Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.83
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
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7: .\" As far as I am concerned, the code I have written for this software
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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.
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1.83 ! jmc 37: .\" $OpenBSD: ssh_config.5,v 1.82 2006/02/24 20:22:16 jmc Exp $
1.1 stevesk 38: .Dd September 25, 1999
39: .Dt SSH_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm ssh_config
43: .Nd OpenSSH SSH client configuration files
44: .Sh SYNOPSIS
45: .Bl -tag -width Ds -compact
1.50 djm 46: .It Pa ~/.ssh/config
1.1 stevesk 47: .It Pa /etc/ssh/ssh_config
48: .El
49: .Sh DESCRIPTION
50: .Nm ssh
51: obtains configuration data from the following sources in
52: the following order:
1.79 jmc 53: .Pp
1.2 stevesk 54: .Bl -enum -offset indent -compact
55: .It
56: command-line options
57: .It
58: user's configuration file
1.50 djm 59: .Pq Pa ~/.ssh/config
1.2 stevesk 60: .It
61: system-wide configuration file
62: .Pq Pa /etc/ssh/ssh_config
63: .El
1.1 stevesk 64: .Pp
65: For each parameter, the first obtained value
66: will be used.
1.41 jmc 67: The configuration files contain sections separated by
1.1 stevesk 68: .Dq Host
69: specifications, and that section is only applied for hosts that
70: match one of the patterns given in the specification.
71: The matched host name is the one given on the command line.
72: .Pp
73: Since the first obtained value for each parameter is used, more
74: host-specific declarations should be given near the beginning of the
75: file, and general defaults at the end.
1.80 jmc 76: .Sh PATTERNS
77: A
78: .Em pattern
79: consists of zero or more non-whitespace characters,
80: .Sq *
81: (a wildcard that matches zero or more characters),
82: or
83: .Sq ?\&
84: (a wildcard that matches exactly one character).
85: For example, to specify a set of declarations for any host in the
86: .Dq .co.uk
87: set of domains,
88: the following pattern could be used:
89: .Pp
90: .Dl Host *.co.uk
91: .Pp
92: The following pattern
93: would match any host in the 192.168.0.[0-9] network range:
94: .Pp
95: .Dl Host 192.168.0.?
96: .Pp
97: A
98: .Em pattern-list
99: is a comma-separated list of patterns.
100: Patterns within pattern-lists may be negated
101: by preceding them with an exclamation mark
102: .Pq Sq !\& .
103: For example,
104: to allow a key to be used from anywhere within an organisation
105: except from the
106: .Dq dialup
107: pool,
108: the following entry (in authorized_keys) could be used:
1.1 stevesk 109: .Pp
1.80 jmc 110: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
111: .Sh FILE FORMAT
1.1 stevesk 112: The configuration file has the following format:
113: .Pp
114: Empty lines and lines starting with
115: .Ql #
116: are comments.
117: Otherwise a line is of the format
118: .Dq keyword arguments .
119: Configuration options may be separated by whitespace or
120: optional whitespace and exactly one
121: .Ql = ;
122: the latter format is useful to avoid the need to quote whitespace
123: when specifying configuration options using the
124: .Nm ssh ,
125: .Nm scp
126: and
127: .Nm sftp
128: .Fl o
129: option.
130: .Pp
131: The possible
132: keywords and their meanings are as follows (note that
133: keywords are case-insensitive and arguments are case-sensitive):
134: .Bl -tag -width Ds
135: .It Cm Host
136: Restricts the following declarations (up to the next
137: .Cm Host
138: keyword) to be only for those hosts that match one of the patterns
139: given after the keyword.
140: A single
1.83 ! jmc 141: .Ql *
1.1 stevesk 142: as a pattern can be used to provide global
143: defaults for all hosts.
144: The host is the
145: .Ar hostname
1.83 ! jmc 146: argument given on the command line (i.e. the name is not converted to
1.1 stevesk 147: a canonicalized host name before matching).
1.81 jmc 148: .Pp
149: See
150: .Sx PATTERNS
151: for more information on patterns.
1.10 djm 152: .It Cm AddressFamily
1.11 jmc 153: Specifies which address family to use when connecting.
154: Valid arguments are
1.10 djm 155: .Dq any ,
156: .Dq inet
1.40 jmc 157: (use IPv4 only) or
1.10 djm 158: .Dq inet6
1.40 jmc 159: (use IPv6 only).
1.1 stevesk 160: .It Cm BatchMode
161: If set to
162: .Dq yes ,
163: passphrase/password querying will be disabled.
164: This option is useful in scripts and other batch jobs where no user
165: is present to supply the password.
166: The argument must be
167: .Dq yes
168: or
169: .Dq no .
170: The default is
171: .Dq no .
172: .It Cm BindAddress
1.60 dtucker 173: Use the specified address on the local machine as the source address of
1.61 jmc 174: the connection.
175: Only useful on systems with more than one address.
1.1 stevesk 176: Note that this option does not work if
177: .Cm UsePrivilegedPort
178: is set to
179: .Dq yes .
180: .It Cm ChallengeResponseAuthentication
1.82 jmc 181: Specifies whether to use challenge-response authentication.
1.1 stevesk 182: The argument to this keyword must be
183: .Dq yes
184: or
185: .Dq no .
186: The default is
187: .Dq yes .
188: .It Cm CheckHostIP
189: If this flag is set to
190: .Dq yes ,
191: ssh will additionally check the host IP address in the
192: .Pa known_hosts
193: file.
194: This allows ssh to detect if a host key changed due to DNS spoofing.
195: If the option is set to
196: .Dq no ,
197: the check will not be executed.
198: The default is
199: .Dq yes .
200: .It Cm Cipher
201: Specifies the cipher to use for encrypting the session
202: in protocol version 1.
203: Currently,
204: .Dq blowfish ,
205: .Dq 3des ,
206: and
207: .Dq des
208: are supported.
209: .Ar des
210: is only supported in the
211: .Nm ssh
212: client for interoperability with legacy protocol 1 implementations
213: that do not support the
214: .Ar 3des
1.7 jmc 215: cipher.
216: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 217: The default is
218: .Dq 3des .
219: .It Cm Ciphers
220: Specifies the ciphers allowed for protocol version 2
221: in order of preference.
222: Multiple ciphers must be comma-separated.
1.35 dtucker 223: The supported ciphers are
224: .Dq 3des-cbc ,
225: .Dq aes128-cbc ,
226: .Dq aes192-cbc ,
227: .Dq aes256-cbc ,
228: .Dq aes128-ctr ,
229: .Dq aes192-ctr ,
230: .Dq aes256-ctr ,
1.54 djm 231: .Dq arcfour128 ,
232: .Dq arcfour256 ,
1.35 dtucker 233: .Dq arcfour ,
234: .Dq blowfish-cbc ,
235: and
236: .Dq cast128-cbc .
1.1 stevesk 237: The default is
238: .Bd -literal
1.54 djm 239: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
240: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
241: aes192-ctr,aes256-ctr''
1.1 stevesk 242: .Ed
243: .It Cm ClearAllForwardings
244: Specifies that all local, remote and dynamic port forwardings
245: specified in the configuration files or on the command line be
1.7 jmc 246: cleared.
247: This option is primarily useful when used from the
1.1 stevesk 248: .Nm ssh
249: command line to clear port forwardings set in
250: configuration files, and is automatically set by
251: .Xr scp 1
252: and
253: .Xr sftp 1 .
254: The argument must be
255: .Dq yes
256: or
257: .Dq no .
258: The default is
259: .Dq no .
260: .It Cm Compression
261: Specifies whether to use compression.
262: The argument must be
263: .Dq yes
264: or
265: .Dq no .
266: The default is
267: .Dq no .
268: .It Cm CompressionLevel
269: Specifies the compression level to use if compression is enabled.
270: The argument must be an integer from 1 (fast) to 9 (slow, best).
271: The default level is 6, which is good for most applications.
272: The meaning of the values is the same as in
273: .Xr gzip 1 .
274: Note that this option applies to protocol version 1 only.
275: .It Cm ConnectionAttempts
276: Specifies the number of tries (one per second) to make before exiting.
277: The argument must be an integer.
278: This may be useful in scripts if the connection sometimes fails.
279: The default is 1.
1.9 djm 280: .It Cm ConnectTimeout
281: Specifies the timeout (in seconds) used when connecting to the ssh
1.11 jmc 282: server, instead of using the default system TCP timeout.
283: This value is used only when the target is down or really unreachable,
284: not when it refuses the connection.
1.36 djm 285: .It Cm ControlMaster
286: Enables the sharing of multiple sessions over a single network connection.
287: When set to
288: .Dq yes
289: .Nm ssh
290: will listen for connections on a control socket specified using the
291: .Cm ControlPath
292: argument.
293: Additional sessions can connect to this socket using the same
294: .Cm ControlPath
295: with
296: .Cm ControlMaster
297: set to
298: .Dq no
1.38 jmc 299: (the default).
1.64 jmc 300: These sessions will try to reuse the master instance's network connection
1.63 djm 301: rather than initiating new ones, but will fall back to connecting normally
302: if the control socket does not exist, or is not listening.
303: .Pp
1.37 djm 304: Setting this to
305: .Dq ask
306: will cause
307: .Nm ssh
308: to listen for control connections, but require confirmation using the
309: .Ev SSH_ASKPASS
310: program before they are accepted (see
311: .Xr ssh-add 1
1.38 jmc 312: for details).
1.51 jakob 313: If the
314: .Cm ControlPath
315: can not be opened,
316: .Nm ssh
317: will continue without connecting to a master instance.
1.58 djm 318: .Pp
319: X11 and
1.59 jmc 320: .Xr ssh-agent 1
1.58 djm 321: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 322: display and agent forwarded will be the one belonging to the master
1.59 jmc 323: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 324: .Pp
325: Two additional options allow for opportunistic multiplexing: try to use a
326: master connection but fall back to creating a new one if one does not already
327: exist.
328: These options are:
329: .Dq auto
330: and
331: .Dq autoask .
332: The latter requires confirmation like the
333: .Dq ask
334: option.
1.36 djm 335: .It Cm ControlPath
1.55 djm 336: Specify the path to the control socket used for connection sharing as described
337: in the
1.36 djm 338: .Cm ControlMaster
1.57 djm 339: section above or the string
340: .Dq none
341: to disable connection sharing.
1.55 djm 342: In the path,
1.77 djm 343: .Ql %l
344: will be substituted by the local host name,
1.55 djm 345: .Ql %h
346: will be substituted by the target host name,
347: .Ql %p
348: the port and
349: .Ql %r
350: by the remote login username.
1.56 djm 351: It is recommended that any
352: .Cm ControlPath
353: used for opportunistic connection sharing include
1.78 jmc 354: at least %h, %p, and %r.
1.56 djm 355: This ensures that shared connections are uniquely identified.
1.38 jmc 356: .It Cm DynamicForward
1.74 jmc 357: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 358: over the secure channel, and the application
359: protocol is then used to determine where to connect to from the
360: remote machine.
1.62 djm 361: .Pp
362: The argument must be
363: .Sm off
364: .Oo Ar bind_address : Oc Ar port .
365: .Sm on
366: IPv6 addresses can be specified by enclosing addresses in square brackets or
367: by using an alternative syntax:
368: .Oo Ar bind_address Ns / Oc Ns Ar port .
369: By default, the local port is bound in accordance with the
370: .Cm GatewayPorts
371: setting.
372: However, an explicit
373: .Ar bind_address
374: may be used to bind the connection to a specific address.
375: The
376: .Ar bind_address
377: of
378: .Dq localhost
379: indicates that the listening port be bound for local use only, while an
380: empty address or
381: .Sq *
382: indicates that the port should be available from all interfaces.
383: .Pp
1.38 jmc 384: Currently the SOCKS4 and SOCKS5 protocols are supported, and
385: .Nm ssh
386: will act as a SOCKS server.
387: Multiple forwardings may be specified, and
388: additional forwardings can be given on the command line.
389: Only the superuser can forward privileged ports.
1.14 markus 390: .It Cm EnableSSHKeysign
391: Setting this option to
392: .Dq yes
393: in the global client configuration file
394: .Pa /etc/ssh/ssh_config
395: enables the use of the helper program
396: .Xr ssh-keysign 8
397: during
398: .Cm HostbasedAuthentication .
399: The argument must be
400: .Dq yes
401: or
402: .Dq no .
403: The default is
404: .Dq no .
1.23 jmc 405: This option should be placed in the non-hostspecific section.
1.14 markus 406: See
407: .Xr ssh-keysign 8
408: for more information.
1.1 stevesk 409: .It Cm EscapeChar
410: Sets the escape character (default:
411: .Ql ~ ) .
412: The escape character can also
413: be set on the command line.
414: The argument should be a single character,
415: .Ql ^
416: followed by a letter, or
417: .Dq none
418: to disable the escape
419: character entirely (making the connection transparent for binary
420: data).
421: .It Cm ForwardAgent
422: Specifies whether the connection to the authentication agent (if any)
423: will be forwarded to the remote machine.
424: The argument must be
425: .Dq yes
426: or
427: .Dq no .
428: The default is
429: .Dq no .
1.3 stevesk 430: .Pp
1.7 jmc 431: Agent forwarding should be enabled with caution.
432: Users with the ability to bypass file permissions on the remote host
433: (for the agent's Unix-domain socket)
434: can access the local agent through the forwarded connection.
435: An attacker cannot obtain key material from the agent,
1.3 stevesk 436: however they can perform operations on the keys that enable them to
437: authenticate using the identities loaded into the agent.
1.1 stevesk 438: .It Cm ForwardX11
439: Specifies whether X11 connections will be automatically redirected
440: over the secure channel and
441: .Ev DISPLAY
442: set.
443: The argument must be
444: .Dq yes
445: or
446: .Dq no .
447: The default is
448: .Dq no .
1.3 stevesk 449: .Pp
1.7 jmc 450: X11 forwarding should be enabled with caution.
451: Users with the ability to bypass file permissions on the remote host
1.22 markus 452: (for the user's X11 authorization database)
1.7 jmc 453: can access the local X11 display through the forwarded connection.
1.22 markus 454: An attacker may then be able to perform activities such as keystroke monitoring
455: if the
456: .Cm ForwardX11Trusted
457: option is also enabled.
458: .It Cm ForwardX11Trusted
1.34 jmc 459: If this option is set to
1.22 markus 460: .Dq yes
461: then remote X11 clients will have full access to the original X11 display.
1.42 djm 462: .Pp
1.22 markus 463: If this option is set to
464: .Dq no
465: then remote X11 clients will be considered untrusted and prevented
466: from stealing or tampering with data belonging to trusted X11
467: clients.
1.42 djm 468: Furthermore, the
469: .Xr xauth 1
470: token used for the session will be set to expire after 20 minutes.
471: Remote clients will be refused access after this time.
1.22 markus 472: .Pp
473: The default is
474: .Dq no .
475: .Pp
476: See the X11 SECURITY extension specification for full details on
477: the restrictions imposed on untrusted clients.
1.1 stevesk 478: .It Cm GatewayPorts
479: Specifies whether remote hosts are allowed to connect to local
480: forwarded ports.
481: By default,
482: .Nm ssh
1.7 jmc 483: binds local port forwardings to the loopback address.
484: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 485: .Cm GatewayPorts
486: can be used to specify that
487: .Nm ssh
488: should bind local port forwardings to the wildcard address,
489: thus allowing remote hosts to connect to forwarded ports.
490: The argument must be
491: .Dq yes
492: or
493: .Dq no .
494: The default is
495: .Dq no .
496: .It Cm GlobalKnownHostsFile
497: Specifies a file to use for the global
498: host key database instead of
499: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 500: .It Cm GSSAPIAuthentication
1.27 markus 501: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 502: The default is
1.21 markus 503: .Dq no .
1.18 markus 504: Note that this option applies to protocol version 2 only.
505: .It Cm GSSAPIDelegateCredentials
506: Forward (delegate) credentials to the server.
507: The default is
508: .Dq no .
509: Note that this option applies to protocol version 2 only.
1.44 djm 510: .It Cm HashKnownHosts
511: Indicates that
512: .Nm ssh
513: should hash host names and addresses when they are added to
1.50 djm 514: .Pa ~/.ssh/known_hosts .
1.44 djm 515: These hashed names may be used normally by
516: .Nm ssh
517: and
518: .Nm sshd ,
519: but they do not reveal identifying information should the file's contents
520: be disclosed.
521: The default is
522: .Dq no .
1.46 jmc 523: Note that hashing of names and addresses will not be retrospectively applied
1.45 djm 524: to existing known hosts files, but these may be manually hashed using
525: .Xr ssh-keygen 1 .
1.1 stevesk 526: .It Cm HostbasedAuthentication
527: Specifies whether to try rhosts based authentication with public key
528: authentication.
529: The argument must be
530: .Dq yes
531: or
532: .Dq no .
533: The default is
534: .Dq no .
535: This option applies to protocol version 2 only and
536: is similar to
537: .Cm RhostsRSAAuthentication .
538: .It Cm HostKeyAlgorithms
539: Specifies the protocol version 2 host key algorithms
540: that the client wants to use in order of preference.
541: The default for this option is:
542: .Dq ssh-rsa,ssh-dss .
543: .It Cm HostKeyAlias
544: Specifies an alias that should be used instead of the
545: real host name when looking up or saving the host key
546: in the host key database files.
547: This option is useful for tunneling ssh connections
548: or for multiple servers running on a single host.
549: .It Cm HostName
550: Specifies the real host name to log into.
551: This can be used to specify nicknames or abbreviations for hosts.
552: Default is the name given on the command line.
553: Numeric IP addresses are also permitted (both on the command line and in
554: .Cm HostName
555: specifications).
1.29 markus 556: .It Cm IdentitiesOnly
557: Specifies that
558: .Nm ssh
559: should only use the authentication identity files configured in the
1.31 jmc 560: .Nm
1.29 markus 561: files,
562: even if the
563: .Nm ssh-agent
564: offers more identities.
565: The argument to this keyword must be
566: .Dq yes
567: or
568: .Dq no .
1.71 stevesk 569: This option is intended for situations where
1.29 markus 570: .Nm ssh-agent
571: offers many different identities.
572: The default is
573: .Dq no .
1.67 jmc 574: .It Cm IdentityFile
575: Specifies a file from which the user's RSA or DSA authentication identity
576: is read.
577: The default is
578: .Pa ~/.ssh/identity
579: for protocol version 1, and
580: .Pa ~/.ssh/id_rsa
581: and
582: .Pa ~/.ssh/id_dsa
583: for protocol version 2.
584: Additionally, any identities represented by the authentication agent
585: will be used for authentication.
586: The file name may use the tilde
587: syntax to refer to a user's home directory.
588: It is possible to have
589: multiple identity files specified in configuration files; all these
590: identities will be tried in sequence.
1.39 djm 591: .It Cm KbdInteractiveDevices
592: Specifies the list of methods to use in keyboard-interactive authentication.
593: Multiple method names must be comma-separated.
594: The default is to use the server specified list.
1.65 reyk 595: .It Cm LocalCommand
596: Specifies a command to execute on the local machine after successfully
597: connecting to the server.
598: The command string extends to the end of the line, and is executed with
599: .Pa /bin/sh .
600: This directive is ignored unless
601: .Cm PermitLocalCommand
602: has been enabled.
1.1 stevesk 603: .It Cm LocalForward
1.74 jmc 604: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 605: the secure channel to the specified host and port from the remote machine.
1.49 jmc 606: The first argument must be
1.43 djm 607: .Sm off
1.49 jmc 608: .Oo Ar bind_address : Oc Ar port
1.43 djm 609: .Sm on
1.49 jmc 610: and the second argument must be
611: .Ar host : Ns Ar hostport .
1.46 jmc 612: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 613: by using an alternative syntax:
1.49 jmc 614: .Oo Ar bind_address Ns / Oc Ns Ar port
615: and
616: .Ar host Ns / Ns Ar hostport .
1.46 jmc 617: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 618: given on the command line.
1.1 stevesk 619: Only the superuser can forward privileged ports.
1.43 djm 620: By default, the local port is bound in accordance with the
621: .Cm GatewayPorts
622: setting.
623: However, an explicit
624: .Ar bind_address
625: may be used to bind the connection to a specific address.
626: The
627: .Ar bind_address
628: of
629: .Dq localhost
1.46 jmc 630: indicates that the listening port be bound for local use only, while an
631: empty address or
632: .Sq *
1.43 djm 633: indicates that the port should be available from all interfaces.
1.1 stevesk 634: .It Cm LogLevel
635: Gives the verbosity level that is used when logging messages from
636: .Nm ssh .
637: The possible values are:
638: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 639: The default is INFO.
640: DEBUG and DEBUG1 are equivalent.
641: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 642: .It Cm MACs
643: Specifies the MAC (message authentication code) algorithms
644: in order of preference.
645: The MAC algorithm is used in protocol version 2
646: for data integrity protection.
647: Multiple algorithms must be comma-separated.
648: The default is
649: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
650: .It Cm NoHostAuthenticationForLocalhost
651: This option can be used if the home directory is shared across machines.
652: In this case localhost will refer to a different machine on each of
653: the machines and the user will get many warnings about changed host keys.
654: However, this option disables host authentication for localhost.
655: The argument to this keyword must be
656: .Dq yes
657: or
658: .Dq no .
659: The default is to check the host key for localhost.
660: .It Cm NumberOfPasswordPrompts
661: Specifies the number of password prompts before giving up.
662: The argument to this keyword must be an integer.
663: Default is 3.
664: .It Cm PasswordAuthentication
665: Specifies whether to use password authentication.
666: The argument to this keyword must be
667: .Dq yes
668: or
669: .Dq no .
670: The default is
671: .Dq yes .
1.65 reyk 672: .It Cm PermitLocalCommand
673: Allow local command execution via the
674: .Ic LocalCommand
675: option or using the
1.66 jmc 676: .Ic !\& Ns Ar command
1.65 reyk 677: escape sequence in
678: .Xr ssh 1 .
679: The argument must be
680: .Dq yes
681: or
682: .Dq no .
683: The default is
684: .Dq no .
1.67 jmc 685: .It Cm Port
686: Specifies the port number to connect on the remote host.
687: Default is 22.
1.1 stevesk 688: .It Cm PreferredAuthentications
689: Specifies the order in which the client should try protocol 2
1.11 jmc 690: authentication methods.
1.48 jmc 691: This allows a client to prefer one method (e.g.\&
1.1 stevesk 692: .Cm keyboard-interactive )
1.48 jmc 693: over another method (e.g.\&
1.1 stevesk 694: .Cm password )
695: The default for this option is:
696: .Dq hostbased,publickey,keyboard-interactive,password .
697: .It Cm Protocol
698: Specifies the protocol versions
699: .Nm ssh
700: should support in order of preference.
701: The possible values are
702: .Dq 1
703: and
704: .Dq 2 .
705: Multiple versions must be comma-separated.
706: The default is
707: .Dq 2,1 .
708: This means that
709: .Nm ssh
710: tries version 2 and falls back to version 1
711: if version 2 is not available.
712: .It Cm ProxyCommand
713: Specifies the command to use to connect to the server.
714: The command
715: string extends to the end of the line, and is executed with
716: .Pa /bin/sh .
717: In the command string,
718: .Ql %h
719: will be substituted by the host name to
720: connect and
721: .Ql %p
722: by the port.
723: The command can be basically anything,
724: and should read from its standard input and write to its standard output.
725: It should eventually connect an
726: .Xr sshd 8
727: server running on some machine, or execute
728: .Ic sshd -i
729: somewhere.
730: Host key management will be done using the
731: HostName of the host being connected (defaulting to the name typed by
732: the user).
1.7 jmc 733: Setting the command to
734: .Dq none
1.6 markus 735: disables this option entirely.
1.1 stevesk 736: Note that
737: .Cm CheckHostIP
738: is not available for connects with a proxy command.
1.52 djm 739: .Pp
740: This directive is useful in conjunction with
741: .Xr nc 1
742: and its proxy support.
1.53 jmc 743: For example, the following directive would connect via an HTTP proxy at
1.52 djm 744: 192.0.2.0:
745: .Bd -literal -offset 3n
746: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
747: .Ed
1.1 stevesk 748: .It Cm PubkeyAuthentication
749: Specifies whether to try public key authentication.
750: The argument to this keyword must be
751: .Dq yes
752: or
753: .Dq no .
754: The default is
755: .Dq yes .
756: This option applies to protocol version 2 only.
1.75 dtucker 757: .It Cm RekeyLimit
758: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 759: session key is renegotiated.
1.75 dtucker 760: The argument is the number of bytes, with an optional suffix of
1.76 jmc 761: .Sq K ,
762: .Sq M ,
1.75 dtucker 763: or
1.76 jmc 764: .Sq G
1.75 dtucker 765: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
766: The default is between
767: .Dq 1G
768: and
769: .Dq 4G ,
770: depending on the cipher.
1.76 jmc 771: This option applies to protocol version 2 only.
1.1 stevesk 772: .It Cm RemoteForward
1.74 jmc 773: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 774: the secure channel to the specified host and port from the local machine.
1.49 jmc 775: The first argument must be
1.43 djm 776: .Sm off
1.49 jmc 777: .Oo Ar bind_address : Oc Ar port
1.43 djm 778: .Sm on
1.49 jmc 779: and the second argument must be
780: .Ar host : Ns Ar hostport .
781: IPv6 addresses can be specified by enclosing addresses in square brackets
782: or by using an alternative syntax:
783: .Oo Ar bind_address Ns / Oc Ns Ar port
784: and
785: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 786: Multiple forwardings may be specified, and additional
787: forwardings can be given on the command line.
788: Only the superuser can forward privileged ports.
1.43 djm 789: .Pp
790: If the
791: .Ar bind_address
792: is not specified, the default is to only bind to loopback addresses.
793: If the
794: .Ar bind_address
795: is
796: .Ql *
797: or an empty string, then the forwarding is requested to listen on all
798: interfaces.
799: Specifying a remote
800: .Ar bind_address
1.46 jmc 801: will only succeed if the server's
802: .Cm GatewayPorts
1.43 djm 803: option is enabled (see
1.46 jmc 804: .Xr sshd_config 5 ) .
1.1 stevesk 805: .It Cm RhostsRSAAuthentication
806: Specifies whether to try rhosts based authentication with RSA host
807: authentication.
808: The argument must be
809: .Dq yes
810: or
811: .Dq no .
812: The default is
813: .Dq no .
814: This option applies to protocol version 1 only and requires
815: .Nm ssh
816: to be setuid root.
817: .It Cm RSAAuthentication
818: Specifies whether to try RSA authentication.
819: The argument to this keyword must be
820: .Dq yes
821: or
822: .Dq no .
823: RSA authentication will only be
824: attempted if the identity file exists, or an authentication agent is
825: running.
826: The default is
827: .Dq yes .
828: Note that this option applies to protocol version 1 only.
1.32 djm 829: .It Cm SendEnv
830: Specifies what variables from the local
831: .Xr environ 7
832: should be sent to the server.
833: Note that environment passing is only supported for protocol 2, the
1.33 djm 834: server must also support it, and the server must be configured to
835: accept these environment variables.
1.32 djm 836: Refer to
837: .Cm AcceptEnv
838: in
839: .Xr sshd_config 5
840: for how to configure the server.
1.80 jmc 841: Variables are specified by name, which may contain wildcard characters.
1.33 djm 842: Multiple environment variables may be separated by whitespace or spread
1.32 djm 843: across multiple
844: .Cm SendEnv
845: directives.
846: The default is not to send any environment variables.
1.81 jmc 847: .Pp
848: See
849: .Sx PATTERNS
850: for more information on patterns.
1.28 markus 851: .It Cm ServerAliveCountMax
1.73 jmc 852: Sets the number of server alive messages (see below) which may be
1.28 markus 853: sent without
854: .Nm ssh
855: receiving any messages back from the server.
856: If this threshold is reached while server alive messages are being sent,
857: .Nm ssh
858: will disconnect from the server, terminating the session.
859: It is important to note that the use of server alive messages is very
860: different from
861: .Cm TCPKeepAlive
862: (below).
863: The server alive messages are sent through the encrypted channel
864: and therefore will not be spoofable.
865: The TCP keepalive option enabled by
866: .Cm TCPKeepAlive
867: is spoofable.
868: The server alive mechanism is valuable when the client or
869: server depend on knowing when a connection has become inactive.
870: .Pp
871: The default value is 3.
872: If, for example,
873: .Cm ServerAliveInterval
1.73 jmc 874: (see below) is set to 15, and
1.28 markus 875: .Cm ServerAliveCountMax
876: is left at the default, if the server becomes unresponsive ssh
877: will disconnect after approximately 45 seconds.
1.67 jmc 878: .It Cm ServerAliveInterval
879: Sets a timeout interval in seconds after which if no data has been received
880: from the server,
881: .Nm ssh
882: will send a message through the encrypted
883: channel to request a response from the server.
884: The default
885: is 0, indicating that these messages will not be sent to the server.
886: This option applies to protocol version 2 only.
1.1 stevesk 887: .It Cm SmartcardDevice
1.11 jmc 888: Specifies which smartcard device to use.
889: The argument to this keyword is the device
1.1 stevesk 890: .Nm ssh
891: should use to communicate with a smartcard used for storing the user's
1.11 jmc 892: private RSA key.
893: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 894: .It Cm StrictHostKeyChecking
895: If this flag is set to
896: .Dq yes ,
897: .Nm ssh
898: will never automatically add host keys to the
1.50 djm 899: .Pa ~/.ssh/known_hosts
1.1 stevesk 900: file, and refuses to connect to hosts whose host key has changed.
901: This provides maximum protection against trojan horse attacks,
902: however, can be annoying when the
903: .Pa /etc/ssh/ssh_known_hosts
904: file is poorly maintained, or connections to new hosts are
905: frequently made.
906: This option forces the user to manually
907: add all new hosts.
908: If this flag is set to
909: .Dq no ,
910: .Nm ssh
911: will automatically add new host keys to the
912: user known hosts files.
913: If this flag is set to
914: .Dq ask ,
915: new host keys
916: will be added to the user known host files only after the user
917: has confirmed that is what they really want to do, and
918: .Nm ssh
919: will refuse to connect to hosts whose host key has changed.
920: The host keys of
921: known hosts will be verified automatically in all cases.
922: The argument must be
923: .Dq yes ,
924: .Dq no
925: or
926: .Dq ask .
927: The default is
928: .Dq ask .
1.26 markus 929: .It Cm TCPKeepAlive
930: Specifies whether the system should send TCP keepalive messages to the
931: other side.
932: If they are sent, death of the connection or crash of one
933: of the machines will be properly noticed.
934: However, this means that
935: connections will die if the route is down temporarily, and some people
936: find it annoying.
937: .Pp
938: The default is
939: .Dq yes
940: (to send TCP keepalive messages), and the client will notice
941: if the network goes down or the remote host dies.
942: This is important in scripts, and many users want it too.
943: .Pp
944: To disable TCP keepalive messages, the value should be set to
945: .Dq no .
1.65 reyk 946: .It Cm Tunnel
947: Request starting
948: .Xr tun 4
1.69 jmc 949: device forwarding between the client and the server.
950: This option also allows requesting layer 2 (ethernet)
951: instead of layer 3 (point-to-point) tunneling from the server.
1.65 reyk 952: The argument must be
1.68 reyk 953: .Dq yes ,
954: .Dq point-to-point ,
955: .Dq ethernet
1.65 reyk 956: or
957: .Dq no .
958: The default is
959: .Dq no .
960: .It Cm TunnelDevice
961: Force a specified
962: .Xr tun 4
963: device on the client.
964: Without this option, the next available device will be used.
1.72 jmc 965: .It Cm UsePrivilegedPort
966: Specifies whether to use a privileged port for outgoing connections.
967: The argument must be
968: .Dq yes
969: or
970: .Dq no .
971: The default is
972: .Dq no .
973: If set to
974: .Dq yes
975: .Nm ssh
976: must be setuid root.
977: Note that this option must be set to
978: .Dq yes
979: for
980: .Cm RhostsRSAAuthentication
981: with older servers.
1.1 stevesk 982: .It Cm User
983: Specifies the user to log in as.
984: This can be useful when a different user name is used on different machines.
985: This saves the trouble of
986: having to remember to give the user name on the command line.
987: .It Cm UserKnownHostsFile
988: Specifies a file to use for the user
989: host key database instead of
1.50 djm 990: .Pa ~/.ssh/known_hosts .
1.8 jakob 991: .It Cm VerifyHostKeyDNS
992: Specifies whether to verify the remote key using DNS and SSHFP resource
993: records.
1.24 jakob 994: If this option is set to
995: .Dq yes ,
1.25 jmc 996: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 997: from DNS.
998: Insecure fingerprints will be handled as if this option was set to
999: .Dq ask .
1000: If this option is set to
1001: .Dq ask ,
1002: information on fingerprint match will be displayed, but the user will still
1003: need to confirm new host keys according to the
1004: .Cm StrictHostKeyChecking
1005: option.
1006: The argument must be
1007: .Dq yes ,
1008: .Dq no
1.25 jmc 1009: or
1010: .Dq ask .
1.8 jakob 1011: The default is
1012: .Dq no .
1.12 jakob 1013: Note that this option applies to protocol version 2 only.
1.1 stevesk 1014: .It Cm XAuthLocation
1.5 stevesk 1015: Specifies the full pathname of the
1.1 stevesk 1016: .Xr xauth 1
1017: program.
1018: The default is
1019: .Pa /usr/X11R6/bin/xauth .
1020: .El
1021: .Sh FILES
1022: .Bl -tag -width Ds
1.50 djm 1023: .It Pa ~/.ssh/config
1.1 stevesk 1024: This is the per-user configuration file.
1025: The format of this file is described above.
1026: This file is used by the
1027: .Nm ssh
1028: client.
1.30 djm 1029: Because of the potential for abuse, this file must have strict permissions:
1030: read/write for the user, and not accessible by others.
1.1 stevesk 1031: .It Pa /etc/ssh/ssh_config
1032: Systemwide configuration file.
1033: This file provides defaults for those
1034: values that are not specified in the user's configuration file, and
1035: for those users who do not have a configuration file.
1036: This file must be world-readable.
1037: .El
1.13 jmc 1038: .Sh SEE ALSO
1039: .Xr ssh 1
1.1 stevesk 1040: .Sh AUTHORS
1041: OpenSSH is a derivative of the original and free
1042: ssh 1.2.12 release by Tatu Ylonen.
1043: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1044: Theo de Raadt and Dug Song
1045: removed many bugs, re-added newer features and
1046: created OpenSSH.
1047: Markus Friedl contributed the support for SSH
1048: protocol versions 1.5 and 2.0.
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