Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.347
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.347 ! markus 36: .\" $OpenBSD: ssh_config.5,v 1.346 2021/02/15 11:09:22 dlg Exp $
! 37: .Dd $Mdocdate: February 15 2021 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
1.310 jmc 42: .Nd OpenSSH client configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.84 jmc 44: .Xr ssh 1
1.1 stevesk 45: obtains configuration data from the following sources in
46: the following order:
1.79 jmc 47: .Pp
1.2 stevesk 48: .Bl -enum -offset indent -compact
49: .It
50: command-line options
51: .It
52: user's configuration file
1.50 djm 53: .Pq Pa ~/.ssh/config
1.2 stevesk 54: .It
55: system-wide configuration file
56: .Pq Pa /etc/ssh/ssh_config
57: .El
1.1 stevesk 58: .Pp
59: For each parameter, the first obtained value
60: will be used.
1.41 jmc 61: The configuration files contain sections separated by
1.240 jmc 62: .Cm Host
1.1 stevesk 63: specifications, and that section is only applied for hosts that
64: match one of the patterns given in the specification.
1.193 djm 65: The matched host name is usually the one given on the command line
66: (see the
67: .Cm CanonicalizeHostname
1.240 jmc 68: option for exceptions).
1.1 stevesk 69: .Pp
70: Since the first obtained value for each parameter is used, more
71: host-specific declarations should be given near the beginning of the
72: file, and general defaults at the end.
1.80 jmc 73: .Pp
1.240 jmc 74: The file contains keyword-argument pairs, one per line.
75: Lines starting with
1.1 stevesk 76: .Ql #
1.240 jmc 77: and empty lines are interpreted as comments.
78: Arguments may optionally be enclosed in double quotes
79: .Pq \&"
80: in order to represent arguments containing spaces.
1.1 stevesk 81: Configuration options may be separated by whitespace or
82: optional whitespace and exactly one
83: .Ql = ;
84: the latter format is useful to avoid the need to quote whitespace
85: when specifying configuration options using the
86: .Nm ssh ,
1.87 jmc 87: .Nm scp ,
1.1 stevesk 88: and
89: .Nm sftp
90: .Fl o
91: option.
92: .Pp
93: The possible
94: keywords and their meanings are as follows (note that
95: keywords are case-insensitive and arguments are case-sensitive):
96: .Bl -tag -width Ds
97: .It Cm Host
98: Restricts the following declarations (up to the next
99: .Cm Host
1.169 djm 100: or
101: .Cm Match
1.1 stevesk 102: keyword) to be only for those hosts that match one of the patterns
103: given after the keyword.
1.112 krw 104: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 105: A single
1.83 jmc 106: .Ql *
1.1 stevesk 107: as a pattern can be used to provide global
108: defaults for all hosts.
1.193 djm 109: The host is usually the
1.1 stevesk 110: .Ar hostname
1.193 djm 111: argument given on the command line
112: (see the
113: .Cm CanonicalizeHostname
1.240 jmc 114: keyword for exceptions).
1.148 djm 115: .Pp
116: A pattern entry may be negated by prefixing it with an exclamation mark
117: .Pq Sq !\& .
118: If a negated entry is matched, then the
119: .Cm Host
120: entry is ignored, regardless of whether any other patterns on the line
121: match.
122: Negated matches are therefore useful to provide exceptions for wildcard
123: matches.
1.81 jmc 124: .Pp
125: See
126: .Sx PATTERNS
127: for more information on patterns.
1.170 jmc 128: .It Cm Match
1.169 djm 129: Restricts the following declarations (up to the next
130: .Cm Host
131: or
132: .Cm Match
133: keyword) to be used only when the conditions following the
134: .Cm Match
135: keyword are satisfied.
1.220 sobrado 136: Match conditions are specified using one or more criteria
1.178 dtucker 137: or the single token
138: .Cm all
1.193 djm 139: which always matches.
140: The available criteria keywords are:
141: .Cm canonical ,
1.287 djm 142: .Cm final ,
1.176 djm 143: .Cm exec ,
1.169 djm 144: .Cm host ,
145: .Cm originalhost ,
146: .Cm user ,
147: and
148: .Cm localuser .
1.193 djm 149: The
150: .Cm all
151: criteria must appear alone or immediately after
1.287 djm 152: .Cm canonical
153: or
154: .Cm final .
1.193 djm 155: Other criteria may be combined arbitrarily.
156: All criteria but
1.288 jmc 157: .Cm all ,
158: .Cm canonical ,
1.193 djm 159: and
1.287 djm 160: .Cm final
1.193 djm 161: require an argument.
162: Criteria may be negated by prepending an exclamation mark
163: .Pq Sq !\& .
1.169 djm 164: .Pp
1.177 jmc 165: The
1.193 djm 166: .Cm canonical
1.210 dtucker 167: keyword matches only when the configuration file is being re-parsed
1.193 djm 168: after hostname canonicalization (see the
169: .Cm CanonicalizeHostname
1.288 jmc 170: option).
1.193 djm 171: This may be useful to specify conditions that work with canonical host
172: names only.
1.287 djm 173: .Pp
174: The
175: .Cm final
176: keyword requests that the configuration be re-parsed (regardless of whether
177: .Cm CanonicalizeHostname
178: is enabled), and matches only during this final pass.
179: If
180: .Cm CanonicalizeHostname
181: is enabled, then
182: .Cm canonical
183: and
184: .Cm final
185: match during the same pass.
186: .Pp
1.193 djm 187: The
1.176 djm 188: .Cm exec
1.177 jmc 189: keyword executes the specified command under the user's shell.
1.169 djm 190: If the command returns a zero exit status then the condition is considered true.
191: Commands containing whitespace characters must be quoted.
1.239 jmc 192: Arguments to
193: .Cm exec
194: accept the tokens described in the
195: .Sx TOKENS
196: section.
1.169 djm 197: .Pp
198: The other keywords' criteria must be single entries or comma-separated
199: lists and may use the wildcard and negation operators described in the
200: .Sx PATTERNS
201: section.
202: The criteria for the
203: .Cm host
204: keyword are matched against the target hostname, after any substitution
205: by the
1.295 jmc 206: .Cm Hostname
1.193 djm 207: or
208: .Cm CanonicalizeHostname
209: options.
1.169 djm 210: The
211: .Cm originalhost
212: keyword matches against the hostname as it was specified on the command-line.
213: The
214: .Cm user
215: keyword matches against the target username on the remote host.
216: The
217: .Cm localuser
218: keyword matches against the name of the local user running
219: .Xr ssh 1
220: (this keyword may be useful in system-wide
221: .Nm
222: files).
1.222 jcs 223: .It Cm AddKeysToAgent
224: Specifies whether keys should be automatically added to a running
1.223 jmc 225: .Xr ssh-agent 1 .
1.222 jcs 226: If this option is set to
1.240 jmc 227: .Cm yes
1.222 jcs 228: and a key is loaded from a file, the key and its passphrase are added to
229: the agent with the default lifetime, as if by
230: .Xr ssh-add 1 .
231: If this option is set to
1.240 jmc 232: .Cm ask ,
233: .Xr ssh 1
1.222 jcs 234: will require confirmation using the
235: .Ev SSH_ASKPASS
236: program before adding a key (see
237: .Xr ssh-add 1
238: for details).
239: If this option is set to
1.240 jmc 240: .Cm confirm ,
1.222 jcs 241: each use of the key must be confirmed, as if the
242: .Fl c
243: option was specified to
244: .Xr ssh-add 1 .
245: If this option is set to
1.240 jmc 246: .Cm no ,
1.222 jcs 247: no keys are added to the agent.
1.332 djm 248: Alternately, this option may be specified as a time interval
249: using the format described in the
250: .Sx TIME FORMATS
251: section of
252: .Xr sshd_config 5
253: to specify the key's lifetime in
254: .Xr ssh-agent 1 ,
255: after which it will automatically be removed.
1.222 jcs 256: The argument must be
1.332 djm 257: .Cm no
258: (the default),
1.240 jmc 259: .Cm yes ,
1.332 djm 260: .Cm confirm
261: (optionally followed by a time interval),
262: .Cm ask
263: or a time interval.
1.10 djm 264: .It Cm AddressFamily
1.11 jmc 265: Specifies which address family to use when connecting.
266: Valid arguments are
1.240 jmc 267: .Cm any
268: (the default),
269: .Cm inet
1.84 jmc 270: (use IPv4 only), or
1.240 jmc 271: .Cm inet6
1.40 jmc 272: (use IPv6 only).
1.1 stevesk 273: .It Cm BatchMode
274: If set to
1.240 jmc 275: .Cm yes ,
1.318 djm 276: user interaction such as password prompts and host key confirmation requests
277: will be disabled.
1.1 stevesk 278: This option is useful in scripts and other batch jobs where no user
1.318 djm 279: is present to interact with
280: .Xr ssh 1 .
1.1 stevesk 281: The argument must be
1.240 jmc 282: .Cm yes
1.1 stevesk 283: or
1.240 jmc 284: .Cm no
285: (the default).
1.268 jmc 286: .It Cm BindAddress
287: Use the specified address on the local machine as the source address of
288: the connection.
289: Only useful on systems with more than one address.
290: .It Cm BindInterface
291: Use the address of the specified interface on the local machine as the
292: source address of the connection.
1.171 djm 293: .It Cm CanonicalDomains
1.172 jmc 294: When
1.173 djm 295: .Cm CanonicalizeHostname
1.171 djm 296: is enabled, this option specifies the list of domain suffixes in which to
297: search for the specified destination host.
1.173 djm 298: .It Cm CanonicalizeFallbackLocal
1.174 djm 299: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 300: The default,
1.240 jmc 301: .Cm yes ,
1.172 jmc 302: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 303: search rules.
304: A value of
1.240 jmc 305: .Cm no
1.171 djm 306: will cause
307: .Xr ssh 1
308: to fail instantly if
1.173 djm 309: .Cm CanonicalizeHostname
1.171 djm 310: is enabled and the target hostname cannot be found in any of the domains
311: specified by
312: .Cm CanonicalDomains .
1.173 djm 313: .It Cm CanonicalizeHostname
1.174 djm 314: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 315: The default,
1.240 jmc 316: .Cm no ,
1.171 djm 317: is not to perform any name rewriting and let the system resolver handle all
318: hostname lookups.
319: If set to
1.240 jmc 320: .Cm yes
1.171 djm 321: then, for connections that do not use a
1.284 djm 322: .Cm ProxyCommand
323: or
324: .Cm ProxyJump ,
1.171 djm 325: .Xr ssh 1
1.173 djm 326: will attempt to canonicalize the hostname specified on the command line
1.171 djm 327: using the
328: .Cm CanonicalDomains
329: suffixes and
1.173 djm 330: .Cm CanonicalizePermittedCNAMEs
1.171 djm 331: rules.
332: If
1.173 djm 333: .Cm CanonicalizeHostname
1.171 djm 334: is set to
1.240 jmc 335: .Cm always ,
1.174 djm 336: then canonicalization is applied to proxied connections too.
1.185 djm 337: .Pp
1.193 djm 338: If this option is enabled, then the configuration files are processed
339: again using the new target name to pick up any new configuration in matching
1.185 djm 340: .Cm Host
1.193 djm 341: and
342: .Cm Match
1.185 djm 343: stanzas.
1.173 djm 344: .It Cm CanonicalizeMaxDots
1.172 jmc 345: Specifies the maximum number of dot characters in a hostname before
1.174 djm 346: canonicalization is disabled.
1.240 jmc 347: The default, 1,
1.172 jmc 348: allows a single dot (i.e. hostname.subdomain).
1.173 djm 349: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 350: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 351: canonicalizing hostnames.
1.171 djm 352: The rules consist of one or more arguments of
1.172 jmc 353: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 354: where
355: .Ar source_domain_list
1.174 djm 356: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 357: and
358: .Ar target_domain_list
1.172 jmc 359: is a pattern-list of domains that they may resolve to.
1.171 djm 360: .Pp
361: For example,
1.240 jmc 362: .Qq *.a.example.com:*.b.example.com,*.c.example.com
1.171 djm 363: will allow hostnames matching
1.240 jmc 364: .Qq *.a.example.com
1.173 djm 365: to be canonicalized to names in the
1.240 jmc 366: .Qq *.b.example.com
1.171 djm 367: or
1.240 jmc 368: .Qq *.c.example.com
1.171 djm 369: domains.
1.283 jmc 370: .It Cm CASignatureAlgorithms
371: Specifies which algorithms are allowed for signing of certificates
372: by certificate authorities (CAs).
373: The default is:
374: .Bd -literal -offset indent
1.333 djm 375: ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,
376: ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.283 jmc 377: .Ed
378: .Pp
379: .Xr ssh 1
380: will not accept host certificates signed using algorithms other than those
381: specified.
1.221 djm 382: .It Cm CertificateFile
383: Specifies a file from which the user's certificate is read.
384: A corresponding private key must be provided separately in order
385: to use this certificate either
386: from an
387: .Cm IdentityFile
388: directive or
389: .Fl i
390: flag to
391: .Xr ssh 1 ,
392: via
393: .Xr ssh-agent 1 ,
394: or via a
1.305 naddy 395: .Cm PKCS11Provider
396: or
397: .Cm SecurityKeyProvider .
1.221 djm 398: .Pp
1.239 jmc 399: Arguments to
400: .Cm CertificateFile
1.326 dtucker 401: may use the tilde syntax to refer to a user's home directory,
402: the tokens described in the
1.239 jmc 403: .Sx TOKENS
1.326 dtucker 404: section and environment variables as described in the
405: .Sx ENVIRONMENT VARIABLES
1.239 jmc 406: section.
1.221 djm 407: .Pp
408: It is possible to have multiple certificate files specified in
409: configuration files; these certificates will be tried in sequence.
410: Multiple
411: .Cm CertificateFile
412: directives will add to the list of certificates used for
413: authentication.
1.1 stevesk 414: .It Cm ChallengeResponseAuthentication
1.82 jmc 415: Specifies whether to use challenge-response authentication.
1.1 stevesk 416: The argument to this keyword must be
1.240 jmc 417: .Cm yes
418: (the default)
1.1 stevesk 419: or
1.240 jmc 420: .Cm no .
1.1 stevesk 421: .It Cm CheckHostIP
1.240 jmc 422: If set to
423: .Cm yes
1.84 jmc 424: .Xr ssh 1
425: will additionally check the host IP address in the
1.1 stevesk 426: .Pa known_hosts
427: file.
1.240 jmc 428: This allows it to detect if a host key changed due to DNS spoofing
1.211 djm 429: and will add addresses of destination hosts to
430: .Pa ~/.ssh/known_hosts
431: in the process, regardless of the setting of
432: .Cm StrictHostKeyChecking .
1.107 grunk 433: If the option is set to
1.341 djm 434: .Cm no
435: (the default),
1.1 stevesk 436: the check will not be executed.
437: .It Cm Ciphers
1.245 djm 438: Specifies the ciphers allowed and their order of preference.
1.1 stevesk 439: Multiple ciphers must be comma-separated.
1.299 kn 440: If the specified list begins with a
1.214 djm 441: .Sq +
442: character, then the specified ciphers will be appended to the default set
443: instead of replacing them.
1.299 kn 444: If the specified list begins with a
1.241 djm 445: .Sq -
446: character, then the specified ciphers (including wildcards) will be removed
447: from the default set instead of replacing them.
1.301 naddy 448: If the specified list begins with a
449: .Sq ^
450: character, then the specified ciphers will be placed at the head of the
451: default set.
1.214 djm 452: .Pp
1.180 djm 453: The supported ciphers are:
1.240 jmc 454: .Bd -literal -offset indent
1.186 naddy 455: 3des-cbc
456: aes128-cbc
457: aes192-cbc
458: aes256-cbc
459: aes128-ctr
460: aes192-ctr
461: aes256-ctr
462: aes128-gcm@openssh.com
463: aes256-gcm@openssh.com
464: chacha20-poly1305@openssh.com
1.240 jmc 465: .Ed
1.180 djm 466: .Pp
1.84 jmc 467: The default is:
1.186 naddy 468: .Bd -literal -offset indent
1.215 jmc 469: chacha20-poly1305@openssh.com,
1.186 naddy 470: aes128-ctr,aes192-ctr,aes256-ctr,
1.270 djm 471: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 472: .Ed
1.180 djm 473: .Pp
1.240 jmc 474: The list of available ciphers may also be obtained using
475: .Qq ssh -Q cipher .
1.1 stevesk 476: .It Cm ClearAllForwardings
1.84 jmc 477: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 478: specified in the configuration files or on the command line be
1.7 jmc 479: cleared.
480: This option is primarily useful when used from the
1.84 jmc 481: .Xr ssh 1
1.1 stevesk 482: command line to clear port forwardings set in
483: configuration files, and is automatically set by
484: .Xr scp 1
485: and
486: .Xr sftp 1 .
487: The argument must be
1.240 jmc 488: .Cm yes
1.1 stevesk 489: or
1.240 jmc 490: .Cm no
491: (the default).
1.1 stevesk 492: .It Cm Compression
493: Specifies whether to use compression.
494: The argument must be
1.240 jmc 495: .Cm yes
1.1 stevesk 496: or
1.240 jmc 497: .Cm no
498: (the default).
1.247 naddy 499: .It Cm ConnectionAttempts
500: Specifies the number of tries (one per second) to make before exiting.
501: The argument must be an integer.
502: This may be useful in scripts if the connection sometimes fails.
503: The default is 1.
1.9 djm 504: .It Cm ConnectTimeout
1.84 jmc 505: Specifies the timeout (in seconds) used when connecting to the
506: SSH server, instead of using the default system TCP timeout.
1.302 djm 507: This timeout is applied both to establishing the connection and to performing
508: the initial SSH protocol handshake and key exchange.
1.36 djm 509: .It Cm ControlMaster
510: Enables the sharing of multiple sessions over a single network connection.
511: When set to
1.240 jmc 512: .Cm yes ,
1.84 jmc 513: .Xr ssh 1
1.36 djm 514: will listen for connections on a control socket specified using the
515: .Cm ControlPath
516: argument.
517: Additional sessions can connect to this socket using the same
518: .Cm ControlPath
519: with
520: .Cm ControlMaster
521: set to
1.240 jmc 522: .Cm no
1.38 jmc 523: (the default).
1.64 jmc 524: These sessions will try to reuse the master instance's network connection
1.63 djm 525: rather than initiating new ones, but will fall back to connecting normally
526: if the control socket does not exist, or is not listening.
527: .Pp
1.37 djm 528: Setting this to
1.240 jmc 529: .Cm ask
530: will cause
531: .Xr ssh 1
1.206 jmc 532: to listen for control connections, but require confirmation using
533: .Xr ssh-askpass 1 .
1.51 jakob 534: If the
535: .Cm ControlPath
1.84 jmc 536: cannot be opened,
1.240 jmc 537: .Xr ssh 1
538: will continue without connecting to a master instance.
1.58 djm 539: .Pp
540: X11 and
1.59 jmc 541: .Xr ssh-agent 1
1.58 djm 542: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 543: display and agent forwarded will be the one belonging to the master
1.59 jmc 544: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 545: .Pp
546: Two additional options allow for opportunistic multiplexing: try to use a
547: master connection but fall back to creating a new one if one does not already
548: exist.
549: These options are:
1.240 jmc 550: .Cm auto
1.56 djm 551: and
1.240 jmc 552: .Cm autoask .
1.56 djm 553: The latter requires confirmation like the
1.240 jmc 554: .Cm ask
1.56 djm 555: option.
1.36 djm 556: .It Cm ControlPath
1.55 djm 557: Specify the path to the control socket used for connection sharing as described
558: in the
1.36 djm 559: .Cm ControlMaster
1.57 djm 560: section above or the string
1.240 jmc 561: .Cm none
1.57 djm 562: to disable connection sharing.
1.239 jmc 563: Arguments to
564: .Cm ControlPath
1.326 dtucker 565: may use the tilde syntax to refer to a user's home directory,
566: the tokens described in the
1.239 jmc 567: .Sx TOKENS
1.326 dtucker 568: section and environment variables as described in the
569: .Sx ENVIRONMENT VARIABLES
1.239 jmc 570: section.
1.56 djm 571: It is recommended that any
572: .Cm ControlPath
573: used for opportunistic connection sharing include
1.195 djm 574: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
575: that is not writable by other users.
1.56 djm 576: This ensures that shared connections are uniquely identified.
1.137 djm 577: .It Cm ControlPersist
578: When used in conjunction with
579: .Cm ControlMaster ,
580: specifies that the master connection should remain open
581: in the background (waiting for future client connections)
582: after the initial client connection has been closed.
583: If set to
1.314 naddy 584: .Cm no
585: (the default),
1.137 djm 586: then the master connection will not be placed into the background,
587: and will close as soon as the initial client connection is closed.
588: If set to
1.240 jmc 589: .Cm yes
590: or 0,
1.137 djm 591: then the master connection will remain in the background indefinitely
592: (until killed or closed via a mechanism such as the
1.240 jmc 593: .Qq ssh -O exit ) .
1.137 djm 594: If set to a time in seconds, or a time in any of the formats documented in
595: .Xr sshd_config 5 ,
596: then the backgrounded master connection will automatically terminate
597: after it has remained idle (with no client connections) for the
598: specified time.
1.38 jmc 599: .It Cm DynamicForward
1.74 jmc 600: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 601: over the secure channel, and the application
602: protocol is then used to determine where to connect to from the
603: remote machine.
1.62 djm 604: .Pp
605: The argument must be
606: .Sm off
607: .Oo Ar bind_address : Oc Ar port .
608: .Sm on
1.138 djm 609: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 610: By default, the local port is bound in accordance with the
611: .Cm GatewayPorts
612: setting.
613: However, an explicit
614: .Ar bind_address
615: may be used to bind the connection to a specific address.
616: The
617: .Ar bind_address
618: of
1.240 jmc 619: .Cm localhost
1.62 djm 620: indicates that the listening port be bound for local use only, while an
621: empty address or
622: .Sq *
623: indicates that the port should be available from all interfaces.
624: .Pp
1.38 jmc 625: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 626: .Xr ssh 1
1.38 jmc 627: will act as a SOCKS server.
628: Multiple forwardings may be specified, and
629: additional forwardings can be given on the command line.
630: Only the superuser can forward privileged ports.
1.14 markus 631: .It Cm EnableSSHKeysign
632: Setting this option to
1.240 jmc 633: .Cm yes
1.14 markus 634: in the global client configuration file
635: .Pa /etc/ssh/ssh_config
636: enables the use of the helper program
637: .Xr ssh-keysign 8
638: during
639: .Cm HostbasedAuthentication .
640: The argument must be
1.240 jmc 641: .Cm yes
1.14 markus 642: or
1.240 jmc 643: .Cm no
644: (the default).
1.23 jmc 645: This option should be placed in the non-hostspecific section.
1.14 markus 646: See
647: .Xr ssh-keysign 8
648: for more information.
1.1 stevesk 649: .It Cm EscapeChar
650: Sets the escape character (default:
651: .Ql ~ ) .
652: The escape character can also
653: be set on the command line.
654: The argument should be a single character,
655: .Ql ^
656: followed by a letter, or
1.240 jmc 657: .Cm none
1.1 stevesk 658: to disable the escape
659: character entirely (making the connection transparent for binary
660: data).
1.96 markus 661: .It Cm ExitOnForwardFailure
662: Specifies whether
663: .Xr ssh 1
664: should terminate the connection if it cannot set up all requested
1.216 djm 665: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 666: if either end is unable to bind and listen on a specified port).
1.216 djm 667: Note that
668: .Cm ExitOnForwardFailure
669: does not apply to connections made over port forwardings and will not,
670: for example, cause
671: .Xr ssh 1
672: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 673: The argument must be
1.240 jmc 674: .Cm yes
1.96 markus 675: or
1.240 jmc 676: .Cm no
677: (the default).
1.197 djm 678: .It Cm FingerprintHash
679: Specifies the hash algorithm used when displaying key fingerprints.
680: Valid options are:
1.240 jmc 681: .Cm md5
1.197 djm 682: and
1.240 jmc 683: .Cm sha256
684: (the default).
1.1 stevesk 685: .It Cm ForwardAgent
686: Specifies whether the connection to the authentication agent (if any)
687: will be forwarded to the remote machine.
1.312 djm 688: The argument may be
689: .Cm yes ,
1.240 jmc 690: .Cm no
1.312 djm 691: (the default),
692: an explicit path to an agent socket or the name of an environment variable
693: (beginning with
694: .Sq $ )
695: in which to find the path.
1.3 stevesk 696: .Pp
1.7 jmc 697: Agent forwarding should be enabled with caution.
698: Users with the ability to bypass file permissions on the remote host
699: (for the agent's Unix-domain socket)
700: can access the local agent through the forwarded connection.
701: An attacker cannot obtain key material from the agent,
1.3 stevesk 702: however they can perform operations on the keys that enable them to
703: authenticate using the identities loaded into the agent.
1.1 stevesk 704: .It Cm ForwardX11
705: Specifies whether X11 connections will be automatically redirected
706: over the secure channel and
707: .Ev DISPLAY
708: set.
709: The argument must be
1.240 jmc 710: .Cm yes
1.1 stevesk 711: or
1.240 jmc 712: .Cm no
713: (the default).
1.3 stevesk 714: .Pp
1.7 jmc 715: X11 forwarding should be enabled with caution.
716: Users with the ability to bypass file permissions on the remote host
1.22 markus 717: (for the user's X11 authorization database)
1.7 jmc 718: can access the local X11 display through the forwarded connection.
1.22 markus 719: An attacker may then be able to perform activities such as keystroke monitoring
720: if the
721: .Cm ForwardX11Trusted
722: option is also enabled.
1.134 djm 723: .It Cm ForwardX11Timeout
1.135 jmc 724: Specify a timeout for untrusted X11 forwarding
725: using the format described in the
1.240 jmc 726: .Sx TIME FORMATS
727: section of
1.134 djm 728: .Xr sshd_config 5 .
729: X11 connections received by
730: .Xr ssh 1
731: after this time will be refused.
1.285 djm 732: Setting
733: .Cm ForwardX11Timeout
734: to zero will disable the timeout and permit X11 forwarding for the life
735: of the connection.
1.134 djm 736: The default is to disable untrusted X11 forwarding after twenty minutes has
737: elapsed.
1.22 markus 738: .It Cm ForwardX11Trusted
1.34 jmc 739: If this option is set to
1.240 jmc 740: .Cm yes ,
1.84 jmc 741: remote X11 clients will have full access to the original X11 display.
1.42 djm 742: .Pp
1.22 markus 743: If this option is set to
1.240 jmc 744: .Cm no
745: (the default),
1.84 jmc 746: remote X11 clients will be considered untrusted and prevented
1.22 markus 747: from stealing or tampering with data belonging to trusted X11
748: clients.
1.42 djm 749: Furthermore, the
750: .Xr xauth 1
751: token used for the session will be set to expire after 20 minutes.
752: Remote clients will be refused access after this time.
1.22 markus 753: .Pp
754: See the X11 SECURITY extension specification for full details on
755: the restrictions imposed on untrusted clients.
1.1 stevesk 756: .It Cm GatewayPorts
757: Specifies whether remote hosts are allowed to connect to local
758: forwarded ports.
759: By default,
1.84 jmc 760: .Xr ssh 1
1.7 jmc 761: binds local port forwardings to the loopback address.
762: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 763: .Cm GatewayPorts
1.84 jmc 764: can be used to specify that ssh
1.1 stevesk 765: should bind local port forwardings to the wildcard address,
766: thus allowing remote hosts to connect to forwarded ports.
767: The argument must be
1.240 jmc 768: .Cm yes
1.1 stevesk 769: or
1.240 jmc 770: .Cm no
771: (the default).
1.1 stevesk 772: .It Cm GlobalKnownHostsFile
1.151 djm 773: Specifies one or more files to use for the global
774: host key database, separated by whitespace.
775: The default is
776: .Pa /etc/ssh/ssh_known_hosts ,
777: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 778: .It Cm GSSAPIAuthentication
1.27 markus 779: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 780: The default is
1.240 jmc 781: .Cm no .
1.18 markus 782: .It Cm GSSAPIDelegateCredentials
783: Forward (delegate) credentials to the server.
784: The default is
1.240 jmc 785: .Cm no .
1.44 djm 786: .It Cm HashKnownHosts
787: Indicates that
1.84 jmc 788: .Xr ssh 1
1.44 djm 789: should hash host names and addresses when they are added to
1.50 djm 790: .Pa ~/.ssh/known_hosts .
1.44 djm 791: These hashed names may be used normally by
1.84 jmc 792: .Xr ssh 1
1.44 djm 793: and
1.84 jmc 794: .Xr sshd 8 ,
1.316 djm 795: but they do not visually reveal identifying information if the
796: file's contents are disclosed.
1.44 djm 797: The default is
1.240 jmc 798: .Cm no .
1.97 jmc 799: Note that existing names and addresses in known hosts files
800: will not be converted automatically,
801: but may be manually hashed using
1.45 djm 802: .Xr ssh-keygen 1 .
1.344 dtucker 803: .It Cm HostbasedAcceptedAlgorithms
1.202 djm 804: Specifies the key types that will be used for hostbased authentication
1.279 djm 805: as a comma-separated list of patterns.
1.300 naddy 806: Alternately if the specified list begins with a
1.214 djm 807: .Sq +
808: character, then the specified key types will be appended to the default set
809: instead of replacing them.
1.300 naddy 810: If the specified list begins with a
1.241 djm 811: .Sq -
812: character, then the specified key types (including wildcards) will be removed
813: from the default set instead of replacing them.
1.301 naddy 814: If the specified list begins with a
815: .Sq ^
816: character, then the specified key types will be placed at the head of the
817: default set.
1.213 markus 818: The default for this option is:
819: .Bd -literal -offset 3n
1.333 djm 820: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 821: ecdsa-sha2-nistp256-cert-v01@openssh.com,
822: ecdsa-sha2-nistp384-cert-v01@openssh.com,
823: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 824: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 825: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 826: rsa-sha2-512-cert-v01@openssh.com,
827: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 828: ssh-rsa-cert-v01@openssh.com,
1.333 djm 829: ssh-ed25519,
1.213 markus 830: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 831: sk-ssh-ed25519@openssh.com,
1.311 naddy 832: sk-ecdsa-sha2-nistp256@openssh.com,
833: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 834: .Ed
835: .Pp
1.202 djm 836: The
837: .Fl Q
838: option of
839: .Xr ssh 1
840: may be used to list supported key types.
1.344 dtucker 841: This was formerly named HostbasedKeyTypes.
1.345 naddy 842: .It Cm HostbasedAuthentication
843: Specifies whether to try rhosts based authentication with public key
844: authentication.
845: The argument must be
846: .Cm yes
847: or
848: .Cm no
849: (the default).
1.1 stevesk 850: .It Cm HostKeyAlgorithms
1.226 jmc 851: Specifies the host key algorithms
1.1 stevesk 852: that the client wants to use in order of preference.
1.300 naddy 853: Alternately if the specified list begins with a
1.214 djm 854: .Sq +
855: character, then the specified key types will be appended to the default set
856: instead of replacing them.
1.300 naddy 857: If the specified list begins with a
1.241 djm 858: .Sq -
859: character, then the specified key types (including wildcards) will be removed
860: from the default set instead of replacing them.
1.301 naddy 861: If the specified list begins with a
862: .Sq ^
863: character, then the specified key types will be placed at the head of the
864: default set.
1.1 stevesk 865: The default for this option is:
1.139 djm 866: .Bd -literal -offset 3n
1.333 djm 867: ssh-ed25519-cert-v01@openssh.com,
1.139 djm 868: ecdsa-sha2-nistp256-cert-v01@openssh.com,
869: ecdsa-sha2-nistp384-cert-v01@openssh.com,
870: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 871: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 872: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 873: rsa-sha2-512-cert-v01@openssh.com,
874: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 875: ssh-rsa-cert-v01@openssh.com,
1.333 djm 876: ssh-ed25519,
1.139 djm 877: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.311 naddy 878: sk-ecdsa-sha2-nistp256@openssh.com,
1.333 djm 879: sk-ssh-ed25519@openssh.com,
1.311 naddy 880: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.139 djm 881: .Ed
1.145 djm 882: .Pp
883: If hostkeys are known for the destination host then this default is modified
884: to prefer their algorithms.
1.198 djm 885: .Pp
1.240 jmc 886: The list of available key types may also be obtained using
1.322 dtucker 887: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 888: .It Cm HostKeyAlias
889: Specifies an alias that should be used instead of the
890: real host name when looking up or saving the host key
1.251 djm 891: in the host key database files and when validating host certificates.
1.84 jmc 892: This option is useful for tunneling SSH connections
1.1 stevesk 893: or for multiple servers running on a single host.
1.295 jmc 894: .It Cm Hostname
1.1 stevesk 895: Specifies the real host name to log into.
896: This can be used to specify nicknames or abbreviations for hosts.
1.239 jmc 897: Arguments to
1.295 jmc 898: .Cm Hostname
1.239 jmc 899: accept the tokens described in the
900: .Sx TOKENS
901: section.
1.1 stevesk 902: Numeric IP addresses are also permitted (both on the command line and in
1.295 jmc 903: .Cm Hostname
1.1 stevesk 904: specifications).
1.239 jmc 905: The default is the name given on the command line.
1.29 markus 906: .It Cm IdentitiesOnly
907: Specifies that
1.84 jmc 908: .Xr ssh 1
1.304 djm 909: should only use the configured authentication identity and certificate files
910: (either the default files, or those explicitly configured in the
1.31 jmc 911: .Nm
1.221 djm 912: files
913: or passed on the
914: .Xr ssh 1
1.304 djm 915: command-line),
1.84 jmc 916: even if
917: .Xr ssh-agent 1
1.159 djm 918: or a
919: .Cm PKCS11Provider
1.305 naddy 920: or
921: .Cm SecurityKeyProvider
1.29 markus 922: offers more identities.
923: The argument to this keyword must be
1.240 jmc 924: .Cm yes
1.29 markus 925: or
1.240 jmc 926: .Cm no
927: (the default).
1.84 jmc 928: This option is intended for situations where ssh-agent
1.29 markus 929: offers many different identities.
1.231 markus 930: .It Cm IdentityAgent
931: Specifies the
932: .Ux Ns -domain
933: socket used to communicate with the authentication agent.
934: .Pp
935: This option overrides the
1.240 jmc 936: .Ev SSH_AUTH_SOCK
1.231 markus 937: environment variable and can be used to select a specific agent.
938: Setting the socket name to
1.240 jmc 939: .Cm none
1.231 markus 940: disables the use of an authentication agent.
1.232 markus 941: If the string
1.240 jmc 942: .Qq SSH_AUTH_SOCK
1.232 markus 943: is specified, the location of the socket will be read from the
944: .Ev SSH_AUTH_SOCK
945: environment variable.
1.286 djm 946: Otherwise if the specified value begins with a
947: .Sq $
948: character, then it will be treated as an environment variable containing
949: the location of the socket.
1.231 markus 950: .Pp
1.239 jmc 951: Arguments to
952: .Cm IdentityAgent
1.326 dtucker 953: may use the tilde syntax to refer to a user's home directory,
954: the tokens described in the
1.239 jmc 955: .Sx TOKENS
1.326 dtucker 956: section and environment variables as described in the
957: .Sx ENVIRONMENT VARIABLES
1.239 jmc 958: section.
1.67 jmc 959: .It Cm IdentityFile
1.313 naddy 960: Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
961: Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1.67 jmc 962: The default is
1.139 djm 963: .Pa ~/.ssh/id_dsa ,
1.183 naddy 964: .Pa ~/.ssh/id_ecdsa ,
1.305 naddy 965: .Pa ~/.ssh/id_ecdsa_sk ,
1.308 naddy 966: .Pa ~/.ssh/id_ed25519 ,
967: .Pa ~/.ssh/id_ed25519_sk
1.139 djm 968: and
1.245 djm 969: .Pa ~/.ssh/id_rsa .
1.67 jmc 970: Additionally, any identities represented by the authentication agent
1.165 djm 971: will be used for authentication unless
972: .Cm IdentitiesOnly
973: is set.
1.221 djm 974: If no certificates have been explicitly specified by
975: .Cm CertificateFile ,
1.129 djm 976: .Xr ssh 1
977: will try to load certificate information from the filename obtained by
978: appending
979: .Pa -cert.pub
980: to the path of a specified
981: .Cm IdentityFile .
1.90 djm 982: .Pp
1.239 jmc 983: Arguments to
984: .Cm IdentityFile
985: may use the tilde syntax to refer to a user's home directory
986: or the tokens described in the
987: .Sx TOKENS
988: section.
1.90 djm 989: .Pp
1.67 jmc 990: It is possible to have
991: multiple identity files specified in configuration files; all these
992: identities will be tried in sequence.
1.152 djm 993: Multiple
994: .Cm IdentityFile
995: directives will add to the list of identities tried (this behaviour
996: differs from that of other configuration directives).
1.165 djm 997: .Pp
998: .Cm IdentityFile
999: may be used in conjunction with
1000: .Cm IdentitiesOnly
1001: to select which identities in an agent are offered during authentication.
1.221 djm 1002: .Cm IdentityFile
1003: may also be used in conjunction with
1004: .Cm CertificateFile
1005: in order to provide any certificate also needed for authentication with
1006: the identity.
1.164 jmc 1007: .It Cm IgnoreUnknown
1008: Specifies a pattern-list of unknown options to be ignored if they are
1009: encountered in configuration parsing.
1010: This may be used to suppress errors if
1011: .Nm
1012: contains options that are unrecognised by
1013: .Xr ssh 1 .
1014: It is recommended that
1015: .Cm IgnoreUnknown
1016: be listed early in the configuration file as it will not be applied
1017: to unknown options that appear before it.
1.229 djm 1018: .It Cm Include
1019: Include the specified configuration file(s).
1.230 jmc 1020: Multiple pathnames may be specified and each pathname may contain
1.281 kn 1021: .Xr glob 7
1.229 djm 1022: wildcards and, for user configurations, shell-like
1.240 jmc 1023: .Sq ~
1.229 djm 1024: references to user home directories.
1.327 djm 1025: Wildcards will be expanded and processed in lexical order.
1.229 djm 1026: Files without absolute paths are assumed to be in
1027: .Pa ~/.ssh
1.230 jmc 1028: if included in a user configuration file or
1.229 djm 1029: .Pa /etc/ssh
1030: if included from the system configuration file.
1031: .Cm Include
1032: directive may appear inside a
1033: .Cm Match
1034: or
1035: .Cm Host
1036: block
1037: to perform conditional inclusion.
1.143 djm 1038: .It Cm IPQoS
1039: Specifies the IPv4 type-of-service or DSCP class for connections.
1040: Accepted values are
1.240 jmc 1041: .Cm af11 ,
1042: .Cm af12 ,
1043: .Cm af13 ,
1044: .Cm af21 ,
1045: .Cm af22 ,
1046: .Cm af23 ,
1047: .Cm af31 ,
1048: .Cm af32 ,
1049: .Cm af33 ,
1050: .Cm af41 ,
1051: .Cm af42 ,
1052: .Cm af43 ,
1053: .Cm cs0 ,
1054: .Cm cs1 ,
1055: .Cm cs2 ,
1056: .Cm cs3 ,
1057: .Cm cs4 ,
1058: .Cm cs5 ,
1059: .Cm cs6 ,
1060: .Cm cs7 ,
1061: .Cm ef ,
1.319 djm 1062: .Cm le ,
1.240 jmc 1063: .Cm lowdelay ,
1064: .Cm throughput ,
1065: .Cm reliability ,
1.253 djm 1066: a numeric value, or
1067: .Cm none
1068: to use the operating system default.
1.146 djm 1069: This option may take one or two arguments, separated by whitespace.
1.143 djm 1070: If one argument is specified, it is used as the packet class unconditionally.
1071: If two values are specified, the first is automatically selected for
1072: interactive sessions and the second for non-interactive sessions.
1073: The default is
1.269 job 1074: .Cm af21
1.272 jmc 1075: (Low-Latency Data)
1.143 djm 1076: for interactive sessions and
1.269 job 1077: .Cm cs1
1.272 jmc 1078: (Lower Effort)
1.143 djm 1079: for non-interactive sessions.
1.103 djm 1080: .It Cm KbdInteractiveAuthentication
1081: Specifies whether to use keyboard-interactive authentication.
1082: The argument to this keyword must be
1.240 jmc 1083: .Cm yes
1084: (the default)
1.103 djm 1085: or
1.240 jmc 1086: .Cm no .
1.39 djm 1087: .It Cm KbdInteractiveDevices
1088: Specifies the list of methods to use in keyboard-interactive authentication.
1089: Multiple method names must be comma-separated.
1090: The default is to use the server specified list.
1.85 jmc 1091: The methods available vary depending on what the server supports.
1092: For an OpenSSH server,
1093: it may be zero or more of:
1.240 jmc 1094: .Cm bsdauth ,
1095: .Cm pam ,
1.85 jmc 1096: and
1.240 jmc 1097: .Cm skey .
1.140 djm 1098: .It Cm KexAlgorithms
1099: Specifies the available KEX (Key Exchange) algorithms.
1100: Multiple algorithms must be comma-separated.
1.299 kn 1101: If the specified list begins with a
1.214 djm 1102: .Sq +
1103: character, then the specified methods will be appended to the default set
1104: instead of replacing them.
1.299 kn 1105: If the specified list begins with a
1.241 djm 1106: .Sq -
1107: character, then the specified methods (including wildcards) will be removed
1108: from the default set instead of replacing them.
1.301 naddy 1109: If the specified list begins with a
1110: .Sq ^
1111: character, then the specified methods will be placed at the head of the
1112: default set.
1.141 jmc 1113: The default is:
1114: .Bd -literal -offset indent
1.238 djm 1115: curve25519-sha256,curve25519-sha256@libssh.org,
1.141 jmc 1116: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1117: diffie-hellman-group-exchange-sha256,
1.266 djm 1118: diffie-hellman-group16-sha512,
1119: diffie-hellman-group18-sha512,
1.317 tedu 1120: diffie-hellman-group14-sha256
1.141 jmc 1121: .Ed
1.198 djm 1122: .Pp
1.240 jmc 1123: The list of available key exchange algorithms may also be obtained using
1124: .Qq ssh -Q kex .
1.339 djm 1125: .It Cm KnownHostsCommand
1.340 jmc 1126: Specifies a command to use to obtain a list of host keys, in addition to
1.339 djm 1127: those listed in
1128: .Cm UserKnownHostsFile
1129: and
1130: .Cm GlobalKnownHostsFile .
1131: This command is executed after the files have been read.
1.340 jmc 1132: It may write host key lines to standard output in identical format to the
1.339 djm 1133: usual files (described in the
1134: .Sx VERIFYING HOST KEYS
1135: section in
1136: .Xr ssh 1 ) .
1137: Arguments to
1138: .Cm KnownHostsCommand
1139: accept the tokens described in the
1140: .Sx TOKENS
1141: section.
1.340 jmc 1142: The command may be invoked multiple times per connection: once when preparing
1.339 djm 1143: the preference list of host key algorithms to use, again to obtain the
1144: host key for the requested host name and, if
1145: .Cm CheckHostIP
1146: is enabled, one more time to obtain the host key matching the server's
1147: address.
1148: If the command exits abnormally or returns a non-zero exit status then the
1149: connection is terminated.
1.65 reyk 1150: .It Cm LocalCommand
1151: Specifies a command to execute on the local machine after successfully
1152: connecting to the server.
1153: The command string extends to the end of the line, and is executed with
1.105 jmc 1154: the user's shell.
1.239 jmc 1155: Arguments to
1156: .Cm LocalCommand
1157: accept the tokens described in the
1158: .Sx TOKENS
1159: section.
1.123 djm 1160: .Pp
1161: The command is run synchronously and does not have access to the
1162: session of the
1163: .Xr ssh 1
1164: that spawned it.
1165: It should not be used for interactive commands.
1166: .Pp
1.65 reyk 1167: This directive is ignored unless
1168: .Cm PermitLocalCommand
1169: has been enabled.
1.1 stevesk 1170: .It Cm LocalForward
1.74 jmc 1171: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1172: the secure channel to the specified host and port from the remote machine.
1.324 dtucker 1173: The first argument specifies the listener and may be
1.43 djm 1174: .Sm off
1.49 jmc 1175: .Oo Ar bind_address : Oc Ar port
1.43 djm 1176: .Sm on
1.324 dtucker 1177: or a Unix domain socket path.
1178: The second argument is the destination and may be
1179: .Ar host : Ns Ar hostport
1180: or a Unix domain socket path if the remote host supports it.
1181: .Pp
1.138 djm 1182: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1183: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1184: given on the command line.
1.1 stevesk 1185: Only the superuser can forward privileged ports.
1.43 djm 1186: By default, the local port is bound in accordance with the
1187: .Cm GatewayPorts
1188: setting.
1189: However, an explicit
1190: .Ar bind_address
1191: may be used to bind the connection to a specific address.
1192: The
1193: .Ar bind_address
1194: of
1.240 jmc 1195: .Cm localhost
1.46 jmc 1196: indicates that the listening port be bound for local use only, while an
1197: empty address or
1198: .Sq *
1.43 djm 1199: indicates that the port should be available from all interfaces.
1.326 dtucker 1200: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1201: .Sx TOKENS
1.326 dtucker 1202: section and environment variables as described in the
1203: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1204: section.
1.1 stevesk 1205: .It Cm LogLevel
1206: Gives the verbosity level that is used when logging messages from
1.84 jmc 1207: .Xr ssh 1 .
1.1 stevesk 1208: The possible values are:
1.84 jmc 1209: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1210: The default is INFO.
1211: DEBUG and DEBUG1 are equivalent.
1212: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.337 djm 1213: .It Cm LogVerbose
1214: Specify one or more overrides to LogLevel.
1215: An override consists of a pattern lists that matches the source file, function
1216: and line number to force detailed logging for.
1217: For example, an override pattern of:
1218: .Bd -literal -offset indent
1219: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1220: .Ed
1221: .Pp
1222: would enable detailed logging for line 1000 of
1.338 jmc 1223: .Pa kex.c ,
1.337 djm 1224: everything in the
1225: .Fn kex_exchange_identification
1226: function, and all code in the
1227: .Pa packet.c
1228: file.
1229: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1230: .It Cm MACs
1231: Specifies the MAC (message authentication code) algorithms
1232: in order of preference.
1.226 jmc 1233: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1234: Multiple algorithms must be comma-separated.
1.299 kn 1235: If the specified list begins with a
1.214 djm 1236: .Sq +
1237: character, then the specified algorithms will be appended to the default set
1238: instead of replacing them.
1.299 kn 1239: If the specified list begins with a
1.241 djm 1240: .Sq -
1241: character, then the specified algorithms (including wildcards) will be removed
1242: from the default set instead of replacing them.
1.301 naddy 1243: If the specified list begins with a
1244: .Sq ^
1245: character, then the specified algorithms will be placed at the head of the
1246: default set.
1.214 djm 1247: .Pp
1.160 markus 1248: The algorithms that contain
1.240 jmc 1249: .Qq -etm
1.160 markus 1250: calculate the MAC after encryption (encrypt-then-mac).
1251: These are considered safer and their use recommended.
1.214 djm 1252: .Pp
1.84 jmc 1253: The default is:
1.101 jmc 1254: .Bd -literal -offset indent
1.160 markus 1255: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1256: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.224 djm 1257: hmac-sha1-etm@openssh.com,
1.186 naddy 1258: umac-64@openssh.com,umac-128@openssh.com,
1.224 djm 1259: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.101 jmc 1260: .Ed
1.198 djm 1261: .Pp
1.240 jmc 1262: The list of available MAC algorithms may also be obtained using
1263: .Qq ssh -Q mac .
1.1 stevesk 1264: .It Cm NoHostAuthenticationForLocalhost
1.264 djm 1265: Disable host authentication for localhost (loopback addresses).
1.1 stevesk 1266: The argument to this keyword must be
1.240 jmc 1267: .Cm yes
1.1 stevesk 1268: or
1.242 jmc 1269: .Cm no
1.240 jmc 1270: (the default).
1.1 stevesk 1271: .It Cm NumberOfPasswordPrompts
1272: Specifies the number of password prompts before giving up.
1273: The argument to this keyword must be an integer.
1.84 jmc 1274: The default is 3.
1.1 stevesk 1275: .It Cm PasswordAuthentication
1276: Specifies whether to use password authentication.
1277: The argument to this keyword must be
1.240 jmc 1278: .Cm yes
1279: (the default)
1.1 stevesk 1280: or
1.240 jmc 1281: .Cm no .
1.65 reyk 1282: .It Cm PermitLocalCommand
1283: Allow local command execution via the
1284: .Ic LocalCommand
1285: option or using the
1.66 jmc 1286: .Ic !\& Ns Ar command
1.65 reyk 1287: escape sequence in
1288: .Xr ssh 1 .
1289: The argument must be
1.240 jmc 1290: .Cm yes
1.65 reyk 1291: or
1.240 jmc 1292: .Cm no
1293: (the default).
1.347 ! markus 1294: .It Cm PermitRemoteOpen
! 1295: Specifies the destinations to which remote TCP port forwarding is permitted when
! 1296: .Cm RemoteForward
! 1297: is used as a SOCKS proxy.
! 1298: The forwarding specification must be one of the following forms:
! 1299: .Pp
! 1300: .Bl -item -offset indent -compact
! 1301: .It
! 1302: .Cm PermitRemoteOpen
! 1303: .Sm off
! 1304: .Ar host : port
! 1305: .Sm on
! 1306: .It
! 1307: .Cm PermitRemoteOpen
! 1308: .Sm off
! 1309: .Ar IPv4_addr : port
! 1310: .Sm on
! 1311: .It
! 1312: .Cm PermitRemoteOpen
! 1313: .Sm off
! 1314: .Ar \&[ IPv6_addr \&] : port
! 1315: .Sm on
! 1316: .El
! 1317: .Pp
! 1318: Multiple forwards may be specified by separating them with whitespace.
! 1319: An argument of
! 1320: .Cm any
! 1321: can be used to remove all restrictions and permit any forwarding requests.
! 1322: An argument of
! 1323: .Cm none
! 1324: can be used to prohibit all forwarding requests.
! 1325: The wildcard
! 1326: .Sq *
! 1327: can be used for host or port to allow all hosts or ports respectively.
! 1328: Otherwise, no pattern matching or address lookups are performed on supplied
! 1329: names.
1.127 markus 1330: .It Cm PKCS11Provider
1.292 djm 1331: Specifies which PKCS#11 provider to use or
1332: .Cm none
1333: to indicate that no provider should be used (the default).
1334: The argument to this keyword is a path to the PKCS#11 shared library
1.127 markus 1335: .Xr ssh 1
1.292 djm 1336: should use to communicate with a PKCS#11 token providing keys for user
1337: authentication.
1.67 jmc 1338: .It Cm Port
1339: Specifies the port number to connect on the remote host.
1.84 jmc 1340: The default is 22.
1.1 stevesk 1341: .It Cm PreferredAuthentications
1.226 jmc 1342: Specifies the order in which the client should try authentication methods.
1.48 jmc 1343: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1344: .Cm keyboard-interactive )
1.48 jmc 1345: over another method (e.g.\&
1.131 jmc 1346: .Cm password ) .
1347: The default is:
1348: .Bd -literal -offset indent
1349: gssapi-with-mic,hostbased,publickey,
1350: keyboard-interactive,password
1351: .Ed
1.1 stevesk 1352: .It Cm ProxyCommand
1353: Specifies the command to use to connect to the server.
1354: The command
1.190 djm 1355: string extends to the end of the line, and is executed
1356: using the user's shell
1357: .Ql exec
1358: directive to avoid a lingering shell process.
1359: .Pp
1.239 jmc 1360: Arguments to
1361: .Cm ProxyCommand
1362: accept the tokens described in the
1363: .Sx TOKENS
1364: section.
1.1 stevesk 1365: The command can be basically anything,
1366: and should read from its standard input and write to its standard output.
1367: It should eventually connect an
1368: .Xr sshd 8
1369: server running on some machine, or execute
1370: .Ic sshd -i
1371: somewhere.
1372: Host key management will be done using the
1.296 jmc 1373: .Cm Hostname
1374: of the host being connected (defaulting to the name typed by the user).
1.7 jmc 1375: Setting the command to
1.240 jmc 1376: .Cm none
1.6 markus 1377: disables this option entirely.
1.1 stevesk 1378: Note that
1379: .Cm CheckHostIP
1380: is not available for connects with a proxy command.
1.52 djm 1381: .Pp
1382: This directive is useful in conjunction with
1383: .Xr nc 1
1384: and its proxy support.
1.53 jmc 1385: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1386: 192.0.2.0:
1387: .Bd -literal -offset 3n
1388: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1389: .Ed
1.233 djm 1390: .It Cm ProxyJump
1.260 millert 1391: Specifies one or more jump proxies as either
1.233 djm 1392: .Xo
1393: .Sm off
1.234 jmc 1394: .Op Ar user No @
1.233 djm 1395: .Ar host
1.234 jmc 1396: .Op : Ns Ar port
1.233 djm 1397: .Sm on
1.260 millert 1398: or an ssh URI
1.233 djm 1399: .Xc .
1.235 djm 1400: Multiple proxies may be separated by comma characters and will be visited
1.236 djm 1401: sequentially.
1.233 djm 1402: Setting this option will cause
1403: .Xr ssh 1
1404: to connect to the target host by first making a
1405: .Xr ssh 1
1406: connection to the specified
1407: .Cm ProxyJump
1408: host and then establishing a
1.234 jmc 1409: TCP forwarding to the ultimate target from there.
1.346 dlg 1410: Setting the host to
1411: .Cm none
1412: disables this option entirely.
1.233 djm 1413: .Pp
1414: Note that this option will compete with the
1415: .Cm ProxyCommand
1416: option - whichever is specified first will prevent later instances of the
1417: other from taking effect.
1.289 djm 1418: .Pp
1419: Note also that the configuration for the destination host (either supplied
1420: via the command-line or the configuration file) is not generally applied
1421: to jump hosts.
1422: .Pa ~/.ssh/config
1423: should be used if specific configuration is required for jump hosts.
1.167 djm 1424: .It Cm ProxyUseFdpass
1.168 jmc 1425: Specifies that
1.167 djm 1426: .Cm ProxyCommand
1427: will pass a connected file descriptor back to
1.168 jmc 1428: .Xr ssh 1
1.167 djm 1429: instead of continuing to execute and pass data.
1430: The default is
1.240 jmc 1431: .Cm no .
1.343 dtucker 1432: .It Cm PubkeyAcceptedAlgorithms
1433: Specifies the signature algorithms that will be used for public key
1434: authentication as a comma-separated list of patterns.
1.299 kn 1435: If the specified list begins with a
1.214 djm 1436: .Sq +
1.343 dtucker 1437: character, then the algorithms after it will be appended to the default
1.214 djm 1438: instead of replacing it.
1.299 kn 1439: If the specified list begins with a
1.241 djm 1440: .Sq -
1.343 dtucker 1441: character, then the specified algorithms (including wildcards) will be removed
1.241 djm 1442: from the default set instead of replacing them.
1.301 naddy 1443: If the specified list begins with a
1444: .Sq ^
1.343 dtucker 1445: character, then the specified algorithms will be placed at the head of the
1.301 naddy 1446: default set.
1.213 markus 1447: The default for this option is:
1448: .Bd -literal -offset 3n
1.333 djm 1449: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 1450: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1451: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1452: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.333 djm 1453: sk-ssh-ed25519-cert-v01@openssh.com,
1.311 naddy 1454: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.305 naddy 1455: rsa-sha2-512-cert-v01@openssh.com,
1456: rsa-sha2-256-cert-v01@openssh.com,
1.213 markus 1457: ssh-rsa-cert-v01@openssh.com,
1.333 djm 1458: ssh-ed25519,
1.311 naddy 1459: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.333 djm 1460: sk-ssh-ed25519@openssh.com,
1.305 naddy 1461: sk-ecdsa-sha2-nistp256@openssh.com,
1.311 naddy 1462: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.213 markus 1463: .Ed
1464: .Pp
1.240 jmc 1465: The list of available key types may also be obtained using
1.343 dtucker 1466: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.1 stevesk 1467: .It Cm PubkeyAuthentication
1468: Specifies whether to try public key authentication.
1469: The argument to this keyword must be
1.240 jmc 1470: .Cm yes
1471: (the default)
1.1 stevesk 1472: or
1.240 jmc 1473: .Cm no .
1.75 dtucker 1474: .It Cm RekeyLimit
1475: Specifies the maximum amount of data that may be transmitted before the
1.342 rob 1476: session key is renegotiated, optionally followed by a maximum amount of
1.162 dtucker 1477: time that may pass before the session key is renegotiated.
1478: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1479: .Sq K ,
1480: .Sq M ,
1.75 dtucker 1481: or
1.76 jmc 1482: .Sq G
1.75 dtucker 1483: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1484: The default is between
1.84 jmc 1485: .Sq 1G
1.75 dtucker 1486: and
1.84 jmc 1487: .Sq 4G ,
1.75 dtucker 1488: depending on the cipher.
1.162 dtucker 1489: The optional second value is specified in seconds and may use any of the
1.293 schwarze 1490: units documented in the TIME FORMATS section of
1.162 dtucker 1491: .Xr sshd_config 5 .
1492: The default value for
1493: .Cm RekeyLimit
1494: is
1.240 jmc 1495: .Cm default none ,
1.162 dtucker 1496: which means that rekeying is performed after the cipher's default amount
1497: of data has been sent or received and no time based rekeying is done.
1.249 bluhm 1498: .It Cm RemoteCommand
1499: Specifies a command to execute on the remote machine after successfully
1500: connecting to the server.
1501: The command string extends to the end of the line, and is executed with
1502: the user's shell.
1.250 jmc 1503: Arguments to
1504: .Cm RemoteCommand
1505: accept the tokens described in the
1506: .Sx TOKENS
1507: section.
1.1 stevesk 1508: .It Cm RemoteForward
1.74 jmc 1509: Specifies that a TCP port on the remote machine be forwarded over
1.256 markus 1510: the secure channel.
1.273 djm 1511: The remote port may either be forwarded to a specified host and port
1.256 markus 1512: from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1513: client to connect to arbitrary destinations from the local machine.
1.324 dtucker 1514: The first argument is the listening specification and may be
1.43 djm 1515: .Sm off
1.49 jmc 1516: .Oo Ar bind_address : Oc Ar port
1.43 djm 1517: .Sm on
1.324 dtucker 1518: or, if the remote host supports it, a Unix domain socket path.
1.256 markus 1519: If forwarding to a specific destination then the second argument must be
1.324 dtucker 1520: .Ar host : Ns Ar hostport
1521: or a Unix domain socket path,
1.256 markus 1522: otherwise if no destination argument is specified then the remote forwarding
1523: will be established as a SOCKS proxy.
1.347 ! markus 1524: When acting as a SOCKS proxy the destination of the connection can be
! 1525: restricted by
! 1526: .Cm PermitRemoteOpen .
1.256 markus 1527: .Pp
1.138 djm 1528: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1529: Multiple forwardings may be specified, and additional
1530: forwardings can be given on the command line.
1.113 stevesk 1531: Privileged ports can be forwarded only when
1532: logging in as root on the remote machine.
1.326 dtucker 1533: Unix domain socket paths may use the tokens described in the
1.324 dtucker 1534: .Sx TOKENS
1.326 dtucker 1535: section and environment variables as described in the
1536: .Sx ENVIRONMENT VARIABLES
1.324 dtucker 1537: section.
1.118 jmc 1538: .Pp
1.117 djm 1539: If the
1540: .Ar port
1.240 jmc 1541: argument is 0,
1.117 djm 1542: the listen port will be dynamically allocated on the server and reported
1543: to the client at run time.
1.43 djm 1544: .Pp
1545: If the
1546: .Ar bind_address
1547: is not specified, the default is to only bind to loopback addresses.
1548: If the
1549: .Ar bind_address
1550: is
1551: .Ql *
1552: or an empty string, then the forwarding is requested to listen on all
1553: interfaces.
1554: Specifying a remote
1555: .Ar bind_address
1.46 jmc 1556: will only succeed if the server's
1557: .Cm GatewayPorts
1.43 djm 1558: option is enabled (see
1.46 jmc 1559: .Xr sshd_config 5 ) .
1.149 djm 1560: .It Cm RequestTTY
1561: Specifies whether to request a pseudo-tty for the session.
1562: The argument may be one of:
1.240 jmc 1563: .Cm no
1.149 djm 1564: (never request a TTY),
1.240 jmc 1565: .Cm yes
1.149 djm 1566: (always request a TTY when standard input is a TTY),
1.240 jmc 1567: .Cm force
1.149 djm 1568: (always request a TTY) or
1.240 jmc 1569: .Cm auto
1.149 djm 1570: (request a TTY when opening a login session).
1571: This option mirrors the
1572: .Fl t
1573: and
1574: .Fl T
1575: flags for
1576: .Xr ssh 1 .
1.196 djm 1577: .It Cm RevokedHostKeys
1578: Specifies revoked host public keys.
1579: Keys listed in this file will be refused for host authentication.
1580: Note that if this file does not exist or is not readable,
1581: then host authentication will be refused for all hosts.
1582: Keys may be specified as a text file, listing one public key per line, or as
1583: an OpenSSH Key Revocation List (KRL) as generated by
1584: .Xr ssh-keygen 1 .
1585: For more information on KRLs, see the KEY REVOCATION LISTS section in
1586: .Xr ssh-keygen 1 .
1.305 naddy 1587: .It Cm SecurityKeyProvider
1.313 naddy 1588: Specifies a path to a library that will be used when loading any
1589: FIDO authenticator-hosted keys, overriding the default of using
1590: the built-in USB HID support.
1.309 naddy 1591: .Pp
1592: If the specified value begins with a
1593: .Sq $
1594: character, then it will be treated as an environment variable containing
1595: the path to the library.
1.32 djm 1596: .It Cm SendEnv
1597: Specifies what variables from the local
1598: .Xr environ 7
1599: should be sent to the server.
1.84 jmc 1600: The server must also support it, and the server must be configured to
1.33 djm 1601: accept these environment variables.
1.207 dtucker 1602: Note that the
1603: .Ev TERM
1.208 jmc 1604: environment variable is always sent whenever a
1.207 dtucker 1605: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1606: Refer to
1607: .Cm AcceptEnv
1608: in
1609: .Xr sshd_config 5
1610: for how to configure the server.
1.80 jmc 1611: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1612: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1613: across multiple
1614: .Cm SendEnv
1615: directives.
1.81 jmc 1616: .Pp
1617: See
1618: .Sx PATTERNS
1619: for more information on patterns.
1.271 djm 1620: .Pp
1.272 jmc 1621: It is possible to clear previously set
1.271 djm 1622: .Cm SendEnv
1623: variable names by prefixing patterns with
1624: .Pa - .
1625: The default is not to send any environment variables.
1.28 markus 1626: .It Cm ServerAliveCountMax
1.73 jmc 1627: Sets the number of server alive messages (see below) which may be
1.28 markus 1628: sent without
1.84 jmc 1629: .Xr ssh 1
1.28 markus 1630: receiving any messages back from the server.
1631: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1632: ssh will disconnect from the server, terminating the session.
1.28 markus 1633: It is important to note that the use of server alive messages is very
1634: different from
1635: .Cm TCPKeepAlive
1636: (below).
1637: The server alive messages are sent through the encrypted channel
1638: and therefore will not be spoofable.
1639: The TCP keepalive option enabled by
1640: .Cm TCPKeepAlive
1641: is spoofable.
1642: The server alive mechanism is valuable when the client or
1.298 dtucker 1643: server depend on knowing when a connection has become unresponsive.
1.28 markus 1644: .Pp
1645: The default value is 3.
1646: If, for example,
1647: .Cm ServerAliveInterval
1.84 jmc 1648: (see below) is set to 15 and
1.28 markus 1649: .Cm ServerAliveCountMax
1.84 jmc 1650: is left at the default, if the server becomes unresponsive,
1651: ssh will disconnect after approximately 45 seconds.
1.67 jmc 1652: .It Cm ServerAliveInterval
1653: Sets a timeout interval in seconds after which if no data has been received
1654: from the server,
1.84 jmc 1655: .Xr ssh 1
1.67 jmc 1656: will send a message through the encrypted
1657: channel to request a response from the server.
1658: The default
1659: is 0, indicating that these messages will not be sent to the server.
1.277 jmc 1660: .It Cm SetEnv
1661: Directly specify one or more environment variables and their contents to
1662: be sent to the server.
1663: Similarly to
1664: .Cm SendEnv ,
1665: the server must be prepared to accept the environment variable.
1.191 millert 1666: .It Cm StreamLocalBindMask
1667: Sets the octal file creation mode mask
1668: .Pq umask
1669: used when creating a Unix-domain socket file for local or remote
1670: port forwarding.
1671: This option is only used for port forwarding to a Unix-domain socket file.
1672: .Pp
1673: The default value is 0177, which creates a Unix-domain socket file that is
1674: readable and writable only by the owner.
1675: Note that not all operating systems honor the file mode on Unix-domain
1676: socket files.
1677: .It Cm StreamLocalBindUnlink
1678: Specifies whether to remove an existing Unix-domain socket file for local
1679: or remote port forwarding before creating a new one.
1680: If the socket file already exists and
1681: .Cm StreamLocalBindUnlink
1682: is not enabled,
1683: .Nm ssh
1684: will be unable to forward the port to the Unix-domain socket file.
1685: This option is only used for port forwarding to a Unix-domain socket file.
1686: .Pp
1687: The argument must be
1.240 jmc 1688: .Cm yes
1.191 millert 1689: or
1.240 jmc 1690: .Cm no
1691: (the default).
1.1 stevesk 1692: .It Cm StrictHostKeyChecking
1693: If this flag is set to
1.240 jmc 1694: .Cm yes ,
1.84 jmc 1695: .Xr ssh 1
1.1 stevesk 1696: will never automatically add host keys to the
1.50 djm 1697: .Pa ~/.ssh/known_hosts
1.1 stevesk 1698: file, and refuses to connect to hosts whose host key has changed.
1.263 dtucker 1699: This provides maximum protection against man-in-the-middle (MITM) attacks,
1.84 jmc 1700: though it can be annoying when the
1.1 stevesk 1701: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1702: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1703: frequently made.
1704: This option forces the user to manually
1705: add all new hosts.
1.255 jmc 1706: .Pp
1.1 stevesk 1707: If this flag is set to
1.254 djm 1708: .Dq accept-new
1.255 jmc 1709: then ssh will automatically add new host keys to the user
1.254 djm 1710: known hosts files, but will not permit connections to hosts with
1711: changed host keys.
1712: If this flag is set to
1713: .Dq no
1714: or
1715: .Dq off ,
1.255 jmc 1716: ssh will automatically add new host keys to the user known hosts files
1717: and allow connections to hosts with changed hostkeys to proceed,
1718: subject to some restrictions.
1.1 stevesk 1719: If this flag is set to
1.240 jmc 1720: .Cm ask
1721: (the default),
1.1 stevesk 1722: new host keys
1723: will be added to the user known host files only after the user
1724: has confirmed that is what they really want to do, and
1.84 jmc 1725: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1726: The host keys of
1727: known hosts will be verified automatically in all cases.
1.244 jmc 1728: .It Cm SyslogFacility
1729: Gives the facility code that is used when logging messages from
1730: .Xr ssh 1 .
1731: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1732: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1733: The default is USER.
1.26 markus 1734: .It Cm TCPKeepAlive
1735: Specifies whether the system should send TCP keepalive messages to the
1736: other side.
1737: If they are sent, death of the connection or crash of one
1738: of the machines will be properly noticed.
1739: However, this means that
1740: connections will die if the route is down temporarily, and some people
1741: find it annoying.
1742: .Pp
1743: The default is
1.240 jmc 1744: .Cm yes
1.26 markus 1745: (to send TCP keepalive messages), and the client will notice
1746: if the network goes down or the remote host dies.
1747: This is important in scripts, and many users want it too.
1748: .Pp
1749: To disable TCP keepalive messages, the value should be set to
1.240 jmc 1750: .Cm no .
1.265 djm 1751: See also
1752: .Cm ServerAliveInterval
1753: for protocol-level keepalives.
1.65 reyk 1754: .It Cm Tunnel
1.95 stevesk 1755: Request
1.65 reyk 1756: .Xr tun 4
1.69 jmc 1757: device forwarding between the client and the server.
1.65 reyk 1758: The argument must be
1.240 jmc 1759: .Cm yes ,
1760: .Cm point-to-point
1.95 stevesk 1761: (layer 3),
1.240 jmc 1762: .Cm ethernet
1.95 stevesk 1763: (layer 2),
1.65 reyk 1764: or
1.240 jmc 1765: .Cm no
1766: (the default).
1.95 stevesk 1767: Specifying
1.240 jmc 1768: .Cm yes
1.95 stevesk 1769: requests the default tunnel mode, which is
1.240 jmc 1770: .Cm point-to-point .
1.65 reyk 1771: .It Cm TunnelDevice
1.95 stevesk 1772: Specifies the
1.65 reyk 1773: .Xr tun 4
1.95 stevesk 1774: devices to open on the client
1775: .Pq Ar local_tun
1776: and the server
1777: .Pq Ar remote_tun .
1778: .Pp
1779: The argument must be
1780: .Sm off
1781: .Ar local_tun Op : Ar remote_tun .
1782: .Sm on
1783: The devices may be specified by numerical ID or the keyword
1.240 jmc 1784: .Cm any ,
1.95 stevesk 1785: which uses the next available tunnel device.
1786: If
1787: .Ar remote_tun
1788: is not specified, it defaults to
1.240 jmc 1789: .Cm any .
1.95 stevesk 1790: The default is
1.240 jmc 1791: .Cm any:any .
1.201 djm 1792: .It Cm UpdateHostKeys
1.200 djm 1793: Specifies whether
1794: .Xr ssh 1
1795: should accept notifications of additional hostkeys from the server sent
1796: after authentication has completed and add them to
1797: .Cm UserKnownHostsFile .
1798: The argument must be
1.240 jmc 1799: .Cm yes ,
1800: .Cm no
1.320 djm 1801: or
1.240 jmc 1802: .Cm ask .
1.320 djm 1803: This option allows learning alternate hostkeys for a server
1.201 djm 1804: and supports graceful key rotation by allowing a server to send replacement
1805: public keys before old ones are removed.
1.336 djm 1806: .Pp
1.200 djm 1807: Additional hostkeys are only accepted if the key used to authenticate the
1.336 djm 1808: host was already trusted or explicitly accepted by the user, the host was
1809: authenticated via
1810: .Cm UserKnownHostsFile
1811: (i.e. not
1812: .Cm GlobalKnownHostsFile )
1813: and the host was authenticated using a plain key and not a certificate.
1.320 djm 1814: .Pp
1815: .Cm UpdateHostKeys
1.321 jmc 1816: is enabled by default if the user has not overridden the default
1.320 djm 1817: .Cm UserKnownHostsFile
1.335 djm 1818: setting and has not enabled
1819: .Cm VerifyHostKeyDNS ,
1820: otherwise
1.320 djm 1821: .Cm UpdateHostKeys
1822: will be set to
1.334 djm 1823: .Cm no .
1.320 djm 1824: .Pp
1.204 djm 1825: If
1826: .Cm UpdateHostKeys
1827: is set to
1.240 jmc 1828: .Cm ask ,
1.204 djm 1829: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1830: Confirmation is currently incompatible with
1831: .Cm ControlPersist ,
1832: and will be disabled if it is enabled.
1.200 djm 1833: .Pp
1834: Presently, only
1835: .Xr sshd 8
1836: from OpenSSH 6.8 and greater support the
1.240 jmc 1837: .Qq hostkeys@openssh.com
1.200 djm 1838: protocol extension used to inform the client of all the server's hostkeys.
1.1 stevesk 1839: .It Cm User
1840: Specifies the user to log in as.
1841: This can be useful when a different user name is used on different machines.
1842: This saves the trouble of
1843: having to remember to give the user name on the command line.
1844: .It Cm UserKnownHostsFile
1.151 djm 1845: Specifies one or more files to use for the user
1846: host key database, separated by whitespace.
1.329 dtucker 1847: Each filename may use tilde notation to refer to the user's home directory,
1848: the tokens described in the
1849: .Sx TOKENS
1850: section and environment variables as described in the
1851: .Sx ENVIRONMENT VARIABLES
1852: section.
1.151 djm 1853: The default is
1854: .Pa ~/.ssh/known_hosts ,
1855: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1856: .It Cm VerifyHostKeyDNS
1857: Specifies whether to verify the remote key using DNS and SSHFP resource
1858: records.
1.24 jakob 1859: If this option is set to
1.240 jmc 1860: .Cm yes ,
1.25 jmc 1861: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1862: from DNS.
1863: Insecure fingerprints will be handled as if this option was set to
1.240 jmc 1864: .Cm ask .
1.24 jakob 1865: If this option is set to
1.240 jmc 1866: .Cm ask ,
1.24 jakob 1867: information on fingerprint match will be displayed, but the user will still
1868: need to confirm new host keys according to the
1869: .Cm StrictHostKeyChecking
1870: option.
1.8 jakob 1871: The default is
1.240 jmc 1872: .Cm no .
1.84 jmc 1873: .Pp
1.240 jmc 1874: See also
1875: .Sx VERIFYING HOST KEYS
1876: in
1.84 jmc 1877: .Xr ssh 1 .
1.111 grunk 1878: .It Cm VisualHostKey
1879: If this flag is set to
1.240 jmc 1880: .Cm yes ,
1.111 grunk 1881: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1882: printed in addition to the fingerprint string at login and
1.114 stevesk 1883: for unknown host keys.
1.111 grunk 1884: If this flag is set to
1.240 jmc 1885: .Cm no
1886: (the default),
1.114 stevesk 1887: no fingerprint strings are printed at login and
1.197 djm 1888: only the fingerprint string will be printed for unknown host keys.
1.1 stevesk 1889: .It Cm XAuthLocation
1.5 stevesk 1890: Specifies the full pathname of the
1.1 stevesk 1891: .Xr xauth 1
1892: program.
1893: The default is
1894: .Pa /usr/X11R6/bin/xauth .
1895: .El
1.86 jmc 1896: .Sh PATTERNS
1897: A
1898: .Em pattern
1899: consists of zero or more non-whitespace characters,
1900: .Sq *
1901: (a wildcard that matches zero or more characters),
1902: or
1903: .Sq ?\&
1904: (a wildcard that matches exactly one character).
1905: For example, to specify a set of declarations for any host in the
1.240 jmc 1906: .Qq .co.uk
1.86 jmc 1907: set of domains,
1908: the following pattern could be used:
1909: .Pp
1910: .Dl Host *.co.uk
1911: .Pp
1912: The following pattern
1913: would match any host in the 192.168.0.[0-9] network range:
1914: .Pp
1915: .Dl Host 192.168.0.?
1916: .Pp
1917: A
1918: .Em pattern-list
1919: is a comma-separated list of patterns.
1920: Patterns within pattern-lists may be negated
1921: by preceding them with an exclamation mark
1922: .Pq Sq !\& .
1923: For example,
1.174 djm 1924: to allow a key to be used from anywhere within an organization
1.86 jmc 1925: except from the
1.240 jmc 1926: .Qq dialup
1.86 jmc 1927: pool,
1928: the following entry (in authorized_keys) could be used:
1929: .Pp
1930: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.258 djm 1931: .Pp
1932: Note that a negated match will never produce a positive result by itself.
1933: For example, attempting to match
1934: .Qq host3
1935: against the following pattern-list will fail:
1936: .Pp
1937: .Dl from=\&"!host1,!host2\&"
1938: .Pp
1939: The solution here is to include a term that will yield a positive match,
1940: such as a wildcard:
1941: .Pp
1942: .Dl from=\&"!host1,!host2,*\&"
1.239 jmc 1943: .Sh TOKENS
1944: Arguments to some keywords can make use of tokens,
1945: which are expanded at runtime:
1946: .Pp
1947: .Bl -tag -width XXXX -offset indent -compact
1948: .It %%
1949: A literal
1950: .Sq % .
1951: .It \&%C
1.257 jmc 1952: Hash of %l%h%p%r.
1.239 jmc 1953: .It %d
1954: Local user's home directory.
1.339 djm 1955: .It %f
1956: The fingerprint of the server's host key.
1957: .It %H
1958: The
1959: .Pa known_hosts
1960: hostname or address that is being searched for.
1.239 jmc 1961: .It %h
1962: The remote hostname.
1.340 jmc 1963: .It \%%I
1.339 djm 1964: A string describing the reason for a
1965: .Cm KnownHostsCommand
1.340 jmc 1966: execution: either
1967: .Cm ADDRESS
1.339 djm 1968: when looking up a host by address (only when
1969: .Cm CheckHostIP
1970: is enabled),
1.340 jmc 1971: .Cm HOSTNAME
1972: when searching by hostname, or
1973: .Cm ORDER
1.339 djm 1974: when preparing the host key algorithm preference list to use for the
1975: destination host.
1.239 jmc 1976: .It %i
1977: The local user ID.
1.339 djm 1978: .It %K
1979: The base64 encoded host key.
1.330 dtucker 1980: .It %k
1981: The host key alias if specified, otherwise the orignal remote hostname given
1982: on the command line.
1.239 jmc 1983: .It %L
1984: The local hostname.
1985: .It %l
1986: The local hostname, including the domain name.
1987: .It %n
1988: The original remote hostname, as given on the command line.
1989: .It %p
1990: The remote port.
1991: .It %r
1992: The remote username.
1.261 djm 1993: .It \&%T
1994: The local
1995: .Xr tun 4
1996: or
1997: .Xr tap 4
1998: network interface assigned if
1.262 jmc 1999: tunnel forwarding was requested, or
2000: .Qq NONE
1.261 djm 2001: otherwise.
1.339 djm 2002: .It %t
2003: The type of the server host key, e.g.
2004: .Cm ssh-ed25519
1.239 jmc 2005: .It %u
2006: The local username.
2007: .El
2008: .Pp
1.323 dtucker 2009: .Cm CertificateFile ,
2010: .Cm ControlPath ,
2011: .Cm IdentityAgent ,
2012: .Cm IdentityFile ,
1.339 djm 2013: .Cm KnownHostsCommand ,
1.325 jmc 2014: .Cm LocalForward ,
1.324 dtucker 2015: .Cm Match exec ,
2016: .Cm RemoteCommand ,
1.329 dtucker 2017: .Cm RemoteForward ,
1.323 dtucker 2018: and
1.331 jmc 2019: .Cm UserKnownHostsFile
1.323 dtucker 2020: accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u.
1.239 jmc 2021: .Pp
1.339 djm 2022: .Cm KnownHostsCommand
2023: additionally accepts the tokens %f, %H, %I, %K and %t.
2024: .Pp
1.295 jmc 2025: .Cm Hostname
1.239 jmc 2026: accepts the tokens %% and %h.
2027: .Pp
2028: .Cm LocalCommand
1.323 dtucker 2029: accepts all tokens.
1.239 jmc 2030: .Pp
2031: .Cm ProxyCommand
1.303 djm 2032: accepts the tokens %%, %h, %n, %p, and %r.
1.326 dtucker 2033: .Sh ENVIRONMENT VARIABLES
2034: Arguments to some keywords can be expanded at runtime from environment
2035: variables on the client by enclosing them in
2036: .Ic ${} ,
2037: for example
2038: .Ic ${HOME}/.ssh
2039: would refer to the user's .ssh directory.
2040: If a specified environment variable does not exist then an error will be
2041: returned and the setting for that keyword will be ignored.
2042: .Pp
2043: The keywords
2044: .Cm CertificateFile ,
2045: .Cm ControlPath ,
1.329 dtucker 2046: .Cm IdentityAgent ,
2047: .Cm IdentityFile
1.339 djm 2048: .Cm KnownHostsCommand ,
1.326 dtucker 2049: and
1.329 dtucker 2050: .Cm UserKnownHostsFile
1.326 dtucker 2051: support environment variables.
2052: The keywords
2053: .Cm LocalForward
2054: and
2055: .Cm RemoteForward
2056: support environment variables only for Unix domain socket paths.
1.1 stevesk 2057: .Sh FILES
2058: .Bl -tag -width Ds
1.50 djm 2059: .It Pa ~/.ssh/config
1.1 stevesk 2060: This is the per-user configuration file.
2061: The format of this file is described above.
1.84 jmc 2062: This file is used by the SSH client.
1.30 djm 2063: Because of the potential for abuse, this file must have strict permissions:
1.290 jmc 2064: read/write for the user, and not writable by others.
1.1 stevesk 2065: .It Pa /etc/ssh/ssh_config
2066: Systemwide configuration file.
2067: This file provides defaults for those
2068: values that are not specified in the user's configuration file, and
2069: for those users who do not have a configuration file.
2070: This file must be world-readable.
2071: .El
1.13 jmc 2072: .Sh SEE ALSO
2073: .Xr ssh 1
1.1 stevesk 2074: .Sh AUTHORS
1.240 jmc 2075: .An -nosplit
1.1 stevesk 2076: OpenSSH is a derivative of the original and free
1.240 jmc 2077: ssh 1.2.12 release by
2078: .An Tatu Ylonen .
2079: .An Aaron Campbell , Bob Beck , Markus Friedl ,
2080: .An Niels Provos , Theo de Raadt
2081: and
2082: .An Dug Song
1.1 stevesk 2083: removed many bugs, re-added newer features and
2084: created OpenSSH.
1.240 jmc 2085: .An Markus Friedl
2086: contributed the support for SSH protocol versions 1.5 and 2.0.
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