Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.3
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
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.
36: .\"
1.3 ! stevesk 37: .\" $OpenBSD: ssh_config.5,v 1.2 2002/08/17 23:55:01 stevesk 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
179: cipher. Its use is strongly discouraged due to cryptographic
180: weaknesses.
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
196: cleared. This option is primarily useful when used from the
197: .Nm ssh
198: command line to clear port forwardings set in
199: configuration files, and is automatically set by
200: .Xr scp 1
201: and
202: .Xr sftp 1 .
203: The argument must be
204: .Dq yes
205: or
206: .Dq no .
207: The default is
208: .Dq no .
209: .It Cm Compression
210: Specifies whether to use compression.
211: The argument must be
212: .Dq yes
213: or
214: .Dq no .
215: The default is
216: .Dq no .
217: .It Cm CompressionLevel
218: Specifies the compression level to use if compression is enabled.
219: The argument must be an integer from 1 (fast) to 9 (slow, best).
220: The default level is 6, which is good for most applications.
221: The meaning of the values is the same as in
222: .Xr gzip 1 .
223: Note that this option applies to protocol version 1 only.
224: .It Cm ConnectionAttempts
225: Specifies the number of tries (one per second) to make before exiting.
226: The argument must be an integer.
227: This may be useful in scripts if the connection sometimes fails.
228: The default is 1.
229: .It Cm DynamicForward
230: Specifies that a TCP/IP port on the local machine be forwarded
231: over the secure channel, and the application
232: protocol is then used to determine where to connect to from the
233: remote machine. The argument must be a port number.
234: Currently the SOCKS4 protocol is supported, and
235: .Nm ssh
236: will act as a SOCKS4 server.
237: Multiple forwardings may be specified, and
238: additional forwardings can be given on the command line. Only
239: the superuser can forward privileged ports.
240: .It Cm EscapeChar
241: Sets the escape character (default:
242: .Ql ~ ) .
243: The escape character can also
244: be set on the command line.
245: The argument should be a single character,
246: .Ql ^
247: followed by a letter, or
248: .Dq none
249: to disable the escape
250: character entirely (making the connection transparent for binary
251: data).
252: .It Cm ForwardAgent
253: Specifies whether the connection to the authentication agent (if any)
254: will be forwarded to the remote machine.
255: The argument must be
256: .Dq yes
257: or
258: .Dq no .
259: The default is
260: .Dq no .
1.3 ! stevesk 261: .Pp
! 262: Agent forwarding should be enabled with caution. Users with the
! 263: ability to bypass file permissions on the remote host (for the agent's
! 264: Unix-domain socket) can access the local agent through the forwarded
! 265: connection. An attacker cannot obtain key material from the agent,
! 266: however they can perform operations on the keys that enable them to
! 267: authenticate using the identities loaded into the agent.
1.1 stevesk 268: .It Cm ForwardX11
269: Specifies whether X11 connections will be automatically redirected
270: over the secure channel and
271: .Ev DISPLAY
272: set.
273: The argument must be
274: .Dq yes
275: or
276: .Dq no .
277: The default is
278: .Dq no .
1.3 ! stevesk 279: .Pp
! 280: X11 forwarding should be enabled with caution. Users with the ability
! 281: to bypass file permissions on the remote host (for the user's X
! 282: authorization database) can access the local X11 display through the
! 283: forwarded connection. An attacker may then be able to perform
! 284: activities such as keystroke monitoring.
1.1 stevesk 285: .It Cm GatewayPorts
286: Specifies whether remote hosts are allowed to connect to local
287: forwarded ports.
288: By default,
289: .Nm ssh
290: binds local port forwardings to the loopback address. This
291: prevents other remote hosts from connecting to forwarded ports.
292: .Cm GatewayPorts
293: can be used to specify that
294: .Nm ssh
295: should bind local port forwardings to the wildcard address,
296: thus allowing remote hosts to connect to forwarded ports.
297: The argument must be
298: .Dq yes
299: or
300: .Dq no .
301: The default is
302: .Dq no .
303: .It Cm GlobalKnownHostsFile
304: Specifies a file to use for the global
305: host key database instead of
306: .Pa /etc/ssh/ssh_known_hosts .
307: .It Cm HostbasedAuthentication
308: Specifies whether to try rhosts based authentication with public key
309: authentication.
310: The argument must be
311: .Dq yes
312: or
313: .Dq no .
314: The default is
315: .Dq no .
316: This option applies to protocol version 2 only and
317: is similar to
318: .Cm RhostsRSAAuthentication .
319: .It Cm HostKeyAlgorithms
320: Specifies the protocol version 2 host key algorithms
321: that the client wants to use in order of preference.
322: The default for this option is:
323: .Dq ssh-rsa,ssh-dss .
324: .It Cm HostKeyAlias
325: Specifies an alias that should be used instead of the
326: real host name when looking up or saving the host key
327: in the host key database files.
328: This option is useful for tunneling ssh connections
329: or for multiple servers running on a single host.
330: .It Cm HostName
331: Specifies the real host name to log into.
332: This can be used to specify nicknames or abbreviations for hosts.
333: Default is the name given on the command line.
334: Numeric IP addresses are also permitted (both on the command line and in
335: .Cm HostName
336: specifications).
337: .It Cm IdentityFile
338: Specifies a file from which the user's RSA or DSA authentication identity
339: is read. The default is
340: .Pa $HOME/.ssh/identity
341: for protocol version 1, and
342: .Pa $HOME/.ssh/id_rsa
343: and
344: .Pa $HOME/.ssh/id_dsa
345: for protocol version 2.
346: Additionally, any identities represented by the authentication agent
347: will be used for authentication.
348: The file name may use the tilde
349: syntax to refer to a user's home directory.
350: It is possible to have
351: multiple identity files specified in configuration files; all these
352: identities will be tried in sequence.
353: .It Cm KeepAlive
354: Specifies whether the system should send TCP keepalive messages to the
355: other side.
356: If they are sent, death of the connection or crash of one
357: of the machines will be properly noticed.
358: However, this means that
359: connections will die if the route is down temporarily, and some people
360: find it annoying.
361: .Pp
362: The default is
363: .Dq yes
364: (to send keepalives), and the client will notice
365: if the network goes down or the remote host dies.
366: This is important in scripts, and many users want it too.
367: .Pp
368: To disable keepalives, the value should be set to
369: .Dq no .
370: .It Cm KerberosAuthentication
371: Specifies whether Kerberos authentication will be used.
372: The argument to this keyword must be
373: .Dq yes
374: or
375: .Dq no .
376: .It Cm KerberosTgtPassing
377: Specifies whether a Kerberos TGT will be forwarded to the server.
378: This will only work if the Kerberos server is actually an AFS kaserver.
379: The argument to this keyword must be
380: .Dq yes
381: or
382: .Dq no .
383: .It Cm LocalForward
384: Specifies that a TCP/IP port on the local machine be forwarded over
385: the secure channel to the specified host and port from the remote machine.
386: The first argument must be a port number, and the second must be
387: .Ar host:port .
388: IPv6 addresses can be specified with an alternative syntax:
389: .Ar host/port .
390: Multiple forwardings may be specified, and additional
391: forwardings can be given on the command line.
392: Only the superuser can forward privileged ports.
393: .It Cm LogLevel
394: Gives the verbosity level that is used when logging messages from
395: .Nm ssh .
396: The possible values are:
397: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
398: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
399: and DEBUG3 each specify higher levels of verbose output.
400: .It Cm MACs
401: Specifies the MAC (message authentication code) algorithms
402: in order of preference.
403: The MAC algorithm is used in protocol version 2
404: for data integrity protection.
405: Multiple algorithms must be comma-separated.
406: The default is
407: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
408: .It Cm NoHostAuthenticationForLocalhost
409: This option can be used if the home directory is shared across machines.
410: In this case localhost will refer to a different machine on each of
411: the machines and the user will get many warnings about changed host keys.
412: However, this option disables host authentication for localhost.
413: The argument to this keyword must be
414: .Dq yes
415: or
416: .Dq no .
417: The default is to check the host key for localhost.
418: .It Cm NumberOfPasswordPrompts
419: Specifies the number of password prompts before giving up.
420: The argument to this keyword must be an integer.
421: Default is 3.
422: .It Cm PasswordAuthentication
423: Specifies whether to use password authentication.
424: The argument to this keyword must be
425: .Dq yes
426: or
427: .Dq no .
428: The default is
429: .Dq yes .
430: .It Cm Port
431: Specifies the port number to connect on the remote host.
432: Default is 22.
433: .It Cm PreferredAuthentications
434: Specifies the order in which the client should try protocol 2
435: authentication methods. This allows a client to prefer one method (e.g.
436: .Cm keyboard-interactive )
437: over another method (e.g.
438: .Cm password )
439: The default for this option is:
440: .Dq hostbased,publickey,keyboard-interactive,password .
441: .It Cm Protocol
442: Specifies the protocol versions
443: .Nm ssh
444: should support in order of preference.
445: The possible values are
446: .Dq 1
447: and
448: .Dq 2 .
449: Multiple versions must be comma-separated.
450: The default is
451: .Dq 2,1 .
452: This means that
453: .Nm ssh
454: tries version 2 and falls back to version 1
455: if version 2 is not available.
456: .It Cm ProxyCommand
457: Specifies the command to use to connect to the server.
458: The command
459: string extends to the end of the line, and is executed with
460: .Pa /bin/sh .
461: In the command string,
462: .Ql %h
463: will be substituted by the host name to
464: connect and
465: .Ql %p
466: by the port.
467: The command can be basically anything,
468: and should read from its standard input and write to its standard output.
469: It should eventually connect an
470: .Xr sshd 8
471: server running on some machine, or execute
472: .Ic sshd -i
473: somewhere.
474: Host key management will be done using the
475: HostName of the host being connected (defaulting to the name typed by
476: the user).
477: Note that
478: .Cm CheckHostIP
479: is not available for connects with a proxy command.
480: .Pp
481: .It Cm PubkeyAuthentication
482: Specifies whether to try public key authentication.
483: The argument to this keyword must be
484: .Dq yes
485: or
486: .Dq no .
487: The default is
488: .Dq yes .
489: This option applies to protocol version 2 only.
490: .It Cm RemoteForward
491: Specifies that a TCP/IP port on the remote machine be forwarded over
492: the secure channel to the specified host and port from the local machine.
493: The first argument must be a port number, and the second must be
494: .Ar host:port .
495: IPv6 addresses can be specified with an alternative syntax:
496: .Ar host/port .
497: Multiple forwardings may be specified, and additional
498: forwardings can be given on the command line.
499: Only the superuser can forward privileged ports.
500: .It Cm RhostsAuthentication
501: Specifies whether to try rhosts based authentication.
502: Note that this
503: declaration only affects the client side and has no effect whatsoever
504: on security.
505: Most servers do not permit RhostsAuthentication because it
506: is not secure (see
507: .Cm RhostsRSAAuthentication ) .
508: The argument to this keyword must be
509: .Dq yes
510: or
511: .Dq no .
512: The default is
513: .Dq no .
514: This option applies to protocol version 1 only.
515: .It Cm RhostsRSAAuthentication
516: Specifies whether to try rhosts based authentication with RSA host
517: authentication.
518: The argument must be
519: .Dq yes
520: or
521: .Dq no .
522: The default is
523: .Dq no .
524: This option applies to protocol version 1 only and requires
525: .Nm ssh
526: to be setuid root.
527: .It Cm RSAAuthentication
528: Specifies whether to try RSA authentication.
529: The argument to this keyword must be
530: .Dq yes
531: or
532: .Dq no .
533: RSA authentication will only be
534: attempted if the identity file exists, or an authentication agent is
535: running.
536: The default is
537: .Dq yes .
538: Note that this option applies to protocol version 1 only.
539: .It Cm SmartcardDevice
540: Specifies which smartcard device to use. The argument to this keyword is
541: the device
542: .Nm ssh
543: should use to communicate with a smartcard used for storing the user's
544: private RSA key. By default, no device is specified and smartcard support
545: is not activated.
546: .It Cm StrictHostKeyChecking
547: If this flag is set to
548: .Dq yes ,
549: .Nm ssh
550: will never automatically add host keys to the
551: .Pa $HOME/.ssh/known_hosts
552: file, and refuses to connect to hosts whose host key has changed.
553: This provides maximum protection against trojan horse attacks,
554: however, can be annoying when the
555: .Pa /etc/ssh/ssh_known_hosts
556: file is poorly maintained, or connections to new hosts are
557: frequently made.
558: This option forces the user to manually
559: add all new hosts.
560: If this flag is set to
561: .Dq no ,
562: .Nm ssh
563: will automatically add new host keys to the
564: user known hosts files.
565: If this flag is set to
566: .Dq ask ,
567: new host keys
568: will be added to the user known host files only after the user
569: has confirmed that is what they really want to do, and
570: .Nm ssh
571: will refuse to connect to hosts whose host key has changed.
572: The host keys of
573: known hosts will be verified automatically in all cases.
574: The argument must be
575: .Dq yes ,
576: .Dq no
577: or
578: .Dq ask .
579: The default is
580: .Dq ask .
581: .It Cm UsePrivilegedPort
582: Specifies whether to use a privileged port for outgoing connections.
583: The argument must be
584: .Dq yes
585: or
586: .Dq no .
587: The default is
588: .Dq no .
589: Note that this option must be set to
590: .Dq yes
591: if
592: .Cm RhostsAuthentication
593: and
594: .Cm RhostsRSAAuthentication
595: authentications are needed with older servers.
596: .It Cm User
597: Specifies the user to log in as.
598: This can be useful when a different user name is used on different machines.
599: This saves the trouble of
600: having to remember to give the user name on the command line.
601: .It Cm UserKnownHostsFile
602: Specifies a file to use for the user
603: host key database instead of
604: .Pa $HOME/.ssh/known_hosts .
605: .It Cm XAuthLocation
606: Specifies the location of the
607: .Xr xauth 1
608: program.
609: The default is
610: .Pa /usr/X11R6/bin/xauth .
611: .El
612: .Sh FILES
613: .Bl -tag -width Ds
614: .It Pa $HOME/.ssh/config
615: This is the per-user configuration file.
616: The format of this file is described above.
617: This file is used by the
618: .Nm ssh
619: client.
620: This file does not usually contain any sensitive information,
621: but the recommended permissions are read/write for the user, and not
622: accessible by others.
623: .It Pa /etc/ssh/ssh_config
624: Systemwide configuration file.
625: This file provides defaults for those
626: values that are not specified in the user's configuration file, and
627: for those users who do not have a configuration file.
628: This file must be world-readable.
629: .El
630: .Sh AUTHORS
631: OpenSSH is a derivative of the original and free
632: ssh 1.2.12 release by Tatu Ylonen.
633: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
634: Theo de Raadt and Dug Song
635: removed many bugs, re-added newer features and
636: created OpenSSH.
637: Markus Friedl contributed the support for SSH
638: protocol versions 1.5 and 2.0.
639: .Sh SEE ALSO
640: .Xr ssh 1
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