Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.135
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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1.135 ! jmc 37: .\" $OpenBSD: ssh_config.5,v 1.134 2010/06/25 23:15:36 djm Exp $
! 38: .Dd $Mdocdate: June 25 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.
1.38 jmc 322: .It Cm DynamicForward
1.74 jmc 323: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 324: over the secure channel, and the application
325: protocol is then used to determine where to connect to from the
326: remote machine.
1.62 djm 327: .Pp
328: The argument must be
329: .Sm off
330: .Oo Ar bind_address : Oc Ar port .
331: .Sm on
332: IPv6 addresses can be specified by enclosing addresses in square brackets or
333: by using an alternative syntax:
334: .Oo Ar bind_address Ns / Oc Ns Ar port .
335: By default, the local port is bound in accordance with the
336: .Cm GatewayPorts
337: setting.
338: However, an explicit
339: .Ar bind_address
340: may be used to bind the connection to a specific address.
341: The
342: .Ar bind_address
343: of
344: .Dq localhost
345: indicates that the listening port be bound for local use only, while an
346: empty address or
347: .Sq *
348: indicates that the port should be available from all interfaces.
349: .Pp
1.38 jmc 350: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 351: .Xr ssh 1
1.38 jmc 352: will act as a SOCKS server.
353: Multiple forwardings may be specified, and
354: additional forwardings can be given on the command line.
355: Only the superuser can forward privileged ports.
1.14 markus 356: .It Cm EnableSSHKeysign
357: Setting this option to
358: .Dq yes
359: in the global client configuration file
360: .Pa /etc/ssh/ssh_config
361: enables the use of the helper program
362: .Xr ssh-keysign 8
363: during
364: .Cm HostbasedAuthentication .
365: The argument must be
366: .Dq yes
367: or
368: .Dq no .
369: The default is
370: .Dq no .
1.23 jmc 371: This option should be placed in the non-hostspecific section.
1.14 markus 372: See
373: .Xr ssh-keysign 8
374: for more information.
1.1 stevesk 375: .It Cm EscapeChar
376: Sets the escape character (default:
377: .Ql ~ ) .
378: The escape character can also
379: be set on the command line.
380: The argument should be a single character,
381: .Ql ^
382: followed by a letter, or
383: .Dq none
384: to disable the escape
385: character entirely (making the connection transparent for binary
386: data).
1.96 markus 387: .It Cm ExitOnForwardFailure
388: Specifies whether
389: .Xr ssh 1
390: should terminate the connection if it cannot set up all requested
1.102 stevesk 391: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 392: The argument must be
393: .Dq yes
394: or
395: .Dq no .
396: The default is
397: .Dq no .
1.1 stevesk 398: .It Cm ForwardAgent
399: Specifies whether the connection to the authentication agent (if any)
400: will be forwarded to the remote machine.
401: The argument must be
402: .Dq yes
403: or
404: .Dq no .
405: The default is
406: .Dq no .
1.3 stevesk 407: .Pp
1.7 jmc 408: Agent forwarding should be enabled with caution.
409: Users with the ability to bypass file permissions on the remote host
410: (for the agent's Unix-domain socket)
411: can access the local agent through the forwarded connection.
412: An attacker cannot obtain key material from the agent,
1.3 stevesk 413: however they can perform operations on the keys that enable them to
414: authenticate using the identities loaded into the agent.
1.1 stevesk 415: .It Cm ForwardX11
416: Specifies whether X11 connections will be automatically redirected
417: over the secure channel and
418: .Ev DISPLAY
419: set.
420: The argument must be
421: .Dq yes
422: or
423: .Dq no .
424: The default is
425: .Dq no .
1.3 stevesk 426: .Pp
1.7 jmc 427: X11 forwarding should be enabled with caution.
428: Users with the ability to bypass file permissions on the remote host
1.22 markus 429: (for the user's X11 authorization database)
1.7 jmc 430: can access the local X11 display through the forwarded connection.
1.22 markus 431: An attacker may then be able to perform activities such as keystroke monitoring
432: if the
433: .Cm ForwardX11Trusted
434: option is also enabled.
1.134 djm 435: .It Cm ForwardX11Timeout
1.135 ! jmc 436: Specify a timeout for untrusted X11 forwarding
! 437: using the format described in the
1.134 djm 438: .Sx TIME FORMATS
439: section of
440: .Xr sshd_config 5 .
441: X11 connections received by
442: .Xr ssh 1
443: after this time will be refused.
444: The default is to disable untrusted X11 forwarding after twenty minutes has
445: elapsed.
1.22 markus 446: .It Cm ForwardX11Trusted
1.34 jmc 447: If this option is set to
1.84 jmc 448: .Dq yes ,
449: remote X11 clients will have full access to the original X11 display.
1.42 djm 450: .Pp
1.22 markus 451: If this option is set to
1.84 jmc 452: .Dq no ,
453: remote X11 clients will be considered untrusted and prevented
1.22 markus 454: from stealing or tampering with data belonging to trusted X11
455: clients.
1.42 djm 456: Furthermore, the
457: .Xr xauth 1
458: token used for the session will be set to expire after 20 minutes.
459: Remote clients will be refused access after this time.
1.22 markus 460: .Pp
461: The default is
462: .Dq no .
463: .Pp
464: See the X11 SECURITY extension specification for full details on
465: the restrictions imposed on untrusted clients.
1.1 stevesk 466: .It Cm GatewayPorts
467: Specifies whether remote hosts are allowed to connect to local
468: forwarded ports.
469: By default,
1.84 jmc 470: .Xr ssh 1
1.7 jmc 471: binds local port forwardings to the loopback address.
472: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 473: .Cm GatewayPorts
1.84 jmc 474: can be used to specify that ssh
1.1 stevesk 475: should bind local port forwardings to the wildcard address,
476: thus allowing remote hosts to connect to forwarded ports.
477: The argument must be
478: .Dq yes
479: or
480: .Dq no .
481: The default is
482: .Dq no .
483: .It Cm GlobalKnownHostsFile
484: Specifies a file to use for the global
485: host key database instead of
486: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 487: .It Cm GSSAPIAuthentication
1.27 markus 488: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 489: The default is
1.21 markus 490: .Dq no .
1.18 markus 491: Note that this option applies to protocol version 2 only.
492: .It Cm GSSAPIDelegateCredentials
493: Forward (delegate) credentials to the server.
494: The default is
495: .Dq no .
496: Note that this option applies to protocol version 2 only.
1.44 djm 497: .It Cm HashKnownHosts
498: Indicates that
1.84 jmc 499: .Xr ssh 1
1.44 djm 500: should hash host names and addresses when they are added to
1.50 djm 501: .Pa ~/.ssh/known_hosts .
1.44 djm 502: These hashed names may be used normally by
1.84 jmc 503: .Xr ssh 1
1.44 djm 504: and
1.84 jmc 505: .Xr sshd 8 ,
1.44 djm 506: but they do not reveal identifying information should the file's contents
507: be disclosed.
508: The default is
509: .Dq no .
1.97 jmc 510: Note that existing names and addresses in known hosts files
511: will not be converted automatically,
512: but may be manually hashed using
1.45 djm 513: .Xr ssh-keygen 1 .
1.1 stevesk 514: .It Cm HostbasedAuthentication
515: Specifies whether to try rhosts based authentication with public key
516: authentication.
517: The argument must be
518: .Dq yes
519: or
520: .Dq no .
521: The default is
522: .Dq no .
523: This option applies to protocol version 2 only and
524: is similar to
525: .Cm RhostsRSAAuthentication .
526: .It Cm HostKeyAlgorithms
527: Specifies the protocol version 2 host key algorithms
528: that the client wants to use in order of preference.
529: The default for this option is:
530: .Dq ssh-rsa,ssh-dss .
531: .It Cm HostKeyAlias
532: Specifies an alias that should be used instead of the
533: real host name when looking up or saving the host key
534: in the host key database files.
1.84 jmc 535: This option is useful for tunneling SSH connections
1.1 stevesk 536: or for multiple servers running on a single host.
537: .It Cm HostName
538: Specifies the real host name to log into.
539: This can be used to specify nicknames or abbreviations for hosts.
1.84 jmc 540: The default is the name given on the command line.
1.1 stevesk 541: Numeric IP addresses are also permitted (both on the command line and in
542: .Cm HostName
543: specifications).
1.29 markus 544: .It Cm IdentitiesOnly
545: Specifies that
1.84 jmc 546: .Xr ssh 1
1.29 markus 547: should only use the authentication identity files configured in the
1.31 jmc 548: .Nm
1.29 markus 549: files,
1.84 jmc 550: even if
551: .Xr ssh-agent 1
1.29 markus 552: offers more identities.
553: The argument to this keyword must be
554: .Dq yes
555: or
556: .Dq no .
1.84 jmc 557: This option is intended for situations where ssh-agent
1.29 markus 558: offers many different identities.
559: The default is
560: .Dq no .
1.67 jmc 561: .It Cm IdentityFile
562: Specifies a file from which the user's RSA or DSA authentication identity
563: is read.
564: The default is
565: .Pa ~/.ssh/identity
566: for protocol version 1, and
567: .Pa ~/.ssh/id_rsa
568: and
569: .Pa ~/.ssh/id_dsa
570: for protocol version 2.
571: Additionally, any identities represented by the authentication agent
572: will be used for authentication.
1.129 djm 573: .Xr ssh 1
574: will try to load certificate information from the filename obtained by
575: appending
576: .Pa -cert.pub
577: to the path of a specified
578: .Cm IdentityFile .
1.90 djm 579: .Pp
1.67 jmc 580: The file name may use the tilde
1.91 jmc 581: syntax to refer to a user's home directory or one of the following
1.90 djm 582: escape characters:
583: .Ql %d
584: (local user's home directory),
585: .Ql %u
586: (local user name),
587: .Ql %l
588: (local host name),
589: .Ql %h
590: (remote host name) or
1.92 djm 591: .Ql %r
1.90 djm 592: (remote user name).
593: .Pp
1.67 jmc 594: It is possible to have
595: multiple identity files specified in configuration files; all these
596: identities will be tried in sequence.
1.103 djm 597: .It Cm KbdInteractiveAuthentication
598: Specifies whether to use keyboard-interactive authentication.
599: The argument to this keyword must be
600: .Dq yes
601: or
602: .Dq no .
603: The default is
604: .Dq yes .
1.39 djm 605: .It Cm KbdInteractiveDevices
606: Specifies the list of methods to use in keyboard-interactive authentication.
607: Multiple method names must be comma-separated.
608: The default is to use the server specified list.
1.85 jmc 609: The methods available vary depending on what the server supports.
610: For an OpenSSH server,
611: it may be zero or more of:
612: .Dq bsdauth ,
613: .Dq pam ,
614: and
615: .Dq skey .
1.65 reyk 616: .It Cm LocalCommand
617: Specifies a command to execute on the local machine after successfully
618: connecting to the server.
619: The command string extends to the end of the line, and is executed with
1.105 jmc 620: the user's shell.
1.109 dtucker 621: The following escape character substitutions will be performed:
622: .Ql %d
623: (local user's home directory),
624: .Ql %h
625: (remote host name),
626: .Ql %l
627: (local host name),
628: .Ql %n
629: (host name as provided on the command line),
630: .Ql %p
631: (remote port),
632: .Ql %r
633: (remote user name) or
634: .Ql %u
635: (local user name).
1.123 djm 636: .Pp
637: The command is run synchronously and does not have access to the
638: session of the
639: .Xr ssh 1
640: that spawned it.
641: It should not be used for interactive commands.
642: .Pp
1.65 reyk 643: This directive is ignored unless
644: .Cm PermitLocalCommand
645: has been enabled.
1.1 stevesk 646: .It Cm LocalForward
1.74 jmc 647: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 648: the secure channel to the specified host and port from the remote machine.
1.49 jmc 649: The first argument must be
1.43 djm 650: .Sm off
1.49 jmc 651: .Oo Ar bind_address : Oc Ar port
1.43 djm 652: .Sm on
1.49 jmc 653: and the second argument must be
654: .Ar host : Ns Ar hostport .
1.46 jmc 655: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 656: by using an alternative syntax:
1.49 jmc 657: .Oo Ar bind_address Ns / Oc Ns Ar port
658: and
659: .Ar host Ns / Ns Ar hostport .
1.46 jmc 660: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 661: given on the command line.
1.1 stevesk 662: Only the superuser can forward privileged ports.
1.43 djm 663: By default, the local port is bound in accordance with the
664: .Cm GatewayPorts
665: setting.
666: However, an explicit
667: .Ar bind_address
668: may be used to bind the connection to a specific address.
669: The
670: .Ar bind_address
671: of
672: .Dq localhost
1.46 jmc 673: indicates that the listening port be bound for local use only, while an
674: empty address or
675: .Sq *
1.43 djm 676: indicates that the port should be available from all interfaces.
1.1 stevesk 677: .It Cm LogLevel
678: Gives the verbosity level that is used when logging messages from
1.84 jmc 679: .Xr ssh 1 .
1.1 stevesk 680: The possible values are:
1.84 jmc 681: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 682: The default is INFO.
683: DEBUG and DEBUG1 are equivalent.
684: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 685: .It Cm MACs
686: Specifies the MAC (message authentication code) algorithms
687: in order of preference.
688: The MAC algorithm is used in protocol version 2
689: for data integrity protection.
690: Multiple algorithms must be comma-separated.
1.84 jmc 691: The default is:
1.101 jmc 692: .Bd -literal -offset indent
693: hmac-md5,hmac-sha1,umac-64@openssh.com,
694: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
695: .Ed
1.1 stevesk 696: .It Cm NoHostAuthenticationForLocalhost
697: This option can be used if the home directory is shared across machines.
698: In this case localhost will refer to a different machine on each of
699: the machines and the user will get many warnings about changed host keys.
700: However, this option disables host authentication for localhost.
701: The argument to this keyword must be
702: .Dq yes
703: or
704: .Dq no .
705: The default is to check the host key for localhost.
706: .It Cm NumberOfPasswordPrompts
707: Specifies the number of password prompts before giving up.
708: The argument to this keyword must be an integer.
1.84 jmc 709: The default is 3.
1.1 stevesk 710: .It Cm PasswordAuthentication
711: Specifies whether to use password authentication.
712: The argument to this keyword must be
713: .Dq yes
714: or
715: .Dq no .
716: The default is
717: .Dq yes .
1.65 reyk 718: .It Cm PermitLocalCommand
719: Allow local command execution via the
720: .Ic LocalCommand
721: option or using the
1.66 jmc 722: .Ic !\& Ns Ar command
1.65 reyk 723: escape sequence in
724: .Xr ssh 1 .
725: The argument must be
726: .Dq yes
727: or
728: .Dq no .
729: The default is
730: .Dq no .
1.127 markus 731: .It Cm PKCS11Provider
732: Specifies which PKCS#11 provider to use.
733: The argument to this keyword is the PKCS#11 shared libary
734: .Xr ssh 1
1.128 markus 735: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 736: private RSA key.
1.67 jmc 737: .It Cm Port
738: Specifies the port number to connect on the remote host.
1.84 jmc 739: The default is 22.
1.1 stevesk 740: .It Cm PreferredAuthentications
741: Specifies the order in which the client should try protocol 2
1.11 jmc 742: authentication methods.
1.48 jmc 743: This allows a client to prefer one method (e.g.\&
1.1 stevesk 744: .Cm keyboard-interactive )
1.48 jmc 745: over another method (e.g.\&
1.131 jmc 746: .Cm password ) .
747: The default is:
748: .Bd -literal -offset indent
749: gssapi-with-mic,hostbased,publickey,
750: keyboard-interactive,password
751: .Ed
1.1 stevesk 752: .It Cm Protocol
753: Specifies the protocol versions
1.84 jmc 754: .Xr ssh 1
1.1 stevesk 755: should support in order of preference.
756: The possible values are
1.84 jmc 757: .Sq 1
1.1 stevesk 758: and
1.84 jmc 759: .Sq 2 .
1.1 stevesk 760: Multiple versions must be comma-separated.
1.120 markus 761: When this option is set to
1.121 jmc 762: .Dq 2,1
1.120 markus 763: .Nm ssh
764: will try version 2 and fall back to version 1
765: if version 2 is not available.
1.1 stevesk 766: The default is
1.121 jmc 767: .Sq 2 .
1.1 stevesk 768: .It Cm ProxyCommand
769: Specifies the command to use to connect to the server.
770: The command
771: string extends to the end of the line, and is executed with
1.105 jmc 772: the user's shell.
1.133 jmc 773: In the command string, any occurrence of
1.1 stevesk 774: .Ql %h
775: will be substituted by the host name to
1.132 djm 776: connect,
1.1 stevesk 777: .Ql %p
1.133 jmc 778: by the port, and
779: .Ql %r
1.132 djm 780: by the remote user name.
1.1 stevesk 781: The command can be basically anything,
782: and should read from its standard input and write to its standard output.
783: It should eventually connect an
784: .Xr sshd 8
785: server running on some machine, or execute
786: .Ic sshd -i
787: somewhere.
788: Host key management will be done using the
789: HostName of the host being connected (defaulting to the name typed by
790: the user).
1.7 jmc 791: Setting the command to
792: .Dq none
1.6 markus 793: disables this option entirely.
1.1 stevesk 794: Note that
795: .Cm CheckHostIP
796: is not available for connects with a proxy command.
1.52 djm 797: .Pp
798: This directive is useful in conjunction with
799: .Xr nc 1
800: and its proxy support.
1.53 jmc 801: For example, the following directive would connect via an HTTP proxy at
1.52 djm 802: 192.0.2.0:
803: .Bd -literal -offset 3n
804: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
805: .Ed
1.1 stevesk 806: .It Cm PubkeyAuthentication
807: Specifies whether to try public key authentication.
808: The argument to this keyword must be
809: .Dq yes
810: or
811: .Dq no .
812: The default is
813: .Dq yes .
814: This option applies to protocol version 2 only.
1.75 dtucker 815: .It Cm RekeyLimit
816: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 817: session key is renegotiated.
1.75 dtucker 818: The argument is the number of bytes, with an optional suffix of
1.76 jmc 819: .Sq K ,
820: .Sq M ,
1.75 dtucker 821: or
1.76 jmc 822: .Sq G
1.75 dtucker 823: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
824: The default is between
1.84 jmc 825: .Sq 1G
1.75 dtucker 826: and
1.84 jmc 827: .Sq 4G ,
1.75 dtucker 828: depending on the cipher.
1.76 jmc 829: This option applies to protocol version 2 only.
1.1 stevesk 830: .It Cm RemoteForward
1.74 jmc 831: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 832: the secure channel to the specified host and port from the local machine.
1.49 jmc 833: The first argument must be
1.43 djm 834: .Sm off
1.49 jmc 835: .Oo Ar bind_address : Oc Ar port
1.43 djm 836: .Sm on
1.49 jmc 837: and the second argument must be
838: .Ar host : Ns Ar hostport .
839: IPv6 addresses can be specified by enclosing addresses in square brackets
840: or by using an alternative syntax:
841: .Oo Ar bind_address Ns / Oc Ns Ar port
842: and
843: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 844: Multiple forwardings may be specified, and additional
845: forwardings can be given on the command line.
1.113 stevesk 846: Privileged ports can be forwarded only when
847: logging in as root on the remote machine.
1.118 jmc 848: .Pp
1.117 djm 849: If the
850: .Ar port
851: argument is
852: .Ql 0 ,
853: the listen port will be dynamically allocated on the server and reported
854: to the client at run time.
1.43 djm 855: .Pp
856: If the
857: .Ar bind_address
858: is not specified, the default is to only bind to loopback addresses.
859: If the
860: .Ar bind_address
861: is
862: .Ql *
863: or an empty string, then the forwarding is requested to listen on all
864: interfaces.
865: Specifying a remote
866: .Ar bind_address
1.46 jmc 867: will only succeed if the server's
868: .Cm GatewayPorts
1.43 djm 869: option is enabled (see
1.46 jmc 870: .Xr sshd_config 5 ) .
1.1 stevesk 871: .It Cm RhostsRSAAuthentication
872: Specifies whether to try rhosts based authentication with RSA host
873: authentication.
874: The argument must be
875: .Dq yes
876: or
877: .Dq no .
878: The default is
879: .Dq no .
880: This option applies to protocol version 1 only and requires
1.84 jmc 881: .Xr ssh 1
1.1 stevesk 882: to be setuid root.
883: .It Cm RSAAuthentication
884: Specifies whether to try RSA authentication.
885: The argument to this keyword must be
886: .Dq yes
887: or
888: .Dq no .
889: RSA authentication will only be
890: attempted if the identity file exists, or an authentication agent is
891: running.
892: The default is
893: .Dq yes .
894: Note that this option applies to protocol version 1 only.
1.32 djm 895: .It Cm SendEnv
896: Specifies what variables from the local
897: .Xr environ 7
898: should be sent to the server.
1.84 jmc 899: Note that environment passing is only supported for protocol 2.
900: The server must also support it, and the server must be configured to
1.33 djm 901: accept these environment variables.
1.32 djm 902: Refer to
903: .Cm AcceptEnv
904: in
905: .Xr sshd_config 5
906: for how to configure the server.
1.80 jmc 907: Variables are specified by name, which may contain wildcard characters.
1.33 djm 908: Multiple environment variables may be separated by whitespace or spread
1.32 djm 909: across multiple
910: .Cm SendEnv
911: directives.
912: The default is not to send any environment variables.
1.81 jmc 913: .Pp
914: See
915: .Sx PATTERNS
916: for more information on patterns.
1.28 markus 917: .It Cm ServerAliveCountMax
1.73 jmc 918: Sets the number of server alive messages (see below) which may be
1.28 markus 919: sent without
1.84 jmc 920: .Xr ssh 1
1.28 markus 921: receiving any messages back from the server.
922: If this threshold is reached while server alive messages are being sent,
1.84 jmc 923: ssh will disconnect from the server, terminating the session.
1.28 markus 924: It is important to note that the use of server alive messages is very
925: different from
926: .Cm TCPKeepAlive
927: (below).
928: The server alive messages are sent through the encrypted channel
929: and therefore will not be spoofable.
930: The TCP keepalive option enabled by
931: .Cm TCPKeepAlive
932: is spoofable.
933: The server alive mechanism is valuable when the client or
934: server depend on knowing when a connection has become inactive.
935: .Pp
936: The default value is 3.
937: If, for example,
938: .Cm ServerAliveInterval
1.84 jmc 939: (see below) is set to 15 and
1.28 markus 940: .Cm ServerAliveCountMax
1.84 jmc 941: is left at the default, if the server becomes unresponsive,
942: ssh will disconnect after approximately 45 seconds.
1.89 markus 943: This option applies to protocol version 2 only.
1.67 jmc 944: .It Cm ServerAliveInterval
945: Sets a timeout interval in seconds after which if no data has been received
946: from the server,
1.84 jmc 947: .Xr ssh 1
1.67 jmc 948: will send a message through the encrypted
949: channel to request a response from the server.
950: The default
951: is 0, indicating that these messages will not be sent to the server.
952: This option applies to protocol version 2 only.
1.1 stevesk 953: .It Cm StrictHostKeyChecking
954: If this flag is set to
955: .Dq yes ,
1.84 jmc 956: .Xr ssh 1
1.1 stevesk 957: will never automatically add host keys to the
1.50 djm 958: .Pa ~/.ssh/known_hosts
1.1 stevesk 959: file, and refuses to connect to hosts whose host key has changed.
960: This provides maximum protection against trojan horse attacks,
1.84 jmc 961: though it can be annoying when the
1.1 stevesk 962: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 963: file is poorly maintained or when connections to new hosts are
1.1 stevesk 964: frequently made.
965: This option forces the user to manually
966: add all new hosts.
967: If this flag is set to
968: .Dq no ,
1.84 jmc 969: ssh will automatically add new host keys to the
1.1 stevesk 970: user known hosts files.
971: If this flag is set to
972: .Dq ask ,
973: new host keys
974: will be added to the user known host files only after the user
975: has confirmed that is what they really want to do, and
1.84 jmc 976: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 977: The host keys of
978: known hosts will be verified automatically in all cases.
979: The argument must be
980: .Dq yes ,
1.84 jmc 981: .Dq no ,
1.1 stevesk 982: or
983: .Dq ask .
984: The default is
985: .Dq ask .
1.26 markus 986: .It Cm TCPKeepAlive
987: Specifies whether the system should send TCP keepalive messages to the
988: other side.
989: If they are sent, death of the connection or crash of one
990: of the machines will be properly noticed.
991: However, this means that
992: connections will die if the route is down temporarily, and some people
993: find it annoying.
994: .Pp
995: The default is
996: .Dq yes
997: (to send TCP keepalive messages), and the client will notice
998: if the network goes down or the remote host dies.
999: This is important in scripts, and many users want it too.
1000: .Pp
1001: To disable TCP keepalive messages, the value should be set to
1002: .Dq no .
1.65 reyk 1003: .It Cm Tunnel
1.95 stevesk 1004: Request
1.65 reyk 1005: .Xr tun 4
1.69 jmc 1006: device forwarding between the client and the server.
1.65 reyk 1007: The argument must be
1.68 reyk 1008: .Dq yes ,
1.95 stevesk 1009: .Dq point-to-point
1010: (layer 3),
1011: .Dq ethernet
1012: (layer 2),
1.65 reyk 1013: or
1014: .Dq no .
1.95 stevesk 1015: Specifying
1016: .Dq yes
1017: requests the default tunnel mode, which is
1018: .Dq point-to-point .
1.65 reyk 1019: The default is
1020: .Dq no .
1021: .It Cm TunnelDevice
1.95 stevesk 1022: Specifies the
1.65 reyk 1023: .Xr tun 4
1.95 stevesk 1024: devices to open on the client
1025: .Pq Ar local_tun
1026: and the server
1027: .Pq Ar remote_tun .
1028: .Pp
1029: The argument must be
1030: .Sm off
1031: .Ar local_tun Op : Ar remote_tun .
1032: .Sm on
1033: The devices may be specified by numerical ID or the keyword
1034: .Dq any ,
1035: which uses the next available tunnel device.
1036: If
1037: .Ar remote_tun
1038: is not specified, it defaults to
1039: .Dq any .
1040: The default is
1041: .Dq any:any .
1.72 jmc 1042: .It Cm UsePrivilegedPort
1043: Specifies whether to use a privileged port for outgoing connections.
1044: The argument must be
1045: .Dq yes
1046: or
1047: .Dq no .
1048: The default is
1049: .Dq no .
1050: If set to
1.84 jmc 1051: .Dq yes ,
1052: .Xr ssh 1
1.72 jmc 1053: must be setuid root.
1054: Note that this option must be set to
1055: .Dq yes
1056: for
1057: .Cm RhostsRSAAuthentication
1058: with older servers.
1.1 stevesk 1059: .It Cm User
1060: Specifies the user to log in as.
1061: This can be useful when a different user name is used on different machines.
1062: This saves the trouble of
1063: having to remember to give the user name on the command line.
1064: .It Cm UserKnownHostsFile
1065: Specifies a file to use for the user
1066: host key database instead of
1.50 djm 1067: .Pa ~/.ssh/known_hosts .
1.8 jakob 1068: .It Cm VerifyHostKeyDNS
1069: Specifies whether to verify the remote key using DNS and SSHFP resource
1070: records.
1.24 jakob 1071: If this option is set to
1072: .Dq yes ,
1.25 jmc 1073: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1074: from DNS.
1075: Insecure fingerprints will be handled as if this option was set to
1076: .Dq ask .
1077: If this option is set to
1078: .Dq ask ,
1079: information on fingerprint match will be displayed, but the user will still
1080: need to confirm new host keys according to the
1081: .Cm StrictHostKeyChecking
1082: option.
1083: The argument must be
1084: .Dq yes ,
1.84 jmc 1085: .Dq no ,
1.25 jmc 1086: or
1087: .Dq ask .
1.8 jakob 1088: The default is
1089: .Dq no .
1.12 jakob 1090: Note that this option applies to protocol version 2 only.
1.84 jmc 1091: .Pp
1092: See also
1093: .Sx VERIFYING HOST KEYS
1094: in
1095: .Xr ssh 1 .
1.111 grunk 1096: .It Cm VisualHostKey
1097: If this flag is set to
1098: .Dq yes ,
1099: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1100: printed in addition to the hex fingerprint string at login and
1101: for unknown host keys.
1.111 grunk 1102: If this flag is set to
1103: .Dq no ,
1.114 stevesk 1104: no fingerprint strings are printed at login and
1105: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1106: The default is
1107: .Dq no .
1.1 stevesk 1108: .It Cm XAuthLocation
1.5 stevesk 1109: Specifies the full pathname of the
1.1 stevesk 1110: .Xr xauth 1
1111: program.
1112: The default is
1113: .Pa /usr/X11R6/bin/xauth .
1114: .El
1.86 jmc 1115: .Sh PATTERNS
1116: A
1117: .Em pattern
1118: consists of zero or more non-whitespace characters,
1119: .Sq *
1120: (a wildcard that matches zero or more characters),
1121: or
1122: .Sq ?\&
1123: (a wildcard that matches exactly one character).
1124: For example, to specify a set of declarations for any host in the
1125: .Dq .co.uk
1126: set of domains,
1127: the following pattern could be used:
1128: .Pp
1129: .Dl Host *.co.uk
1130: .Pp
1131: The following pattern
1132: would match any host in the 192.168.0.[0-9] network range:
1133: .Pp
1134: .Dl Host 192.168.0.?
1135: .Pp
1136: A
1137: .Em pattern-list
1138: is a comma-separated list of patterns.
1139: Patterns within pattern-lists may be negated
1140: by preceding them with an exclamation mark
1141: .Pq Sq !\& .
1142: For example,
1143: to allow a key to be used from anywhere within an organisation
1144: except from the
1145: .Dq dialup
1146: pool,
1147: the following entry (in authorized_keys) could be used:
1148: .Pp
1149: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1150: .Sh FILES
1151: .Bl -tag -width Ds
1.50 djm 1152: .It Pa ~/.ssh/config
1.1 stevesk 1153: This is the per-user configuration file.
1154: The format of this file is described above.
1.84 jmc 1155: This file is used by the SSH client.
1.30 djm 1156: Because of the potential for abuse, this file must have strict permissions:
1157: read/write for the user, and not accessible by others.
1.1 stevesk 1158: .It Pa /etc/ssh/ssh_config
1159: Systemwide configuration file.
1160: This file provides defaults for those
1161: values that are not specified in the user's configuration file, and
1162: for those users who do not have a configuration file.
1163: This file must be world-readable.
1164: .El
1.13 jmc 1165: .Sh SEE ALSO
1166: .Xr ssh 1
1.1 stevesk 1167: .Sh AUTHORS
1168: OpenSSH is a derivative of the original and free
1169: ssh 1.2.12 release by Tatu Ylonen.
1170: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1171: Theo de Raadt and Dug Song
1172: removed many bugs, re-added newer features and
1173: created OpenSSH.
1174: Markus Friedl contributed the support for SSH
1175: protocol versions 1.5 and 2.0.