Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.38.2.3
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: .\"
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18: .\" modification, are permitted provided that the following conditions
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.38.2.3! brad 37: .\" $OpenBSD: ssh_config.5,v 1.61 2005/07/08 12:53:10 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
1.38.2.3! brad 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
1.38.2.3! brad 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.
1.38.2.1 brad 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
1.38.2.1 brad 123: (use IPv4 only) or
1.10 djm 124: .Dq inet6
1.38.2.1 brad 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
1.38.2.3! brad 139: Use the specified address on the local machine as the source address of
! 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 ,
1.38.2.3! brad 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
1.38.2.3! brad 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).
1.36 djm 266: These sessions will reuse the master instance's network connection rather
267: than initiating new ones.
1.37 djm 268: Setting this to
269: .Dq ask
270: will cause
271: .Nm ssh
272: to listen for control connections, but require confirmation using the
273: .Ev SSH_ASKPASS
274: program before they are accepted (see
275: .Xr ssh-add 1
1.38 jmc 276: for details).
1.38.2.3! brad 277: If the
! 278: .Cm ControlPath
! 279: can not be opened,
! 280: .Nm ssh
! 281: will continue without connecting to a master instance.
! 282: .Pp
! 283: X11 and
! 284: .Xr ssh-agent 1
! 285: forwarding is supported over these multiplexed connections, however the
! 286: display and agent fowarded will be the one belonging to the master
! 287: connection i.e. it is not possible to forward multiple displays or agents.
! 288: .Pp
! 289: Two additional options allow for opportunistic multiplexing: try to use a
! 290: master connection but fall back to creating a new one if one does not already
! 291: exist.
! 292: These options are:
! 293: .Dq auto
! 294: and
! 295: .Dq autoask .
! 296: The latter requires confirmation like the
! 297: .Dq ask
! 298: option.
1.36 djm 299: .It Cm ControlPath
1.38.2.3! brad 300: Specify the path to the control socket used for connection sharing as described
! 301: in the
1.36 djm 302: .Cm ControlMaster
1.38.2.3! brad 303: section above or the string
! 304: .Dq none
! 305: to disable connection sharing.
! 306: In the path,
! 307: .Ql %h
! 308: will be substituted by the target host name,
! 309: .Ql %p
! 310: the port and
! 311: .Ql %r
! 312: by the remote login username.
! 313: It is recommended that any
! 314: .Cm ControlPath
! 315: used for opportunistic connection sharing include
! 316: all three of these escape sequences.
! 317: This ensures that shared connections are uniquely identified.
1.38 jmc 318: .It Cm DynamicForward
319: Specifies that a TCP/IP port on the local machine be forwarded
320: over the secure channel, and the application
321: protocol is then used to determine where to connect to from the
322: remote machine.
323: The argument must be a port number.
324: Currently the SOCKS4 and SOCKS5 protocols are supported, and
325: .Nm ssh
326: will act as a SOCKS server.
327: Multiple forwardings may be specified, and
328: additional forwardings can be given on the command line.
329: Only the superuser can forward privileged ports.
1.14 markus 330: .It Cm EnableSSHKeysign
331: Setting this option to
332: .Dq yes
333: in the global client configuration file
334: .Pa /etc/ssh/ssh_config
335: enables the use of the helper program
336: .Xr ssh-keysign 8
337: during
338: .Cm HostbasedAuthentication .
339: The argument must be
340: .Dq yes
341: or
342: .Dq no .
343: The default is
344: .Dq no .
1.23 jmc 345: This option should be placed in the non-hostspecific section.
1.14 markus 346: See
347: .Xr ssh-keysign 8
348: for more information.
1.1 stevesk 349: .It Cm EscapeChar
350: Sets the escape character (default:
351: .Ql ~ ) .
352: The escape character can also
353: be set on the command line.
354: The argument should be a single character,
355: .Ql ^
356: followed by a letter, or
357: .Dq none
358: to disable the escape
359: character entirely (making the connection transparent for binary
360: data).
361: .It Cm ForwardAgent
362: Specifies whether the connection to the authentication agent (if any)
363: will be forwarded to the remote machine.
364: The argument must be
365: .Dq yes
366: or
367: .Dq no .
368: The default is
369: .Dq no .
1.3 stevesk 370: .Pp
1.7 jmc 371: Agent forwarding should be enabled with caution.
372: Users with the ability to bypass file permissions on the remote host
373: (for the agent's Unix-domain socket)
374: can access the local agent through the forwarded connection.
375: An attacker cannot obtain key material from the agent,
1.3 stevesk 376: however they can perform operations on the keys that enable them to
377: authenticate using the identities loaded into the agent.
1.1 stevesk 378: .It Cm ForwardX11
379: Specifies whether X11 connections will be automatically redirected
380: over the secure channel and
381: .Ev DISPLAY
382: set.
383: The argument must be
384: .Dq yes
385: or
386: .Dq no .
387: The default is
388: .Dq no .
1.3 stevesk 389: .Pp
1.7 jmc 390: X11 forwarding should be enabled with caution.
391: Users with the ability to bypass file permissions on the remote host
1.22 markus 392: (for the user's X11 authorization database)
1.7 jmc 393: can access the local X11 display through the forwarded connection.
1.22 markus 394: An attacker may then be able to perform activities such as keystroke monitoring
395: if the
396: .Cm ForwardX11Trusted
397: option is also enabled.
398: .It Cm ForwardX11Trusted
1.34 jmc 399: If this option is set to
1.22 markus 400: .Dq yes
401: then remote X11 clients will have full access to the original X11 display.
1.38.2.1 brad 402: .Pp
1.22 markus 403: If this option is set to
404: .Dq no
405: then remote X11 clients will be considered untrusted and prevented
406: from stealing or tampering with data belonging to trusted X11
407: clients.
1.38.2.1 brad 408: Furthermore, the
409: .Xr xauth 1
410: token used for the session will be set to expire after 20 minutes.
411: Remote clients will be refused access after this time.
1.22 markus 412: .Pp
413: The default is
414: .Dq no .
415: .Pp
416: See the X11 SECURITY extension specification for full details on
417: the restrictions imposed on untrusted clients.
1.1 stevesk 418: .It Cm GatewayPorts
419: Specifies whether remote hosts are allowed to connect to local
420: forwarded ports.
421: By default,
422: .Nm ssh
1.7 jmc 423: binds local port forwardings to the loopback address.
424: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 425: .Cm GatewayPorts
426: can be used to specify that
427: .Nm ssh
428: should bind local port forwardings to the wildcard address,
429: thus allowing remote hosts to connect to forwarded ports.
430: The argument must be
431: .Dq yes
432: or
433: .Dq no .
434: The default is
435: .Dq no .
436: .It Cm GlobalKnownHostsFile
437: Specifies a file to use for the global
438: host key database instead of
439: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 440: .It Cm GSSAPIAuthentication
1.27 markus 441: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 442: The default is
1.21 markus 443: .Dq no .
1.18 markus 444: Note that this option applies to protocol version 2 only.
445: .It Cm GSSAPIDelegateCredentials
446: Forward (delegate) credentials to the server.
447: The default is
448: .Dq no .
449: Note that this option applies to protocol version 2 only.
1.38.2.1 brad 450: .It Cm HashKnownHosts
451: Indicates that
452: .Nm ssh
453: should hash host names and addresses when they are added to
1.38.2.3! brad 454: .Pa ~/.ssh/known_hosts .
1.38.2.1 brad 455: These hashed names may be used normally by
456: .Nm ssh
457: and
458: .Nm sshd ,
459: but they do not reveal identifying information should the file's contents
460: be disclosed.
461: The default is
462: .Dq no .
463: Note that hashing of names and addresses will not be retrospectively applied
464: to existing known hosts files, but these may be manually hashed using
465: .Xr ssh-keygen 1 .
1.1 stevesk 466: .It Cm HostbasedAuthentication
467: Specifies whether to try rhosts based authentication with public key
468: authentication.
469: The argument must be
470: .Dq yes
471: or
472: .Dq no .
473: The default is
474: .Dq no .
475: This option applies to protocol version 2 only and
476: is similar to
477: .Cm RhostsRSAAuthentication .
478: .It Cm HostKeyAlgorithms
479: Specifies the protocol version 2 host key algorithms
480: that the client wants to use in order of preference.
481: The default for this option is:
482: .Dq ssh-rsa,ssh-dss .
483: .It Cm HostKeyAlias
484: Specifies an alias that should be used instead of the
485: real host name when looking up or saving the host key
486: in the host key database files.
487: This option is useful for tunneling ssh connections
488: or for multiple servers running on a single host.
489: .It Cm HostName
490: Specifies the real host name to log into.
491: This can be used to specify nicknames or abbreviations for hosts.
492: Default is the name given on the command line.
493: Numeric IP addresses are also permitted (both on the command line and in
494: .Cm HostName
495: specifications).
496: .It Cm IdentityFile
497: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 498: is read.
499: The default is
1.38.2.3! brad 500: .Pa ~/.ssh/identity
1.1 stevesk 501: for protocol version 1, and
1.38.2.3! brad 502: .Pa ~/.ssh/id_rsa
1.1 stevesk 503: and
1.38.2.3! brad 504: .Pa ~/.ssh/id_dsa
1.1 stevesk 505: for protocol version 2.
506: Additionally, any identities represented by the authentication agent
507: will be used for authentication.
508: The file name may use the tilde
509: syntax to refer to a user's home directory.
510: It is possible to have
511: multiple identity files specified in configuration files; all these
512: identities will be tried in sequence.
1.29 markus 513: .It Cm IdentitiesOnly
514: Specifies that
515: .Nm ssh
516: should only use the authentication identity files configured in the
1.31 jmc 517: .Nm
1.29 markus 518: files,
519: even if the
520: .Nm ssh-agent
521: offers more identities.
522: The argument to this keyword must be
523: .Dq yes
524: or
525: .Dq no .
526: This option is intented for situations where
527: .Nm ssh-agent
528: offers many different identities.
529: The default is
530: .Dq no .
1.38.2.1 brad 531: .It Cm KbdInteractiveDevices
532: Specifies the list of methods to use in keyboard-interactive authentication.
533: Multiple method names must be comma-separated.
534: The default is to use the server specified list.
1.1 stevesk 535: .It Cm LocalForward
536: Specifies that a TCP/IP port on the local machine be forwarded over
537: the secure channel to the specified host and port from the remote machine.
1.38.2.2 brad 538: The first argument must be
1.38.2.1 brad 539: .Sm off
1.38.2.2 brad 540: .Oo Ar bind_address : Oc Ar port
1.38.2.1 brad 541: .Sm on
1.38.2.2 brad 542: and the second argument must be
543: .Ar host : Ns Ar hostport .
1.38.2.1 brad 544: IPv6 addresses can be specified by enclosing addresses in square brackets or
545: by using an alternative syntax:
1.38.2.2 brad 546: .Oo Ar bind_address Ns / Oc Ns Ar port
547: and
548: .Ar host Ns / Ns Ar hostport .
1.38.2.1 brad 549: Multiple forwardings may be specified, and additional forwardings can be
550: given on the command line.
1.1 stevesk 551: Only the superuser can forward privileged ports.
1.38.2.1 brad 552: By default, the local port is bound in accordance with the
553: .Cm GatewayPorts
554: setting.
555: However, an explicit
556: .Ar bind_address
557: may be used to bind the connection to a specific address.
558: The
559: .Ar bind_address
560: of
561: .Dq localhost
562: indicates that the listening port be bound for local use only, while an
563: empty address or
564: .Sq *
565: indicates that the port should be available from all interfaces.
1.1 stevesk 566: .It Cm LogLevel
567: Gives the verbosity level that is used when logging messages from
568: .Nm ssh .
569: The possible values are:
570: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 571: The default is INFO.
572: DEBUG and DEBUG1 are equivalent.
573: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 574: .It Cm MACs
575: Specifies the MAC (message authentication code) algorithms
576: in order of preference.
577: The MAC algorithm is used in protocol version 2
578: for data integrity protection.
579: Multiple algorithms must be comma-separated.
580: The default is
581: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
582: .It Cm NoHostAuthenticationForLocalhost
583: This option can be used if the home directory is shared across machines.
584: In this case localhost will refer to a different machine on each of
585: the machines and the user will get many warnings about changed host keys.
586: However, this option disables host authentication for localhost.
587: The argument to this keyword must be
588: .Dq yes
589: or
590: .Dq no .
591: The default is to check the host key for localhost.
592: .It Cm NumberOfPasswordPrompts
593: Specifies the number of password prompts before giving up.
594: The argument to this keyword must be an integer.
595: Default is 3.
596: .It Cm PasswordAuthentication
597: Specifies whether to use password authentication.
598: The argument to this keyword must be
599: .Dq yes
600: or
601: .Dq no .
602: The default is
603: .Dq yes .
604: .It Cm Port
605: Specifies the port number to connect on the remote host.
606: Default is 22.
607: .It Cm PreferredAuthentications
608: Specifies the order in which the client should try protocol 2
1.11 jmc 609: authentication methods.
1.38.2.2 brad 610: This allows a client to prefer one method (e.g.\&
1.1 stevesk 611: .Cm keyboard-interactive )
1.38.2.2 brad 612: over another method (e.g.\&
1.1 stevesk 613: .Cm password )
614: The default for this option is:
615: .Dq hostbased,publickey,keyboard-interactive,password .
616: .It Cm Protocol
617: Specifies the protocol versions
618: .Nm ssh
619: should support in order of preference.
620: The possible values are
621: .Dq 1
622: and
623: .Dq 2 .
624: Multiple versions must be comma-separated.
625: The default is
626: .Dq 2,1 .
627: This means that
628: .Nm ssh
629: tries version 2 and falls back to version 1
630: if version 2 is not available.
631: .It Cm ProxyCommand
632: Specifies the command to use to connect to the server.
633: The command
634: string extends to the end of the line, and is executed with
635: .Pa /bin/sh .
636: In the command string,
637: .Ql %h
638: will be substituted by the host name to
639: connect and
640: .Ql %p
641: by the port.
642: The command can be basically anything,
643: and should read from its standard input and write to its standard output.
644: It should eventually connect an
645: .Xr sshd 8
646: server running on some machine, or execute
647: .Ic sshd -i
648: somewhere.
649: Host key management will be done using the
650: HostName of the host being connected (defaulting to the name typed by
651: the user).
1.7 jmc 652: Setting the command to
653: .Dq none
1.6 markus 654: disables this option entirely.
1.1 stevesk 655: Note that
656: .Cm CheckHostIP
657: is not available for connects with a proxy command.
658: .Pp
1.38.2.3! brad 659: This directive is useful in conjunction with
! 660: .Xr nc 1
! 661: and its proxy support.
! 662: For example, the following directive would connect via an HTTP proxy at
! 663: 192.0.2.0:
! 664: .Bd -literal -offset 3n
! 665: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
! 666: .Ed
1.1 stevesk 667: .It Cm PubkeyAuthentication
668: Specifies whether to try public key authentication.
669: The argument to this keyword must be
670: .Dq yes
671: or
672: .Dq no .
673: The default is
674: .Dq yes .
675: This option applies to protocol version 2 only.
676: .It Cm RemoteForward
677: Specifies that a TCP/IP port on the remote machine be forwarded over
678: the secure channel to the specified host and port from the local machine.
1.38.2.2 brad 679: The first argument must be
1.38.2.1 brad 680: .Sm off
1.38.2.2 brad 681: .Oo Ar bind_address : Oc Ar port
1.38.2.1 brad 682: .Sm on
1.38.2.2 brad 683: and the second argument must be
684: .Ar host : Ns Ar hostport .
685: IPv6 addresses can be specified by enclosing addresses in square brackets
686: or by using an alternative syntax:
687: .Oo Ar bind_address Ns / Oc Ns Ar port
688: and
689: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 690: Multiple forwardings may be specified, and additional
691: forwardings can be given on the command line.
692: Only the superuser can forward privileged ports.
1.38.2.1 brad 693: .Pp
694: If the
695: .Ar bind_address
696: is not specified, the default is to only bind to loopback addresses.
697: If the
698: .Ar bind_address
699: is
700: .Ql *
701: or an empty string, then the forwarding is requested to listen on all
702: interfaces.
703: Specifying a remote
704: .Ar bind_address
705: will only succeed if the server's
706: .Cm GatewayPorts
707: option is enabled (see
708: .Xr sshd_config 5 ) .
1.1 stevesk 709: .It Cm RhostsRSAAuthentication
710: Specifies whether to try rhosts based authentication with RSA host
711: authentication.
712: The argument must be
713: .Dq yes
714: or
715: .Dq no .
716: The default is
717: .Dq no .
718: This option applies to protocol version 1 only and requires
719: .Nm ssh
720: to be setuid root.
721: .It Cm RSAAuthentication
722: Specifies whether to try RSA authentication.
723: The argument to this keyword must be
724: .Dq yes
725: or
726: .Dq no .
727: RSA authentication will only be
728: attempted if the identity file exists, or an authentication agent is
729: running.
730: The default is
731: .Dq yes .
732: Note that this option applies to protocol version 1 only.
1.32 djm 733: .It Cm SendEnv
734: Specifies what variables from the local
735: .Xr environ 7
736: should be sent to the server.
737: Note that environment passing is only supported for protocol 2, the
1.33 djm 738: server must also support it, and the server must be configured to
739: accept these environment variables.
1.32 djm 740: Refer to
741: .Cm AcceptEnv
742: in
743: .Xr sshd_config 5
744: for how to configure the server.
745: Variables are specified by name, which may contain the wildcard characters
746: .Ql \&*
747: and
748: .Ql \&? .
1.33 djm 749: Multiple environment variables may be separated by whitespace or spread
1.32 djm 750: across multiple
751: .Cm SendEnv
752: directives.
753: The default is not to send any environment variables.
1.28 markus 754: .It Cm ServerAliveInterval
755: Sets a timeout interval in seconds after which if no data has been received
756: from the server,
757: .Nm ssh
758: will send a message through the encrypted
759: channel to request a response from the server.
760: The default
761: is 0, indicating that these messages will not be sent to the server.
762: This option applies to protocol version 2 only.
763: .It Cm ServerAliveCountMax
764: Sets the number of server alive messages (see above) which may be
765: sent without
766: .Nm ssh
767: receiving any messages back from the server.
768: If this threshold is reached while server alive messages are being sent,
769: .Nm ssh
770: will disconnect from the server, terminating the session.
771: It is important to note that the use of server alive messages is very
772: different from
773: .Cm TCPKeepAlive
774: (below).
775: The server alive messages are sent through the encrypted channel
776: and therefore will not be spoofable.
777: The TCP keepalive option enabled by
778: .Cm TCPKeepAlive
779: is spoofable.
780: The server alive mechanism is valuable when the client or
781: server depend on knowing when a connection has become inactive.
782: .Pp
783: The default value is 3.
784: If, for example,
785: .Cm ServerAliveInterval
786: (above) is set to 15, and
787: .Cm ServerAliveCountMax
788: is left at the default, if the server becomes unresponsive ssh
789: will disconnect after approximately 45 seconds.
1.1 stevesk 790: .It Cm SmartcardDevice
1.11 jmc 791: Specifies which smartcard device to use.
792: The argument to this keyword is the device
1.1 stevesk 793: .Nm ssh
794: should use to communicate with a smartcard used for storing the user's
1.11 jmc 795: private RSA key.
796: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 797: .It Cm StrictHostKeyChecking
798: If this flag is set to
799: .Dq yes ,
800: .Nm ssh
801: will never automatically add host keys to the
1.38.2.3! brad 802: .Pa ~/.ssh/known_hosts
1.1 stevesk 803: file, and refuses to connect to hosts whose host key has changed.
804: This provides maximum protection against trojan horse attacks,
805: however, can be annoying when the
806: .Pa /etc/ssh/ssh_known_hosts
807: file is poorly maintained, or connections to new hosts are
808: frequently made.
809: This option forces the user to manually
810: add all new hosts.
811: If this flag is set to
812: .Dq no ,
813: .Nm ssh
814: will automatically add new host keys to the
815: user known hosts files.
816: If this flag is set to
817: .Dq ask ,
818: new host keys
819: will be added to the user known host files only after the user
820: has confirmed that is what they really want to do, and
821: .Nm ssh
822: will refuse to connect to hosts whose host key has changed.
823: The host keys of
824: known hosts will be verified automatically in all cases.
825: The argument must be
826: .Dq yes ,
827: .Dq no
828: or
829: .Dq ask .
830: The default is
831: .Dq ask .
1.26 markus 832: .It Cm TCPKeepAlive
833: Specifies whether the system should send TCP keepalive messages to the
834: other side.
835: If they are sent, death of the connection or crash of one
836: of the machines will be properly noticed.
837: However, this means that
838: connections will die if the route is down temporarily, and some people
839: find it annoying.
840: .Pp
841: The default is
842: .Dq yes
843: (to send TCP keepalive messages), and the client will notice
844: if the network goes down or the remote host dies.
845: This is important in scripts, and many users want it too.
846: .Pp
847: To disable TCP keepalive messages, the value should be set to
848: .Dq no .
1.1 stevesk 849: .It Cm UsePrivilegedPort
850: Specifies whether to use a privileged port for outgoing connections.
851: The argument must be
852: .Dq yes
853: or
854: .Dq no .
855: The default is
856: .Dq no .
1.4 stevesk 857: If set to
858: .Dq yes
859: .Nm ssh
860: must be setuid root.
1.1 stevesk 861: Note that this option must be set to
862: .Dq yes
1.17 markus 863: for
1.1 stevesk 864: .Cm RhostsRSAAuthentication
1.17 markus 865: with older servers.
1.1 stevesk 866: .It Cm User
867: Specifies the user to log in as.
868: This can be useful when a different user name is used on different machines.
869: This saves the trouble of
870: having to remember to give the user name on the command line.
871: .It Cm UserKnownHostsFile
872: Specifies a file to use for the user
873: host key database instead of
1.38.2.3! brad 874: .Pa ~/.ssh/known_hosts .
1.8 jakob 875: .It Cm VerifyHostKeyDNS
876: Specifies whether to verify the remote key using DNS and SSHFP resource
877: records.
1.24 jakob 878: If this option is set to
879: .Dq yes ,
1.25 jmc 880: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 881: from DNS.
882: Insecure fingerprints will be handled as if this option was set to
883: .Dq ask .
884: If this option is set to
885: .Dq ask ,
886: information on fingerprint match will be displayed, but the user will still
887: need to confirm new host keys according to the
888: .Cm StrictHostKeyChecking
889: option.
890: The argument must be
891: .Dq yes ,
892: .Dq no
1.25 jmc 893: or
894: .Dq ask .
1.8 jakob 895: The default is
896: .Dq no .
1.12 jakob 897: Note that this option applies to protocol version 2 only.
1.1 stevesk 898: .It Cm XAuthLocation
1.5 stevesk 899: Specifies the full pathname of the
1.1 stevesk 900: .Xr xauth 1
901: program.
902: The default is
903: .Pa /usr/X11R6/bin/xauth .
904: .El
905: .Sh FILES
906: .Bl -tag -width Ds
1.38.2.3! brad 907: .It Pa ~/.ssh/config
1.1 stevesk 908: This is the per-user configuration file.
909: The format of this file is described above.
910: This file is used by the
911: .Nm ssh
912: client.
1.30 djm 913: Because of the potential for abuse, this file must have strict permissions:
914: read/write for the user, and not accessible by others.
1.1 stevesk 915: .It Pa /etc/ssh/ssh_config
916: Systemwide configuration file.
917: This file provides defaults for those
918: values that are not specified in the user's configuration file, and
919: for those users who do not have a configuration file.
920: This file must be world-readable.
921: .El
1.13 jmc 922: .Sh SEE ALSO
923: .Xr ssh 1
1.1 stevesk 924: .Sh AUTHORS
925: OpenSSH is a derivative of the original and free
926: ssh 1.2.12 release by Tatu Ylonen.
927: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
928: Theo de Raadt and Dug Song
929: removed many bugs, re-added newer features and
930: created OpenSSH.
931: Markus Friedl contributed the support for SSH
932: protocol versions 1.5 and 2.0.
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