Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.77
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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1.77 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.76 2006/01/20 11:21:45 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:
1.2 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).
1.64 jmc 266: These sessions will try to reuse the master instance's network connection
1.63 djm 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
1.70 stevesk 288: display and agent forwarded will be the one belonging to the master
1.59 jmc 289: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 290: .Pp
291: Two additional options allow for opportunistic multiplexing: try to use a
292: master connection but fall back to creating a new one if one does not already
293: exist.
294: These options are:
295: .Dq auto
296: and
297: .Dq autoask .
298: The latter requires confirmation like the
299: .Dq ask
300: option.
1.36 djm 301: .It Cm ControlPath
1.55 djm 302: Specify the path to the control socket used for connection sharing as described
303: in the
1.36 djm 304: .Cm ControlMaster
1.57 djm 305: section above or the string
306: .Dq none
307: to disable connection sharing.
1.55 djm 308: In the path,
1.77 ! djm 309: .Ql %l
! 310: will be substituted by the local host name,
1.55 djm 311: .Ql %h
312: will be substituted by the target host name,
313: .Ql %p
314: the port and
315: .Ql %r
316: by the remote login username.
1.56 djm 317: It is recommended that any
318: .Cm ControlPath
319: used for opportunistic connection sharing include
1.77 ! djm 320: at least the last three of these escape sequences (%h, %p and %r).
1.56 djm 321: This ensures that shared connections are uniquely identified.
1.38 jmc 322: .It Cm DynamicForward
1.74 jmc 323: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 324: over the secure channel, and the application
325: protocol is then used to determine where to connect to from the
326: remote machine.
1.62 djm 327: .Pp
328: The argument must be
329: .Sm off
330: .Oo Ar bind_address : Oc Ar port .
331: .Sm on
332: IPv6 addresses can be specified by enclosing addresses in square brackets or
333: by using an alternative syntax:
334: .Oo Ar bind_address Ns / Oc Ns Ar port .
335: By default, the local port is bound in accordance with the
336: .Cm GatewayPorts
337: setting.
338: However, an explicit
339: .Ar bind_address
340: may be used to bind the connection to a specific address.
341: The
342: .Ar bind_address
343: of
344: .Dq localhost
345: indicates that the listening port be bound for local use only, while an
346: empty address or
347: .Sq *
348: indicates that the port should be available from all interfaces.
349: .Pp
1.38 jmc 350: Currently the SOCKS4 and SOCKS5 protocols are supported, and
351: .Nm ssh
352: will act as a SOCKS server.
353: Multiple forwardings may be specified, and
354: additional forwardings can be given on the command line.
355: Only the superuser can forward privileged ports.
1.14 markus 356: .It Cm EnableSSHKeysign
357: Setting this option to
358: .Dq yes
359: in the global client configuration file
360: .Pa /etc/ssh/ssh_config
361: enables the use of the helper program
362: .Xr ssh-keysign 8
363: during
364: .Cm HostbasedAuthentication .
365: The argument must be
366: .Dq yes
367: or
368: .Dq no .
369: The default is
370: .Dq no .
1.23 jmc 371: This option should be placed in the non-hostspecific section.
1.14 markus 372: See
373: .Xr ssh-keysign 8
374: for more information.
1.1 stevesk 375: .It Cm EscapeChar
376: Sets the escape character (default:
377: .Ql ~ ) .
378: The escape character can also
379: be set on the command line.
380: The argument should be a single character,
381: .Ql ^
382: followed by a letter, or
383: .Dq none
384: to disable the escape
385: character entirely (making the connection transparent for binary
386: data).
387: .It Cm ForwardAgent
388: Specifies whether the connection to the authentication agent (if any)
389: will be forwarded to the remote machine.
390: The argument must be
391: .Dq yes
392: or
393: .Dq no .
394: The default is
395: .Dq no .
1.3 stevesk 396: .Pp
1.7 jmc 397: Agent forwarding should be enabled with caution.
398: Users with the ability to bypass file permissions on the remote host
399: (for the agent's Unix-domain socket)
400: can access the local agent through the forwarded connection.
401: An attacker cannot obtain key material from the agent,
1.3 stevesk 402: however they can perform operations on the keys that enable them to
403: authenticate using the identities loaded into the agent.
1.1 stevesk 404: .It Cm ForwardX11
405: Specifies whether X11 connections will be automatically redirected
406: over the secure channel and
407: .Ev DISPLAY
408: set.
409: The argument must be
410: .Dq yes
411: or
412: .Dq no .
413: The default is
414: .Dq no .
1.3 stevesk 415: .Pp
1.7 jmc 416: X11 forwarding should be enabled with caution.
417: Users with the ability to bypass file permissions on the remote host
1.22 markus 418: (for the user's X11 authorization database)
1.7 jmc 419: can access the local X11 display through the forwarded connection.
1.22 markus 420: An attacker may then be able to perform activities such as keystroke monitoring
421: if the
422: .Cm ForwardX11Trusted
423: option is also enabled.
424: .It Cm ForwardX11Trusted
1.34 jmc 425: If this option is set to
1.22 markus 426: .Dq yes
427: then remote X11 clients will have full access to the original X11 display.
1.42 djm 428: .Pp
1.22 markus 429: If this option is set to
430: .Dq no
431: then remote X11 clients will be considered untrusted and prevented
432: from stealing or tampering with data belonging to trusted X11
433: clients.
1.42 djm 434: Furthermore, the
435: .Xr xauth 1
436: token used for the session will be set to expire after 20 minutes.
437: Remote clients will be refused access after this time.
1.22 markus 438: .Pp
439: The default is
440: .Dq no .
441: .Pp
442: See the X11 SECURITY extension specification for full details on
443: the restrictions imposed on untrusted clients.
1.1 stevesk 444: .It Cm GatewayPorts
445: Specifies whether remote hosts are allowed to connect to local
446: forwarded ports.
447: By default,
448: .Nm ssh
1.7 jmc 449: binds local port forwardings to the loopback address.
450: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 451: .Cm GatewayPorts
452: can be used to specify that
453: .Nm ssh
454: should bind local port forwardings to the wildcard address,
455: thus allowing remote hosts to connect to forwarded ports.
456: The argument must be
457: .Dq yes
458: or
459: .Dq no .
460: The default is
461: .Dq no .
462: .It Cm GlobalKnownHostsFile
463: Specifies a file to use for the global
464: host key database instead of
465: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 466: .It Cm GSSAPIAuthentication
1.27 markus 467: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 468: The default is
1.21 markus 469: .Dq no .
1.18 markus 470: Note that this option applies to protocol version 2 only.
471: .It Cm GSSAPIDelegateCredentials
472: Forward (delegate) credentials to the server.
473: The default is
474: .Dq no .
475: Note that this option applies to protocol version 2 only.
1.44 djm 476: .It Cm HashKnownHosts
477: Indicates that
478: .Nm ssh
479: should hash host names and addresses when they are added to
1.50 djm 480: .Pa ~/.ssh/known_hosts .
1.44 djm 481: These hashed names may be used normally by
482: .Nm ssh
483: and
484: .Nm sshd ,
485: but they do not reveal identifying information should the file's contents
486: be disclosed.
487: The default is
488: .Dq no .
1.46 jmc 489: Note that hashing of names and addresses will not be retrospectively applied
1.45 djm 490: to existing known hosts files, but these may be manually hashed using
491: .Xr ssh-keygen 1 .
1.1 stevesk 492: .It Cm HostbasedAuthentication
493: Specifies whether to try rhosts based authentication with public key
494: authentication.
495: The argument must be
496: .Dq yes
497: or
498: .Dq no .
499: The default is
500: .Dq no .
501: This option applies to protocol version 2 only and
502: is similar to
503: .Cm RhostsRSAAuthentication .
504: .It Cm HostKeyAlgorithms
505: Specifies the protocol version 2 host key algorithms
506: that the client wants to use in order of preference.
507: The default for this option is:
508: .Dq ssh-rsa,ssh-dss .
509: .It Cm HostKeyAlias
510: Specifies an alias that should be used instead of the
511: real host name when looking up or saving the host key
512: in the host key database files.
513: This option is useful for tunneling ssh connections
514: or for multiple servers running on a single host.
515: .It Cm HostName
516: Specifies the real host name to log into.
517: This can be used to specify nicknames or abbreviations for hosts.
518: Default is the name given on the command line.
519: Numeric IP addresses are also permitted (both on the command line and in
520: .Cm HostName
521: specifications).
1.29 markus 522: .It Cm IdentitiesOnly
523: Specifies that
524: .Nm ssh
525: should only use the authentication identity files configured in the
1.31 jmc 526: .Nm
1.29 markus 527: files,
528: even if the
529: .Nm ssh-agent
530: offers more identities.
531: The argument to this keyword must be
532: .Dq yes
533: or
534: .Dq no .
1.71 stevesk 535: This option is intended for situations where
1.29 markus 536: .Nm ssh-agent
537: offers many different identities.
538: The default is
539: .Dq no .
1.67 jmc 540: .It Cm IdentityFile
541: Specifies a file from which the user's RSA or DSA authentication identity
542: is read.
543: The default is
544: .Pa ~/.ssh/identity
545: for protocol version 1, and
546: .Pa ~/.ssh/id_rsa
547: and
548: .Pa ~/.ssh/id_dsa
549: for protocol version 2.
550: Additionally, any identities represented by the authentication agent
551: will be used for authentication.
552: The file name may use the tilde
553: syntax to refer to a user's home directory.
554: It is possible to have
555: multiple identity files specified in configuration files; all these
556: identities will be tried in sequence.
1.39 djm 557: .It Cm KbdInteractiveDevices
558: Specifies the list of methods to use in keyboard-interactive authentication.
559: Multiple method names must be comma-separated.
560: The default is to use the server specified list.
1.65 reyk 561: .It Cm LocalCommand
562: Specifies a command to execute on the local machine after successfully
563: connecting to the server.
564: The command string extends to the end of the line, and is executed with
565: .Pa /bin/sh .
566: This directive is ignored unless
567: .Cm PermitLocalCommand
568: has been enabled.
1.1 stevesk 569: .It Cm LocalForward
1.74 jmc 570: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 571: the secure channel to the specified host and port from the remote machine.
1.49 jmc 572: The first argument must be
1.43 djm 573: .Sm off
1.49 jmc 574: .Oo Ar bind_address : Oc Ar port
1.43 djm 575: .Sm on
1.49 jmc 576: and the second argument must be
577: .Ar host : Ns Ar hostport .
1.46 jmc 578: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 579: by using an alternative syntax:
1.49 jmc 580: .Oo Ar bind_address Ns / Oc Ns Ar port
581: and
582: .Ar host Ns / Ns Ar hostport .
1.46 jmc 583: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 584: given on the command line.
1.1 stevesk 585: Only the superuser can forward privileged ports.
1.43 djm 586: By default, the local port is bound in accordance with the
587: .Cm GatewayPorts
588: setting.
589: However, an explicit
590: .Ar bind_address
591: may be used to bind the connection to a specific address.
592: The
593: .Ar bind_address
594: of
595: .Dq localhost
1.46 jmc 596: indicates that the listening port be bound for local use only, while an
597: empty address or
598: .Sq *
1.43 djm 599: indicates that the port should be available from all interfaces.
1.1 stevesk 600: .It Cm LogLevel
601: Gives the verbosity level that is used when logging messages from
602: .Nm ssh .
603: The possible values are:
604: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 605: The default is INFO.
606: DEBUG and DEBUG1 are equivalent.
607: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 608: .It Cm MACs
609: Specifies the MAC (message authentication code) algorithms
610: in order of preference.
611: The MAC algorithm is used in protocol version 2
612: for data integrity protection.
613: Multiple algorithms must be comma-separated.
614: The default is
615: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
616: .It Cm NoHostAuthenticationForLocalhost
617: This option can be used if the home directory is shared across machines.
618: In this case localhost will refer to a different machine on each of
619: the machines and the user will get many warnings about changed host keys.
620: However, this option disables host authentication for localhost.
621: The argument to this keyword must be
622: .Dq yes
623: or
624: .Dq no .
625: The default is to check the host key for localhost.
626: .It Cm NumberOfPasswordPrompts
627: Specifies the number of password prompts before giving up.
628: The argument to this keyword must be an integer.
629: Default is 3.
630: .It Cm PasswordAuthentication
631: Specifies whether to use password authentication.
632: The argument to this keyword must be
633: .Dq yes
634: or
635: .Dq no .
636: The default is
637: .Dq yes .
1.65 reyk 638: .It Cm PermitLocalCommand
639: Allow local command execution via the
640: .Ic LocalCommand
641: option or using the
1.66 jmc 642: .Ic !\& Ns Ar command
1.65 reyk 643: escape sequence in
644: .Xr ssh 1 .
645: The argument must be
646: .Dq yes
647: or
648: .Dq no .
649: The default is
650: .Dq no .
1.67 jmc 651: .It Cm Port
652: Specifies the port number to connect on the remote host.
653: Default is 22.
1.1 stevesk 654: .It Cm PreferredAuthentications
655: Specifies the order in which the client should try protocol 2
1.11 jmc 656: authentication methods.
1.48 jmc 657: This allows a client to prefer one method (e.g.\&
1.1 stevesk 658: .Cm keyboard-interactive )
1.48 jmc 659: over another method (e.g.\&
1.1 stevesk 660: .Cm password )
661: The default for this option is:
662: .Dq hostbased,publickey,keyboard-interactive,password .
663: .It Cm Protocol
664: Specifies the protocol versions
665: .Nm ssh
666: should support in order of preference.
667: The possible values are
668: .Dq 1
669: and
670: .Dq 2 .
671: Multiple versions must be comma-separated.
672: The default is
673: .Dq 2,1 .
674: This means that
675: .Nm ssh
676: tries version 2 and falls back to version 1
677: if version 2 is not available.
678: .It Cm ProxyCommand
679: Specifies the command to use to connect to the server.
680: The command
681: string extends to the end of the line, and is executed with
682: .Pa /bin/sh .
683: In the command string,
684: .Ql %h
685: will be substituted by the host name to
686: connect and
687: .Ql %p
688: by the port.
689: The command can be basically anything,
690: and should read from its standard input and write to its standard output.
691: It should eventually connect an
692: .Xr sshd 8
693: server running on some machine, or execute
694: .Ic sshd -i
695: somewhere.
696: Host key management will be done using the
697: HostName of the host being connected (defaulting to the name typed by
698: the user).
1.7 jmc 699: Setting the command to
700: .Dq none
1.6 markus 701: disables this option entirely.
1.1 stevesk 702: Note that
703: .Cm CheckHostIP
704: is not available for connects with a proxy command.
1.52 djm 705: .Pp
706: This directive is useful in conjunction with
707: .Xr nc 1
708: and its proxy support.
1.53 jmc 709: For example, the following directive would connect via an HTTP proxy at
1.52 djm 710: 192.0.2.0:
711: .Bd -literal -offset 3n
712: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
713: .Ed
1.1 stevesk 714: .It Cm PubkeyAuthentication
715: Specifies whether to try public key authentication.
716: The argument to this keyword must be
717: .Dq yes
718: or
719: .Dq no .
720: The default is
721: .Dq yes .
722: This option applies to protocol version 2 only.
1.75 dtucker 723: .It Cm RekeyLimit
724: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 725: session key is renegotiated.
1.75 dtucker 726: The argument is the number of bytes, with an optional suffix of
1.76 jmc 727: .Sq K ,
728: .Sq M ,
1.75 dtucker 729: or
1.76 jmc 730: .Sq G
1.75 dtucker 731: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
732: The default is between
733: .Dq 1G
734: and
735: .Dq 4G ,
736: depending on the cipher.
1.76 jmc 737: This option applies to protocol version 2 only.
1.1 stevesk 738: .It Cm RemoteForward
1.74 jmc 739: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 740: the secure channel to the specified host and port from the local machine.
1.49 jmc 741: The first argument must be
1.43 djm 742: .Sm off
1.49 jmc 743: .Oo Ar bind_address : Oc Ar port
1.43 djm 744: .Sm on
1.49 jmc 745: and the second argument must be
746: .Ar host : Ns Ar hostport .
747: IPv6 addresses can be specified by enclosing addresses in square brackets
748: or by using an alternative syntax:
749: .Oo Ar bind_address Ns / Oc Ns Ar port
750: and
751: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 752: Multiple forwardings may be specified, and additional
753: forwardings can be given on the command line.
754: Only the superuser can forward privileged ports.
1.43 djm 755: .Pp
756: If the
757: .Ar bind_address
758: is not specified, the default is to only bind to loopback addresses.
759: If the
760: .Ar bind_address
761: is
762: .Ql *
763: or an empty string, then the forwarding is requested to listen on all
764: interfaces.
765: Specifying a remote
766: .Ar bind_address
1.46 jmc 767: will only succeed if the server's
768: .Cm GatewayPorts
1.43 djm 769: option is enabled (see
1.46 jmc 770: .Xr sshd_config 5 ) .
1.1 stevesk 771: .It Cm RhostsRSAAuthentication
772: Specifies whether to try rhosts based authentication with RSA host
773: authentication.
774: The argument must be
775: .Dq yes
776: or
777: .Dq no .
778: The default is
779: .Dq no .
780: This option applies to protocol version 1 only and requires
781: .Nm ssh
782: to be setuid root.
783: .It Cm RSAAuthentication
784: Specifies whether to try RSA authentication.
785: The argument to this keyword must be
786: .Dq yes
787: or
788: .Dq no .
789: RSA authentication will only be
790: attempted if the identity file exists, or an authentication agent is
791: running.
792: The default is
793: .Dq yes .
794: Note that this option applies to protocol version 1 only.
1.32 djm 795: .It Cm SendEnv
796: Specifies what variables from the local
797: .Xr environ 7
798: should be sent to the server.
799: Note that environment passing is only supported for protocol 2, the
1.33 djm 800: server must also support it, and the server must be configured to
801: accept these environment variables.
1.32 djm 802: Refer to
803: .Cm AcceptEnv
804: in
805: .Xr sshd_config 5
806: for how to configure the server.
807: Variables are specified by name, which may contain the wildcard characters
808: .Ql \&*
809: and
810: .Ql \&? .
1.33 djm 811: Multiple environment variables may be separated by whitespace or spread
1.32 djm 812: across multiple
813: .Cm SendEnv
814: directives.
815: The default is not to send any environment variables.
1.28 markus 816: .It Cm ServerAliveCountMax
1.73 jmc 817: Sets the number of server alive messages (see below) which may be
1.28 markus 818: sent without
819: .Nm ssh
820: receiving any messages back from the server.
821: If this threshold is reached while server alive messages are being sent,
822: .Nm ssh
823: will disconnect from the server, terminating the session.
824: It is important to note that the use of server alive messages is very
825: different from
826: .Cm TCPKeepAlive
827: (below).
828: The server alive messages are sent through the encrypted channel
829: and therefore will not be spoofable.
830: The TCP keepalive option enabled by
831: .Cm TCPKeepAlive
832: is spoofable.
833: The server alive mechanism is valuable when the client or
834: server depend on knowing when a connection has become inactive.
835: .Pp
836: The default value is 3.
837: If, for example,
838: .Cm ServerAliveInterval
1.73 jmc 839: (see below) is set to 15, and
1.28 markus 840: .Cm ServerAliveCountMax
841: is left at the default, if the server becomes unresponsive ssh
842: will disconnect after approximately 45 seconds.
1.67 jmc 843: .It Cm ServerAliveInterval
844: Sets a timeout interval in seconds after which if no data has been received
845: from the server,
846: .Nm ssh
847: will send a message through the encrypted
848: channel to request a response from the server.
849: The default
850: is 0, indicating that these messages will not be sent to the server.
851: This option applies to protocol version 2 only.
1.1 stevesk 852: .It Cm SmartcardDevice
1.11 jmc 853: Specifies which smartcard device to use.
854: The argument to this keyword is the device
1.1 stevesk 855: .Nm ssh
856: should use to communicate with a smartcard used for storing the user's
1.11 jmc 857: private RSA key.
858: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 859: .It Cm StrictHostKeyChecking
860: If this flag is set to
861: .Dq yes ,
862: .Nm ssh
863: will never automatically add host keys to the
1.50 djm 864: .Pa ~/.ssh/known_hosts
1.1 stevesk 865: file, and refuses to connect to hosts whose host key has changed.
866: This provides maximum protection against trojan horse attacks,
867: however, can be annoying when the
868: .Pa /etc/ssh/ssh_known_hosts
869: file is poorly maintained, or connections to new hosts are
870: frequently made.
871: This option forces the user to manually
872: add all new hosts.
873: If this flag is set to
874: .Dq no ,
875: .Nm ssh
876: will automatically add new host keys to the
877: user known hosts files.
878: If this flag is set to
879: .Dq ask ,
880: new host keys
881: will be added to the user known host files only after the user
882: has confirmed that is what they really want to do, and
883: .Nm ssh
884: will refuse to connect to hosts whose host key has changed.
885: The host keys of
886: known hosts will be verified automatically in all cases.
887: The argument must be
888: .Dq yes ,
889: .Dq no
890: or
891: .Dq ask .
892: The default is
893: .Dq ask .
1.26 markus 894: .It Cm TCPKeepAlive
895: Specifies whether the system should send TCP keepalive messages to the
896: other side.
897: If they are sent, death of the connection or crash of one
898: of the machines will be properly noticed.
899: However, this means that
900: connections will die if the route is down temporarily, and some people
901: find it annoying.
902: .Pp
903: The default is
904: .Dq yes
905: (to send TCP keepalive messages), and the client will notice
906: if the network goes down or the remote host dies.
907: This is important in scripts, and many users want it too.
908: .Pp
909: To disable TCP keepalive messages, the value should be set to
910: .Dq no .
1.65 reyk 911: .It Cm Tunnel
912: Request starting
913: .Xr tun 4
1.69 jmc 914: device forwarding between the client and the server.
915: This option also allows requesting layer 2 (ethernet)
916: instead of layer 3 (point-to-point) tunneling from the server.
1.65 reyk 917: The argument must be
1.68 reyk 918: .Dq yes ,
919: .Dq point-to-point ,
920: .Dq ethernet
1.65 reyk 921: or
922: .Dq no .
923: The default is
924: .Dq no .
925: .It Cm TunnelDevice
926: Force a specified
927: .Xr tun 4
928: device on the client.
929: Without this option, the next available device will be used.
1.72 jmc 930: .It Cm UsePrivilegedPort
931: Specifies whether to use a privileged port for outgoing connections.
932: The argument must be
933: .Dq yes
934: or
935: .Dq no .
936: The default is
937: .Dq no .
938: If set to
939: .Dq yes
940: .Nm ssh
941: must be setuid root.
942: Note that this option must be set to
943: .Dq yes
944: for
945: .Cm RhostsRSAAuthentication
946: with older servers.
1.1 stevesk 947: .It Cm User
948: Specifies the user to log in as.
949: This can be useful when a different user name is used on different machines.
950: This saves the trouble of
951: having to remember to give the user name on the command line.
952: .It Cm UserKnownHostsFile
953: Specifies a file to use for the user
954: host key database instead of
1.50 djm 955: .Pa ~/.ssh/known_hosts .
1.8 jakob 956: .It Cm VerifyHostKeyDNS
957: Specifies whether to verify the remote key using DNS and SSHFP resource
958: records.
1.24 jakob 959: If this option is set to
960: .Dq yes ,
1.25 jmc 961: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 962: from DNS.
963: Insecure fingerprints will be handled as if this option was set to
964: .Dq ask .
965: If this option is set to
966: .Dq ask ,
967: information on fingerprint match will be displayed, but the user will still
968: need to confirm new host keys according to the
969: .Cm StrictHostKeyChecking
970: option.
971: The argument must be
972: .Dq yes ,
973: .Dq no
1.25 jmc 974: or
975: .Dq ask .
1.8 jakob 976: The default is
977: .Dq no .
1.12 jakob 978: Note that this option applies to protocol version 2 only.
1.1 stevesk 979: .It Cm XAuthLocation
1.5 stevesk 980: Specifies the full pathname of the
1.1 stevesk 981: .Xr xauth 1
982: program.
983: The default is
984: .Pa /usr/X11R6/bin/xauth .
985: .El
986: .Sh FILES
987: .Bl -tag -width Ds
1.50 djm 988: .It Pa ~/.ssh/config
1.1 stevesk 989: This is the per-user configuration file.
990: The format of this file is described above.
991: This file is used by the
992: .Nm ssh
993: client.
1.30 djm 994: Because of the potential for abuse, this file must have strict permissions:
995: read/write for the user, and not accessible by others.
1.1 stevesk 996: .It Pa /etc/ssh/ssh_config
997: Systemwide configuration file.
998: This file provides defaults for those
999: values that are not specified in the user's configuration file, and
1000: for those users who do not have a configuration file.
1001: This file must be world-readable.
1002: .El
1.13 jmc 1003: .Sh SEE ALSO
1004: .Xr ssh 1
1.1 stevesk 1005: .Sh AUTHORS
1006: OpenSSH is a derivative of the original and free
1007: ssh 1.2.12 release by Tatu Ylonen.
1008: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1009: Theo de Raadt and Dug Song
1010: removed many bugs, re-added newer features and
1011: created OpenSSH.
1012: Markus Friedl contributed the support for SSH
1013: protocol versions 1.5 and 2.0.
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