Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.219
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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1.219 ! jmc 36: .\" $OpenBSD: ssh_config.5,v 1.218 2015/09/11 03:47:28 djm Exp $
! 37: .Dd $Mdocdate: September 11 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
1.219 ! jmc 543: by the numeric user ID (uid) of the user running
! 544: .Xr ssh 1 ,
! 545: and
1.189 jmc 546: .Ql \&%C
1.188 djm 547: by a hash of the concatenation: %l%h%p%r.
1.56 djm 548: It is recommended that any
549: .Cm ControlPath
550: used for opportunistic connection sharing include
1.195 djm 551: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
552: that is not writable by other users.
1.56 djm 553: This ensures that shared connections are uniquely identified.
1.137 djm 554: .It Cm ControlPersist
555: When used in conjunction with
556: .Cm ControlMaster ,
557: specifies that the master connection should remain open
558: in the background (waiting for future client connections)
559: after the initial client connection has been closed.
560: If set to
561: .Dq no ,
562: then the master connection will not be placed into the background,
563: and will close as soon as the initial client connection is closed.
564: If set to
1.195 djm 565: .Dq yes
566: or
567: .Dq 0 ,
1.137 djm 568: then the master connection will remain in the background indefinitely
569: (until killed or closed via a mechanism such as the
570: .Xr ssh 1
571: .Dq Fl O No exit
572: option).
573: If set to a time in seconds, or a time in any of the formats documented in
574: .Xr sshd_config 5 ,
575: then the backgrounded master connection will automatically terminate
576: after it has remained idle (with no client connections) for the
577: specified time.
1.38 jmc 578: .It Cm DynamicForward
1.74 jmc 579: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 580: over the secure channel, and the application
581: protocol is then used to determine where to connect to from the
582: remote machine.
1.62 djm 583: .Pp
584: The argument must be
585: .Sm off
586: .Oo Ar bind_address : Oc Ar port .
587: .Sm on
1.138 djm 588: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 589: By default, the local port is bound in accordance with the
590: .Cm GatewayPorts
591: setting.
592: However, an explicit
593: .Ar bind_address
594: may be used to bind the connection to a specific address.
595: The
596: .Ar bind_address
597: of
598: .Dq localhost
599: indicates that the listening port be bound for local use only, while an
600: empty address or
601: .Sq *
602: indicates that the port should be available from all interfaces.
603: .Pp
1.38 jmc 604: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 605: .Xr ssh 1
1.38 jmc 606: will act as a SOCKS server.
607: Multiple forwardings may be specified, and
608: additional forwardings can be given on the command line.
609: Only the superuser can forward privileged ports.
1.14 markus 610: .It Cm EnableSSHKeysign
611: Setting this option to
612: .Dq yes
613: in the global client configuration file
614: .Pa /etc/ssh/ssh_config
615: enables the use of the helper program
616: .Xr ssh-keysign 8
617: during
618: .Cm HostbasedAuthentication .
619: The argument must be
620: .Dq yes
621: or
622: .Dq no .
623: The default is
624: .Dq no .
1.23 jmc 625: This option should be placed in the non-hostspecific section.
1.14 markus 626: See
627: .Xr ssh-keysign 8
628: for more information.
1.1 stevesk 629: .It Cm EscapeChar
630: Sets the escape character (default:
631: .Ql ~ ) .
632: The escape character can also
633: be set on the command line.
634: The argument should be a single character,
635: .Ql ^
636: followed by a letter, or
637: .Dq none
638: to disable the escape
639: character entirely (making the connection transparent for binary
640: data).
1.96 markus 641: .It Cm ExitOnForwardFailure
642: Specifies whether
643: .Xr ssh 1
644: should terminate the connection if it cannot set up all requested
1.216 djm 645: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 646: if either end is unable to bind and listen on a specified port).
1.216 djm 647: Note that
648: .Cm ExitOnForwardFailure
649: does not apply to connections made over port forwardings and will not,
650: for example, cause
651: .Xr ssh 1
652: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 653: The argument must be
654: .Dq yes
655: or
656: .Dq no .
657: The default is
658: .Dq no .
1.197 djm 659: .It Cm FingerprintHash
660: Specifies the hash algorithm used when displaying key fingerprints.
661: Valid options are:
662: .Dq md5
663: and
664: .Dq sha256 .
665: The default is
666: .Dq sha256 .
1.1 stevesk 667: .It Cm ForwardAgent
668: Specifies whether the connection to the authentication agent (if any)
669: will be forwarded to the remote machine.
670: The argument must be
671: .Dq yes
672: or
673: .Dq no .
674: The default is
675: .Dq no .
1.3 stevesk 676: .Pp
1.7 jmc 677: Agent forwarding should be enabled with caution.
678: Users with the ability to bypass file permissions on the remote host
679: (for the agent's Unix-domain socket)
680: can access the local agent through the forwarded connection.
681: An attacker cannot obtain key material from the agent,
1.3 stevesk 682: however they can perform operations on the keys that enable them to
683: authenticate using the identities loaded into the agent.
1.1 stevesk 684: .It Cm ForwardX11
685: Specifies whether X11 connections will be automatically redirected
686: over the secure channel and
687: .Ev DISPLAY
688: set.
689: The argument must be
690: .Dq yes
691: or
692: .Dq no .
693: The default is
694: .Dq no .
1.3 stevesk 695: .Pp
1.7 jmc 696: X11 forwarding should be enabled with caution.
697: Users with the ability to bypass file permissions on the remote host
1.22 markus 698: (for the user's X11 authorization database)
1.7 jmc 699: can access the local X11 display through the forwarded connection.
1.22 markus 700: An attacker may then be able to perform activities such as keystroke monitoring
701: if the
702: .Cm ForwardX11Trusted
703: option is also enabled.
1.134 djm 704: .It Cm ForwardX11Timeout
1.135 jmc 705: Specify a timeout for untrusted X11 forwarding
706: using the format described in the
1.166 jmc 707: TIME FORMATS section of
1.134 djm 708: .Xr sshd_config 5 .
709: X11 connections received by
710: .Xr ssh 1
711: after this time will be refused.
712: The default is to disable untrusted X11 forwarding after twenty minutes has
713: elapsed.
1.22 markus 714: .It Cm ForwardX11Trusted
1.34 jmc 715: If this option is set to
1.84 jmc 716: .Dq yes ,
717: remote X11 clients will have full access to the original X11 display.
1.42 djm 718: .Pp
1.22 markus 719: If this option is set to
1.84 jmc 720: .Dq no ,
721: remote X11 clients will be considered untrusted and prevented
1.22 markus 722: from stealing or tampering with data belonging to trusted X11
723: clients.
1.42 djm 724: Furthermore, the
725: .Xr xauth 1
726: token used for the session will be set to expire after 20 minutes.
727: Remote clients will be refused access after this time.
1.22 markus 728: .Pp
729: The default is
730: .Dq no .
731: .Pp
732: See the X11 SECURITY extension specification for full details on
733: the restrictions imposed on untrusted clients.
1.1 stevesk 734: .It Cm GatewayPorts
735: Specifies whether remote hosts are allowed to connect to local
736: forwarded ports.
737: By default,
1.84 jmc 738: .Xr ssh 1
1.7 jmc 739: binds local port forwardings to the loopback address.
740: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 741: .Cm GatewayPorts
1.84 jmc 742: can be used to specify that ssh
1.1 stevesk 743: should bind local port forwardings to the wildcard address,
744: thus allowing remote hosts to connect to forwarded ports.
745: The argument must be
746: .Dq yes
747: or
748: .Dq no .
749: The default is
750: .Dq no .
751: .It Cm GlobalKnownHostsFile
1.151 djm 752: Specifies one or more files to use for the global
753: host key database, separated by whitespace.
754: The default is
755: .Pa /etc/ssh/ssh_known_hosts ,
756: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 757: .It Cm GSSAPIAuthentication
1.27 markus 758: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 759: The default is
1.21 markus 760: .Dq no .
1.18 markus 761: Note that this option applies to protocol version 2 only.
762: .It Cm GSSAPIDelegateCredentials
763: Forward (delegate) credentials to the server.
764: The default is
765: .Dq no .
766: Note that this option applies to protocol version 2 only.
1.44 djm 767: .It Cm HashKnownHosts
768: Indicates that
1.84 jmc 769: .Xr ssh 1
1.44 djm 770: should hash host names and addresses when they are added to
1.50 djm 771: .Pa ~/.ssh/known_hosts .
1.44 djm 772: These hashed names may be used normally by
1.84 jmc 773: .Xr ssh 1
1.44 djm 774: and
1.84 jmc 775: .Xr sshd 8 ,
1.44 djm 776: but they do not reveal identifying information should the file's contents
777: be disclosed.
778: The default is
779: .Dq no .
1.97 jmc 780: Note that existing names and addresses in known hosts files
781: will not be converted automatically,
782: but may be manually hashed using
1.45 djm 783: .Xr ssh-keygen 1 .
1.1 stevesk 784: .It Cm HostbasedAuthentication
785: Specifies whether to try rhosts based authentication with public key
786: authentication.
787: The argument must be
788: .Dq yes
789: or
790: .Dq no .
791: The default is
792: .Dq no .
793: This option applies to protocol version 2 only and
794: is similar to
795: .Cm RhostsRSAAuthentication .
1.202 djm 796: .It Cm HostbasedKeyTypes
797: Specifies the key types that will be used for hostbased authentication
798: as a comma-separated pattern list.
1.214 djm 799: Alternately if the specified value begins with a
800: .Sq +
801: character, then the specified key types will be appended to the default set
802: instead of replacing them.
1.213 markus 803: The default for this option is:
804: .Bd -literal -offset 3n
805: ecdsa-sha2-nistp256-cert-v01@openssh.com,
806: ecdsa-sha2-nistp384-cert-v01@openssh.com,
807: ecdsa-sha2-nistp521-cert-v01@openssh.com,
808: ssh-ed25519-cert-v01@openssh.com,
809: ssh-rsa-cert-v01@openssh.com,
810: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
811: ssh-ed25519,ssh-rsa
812: .Ed
813: .Pp
1.202 djm 814: The
815: .Fl Q
816: option of
817: .Xr ssh 1
818: may be used to list supported key types.
1.1 stevesk 819: .It Cm HostKeyAlgorithms
820: Specifies the protocol version 2 host key algorithms
821: that the client wants to use in order of preference.
1.214 djm 822: Alternately if the specified value begins with a
823: .Sq +
824: character, then the specified key types will be appended to the default set
825: instead of replacing them.
1.1 stevesk 826: The default for this option is:
1.139 djm 827: .Bd -literal -offset 3n
828: ecdsa-sha2-nistp256-cert-v01@openssh.com,
829: ecdsa-sha2-nistp384-cert-v01@openssh.com,
830: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 831: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 832: ssh-rsa-cert-v01@openssh.com,
1.139 djm 833: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.213 markus 834: ssh-ed25519,ssh-rsa
1.139 djm 835: .Ed
1.145 djm 836: .Pp
837: If hostkeys are known for the destination host then this default is modified
838: to prefer their algorithms.
1.198 djm 839: .Pp
840: The list of available key types may also be obtained using the
841: .Fl Q
842: option of
843: .Xr ssh 1
844: with an argument of
845: .Dq key .
1.1 stevesk 846: .It Cm HostKeyAlias
847: Specifies an alias that should be used instead of the
848: real host name when looking up or saving the host key
849: in the host key database files.
1.84 jmc 850: This option is useful for tunneling SSH connections
1.1 stevesk 851: or for multiple servers running on a single host.
852: .It Cm HostName
853: Specifies the real host name to log into.
854: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 855: If the hostname contains the character sequence
856: .Ql %h ,
1.150 jmc 857: then this will be replaced with the host name specified on the command line
1.136 djm 858: (this is useful for manipulating unqualified names).
1.187 djm 859: The character sequence
860: .Ql %%
861: will be replaced by a single
862: .Ql %
863: character, which may be used when specifying IPv6 link-local addresses.
864: .Pp
1.84 jmc 865: The default is the name given on the command line.
1.1 stevesk 866: Numeric IP addresses are also permitted (both on the command line and in
867: .Cm HostName
868: specifications).
1.29 markus 869: .It Cm IdentitiesOnly
870: Specifies that
1.84 jmc 871: .Xr ssh 1
1.29 markus 872: should only use the authentication identity files configured in the
1.31 jmc 873: .Nm
1.29 markus 874: files,
1.84 jmc 875: even if
876: .Xr ssh-agent 1
1.159 djm 877: or a
878: .Cm PKCS11Provider
1.29 markus 879: offers more identities.
880: The argument to this keyword must be
881: .Dq yes
882: or
883: .Dq no .
1.84 jmc 884: This option is intended for situations where ssh-agent
1.29 markus 885: offers many different identities.
886: The default is
887: .Dq no .
1.67 jmc 888: .It Cm IdentityFile
1.192 sobrado 889: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 890: identity is read.
1.67 jmc 891: The default is
892: .Pa ~/.ssh/identity
893: for protocol version 1, and
1.139 djm 894: .Pa ~/.ssh/id_dsa ,
1.183 naddy 895: .Pa ~/.ssh/id_ecdsa ,
896: .Pa ~/.ssh/id_ed25519
1.139 djm 897: and
1.67 jmc 898: .Pa ~/.ssh/id_rsa
899: for protocol version 2.
900: Additionally, any identities represented by the authentication agent
1.165 djm 901: will be used for authentication unless
902: .Cm IdentitiesOnly
903: is set.
1.129 djm 904: .Xr ssh 1
905: will try to load certificate information from the filename obtained by
906: appending
907: .Pa -cert.pub
908: to the path of a specified
909: .Cm IdentityFile .
1.90 djm 910: .Pp
1.67 jmc 911: The file name may use the tilde
1.91 jmc 912: syntax to refer to a user's home directory or one of the following
1.90 djm 913: escape characters:
914: .Ql %d
915: (local user's home directory),
916: .Ql %u
917: (local user name),
918: .Ql %l
919: (local host name),
920: .Ql %h
921: (remote host name) or
1.92 djm 922: .Ql %r
1.90 djm 923: (remote user name).
924: .Pp
1.67 jmc 925: It is possible to have
926: multiple identity files specified in configuration files; all these
927: identities will be tried in sequence.
1.152 djm 928: Multiple
929: .Cm IdentityFile
930: directives will add to the list of identities tried (this behaviour
931: differs from that of other configuration directives).
1.165 djm 932: .Pp
933: .Cm IdentityFile
934: may be used in conjunction with
935: .Cm IdentitiesOnly
936: to select which identities in an agent are offered during authentication.
1.164 jmc 937: .It Cm IgnoreUnknown
938: Specifies a pattern-list of unknown options to be ignored if they are
939: encountered in configuration parsing.
940: This may be used to suppress errors if
941: .Nm
942: contains options that are unrecognised by
943: .Xr ssh 1 .
944: It is recommended that
945: .Cm IgnoreUnknown
946: be listed early in the configuration file as it will not be applied
947: to unknown options that appear before it.
1.143 djm 948: .It Cm IPQoS
949: Specifies the IPv4 type-of-service or DSCP class for connections.
950: Accepted values are
951: .Dq af11 ,
952: .Dq af12 ,
953: .Dq af13 ,
1.154 djm 954: .Dq af21 ,
1.143 djm 955: .Dq af22 ,
956: .Dq af23 ,
957: .Dq af31 ,
958: .Dq af32 ,
959: .Dq af33 ,
960: .Dq af41 ,
961: .Dq af42 ,
962: .Dq af43 ,
963: .Dq cs0 ,
964: .Dq cs1 ,
965: .Dq cs2 ,
966: .Dq cs3 ,
967: .Dq cs4 ,
968: .Dq cs5 ,
969: .Dq cs6 ,
970: .Dq cs7 ,
971: .Dq ef ,
972: .Dq lowdelay ,
973: .Dq throughput ,
974: .Dq reliability ,
975: or a numeric value.
1.146 djm 976: This option may take one or two arguments, separated by whitespace.
1.143 djm 977: If one argument is specified, it is used as the packet class unconditionally.
978: If two values are specified, the first is automatically selected for
979: interactive sessions and the second for non-interactive sessions.
980: The default is
981: .Dq lowdelay
982: for interactive sessions and
983: .Dq throughput
984: for non-interactive sessions.
1.103 djm 985: .It Cm KbdInteractiveAuthentication
986: Specifies whether to use keyboard-interactive authentication.
987: The argument to this keyword must be
988: .Dq yes
989: or
990: .Dq no .
991: The default is
992: .Dq yes .
1.39 djm 993: .It Cm KbdInteractiveDevices
994: Specifies the list of methods to use in keyboard-interactive authentication.
995: Multiple method names must be comma-separated.
996: The default is to use the server specified list.
1.85 jmc 997: The methods available vary depending on what the server supports.
998: For an OpenSSH server,
999: it may be zero or more of:
1000: .Dq bsdauth ,
1001: .Dq pam ,
1002: and
1003: .Dq skey .
1.140 djm 1004: .It Cm KexAlgorithms
1005: Specifies the available KEX (Key Exchange) algorithms.
1006: Multiple algorithms must be comma-separated.
1.214 djm 1007: Alternately if the specified value begins with a
1008: .Sq +
1009: character, then the specified methods will be appended to the default set
1010: instead of replacing them.
1.141 jmc 1011: The default is:
1012: .Bd -literal -offset indent
1.179 markus 1013: curve25519-sha256@libssh.org,
1.141 jmc 1014: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1015: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1016: diffie-hellman-group-exchange-sha1,
1.212 djm 1017: diffie-hellman-group14-sha1
1.141 jmc 1018: .Ed
1.198 djm 1019: .Pp
1020: The list of available key exchange algorithms may also be obtained using the
1021: .Fl Q
1022: option of
1023: .Xr ssh 1
1024: with an argument of
1025: .Dq kex .
1.65 reyk 1026: .It Cm LocalCommand
1027: Specifies a command to execute on the local machine after successfully
1028: connecting to the server.
1029: The command string extends to the end of the line, and is executed with
1.105 jmc 1030: the user's shell.
1.109 dtucker 1031: The following escape character substitutions will be performed:
1032: .Ql %d
1033: (local user's home directory),
1034: .Ql %h
1035: (remote host name),
1036: .Ql %l
1037: (local host name),
1038: .Ql %n
1039: (host name as provided on the command line),
1040: .Ql %p
1041: (remote port),
1042: .Ql %r
1043: (remote user name) or
1044: .Ql %u
1.188 djm 1045: (local user name) or
1.189 jmc 1046: .Ql \&%C
1.188 djm 1047: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1048: .Pp
1049: The command is run synchronously and does not have access to the
1050: session of the
1051: .Xr ssh 1
1052: that spawned it.
1053: It should not be used for interactive commands.
1054: .Pp
1.65 reyk 1055: This directive is ignored unless
1056: .Cm PermitLocalCommand
1057: has been enabled.
1.1 stevesk 1058: .It Cm LocalForward
1.74 jmc 1059: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1060: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1061: The first argument must be
1.43 djm 1062: .Sm off
1.49 jmc 1063: .Oo Ar bind_address : Oc Ar port
1.43 djm 1064: .Sm on
1.49 jmc 1065: and the second argument must be
1066: .Ar host : Ns Ar hostport .
1.138 djm 1067: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1068: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1069: given on the command line.
1.1 stevesk 1070: Only the superuser can forward privileged ports.
1.43 djm 1071: By default, the local port is bound in accordance with the
1072: .Cm GatewayPorts
1073: setting.
1074: However, an explicit
1075: .Ar bind_address
1076: may be used to bind the connection to a specific address.
1077: The
1078: .Ar bind_address
1079: of
1080: .Dq localhost
1.46 jmc 1081: indicates that the listening port be bound for local use only, while an
1082: empty address or
1083: .Sq *
1.43 djm 1084: indicates that the port should be available from all interfaces.
1.1 stevesk 1085: .It Cm LogLevel
1086: Gives the verbosity level that is used when logging messages from
1.84 jmc 1087: .Xr ssh 1 .
1.1 stevesk 1088: The possible values are:
1.84 jmc 1089: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1090: The default is INFO.
1091: DEBUG and DEBUG1 are equivalent.
1092: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1093: .It Cm MACs
1094: Specifies the MAC (message authentication code) algorithms
1095: in order of preference.
1096: The MAC algorithm is used in protocol version 2
1097: for data integrity protection.
1098: Multiple algorithms must be comma-separated.
1.214 djm 1099: If the specified value begins with a
1100: .Sq +
1101: character, then the specified algorithms will be appended to the default set
1102: instead of replacing them.
1103: .Pp
1.160 markus 1104: The algorithms that contain
1105: .Dq -etm
1106: calculate the MAC after encryption (encrypt-then-mac).
1107: These are considered safer and their use recommended.
1.214 djm 1108: .Pp
1.84 jmc 1109: The default is:
1.101 jmc 1110: .Bd -literal -offset indent
1.160 markus 1111: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1112: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1113: umac-64@openssh.com,umac-128@openssh.com,
1114: hmac-sha2-256,hmac-sha2-512,
1115: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1116: hmac-ripemd160-etm@openssh.com,
1117: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1118: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1119: hmac-sha1-96,hmac-md5-96
1.101 jmc 1120: .Ed
1.198 djm 1121: .Pp
1122: The list of available MAC algorithms may also be obtained using the
1123: .Fl Q
1124: option of
1125: .Xr ssh 1
1126: with an argument of
1127: .Dq mac .
1.1 stevesk 1128: .It Cm NoHostAuthenticationForLocalhost
1129: This option can be used if the home directory is shared across machines.
1130: In this case localhost will refer to a different machine on each of
1131: the machines and the user will get many warnings about changed host keys.
1132: However, this option disables host authentication for localhost.
1133: The argument to this keyword must be
1134: .Dq yes
1135: or
1136: .Dq no .
1137: The default is to check the host key for localhost.
1138: .It Cm NumberOfPasswordPrompts
1139: Specifies the number of password prompts before giving up.
1140: The argument to this keyword must be an integer.
1.84 jmc 1141: The default is 3.
1.1 stevesk 1142: .It Cm PasswordAuthentication
1143: Specifies whether to use password authentication.
1144: The argument to this keyword must be
1145: .Dq yes
1146: or
1147: .Dq no .
1148: The default is
1149: .Dq yes .
1.65 reyk 1150: .It Cm PermitLocalCommand
1151: Allow local command execution via the
1152: .Ic LocalCommand
1153: option or using the
1.66 jmc 1154: .Ic !\& Ns Ar command
1.65 reyk 1155: escape sequence in
1156: .Xr ssh 1 .
1157: The argument must be
1158: .Dq yes
1159: or
1160: .Dq no .
1161: The default is
1162: .Dq no .
1.127 markus 1163: .It Cm PKCS11Provider
1164: Specifies which PKCS#11 provider to use.
1.144 jmc 1165: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1166: .Xr ssh 1
1.128 markus 1167: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1168: private RSA key.
1.67 jmc 1169: .It Cm Port
1170: Specifies the port number to connect on the remote host.
1.84 jmc 1171: The default is 22.
1.1 stevesk 1172: .It Cm PreferredAuthentications
1173: Specifies the order in which the client should try protocol 2
1.11 jmc 1174: authentication methods.
1.48 jmc 1175: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1176: .Cm keyboard-interactive )
1.48 jmc 1177: over another method (e.g.\&
1.131 jmc 1178: .Cm password ) .
1179: The default is:
1180: .Bd -literal -offset indent
1181: gssapi-with-mic,hostbased,publickey,
1182: keyboard-interactive,password
1183: .Ed
1.1 stevesk 1184: .It Cm Protocol
1185: Specifies the protocol versions
1.84 jmc 1186: .Xr ssh 1
1.1 stevesk 1187: should support in order of preference.
1188: The possible values are
1.84 jmc 1189: .Sq 1
1.1 stevesk 1190: and
1.84 jmc 1191: .Sq 2 .
1.1 stevesk 1192: Multiple versions must be comma-separated.
1.120 markus 1193: When this option is set to
1.121 jmc 1194: .Dq 2,1
1.120 markus 1195: .Nm ssh
1196: will try version 2 and fall back to version 1
1197: if version 2 is not available.
1.1 stevesk 1198: The default is
1.121 jmc 1199: .Sq 2 .
1.1 stevesk 1200: .It Cm ProxyCommand
1201: Specifies the command to use to connect to the server.
1202: The command
1.190 djm 1203: string extends to the end of the line, and is executed
1204: using the user's shell
1205: .Ql exec
1206: directive to avoid a lingering shell process.
1207: .Pp
1.133 jmc 1208: In the command string, any occurrence of
1.1 stevesk 1209: .Ql %h
1210: will be substituted by the host name to
1.132 djm 1211: connect,
1.1 stevesk 1212: .Ql %p
1.133 jmc 1213: by the port, and
1214: .Ql %r
1.132 djm 1215: by the remote user name.
1.1 stevesk 1216: The command can be basically anything,
1217: and should read from its standard input and write to its standard output.
1218: It should eventually connect an
1219: .Xr sshd 8
1220: server running on some machine, or execute
1221: .Ic sshd -i
1222: somewhere.
1223: Host key management will be done using the
1224: HostName of the host being connected (defaulting to the name typed by
1225: the user).
1.7 jmc 1226: Setting the command to
1227: .Dq none
1.6 markus 1228: disables this option entirely.
1.1 stevesk 1229: Note that
1230: .Cm CheckHostIP
1231: is not available for connects with a proxy command.
1.52 djm 1232: .Pp
1233: This directive is useful in conjunction with
1234: .Xr nc 1
1235: and its proxy support.
1.53 jmc 1236: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1237: 192.0.2.0:
1238: .Bd -literal -offset 3n
1239: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1240: .Ed
1.167 djm 1241: .It Cm ProxyUseFdpass
1.168 jmc 1242: Specifies that
1.167 djm 1243: .Cm ProxyCommand
1244: will pass a connected file descriptor back to
1.168 jmc 1245: .Xr ssh 1
1.167 djm 1246: instead of continuing to execute and pass data.
1247: The default is
1248: .Dq no .
1.213 markus 1249: .It Cm PubkeyAcceptedKeyTypes
1250: Specifies the key types that will be used for public key authentication
1251: as a comma-separated pattern list.
1.214 djm 1252: Alternately if the specified value begins with a
1253: .Sq +
1254: character, then the key types after it will be appended to the default
1255: instead of replacing it.
1.213 markus 1256: The default for this option is:
1257: .Bd -literal -offset 3n
1258: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1259: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1260: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1261: ssh-ed25519-cert-v01@openssh.com,
1262: ssh-rsa-cert-v01@openssh.com,
1263: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1264: ssh-ed25519,ssh-rsa
1265: .Ed
1266: .Pp
1267: The
1268: .Fl Q
1269: option of
1270: .Xr ssh 1
1271: may be used to list supported key types.
1.1 stevesk 1272: .It Cm PubkeyAuthentication
1273: Specifies whether to try public key authentication.
1274: The argument to this keyword must be
1275: .Dq yes
1276: or
1277: .Dq no .
1278: The default is
1279: .Dq yes .
1280: This option applies to protocol version 2 only.
1.75 dtucker 1281: .It Cm RekeyLimit
1282: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1283: session key is renegotiated, optionally followed a maximum amount of
1284: time that may pass before the session key is renegotiated.
1285: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1286: .Sq K ,
1287: .Sq M ,
1.75 dtucker 1288: or
1.76 jmc 1289: .Sq G
1.75 dtucker 1290: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1291: The default is between
1.84 jmc 1292: .Sq 1G
1.75 dtucker 1293: and
1.84 jmc 1294: .Sq 4G ,
1.75 dtucker 1295: depending on the cipher.
1.162 dtucker 1296: The optional second value is specified in seconds and may use any of the
1297: units documented in the
1.166 jmc 1298: TIME FORMATS section of
1.162 dtucker 1299: .Xr sshd_config 5 .
1300: The default value for
1301: .Cm RekeyLimit
1302: is
1303: .Dq default none ,
1304: which means that rekeying is performed after the cipher's default amount
1305: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1306: This option applies to protocol version 2 only.
1.1 stevesk 1307: .It Cm RemoteForward
1.74 jmc 1308: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1309: the secure channel to the specified host and port from the local machine.
1.49 jmc 1310: The first argument must be
1.43 djm 1311: .Sm off
1.49 jmc 1312: .Oo Ar bind_address : Oc Ar port
1.43 djm 1313: .Sm on
1.49 jmc 1314: and the second argument must be
1315: .Ar host : Ns Ar hostport .
1.138 djm 1316: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1317: Multiple forwardings may be specified, and additional
1318: forwardings can be given on the command line.
1.113 stevesk 1319: Privileged ports can be forwarded only when
1320: logging in as root on the remote machine.
1.118 jmc 1321: .Pp
1.117 djm 1322: If the
1323: .Ar port
1324: argument is
1325: .Ql 0 ,
1326: the listen port will be dynamically allocated on the server and reported
1327: to the client at run time.
1.43 djm 1328: .Pp
1329: If the
1330: .Ar bind_address
1331: is not specified, the default is to only bind to loopback addresses.
1332: If the
1333: .Ar bind_address
1334: is
1335: .Ql *
1336: or an empty string, then the forwarding is requested to listen on all
1337: interfaces.
1338: Specifying a remote
1339: .Ar bind_address
1.46 jmc 1340: will only succeed if the server's
1341: .Cm GatewayPorts
1.43 djm 1342: option is enabled (see
1.46 jmc 1343: .Xr sshd_config 5 ) .
1.149 djm 1344: .It Cm RequestTTY
1345: Specifies whether to request a pseudo-tty for the session.
1346: The argument may be one of:
1347: .Dq no
1348: (never request a TTY),
1349: .Dq yes
1350: (always request a TTY when standard input is a TTY),
1351: .Dq force
1352: (always request a TTY) or
1353: .Dq auto
1354: (request a TTY when opening a login session).
1355: This option mirrors the
1356: .Fl t
1357: and
1358: .Fl T
1359: flags for
1360: .Xr ssh 1 .
1.196 djm 1361: .It Cm RevokedHostKeys
1362: Specifies revoked host public keys.
1363: Keys listed in this file will be refused for host authentication.
1364: Note that if this file does not exist or is not readable,
1365: then host authentication will be refused for all hosts.
1366: Keys may be specified as a text file, listing one public key per line, or as
1367: an OpenSSH Key Revocation List (KRL) as generated by
1368: .Xr ssh-keygen 1 .
1369: For more information on KRLs, see the KEY REVOCATION LISTS section in
1370: .Xr ssh-keygen 1 .
1.1 stevesk 1371: .It Cm RhostsRSAAuthentication
1372: Specifies whether to try rhosts based authentication with RSA host
1373: authentication.
1374: The argument must be
1375: .Dq yes
1376: or
1377: .Dq no .
1378: The default is
1379: .Dq no .
1380: This option applies to protocol version 1 only and requires
1.84 jmc 1381: .Xr ssh 1
1.1 stevesk 1382: to be setuid root.
1383: .It Cm RSAAuthentication
1384: Specifies whether to try RSA authentication.
1385: The argument to this keyword must be
1386: .Dq yes
1387: or
1388: .Dq no .
1389: RSA authentication will only be
1390: attempted if the identity file exists, or an authentication agent is
1391: running.
1392: The default is
1393: .Dq yes .
1394: Note that this option applies to protocol version 1 only.
1.32 djm 1395: .It Cm SendEnv
1396: Specifies what variables from the local
1397: .Xr environ 7
1398: should be sent to the server.
1.84 jmc 1399: Note that environment passing is only supported for protocol 2.
1400: The server must also support it, and the server must be configured to
1.33 djm 1401: accept these environment variables.
1.207 dtucker 1402: Note that the
1403: .Ev TERM
1.208 jmc 1404: environment variable is always sent whenever a
1.207 dtucker 1405: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1406: Refer to
1407: .Cm AcceptEnv
1408: in
1409: .Xr sshd_config 5
1410: for how to configure the server.
1.80 jmc 1411: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1412: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1413: across multiple
1414: .Cm SendEnv
1415: directives.
1416: The default is not to send any environment variables.
1.81 jmc 1417: .Pp
1418: See
1419: .Sx PATTERNS
1420: for more information on patterns.
1.28 markus 1421: .It Cm ServerAliveCountMax
1.73 jmc 1422: Sets the number of server alive messages (see below) which may be
1.28 markus 1423: sent without
1.84 jmc 1424: .Xr ssh 1
1.28 markus 1425: receiving any messages back from the server.
1426: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1427: ssh will disconnect from the server, terminating the session.
1.28 markus 1428: It is important to note that the use of server alive messages is very
1429: different from
1430: .Cm TCPKeepAlive
1431: (below).
1432: The server alive messages are sent through the encrypted channel
1433: and therefore will not be spoofable.
1434: The TCP keepalive option enabled by
1435: .Cm TCPKeepAlive
1436: is spoofable.
1437: The server alive mechanism is valuable when the client or
1438: server depend on knowing when a connection has become inactive.
1439: .Pp
1440: The default value is 3.
1441: If, for example,
1442: .Cm ServerAliveInterval
1.84 jmc 1443: (see below) is set to 15 and
1.28 markus 1444: .Cm ServerAliveCountMax
1.84 jmc 1445: is left at the default, if the server becomes unresponsive,
1446: ssh will disconnect after approximately 45 seconds.
1.89 markus 1447: This option applies to protocol version 2 only.
1.67 jmc 1448: .It Cm ServerAliveInterval
1449: Sets a timeout interval in seconds after which if no data has been received
1450: from the server,
1.84 jmc 1451: .Xr ssh 1
1.67 jmc 1452: will send a message through the encrypted
1453: channel to request a response from the server.
1454: The default
1455: is 0, indicating that these messages will not be sent to the server.
1456: This option applies to protocol version 2 only.
1.191 millert 1457: .It Cm StreamLocalBindMask
1458: Sets the octal file creation mode mask
1459: .Pq umask
1460: used when creating a Unix-domain socket file for local or remote
1461: port forwarding.
1462: This option is only used for port forwarding to a Unix-domain socket file.
1463: .Pp
1464: The default value is 0177, which creates a Unix-domain socket file that is
1465: readable and writable only by the owner.
1466: Note that not all operating systems honor the file mode on Unix-domain
1467: socket files.
1468: .It Cm StreamLocalBindUnlink
1469: Specifies whether to remove an existing Unix-domain socket file for local
1470: or remote port forwarding before creating a new one.
1471: If the socket file already exists and
1472: .Cm StreamLocalBindUnlink
1473: is not enabled,
1474: .Nm ssh
1475: will be unable to forward the port to the Unix-domain socket file.
1476: This option is only used for port forwarding to a Unix-domain socket file.
1477: .Pp
1478: The argument must be
1479: .Dq yes
1480: or
1481: .Dq no .
1482: The default is
1483: .Dq no .
1.1 stevesk 1484: .It Cm StrictHostKeyChecking
1485: If this flag is set to
1486: .Dq yes ,
1.84 jmc 1487: .Xr ssh 1
1.1 stevesk 1488: will never automatically add host keys to the
1.50 djm 1489: .Pa ~/.ssh/known_hosts
1.1 stevesk 1490: file, and refuses to connect to hosts whose host key has changed.
1491: This provides maximum protection against trojan horse attacks,
1.84 jmc 1492: though it can be annoying when the
1.1 stevesk 1493: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1494: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1495: frequently made.
1496: This option forces the user to manually
1497: add all new hosts.
1498: If this flag is set to
1499: .Dq no ,
1.84 jmc 1500: ssh will automatically add new host keys to the
1.1 stevesk 1501: user known hosts files.
1502: If this flag is set to
1503: .Dq ask ,
1504: new host keys
1505: will be added to the user known host files only after the user
1506: has confirmed that is what they really want to do, and
1.84 jmc 1507: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1508: The host keys of
1509: known hosts will be verified automatically in all cases.
1510: The argument must be
1511: .Dq yes ,
1.84 jmc 1512: .Dq no ,
1.1 stevesk 1513: or
1514: .Dq ask .
1515: The default is
1516: .Dq ask .
1.26 markus 1517: .It Cm TCPKeepAlive
1518: Specifies whether the system should send TCP keepalive messages to the
1519: other side.
1520: If they are sent, death of the connection or crash of one
1521: of the machines will be properly noticed.
1522: However, this means that
1523: connections will die if the route is down temporarily, and some people
1524: find it annoying.
1525: .Pp
1526: The default is
1527: .Dq yes
1528: (to send TCP keepalive messages), and the client will notice
1529: if the network goes down or the remote host dies.
1530: This is important in scripts, and many users want it too.
1531: .Pp
1532: To disable TCP keepalive messages, the value should be set to
1533: .Dq no .
1.65 reyk 1534: .It Cm Tunnel
1.95 stevesk 1535: Request
1.65 reyk 1536: .Xr tun 4
1.69 jmc 1537: device forwarding between the client and the server.
1.65 reyk 1538: The argument must be
1.68 reyk 1539: .Dq yes ,
1.95 stevesk 1540: .Dq point-to-point
1541: (layer 3),
1542: .Dq ethernet
1543: (layer 2),
1.65 reyk 1544: or
1545: .Dq no .
1.95 stevesk 1546: Specifying
1547: .Dq yes
1548: requests the default tunnel mode, which is
1549: .Dq point-to-point .
1.65 reyk 1550: The default is
1551: .Dq no .
1552: .It Cm TunnelDevice
1.95 stevesk 1553: Specifies the
1.65 reyk 1554: .Xr tun 4
1.95 stevesk 1555: devices to open on the client
1556: .Pq Ar local_tun
1557: and the server
1558: .Pq Ar remote_tun .
1559: .Pp
1560: The argument must be
1561: .Sm off
1562: .Ar local_tun Op : Ar remote_tun .
1563: .Sm on
1564: The devices may be specified by numerical ID or the keyword
1565: .Dq any ,
1566: which uses the next available tunnel device.
1567: If
1568: .Ar remote_tun
1569: is not specified, it defaults to
1570: .Dq any .
1571: The default is
1572: .Dq any:any .
1.201 djm 1573: .It Cm UpdateHostKeys
1.200 djm 1574: Specifies whether
1575: .Xr ssh 1
1576: should accept notifications of additional hostkeys from the server sent
1577: after authentication has completed and add them to
1578: .Cm UserKnownHostsFile .
1579: The argument must be
1.204 djm 1580: .Dq yes ,
1.203 djm 1581: .Dq no
1.204 djm 1582: (the default) or
1583: .Dq ask .
1.200 djm 1584: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1585: and supports graceful key rotation by allowing a server to send replacement
1586: public keys before old ones are removed.
1.200 djm 1587: Additional hostkeys are only accepted if the key used to authenticate the
1588: host was already trusted or explicity accepted by the user.
1.204 djm 1589: If
1590: .Cm UpdateHostKeys
1591: is set to
1592: .Dq ask ,
1593: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1594: Confirmation is currently incompatible with
1595: .Cm ControlPersist ,
1596: and will be disabled if it is enabled.
1.200 djm 1597: .Pp
1598: Presently, only
1599: .Xr sshd 8
1600: from OpenSSH 6.8 and greater support the
1601: .Dq hostkeys@openssh.com
1602: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1603: .It Cm UsePrivilegedPort
1604: Specifies whether to use a privileged port for outgoing connections.
1605: The argument must be
1606: .Dq yes
1607: or
1608: .Dq no .
1609: The default is
1610: .Dq no .
1611: If set to
1.84 jmc 1612: .Dq yes ,
1613: .Xr ssh 1
1.72 jmc 1614: must be setuid root.
1615: Note that this option must be set to
1616: .Dq yes
1617: for
1618: .Cm RhostsRSAAuthentication
1619: with older servers.
1.1 stevesk 1620: .It Cm User
1621: Specifies the user to log in as.
1622: This can be useful when a different user name is used on different machines.
1623: This saves the trouble of
1624: having to remember to give the user name on the command line.
1625: .It Cm UserKnownHostsFile
1.151 djm 1626: Specifies one or more files to use for the user
1627: host key database, separated by whitespace.
1628: The default is
1629: .Pa ~/.ssh/known_hosts ,
1630: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1631: .It Cm VerifyHostKeyDNS
1632: Specifies whether to verify the remote key using DNS and SSHFP resource
1633: records.
1.24 jakob 1634: If this option is set to
1635: .Dq yes ,
1.25 jmc 1636: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1637: from DNS.
1638: Insecure fingerprints will be handled as if this option was set to
1639: .Dq ask .
1640: If this option is set to
1641: .Dq ask ,
1642: information on fingerprint match will be displayed, but the user will still
1643: need to confirm new host keys according to the
1644: .Cm StrictHostKeyChecking
1645: option.
1646: The argument must be
1647: .Dq yes ,
1.84 jmc 1648: .Dq no ,
1.25 jmc 1649: or
1650: .Dq ask .
1.8 jakob 1651: The default is
1652: .Dq no .
1.12 jakob 1653: Note that this option applies to protocol version 2 only.
1.84 jmc 1654: .Pp
1.166 jmc 1655: See also VERIFYING HOST KEYS in
1.84 jmc 1656: .Xr ssh 1 .
1.111 grunk 1657: .It Cm VisualHostKey
1658: If this flag is set to
1659: .Dq yes ,
1660: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1661: printed in addition to the fingerprint string at login and
1.114 stevesk 1662: for unknown host keys.
1.111 grunk 1663: If this flag is set to
1664: .Dq no ,
1.114 stevesk 1665: no fingerprint strings are printed at login and
1.197 djm 1666: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1667: The default is
1668: .Dq no .
1.1 stevesk 1669: .It Cm XAuthLocation
1.5 stevesk 1670: Specifies the full pathname of the
1.1 stevesk 1671: .Xr xauth 1
1672: program.
1673: The default is
1674: .Pa /usr/X11R6/bin/xauth .
1675: .El
1.86 jmc 1676: .Sh PATTERNS
1677: A
1678: .Em pattern
1679: consists of zero or more non-whitespace characters,
1680: .Sq *
1681: (a wildcard that matches zero or more characters),
1682: or
1683: .Sq ?\&
1684: (a wildcard that matches exactly one character).
1685: For example, to specify a set of declarations for any host in the
1686: .Dq .co.uk
1687: set of domains,
1688: the following pattern could be used:
1689: .Pp
1690: .Dl Host *.co.uk
1691: .Pp
1692: The following pattern
1693: would match any host in the 192.168.0.[0-9] network range:
1694: .Pp
1695: .Dl Host 192.168.0.?
1696: .Pp
1697: A
1698: .Em pattern-list
1699: is a comma-separated list of patterns.
1700: Patterns within pattern-lists may be negated
1701: by preceding them with an exclamation mark
1702: .Pq Sq !\& .
1703: For example,
1.174 djm 1704: to allow a key to be used from anywhere within an organization
1.86 jmc 1705: except from the
1706: .Dq dialup
1707: pool,
1708: the following entry (in authorized_keys) could be used:
1709: .Pp
1710: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1711: .Sh FILES
1712: .Bl -tag -width Ds
1.50 djm 1713: .It Pa ~/.ssh/config
1.1 stevesk 1714: This is the per-user configuration file.
1715: The format of this file is described above.
1.84 jmc 1716: This file is used by the SSH client.
1.30 djm 1717: Because of the potential for abuse, this file must have strict permissions:
1718: read/write for the user, and not accessible by others.
1.1 stevesk 1719: .It Pa /etc/ssh/ssh_config
1720: Systemwide configuration file.
1721: This file provides defaults for those
1722: values that are not specified in the user's configuration file, and
1723: for those users who do not have a configuration file.
1724: This file must be world-readable.
1725: .El
1.13 jmc 1726: .Sh SEE ALSO
1727: .Xr ssh 1
1.1 stevesk 1728: .Sh AUTHORS
1729: OpenSSH is a derivative of the original and free
1730: ssh 1.2.12 release by Tatu Ylonen.
1731: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1732: Theo de Raadt and Dug Song
1733: removed many bugs, re-added newer features and
1734: created OpenSSH.
1735: Markus Friedl contributed the support for SSH
1736: protocol versions 1.5 and 2.0.