminate the connection if it cannot set up all requested
1.216 djm 686: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 687: if either end is unable to bind and listen on a specified port).
1.216 djm 688: Note that
689: .Cm ExitOnForwardFailure
690: does not apply to connections made over port forwardings and will not,
691: for example, cause
692: .Xr ssh 1
693: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 694: The argument must be
1.240 jmc 695: .Cm yes
1.96 markus 696: or
1.240 jmc 697: .Cm no
698: (the default).
1.197 djm 699: .It Cm FingerprintHash
700: Specifies the hash algorithm used when displaying key fingerprints.
701: Valid options are:
1.240 jmc 702: .Cm md5
1.197 djm 703: and
1.240 jmc 704: .Cm sha256
1.359 djm 705: (the default).
706: .It Cm ForkAfterAuthentication
707: Requests
708: .Nm ssh
709: to go to background just before command execution.
710: This is useful if
711: .Nm ssh
712: is going to ask for passwords or passphrases, but the user
713: wants it in the background.
714: This implies the
715: .Cm StdinNull
716: configuration option being set to
717: .Dq yes .
718: The recommended way to start X11 programs at a remote site is with
719: something like
720: .Ic ssh -f host xterm ,
721: which is the same as
722: .Ic ssh host xterm
723: if the
724: .Cm ForkAfterAuthentication
725: configuration option is set to
726: .Dq yes .
727: .Pp
728: If the
729: .Cm ExitOnForwardFailure
730: configuration option is set to
731: .Dq yes ,
732: then a client started with the
733: .Cm ForkAfterAuthentication
734: configuration option being set to
735: .Dq yes
736: will wait for all remote port forwards to be successfully established
737: before placing itself in the background.
738: The argument to this keyword must be
739: .Cm yes
740: (same as the
741: .Fl f
742: option) or
743: .Cm no
1.240 jmc 744: (the default).
1.1 stevesk 745: .It Cm ForwardAgent
746: Specifies whether the connection to the authentication agent (if any)
747: will be forwarded to the remote machine.
1.312 djm 748: The argument may be
749: .Cm yes ,
1.240 jmc 750: .Cm no
1.312 djm 751: (the default),
752: an explicit path to an agent socket or the name of an environment variable
753: (beginning with
754: .Sq $ )
755: in which to find the path.
1.3 stevesk 756: .Pp
1.7 jmc 757: Agent forwarding should be enabled with caution.
758: Users with the ability to bypass file permissions on the remote host
759: (for the agent's Unix-domain socket)
760: can access the local agent through the forwarded connection.
761: An attacker cannot obtain key material from the agent,
1.3 stevesk 762: however they can perform operations on the keys that enable them to
763: authenticate using the identities loaded into the agent.
1.1 stevesk 764: .It Cm ForwardX11
765: Specifies whether X11 connections will be automatically redirected
766: over the secure channel and
767: .Ev DISPLAY
768: set.
769: The argument must be
1.240 jmc 770: .Cm yes
1.1 stevesk 771: or
1.240 jmc 772: .Cm no
773: (the default).
1.3 stevesk 774: .Pp
1.7 jmc 775: X11 forwarding should be enabled with caution.
776: Users with the ability to bypass file permissions on the remote host
1.22 markus 777: (for the user's X11 authorization database)
1.7 jmc 778: can access the local X11 display through the forwarded connection.
1.22 markus 779: An attacker may then be able to perform activities such as keystroke monitoring
780: if the
781: .Cm ForwardX11Trusted
782: option is also enabled.
1.134 djm 783: .It Cm ForwardX11Timeout
1.135 jmc 784: Specify a timeout for untrusted X11 forwarding
785: using the format described in the
1.240 jmc 786: .Sx TIME FORMATS
787: section of
1.134 djm 788: .Xr sshd_config 5 .
789: X11 connections received by
790: .Xr ssh 1
791: after this time will be refused.
1.285 djm 792: Setting
793: .Cm ForwardX11Timeout
794: to zero will disable the timeout and permit X11 forwarding for the life
795: of the connection.
1.134 djm 796: The default is to disable untrusted X11 forwarding after twenty minutes has
797: elapsed.
1.22 markus 798: .It Cm ForwardX11Trusted
1.34 jmc 799: If this option is set to
1.240 jmc 800: .Cm yes ,
1.84 jmc 801: remote X11 clients will have full access to the original X11 display.
1.42 djm 802: .Pp
1.22 markus 803: If this option is set to
1.240 jmc 804: .Cm no
805: (the default),
1.84 jmc 806: remote X11 clients will be considered untrusted and prevented
1.22 markus 807: from stealing or tampering with data belonging to trusted X11
808: clients.
1.42 djm 809: Furthermore, the
810: .Xr xauth 1
811: token used for the session will be set to expire after 20 minutes.
812: Remote clients will be refused access after this time.
1.22 markus 813: .Pp
814: See the X11 SECURITY extension specification for full details on
815: the restrictions imposed on untrusted clients.
1.1 stevesk 816: .It Cm GatewayPorts
817: Specifies whether remote hosts are allowed to connect to local
818: forwarded ports.
819: By default,
1.84 jmc 820: .Xr ssh 1
1.7 jmc 821: binds local port forwardings to the loopback address.
822: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 823: .Cm GatewayPorts
1.84 jmc 824: can be used to specify that ssh
1.1 stevesk 825: should bind local port forwardings to the wildcard address,
826: thus allowing remote hosts to connect to forwarded ports.
827: The argument must be
1.240 jmc 828: .Cm yes
1.1 stevesk 829: or
1.240 jmc 830: .Cm no
831: (the default).
1.1 stevesk 832: .It Cm GlobalKnownHostsFile
1.151 djm 833: Specifies one or more files to use for the global
834: host key database, separated by whitespace.
835: The default is
836: .Pa /etc/ssh/ssh_known_hosts ,
837: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 838: .It Cm GSSAPIAuthentication
1.27 markus 839: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 840: The default is
1.240 jmc 841: .Cm no .
1.18 markus 842: .It Cm GSSAPIDelegateCredentials
843: Forward (delegate) credentials to the server.
844: The default is
1.240 jmc 845: .Cm no .
1.44 djm 846: .It Cm HashKnownHosts
847: Indicates that
1.84 jmc 848: .Xr ssh 1
1.44 djm 849: should hash host names and addresses when they are added to
1.50 djm 850: .Pa ~/.ssh/known_hosts .
1.44 djm 851: These hashed names may be used normally by
1.84 jmc 852: .Xr ssh 1
1.44 djm 853: and
1.84 jmc 854: .Xr sshd 8 ,
1.316 djm 855: but they do not visually reveal identifying information if the
856: file's contents are disclosed.
1.44 djm 857: The default is
1.240 jmc 858: .Cm no .
1.97 jmc 859: Note that existing names and addresses in known hosts files
860: will not be converted automatically,
861: but may be manually hashed using
1.45 djm 862: .Xr ssh-keygen 1 .
1.344 dtucker 863: .It Cm HostbasedAcceptedAlgorithms
1.348 djm 864: Specifies the signature algorithms that will be used for hostbased
865: authentication as a comma-separated list of patterns.
1.300 naddy 866: Alternately if the specified list begins with a
1.214 djm 867: .Sq +
1.348 djm 868: character, then the specified signature algorithms will be appended
869: to the default set instead of replacing them.
1.300 naddy 870: If the specified list begins with a
1.241 djm 871: .Sq -
1.348 djm 872: character, then the specified signature algorithms (including wildcards)
873: will be removed from the default set instead of replacing them.
1.301 naddy 874: If the specified list begins with a
875: .Sq ^
1.348 djm 876: character, then the specified signature algorithms will be placed
877: at the head of the default set.
1.213 markus 878: The default for this option is:
879: .Bd -literal -offset 3n
1.333 djm 880: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 881: ecdsa-sha2-nistp256-cert-v01@openssh.com,
882: ecdsa-sha2-nistp384-cert-v01@openssh.com,
883: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 884: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 885: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 886: rsa-sha2-512-cert-v01@openssh.com,
887: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 888: ssh-ed25519,
1.213 markus 889: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 890: sk-ssh-ed25519@openssh.com,
1.311 naddy 891: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 892: rsa-sha2-512,rsa-sha2-256
1.213 markus 893: .Ed
894: .Pp
1.202 djm 895: The
896: .Fl Q
897: option of
898: .Xr ssh 1
1.348 djm 899: may be used to list supported signature algorithms.
1.344 dtucker 900: This was formerly named HostbasedKeyTypes.
1.345 naddy 901: .It Cm HostbasedAuthentication
902: Specifies whether to try rhosts based authentication with public key
903: authentication.
904: The argument must be
905: .Cm yes
906: or
907: .Cm no
908: (the default).
1.1 stevesk 909: .It Cm HostKeyAlgorithms
1.348 djm 910: Specifies the host key signature algorithms
1.1 stevesk 911: that the client wants to use in order of preference.
1.300 naddy 912: Alternately if the specified list begins with a
1.214 djm 913: .Sq +
1.348 djm 914: character, then the specified signature algorithms will be appended to
915: the default set instead of replacing them.
1.300 naddy 916: If the specified list begins with a
1.241 djm 917: .Sq -
1.348 djm 918: character, then the specified signature algorithms (including wildcards)
919: will be removed from the default set instead of replacing them.
1.301 naddy 920: If the specified list begins with a
921: .Sq ^
1.348 djm 922: character, then the specified signature algorithms will be placed
923: at the head of the default set.
1.1 stevesk 924: The default for this option is:
1.139 djm 925: .Bd -literal -offset 3n
1.333 djm 926: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 927: ecdsa-sha2-nistp256-cert-v01@openssh.com,
928: ecdsa-sha2-nistp384-cert-v01@openssh.com,
929: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 930: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 931: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 932: rsa-sha2-512-cert-v01@openssh.com,
933: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 934: ssh-ed25519,
1.139 djm 935: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 936: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 937: sk-ssh-ed25519@openssh.com,
1.366 kn 938: rsa-sha2-512,rsa-sha2-256
1.139 djm 939: .Ed
1.145 djm 940: .Pp
941: If hostkeys are known for the destination host then this default is modified
942: to prefer their algorithms.
1.198 djm 943: .Pp
1.348 djm 944: The list of available signature algorithms may also be obtained using
1.322 dtucker 945: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 946: .It Cm HostKeyAlias
947: Specifies an alias that should be used instead of the
948: real host name when looking up or saving the host key
1.251 djm 949: in the host key database files and when validating host certificates.
1.84 jmc 950: This option is useful for tunneling SSH connections
1.1 stevesk 951: or for multiple servers running on a single host.
1.295 jmc 952: .It Cm Hostname
1.1 stevesk 953: Specifies the real host name to log into.
954: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 955: Arguments to
1.295 jmc 956: .Cm Hostname
1.239 jmc 957: accept the tokens described in the
958: .Sx TOKENS
959: section.
1.1 stevesk 960: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 961: .Cm Hostname
1.1 stevesk 962: specifications).
1.239 jmc 963: The default is the name given on the command line.
1.29 markus 964: .It Cm IdentitiesOnly
965: Specifies that
1.84 jmc 966: .Xr ssh 1
1.304 djm 967: should only use the configured authentication identity and certificate files
968: (either the default files, or those explicitly configured in the
1.31 jmc 969: .Nm
1.221 djm 970: files
971: or passed on the
972: .Xr ssh 1
1.304 djm 973: command-line),
1.84 jmc 974: even if
975: .Xr ssh-agent 1
1.159 djm 976: or a
977: .Cm PKCS11Provider
1.305 naddy 978: or
979: .Cm SecurityKeyProvider
1.29 markus 980: offers more identities.
981: The argument to this keyword must be
1.240 jmc 982: .Cm yes
1.29 markus 983: or
1.240 jmc 984: .Cm no
985: (the default).
1.84 jmc 986: This option is intended for situations where ssh-agent
1.29 markus 987: offers many different identities.
1.231 markus 988: .It Cm IdentityAgent
989: Specifies the
990: .Ux Ns -domain
991: socket used to communicate with the authentication agent.
992: .Pp
993: This option overrides the
1.240 jmc 994: .Ev SSH_AUTH_SOCK
1.231 markus 995: environment variable and can be used to select a specific agent.
996: Setting the socket name to
1.240 jmc 997: .Cm none
1.231 markus 998: disables the use of an authentication agent.
1.232 markus 999: If the string
1.240 jmc 1000: .Qq SSH_AUTH_SOCK
1.232 markus 1001: is specified, the location of the socket will be read from the
1002: .Ev SSH_AUTH_SOCK
1003: environment variable.
1.286 djm 1004: Otherwise if the specified value begins with a
1005: .Sq $
1006: character, then it will be treated as an environment variable containing
1007: the location of the socket.
1.231 markus 1008: .Pp
1.239 jmc 1009: Arguments to
1010: .Cm IdentityAgent
1.326 dtucker 1011: may use the tilde syntax to refer to a user's home directory,
1012: the tokens described in the
1.239 jmc 1013: .Sx TOKENS
1.326 dtucker 1014: section and environment variables as described in the
1015: .Sx ENVIRONMENT VARIABLES
1.239 jmc 1016: section.
1.67 jmc 1017: .It Cm IdentityFile
1.313 naddy 1018: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1019: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.375 dtucker 1020: You can also specify a public key file to use the corresponding
1021: private key that is loaded in
1022: .Xr ssh-agent 1
1023: when the private key file is not present locally.
1.67 jmc 1024: The default is
1.368 dtucker 1025: .Pa ~/.ssh/id_rsa ,
1.183 naddy 1026: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 1027: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 1028: .Pa ~/.ssh/id_ed25519 ,
1029: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 1030: and
1.368 dtucker 1031: .Pa ~/.ssh/id_dsa .
1.67 jmc 1032: Additionally, any identities represented by the authentication agent
1.165 djm 1033: will be used for authentication unless
1034: .Cm IdentitiesOnly
1035: is set.
1.221 djm 1036: If no certificates have been explicitly specified by
1037: .Cm CertificateFile ,
1.129 djm 1038: .Xr ssh 1
1039: will try to load certificate information from the filename obtained by
1040: appending
1041: .Pa -cert.pub
1042: to the path of a specified
1043: .Cm IdentityFile .
1.90 djm 1044: .Pp
1.239 jmc 1045: Arguments to
1046: .Cm IdentityFile
1047: may use the tilde syntax to refer to a user's home directory
1048: or the tokens described in the
1049: .Sx TOKENS
1050: section.
1.90 djm 1051: .Pp
1.67 jmc 1052: It is possible to have
1053: multiple identity files specified in configuration files; all these
1054: identities will be tried in sequence.
1.152 djm 1055: Multiple
1056: .Cm IdentityFile
1057: directives will add to the list of identities tried (this behaviour
1058: differs from that of other configuration directives).
1.165 djm 1059: .Pp
1060: .Cm IdentityFile
1061: may be used in conjunction with
1062: .Cm IdentitiesOnly
1063: to select which identities in an agent are offered during authentication.
1.221 djm 1064: .Cm IdentityFile
1065: may also be used in conjunction with
1066: .Cm CertificateFile
1067: in order to provide any certificate also needed for authentication with
1068: the identity.
1.164 jmc 1069: .It Cm IgnoreUnknown
1070: Specifies a pattern-list of unknown options to be ignored if they are
1071: encountered in configuration parsing.
1072: This may be used to suppress errors if
1073: .Nm
1074: contains options that are unrecognised by
1075: .Xr ssh 1 .
1076: It is recommended that
1077: .Cm IgnoreUnknown
1078: be listed early in the configuration file as it will not be applied
1079: to unknown options that appear before it.
1.229 djm 1080: .It Cm Include
1081: Include the specified configuration file(s).
1.230 jmc 1082: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1083: .Xr glob 7
1.229 djm 1084: wildcards and, for user configurations, shell-like
1.240 jmc 1085: .Sq ~
1.229 djm 1086: references to user home directories.
1.327 djm 1087: Wildcards will be expanded and processed in lexical order.
1.229 djm 1088: Files without absolute paths are assumed to be in
1089: .Pa ~/.ssh
1.230 jmc 1090: if included in a user configuration file or
1.229 djm 1091: .Pa /etc/ssh
1092: if included from the system configuration file.
1093: .Cm Include
1094: directive may appear inside a
1095: .Cm Match
1096: or
1097: .Cm Host
1098: block
1099: to perform conditional inclusion.
1.143 djm 1100: .It Cm IPQoS
1101: Specifies the IPv4 type-of-service or DSCP class for connections.
1102: Accepted values are
1.240 jmc 1103: .Cm af11 ,
1104: .Cm af12 ,
1105: .Cm af13 ,
1106: .Cm af21 ,
1107: .Cm af22 ,
1108: .Cm af23 ,
1109: .Cm af31 ,
1110: .Cm af32 ,
1111: .Cm af33 ,
1112: .Cm af41 ,
1113: .Cm af42 ,
1114: .Cm af43 ,
1115: .Cm cs0 ,
1116: .Cm cs1 ,
1117: .Cm cs2 ,
1118: .Cm cs3 ,
1119: .Cm cs4 ,
1120: .Cm cs5 ,
1121: .Cm cs6 ,
1122: .Cm cs7 ,
1123: .Cm ef ,
1.319 djm 1124: .Cm le ,
1.240 jmc 1125: .Cm lowdelay ,
1126: .Cm throughput ,
1127: .Cm reliability ,
1.253 djm 1128: a numeric value, or
1129: .Cm none
1130: to use the operating system default.
1.146 djm 1131: This option may take one or two arguments, separated by whitespace.
1.143 djm 1132: If one argument is specified, it is used as the packet class unconditionally.
1133: If two values are specified, the first is automatically selected for
1134: interactive sessions and the second for non-interactive sessions.
1135: The default is
1.269 job 1136: .Cm af21
1.272 jmc 1137: (Low-Latency Data)
1.143 djm 1138: for interactive sessions and
1.269 job 1139: .Cm cs1
1.272 jmc 1140: (Lower Effort)
1.143 djm 1141: for non-interactive sessions.
1.103 djm 1142: .It Cm KbdInteractiveAuthentication
1143: Specifies whether to use keyboard-interactive authentication.
1144: The argument to this keyword must be
1.240 jmc 1145: .Cm yes
1146: (the default)
1.103 djm 1147: or
1.240 jmc 1148: .Cm no .
1.355 dtucker 1149: .Cm ChallengeResponseAuthentication
1150: is a deprecated alias for this.
1.39 djm 1151: .It Cm KbdInteractiveDevices
1152: Specifies the list of methods to use in keyboard-interactive authentication.
1153: Multiple method names must be comma-separated.
1154: The default is to use the server specified list.
1.85 jmc 1155: The methods available vary depending on what the server supports.
1156: For an OpenSSH server,
1157: it may be zero or more of:
1.240 jmc 1158: .Cm bsdauth ,
1159: .Cm pam ,
1.85 jmc 1160: and
1.240 jmc 1161: .Cm skey .
1.140 djm 1162: .It Cm KexAlgorithms
1163: Specifies the available KEX (Key Exchange) algorithms.
1164: Multiple algorithms must be comma-separated.
1.299 kn 1165: If the specified list begins with a
1.214 djm 1166: .Sq +
1.363 dtucker 1167: character, then the specified algorithms will be appended to the default set
1.214 djm 1168: instead of replacing them.
1.299 kn 1169: If the specified list begins with a
1.241 djm 1170: .Sq -
1.363 dtucker 1171: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1172: from the default set instead of replacing them.
1.301 naddy 1173: If the specified list begins with a
1174: .Sq ^
1.363 dtucker 1175: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1176: default set.
1.141 jmc 1177: The default is:
1178: .Bd -literal -offset indent
1.371 naddy 1179: sntrup761x25519-sha512@openssh.com,
1.238 djm 1180: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1181: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1182: diffie-hellman-group-exchange-sha256,
1.266 djm 1183: diffie-hellman-group16-sha512,
1184: diffie-hellman-group18-sha512,
1.317 tedu 1185: diffie-hellman-group14-sha256
1.141 jmc 1186: .Ed
1.198 djm 1187: .Pp
1.240 jmc 1188: The list of available key exchange algorithms may also be obtained using
1189: .Qq ssh -Q kex .
1.339 djm 1190: .It Cm KnownHostsCommand
1.340 jmc 1191: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1192: those listed in
1193: .Cm UserKnownHostsFile
1194: and
1195: .Cm GlobalKnownHostsFile .
1196: This command is executed after the files have been read.
1.340 jmc 1197: It may write host key lines to standard output in identical format to the
1.339 djm 1198: usual files (described in the
1199: .Sx VERIFYING HOST KEYS
1200: section in
1201: .Xr ssh 1 ) .
1202: Arguments to
1203: .Cm KnownHostsCommand
1204: accept the tokens described in the
1205: .Sx TOKENS
1206: section.
1.340 jmc 1207: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1208: the preference list of host key algorithms to use, again to obtain the
1209: host key for the requested host name and, if
1210: .Cm CheckHostIP
1211: is enabled, one more time to obtain the host key matching the server's
1212: address.
1213: If the command exits abnormally or returns a non-zero exit status then the
1214: connection is terminated.
1.65 reyk 1215: .It Cm LocalCommand
1216: Specifies a command to execute on the local machine after successfully
1217: connecting to the server.
1218: The command string extends to the end of the line, and is executed with
1.105 jmc 1219: the user's shell.
1.239 jmc 1220: Arguments to
1221: .Cm LocalCommand
1222: accept the tokens described in the
1223: .Sx TOKENS
1224: section.
1.123 djm 1225: .Pp
1226: The command is run synchronously and does not have access to the
1227: session of the
1228: .Xr ssh 1
1229: that spawned it.
1230: It should not be used for interactive commands.
1231: .Pp
1.65 reyk 1232: This directive is ignored unless
1233: .Cm PermitLocalCommand
1234: has been enabled.
1.1 stevesk 1235: .It Cm LocalForward
1.74 jmc 1236: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1237: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1238: The first argument specifies the listener and may be
1.43 djm 1239: .Sm off
1.49 jmc 1240: .Oo Ar bind_address : Oc Ar port
1.43 djm 1241: .Sm on
1.324 dtucker 1242: or a Unix domain socket path.
1243: The second argument is the destination and may be
1244: .Ar host : Ns Ar hostport
1245: or a Unix domain socket path if the remote host supports it.
1246: .Pp
1.138 djm 1247: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1248: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1249: given on the command line.
1.1 stevesk 1250: Only the superuser can forward privileged ports.
1.43 djm 1251: By default, the local port is bound in accordance with the
1252: .Cm GatewayPorts
1253: setting.
1254: However, an explicit
1255: .Ar bind_address
1256: may be used to bind the connection to a specific address.
1257: The
1258: .Ar bind_address
1259: of
1.240 jmc 1260: .Cm localhost
1.46 jmc 1261: indicates that the listening port be bound for local use only, while an
1262: empty address or
1263: .Sq *
1.43 djm 1264: indicates that the port should be available from all interfaces.
1.326 dtucker 1265: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1266: .Sx TOKENS
1.326 dtucker 1267: section and environment variables as described in the
1268: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1269: section.
1.1 stevesk 1270: .It Cm LogLevel
1271: Gives the verbosity level that is used when logging messages from
1.84 jmc 1272: .Xr ssh 1 .
1.1 stevesk 1273: The possible values are:
1.84 jmc 1274: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1275: The default is INFO.
1276: DEBUG and DEBUG1 are equivalent.
1277: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1278: .It Cm LogVerbose
1279: Specify one or more overrides to LogLevel.
1280: An override consists of a pattern lists that matches the source file, function
1281: and line number to force detailed logging for.
1282: For example, an override pattern of:
1283: .Bd -literal -offset indent
1284: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1285: .Ed
1286: .Pp
1287: would enable detailed logging for line 1000 of
1.338 jmc 1288: .Pa kex.c ,
1.337 djm 1289: everything in the
1290: .Fn kex_exchange_identification
1291: function, and all code in the
1292: .Pa packet.c
1293: file.
1294: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1295: .It Cm MACs
1296: Specifies the MAC (message authentication code) algorithms
1297: in order of preference.
1.226 jmc 1298: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1299: Multiple algorithms must be comma-separated.
1.299 kn 1300: If the specified list begins with a
1.214 djm 1301: .Sq +
1302: character, then the specified algorithms will be appended to the default set
1303: instead of replacing them.
1.299 kn 1304: If the specified list begins with a
1.241 djm 1305: .Sq -
1306: character, then the specified algorithms (including wildcards) will be removed
1307: from the default set instead of replacing them.
1.301 naddy 1308: If the specified list begins with a
1309: .Sq ^
1310: character, then the specified algorithms will be placed at the head of the
1311: default set.
1.214 djm 1312: .Pp
1.160 markus 1313: The algorithms that contain
1.240 jmc 1314: .Qq -etm
1.160 markus 1315: calculate the MAC after encryption (encrypt-then-mac).
1316: These are considered safer and their use recommended.
1.214 djm 1317: .Pp
1.84 jmc 1318: The default is:
1.101 jmc 1319: .Bd -literal -offset indent
1.160 markus 1320: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1321: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1322: hmac-sha1-etm@openssh.com,
1.186 naddy 1323: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1324: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1325: .Ed
1.198 djm 1326: .Pp
1.240 jmc 1327: The list of available MAC algorithms may also be obtained using
1328: .Qq ssh -Q mac .
1.1 stevesk 1329: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1330: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1331: The argument to this keyword must be
1.240 jmc 1332: .Cm yes
1.1 stevesk 1333: or
1.242 jmc 1334: .Cm no
1.240 jmc 1335: (the default).
1.1 stevesk 1336: .It Cm NumberOfPasswordPrompts
1337: Specifies the number of password prompts before giving up.
1338: The argument to this keyword must be an integer.
1.84 jmc 1339: The default is 3.
1.1 stevesk 1340: .It Cm PasswordAuthentication
1341: Specifies whether to use password authentication.
1342: The argument to this keyword must be
1.240 jmc 1343: .Cm yes
1344: (the default)
1.1 stevesk 1345: or
1.240 jmc 1346: .Cm no .
1.65 reyk 1347: .It Cm PermitLocalCommand
1348: Allow local command execution via the
1349: .Ic LocalCommand
1350: option or using the
1.66 jmc 1351: .Ic !\& Ns Ar command
1.65 reyk 1352: escape sequence in
1353: .Xr ssh 1 .
1354: The argument must be
1.240 jmc 1355: .Cm yes
1.65 reyk 1356: or
1.240 jmc 1357: .Cm no
1358: (the default).
1.347 markus 1359: .It Cm PermitRemoteOpen
1360: Specifies the destinations to which remote TCP port forwarding is permitted when
1361: .Cm RemoteForward
1362: is used as a SOCKS proxy.
1363: The forwarding specification must be one of the following forms:
1364: .Pp
1365: .Bl -item -offset indent -compact
1366: .It
1367: .Cm PermitRemoteOpen
1368: .Sm off
1369: .Ar host : port
1370: .Sm on
1371: .It
1372: .Cm PermitRemoteOpen
1373: .Sm off
1374: .Ar IPv4_addr : port
1375: .Sm on
1376: .It
1377: .Cm PermitRemoteOpen
1378: .Sm off
1379: .Ar \&[ IPv6_addr \&] : port
1380: .Sm on
1381: .El
1382: .Pp
1383: Multiple forwards may be specified by separating them with whitespace.
1384: An argument of
1385: .Cm any
1386: can be used to remove all restrictions and permit any forwarding requests.
1387: An argument of
1388: .Cm none
1389: can be used to prohibit all forwarding requests.
1390: The wildcard
1391: .Sq *
1392: can be used for host or port to allow all hosts or ports respectively.
1393: Otherwise, no pattern matching or address lookups are performed on supplied
1394: names.
1.127 markus 1395: .It Cm PKCS11Provider
1.292 djm 1396: Specifies which PKCS#11 provider to use or
1397: .Cm none
1398: to indicate that no provider should be used (the default).
1399: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1400: .Xr ssh 1
1.292 djm 1401: should use to communicate with a PKCS#11 token providing keys for user
1402: authentication.
1.67 jmc 1403: .It Cm Port
1404: Specifies the port number to connect on the remote host.
1.84 jmc 1405: The default is 22.
1.1 stevesk 1406: .It Cm PreferredAuthentications
1.226 jmc 1407: Specifies the order in which the client should try authentication methods.
1.48 jmc 1408: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1409: .Cm keyboard-interactive )
1.48 jmc 1410: over another method (e.g.\&
1.131 jmc 1411: .Cm password ) .
1412: The default is:
1413: .Bd -literal -offset indent
1414: gssapi-with-mic,hostbased,publickey,
1415: keyboard-interactive,password
1416: .Ed
1.1 stevesk 1417: .It Cm ProxyCommand
1418: Specifies the command to use to connect to the server.
1419: The command
1.190 djm 1420: string extends to the end of the line, and is executed
1421: using the user's shell
1422: .Ql exec
1423: directive to avoid a lingering shell process.
1424: .Pp
1.239 jmc 1425: Arguments to
1426: .Cm ProxyCommand
1427: accept the tokens described in the
1428: .Sx TOKENS
1429: section.
1.1 stevesk 1430: The command can be basically anything,
1431: and should read from its standard input and write to its standard output.
1432: It should eventually connect an
1433: .Xr sshd 8
1434: server running on some machine, or execute
1435: .Ic sshd -i
1436: somewhere.
1437: Host key management will be done using the
1.296 jmc 1438: .Cm Hostname
1439: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1440: Setting the command to
1.240 jmc 1441: .Cm none
1.6 markus 1442: disables this option entirely.
1.1 stevesk 1443: Note that
1444: .Cm CheckHostIP
1445: is not available for connects with a proxy command.
1.52 djm 1446: .Pp
1447: This directive is useful in conjunction with
1448: .Xr nc 1
1449: and its proxy support.
1.53 jmc 1450: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1451: 192.0.2.0:
1452: .Bd -literal -offset 3n
1453: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1454: .Ed
1.233 djm 1455: .It Cm ProxyJump
1.260 millert 1456: Specifies one or more jump proxies as either
1.233 djm 1457: .Xo
1458: .Sm off
1.234 jmc 1459: .Op Ar user No @
1.233 djm 1460: .Ar host
1.234 jmc 1461: .Op : Ns Ar port
1.233 djm 1462: .Sm on
1.260 millert 1463: or an ssh URI
1.233 djm 1464: .Xc .
1.235 djm 1465: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1466: sequentially.
1.233 djm 1467: Setting this option will cause
1468: .Xr ssh 1
1469: to connect to the target host by first making a
1470: .Xr ssh 1
1471: connection to the specified
1472: .Cm ProxyJump
1473: host and then establishing a
1.234 jmc 1474: TCP forwarding to the ultimate target from there.
1.346 dlg 1475: Setting the host to
1476: .Cm none
1477: disables this option entirely.
1.233 djm 1478: .Pp
1479: Note that this option will compete with the
1480: .Cm ProxyCommand
1481: option - whichever is specified first will prevent later instances of the
1482: other from taking effect.
1.289 djm 1483: .Pp
1484: Note also that the configuration for the destination host (either supplied
1485: via the command-line or the configuration file) is not generally applied
1486: to jump hosts.
1487: .Pa ~/.ssh/config
1488: should be used if specific configuration is required for jump hosts.
1.167 djm 1489: .It Cm ProxyUseFdpass
1.168 jmc 1490: Specifies that
1.167 djm 1491: .Cm ProxyCommand
1492: will pass a connected file descriptor back to
1.168 jmc 1493: .Xr ssh 1
1.167 djm 1494: instead of continuing to execute and pass data.
1495: The default is
1.240 jmc 1496: .Cm no .
1.343 dtucker 1497: .It Cm PubkeyAcceptedAlgorithms
1498: Specifies the signature algorithms that will be used for public key
1499: authentication as a comma-separated list of patterns.
1.299 kn 1500: If the specified list begins with a
1.214 djm 1501: .Sq +
1.343 dtucker 1502: character, then the algorithms after it will be appended to the default
1.214 djm 1503: instead of replacing it.
1.299 kn 1504: If the specified list begins with a
1.241 djm 1505: .Sq -
1.343 dtucker 1506: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1507: from the default set instead of replacing them.
1.301 naddy 1508: If the specified list begins with a
1509: .Sq ^
1.343 dtucker 1510: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1511: default set.
1.213 markus 1512: The default for this option is:
1513: .Bd -literal -offset 3n
1.333 djm 1514: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1515: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1516: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1517: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1518: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1519: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1520: rsa-sha2-512-cert-v01@openssh.com,
1521: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 1522: ssh-ed25519,
1.311 naddy 1523: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1524: sk-ssh-ed25519@openssh.com,
1.305 naddy 1525: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 1526: rsa-sha2-512,rsa-sha2-256
1.213 markus 1527: .Ed
1528: .Pp
1.348 djm 1529: The list of available signature algorithms may also be obtained using
1.343 dtucker 1530: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1531: .It Cm PubkeyAuthentication
1532: Specifies whether to try public key authentication.
1533: The argument to this keyword must be
1.240 jmc 1534: .Cm yes
1.369 djm 1535: (the default),
1536: .Cm no ,
1537: .Cm unbound
1.1 stevesk 1538: or
1.369 djm 1539: .Cm host-bound .
1540: The final two options enable public key authentication while respectively
1541: disabling or enabling the OpenSSH host-bound authentication protocol
1542: extension required for restricted
1543: .Xr ssh-agent 1
1544: forwarding.
1.75 dtucker 1545: .It Cm RekeyLimit
1.373 djm 1546: Specifies the maximum amount of data that may be transmitted or received
1547: before the session key is renegotiated, optionally followed by a maximum
1548: amount of time that may pass before the session key is renegotiated.
1.162 dtucker 1549: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1550: .Sq K ,
1551: .Sq M ,
1.75 dtucker 1552: or
1.76 jmc 1553: .Sq G
1.75 dtucker 1554: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1555: The default is between
1.84 jmc 1556: .Sq 1G
1.75 dtucker 1557: and
1.84 jmc 1558: .Sq 4G ,
1.75 dtucker 1559: depending on the cipher.
1.162 dtucker 1560: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1561: units documented in the TIME FORMATS section of
1.162 dtucker 1562: .Xr sshd_config 5 .
1563: The default value for
1564: .Cm RekeyLimit
1565: is
1.240 jmc 1566: .Cm default none ,
1.162 dtucker 1567: which means that rekeying is performed after the cipher's default amount
1568: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1569: .It Cm RemoteCommand
1570: Specifies a command to execute on the remote machine after successfully
1571: connecting to the server.
1572: The command string extends to the end of the line, and is executed with
1573: the user's shell.
1.250 jmc 1574: Arguments to
1575: .Cm RemoteCommand
1576: accept the tokens described in the
1577: .Sx TOKENS
1578: section.
1.1 stevesk 1579: .It Cm RemoteForward
1.74 jmc 1580: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1581: the secure channel.
1.273 djm 1582: The remote port may either be forwarded to a specified host and port
1.256 markus 1583: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1584: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1585: The first argument is the listening specification and may be
1.43 djm 1586: .Sm off
1.49 jmc 1587: .Oo Ar bind_address : Oc Ar port
1.43 djm 1588: .Sm on
1.324 dtucker 1589: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1590: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1591: .Ar host : Ns Ar hostport
1592: or a Unix domain socket path,
1.256 markus 1593: otherwise if no destination argument is specified then the remote forwarding
1594: will be established as a SOCKS proxy.
1.370 naddy 1595: When acting as a SOCKS proxy, the destination of the connection can be
1.347 markus 1596: restricted by
1597: .Cm PermitRemoteOpen .
1.256 markus 1598: .Pp
1.138 djm 1599: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1600: Multiple forwardings may be specified, and additional
1601: forwardings can be given on the command line.
1.113 stevesk 1602: Privileged ports can be forwarded only when
1603: logging in as root on the remote machine.
1.326 dtucker 1604: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1605: .Sx TOKENS
1.326 dtucker 1606: section and environment variables as described in the
1607: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1608: section.
1.118 jmc 1609: .Pp
1.117 djm 1610: If the
1611: .Ar port
1.240 jmc 1612: argument is 0,
1.117 djm 1613: the listen port will be dynamically allocated on the server and reported
1614: to the client at run time.
1.43 djm 1615: .Pp
1616: If the
1617: .Ar bind_address
1618: is not specified, the default is to only bind to loopback addresses.
1619: If the
1620: .Ar bind_address
1621: is
1622: .Ql *
1623: or an empty string, then the forwarding is requested to listen on all
1624: interfaces.
1625: Specifying a remote
1626: .Ar bind_address
1.46 jmc 1627: will only succeed if the server's
1628: .Cm GatewayPorts
1.43 djm 1629: option is enabled (see
1.46 jmc 1630: .Xr sshd_config 5 ) .
1.149 djm 1631: .It Cm RequestTTY
1632: Specifies whether to request a pseudo-tty for the session.
1633: The argument may be one of:
1.240 jmc 1634: .Cm no
1.149 djm 1635: (never request a TTY),
1.240 jmc 1636: .Cm yes
1.149 djm 1637: (always request a TTY when standard input is a TTY),
1.240 jmc 1638: .Cm force
1.149 djm 1639: (always request a TTY) or
1.240 jmc 1640: .Cm auto
1.149 djm 1641: (request a TTY when opening a login session).
1642: This option mirrors the
1643: .Fl t
1644: and
1645: .Fl T
1646: flags for
1647: .Xr ssh 1 .
1.374 djm 1648: .It Cm RequiredRSASize
1649: Specifies the minimum RSA key size (in bits) that
1650: .Xr ssh 1
1651: will accept.
1652: User authentication keys smaller than this limit will be ignored.
1653: Servers that present host keys smaller than this limit will cause the
1654: connection to be terminated.
1655: The default is
1656: .Cm 1024
1657: bits.
1658: Note that this limit may only be raised from the default.
1.196 djm 1659: .It Cm RevokedHostKeys
1660: Specifies revoked host public keys.
1661: Keys listed in this file will be refused for host authentication.
1662: Note that if this file does not exist or is not readable,
1663: then host authentication will be refused for all hosts.
1664: Keys may be specified as a text file, listing one public key per line, or as
1665: an OpenSSH Key Revocation List (KRL) as generated by
1666: .Xr ssh-keygen 1 .
1667: For more information on KRLs, see the KEY REVOCATION LISTS section in
1668: .Xr ssh-keygen 1 .
1.305 naddy 1669: .It Cm SecurityKeyProvider
1.313 naddy 1670: Specifies a path to a library that will be used when loading any
1671: FIDO authenticator-hosted keys, overriding the default of using
1672: the built-in USB HID support.
1.309 naddy 1673: .Pp
1674: If the specified value begins with a
1675: .Sq $
1676: character, then it will be treated as an environment variable containing
1677: the path to the library.
1.32 djm 1678: .It Cm SendEnv
1679: Specifies what variables from the local
1680: .Xr environ 7
1681: should be sent to the server.
1.84 jmc 1682: The server must also support it, and the server must be configured to
1.33 djm 1683: accept these environment variables.
1.207 dtucker 1684: Note that the
1685: .Ev TERM
1.208 jmc 1686: environment variable is always sent whenever a
1.207 dtucker 1687: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1688: Refer to
1689: .Cm AcceptEnv
1690: in
1691: .Xr sshd_config 5
1692: for how to configure the server.
1.80 jmc 1693: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1694: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1695: across multiple
1696: .Cm SendEnv
1697: directives.
1.81 jmc 1698: .Pp
1699: See
1700: .Sx PATTERNS
1701: for more information on patterns.
1.271 djm 1702: .Pp
1.272 jmc 1703: It is possible to clear previously set
1.271 djm 1704: .Cm SendEnv
1705: variable names by prefixing patterns with
1706: .Pa - .
1707: The default is not to send any environment variables.
1.28 markus 1708: .It Cm ServerAliveCountMax
1.73 jmc 1709: Sets the number of server alive messages (see below) which may be
1.28 markus 1710: sent without
1.84 jmc 1711: .Xr ssh 1
1.28 markus 1712: receiving any messages back from the server.
1713: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1714: ssh will disconnect from the server, terminating the session.
1.28 markus 1715: It is important to note that the use of server alive messages is very
1716: different from
1717: .Cm TCPKeepAlive
1718: (below).
1719: The server alive messages are sent through the encrypted channel
1720: and therefore will not be spoofable.
1721: The TCP keepalive option enabled by
1722: .Cm TCPKeepAlive
1723: is spoofable.
1724: The server alive mechanism is valuable when the client or
1.298 dtucker 1725: server depend on knowing when a connection has become unresponsive.
1.28 markus 1726: .Pp
1727: The default value is 3.
1728: If, for example,
1729: .Cm ServerAliveInterval
1.84 jmc 1730: (see below) is set to 15 and
1.28 markus 1731: .Cm ServerAliveCountMax
1.84 jmc 1732: is left at the default, if the server becomes unresponsive,
1733: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1734: .It Cm ServerAliveInterval
1735: Sets a timeout interval in seconds after which if no data has been received
1736: from the server,
1.84 jmc 1737: .Xr ssh 1
1.67 jmc 1738: will send a message through the encrypted
1739: channel to request a response from the server.
1740: The default
1741: is 0, indicating that these messages will not be sent to the server.
1.357 jmc 1742: .It Cm SessionType
1743: May be used to either request invocation of a subsystem on the remote system,
1744: or to prevent the execution of a remote command at all.
1745: The latter is useful for just forwarding ports.
1746: The argument to this keyword must be
1747: .Cm none
1748: (same as the
1749: .Fl N
1750: option),
1751: .Cm subsystem
1752: (same as the
1753: .Fl s
1754: option) or
1755: .Cm default
1756: (shell or command execution).
1.277 jmc 1757: .It Cm SetEnv
1758: Directly specify one or more environment variables and their contents to
1759: be sent to the server.
1760: Similarly to
1761: .Cm SendEnv ,
1.354 djm 1762: with the exception of the
1763: .Ev TERM
1764: variable, the server must be prepared to accept the environment variable.
1.358 djm 1765: .It Cm StdinNull
1766: Redirects stdin from
1767: .Pa /dev/null
1768: (actually, prevents reading from stdin).
1769: Either this or the equivalent
1770: .Fl n
1771: option must be used when
1772: .Nm ssh
1773: is run in the background.
1774: The argument to this keyword must be
1775: .Cm yes
1776: (same as the
1777: .Fl n
1778: option) or
1779: .Cm no
1780: (the default).
1.191 millert 1781: .It Cm StreamLocalBindMask
1782: Sets the octal file creation mode mask
1783: .Pq umask
1784: used when creating a Unix-domain socket file for local or remote
1785: port forwarding.
1786: This option is only used for port forwarding to a Unix-domain socket file.
1787: .Pp
1788: The default value is 0177, which creates a Unix-domain socket file that is
1789: readable and writable only by the owner.
1790: Note that not all operating systems honor the file mode on Unix-domain
1791: socket files.
1792: .It Cm StreamLocalBindUnlink
1793: Specifies whether to remove an existing Unix-domain socket file for local
1794: or remote port forwarding before creating a new one.
1795: If the socket file already exists and
1796: .Cm StreamLocalBindUnlink
1797: is not enabled,
1798: .Nm ssh
1799: will be unable to forward the port to the Unix-domain socket file.
1800: This option is only used for port forwarding to a Unix-domain socket file.
1801: .Pp
1802: The argument must be
1.240 jmc 1803: .Cm yes
1.191 millert 1804: or
1.240 jmc 1805: .Cm no
1806: (the default).
1.1 stevesk 1807: .It Cm StrictHostKeyChecking
1808: If this flag is set to
1.240 jmc 1809: .Cm yes ,
1.84 jmc 1810: .Xr ssh 1
1.1 stevesk 1811: will never automatically add host keys to the
1.50 djm 1812: .Pa ~/.ssh/known_hosts
1.1 stevesk 1813: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1814: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1815: though it can be annoying when the
1.1 stevesk 1816: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1817: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1818: frequently made.
1819: This option forces the user to manually
1820: add all new hosts.
1.255 jmc 1821: .Pp
1.1 stevesk 1822: If this flag is set to
1.364 dtucker 1823: .Cm accept-new
1.360 jmc 1824: then ssh will automatically add new host keys to the user's
1825: .Pa known_hosts
1826: file, but will not permit connections to hosts with
1.254 djm 1827: changed host keys.
1828: If this flag is set to
1.364 dtucker 1829: .Cm no
1.254 djm 1830: or
1.364 dtucker 1831: .Cm off ,
1.255 jmc 1832: ssh will automatically add new host keys to the user known hosts files
1833: and allow connections to hosts with changed hostkeys to proceed,
1834: subject to some restrictions.
1.1 stevesk 1835: If this flag is set to
1.240 jmc 1836: .Cm ask
1837: (the default),
1.1 stevesk 1838: new host keys
1839: will be added to the user known host files only after the user
1840: has confirmed that is what they really want to do, and
1.84 jmc 1841: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1842: The host keys of
1843: known hosts will be verified automatically in all cases.
1.244 jmc 1844: .It Cm SyslogFacility
1845: Gives the facility code that is used when logging messages from
1846: .Xr ssh 1 .
1847: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1848: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1849: The default is USER.
1.26 markus 1850: .It Cm TCPKeepAlive
1851: Specifies whether the system should send TCP keepalive messages to the
1852: other side.
1853: If they are sent, death of the connection or crash of one
1854: of the machines will be properly noticed.
1855: However, this means that
1856: connections will die if the route is down temporarily, and some people
1857: find it annoying.
1858: .Pp
1859: The default is
1.240 jmc 1860: .Cm yes
1.26 markus 1861: (to send TCP keepalive messages), and the client will notice
1862: if the network goes down or the remote host dies.
1863: This is important in scripts, and many users want it too.
1864: .Pp
1865: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1866: .Cm no .
1.265 djm 1867: See also
1868: .Cm ServerAliveInterval
1869: for protocol-level keepalives.
1.65 reyk 1870: .It Cm Tunnel
1.95 stevesk 1871: Request
1.65 reyk 1872: .Xr tun 4
1.69 jmc 1873: device forwarding between the client and the server.
1.65 reyk 1874: The argument must be
1.240 jmc 1875: .Cm yes ,
1876: .Cm point-to-point
1.95 stevesk 1877: (layer 3),
1.240 jmc 1878: .Cm ethernet
1.95 stevesk 1879: (layer 2),
1.65 reyk 1880: or
1.240 jmc 1881: .Cm no
1882: (the default).
1.95 stevesk 1883: Specifying
1.240 jmc 1884: .Cm yes
1.95 stevesk 1885: requests the default tunnel mode, which is
1.240 jmc 1886: .Cm point-to-point .
1.65 reyk 1887: .It Cm TunnelDevice
1.95 stevesk 1888: Specifies the
1.65 reyk 1889: .Xr tun 4
1.95 stevesk 1890: devices to open on the client
1891: .Pq Ar local_tun
1892: and the server
1893: .Pq Ar remote_tun .
1894: .Pp
1895: The argument must be
1896: .Sm off
1897: .Ar local_tun Op : Ar remote_tun .
1898: .Sm on
1899: The devices may be specified by numerical ID or the keyword
1.240 jmc 1900: .Cm any ,
1.95 stevesk 1901: which uses the next available tunnel device.
1902: If
1903: .Ar remote_tun
1904: is not specified, it defaults to
1.240 jmc 1905: .Cm any .
1.95 stevesk 1906: The default is
1.240 jmc 1907: .Cm any:any .
1.201 djm 1908: .It Cm UpdateHostKeys
1.200 djm 1909: Specifies whether
1910: .Xr ssh 1
1911: should accept notifications of additional hostkeys from the server sent
1912: after authentication has completed and add them to
1913: .Cm UserKnownHostsFile .
1914: The argument must be
1.240 jmc 1915: .Cm yes ,
1916: .Cm no
1.320 djm 1917: or
1.240 jmc 1918: .Cm ask .
1.320 djm 1919: This option allows learning alternate hostkeys for a server
1.201 djm 1920: and supports graceful key rotation by allowing a server to send replacement
1921: public keys before old ones are removed.
1.336 djm 1922: .Pp
1.200 djm 1923: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1924: host was already trusted or explicitly accepted by the user, the host was
1925: authenticated via
1926: .Cm UserKnownHostsFile
1927: (i.e. not
1928: .Cm GlobalKnownHostsFile )
1929: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1930: .Pp
1931: .Cm UpdateHostKeys
1.321 jmc 1932: is enabled by default if the user has not overridden the default
1.320 djm 1933: .Cm UserKnownHostsFile
1.335 djm 1934: setting and has not enabled
1935: .Cm VerifyHostKeyDNS ,
1936: otherwise
1.320 djm 1937: .Cm UpdateHostKeys
1938: will be set to
1.334 djm 1939: .Cm no .
1.320 djm 1940: .Pp
1.204 djm 1941: If
1942: .Cm UpdateHostKeys
1943: is set to
1.240 jmc 1944: .Cm ask ,
1.204 djm 1945: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1946: Confirmation is currently incompatible with
1947: .Cm ControlPersist ,
1948: and will be disabled if it is enabled.
1.200 djm 1949: .Pp
1950: Presently, only
1951: .Xr sshd 8
1952: from OpenSSH 6.8 and greater support the
1.240 jmc 1953: .Qq hostkeys@openssh.com
1.200 djm 1954: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1955: .It Cm User
1956: Specifies the user to log in as.
1957: This can be useful when a different user name is used on different machines.
1958: This saves the trouble of
1959: having to remember to give the user name on the command line.
1960: .It Cm UserKnownHostsFile
1.151 djm 1961: Specifies one or more files to use for the user
1962: host key database, separated by whitespace.
1.329 dtucker 1963: Each filename may use tilde notation to refer to the user's home directory,
1964: the tokens described in the
1965: .Sx TOKENS
1966: section and environment variables as described in the
1967: .Sx ENVIRONMENT VARIABLES
1968: section.
1.377 dtucker 1969: A value of
1970: .Cm none
1971: causes
1972: .Xr ssh 1
1.378 ! jmc 1973: to ignore any user-specific known hosts files.
1.151 djm 1974: The default is
1975: .Pa ~/.ssh/known_hosts ,
1976: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1977: .It Cm VerifyHostKeyDNS
1978: Specifies whether to verify the remote key using DNS and SSHFP resource
1979: records.
1.24 jakob 1980: If this option is set to
1.240 jmc 1981: .Cm yes ,
1.25 jmc 1982: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1983: from DNS.
1984: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 1985: .Cm ask .
1.24 jakob 1986: If this option is set to
1.240 jmc 1987: .Cm ask ,
1.24 jakob 1988: information on fingerprint match will be displayed, but the user will still
1989: need to confirm new host keys according to the
1990: .Cm StrictHostKeyChecking
1991: option.
1.8 jakob 1992: The default is
1.240 jmc 1993: .Cm no .
1.84 jmc 1994: .Pp
1.240 jmc 1995: See also
1996: .Sx VERIFYING HOST KEYS
1997: in
1.84 jmc 1998: .Xr ssh 1 .
1.111 grunk 1999: .It Cm VisualHostKey
2000: If this flag is set to
1.240 jmc 2001: .Cm yes ,
1.111 grunk 2002: an ASCII art representation of the remote host key fingerprint is
1.197 djm 2003: printed in addition to the fingerprint string at login and
1.114 stevesk 2004: for unknown host keys.
1.111 grunk 2005: If this flag is set to
1.240 jmc 2006: .Cm no
2007: (the default),
1.114 stevesk 2008: no fingerprint strings are printed at login and
1.197 djm 2009: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 2010: .It Cm XAuthLocation
1.5 stevesk 2011: Specifies the full pathname of the
1.1 stevesk 2012: .Xr xauth 1
2013: program.
2014: The default is
2015: .Pa /usr/X11R6/bin/xauth .
2016: .El
1.86 jmc 2017: .Sh PATTERNS
2018: A
2019: .Em pattern
2020: consists of zero or more non-whitespace characters,
2021: .Sq *
2022: (a wildcard that matches zero or more characters),
2023: or
2024: .Sq ?\&
2025: (a wildcard that matches exactly one character).
2026: For example, to specify a set of declarations for any host in the
1.240 jmc 2027: .Qq .co.uk
1.86 jmc 2028: set of domains,
2029: the following pattern could be used:
2030: .Pp
2031: .Dl Host *.co.uk
2032: .Pp
2033: The following pattern
2034: would match any host in the 192.168.0.[0-9] network range:
2035: .Pp
2036: .Dl Host 192.168.0.?
2037: .Pp
2038: A
2039: .Em pattern-list
2040: is a comma-separated list of patterns.
2041: Patterns within pattern-lists may be negated
2042: by preceding them with an exclamation mark
2043: .Pq Sq !\& .
2044: For example,
1.174 djm 2045: to allow a key to be used from anywhere within an organization
1.86 jmc 2046: except from the
1.240 jmc 2047: .Qq dialup
1.86 jmc 2048: pool,
2049: the following entry (in authorized_keys) could be used:
2050: .Pp
2051: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 2052: .Pp
2053: Note that a negated match will never produce a positive result by itself.
2054: For example, attempting to match
2055: .Qq host3
2056: against the following pattern-list will fail:
2057: .Pp
2058: .Dl from=\&"!host1,!host2\&"
2059: .Pp
2060: The solution here is to include a term that will yield a positive match,
2061: such as a wildcard:
2062: .Pp
2063: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 2064: .Sh TOKENS
2065: Arguments to some keywords can make use of tokens,
2066: which are expanded at runtime:
2067: .Pp
2068: .Bl -tag -width XXXX -offset indent -compact
2069: .It %%
2070: A literal
2071: .Sq % .
2072: .It \&%C
1.257 jmc 2073: Hash of %l%h%p%r.
1.239 jmc 2074: .It %d
2075: Local user's home directory.
1.339 djm 2076: .It %f
2077: The fingerprint of the server's host key.
2078: .It %H
2079: The
2080: .Pa known_hosts
2081: hostname or address that is being searched for.
1.239 jmc 2082: .It %h
2083: The remote hostname.
1.340 jmc 2084: .It \%%I
1.339 djm 2085: A string describing the reason for a
2086: .Cm KnownHostsCommand
1.340 jmc 2087: execution: either
2088: .Cm ADDRESS
1.339 djm 2089: when looking up a host by address (only when
2090: .Cm CheckHostIP
2091: is enabled),
1.340 jmc 2092: .Cm HOSTNAME
2093: when searching by hostname, or
2094: .Cm ORDER
1.339 djm 2095: when preparing the host key algorithm preference list to use for the
2096: destination host.
1.239 jmc 2097: .It %i
2098: The local user ID.
1.339 djm 2099: .It %K
2100: The base64 encoded host key.
1.330 dtucker 2101: .It %k
1.350 jsg 2102: The host key alias if specified, otherwise the original remote hostname given
1.330 dtucker 2103: on the command line.
1.239 jmc 2104: .It %L
2105: The local hostname.
2106: .It %l
2107: The local hostname, including the domain name.
2108: .It %n
2109: The original remote hostname, as given on the command line.
2110: .It %p
2111: The remote port.
2112: .It %r
2113: The remote username.
1.261 djm 2114: .It \&%T
2115: The local
2116: .Xr tun 4
2117: or
2118: .Xr tap 4
2119: network interface assigned if
1.262 jmc 2120: tunnel forwarding was requested, or
2121: .Qq NONE
1.261 djm 2122: otherwise.
1.339 djm 2123: .It %t
2124: The type of the server host key, e.g.
1.360 jmc 2125: .Cm ssh-ed25519 .
1.239 jmc 2126: .It %u
2127: The local username.
2128: .El
2129: .Pp
1.323 dtucker 2130: .Cm CertificateFile ,
2131: .Cm ControlPath ,
2132: .Cm IdentityAgent ,
2133: .Cm IdentityFile ,
1.339 djm 2134: .Cm KnownHostsCommand ,
1.325 jmc 2135: .Cm LocalForward ,
1.324 dtucker 2136: .Cm Match exec ,
2137: .Cm RemoteCommand ,
1.329 dtucker 2138: .Cm RemoteForward ,
1.323 dtucker 2139: and
1.331 jmc 2140: .Cm UserKnownHostsFile
1.349 dtucker 2141: accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2142: .Pp
1.339 djm 2143: .Cm KnownHostsCommand
2144: additionally accepts the tokens %f, %H, %I, %K and %t.
2145: .Pp
1.295 jmc 2146: .Cm Hostname
1.239 jmc 2147: accepts the tokens %% and %h.
2148: .Pp
2149: .Cm LocalCommand
1.323 dtucker 2150: accepts all tokens.
1.239 jmc 2151: .Pp
2152: .Cm ProxyCommand
1.372 dtucker 2153: and
2154: .Cm ProxyJump
2155: accept the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2156: .Sh ENVIRONMENT VARIABLES
2157: Arguments to some keywords can be expanded at runtime from environment
2158: variables on the client by enclosing them in
2159: .Ic ${} ,
2160: for example
2161: .Ic ${HOME}/.ssh
2162: would refer to the user's .ssh directory.
2163: If a specified environment variable does not exist then an error will be
2164: returned and the setting for that keyword will be ignored.
2165: .Pp
2166: The keywords
2167: .Cm CertificateFile ,
2168: .Cm ControlPath ,
1.329 dtucker 2169: .Cm IdentityAgent ,
1.352 jmc 2170: .Cm IdentityFile ,
1.339 djm 2171: .Cm KnownHostsCommand ,
1.326 dtucker 2172: and
1.329 dtucker 2173: .Cm UserKnownHostsFile
1.326 dtucker 2174: support environment variables.
2175: The keywords
2176: .Cm LocalForward
2177: and
2178: .Cm RemoteForward
2179: support environment variables only for Unix domain socket paths.
1.1 stevesk 2180: .Sh FILES
2181: .Bl -tag -width Ds
1.50 djm 2182: .It Pa ~/.ssh/config
1.1 stevesk 2183: This is the per-user configuration file.
2184: The format of this file is described above.
1.84 jmc 2185: This file is used by the SSH client.
1.30 djm 2186: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2187: read/write for the user, and not writable by others.
1.1 stevesk 2188: .It Pa /etc/ssh/ssh_config
2189: Systemwide configuration file.
2190: This file provides defaults for those
2191: values that are not specified in the user's configuration file, and
2192: for those users who do not have a configuration file.
2193: This file must be world-readable.
2194: .El
1.13 jmc 2195: .Sh SEE ALSO
2196: .Xr ssh 1
1.1 stevesk 2197: .Sh AUTHORS
1.240 jmc 2198: .An -nosplit
1.1 stevesk 2199: OpenSSH is a derivative of the original and free
1.240 jmc 2200: ssh 1.2.12 release by
2201: .An Tatu Ylonen .
2202: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2203: .An Niels Provos , Theo de Raadt
2204: and
2205: .An Dug Song
1.1 stevesk 2206: removed many bugs, re-added newer features and
2207: created OpenSSH.
1.240 jmc 2208: .An Markus Friedl
2209: contributed the support for SSH protocol versions 1.5 and 2.0.
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