Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.63
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.
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1.63 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.62 2005/09/19 11:37:34 djm 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:
1.2 stevesk 53: .Bl -enum -offset indent -compact
54: .It
55: command-line options
56: .It
57: user's configuration file
1.50 djm 58: .Pq Pa ~/.ssh/config
1.2 stevesk 59: .It
60: system-wide configuration file
61: .Pq Pa /etc/ssh/ssh_config
62: .El
1.1 stevesk 63: .Pp
64: For each parameter, the first obtained value
65: will be used.
1.41 jmc 66: The configuration files contain sections separated by
1.1 stevesk 67: .Dq Host
68: specifications, and that section is only applied for hosts that
69: match one of the patterns given in the specification.
70: The matched host name is the one given on the command line.
71: .Pp
72: Since the first obtained value for each parameter is used, more
73: host-specific declarations should be given near the beginning of the
74: file, and general defaults at the end.
75: .Pp
76: The configuration file has the following format:
77: .Pp
78: Empty lines and lines starting with
79: .Ql #
80: are comments.
81: .Pp
82: Otherwise a line is of the format
83: .Dq keyword arguments .
84: Configuration options may be separated by whitespace or
85: optional whitespace and exactly one
86: .Ql = ;
87: the latter format is useful to avoid the need to quote whitespace
88: when specifying configuration options using the
89: .Nm ssh ,
90: .Nm scp
91: and
92: .Nm sftp
93: .Fl o
94: option.
95: .Pp
96: The possible
97: keywords and their meanings are as follows (note that
98: keywords are case-insensitive and arguments are case-sensitive):
99: .Bl -tag -width Ds
100: .It Cm Host
101: Restricts the following declarations (up to the next
102: .Cm Host
103: keyword) to be only for those hosts that match one of the patterns
104: given after the keyword.
105: .Ql \&*
106: and
1.20 jmc 107: .Ql \&?
1.1 stevesk 108: can be used as wildcards in the
109: patterns.
110: A single
111: .Ql \&*
112: as a pattern can be used to provide global
113: defaults for all hosts.
114: The host is the
115: .Ar hostname
116: argument given on the command line (i.e., the name is not converted to
117: a canonicalized host name before matching).
1.10 djm 118: .It Cm AddressFamily
1.11 jmc 119: Specifies which address family to use when connecting.
120: Valid arguments are
1.10 djm 121: .Dq any ,
122: .Dq inet
1.40 jmc 123: (use IPv4 only) or
1.10 djm 124: .Dq inet6
1.40 jmc 125: (use IPv6 only).
1.1 stevesk 126: .It Cm BatchMode
127: If set to
128: .Dq yes ,
129: passphrase/password querying will be disabled.
130: This option is useful in scripts and other batch jobs where no user
131: is present to supply the password.
132: The argument must be
133: .Dq yes
134: or
135: .Dq no .
136: The default is
137: .Dq no .
138: .It Cm BindAddress
1.60 dtucker 139: Use the specified address on the local machine as the source address of
1.61 jmc 140: the connection.
141: Only useful on systems with more than one address.
1.1 stevesk 142: Note that this option does not work if
143: .Cm UsePrivilegedPort
144: is set to
145: .Dq yes .
146: .It Cm ChallengeResponseAuthentication
147: Specifies whether to use challenge response authentication.
148: The argument to this keyword must be
149: .Dq yes
150: or
151: .Dq no .
152: The default is
153: .Dq yes .
154: .It Cm CheckHostIP
155: If this flag is set to
156: .Dq yes ,
157: ssh will additionally check the host IP address in the
158: .Pa known_hosts
159: file.
160: This allows ssh to detect if a host key changed due to DNS spoofing.
161: If the option is set to
162: .Dq no ,
163: the check will not be executed.
164: The default is
165: .Dq yes .
166: .It Cm Cipher
167: Specifies the cipher to use for encrypting the session
168: in protocol version 1.
169: Currently,
170: .Dq blowfish ,
171: .Dq 3des ,
172: and
173: .Dq des
174: are supported.
175: .Ar des
176: is only supported in the
177: .Nm ssh
178: client for interoperability with legacy protocol 1 implementations
179: that do not support the
180: .Ar 3des
1.7 jmc 181: cipher.
182: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 183: The default is
184: .Dq 3des .
185: .It Cm Ciphers
186: Specifies the ciphers allowed for protocol version 2
187: in order of preference.
188: Multiple ciphers must be comma-separated.
1.35 dtucker 189: The supported ciphers are
190: .Dq 3des-cbc ,
191: .Dq aes128-cbc ,
192: .Dq aes192-cbc ,
193: .Dq aes256-cbc ,
194: .Dq aes128-ctr ,
195: .Dq aes192-ctr ,
196: .Dq aes256-ctr ,
1.54 djm 197: .Dq arcfour128 ,
198: .Dq arcfour256 ,
1.35 dtucker 199: .Dq arcfour ,
200: .Dq blowfish-cbc ,
201: and
202: .Dq cast128-cbc .
1.1 stevesk 203: The default is
204: .Bd -literal
1.54 djm 205: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
206: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
207: aes192-ctr,aes256-ctr''
1.1 stevesk 208: .Ed
209: .It Cm ClearAllForwardings
210: Specifies that all local, remote and dynamic port forwardings
211: specified in the configuration files or on the command line be
1.7 jmc 212: cleared.
213: This option is primarily useful when used from the
1.1 stevesk 214: .Nm ssh
215: command line to clear port forwardings set in
216: configuration files, and is automatically set by
217: .Xr scp 1
218: and
219: .Xr sftp 1 .
220: The argument must be
221: .Dq yes
222: or
223: .Dq no .
224: The default is
225: .Dq no .
226: .It Cm Compression
227: Specifies whether to use compression.
228: The argument must be
229: .Dq yes
230: or
231: .Dq no .
232: The default is
233: .Dq no .
234: .It Cm CompressionLevel
235: Specifies the compression level to use if compression is enabled.
236: The argument must be an integer from 1 (fast) to 9 (slow, best).
237: The default level is 6, which is good for most applications.
238: The meaning of the values is the same as in
239: .Xr gzip 1 .
240: Note that this option applies to protocol version 1 only.
241: .It Cm ConnectionAttempts
242: Specifies the number of tries (one per second) to make before exiting.
243: The argument must be an integer.
244: This may be useful in scripts if the connection sometimes fails.
245: The default is 1.
1.9 djm 246: .It Cm ConnectTimeout
247: Specifies the timeout (in seconds) used when connecting to the ssh
1.11 jmc 248: server, instead of using the default system TCP timeout.
249: This value is used only when the target is down or really unreachable,
250: not when it refuses the connection.
1.36 djm 251: .It Cm ControlMaster
252: Enables the sharing of multiple sessions over a single network connection.
253: When set to
254: .Dq yes
255: .Nm ssh
256: will listen for connections on a control socket specified using the
257: .Cm ControlPath
258: argument.
259: Additional sessions can connect to this socket using the same
260: .Cm ControlPath
261: with
262: .Cm ControlMaster
263: set to
264: .Dq no
1.38 jmc 265: (the default).
1.63 ! djm 266: These sessions will try to reuse the master instance's network connection
! 267: rather than initiating new ones, but will fall back to connecting normally
! 268: if the control socket does not exist, or is not listening.
! 269: .Pp
1.37 djm 270: Setting this to
271: .Dq ask
272: will cause
273: .Nm ssh
274: to listen for control connections, but require confirmation using the
275: .Ev SSH_ASKPASS
276: program before they are accepted (see
277: .Xr ssh-add 1
1.38 jmc 278: for details).
1.51 jakob 279: If the
280: .Cm ControlPath
281: can not be opened,
282: .Nm ssh
283: 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
288: display and agent fowarded 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,
309: .Ql %h
310: will be substituted by the target host name,
311: .Ql %p
312: the port and
313: .Ql %r
314: by the remote login username.
1.56 djm 315: It is recommended that any
316: .Cm ControlPath
317: used for opportunistic connection sharing include
318: all three of these escape sequences.
319: This ensures that shared connections are uniquely identified.
1.38 jmc 320: .It Cm DynamicForward
321: Specifies that a TCP/IP port on the local machine be forwarded
322: over the secure channel, and the application
323: protocol is then used to determine where to connect to from the
324: remote machine.
1.62 djm 325: .Pp
326: The argument must be
327: .Sm off
328: .Oo Ar bind_address : Oc Ar port .
329: .Sm on
330: IPv6 addresses can be specified by enclosing addresses in square brackets or
331: by using an alternative syntax:
332: .Oo Ar bind_address Ns / Oc Ns Ar port .
333: By default, the local port is bound in accordance with the
334: .Cm GatewayPorts
335: setting.
336: However, an explicit
337: .Ar bind_address
338: may be used to bind the connection to a specific address.
339: The
340: .Ar bind_address
341: of
342: .Dq localhost
343: indicates that the listening port be bound for local use only, while an
344: empty address or
345: .Sq *
346: indicates that the port should be available from all interfaces.
347: .Pp
1.38 jmc 348: Currently the SOCKS4 and SOCKS5 protocols are supported, and
349: .Nm ssh
350: will act as a SOCKS server.
351: Multiple forwardings may be specified, and
352: additional forwardings can be given on the command line.
353: Only the superuser can forward privileged ports.
1.14 markus 354: .It Cm EnableSSHKeysign
355: Setting this option to
356: .Dq yes
357: in the global client configuration file
358: .Pa /etc/ssh/ssh_config
359: enables the use of the helper program
360: .Xr ssh-keysign 8
361: during
362: .Cm HostbasedAuthentication .
363: The argument must be
364: .Dq yes
365: or
366: .Dq no .
367: The default is
368: .Dq no .
1.23 jmc 369: This option should be placed in the non-hostspecific section.
1.14 markus 370: See
371: .Xr ssh-keysign 8
372: for more information.
1.1 stevesk 373: .It Cm EscapeChar
374: Sets the escape character (default:
375: .Ql ~ ) .
376: The escape character can also
377: be set on the command line.
378: The argument should be a single character,
379: .Ql ^
380: followed by a letter, or
381: .Dq none
382: to disable the escape
383: character entirely (making the connection transparent for binary
384: data).
385: .It Cm ForwardAgent
386: Specifies whether the connection to the authentication agent (if any)
387: will be forwarded to the remote machine.
388: The argument must be
389: .Dq yes
390: or
391: .Dq no .
392: The default is
393: .Dq no .
1.3 stevesk 394: .Pp
1.7 jmc 395: Agent forwarding should be enabled with caution.
396: Users with the ability to bypass file permissions on the remote host
397: (for the agent's Unix-domain socket)
398: can access the local agent through the forwarded connection.
399: An attacker cannot obtain key material from the agent,
1.3 stevesk 400: however they can perform operations on the keys that enable them to
401: authenticate using the identities loaded into the agent.
1.1 stevesk 402: .It Cm ForwardX11
403: Specifies whether X11 connections will be automatically redirected
404: over the secure channel and
405: .Ev DISPLAY
406: set.
407: The argument must be
408: .Dq yes
409: or
410: .Dq no .
411: The default is
412: .Dq no .
1.3 stevesk 413: .Pp
1.7 jmc 414: X11 forwarding should be enabled with caution.
415: Users with the ability to bypass file permissions on the remote host
1.22 markus 416: (for the user's X11 authorization database)
1.7 jmc 417: can access the local X11 display through the forwarded connection.
1.22 markus 418: An attacker may then be able to perform activities such as keystroke monitoring
419: if the
420: .Cm ForwardX11Trusted
421: option is also enabled.
422: .It Cm ForwardX11Trusted
1.34 jmc 423: If this option is set to
1.22 markus 424: .Dq yes
425: then remote X11 clients will have full access to the original X11 display.
1.42 djm 426: .Pp
1.22 markus 427: If this option is set to
428: .Dq no
429: then remote X11 clients will be considered untrusted and prevented
430: from stealing or tampering with data belonging to trusted X11
431: clients.
1.42 djm 432: Furthermore, the
433: .Xr xauth 1
434: token used for the session will be set to expire after 20 minutes.
435: Remote clients will be refused access after this time.
1.22 markus 436: .Pp
437: The default is
438: .Dq no .
439: .Pp
440: See the X11 SECURITY extension specification for full details on
441: the restrictions imposed on untrusted clients.
1.1 stevesk 442: .It Cm GatewayPorts
443: Specifies whether remote hosts are allowed to connect to local
444: forwarded ports.
445: By default,
446: .Nm ssh
1.7 jmc 447: binds local port forwardings to the loopback address.
448: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 449: .Cm GatewayPorts
450: can be used to specify that
451: .Nm ssh
452: should bind local port forwardings to the wildcard address,
453: thus allowing remote hosts to connect to forwarded ports.
454: The argument must be
455: .Dq yes
456: or
457: .Dq no .
458: The default is
459: .Dq no .
460: .It Cm GlobalKnownHostsFile
461: Specifies a file to use for the global
462: host key database instead of
463: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 464: .It Cm GSSAPIAuthentication
1.27 markus 465: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 466: The default is
1.21 markus 467: .Dq no .
1.18 markus 468: Note that this option applies to protocol version 2 only.
469: .It Cm GSSAPIDelegateCredentials
470: Forward (delegate) credentials to the server.
471: The default is
472: .Dq no .
473: Note that this option applies to protocol version 2 only.
1.44 djm 474: .It Cm HashKnownHosts
475: Indicates that
476: .Nm ssh
477: should hash host names and addresses when they are added to
1.50 djm 478: .Pa ~/.ssh/known_hosts .
1.44 djm 479: These hashed names may be used normally by
480: .Nm ssh
481: and
482: .Nm sshd ,
483: but they do not reveal identifying information should the file's contents
484: be disclosed.
485: The default is
486: .Dq no .
1.46 jmc 487: Note that hashing of names and addresses will not be retrospectively applied
1.45 djm 488: to existing known hosts files, but these may be manually hashed using
489: .Xr ssh-keygen 1 .
1.1 stevesk 490: .It Cm HostbasedAuthentication
491: Specifies whether to try rhosts based authentication with public key
492: authentication.
493: The argument must be
494: .Dq yes
495: or
496: .Dq no .
497: The default is
498: .Dq no .
499: This option applies to protocol version 2 only and
500: is similar to
501: .Cm RhostsRSAAuthentication .
502: .It Cm HostKeyAlgorithms
503: Specifies the protocol version 2 host key algorithms
504: that the client wants to use in order of preference.
505: The default for this option is:
506: .Dq ssh-rsa,ssh-dss .
507: .It Cm HostKeyAlias
508: Specifies an alias that should be used instead of the
509: real host name when looking up or saving the host key
510: in the host key database files.
511: This option is useful for tunneling ssh connections
512: or for multiple servers running on a single host.
513: .It Cm HostName
514: Specifies the real host name to log into.
515: This can be used to specify nicknames or abbreviations for hosts.
516: Default is the name given on the command line.
517: Numeric IP addresses are also permitted (both on the command line and in
518: .Cm HostName
519: specifications).
520: .It Cm IdentityFile
521: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 522: is read.
523: The default is
1.50 djm 524: .Pa ~/.ssh/identity
1.1 stevesk 525: for protocol version 1, and
1.50 djm 526: .Pa ~/.ssh/id_rsa
1.1 stevesk 527: and
1.50 djm 528: .Pa ~/.ssh/id_dsa
1.1 stevesk 529: for protocol version 2.
530: Additionally, any identities represented by the authentication agent
531: will be used for authentication.
532: The file name may use the tilde
533: syntax to refer to a user's home directory.
534: It is possible to have
535: multiple identity files specified in configuration files; all these
536: identities will be tried in sequence.
1.29 markus 537: .It Cm IdentitiesOnly
538: Specifies that
539: .Nm ssh
540: should only use the authentication identity files configured in the
1.31 jmc 541: .Nm
1.29 markus 542: files,
543: even if the
544: .Nm ssh-agent
545: offers more identities.
546: The argument to this keyword must be
547: .Dq yes
548: or
549: .Dq no .
550: This option is intented for situations where
551: .Nm ssh-agent
552: offers many different identities.
553: The default is
554: .Dq no .
1.39 djm 555: .It Cm KbdInteractiveDevices
556: Specifies the list of methods to use in keyboard-interactive authentication.
557: Multiple method names must be comma-separated.
558: The default is to use the server specified list.
1.1 stevesk 559: .It Cm LocalForward
560: Specifies that a TCP/IP port on the local machine be forwarded over
561: the secure channel to the specified host and port from the remote machine.
1.49 jmc 562: The first argument must be
1.43 djm 563: .Sm off
1.49 jmc 564: .Oo Ar bind_address : Oc Ar port
1.43 djm 565: .Sm on
1.49 jmc 566: and the second argument must be
567: .Ar host : Ns Ar hostport .
1.46 jmc 568: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 569: by using an alternative syntax:
1.49 jmc 570: .Oo Ar bind_address Ns / Oc Ns Ar port
571: and
572: .Ar host Ns / Ns Ar hostport .
1.46 jmc 573: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 574: given on the command line.
1.1 stevesk 575: Only the superuser can forward privileged ports.
1.43 djm 576: By default, the local port is bound in accordance with the
577: .Cm GatewayPorts
578: setting.
579: However, an explicit
580: .Ar bind_address
581: may be used to bind the connection to a specific address.
582: The
583: .Ar bind_address
584: of
585: .Dq localhost
1.46 jmc 586: indicates that the listening port be bound for local use only, while an
587: empty address or
588: .Sq *
1.43 djm 589: indicates that the port should be available from all interfaces.
1.1 stevesk 590: .It Cm LogLevel
591: Gives the verbosity level that is used when logging messages from
592: .Nm ssh .
593: The possible values are:
594: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 595: The default is INFO.
596: DEBUG and DEBUG1 are equivalent.
597: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 598: .It Cm MACs
599: Specifies the MAC (message authentication code) algorithms
600: in order of preference.
601: The MAC algorithm is used in protocol version 2
602: for data integrity protection.
603: Multiple algorithms must be comma-separated.
604: The default is
605: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
606: .It Cm NoHostAuthenticationForLocalhost
607: This option can be used if the home directory is shared across machines.
608: In this case localhost will refer to a different machine on each of
609: the machines and the user will get many warnings about changed host keys.
610: However, this option disables host authentication for localhost.
611: The argument to this keyword must be
612: .Dq yes
613: or
614: .Dq no .
615: The default is to check the host key for localhost.
616: .It Cm NumberOfPasswordPrompts
617: Specifies the number of password prompts before giving up.
618: The argument to this keyword must be an integer.
619: Default is 3.
620: .It Cm PasswordAuthentication
621: Specifies whether to use password authentication.
622: The argument to this keyword must be
623: .Dq yes
624: or
625: .Dq no .
626: The default is
627: .Dq yes .
628: .It Cm Port
629: Specifies the port number to connect on the remote host.
630: Default is 22.
631: .It Cm PreferredAuthentications
632: Specifies the order in which the client should try protocol 2
1.11 jmc 633: authentication methods.
1.48 jmc 634: This allows a client to prefer one method (e.g.\&
1.1 stevesk 635: .Cm keyboard-interactive )
1.48 jmc 636: over another method (e.g.\&
1.1 stevesk 637: .Cm password )
638: The default for this option is:
639: .Dq hostbased,publickey,keyboard-interactive,password .
640: .It Cm Protocol
641: Specifies the protocol versions
642: .Nm ssh
643: should support in order of preference.
644: The possible values are
645: .Dq 1
646: and
647: .Dq 2 .
648: Multiple versions must be comma-separated.
649: The default is
650: .Dq 2,1 .
651: This means that
652: .Nm ssh
653: tries version 2 and falls back to version 1
654: if version 2 is not available.
655: .It Cm ProxyCommand
656: Specifies the command to use to connect to the server.
657: The command
658: string extends to the end of the line, and is executed with
659: .Pa /bin/sh .
660: In the command string,
661: .Ql %h
662: will be substituted by the host name to
663: connect and
664: .Ql %p
665: by the port.
666: The command can be basically anything,
667: and should read from its standard input and write to its standard output.
668: It should eventually connect an
669: .Xr sshd 8
670: server running on some machine, or execute
671: .Ic sshd -i
672: somewhere.
673: Host key management will be done using the
674: HostName of the host being connected (defaulting to the name typed by
675: the user).
1.7 jmc 676: Setting the command to
677: .Dq none
1.6 markus 678: disables this option entirely.
1.1 stevesk 679: Note that
680: .Cm CheckHostIP
681: is not available for connects with a proxy command.
1.52 djm 682: .Pp
683: This directive is useful in conjunction with
684: .Xr nc 1
685: and its proxy support.
1.53 jmc 686: For example, the following directive would connect via an HTTP proxy at
1.52 djm 687: 192.0.2.0:
688: .Bd -literal -offset 3n
689: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
690: .Ed
1.1 stevesk 691: .It Cm PubkeyAuthentication
692: Specifies whether to try public key authentication.
693: The argument to this keyword must be
694: .Dq yes
695: or
696: .Dq no .
697: The default is
698: .Dq yes .
699: This option applies to protocol version 2 only.
700: .It Cm RemoteForward
701: Specifies that a TCP/IP port on the remote machine be forwarded over
702: the secure channel to the specified host and port from the local machine.
1.49 jmc 703: The first argument must be
1.43 djm 704: .Sm off
1.49 jmc 705: .Oo Ar bind_address : Oc Ar port
1.43 djm 706: .Sm on
1.49 jmc 707: and the second argument must be
708: .Ar host : Ns Ar hostport .
709: IPv6 addresses can be specified by enclosing addresses in square brackets
710: or by using an alternative syntax:
711: .Oo Ar bind_address Ns / Oc Ns Ar port
712: and
713: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 714: Multiple forwardings may be specified, and additional
715: forwardings can be given on the command line.
716: Only the superuser can forward privileged ports.
1.43 djm 717: .Pp
718: If the
719: .Ar bind_address
720: is not specified, the default is to only bind to loopback addresses.
721: If the
722: .Ar bind_address
723: is
724: .Ql *
725: or an empty string, then the forwarding is requested to listen on all
726: interfaces.
727: Specifying a remote
728: .Ar bind_address
1.46 jmc 729: will only succeed if the server's
730: .Cm GatewayPorts
1.43 djm 731: option is enabled (see
1.46 jmc 732: .Xr sshd_config 5 ) .
1.1 stevesk 733: .It Cm RhostsRSAAuthentication
734: Specifies whether to try rhosts based authentication with RSA host
735: authentication.
736: The argument must be
737: .Dq yes
738: or
739: .Dq no .
740: The default is
741: .Dq no .
742: This option applies to protocol version 1 only and requires
743: .Nm ssh
744: to be setuid root.
745: .It Cm RSAAuthentication
746: Specifies whether to try RSA authentication.
747: The argument to this keyword must be
748: .Dq yes
749: or
750: .Dq no .
751: RSA authentication will only be
752: attempted if the identity file exists, or an authentication agent is
753: running.
754: The default is
755: .Dq yes .
756: Note that this option applies to protocol version 1 only.
1.32 djm 757: .It Cm SendEnv
758: Specifies what variables from the local
759: .Xr environ 7
760: should be sent to the server.
761: Note that environment passing is only supported for protocol 2, the
1.33 djm 762: server must also support it, and the server must be configured to
763: accept these environment variables.
1.32 djm 764: Refer to
765: .Cm AcceptEnv
766: in
767: .Xr sshd_config 5
768: for how to configure the server.
769: Variables are specified by name, which may contain the wildcard characters
770: .Ql \&*
771: and
772: .Ql \&? .
1.33 djm 773: Multiple environment variables may be separated by whitespace or spread
1.32 djm 774: across multiple
775: .Cm SendEnv
776: directives.
777: The default is not to send any environment variables.
1.28 markus 778: .It Cm ServerAliveInterval
779: Sets a timeout interval in seconds after which if no data has been received
780: from the server,
781: .Nm ssh
782: will send a message through the encrypted
783: channel to request a response from the server.
784: The default
785: is 0, indicating that these messages will not be sent to the server.
786: This option applies to protocol version 2 only.
787: .It Cm ServerAliveCountMax
788: Sets the number of server alive messages (see above) which may be
789: sent without
790: .Nm ssh
791: receiving any messages back from the server.
792: If this threshold is reached while server alive messages are being sent,
793: .Nm ssh
794: will disconnect from the server, terminating the session.
795: It is important to note that the use of server alive messages is very
796: different from
797: .Cm TCPKeepAlive
798: (below).
799: The server alive messages are sent through the encrypted channel
800: and therefore will not be spoofable.
801: The TCP keepalive option enabled by
802: .Cm TCPKeepAlive
803: is spoofable.
804: The server alive mechanism is valuable when the client or
805: server depend on knowing when a connection has become inactive.
806: .Pp
807: The default value is 3.
808: If, for example,
809: .Cm ServerAliveInterval
810: (above) is set to 15, and
811: .Cm ServerAliveCountMax
812: is left at the default, if the server becomes unresponsive ssh
813: will disconnect after approximately 45 seconds.
1.1 stevesk 814: .It Cm SmartcardDevice
1.11 jmc 815: Specifies which smartcard device to use.
816: The argument to this keyword is the device
1.1 stevesk 817: .Nm ssh
818: should use to communicate with a smartcard used for storing the user's
1.11 jmc 819: private RSA key.
820: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 821: .It Cm StrictHostKeyChecking
822: If this flag is set to
823: .Dq yes ,
824: .Nm ssh
825: will never automatically add host keys to the
1.50 djm 826: .Pa ~/.ssh/known_hosts
1.1 stevesk 827: file, and refuses to connect to hosts whose host key has changed.
828: This provides maximum protection against trojan horse attacks,
829: however, can be annoying when the
830: .Pa /etc/ssh/ssh_known_hosts
831: file is poorly maintained, or connections to new hosts are
832: frequently made.
833: This option forces the user to manually
834: add all new hosts.
835: If this flag is set to
836: .Dq no ,
837: .Nm ssh
838: will automatically add new host keys to the
839: user known hosts files.
840: If this flag is set to
841: .Dq ask ,
842: new host keys
843: will be added to the user known host files only after the user
844: has confirmed that is what they really want to do, and
845: .Nm ssh
846: will refuse to connect to hosts whose host key has changed.
847: The host keys of
848: known hosts will be verified automatically in all cases.
849: The argument must be
850: .Dq yes ,
851: .Dq no
852: or
853: .Dq ask .
854: The default is
855: .Dq ask .
1.26 markus 856: .It Cm TCPKeepAlive
857: Specifies whether the system should send TCP keepalive messages to the
858: other side.
859: If they are sent, death of the connection or crash of one
860: of the machines will be properly noticed.
861: However, this means that
862: connections will die if the route is down temporarily, and some people
863: find it annoying.
864: .Pp
865: The default is
866: .Dq yes
867: (to send TCP keepalive messages), and the client will notice
868: if the network goes down or the remote host dies.
869: This is important in scripts, and many users want it too.
870: .Pp
871: To disable TCP keepalive messages, the value should be set to
872: .Dq no .
1.1 stevesk 873: .It Cm UsePrivilegedPort
874: Specifies whether to use a privileged port for outgoing connections.
875: The argument must be
876: .Dq yes
877: or
878: .Dq no .
879: The default is
880: .Dq no .
1.4 stevesk 881: If set to
882: .Dq yes
883: .Nm ssh
884: must be setuid root.
1.1 stevesk 885: Note that this option must be set to
886: .Dq yes
1.17 markus 887: for
1.1 stevesk 888: .Cm RhostsRSAAuthentication
1.17 markus 889: with older servers.
1.1 stevesk 890: .It Cm User
891: Specifies the user to log in as.
892: This can be useful when a different user name is used on different machines.
893: This saves the trouble of
894: having to remember to give the user name on the command line.
895: .It Cm UserKnownHostsFile
896: Specifies a file to use for the user
897: host key database instead of
1.50 djm 898: .Pa ~/.ssh/known_hosts .
1.8 jakob 899: .It Cm VerifyHostKeyDNS
900: Specifies whether to verify the remote key using DNS and SSHFP resource
901: records.
1.24 jakob 902: If this option is set to
903: .Dq yes ,
1.25 jmc 904: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 905: from DNS.
906: Insecure fingerprints will be handled as if this option was set to
907: .Dq ask .
908: If this option is set to
909: .Dq ask ,
910: information on fingerprint match will be displayed, but the user will still
911: need to confirm new host keys according to the
912: .Cm StrictHostKeyChecking
913: option.
914: The argument must be
915: .Dq yes ,
916: .Dq no
1.25 jmc 917: or
918: .Dq ask .
1.8 jakob 919: The default is
920: .Dq no .
1.12 jakob 921: Note that this option applies to protocol version 2 only.
1.1 stevesk 922: .It Cm XAuthLocation
1.5 stevesk 923: Specifies the full pathname of the
1.1 stevesk 924: .Xr xauth 1
925: program.
926: The default is
927: .Pa /usr/X11R6/bin/xauth .
928: .El
929: .Sh FILES
930: .Bl -tag -width Ds
1.50 djm 931: .It Pa ~/.ssh/config
1.1 stevesk 932: This is the per-user configuration file.
933: The format of this file is described above.
934: This file is used by the
935: .Nm ssh
936: client.
1.30 djm 937: Because of the potential for abuse, this file must have strict permissions:
938: read/write for the user, and not accessible by others.
1.1 stevesk 939: .It Pa /etc/ssh/ssh_config
940: Systemwide configuration file.
941: This file provides defaults for those
942: values that are not specified in the user's configuration file, and
943: for those users who do not have a configuration file.
944: This file must be world-readable.
945: .El
1.13 jmc 946: .Sh SEE ALSO
947: .Xr ssh 1
1.1 stevesk 948: .Sh AUTHORS
949: OpenSSH is a derivative of the original and free
950: ssh 1.2.12 release by Tatu Ylonen.
951: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
952: Theo de Raadt and Dug Song
953: removed many bugs, re-added newer features and
954: created OpenSSH.
955: Markus Friedl contributed the support for SSH
956: protocol versions 1.5 and 2.0.
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