Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.9
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: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
25: .\"
26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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1.9 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.8 2003/05/14 18:16:20 jakob 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
46: .It Pa $HOME/.ssh/config
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
58: .Pq Pa $HOME/.ssh/config
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.
66: The configuration files contain sections bracketed by
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
107: .Ql ?
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).
118: .It Cm AFSTokenPassing
119: Specifies whether to pass AFS tokens to remote host.
120: The argument to this keyword must be
121: .Dq yes
122: or
123: .Dq no .
124: This option applies to protocol version 1 only.
125: .It Cm BatchMode
126: If set to
127: .Dq yes ,
128: passphrase/password querying will be disabled.
129: This option is useful in scripts and other batch jobs where no user
130: is present to supply the password.
131: The argument must be
132: .Dq yes
133: or
134: .Dq no .
135: The default is
136: .Dq no .
137: .It Cm BindAddress
138: Specify the interface to transmit from on machines with multiple
139: interfaces or aliased addresses.
140: Note that this option does not work if
141: .Cm UsePrivilegedPort
142: is set to
143: .Dq yes .
144: .It Cm ChallengeResponseAuthentication
145: Specifies whether to use challenge response authentication.
146: The argument to this keyword must be
147: .Dq yes
148: or
149: .Dq no .
150: The default is
151: .Dq yes .
152: .It Cm CheckHostIP
153: If this flag is set to
154: .Dq yes ,
155: ssh will additionally check the host IP address in the
156: .Pa known_hosts
157: file.
158: This allows ssh to detect if a host key changed due to DNS spoofing.
159: If the option is set to
160: .Dq no ,
161: the check will not be executed.
162: The default is
163: .Dq yes .
164: .It Cm Cipher
165: Specifies the cipher to use for encrypting the session
166: in protocol version 1.
167: Currently,
168: .Dq blowfish ,
169: .Dq 3des ,
170: and
171: .Dq des
172: are supported.
173: .Ar des
174: is only supported in the
175: .Nm ssh
176: client for interoperability with legacy protocol 1 implementations
177: that do not support the
178: .Ar 3des
1.7 jmc 179: cipher.
180: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 181: The default is
182: .Dq 3des .
183: .It Cm Ciphers
184: Specifies the ciphers allowed for protocol version 2
185: in order of preference.
186: Multiple ciphers must be comma-separated.
187: The default is
188: .Pp
189: .Bd -literal
190: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
191: aes192-cbc,aes256-cbc''
192: .Ed
193: .It Cm ClearAllForwardings
194: Specifies that all local, remote and dynamic port forwardings
195: specified in the configuration files or on the command line be
1.7 jmc 196: cleared.
197: This option is primarily useful when used from the
1.1 stevesk 198: .Nm ssh
199: command line to clear port forwardings set in
200: configuration files, and is automatically set by
201: .Xr scp 1
202: and
203: .Xr sftp 1 .
204: The argument must be
205: .Dq yes
206: or
207: .Dq no .
208: The default is
209: .Dq no .
210: .It Cm Compression
211: Specifies whether to use compression.
212: The argument must be
213: .Dq yes
214: or
215: .Dq no .
216: The default is
217: .Dq no .
218: .It Cm CompressionLevel
219: Specifies the compression level to use if compression is enabled.
220: The argument must be an integer from 1 (fast) to 9 (slow, best).
221: The default level is 6, which is good for most applications.
222: The meaning of the values is the same as in
223: .Xr gzip 1 .
224: Note that this option applies to protocol version 1 only.
225: .It Cm ConnectionAttempts
226: Specifies the number of tries (one per second) to make before exiting.
227: The argument must be an integer.
228: This may be useful in scripts if the connection sometimes fails.
229: The default is 1.
1.9 ! djm 230: .It Cm ConnectTimeout
! 231: Specifies the timeout (in seconds) used when connecting to the ssh
! 232: server, instead of using the default system TCP timeout. This value is
! 233: used only when the target is down or really unreachable, not when it
! 234: refuses the connection.
1.1 stevesk 235: .It Cm DynamicForward
236: Specifies that a TCP/IP port on the local machine be forwarded
237: over the secure channel, and the application
238: protocol is then used to determine where to connect to from the
1.7 jmc 239: remote machine.
240: The argument must be a port number.
1.1 stevesk 241: Currently the SOCKS4 protocol is supported, and
242: .Nm ssh
243: will act as a SOCKS4 server.
244: Multiple forwardings may be specified, and
1.7 jmc 245: additional forwardings can be given on the command line.
246: Only the superuser can forward privileged ports.
1.1 stevesk 247: .It Cm EscapeChar
248: Sets the escape character (default:
249: .Ql ~ ) .
250: The escape character can also
251: be set on the command line.
252: The argument should be a single character,
253: .Ql ^
254: followed by a letter, or
255: .Dq none
256: to disable the escape
257: character entirely (making the connection transparent for binary
258: data).
259: .It Cm ForwardAgent
260: Specifies whether the connection to the authentication agent (if any)
261: will be forwarded to the remote machine.
262: The argument must be
263: .Dq yes
264: or
265: .Dq no .
266: The default is
267: .Dq no .
1.3 stevesk 268: .Pp
1.7 jmc 269: Agent forwarding should be enabled with caution.
270: Users with the ability to bypass file permissions on the remote host
271: (for the agent's Unix-domain socket)
272: can access the local agent through the forwarded connection.
273: An attacker cannot obtain key material from the agent,
1.3 stevesk 274: however they can perform operations on the keys that enable them to
275: authenticate using the identities loaded into the agent.
1.1 stevesk 276: .It Cm ForwardX11
277: Specifies whether X11 connections will be automatically redirected
278: over the secure channel and
279: .Ev DISPLAY
280: set.
281: The argument must be
282: .Dq yes
283: or
284: .Dq no .
285: The default is
286: .Dq no .
1.3 stevesk 287: .Pp
1.7 jmc 288: X11 forwarding should be enabled with caution.
289: Users with the ability to bypass file permissions on the remote host
290: (for the user's X authorization database)
291: can access the local X11 display through the forwarded connection.
292: An attacker may then be able to perform activities such as keystroke monitoring.
1.1 stevesk 293: .It Cm GatewayPorts
294: Specifies whether remote hosts are allowed to connect to local
295: forwarded ports.
296: By default,
297: .Nm ssh
1.7 jmc 298: binds local port forwardings to the loopback address.
299: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 300: .Cm GatewayPorts
301: can be used to specify that
302: .Nm ssh
303: should bind local port forwardings to the wildcard address,
304: thus allowing remote hosts to connect to forwarded ports.
305: The argument must be
306: .Dq yes
307: or
308: .Dq no .
309: The default is
310: .Dq no .
311: .It Cm GlobalKnownHostsFile
312: Specifies a file to use for the global
313: host key database instead of
314: .Pa /etc/ssh/ssh_known_hosts .
315: .It Cm HostbasedAuthentication
316: Specifies whether to try rhosts based authentication with public key
317: authentication.
318: The argument must be
319: .Dq yes
320: or
321: .Dq no .
322: The default is
323: .Dq no .
324: This option applies to protocol version 2 only and
325: is similar to
326: .Cm RhostsRSAAuthentication .
327: .It Cm HostKeyAlgorithms
328: Specifies the protocol version 2 host key algorithms
329: that the client wants to use in order of preference.
330: The default for this option is:
331: .Dq ssh-rsa,ssh-dss .
332: .It Cm HostKeyAlias
333: Specifies an alias that should be used instead of the
334: real host name when looking up or saving the host key
335: in the host key database files.
336: This option is useful for tunneling ssh connections
337: or for multiple servers running on a single host.
338: .It Cm HostName
339: Specifies the real host name to log into.
340: This can be used to specify nicknames or abbreviations for hosts.
341: Default is the name given on the command line.
342: Numeric IP addresses are also permitted (both on the command line and in
343: .Cm HostName
344: specifications).
345: .It Cm IdentityFile
346: Specifies a file from which the user's RSA or DSA authentication identity
347: is read. The default is
348: .Pa $HOME/.ssh/identity
349: for protocol version 1, and
350: .Pa $HOME/.ssh/id_rsa
351: and
352: .Pa $HOME/.ssh/id_dsa
353: for protocol version 2.
354: Additionally, any identities represented by the authentication agent
355: will be used for authentication.
356: The file name may use the tilde
357: syntax to refer to a user's home directory.
358: It is possible to have
359: multiple identity files specified in configuration files; all these
360: identities will be tried in sequence.
361: .It Cm KeepAlive
362: Specifies whether the system should send TCP keepalive messages to the
363: other side.
364: If they are sent, death of the connection or crash of one
365: of the machines will be properly noticed.
366: However, this means that
367: connections will die if the route is down temporarily, and some people
368: find it annoying.
369: .Pp
370: The default is
371: .Dq yes
372: (to send keepalives), and the client will notice
373: if the network goes down or the remote host dies.
374: This is important in scripts, and many users want it too.
375: .Pp
376: To disable keepalives, the value should be set to
377: .Dq no .
378: .It Cm KerberosAuthentication
379: Specifies whether Kerberos authentication will be used.
380: The argument to this keyword must be
381: .Dq yes
382: or
383: .Dq no .
384: .It Cm KerberosTgtPassing
385: Specifies whether a Kerberos TGT will be forwarded to the server.
386: This will only work if the Kerberos server is actually an AFS kaserver.
387: The argument to this keyword must be
388: .Dq yes
389: or
390: .Dq no .
391: .It Cm LocalForward
392: Specifies that a TCP/IP port on the local machine be forwarded over
393: the secure channel to the specified host and port from the remote machine.
394: The first argument must be a port number, and the second must be
395: .Ar host:port .
396: IPv6 addresses can be specified with an alternative syntax:
397: .Ar host/port .
398: Multiple forwardings may be specified, and additional
399: forwardings can be given on the command line.
400: Only the superuser can forward privileged ports.
401: .It Cm LogLevel
402: Gives the verbosity level that is used when logging messages from
403: .Nm ssh .
404: The possible values are:
405: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 406: The default is INFO.
407: DEBUG and DEBUG1 are equivalent.
408: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 409: .It Cm MACs
410: Specifies the MAC (message authentication code) algorithms
411: in order of preference.
412: The MAC algorithm is used in protocol version 2
413: for data integrity protection.
414: Multiple algorithms must be comma-separated.
415: The default is
416: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
417: .It Cm NoHostAuthenticationForLocalhost
418: This option can be used if the home directory is shared across machines.
419: In this case localhost will refer to a different machine on each of
420: the machines and the user will get many warnings about changed host keys.
421: However, this option disables host authentication for localhost.
422: The argument to this keyword must be
423: .Dq yes
424: or
425: .Dq no .
426: The default is to check the host key for localhost.
427: .It Cm NumberOfPasswordPrompts
428: Specifies the number of password prompts before giving up.
429: The argument to this keyword must be an integer.
430: Default is 3.
431: .It Cm PasswordAuthentication
432: Specifies whether to use password authentication.
433: The argument to this keyword must be
434: .Dq yes
435: or
436: .Dq no .
437: The default is
438: .Dq yes .
439: .It Cm Port
440: Specifies the port number to connect on the remote host.
441: Default is 22.
442: .It Cm PreferredAuthentications
443: Specifies the order in which the client should try protocol 2
444: authentication methods. This allows a client to prefer one method (e.g.
445: .Cm keyboard-interactive )
446: over another method (e.g.
447: .Cm password )
448: The default for this option is:
449: .Dq hostbased,publickey,keyboard-interactive,password .
450: .It Cm Protocol
451: Specifies the protocol versions
452: .Nm ssh
453: should support in order of preference.
454: The possible values are
455: .Dq 1
456: and
457: .Dq 2 .
458: Multiple versions must be comma-separated.
459: The default is
460: .Dq 2,1 .
461: This means that
462: .Nm ssh
463: tries version 2 and falls back to version 1
464: if version 2 is not available.
465: .It Cm ProxyCommand
466: Specifies the command to use to connect to the server.
467: The command
468: string extends to the end of the line, and is executed with
469: .Pa /bin/sh .
470: In the command string,
471: .Ql %h
472: will be substituted by the host name to
473: connect and
474: .Ql %p
475: by the port.
476: The command can be basically anything,
477: and should read from its standard input and write to its standard output.
478: It should eventually connect an
479: .Xr sshd 8
480: server running on some machine, or execute
481: .Ic sshd -i
482: somewhere.
483: Host key management will be done using the
484: HostName of the host being connected (defaulting to the name typed by
485: the user).
1.7 jmc 486: Setting the command to
487: .Dq none
1.6 markus 488: disables this option entirely.
1.1 stevesk 489: Note that
490: .Cm CheckHostIP
491: is not available for connects with a proxy command.
492: .Pp
493: .It Cm PubkeyAuthentication
494: Specifies whether to try public key authentication.
495: The argument to this keyword must be
496: .Dq yes
497: or
498: .Dq no .
499: The default is
500: .Dq yes .
501: This option applies to protocol version 2 only.
502: .It Cm RemoteForward
503: Specifies that a TCP/IP port on the remote machine be forwarded over
504: the secure channel to the specified host and port from the local machine.
505: The first argument must be a port number, and the second must be
506: .Ar host:port .
507: IPv6 addresses can be specified with an alternative syntax:
508: .Ar host/port .
509: Multiple forwardings may be specified, and additional
510: forwardings can be given on the command line.
511: Only the superuser can forward privileged ports.
512: .It Cm RhostsAuthentication
513: Specifies whether to try rhosts based authentication.
514: Note that this
515: declaration only affects the client side and has no effect whatsoever
516: on security.
517: Most servers do not permit RhostsAuthentication because it
518: is not secure (see
519: .Cm RhostsRSAAuthentication ) .
520: The argument to this keyword must be
521: .Dq yes
522: or
523: .Dq no .
524: The default is
525: .Dq no .
1.4 stevesk 526: This option applies to protocol version 1 only and requires
527: .Nm ssh
528: to be setuid root and
529: .Cm UsePrivilegedPort
530: to be set to
531: .Dq yes .
1.1 stevesk 532: .It Cm RhostsRSAAuthentication
533: Specifies whether to try rhosts based authentication with RSA host
534: authentication.
535: The argument must be
536: .Dq yes
537: or
538: .Dq no .
539: The default is
540: .Dq no .
541: This option applies to protocol version 1 only and requires
542: .Nm ssh
543: to be setuid root.
544: .It Cm RSAAuthentication
545: Specifies whether to try RSA authentication.
546: The argument to this keyword must be
547: .Dq yes
548: or
549: .Dq no .
550: RSA authentication will only be
551: attempted if the identity file exists, or an authentication agent is
552: running.
553: The default is
554: .Dq yes .
555: Note that this option applies to protocol version 1 only.
556: .It Cm SmartcardDevice
557: Specifies which smartcard device to use. The argument to this keyword is
558: the device
559: .Nm ssh
560: should use to communicate with a smartcard used for storing the user's
561: private RSA key. By default, no device is specified and smartcard support
562: is not activated.
563: .It Cm StrictHostKeyChecking
564: If this flag is set to
565: .Dq yes ,
566: .Nm ssh
567: will never automatically add host keys to the
568: .Pa $HOME/.ssh/known_hosts
569: file, and refuses to connect to hosts whose host key has changed.
570: This provides maximum protection against trojan horse attacks,
571: however, can be annoying when the
572: .Pa /etc/ssh/ssh_known_hosts
573: file is poorly maintained, or connections to new hosts are
574: frequently made.
575: This option forces the user to manually
576: add all new hosts.
577: If this flag is set to
578: .Dq no ,
579: .Nm ssh
580: will automatically add new host keys to the
581: user known hosts files.
582: If this flag is set to
583: .Dq ask ,
584: new host keys
585: will be added to the user known host files only after the user
586: has confirmed that is what they really want to do, and
587: .Nm ssh
588: will refuse to connect to hosts whose host key has changed.
589: The host keys of
590: known hosts will be verified automatically in all cases.
591: The argument must be
592: .Dq yes ,
593: .Dq no
594: or
595: .Dq ask .
596: The default is
597: .Dq ask .
598: .It Cm UsePrivilegedPort
599: Specifies whether to use a privileged port for outgoing connections.
600: The argument must be
601: .Dq yes
602: or
603: .Dq no .
604: The default is
605: .Dq no .
1.4 stevesk 606: If set to
607: .Dq yes
608: .Nm ssh
609: must be setuid root.
1.1 stevesk 610: Note that this option must be set to
611: .Dq yes
612: if
613: .Cm RhostsAuthentication
614: and
615: .Cm RhostsRSAAuthentication
616: authentications are needed with older servers.
617: .It Cm User
618: Specifies the user to log in as.
619: This can be useful when a different user name is used on different machines.
620: This saves the trouble of
621: having to remember to give the user name on the command line.
622: .It Cm UserKnownHostsFile
623: Specifies a file to use for the user
624: host key database instead of
625: .Pa $HOME/.ssh/known_hosts .
1.8 jakob 626: .It Cm VerifyHostKeyDNS
627: Specifies whether to verify the remote key using DNS and SSHFP resource
628: records.
629: The default is
630: .Dq no .
1.1 stevesk 631: .It Cm XAuthLocation
1.5 stevesk 632: Specifies the full pathname of the
1.1 stevesk 633: .Xr xauth 1
634: program.
635: The default is
636: .Pa /usr/X11R6/bin/xauth .
637: .El
638: .Sh FILES
639: .Bl -tag -width Ds
640: .It Pa $HOME/.ssh/config
641: This is the per-user configuration file.
642: The format of this file is described above.
643: This file is used by the
644: .Nm ssh
645: client.
646: This file does not usually contain any sensitive information,
647: but the recommended permissions are read/write for the user, and not
648: accessible by others.
649: .It Pa /etc/ssh/ssh_config
650: Systemwide configuration file.
651: This file provides defaults for those
652: values that are not specified in the user's configuration file, and
653: for those users who do not have a configuration file.
654: This file must be world-readable.
655: .El
656: .Sh AUTHORS
657: OpenSSH is a derivative of the original and free
658: ssh 1.2.12 release by Tatu Ylonen.
659: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
660: Theo de Raadt and Dug Song
661: removed many bugs, re-added newer features and
662: created OpenSSH.
663: Markus Friedl contributed the support for SSH
664: protocol versions 1.5 and 2.0.
665: .Sh SEE ALSO
666: .Xr ssh 1
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