Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.375
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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1.375 ! dtucker 36: .\" $OpenBSD: ssh_config.5,v 1.374 2022/09/17 10:33:18 djm Exp $
! 37: .Dd $Mdocdate: September 17 2022 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
1.310 jmc 42: .Nd OpenSSH client configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.84 jmc 44: .Xr ssh 1
1.1 stevesk 45: obtains configuration data from the following sources in
46: the following order:
1.79 jmc 47: .Pp
1.2 stevesk 48: .Bl -enum -offset indent -compact
49: .It
50: command-line options
51: .It
52: user's configuration file
1.50 djm 53: .Pq Pa ~/.ssh/config
1.2 stevesk 54: .It
55: system-wide configuration file
56: .Pq Pa /etc/ssh/ssh_config
57: .El
1.1 stevesk 58: .Pp
59: For each parameter, the first obtained value
60: will be used.
1.41 jmc 61: The configuration files contain sections separated by
1.240 jmc 62: .Cm Host
1.1 stevesk 63: specifications, and that section is only applied for hosts that
64: match one of the patterns given in the specification.
1.193 djm 65: The matched host name is usually the one given on the command line
66: (see the
67: .Cm CanonicalizeHostname
1.240 jmc 68: option for exceptions).
1.1 stevesk 69: .Pp
70: Since the first obtained value for each parameter is used, more
71: host-specific declarations should be given near the beginning of the
72: file, and general defaults at the end.
1.80 jmc 73: .Pp
1.240 jmc 74: The file contains keyword-argument pairs, one per line.
75: Lines starting with
1.1 stevesk 76: .Ql #
1.240 jmc 77: and empty lines are interpreted as comments.
78: Arguments may optionally be enclosed in double quotes
79: .Pq \&"
80: in order to represent arguments containing spaces.
1.1 stevesk 81: Configuration options may be separated by whitespace or
82: optional whitespace and exactly one
83: .Ql = ;
84: the latter format is useful to avoid the need to quote whitespace
85: when specifying configuration options using the
86: .Nm ssh ,
1.87 jmc 87: .Nm scp ,
1.1 stevesk 88: and
89: .Nm sftp
90: .Fl o
91: option.
92: .Pp
93: The possible
94: keywords and their meanings are as follows (note that
95: keywords are case-insensitive and arguments are case-sensitive):
96: .Bl -tag -width Ds
97: .It Cm Host
98: Restricts the following declarations (up to the next
99: .Cm Host
1.169 djm 100: or
101: .Cm Match
1.1 stevesk 102: keyword) to be only for those hosts that match one of the patterns
103: given after the keyword.
1.112 krw 104: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 105: A single
1.83 jmc 106: .Ql *
1.1 stevesk 107: as a pattern can be used to provide global
108: defaults for all hosts.
1.193 djm 109: The host is usually the
1.1 stevesk 110: .Ar hostname
1.193 djm 111: argument given on the command line
112: (see the
113: .Cm CanonicalizeHostname
1.240 jmc 114: keyword for exceptions).
1.148 djm 115: .Pp
116: A pattern entry may be negated by prefixing it with an exclamation mark
117: .Pq Sq !\& .
118: If a negated entry is matched, then the
119: .Cm Host
120: entry is ignored, regardless of whether any other patterns on the line
121: match.
122: Negated matches are therefore useful to provide exceptions for wildcard
123: matches.
1.81 jmc 124: .Pp
125: See
126: .Sx PATTERNS
127: for more information on patterns.
1.170 jmc 128: .It Cm Match
1.169 djm 129: Restricts the following declarations (up to the next
130: .Cm Host
131: or
132: .Cm Match
133: keyword) to be used only when the conditions following the
134: .Cm Match
135: keyword are satisfied.
1.220 sobrado 136: Match conditions are specified using one or more criteria
1.178 dtucker 137: or the single token
138: .Cm all
1.193 djm 139: which always matches.
140: The available criteria keywords are:
141: .Cm canonical ,
1.287 djm 142: .Cm final ,
1.176 djm 143: .Cm exec ,
1.169 djm 144: .Cm host ,
145: .Cm originalhost ,
146: .Cm user ,
147: and
148: .Cm localuser .
1.193 djm 149: The
150: .Cm all
151: criteria must appear alone or immediately after
1.287 djm 152: .Cm canonical
153: or
154: .Cm final .
1.193 djm 155: Other criteria may be combined arbitrarily.
156: All criteria but
1.288 jmc 157: .Cm all ,
158: .Cm canonical ,
1.193 djm 159: and
1.287 djm 160: .Cm final
1.193 djm 161: require an argument.
162: Criteria may be negated by prepending an exclamation mark
163: .Pq Sq !\& .
1.169 djm 164: .Pp
1.177 jmc 165: The
1.193 djm 166: .Cm canonical
1.210 dtucker 167: keyword matches only when the configuration file is being re-parsed
1.193 djm 168: after hostname canonicalization (see the
169: .Cm CanonicalizeHostname
1.288 jmc 170: option).
1.193 djm 171: This may be useful to specify conditions that work with canonical host
172: names only.
1.287 djm 173: .Pp
174: The
175: .Cm final
176: keyword requests that the configuration be re-parsed (regardless of whether
177: .Cm CanonicalizeHostname
178: is enabled), and matches only during this final pass.
179: If
180: .Cm CanonicalizeHostname
181: is enabled, then
182: .Cm canonical
183: and
184: .Cm final
185: match during the same pass.
186: .Pp
1.193 djm 187: The
1.176 djm 188: .Cm exec
1.177 jmc 189: keyword executes the specified command under the user's shell.
1.169 djm 190: If the command returns a zero exit status then the condition is considered true.
191: Commands containing whitespace characters must be quoted.
1.239 jmc 192: Arguments to
193: .Cm exec
194: accept the tokens described in the
195: .Sx TOKENS
196: section.
1.169 djm 197: .Pp
198: The other keywords' criteria must be single entries or comma-separated
199: lists and may use the wildcard and negation operators described in the
200: .Sx PATTERNS
201: section.
202: The criteria for the
203: .Cm host
204: keyword are matched against the target hostname, after any substitution
205: by the
1.295 jmc 206: .Cm Hostname
1.193 djm 207: or
208: .Cm CanonicalizeHostname
209: options.
1.169 djm 210: The
211: .Cm originalhost
212: keyword matches against the hostname as it was specified on the command-line.
213: The
214: .Cm user
215: keyword matches against the target username on the remote host.
216: The
217: .Cm localuser
218: keyword matches against the name of the local user running
219: .Xr ssh 1
220: (this keyword may be useful in system-wide
221: .Nm
222: files).
1.222 jcs 223: .It Cm AddKeysToAgent
224: Specifies whether keys should be automatically added to a running
1.223 jmc 225: .Xr ssh-agent 1 .
1.222 jcs 226: If this option is set to
1.240 jmc 227: .Cm yes
1.222 jcs 228: and a key is loaded from a file, the key and its passphrase are added to
229: the agent with the default lifetime, as if by
230: .Xr ssh-add 1 .
231: If this option is set to
1.240 jmc 232: .Cm ask ,
233: .Xr ssh 1
1.222 jcs 234: will require confirmation using the
235: .Ev SSH_ASKPASS
236: program before adding a key (see
237: .Xr ssh-add 1
238: for details).
239: If this option is set to
1.240 jmc 240: .Cm confirm ,
1.222 jcs 241: each use of the key must be confirmed, as if the
242: .Fl c
243: option was specified to
244: .Xr ssh-add 1 .
245: If this option is set to
1.240 jmc 246: .Cm no ,
1.222 jcs 247: no keys are added to the agent.
1.332 djm 248: Alternately, this option may be specified as a time interval
249: using the format described in the
250: .Sx TIME FORMATS
251: section of
252: .Xr sshd_config 5
253: to specify the key's lifetime in
254: .Xr ssh-agent 1 ,
255: after which it will automatically be removed.
1.222 jcs 256: The argument must be
1.332 djm 257: .Cm no
258: (the default),
1.240 jmc 259: .Cm yes ,
1.332 djm 260: .Cm confirm
261: (optionally followed by a time interval),
262: .Cm ask
263: or a time interval.
1.10 djm 264: .It Cm AddressFamily
1.11 jmc 265: Specifies which address family to use when connecting.
266: Valid arguments are
1.240 jmc 267: .Cm any
268: (the default),
269: .Cm inet
1.84 jmc 270: (use IPv4 only), or
1.240 jmc 271: .Cm inet6
1.40 jmc 272: (use IPv6 only).
1.1 stevesk 273: .It Cm BatchMode
274: If set to
1.240 jmc 275: .Cm yes ,
1.318 djm 276: user interaction such as password prompts and host key confirmation requests
277: will be disabled.
1.1 stevesk 278: This option is useful in scripts and other batch jobs where no user
1.318 djm 279: is present to interact with
280: .Xr ssh 1 .
1.1 stevesk 281: The argument must be
1.240 jmc 282: .Cm yes
1.1 stevesk 283: or
1.240 jmc 284: .Cm no
285: (the default).
1.268 jmc 286: .It Cm BindAddress
287: Use the specified address on the local machine as the source address of
288: the connection.
289: Only useful on systems with more than one address.
290: .It Cm BindInterface
291: Use the address of the specified interface on the local machine as the
292: source address of the connection.
1.171 djm 293: .It Cm CanonicalDomains
1.172 jmc 294: When
1.173 djm 295: .Cm CanonicalizeHostname
1.171 djm 296: is enabled, this option specifies the list of domain suffixes in which to
297: search for the specified destination host.
1.173 djm 298: .It Cm CanonicalizeFallbackLocal
1.174 djm 299: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 300: The default,
1.240 jmc 301: .Cm yes ,
1.172 jmc 302: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 303: search rules.
304: A value of
1.240 jmc 305: .Cm no
1.171 djm 306: will cause
307: .Xr ssh 1
308: to fail instantly if
1.173 djm 309: .Cm CanonicalizeHostname
1.171 djm 310: is enabled and the target hostname cannot be found in any of the domains
311: specified by
312: .Cm CanonicalDomains .
1.173 djm 313: .It Cm CanonicalizeHostname
1.174 djm 314: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 315: The default,
1.240 jmc 316: .Cm no ,
1.171 djm 317: is not to perform any name rewriting and let the system resolver handle all
318: hostname lookups.
319: If set to
1.240 jmc 320: .Cm yes
1.171 djm 321: then, for connections that do not use a
1.284 djm 322: .Cm ProxyCommand
323: or
324: .Cm ProxyJump ,
1.171 djm 325: .Xr ssh 1
1.173 djm 326: will attempt to canonicalize the hostname specified on the command line
1.171 djm 327: using the
328: .Cm CanonicalDomains
329: suffixes and
1.173 djm 330: .Cm CanonicalizePermittedCNAMEs
1.171 djm 331: rules.
332: If
1.173 djm 333: .Cm CanonicalizeHostname
1.171 djm 334: is set to
1.240 jmc 335: .Cm always ,
1.174 djm 336: then canonicalization is applied to proxied connections too.
1.185 djm 337: .Pp
1.193 djm 338: If this option is enabled, then the configuration files are processed
339: again using the new target name to pick up any new configuration in matching
1.185 djm 340: .Cm Host
1.193 djm 341: and
342: .Cm Match
1.185 djm 343: stanzas.
1.361 dtucker 344: A value of
345: .Cm none
346: disables the use of a
347: .Cm ProxyJump
348: host.
1.173 djm 349: .It Cm CanonicalizeMaxDots
1.172 jmc 350: Specifies the maximum number of dot characters in a hostname before
1.174 djm 351: canonicalization is disabled.
1.240 jmc 352: The default, 1,
1.172 jmc 353: allows a single dot (i.e. hostname.subdomain).
1.173 djm 354: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 355: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 356: canonicalizing hostnames.
1.171 djm 357: The rules consist of one or more arguments of
1.172 jmc 358: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 359: where
360: .Ar source_domain_list
1.174 djm 361: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 362: and
363: .Ar target_domain_list
1.172 jmc 364: is a pattern-list of domains that they may resolve to.
1.171 djm 365: .Pp
366: For example,
1.240 jmc 367: .Qq *.a.example.com:*.b.example.com,*.c.example.com
1.171 djm 368: will allow hostnames matching
1.240 jmc 369: .Qq *.a.example.com
1.173 djm 370: to be canonicalized to names in the
1.240 jmc 371: .Qq *.b.example.com
1.171 djm 372: or
1.240 jmc 373: .Qq *.c.example.com
1.171 djm 374: domains.
1.365 djm 375: .Pp
376: A single argument of
377: .Qq none
378: causes no CNAMEs to be considered for canonicalization.
379: This is the default behaviour.
1.283 jmc 380: .It Cm CASignatureAlgorithms
381: Specifies which algorithms are allowed for signing of certificates
382: by certificate authorities (CAs).
383: The default is:
384: .Bd -literal -offset indent
1.362 djm 385: ssh-ed25519,ecdsa-sha2-nistp256,
386: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
387: sk-ssh-ed25519@openssh.com,
388: sk-ecdsa-sha2-nistp256@openssh.com,
1.351 djm 389: rsa-sha2-512,rsa-sha2-256
1.283 jmc 390: .Ed
1.362 djm 391: .Pp
392: If the specified list begins with a
393: .Sq +
394: character, then the specified algorithms will be appended to the default set
395: instead of replacing them.
396: If the specified list begins with a
397: .Sq -
398: character, then the specified algorithms (including wildcards) will be removed
399: from the default set instead of replacing them.
1.283 jmc 400: .Pp
401: .Xr ssh 1
402: will not accept host certificates signed using algorithms other than those
403: specified.
1.221 djm 404: .It Cm CertificateFile
405: Specifies a file from which the user's certificate is read.
406: A corresponding private key must be provided separately in order
407: to use this certificate either
408: from an
409: .Cm IdentityFile
410: directive or
411: .Fl i
412: flag to
413: .Xr ssh 1 ,
414: via
415: .Xr ssh-agent 1 ,
416: or via a
1.305 naddy 417: .Cm PKCS11Provider
418: or
419: .Cm SecurityKeyProvider .
1.221 djm 420: .Pp
1.239 jmc 421: Arguments to
422: .Cm CertificateFile
1.326 dtucker 423: may use the tilde syntax to refer to a user's home directory,
424: the tokens described in the
1.239 jmc 425: .Sx TOKENS
1.326 dtucker 426: section and environment variables as described in the
427: .Sx ENVIRONMENT VARIABLES
1.239 jmc 428: section.
1.221 djm 429: .Pp
430: It is possible to have multiple certificate files specified in
431: configuration files; these certificates will be tried in sequence.
432: Multiple
433: .Cm CertificateFile
434: directives will add to the list of certificates used for
435: authentication.
1.1 stevesk 436: .It Cm CheckHostIP
1.240 jmc 437: If set to
1.370 naddy 438: .Cm yes ,
1.84 jmc 439: .Xr ssh 1
440: will additionally check the host IP address in the
1.1 stevesk 441: .Pa known_hosts
442: file.
1.240 jmc 443: This allows it to detect if a host key changed due to DNS spoofing
1.211 djm 444: and will add addresses of destination hosts to
445: .Pa ~/.ssh/known_hosts
446: in the process, regardless of the setting of
447: .Cm StrictHostKeyChecking .
1.107 grunk 448: If the option is set to
1.341 djm 449: .Cm no
450: (the default),
1.1 stevesk 451: the check will not be executed.
452: .It Cm Ciphers
1.245 djm 453: Specifies the ciphers allowed and their order of preference.
1.1 stevesk 454: Multiple ciphers must be comma-separated.
1.299 kn 455: If the specified list begins with a
1.214 djm 456: .Sq +
457: character, then the specified ciphers will be appended to the default set
458: instead of replacing them.
1.299 kn 459: If the specified list begins with a
1.241 djm 460: .Sq -
461: character, then the specified ciphers (including wildcards) will be removed
462: from the default set instead of replacing them.
1.301 naddy 463: If the specified list begins with a
464: .Sq ^
465: character, then the specified ciphers will be placed at the head of the
466: default set.
1.214 djm 467: .Pp
1.180 djm 468: The supported ciphers are:
1.240 jmc 469: .Bd -literal -offset indent
1.186 naddy 470: 3des-cbc
471: aes128-cbc
472: aes192-cbc
473: aes256-cbc
474: aes128-ctr
475: aes192-ctr
476: aes256-ctr
477: aes128-gcm@openssh.com
478: aes256-gcm@openssh.com
479: chacha20-poly1305@openssh.com
1.240 jmc 480: .Ed
1.180 djm 481: .Pp
1.84 jmc 482: The default is:
1.186 naddy 483: .Bd -literal -offset indent
1.215 jmc 484: chacha20-poly1305@openssh.com,
1.186 naddy 485: aes128-ctr,aes192-ctr,aes256-ctr,
1.270 djm 486: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 487: .Ed
1.180 djm 488: .Pp
1.240 jmc 489: The list of available ciphers may also be obtained using
490: .Qq ssh -Q cipher .
1.1 stevesk 491: .It Cm ClearAllForwardings
1.84 jmc 492: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 493: specified in the configuration files or on the command line be
1.7 jmc 494: cleared.
495: This option is primarily useful when used from the
1.84 jmc 496: .Xr ssh 1
1.1 stevesk 497: command line to clear port forwardings set in
498: configuration files, and is automatically set by
499: .Xr scp 1
500: and
501: .Xr sftp 1 .
502: The argument must be
1.240 jmc 503: .Cm yes
1.1 stevesk 504: or
1.240 jmc 505: .Cm no
506: (the default).
1.1 stevesk 507: .It Cm Compression
508: Specifies whether to use compression.
509: The argument must be
1.240 jmc 510: .Cm yes
1.1 stevesk 511: or
1.240 jmc 512: .Cm no
513: (the default).
1.247 naddy 514: .It Cm ConnectionAttempts
515: Specifies the number of tries (one per second) to make before exiting.
516: The argument must be an integer.
517: This may be useful in scripts if the connection sometimes fails.
518: The default is 1.
1.9 djm 519: .It Cm ConnectTimeout
1.84 jmc 520: Specifies the timeout (in seconds) used when connecting to the
521: SSH server, instead of using the default system TCP timeout.
1.302 djm 522: This timeout is applied both to establishing the connection and to performing
523: the initial SSH protocol handshake and key exchange.
1.36 djm 524: .It Cm ControlMaster
525: Enables the sharing of multiple sessions over a single network connection.
526: When set to
1.240 jmc 527: .Cm yes ,
1.84 jmc 528: .Xr ssh 1
1.36 djm 529: will listen for connections on a control socket specified using the
530: .Cm ControlPath
531: argument.
532: Additional sessions can connect to this socket using the same
533: .Cm ControlPath
534: with
535: .Cm ControlMaster
536: set to
1.240 jmc 537: .Cm no
1.38 jmc 538: (the default).
1.64 jmc 539: These sessions will try to reuse the master instance's network connection
1.63 djm 540: rather than initiating new ones, but will fall back to connecting normally
541: if the control socket does not exist, or is not listening.
542: .Pp
1.37 djm 543: Setting this to
1.240 jmc 544: .Cm ask
545: will cause
546: .Xr ssh 1
1.206 jmc 547: to listen for control connections, but require confirmation using
548: .Xr ssh-askpass 1 .
1.51 jakob 549: If the
550: .Cm ControlPath
1.84 jmc 551: cannot be opened,
1.240 jmc 552: .Xr ssh 1
553: will continue without connecting to a master instance.
1.58 djm 554: .Pp
555: X11 and
1.59 jmc 556: .Xr ssh-agent 1
1.58 djm 557: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 558: display and agent forwarded will be the one belonging to the master
1.59 jmc 559: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 560: .Pp
561: Two additional options allow for opportunistic multiplexing: try to use a
562: master connection but fall back to creating a new one if one does not already
563: exist.
564: These options are:
1.240 jmc 565: .Cm auto
1.56 djm 566: and
1.240 jmc 567: .Cm autoask .
1.56 djm 568: The latter requires confirmation like the
1.240 jmc 569: .Cm ask
1.56 djm 570: option.
1.36 djm 571: .It Cm ControlPath
1.55 djm 572: Specify the path to the control socket used for connection sharing as described
573: in the
1.36 djm 574: .Cm ControlMaster
1.57 djm 575: section above or the string
1.240 jmc 576: .Cm none
1.57 djm 577: to disable connection sharing.
1.239 jmc 578: Arguments to
579: .Cm ControlPath
1.326 dtucker 580: may use the tilde syntax to refer to a user's home directory,
581: the tokens described in the
1.239 jmc 582: .Sx TOKENS
1.326 dtucker 583: section and environment variables as described in the
584: .Sx ENVIRONMENT VARIABLES
1.239 jmc 585: section.
1.56 djm 586: It is recommended that any
587: .Cm ControlPath
588: used for opportunistic connection sharing include
1.195 djm 589: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
590: that is not writable by other users.
1.56 djm 591: This ensures that shared connections are uniquely identified.
1.137 djm 592: .It Cm ControlPersist
593: When used in conjunction with
594: .Cm ControlMaster ,
595: specifies that the master connection should remain open
596: in the background (waiting for future client connections)
597: after the initial client connection has been closed.
598: If set to
1.314 naddy 599: .Cm no
600: (the default),
1.137 djm 601: then the master connection will not be placed into the background,
602: and will close as soon as the initial client connection is closed.
603: If set to
1.240 jmc 604: .Cm yes
605: or 0,
1.137 djm 606: then the master connection will remain in the background indefinitely
607: (until killed or closed via a mechanism such as the
1.240 jmc 608: .Qq ssh -O exit ) .
1.137 djm 609: If set to a time in seconds, or a time in any of the formats documented in
610: .Xr sshd_config 5 ,
611: then the backgrounded master connection will automatically terminate
612: after it has remained idle (with no client connections) for the
613: specified time.
1.38 jmc 614: .It Cm DynamicForward
1.74 jmc 615: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 616: over the secure channel, and the application
617: protocol is then used to determine where to connect to from the
618: remote machine.
1.62 djm 619: .Pp
620: The argument must be
621: .Sm off
622: .Oo Ar bind_address : Oc Ar port .
623: .Sm on
1.138 djm 624: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 625: By default, the local port is bound in accordance with the
626: .Cm GatewayPorts
627: setting.
628: However, an explicit
629: .Ar bind_address
630: may be used to bind the connection to a specific address.
631: The
632: .Ar bind_address
633: of
1.240 jmc 634: .Cm localhost
1.62 djm 635: indicates that the listening port be bound for local use only, while an
636: empty address or
637: .Sq *
638: indicates that the port should be available from all interfaces.
639: .Pp
1.38 jmc 640: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 641: .Xr ssh 1
1.38 jmc 642: will act as a SOCKS server.
643: Multiple forwardings may be specified, and
644: additional forwardings can be given on the command line.
645: Only the superuser can forward privileged ports.
1.14 markus 646: .It Cm EnableSSHKeysign
647: Setting this option to
1.240 jmc 648: .Cm yes
1.14 markus 649: in the global client configuration file
650: .Pa /etc/ssh/ssh_config
651: enables the use of the helper program
652: .Xr ssh-keysign 8
653: during
654: .Cm HostbasedAuthentication .
655: The argument must be
1.240 jmc 656: .Cm yes
1.14 markus 657: or
1.240 jmc 658: .Cm no
659: (the default).
1.23 jmc 660: This option should be placed in the non-hostspecific section.
1.14 markus 661: See
662: .Xr ssh-keysign 8
663: for more information.
1.1 stevesk 664: .It Cm EscapeChar
665: Sets the escape character (default:
666: .Ql ~ ) .
667: The escape character can also
668: be set on the command line.
669: The argument should be a single character,
670: .Ql ^
671: followed by a letter, or
1.240 jmc 672: .Cm none
1.1 stevesk 673: to disable the escape
674: character entirely (making the connection transparent for binary
675: data).
1.96 markus 676: .It Cm ExitOnForwardFailure
677: Specifies whether
678: .Xr ssh 1
679: should terminate the connection if it cannot set up all requested
1.216 djm 680: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 681: if either end is unable to bind and listen on a specified port).
1.216 djm 682: Note that
683: .Cm ExitOnForwardFailure
684: does not apply to connections made over port forwardings and will not,
685: for example, cause
686: .Xr ssh 1
687: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 688: The argument must be
1.240 jmc 689: .Cm yes
1.96 markus 690: or
1.240 jmc 691: .Cm no
692: (the default).
1.197 djm 693: .It Cm FingerprintHash
694: Specifies the hash algorithm used when displaying key fingerprints.
695: Valid options are:
1.240 jmc 696: .Cm md5
1.197 djm 697: and
1.240 jmc 698: .Cm sha256
1.359 djm 699: (the default).
700: .It Cm ForkAfterAuthentication
701: Requests
702: .Nm ssh
703: to go to background just before command execution.
704: This is useful if
705: .Nm ssh
706: is going to ask for passwords or passphrases, but the user
707: wants it in the background.
708: This implies the
709: .Cm StdinNull
710: configuration option being set to
711: .Dq yes .
712: The recommended way to start X11 programs at a remote site is with
713: something like
714: .Ic ssh -f host xterm ,
715: which is the same as
716: .Ic ssh host xterm
717: if the
718: .Cm ForkAfterAuthentication
719: configuration option is set to
720: .Dq yes .
721: .Pp
722: If the
723: .Cm ExitOnForwardFailure
724: configuration option is set to
725: .Dq yes ,
726: then a client started with the
727: .Cm ForkAfterAuthentication
728: configuration option being set to
729: .Dq yes
730: will wait for all remote port forwards to be successfully established
731: before placing itself in the background.
732: The argument to this keyword must be
733: .Cm yes
734: (same as the
735: .Fl f
736: option) or
737: .Cm no
1.240 jmc 738: (the default).
1.1 stevesk 739: .It Cm ForwardAgent
740: Specifies whether the connection to the authentication agent (if any)
741: will be forwarded to the remote machine.
1.312 djm 742: The argument may be
743: .Cm yes ,
1.240 jmc 744: .Cm no
1.312 djm 745: (the default),
746: an explicit path to an agent socket or the name of an environment variable
747: (beginning with
748: .Sq $ )
749: in which to find the path.
1.3 stevesk 750: .Pp
1.7 jmc 751: Agent forwarding should be enabled with caution.
752: Users with the ability to bypass file permissions on the remote host
753: (for the agent's Unix-domain socket)
754: can access the local agent through the forwarded connection.
755: An attacker cannot obtain key material from the agent,
1.3 stevesk 756: however they can perform operations on the keys that enable them to
757: authenticate using the identities loaded into the agent.
1.1 stevesk 758: .It Cm ForwardX11
759: Specifies whether X11 connections will be automatically redirected
760: over the secure channel and
761: .Ev DISPLAY
762: set.
763: The argument must be
1.240 jmc 764: .Cm yes
1.1 stevesk 765: or
1.240 jmc 766: .Cm no
767: (the default).
1.3 stevesk 768: .Pp
1.7 jmc 769: X11 forwarding should be enabled with caution.
770: Users with the ability to bypass file permissions on the remote host
1.22 markus 771: (for the user's X11 authorization database)
1.7 jmc 772: can access the local X11 display through the forwarded connection.
1.22 markus 773: An attacker may then be able to perform activities such as keystroke monitoring
774: if the
775: .Cm ForwardX11Trusted
776: option is also enabled.
1.134 djm 777: .It Cm ForwardX11Timeout
1.135 jmc 778: Specify a timeout for untrusted X11 forwarding
779: using the format described in the
1.240 jmc 780: .Sx TIME FORMATS
781: section of
1.134 djm 782: .Xr sshd_config 5 .
783: X11 connections received by
784: .Xr ssh 1
785: after this time will be refused.
1.285 djm 786: Setting
787: .Cm ForwardX11Timeout
788: to zero will disable the timeout and permit X11 forwarding for the life
789: of the connection.
1.134 djm 790: The default is to disable untrusted X11 forwarding after twenty minutes has
791: elapsed.
1.22 markus 792: .It Cm ForwardX11Trusted
1.34 jmc 793: If this option is set to
1.240 jmc 794: .Cm yes ,
1.84 jmc 795: remote X11 clients will have full access to the original X11 display.
1.42 djm 796: .Pp
1.22 markus 797: If this option is set to
1.240 jmc 798: .Cm no
799: (the default),
1.84 jmc 800: remote X11 clients will be considered untrusted and prevented
1.22 markus 801: from stealing or tampering with data belonging to trusted X11
802: clients.
1.42 djm 803: Furthermore, the
804: .Xr xauth 1
805: token used for the session will be set to expire after 20 minutes.
806: Remote clients will be refused access after this time.
1.22 markus 807: .Pp
808: See the X11 SECURITY extension specification for full details on
809: the restrictions imposed on untrusted clients.
1.1 stevesk 810: .It Cm GatewayPorts
811: Specifies whether remote hosts are allowed to connect to local
812: forwarded ports.
813: By default,
1.84 jmc 814: .Xr ssh 1
1.7 jmc 815: binds local port forwardings to the loopback address.
816: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 817: .Cm GatewayPorts
1.84 jmc 818: can be used to specify that ssh
1.1 stevesk 819: should bind local port forwardings to the wildcard address,
820: thus allowing remote hosts to connect to forwarded ports.
821: The argument must be
1.240 jmc 822: .Cm yes
1.1 stevesk 823: or
1.240 jmc 824: .Cm no
825: (the default).
1.1 stevesk 826: .It Cm GlobalKnownHostsFile
1.151 djm 827: Specifies one or more files to use for the global
828: host key database, separated by whitespace.
829: The default is
830: .Pa /etc/ssh/ssh_known_hosts ,
831: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 832: .It Cm GSSAPIAuthentication
1.27 markus 833: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 834: The default is
1.240 jmc 835: .Cm no .
1.18 markus 836: .It Cm GSSAPIDelegateCredentials
837: Forward (delegate) credentials to the server.
838: The default is
1.240 jmc 839: .Cm no .
1.44 djm 840: .It Cm HashKnownHosts
841: Indicates that
1.84 jmc 842: .Xr ssh 1
1.44 djm 843: should hash host names and addresses when they are added to
1.50 djm 844: .Pa ~/.ssh/known_hosts .
1.44 djm 845: These hashed names may be used normally by
1.84 jmc 846: .Xr ssh 1
1.44 djm 847: and
1.84 jmc 848: .Xr sshd 8 ,
1.316 djm 849: but they do not visually reveal identifying information if the
850: file's contents are disclosed.
1.44 djm 851: The default is
1.240 jmc 852: .Cm no .
1.97 jmc 853: Note that existing names and addresses in known hosts files
854: will not be converted automatically,
855: but may be manually hashed using
1.45 djm 856: .Xr ssh-keygen 1 .
1.344 dtucker 857: .It Cm HostbasedAcceptedAlgorithms
1.348 djm 858: Specifies the signature algorithms that will be used for hostbased
859: authentication as a comma-separated list of patterns.
1.300 naddy 860: Alternately if the specified list begins with a
1.214 djm 861: .Sq +
1.348 djm 862: character, then the specified signature algorithms will be appended
863: to the default set instead of replacing them.
1.300 naddy 864: If the specified list begins with a
1.241 djm 865: .Sq -
1.348 djm 866: character, then the specified signature algorithms (including wildcards)
867: will be removed from the default set instead of replacing them.
1.301 naddy 868: If the specified list begins with a
869: .Sq ^
1.348 djm 870: character, then the specified signature algorithms will be placed
871: at the head of the default set.
1.213 markus 872: The default for this option is:
873: .Bd -literal -offset 3n
1.333 djm 874: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 875: ecdsa-sha2-nistp256-cert-v01@openssh.com,
876: ecdsa-sha2-nistp384-cert-v01@openssh.com,
877: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 878: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 879: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 880: rsa-sha2-512-cert-v01@openssh.com,
881: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 882: ssh-ed25519,
1.213 markus 883: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 884: sk-ssh-ed25519@openssh.com,
1.311 naddy 885: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 886: rsa-sha2-512,rsa-sha2-256
1.213 markus 887: .Ed
888: .Pp
1.202 djm 889: The
890: .Fl Q
891: option of
892: .Xr ssh 1
1.348 djm 893: may be used to list supported signature algorithms.
1.344 dtucker 894: This was formerly named HostbasedKeyTypes.
1.345 naddy 895: .It Cm HostbasedAuthentication
896: Specifies whether to try rhosts based authentication with public key
897: authentication.
898: The argument must be
899: .Cm yes
900: or
901: .Cm no
902: (the default).
1.1 stevesk 903: .It Cm HostKeyAlgorithms
1.348 djm 904: Specifies the host key signature algorithms
1.1 stevesk 905: that the client wants to use in order of preference.
1.300 naddy 906: Alternately if the specified list begins with a
1.214 djm 907: .Sq +
1.348 djm 908: character, then the specified signature algorithms will be appended to
909: the default set instead of replacing them.
1.300 naddy 910: If the specified list begins with a
1.241 djm 911: .Sq -
1.348 djm 912: character, then the specified signature algorithms (including wildcards)
913: will be removed from the default set instead of replacing them.
1.301 naddy 914: If the specified list begins with a
915: .Sq ^
1.348 djm 916: character, then the specified signature algorithms will be placed
917: at the head of the default set.
1.1 stevesk 918: The default for this option is:
1.139 djm 919: .Bd -literal -offset 3n
1.333 djm 920: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 921: ecdsa-sha2-nistp256-cert-v01@openssh.com,
922: ecdsa-sha2-nistp384-cert-v01@openssh.com,
923: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 924: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 925: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 926: rsa-sha2-512-cert-v01@openssh.com,
927: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 928: ssh-ed25519,
1.139 djm 929: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 930: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 931: sk-ssh-ed25519@openssh.com,
1.366 kn 932: rsa-sha2-512,rsa-sha2-256
1.139 djm 933: .Ed
1.145 djm 934: .Pp
935: If hostkeys are known for the destination host then this default is modified
936: to prefer their algorithms.
1.198 djm 937: .Pp
1.348 djm 938: The list of available signature algorithms may also be obtained using
1.322 dtucker 939: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 940: .It Cm HostKeyAlias
941: Specifies an alias that should be used instead of the
942: real host name when looking up or saving the host key
1.251 djm 943: in the host key database files and when validating host certificates.
1.84 jmc 944: This option is useful for tunneling SSH connections
1.1 stevesk 945: or for multiple servers running on a single host.
1.295 jmc 946: .It Cm Hostname
1.1 stevesk 947: Specifies the real host name to log into.
948: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 949: Arguments to
1.295 jmc 950: .Cm Hostname
1.239 jmc 951: accept the tokens described in the
952: .Sx TOKENS
953: section.
1.1 stevesk 954: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 955: .Cm Hostname
1.1 stevesk 956: specifications).
1.239 jmc 957: The default is the name given on the command line.
1.29 markus 958: .It Cm IdentitiesOnly
959: Specifies that
1.84 jmc 960: .Xr ssh 1
1.304 djm 961: should only use the configured authentication identity and certificate files
962: (either the default files, or those explicitly configured in the
1.31 jmc 963: .Nm
1.221 djm 964: files
965: or passed on the
966: .Xr ssh 1
1.304 djm 967: command-line),
1.84 jmc 968: even if
969: .Xr ssh-agent 1
1.159 djm 970: or a
971: .Cm PKCS11Provider
1.305 naddy 972: or
973: .Cm SecurityKeyProvider
1.29 markus 974: offers more identities.
975: The argument to this keyword must be
1.240 jmc 976: .Cm yes
1.29 markus 977: or
1.240 jmc 978: .Cm no
979: (the default).
1.84 jmc 980: This option is intended for situations where ssh-agent
1.29 markus 981: offers many different identities.
1.231 markus 982: .It Cm IdentityAgent
983: Specifies the
984: .Ux Ns -domain
985: socket used to communicate with the authentication agent.
986: .Pp
987: This option overrides the
1.240 jmc 988: .Ev SSH_AUTH_SOCK
1.231 markus 989: environment variable and can be used to select a specific agent.
990: Setting the socket name to
1.240 jmc 991: .Cm none
1.231 markus 992: disables the use of an authentication agent.
1.232 markus 993: If the string
1.240 jmc 994: .Qq SSH_AUTH_SOCK
1.232 markus 995: is specified, the location of the socket will be read from the
996: .Ev SSH_AUTH_SOCK
997: environment variable.
1.286 djm 998: Otherwise if the specified value begins with a
999: .Sq $
1000: character, then it will be treated as an environment variable containing
1001: the location of the socket.
1.231 markus 1002: .Pp
1.239 jmc 1003: Arguments to
1004: .Cm IdentityAgent
1.326 dtucker 1005: may use the tilde syntax to refer to a user's home directory,
1006: the tokens described in the
1.239 jmc 1007: .Sx TOKENS
1.326 dtucker 1008: section and environment variables as described in the
1009: .Sx ENVIRONMENT VARIABLES
1.239 jmc 1010: section.
1.67 jmc 1011: .It Cm IdentityFile
1.313 naddy 1012: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1013: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.375 ! dtucker 1014: You can also specify a public key file to use the corresponding
! 1015: private key that is loaded in
! 1016: .Xr ssh-agent 1
! 1017: when the private key file is not present locally.
1.67 jmc 1018: The default is
1.368 dtucker 1019: .Pa ~/.ssh/id_rsa ,
1.183 naddy 1020: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 1021: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 1022: .Pa ~/.ssh/id_ed25519 ,
1023: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 1024: and
1.368 dtucker 1025: .Pa ~/.ssh/id_dsa .
1.67 jmc 1026: Additionally, any identities represented by the authentication agent
1.165 djm 1027: will be used for authentication unless
1028: .Cm IdentitiesOnly
1029: is set.
1.221 djm 1030: If no certificates have been explicitly specified by
1031: .Cm CertificateFile ,
1.129 djm 1032: .Xr ssh 1
1033: will try to load certificate information from the filename obtained by
1034: appending
1035: .Pa -cert.pub
1036: to the path of a specified
1037: .Cm IdentityFile .
1.90 djm 1038: .Pp
1.239 jmc 1039: Arguments to
1040: .Cm IdentityFile
1041: may use the tilde syntax to refer to a user's home directory
1042: or the tokens described in the
1043: .Sx TOKENS
1044: section.
1.90 djm 1045: .Pp
1.67 jmc 1046: It is possible to have
1047: multiple identity files specified in configuration files; all these
1048: identities will be tried in sequence.
1.152 djm 1049: Multiple
1050: .Cm IdentityFile
1051: directives will add to the list of identities tried (this behaviour
1052: differs from that of other configuration directives).
1.165 djm 1053: .Pp
1054: .Cm IdentityFile
1055: may be used in conjunction with
1056: .Cm IdentitiesOnly
1057: to select which identities in an agent are offered during authentication.
1.221 djm 1058: .Cm IdentityFile
1059: may also be used in conjunction with
1060: .Cm CertificateFile
1061: in order to provide any certificate also needed for authentication with
1062: the identity.
1.164 jmc 1063: .It Cm IgnoreUnknown
1064: Specifies a pattern-list of unknown options to be ignored if they are
1065: encountered in configuration parsing.
1066: This may be used to suppress errors if
1067: .Nm
1068: contains options that are unrecognised by
1069: .Xr ssh 1 .
1070: It is recommended that
1071: .Cm IgnoreUnknown
1072: be listed early in the configuration file as it will not be applied
1073: to unknown options that appear before it.
1.229 djm 1074: .It Cm Include
1075: Include the specified configuration file(s).
1.230 jmc 1076: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1077: .Xr glob 7
1.229 djm 1078: wildcards and, for user configurations, shell-like
1.240 jmc 1079: .Sq ~
1.229 djm 1080: references to user home directories.
1.327 djm 1081: Wildcards will be expanded and processed in lexical order.
1.229 djm 1082: Files without absolute paths are assumed to be in
1083: .Pa ~/.ssh
1.230 jmc 1084: if included in a user configuration file or
1.229 djm 1085: .Pa /etc/ssh
1086: if included from the system configuration file.
1087: .Cm Include
1088: directive may appear inside a
1089: .Cm Match
1090: or
1091: .Cm Host
1092: block
1093: to perform conditional inclusion.
1.143 djm 1094: .It Cm IPQoS
1095: Specifies the IPv4 type-of-service or DSCP class for connections.
1096: Accepted values are
1.240 jmc 1097: .Cm af11 ,
1098: .Cm af12 ,
1099: .Cm af13 ,
1100: .Cm af21 ,
1101: .Cm af22 ,
1102: .Cm af23 ,
1103: .Cm af31 ,
1104: .Cm af32 ,
1105: .Cm af33 ,
1106: .Cm af41 ,
1107: .Cm af42 ,
1108: .Cm af43 ,
1109: .Cm cs0 ,
1110: .Cm cs1 ,
1111: .Cm cs2 ,
1112: .Cm cs3 ,
1113: .Cm cs4 ,
1114: .Cm cs5 ,
1115: .Cm cs6 ,
1116: .Cm cs7 ,
1117: .Cm ef ,
1.319 djm 1118: .Cm le ,
1.240 jmc 1119: .Cm lowdelay ,
1120: .Cm throughput ,
1121: .Cm reliability ,
1.253 djm 1122: a numeric value, or
1123: .Cm none
1124: to use the operating system default.
1.146 djm 1125: This option may take one or two arguments, separated by whitespace.
1.143 djm 1126: If one argument is specified, it is used as the packet class unconditionally.
1127: If two values are specified, the first is automatically selected for
1128: interactive sessions and the second for non-interactive sessions.
1129: The default is
1.269 job 1130: .Cm af21
1.272 jmc 1131: (Low-Latency Data)
1.143 djm 1132: for interactive sessions and
1.269 job 1133: .Cm cs1
1.272 jmc 1134: (Lower Effort)
1.143 djm 1135: for non-interactive sessions.
1.103 djm 1136: .It Cm KbdInteractiveAuthentication
1137: Specifies whether to use keyboard-interactive authentication.
1138: The argument to this keyword must be
1.240 jmc 1139: .Cm yes
1140: (the default)
1.103 djm 1141: or
1.240 jmc 1142: .Cm no .
1.355 dtucker 1143: .Cm ChallengeResponseAuthentication
1144: is a deprecated alias for this.
1.39 djm 1145: .It Cm KbdInteractiveDevices
1146: Specifies the list of methods to use in keyboard-interactive authentication.
1147: Multiple method names must be comma-separated.
1148: The default is to use the server specified list.
1.85 jmc 1149: The methods available vary depending on what the server supports.
1150: For an OpenSSH server,
1151: it may be zero or more of:
1.240 jmc 1152: .Cm bsdauth ,
1153: .Cm pam ,
1.85 jmc 1154: and
1.240 jmc 1155: .Cm skey .
1.140 djm 1156: .It Cm KexAlgorithms
1157: Specifies the available KEX (Key Exchange) algorithms.
1158: Multiple algorithms must be comma-separated.
1.299 kn 1159: If the specified list begins with a
1.214 djm 1160: .Sq +
1.363 dtucker 1161: character, then the specified algorithms will be appended to the default set
1.214 djm 1162: instead of replacing them.
1.299 kn 1163: If the specified list begins with a
1.241 djm 1164: .Sq -
1.363 dtucker 1165: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1166: from the default set instead of replacing them.
1.301 naddy 1167: If the specified list begins with a
1168: .Sq ^
1.363 dtucker 1169: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1170: default set.
1.141 jmc 1171: The default is:
1172: .Bd -literal -offset indent
1.371 naddy 1173: sntrup761x25519-sha512@openssh.com,
1.238 djm 1174: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1175: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1176: diffie-hellman-group-exchange-sha256,
1.266 djm 1177: diffie-hellman-group16-sha512,
1178: diffie-hellman-group18-sha512,
1.317 tedu 1179: diffie-hellman-group14-sha256
1.141 jmc 1180: .Ed
1.198 djm 1181: .Pp
1.240 jmc 1182: The list of available key exchange algorithms may also be obtained using
1183: .Qq ssh -Q kex .
1.339 djm 1184: .It Cm KnownHostsCommand
1.340 jmc 1185: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1186: those listed in
1187: .Cm UserKnownHostsFile
1188: and
1189: .Cm GlobalKnownHostsFile .
1190: This command is executed after the files have been read.
1.340 jmc 1191: It may write host key lines to standard output in identical format to the
1.339 djm 1192: usual files (described in the
1193: .Sx VERIFYING HOST KEYS
1194: section in
1195: .Xr ssh 1 ) .
1196: Arguments to
1197: .Cm KnownHostsCommand
1198: accept the tokens described in the
1199: .Sx TOKENS
1200: section.
1.340 jmc 1201: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1202: the preference list of host key algorithms to use, again to obtain the
1203: host key for the requested host name and, if
1204: .Cm CheckHostIP
1205: is enabled, one more time to obtain the host key matching the server's
1206: address.
1207: If the command exits abnormally or returns a non-zero exit status then the
1208: connection is terminated.
1.65 reyk 1209: .It Cm LocalCommand
1210: Specifies a command to execute on the local machine after successfully
1211: connecting to the server.
1212: The command string extends to the end of the line, and is executed with
1.105 jmc 1213: the user's shell.
1.239 jmc 1214: Arguments to
1215: .Cm LocalCommand
1216: accept the tokens described in the
1217: .Sx TOKENS
1218: section.
1.123 djm 1219: .Pp
1220: The command is run synchronously and does not have access to the
1221: session of the
1222: .Xr ssh 1
1223: that spawned it.
1224: It should not be used for interactive commands.
1225: .Pp
1.65 reyk 1226: This directive is ignored unless
1227: .Cm PermitLocalCommand
1228: has been enabled.
1.1 stevesk 1229: .It Cm LocalForward
1.74 jmc 1230: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1231: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1232: The first argument specifies the listener and may be
1.43 djm 1233: .Sm off
1.49 jmc 1234: .Oo Ar bind_address : Oc Ar port
1.43 djm 1235: .Sm on
1.324 dtucker 1236: or a Unix domain socket path.
1237: The second argument is the destination and may be
1238: .Ar host : Ns Ar hostport
1239: or a Unix domain socket path if the remote host supports it.
1240: .Pp
1.138 djm 1241: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1242: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1243: given on the command line.
1.1 stevesk 1244: Only the superuser can forward privileged ports.
1.43 djm 1245: By default, the local port is bound in accordance with the
1246: .Cm GatewayPorts
1247: setting.
1248: However, an explicit
1249: .Ar bind_address
1250: may be used to bind the connection to a specific address.
1251: The
1252: .Ar bind_address
1253: of
1.240 jmc 1254: .Cm localhost
1.46 jmc 1255: indicates that the listening port be bound for local use only, while an
1256: empty address or
1257: .Sq *
1.43 djm 1258: indicates that the port should be available from all interfaces.
1.326 dtucker 1259: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1260: .Sx TOKENS
1.326 dtucker 1261: section and environment variables as described in the
1262: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1263: section.
1.1 stevesk 1264: .It Cm LogLevel
1265: Gives the verbosity level that is used when logging messages from
1.84 jmc 1266: .Xr ssh 1 .
1.1 stevesk 1267: The possible values are:
1.84 jmc 1268: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1269: The default is INFO.
1270: DEBUG and DEBUG1 are equivalent.
1271: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1272: .It Cm LogVerbose
1273: Specify one or more overrides to LogLevel.
1274: An override consists of a pattern lists that matches the source file, function
1275: and line number to force detailed logging for.
1276: For example, an override pattern of:
1277: .Bd -literal -offset indent
1278: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1279: .Ed
1280: .Pp
1281: would enable detailed logging for line 1000 of
1.338 jmc 1282: .Pa kex.c ,
1.337 djm 1283: everything in the
1284: .Fn kex_exchange_identification
1285: function, and all code in the
1286: .Pa packet.c
1287: file.
1288: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1289: .It Cm MACs
1290: Specifies the MAC (message authentication code) algorithms
1291: in order of preference.
1.226 jmc 1292: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1293: Multiple algorithms must be comma-separated.
1.299 kn 1294: If the specified list begins with a
1.214 djm 1295: .Sq +
1296: character, then the specified algorithms will be appended to the default set
1297: instead of replacing them.
1.299 kn 1298: If the specified list begins with a
1.241 djm 1299: .Sq -
1300: character, then the specified algorithms (including wildcards) will be removed
1301: from the default set instead of replacing them.
1.301 naddy 1302: If the specified list begins with a
1303: .Sq ^
1304: character, then the specified algorithms will be placed at the head of the
1305: default set.
1.214 djm 1306: .Pp
1.160 markus 1307: The algorithms that contain
1.240 jmc 1308: .Qq -etm
1.160 markus 1309: calculate the MAC after encryption (encrypt-then-mac).
1310: These are considered safer and their use recommended.
1.214 djm 1311: .Pp
1.84 jmc 1312: The default is:
1.101 jmc 1313: .Bd -literal -offset indent
1.160 markus 1314: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1315: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1316: hmac-sha1-etm@openssh.com,
1.186 naddy 1317: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1318: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1319: .Ed
1.198 djm 1320: .Pp
1.240 jmc 1321: The list of available MAC algorithms may also be obtained using
1322: .Qq ssh -Q mac .
1.1 stevesk 1323: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1324: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1325: The argument to this keyword must be
1.240 jmc 1326: .Cm yes
1.1 stevesk 1327: or
1.242 jmc 1328: .Cm no
1.240 jmc 1329: (the default).
1.1 stevesk 1330: .It Cm NumberOfPasswordPrompts
1331: Specifies the number of password prompts before giving up.
1332: The argument to this keyword must be an integer.
1.84 jmc 1333: The default is 3.
1.1 stevesk 1334: .It Cm PasswordAuthentication
1335: Specifies whether to use password authentication.
1336: The argument to this keyword must be
1.240 jmc 1337: .Cm yes
1338: (the default)
1.1 stevesk 1339: or
1.240 jmc 1340: .Cm no .
1.65 reyk 1341: .It Cm PermitLocalCommand
1342: Allow local command execution via the
1343: .Ic LocalCommand
1344: option or using the
1.66 jmc 1345: .Ic !\& Ns Ar command
1.65 reyk 1346: escape sequence in
1347: .Xr ssh 1 .
1348: The argument must be
1.240 jmc 1349: .Cm yes
1.65 reyk 1350: or
1.240 jmc 1351: .Cm no
1352: (the default).
1.347 markus 1353: .It Cm PermitRemoteOpen
1354: Specifies the destinations to which remote TCP port forwarding is permitted when
1355: .Cm RemoteForward
1356: is used as a SOCKS proxy.
1357: The forwarding specification must be one of the following forms:
1358: .Pp
1359: .Bl -item -offset indent -compact
1360: .It
1361: .Cm PermitRemoteOpen
1362: .Sm off
1363: .Ar host : port
1364: .Sm on
1365: .It
1366: .Cm PermitRemoteOpen
1367: .Sm off
1368: .Ar IPv4_addr : port
1369: .Sm on
1370: .It
1371: .Cm PermitRemoteOpen
1372: .Sm off
1373: .Ar \&[ IPv6_addr \&] : port
1374: .Sm on
1375: .El
1376: .Pp
1377: Multiple forwards may be specified by separating them with whitespace.
1378: An argument of
1379: .Cm any
1380: can be used to remove all restrictions and permit any forwarding requests.
1381: An argument of
1382: .Cm none
1383: can be used to prohibit all forwarding requests.
1384: The wildcard
1385: .Sq *
1386: can be used for host or port to allow all hosts or ports respectively.
1387: Otherwise, no pattern matching or address lookups are performed on supplied
1388: names.
1.127 markus 1389: .It Cm PKCS11Provider
1.292 djm 1390: Specifies which PKCS#11 provider to use or
1391: .Cm none
1392: to indicate that no provider should be used (the default).
1393: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1394: .Xr ssh 1
1.292 djm 1395: should use to communicate with a PKCS#11 token providing keys for user
1396: authentication.
1.67 jmc 1397: .It Cm Port
1398: Specifies the port number to connect on the remote host.
1.84 jmc 1399: The default is 22.
1.1 stevesk 1400: .It Cm PreferredAuthentications
1.226 jmc 1401: Specifies the order in which the client should try authentication methods.
1.48 jmc 1402: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1403: .Cm keyboard-interactive )
1.48 jmc 1404: over another method (e.g.\&
1.131 jmc 1405: .Cm password ) .
1406: The default is:
1407: .Bd -literal -offset indent
1408: gssapi-with-mic,hostbased,publickey,
1409: keyboard-interactive,password
1410: .Ed
1.1 stevesk 1411: .It Cm ProxyCommand
1412: Specifies the command to use to connect to the server.
1413: The command
1.190 djm 1414: string extends to the end of the line, and is executed
1415: using the user's shell
1416: .Ql exec
1417: directive to avoid a lingering shell process.
1418: .Pp
1.239 jmc 1419: Arguments to
1420: .Cm ProxyCommand
1421: accept the tokens described in the
1422: .Sx TOKENS
1423: section.
1.1 stevesk 1424: The command can be basically anything,
1425: and should read from its standard input and write to its standard output.
1426: It should eventually connect an
1427: .Xr sshd 8
1428: server running on some machine, or execute
1429: .Ic sshd -i
1430: somewhere.
1431: Host key management will be done using the
1.296 jmc 1432: .Cm Hostname
1433: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1434: Setting the command to
1.240 jmc 1435: .Cm none
1.6 markus 1436: disables this option entirely.
1.1 stevesk 1437: Note that
1438: .Cm CheckHostIP
1439: is not available for connects with a proxy command.
1.52 djm 1440: .Pp
1441: This directive is useful in conjunction with
1442: .Xr nc 1
1443: and its proxy support.
1.53 jmc 1444: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1445: 192.0.2.0:
1446: .Bd -literal -offset 3n
1447: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1448: .Ed
1.233 djm 1449: .It Cm ProxyJump
1.260 millert 1450: Specifies one or more jump proxies as either
1.233 djm 1451: .Xo
1452: .Sm off
1.234 jmc 1453: .Op Ar user No @
1.233 djm 1454: .Ar host
1.234 jmc 1455: .Op : Ns Ar port
1.233 djm 1456: .Sm on
1.260 millert 1457: or an ssh URI
1.233 djm 1458: .Xc .
1.235 djm 1459: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1460: sequentially.
1.233 djm 1461: Setting this option will cause
1462: .Xr ssh 1
1463: to connect to the target host by first making a
1464: .Xr ssh 1
1465: connection to the specified
1466: .Cm ProxyJump
1467: host and then establishing a
1.234 jmc 1468: TCP forwarding to the ultimate target from there.
1.346 dlg 1469: Setting the host to
1470: .Cm none
1471: disables this option entirely.
1.233 djm 1472: .Pp
1473: Note that this option will compete with the
1474: .Cm ProxyCommand
1475: option - whichever is specified first will prevent later instances of the
1476: other from taking effect.
1.289 djm 1477: .Pp
1478: Note also that the configuration for the destination host (either supplied
1479: via the command-line or the configuration file) is not generally applied
1480: to jump hosts.
1481: .Pa ~/.ssh/config
1482: should be used if specific configuration is required for jump hosts.
1.167 djm 1483: .It Cm ProxyUseFdpass
1.168 jmc 1484: Specifies that
1.167 djm 1485: .Cm ProxyCommand
1486: will pass a connected file descriptor back to
1.168 jmc 1487: .Xr ssh 1
1.167 djm 1488: instead of continuing to execute and pass data.
1489: The default is
1.240 jmc 1490: .Cm no .
1.343 dtucker 1491: .It Cm PubkeyAcceptedAlgorithms
1492: Specifies the signature algorithms that will be used for public key
1493: authentication as a comma-separated list of patterns.
1.299 kn 1494: If the specified list begins with a
1.214 djm 1495: .Sq +
1.343 dtucker 1496: character, then the algorithms after it will be appended to the default
1.214 djm 1497: instead of replacing it.
1.299 kn 1498: If the specified list begins with a
1.241 djm 1499: .Sq -
1.343 dtucker 1500: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1501: from the default set instead of replacing them.
1.301 naddy 1502: If the specified list begins with a
1503: .Sq ^
1.343 dtucker 1504: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1505: default set.
1.213 markus 1506: The default for this option is:
1507: .Bd -literal -offset 3n
1.333 djm 1508: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1509: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1510: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1511: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1512: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1513: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1514: rsa-sha2-512-cert-v01@openssh.com,
1515: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 1516: ssh-ed25519,
1.311 naddy 1517: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1518: sk-ssh-ed25519@openssh.com,
1.305 naddy 1519: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 1520: rsa-sha2-512,rsa-sha2-256
1.213 markus 1521: .Ed
1522: .Pp
1.348 djm 1523: The list of available signature algorithms may also be obtained using
1.343 dtucker 1524: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1525: .It Cm PubkeyAuthentication
1526: Specifies whether to try public key authentication.
1527: The argument to this keyword must be
1.240 jmc 1528: .Cm yes
1.369 djm 1529: (the default),
1530: .Cm no ,
1531: .Cm unbound
1.1 stevesk 1532: or
1.369 djm 1533: .Cm host-bound .
1534: The final two options enable public key authentication while respectively
1535: disabling or enabling the OpenSSH host-bound authentication protocol
1536: extension required for restricted
1537: .Xr ssh-agent 1
1538: forwarding.
1.75 dtucker 1539: .It Cm RekeyLimit
1.373 djm 1540: Specifies the maximum amount of data that may be transmitted or received
1541: before the session key is renegotiated, optionally followed by a maximum
1542: amount of time that may pass before the session key is renegotiated.
1.162 dtucker 1543: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1544: .Sq K ,
1545: .Sq M ,
1.75 dtucker 1546: or
1.76 jmc 1547: .Sq G
1.75 dtucker 1548: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1549: The default is between
1.84 jmc 1550: .Sq 1G
1.75 dtucker 1551: and
1.84 jmc 1552: .Sq 4G ,
1.75 dtucker 1553: depending on the cipher.
1.162 dtucker 1554: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1555: units documented in the TIME FORMATS section of
1.162 dtucker 1556: .Xr sshd_config 5 .
1557: The default value for
1558: .Cm RekeyLimit
1559: is
1.240 jmc 1560: .Cm default none ,
1.162 dtucker 1561: which means that rekeying is performed after the cipher's default amount
1562: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1563: .It Cm RemoteCommand
1564: Specifies a command to execute on the remote machine after successfully
1565: connecting to the server.
1566: The command string extends to the end of the line, and is executed with
1567: the user's shell.
1.250 jmc 1568: Arguments to
1569: .Cm RemoteCommand
1570: accept the tokens described in the
1571: .Sx TOKENS
1572: section.
1.1 stevesk 1573: .It Cm RemoteForward
1.74 jmc 1574: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1575: the secure channel.
1.273 djm 1576: The remote port may either be forwarded to a specified host and port
1.256 markus 1577: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1578: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1579: The first argument is the listening specification and may be
1.43 djm 1580: .Sm off
1.49 jmc 1581: .Oo Ar bind_address : Oc Ar port
1.43 djm 1582: .Sm on
1.324 dtucker 1583: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1584: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1585: .Ar host : Ns Ar hostport
1586: or a Unix domain socket path,
1.256 markus 1587: otherwise if no destination argument is specified then the remote forwarding
1588: will be established as a SOCKS proxy.
1.370 naddy 1589: When acting as a SOCKS proxy, the destination of the connection can be
1.347 markus 1590: restricted by
1591: .Cm PermitRemoteOpen .
1.256 markus 1592: .Pp
1.138 djm 1593: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1594: Multiple forwardings may be specified, and additional
1595: forwardings can be given on the command line.
1.113 stevesk 1596: Privileged ports can be forwarded only when
1597: logging in as root on the remote machine.
1.326 dtucker 1598: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1599: .Sx TOKENS
1.326 dtucker 1600: section and environment variables as described in the
1601: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1602: section.
1.118 jmc 1603: .Pp
1.117 djm 1604: If the
1605: .Ar port
1.240 jmc 1606: argument is 0,
1.117 djm 1607: the listen port will be dynamically allocated on the server and reported
1608: to the client at run time.
1.43 djm 1609: .Pp
1610: If the
1611: .Ar bind_address
1612: is not specified, the default is to only bind to loopback addresses.
1613: If the
1614: .Ar bind_address
1615: is
1616: .Ql *
1617: or an empty string, then the forwarding is requested to listen on all
1618: interfaces.
1619: Specifying a remote
1620: .Ar bind_address
1.46 jmc 1621: will only succeed if the server's
1622: .Cm GatewayPorts
1.43 djm 1623: option is enabled (see
1.46 jmc 1624: .Xr sshd_config 5 ) .
1.149 djm 1625: .It Cm RequestTTY
1626: Specifies whether to request a pseudo-tty for the session.
1627: The argument may be one of:
1.240 jmc 1628: .Cm no
1.149 djm 1629: (never request a TTY),
1.240 jmc 1630: .Cm yes
1.149 djm 1631: (always request a TTY when standard input is a TTY),
1.240 jmc 1632: .Cm force
1.149 djm 1633: (always request a TTY) or
1.240 jmc 1634: .Cm auto
1.149 djm 1635: (request a TTY when opening a login session).
1636: This option mirrors the
1637: .Fl t
1638: and
1639: .Fl T
1640: flags for
1641: .Xr ssh 1 .
1.374 djm 1642: .It Cm RequiredRSASize
1643: Specifies the minimum RSA key size (in bits) that
1644: .Xr ssh 1
1645: will accept.
1646: User authentication keys smaller than this limit will be ignored.
1647: Servers that present host keys smaller than this limit will cause the
1648: connection to be terminated.
1649: The default is
1650: .Cm 1024
1651: bits.
1652: Note that this limit may only be raised from the default.
1.196 djm 1653: .It Cm RevokedHostKeys
1654: Specifies revoked host public keys.
1655: Keys listed in this file will be refused for host authentication.
1656: Note that if this file does not exist or is not readable,
1657: then host authentication will be refused for all hosts.
1658: Keys may be specified as a text file, listing one public key per line, or as
1659: an OpenSSH Key Revocation List (KRL) as generated by
1660: .Xr ssh-keygen 1 .
1661: For more information on KRLs, see the KEY REVOCATION LISTS section in
1662: .Xr ssh-keygen 1 .
1.305 naddy 1663: .It Cm SecurityKeyProvider
1.313 naddy 1664: Specifies a path to a library that will be used when loading any
1665: FIDO authenticator-hosted keys, overriding the default of using
1666: the built-in USB HID support.
1.309 naddy 1667: .Pp
1668: If the specified value begins with a
1669: .Sq $
1670: character, then it will be treated as an environment variable containing
1671: the path to the library.
1.32 djm 1672: .It Cm SendEnv
1673: Specifies what variables from the local
1674: .Xr environ 7
1675: should be sent to the server.
1.84 jmc 1676: The server must also support it, and the server must be configured to
1.33 djm 1677: accept these environment variables.
1.207 dtucker 1678: Note that the
1679: .Ev TERM
1.208 jmc 1680: environment variable is always sent whenever a
1.207 dtucker 1681: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1682: Refer to
1683: .Cm AcceptEnv
1684: in
1685: .Xr sshd_config 5
1686: for how to configure the server.
1.80 jmc 1687: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1688: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1689: across multiple
1690: .Cm SendEnv
1691: directives.
1.81 jmc 1692: .Pp
1693: See
1694: .Sx PATTERNS
1695: for more information on patterns.
1.271 djm 1696: .Pp
1.272 jmc 1697: It is possible to clear previously set
1.271 djm 1698: .Cm SendEnv
1699: variable names by prefixing patterns with
1700: .Pa - .
1701: The default is not to send any environment variables.
1.28 markus 1702: .It Cm ServerAliveCountMax
1.73 jmc 1703: Sets the number of server alive messages (see below) which may be
1.28 markus 1704: sent without
1.84 jmc 1705: .Xr ssh 1
1.28 markus 1706: receiving any messages back from the server.
1707: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1708: ssh will disconnect from the server, terminating the session.
1.28 markus 1709: It is important to note that the use of server alive messages is very
1710: different from
1711: .Cm TCPKeepAlive
1712: (below).
1713: The server alive messages are sent through the encrypted channel
1714: and therefore will not be spoofable.
1715: The TCP keepalive option enabled by
1716: .Cm TCPKeepAlive
1717: is spoofable.
1718: The server alive mechanism is valuable when the client or
1.298 dtucker 1719: server depend on knowing when a connection has become unresponsive.
1.28 markus 1720: .Pp
1721: The default value is 3.
1722: If, for example,
1723: .Cm ServerAliveInterval
1.84 jmc 1724: (see below) is set to 15 and
1.28 markus 1725: .Cm ServerAliveCountMax
1.84 jmc 1726: is left at the default, if the server becomes unresponsive,
1727: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1728: .It Cm ServerAliveInterval
1729: Sets a timeout interval in seconds after which if no data has been received
1730: from the server,
1.84 jmc 1731: .Xr ssh 1
1.67 jmc 1732: will send a message through the encrypted
1733: channel to request a response from the server.
1734: The default
1735: is 0, indicating that these messages will not be sent to the server.
1.357 jmc 1736: .It Cm SessionType
1737: May be used to either request invocation of a subsystem on the remote system,
1738: or to prevent the execution of a remote command at all.
1739: The latter is useful for just forwarding ports.
1740: The argument to this keyword must be
1741: .Cm none
1742: (same as the
1743: .Fl N
1744: option),
1745: .Cm subsystem
1746: (same as the
1747: .Fl s
1748: option) or
1749: .Cm default
1750: (shell or command execution).
1.277 jmc 1751: .It Cm SetEnv
1752: Directly specify one or more environment variables and their contents to
1753: be sent to the server.
1754: Similarly to
1755: .Cm SendEnv ,
1.354 djm 1756: with the exception of the
1757: .Ev TERM
1758: variable, the server must be prepared to accept the environment variable.
1.358 djm 1759: .It Cm StdinNull
1760: Redirects stdin from
1761: .Pa /dev/null
1762: (actually, prevents reading from stdin).
1763: Either this or the equivalent
1764: .Fl n
1765: option must be used when
1766: .Nm ssh
1767: is run in the background.
1768: The argument to this keyword must be
1769: .Cm yes
1770: (same as the
1771: .Fl n
1772: option) or
1773: .Cm no
1774: (the default).
1.191 millert 1775: .It Cm StreamLocalBindMask
1776: Sets the octal file creation mode mask
1777: .Pq umask
1778: used when creating a Unix-domain socket file for local or remote
1779: port forwarding.
1780: This option is only used for port forwarding to a Unix-domain socket file.
1781: .Pp
1782: The default value is 0177, which creates a Unix-domain socket file that is
1783: readable and writable only by the owner.
1784: Note that not all operating systems honor the file mode on Unix-domain
1785: socket files.
1786: .It Cm StreamLocalBindUnlink
1787: Specifies whether to remove an existing Unix-domain socket file for local
1788: or remote port forwarding before creating a new one.
1789: If the socket file already exists and
1790: .Cm StreamLocalBindUnlink
1791: is not enabled,
1792: .Nm ssh
1793: will be unable to forward the port to the Unix-domain socket file.
1794: This option is only used for port forwarding to a Unix-domain socket file.
1795: .Pp
1796: The argument must be
1.240 jmc 1797: .Cm yes
1.191 millert 1798: or
1.240 jmc 1799: .Cm no
1800: (the default).
1.1 stevesk 1801: .It Cm StrictHostKeyChecking
1802: If this flag is set to
1.240 jmc 1803: .Cm yes ,
1.84 jmc 1804: .Xr ssh 1
1.1 stevesk 1805: will never automatically add host keys to the
1.50 djm 1806: .Pa ~/.ssh/known_hosts
1.1 stevesk 1807: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1808: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1809: though it can be annoying when the
1.1 stevesk 1810: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1811: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1812: frequently made.
1813: This option forces the user to manually
1814: add all new hosts.
1.255 jmc 1815: .Pp
1.1 stevesk 1816: If this flag is set to
1.364 dtucker 1817: .Cm accept-new
1.360 jmc 1818: then ssh will automatically add new host keys to the user's
1819: .Pa known_hosts
1820: file, but will not permit connections to hosts with
1.254 djm 1821: changed host keys.
1822: If this flag is set to
1.364 dtucker 1823: .Cm no
1.254 djm 1824: or
1.364 dtucker 1825: .Cm off ,
1.255 jmc 1826: ssh will automatically add new host keys to the user known hosts files
1827: and allow connections to hosts with changed hostkeys to proceed,
1828: subject to some restrictions.
1.1 stevesk 1829: If this flag is set to
1.240 jmc 1830: .Cm ask
1831: (the default),
1.1 stevesk 1832: new host keys
1833: will be added to the user known host files only after the user
1834: has confirmed that is what they really want to do, and
1.84 jmc 1835: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1836: The host keys of
1837: known hosts will be verified automatically in all cases.
1.244 jmc 1838: .It Cm SyslogFacility
1839: Gives the facility code that is used when logging messages from
1840: .Xr ssh 1 .
1841: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1842: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1843: The default is USER.
1.26 markus 1844: .It Cm TCPKeepAlive
1845: Specifies whether the system should send TCP keepalive messages to the
1846: other side.
1847: If they are sent, death of the connection or crash of one
1848: of the machines will be properly noticed.
1849: However, this means that
1850: connections will die if the route is down temporarily, and some people
1851: find it annoying.
1852: .Pp
1853: The default is
1.240 jmc 1854: .Cm yes
1.26 markus 1855: (to send TCP keepalive messages), and the client will notice
1856: if the network goes down or the remote host dies.
1857: This is important in scripts, and many users want it too.
1858: .Pp
1859: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1860: .Cm no .
1.265 djm 1861: See also
1862: .Cm ServerAliveInterval
1863: for protocol-level keepalives.
1.65 reyk 1864: .It Cm Tunnel
1.95 stevesk 1865: Request
1.65 reyk 1866: .Xr tun 4
1.69 jmc 1867: device forwarding between the client and the server.
1.65 reyk 1868: The argument must be
1.240 jmc 1869: .Cm yes ,
1870: .Cm point-to-point
1.95 stevesk 1871: (layer 3),
1.240 jmc 1872: .Cm ethernet
1.95 stevesk 1873: (layer 2),
1.65 reyk 1874: or
1.240 jmc 1875: .Cm no
1876: (the default).
1.95 stevesk 1877: Specifying
1.240 jmc 1878: .Cm yes
1.95 stevesk 1879: requests the default tunnel mode, which is
1.240 jmc 1880: .Cm point-to-point .
1.65 reyk 1881: .It Cm TunnelDevice
1.95 stevesk 1882: Specifies the
1.65 reyk 1883: .Xr tun 4
1.95 stevesk 1884: devices to open on the client
1885: .Pq Ar local_tun
1886: and the server
1887: .Pq Ar remote_tun .
1888: .Pp
1889: The argument must be
1890: .Sm off
1891: .Ar local_tun Op : Ar remote_tun .
1892: .Sm on
1893: The devices may be specified by numerical ID or the keyword
1.240 jmc 1894: .Cm any ,
1.95 stevesk 1895: which uses the next available tunnel device.
1896: If
1897: .Ar remote_tun
1898: is not specified, it defaults to
1.240 jmc 1899: .Cm any .
1.95 stevesk 1900: The default is
1.240 jmc 1901: .Cm any:any .
1.201 djm 1902: .It Cm UpdateHostKeys
1.200 djm 1903: Specifies whether
1904: .Xr ssh 1
1905: should accept notifications of additional hostkeys from the server sent
1906: after authentication has completed and add them to
1907: .Cm UserKnownHostsFile .
1908: The argument must be
1.240 jmc 1909: .Cm yes ,
1910: .Cm no
1.320 djm 1911: or
1.240 jmc 1912: .Cm ask .
1.320 djm 1913: This option allows learning alternate hostkeys for a server
1.201 djm 1914: and supports graceful key rotation by allowing a server to send replacement
1915: public keys before old ones are removed.
1.336 djm 1916: .Pp
1.200 djm 1917: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1918: host was already trusted or explicitly accepted by the user, the host was
1919: authenticated via
1920: .Cm UserKnownHostsFile
1921: (i.e. not
1922: .Cm GlobalKnownHostsFile )
1923: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1924: .Pp
1925: .Cm UpdateHostKeys
1.321 jmc 1926: is enabled by default if the user has not overridden the default
1.320 djm 1927: .Cm UserKnownHostsFile
1.335 djm 1928: setting and has not enabled
1929: .Cm VerifyHostKeyDNS ,
1930: otherwise
1.320 djm 1931: .Cm UpdateHostKeys
1932: will be set to
1.334 djm 1933: .Cm no .
1.320 djm 1934: .Pp
1.204 djm 1935: If
1936: .Cm UpdateHostKeys
1937: is set to
1.240 jmc 1938: .Cm ask ,
1.204 djm 1939: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1940: Confirmation is currently incompatible with
1941: .Cm ControlPersist ,
1942: and will be disabled if it is enabled.
1.200 djm 1943: .Pp
1944: Presently, only
1945: .Xr sshd 8
1946: from OpenSSH 6.8 and greater support the
1.240 jmc 1947: .Qq hostkeys@openssh.com
1.200 djm 1948: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1949: .It Cm User
1950: Specifies the user to log in as.
1951: This can be useful when a different user name is used on different machines.
1952: This saves the trouble of
1953: having to remember to give the user name on the command line.
1954: .It Cm UserKnownHostsFile
1.151 djm 1955: Specifies one or more files to use for the user
1956: host key database, separated by whitespace.
1.329 dtucker 1957: Each filename may use tilde notation to refer to the user's home directory,
1958: the tokens described in the
1959: .Sx TOKENS
1960: section and environment variables as described in the
1961: .Sx ENVIRONMENT VARIABLES
1962: section.
1.151 djm 1963: The default is
1964: .Pa ~/.ssh/known_hosts ,
1965: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1966: .It Cm VerifyHostKeyDNS
1967: Specifies whether to verify the remote key using DNS and SSHFP resource
1968: records.
1.24 jakob 1969: If this option is set to
1.240 jmc 1970: .Cm yes ,
1.25 jmc 1971: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1972: from DNS.
1973: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 1974: .Cm ask .
1.24 jakob 1975: If this option is set to
1.240 jmc 1976: .Cm ask ,
1.24 jakob 1977: information on fingerprint match will be displayed, but the user will still
1978: need to confirm new host keys according to the
1979: .Cm StrictHostKeyChecking
1980: option.
1.8 jakob 1981: The default is
1.240 jmc 1982: .Cm no .
1.84 jmc 1983: .Pp
1.240 jmc 1984: See also
1985: .Sx VERIFYING HOST KEYS
1986: in
1.84 jmc 1987: .Xr ssh 1 .
1.111 grunk 1988: .It Cm VisualHostKey
1989: If this flag is set to
1.240 jmc 1990: .Cm yes ,
1.111 grunk 1991: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1992: printed in addition to the fingerprint string at login and
1.114 stevesk 1993: for unknown host keys.
1.111 grunk 1994: If this flag is set to
1.240 jmc 1995: .Cm no
1996: (the default),
1.114 stevesk 1997: no fingerprint strings are printed at login and
1.197 djm 1998: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 1999: .It Cm XAuthLocation
1.5 stevesk 2000: Specifies the full pathname of the
1.1 stevesk 2001: .Xr xauth 1
2002: program.
2003: The default is
2004: .Pa /usr/X11R6/bin/xauth .
2005: .El
1.86 jmc 2006: .Sh PATTERNS
2007: A
2008: .Em pattern
2009: consists of zero or more non-whitespace characters,
2010: .Sq *
2011: (a wildcard that matches zero or more characters),
2012: or
2013: .Sq ?\&
2014: (a wildcard that matches exactly one character).
2015: For example, to specify a set of declarations for any host in the
1.240 jmc 2016: .Qq .co.uk
1.86 jmc 2017: set of domains,
2018: the following pattern could be used:
2019: .Pp
2020: .Dl Host *.co.uk
2021: .Pp
2022: The following pattern
2023: would match any host in the 192.168.0.[0-9] network range:
2024: .Pp
2025: .Dl Host 192.168.0.?
2026: .Pp
2027: A
2028: .Em pattern-list
2029: is a comma-separated list of patterns.
2030: Patterns within pattern-lists may be negated
2031: by preceding them with an exclamation mark
2032: .Pq Sq !\& .
2033: For example,
1.174 djm 2034: to allow a key to be used from anywhere within an organization
1.86 jmc 2035: except from the
1.240 jmc 2036: .Qq dialup
1.86 jmc 2037: pool,
2038: the following entry (in authorized_keys) could be used:
2039: .Pp
2040: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 2041: .Pp
2042: Note that a negated match will never produce a positive result by itself.
2043: For example, attempting to match
2044: .Qq host3
2045: against the following pattern-list will fail:
2046: .Pp
2047: .Dl from=\&"!host1,!host2\&"
2048: .Pp
2049: The solution here is to include a term that will yield a positive match,
2050: such as a wildcard:
2051: .Pp
2052: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 2053: .Sh TOKENS
2054: Arguments to some keywords can make use of tokens,
2055: which are expanded at runtime:
2056: .Pp
2057: .Bl -tag -width XXXX -offset indent -compact
2058: .It %%
2059: A literal
2060: .Sq % .
2061: .It \&%C
1.257 jmc 2062: Hash of %l%h%p%r.
1.239 jmc 2063: .It %d
2064: Local user's home directory.
1.339 djm 2065: .It %f
2066: The fingerprint of the server's host key.
2067: .It %H
2068: The
2069: .Pa known_hosts
2070: hostname or address that is being searched for.
1.239 jmc 2071: .It %h
2072: The remote hostname.
1.340 jmc 2073: .It \%%I
1.339 djm 2074: A string describing the reason for a
2075: .Cm KnownHostsCommand
1.340 jmc 2076: execution: either
2077: .Cm ADDRESS
1.339 djm 2078: when looking up a host by address (only when
2079: .Cm CheckHostIP
2080: is enabled),
1.340 jmc 2081: .Cm HOSTNAME
2082: when searching by hostname, or
2083: .Cm ORDER
1.339 djm 2084: when preparing the host key algorithm preference list to use for the
2085: destination host.
1.239 jmc 2086: .It %i
2087: The local user ID.
1.339 djm 2088: .It %K
2089: The base64 encoded host key.
1.330 dtucker 2090: .It %k
1.350 jsg 2091: The host key alias if specified, otherwise the original remote hostname given
1.330 dtucker 2092: on the command line.
1.239 jmc 2093: .It %L
2094: The local hostname.
2095: .It %l
2096: The local hostname, including the domain name.
2097: .It %n
2098: The original remote hostname, as given on the command line.
2099: .It %p
2100: The remote port.
2101: .It %r
2102: The remote username.
1.261 djm 2103: .It \&%T
2104: The local
2105: .Xr tun 4
2106: or
2107: .Xr tap 4
2108: network interface assigned if
1.262 jmc 2109: tunnel forwarding was requested, or
2110: .Qq NONE
1.261 djm 2111: otherwise.
1.339 djm 2112: .It %t
2113: The type of the server host key, e.g.
1.360 jmc 2114: .Cm ssh-ed25519 .
1.239 jmc 2115: .It %u
2116: The local username.
2117: .El
2118: .Pp
1.323 dtucker 2119: .Cm CertificateFile ,
2120: .Cm ControlPath ,
2121: .Cm IdentityAgent ,
2122: .Cm IdentityFile ,
1.339 djm 2123: .Cm KnownHostsCommand ,
1.325 jmc 2124: .Cm LocalForward ,
1.324 dtucker 2125: .Cm Match exec ,
2126: .Cm RemoteCommand ,
1.329 dtucker 2127: .Cm RemoteForward ,
1.323 dtucker 2128: and
1.331 jmc 2129: .Cm UserKnownHostsFile
1.349 dtucker 2130: accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2131: .Pp
1.339 djm 2132: .Cm KnownHostsCommand
2133: additionally accepts the tokens %f, %H, %I, %K and %t.
2134: .Pp
1.295 jmc 2135: .Cm Hostname
1.239 jmc 2136: accepts the tokens %% and %h.
2137: .Pp
2138: .Cm LocalCommand
1.323 dtucker 2139: accepts all tokens.
1.239 jmc 2140: .Pp
2141: .Cm ProxyCommand
1.372 dtucker 2142: and
2143: .Cm ProxyJump
2144: accept the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2145: .Sh ENVIRONMENT VARIABLES
2146: Arguments to some keywords can be expanded at runtime from environment
2147: variables on the client by enclosing them in
2148: .Ic ${} ,
2149: for example
2150: .Ic ${HOME}/.ssh
2151: would refer to the user's .ssh directory.
2152: If a specified environment variable does not exist then an error will be
2153: returned and the setting for that keyword will be ignored.
2154: .Pp
2155: The keywords
2156: .Cm CertificateFile ,
2157: .Cm ControlPath ,
1.329 dtucker 2158: .Cm IdentityAgent ,
1.352 jmc 2159: .Cm IdentityFile ,
1.339 djm 2160: .Cm KnownHostsCommand ,
1.326 dtucker 2161: and
1.329 dtucker 2162: .Cm UserKnownHostsFile
1.326 dtucker 2163: support environment variables.
2164: The keywords
2165: .Cm LocalForward
2166: and
2167: .Cm RemoteForward
2168: support environment variables only for Unix domain socket paths.
1.1 stevesk 2169: .Sh FILES
2170: .Bl -tag -width Ds
1.50 djm 2171: .It Pa ~/.ssh/config
1.1 stevesk 2172: This is the per-user configuration file.
2173: The format of this file is described above.
1.84 jmc 2174: This file is used by the SSH client.
1.30 djm 2175: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2176: read/write for the user, and not writable by others.
1.1 stevesk 2177: .It Pa /etc/ssh/ssh_config
2178: Systemwide configuration file.
2179: This file provides defaults for those
2180: values that are not specified in the user's configuration file, and
2181: for those users who do not have a configuration file.
2182: This file must be world-readable.
2183: .El
1.13 jmc 2184: .Sh SEE ALSO
2185: .Xr ssh 1
1.1 stevesk 2186: .Sh AUTHORS
1.240 jmc 2187: .An -nosplit
1.1 stevesk 2188: OpenSSH is a derivative of the original and free
1.240 jmc 2189: ssh 1.2.12 release by
2190: .An Tatu Ylonen .
2191: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2192: .An Niels Provos , Theo de Raadt
2193: and
2194: .An Dug Song
1.1 stevesk 2195: removed many bugs, re-added newer features and
2196: created OpenSSH.
1.240 jmc 2197: .An Markus Friedl
2198: contributed the support for SSH protocol versions 1.5 and 2.0.
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