Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.218
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.218 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.217 2015/09/04 06:40:45 jmc Exp $
1.217 jmc 37: .Dd $Mdocdate: September 4 2015 $
1.1 stevesk 38: .Dt SSH_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm ssh_config
42: .Nd OpenSSH SSH client configuration files
43: .Sh SYNOPSIS
1.98 jmc 44: .Nm ~/.ssh/config
45: .Nm /etc/ssh/ssh_config
1.1 stevesk 46: .Sh DESCRIPTION
1.84 jmc 47: .Xr ssh 1
1.1 stevesk 48: obtains configuration data from the following sources in
49: the following order:
1.79 jmc 50: .Pp
1.2 stevesk 51: .Bl -enum -offset indent -compact
52: .It
53: command-line options
54: .It
55: user's configuration file
1.50 djm 56: .Pq Pa ~/.ssh/config
1.2 stevesk 57: .It
58: system-wide configuration file
59: .Pq Pa /etc/ssh/ssh_config
60: .El
1.1 stevesk 61: .Pp
62: For each parameter, the first obtained value
63: will be used.
1.41 jmc 64: The configuration files contain sections separated by
1.1 stevesk 65: .Dq Host
66: specifications, and that section is only applied for hosts that
67: match one of the patterns given in the specification.
1.193 djm 68: The matched host name is usually the one given on the command line
69: (see the
70: .Cm CanonicalizeHostname
71: option for exceptions.)
1.1 stevesk 72: .Pp
73: Since the first obtained value for each parameter is used, more
74: host-specific declarations should be given near the beginning of the
75: file, and general defaults at the end.
1.80 jmc 76: .Pp
1.1 stevesk 77: The configuration file has the following format:
78: .Pp
79: Empty lines and lines starting with
80: .Ql #
81: are comments.
82: Otherwise a line is of the format
83: .Dq keyword arguments .
84: Configuration options may be separated by whitespace or
85: optional whitespace and exactly one
86: .Ql = ;
87: the latter format is useful to avoid the need to quote whitespace
88: when specifying configuration options using the
89: .Nm ssh ,
1.87 jmc 90: .Nm scp ,
1.1 stevesk 91: and
92: .Nm sftp
93: .Fl o
94: option.
1.88 dtucker 95: Arguments may optionally be enclosed in double quotes
96: .Pq \&"
97: in order to represent arguments containing spaces.
1.1 stevesk 98: .Pp
99: The possible
100: keywords and their meanings are as follows (note that
101: keywords are case-insensitive and arguments are case-sensitive):
102: .Bl -tag -width Ds
103: .It Cm Host
104: Restricts the following declarations (up to the next
105: .Cm Host
1.169 djm 106: or
107: .Cm Match
1.1 stevesk 108: keyword) to be only for those hosts that match one of the patterns
109: given after the keyword.
1.112 krw 110: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 111: A single
1.83 jmc 112: .Ql *
1.1 stevesk 113: as a pattern can be used to provide global
114: defaults for all hosts.
1.193 djm 115: The host is usually the
1.1 stevesk 116: .Ar hostname
1.193 djm 117: argument given on the command line
118: (see the
119: .Cm CanonicalizeHostname
120: option for exceptions.)
1.148 djm 121: .Pp
122: A pattern entry may be negated by prefixing it with an exclamation mark
123: .Pq Sq !\& .
124: If a negated entry is matched, then the
125: .Cm Host
126: entry is ignored, regardless of whether any other patterns on the line
127: match.
128: Negated matches are therefore useful to provide exceptions for wildcard
129: matches.
1.81 jmc 130: .Pp
131: See
132: .Sx PATTERNS
133: for more information on patterns.
1.170 jmc 134: .It Cm Match
1.169 djm 135: Restricts the following declarations (up to the next
136: .Cm Host
137: or
138: .Cm Match
139: keyword) to be used only when the conditions following the
140: .Cm Match
141: keyword are satisfied.
1.193 djm 142: Match conditions are specified using one or more critera
1.178 dtucker 143: or the single token
144: .Cm all
1.193 djm 145: which always matches.
146: The available criteria keywords are:
147: .Cm canonical ,
1.176 djm 148: .Cm exec ,
1.169 djm 149: .Cm host ,
150: .Cm originalhost ,
151: .Cm user ,
152: and
153: .Cm localuser .
1.193 djm 154: The
155: .Cm all
156: criteria must appear alone or immediately after
1.194 jmc 157: .Cm canonical .
1.193 djm 158: Other criteria may be combined arbitrarily.
159: All criteria but
160: .Cm all
161: and
162: .Cm canonical
163: require an argument.
164: Criteria may be negated by prepending an exclamation mark
165: .Pq Sq !\& .
1.169 djm 166: .Pp
1.177 jmc 167: The
1.193 djm 168: .Cm canonical
1.210 dtucker 169: keyword matches only when the configuration file is being re-parsed
1.193 djm 170: after hostname canonicalization (see the
171: .Cm CanonicalizeHostname
172: option.)
173: This may be useful to specify conditions that work with canonical host
174: names only.
175: The
1.176 djm 176: .Cm exec
1.177 jmc 177: keyword executes the specified command under the user's shell.
1.169 djm 178: If the command returns a zero exit status then the condition is considered true.
179: Commands containing whitespace characters must be quoted.
1.175 djm 180: The following character sequences in the command will be expanded prior to
181: execution:
182: .Ql %L
183: will be substituted by the first component of the local host name,
184: .Ql %l
185: will be substituted by the local host name (including any domain name),
186: .Ql %h
187: will be substituted by the target host name,
188: .Ql %n
189: will be substituted by the original target host name
1.176 djm 190: specified on the command-line,
1.175 djm 191: .Ql %p
192: the destination port,
193: .Ql %r
194: by the remote login username, and
195: .Ql %u
196: by the username of the user running
197: .Xr ssh 1 .
1.169 djm 198: .Pp
199: The other keywords' criteria must be single entries or comma-separated
200: lists and may use the wildcard and negation operators described in the
201: .Sx PATTERNS
202: section.
203: The criteria for the
204: .Cm host
205: keyword are matched against the target hostname, after any substitution
206: by the
207: .Cm Hostname
1.193 djm 208: or
209: .Cm CanonicalizeHostname
210: options.
1.169 djm 211: The
212: .Cm originalhost
213: keyword matches against the hostname as it was specified on the command-line.
214: The
215: .Cm user
216: keyword matches against the target username on the remote host.
217: The
218: .Cm localuser
219: keyword matches against the name of the local user running
220: .Xr ssh 1
221: (this keyword may be useful in system-wide
222: .Nm
223: files).
1.10 djm 224: .It Cm AddressFamily
1.11 jmc 225: Specifies which address family to use when connecting.
226: Valid arguments are
1.10 djm 227: .Dq any ,
228: .Dq inet
1.84 jmc 229: (use IPv4 only), or
1.10 djm 230: .Dq inet6
1.40 jmc 231: (use IPv6 only).
1.1 stevesk 232: .It Cm BatchMode
233: If set to
234: .Dq yes ,
235: passphrase/password querying will be disabled.
236: This option is useful in scripts and other batch jobs where no user
237: is present to supply the password.
238: The argument must be
239: .Dq yes
240: or
241: .Dq no .
242: The default is
243: .Dq no .
244: .It Cm BindAddress
1.60 dtucker 245: Use the specified address on the local machine as the source address of
1.61 jmc 246: the connection.
247: Only useful on systems with more than one address.
1.1 stevesk 248: Note that this option does not work if
249: .Cm UsePrivilegedPort
250: is set to
251: .Dq yes .
1.171 djm 252: .It Cm CanonicalDomains
1.172 jmc 253: When
1.173 djm 254: .Cm CanonicalizeHostname
1.171 djm 255: is enabled, this option specifies the list of domain suffixes in which to
256: search for the specified destination host.
1.173 djm 257: .It Cm CanonicalizeFallbackLocal
1.174 djm 258: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 259: The default,
1.184 djm 260: .Dq yes ,
1.172 jmc 261: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 262: search rules.
263: A value of
1.184 djm 264: .Dq no
1.171 djm 265: will cause
266: .Xr ssh 1
267: to fail instantly if
1.173 djm 268: .Cm CanonicalizeHostname
1.171 djm 269: is enabled and the target hostname cannot be found in any of the domains
270: specified by
271: .Cm CanonicalDomains .
1.173 djm 272: .It Cm CanonicalizeHostname
1.174 djm 273: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 274: The default,
275: .Dq no ,
1.171 djm 276: is not to perform any name rewriting and let the system resolver handle all
277: hostname lookups.
278: If set to
279: .Dq yes
280: then, for connections that do not use a
281: .Cm ProxyCommand ,
282: .Xr ssh 1
1.173 djm 283: will attempt to canonicalize the hostname specified on the command line
1.171 djm 284: using the
285: .Cm CanonicalDomains
286: suffixes and
1.173 djm 287: .Cm CanonicalizePermittedCNAMEs
1.171 djm 288: rules.
289: If
1.173 djm 290: .Cm CanonicalizeHostname
1.171 djm 291: is set to
292: .Dq always ,
1.174 djm 293: then canonicalization is applied to proxied connections too.
1.185 djm 294: .Pp
1.193 djm 295: If this option is enabled, then the configuration files are processed
296: again using the new target name to pick up any new configuration in matching
1.185 djm 297: .Cm Host
1.193 djm 298: and
299: .Cm Match
1.185 djm 300: stanzas.
1.173 djm 301: .It Cm CanonicalizeMaxDots
1.172 jmc 302: Specifies the maximum number of dot characters in a hostname before
1.174 djm 303: canonicalization is disabled.
1.172 jmc 304: The default,
305: .Dq 1 ,
306: allows a single dot (i.e. hostname.subdomain).
1.173 djm 307: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 308: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 309: canonicalizing hostnames.
1.171 djm 310: The rules consist of one or more arguments of
1.172 jmc 311: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 312: where
313: .Ar source_domain_list
1.174 djm 314: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 315: and
316: .Ar target_domain_list
1.172 jmc 317: is a pattern-list of domains that they may resolve to.
1.171 djm 318: .Pp
319: For example,
320: .Dq *.a.example.com:*.b.example.com,*.c.example.com
321: will allow hostnames matching
322: .Dq *.a.example.com
1.173 djm 323: to be canonicalized to names in the
1.171 djm 324: .Dq *.b.example.com
325: or
326: .Dq *.c.example.com
327: domains.
1.1 stevesk 328: .It Cm ChallengeResponseAuthentication
1.82 jmc 329: Specifies whether to use challenge-response authentication.
1.1 stevesk 330: The argument to this keyword must be
331: .Dq yes
332: or
333: .Dq no .
334: The default is
335: .Dq yes .
336: .It Cm CheckHostIP
337: If this flag is set to
338: .Dq yes ,
1.84 jmc 339: .Xr ssh 1
340: will additionally check the host IP address in the
1.1 stevesk 341: .Pa known_hosts
342: file.
1.211 djm 343: This allows ssh to detect if a host key changed due to DNS spoofing
344: and will add addresses of destination hosts to
345: .Pa ~/.ssh/known_hosts
346: in the process, regardless of the setting of
347: .Cm StrictHostKeyChecking .
1.107 grunk 348: If the option is set to
1.1 stevesk 349: .Dq no ,
350: the check will not be executed.
351: The default is
352: .Dq yes .
353: .It Cm Cipher
354: Specifies the cipher to use for encrypting the session
355: in protocol version 1.
356: Currently,
357: .Dq blowfish ,
358: .Dq 3des ,
359: and
360: .Dq des
361: are supported.
362: .Ar des
363: is only supported in the
1.84 jmc 364: .Xr ssh 1
1.1 stevesk 365: client for interoperability with legacy protocol 1 implementations
366: that do not support the
367: .Ar 3des
1.7 jmc 368: cipher.
369: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 370: The default is
371: .Dq 3des .
372: .It Cm Ciphers
373: Specifies the ciphers allowed for protocol version 2
374: in order of preference.
375: Multiple ciphers must be comma-separated.
1.214 djm 376: If the specified value begins with a
377: .Sq +
378: character, then the specified ciphers will be appended to the default set
379: instead of replacing them.
380: .Pp
1.180 djm 381: The supported ciphers are:
382: .Pp
1.186 naddy 383: .Bl -item -compact -offset indent
384: .It
385: 3des-cbc
386: .It
387: aes128-cbc
388: .It
389: aes192-cbc
390: .It
391: aes256-cbc
392: .It
393: aes128-ctr
394: .It
395: aes192-ctr
396: .It
397: aes256-ctr
398: .It
399: aes128-gcm@openssh.com
400: .It
401: aes256-gcm@openssh.com
402: .It
403: arcfour
404: .It
405: arcfour128
406: .It
407: arcfour256
408: .It
409: blowfish-cbc
410: .It
411: cast128-cbc
412: .It
413: chacha20-poly1305@openssh.com
414: .El
1.180 djm 415: .Pp
1.84 jmc 416: The default is:
1.186 naddy 417: .Bd -literal -offset indent
1.215 jmc 418: chacha20-poly1305@openssh.com,
1.186 naddy 419: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 420: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.186 naddy 421: arcfour256,arcfour128,
422: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
423: aes192-cbc,aes256-cbc,arcfour
1.1 stevesk 424: .Ed
1.180 djm 425: .Pp
426: The list of available ciphers may also be obtained using the
427: .Fl Q
428: option of
1.198 djm 429: .Xr ssh 1
430: with an argument of
431: .Dq cipher .
1.1 stevesk 432: .It Cm ClearAllForwardings
1.84 jmc 433: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 434: specified in the configuration files or on the command line be
1.7 jmc 435: cleared.
436: This option is primarily useful when used from the
1.84 jmc 437: .Xr ssh 1
1.1 stevesk 438: command line to clear port forwardings set in
439: configuration files, and is automatically set by
440: .Xr scp 1
441: and
442: .Xr sftp 1 .
443: The argument must be
444: .Dq yes
445: or
446: .Dq no .
447: The default is
448: .Dq no .
449: .It Cm Compression
450: Specifies whether to use compression.
451: The argument must be
452: .Dq yes
453: or
454: .Dq no .
455: The default is
456: .Dq no .
457: .It Cm CompressionLevel
458: Specifies the compression level to use if compression is enabled.
459: The argument must be an integer from 1 (fast) to 9 (slow, best).
460: The default level is 6, which is good for most applications.
461: The meaning of the values is the same as in
462: .Xr gzip 1 .
463: Note that this option applies to protocol version 1 only.
464: .It Cm ConnectionAttempts
465: Specifies the number of tries (one per second) to make before exiting.
466: The argument must be an integer.
467: This may be useful in scripts if the connection sometimes fails.
468: The default is 1.
1.9 djm 469: .It Cm ConnectTimeout
1.84 jmc 470: Specifies the timeout (in seconds) used when connecting to the
471: SSH server, instead of using the default system TCP timeout.
1.11 jmc 472: This value is used only when the target is down or really unreachable,
473: not when it refuses the connection.
1.36 djm 474: .It Cm ControlMaster
475: Enables the sharing of multiple sessions over a single network connection.
476: When set to
1.84 jmc 477: .Dq yes ,
478: .Xr ssh 1
1.36 djm 479: will listen for connections on a control socket specified using the
480: .Cm ControlPath
481: argument.
482: Additional sessions can connect to this socket using the same
483: .Cm ControlPath
484: with
485: .Cm ControlMaster
486: set to
487: .Dq no
1.38 jmc 488: (the default).
1.64 jmc 489: These sessions will try to reuse the master instance's network connection
1.63 djm 490: rather than initiating new ones, but will fall back to connecting normally
491: if the control socket does not exist, or is not listening.
492: .Pp
1.37 djm 493: Setting this to
494: .Dq ask
1.84 jmc 495: will cause ssh
1.206 jmc 496: to listen for control connections, but require confirmation using
497: .Xr ssh-askpass 1 .
1.51 jakob 498: If the
499: .Cm ControlPath
1.84 jmc 500: cannot be opened,
501: ssh will continue without connecting to a master instance.
1.58 djm 502: .Pp
503: X11 and
1.59 jmc 504: .Xr ssh-agent 1
1.58 djm 505: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 506: display and agent forwarded will be the one belonging to the master
1.59 jmc 507: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 508: .Pp
509: Two additional options allow for opportunistic multiplexing: try to use a
510: master connection but fall back to creating a new one if one does not already
511: exist.
512: These options are:
513: .Dq auto
514: and
515: .Dq autoask .
516: The latter requires confirmation like the
517: .Dq ask
518: option.
1.36 djm 519: .It Cm ControlPath
1.55 djm 520: Specify the path to the control socket used for connection sharing as described
521: in the
1.36 djm 522: .Cm ControlMaster
1.57 djm 523: section above or the string
524: .Dq none
525: to disable connection sharing.
1.55 djm 526: In the path,
1.147 djm 527: .Ql %L
528: will be substituted by the first component of the local host name,
1.77 djm 529: .Ql %l
1.147 djm 530: will be substituted by the local host name (including any domain name),
1.55 djm 531: .Ql %h
532: will be substituted by the target host name,
1.150 jmc 533: .Ql %n
534: will be substituted by the original target host name
535: specified on the command line,
1.55 djm 536: .Ql %p
1.175 djm 537: the destination port,
1.55 djm 538: .Ql %r
1.188 djm 539: by the remote login username,
1.147 djm 540: .Ql %u
1.218 ! djm 541: by the username and
! 542: .Ql %i
! 543: by the numeric user id (uid) of the user running
1.188 djm 544: .Xr ssh 1 , and
1.189 jmc 545: .Ql \&%C
1.188 djm 546: by a hash of the concatenation: %l%h%p%r.
1.56 djm 547: It is recommended that any
548: .Cm ControlPath
549: used for opportunistic connection sharing include
1.195 djm 550: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
551: that is not writable by other users.
1.56 djm 552: This ensures that shared connections are uniquely identified.
1.137 djm 553: .It Cm ControlPersist
554: When used in conjunction with
555: .Cm ControlMaster ,
556: specifies that the master connection should remain open
557: in the background (waiting for future client connections)
558: after the initial client connection has been closed.
559: If set to
560: .Dq no ,
561: then the master connection will not be placed into the background,
562: and will close as soon as the initial client connection is closed.
563: If set to
1.195 djm 564: .Dq yes
565: or
566: .Dq 0 ,
1.137 djm 567: then the master connection will remain in the background indefinitely
568: (until killed or closed via a mechanism such as the
569: .Xr ssh 1
570: .Dq Fl O No exit
571: option).
572: If set to a time in seconds, or a time in any of the formats documented in
573: .Xr sshd_config 5 ,
574: then the backgrounded master connection will automatically terminate
575: after it has remained idle (with no client connections) for the
576: specified time.
1.38 jmc 577: .It Cm DynamicForward
1.74 jmc 578: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 579: over the secure channel, and the application
580: protocol is then used to determine where to connect to from the
581: remote machine.
1.62 djm 582: .Pp
583: The argument must be
584: .Sm off
585: .Oo Ar bind_address : Oc Ar port .
586: .Sm on
1.138 djm 587: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 588: By default, the local port is bound in accordance with the
589: .Cm GatewayPorts
590: setting.
591: However, an explicit
592: .Ar bind_address
593: may be used to bind the connection to a specific address.
594: The
595: .Ar bind_address
596: of
597: .Dq localhost
598: indicates that the listening port be bound for local use only, while an
599: empty address or
600: .Sq *
601: indicates that the port should be available from all interfaces.
602: .Pp
1.38 jmc 603: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 604: .Xr ssh 1
1.38 jmc 605: will act as a SOCKS server.
606: Multiple forwardings may be specified, and
607: additional forwardings can be given on the command line.
608: Only the superuser can forward privileged ports.
1.14 markus 609: .It Cm EnableSSHKeysign
610: Setting this option to
611: .Dq yes
612: in the global client configuration file
613: .Pa /etc/ssh/ssh_config
614: enables the use of the helper program
615: .Xr ssh-keysign 8
616: during
617: .Cm HostbasedAuthentication .
618: The argument must be
619: .Dq yes
620: or
621: .Dq no .
622: The default is
623: .Dq no .
1.23 jmc 624: This option should be placed in the non-hostspecific section.
1.14 markus 625: See
626: .Xr ssh-keysign 8
627: for more information.
1.1 stevesk 628: .It Cm EscapeChar
629: Sets the escape character (default:
630: .Ql ~ ) .
631: The escape character can also
632: be set on the command line.
633: The argument should be a single character,
634: .Ql ^
635: followed by a letter, or
636: .Dq none
637: to disable the escape
638: character entirely (making the connection transparent for binary
639: data).
1.96 markus 640: .It Cm ExitOnForwardFailure
641: Specifies whether
642: .Xr ssh 1
643: should terminate the connection if it cannot set up all requested
1.216 djm 644: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 645: if either end is unable to bind and listen on a specified port).
1.216 djm 646: Note that
647: .Cm ExitOnForwardFailure
648: does not apply to connections made over port forwardings and will not,
649: for example, cause
650: .Xr ssh 1
651: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 652: The argument must be
653: .Dq yes
654: or
655: .Dq no .
656: The default is
657: .Dq no .
1.197 djm 658: .It Cm FingerprintHash
659: Specifies the hash algorithm used when displaying key fingerprints.
660: Valid options are:
661: .Dq md5
662: and
663: .Dq sha256 .
664: The default is
665: .Dq sha256 .
1.1 stevesk 666: .It Cm ForwardAgent
667: Specifies whether the connection to the authentication agent (if any)
668: will be forwarded to the remote machine.
669: The argument must be
670: .Dq yes
671: or
672: .Dq no .
673: The default is
674: .Dq no .
1.3 stevesk 675: .Pp
1.7 jmc 676: Agent forwarding should be enabled with caution.
677: Users with the ability to bypass file permissions on the remote host
678: (for the agent's Unix-domain socket)
679: can access the local agent through the forwarded connection.
680: An attacker cannot obtain key material from the agent,
1.3 stevesk 681: however they can perform operations on the keys that enable them to
682: authenticate using the identities loaded into the agent.
1.1 stevesk 683: .It Cm ForwardX11
684: Specifies whether X11 connections will be automatically redirected
685: over the secure channel and
686: .Ev DISPLAY
687: set.
688: The argument must be
689: .Dq yes
690: or
691: .Dq no .
692: The default is
693: .Dq no .
1.3 stevesk 694: .Pp
1.7 jmc 695: X11 forwarding should be enabled with caution.
696: Users with the ability to bypass file permissions on the remote host
1.22 markus 697: (for the user's X11 authorization database)
1.7 jmc 698: can access the local X11 display through the forwarded connection.
1.22 markus 699: An attacker may then be able to perform activities such as keystroke monitoring
700: if the
701: .Cm ForwardX11Trusted
702: option is also enabled.
1.134 djm 703: .It Cm ForwardX11Timeout
1.135 jmc 704: Specify a timeout for untrusted X11 forwarding
705: using the format described in the
1.166 jmc 706: TIME FORMATS section of
1.134 djm 707: .Xr sshd_config 5 .
708: X11 connections received by
709: .Xr ssh 1
710: after this time will be refused.
711: The default is to disable untrusted X11 forwarding after twenty minutes has
712: elapsed.
1.22 markus 713: .It Cm ForwardX11Trusted
1.34 jmc 714: If this option is set to
1.84 jmc 715: .Dq yes ,
716: remote X11 clients will have full access to the original X11 display.
1.42 djm 717: .Pp
1.22 markus 718: If this option is set to
1.84 jmc 719: .Dq no ,
720: remote X11 clients will be considered untrusted and prevented
1.22 markus 721: from stealing or tampering with data belonging to trusted X11
722: clients.
1.42 djm 723: Furthermore, the
724: .Xr xauth 1
725: token used for the session will be set to expire after 20 minutes.
726: Remote clients will be refused access after this time.
1.22 markus 727: .Pp
728: The default is
729: .Dq no .
730: .Pp
731: See the X11 SECURITY extension specification for full details on
732: the restrictions imposed on untrusted clients.
1.1 stevesk 733: .It Cm GatewayPorts
734: Specifies whether remote hosts are allowed to connect to local
735: forwarded ports.
736: By default,
1.84 jmc 737: .Xr ssh 1
1.7 jmc 738: binds local port forwardings to the loopback address.
739: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 740: .Cm GatewayPorts
1.84 jmc 741: can be used to specify that ssh
1.1 stevesk 742: should bind local port forwardings to the wildcard address,
743: thus allowing remote hosts to connect to forwarded ports.
744: The argument must be
745: .Dq yes
746: or
747: .Dq no .
748: The default is
749: .Dq no .
750: .It Cm GlobalKnownHostsFile
1.151 djm 751: Specifies one or more files to use for the global
752: host key database, separated by whitespace.
753: The default is
754: .Pa /etc/ssh/ssh_known_hosts ,
755: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 756: .It Cm GSSAPIAuthentication
1.27 markus 757: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 758: The default is
1.21 markus 759: .Dq no .
1.18 markus 760: Note that this option applies to protocol version 2 only.
761: .It Cm GSSAPIDelegateCredentials
762: Forward (delegate) credentials to the server.
763: The default is
764: .Dq no .
765: Note that this option applies to protocol version 2 only.
1.44 djm 766: .It Cm HashKnownHosts
767: Indicates that
1.84 jmc 768: .Xr ssh 1
1.44 djm 769: should hash host names and addresses when they are added to
1.50 djm 770: .Pa ~/.ssh/known_hosts .
1.44 djm 771: These hashed names may be used normally by
1.84 jmc 772: .Xr ssh 1
1.44 djm 773: and
1.84 jmc 774: .Xr sshd 8 ,
1.44 djm 775: but they do not reveal identifying information should the file's contents
776: be disclosed.
777: The default is
778: .Dq no .
1.97 jmc 779: Note that existing names and addresses in known hosts files
780: will not be converted automatically,
781: but may be manually hashed using
1.45 djm 782: .Xr ssh-keygen 1 .
1.1 stevesk 783: .It Cm HostbasedAuthentication
784: Specifies whether to try rhosts based authentication with public key
785: authentication.
786: The argument must be
787: .Dq yes
788: or
789: .Dq no .
790: The default is
791: .Dq no .
792: This option applies to protocol version 2 only and
793: is similar to
794: .Cm RhostsRSAAuthentication .
1.202 djm 795: .It Cm HostbasedKeyTypes
796: Specifies the key types that will be used for hostbased authentication
797: as a comma-separated pattern list.
1.214 djm 798: Alternately if the specified value begins with a
799: .Sq +
800: character, then the specified key types will be appended to the default set
801: instead of replacing them.
1.213 markus 802: The default for this option is:
803: .Bd -literal -offset 3n
804: ecdsa-sha2-nistp256-cert-v01@openssh.com,
805: ecdsa-sha2-nistp384-cert-v01@openssh.com,
806: ecdsa-sha2-nistp521-cert-v01@openssh.com,
807: ssh-ed25519-cert-v01@openssh.com,
808: ssh-rsa-cert-v01@openssh.com,
809: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
810: ssh-ed25519,ssh-rsa
811: .Ed
812: .Pp
1.202 djm 813: The
814: .Fl Q
815: option of
816: .Xr ssh 1
817: may be used to list supported key types.
1.1 stevesk 818: .It Cm HostKeyAlgorithms
819: Specifies the protocol version 2 host key algorithms
820: that the client wants to use in order of preference.
1.214 djm 821: Alternately if the specified value begins with a
822: .Sq +
823: character, then the specified key types will be appended to the default set
824: instead of replacing them.
1.1 stevesk 825: The default for this option is:
1.139 djm 826: .Bd -literal -offset 3n
827: ecdsa-sha2-nistp256-cert-v01@openssh.com,
828: ecdsa-sha2-nistp384-cert-v01@openssh.com,
829: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 830: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 831: ssh-rsa-cert-v01@openssh.com,
1.139 djm 832: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.213 markus 833: ssh-ed25519,ssh-rsa
1.139 djm 834: .Ed
1.145 djm 835: .Pp
836: If hostkeys are known for the destination host then this default is modified
837: to prefer their algorithms.
1.198 djm 838: .Pp
839: The list of available key types may also be obtained using the
840: .Fl Q
841: option of
842: .Xr ssh 1
843: with an argument of
844: .Dq key .
1.1 stevesk 845: .It Cm HostKeyAlias
846: Specifies an alias that should be used instead of the
847: real host name when looking up or saving the host key
848: in the host key database files.
1.84 jmc 849: This option is useful for tunneling SSH connections
1.1 stevesk 850: or for multiple servers running on a single host.
851: .It Cm HostName
852: Specifies the real host name to log into.
853: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 854: If the hostname contains the character sequence
855: .Ql %h ,
1.150 jmc 856: then this will be replaced with the host name specified on the command line
1.136 djm 857: (this is useful for manipulating unqualified names).
1.187 djm 858: The character sequence
859: .Ql %%
860: will be replaced by a single
861: .Ql %
862: character, which may be used when specifying IPv6 link-local addresses.
863: .Pp
1.84 jmc 864: The default is the name given on the command line.
1.1 stevesk 865: Numeric IP addresses are also permitted (both on the command line and in
866: .Cm HostName
867: specifications).
1.29 markus 868: .It Cm IdentitiesOnly
869: Specifies that
1.84 jmc 870: .Xr ssh 1
1.29 markus 871: should only use the authentication identity files configured in the
1.31 jmc 872: .Nm
1.29 markus 873: files,
1.84 jmc 874: even if
875: .Xr ssh-agent 1
1.159 djm 876: or a
877: .Cm PKCS11Provider
1.29 markus 878: offers more identities.
879: The argument to this keyword must be
880: .Dq yes
881: or
882: .Dq no .
1.84 jmc 883: This option is intended for situations where ssh-agent
1.29 markus 884: offers many different identities.
885: The default is
886: .Dq no .
1.67 jmc 887: .It Cm IdentityFile
1.192 sobrado 888: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 889: identity is read.
1.67 jmc 890: The default is
891: .Pa ~/.ssh/identity
892: for protocol version 1, and
1.139 djm 893: .Pa ~/.ssh/id_dsa ,
1.183 naddy 894: .Pa ~/.ssh/id_ecdsa ,
895: .Pa ~/.ssh/id_ed25519
1.139 djm 896: and
1.67 jmc 897: .Pa ~/.ssh/id_rsa
898: for protocol version 2.
899: Additionally, any identities represented by the authentication agent
1.165 djm 900: will be used for authentication unless
901: .Cm IdentitiesOnly
902: is set.
1.129 djm 903: .Xr ssh 1
904: will try to load certificate information from the filename obtained by
905: appending
906: .Pa -cert.pub
907: to the path of a specified
908: .Cm IdentityFile .
1.90 djm 909: .Pp
1.67 jmc 910: The file name may use the tilde
1.91 jmc 911: syntax to refer to a user's home directory or one of the following
1.90 djm 912: escape characters:
913: .Ql %d
914: (local user's home directory),
915: .Ql %u
916: (local user name),
917: .Ql %l
918: (local host name),
919: .Ql %h
920: (remote host name) or
1.92 djm 921: .Ql %r
1.90 djm 922: (remote user name).
923: .Pp
1.67 jmc 924: It is possible to have
925: multiple identity files specified in configuration files; all these
926: identities will be tried in sequence.
1.152 djm 927: Multiple
928: .Cm IdentityFile
929: directives will add to the list of identities tried (this behaviour
930: differs from that of other configuration directives).
1.165 djm 931: .Pp
932: .Cm IdentityFile
933: may be used in conjunction with
934: .Cm IdentitiesOnly
935: to select which identities in an agent are offered during authentication.
1.164 jmc 936: .It Cm IgnoreUnknown
937: Specifies a pattern-list of unknown options to be ignored if they are
938: encountered in configuration parsing.
939: This may be used to suppress errors if
940: .Nm
941: contains options that are unrecognised by
942: .Xr ssh 1 .
943: It is recommended that
944: .Cm IgnoreUnknown
945: be listed early in the configuration file as it will not be applied
946: to unknown options that appear before it.
1.143 djm 947: .It Cm IPQoS
948: Specifies the IPv4 type-of-service or DSCP class for connections.
949: Accepted values are
950: .Dq af11 ,
951: .Dq af12 ,
952: .Dq af13 ,
1.154 djm 953: .Dq af21 ,
1.143 djm 954: .Dq af22 ,
955: .Dq af23 ,
956: .Dq af31 ,
957: .Dq af32 ,
958: .Dq af33 ,
959: .Dq af41 ,
960: .Dq af42 ,
961: .Dq af43 ,
962: .Dq cs0 ,
963: .Dq cs1 ,
964: .Dq cs2 ,
965: .Dq cs3 ,
966: .Dq cs4 ,
967: .Dq cs5 ,
968: .Dq cs6 ,
969: .Dq cs7 ,
970: .Dq ef ,
971: .Dq lowdelay ,
972: .Dq throughput ,
973: .Dq reliability ,
974: or a numeric value.
1.146 djm 975: This option may take one or two arguments, separated by whitespace.
1.143 djm 976: If one argument is specified, it is used as the packet class unconditionally.
977: If two values are specified, the first is automatically selected for
978: interactive sessions and the second for non-interactive sessions.
979: The default is
980: .Dq lowdelay
981: for interactive sessions and
982: .Dq throughput
983: for non-interactive sessions.
1.103 djm 984: .It Cm KbdInteractiveAuthentication
985: Specifies whether to use keyboard-interactive authentication.
986: The argument to this keyword must be
987: .Dq yes
988: or
989: .Dq no .
990: The default is
991: .Dq yes .
1.39 djm 992: .It Cm KbdInteractiveDevices
993: Specifies the list of methods to use in keyboard-interactive authentication.
994: Multiple method names must be comma-separated.
995: The default is to use the server specified list.
1.85 jmc 996: The methods available vary depending on what the server supports.
997: For an OpenSSH server,
998: it may be zero or more of:
999: .Dq bsdauth ,
1000: .Dq pam ,
1001: and
1002: .Dq skey .
1.140 djm 1003: .It Cm KexAlgorithms
1004: Specifies the available KEX (Key Exchange) algorithms.
1005: Multiple algorithms must be comma-separated.
1.214 djm 1006: Alternately if the specified value begins with a
1007: .Sq +
1008: character, then the specified methods will be appended to the default set
1009: instead of replacing them.
1.141 jmc 1010: The default is:
1011: .Bd -literal -offset indent
1.179 markus 1012: curve25519-sha256@libssh.org,
1.141 jmc 1013: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1014: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1015: diffie-hellman-group-exchange-sha1,
1.212 djm 1016: diffie-hellman-group14-sha1
1.141 jmc 1017: .Ed
1.198 djm 1018: .Pp
1019: The list of available key exchange algorithms may also be obtained using the
1020: .Fl Q
1021: option of
1022: .Xr ssh 1
1023: with an argument of
1024: .Dq kex .
1.65 reyk 1025: .It Cm LocalCommand
1026: Specifies a command to execute on the local machine after successfully
1027: connecting to the server.
1028: The command string extends to the end of the line, and is executed with
1.105 jmc 1029: the user's shell.
1.109 dtucker 1030: The following escape character substitutions will be performed:
1031: .Ql %d
1032: (local user's home directory),
1033: .Ql %h
1034: (remote host name),
1035: .Ql %l
1036: (local host name),
1037: .Ql %n
1038: (host name as provided on the command line),
1039: .Ql %p
1040: (remote port),
1041: .Ql %r
1042: (remote user name) or
1043: .Ql %u
1.188 djm 1044: (local user name) or
1.189 jmc 1045: .Ql \&%C
1.188 djm 1046: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1047: .Pp
1048: The command is run synchronously and does not have access to the
1049: session of the
1050: .Xr ssh 1
1051: that spawned it.
1052: It should not be used for interactive commands.
1053: .Pp
1.65 reyk 1054: This directive is ignored unless
1055: .Cm PermitLocalCommand
1056: has been enabled.
1.1 stevesk 1057: .It Cm LocalForward
1.74 jmc 1058: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1059: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1060: The first argument must be
1.43 djm 1061: .Sm off
1.49 jmc 1062: .Oo Ar bind_address : Oc Ar port
1.43 djm 1063: .Sm on
1.49 jmc 1064: and the second argument must be
1065: .Ar host : Ns Ar hostport .
1.138 djm 1066: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1067: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1068: given on the command line.
1.1 stevesk 1069: Only the superuser can forward privileged ports.
1.43 djm 1070: By default, the local port is bound in accordance with the
1071: .Cm GatewayPorts
1072: setting.
1073: However, an explicit
1074: .Ar bind_address
1075: may be used to bind the connection to a specific address.
1076: The
1077: .Ar bind_address
1078: of
1079: .Dq localhost
1.46 jmc 1080: indicates that the listening port be bound for local use only, while an
1081: empty address or
1082: .Sq *
1.43 djm 1083: indicates that the port should be available from all interfaces.
1.1 stevesk 1084: .It Cm LogLevel
1085: Gives the verbosity level that is used when logging messages from
1.84 jmc 1086: .Xr ssh 1 .
1.1 stevesk 1087: The possible values are:
1.84 jmc 1088: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1089: The default is INFO.
1090: DEBUG and DEBUG1 are equivalent.
1091: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1092: .It Cm MACs
1093: Specifies the MAC (message authentication code) algorithms
1094: in order of preference.
1095: The MAC algorithm is used in protocol version 2
1096: for data integrity protection.
1097: Multiple algorithms must be comma-separated.
1.214 djm 1098: If the specified value begins with a
1099: .Sq +
1100: character, then the specified algorithms will be appended to the default set
1101: instead of replacing them.
1102: .Pp
1.160 markus 1103: The algorithms that contain
1104: .Dq -etm
1105: calculate the MAC after encryption (encrypt-then-mac).
1106: These are considered safer and their use recommended.
1.214 djm 1107: .Pp
1.84 jmc 1108: The default is:
1.101 jmc 1109: .Bd -literal -offset indent
1.160 markus 1110: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1111: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1112: umac-64@openssh.com,umac-128@openssh.com,
1113: hmac-sha2-256,hmac-sha2-512,
1114: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1115: hmac-ripemd160-etm@openssh.com,
1116: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1117: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1118: hmac-sha1-96,hmac-md5-96
1.101 jmc 1119: .Ed
1.198 djm 1120: .Pp
1121: The list of available MAC algorithms may also be obtained using the
1122: .Fl Q
1123: option of
1124: .Xr ssh 1
1125: with an argument of
1126: .Dq mac .
1.1 stevesk 1127: .It Cm NoHostAuthenticationForLocalhost
1128: This option can be used if the home directory is shared across machines.
1129: In this case localhost will refer to a different machine on each of
1130: the machines and the user will get many warnings about changed host keys.
1131: However, this option disables host authentication for localhost.
1132: The argument to this keyword must be
1133: .Dq yes
1134: or
1135: .Dq no .
1136: The default is to check the host key for localhost.
1137: .It Cm NumberOfPasswordPrompts
1138: Specifies the number of password prompts before giving up.
1139: The argument to this keyword must be an integer.
1.84 jmc 1140: The default is 3.
1.1 stevesk 1141: .It Cm PasswordAuthentication
1142: Specifies whether to use password authentication.
1143: The argument to this keyword must be
1144: .Dq yes
1145: or
1146: .Dq no .
1147: The default is
1148: .Dq yes .
1.65 reyk 1149: .It Cm PermitLocalCommand
1150: Allow local command execution via the
1151: .Ic LocalCommand
1152: option or using the
1.66 jmc 1153: .Ic !\& Ns Ar command
1.65 reyk 1154: escape sequence in
1155: .Xr ssh 1 .
1156: The argument must be
1157: .Dq yes
1158: or
1159: .Dq no .
1160: The default is
1161: .Dq no .
1.127 markus 1162: .It Cm PKCS11Provider
1163: Specifies which PKCS#11 provider to use.
1.144 jmc 1164: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1165: .Xr ssh 1
1.128 markus 1166: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1167: private RSA key.
1.67 jmc 1168: .It Cm Port
1169: Specifies the port number to connect on the remote host.
1.84 jmc 1170: The default is 22.
1.1 stevesk 1171: .It Cm PreferredAuthentications
1172: Specifies the order in which the client should try protocol 2
1.11 jmc 1173: authentication methods.
1.48 jmc 1174: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1175: .Cm keyboard-interactive )
1.48 jmc 1176: over another method (e.g.\&
1.131 jmc 1177: .Cm password ) .
1178: The default is:
1179: .Bd -literal -offset indent
1180: gssapi-with-mic,hostbased,publickey,
1181: keyboard-interactive,password
1182: .Ed
1.1 stevesk 1183: .It Cm Protocol
1184: Specifies the protocol versions
1.84 jmc 1185: .Xr ssh 1
1.1 stevesk 1186: should support in order of preference.
1187: The possible values are
1.84 jmc 1188: .Sq 1
1.1 stevesk 1189: and
1.84 jmc 1190: .Sq 2 .
1.1 stevesk 1191: Multiple versions must be comma-separated.
1.120 markus 1192: When this option is set to
1.121 jmc 1193: .Dq 2,1
1.120 markus 1194: .Nm ssh
1195: will try version 2 and fall back to version 1
1196: if version 2 is not available.
1.1 stevesk 1197: The default is
1.121 jmc 1198: .Sq 2 .
1.1 stevesk 1199: .It Cm ProxyCommand
1200: Specifies the command to use to connect to the server.
1201: The command
1.190 djm 1202: string extends to the end of the line, and is executed
1203: using the user's shell
1204: .Ql exec
1205: directive to avoid a lingering shell process.
1206: .Pp
1.133 jmc 1207: In the command string, any occurrence of
1.1 stevesk 1208: .Ql %h
1209: will be substituted by the host name to
1.132 djm 1210: connect,
1.1 stevesk 1211: .Ql %p
1.133 jmc 1212: by the port, and
1213: .Ql %r
1.132 djm 1214: by the remote user name.
1.1 stevesk 1215: The command can be basically anything,
1216: and should read from its standard input and write to its standard output.
1217: It should eventually connect an
1218: .Xr sshd 8
1219: server running on some machine, or execute
1220: .Ic sshd -i
1221: somewhere.
1222: Host key management will be done using the
1223: HostName of the host being connected (defaulting to the name typed by
1224: the user).
1.7 jmc 1225: Setting the command to
1226: .Dq none
1.6 markus 1227: disables this option entirely.
1.1 stevesk 1228: Note that
1229: .Cm CheckHostIP
1230: is not available for connects with a proxy command.
1.52 djm 1231: .Pp
1232: This directive is useful in conjunction with
1233: .Xr nc 1
1234: and its proxy support.
1.53 jmc 1235: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1236: 192.0.2.0:
1237: .Bd -literal -offset 3n
1238: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1239: .Ed
1.167 djm 1240: .It Cm ProxyUseFdpass
1.168 jmc 1241: Specifies that
1.167 djm 1242: .Cm ProxyCommand
1243: will pass a connected file descriptor back to
1.168 jmc 1244: .Xr ssh 1
1.167 djm 1245: instead of continuing to execute and pass data.
1246: The default is
1247: .Dq no .
1.213 markus 1248: .It Cm PubkeyAcceptedKeyTypes
1249: Specifies the key types that will be used for public key authentication
1250: as a comma-separated pattern list.
1.214 djm 1251: Alternately if the specified value begins with a
1252: .Sq +
1253: character, then the key types after it will be appended to the default
1254: instead of replacing it.
1.213 markus 1255: The default for this option is:
1256: .Bd -literal -offset 3n
1257: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1258: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1259: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1260: ssh-ed25519-cert-v01@openssh.com,
1261: ssh-rsa-cert-v01@openssh.com,
1262: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1263: ssh-ed25519,ssh-rsa
1264: .Ed
1265: .Pp
1266: The
1267: .Fl Q
1268: option of
1269: .Xr ssh 1
1270: may be used to list supported key types.
1.1 stevesk 1271: .It Cm PubkeyAuthentication
1272: Specifies whether to try public key authentication.
1273: The argument to this keyword must be
1274: .Dq yes
1275: or
1276: .Dq no .
1277: The default is
1278: .Dq yes .
1279: This option applies to protocol version 2 only.
1.75 dtucker 1280: .It Cm RekeyLimit
1281: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1282: session key is renegotiated, optionally followed a maximum amount of
1283: time that may pass before the session key is renegotiated.
1284: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1285: .Sq K ,
1286: .Sq M ,
1.75 dtucker 1287: or
1.76 jmc 1288: .Sq G
1.75 dtucker 1289: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1290: The default is between
1.84 jmc 1291: .Sq 1G
1.75 dtucker 1292: and
1.84 jmc 1293: .Sq 4G ,
1.75 dtucker 1294: depending on the cipher.
1.162 dtucker 1295: The optional second value is specified in seconds and may use any of the
1296: units documented in the
1.166 jmc 1297: TIME FORMATS section of
1.162 dtucker 1298: .Xr sshd_config 5 .
1299: The default value for
1300: .Cm RekeyLimit
1301: is
1302: .Dq default none ,
1303: which means that rekeying is performed after the cipher's default amount
1304: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1305: This option applies to protocol version 2 only.
1.1 stevesk 1306: .It Cm RemoteForward
1.74 jmc 1307: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1308: the secure channel to the specified host and port from the local machine.
1.49 jmc 1309: The first argument must be
1.43 djm 1310: .Sm off
1.49 jmc 1311: .Oo Ar bind_address : Oc Ar port
1.43 djm 1312: .Sm on
1.49 jmc 1313: and the second argument must be
1314: .Ar host : Ns Ar hostport .
1.138 djm 1315: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1316: Multiple forwardings may be specified, and additional
1317: forwardings can be given on the command line.
1.113 stevesk 1318: Privileged ports can be forwarded only when
1319: logging in as root on the remote machine.
1.118 jmc 1320: .Pp
1.117 djm 1321: If the
1322: .Ar port
1323: argument is
1324: .Ql 0 ,
1325: the listen port will be dynamically allocated on the server and reported
1326: to the client at run time.
1.43 djm 1327: .Pp
1328: If the
1329: .Ar bind_address
1330: is not specified, the default is to only bind to loopback addresses.
1331: If the
1332: .Ar bind_address
1333: is
1334: .Ql *
1335: or an empty string, then the forwarding is requested to listen on all
1336: interfaces.
1337: Specifying a remote
1338: .Ar bind_address
1.46 jmc 1339: will only succeed if the server's
1340: .Cm GatewayPorts
1.43 djm 1341: option is enabled (see
1.46 jmc 1342: .Xr sshd_config 5 ) .
1.149 djm 1343: .It Cm RequestTTY
1344: Specifies whether to request a pseudo-tty for the session.
1345: The argument may be one of:
1346: .Dq no
1347: (never request a TTY),
1348: .Dq yes
1349: (always request a TTY when standard input is a TTY),
1350: .Dq force
1351: (always request a TTY) or
1352: .Dq auto
1353: (request a TTY when opening a login session).
1354: This option mirrors the
1355: .Fl t
1356: and
1357: .Fl T
1358: flags for
1359: .Xr ssh 1 .
1.196 djm 1360: .It Cm RevokedHostKeys
1361: Specifies revoked host public keys.
1362: Keys listed in this file will be refused for host authentication.
1363: Note that if this file does not exist or is not readable,
1364: then host authentication will be refused for all hosts.
1365: Keys may be specified as a text file, listing one public key per line, or as
1366: an OpenSSH Key Revocation List (KRL) as generated by
1367: .Xr ssh-keygen 1 .
1368: For more information on KRLs, see the KEY REVOCATION LISTS section in
1369: .Xr ssh-keygen 1 .
1.1 stevesk 1370: .It Cm RhostsRSAAuthentication
1371: Specifies whether to try rhosts based authentication with RSA host
1372: authentication.
1373: The argument must be
1374: .Dq yes
1375: or
1376: .Dq no .
1377: The default is
1378: .Dq no .
1379: This option applies to protocol version 1 only and requires
1.84 jmc 1380: .Xr ssh 1
1.1 stevesk 1381: to be setuid root.
1382: .It Cm RSAAuthentication
1383: Specifies whether to try RSA authentication.
1384: The argument to this keyword must be
1385: .Dq yes
1386: or
1387: .Dq no .
1388: RSA authentication will only be
1389: attempted if the identity file exists, or an authentication agent is
1390: running.
1391: The default is
1392: .Dq yes .
1393: Note that this option applies to protocol version 1 only.
1.32 djm 1394: .It Cm SendEnv
1395: Specifies what variables from the local
1396: .Xr environ 7
1397: should be sent to the server.
1.84 jmc 1398: Note that environment passing is only supported for protocol 2.
1399: The server must also support it, and the server must be configured to
1.33 djm 1400: accept these environment variables.
1.207 dtucker 1401: Note that the
1402: .Ev TERM
1.208 jmc 1403: environment variable is always sent whenever a
1.207 dtucker 1404: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1405: Refer to
1406: .Cm AcceptEnv
1407: in
1408: .Xr sshd_config 5
1409: for how to configure the server.
1.80 jmc 1410: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1411: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1412: across multiple
1413: .Cm SendEnv
1414: directives.
1415: The default is not to send any environment variables.
1.81 jmc 1416: .Pp
1417: See
1418: .Sx PATTERNS
1419: for more information on patterns.
1.28 markus 1420: .It Cm ServerAliveCountMax
1.73 jmc 1421: Sets the number of server alive messages (see below) which may be
1.28 markus 1422: sent without
1.84 jmc 1423: .Xr ssh 1
1.28 markus 1424: receiving any messages back from the server.
1425: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1426: ssh will disconnect from the server, terminating the session.
1.28 markus 1427: It is important to note that the use of server alive messages is very
1428: different from
1429: .Cm TCPKeepAlive
1430: (below).
1431: The server alive messages are sent through the encrypted channel
1432: and therefore will not be spoofable.
1433: The TCP keepalive option enabled by
1434: .Cm TCPKeepAlive
1435: is spoofable.
1436: The server alive mechanism is valuable when the client or
1437: server depend on knowing when a connection has become inactive.
1438: .Pp
1439: The default value is 3.
1440: If, for example,
1441: .Cm ServerAliveInterval
1.84 jmc 1442: (see below) is set to 15 and
1.28 markus 1443: .Cm ServerAliveCountMax
1.84 jmc 1444: is left at the default, if the server becomes unresponsive,
1445: ssh will disconnect after approximately 45 seconds.
1.89 markus 1446: This option applies to protocol version 2 only.
1.67 jmc 1447: .It Cm ServerAliveInterval
1448: Sets a timeout interval in seconds after which if no data has been received
1449: from the server,
1.84 jmc 1450: .Xr ssh 1
1.67 jmc 1451: will send a message through the encrypted
1452: channel to request a response from the server.
1453: The default
1454: is 0, indicating that these messages will not be sent to the server.
1455: This option applies to protocol version 2 only.
1.191 millert 1456: .It Cm StreamLocalBindMask
1457: Sets the octal file creation mode mask
1458: .Pq umask
1459: used when creating a Unix-domain socket file for local or remote
1460: port forwarding.
1461: This option is only used for port forwarding to a Unix-domain socket file.
1462: .Pp
1463: The default value is 0177, which creates a Unix-domain socket file that is
1464: readable and writable only by the owner.
1465: Note that not all operating systems honor the file mode on Unix-domain
1466: socket files.
1467: .It Cm StreamLocalBindUnlink
1468: Specifies whether to remove an existing Unix-domain socket file for local
1469: or remote port forwarding before creating a new one.
1470: If the socket file already exists and
1471: .Cm StreamLocalBindUnlink
1472: is not enabled,
1473: .Nm ssh
1474: will be unable to forward the port to the Unix-domain socket file.
1475: This option is only used for port forwarding to a Unix-domain socket file.
1476: .Pp
1477: The argument must be
1478: .Dq yes
1479: or
1480: .Dq no .
1481: The default is
1482: .Dq no .
1.1 stevesk 1483: .It Cm StrictHostKeyChecking
1484: If this flag is set to
1485: .Dq yes ,
1.84 jmc 1486: .Xr ssh 1
1.1 stevesk 1487: will never automatically add host keys to the
1.50 djm 1488: .Pa ~/.ssh/known_hosts
1.1 stevesk 1489: file, and refuses to connect to hosts whose host key has changed.
1490: This provides maximum protection against trojan horse attacks,
1.84 jmc 1491: though it can be annoying when the
1.1 stevesk 1492: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1493: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1494: frequently made.
1495: This option forces the user to manually
1496: add all new hosts.
1497: If this flag is set to
1498: .Dq no ,
1.84 jmc 1499: ssh will automatically add new host keys to the
1.1 stevesk 1500: user known hosts files.
1501: If this flag is set to
1502: .Dq ask ,
1503: new host keys
1504: will be added to the user known host files only after the user
1505: has confirmed that is what they really want to do, and
1.84 jmc 1506: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1507: The host keys of
1508: known hosts will be verified automatically in all cases.
1509: The argument must be
1510: .Dq yes ,
1.84 jmc 1511: .Dq no ,
1.1 stevesk 1512: or
1513: .Dq ask .
1514: The default is
1515: .Dq ask .
1.26 markus 1516: .It Cm TCPKeepAlive
1517: Specifies whether the system should send TCP keepalive messages to the
1518: other side.
1519: If they are sent, death of the connection or crash of one
1520: of the machines will be properly noticed.
1521: However, this means that
1522: connections will die if the route is down temporarily, and some people
1523: find it annoying.
1524: .Pp
1525: The default is
1526: .Dq yes
1527: (to send TCP keepalive messages), and the client will notice
1528: if the network goes down or the remote host dies.
1529: This is important in scripts, and many users want it too.
1530: .Pp
1531: To disable TCP keepalive messages, the value should be set to
1532: .Dq no .
1.65 reyk 1533: .It Cm Tunnel
1.95 stevesk 1534: Request
1.65 reyk 1535: .Xr tun 4
1.69 jmc 1536: device forwarding between the client and the server.
1.65 reyk 1537: The argument must be
1.68 reyk 1538: .Dq yes ,
1.95 stevesk 1539: .Dq point-to-point
1540: (layer 3),
1541: .Dq ethernet
1542: (layer 2),
1.65 reyk 1543: or
1544: .Dq no .
1.95 stevesk 1545: Specifying
1546: .Dq yes
1547: requests the default tunnel mode, which is
1548: .Dq point-to-point .
1.65 reyk 1549: The default is
1550: .Dq no .
1551: .It Cm TunnelDevice
1.95 stevesk 1552: Specifies the
1.65 reyk 1553: .Xr tun 4
1.95 stevesk 1554: devices to open on the client
1555: .Pq Ar local_tun
1556: and the server
1557: .Pq Ar remote_tun .
1558: .Pp
1559: The argument must be
1560: .Sm off
1561: .Ar local_tun Op : Ar remote_tun .
1562: .Sm on
1563: The devices may be specified by numerical ID or the keyword
1564: .Dq any ,
1565: which uses the next available tunnel device.
1566: If
1567: .Ar remote_tun
1568: is not specified, it defaults to
1569: .Dq any .
1570: The default is
1571: .Dq any:any .
1.201 djm 1572: .It Cm UpdateHostKeys
1.200 djm 1573: Specifies whether
1574: .Xr ssh 1
1575: should accept notifications of additional hostkeys from the server sent
1576: after authentication has completed and add them to
1577: .Cm UserKnownHostsFile .
1578: The argument must be
1.204 djm 1579: .Dq yes ,
1.203 djm 1580: .Dq no
1.204 djm 1581: (the default) or
1582: .Dq ask .
1.200 djm 1583: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1584: and supports graceful key rotation by allowing a server to send replacement
1585: public keys before old ones are removed.
1.200 djm 1586: Additional hostkeys are only accepted if the key used to authenticate the
1587: host was already trusted or explicity accepted by the user.
1.204 djm 1588: If
1589: .Cm UpdateHostKeys
1590: is set to
1591: .Dq ask ,
1592: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1593: Confirmation is currently incompatible with
1594: .Cm ControlPersist ,
1595: and will be disabled if it is enabled.
1.200 djm 1596: .Pp
1597: Presently, only
1598: .Xr sshd 8
1599: from OpenSSH 6.8 and greater support the
1600: .Dq hostkeys@openssh.com
1601: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1602: .It Cm UsePrivilegedPort
1603: Specifies whether to use a privileged port for outgoing connections.
1604: The argument must be
1605: .Dq yes
1606: or
1607: .Dq no .
1608: The default is
1609: .Dq no .
1610: If set to
1.84 jmc 1611: .Dq yes ,
1612: .Xr ssh 1
1.72 jmc 1613: must be setuid root.
1614: Note that this option must be set to
1615: .Dq yes
1616: for
1617: .Cm RhostsRSAAuthentication
1618: with older servers.
1.1 stevesk 1619: .It Cm User
1620: Specifies the user to log in as.
1621: This can be useful when a different user name is used on different machines.
1622: This saves the trouble of
1623: having to remember to give the user name on the command line.
1624: .It Cm UserKnownHostsFile
1.151 djm 1625: Specifies one or more files to use for the user
1626: host key database, separated by whitespace.
1627: The default is
1628: .Pa ~/.ssh/known_hosts ,
1629: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1630: .It Cm VerifyHostKeyDNS
1631: Specifies whether to verify the remote key using DNS and SSHFP resource
1632: records.
1.24 jakob 1633: If this option is set to
1634: .Dq yes ,
1.25 jmc 1635: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1636: from DNS.
1637: Insecure fingerprints will be handled as if this option was set to
1638: .Dq ask .
1639: If this option is set to
1640: .Dq ask ,
1641: information on fingerprint match will be displayed, but the user will still
1642: need to confirm new host keys according to the
1643: .Cm StrictHostKeyChecking
1644: option.
1645: The argument must be
1646: .Dq yes ,
1.84 jmc 1647: .Dq no ,
1.25 jmc 1648: or
1649: .Dq ask .
1.8 jakob 1650: The default is
1651: .Dq no .
1.12 jakob 1652: Note that this option applies to protocol version 2 only.
1.84 jmc 1653: .Pp
1.166 jmc 1654: See also VERIFYING HOST KEYS in
1.84 jmc 1655: .Xr ssh 1 .
1.111 grunk 1656: .It Cm VisualHostKey
1657: If this flag is set to
1658: .Dq yes ,
1659: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1660: printed in addition to the fingerprint string at login and
1.114 stevesk 1661: for unknown host keys.
1.111 grunk 1662: If this flag is set to
1663: .Dq no ,
1.114 stevesk 1664: no fingerprint strings are printed at login and
1.197 djm 1665: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1666: The default is
1667: .Dq no .
1.1 stevesk 1668: .It Cm XAuthLocation
1.5 stevesk 1669: Specifies the full pathname of the
1.1 stevesk 1670: .Xr xauth 1
1671: program.
1672: The default is
1673: .Pa /usr/X11R6/bin/xauth .
1674: .El
1.86 jmc 1675: .Sh PATTERNS
1676: A
1677: .Em pattern
1678: consists of zero or more non-whitespace characters,
1679: .Sq *
1680: (a wildcard that matches zero or more characters),
1681: or
1682: .Sq ?\&
1683: (a wildcard that matches exactly one character).
1684: For example, to specify a set of declarations for any host in the
1685: .Dq .co.uk
1686: set of domains,
1687: the following pattern could be used:
1688: .Pp
1689: .Dl Host *.co.uk
1690: .Pp
1691: The following pattern
1692: would match any host in the 192.168.0.[0-9] network range:
1693: .Pp
1694: .Dl Host 192.168.0.?
1695: .Pp
1696: A
1697: .Em pattern-list
1698: is a comma-separated list of patterns.
1699: Patterns within pattern-lists may be negated
1700: by preceding them with an exclamation mark
1701: .Pq Sq !\& .
1702: For example,
1.174 djm 1703: to allow a key to be used from anywhere within an organization
1.86 jmc 1704: except from the
1705: .Dq dialup
1706: pool,
1707: the following entry (in authorized_keys) could be used:
1708: .Pp
1709: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1710: .Sh FILES
1711: .Bl -tag -width Ds
1.50 djm 1712: .It Pa ~/.ssh/config
1.1 stevesk 1713: This is the per-user configuration file.
1714: The format of this file is described above.
1.84 jmc 1715: This file is used by the SSH client.
1.30 djm 1716: Because of the potential for abuse, this file must have strict permissions:
1717: read/write for the user, and not accessible by others.
1.1 stevesk 1718: .It Pa /etc/ssh/ssh_config
1719: Systemwide configuration file.
1720: This file provides defaults for those
1721: values that are not specified in the user's configuration file, and
1722: for those users who do not have a configuration file.
1723: This file must be world-readable.
1724: .El
1.13 jmc 1725: .Sh SEE ALSO
1726: .Xr ssh 1
1.1 stevesk 1727: .Sh AUTHORS
1728: OpenSSH is a derivative of the original and free
1729: ssh 1.2.12 release by Tatu Ylonen.
1730: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1731: Theo de Raadt and Dug Song
1732: removed many bugs, re-added newer features and
1733: created OpenSSH.
1734: Markus Friedl contributed the support for SSH
1735: protocol versions 1.5 and 2.0.
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