Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.381
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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1.381 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.380 2023/03/27 03:56:11 dtucker Exp $
! 37: .Dd $Mdocdate: March 27 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
1.379 djm 59: Unless noted otherwise, for each parameter, the first obtained value
1.1 stevesk 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 ,
1.381 ! djm 144: .Cm localnetwork ,
1.169 djm 145: .Cm host ,
146: .Cm originalhost ,
147: .Cm user ,
148: and
149: .Cm localuser .
1.193 djm 150: The
151: .Cm all
152: criteria must appear alone or immediately after
1.287 djm 153: .Cm canonical
154: or
155: .Cm final .
1.193 djm 156: Other criteria may be combined arbitrarily.
157: All criteria but
1.288 jmc 158: .Cm all ,
159: .Cm canonical ,
1.193 djm 160: and
1.287 djm 161: .Cm final
1.193 djm 162: require an argument.
163: Criteria may be negated by prepending an exclamation mark
164: .Pq Sq !\& .
1.169 djm 165: .Pp
1.177 jmc 166: The
1.193 djm 167: .Cm canonical
1.210 dtucker 168: keyword matches only when the configuration file is being re-parsed
1.193 djm 169: after hostname canonicalization (see the
170: .Cm CanonicalizeHostname
1.288 jmc 171: option).
1.193 djm 172: This may be useful to specify conditions that work with canonical host
173: names only.
1.287 djm 174: .Pp
175: The
176: .Cm final
177: keyword requests that the configuration be re-parsed (regardless of whether
178: .Cm CanonicalizeHostname
179: is enabled), and matches only during this final pass.
180: If
181: .Cm CanonicalizeHostname
182: is enabled, then
183: .Cm canonical
184: and
185: .Cm final
186: match during the same pass.
187: .Pp
1.193 djm 188: The
1.176 djm 189: .Cm exec
1.177 jmc 190: keyword executes the specified command under the user's shell.
1.169 djm 191: If the command returns a zero exit status then the condition is considered true.
192: Commands containing whitespace characters must be quoted.
1.239 jmc 193: Arguments to
194: .Cm exec
195: accept the tokens described in the
196: .Sx TOKENS
197: section.
1.381 ! djm 198: .Pp
! 199: The
! 200: .Cm localnetwork
! 201: keyword matches the addresses of active local network interfaces against the
! 202: supplied list of networks in CIDR format.
! 203: This may be convenient for varying the effective configuration on devices that
! 204: roam between networks.
! 205: Note that network address is not a trustworthy criteria in many
! 206: situations (e.g. when the network is automatically configured using DHCP)
! 207: and so caution should be applied if using it to control security-sensitive
! 208: configuration.
1.169 djm 209: .Pp
210: The other keywords' criteria must be single entries or comma-separated
211: lists and may use the wildcard and negation operators described in the
212: .Sx PATTERNS
213: section.
214: The criteria for the
215: .Cm host
216: keyword are matched against the target hostname, after any substitution
217: by the
1.295 jmc 218: .Cm Hostname
1.193 djm 219: or
220: .Cm CanonicalizeHostname
221: options.
1.169 djm 222: The
223: .Cm originalhost
224: keyword matches against the hostname as it was specified on the command-line.
225: The
226: .Cm user
227: keyword matches against the target username on the remote host.
228: The
229: .Cm localuser
230: keyword matches against the name of the local user running
231: .Xr ssh 1
232: (this keyword may be useful in system-wide
233: .Nm
234: files).
1.222 jcs 235: .It Cm AddKeysToAgent
236: Specifies whether keys should be automatically added to a running
1.223 jmc 237: .Xr ssh-agent 1 .
1.222 jcs 238: If this option is set to
1.240 jmc 239: .Cm yes
1.222 jcs 240: and a key is loaded from a file, the key and its passphrase are added to
241: the agent with the default lifetime, as if by
242: .Xr ssh-add 1 .
243: If this option is set to
1.240 jmc 244: .Cm ask ,
245: .Xr ssh 1
1.222 jcs 246: will require confirmation using the
247: .Ev SSH_ASKPASS
248: program before adding a key (see
249: .Xr ssh-add 1
250: for details).
251: If this option is set to
1.240 jmc 252: .Cm confirm ,
1.222 jcs 253: each use of the key must be confirmed, as if the
254: .Fl c
255: option was specified to
256: .Xr ssh-add 1 .
257: If this option is set to
1.240 jmc 258: .Cm no ,
1.222 jcs 259: no keys are added to the agent.
1.332 djm 260: Alternately, this option may be specified as a time interval
261: using the format described in the
262: .Sx TIME FORMATS
263: section of
264: .Xr sshd_config 5
265: to specify the key's lifetime in
266: .Xr ssh-agent 1 ,
267: after which it will automatically be removed.
1.222 jcs 268: The argument must be
1.332 djm 269: .Cm no
270: (the default),
1.240 jmc 271: .Cm yes ,
1.332 djm 272: .Cm confirm
273: (optionally followed by a time interval),
274: .Cm ask
275: or a time interval.
1.10 djm 276: .It Cm AddressFamily
1.11 jmc 277: Specifies which address family to use when connecting.
278: Valid arguments are
1.240 jmc 279: .Cm any
280: (the default),
281: .Cm inet
1.84 jmc 282: (use IPv4 only), or
1.240 jmc 283: .Cm inet6
1.40 jmc 284: (use IPv6 only).
1.1 stevesk 285: .It Cm BatchMode
286: If set to
1.240 jmc 287: .Cm yes ,
1.318 djm 288: user interaction such as password prompts and host key confirmation requests
289: will be disabled.
1.1 stevesk 290: This option is useful in scripts and other batch jobs where no user
1.318 djm 291: is present to interact with
292: .Xr ssh 1 .
1.1 stevesk 293: The argument must be
1.240 jmc 294: .Cm yes
1.1 stevesk 295: or
1.240 jmc 296: .Cm no
297: (the default).
1.268 jmc 298: .It Cm BindAddress
299: Use the specified address on the local machine as the source address of
300: the connection.
301: Only useful on systems with more than one address.
302: .It Cm BindInterface
303: Use the address of the specified interface on the local machine as the
304: source address of the connection.
1.171 djm 305: .It Cm CanonicalDomains
1.172 jmc 306: When
1.173 djm 307: .Cm CanonicalizeHostname
1.171 djm 308: is enabled, this option specifies the list of domain suffixes in which to
309: search for the specified destination host.
1.173 djm 310: .It Cm CanonicalizeFallbackLocal
1.174 djm 311: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 312: The default,
1.240 jmc 313: .Cm yes ,
1.172 jmc 314: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 315: search rules.
316: A value of
1.240 jmc 317: .Cm no
1.171 djm 318: will cause
319: .Xr ssh 1
320: to fail instantly if
1.173 djm 321: .Cm CanonicalizeHostname
1.171 djm 322: is enabled and the target hostname cannot be found in any of the domains
323: specified by
324: .Cm CanonicalDomains .
1.173 djm 325: .It Cm CanonicalizeHostname
1.174 djm 326: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 327: The default,
1.240 jmc 328: .Cm no ,
1.171 djm 329: is not to perform any name rewriting and let the system resolver handle all
330: hostname lookups.
331: If set to
1.240 jmc 332: .Cm yes
1.171 djm 333: then, for connections that do not use a
1.284 djm 334: .Cm ProxyCommand
335: or
336: .Cm ProxyJump ,
1.171 djm 337: .Xr ssh 1
1.173 djm 338: will attempt to canonicalize the hostname specified on the command line
1.171 djm 339: using the
340: .Cm CanonicalDomains
341: suffixes and
1.173 djm 342: .Cm CanonicalizePermittedCNAMEs
1.171 djm 343: rules.
344: If
1.173 djm 345: .Cm CanonicalizeHostname
1.171 djm 346: is set to
1.240 jmc 347: .Cm always ,
1.174 djm 348: then canonicalization is applied to proxied connections too.
1.185 djm 349: .Pp
1.193 djm 350: If this option is enabled, then the configuration files are processed
351: again using the new target name to pick up any new configuration in matching
1.185 djm 352: .Cm Host
1.193 djm 353: and
354: .Cm Match
1.185 djm 355: stanzas.
1.361 dtucker 356: A value of
357: .Cm none
358: disables the use of a
359: .Cm ProxyJump
360: host.
1.173 djm 361: .It Cm CanonicalizeMaxDots
1.172 jmc 362: Specifies the maximum number of dot characters in a hostname before
1.174 djm 363: canonicalization is disabled.
1.240 jmc 364: The default, 1,
1.172 jmc 365: allows a single dot (i.e. hostname.subdomain).
1.173 djm 366: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 367: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 368: canonicalizing hostnames.
1.171 djm 369: The rules consist of one or more arguments of
1.172 jmc 370: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 371: where
372: .Ar source_domain_list
1.174 djm 373: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 374: and
375: .Ar target_domain_list
1.172 jmc 376: is a pattern-list of domains that they may resolve to.
1.171 djm 377: .Pp
378: For example,
1.240 jmc 379: .Qq *.a.example.com:*.b.example.com,*.c.example.com
1.171 djm 380: will allow hostnames matching
1.240 jmc 381: .Qq *.a.example.com
1.173 djm 382: to be canonicalized to names in the
1.240 jmc 383: .Qq *.b.example.com
1.171 djm 384: or
1.240 jmc 385: .Qq *.c.example.com
1.171 djm 386: domains.
1.365 djm 387: .Pp
388: A single argument of
389: .Qq none
390: causes no CNAMEs to be considered for canonicalization.
391: This is the default behaviour.
1.283 jmc 392: .It Cm CASignatureAlgorithms
393: Specifies which algorithms are allowed for signing of certificates
394: by certificate authorities (CAs).
395: The default is:
396: .Bd -literal -offset indent
1.362 djm 397: ssh-ed25519,ecdsa-sha2-nistp256,
398: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
399: sk-ssh-ed25519@openssh.com,
400: sk-ecdsa-sha2-nistp256@openssh.com,
1.351 djm 401: rsa-sha2-512,rsa-sha2-256
1.283 jmc 402: .Ed
1.362 djm 403: .Pp
404: If the specified list begins with a
405: .Sq +
406: character, then the specified algorithms will be appended to the default set
407: instead of replacing them.
408: If the specified list begins with a
409: .Sq -
410: character, then the specified algorithms (including wildcards) will be removed
411: from the default set instead of replacing them.
1.283 jmc 412: .Pp
413: .Xr ssh 1
414: will not accept host certificates signed using algorithms other than those
415: specified.
1.221 djm 416: .It Cm CertificateFile
417: Specifies a file from which the user's certificate is read.
418: A corresponding private key must be provided separately in order
419: to use this certificate either
420: from an
421: .Cm IdentityFile
422: directive or
423: .Fl i
424: flag to
425: .Xr ssh 1 ,
426: via
427: .Xr ssh-agent 1 ,
428: or via a
1.305 naddy 429: .Cm PKCS11Provider
430: or
431: .Cm SecurityKeyProvider .
1.221 djm 432: .Pp
1.239 jmc 433: Arguments to
434: .Cm CertificateFile
1.326 dtucker 435: may use the tilde syntax to refer to a user's home directory,
436: the tokens described in the
1.239 jmc 437: .Sx TOKENS
1.326 dtucker 438: section and environment variables as described in the
439: .Sx ENVIRONMENT VARIABLES
1.239 jmc 440: section.
1.221 djm 441: .Pp
442: It is possible to have multiple certificate files specified in
443: configuration files; these certificates will be tried in sequence.
444: Multiple
445: .Cm CertificateFile
446: directives will add to the list of certificates used for
447: authentication.
1.1 stevesk 448: .It Cm CheckHostIP
1.240 jmc 449: If set to
1.370 naddy 450: .Cm yes ,
1.84 jmc 451: .Xr ssh 1
452: will additionally check the host IP address in the
1.1 stevesk 453: .Pa known_hosts
454: file.
1.240 jmc 455: This allows it to detect if a host key changed due to DNS spoofing
1.211 djm 456: and will add addresses of destination hosts to
457: .Pa ~/.ssh/known_hosts
458: in the process, regardless of the setting of
459: .Cm StrictHostKeyChecking .
1.107 grunk 460: If the option is set to
1.341 djm 461: .Cm no
462: (the default),
1.1 stevesk 463: the check will not be executed.
464: .It Cm Ciphers
1.245 djm 465: Specifies the ciphers allowed and their order of preference.
1.1 stevesk 466: Multiple ciphers must be comma-separated.
1.299 kn 467: If the specified list begins with a
1.214 djm 468: .Sq +
469: character, then the specified ciphers will be appended to the default set
470: instead of replacing them.
1.299 kn 471: If the specified list begins with a
1.241 djm 472: .Sq -
473: character, then the specified ciphers (including wildcards) will be removed
474: from the default set instead of replacing them.
1.301 naddy 475: If the specified list begins with a
476: .Sq ^
477: character, then the specified ciphers will be placed at the head of the
478: default set.
1.214 djm 479: .Pp
1.180 djm 480: The supported ciphers are:
1.240 jmc 481: .Bd -literal -offset indent
1.186 naddy 482: 3des-cbc
483: aes128-cbc
484: aes192-cbc
485: aes256-cbc
486: aes128-ctr
487: aes192-ctr
488: aes256-ctr
489: aes128-gcm@openssh.com
490: aes256-gcm@openssh.com
491: chacha20-poly1305@openssh.com
1.240 jmc 492: .Ed
1.180 djm 493: .Pp
1.84 jmc 494: The default is:
1.186 naddy 495: .Bd -literal -offset indent
1.215 jmc 496: chacha20-poly1305@openssh.com,
1.186 naddy 497: aes128-ctr,aes192-ctr,aes256-ctr,
1.270 djm 498: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 499: .Ed
1.180 djm 500: .Pp
1.240 jmc 501: The list of available ciphers may also be obtained using
502: .Qq ssh -Q cipher .
1.1 stevesk 503: .It Cm ClearAllForwardings
1.84 jmc 504: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 505: specified in the configuration files or on the command line be
1.7 jmc 506: cleared.
507: This option is primarily useful when used from the
1.84 jmc 508: .Xr ssh 1
1.1 stevesk 509: command line to clear port forwardings set in
510: configuration files, and is automatically set by
511: .Xr scp 1
512: and
513: .Xr sftp 1 .
514: The argument must be
1.240 jmc 515: .Cm yes
1.1 stevesk 516: or
1.240 jmc 517: .Cm no
518: (the default).
1.1 stevesk 519: .It Cm Compression
520: Specifies whether to use compression.
521: The argument must be
1.240 jmc 522: .Cm yes
1.1 stevesk 523: or
1.240 jmc 524: .Cm no
525: (the default).
1.247 naddy 526: .It Cm ConnectionAttempts
527: Specifies the number of tries (one per second) to make before exiting.
528: The argument must be an integer.
529: This may be useful in scripts if the connection sometimes fails.
530: The default is 1.
1.9 djm 531: .It Cm ConnectTimeout
1.84 jmc 532: Specifies the timeout (in seconds) used when connecting to the
533: SSH server, instead of using the default system TCP timeout.
1.302 djm 534: This timeout is applied both to establishing the connection and to performing
535: the initial SSH protocol handshake and key exchange.
1.36 djm 536: .It Cm ControlMaster
537: Enables the sharing of multiple sessions over a single network connection.
538: When set to
1.240 jmc 539: .Cm yes ,
1.84 jmc 540: .Xr ssh 1
1.36 djm 541: will listen for connections on a control socket specified using the
542: .Cm ControlPath
543: argument.
544: Additional sessions can connect to this socket using the same
545: .Cm ControlPath
546: with
547: .Cm ControlMaster
548: set to
1.240 jmc 549: .Cm no
1.38 jmc 550: (the default).
1.64 jmc 551: These sessions will try to reuse the master instance's network connection
1.63 djm 552: rather than initiating new ones, but will fall back to connecting normally
553: if the control socket does not exist, or is not listening.
554: .Pp
1.37 djm 555: Setting this to
1.240 jmc 556: .Cm ask
557: will cause
558: .Xr ssh 1
1.206 jmc 559: to listen for control connections, but require confirmation using
560: .Xr ssh-askpass 1 .
1.51 jakob 561: If the
562: .Cm ControlPath
1.84 jmc 563: cannot be opened,
1.240 jmc 564: .Xr ssh 1
565: will continue without connecting to a master instance.
1.58 djm 566: .Pp
567: X11 and
1.59 jmc 568: .Xr ssh-agent 1
1.58 djm 569: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 570: display and agent forwarded will be the one belonging to the master
1.59 jmc 571: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 572: .Pp
573: Two additional options allow for opportunistic multiplexing: try to use a
574: master connection but fall back to creating a new one if one does not already
575: exist.
576: These options are:
1.240 jmc 577: .Cm auto
1.56 djm 578: and
1.240 jmc 579: .Cm autoask .
1.56 djm 580: The latter requires confirmation like the
1.240 jmc 581: .Cm ask
1.56 djm 582: option.
1.36 djm 583: .It Cm ControlPath
1.55 djm 584: Specify the path to the control socket used for connection sharing as described
585: in the
1.36 djm 586: .Cm ControlMaster
1.57 djm 587: section above or the string
1.240 jmc 588: .Cm none
1.57 djm 589: to disable connection sharing.
1.239 jmc 590: Arguments to
591: .Cm ControlPath
1.326 dtucker 592: may use the tilde syntax to refer to a user's home directory,
593: the tokens described in the
1.239 jmc 594: .Sx TOKENS
1.326 dtucker 595: section and environment variables as described in the
596: .Sx ENVIRONMENT VARIABLES
1.239 jmc 597: section.
1.56 djm 598: It is recommended that any
599: .Cm ControlPath
600: used for opportunistic connection sharing include
1.195 djm 601: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
602: that is not writable by other users.
1.56 djm 603: This ensures that shared connections are uniquely identified.
1.137 djm 604: .It Cm ControlPersist
605: When used in conjunction with
606: .Cm ControlMaster ,
607: specifies that the master connection should remain open
608: in the background (waiting for future client connections)
609: after the initial client connection has been closed.
610: If set to
1.314 naddy 611: .Cm no
612: (the default),
1.137 djm 613: then the master connection will not be placed into the background,
614: and will close as soon as the initial client connection is closed.
615: If set to
1.240 jmc 616: .Cm yes
617: or 0,
1.137 djm 618: then the master connection will remain in the background indefinitely
619: (until killed or closed via a mechanism such as the
1.240 jmc 620: .Qq ssh -O exit ) .
1.137 djm 621: If set to a time in seconds, or a time in any of the formats documented in
622: .Xr sshd_config 5 ,
623: then the backgrounded master connection will automatically terminate
624: after it has remained idle (with no client connections) for the
625: specified time.
1.38 jmc 626: .It Cm DynamicForward
1.74 jmc 627: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 628: over the secure channel, and the application
629: protocol is then used to determine where to connect to from the
630: remote machine.
1.62 djm 631: .Pp
632: The argument must be
633: .Sm off
634: .Oo Ar bind_address : Oc Ar port .
635: .Sm on
1.138 djm 636: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 637: By default, the local port is bound in accordance with the
638: .Cm GatewayPorts
639: setting.
640: However, an explicit
641: .Ar bind_address
642: may be used to bind the connection to a specific address.
643: The
644: .Ar bind_address
645: of
1.240 jmc 646: .Cm localhost
1.62 djm 647: indicates that the listening port be bound for local use only, while an
648: empty address or
649: .Sq *
650: indicates that the port should be available from all interfaces.
651: .Pp
1.38 jmc 652: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 653: .Xr ssh 1
1.38 jmc 654: will act as a SOCKS server.
655: Multiple forwardings may be specified, and
656: additional forwardings can be given on the command line.
657: Only the superuser can forward privileged ports.
1.376 djm 658: .It Cm EnableEscapeCommandline
659: Enables the command line option in the
660: .Cm EscapeChar
661: menu for interactive sessions (default
662: .Ql ~C ) .
663: By default, the command line is disabled.
1.14 markus 664: .It Cm EnableSSHKeysign
665: Setting this option to
1.240 jmc 666: .Cm yes
1.14 markus 667: in the global client configuration file
668: .Pa /etc/ssh/ssh_config
669: enables the use of the helper program
670: .Xr ssh-keysign 8
671: during
672: .Cm HostbasedAuthentication .
673: The argument must be
1.240 jmc 674: .Cm yes
1.14 markus 675: or
1.240 jmc 676: .Cm no
677: (the default).
1.23 jmc 678: This option should be placed in the non-hostspecific section.
1.14 markus 679: See
680: .Xr ssh-keysign 8
681: for more information.
1.1 stevesk 682: .It Cm EscapeChar
683: Sets the escape character (default:
684: .Ql ~ ) .
685: The escape character can also
686: be set on the command line.
687: The argument should be a single character,
688: .Ql ^
689: followed by a letter, or
1.240 jmc 690: .Cm none
1.1 stevesk 691: to disable the escape
692: character entirely (making the connection transparent for binary
693: data).
1.96 markus 694: .It Cm ExitOnForwardFailure
695: Specifies whether
696: .Xr ssh 1
697: should terminate the connection if it cannot set up all requested
1.216 djm 698: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 699: if either end is unable to bind and listen on a specified port).
1.216 djm 700: Note that
701: .Cm ExitOnForwardFailure
702: does not apply to connections made over port forwardings and will not,
703: for example, cause
704: .Xr ssh 1
705: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 706: The argument must be
1.240 jmc 707: .Cm yes
1.96 markus 708: or
1.240 jmc 709: .Cm no
710: (the default).
1.197 djm 711: .It Cm FingerprintHash
712: Specifies the hash algorithm used when displaying key fingerprints.
713: Valid options are:
1.240 jmc 714: .Cm md5
1.197 djm 715: and
1.240 jmc 716: .Cm sha256
1.359 djm 717: (the default).
718: .It Cm ForkAfterAuthentication
719: Requests
720: .Nm ssh
721: to go to background just before command execution.
722: This is useful if
723: .Nm ssh
724: is going to ask for passwords or passphrases, but the user
725: wants it in the background.
726: This implies the
727: .Cm StdinNull
728: configuration option being set to
729: .Dq yes .
730: The recommended way to start X11 programs at a remote site is with
731: something like
732: .Ic ssh -f host xterm ,
733: which is the same as
734: .Ic ssh host xterm
735: if the
736: .Cm ForkAfterAuthentication
737: configuration option is set to
738: .Dq yes .
739: .Pp
740: If the
741: .Cm ExitOnForwardFailure
742: configuration option is set to
743: .Dq yes ,
744: then a client started with the
745: .Cm ForkAfterAuthentication
746: configuration option being set to
747: .Dq yes
748: will wait for all remote port forwards to be successfully established
749: before placing itself in the background.
750: The argument to this keyword must be
751: .Cm yes
752: (same as the
753: .Fl f
754: option) or
755: .Cm no
1.240 jmc 756: (the default).
1.1 stevesk 757: .It Cm ForwardAgent
758: Specifies whether the connection to the authentication agent (if any)
759: will be forwarded to the remote machine.
1.312 djm 760: The argument may be
761: .Cm yes ,
1.240 jmc 762: .Cm no
1.312 djm 763: (the default),
764: an explicit path to an agent socket or the name of an environment variable
765: (beginning with
766: .Sq $ )
767: in which to find the path.
1.3 stevesk 768: .Pp
1.7 jmc 769: Agent forwarding should be enabled with caution.
770: Users with the ability to bypass file permissions on the remote host
771: (for the agent's Unix-domain socket)
772: can access the local agent through the forwarded connection.
773: An attacker cannot obtain key material from the agent,
1.3 stevesk 774: however they can perform operations on the keys that enable them to
775: authenticate using the identities loaded into the agent.
1.1 stevesk 776: .It Cm ForwardX11
777: Specifies whether X11 connections will be automatically redirected
778: over the secure channel and
779: .Ev DISPLAY
780: set.
781: The argument must be
1.240 jmc 782: .Cm yes
1.1 stevesk 783: or
1.240 jmc 784: .Cm no
785: (the default).
1.3 stevesk 786: .Pp
1.7 jmc 787: X11 forwarding should be enabled with caution.
788: Users with the ability to bypass file permissions on the remote host
1.22 markus 789: (for the user's X11 authorization database)
1.7 jmc 790: can access the local X11 display through the forwarded connection.
1.22 markus 791: An attacker may then be able to perform activities such as keystroke monitoring
792: if the
793: .Cm ForwardX11Trusted
794: option is also enabled.
1.134 djm 795: .It Cm ForwardX11Timeout
1.135 jmc 796: Specify a timeout for untrusted X11 forwarding
797: using the format described in the
1.240 jmc 798: .Sx TIME FORMATS
799: section of
1.134 djm 800: .Xr sshd_config 5 .
801: X11 connections received by
802: .Xr ssh 1
803: after this time will be refused.
1.285 djm 804: Setting
805: .Cm ForwardX11Timeout
806: to zero will disable the timeout and permit X11 forwarding for the life
807: of the connection.
1.134 djm 808: The default is to disable untrusted X11 forwarding after twenty minutes has
809: elapsed.
1.22 markus 810: .It Cm ForwardX11Trusted
1.34 jmc 811: If this option is set to
1.240 jmc 812: .Cm yes ,
1.84 jmc 813: remote X11 clients will have full access to the original X11 display.
1.42 djm 814: .Pp
1.22 markus 815: If this option is set to
1.240 jmc 816: .Cm no
817: (the default),
1.84 jmc 818: remote X11 clients will be considered untrusted and prevented
1.22 markus 819: from stealing or tampering with data belonging to trusted X11
820: clients.
1.42 djm 821: Furthermore, the
822: .Xr xauth 1
823: token used for the session will be set to expire after 20 minutes.
824: Remote clients will be refused access after this time.
1.22 markus 825: .Pp
826: See the X11 SECURITY extension specification for full details on
827: the restrictions imposed on untrusted clients.
1.1 stevesk 828: .It Cm GatewayPorts
829: Specifies whether remote hosts are allowed to connect to local
830: forwarded ports.
831: By default,
1.84 jmc 832: .Xr ssh 1
1.7 jmc 833: binds local port forwardings to the loopback address.
834: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 835: .Cm GatewayPorts
1.84 jmc 836: can be used to specify that ssh
1.1 stevesk 837: should bind local port forwardings to the wildcard address,
838: thus allowing remote hosts to connect to forwarded ports.
839: The argument must be
1.240 jmc 840: .Cm yes
1.1 stevesk 841: or
1.240 jmc 842: .Cm no
843: (the default).
1.1 stevesk 844: .It Cm GlobalKnownHostsFile
1.151 djm 845: Specifies one or more files to use for the global
846: host key database, separated by whitespace.
847: The default is
848: .Pa /etc/ssh/ssh_known_hosts ,
849: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 850: .It Cm GSSAPIAuthentication
1.27 markus 851: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 852: The default is
1.240 jmc 853: .Cm no .
1.18 markus 854: .It Cm GSSAPIDelegateCredentials
855: Forward (delegate) credentials to the server.
856: The default is
1.240 jmc 857: .Cm no .
1.44 djm 858: .It Cm HashKnownHosts
859: Indicates that
1.84 jmc 860: .Xr ssh 1
1.44 djm 861: should hash host names and addresses when they are added to
1.50 djm 862: .Pa ~/.ssh/known_hosts .
1.44 djm 863: These hashed names may be used normally by
1.84 jmc 864: .Xr ssh 1
1.44 djm 865: and
1.84 jmc 866: .Xr sshd 8 ,
1.316 djm 867: but they do not visually reveal identifying information if the
868: file's contents are disclosed.
1.44 djm 869: The default is
1.240 jmc 870: .Cm no .
1.97 jmc 871: Note that existing names and addresses in known hosts files
872: will not be converted automatically,
873: but may be manually hashed using
1.45 djm 874: .Xr ssh-keygen 1 .
1.344 dtucker 875: .It Cm HostbasedAcceptedAlgorithms
1.348 djm 876: Specifies the signature algorithms that will be used for hostbased
877: authentication as a comma-separated list of patterns.
1.300 naddy 878: Alternately if the specified list begins with a
1.214 djm 879: .Sq +
1.348 djm 880: character, then the specified signature algorithms will be appended
881: to the default set instead of replacing them.
1.300 naddy 882: If the specified list begins with a
1.241 djm 883: .Sq -
1.348 djm 884: character, then the specified signature algorithms (including wildcards)
885: will be removed from the default set instead of replacing them.
1.301 naddy 886: If the specified list begins with a
887: .Sq ^
1.348 djm 888: character, then the specified signature algorithms will be placed
889: at the head of the default set.
1.213 markus 890: The default for this option is:
891: .Bd -literal -offset 3n
1.333 djm 892: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 893: ecdsa-sha2-nistp256-cert-v01@openssh.com,
894: ecdsa-sha2-nistp384-cert-v01@openssh.com,
895: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 896: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 897: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 898: rsa-sha2-512-cert-v01@openssh.com,
899: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 900: ssh-ed25519,
1.213 markus 901: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 902: sk-ssh-ed25519@openssh.com,
1.311 naddy 903: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 904: rsa-sha2-512,rsa-sha2-256
1.213 markus 905: .Ed
906: .Pp
1.202 djm 907: The
908: .Fl Q
909: option of
910: .Xr ssh 1
1.348 djm 911: may be used to list supported signature algorithms.
1.344 dtucker 912: This was formerly named HostbasedKeyTypes.
1.345 naddy 913: .It Cm HostbasedAuthentication
914: Specifies whether to try rhosts based authentication with public key
915: authentication.
916: The argument must be
917: .Cm yes
918: or
919: .Cm no
920: (the default).
1.1 stevesk 921: .It Cm HostKeyAlgorithms
1.348 djm 922: Specifies the host key signature algorithms
1.1 stevesk 923: that the client wants to use in order of preference.
1.300 naddy 924: Alternately if the specified list begins with a
1.214 djm 925: .Sq +
1.348 djm 926: character, then the specified signature algorithms will be appended to
927: the default set instead of replacing them.
1.300 naddy 928: If the specified list begins with a
1.241 djm 929: .Sq -
1.348 djm 930: character, then the specified signature algorithms (including wildcards)
931: will be removed from the default set instead of replacing them.
1.301 naddy 932: If the specified list begins with a
933: .Sq ^
1.348 djm 934: character, then the specified signature algorithms will be placed
935: at the head of the default set.
1.1 stevesk 936: The default for this option is:
1.139 djm 937: .Bd -literal -offset 3n
1.333 djm 938: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 939: ecdsa-sha2-nistp256-cert-v01@openssh.com,
940: ecdsa-sha2-nistp384-cert-v01@openssh.com,
941: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 942: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 943: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 944: rsa-sha2-512-cert-v01@openssh.com,
945: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 946: ssh-ed25519,
1.139 djm 947: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 948: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 949: sk-ssh-ed25519@openssh.com,
1.366 kn 950: rsa-sha2-512,rsa-sha2-256
1.139 djm 951: .Ed
1.145 djm 952: .Pp
953: If hostkeys are known for the destination host then this default is modified
954: to prefer their algorithms.
1.198 djm 955: .Pp
1.348 djm 956: The list of available signature algorithms may also be obtained using
1.322 dtucker 957: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 958: .It Cm HostKeyAlias
959: Specifies an alias that should be used instead of the
960: real host name when looking up or saving the host key
1.251 djm 961: in the host key database files and when validating host certificates.
1.84 jmc 962: This option is useful for tunneling SSH connections
1.1 stevesk 963: or for multiple servers running on a single host.
1.295 jmc 964: .It Cm Hostname
1.1 stevesk 965: Specifies the real host name to log into.
966: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 967: Arguments to
1.295 jmc 968: .Cm Hostname
1.239 jmc 969: accept the tokens described in the
970: .Sx TOKENS
971: section.
1.1 stevesk 972: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 973: .Cm Hostname
1.1 stevesk 974: specifications).
1.239 jmc 975: The default is the name given on the command line.
1.29 markus 976: .It Cm IdentitiesOnly
977: Specifies that
1.84 jmc 978: .Xr ssh 1
1.304 djm 979: should only use the configured authentication identity and certificate files
980: (either the default files, or those explicitly configured in the
1.31 jmc 981: .Nm
1.221 djm 982: files
983: or passed on the
984: .Xr ssh 1
1.304 djm 985: command-line),
1.84 jmc 986: even if
987: .Xr ssh-agent 1
1.159 djm 988: or a
989: .Cm PKCS11Provider
1.305 naddy 990: or
991: .Cm SecurityKeyProvider
1.29 markus 992: offers more identities.
993: The argument to this keyword must be
1.240 jmc 994: .Cm yes
1.29 markus 995: or
1.240 jmc 996: .Cm no
997: (the default).
1.84 jmc 998: This option is intended for situations where ssh-agent
1.29 markus 999: offers many different identities.
1.231 markus 1000: .It Cm IdentityAgent
1001: Specifies the
1002: .Ux Ns -domain
1003: socket used to communicate with the authentication agent.
1004: .Pp
1005: This option overrides the
1.240 jmc 1006: .Ev SSH_AUTH_SOCK
1.231 markus 1007: environment variable and can be used to select a specific agent.
1008: Setting the socket name to
1.240 jmc 1009: .Cm none
1.231 markus 1010: disables the use of an authentication agent.
1.232 markus 1011: If the string
1.240 jmc 1012: .Qq SSH_AUTH_SOCK
1.232 markus 1013: is specified, the location of the socket will be read from the
1014: .Ev SSH_AUTH_SOCK
1015: environment variable.
1.286 djm 1016: Otherwise if the specified value begins with a
1017: .Sq $
1018: character, then it will be treated as an environment variable containing
1019: the location of the socket.
1.231 markus 1020: .Pp
1.239 jmc 1021: Arguments to
1022: .Cm IdentityAgent
1.326 dtucker 1023: may use the tilde syntax to refer to a user's home directory,
1024: the tokens described in the
1.239 jmc 1025: .Sx TOKENS
1.326 dtucker 1026: section and environment variables as described in the
1027: .Sx ENVIRONMENT VARIABLES
1.239 jmc 1028: section.
1.67 jmc 1029: .It Cm IdentityFile
1.313 naddy 1030: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1031: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.375 dtucker 1032: You can also specify a public key file to use the corresponding
1033: private key that is loaded in
1034: .Xr ssh-agent 1
1035: when the private key file is not present locally.
1.67 jmc 1036: The default is
1.368 dtucker 1037: .Pa ~/.ssh/id_rsa ,
1.183 naddy 1038: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 1039: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 1040: .Pa ~/.ssh/id_ed25519 ,
1041: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 1042: and
1.368 dtucker 1043: .Pa ~/.ssh/id_dsa .
1.67 jmc 1044: Additionally, any identities represented by the authentication agent
1.165 djm 1045: will be used for authentication unless
1046: .Cm IdentitiesOnly
1047: is set.
1.221 djm 1048: If no certificates have been explicitly specified by
1049: .Cm CertificateFile ,
1.129 djm 1050: .Xr ssh 1
1051: will try to load certificate information from the filename obtained by
1052: appending
1053: .Pa -cert.pub
1054: to the path of a specified
1055: .Cm IdentityFile .
1.90 djm 1056: .Pp
1.239 jmc 1057: Arguments to
1058: .Cm IdentityFile
1059: may use the tilde syntax to refer to a user's home directory
1060: or the tokens described in the
1061: .Sx TOKENS
1062: section.
1.90 djm 1063: .Pp
1.67 jmc 1064: It is possible to have
1065: multiple identity files specified in configuration files; all these
1066: identities will be tried in sequence.
1.152 djm 1067: Multiple
1068: .Cm IdentityFile
1069: directives will add to the list of identities tried (this behaviour
1070: differs from that of other configuration directives).
1.165 djm 1071: .Pp
1072: .Cm IdentityFile
1073: may be used in conjunction with
1074: .Cm IdentitiesOnly
1075: to select which identities in an agent are offered during authentication.
1.221 djm 1076: .Cm IdentityFile
1077: may also be used in conjunction with
1078: .Cm CertificateFile
1079: in order to provide any certificate also needed for authentication with
1080: the identity.
1.164 jmc 1081: .It Cm IgnoreUnknown
1082: Specifies a pattern-list of unknown options to be ignored if they are
1083: encountered in configuration parsing.
1084: This may be used to suppress errors if
1085: .Nm
1086: contains options that are unrecognised by
1087: .Xr ssh 1 .
1088: It is recommended that
1089: .Cm IgnoreUnknown
1090: be listed early in the configuration file as it will not be applied
1091: to unknown options that appear before it.
1.229 djm 1092: .It Cm Include
1093: Include the specified configuration file(s).
1.230 jmc 1094: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1095: .Xr glob 7
1.229 djm 1096: wildcards and, for user configurations, shell-like
1.240 jmc 1097: .Sq ~
1.229 djm 1098: references to user home directories.
1.327 djm 1099: Wildcards will be expanded and processed in lexical order.
1.229 djm 1100: Files without absolute paths are assumed to be in
1101: .Pa ~/.ssh
1.230 jmc 1102: if included in a user configuration file or
1.229 djm 1103: .Pa /etc/ssh
1104: if included from the system configuration file.
1105: .Cm Include
1106: directive may appear inside a
1107: .Cm Match
1108: or
1109: .Cm Host
1110: block
1111: to perform conditional inclusion.
1.143 djm 1112: .It Cm IPQoS
1113: Specifies the IPv4 type-of-service or DSCP class for connections.
1114: Accepted values are
1.240 jmc 1115: .Cm af11 ,
1116: .Cm af12 ,
1117: .Cm af13 ,
1118: .Cm af21 ,
1119: .Cm af22 ,
1120: .Cm af23 ,
1121: .Cm af31 ,
1122: .Cm af32 ,
1123: .Cm af33 ,
1124: .Cm af41 ,
1125: .Cm af42 ,
1126: .Cm af43 ,
1127: .Cm cs0 ,
1128: .Cm cs1 ,
1129: .Cm cs2 ,
1130: .Cm cs3 ,
1131: .Cm cs4 ,
1132: .Cm cs5 ,
1133: .Cm cs6 ,
1134: .Cm cs7 ,
1135: .Cm ef ,
1.319 djm 1136: .Cm le ,
1.240 jmc 1137: .Cm lowdelay ,
1138: .Cm throughput ,
1139: .Cm reliability ,
1.253 djm 1140: a numeric value, or
1141: .Cm none
1142: to use the operating system default.
1.146 djm 1143: This option may take one or two arguments, separated by whitespace.
1.143 djm 1144: If one argument is specified, it is used as the packet class unconditionally.
1145: If two values are specified, the first is automatically selected for
1146: interactive sessions and the second for non-interactive sessions.
1147: The default is
1.269 job 1148: .Cm af21
1.272 jmc 1149: (Low-Latency Data)
1.143 djm 1150: for interactive sessions and
1.269 job 1151: .Cm cs1
1.272 jmc 1152: (Lower Effort)
1.143 djm 1153: for non-interactive sessions.
1.103 djm 1154: .It Cm KbdInteractiveAuthentication
1155: Specifies whether to use keyboard-interactive authentication.
1156: The argument to this keyword must be
1.240 jmc 1157: .Cm yes
1158: (the default)
1.103 djm 1159: or
1.240 jmc 1160: .Cm no .
1.355 dtucker 1161: .Cm ChallengeResponseAuthentication
1162: is a deprecated alias for this.
1.39 djm 1163: .It Cm KbdInteractiveDevices
1164: Specifies the list of methods to use in keyboard-interactive authentication.
1165: Multiple method names must be comma-separated.
1166: The default is to use the server specified list.
1.85 jmc 1167: The methods available vary depending on what the server supports.
1168: For an OpenSSH server,
1169: it may be zero or more of:
1.240 jmc 1170: .Cm bsdauth ,
1171: .Cm pam ,
1.85 jmc 1172: and
1.240 jmc 1173: .Cm skey .
1.140 djm 1174: .It Cm KexAlgorithms
1175: Specifies the available KEX (Key Exchange) algorithms.
1176: Multiple algorithms must be comma-separated.
1.299 kn 1177: If the specified list begins with a
1.214 djm 1178: .Sq +
1.363 dtucker 1179: character, then the specified algorithms will be appended to the default set
1.214 djm 1180: instead of replacing them.
1.299 kn 1181: If the specified list begins with a
1.241 djm 1182: .Sq -
1.363 dtucker 1183: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1184: from the default set instead of replacing them.
1.301 naddy 1185: If the specified list begins with a
1186: .Sq ^
1.363 dtucker 1187: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1188: default set.
1.141 jmc 1189: The default is:
1190: .Bd -literal -offset indent
1.371 naddy 1191: sntrup761x25519-sha512@openssh.com,
1.238 djm 1192: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1193: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1194: diffie-hellman-group-exchange-sha256,
1.266 djm 1195: diffie-hellman-group16-sha512,
1196: diffie-hellman-group18-sha512,
1.317 tedu 1197: diffie-hellman-group14-sha256
1.141 jmc 1198: .Ed
1.198 djm 1199: .Pp
1.240 jmc 1200: The list of available key exchange algorithms may also be obtained using
1201: .Qq ssh -Q kex .
1.339 djm 1202: .It Cm KnownHostsCommand
1.340 jmc 1203: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1204: those listed in
1205: .Cm UserKnownHostsFile
1206: and
1207: .Cm GlobalKnownHostsFile .
1208: This command is executed after the files have been read.
1.340 jmc 1209: It may write host key lines to standard output in identical format to the
1.339 djm 1210: usual files (described in the
1211: .Sx VERIFYING HOST KEYS
1212: section in
1213: .Xr ssh 1 ) .
1214: Arguments to
1215: .Cm KnownHostsCommand
1216: accept the tokens described in the
1217: .Sx TOKENS
1218: section.
1.340 jmc 1219: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1220: the preference list of host key algorithms to use, again to obtain the
1221: host key for the requested host name and, if
1222: .Cm CheckHostIP
1223: is enabled, one more time to obtain the host key matching the server's
1224: address.
1225: If the command exits abnormally or returns a non-zero exit status then the
1226: connection is terminated.
1.65 reyk 1227: .It Cm LocalCommand
1228: Specifies a command to execute on the local machine after successfully
1229: connecting to the server.
1230: The command string extends to the end of the line, and is executed with
1.105 jmc 1231: the user's shell.
1.239 jmc 1232: Arguments to
1233: .Cm LocalCommand
1234: accept the tokens described in the
1235: .Sx TOKENS
1236: section.
1.123 djm 1237: .Pp
1238: The command is run synchronously and does not have access to the
1239: session of the
1240: .Xr ssh 1
1241: that spawned it.
1242: It should not be used for interactive commands.
1243: .Pp
1.65 reyk 1244: This directive is ignored unless
1245: .Cm PermitLocalCommand
1246: has been enabled.
1.1 stevesk 1247: .It Cm LocalForward
1.74 jmc 1248: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1249: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1250: The first argument specifies the listener and may be
1.43 djm 1251: .Sm off
1.49 jmc 1252: .Oo Ar bind_address : Oc Ar port
1.43 djm 1253: .Sm on
1.324 dtucker 1254: or a Unix domain socket path.
1255: The second argument is the destination and may be
1256: .Ar host : Ns Ar hostport
1257: or a Unix domain socket path if the remote host supports it.
1258: .Pp
1.138 djm 1259: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1260: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1261: given on the command line.
1.1 stevesk 1262: Only the superuser can forward privileged ports.
1.43 djm 1263: By default, the local port is bound in accordance with the
1264: .Cm GatewayPorts
1265: setting.
1266: However, an explicit
1267: .Ar bind_address
1268: may be used to bind the connection to a specific address.
1269: The
1270: .Ar bind_address
1271: of
1.240 jmc 1272: .Cm localhost
1.46 jmc 1273: indicates that the listening port be bound for local use only, while an
1274: empty address or
1275: .Sq *
1.43 djm 1276: indicates that the port should be available from all interfaces.
1.326 dtucker 1277: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1278: .Sx TOKENS
1.326 dtucker 1279: section and environment variables as described in the
1280: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1281: section.
1.1 stevesk 1282: .It Cm LogLevel
1283: Gives the verbosity level that is used when logging messages from
1.84 jmc 1284: .Xr ssh 1 .
1.1 stevesk 1285: The possible values are:
1.84 jmc 1286: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1287: The default is INFO.
1288: DEBUG and DEBUG1 are equivalent.
1289: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1290: .It Cm LogVerbose
1291: Specify one or more overrides to LogLevel.
1292: An override consists of a pattern lists that matches the source file, function
1293: and line number to force detailed logging for.
1294: For example, an override pattern of:
1295: .Bd -literal -offset indent
1296: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1297: .Ed
1298: .Pp
1299: would enable detailed logging for line 1000 of
1.338 jmc 1300: .Pa kex.c ,
1.337 djm 1301: everything in the
1302: .Fn kex_exchange_identification
1303: function, and all code in the
1304: .Pa packet.c
1305: file.
1306: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1307: .It Cm MACs
1308: Specifies the MAC (message authentication code) algorithms
1309: in order of preference.
1.226 jmc 1310: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1311: Multiple algorithms must be comma-separated.
1.299 kn 1312: If the specified list begins with a
1.214 djm 1313: .Sq +
1314: character, then the specified algorithms will be appended to the default set
1315: instead of replacing them.
1.299 kn 1316: If the specified list begins with a
1.241 djm 1317: .Sq -
1318: character, then the specified algorithms (including wildcards) will be removed
1319: from the default set instead of replacing them.
1.301 naddy 1320: If the specified list begins with a
1321: .Sq ^
1322: character, then the specified algorithms will be placed at the head of the
1323: default set.
1.214 djm 1324: .Pp
1.160 markus 1325: The algorithms that contain
1.240 jmc 1326: .Qq -etm
1.160 markus 1327: calculate the MAC after encryption (encrypt-then-mac).
1328: These are considered safer and their use recommended.
1.214 djm 1329: .Pp
1.84 jmc 1330: The default is:
1.101 jmc 1331: .Bd -literal -offset indent
1.160 markus 1332: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1333: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1334: hmac-sha1-etm@openssh.com,
1.186 naddy 1335: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1336: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1337: .Ed
1.198 djm 1338: .Pp
1.240 jmc 1339: The list of available MAC algorithms may also be obtained using
1340: .Qq ssh -Q mac .
1.1 stevesk 1341: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1342: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1343: The argument to this keyword must be
1.240 jmc 1344: .Cm yes
1.1 stevesk 1345: or
1.242 jmc 1346: .Cm no
1.240 jmc 1347: (the default).
1.1 stevesk 1348: .It Cm NumberOfPasswordPrompts
1349: Specifies the number of password prompts before giving up.
1350: The argument to this keyword must be an integer.
1.84 jmc 1351: The default is 3.
1.1 stevesk 1352: .It Cm PasswordAuthentication
1353: Specifies whether to use password authentication.
1354: The argument to this keyword must be
1.240 jmc 1355: .Cm yes
1356: (the default)
1.1 stevesk 1357: or
1.240 jmc 1358: .Cm no .
1.65 reyk 1359: .It Cm PermitLocalCommand
1360: Allow local command execution via the
1361: .Ic LocalCommand
1362: option or using the
1.66 jmc 1363: .Ic !\& Ns Ar command
1.65 reyk 1364: escape sequence in
1365: .Xr ssh 1 .
1366: The argument must be
1.240 jmc 1367: .Cm yes
1.65 reyk 1368: or
1.240 jmc 1369: .Cm no
1370: (the default).
1.347 markus 1371: .It Cm PermitRemoteOpen
1372: Specifies the destinations to which remote TCP port forwarding is permitted when
1373: .Cm RemoteForward
1374: is used as a SOCKS proxy.
1375: The forwarding specification must be one of the following forms:
1376: .Pp
1377: .Bl -item -offset indent -compact
1378: .It
1379: .Cm PermitRemoteOpen
1380: .Sm off
1381: .Ar host : port
1382: .Sm on
1383: .It
1384: .Cm PermitRemoteOpen
1385: .Sm off
1386: .Ar IPv4_addr : port
1387: .Sm on
1388: .It
1389: .Cm PermitRemoteOpen
1390: .Sm off
1391: .Ar \&[ IPv6_addr \&] : port
1392: .Sm on
1393: .El
1394: .Pp
1395: Multiple forwards may be specified by separating them with whitespace.
1396: An argument of
1397: .Cm any
1398: can be used to remove all restrictions and permit any forwarding requests.
1399: An argument of
1400: .Cm none
1401: can be used to prohibit all forwarding requests.
1402: The wildcard
1403: .Sq *
1404: can be used for host or port to allow all hosts or ports respectively.
1405: Otherwise, no pattern matching or address lookups are performed on supplied
1406: names.
1.127 markus 1407: .It Cm PKCS11Provider
1.292 djm 1408: Specifies which PKCS#11 provider to use or
1409: .Cm none
1410: to indicate that no provider should be used (the default).
1411: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1412: .Xr ssh 1
1.292 djm 1413: should use to communicate with a PKCS#11 token providing keys for user
1414: authentication.
1.67 jmc 1415: .It Cm Port
1416: Specifies the port number to connect on the remote host.
1.84 jmc 1417: The default is 22.
1.1 stevesk 1418: .It Cm PreferredAuthentications
1.226 jmc 1419: Specifies the order in which the client should try authentication methods.
1.48 jmc 1420: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1421: .Cm keyboard-interactive )
1.48 jmc 1422: over another method (e.g.\&
1.131 jmc 1423: .Cm password ) .
1424: The default is:
1425: .Bd -literal -offset indent
1426: gssapi-with-mic,hostbased,publickey,
1427: keyboard-interactive,password
1428: .Ed
1.1 stevesk 1429: .It Cm ProxyCommand
1430: Specifies the command to use to connect to the server.
1431: The command
1.190 djm 1432: string extends to the end of the line, and is executed
1433: using the user's shell
1434: .Ql exec
1435: directive to avoid a lingering shell process.
1436: .Pp
1.239 jmc 1437: Arguments to
1438: .Cm ProxyCommand
1439: accept the tokens described in the
1440: .Sx TOKENS
1441: section.
1.1 stevesk 1442: The command can be basically anything,
1443: and should read from its standard input and write to its standard output.
1444: It should eventually connect an
1445: .Xr sshd 8
1446: server running on some machine, or execute
1447: .Ic sshd -i
1448: somewhere.
1449: Host key management will be done using the
1.296 jmc 1450: .Cm Hostname
1451: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1452: Setting the command to
1.240 jmc 1453: .Cm none
1.6 markus 1454: disables this option entirely.
1.1 stevesk 1455: Note that
1456: .Cm CheckHostIP
1457: is not available for connects with a proxy command.
1.52 djm 1458: .Pp
1459: This directive is useful in conjunction with
1460: .Xr nc 1
1461: and its proxy support.
1.53 jmc 1462: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1463: 192.0.2.0:
1464: .Bd -literal -offset 3n
1465: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1466: .Ed
1.233 djm 1467: .It Cm ProxyJump
1.260 millert 1468: Specifies one or more jump proxies as either
1.233 djm 1469: .Xo
1470: .Sm off
1.234 jmc 1471: .Op Ar user No @
1.233 djm 1472: .Ar host
1.234 jmc 1473: .Op : Ns Ar port
1.233 djm 1474: .Sm on
1.260 millert 1475: or an ssh URI
1.233 djm 1476: .Xc .
1.235 djm 1477: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1478: sequentially.
1.233 djm 1479: Setting this option will cause
1480: .Xr ssh 1
1481: to connect to the target host by first making a
1482: .Xr ssh 1
1483: connection to the specified
1484: .Cm ProxyJump
1485: host and then establishing a
1.234 jmc 1486: TCP forwarding to the ultimate target from there.
1.346 dlg 1487: Setting the host to
1488: .Cm none
1489: disables this option entirely.
1.233 djm 1490: .Pp
1491: Note that this option will compete with the
1492: .Cm ProxyCommand
1493: option - whichever is specified first will prevent later instances of the
1494: other from taking effect.
1.289 djm 1495: .Pp
1496: Note also that the configuration for the destination host (either supplied
1497: via the command-line or the configuration file) is not generally applied
1498: to jump hosts.
1499: .Pa ~/.ssh/config
1500: should be used if specific configuration is required for jump hosts.
1.167 djm 1501: .It Cm ProxyUseFdpass
1.168 jmc 1502: Specifies that
1.167 djm 1503: .Cm ProxyCommand
1504: will pass a connected file descriptor back to
1.168 jmc 1505: .Xr ssh 1
1.167 djm 1506: instead of continuing to execute and pass data.
1507: The default is
1.240 jmc 1508: .Cm no .
1.343 dtucker 1509: .It Cm PubkeyAcceptedAlgorithms
1510: Specifies the signature algorithms that will be used for public key
1511: authentication as a comma-separated list of patterns.
1.299 kn 1512: If the specified list begins with a
1.214 djm 1513: .Sq +
1.343 dtucker 1514: character, then the algorithms after it will be appended to the default
1.214 djm 1515: instead of replacing it.
1.299 kn 1516: If the specified list begins with a
1.241 djm 1517: .Sq -
1.343 dtucker 1518: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1519: from the default set instead of replacing them.
1.301 naddy 1520: If the specified list begins with a
1521: .Sq ^
1.343 dtucker 1522: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1523: default set.
1.213 markus 1524: The default for this option is:
1525: .Bd -literal -offset 3n
1.333 djm 1526: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1527: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1528: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1529: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1530: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1531: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1532: rsa-sha2-512-cert-v01@openssh.com,
1533: rsa-sha2-256-cert-v01@openssh.com,
1.333 djm 1534: ssh-ed25519,
1.311 naddy 1535: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1536: sk-ssh-ed25519@openssh.com,
1.305 naddy 1537: sk-ecdsa-sha2-nistp256@openssh.com,
1.366 kn 1538: rsa-sha2-512,rsa-sha2-256
1.213 markus 1539: .Ed
1540: .Pp
1.348 djm 1541: The list of available signature algorithms may also be obtained using
1.343 dtucker 1542: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1543: .It Cm PubkeyAuthentication
1544: Specifies whether to try public key authentication.
1545: The argument to this keyword must be
1.240 jmc 1546: .Cm yes
1.369 djm 1547: (the default),
1548: .Cm no ,
1549: .Cm unbound
1.1 stevesk 1550: or
1.369 djm 1551: .Cm host-bound .
1552: The final two options enable public key authentication while respectively
1553: disabling or enabling the OpenSSH host-bound authentication protocol
1554: extension required for restricted
1555: .Xr ssh-agent 1
1556: forwarding.
1.75 dtucker 1557: .It Cm RekeyLimit
1.373 djm 1558: Specifies the maximum amount of data that may be transmitted or received
1559: before the session key is renegotiated, optionally followed by a maximum
1560: amount of time that may pass before the session key is renegotiated.
1.162 dtucker 1561: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1562: .Sq K ,
1563: .Sq M ,
1.75 dtucker 1564: or
1.76 jmc 1565: .Sq G
1.75 dtucker 1566: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1567: The default is between
1.84 jmc 1568: .Sq 1G
1.75 dtucker 1569: and
1.84 jmc 1570: .Sq 4G ,
1.75 dtucker 1571: depending on the cipher.
1.162 dtucker 1572: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1573: units documented in the TIME FORMATS section of
1.162 dtucker 1574: .Xr sshd_config 5 .
1575: The default value for
1576: .Cm RekeyLimit
1577: is
1.240 jmc 1578: .Cm default none ,
1.162 dtucker 1579: which means that rekeying is performed after the cipher's default amount
1580: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1581: .It Cm RemoteCommand
1582: Specifies a command to execute on the remote machine after successfully
1583: connecting to the server.
1584: The command string extends to the end of the line, and is executed with
1585: the user's shell.
1.250 jmc 1586: Arguments to
1587: .Cm RemoteCommand
1588: accept the tokens described in the
1589: .Sx TOKENS
1590: section.
1.1 stevesk 1591: .It Cm RemoteForward
1.74 jmc 1592: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1593: the secure channel.
1.273 djm 1594: The remote port may either be forwarded to a specified host and port
1.256 markus 1595: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1596: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1597: The first argument is the listening specification and may be
1.43 djm 1598: .Sm off
1.49 jmc 1599: .Oo Ar bind_address : Oc Ar port
1.43 djm 1600: .Sm on
1.324 dtucker 1601: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1602: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1603: .Ar host : Ns Ar hostport
1604: or a Unix domain socket path,
1.256 markus 1605: otherwise if no destination argument is specified then the remote forwarding
1606: will be established as a SOCKS proxy.
1.370 naddy 1607: When acting as a SOCKS proxy, the destination of the connection can be
1.347 markus 1608: restricted by
1609: .Cm PermitRemoteOpen .
1.256 markus 1610: .Pp
1.138 djm 1611: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1612: Multiple forwardings may be specified, and additional
1613: forwardings can be given on the command line.
1.113 stevesk 1614: Privileged ports can be forwarded only when
1615: logging in as root on the remote machine.
1.326 dtucker 1616: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1617: .Sx TOKENS
1.326 dtucker 1618: section and environment variables as described in the
1619: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1620: section.
1.118 jmc 1621: .Pp
1.117 djm 1622: If the
1623: .Ar port
1.240 jmc 1624: argument is 0,
1.117 djm 1625: the listen port will be dynamically allocated on the server and reported
1626: to the client at run time.
1.43 djm 1627: .Pp
1628: If the
1629: .Ar bind_address
1630: is not specified, the default is to only bind to loopback addresses.
1631: If the
1632: .Ar bind_address
1633: is
1634: .Ql *
1635: or an empty string, then the forwarding is requested to listen on all
1636: interfaces.
1637: Specifying a remote
1638: .Ar bind_address
1.46 jmc 1639: will only succeed if the server's
1640: .Cm GatewayPorts
1.43 djm 1641: option is enabled (see
1.46 jmc 1642: .Xr sshd_config 5 ) .
1.149 djm 1643: .It Cm RequestTTY
1644: Specifies whether to request a pseudo-tty for the session.
1645: The argument may be one of:
1.240 jmc 1646: .Cm no
1.149 djm 1647: (never request a TTY),
1.240 jmc 1648: .Cm yes
1.149 djm 1649: (always request a TTY when standard input is a TTY),
1.240 jmc 1650: .Cm force
1.149 djm 1651: (always request a TTY) or
1.240 jmc 1652: .Cm auto
1.149 djm 1653: (request a TTY when opening a login session).
1654: This option mirrors the
1655: .Fl t
1656: and
1657: .Fl T
1658: flags for
1659: .Xr ssh 1 .
1.374 djm 1660: .It Cm RequiredRSASize
1661: Specifies the minimum RSA key size (in bits) that
1662: .Xr ssh 1
1663: will accept.
1664: User authentication keys smaller than this limit will be ignored.
1665: Servers that present host keys smaller than this limit will cause the
1666: connection to be terminated.
1667: The default is
1668: .Cm 1024
1669: bits.
1670: Note that this limit may only be raised from the default.
1.196 djm 1671: .It Cm RevokedHostKeys
1672: Specifies revoked host public keys.
1673: Keys listed in this file will be refused for host authentication.
1674: Note that if this file does not exist or is not readable,
1675: then host authentication will be refused for all hosts.
1676: Keys may be specified as a text file, listing one public key per line, or as
1677: an OpenSSH Key Revocation List (KRL) as generated by
1678: .Xr ssh-keygen 1 .
1679: For more information on KRLs, see the KEY REVOCATION LISTS section in
1680: .Xr ssh-keygen 1 .
1.380 dtucker 1681: Arguments to
1682: .Cm RevokedHostKeys
1683: may use the tilde syntax to refer to a user's home directory,
1684: the tokens described in the
1685: .Sx TOKENS
1686: section and environment variables as described in the
1687: .Sx ENVIRONMENT VARIABLES
1688: section.
1.305 naddy 1689: .It Cm SecurityKeyProvider
1.313 naddy 1690: Specifies a path to a library that will be used when loading any
1691: FIDO authenticator-hosted keys, overriding the default of using
1692: the built-in USB HID support.
1.309 naddy 1693: .Pp
1694: If the specified value begins with a
1695: .Sq $
1696: character, then it will be treated as an environment variable containing
1697: the path to the library.
1.32 djm 1698: .It Cm SendEnv
1699: Specifies what variables from the local
1700: .Xr environ 7
1701: should be sent to the server.
1.84 jmc 1702: The server must also support it, and the server must be configured to
1.33 djm 1703: accept these environment variables.
1.207 dtucker 1704: Note that the
1705: .Ev TERM
1.208 jmc 1706: environment variable is always sent whenever a
1.207 dtucker 1707: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1708: Refer to
1709: .Cm AcceptEnv
1710: in
1711: .Xr sshd_config 5
1712: for how to configure the server.
1.80 jmc 1713: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1714: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1715: across multiple
1716: .Cm SendEnv
1717: directives.
1.81 jmc 1718: .Pp
1719: See
1720: .Sx PATTERNS
1721: for more information on patterns.
1.271 djm 1722: .Pp
1.272 jmc 1723: It is possible to clear previously set
1.271 djm 1724: .Cm SendEnv
1725: variable names by prefixing patterns with
1726: .Pa - .
1727: The default is not to send any environment variables.
1.28 markus 1728: .It Cm ServerAliveCountMax
1.73 jmc 1729: Sets the number of server alive messages (see below) which may be
1.28 markus 1730: sent without
1.84 jmc 1731: .Xr ssh 1
1.28 markus 1732: receiving any messages back from the server.
1733: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1734: ssh will disconnect from the server, terminating the session.
1.28 markus 1735: It is important to note that the use of server alive messages is very
1736: different from
1737: .Cm TCPKeepAlive
1738: (below).
1739: The server alive messages are sent through the encrypted channel
1740: and therefore will not be spoofable.
1741: The TCP keepalive option enabled by
1742: .Cm TCPKeepAlive
1743: is spoofable.
1744: The server alive mechanism is valuable when the client or
1.298 dtucker 1745: server depend on knowing when a connection has become unresponsive.
1.28 markus 1746: .Pp
1747: The default value is 3.
1748: If, for example,
1749: .Cm ServerAliveInterval
1.84 jmc 1750: (see below) is set to 15 and
1.28 markus 1751: .Cm ServerAliveCountMax
1.84 jmc 1752: is left at the default, if the server becomes unresponsive,
1753: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1754: .It Cm ServerAliveInterval
1755: Sets a timeout interval in seconds after which if no data has been received
1756: from the server,
1.84 jmc 1757: .Xr ssh 1
1.67 jmc 1758: will send a message through the encrypted
1759: channel to request a response from the server.
1760: The default
1761: is 0, indicating that these messages will not be sent to the server.
1.357 jmc 1762: .It Cm SessionType
1763: May be used to either request invocation of a subsystem on the remote system,
1764: or to prevent the execution of a remote command at all.
1765: The latter is useful for just forwarding ports.
1766: The argument to this keyword must be
1767: .Cm none
1768: (same as the
1769: .Fl N
1770: option),
1771: .Cm subsystem
1772: (same as the
1773: .Fl s
1774: option) or
1775: .Cm default
1776: (shell or command execution).
1.277 jmc 1777: .It Cm SetEnv
1778: Directly specify one or more environment variables and their contents to
1779: be sent to the server.
1780: Similarly to
1781: .Cm SendEnv ,
1.354 djm 1782: with the exception of the
1783: .Ev TERM
1784: variable, the server must be prepared to accept the environment variable.
1.358 djm 1785: .It Cm StdinNull
1786: Redirects stdin from
1787: .Pa /dev/null
1788: (actually, prevents reading from stdin).
1789: Either this or the equivalent
1790: .Fl n
1791: option must be used when
1792: .Nm ssh
1793: is run in the background.
1794: The argument to this keyword must be
1795: .Cm yes
1796: (same as the
1797: .Fl n
1798: option) or
1799: .Cm no
1800: (the default).
1.191 millert 1801: .It Cm StreamLocalBindMask
1802: Sets the octal file creation mode mask
1803: .Pq umask
1804: used when creating a Unix-domain socket file for local or remote
1805: port forwarding.
1806: This option is only used for port forwarding to a Unix-domain socket file.
1807: .Pp
1808: The default value is 0177, which creates a Unix-domain socket file that is
1809: readable and writable only by the owner.
1810: Note that not all operating systems honor the file mode on Unix-domain
1811: socket files.
1812: .It Cm StreamLocalBindUnlink
1813: Specifies whether to remove an existing Unix-domain socket file for local
1814: or remote port forwarding before creating a new one.
1815: If the socket file already exists and
1816: .Cm StreamLocalBindUnlink
1817: is not enabled,
1818: .Nm ssh
1819: will be unable to forward the port to the Unix-domain socket file.
1820: This option is only used for port forwarding to a Unix-domain socket file.
1821: .Pp
1822: The argument must be
1.240 jmc 1823: .Cm yes
1.191 millert 1824: or
1.240 jmc 1825: .Cm no
1826: (the default).
1.1 stevesk 1827: .It Cm StrictHostKeyChecking
1828: If this flag is set to
1.240 jmc 1829: .Cm yes ,
1.84 jmc 1830: .Xr ssh 1
1.1 stevesk 1831: will never automatically add host keys to the
1.50 djm 1832: .Pa ~/.ssh/known_hosts
1.1 stevesk 1833: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1834: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1835: though it can be annoying when the
1.1 stevesk 1836: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1837: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1838: frequently made.
1839: This option forces the user to manually
1840: add all new hosts.
1.255 jmc 1841: .Pp
1.1 stevesk 1842: If this flag is set to
1.364 dtucker 1843: .Cm accept-new
1.360 jmc 1844: then ssh will automatically add new host keys to the user's
1845: .Pa known_hosts
1846: file, but will not permit connections to hosts with
1.254 djm 1847: changed host keys.
1848: If this flag is set to
1.364 dtucker 1849: .Cm no
1.254 djm 1850: or
1.364 dtucker 1851: .Cm off ,
1.255 jmc 1852: ssh will automatically add new host keys to the user known hosts files
1853: and allow connections to hosts with changed hostkeys to proceed,
1854: subject to some restrictions.
1.1 stevesk 1855: If this flag is set to
1.240 jmc 1856: .Cm ask
1857: (the default),
1.1 stevesk 1858: new host keys
1859: will be added to the user known host files only after the user
1860: has confirmed that is what they really want to do, and
1.84 jmc 1861: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1862: The host keys of
1863: known hosts will be verified automatically in all cases.
1.244 jmc 1864: .It Cm SyslogFacility
1865: Gives the facility code that is used when logging messages from
1866: .Xr ssh 1 .
1867: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1868: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1869: The default is USER.
1.26 markus 1870: .It Cm TCPKeepAlive
1871: Specifies whether the system should send TCP keepalive messages to the
1872: other side.
1873: If they are sent, death of the connection or crash of one
1874: of the machines will be properly noticed.
1875: However, this means that
1876: connections will die if the route is down temporarily, and some people
1877: find it annoying.
1878: .Pp
1879: The default is
1.240 jmc 1880: .Cm yes
1.26 markus 1881: (to send TCP keepalive messages), and the client will notice
1882: if the network goes down or the remote host dies.
1883: This is important in scripts, and many users want it too.
1884: .Pp
1885: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1886: .Cm no .
1.265 djm 1887: See also
1888: .Cm ServerAliveInterval
1889: for protocol-level keepalives.
1.65 reyk 1890: .It Cm Tunnel
1.95 stevesk 1891: Request
1.65 reyk 1892: .Xr tun 4
1.69 jmc 1893: device forwarding between the client and the server.
1.65 reyk 1894: The argument must be
1.240 jmc 1895: .Cm yes ,
1896: .Cm point-to-point
1.95 stevesk 1897: (layer 3),
1.240 jmc 1898: .Cm ethernet
1.95 stevesk 1899: (layer 2),
1.65 reyk 1900: or
1.240 jmc 1901: .Cm no
1902: (the default).
1.95 stevesk 1903: Specifying
1.240 jmc 1904: .Cm yes
1.95 stevesk 1905: requests the default tunnel mode, which is
1.240 jmc 1906: .Cm point-to-point .
1.65 reyk 1907: .It Cm TunnelDevice
1.95 stevesk 1908: Specifies the
1.65 reyk 1909: .Xr tun 4
1.95 stevesk 1910: devices to open on the client
1911: .Pq Ar local_tun
1912: and the server
1913: .Pq Ar remote_tun .
1914: .Pp
1915: The argument must be
1916: .Sm off
1917: .Ar local_tun Op : Ar remote_tun .
1918: .Sm on
1919: The devices may be specified by numerical ID or the keyword
1.240 jmc 1920: .Cm any ,
1.95 stevesk 1921: which uses the next available tunnel device.
1922: If
1923: .Ar remote_tun
1924: is not specified, it defaults to
1.240 jmc 1925: .Cm any .
1.95 stevesk 1926: The default is
1.240 jmc 1927: .Cm any:any .
1.201 djm 1928: .It Cm UpdateHostKeys
1.200 djm 1929: Specifies whether
1930: .Xr ssh 1
1931: should accept notifications of additional hostkeys from the server sent
1932: after authentication has completed and add them to
1933: .Cm UserKnownHostsFile .
1934: The argument must be
1.240 jmc 1935: .Cm yes ,
1936: .Cm no
1.320 djm 1937: or
1.240 jmc 1938: .Cm ask .
1.320 djm 1939: This option allows learning alternate hostkeys for a server
1.201 djm 1940: and supports graceful key rotation by allowing a server to send replacement
1941: public keys before old ones are removed.
1.336 djm 1942: .Pp
1.200 djm 1943: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1944: host was already trusted or explicitly accepted by the user, the host was
1945: authenticated via
1946: .Cm UserKnownHostsFile
1947: (i.e. not
1948: .Cm GlobalKnownHostsFile )
1949: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1950: .Pp
1951: .Cm UpdateHostKeys
1.321 jmc 1952: is enabled by default if the user has not overridden the default
1.320 djm 1953: .Cm UserKnownHostsFile
1.335 djm 1954: setting and has not enabled
1955: .Cm VerifyHostKeyDNS ,
1956: otherwise
1.320 djm 1957: .Cm UpdateHostKeys
1958: will be set to
1.334 djm 1959: .Cm no .
1.320 djm 1960: .Pp
1.204 djm 1961: If
1962: .Cm UpdateHostKeys
1963: is set to
1.240 jmc 1964: .Cm ask ,
1.204 djm 1965: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1966: Confirmation is currently incompatible with
1967: .Cm ControlPersist ,
1968: and will be disabled if it is enabled.
1.200 djm 1969: .Pp
1970: Presently, only
1971: .Xr sshd 8
1972: from OpenSSH 6.8 and greater support the
1.240 jmc 1973: .Qq hostkeys@openssh.com
1.200 djm 1974: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1975: .It Cm User
1976: Specifies the user to log in as.
1977: This can be useful when a different user name is used on different machines.
1978: This saves the trouble of
1979: having to remember to give the user name on the command line.
1980: .It Cm UserKnownHostsFile
1.151 djm 1981: Specifies one or more files to use for the user
1982: host key database, separated by whitespace.
1.329 dtucker 1983: Each filename may use tilde notation to refer to the user's home directory,
1984: the tokens described in the
1985: .Sx TOKENS
1986: section and environment variables as described in the
1987: .Sx ENVIRONMENT VARIABLES
1988: section.
1.377 dtucker 1989: A value of
1990: .Cm none
1991: causes
1992: .Xr ssh 1
1.378 jmc 1993: to ignore any user-specific known hosts files.
1.151 djm 1994: The default is
1995: .Pa ~/.ssh/known_hosts ,
1996: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1997: .It Cm VerifyHostKeyDNS
1998: Specifies whether to verify the remote key using DNS and SSHFP resource
1999: records.
1.24 jakob 2000: If this option is set to
1.240 jmc 2001: .Cm yes ,
1.25 jmc 2002: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 2003: from DNS.
2004: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 2005: .Cm ask .
1.24 jakob 2006: If this option is set to
1.240 jmc 2007: .Cm ask ,
1.24 jakob 2008: information on fingerprint match will be displayed, but the user will still
2009: need to confirm new host keys according to the
2010: .Cm StrictHostKeyChecking
2011: option.
1.8 jakob 2012: The default is
1.240 jmc 2013: .Cm no .
1.84 jmc 2014: .Pp
1.240 jmc 2015: See also
2016: .Sx VERIFYING HOST KEYS
2017: in
1.84 jmc 2018: .Xr ssh 1 .
1.111 grunk 2019: .It Cm VisualHostKey
2020: If this flag is set to
1.240 jmc 2021: .Cm yes ,
1.111 grunk 2022: an ASCII art representation of the remote host key fingerprint is
1.197 djm 2023: printed in addition to the fingerprint string at login and
1.114 stevesk 2024: for unknown host keys.
1.111 grunk 2025: If this flag is set to
1.240 jmc 2026: .Cm no
2027: (the default),
1.114 stevesk 2028: no fingerprint strings are printed at login and
1.197 djm 2029: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 2030: .It Cm XAuthLocation
1.5 stevesk 2031: Specifies the full pathname of the
1.1 stevesk 2032: .Xr xauth 1
2033: program.
2034: The default is
2035: .Pa /usr/X11R6/bin/xauth .
2036: .El
1.86 jmc 2037: .Sh PATTERNS
2038: A
2039: .Em pattern
2040: consists of zero or more non-whitespace characters,
2041: .Sq *
2042: (a wildcard that matches zero or more characters),
2043: or
2044: .Sq ?\&
2045: (a wildcard that matches exactly one character).
2046: For example, to specify a set of declarations for any host in the
1.240 jmc 2047: .Qq .co.uk
1.86 jmc 2048: set of domains,
2049: the following pattern could be used:
2050: .Pp
2051: .Dl Host *.co.uk
2052: .Pp
2053: The following pattern
2054: would match any host in the 192.168.0.[0-9] network range:
2055: .Pp
2056: .Dl Host 192.168.0.?
2057: .Pp
2058: A
2059: .Em pattern-list
2060: is a comma-separated list of patterns.
2061: Patterns within pattern-lists may be negated
2062: by preceding them with an exclamation mark
2063: .Pq Sq !\& .
2064: For example,
1.174 djm 2065: to allow a key to be used from anywhere within an organization
1.86 jmc 2066: except from the
1.240 jmc 2067: .Qq dialup
1.86 jmc 2068: pool,
2069: the following entry (in authorized_keys) could be used:
2070: .Pp
2071: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 2072: .Pp
2073: Note that a negated match will never produce a positive result by itself.
2074: For example, attempting to match
2075: .Qq host3
2076: against the following pattern-list will fail:
2077: .Pp
2078: .Dl from=\&"!host1,!host2\&"
2079: .Pp
2080: The solution here is to include a term that will yield a positive match,
2081: such as a wildcard:
2082: .Pp
2083: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 2084: .Sh TOKENS
2085: Arguments to some keywords can make use of tokens,
2086: which are expanded at runtime:
2087: .Pp
2088: .Bl -tag -width XXXX -offset indent -compact
2089: .It %%
2090: A literal
2091: .Sq % .
2092: .It \&%C
1.257 jmc 2093: Hash of %l%h%p%r.
1.239 jmc 2094: .It %d
2095: Local user's home directory.
1.339 djm 2096: .It %f
2097: The fingerprint of the server's host key.
2098: .It %H
2099: The
2100: .Pa known_hosts
2101: hostname or address that is being searched for.
1.239 jmc 2102: .It %h
2103: The remote hostname.
1.340 jmc 2104: .It \%%I
1.339 djm 2105: A string describing the reason for a
2106: .Cm KnownHostsCommand
1.340 jmc 2107: execution: either
2108: .Cm ADDRESS
1.339 djm 2109: when looking up a host by address (only when
2110: .Cm CheckHostIP
2111: is enabled),
1.340 jmc 2112: .Cm HOSTNAME
2113: when searching by hostname, or
2114: .Cm ORDER
1.339 djm 2115: when preparing the host key algorithm preference list to use for the
2116: destination host.
1.239 jmc 2117: .It %i
2118: The local user ID.
1.339 djm 2119: .It %K
2120: The base64 encoded host key.
1.330 dtucker 2121: .It %k
1.350 jsg 2122: The host key alias if specified, otherwise the original remote hostname given
1.330 dtucker 2123: on the command line.
1.239 jmc 2124: .It %L
2125: The local hostname.
2126: .It %l
2127: The local hostname, including the domain name.
2128: .It %n
2129: The original remote hostname, as given on the command line.
2130: .It %p
2131: The remote port.
2132: .It %r
2133: The remote username.
1.261 djm 2134: .It \&%T
2135: The local
2136: .Xr tun 4
2137: or
2138: .Xr tap 4
2139: network interface assigned if
1.262 jmc 2140: tunnel forwarding was requested, or
2141: .Qq NONE
1.261 djm 2142: otherwise.
1.339 djm 2143: .It %t
2144: The type of the server host key, e.g.
1.360 jmc 2145: .Cm ssh-ed25519 .
1.239 jmc 2146: .It %u
2147: The local username.
2148: .El
2149: .Pp
1.323 dtucker 2150: .Cm CertificateFile ,
2151: .Cm ControlPath ,
2152: .Cm IdentityAgent ,
2153: .Cm IdentityFile ,
1.339 djm 2154: .Cm KnownHostsCommand ,
1.325 jmc 2155: .Cm LocalForward ,
1.324 dtucker 2156: .Cm Match exec ,
2157: .Cm RemoteCommand ,
1.329 dtucker 2158: .Cm RemoteForward ,
1.380 dtucker 2159: .Cm RevokedHostKeys ,
1.323 dtucker 2160: and
1.331 jmc 2161: .Cm UserKnownHostsFile
1.349 dtucker 2162: accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2163: .Pp
1.339 djm 2164: .Cm KnownHostsCommand
2165: additionally accepts the tokens %f, %H, %I, %K and %t.
2166: .Pp
1.295 jmc 2167: .Cm Hostname
1.239 jmc 2168: accepts the tokens %% and %h.
2169: .Pp
2170: .Cm LocalCommand
1.323 dtucker 2171: accepts all tokens.
1.239 jmc 2172: .Pp
2173: .Cm ProxyCommand
1.372 dtucker 2174: and
2175: .Cm ProxyJump
2176: accept the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2177: .Sh ENVIRONMENT VARIABLES
2178: Arguments to some keywords can be expanded at runtime from environment
2179: variables on the client by enclosing them in
2180: .Ic ${} ,
2181: for example
2182: .Ic ${HOME}/.ssh
2183: would refer to the user's .ssh directory.
2184: If a specified environment variable does not exist then an error will be
2185: returned and the setting for that keyword will be ignored.
2186: .Pp
2187: The keywords
2188: .Cm CertificateFile ,
2189: .Cm ControlPath ,
1.329 dtucker 2190: .Cm IdentityAgent ,
1.352 jmc 2191: .Cm IdentityFile ,
1.339 djm 2192: .Cm KnownHostsCommand ,
1.326 dtucker 2193: and
1.329 dtucker 2194: .Cm UserKnownHostsFile
1.326 dtucker 2195: support environment variables.
2196: The keywords
2197: .Cm LocalForward
2198: and
2199: .Cm RemoteForward
2200: support environment variables only for Unix domain socket paths.
1.1 stevesk 2201: .Sh FILES
2202: .Bl -tag -width Ds
1.50 djm 2203: .It Pa ~/.ssh/config
1.1 stevesk 2204: This is the per-user configuration file.
2205: The format of this file is described above.
1.84 jmc 2206: This file is used by the SSH client.
1.30 djm 2207: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2208: read/write for the user, and not writable by others.
1.1 stevesk 2209: .It Pa /etc/ssh/ssh_config
2210: Systemwide configuration file.
2211: This file provides defaults for those
2212: values that are not specified in the user's configuration file, and
2213: for those users who do not have a configuration file.
2214: This file must be world-readable.
2215: .El
1.13 jmc 2216: .Sh SEE ALSO
2217: .Xr ssh 1
1.1 stevesk 2218: .Sh AUTHORS
1.240 jmc 2219: .An -nosplit
1.1 stevesk 2220: OpenSSH is a derivative of the original and free
1.240 jmc 2221: ssh 1.2.12 release by
2222: .An Tatu Ylonen .
2223: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2224: .An Niels Provos , Theo de Raadt
2225: and
2226: .An Dug Song
1.1 stevesk 2227: removed many bugs, re-added newer features and
2228: created OpenSSH.
1.240 jmc 2229: .An Markus Friedl
2230: contributed the support for SSH protocol versions 1.5 and 2.0.
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