Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.181
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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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30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.181 ! jmc 36: .\" $OpenBSD: ssh_config.5,v 1.180 2013/11/21 00:45:44 djm Exp $
! 37: .Dd $Mdocdate: November 21 2013 $
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.
68: The matched host name is the one given on the command line.
69: .Pp
70: Since the first obtained value for each parameter is used, more
71: host-specific declarations should be given near the beginning of the
72: file, and general defaults at the end.
1.80 jmc 73: .Pp
1.1 stevesk 74: The configuration file has the following format:
75: .Pp
76: Empty lines and lines starting with
77: .Ql #
78: are comments.
79: Otherwise a line is of the format
80: .Dq keyword arguments .
81: Configuration options may be separated by whitespace or
82: optional whitespace and exactly one
83: .Ql = ;
84: the latter format is useful to avoid the need to quote whitespace
85: when specifying configuration options using the
86: .Nm ssh ,
1.87 jmc 87: .Nm scp ,
1.1 stevesk 88: and
89: .Nm sftp
90: .Fl o
91: option.
1.88 dtucker 92: Arguments may optionally be enclosed in double quotes
93: .Pq \&"
94: in order to represent arguments containing spaces.
1.1 stevesk 95: .Pp
96: The possible
97: keywords and their meanings are as follows (note that
98: keywords are case-insensitive and arguments are case-sensitive):
99: .Bl -tag -width Ds
100: .It Cm Host
101: Restricts the following declarations (up to the next
102: .Cm Host
1.169 djm 103: or
104: .Cm Match
1.1 stevesk 105: keyword) to be only for those hosts that match one of the patterns
106: given after the keyword.
1.112 krw 107: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 108: A single
1.83 jmc 109: .Ql *
1.1 stevesk 110: as a pattern can be used to provide global
111: defaults for all hosts.
112: The host is the
113: .Ar hostname
1.83 jmc 114: argument given on the command line (i.e. the name is not converted to
1.1 stevesk 115: a canonicalized host name before matching).
1.148 djm 116: .Pp
117: A pattern entry may be negated by prefixing it with an exclamation mark
118: .Pq Sq !\& .
119: If a negated entry is matched, then the
120: .Cm Host
121: entry is ignored, regardless of whether any other patterns on the line
122: match.
123: Negated matches are therefore useful to provide exceptions for wildcard
124: matches.
1.81 jmc 125: .Pp
126: See
127: .Sx PATTERNS
128: for more information on patterns.
1.170 jmc 129: .It Cm Match
1.169 djm 130: Restricts the following declarations (up to the next
131: .Cm Host
132: or
133: .Cm Match
134: keyword) to be used only when the conditions following the
135: .Cm Match
136: keyword are satisfied.
1.178 dtucker 137: Match conditions are specified using one or more keyword/criteria pairs
138: or the single token
139: .Cm all
140: which matches all criteria.
1.169 djm 141: The available keywords are:
1.176 djm 142: .Cm exec ,
1.169 djm 143: .Cm host ,
144: .Cm originalhost ,
145: .Cm user ,
146: and
147: .Cm localuser .
148: .Pp
1.177 jmc 149: The
1.176 djm 150: .Cm exec
1.177 jmc 151: keyword executes the specified command under the user's shell.
1.169 djm 152: If the command returns a zero exit status then the condition is considered true.
153: Commands containing whitespace characters must be quoted.
1.175 djm 154: The following character sequences in the command will be expanded prior to
155: execution:
156: .Ql %L
157: will be substituted by the first component of the local host name,
158: .Ql %l
159: will be substituted by the local host name (including any domain name),
160: .Ql %h
161: will be substituted by the target host name,
162: .Ql %n
163: will be substituted by the original target host name
1.176 djm 164: specified on the command-line,
1.175 djm 165: .Ql %p
166: the destination port,
167: .Ql %r
168: by the remote login username, and
169: .Ql %u
170: by the username of the user running
171: .Xr ssh 1 .
1.169 djm 172: .Pp
173: The other keywords' criteria must be single entries or comma-separated
174: lists and may use the wildcard and negation operators described in the
175: .Sx PATTERNS
176: section.
177: The criteria for the
178: .Cm host
179: keyword are matched against the target hostname, after any substitution
180: by the
181: .Cm Hostname
182: option.
183: The
184: .Cm originalhost
185: keyword matches against the hostname as it was specified on the command-line.
186: The
187: .Cm user
188: keyword matches against the target username on the remote host.
189: The
190: .Cm localuser
191: keyword matches against the name of the local user running
192: .Xr ssh 1
193: (this keyword may be useful in system-wide
194: .Nm
195: files).
1.10 djm 196: .It Cm AddressFamily
1.11 jmc 197: Specifies which address family to use when connecting.
198: Valid arguments are
1.10 djm 199: .Dq any ,
200: .Dq inet
1.84 jmc 201: (use IPv4 only), or
1.10 djm 202: .Dq inet6
1.40 jmc 203: (use IPv6 only).
1.1 stevesk 204: .It Cm BatchMode
205: If set to
206: .Dq yes ,
207: passphrase/password querying will be disabled.
208: This option is useful in scripts and other batch jobs where no user
209: is present to supply the password.
210: The argument must be
211: .Dq yes
212: or
213: .Dq no .
214: The default is
215: .Dq no .
216: .It Cm BindAddress
1.60 dtucker 217: Use the specified address on the local machine as the source address of
1.61 jmc 218: the connection.
219: Only useful on systems with more than one address.
1.1 stevesk 220: Note that this option does not work if
221: .Cm UsePrivilegedPort
222: is set to
223: .Dq yes .
1.171 djm 224: .It Cm CanonicalDomains
1.172 jmc 225: When
1.173 djm 226: .Cm CanonicalizeHostname
1.171 djm 227: is enabled, this option specifies the list of domain suffixes in which to
228: search for the specified destination host.
1.173 djm 229: .It Cm CanonicalizeFallbackLocal
1.174 djm 230: Specifies whether to fail with an error when hostname canonicalization fails.
1.172 jmc 231: The default,
232: .Dq no ,
233: will attempt to look up the unqualified hostname using the system resolver's
1.171 djm 234: search rules.
235: A value of
236: .Dq yes
237: will cause
238: .Xr ssh 1
239: to fail instantly if
1.173 djm 240: .Cm CanonicalizeHostname
1.171 djm 241: is enabled and the target hostname cannot be found in any of the domains
242: specified by
243: .Cm CanonicalDomains .
1.173 djm 244: .It Cm CanonicalizeHostname
1.174 djm 245: Controls whether explicit hostname canonicalization is performed.
1.172 jmc 246: The default,
247: .Dq no ,
1.171 djm 248: is not to perform any name rewriting and let the system resolver handle all
249: hostname lookups.
250: If set to
251: .Dq yes
252: then, for connections that do not use a
253: .Cm ProxyCommand ,
254: .Xr ssh 1
1.173 djm 255: will attempt to canonicalize the hostname specified on the command line
1.171 djm 256: using the
257: .Cm CanonicalDomains
258: suffixes and
1.173 djm 259: .Cm CanonicalizePermittedCNAMEs
1.171 djm 260: rules.
261: If
1.173 djm 262: .Cm CanonicalizeHostname
1.171 djm 263: is set to
264: .Dq always ,
1.174 djm 265: then canonicalization is applied to proxied connections too.
1.173 djm 266: .It Cm CanonicalizeMaxDots
1.172 jmc 267: Specifies the maximum number of dot characters in a hostname before
1.174 djm 268: canonicalization is disabled.
1.172 jmc 269: The default,
270: .Dq 1 ,
271: allows a single dot (i.e. hostname.subdomain).
1.173 djm 272: .It Cm CanonicalizePermittedCNAMEs
1.172 jmc 273: Specifies rules to determine whether CNAMEs should be followed when
1.173 djm 274: canonicalizing hostnames.
1.171 djm 275: The rules consist of one or more arguments of
1.172 jmc 276: .Ar source_domain_list : Ns Ar target_domain_list ,
1.171 djm 277: where
278: .Ar source_domain_list
1.174 djm 279: is a pattern-list of domains that may follow CNAMEs in canonicalization,
1.171 djm 280: and
281: .Ar target_domain_list
1.172 jmc 282: is a pattern-list of domains that they may resolve to.
1.171 djm 283: .Pp
284: For example,
285: .Dq *.a.example.com:*.b.example.com,*.c.example.com
286: will allow hostnames matching
287: .Dq *.a.example.com
1.173 djm 288: to be canonicalized to names in the
1.171 djm 289: .Dq *.b.example.com
290: or
291: .Dq *.c.example.com
292: domains.
1.1 stevesk 293: .It Cm ChallengeResponseAuthentication
1.82 jmc 294: Specifies whether to use challenge-response authentication.
1.1 stevesk 295: The argument to this keyword must be
296: .Dq yes
297: or
298: .Dq no .
299: The default is
300: .Dq yes .
301: .It Cm CheckHostIP
302: If this flag is set to
303: .Dq yes ,
1.84 jmc 304: .Xr ssh 1
305: will additionally check the host IP address in the
1.1 stevesk 306: .Pa known_hosts
307: file.
308: This allows ssh to detect if a host key changed due to DNS spoofing.
1.107 grunk 309: If the option is set to
1.1 stevesk 310: .Dq no ,
311: the check will not be executed.
312: The default is
313: .Dq yes .
314: .It Cm Cipher
315: Specifies the cipher to use for encrypting the session
316: in protocol version 1.
317: Currently,
318: .Dq blowfish ,
319: .Dq 3des ,
320: and
321: .Dq des
322: are supported.
323: .Ar des
324: is only supported in the
1.84 jmc 325: .Xr ssh 1
1.1 stevesk 326: client for interoperability with legacy protocol 1 implementations
327: that do not support the
328: .Ar 3des
1.7 jmc 329: cipher.
330: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 331: The default is
332: .Dq 3des .
333: .It Cm Ciphers
334: Specifies the ciphers allowed for protocol version 2
335: in order of preference.
336: Multiple ciphers must be comma-separated.
1.180 djm 337: The supported ciphers are:
338: .Pp
1.35 dtucker 339: .Dq 3des-cbc ,
340: .Dq aes128-cbc ,
341: .Dq aes192-cbc ,
342: .Dq aes256-cbc ,
343: .Dq aes128-ctr ,
344: .Dq aes192-ctr ,
345: .Dq aes256-ctr ,
1.161 markus 346: .Dq aes128-gcm@openssh.com ,
347: .Dq aes256-gcm@openssh.com ,
1.54 djm 348: .Dq arcfour128 ,
349: .Dq arcfour256 ,
1.35 dtucker 350: .Dq arcfour ,
351: .Dq blowfish-cbc ,
1.180 djm 352: .Dq cast128-cbc ,
1.35 dtucker 353: and
1.180 djm 354: .Dq chacha20-poly1305@openssh.com .
355: .Pp
1.84 jmc 356: The default is:
357: .Bd -literal -offset 3n
1.116 naddy 358: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
1.161 markus 359: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.180 djm 360: chacha20-poly1305@openssh.com,
1.116 naddy 361: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
362: aes256-cbc,arcfour
1.1 stevesk 363: .Ed
1.180 djm 364: .Pp
365: The list of available ciphers may also be obtained using the
366: .Fl Q
367: option of
368: .Xr ssh 1 .
1.1 stevesk 369: .It Cm ClearAllForwardings
1.84 jmc 370: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 371: specified in the configuration files or on the command line be
1.7 jmc 372: cleared.
373: This option is primarily useful when used from the
1.84 jmc 374: .Xr ssh 1
1.1 stevesk 375: command line to clear port forwardings set in
376: configuration files, and is automatically set by
377: .Xr scp 1
378: and
379: .Xr sftp 1 .
380: The argument must be
381: .Dq yes
382: or
383: .Dq no .
384: The default is
385: .Dq no .
386: .It Cm Compression
387: Specifies whether to use compression.
388: The argument must be
389: .Dq yes
390: or
391: .Dq no .
392: The default is
393: .Dq no .
394: .It Cm CompressionLevel
395: Specifies the compression level to use if compression is enabled.
396: The argument must be an integer from 1 (fast) to 9 (slow, best).
397: The default level is 6, which is good for most applications.
398: The meaning of the values is the same as in
399: .Xr gzip 1 .
400: Note that this option applies to protocol version 1 only.
401: .It Cm ConnectionAttempts
402: Specifies the number of tries (one per second) to make before exiting.
403: The argument must be an integer.
404: This may be useful in scripts if the connection sometimes fails.
405: The default is 1.
1.9 djm 406: .It Cm ConnectTimeout
1.84 jmc 407: Specifies the timeout (in seconds) used when connecting to the
408: SSH server, instead of using the default system TCP timeout.
1.11 jmc 409: This value is used only when the target is down or really unreachable,
410: not when it refuses the connection.
1.36 djm 411: .It Cm ControlMaster
412: Enables the sharing of multiple sessions over a single network connection.
413: When set to
1.84 jmc 414: .Dq yes ,
415: .Xr ssh 1
1.36 djm 416: will listen for connections on a control socket specified using the
417: .Cm ControlPath
418: argument.
419: Additional sessions can connect to this socket using the same
420: .Cm ControlPath
421: with
422: .Cm ControlMaster
423: set to
424: .Dq no
1.38 jmc 425: (the default).
1.64 jmc 426: These sessions will try to reuse the master instance's network connection
1.63 djm 427: rather than initiating new ones, but will fall back to connecting normally
428: if the control socket does not exist, or is not listening.
429: .Pp
1.37 djm 430: Setting this to
431: .Dq ask
1.84 jmc 432: will cause ssh
1.37 djm 433: to listen for control connections, but require confirmation using the
434: .Ev SSH_ASKPASS
435: program before they are accepted (see
436: .Xr ssh-add 1
1.38 jmc 437: for details).
1.51 jakob 438: If the
439: .Cm ControlPath
1.84 jmc 440: cannot be opened,
441: ssh will continue without connecting to a master instance.
1.58 djm 442: .Pp
443: X11 and
1.59 jmc 444: .Xr ssh-agent 1
1.58 djm 445: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 446: display and agent forwarded will be the one belonging to the master
1.59 jmc 447: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 448: .Pp
449: Two additional options allow for opportunistic multiplexing: try to use a
450: master connection but fall back to creating a new one if one does not already
451: exist.
452: These options are:
453: .Dq auto
454: and
455: .Dq autoask .
456: The latter requires confirmation like the
457: .Dq ask
458: option.
1.36 djm 459: .It Cm ControlPath
1.55 djm 460: Specify the path to the control socket used for connection sharing as described
461: in the
1.36 djm 462: .Cm ControlMaster
1.57 djm 463: section above or the string
464: .Dq none
465: to disable connection sharing.
1.55 djm 466: In the path,
1.147 djm 467: .Ql %L
468: will be substituted by the first component of the local host name,
1.77 djm 469: .Ql %l
1.147 djm 470: will be substituted by the local host name (including any domain name),
1.55 djm 471: .Ql %h
472: will be substituted by the target host name,
1.150 jmc 473: .Ql %n
474: will be substituted by the original target host name
475: specified on the command line,
1.55 djm 476: .Ql %p
1.175 djm 477: the destination port,
1.55 djm 478: .Ql %r
1.147 djm 479: by the remote login username, and
480: .Ql %u
481: by the username of the user running
482: .Xr ssh 1 .
1.56 djm 483: It is recommended that any
484: .Cm ControlPath
485: used for opportunistic connection sharing include
1.78 jmc 486: at least %h, %p, and %r.
1.56 djm 487: This ensures that shared connections are uniquely identified.
1.137 djm 488: .It Cm ControlPersist
489: When used in conjunction with
490: .Cm ControlMaster ,
491: specifies that the master connection should remain open
492: in the background (waiting for future client connections)
493: after the initial client connection has been closed.
494: If set to
495: .Dq no ,
496: then the master connection will not be placed into the background,
497: and will close as soon as the initial client connection is closed.
498: If set to
499: .Dq yes ,
500: then the master connection will remain in the background indefinitely
501: (until killed or closed via a mechanism such as the
502: .Xr ssh 1
503: .Dq Fl O No exit
504: option).
505: If set to a time in seconds, or a time in any of the formats documented in
506: .Xr sshd_config 5 ,
507: then the backgrounded master connection will automatically terminate
508: after it has remained idle (with no client connections) for the
509: specified time.
1.38 jmc 510: .It Cm DynamicForward
1.74 jmc 511: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 512: over the secure channel, and the application
513: protocol is then used to determine where to connect to from the
514: remote machine.
1.62 djm 515: .Pp
516: The argument must be
517: .Sm off
518: .Oo Ar bind_address : Oc Ar port .
519: .Sm on
1.138 djm 520: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 521: By default, the local port is bound in accordance with the
522: .Cm GatewayPorts
523: setting.
524: However, an explicit
525: .Ar bind_address
526: may be used to bind the connection to a specific address.
527: The
528: .Ar bind_address
529: of
530: .Dq localhost
531: indicates that the listening port be bound for local use only, while an
532: empty address or
533: .Sq *
534: indicates that the port should be available from all interfaces.
535: .Pp
1.38 jmc 536: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 537: .Xr ssh 1
1.38 jmc 538: will act as a SOCKS server.
539: Multiple forwardings may be specified, and
540: additional forwardings can be given on the command line.
541: Only the superuser can forward privileged ports.
1.14 markus 542: .It Cm EnableSSHKeysign
543: Setting this option to
544: .Dq yes
545: in the global client configuration file
546: .Pa /etc/ssh/ssh_config
547: enables the use of the helper program
548: .Xr ssh-keysign 8
549: during
550: .Cm HostbasedAuthentication .
551: The argument must be
552: .Dq yes
553: or
554: .Dq no .
555: The default is
556: .Dq no .
1.23 jmc 557: This option should be placed in the non-hostspecific section.
1.14 markus 558: See
559: .Xr ssh-keysign 8
560: for more information.
1.1 stevesk 561: .It Cm EscapeChar
562: Sets the escape character (default:
563: .Ql ~ ) .
564: The escape character can also
565: be set on the command line.
566: The argument should be a single character,
567: .Ql ^
568: followed by a letter, or
569: .Dq none
570: to disable the escape
571: character entirely (making the connection transparent for binary
572: data).
1.96 markus 573: .It Cm ExitOnForwardFailure
574: Specifies whether
575: .Xr ssh 1
576: should terminate the connection if it cannot set up all requested
1.102 stevesk 577: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 578: The argument must be
579: .Dq yes
580: or
581: .Dq no .
582: The default is
583: .Dq no .
1.1 stevesk 584: .It Cm ForwardAgent
585: Specifies whether the connection to the authentication agent (if any)
586: will be forwarded to the remote machine.
587: The argument must be
588: .Dq yes
589: or
590: .Dq no .
591: The default is
592: .Dq no .
1.3 stevesk 593: .Pp
1.7 jmc 594: Agent forwarding should be enabled with caution.
595: Users with the ability to bypass file permissions on the remote host
596: (for the agent's Unix-domain socket)
597: can access the local agent through the forwarded connection.
598: An attacker cannot obtain key material from the agent,
1.3 stevesk 599: however they can perform operations on the keys that enable them to
600: authenticate using the identities loaded into the agent.
1.1 stevesk 601: .It Cm ForwardX11
602: Specifies whether X11 connections will be automatically redirected
603: over the secure channel and
604: .Ev DISPLAY
605: set.
606: The argument must be
607: .Dq yes
608: or
609: .Dq no .
610: The default is
611: .Dq no .
1.3 stevesk 612: .Pp
1.7 jmc 613: X11 forwarding should be enabled with caution.
614: Users with the ability to bypass file permissions on the remote host
1.22 markus 615: (for the user's X11 authorization database)
1.7 jmc 616: can access the local X11 display through the forwarded connection.
1.22 markus 617: An attacker may then be able to perform activities such as keystroke monitoring
618: if the
619: .Cm ForwardX11Trusted
620: option is also enabled.
1.134 djm 621: .It Cm ForwardX11Timeout
1.135 jmc 622: Specify a timeout for untrusted X11 forwarding
623: using the format described in the
1.166 jmc 624: TIME FORMATS section of
1.134 djm 625: .Xr sshd_config 5 .
626: X11 connections received by
627: .Xr ssh 1
628: after this time will be refused.
629: The default is to disable untrusted X11 forwarding after twenty minutes has
630: elapsed.
1.22 markus 631: .It Cm ForwardX11Trusted
1.34 jmc 632: If this option is set to
1.84 jmc 633: .Dq yes ,
634: remote X11 clients will have full access to the original X11 display.
1.42 djm 635: .Pp
1.22 markus 636: If this option is set to
1.84 jmc 637: .Dq no ,
638: remote X11 clients will be considered untrusted and prevented
1.22 markus 639: from stealing or tampering with data belonging to trusted X11
640: clients.
1.42 djm 641: Furthermore, the
642: .Xr xauth 1
643: token used for the session will be set to expire after 20 minutes.
644: Remote clients will be refused access after this time.
1.22 markus 645: .Pp
646: The default is
647: .Dq no .
648: .Pp
649: See the X11 SECURITY extension specification for full details on
650: the restrictions imposed on untrusted clients.
1.1 stevesk 651: .It Cm GatewayPorts
652: Specifies whether remote hosts are allowed to connect to local
653: forwarded ports.
654: By default,
1.84 jmc 655: .Xr ssh 1
1.7 jmc 656: binds local port forwardings to the loopback address.
657: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 658: .Cm GatewayPorts
1.84 jmc 659: can be used to specify that ssh
1.1 stevesk 660: should bind local port forwardings to the wildcard address,
661: thus allowing remote hosts to connect to forwarded ports.
662: The argument must be
663: .Dq yes
664: or
665: .Dq no .
666: The default is
667: .Dq no .
668: .It Cm GlobalKnownHostsFile
1.151 djm 669: Specifies one or more files to use for the global
670: host key database, separated by whitespace.
671: The default is
672: .Pa /etc/ssh/ssh_known_hosts ,
673: .Pa /etc/ssh/ssh_known_hosts2 .
1.18 markus 674: .It Cm GSSAPIAuthentication
1.27 markus 675: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 676: The default is
1.21 markus 677: .Dq no .
1.18 markus 678: Note that this option applies to protocol version 2 only.
679: .It Cm GSSAPIDelegateCredentials
680: Forward (delegate) credentials to the server.
681: The default is
682: .Dq no .
683: Note that this option applies to protocol version 2 only.
1.44 djm 684: .It Cm HashKnownHosts
685: Indicates that
1.84 jmc 686: .Xr ssh 1
1.44 djm 687: should hash host names and addresses when they are added to
1.50 djm 688: .Pa ~/.ssh/known_hosts .
1.44 djm 689: These hashed names may be used normally by
1.84 jmc 690: .Xr ssh 1
1.44 djm 691: and
1.84 jmc 692: .Xr sshd 8 ,
1.44 djm 693: but they do not reveal identifying information should the file's contents
694: be disclosed.
695: The default is
696: .Dq no .
1.97 jmc 697: Note that existing names and addresses in known hosts files
698: will not be converted automatically,
699: but may be manually hashed using
1.45 djm 700: .Xr ssh-keygen 1 .
1.1 stevesk 701: .It Cm HostbasedAuthentication
702: Specifies whether to try rhosts based authentication with public key
703: authentication.
704: The argument must be
705: .Dq yes
706: or
707: .Dq no .
708: The default is
709: .Dq no .
710: This option applies to protocol version 2 only and
711: is similar to
712: .Cm RhostsRSAAuthentication .
713: .It Cm HostKeyAlgorithms
714: Specifies the protocol version 2 host key algorithms
715: that the client wants to use in order of preference.
716: The default for this option is:
1.139 djm 717: .Bd -literal -offset 3n
718: ecdsa-sha2-nistp256-cert-v01@openssh.com,
719: ecdsa-sha2-nistp384-cert-v01@openssh.com,
720: ecdsa-sha2-nistp521-cert-v01@openssh.com,
721: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
722: ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
723: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
724: ssh-rsa,ssh-dss
725: .Ed
1.145 djm 726: .Pp
727: If hostkeys are known for the destination host then this default is modified
728: to prefer their algorithms.
1.1 stevesk 729: .It Cm HostKeyAlias
730: Specifies an alias that should be used instead of the
731: real host name when looking up or saving the host key
732: in the host key database files.
1.84 jmc 733: This option is useful for tunneling SSH connections
1.1 stevesk 734: or for multiple servers running on a single host.
735: .It Cm HostName
736: Specifies the real host name to log into.
737: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 738: If the hostname contains the character sequence
739: .Ql %h ,
1.150 jmc 740: then this will be replaced with the host name specified on the command line
1.136 djm 741: (this is useful for manipulating unqualified names).
1.84 jmc 742: The default is the name given on the command line.
1.1 stevesk 743: Numeric IP addresses are also permitted (both on the command line and in
744: .Cm HostName
745: specifications).
1.29 markus 746: .It Cm IdentitiesOnly
747: Specifies that
1.84 jmc 748: .Xr ssh 1
1.29 markus 749: should only use the authentication identity files configured in the
1.31 jmc 750: .Nm
1.29 markus 751: files,
1.84 jmc 752: even if
753: .Xr ssh-agent 1
1.159 djm 754: or a
755: .Cm PKCS11Provider
1.29 markus 756: offers more identities.
757: The argument to this keyword must be
758: .Dq yes
759: or
760: .Dq no .
1.84 jmc 761: This option is intended for situations where ssh-agent
1.29 markus 762: offers many different identities.
763: The default is
764: .Dq no .
1.67 jmc 765: .It Cm IdentityFile
1.155 dtucker 766: Specifies a file from which the user's DSA, ECDSA or RSA authentication
1.139 djm 767: identity is read.
1.67 jmc 768: The default is
769: .Pa ~/.ssh/identity
770: for protocol version 1, and
1.139 djm 771: .Pa ~/.ssh/id_dsa ,
772: .Pa ~/.ssh/id_ecdsa
773: and
1.67 jmc 774: .Pa ~/.ssh/id_rsa
775: for protocol version 2.
776: Additionally, any identities represented by the authentication agent
1.165 djm 777: will be used for authentication unless
778: .Cm IdentitiesOnly
779: is set.
1.129 djm 780: .Xr ssh 1
781: will try to load certificate information from the filename obtained by
782: appending
783: .Pa -cert.pub
784: to the path of a specified
785: .Cm IdentityFile .
1.90 djm 786: .Pp
1.67 jmc 787: The file name may use the tilde
1.91 jmc 788: syntax to refer to a user's home directory or one of the following
1.90 djm 789: escape characters:
790: .Ql %d
791: (local user's home directory),
792: .Ql %u
793: (local user name),
794: .Ql %l
795: (local host name),
796: .Ql %h
797: (remote host name) or
1.92 djm 798: .Ql %r
1.90 djm 799: (remote user name).
800: .Pp
1.67 jmc 801: It is possible to have
802: multiple identity files specified in configuration files; all these
803: identities will be tried in sequence.
1.152 djm 804: Multiple
805: .Cm IdentityFile
806: directives will add to the list of identities tried (this behaviour
807: differs from that of other configuration directives).
1.165 djm 808: .Pp
809: .Cm IdentityFile
810: may be used in conjunction with
811: .Cm IdentitiesOnly
812: to select which identities in an agent are offered during authentication.
1.164 jmc 813: .It Cm IgnoreUnknown
814: Specifies a pattern-list of unknown options to be ignored if they are
815: encountered in configuration parsing.
816: This may be used to suppress errors if
817: .Nm
818: contains options that are unrecognised by
819: .Xr ssh 1 .
820: It is recommended that
821: .Cm IgnoreUnknown
822: be listed early in the configuration file as it will not be applied
823: to unknown options that appear before it.
1.143 djm 824: .It Cm IPQoS
825: Specifies the IPv4 type-of-service or DSCP class for connections.
826: Accepted values are
827: .Dq af11 ,
828: .Dq af12 ,
829: .Dq af13 ,
1.154 djm 830: .Dq af21 ,
1.143 djm 831: .Dq af22 ,
832: .Dq af23 ,
833: .Dq af31 ,
834: .Dq af32 ,
835: .Dq af33 ,
836: .Dq af41 ,
837: .Dq af42 ,
838: .Dq af43 ,
839: .Dq cs0 ,
840: .Dq cs1 ,
841: .Dq cs2 ,
842: .Dq cs3 ,
843: .Dq cs4 ,
844: .Dq cs5 ,
845: .Dq cs6 ,
846: .Dq cs7 ,
847: .Dq ef ,
848: .Dq lowdelay ,
849: .Dq throughput ,
850: .Dq reliability ,
851: or a numeric value.
1.146 djm 852: This option may take one or two arguments, separated by whitespace.
1.143 djm 853: If one argument is specified, it is used as the packet class unconditionally.
854: If two values are specified, the first is automatically selected for
855: interactive sessions and the second for non-interactive sessions.
856: The default is
857: .Dq lowdelay
858: for interactive sessions and
859: .Dq throughput
860: for non-interactive sessions.
1.103 djm 861: .It Cm KbdInteractiveAuthentication
862: Specifies whether to use keyboard-interactive authentication.
863: The argument to this keyword must be
864: .Dq yes
865: or
866: .Dq no .
867: The default is
868: .Dq yes .
1.39 djm 869: .It Cm KbdInteractiveDevices
870: Specifies the list of methods to use in keyboard-interactive authentication.
871: Multiple method names must be comma-separated.
872: The default is to use the server specified list.
1.85 jmc 873: The methods available vary depending on what the server supports.
874: For an OpenSSH server,
875: it may be zero or more of:
876: .Dq bsdauth ,
877: .Dq pam ,
878: and
879: .Dq skey .
1.140 djm 880: .It Cm KexAlgorithms
881: Specifies the available KEX (Key Exchange) algorithms.
882: Multiple algorithms must be comma-separated.
1.141 jmc 883: The default is:
884: .Bd -literal -offset indent
1.179 markus 885: curve25519-sha256@libssh.org,
1.141 jmc 886: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
887: diffie-hellman-group-exchange-sha256,
888: diffie-hellman-group-exchange-sha1,
889: diffie-hellman-group14-sha1,
890: diffie-hellman-group1-sha1
891: .Ed
1.65 reyk 892: .It Cm LocalCommand
893: Specifies a command to execute on the local machine after successfully
894: connecting to the server.
895: The command string extends to the end of the line, and is executed with
1.105 jmc 896: the user's shell.
1.109 dtucker 897: The following escape character substitutions will be performed:
898: .Ql %d
899: (local user's home directory),
900: .Ql %h
901: (remote host name),
902: .Ql %l
903: (local host name),
904: .Ql %n
905: (host name as provided on the command line),
906: .Ql %p
907: (remote port),
908: .Ql %r
909: (remote user name) or
910: .Ql %u
911: (local user name).
1.123 djm 912: .Pp
913: The command is run synchronously and does not have access to the
914: session of the
915: .Xr ssh 1
916: that spawned it.
917: It should not be used for interactive commands.
918: .Pp
1.65 reyk 919: This directive is ignored unless
920: .Cm PermitLocalCommand
921: has been enabled.
1.1 stevesk 922: .It Cm LocalForward
1.74 jmc 923: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 924: the secure channel to the specified host and port from the remote machine.
1.49 jmc 925: The first argument must be
1.43 djm 926: .Sm off
1.49 jmc 927: .Oo Ar bind_address : Oc Ar port
1.43 djm 928: .Sm on
1.49 jmc 929: and the second argument must be
930: .Ar host : Ns Ar hostport .
1.138 djm 931: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 932: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 933: given on the command line.
1.1 stevesk 934: Only the superuser can forward privileged ports.
1.43 djm 935: By default, the local port is bound in accordance with the
936: .Cm GatewayPorts
937: setting.
938: However, an explicit
939: .Ar bind_address
940: may be used to bind the connection to a specific address.
941: The
942: .Ar bind_address
943: of
944: .Dq localhost
1.46 jmc 945: indicates that the listening port be bound for local use only, while an
946: empty address or
947: .Sq *
1.43 djm 948: indicates that the port should be available from all interfaces.
1.1 stevesk 949: .It Cm LogLevel
950: Gives the verbosity level that is used when logging messages from
1.84 jmc 951: .Xr ssh 1 .
1.1 stevesk 952: The possible values are:
1.84 jmc 953: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 954: The default is INFO.
955: DEBUG and DEBUG1 are equivalent.
956: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 957: .It Cm MACs
958: Specifies the MAC (message authentication code) algorithms
959: in order of preference.
960: The MAC algorithm is used in protocol version 2
961: for data integrity protection.
962: Multiple algorithms must be comma-separated.
1.160 markus 963: The algorithms that contain
964: .Dq -etm
965: calculate the MAC after encryption (encrypt-then-mac).
966: These are considered safer and their use recommended.
1.84 jmc 967: The default is:
1.101 jmc 968: .Bd -literal -offset indent
1.160 markus 969: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
970: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
971: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
972: hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
973: hmac-md5-96-etm@openssh.com,
1.158 markus 974: hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
1.157 naddy 975: hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
976: hmac-sha1-96,hmac-md5-96
1.101 jmc 977: .Ed
1.1 stevesk 978: .It Cm NoHostAuthenticationForLocalhost
979: This option can be used if the home directory is shared across machines.
980: In this case localhost will refer to a different machine on each of
981: the machines and the user will get many warnings about changed host keys.
982: However, this option disables host authentication for localhost.
983: The argument to this keyword must be
984: .Dq yes
985: or
986: .Dq no .
987: The default is to check the host key for localhost.
988: .It Cm NumberOfPasswordPrompts
989: Specifies the number of password prompts before giving up.
990: The argument to this keyword must be an integer.
1.84 jmc 991: The default is 3.
1.1 stevesk 992: .It Cm PasswordAuthentication
993: Specifies whether to use password authentication.
994: The argument to this keyword must be
995: .Dq yes
996: or
997: .Dq no .
998: The default is
999: .Dq yes .
1.65 reyk 1000: .It Cm PermitLocalCommand
1001: Allow local command execution via the
1002: .Ic LocalCommand
1003: option or using the
1.66 jmc 1004: .Ic !\& Ns Ar command
1.65 reyk 1005: escape sequence in
1006: .Xr ssh 1 .
1007: The argument must be
1008: .Dq yes
1009: or
1010: .Dq no .
1011: The default is
1012: .Dq no .
1.127 markus 1013: .It Cm PKCS11Provider
1014: Specifies which PKCS#11 provider to use.
1.144 jmc 1015: The argument to this keyword is the PKCS#11 shared library
1.127 markus 1016: .Xr ssh 1
1.128 markus 1017: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 1018: private RSA key.
1.67 jmc 1019: .It Cm Port
1020: Specifies the port number to connect on the remote host.
1.84 jmc 1021: The default is 22.
1.1 stevesk 1022: .It Cm PreferredAuthentications
1023: Specifies the order in which the client should try protocol 2
1.11 jmc 1024: authentication methods.
1.48 jmc 1025: This allows a client to prefer one method (e.g.\&
1.1 stevesk 1026: .Cm keyboard-interactive )
1.48 jmc 1027: over another method (e.g.\&
1.131 jmc 1028: .Cm password ) .
1029: The default is:
1030: .Bd -literal -offset indent
1031: gssapi-with-mic,hostbased,publickey,
1032: keyboard-interactive,password
1033: .Ed
1.1 stevesk 1034: .It Cm Protocol
1035: Specifies the protocol versions
1.84 jmc 1036: .Xr ssh 1
1.1 stevesk 1037: should support in order of preference.
1038: The possible values are
1.84 jmc 1039: .Sq 1
1.1 stevesk 1040: and
1.84 jmc 1041: .Sq 2 .
1.1 stevesk 1042: Multiple versions must be comma-separated.
1.120 markus 1043: When this option is set to
1.121 jmc 1044: .Dq 2,1
1.120 markus 1045: .Nm ssh
1046: will try version 2 and fall back to version 1
1047: if version 2 is not available.
1.1 stevesk 1048: The default is
1.121 jmc 1049: .Sq 2 .
1.1 stevesk 1050: .It Cm ProxyCommand
1051: Specifies the command to use to connect to the server.
1052: The command
1053: string extends to the end of the line, and is executed with
1.105 jmc 1054: the user's shell.
1.133 jmc 1055: In the command string, any occurrence of
1.1 stevesk 1056: .Ql %h
1057: will be substituted by the host name to
1.132 djm 1058: connect,
1.1 stevesk 1059: .Ql %p
1.133 jmc 1060: by the port, and
1061: .Ql %r
1.132 djm 1062: by the remote user name.
1.1 stevesk 1063: The command can be basically anything,
1064: and should read from its standard input and write to its standard output.
1065: It should eventually connect an
1066: .Xr sshd 8
1067: server running on some machine, or execute
1068: .Ic sshd -i
1069: somewhere.
1070: Host key management will be done using the
1071: HostName of the host being connected (defaulting to the name typed by
1072: the user).
1.7 jmc 1073: Setting the command to
1074: .Dq none
1.6 markus 1075: disables this option entirely.
1.1 stevesk 1076: Note that
1077: .Cm CheckHostIP
1078: is not available for connects with a proxy command.
1.52 djm 1079: .Pp
1080: This directive is useful in conjunction with
1081: .Xr nc 1
1082: and its proxy support.
1.53 jmc 1083: For example, the following directive would connect via an HTTP proxy at
1.52 djm 1084: 192.0.2.0:
1085: .Bd -literal -offset 3n
1086: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1087: .Ed
1.167 djm 1088: .It Cm ProxyUseFdpass
1.168 jmc 1089: Specifies that
1.167 djm 1090: .Cm ProxyCommand
1091: will pass a connected file descriptor back to
1.168 jmc 1092: .Xr ssh 1
1.167 djm 1093: instead of continuing to execute and pass data.
1094: The default is
1095: .Dq no .
1.1 stevesk 1096: .It Cm PubkeyAuthentication
1097: Specifies whether to try public key authentication.
1098: The argument to this keyword must be
1099: .Dq yes
1100: or
1101: .Dq no .
1102: The default is
1103: .Dq yes .
1104: This option applies to protocol version 2 only.
1.75 dtucker 1105: .It Cm RekeyLimit
1106: Specifies the maximum amount of data that may be transmitted before the
1.162 dtucker 1107: session key is renegotiated, optionally followed a maximum amount of
1108: time that may pass before the session key is renegotiated.
1109: The first argument is specified in bytes and may have a suffix of
1.76 jmc 1110: .Sq K ,
1111: .Sq M ,
1.75 dtucker 1112: or
1.76 jmc 1113: .Sq G
1.75 dtucker 1114: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1115: The default is between
1.84 jmc 1116: .Sq 1G
1.75 dtucker 1117: and
1.84 jmc 1118: .Sq 4G ,
1.75 dtucker 1119: depending on the cipher.
1.162 dtucker 1120: The optional second value is specified in seconds and may use any of the
1121: units documented in the
1.166 jmc 1122: TIME FORMATS section of
1.162 dtucker 1123: .Xr sshd_config 5 .
1124: The default value for
1125: .Cm RekeyLimit
1126: is
1127: .Dq default none ,
1128: which means that rekeying is performed after the cipher's default amount
1129: of data has been sent or received and no time based rekeying is done.
1.76 jmc 1130: This option applies to protocol version 2 only.
1.1 stevesk 1131: .It Cm RemoteForward
1.74 jmc 1132: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 1133: the secure channel to the specified host and port from the local machine.
1.49 jmc 1134: The first argument must be
1.43 djm 1135: .Sm off
1.49 jmc 1136: .Oo Ar bind_address : Oc Ar port
1.43 djm 1137: .Sm on
1.49 jmc 1138: and the second argument must be
1139: .Ar host : Ns Ar hostport .
1.138 djm 1140: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 1141: Multiple forwardings may be specified, and additional
1142: forwardings can be given on the command line.
1.113 stevesk 1143: Privileged ports can be forwarded only when
1144: logging in as root on the remote machine.
1.118 jmc 1145: .Pp
1.117 djm 1146: If the
1147: .Ar port
1148: argument is
1149: .Ql 0 ,
1150: the listen port will be dynamically allocated on the server and reported
1151: to the client at run time.
1.43 djm 1152: .Pp
1153: If the
1154: .Ar bind_address
1155: is not specified, the default is to only bind to loopback addresses.
1156: If the
1157: .Ar bind_address
1158: is
1159: .Ql *
1160: or an empty string, then the forwarding is requested to listen on all
1161: interfaces.
1162: Specifying a remote
1163: .Ar bind_address
1.46 jmc 1164: will only succeed if the server's
1165: .Cm GatewayPorts
1.43 djm 1166: option is enabled (see
1.46 jmc 1167: .Xr sshd_config 5 ) .
1.149 djm 1168: .It Cm RequestTTY
1169: Specifies whether to request a pseudo-tty for the session.
1170: The argument may be one of:
1171: .Dq no
1172: (never request a TTY),
1173: .Dq yes
1174: (always request a TTY when standard input is a TTY),
1175: .Dq force
1176: (always request a TTY) or
1177: .Dq auto
1178: (request a TTY when opening a login session).
1179: This option mirrors the
1180: .Fl t
1181: and
1182: .Fl T
1183: flags for
1184: .Xr ssh 1 .
1.1 stevesk 1185: .It Cm RhostsRSAAuthentication
1186: Specifies whether to try rhosts based authentication with RSA host
1187: authentication.
1188: The argument must be
1189: .Dq yes
1190: or
1191: .Dq no .
1192: The default is
1193: .Dq no .
1194: This option applies to protocol version 1 only and requires
1.84 jmc 1195: .Xr ssh 1
1.1 stevesk 1196: to be setuid root.
1197: .It Cm RSAAuthentication
1198: Specifies whether to try RSA authentication.
1199: The argument to this keyword must be
1200: .Dq yes
1201: or
1202: .Dq no .
1203: RSA authentication will only be
1204: attempted if the identity file exists, or an authentication agent is
1205: running.
1206: The default is
1207: .Dq yes .
1208: Note that this option applies to protocol version 1 only.
1.32 djm 1209: .It Cm SendEnv
1210: Specifies what variables from the local
1211: .Xr environ 7
1212: should be sent to the server.
1.84 jmc 1213: Note that environment passing is only supported for protocol 2.
1214: The server must also support it, and the server must be configured to
1.33 djm 1215: accept these environment variables.
1.32 djm 1216: Refer to
1217: .Cm AcceptEnv
1218: in
1219: .Xr sshd_config 5
1220: for how to configure the server.
1.80 jmc 1221: Variables are specified by name, which may contain wildcard characters.
1.33 djm 1222: Multiple environment variables may be separated by whitespace or spread
1.32 djm 1223: across multiple
1224: .Cm SendEnv
1225: directives.
1226: The default is not to send any environment variables.
1.81 jmc 1227: .Pp
1228: See
1229: .Sx PATTERNS
1230: for more information on patterns.
1.28 markus 1231: .It Cm ServerAliveCountMax
1.73 jmc 1232: Sets the number of server alive messages (see below) which may be
1.28 markus 1233: sent without
1.84 jmc 1234: .Xr ssh 1
1.28 markus 1235: receiving any messages back from the server.
1236: If this threshold is reached while server alive messages are being sent,
1.84 jmc 1237: ssh will disconnect from the server, terminating the session.
1.28 markus 1238: It is important to note that the use of server alive messages is very
1239: different from
1240: .Cm TCPKeepAlive
1241: (below).
1242: The server alive messages are sent through the encrypted channel
1243: and therefore will not be spoofable.
1244: The TCP keepalive option enabled by
1245: .Cm TCPKeepAlive
1246: is spoofable.
1247: The server alive mechanism is valuable when the client or
1248: server depend on knowing when a connection has become inactive.
1249: .Pp
1250: The default value is 3.
1251: If, for example,
1252: .Cm ServerAliveInterval
1.84 jmc 1253: (see below) is set to 15 and
1.28 markus 1254: .Cm ServerAliveCountMax
1.84 jmc 1255: is left at the default, if the server becomes unresponsive,
1256: ssh will disconnect after approximately 45 seconds.
1.89 markus 1257: This option applies to protocol version 2 only.
1.67 jmc 1258: .It Cm ServerAliveInterval
1259: Sets a timeout interval in seconds after which if no data has been received
1260: from the server,
1.84 jmc 1261: .Xr ssh 1
1.67 jmc 1262: will send a message through the encrypted
1263: channel to request a response from the server.
1264: The default
1265: is 0, indicating that these messages will not be sent to the server.
1266: This option applies to protocol version 2 only.
1.1 stevesk 1267: .It Cm StrictHostKeyChecking
1268: If this flag is set to
1269: .Dq yes ,
1.84 jmc 1270: .Xr ssh 1
1.1 stevesk 1271: will never automatically add host keys to the
1.50 djm 1272: .Pa ~/.ssh/known_hosts
1.1 stevesk 1273: file, and refuses to connect to hosts whose host key has changed.
1274: This provides maximum protection against trojan horse attacks,
1.84 jmc 1275: though it can be annoying when the
1.1 stevesk 1276: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 1277: file is poorly maintained or when connections to new hosts are
1.1 stevesk 1278: frequently made.
1279: This option forces the user to manually
1280: add all new hosts.
1281: If this flag is set to
1282: .Dq no ,
1.84 jmc 1283: ssh will automatically add new host keys to the
1.1 stevesk 1284: user known hosts files.
1285: If this flag is set to
1286: .Dq ask ,
1287: new host keys
1288: will be added to the user known host files only after the user
1289: has confirmed that is what they really want to do, and
1.84 jmc 1290: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 1291: The host keys of
1292: known hosts will be verified automatically in all cases.
1293: The argument must be
1294: .Dq yes ,
1.84 jmc 1295: .Dq no ,
1.1 stevesk 1296: or
1297: .Dq ask .
1298: The default is
1299: .Dq ask .
1.26 markus 1300: .It Cm TCPKeepAlive
1301: Specifies whether the system should send TCP keepalive messages to the
1302: other side.
1303: If they are sent, death of the connection or crash of one
1304: of the machines will be properly noticed.
1305: However, this means that
1306: connections will die if the route is down temporarily, and some people
1307: find it annoying.
1308: .Pp
1309: The default is
1310: .Dq yes
1311: (to send TCP keepalive messages), and the client will notice
1312: if the network goes down or the remote host dies.
1313: This is important in scripts, and many users want it too.
1314: .Pp
1315: To disable TCP keepalive messages, the value should be set to
1316: .Dq no .
1.65 reyk 1317: .It Cm Tunnel
1.95 stevesk 1318: Request
1.65 reyk 1319: .Xr tun 4
1.69 jmc 1320: device forwarding between the client and the server.
1.65 reyk 1321: The argument must be
1.68 reyk 1322: .Dq yes ,
1.95 stevesk 1323: .Dq point-to-point
1324: (layer 3),
1325: .Dq ethernet
1326: (layer 2),
1.65 reyk 1327: or
1328: .Dq no .
1.95 stevesk 1329: Specifying
1330: .Dq yes
1331: requests the default tunnel mode, which is
1332: .Dq point-to-point .
1.65 reyk 1333: The default is
1334: .Dq no .
1335: .It Cm TunnelDevice
1.95 stevesk 1336: Specifies the
1.65 reyk 1337: .Xr tun 4
1.95 stevesk 1338: devices to open on the client
1339: .Pq Ar local_tun
1340: and the server
1341: .Pq Ar remote_tun .
1342: .Pp
1343: The argument must be
1344: .Sm off
1345: .Ar local_tun Op : Ar remote_tun .
1346: .Sm on
1347: The devices may be specified by numerical ID or the keyword
1348: .Dq any ,
1349: which uses the next available tunnel device.
1350: If
1351: .Ar remote_tun
1352: is not specified, it defaults to
1353: .Dq any .
1354: The default is
1355: .Dq any:any .
1.72 jmc 1356: .It Cm UsePrivilegedPort
1357: Specifies whether to use a privileged port for outgoing connections.
1358: The argument must be
1359: .Dq yes
1360: or
1361: .Dq no .
1362: The default is
1363: .Dq no .
1364: If set to
1.84 jmc 1365: .Dq yes ,
1366: .Xr ssh 1
1.72 jmc 1367: must be setuid root.
1368: Note that this option must be set to
1369: .Dq yes
1370: for
1371: .Cm RhostsRSAAuthentication
1372: with older servers.
1.1 stevesk 1373: .It Cm User
1374: Specifies the user to log in as.
1375: This can be useful when a different user name is used on different machines.
1376: This saves the trouble of
1377: having to remember to give the user name on the command line.
1378: .It Cm UserKnownHostsFile
1.151 djm 1379: Specifies one or more files to use for the user
1380: host key database, separated by whitespace.
1381: The default is
1382: .Pa ~/.ssh/known_hosts ,
1383: .Pa ~/.ssh/known_hosts2 .
1.8 jakob 1384: .It Cm VerifyHostKeyDNS
1385: Specifies whether to verify the remote key using DNS and SSHFP resource
1386: records.
1.24 jakob 1387: If this option is set to
1388: .Dq yes ,
1.25 jmc 1389: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1390: from DNS.
1391: Insecure fingerprints will be handled as if this option was set to
1392: .Dq ask .
1393: If this option is set to
1394: .Dq ask ,
1395: information on fingerprint match will be displayed, but the user will still
1396: need to confirm new host keys according to the
1397: .Cm StrictHostKeyChecking
1398: option.
1399: The argument must be
1400: .Dq yes ,
1.84 jmc 1401: .Dq no ,
1.25 jmc 1402: or
1403: .Dq ask .
1.8 jakob 1404: The default is
1405: .Dq no .
1.12 jakob 1406: Note that this option applies to protocol version 2 only.
1.84 jmc 1407: .Pp
1.166 jmc 1408: See also VERIFYING HOST KEYS in
1.84 jmc 1409: .Xr ssh 1 .
1.111 grunk 1410: .It Cm VisualHostKey
1411: If this flag is set to
1412: .Dq yes ,
1413: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1414: printed in addition to the hex fingerprint string at login and
1415: for unknown host keys.
1.111 grunk 1416: If this flag is set to
1417: .Dq no ,
1.114 stevesk 1418: no fingerprint strings are printed at login and
1419: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1420: The default is
1421: .Dq no .
1.1 stevesk 1422: .It Cm XAuthLocation
1.5 stevesk 1423: Specifies the full pathname of the
1.1 stevesk 1424: .Xr xauth 1
1425: program.
1426: The default is
1427: .Pa /usr/X11R6/bin/xauth .
1428: .El
1.86 jmc 1429: .Sh PATTERNS
1430: A
1431: .Em pattern
1432: consists of zero or more non-whitespace characters,
1433: .Sq *
1434: (a wildcard that matches zero or more characters),
1435: or
1436: .Sq ?\&
1437: (a wildcard that matches exactly one character).
1438: For example, to specify a set of declarations for any host in the
1439: .Dq .co.uk
1440: set of domains,
1441: the following pattern could be used:
1442: .Pp
1443: .Dl Host *.co.uk
1444: .Pp
1445: The following pattern
1446: would match any host in the 192.168.0.[0-9] network range:
1447: .Pp
1448: .Dl Host 192.168.0.?
1449: .Pp
1450: A
1451: .Em pattern-list
1452: is a comma-separated list of patterns.
1453: Patterns within pattern-lists may be negated
1454: by preceding them with an exclamation mark
1455: .Pq Sq !\& .
1456: For example,
1.174 djm 1457: to allow a key to be used from anywhere within an organization
1.86 jmc 1458: except from the
1459: .Dq dialup
1460: pool,
1461: the following entry (in authorized_keys) could be used:
1462: .Pp
1463: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1464: .Sh FILES
1465: .Bl -tag -width Ds
1.50 djm 1466: .It Pa ~/.ssh/config
1.1 stevesk 1467: This is the per-user configuration file.
1468: The format of this file is described above.
1.84 jmc 1469: This file is used by the SSH client.
1.30 djm 1470: Because of the potential for abuse, this file must have strict permissions:
1471: read/write for the user, and not accessible by others.
1.1 stevesk 1472: .It Pa /etc/ssh/ssh_config
1473: Systemwide configuration file.
1474: This file provides defaults for those
1475: values that are not specified in the user's configuration file, and
1476: for those users who do not have a configuration file.
1477: This file must be world-readable.
1478: .El
1.13 jmc 1479: .Sh SEE ALSO
1480: .Xr ssh 1
1.1 stevesk 1481: .Sh AUTHORS
1482: OpenSSH is a derivative of the original and free
1483: ssh 1.2.12 release by Tatu Ylonen.
1484: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1485: Theo de Raadt and Dug Song
1486: removed many bugs, re-added newer features and
1487: created OpenSSH.
1488: Markus Friedl contributed the support for SSH
1489: protocol versions 1.5 and 2.0.