Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.138
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
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26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.138 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.137 2010/07/19 09:15:12 djm Exp $
! 38: .Dd $Mdocdate: July 19 2010 $
1.1 stevesk 39: .Dt SSH_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm ssh_config
43: .Nd OpenSSH SSH client configuration files
44: .Sh SYNOPSIS
1.98 jmc 45: .Nm ~/.ssh/config
46: .Nm /etc/ssh/ssh_config
1.1 stevesk 47: .Sh DESCRIPTION
1.84 jmc 48: .Xr ssh 1
1.1 stevesk 49: obtains configuration data from the following sources in
50: the following order:
1.79 jmc 51: .Pp
1.2 stevesk 52: .Bl -enum -offset indent -compact
53: .It
54: command-line options
55: .It
56: user's configuration file
1.50 djm 57: .Pq Pa ~/.ssh/config
1.2 stevesk 58: .It
59: system-wide configuration file
60: .Pq Pa /etc/ssh/ssh_config
61: .El
1.1 stevesk 62: .Pp
63: For each parameter, the first obtained value
64: will be used.
1.41 jmc 65: The configuration files contain sections separated by
1.1 stevesk 66: .Dq Host
67: specifications, and that section is only applied for hosts that
68: match one of the patterns given in the specification.
69: The matched host name is the one given on the command line.
70: .Pp
71: Since the first obtained value for each parameter is used, more
72: host-specific declarations should be given near the beginning of the
73: file, and general defaults at the end.
1.80 jmc 74: .Pp
1.1 stevesk 75: The configuration file has the following format:
76: .Pp
77: Empty lines and lines starting with
78: .Ql #
79: are comments.
80: Otherwise a line is of the format
81: .Dq keyword arguments .
82: Configuration options may be separated by whitespace or
83: optional whitespace and exactly one
84: .Ql = ;
85: the latter format is useful to avoid the need to quote whitespace
86: when specifying configuration options using the
87: .Nm ssh ,
1.87 jmc 88: .Nm scp ,
1.1 stevesk 89: and
90: .Nm sftp
91: .Fl o
92: option.
1.88 dtucker 93: Arguments may optionally be enclosed in double quotes
94: .Pq \&"
95: in order to represent arguments containing spaces.
1.1 stevesk 96: .Pp
97: The possible
98: keywords and their meanings are as follows (note that
99: keywords are case-insensitive and arguments are case-sensitive):
100: .Bl -tag -width Ds
101: .It Cm Host
102: Restricts the following declarations (up to the next
103: .Cm Host
104: keyword) to be only for those hosts that match one of the patterns
105: given after the keyword.
1.112 krw 106: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 107: A single
1.83 jmc 108: .Ql *
1.1 stevesk 109: as a pattern can be used to provide global
110: defaults for all hosts.
111: The host is the
112: .Ar hostname
1.83 jmc 113: argument given on the command line (i.e. the name is not converted to
1.1 stevesk 114: a canonicalized host name before matching).
1.81 jmc 115: .Pp
116: See
117: .Sx PATTERNS
118: for more information on patterns.
1.10 djm 119: .It Cm AddressFamily
1.11 jmc 120: Specifies which address family to use when connecting.
121: Valid arguments are
1.10 djm 122: .Dq any ,
123: .Dq inet
1.84 jmc 124: (use IPv4 only), or
1.10 djm 125: .Dq inet6
1.40 jmc 126: (use IPv6 only).
1.1 stevesk 127: .It Cm BatchMode
128: If set to
129: .Dq yes ,
130: passphrase/password querying will be disabled.
131: This option is useful in scripts and other batch jobs where no user
132: is present to supply the password.
133: The argument must be
134: .Dq yes
135: or
136: .Dq no .
137: The default is
138: .Dq no .
139: .It Cm BindAddress
1.60 dtucker 140: Use the specified address on the local machine as the source address of
1.61 jmc 141: the connection.
142: Only useful on systems with more than one address.
1.1 stevesk 143: Note that this option does not work if
144: .Cm UsePrivilegedPort
145: is set to
146: .Dq yes .
147: .It Cm ChallengeResponseAuthentication
1.82 jmc 148: Specifies whether to use challenge-response authentication.
1.1 stevesk 149: The argument to this keyword must be
150: .Dq yes
151: or
152: .Dq no .
153: The default is
154: .Dq yes .
155: .It Cm CheckHostIP
156: If this flag is set to
157: .Dq yes ,
1.84 jmc 158: .Xr ssh 1
159: will additionally check the host IP address in the
1.1 stevesk 160: .Pa known_hosts
161: file.
162: This allows ssh to detect if a host key changed due to DNS spoofing.
1.107 grunk 163: If the option is set to
1.1 stevesk 164: .Dq no ,
165: the check will not be executed.
166: The default is
167: .Dq yes .
168: .It Cm Cipher
169: Specifies the cipher to use for encrypting the session
170: in protocol version 1.
171: Currently,
172: .Dq blowfish ,
173: .Dq 3des ,
174: and
175: .Dq des
176: are supported.
177: .Ar des
178: is only supported in the
1.84 jmc 179: .Xr ssh 1
1.1 stevesk 180: client for interoperability with legacy protocol 1 implementations
181: that do not support the
182: .Ar 3des
1.7 jmc 183: cipher.
184: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 185: The default is
186: .Dq 3des .
187: .It Cm Ciphers
188: Specifies the ciphers allowed for protocol version 2
189: in order of preference.
190: Multiple ciphers must be comma-separated.
1.35 dtucker 191: The supported ciphers are
192: .Dq 3des-cbc ,
193: .Dq aes128-cbc ,
194: .Dq aes192-cbc ,
195: .Dq aes256-cbc ,
196: .Dq aes128-ctr ,
197: .Dq aes192-ctr ,
198: .Dq aes256-ctr ,
1.54 djm 199: .Dq arcfour128 ,
200: .Dq arcfour256 ,
1.35 dtucker 201: .Dq arcfour ,
202: .Dq blowfish-cbc ,
203: and
204: .Dq cast128-cbc .
1.84 jmc 205: The default is:
206: .Bd -literal -offset 3n
1.116 naddy 207: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
208: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
209: aes256-cbc,arcfour
1.1 stevesk 210: .Ed
211: .It Cm ClearAllForwardings
1.84 jmc 212: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 213: specified in the configuration files or on the command line be
1.7 jmc 214: cleared.
215: This option is primarily useful when used from the
1.84 jmc 216: .Xr ssh 1
1.1 stevesk 217: command line to clear port forwardings set in
218: configuration files, and is automatically set by
219: .Xr scp 1
220: and
221: .Xr sftp 1 .
222: The argument must be
223: .Dq yes
224: or
225: .Dq no .
226: The default is
227: .Dq no .
228: .It Cm Compression
229: Specifies whether to use compression.
230: The argument must be
231: .Dq yes
232: or
233: .Dq no .
234: The default is
235: .Dq no .
236: .It Cm CompressionLevel
237: Specifies the compression level to use if compression is enabled.
238: The argument must be an integer from 1 (fast) to 9 (slow, best).
239: The default level is 6, which is good for most applications.
240: The meaning of the values is the same as in
241: .Xr gzip 1 .
242: Note that this option applies to protocol version 1 only.
243: .It Cm ConnectionAttempts
244: Specifies the number of tries (one per second) to make before exiting.
245: The argument must be an integer.
246: This may be useful in scripts if the connection sometimes fails.
247: The default is 1.
1.9 djm 248: .It Cm ConnectTimeout
1.84 jmc 249: Specifies the timeout (in seconds) used when connecting to the
250: SSH server, instead of using the default system TCP timeout.
1.11 jmc 251: This value is used only when the target is down or really unreachable,
252: not when it refuses the connection.
1.36 djm 253: .It Cm ControlMaster
254: Enables the sharing of multiple sessions over a single network connection.
255: When set to
1.84 jmc 256: .Dq yes ,
257: .Xr ssh 1
1.36 djm 258: will listen for connections on a control socket specified using the
259: .Cm ControlPath
260: argument.
261: Additional sessions can connect to this socket using the same
262: .Cm ControlPath
263: with
264: .Cm ControlMaster
265: set to
266: .Dq no
1.38 jmc 267: (the default).
1.64 jmc 268: These sessions will try to reuse the master instance's network connection
1.63 djm 269: rather than initiating new ones, but will fall back to connecting normally
270: if the control socket does not exist, or is not listening.
271: .Pp
1.37 djm 272: Setting this to
273: .Dq ask
1.84 jmc 274: will cause ssh
1.37 djm 275: to listen for control connections, but require confirmation using the
276: .Ev SSH_ASKPASS
277: program before they are accepted (see
278: .Xr ssh-add 1
1.38 jmc 279: for details).
1.51 jakob 280: If the
281: .Cm ControlPath
1.84 jmc 282: cannot be opened,
283: ssh will continue without connecting to a master instance.
1.58 djm 284: .Pp
285: X11 and
1.59 jmc 286: .Xr ssh-agent 1
1.58 djm 287: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 288: display and agent forwarded will be the one belonging to the master
1.59 jmc 289: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 290: .Pp
291: Two additional options allow for opportunistic multiplexing: try to use a
292: master connection but fall back to creating a new one if one does not already
293: exist.
294: These options are:
295: .Dq auto
296: and
297: .Dq autoask .
298: The latter requires confirmation like the
299: .Dq ask
300: option.
1.36 djm 301: .It Cm ControlPath
1.55 djm 302: Specify the path to the control socket used for connection sharing as described
303: in the
1.36 djm 304: .Cm ControlMaster
1.57 djm 305: section above or the string
306: .Dq none
307: to disable connection sharing.
1.55 djm 308: In the path,
1.77 djm 309: .Ql %l
310: will be substituted by the local host name,
1.55 djm 311: .Ql %h
312: will be substituted by the target host name,
313: .Ql %p
1.84 jmc 314: the port, and
1.55 djm 315: .Ql %r
316: by the remote login username.
1.56 djm 317: It is recommended that any
318: .Cm ControlPath
319: used for opportunistic connection sharing include
1.78 jmc 320: at least %h, %p, and %r.
1.56 djm 321: This ensures that shared connections are uniquely identified.
1.137 djm 322: .It Cm ControlPersist
323: When used in conjunction with
324: .Cm ControlMaster ,
325: specifies that the master connection should remain open
326: in the background (waiting for future client connections)
327: after the initial client connection has been closed.
328: If set to
329: .Dq no ,
330: then the master connection will not be placed into the background,
331: and will close as soon as the initial client connection is closed.
332: If set to
333: .Dq yes ,
334: then the master connection will remain in the background indefinitely
335: (until killed or closed via a mechanism such as the
336: .Xr ssh 1
337: .Dq Fl O No exit
338: option).
339: If set to a time in seconds, or a time in any of the formats documented in
340: .Xr sshd_config 5 ,
341: then the backgrounded master connection will automatically terminate
342: after it has remained idle (with no client connections) for the
343: specified time.
1.38 jmc 344: .It Cm DynamicForward
1.74 jmc 345: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 346: over the secure channel, and the application
347: protocol is then used to determine where to connect to from the
348: remote machine.
1.62 djm 349: .Pp
350: The argument must be
351: .Sm off
352: .Oo Ar bind_address : Oc Ar port .
353: .Sm on
1.138 ! djm 354: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.62 djm 355: By default, the local port is bound in accordance with the
356: .Cm GatewayPorts
357: setting.
358: However, an explicit
359: .Ar bind_address
360: may be used to bind the connection to a specific address.
361: The
362: .Ar bind_address
363: of
364: .Dq localhost
365: indicates that the listening port be bound for local use only, while an
366: empty address or
367: .Sq *
368: indicates that the port should be available from all interfaces.
369: .Pp
1.38 jmc 370: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 371: .Xr ssh 1
1.38 jmc 372: will act as a SOCKS server.
373: Multiple forwardings may be specified, and
374: additional forwardings can be given on the command line.
375: Only the superuser can forward privileged ports.
1.14 markus 376: .It Cm EnableSSHKeysign
377: Setting this option to
378: .Dq yes
379: in the global client configuration file
380: .Pa /etc/ssh/ssh_config
381: enables the use of the helper program
382: .Xr ssh-keysign 8
383: during
384: .Cm HostbasedAuthentication .
385: The argument must be
386: .Dq yes
387: or
388: .Dq no .
389: The default is
390: .Dq no .
1.23 jmc 391: This option should be placed in the non-hostspecific section.
1.14 markus 392: See
393: .Xr ssh-keysign 8
394: for more information.
1.1 stevesk 395: .It Cm EscapeChar
396: Sets the escape character (default:
397: .Ql ~ ) .
398: The escape character can also
399: be set on the command line.
400: The argument should be a single character,
401: .Ql ^
402: followed by a letter, or
403: .Dq none
404: to disable the escape
405: character entirely (making the connection transparent for binary
406: data).
1.96 markus 407: .It Cm ExitOnForwardFailure
408: Specifies whether
409: .Xr ssh 1
410: should terminate the connection if it cannot set up all requested
1.102 stevesk 411: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 412: The argument must be
413: .Dq yes
414: or
415: .Dq no .
416: The default is
417: .Dq no .
1.1 stevesk 418: .It Cm ForwardAgent
419: Specifies whether the connection to the authentication agent (if any)
420: will be forwarded to the remote machine.
421: The argument must be
422: .Dq yes
423: or
424: .Dq no .
425: The default is
426: .Dq no .
1.3 stevesk 427: .Pp
1.7 jmc 428: Agent forwarding should be enabled with caution.
429: Users with the ability to bypass file permissions on the remote host
430: (for the agent's Unix-domain socket)
431: can access the local agent through the forwarded connection.
432: An attacker cannot obtain key material from the agent,
1.3 stevesk 433: however they can perform operations on the keys that enable them to
434: authenticate using the identities loaded into the agent.
1.1 stevesk 435: .It Cm ForwardX11
436: Specifies whether X11 connections will be automatically redirected
437: over the secure channel and
438: .Ev DISPLAY
439: set.
440: The argument must be
441: .Dq yes
442: or
443: .Dq no .
444: The default is
445: .Dq no .
1.3 stevesk 446: .Pp
1.7 jmc 447: X11 forwarding should be enabled with caution.
448: Users with the ability to bypass file permissions on the remote host
1.22 markus 449: (for the user's X11 authorization database)
1.7 jmc 450: can access the local X11 display through the forwarded connection.
1.22 markus 451: An attacker may then be able to perform activities such as keystroke monitoring
452: if the
453: .Cm ForwardX11Trusted
454: option is also enabled.
1.134 djm 455: .It Cm ForwardX11Timeout
1.135 jmc 456: Specify a timeout for untrusted X11 forwarding
457: using the format described in the
1.134 djm 458: .Sx TIME FORMATS
459: section of
460: .Xr sshd_config 5 .
461: X11 connections received by
462: .Xr ssh 1
463: after this time will be refused.
464: The default is to disable untrusted X11 forwarding after twenty minutes has
465: elapsed.
1.22 markus 466: .It Cm ForwardX11Trusted
1.34 jmc 467: If this option is set to
1.84 jmc 468: .Dq yes ,
469: remote X11 clients will have full access to the original X11 display.
1.42 djm 470: .Pp
1.22 markus 471: If this option is set to
1.84 jmc 472: .Dq no ,
473: remote X11 clients will be considered untrusted and prevented
1.22 markus 474: from stealing or tampering with data belonging to trusted X11
475: clients.
1.42 djm 476: Furthermore, the
477: .Xr xauth 1
478: token used for the session will be set to expire after 20 minutes.
479: Remote clients will be refused access after this time.
1.22 markus 480: .Pp
481: The default is
482: .Dq no .
483: .Pp
484: See the X11 SECURITY extension specification for full details on
485: the restrictions imposed on untrusted clients.
1.1 stevesk 486: .It Cm GatewayPorts
487: Specifies whether remote hosts are allowed to connect to local
488: forwarded ports.
489: By default,
1.84 jmc 490: .Xr ssh 1
1.7 jmc 491: binds local port forwardings to the loopback address.
492: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 493: .Cm GatewayPorts
1.84 jmc 494: can be used to specify that ssh
1.1 stevesk 495: should bind local port forwardings to the wildcard address,
496: thus allowing remote hosts to connect to forwarded ports.
497: The argument must be
498: .Dq yes
499: or
500: .Dq no .
501: The default is
502: .Dq no .
503: .It Cm GlobalKnownHostsFile
504: Specifies a file to use for the global
505: host key database instead of
506: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 507: .It Cm GSSAPIAuthentication
1.27 markus 508: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 509: The default is
1.21 markus 510: .Dq no .
1.18 markus 511: Note that this option applies to protocol version 2 only.
512: .It Cm GSSAPIDelegateCredentials
513: Forward (delegate) credentials to the server.
514: The default is
515: .Dq no .
516: Note that this option applies to protocol version 2 only.
1.44 djm 517: .It Cm HashKnownHosts
518: Indicates that
1.84 jmc 519: .Xr ssh 1
1.44 djm 520: should hash host names and addresses when they are added to
1.50 djm 521: .Pa ~/.ssh/known_hosts .
1.44 djm 522: These hashed names may be used normally by
1.84 jmc 523: .Xr ssh 1
1.44 djm 524: and
1.84 jmc 525: .Xr sshd 8 ,
1.44 djm 526: but they do not reveal identifying information should the file's contents
527: be disclosed.
528: The default is
529: .Dq no .
1.97 jmc 530: Note that existing names and addresses in known hosts files
531: will not be converted automatically,
532: but may be manually hashed using
1.45 djm 533: .Xr ssh-keygen 1 .
1.1 stevesk 534: .It Cm HostbasedAuthentication
535: Specifies whether to try rhosts based authentication with public key
536: authentication.
537: The argument must be
538: .Dq yes
539: or
540: .Dq no .
541: The default is
542: .Dq no .
543: This option applies to protocol version 2 only and
544: is similar to
545: .Cm RhostsRSAAuthentication .
546: .It Cm HostKeyAlgorithms
547: Specifies the protocol version 2 host key algorithms
548: that the client wants to use in order of preference.
549: The default for this option is:
550: .Dq ssh-rsa,ssh-dss .
551: .It Cm HostKeyAlias
552: Specifies an alias that should be used instead of the
553: real host name when looking up or saving the host key
554: in the host key database files.
1.84 jmc 555: This option is useful for tunneling SSH connections
1.1 stevesk 556: or for multiple servers running on a single host.
557: .It Cm HostName
558: Specifies the real host name to log into.
559: This can be used to specify nicknames or abbreviations for hosts.
1.136 djm 560: If the hostname contains the character sequence
561: .Ql %h ,
562: then this will be replaced with the host name specified on the commandline
563: (this is useful for manipulating unqualified names).
1.84 jmc 564: The default is the name given on the command line.
1.1 stevesk 565: Numeric IP addresses are also permitted (both on the command line and in
566: .Cm HostName
567: specifications).
1.29 markus 568: .It Cm IdentitiesOnly
569: Specifies that
1.84 jmc 570: .Xr ssh 1
1.29 markus 571: should only use the authentication identity files configured in the
1.31 jmc 572: .Nm
1.29 markus 573: files,
1.84 jmc 574: even if
575: .Xr ssh-agent 1
1.29 markus 576: offers more identities.
577: The argument to this keyword must be
578: .Dq yes
579: or
580: .Dq no .
1.84 jmc 581: This option is intended for situations where ssh-agent
1.29 markus 582: offers many different identities.
583: The default is
584: .Dq no .
1.67 jmc 585: .It Cm IdentityFile
586: Specifies a file from which the user's RSA or DSA authentication identity
587: is read.
588: The default is
589: .Pa ~/.ssh/identity
590: for protocol version 1, and
591: .Pa ~/.ssh/id_rsa
592: and
593: .Pa ~/.ssh/id_dsa
594: for protocol version 2.
595: Additionally, any identities represented by the authentication agent
596: will be used for authentication.
1.129 djm 597: .Xr ssh 1
598: will try to load certificate information from the filename obtained by
599: appending
600: .Pa -cert.pub
601: to the path of a specified
602: .Cm IdentityFile .
1.90 djm 603: .Pp
1.67 jmc 604: The file name may use the tilde
1.91 jmc 605: syntax to refer to a user's home directory or one of the following
1.90 djm 606: escape characters:
607: .Ql %d
608: (local user's home directory),
609: .Ql %u
610: (local user name),
611: .Ql %l
612: (local host name),
613: .Ql %h
614: (remote host name) or
1.92 djm 615: .Ql %r
1.90 djm 616: (remote user name).
617: .Pp
1.67 jmc 618: It is possible to have
619: multiple identity files specified in configuration files; all these
620: identities will be tried in sequence.
1.103 djm 621: .It Cm KbdInteractiveAuthentication
622: Specifies whether to use keyboard-interactive authentication.
623: The argument to this keyword must be
624: .Dq yes
625: or
626: .Dq no .
627: The default is
628: .Dq yes .
1.39 djm 629: .It Cm KbdInteractiveDevices
630: Specifies the list of methods to use in keyboard-interactive authentication.
631: Multiple method names must be comma-separated.
632: The default is to use the server specified list.
1.85 jmc 633: The methods available vary depending on what the server supports.
634: For an OpenSSH server,
635: it may be zero or more of:
636: .Dq bsdauth ,
637: .Dq pam ,
638: and
639: .Dq skey .
1.65 reyk 640: .It Cm LocalCommand
641: Specifies a command to execute on the local machine after successfully
642: connecting to the server.
643: The command string extends to the end of the line, and is executed with
1.105 jmc 644: the user's shell.
1.109 dtucker 645: The following escape character substitutions will be performed:
646: .Ql %d
647: (local user's home directory),
648: .Ql %h
649: (remote host name),
650: .Ql %l
651: (local host name),
652: .Ql %n
653: (host name as provided on the command line),
654: .Ql %p
655: (remote port),
656: .Ql %r
657: (remote user name) or
658: .Ql %u
659: (local user name).
1.123 djm 660: .Pp
661: The command is run synchronously and does not have access to the
662: session of the
663: .Xr ssh 1
664: that spawned it.
665: It should not be used for interactive commands.
666: .Pp
1.65 reyk 667: This directive is ignored unless
668: .Cm PermitLocalCommand
669: has been enabled.
1.1 stevesk 670: .It Cm LocalForward
1.74 jmc 671: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 672: the secure channel to the specified host and port from the remote machine.
1.49 jmc 673: The first argument must be
1.43 djm 674: .Sm off
1.49 jmc 675: .Oo Ar bind_address : Oc Ar port
1.43 djm 676: .Sm on
1.49 jmc 677: and the second argument must be
678: .Ar host : Ns Ar hostport .
1.138 ! djm 679: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.46 jmc 680: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 681: given on the command line.
1.1 stevesk 682: Only the superuser can forward privileged ports.
1.43 djm 683: By default, the local port is bound in accordance with the
684: .Cm GatewayPorts
685: setting.
686: However, an explicit
687: .Ar bind_address
688: may be used to bind the connection to a specific address.
689: The
690: .Ar bind_address
691: of
692: .Dq localhost
1.46 jmc 693: indicates that the listening port be bound for local use only, while an
694: empty address or
695: .Sq *
1.43 djm 696: indicates that the port should be available from all interfaces.
1.1 stevesk 697: .It Cm LogLevel
698: Gives the verbosity level that is used when logging messages from
1.84 jmc 699: .Xr ssh 1 .
1.1 stevesk 700: The possible values are:
1.84 jmc 701: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 702: The default is INFO.
703: DEBUG and DEBUG1 are equivalent.
704: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 705: .It Cm MACs
706: Specifies the MAC (message authentication code) algorithms
707: in order of preference.
708: The MAC algorithm is used in protocol version 2
709: for data integrity protection.
710: Multiple algorithms must be comma-separated.
1.84 jmc 711: The default is:
1.101 jmc 712: .Bd -literal -offset indent
713: hmac-md5,hmac-sha1,umac-64@openssh.com,
714: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
715: .Ed
1.1 stevesk 716: .It Cm NoHostAuthenticationForLocalhost
717: This option can be used if the home directory is shared across machines.
718: In this case localhost will refer to a different machine on each of
719: the machines and the user will get many warnings about changed host keys.
720: However, this option disables host authentication for localhost.
721: The argument to this keyword must be
722: .Dq yes
723: or
724: .Dq no .
725: The default is to check the host key for localhost.
726: .It Cm NumberOfPasswordPrompts
727: Specifies the number of password prompts before giving up.
728: The argument to this keyword must be an integer.
1.84 jmc 729: The default is 3.
1.1 stevesk 730: .It Cm PasswordAuthentication
731: Specifies whether to use password authentication.
732: The argument to this keyword must be
733: .Dq yes
734: or
735: .Dq no .
736: The default is
737: .Dq yes .
1.65 reyk 738: .It Cm PermitLocalCommand
739: Allow local command execution via the
740: .Ic LocalCommand
741: option or using the
1.66 jmc 742: .Ic !\& Ns Ar command
1.65 reyk 743: escape sequence in
744: .Xr ssh 1 .
745: The argument must be
746: .Dq yes
747: or
748: .Dq no .
749: The default is
750: .Dq no .
1.127 markus 751: .It Cm PKCS11Provider
752: Specifies which PKCS#11 provider to use.
753: The argument to this keyword is the PKCS#11 shared libary
754: .Xr ssh 1
1.128 markus 755: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 756: private RSA key.
1.67 jmc 757: .It Cm Port
758: Specifies the port number to connect on the remote host.
1.84 jmc 759: The default is 22.
1.1 stevesk 760: .It Cm PreferredAuthentications
761: Specifies the order in which the client should try protocol 2
1.11 jmc 762: authentication methods.
1.48 jmc 763: This allows a client to prefer one method (e.g.\&
1.1 stevesk 764: .Cm keyboard-interactive )
1.48 jmc 765: over another method (e.g.\&
1.131 jmc 766: .Cm password ) .
767: The default is:
768: .Bd -literal -offset indent
769: gssapi-with-mic,hostbased,publickey,
770: keyboard-interactive,password
771: .Ed
1.1 stevesk 772: .It Cm Protocol
773: Specifies the protocol versions
1.84 jmc 774: .Xr ssh 1
1.1 stevesk 775: should support in order of preference.
776: The possible values are
1.84 jmc 777: .Sq 1
1.1 stevesk 778: and
1.84 jmc 779: .Sq 2 .
1.1 stevesk 780: Multiple versions must be comma-separated.
1.120 markus 781: When this option is set to
1.121 jmc 782: .Dq 2,1
1.120 markus 783: .Nm ssh
784: will try version 2 and fall back to version 1
785: if version 2 is not available.
1.1 stevesk 786: The default is
1.121 jmc 787: .Sq 2 .
1.1 stevesk 788: .It Cm ProxyCommand
789: Specifies the command to use to connect to the server.
790: The command
791: string extends to the end of the line, and is executed with
1.105 jmc 792: the user's shell.
1.133 jmc 793: In the command string, any occurrence of
1.1 stevesk 794: .Ql %h
795: will be substituted by the host name to
1.132 djm 796: connect,
1.1 stevesk 797: .Ql %p
1.133 jmc 798: by the port, and
799: .Ql %r
1.132 djm 800: by the remote user name.
1.1 stevesk 801: The command can be basically anything,
802: and should read from its standard input and write to its standard output.
803: It should eventually connect an
804: .Xr sshd 8
805: server running on some machine, or execute
806: .Ic sshd -i
807: somewhere.
808: Host key management will be done using the
809: HostName of the host being connected (defaulting to the name typed by
810: the user).
1.7 jmc 811: Setting the command to
812: .Dq none
1.6 markus 813: disables this option entirely.
1.1 stevesk 814: Note that
815: .Cm CheckHostIP
816: is not available for connects with a proxy command.
1.52 djm 817: .Pp
818: This directive is useful in conjunction with
819: .Xr nc 1
820: and its proxy support.
1.53 jmc 821: For example, the following directive would connect via an HTTP proxy at
1.52 djm 822: 192.0.2.0:
823: .Bd -literal -offset 3n
824: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
825: .Ed
1.1 stevesk 826: .It Cm PubkeyAuthentication
827: Specifies whether to try public key authentication.
828: The argument to this keyword must be
829: .Dq yes
830: or
831: .Dq no .
832: The default is
833: .Dq yes .
834: This option applies to protocol version 2 only.
1.75 dtucker 835: .It Cm RekeyLimit
836: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 837: session key is renegotiated.
1.75 dtucker 838: The argument is the number of bytes, with an optional suffix of
1.76 jmc 839: .Sq K ,
840: .Sq M ,
1.75 dtucker 841: or
1.76 jmc 842: .Sq G
1.75 dtucker 843: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
844: The default is between
1.84 jmc 845: .Sq 1G
1.75 dtucker 846: and
1.84 jmc 847: .Sq 4G ,
1.75 dtucker 848: depending on the cipher.
1.76 jmc 849: This option applies to protocol version 2 only.
1.1 stevesk 850: .It Cm RemoteForward
1.74 jmc 851: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 852: the secure channel to the specified host and port from the local machine.
1.49 jmc 853: The first argument must be
1.43 djm 854: .Sm off
1.49 jmc 855: .Oo Ar bind_address : Oc Ar port
1.43 djm 856: .Sm on
1.49 jmc 857: and the second argument must be
858: .Ar host : Ns Ar hostport .
1.138 ! djm 859: IPv6 addresses can be specified by enclosing addresses in square brackets.
1.1 stevesk 860: Multiple forwardings may be specified, and additional
861: forwardings can be given on the command line.
1.113 stevesk 862: Privileged ports can be forwarded only when
863: logging in as root on the remote machine.
1.118 jmc 864: .Pp
1.117 djm 865: If the
866: .Ar port
867: argument is
868: .Ql 0 ,
869: the listen port will be dynamically allocated on the server and reported
870: to the client at run time.
1.43 djm 871: .Pp
872: If the
873: .Ar bind_address
874: is not specified, the default is to only bind to loopback addresses.
875: If the
876: .Ar bind_address
877: is
878: .Ql *
879: or an empty string, then the forwarding is requested to listen on all
880: interfaces.
881: Specifying a remote
882: .Ar bind_address
1.46 jmc 883: will only succeed if the server's
884: .Cm GatewayPorts
1.43 djm 885: option is enabled (see
1.46 jmc 886: .Xr sshd_config 5 ) .
1.1 stevesk 887: .It Cm RhostsRSAAuthentication
888: Specifies whether to try rhosts based authentication with RSA host
889: authentication.
890: The argument must be
891: .Dq yes
892: or
893: .Dq no .
894: The default is
895: .Dq no .
896: This option applies to protocol version 1 only and requires
1.84 jmc 897: .Xr ssh 1
1.1 stevesk 898: to be setuid root.
899: .It Cm RSAAuthentication
900: Specifies whether to try RSA authentication.
901: The argument to this keyword must be
902: .Dq yes
903: or
904: .Dq no .
905: RSA authentication will only be
906: attempted if the identity file exists, or an authentication agent is
907: running.
908: The default is
909: .Dq yes .
910: Note that this option applies to protocol version 1 only.
1.32 djm 911: .It Cm SendEnv
912: Specifies what variables from the local
913: .Xr environ 7
914: should be sent to the server.
1.84 jmc 915: Note that environment passing is only supported for protocol 2.
916: The server must also support it, and the server must be configured to
1.33 djm 917: accept these environment variables.
1.32 djm 918: Refer to
919: .Cm AcceptEnv
920: in
921: .Xr sshd_config 5
922: for how to configure the server.
1.80 jmc 923: Variables are specified by name, which may contain wildcard characters.
1.33 djm 924: Multiple environment variables may be separated by whitespace or spread
1.32 djm 925: across multiple
926: .Cm SendEnv
927: directives.
928: The default is not to send any environment variables.
1.81 jmc 929: .Pp
930: See
931: .Sx PATTERNS
932: for more information on patterns.
1.28 markus 933: .It Cm ServerAliveCountMax
1.73 jmc 934: Sets the number of server alive messages (see below) which may be
1.28 markus 935: sent without
1.84 jmc 936: .Xr ssh 1
1.28 markus 937: receiving any messages back from the server.
938: If this threshold is reached while server alive messages are being sent,
1.84 jmc 939: ssh will disconnect from the server, terminating the session.
1.28 markus 940: It is important to note that the use of server alive messages is very
941: different from
942: .Cm TCPKeepAlive
943: (below).
944: The server alive messages are sent through the encrypted channel
945: and therefore will not be spoofable.
946: The TCP keepalive option enabled by
947: .Cm TCPKeepAlive
948: is spoofable.
949: The server alive mechanism is valuable when the client or
950: server depend on knowing when a connection has become inactive.
951: .Pp
952: The default value is 3.
953: If, for example,
954: .Cm ServerAliveInterval
1.84 jmc 955: (see below) is set to 15 and
1.28 markus 956: .Cm ServerAliveCountMax
1.84 jmc 957: is left at the default, if the server becomes unresponsive,
958: ssh will disconnect after approximately 45 seconds.
1.89 markus 959: This option applies to protocol version 2 only.
1.67 jmc 960: .It Cm ServerAliveInterval
961: Sets a timeout interval in seconds after which if no data has been received
962: from the server,
1.84 jmc 963: .Xr ssh 1
1.67 jmc 964: will send a message through the encrypted
965: channel to request a response from the server.
966: The default
967: is 0, indicating that these messages will not be sent to the server.
968: This option applies to protocol version 2 only.
1.1 stevesk 969: .It Cm StrictHostKeyChecking
970: If this flag is set to
971: .Dq yes ,
1.84 jmc 972: .Xr ssh 1
1.1 stevesk 973: will never automatically add host keys to the
1.50 djm 974: .Pa ~/.ssh/known_hosts
1.1 stevesk 975: file, and refuses to connect to hosts whose host key has changed.
976: This provides maximum protection against trojan horse attacks,
1.84 jmc 977: though it can be annoying when the
1.1 stevesk 978: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 979: file is poorly maintained or when connections to new hosts are
1.1 stevesk 980: frequently made.
981: This option forces the user to manually
982: add all new hosts.
983: If this flag is set to
984: .Dq no ,
1.84 jmc 985: ssh will automatically add new host keys to the
1.1 stevesk 986: user known hosts files.
987: If this flag is set to
988: .Dq ask ,
989: new host keys
990: will be added to the user known host files only after the user
991: has confirmed that is what they really want to do, and
1.84 jmc 992: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 993: The host keys of
994: known hosts will be verified automatically in all cases.
995: The argument must be
996: .Dq yes ,
1.84 jmc 997: .Dq no ,
1.1 stevesk 998: or
999: .Dq ask .
1000: The default is
1001: .Dq ask .
1.26 markus 1002: .It Cm TCPKeepAlive
1003: Specifies whether the system should send TCP keepalive messages to the
1004: other side.
1005: If they are sent, death of the connection or crash of one
1006: of the machines will be properly noticed.
1007: However, this means that
1008: connections will die if the route is down temporarily, and some people
1009: find it annoying.
1010: .Pp
1011: The default is
1012: .Dq yes
1013: (to send TCP keepalive messages), and the client will notice
1014: if the network goes down or the remote host dies.
1015: This is important in scripts, and many users want it too.
1016: .Pp
1017: To disable TCP keepalive messages, the value should be set to
1018: .Dq no .
1.65 reyk 1019: .It Cm Tunnel
1.95 stevesk 1020: Request
1.65 reyk 1021: .Xr tun 4
1.69 jmc 1022: device forwarding between the client and the server.
1.65 reyk 1023: The argument must be
1.68 reyk 1024: .Dq yes ,
1.95 stevesk 1025: .Dq point-to-point
1026: (layer 3),
1027: .Dq ethernet
1028: (layer 2),
1.65 reyk 1029: or
1030: .Dq no .
1.95 stevesk 1031: Specifying
1032: .Dq yes
1033: requests the default tunnel mode, which is
1034: .Dq point-to-point .
1.65 reyk 1035: The default is
1036: .Dq no .
1037: .It Cm TunnelDevice
1.95 stevesk 1038: Specifies the
1.65 reyk 1039: .Xr tun 4
1.95 stevesk 1040: devices to open on the client
1041: .Pq Ar local_tun
1042: and the server
1043: .Pq Ar remote_tun .
1044: .Pp
1045: The argument must be
1046: .Sm off
1047: .Ar local_tun Op : Ar remote_tun .
1048: .Sm on
1049: The devices may be specified by numerical ID or the keyword
1050: .Dq any ,
1051: which uses the next available tunnel device.
1052: If
1053: .Ar remote_tun
1054: is not specified, it defaults to
1055: .Dq any .
1056: The default is
1057: .Dq any:any .
1.72 jmc 1058: .It Cm UsePrivilegedPort
1059: Specifies whether to use a privileged port for outgoing connections.
1060: The argument must be
1061: .Dq yes
1062: or
1063: .Dq no .
1064: The default is
1065: .Dq no .
1066: If set to
1.84 jmc 1067: .Dq yes ,
1068: .Xr ssh 1
1.72 jmc 1069: must be setuid root.
1070: Note that this option must be set to
1071: .Dq yes
1072: for
1073: .Cm RhostsRSAAuthentication
1074: with older servers.
1.1 stevesk 1075: .It Cm User
1076: Specifies the user to log in as.
1077: This can be useful when a different user name is used on different machines.
1078: This saves the trouble of
1079: having to remember to give the user name on the command line.
1080: .It Cm UserKnownHostsFile
1081: Specifies a file to use for the user
1082: host key database instead of
1.50 djm 1083: .Pa ~/.ssh/known_hosts .
1.8 jakob 1084: .It Cm VerifyHostKeyDNS
1085: Specifies whether to verify the remote key using DNS and SSHFP resource
1086: records.
1.24 jakob 1087: If this option is set to
1088: .Dq yes ,
1.25 jmc 1089: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1090: from DNS.
1091: Insecure fingerprints will be handled as if this option was set to
1092: .Dq ask .
1093: If this option is set to
1094: .Dq ask ,
1095: information on fingerprint match will be displayed, but the user will still
1096: need to confirm new host keys according to the
1097: .Cm StrictHostKeyChecking
1098: option.
1099: The argument must be
1100: .Dq yes ,
1.84 jmc 1101: .Dq no ,
1.25 jmc 1102: or
1103: .Dq ask .
1.8 jakob 1104: The default is
1105: .Dq no .
1.12 jakob 1106: Note that this option applies to protocol version 2 only.
1.84 jmc 1107: .Pp
1108: See also
1109: .Sx VERIFYING HOST KEYS
1110: in
1111: .Xr ssh 1 .
1.111 grunk 1112: .It Cm VisualHostKey
1113: If this flag is set to
1114: .Dq yes ,
1115: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1116: printed in addition to the hex fingerprint string at login and
1117: for unknown host keys.
1.111 grunk 1118: If this flag is set to
1119: .Dq no ,
1.114 stevesk 1120: no fingerprint strings are printed at login and
1121: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1122: The default is
1123: .Dq no .
1.1 stevesk 1124: .It Cm XAuthLocation
1.5 stevesk 1125: Specifies the full pathname of the
1.1 stevesk 1126: .Xr xauth 1
1127: program.
1128: The default is
1129: .Pa /usr/X11R6/bin/xauth .
1130: .El
1.86 jmc 1131: .Sh PATTERNS
1132: A
1133: .Em pattern
1134: consists of zero or more non-whitespace characters,
1135: .Sq *
1136: (a wildcard that matches zero or more characters),
1137: or
1138: .Sq ?\&
1139: (a wildcard that matches exactly one character).
1140: For example, to specify a set of declarations for any host in the
1141: .Dq .co.uk
1142: set of domains,
1143: the following pattern could be used:
1144: .Pp
1145: .Dl Host *.co.uk
1146: .Pp
1147: The following pattern
1148: would match any host in the 192.168.0.[0-9] network range:
1149: .Pp
1150: .Dl Host 192.168.0.?
1151: .Pp
1152: A
1153: .Em pattern-list
1154: is a comma-separated list of patterns.
1155: Patterns within pattern-lists may be negated
1156: by preceding them with an exclamation mark
1157: .Pq Sq !\& .
1158: For example,
1159: to allow a key to be used from anywhere within an organisation
1160: except from the
1161: .Dq dialup
1162: pool,
1163: the following entry (in authorized_keys) could be used:
1164: .Pp
1165: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1166: .Sh FILES
1167: .Bl -tag -width Ds
1.50 djm 1168: .It Pa ~/.ssh/config
1.1 stevesk 1169: This is the per-user configuration file.
1170: The format of this file is described above.
1.84 jmc 1171: This file is used by the SSH client.
1.30 djm 1172: Because of the potential for abuse, this file must have strict permissions:
1173: read/write for the user, and not accessible by others.
1.1 stevesk 1174: .It Pa /etc/ssh/ssh_config
1175: Systemwide configuration file.
1176: This file provides defaults for those
1177: values that are not specified in the user's configuration file, and
1178: for those users who do not have a configuration file.
1179: This file must be world-readable.
1180: .El
1.13 jmc 1181: .Sh SEE ALSO
1182: .Xr ssh 1
1.1 stevesk 1183: .Sh AUTHORS
1184: OpenSSH is a derivative of the original and free
1185: ssh 1.2.12 release by Tatu Ylonen.
1186: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1187: Theo de Raadt and Dug Song
1188: removed many bugs, re-added newer features and
1189: created OpenSSH.
1190: Markus Friedl contributed the support for SSH
1191: protocol versions 1.5 and 2.0.
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