Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.79
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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1.79 ! jmc 37: .\" $OpenBSD: ssh_config.5,v 1.78 2006/02/12 10:49:44 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.
76: .Pp
77: The configuration file has the following format:
78: .Pp
79: Empty lines and lines starting with
80: .Ql #
81: are comments.
82: .Pp
83: Otherwise a line is of the format
84: .Dq keyword arguments .
85: Configuration options may be separated by whitespace or
86: optional whitespace and exactly one
87: .Ql = ;
88: the latter format is useful to avoid the need to quote whitespace
89: when specifying configuration options using the
90: .Nm ssh ,
91: .Nm scp
92: and
93: .Nm sftp
94: .Fl o
95: option.
96: .Pp
97: The possible
98: keywords and their meanings are as follows (note that
99: keywords are case-insensitive and arguments are case-sensitive):
100: .Bl -tag -width Ds
101: .It Cm Host
102: Restricts the following declarations (up to the next
103: .Cm Host
104: keyword) to be only for those hosts that match one of the patterns
105: given after the keyword.
106: .Ql \&*
107: and
1.20 jmc 108: .Ql \&?
1.1 stevesk 109: can be used as wildcards in the
110: patterns.
111: A single
112: .Ql \&*
113: as a pattern can be used to provide global
114: defaults for all hosts.
115: The host is the
116: .Ar hostname
117: argument given on the command line (i.e., the name is not converted to
118: a canonicalized host name before matching).
1.10 djm 119: .It Cm AddressFamily
1.11 jmc 120: Specifies which address family to use when connecting.
121: Valid arguments are
1.10 djm 122: .Dq any ,
123: .Dq inet
1.40 jmc 124: (use IPv4 only) or
1.10 djm 125: .Dq inet6
1.40 jmc 126: (use IPv6 only).
1.1 stevesk 127: .It Cm BatchMode
128: If set to
129: .Dq yes ,
130: passphrase/password querying will be disabled.
131: This option is useful in scripts and other batch jobs where no user
132: is present to supply the password.
133: The argument must be
134: .Dq yes
135: or
136: .Dq no .
137: The default is
138: .Dq no .
139: .It Cm BindAddress
1.60 dtucker 140: Use the specified address on the local machine as the source address of
1.61 jmc 141: the connection.
142: Only useful on systems with more than one address.
1.1 stevesk 143: Note that this option does not work if
144: .Cm UsePrivilegedPort
145: is set to
146: .Dq yes .
147: .It Cm ChallengeResponseAuthentication
148: Specifies whether to use challenge response authentication.
149: The argument to this keyword must be
150: .Dq yes
151: or
152: .Dq no .
153: The default is
154: .Dq yes .
155: .It Cm CheckHostIP
156: If this flag is set to
157: .Dq yes ,
158: ssh will additionally check the host IP address in the
159: .Pa known_hosts
160: file.
161: This allows ssh to detect if a host key changed due to DNS spoofing.
162: If the option is set to
163: .Dq no ,
164: the check will not be executed.
165: The default is
166: .Dq yes .
167: .It Cm Cipher
168: Specifies the cipher to use for encrypting the session
169: in protocol version 1.
170: Currently,
171: .Dq blowfish ,
172: .Dq 3des ,
173: and
174: .Dq des
175: are supported.
176: .Ar des
177: is only supported in the
178: .Nm ssh
179: client for interoperability with legacy protocol 1 implementations
180: that do not support the
181: .Ar 3des
1.7 jmc 182: cipher.
183: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 184: The default is
185: .Dq 3des .
186: .It Cm Ciphers
187: Specifies the ciphers allowed for protocol version 2
188: in order of preference.
189: Multiple ciphers must be comma-separated.
1.35 dtucker 190: The supported ciphers are
191: .Dq 3des-cbc ,
192: .Dq aes128-cbc ,
193: .Dq aes192-cbc ,
194: .Dq aes256-cbc ,
195: .Dq aes128-ctr ,
196: .Dq aes192-ctr ,
197: .Dq aes256-ctr ,
1.54 djm 198: .Dq arcfour128 ,
199: .Dq arcfour256 ,
1.35 dtucker 200: .Dq arcfour ,
201: .Dq blowfish-cbc ,
202: and
203: .Dq cast128-cbc .
1.1 stevesk 204: The default is
205: .Bd -literal
1.54 djm 206: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
207: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
208: aes192-ctr,aes256-ctr''
1.1 stevesk 209: .Ed
210: .It Cm ClearAllForwardings
211: Specifies that all local, remote and dynamic port forwardings
212: specified in the configuration files or on the command line be
1.7 jmc 213: cleared.
214: This option is primarily useful when used from the
1.1 stevesk 215: .Nm ssh
216: command line to clear port forwardings set in
217: configuration files, and is automatically set by
218: .Xr scp 1
219: and
220: .Xr sftp 1 .
221: The argument must be
222: .Dq yes
223: or
224: .Dq no .
225: The default is
226: .Dq no .
227: .It Cm Compression
228: Specifies whether to use compression.
229: The argument must be
230: .Dq yes
231: or
232: .Dq no .
233: The default is
234: .Dq no .
235: .It Cm CompressionLevel
236: Specifies the compression level to use if compression is enabled.
237: The argument must be an integer from 1 (fast) to 9 (slow, best).
238: The default level is 6, which is good for most applications.
239: The meaning of the values is the same as in
240: .Xr gzip 1 .
241: Note that this option applies to protocol version 1 only.
242: .It Cm ConnectionAttempts
243: Specifies the number of tries (one per second) to make before exiting.
244: The argument must be an integer.
245: This may be useful in scripts if the connection sometimes fails.
246: The default is 1.
1.9 djm 247: .It Cm ConnectTimeout
248: Specifies the timeout (in seconds) used when connecting to the ssh
1.11 jmc 249: server, instead of using the default system TCP timeout.
250: This value is used only when the target is down or really unreachable,
251: not when it refuses the connection.
1.36 djm 252: .It Cm ControlMaster
253: Enables the sharing of multiple sessions over a single network connection.
254: When set to
255: .Dq yes
256: .Nm ssh
257: will listen for connections on a control socket specified using the
258: .Cm ControlPath
259: argument.
260: Additional sessions can connect to this socket using the same
261: .Cm ControlPath
262: with
263: .Cm ControlMaster
264: set to
265: .Dq no
1.38 jmc 266: (the default).
1.64 jmc 267: These sessions will try to reuse the master instance's network connection
1.63 djm 268: rather than initiating new ones, but will fall back to connecting normally
269: if the control socket does not exist, or is not listening.
270: .Pp
1.37 djm 271: Setting this to
272: .Dq ask
273: will cause
274: .Nm ssh
275: to listen for control connections, but require confirmation using the
276: .Ev SSH_ASKPASS
277: program before they are accepted (see
278: .Xr ssh-add 1
1.38 jmc 279: for details).
1.51 jakob 280: If the
281: .Cm ControlPath
282: can not be opened,
283: .Nm ssh
284: will continue without connecting to a master instance.
1.58 djm 285: .Pp
286: X11 and
1.59 jmc 287: .Xr ssh-agent 1
1.58 djm 288: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 289: display and agent forwarded will be the one belonging to the master
1.59 jmc 290: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 291: .Pp
292: Two additional options allow for opportunistic multiplexing: try to use a
293: master connection but fall back to creating a new one if one does not already
294: exist.
295: These options are:
296: .Dq auto
297: and
298: .Dq autoask .
299: The latter requires confirmation like the
300: .Dq ask
301: option.
1.36 djm 302: .It Cm ControlPath
1.55 djm 303: Specify the path to the control socket used for connection sharing as described
304: in the
1.36 djm 305: .Cm ControlMaster
1.57 djm 306: section above or the string
307: .Dq none
308: to disable connection sharing.
1.55 djm 309: In the path,
1.77 djm 310: .Ql %l
311: will be substituted by the local host name,
1.55 djm 312: .Ql %h
313: will be substituted by the target host name,
314: .Ql %p
315: the port and
316: .Ql %r
317: by the remote login username.
1.56 djm 318: It is recommended that any
319: .Cm ControlPath
320: used for opportunistic connection sharing include
1.78 jmc 321: at least %h, %p, and %r.
1.56 djm 322: This ensures that shared connections are uniquely identified.
1.38 jmc 323: .It Cm DynamicForward
1.74 jmc 324: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 325: over the secure channel, and the application
326: protocol is then used to determine where to connect to from the
327: remote machine.
1.62 djm 328: .Pp
329: The argument must be
330: .Sm off
331: .Oo Ar bind_address : Oc Ar port .
332: .Sm on
333: IPv6 addresses can be specified by enclosing addresses in square brackets or
334: by using an alternative syntax:
335: .Oo Ar bind_address Ns / Oc Ns Ar port .
336: By default, the local port is bound in accordance with the
337: .Cm GatewayPorts
338: setting.
339: However, an explicit
340: .Ar bind_address
341: may be used to bind the connection to a specific address.
342: The
343: .Ar bind_address
344: of
345: .Dq localhost
346: indicates that the listening port be bound for local use only, while an
347: empty address or
348: .Sq *
349: indicates that the port should be available from all interfaces.
350: .Pp
1.38 jmc 351: Currently the SOCKS4 and SOCKS5 protocols are supported, and
352: .Nm ssh
353: will act as a SOCKS server.
354: Multiple forwardings may be specified, and
355: additional forwardings can be given on the command line.
356: Only the superuser can forward privileged ports.
1.14 markus 357: .It Cm EnableSSHKeysign
358: Setting this option to
359: .Dq yes
360: in the global client configuration file
361: .Pa /etc/ssh/ssh_config
362: enables the use of the helper program
363: .Xr ssh-keysign 8
364: during
365: .Cm HostbasedAuthentication .
366: The argument must be
367: .Dq yes
368: or
369: .Dq no .
370: The default is
371: .Dq no .
1.23 jmc 372: This option should be placed in the non-hostspecific section.
1.14 markus 373: See
374: .Xr ssh-keysign 8
375: for more information.
1.1 stevesk 376: .It Cm EscapeChar
377: Sets the escape character (default:
378: .Ql ~ ) .
379: The escape character can also
380: be set on the command line.
381: The argument should be a single character,
382: .Ql ^
383: followed by a letter, or
384: .Dq none
385: to disable the escape
386: character entirely (making the connection transparent for binary
387: data).
388: .It Cm ForwardAgent
389: Specifies whether the connection to the authentication agent (if any)
390: will be forwarded to the remote machine.
391: The argument must be
392: .Dq yes
393: or
394: .Dq no .
395: The default is
396: .Dq no .
1.3 stevesk 397: .Pp
1.7 jmc 398: Agent forwarding should be enabled with caution.
399: Users with the ability to bypass file permissions on the remote host
400: (for the agent's Unix-domain socket)
401: can access the local agent through the forwarded connection.
402: An attacker cannot obtain key material from the agent,
1.3 stevesk 403: however they can perform operations on the keys that enable them to
404: authenticate using the identities loaded into the agent.
1.1 stevesk 405: .It Cm ForwardX11
406: Specifies whether X11 connections will be automatically redirected
407: over the secure channel and
408: .Ev DISPLAY
409: set.
410: The argument must be
411: .Dq yes
412: or
413: .Dq no .
414: The default is
415: .Dq no .
1.3 stevesk 416: .Pp
1.7 jmc 417: X11 forwarding should be enabled with caution.
418: Users with the ability to bypass file permissions on the remote host
1.22 markus 419: (for the user's X11 authorization database)
1.7 jmc 420: can access the local X11 display through the forwarded connection.
1.22 markus 421: An attacker may then be able to perform activities such as keystroke monitoring
422: if the
423: .Cm ForwardX11Trusted
424: option is also enabled.
425: .It Cm ForwardX11Trusted
1.34 jmc 426: If this option is set to
1.22 markus 427: .Dq yes
428: then remote X11 clients will have full access to the original X11 display.
1.42 djm 429: .Pp
1.22 markus 430: If this option is set to
431: .Dq no
432: then remote X11 clients will be considered untrusted and prevented
433: from stealing or tampering with data belonging to trusted X11
434: clients.
1.42 djm 435: Furthermore, the
436: .Xr xauth 1
437: token used for the session will be set to expire after 20 minutes.
438: Remote clients will be refused access after this time.
1.22 markus 439: .Pp
440: The default is
441: .Dq no .
442: .Pp
443: See the X11 SECURITY extension specification for full details on
444: the restrictions imposed on untrusted clients.
1.1 stevesk 445: .It Cm GatewayPorts
446: Specifies whether remote hosts are allowed to connect to local
447: forwarded ports.
448: By default,
449: .Nm ssh
1.7 jmc 450: binds local port forwardings to the loopback address.
451: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 452: .Cm GatewayPorts
453: can be used to specify that
454: .Nm ssh
455: should bind local port forwardings to the wildcard address,
456: thus allowing remote hosts to connect to forwarded ports.
457: The argument must be
458: .Dq yes
459: or
460: .Dq no .
461: The default is
462: .Dq no .
463: .It Cm GlobalKnownHostsFile
464: Specifies a file to use for the global
465: host key database instead of
466: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 467: .It Cm GSSAPIAuthentication
1.27 markus 468: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 469: The default is
1.21 markus 470: .Dq no .
1.18 markus 471: Note that this option applies to protocol version 2 only.
472: .It Cm GSSAPIDelegateCredentials
473: Forward (delegate) credentials to the server.
474: The default is
475: .Dq no .
476: Note that this option applies to protocol version 2 only.
1.44 djm 477: .It Cm HashKnownHosts
478: Indicates that
479: .Nm ssh
480: should hash host names and addresses when they are added to
1.50 djm 481: .Pa ~/.ssh/known_hosts .
1.44 djm 482: These hashed names may be used normally by
483: .Nm ssh
484: and
485: .Nm sshd ,
486: but they do not reveal identifying information should the file's contents
487: be disclosed.
488: The default is
489: .Dq no .
1.46 jmc 490: Note that hashing of names and addresses will not be retrospectively applied
1.45 djm 491: to existing known hosts files, but these may be manually hashed using
492: .Xr ssh-keygen 1 .
1.1 stevesk 493: .It Cm HostbasedAuthentication
494: Specifies whether to try rhosts based authentication with public key
495: authentication.
496: The argument must be
497: .Dq yes
498: or
499: .Dq no .
500: The default is
501: .Dq no .
502: This option applies to protocol version 2 only and
503: is similar to
504: .Cm RhostsRSAAuthentication .
505: .It Cm HostKeyAlgorithms
506: Specifies the protocol version 2 host key algorithms
507: that the client wants to use in order of preference.
508: The default for this option is:
509: .Dq ssh-rsa,ssh-dss .
510: .It Cm HostKeyAlias
511: Specifies an alias that should be used instead of the
512: real host name when looking up or saving the host key
513: in the host key database files.
514: This option is useful for tunneling ssh connections
515: or for multiple servers running on a single host.
516: .It Cm HostName
517: Specifies the real host name to log into.
518: This can be used to specify nicknames or abbreviations for hosts.
519: Default is the name given on the command line.
520: Numeric IP addresses are also permitted (both on the command line and in
521: .Cm HostName
522: specifications).
1.29 markus 523: .It Cm IdentitiesOnly
524: Specifies that
525: .Nm ssh
526: should only use the authentication identity files configured in the
1.31 jmc 527: .Nm
1.29 markus 528: files,
529: even if the
530: .Nm ssh-agent
531: offers more identities.
532: The argument to this keyword must be
533: .Dq yes
534: or
535: .Dq no .
1.71 stevesk 536: This option is intended for situations where
1.29 markus 537: .Nm ssh-agent
538: offers many different identities.
539: The default is
540: .Dq no .
1.67 jmc 541: .It Cm IdentityFile
542: Specifies a file from which the user's RSA or DSA authentication identity
543: is read.
544: The default is
545: .Pa ~/.ssh/identity
546: for protocol version 1, and
547: .Pa ~/.ssh/id_rsa
548: and
549: .Pa ~/.ssh/id_dsa
550: for protocol version 2.
551: Additionally, any identities represented by the authentication agent
552: will be used for authentication.
553: The file name may use the tilde
554: syntax to refer to a user's home directory.
555: It is possible to have
556: multiple identity files specified in configuration files; all these
557: identities will be tried in sequence.
1.39 djm 558: .It Cm KbdInteractiveDevices
559: Specifies the list of methods to use in keyboard-interactive authentication.
560: Multiple method names must be comma-separated.
561: The default is to use the server specified list.
1.65 reyk 562: .It Cm LocalCommand
563: Specifies a command to execute on the local machine after successfully
564: connecting to the server.
565: The command string extends to the end of the line, and is executed with
566: .Pa /bin/sh .
567: This directive is ignored unless
568: .Cm PermitLocalCommand
569: has been enabled.
1.1 stevesk 570: .It Cm LocalForward
1.74 jmc 571: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 572: the secure channel to the specified host and port from the remote machine.
1.49 jmc 573: The first argument must be
1.43 djm 574: .Sm off
1.49 jmc 575: .Oo Ar bind_address : Oc Ar port
1.43 djm 576: .Sm on
1.49 jmc 577: and the second argument must be
578: .Ar host : Ns Ar hostport .
1.46 jmc 579: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 580: by using an alternative syntax:
1.49 jmc 581: .Oo Ar bind_address Ns / Oc Ns Ar port
582: and
583: .Ar host Ns / Ns Ar hostport .
1.46 jmc 584: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 585: given on the command line.
1.1 stevesk 586: Only the superuser can forward privileged ports.
1.43 djm 587: By default, the local port is bound in accordance with the
588: .Cm GatewayPorts
589: setting.
590: However, an explicit
591: .Ar bind_address
592: may be used to bind the connection to a specific address.
593: The
594: .Ar bind_address
595: of
596: .Dq localhost
1.46 jmc 597: indicates that the listening port be bound for local use only, while an
598: empty address or
599: .Sq *
1.43 djm 600: indicates that the port should be available from all interfaces.
1.1 stevesk 601: .It Cm LogLevel
602: Gives the verbosity level that is used when logging messages from
603: .Nm ssh .
604: The possible values are:
605: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 606: The default is INFO.
607: DEBUG and DEBUG1 are equivalent.
608: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 609: .It Cm MACs
610: Specifies the MAC (message authentication code) algorithms
611: in order of preference.
612: The MAC algorithm is used in protocol version 2
613: for data integrity protection.
614: Multiple algorithms must be comma-separated.
615: The default is
616: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
617: .It Cm NoHostAuthenticationForLocalhost
618: This option can be used if the home directory is shared across machines.
619: In this case localhost will refer to a different machine on each of
620: the machines and the user will get many warnings about changed host keys.
621: However, this option disables host authentication for localhost.
622: The argument to this keyword must be
623: .Dq yes
624: or
625: .Dq no .
626: The default is to check the host key for localhost.
627: .It Cm NumberOfPasswordPrompts
628: Specifies the number of password prompts before giving up.
629: The argument to this keyword must be an integer.
630: Default is 3.
631: .It Cm PasswordAuthentication
632: Specifies whether to use password authentication.
633: The argument to this keyword must be
634: .Dq yes
635: or
636: .Dq no .
637: The default is
638: .Dq yes .
1.65 reyk 639: .It Cm PermitLocalCommand
640: Allow local command execution via the
641: .Ic LocalCommand
642: option or using the
1.66 jmc 643: .Ic !\& Ns Ar command
1.65 reyk 644: escape sequence in
645: .Xr ssh 1 .
646: The argument must be
647: .Dq yes
648: or
649: .Dq no .
650: The default is
651: .Dq no .
1.67 jmc 652: .It Cm Port
653: Specifies the port number to connect on the remote host.
654: Default is 22.
1.1 stevesk 655: .It Cm PreferredAuthentications
656: Specifies the order in which the client should try protocol 2
1.11 jmc 657: authentication methods.
1.48 jmc 658: This allows a client to prefer one method (e.g.\&
1.1 stevesk 659: .Cm keyboard-interactive )
1.48 jmc 660: over another method (e.g.\&
1.1 stevesk 661: .Cm password )
662: The default for this option is:
663: .Dq hostbased,publickey,keyboard-interactive,password .
664: .It Cm Protocol
665: Specifies the protocol versions
666: .Nm ssh
667: should support in order of preference.
668: The possible values are
669: .Dq 1
670: and
671: .Dq 2 .
672: Multiple versions must be comma-separated.
673: The default is
674: .Dq 2,1 .
675: This means that
676: .Nm ssh
677: tries version 2 and falls back to version 1
678: if version 2 is not available.
679: .It Cm ProxyCommand
680: Specifies the command to use to connect to the server.
681: The command
682: string extends to the end of the line, and is executed with
683: .Pa /bin/sh .
684: In the command string,
685: .Ql %h
686: will be substituted by the host name to
687: connect and
688: .Ql %p
689: by the port.
690: The command can be basically anything,
691: and should read from its standard input and write to its standard output.
692: It should eventually connect an
693: .Xr sshd 8
694: server running on some machine, or execute
695: .Ic sshd -i
696: somewhere.
697: Host key management will be done using the
698: HostName of the host being connected (defaulting to the name typed by
699: the user).
1.7 jmc 700: Setting the command to
701: .Dq none
1.6 markus 702: disables this option entirely.
1.1 stevesk 703: Note that
704: .Cm CheckHostIP
705: is not available for connects with a proxy command.
1.52 djm 706: .Pp
707: This directive is useful in conjunction with
708: .Xr nc 1
709: and its proxy support.
1.53 jmc 710: For example, the following directive would connect via an HTTP proxy at
1.52 djm 711: 192.0.2.0:
712: .Bd -literal -offset 3n
713: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
714: .Ed
1.1 stevesk 715: .It Cm PubkeyAuthentication
716: Specifies whether to try public key authentication.
717: The argument to this keyword must be
718: .Dq yes
719: or
720: .Dq no .
721: The default is
722: .Dq yes .
723: This option applies to protocol version 2 only.
1.75 dtucker 724: .It Cm RekeyLimit
725: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 726: session key is renegotiated.
1.75 dtucker 727: The argument is the number of bytes, with an optional suffix of
1.76 jmc 728: .Sq K ,
729: .Sq M ,
1.75 dtucker 730: or
1.76 jmc 731: .Sq G
1.75 dtucker 732: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
733: The default is between
734: .Dq 1G
735: and
736: .Dq 4G ,
737: depending on the cipher.
1.76 jmc 738: This option applies to protocol version 2 only.
1.1 stevesk 739: .It Cm RemoteForward
1.74 jmc 740: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 741: the secure channel to the specified host and port from the local machine.
1.49 jmc 742: The first argument must be
1.43 djm 743: .Sm off
1.49 jmc 744: .Oo Ar bind_address : Oc Ar port
1.43 djm 745: .Sm on
1.49 jmc 746: and the second argument must be
747: .Ar host : Ns Ar hostport .
748: IPv6 addresses can be specified by enclosing addresses in square brackets
749: or by using an alternative syntax:
750: .Oo Ar bind_address Ns / Oc Ns Ar port
751: and
752: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 753: Multiple forwardings may be specified, and additional
754: forwardings can be given on the command line.
755: Only the superuser can forward privileged ports.
1.43 djm 756: .Pp
757: If the
758: .Ar bind_address
759: is not specified, the default is to only bind to loopback addresses.
760: If the
761: .Ar bind_address
762: is
763: .Ql *
764: or an empty string, then the forwarding is requested to listen on all
765: interfaces.
766: Specifying a remote
767: .Ar bind_address
1.46 jmc 768: will only succeed if the server's
769: .Cm GatewayPorts
1.43 djm 770: option is enabled (see
1.46 jmc 771: .Xr sshd_config 5 ) .
1.1 stevesk 772: .It Cm RhostsRSAAuthentication
773: Specifies whether to try rhosts based authentication with RSA host
774: authentication.
775: The argument must be
776: .Dq yes
777: or
778: .Dq no .
779: The default is
780: .Dq no .
781: This option applies to protocol version 1 only and requires
782: .Nm ssh
783: to be setuid root.
784: .It Cm RSAAuthentication
785: Specifies whether to try RSA authentication.
786: The argument to this keyword must be
787: .Dq yes
788: or
789: .Dq no .
790: RSA authentication will only be
791: attempted if the identity file exists, or an authentication agent is
792: running.
793: The default is
794: .Dq yes .
795: Note that this option applies to protocol version 1 only.
1.32 djm 796: .It Cm SendEnv
797: Specifies what variables from the local
798: .Xr environ 7
799: should be sent to the server.
800: Note that environment passing is only supported for protocol 2, the
1.33 djm 801: server must also support it, and the server must be configured to
802: accept these environment variables.
1.32 djm 803: Refer to
804: .Cm AcceptEnv
805: in
806: .Xr sshd_config 5
807: for how to configure the server.
808: Variables are specified by name, which may contain the wildcard characters
809: .Ql \&*
810: and
811: .Ql \&? .
1.33 djm 812: Multiple environment variables may be separated by whitespace or spread
1.32 djm 813: across multiple
814: .Cm SendEnv
815: directives.
816: The default is not to send any environment variables.
1.28 markus 817: .It Cm ServerAliveCountMax
1.73 jmc 818: Sets the number of server alive messages (see below) which may be
1.28 markus 819: sent without
820: .Nm ssh
821: receiving any messages back from the server.
822: If this threshold is reached while server alive messages are being sent,
823: .Nm ssh
824: will disconnect from the server, terminating the session.
825: It is important to note that the use of server alive messages is very
826: different from
827: .Cm TCPKeepAlive
828: (below).
829: The server alive messages are sent through the encrypted channel
830: and therefore will not be spoofable.
831: The TCP keepalive option enabled by
832: .Cm TCPKeepAlive
833: is spoofable.
834: The server alive mechanism is valuable when the client or
835: server depend on knowing when a connection has become inactive.
836: .Pp
837: The default value is 3.
838: If, for example,
839: .Cm ServerAliveInterval
1.73 jmc 840: (see below) is set to 15, and
1.28 markus 841: .Cm ServerAliveCountMax
842: is left at the default, if the server becomes unresponsive ssh
843: will disconnect after approximately 45 seconds.
1.67 jmc 844: .It Cm ServerAliveInterval
845: Sets a timeout interval in seconds after which if no data has been received
846: from the server,
847: .Nm ssh
848: will send a message through the encrypted
849: channel to request a response from the server.
850: The default
851: is 0, indicating that these messages will not be sent to the server.
852: This option applies to protocol version 2 only.
1.1 stevesk 853: .It Cm SmartcardDevice
1.11 jmc 854: Specifies which smartcard device to use.
855: The argument to this keyword is the device
1.1 stevesk 856: .Nm ssh
857: should use to communicate with a smartcard used for storing the user's
1.11 jmc 858: private RSA key.
859: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 860: .It Cm StrictHostKeyChecking
861: If this flag is set to
862: .Dq yes ,
863: .Nm ssh
864: will never automatically add host keys to the
1.50 djm 865: .Pa ~/.ssh/known_hosts
1.1 stevesk 866: file, and refuses to connect to hosts whose host key has changed.
867: This provides maximum protection against trojan horse attacks,
868: however, can be annoying when the
869: .Pa /etc/ssh/ssh_known_hosts
870: file is poorly maintained, or connections to new hosts are
871: frequently made.
872: This option forces the user to manually
873: add all new hosts.
874: If this flag is set to
875: .Dq no ,
876: .Nm ssh
877: will automatically add new host keys to the
878: user known hosts files.
879: If this flag is set to
880: .Dq ask ,
881: new host keys
882: will be added to the user known host files only after the user
883: has confirmed that is what they really want to do, and
884: .Nm ssh
885: will refuse to connect to hosts whose host key has changed.
886: The host keys of
887: known hosts will be verified automatically in all cases.
888: The argument must be
889: .Dq yes ,
890: .Dq no
891: or
892: .Dq ask .
893: The default is
894: .Dq ask .
1.26 markus 895: .It Cm TCPKeepAlive
896: Specifies whether the system should send TCP keepalive messages to the
897: other side.
898: If they are sent, death of the connection or crash of one
899: of the machines will be properly noticed.
900: However, this means that
901: connections will die if the route is down temporarily, and some people
902: find it annoying.
903: .Pp
904: The default is
905: .Dq yes
906: (to send TCP keepalive messages), and the client will notice
907: if the network goes down or the remote host dies.
908: This is important in scripts, and many users want it too.
909: .Pp
910: To disable TCP keepalive messages, the value should be set to
911: .Dq no .
1.65 reyk 912: .It Cm Tunnel
913: Request starting
914: .Xr tun 4
1.69 jmc 915: device forwarding between the client and the server.
916: This option also allows requesting layer 2 (ethernet)
917: instead of layer 3 (point-to-point) tunneling from the server.
1.65 reyk 918: The argument must be
1.68 reyk 919: .Dq yes ,
920: .Dq point-to-point ,
921: .Dq ethernet
1.65 reyk 922: or
923: .Dq no .
924: The default is
925: .Dq no .
926: .It Cm TunnelDevice
927: Force a specified
928: .Xr tun 4
929: device on the client.
930: Without this option, the next available device will be used.
1.72 jmc 931: .It Cm UsePrivilegedPort
932: Specifies whether to use a privileged port for outgoing connections.
933: The argument must be
934: .Dq yes
935: or
936: .Dq no .
937: The default is
938: .Dq no .
939: If set to
940: .Dq yes
941: .Nm ssh
942: must be setuid root.
943: Note that this option must be set to
944: .Dq yes
945: for
946: .Cm RhostsRSAAuthentication
947: with older servers.
1.1 stevesk 948: .It Cm User
949: Specifies the user to log in as.
950: This can be useful when a different user name is used on different machines.
951: This saves the trouble of
952: having to remember to give the user name on the command line.
953: .It Cm UserKnownHostsFile
954: Specifies a file to use for the user
955: host key database instead of
1.50 djm 956: .Pa ~/.ssh/known_hosts .
1.8 jakob 957: .It Cm VerifyHostKeyDNS
958: Specifies whether to verify the remote key using DNS and SSHFP resource
959: records.
1.24 jakob 960: If this option is set to
961: .Dq yes ,
1.25 jmc 962: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 963: from DNS.
964: Insecure fingerprints will be handled as if this option was set to
965: .Dq ask .
966: If this option is set to
967: .Dq ask ,
968: information on fingerprint match will be displayed, but the user will still
969: need to confirm new host keys according to the
970: .Cm StrictHostKeyChecking
971: option.
972: The argument must be
973: .Dq yes ,
974: .Dq no
1.25 jmc 975: or
976: .Dq ask .
1.8 jakob 977: The default is
978: .Dq no .
1.12 jakob 979: Note that this option applies to protocol version 2 only.
1.1 stevesk 980: .It Cm XAuthLocation
1.5 stevesk 981: Specifies the full pathname of the
1.1 stevesk 982: .Xr xauth 1
983: program.
984: The default is
985: .Pa /usr/X11R6/bin/xauth .
986: .El
987: .Sh FILES
988: .Bl -tag -width Ds
1.50 djm 989: .It Pa ~/.ssh/config
1.1 stevesk 990: This is the per-user configuration file.
991: The format of this file is described above.
992: This file is used by the
993: .Nm ssh
994: client.
1.30 djm 995: Because of the potential for abuse, this file must have strict permissions:
996: read/write for the user, and not accessible by others.
1.1 stevesk 997: .It Pa /etc/ssh/ssh_config
998: Systemwide configuration file.
999: This file provides defaults for those
1000: values that are not specified in the user's configuration file, and
1001: for those users who do not have a configuration file.
1002: This file must be world-readable.
1003: .El
1.13 jmc 1004: .Sh SEE ALSO
1005: .Xr ssh 1
1.1 stevesk 1006: .Sh AUTHORS
1007: OpenSSH is a derivative of the original and free
1008: ssh 1.2.12 release by Tatu Ylonen.
1009: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1010: Theo de Raadt and Dug Song
1011: removed many bugs, re-added newer features and
1012: created OpenSSH.
1013: Markus Friedl contributed the support for SSH
1014: protocol versions 1.5 and 2.0.
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