Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.221
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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1.221 ! djm 36: .\" $OpenBSD: ssh_config.5,v 1.220 2015/09/22 08:33:23 sobrado Exp $
! 37: .Dd $Mdocdate: September 22 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.
1.220 sobrado 142: Match conditions are specified using one or more criteria
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.
1.221 ! djm 328: .It Cm CertificateFile
! 329: Specifies a file from which the user's certificate is read.
! 330: A corresponding private key must be provided separately in order
! 331: to use this certificate either
! 332: from an
! 333: .Cm IdentityFile
! 334: directive or
! 335: .Fl i
! 336: flag to
! 337: .Xr ssh 1 ,
! 338: via
! 339: .Xr ssh-agent 1 ,
! 340: or via a
! 341: .Cm PKCS11Provider .
! 342: .Pp
! 343: The file name may use the tilde
! 344: syntax to refer to a user's home directory or one of the following
! 345: escape characters:
! 346: .Ql %d
! 347: (local user's home directory),
! 348: .Ql %u
! 349: (local user name),
! 350: .Ql %l
! 351: (local host name),
! 352: .Ql %h
! 353: (remote host name) or
! 354: .Ql %r
! 355: (remote user name).
! 356: .Pp
! 357: It is possible to have multiple certificate files specified in
! 358: configuration files; these certificates will be tried in sequence.
! 359: Multiple
! 360: .Cm CertificateFile
! 361: directives will add to the list of certificates used for
! 362: authentication.
1.1 stevesk 363: .It Cm ChallengeResponseAuthentication
1.82 jmc 364: Specifies whether to use challenge-response authentication.
1.1 stevesk 365: The argument to this keyword must be
366: .Dq yes
367: or
368: .Dq no .
369: The default is
370: .Dq yes .
371: .It Cm CheckHostIP
372: If this flag is set to
373: .Dq yes ,
1.84 jmc 374: .Xr ssh 1
375: will additionally check the host IP address in the
1.1 stevesk 376: .Pa known_hosts
377: file.
1.211 djm 378: This allows ssh to detect if a host key changed due to DNS spoofing
379: and will add addresses of destination hosts to
380: .Pa ~/.ssh/known_hosts
381: in the process, regardless of the setting of
382: .Cm StrictHostKeyChecking .
1.107 grunk 383: If the option is set to
1.1 stevesk 384: .Dq no ,
385: the check will not be executed.
386: The default is
387: .Dq yes .
388: .It Cm Cipher
389: Specifies the cipher to use for encrypting the session
390: in protocol version 1.
391: Currently,
392: .Dq blowfish ,
393: .Dq 3des ,
394: and
395: .Dq des
396: are supported.
397: .Ar des
398: is only supported in the
1.84 jmc 399: .Xr ssh 1
1.1 stevesk 400: client for interoperability with legacy protocol 1 implementations
401: that do not support the
402: .Ar 3des
1.7 jmc 403: cipher.
404: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 405: The default is
406: .Dq 3des .
407: .It Cm Ciphers
408: Specifies the ciphers allowed for protocol version 2
409: in order of preference.
410: Multiple ciphers must be comma-separated.
1.214 djm 411: If the specified value begins with a
412: .Sq +
413: character, then the specified ciphers will be appended to the default set
414: instead of replacing them.
415: .Pp
1.180 djm 416: The supported ciphers are:
417: .Pp
1.186 naddy 418: .Bl -item -compact -offset indent
419: .It
420: 3des-cbc
421: .It
422: aes128-cbc
423: .It
424: aes192-cbc
425: .It
426: aes256-cbc
427: .It
428: aes128-ctr
429: .It
430: aes192-ctr
431: .It
432: aes256-ctr
433: .It
434: aes128-gcm@openssh.com
435: .It
436: aes256-gcm@openssh.com
437: .It
438: arcfour
439: .It
440: arcfour128
441: .It
442: arcfour256
443: .It
444: blowfish-cbc
445: .It
446: cast128-cbc
447: .It
448: chacha20-poly1305@openssh.com
449: .El
1.180 djm 450: .Pp
1.84 jmc 451: The default is:
1.186 naddy 452: .Bd -literal -offset indent
1.215 jmc 453: chacha20-poly1305@openssh.com,
1.186 naddy 454: aes128-ctr,aes192-ctr,aes256-ctr,
1.161 markus 455: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.186 naddy 456: arcfour256,arcfour128,
457: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
458: aes192-cbc,aes256-cbc,arcfour
1.1 stevesk 459: .Ed
1.180 djm 460: .Pp
461: The list of available ciphers may also be obtained using the
462: .Fl Q
463: option of
1.198 djm 464: .Xr ssh 1
465: with an argument of
466: .Dq cipher .
1.1 stevesk 467: .It Cm ClearAllForwardings
1.84 jmc 468: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 469: specified in the configuration files or on the command line be
1.7 jmc 470: cleared.
471: This option is primarily useful when used from the
1.84 jmc 472: .Xr ssh 1
1.1 stevesk 473: command line to clear port forwardings set in
474: configuration files, and is automatically set by
475: .Xr scp 1
476: and
477: .Xr sftp 1 .
478: The argument must be
479: .Dq yes
480: or
481: .Dq no .
482: The default is
483: .Dq no .
484: .It Cm Compression
485: Specifies whether to use compression.
486: The argument must be
487: .Dq yes
488: or
489: .Dq no .
490: The default is
491: .Dq no .
492: .It Cm CompressionLevel
493: Specifies the compression level to use if compression is enabled.
494: The argument must be an integer from 1 (fast) to 9 (slow, best).
495: The default level is 6, which is good for most applications.
496: The meaning of the values is the same as in
497: .Xr gzip 1 .
498: Note that this option applies to protocol version 1 only.
499: .It Cm ConnectionAttempts
500: Specifies the number of tries (one per second) to make before exiting.
501: The argument must be an integer.
502: This may be useful in scripts if the connection sometimes fails.
503: The default is 1.
1.9 djm 504: .It Cm ConnectTimeout
1.84 jmc 505: Specifies the timeout (in seconds) used when connecting to the
506: SSH server, instead of using the default system TCP timeout.
1.11 jmc 507: This value is used only when the target is down or really unreachable,
508: not when it refuses the connection.
1.36 djm 509: .It Cm ControlMaster
510: Enables the sharing of multiple sessions over a single network connection.
511: When set to
1.84 jmc 512: .Dq yes ,
513: .Xr ssh 1
1.36 djm 514: will listen for connections on a control socket specified using the
515: .Cm ControlPath
516: argument.
517: Additional sessions can connect to this socket using the same
518: .Cm ControlPath
519: with
520: .Cm ControlMaster
521: set to
522: .Dq no
1.38 jmc 523: (the default).
1.64 jmc 524: These sessions will try to reuse the master instance's network connection
1.63 djm 525: rather than initiating new ones, but will fall back to connecting normally
526: if the control socket does not exist, or is not listening.
527: .Pp
1.37 djm 528: Setting this to
529: .Dq ask
1.84 jmc 530: will cause ssh
1.206 jmc 531: to listen for control connections, but require confirmation using
532: .Xr ssh-askpass 1 .
1.51 jakob 533: If the
534: .Cm ControlPath
1.84 jmc 535: cannot be opened,
536: ssh will continue without connecting to a master instance.
1.58 djm 537: .Pp
538: X11 and
1.59 jmc 539: .Xr ssh-agent 1
1.58 djm 540: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 541: display and agent forwarded will be the one belonging to the master
1.59 jmc 542: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 543: .Pp
544: Two additional options allow for opportunistic multiplexing: try to use a
545: master connection but fall back to creating a new one if one does not already
546: exist.
547: These options are:
548: .Dq auto
549: and
550: .Dq autoask .
551: The latter requires confirmation like the
552: .Dq ask
553: option.
1.36 djm 554: .It Cm ControlPath
1.55 djm 555: Specify the path to the control socket used for connection sharing as described
556: in the
1.36 djm 557: .Cm ControlMaster
1.57 djm 558: section above or the string
559: .Dq none
560: to disable connection sharing.
1.55 djm 561: In the path,
1.147 djm 562: .Ql %L
563: will be substituted by the first component of the local host name,
1.77 djm 564: .Ql %l
1.147 djm 565: will be substituted by the local host name (including any domain name),
1.55 djm 566: .Ql %h
567: will be substituted by the target host name,
1.150 jmc 568: .Ql %n
569: will be substituted by the original target host name
570: specified on the command line,
1.55 djm 571: .Ql %p
1.175 djm 572: the destination port,
1.55 djm 573: .Ql %r
1.188 djm 574: by the remote login username,
1.147 djm 575: .Ql %u
1.218 djm 576: by the username and
577: .Ql %i
1.219 jmc 578: by the numeric user ID (uid) of the user running
579: .Xr ssh 1 ,
580: and
1.189 jmc 581: .Ql \&%C
1.188 djm 582: by a hash of the concatenation: %l%h%p%r.
1.56 djm 583: It is recommended that any
584: .Cm ControlPath
585: used for opportunistic connection sharing include
1.195 djm 586: at least %h, %p, and %r (or alternatively %C) and be placed in a directory
587: that is not writable by other users.
1.56 djm 588: This ensures that shared connections are uniquely identified.
1.137 djm 589: .It Cm ControlPersist
590: When used in conjunction with
591: .Cm ControlMaster ,
592: specifies that the master connection should remain open
593: in the background (waiting for future client connections)
594: after the initial client connection has been closed.
595: If set to
596: .Dq no ,
597: then the master connection will not be placed into the background,
598: and will close as soon as the initial client connection is closed.
599: If set to
1.195 djm 600: .Dq yes
601: or
602: .Dq 0 ,
1.137 djm 603: then the master connection will remain in the background indefinitely
604: (until killed or closed via a mechanism such as the
605: .Xr ssh 1
606: .Dq Fl O No exit
607: option).
608: If set to a time in seconds, or a time in any of the formats documented in
609: .Xr sshd_config 5 ,
610: then the backgrounded master connection will automatically terminate
611: after it has remained idle (with no client connections) for the
612: specified time.
1.38 jmc 613: .It Cm DynamicForward
1.74 jmc 614: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 615: over the secure channel, and the application
616: protocol is then used to determine where to connect to from the
617: remote machine.
1.62 djm 618: .Pp
619: The argument must be
620: .Sm off
621: .Oo Ar bind_address : Oc Ar port .
622: .Sm on
1.138 djm 623: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 624: By default, the local port is bound in accordance with the
625: .Cm GatewayPorts
626: setting.
627: However, an explicit
628: .Ar bind_address
629: may be used to bind the connection to a specific address.
630: The
631: .Ar bind_address
632: of
633: .Dq localhost
634: indicates that the listening port be bound for local use only, while an
635: empty address or
636: .Sq *
637: indicates that the port should be available from all interfaces.
638: .Pp
1.38 jmc 639: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 640: .Xr ssh 1
1.38 jmc 641: will act as a SOCKS server.
642: Multiple forwardings may be specified, and
643: additional forwardings can be given on the command line.
644: Only the superuser can forward privileged ports.
1.14 markus 645: .It Cm EnableSSHKeysign
646: Setting this option to
647: .Dq yes
648: in the global client configuration file
649: .Pa /etc/ssh/ssh_config
650: enables the use of the helper program
651: .Xr ssh-keysign 8
652: during
653: .Cm HostbasedAuthentication .
654: The argument must be
655: .Dq yes
656: or
657: .Dq no .
658: The default is
659: .Dq no .
1.23 jmc 660: This option should be placed in the non-hostspecific section.
1.14 markus 661: See
662: .Xr ssh-keysign 8
663: for more information.
1.1 stevesk 664: .It Cm EscapeChar
665: Sets the escape character (default:
666: .Ql ~ ) .
667: The escape character can also
668: be set on the command line.
669: The argument should be a single character,
670: .Ql ^
671: followed by a letter, or
672: .Dq none
673: to disable the escape
674: character entirely (making the connection transparent for binary
675: data).
1.96 markus 676: .It Cm ExitOnForwardFailure
677: Specifies whether
678: .Xr ssh 1
679: should terminate the connection if it cannot set up all requested
1.216 djm 680: dynamic, tunnel, local, and remote port forwardings, (e.g.\&
1.217 jmc 681: if either end is unable to bind and listen on a specified port).
1.216 djm 682: Note that
683: .Cm ExitOnForwardFailure
684: does not apply to connections made over port forwardings and will not,
685: for example, cause
686: .Xr ssh 1
687: to exit if TCP connections to the ultimate forwarding destination fail.
1.96 markus 688: The argument must be
689: .Dq yes
690: or
691: .Dq no .
692: The default is
693: .Dq no .
1.197 djm 694: .It Cm FingerprintHash
695: Specifies the hash algorithm used when displaying key fingerprints.
696: Valid options are:
697: .Dq md5
698: and
699: .Dq sha256 .
700: The default is
701: .Dq sha256 .
1.1 stevesk 702: .It Cm ForwardAgent
703: Specifies whether the connection to the authentication agent (if any)
704: will be forwarded to the remote machine.
705: The argument must be
706: .Dq yes
707: or
708: .Dq no .
709: The default is
710: .Dq no .
1.3 stevesk 711: .Pp
1.7 jmc 712: Agent forwarding should be enabled with caution.
713: Users with the ability to bypass file permissions on the remote host
714: (for the agent's Unix-domain socket)
715: can access the local agent through the forwarded connection.
716: An attacker cannot obtain key material from the agent,
1.3 stevesk 717: however they can perform operations on the keys that enable them to
718: authenticate using the identities loaded into the agent.
1.1 stevesk 719: .It Cm ForwardX11
720: Specifies whether X11 connections will be automatically redirected
721: over the secure channel and
722: .Ev DISPLAY
723: set.
724: The argument must be
725: .Dq yes
726: or
727: .Dq no .
728: The default is
729: .Dq no .
1.3 stevesk 730: .Pp
1.7 jmc 731: X11 forwarding should be enabled with caution.
732: Users with the ability to bypass file permissions on the remote host
1.22 markus 733: (for the user's X11 authorization database)
1.7 jmc 734: can access the local X11 display through the forwarded connection.
1.22 markus 735: An attacker may then be able to perform activities such as keystroke monitoring
736: if the
737: .Cm ForwardX11Trusted
738: option is also enabled.
1.134 djm 739: .It Cm ForwardX11Timeout
1.135 jmc 740: Specify a timeout for untrusted X11 forwarding
741: using the format described in the
1.166 jmc 742: TIME FORMATS section of
1.134 djm 743: .Xr sshd_config 5 .
744: X11 connections received by
745: .Xr ssh 1
746: after this time will be refused.
747: The default is to disable untrusted X11 forwarding after twenty minutes has
748: elapsed.
1.22 markus 749: .It Cm ForwardX11Trusted
1.34 jmc 750: If this option is set to
1.84 jmc 751: .Dq yes ,
752: remote X11 clients will have full access to the original X11 display.
1.42 djm 753: .Pp
1.22 markus 754: If this option is set to
1.84 jmc 755: .Dq no ,
756: remote X11 clients will be considered untrusted and prevented
1.22 markus 757: from stealing or tampering with data belonging to trusted X11
758: clients.
1.42 djm 759: Furthermore, the
760: .Xr xauth 1
761: token used for the session will be set to expire after 20 minutes.
762: Remote clients will be refused access after this time.
1.22 markus 763: .Pp
764: The default is
765: .Dq no .
766: .Pp
767: See the X11 SECURITY extension specification for full details on
768: the restrictions imposed on untrusted clients.
1.1 stevesk 769: .It Cm GatewayPorts
770: Specifies whether remote hosts are allowed to connect to local
771: forwarded ports.
772: By default,
1.84 jmc 773: .Xr ssh 1
1.7 jmc 774: binds local port forwardings to the loopback address.
775: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 776: .Cm GatewayPorts
1.84 jmc 777: can be used to specify that ssh
1.1 stevesk 778: should bind local port forwardings to the wildcard address,
779: thus allowing remote hosts to connect to forwarded ports.
780: The argument must be
781: .Dq yes
782: or
783: .Dq no .
784: The default is
785: .Dq no .
786: .It Cm GlobalKnownHostsFile
1.151 djm 787: Specifies one or more files to use for the global
788: host key database, separated by whitespace.
789: The default is
790: .Pa /etc/ssh/ssh_known_hosts ,
791: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 792: .It Cm GSSAPIAuthentication
1.27 markus 793: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 794: The default is
1.21 markus 795: .Dq no .
1.18 markus 796: Note that this option applies to protocol version 2 only.
797: .It Cm GSSAPIDelegateCredentials
798: Forward (delegate) credentials to the server.
799: The default is
800: .Dq no .
801: Note that this option applies to protocol version 2 only.
1.44 djm 802: .It Cm HashKnownHosts
803: Indicates that
1.84 jmc 804: .Xr ssh 1
1.44 djm 805: should hash host names and addresses when they are added to
1.50 djm 806: .Pa ~/.ssh/known_hosts .
1.44 djm 807: These hashed names may be used normally by
1.84 jmc 808: .Xr ssh 1
1.44 djm 809: and
1.84 jmc 810: .Xr sshd 8 ,
1.44 djm 811: but they do not reveal identifying information should the file's contents
812: be disclosed.
813: The default is
814: .Dq no .
1.97 jmc 815: Note that existing names and addresses in known hosts files
816: will not be converted automatically,
817: but may be manually hashed using
1.45 djm 818: .Xr ssh-keygen 1 .
1.1 stevesk 819: .It Cm HostbasedAuthentication
820: Specifies whether to try rhosts based authentication with public key
821: authentication.
822: The argument must be
823: .Dq yes
824: or
825: .Dq no .
826: The default is
827: .Dq no .
828: This option applies to protocol version 2 only and
829: is similar to
830: .Cm RhostsRSAAuthentication .
1.202 djm 831: .It Cm HostbasedKeyTypes
832: Specifies the key types that will be used for hostbased authentication
833: as a comma-separated pattern list.
1.214 djm 834: Alternately if the specified value begins with a
835: .Sq +
836: character, then the specified key types will be appended to the default set
837: instead of replacing them.
1.213 markus 838: The default for this option is:
839: .Bd -literal -offset 3n
840: ecdsa-sha2-nistp256-cert-v01@openssh.com,
841: ecdsa-sha2-nistp384-cert-v01@openssh.com,
842: ecdsa-sha2-nistp521-cert-v01@openssh.com,
843: ssh-ed25519-cert-v01@openssh.com,
844: ssh-rsa-cert-v01@openssh.com,
845: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
846: ssh-ed25519,ssh-rsa
847: .Ed
848: .Pp
1.202 djm 849: The
850: .Fl Q
851: option of
852: .Xr ssh 1
853: may be used to list supported key types.
1.1 stevesk 854: .It Cm HostKeyAlgorithms
855: Specifies the protocol version 2 host key algorithms
856: that the client wants to use in order of preference.
1.214 djm 857: Alternately if the specified value begins with a
858: .Sq +
859: character, then the specified key types will be appended to the default set
860: instead of replacing them.
1.1 stevesk 861: The default for this option is:
1.139 djm 862: .Bd -literal -offset 3n
863: ecdsa-sha2-nistp256-cert-v01@openssh.com,
864: ecdsa-sha2-nistp384-cert-v01@openssh.com,
865: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.183 naddy 866: ssh-ed25519-cert-v01@openssh.com,
1.213 markus 867: ssh-rsa-cert-v01@openssh.com,
1.139 djm 868: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.213 markus 869: ssh-ed25519,ssh-rsa
1.139 djm 870: .Ed
1.145 djm 871: .Pp
872: If hostkeys are known for the destination host then this default is modified
873: to prefer their algorithms.
1.198 djm 874: .Pp
875: The list of available key types may also be obtained using the
876: .Fl Q
877: option of
878: .Xr ssh 1
879: with an argument of
880: .Dq key .
1.1 stevesk 881: .It Cm HostKeyAlias
882: Specifies an alias that should be used instead of the
883: real host name when looking up or saving the host key
884: in the host key database files.
1.84 jmc 885: This option is useful for tunneling SSH connections
1.1 stevesk 886: or for multiple servers running on a single host.
887: .It Cm HostName
888: Specifies the real host name to log into.
889: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 890: If the hostname contains the character sequence
891: .Ql %h ,
1.150 jmc 892: then this will be replaced with the host name specified on the command line
1.136 djm 893: (this is useful for manipulating unqualified names).
1.187 djm 894: The character sequence
895: .Ql %%
896: will be replaced by a single
897: .Ql %
898: character, which may be used when specifying IPv6 link-local addresses.
899: .Pp
1.84 jmc 900: The default is the name given on the command line.
1.1 stevesk 901: Numeric IP addresses are also permitted (both on the command line and in
902: .Cm HostName
903: specifications).
1.29 markus 904: .It Cm IdentitiesOnly
905: Specifies that
1.84 jmc 906: .Xr ssh 1
1.221 ! djm 907: should only use the authentication identity and certificate files explicitly
! 908: configured in the
1.31 jmc 909: .Nm
1.221 ! djm 910: files
! 911: or passed on the
! 912: .Xr ssh 1
! 913: command-line,
1.84 jmc 914: even if
915: .Xr ssh-agent 1
1.159 djm 916: or a
917: .Cm PKCS11Provider
1.29 markus 918: offers more identities.
919: The argument to this keyword must be
920: .Dq yes
921: or
922: .Dq no .
1.84 jmc 923: This option is intended for situations where ssh-agent
1.29 markus 924: offers many different identities.
925: The default is
926: .Dq no .
1.67 jmc 927: .It Cm IdentityFile
1.192 sobrado 928: Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
1.139 djm 929: identity is read.
1.67 jmc 930: The default is
931: .Pa ~/.ssh/identity
932: for protocol version 1, and
1.139 djm 933: .Pa ~/.ssh/id_dsa ,
1.183 naddy 934: .Pa ~/.ssh/id_ecdsa ,
935: .Pa ~/.ssh/id_ed25519
1.139 djm 936: and
1.67 jmc 937: .Pa ~/.ssh/id_rsa
938: for protocol version 2.
939: Additionally, any identities represented by the authentication agent
1.165 djm 940: will be used for authentication unless
941: .Cm IdentitiesOnly
942: is set.
1.221 ! djm 943: If no certificates have been explicitly specified by
! 944: .Cm CertificateFile ,
1.129 djm 945: .Xr ssh 1
946: will try to load certificate information from the filename obtained by
947: appending
948: .Pa -cert.pub
949: to the path of a specified
950: .Cm IdentityFile .
1.90 djm 951: .Pp
1.67 jmc 952: The file name may use the tilde
1.91 jmc 953: syntax to refer to a user's home directory or one of the following
1.90 djm 954: escape characters:
955: .Ql %d
956: (local user's home directory),
957: .Ql %u
958: (local user name),
959: .Ql %l
960: (local host name),
961: .Ql %h
962: (remote host name) or
1.92 djm 963: .Ql %r
1.90 djm 964: (remote user name).
965: .Pp
1.67 jmc 966: It is possible to have
967: multiple identity files specified in configuration files; all these
968: identities will be tried in sequence.
1.152 djm 969: Multiple
970: .Cm IdentityFile
971: directives will add to the list of identities tried (this behaviour
972: differs from that of other configuration directives).
1.165 djm 973: .Pp
974: .Cm IdentityFile
975: may be used in conjunction with
976: .Cm IdentitiesOnly
977: to select which identities in an agent are offered during authentication.
1.221 ! djm 978: .Cm IdentityFile
! 979: may also be used in conjunction with
! 980: .Cm CertificateFile
! 981: in order to provide any certificate also needed for authentication with
! 982: the identity.
1.164 jmc 983: .It Cm IgnoreUnknown
984: Specifies a pattern-list of unknown options to be ignored if they are
985: encountered in configuration parsing.
986: This may be used to suppress errors if
987: .Nm
988: contains options that are unrecognised by
989: .Xr ssh 1 .
990: It is recommended that
991: .Cm IgnoreUnknown
992: be listed early in the configuration file as it will not be applied
993: to unknown options that appear before it.
1.143 djm 994: .It Cm IPQoS
995: Specifies the IPv4 type-of-service or DSCP class for connections.
996: Accepted values are
997: .Dq af11 ,
998: .Dq af12 ,
999: .Dq af13 ,
1.154 djm 1000: .Dq af21 ,
1.143 djm 1001: .Dq af22 ,
1002: .Dq af23 ,
1003: .Dq af31 ,
1004: .Dq af32 ,
1005: .Dq af33 ,
1006: .Dq af41 ,
1007: .Dq af42 ,
1008: .Dq af43 ,
1009: .Dq cs0 ,
1010: .Dq cs1 ,
1011: .Dq cs2 ,
1012: .Dq cs3 ,
1013: .Dq cs4 ,
1014: .Dq cs5 ,
1015: .Dq cs6 ,
1016: .Dq cs7 ,
1017: .Dq ef ,
1018: .Dq lowdelay ,
1019: .Dq throughput ,
1020: .Dq reliability ,
1021: or a numeric value.
1.146 djm 1022: This option may take one or two arguments, separated by whitespace.
1.143 djm 1023: If one argument is specified, it is used as the packet class unconditionally.
1024: If two values are specified, the first is automatically selected for
1025: interactive sessions and the second for non-interactive sessions.
1026: The default is
1027: .Dq lowdelay
1028: for interactive sessions and
1029: .Dq throughput
1030: for non-interactive sessions.
1.103 djm 1031: .It Cm KbdInteractiveAuthentication
1032: Specifies whether to use keyboard-interactive authentication.
1033: The argument to this keyword must be
1034: .Dq yes
1035: or
1036: .Dq no .
1037: The default is
1038: .Dq yes .
1.39 djm 1039: .It Cm KbdInteractiveDevices
1040: Specifies the list of methods to use in keyboard-interactive authentication.
1041: Multiple method names must be comma-separated.
1042: The default is to use the server specified list.
1.85 jmc 1043: The methods available vary depending on what the server supports.
1044: For an OpenSSH server,
1045: it may be zero or more of:
1046: .Dq bsdauth ,
1047: .Dq pam ,
1048: and
1049: .Dq skey .
1.140 djm 1050: .It Cm KexAlgorithms
1051: Specifies the available KEX (Key Exchange) algorithms.
1052: Multiple algorithms must be comma-separated.
1.214 djm 1053: Alternately if the specified value begins with a
1054: .Sq +
1055: character, then the specified methods will be appended to the default set
1056: instead of replacing them.
1.141 jmc 1057: The default is:
1058: .Bd -literal -offset indent
1.179 markus 1059: curve25519-sha256@libssh.org,
1.141 jmc 1060: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1061: diffie-hellman-group-exchange-sha256,
1.209 dtucker 1062: diffie-hellman-group-exchange-sha1,
1.212 djm 1063: diffie-hellman-group14-sha1
1.141 jmc 1064: .Ed
1.198 djm 1065: .Pp
1066: The list of available key exchange algorithms may also be obtained using the
1067: .Fl Q
1068: option of
1069: .Xr ssh 1
1070: with an argument of
1071: .Dq kex .
1.65 reyk 1072: .It Cm LocalCommand
1073: Specifies a command to execute on the local machine after successfully
1074: connecting to the server.
1075: The command string extends to the end of the line, and is executed with
1.105 jmc 1076: the user's shell.
1.109 dtucker 1077: The following escape character substitutions will be performed:
1078: .Ql %d
1079: (local user's home directory),
1080: .Ql %h
1081: (remote host name),
1082: .Ql %l
1083: (local host name),
1084: .Ql %n
1085: (host name as provided on the command line),
1086: .Ql %p
1087: (remote port),
1088: .Ql %r
1089: (remote user name) or
1090: .Ql %u
1.188 djm 1091: (local user name) or
1.189 jmc 1092: .Ql \&%C
1.188 djm 1093: by a hash of the concatenation: %l%h%p%r.
1.123 djm 1094: .Pp
1095: The command is run synchronously and does not have access to the
1096: session of the
1097: .Xr ssh 1
1098: that spawned it.
1099: It should not be used for interactive commands.
1100: .Pp
1.65 reyk 1101: This directive is ignored unless
1102: .Cm PermitLocalCommand
1103: has been enabled.
1.1 stevesk 1104: .It Cm LocalForward
1.74 jmc 1105: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 1106: the secure channel to the specified host and port from the remote machine.
1.49 jmc 1107: The first argument must be
1.43 djm 1108: .Sm off
1.49 jmc 1109: .Oo Ar bind_address : Oc Ar port
1.43 djm 1110: .Sm on
1.49 jmc 1111: and the second argument must be
1112: .Ar host : Ns Ar hostport .
1.138 djm 1113: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 1114: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 1115: given on the command line.
1.1 stevesk 1116: Only the superuser can forward privileged ports.
1.43 djm 1117: By default, the local port is bound in accordance with the
1118: .Cm GatewayPorts
1119: setting.
1120: However, an explicit
1121: .Ar bind_address
1122: may be used to bind the connection to a specific address.
1123: The
1124: .Ar bind_address
1125: of
1126: .Dq localhost
1.46 jmc 1127: indicates that the listening port be bound for local use only, while an
1128: empty address or
1129: .Sq *
1.43 djm 1130: indicates that the port should be available from all interfaces.
1.1 stevesk 1131: .It Cm LogLevel
1132: Gives the verbosity level that is used when logging messages from
1.84 jmc 1133: .Xr ssh 1 .
1.1 stevesk 1134: The possible values are:
1.84 jmc 1135: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 1136: The default is INFO.
1137: DEBUG and DEBUG1 are equivalent.
1138: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 1139: .It Cm MACs
1140: Specifies the MAC (message authentication code) algorithms
1141: in order of preference.
1142: The MAC algorithm is used in protocol version 2
1143: for data integrity protection.
1144: Multiple algorithms must be comma-separated.
1.214 djm 1145: If the specified value begins with a
1146: .Sq +
1147: character, then the specified algorithms will be appended to the default set
1148: instead of replacing them.
1149: .Pp
1.160 markus 1150: The algorithms that contain
1151: .Dq -etm
1152: calculate the MAC after encryption (encrypt-then-mac).
1153: These are considered safer and their use recommended.
1.214 djm 1154: .Pp
1.84 jmc 1155: The default is:
1.101 jmc 1156: .Bd -literal -offset indent
1.160 markus 1157: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1158: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.186 naddy 1159: umac-64@openssh.com,umac-128@openssh.com,
1160: hmac-sha2-256,hmac-sha2-512,
1161: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1162: hmac-ripemd160-etm@openssh.com,
1163: hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1164: hmac-md5,hmac-sha1,hmac-ripemd160,
1.157 naddy 1165: hmac-sha1-96,hmac-md5-96
1.101 jmc 1166: .Ed
1.198 djm 1167: .Pp
1168: The list of available MAC algorithms may also be obtained using the
1169: .Fl Q
1170: option of
1171: .Xr ssh 1
1172: with an argument of
1173: .Dq mac .
1.1 stevesk 1174: .It Cm NoHostAuthenticationForLocalhost
1175: This option can be used if the home directory is shared across machines.
1176: In this case localhost will refer to a different machine on each of
1177: the machines and the user will get many warnings about changed host keys.
1178: However, this option disables host authentication for localhost.
1179: The argument to this keyword must be
1180: .Dq yes
1181: or
1182: .Dq no .
1183: The default is to check the host key for localhost.
1184: .It Cm NumberOfPasswordPrompts
1185: Specifies the number of password prompts before giving up.
1186: The argument to this keyword must be an integer.
1.84 jmc 1187: The default is 3.
1.1 stevesk 1188: .It Cm PasswordAuthentication
1189: Specifies whether to use password authentication.
1190: The argument to this keyword must be
1191: .Dq yes
1192: or
1193: .Dq no .
1194: The default is
1195: .Dq yes .
1.65 reyk 1196: .It Cm PermitLocalCommand
1197: Allow local command execution via the
1198: .Ic LocalCommand
1199: option or using the
1.66 jmc 1200: .Ic !\& Ns Ar command
1.65 reyk 1201: escape sequence in
1202: .Xr ssh 1 .
1203: The argument must be
1204: .Dq yes
1205: or
1206: .Dq no .
1207: The default is
1208: .Dq no .
1.127 markus 1209: .It Cm PKCS11Provider
1210: Specifies which PKCS#11 provider to use.
1.144 jmc 1211: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1212: .Xr ssh 1
1.128 markus 1213: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1214: private RSA key.
1.67 jmc 1215: .It Cm Port
1216: Specifies the port number to connect on the remote host.
1.84 jmc 1217: The default is 22.
1.1 stevesk 1218: .It Cm PreferredAuthentications
1219: Specifies the order in which the client should try protocol 2
1.11 jmc 1220: authentication methods.
1.48 jmc 1221: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1222: .Cm keyboard-interactive )
1.48 jmc 1223: over another method (e.g.\&
1.131 jmc 1224: .Cm password ) .
1225: The default is:
1226: .Bd -literal -offset indent
1227: gssapi-with-mic,hostbased,publickey,
1228: keyboard-interactive,password
1229: .Ed
1.1 stevesk 1230: .It Cm Protocol
1231: Specifies the protocol versions
1.84 jmc 1232: .Xr ssh 1
1.1 stevesk 1233: should support in order of preference.
1234: The possible values are
1.84 jmc 1235: .Sq 1
1.1 stevesk 1236: and
1.84 jmc 1237: .Sq 2 .
1.1 stevesk 1238: Multiple versions must be comma-separated.
1.120 markus 1239: When this option is set to
1.121 jmc 1240: .Dq 2,1
1.120 markus 1241: .Nm ssh
1242: will try version 2 and fall back to version 1
1243: if version 2 is not available.
1.1 stevesk 1244: The default is
1.121 jmc 1245: .Sq 2 .
1.1 stevesk 1246: .It Cm ProxyCommand
1247: Specifies the command to use to connect to the server.
1248: The command
1.190 djm 1249: string extends to the end of the line, and is executed
1250: using the user's shell
1251: .Ql exec
1252: directive to avoid a lingering shell process.
1253: .Pp
1.133 jmc 1254: In the command string, any occurrence of
1.1 stevesk 1255: .Ql %h
1256: will be substituted by the host name to
1.132 djm 1257: connect,
1.1 stevesk 1258: .Ql %p
1.133 jmc 1259: by the port, and
1260: .Ql %r
1.132 djm 1261: by the remote user name.
1.1 stevesk 1262: The command can be basically anything,
1263: and should read from its standard input and write to its standard output.
1264: It should eventually connect an
1265: .Xr sshd 8
1266: server running on some machine, or execute
1267: .Ic sshd -i
1268: somewhere.
1269: Host key management will be done using the
1270: HostName of the host being connected (defaulting to the name typed by
1271: the user).
1.7 jmc 1272: Setting the command to
1273: .Dq none
1.6 markus 1274: disables this option entirely.
1.1 stevesk 1275: Note that
1276: .Cm CheckHostIP
1277: is not available for connects with a proxy command.
1.52 djm 1278: .Pp
1279: This directive is useful in conjunction with
1280: .Xr nc 1
1281: and its proxy support.
1.53 jmc 1282: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1283: 192.0.2.0:
1284: .Bd -literal -offset 3n
1285: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1286: .Ed
1.167 djm 1287: .It Cm ProxyUseFdpass
1.168 jmc 1288: Specifies that
1.167 djm 1289: .Cm ProxyCommand
1290: will pass a connected file descriptor back to
1.168 jmc 1291: .Xr ssh 1
1.167 djm 1292: instead of continuing to execute and pass data.
1293: The default is
1294: .Dq no .
1.213 markus 1295: .It Cm PubkeyAcceptedKeyTypes
1296: Specifies the key types that will be used for public key authentication
1297: as a comma-separated pattern list.
1.214 djm 1298: Alternately if the specified value begins with a
1299: .Sq +
1300: character, then the key types after it will be appended to the default
1301: instead of replacing it.
1.213 markus 1302: The default for this option is:
1303: .Bd -literal -offset 3n
1304: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1305: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1306: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1307: ssh-ed25519-cert-v01@openssh.com,
1308: ssh-rsa-cert-v01@openssh.com,
1309: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1310: ssh-ed25519,ssh-rsa
1311: .Ed
1312: .Pp
1313: The
1314: .Fl Q
1315: option of
1316: .Xr ssh 1
1317: may be used to list supported key types.
1.1 stevesk 1318: .It Cm PubkeyAuthentication
1319: Specifies whether to try public key authentication.
1320: The argument to this keyword must be
1321: .Dq yes
1322: or
1323: .Dq no .
1324: The default is
1325: .Dq yes .
1326: This option applies to protocol version 2 only.
1.75 dtucker 1327: .It Cm RekeyLimit
1328: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1329: session key is renegotiated, optionally followed a maximum amount of
1330: time that may pass before the session key is renegotiated.
1331: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1332: .Sq K ,
1333: .Sq M ,
1.75 dtucker 1334: or
1.76 jmc 1335: .Sq G
1.75 dtucker 1336: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1337: The default is between
1.84 jmc 1338: .Sq 1G
1.75 dtucker 1339: and
1.84 jmc 1340: .Sq 4G ,
1.75 dtucker 1341: depending on the cipher.
1.162 dtucker 1342: The optional second value is specified in seconds and may use any of the
1343: units documented in the
1.166 jmc 1344: TIME FORMATS section of
1.162 dtucker 1345: .Xr sshd_config 5 .
1346: The default value for
1347: .Cm RekeyLimit
1348: is
1349: .Dq default none ,
1350: which means that rekeying is performed after the cipher's default amount
1351: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1352: This option applies to protocol version 2 only.
1.1 stevesk 1353: .It Cm RemoteForward
1.74 jmc 1354: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1355: the secure channel to the specified host and port from the local machine.
1.49 jmc 1356: The first argument must be
1.43 djm 1357: .Sm off
1.49 jmc 1358: .Oo Ar bind_address : Oc Ar port
1.43 djm 1359: .Sm on
1.49 jmc 1360: and the second argument must be
1361: .Ar host : Ns Ar hostport .
1.138 djm 1362: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1363: Multiple forwardings may be specified, and additional
1364: forwardings can be given on the command line.
1.113 stevesk 1365: Privileged ports can be forwarded only when
1366: logging in as root on the remote machine.
1.118 jmc 1367: .Pp
1.117 djm 1368: If the
1369: .Ar port
1370: argument is
1371: .Ql 0 ,
1372: the listen port will be dynamically allocated on the server and reported
1373: to the client at run time.
1.43 djm 1374: .Pp
1375: If the
1376: .Ar bind_address
1377: is not specified, the default is to only bind to loopback addresses.
1378: If the
1379: .Ar bind_address
1380: is
1381: .Ql *
1382: or an empty string, then the forwarding is requested to listen on all
1383: interfaces.
1384: Specifying a remote
1385: .Ar bind_address
1.46 jmc 1386: will only succeed if the server's
1387: .Cm GatewayPorts
1.43 djm 1388: option is enabled (see
1.46 jmc 1389: .Xr sshd_config 5 ) .
1.149 djm 1390: .It Cm RequestTTY
1391: Specifies whether to request a pseudo-tty for the session.
1392: The argument may be one of:
1393: .Dq no
1394: (never request a TTY),
1395: .Dq yes
1396: (always request a TTY when standard input is a TTY),
1397: .Dq force
1398: (always request a TTY) or
1399: .Dq auto
1400: (request a TTY when opening a login session).
1401: This option mirrors the
1402: .Fl t
1403: and
1404: .Fl T
1405: flags for
1406: .Xr ssh 1 .
1.196 djm 1407: .It Cm RevokedHostKeys
1408: Specifies revoked host public keys.
1409: Keys listed in this file will be refused for host authentication.
1410: Note that if this file does not exist or is not readable,
1411: then host authentication will be refused for all hosts.
1412: Keys may be specified as a text file, listing one public key per line, or as
1413: an OpenSSH Key Revocation List (KRL) as generated by
1414: .Xr ssh-keygen 1 .
1415: For more information on KRLs, see the KEY REVOCATION LISTS section in
1416: .Xr ssh-keygen 1 .
1.1 stevesk 1417: .It Cm RhostsRSAAuthentication
1418: Specifies whether to try rhosts based authentication with RSA host
1419: authentication.
1420: The argument must be
1421: .Dq yes
1422: or
1423: .Dq no .
1424: The default is
1425: .Dq no .
1426: This option applies to protocol version 1 only and requires
1.84 jmc 1427: .Xr ssh 1
1.1 stevesk 1428: to be setuid root.
1429: .It Cm RSAAuthentication
1430: Specifies whether to try RSA authentication.
1431: The argument to this keyword must be
1432: .Dq yes
1433: or
1434: .Dq no .
1435: RSA authentication will only be
1436: attempted if the identity file exists, or an authentication agent is
1437: running.
1438: The default is
1439: .Dq yes .
1440: Note that this option applies to protocol version 1 only.
1.32 djm 1441: .It Cm SendEnv
1442: Specifies what variables from the local
1443: .Xr environ 7
1444: should be sent to the server.
1.84 jmc 1445: Note that environment passing is only supported for protocol 2.
1446: The server must also support it, and the server must be configured to
1.33 djm 1447: accept these environment variables.
1.207 dtucker 1448: Note that the
1449: .Ev TERM
1.208 jmc 1450: environment variable is always sent whenever a
1.207 dtucker 1451: pseudo-terminal is requested as it is required by the protocol.
1.32 djm 1452: Refer to
1453: .Cm AcceptEnv
1454: in
1455: .Xr sshd_config 5
1456: for how to configure the server.
1.80 jmc 1457: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1458: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1459: across multiple
1460: .Cm SendEnv
1461: directives.
1462: The default is not to send any environment variables.
1.81 jmc 1463: .Pp
1464: See
1465: .Sx PATTERNS
1466: for more information on patterns.
1.28 markus 1467: .It Cm ServerAliveCountMax
1.73 jmc 1468: Sets the number of server alive messages (see below) which may be
1.28 markus 1469: sent without
1.84 jmc 1470: .Xr ssh 1
1.28 markus 1471: receiving any messages back from the server.
1472: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1473: ssh will disconnect from the server, terminating the session.
1.28 markus 1474: It is important to note that the use of server alive messages is very
1475: different from
1476: .Cm TCPKeepAlive
1477: (below).
1478: The server alive messages are sent through the encrypted channel
1479: and therefore will not be spoofable.
1480: The TCP keepalive option enabled by
1481: .Cm TCPKeepAlive
1482: is spoofable.
1483: The server alive mechanism is valuable when the client or
1484: server depend on knowing when a connection has become inactive.
1485: .Pp
1486: The default value is 3.
1487: If, for example,
1488: .Cm ServerAliveInterval
1.84 jmc 1489: (see below) is set to 15 and
1.28 markus 1490: .Cm ServerAliveCountMax
1.84 jmc 1491: is left at the default, if the server becomes unresponsive,
1492: ssh will disconnect after approximately 45 seconds.
1.89 markus 1493: This option applies to protocol version 2 only.
1.67 jmc 1494: .It Cm ServerAliveInterval
1495: Sets a timeout interval in seconds after which if no data has been received
1496: from the server,
1.84 jmc 1497: .Xr ssh 1
1.67 jmc 1498: will send a message through the encrypted
1499: channel to request a response from the server.
1500: The default
1501: is 0, indicating that these messages will not be sent to the server.
1502: This option applies to protocol version 2 only.
1.191 millert 1503: .It Cm StreamLocalBindMask
1504: Sets the octal file creation mode mask
1505: .Pq umask
1506: used when creating a Unix-domain socket file for local or remote
1507: port forwarding.
1508: This option is only used for port forwarding to a Unix-domain socket file.
1509: .Pp
1510: The default value is 0177, which creates a Unix-domain socket file that is
1511: readable and writable only by the owner.
1512: Note that not all operating systems honor the file mode on Unix-domain
1513: socket files.
1514: .It Cm StreamLocalBindUnlink
1515: Specifies whether to remove an existing Unix-domain socket file for local
1516: or remote port forwarding before creating a new one.
1517: If the socket file already exists and
1518: .Cm StreamLocalBindUnlink
1519: is not enabled,
1520: .Nm ssh
1521: will be unable to forward the port to the Unix-domain socket file.
1522: This option is only used for port forwarding to a Unix-domain socket file.
1523: .Pp
1524: The argument must be
1525: .Dq yes
1526: or
1527: .Dq no .
1528: The default is
1529: .Dq no .
1.1 stevesk 1530: .It Cm StrictHostKeyChecking
1531: If this flag is set to
1532: .Dq yes ,
1.84 jmc 1533: .Xr ssh 1
1.1 stevesk 1534: will never automatically add host keys to the
1.50 djm 1535: .Pa ~/.ssh/known_hosts
1.1 stevesk 1536: file, and refuses to connect to hosts whose host key has changed.
1537: This provides maximum protection against trojan horse attacks,
1.84 jmc 1538: though it can be annoying when the
1.1 stevesk 1539: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1540: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1541: frequently made.
1542: This option forces the user to manually
1543: add all new hosts.
1544: If this flag is set to
1545: .Dq no ,
1.84 jmc 1546: ssh will automatically add new host keys to the
1.1 stevesk 1547: user known hosts files.
1548: If this flag is set to
1549: .Dq ask ,
1550: new host keys
1551: will be added to the user known host files only after the user
1552: has confirmed that is what they really want to do, and
1.84 jmc 1553: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1554: The host keys of
1555: known hosts will be verified automatically in all cases.
1556: The argument must be
1557: .Dq yes ,
1.84 jmc 1558: .Dq no ,
1.1 stevesk 1559: or
1560: .Dq ask .
1561: The default is
1562: .Dq ask .
1.26 markus 1563: .It Cm TCPKeepAlive
1564: Specifies whether the system should send TCP keepalive messages to the
1565: other side.
1566: If they are sent, death of the connection or crash of one
1567: of the machines will be properly noticed.
1568: However, this means that
1569: connections will die if the route is down temporarily, and some people
1570: find it annoying.
1571: .Pp
1572: The default is
1573: .Dq yes
1574: (to send TCP keepalive messages), and the client will notice
1575: if the network goes down or the remote host dies.
1576: This is important in scripts, and many users want it too.
1577: .Pp
1578: To disable TCP keepalive messages, the value should be set to
1579: .Dq no .
1.65 reyk 1580: .It Cm Tunnel
1.95 stevesk 1581: Request
1.65 reyk 1582: .Xr tun 4
1.69 jmc 1583: device forwarding between the client and the server.
1.65 reyk 1584: The argument must be
1.68 reyk 1585: .Dq yes ,
1.95 stevesk 1586: .Dq point-to-point
1587: (layer 3),
1588: .Dq ethernet
1589: (layer 2),
1.65 reyk 1590: or
1591: .Dq no .
1.95 stevesk 1592: Specifying
1593: .Dq yes
1594: requests the default tunnel mode, which is
1595: .Dq point-to-point .
1.65 reyk 1596: The default is
1597: .Dq no .
1598: .It Cm TunnelDevice
1.95 stevesk 1599: Specifies the
1.65 reyk 1600: .Xr tun 4
1.95 stevesk 1601: devices to open on the client
1602: .Pq Ar local_tun
1603: and the server
1604: .Pq Ar remote_tun .
1605: .Pp
1606: The argument must be
1607: .Sm off
1608: .Ar local_tun Op : Ar remote_tun .
1609: .Sm on
1610: The devices may be specified by numerical ID or the keyword
1611: .Dq any ,
1612: which uses the next available tunnel device.
1613: If
1614: .Ar remote_tun
1615: is not specified, it defaults to
1616: .Dq any .
1617: The default is
1618: .Dq any:any .
1.201 djm 1619: .It Cm UpdateHostKeys
1.200 djm 1620: Specifies whether
1621: .Xr ssh 1
1622: should accept notifications of additional hostkeys from the server sent
1623: after authentication has completed and add them to
1624: .Cm UserKnownHostsFile .
1625: The argument must be
1.204 djm 1626: .Dq yes ,
1.203 djm 1627: .Dq no
1.204 djm 1628: (the default) or
1629: .Dq ask .
1.200 djm 1630: Enabling this option allows learning alternate hostkeys for a server
1.201 djm 1631: and supports graceful key rotation by allowing a server to send replacement
1632: public keys before old ones are removed.
1.200 djm 1633: Additional hostkeys are only accepted if the key used to authenticate the
1.220 sobrado 1634: host was already trusted or explicitly accepted by the user.
1.204 djm 1635: If
1636: .Cm UpdateHostKeys
1637: is set to
1638: .Dq ask ,
1639: then the user is asked to confirm the modifications to the known_hosts file.
1.205 djm 1640: Confirmation is currently incompatible with
1641: .Cm ControlPersist ,
1642: and will be disabled if it is enabled.
1.200 djm 1643: .Pp
1644: Presently, only
1645: .Xr sshd 8
1646: from OpenSSH 6.8 and greater support the
1647: .Dq hostkeys@openssh.com
1648: protocol extension used to inform the client of all the server's hostkeys.
1.72 jmc 1649: .It Cm UsePrivilegedPort
1650: Specifies whether to use a privileged port for outgoing connections.
1651: The argument must be
1652: .Dq yes
1653: or
1654: .Dq no .
1655: The default is
1656: .Dq no .
1657: If set to
1.84 jmc 1658: .Dq yes ,
1659: .Xr ssh 1
1.72 jmc 1660: must be setuid root.
1661: Note that this option must be set to
1662: .Dq yes
1663: for
1664: .Cm RhostsRSAAuthentication
1665: with older servers.
1.1 stevesk 1666: .It Cm User
1667: Specifies the user to log in as.
1668: This can be useful when a different user name is used on different machines.
1669: This saves the trouble of
1670: having to remember to give the user name on the command line.
1671: .It Cm UserKnownHostsFile
1.151 djm 1672: Specifies one or more files to use for the user
1673: host key database, separated by whitespace.
1674: The default is
1675: .Pa ~/.ssh/known_hosts ,
1676: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1677: .It Cm VerifyHostKeyDNS
1678: Specifies whether to verify the remote key using DNS and SSHFP resource
1679: records.
1.24 jakob 1680: If this option is set to
1681: .Dq yes ,
1.25 jmc 1682: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1683: from DNS.
1684: Insecure fingerprints will be handled as if this option was set to
1685: .Dq ask .
1686: If this option is set to
1687: .Dq ask ,
1688: information on fingerprint match will be displayed, but the user will still
1689: need to confirm new host keys according to the
1690: .Cm StrictHostKeyChecking
1691: option.
1692: The argument must be
1693: .Dq yes ,
1.84 jmc 1694: .Dq no ,
1.25 jmc 1695: or
1696: .Dq ask .
1.8 jakob 1697: The default is
1698: .Dq no .
1.12 jakob 1699: Note that this option applies to protocol version 2 only.
1.84 jmc 1700: .Pp
1.166 jmc 1701: See also VERIFYING HOST KEYS in
1.84 jmc 1702: .Xr ssh 1 .
1.111 grunk 1703: .It Cm VisualHostKey
1704: If this flag is set to
1705: .Dq yes ,
1706: an ASCII art representation of the remote host key fingerprint is
1.197 djm 1707: printed in addition to the fingerprint string at login and
1.114 stevesk 1708: for unknown host keys.
1.111 grunk 1709: If this flag is set to
1710: .Dq no ,
1.114 stevesk 1711: no fingerprint strings are printed at login and
1.197 djm 1712: only the fingerprint string will be printed for unknown host keys.
1.111 grunk 1713: The default is
1714: .Dq no .
1.1 stevesk 1715: .It Cm XAuthLocation
1.5 stevesk 1716: Specifies the full pathname of the
1.1 stevesk 1717: .Xr xauth 1
1718: program.
1719: The default is
1720: .Pa /usr/X11R6/bin/xauth .
1721: .El
1.86 jmc 1722: .Sh PATTERNS
1723: A
1724: .Em pattern
1725: consists of zero or more non-whitespace characters,
1726: .Sq *
1727: (a wildcard that matches zero or more characters),
1728: or
1729: .Sq ?\&
1730: (a wildcard that matches exactly one character).
1731: For example, to specify a set of declarations for any host in the
1732: .Dq .co.uk
1733: set of domains,
1734: the following pattern could be used:
1735: .Pp
1736: .Dl Host *.co.uk
1737: .Pp
1738: The following pattern
1739: would match any host in the 192.168.0.[0-9] network range:
1740: .Pp
1741: .Dl Host 192.168.0.?
1742: .Pp
1743: A
1744: .Em pattern-list
1745: is a comma-separated list of patterns.
1746: Patterns within pattern-lists may be negated
1747: by preceding them with an exclamation mark
1748: .Pq Sq !\& .
1749: For example,
1.174 djm 1750: to allow a key to be used from anywhere within an organization
1.86 jmc 1751: except from the
1752: .Dq dialup
1753: pool,
1754: the following entry (in authorized_keys) could be used:
1755: .Pp
1756: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1757: .Sh FILES
1758: .Bl -tag -width Ds
1.50 djm 1759: .It Pa ~/.ssh/config
1.1 stevesk 1760: This is the per-user configuration file.
1761: The format of this file is described above.
1.84 jmc 1762: This file is used by the SSH client.
1.30 djm 1763: Because of the potential for abuse, this file must have strict permissions:
1764: read/write for the user, and not accessible by others.
1.1 stevesk 1765: .It Pa /etc/ssh/ssh_config
1766: Systemwide configuration file.
1767: This file provides defaults for those
1768: values that are not specified in the user's configuration file, and
1769: for those users who do not have a configuration file.
1770: This file must be world-readable.
1771: .El
1.13 jmc 1772: .Sh SEE ALSO
1773: .Xr ssh 1
1.1 stevesk 1774: .Sh AUTHORS
1775: OpenSSH is a derivative of the original and free
1776: ssh 1.2.12 release by Tatu Ylonen.
1777: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1778: Theo de Raadt and Dug Song
1779: removed many bugs, re-added newer features and
1780: created OpenSSH.
1781: Markus Friedl contributed the support for SSH
1782: protocol versions 1.5 and 2.0.
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