Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.57
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.57 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.56 2005/06/08 11:25:09 djm 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
139: Specify the interface to transmit from on machines with multiple
140: interfaces or aliased addresses.
141: Note that this option does not work if
142: .Cm UsePrivilegedPort
143: is set to
144: .Dq yes .
145: .It Cm ChallengeResponseAuthentication
146: Specifies whether to use challenge response authentication.
147: The argument to this keyword must be
148: .Dq yes
149: or
150: .Dq no .
151: The default is
152: .Dq yes .
153: .It Cm CheckHostIP
154: If this flag is set to
155: .Dq yes ,
156: ssh will additionally check the host IP address in the
157: .Pa known_hosts
158: file.
159: This allows ssh to detect if a host key changed due to DNS spoofing.
160: If the option is set to
161: .Dq no ,
162: the check will not be executed.
163: The default is
164: .Dq yes .
165: .It Cm Cipher
166: Specifies the cipher to use for encrypting the session
167: in protocol version 1.
168: Currently,
169: .Dq blowfish ,
170: .Dq 3des ,
171: and
172: .Dq des
173: are supported.
174: .Ar des
175: is only supported in the
176: .Nm ssh
177: client for interoperability with legacy protocol 1 implementations
178: that do not support the
179: .Ar 3des
1.7 jmc 180: cipher.
181: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 182: The default is
183: .Dq 3des .
184: .It Cm Ciphers
185: Specifies the ciphers allowed for protocol version 2
186: in order of preference.
187: Multiple ciphers must be comma-separated.
1.35 dtucker 188: The supported ciphers are
189: .Dq 3des-cbc ,
190: .Dq aes128-cbc ,
191: .Dq aes192-cbc ,
192: .Dq aes256-cbc ,
193: .Dq aes128-ctr ,
194: .Dq aes192-ctr ,
195: .Dq aes256-ctr ,
1.54 djm 196: .Dq arcfour128 ,
197: .Dq arcfour256 ,
1.35 dtucker 198: .Dq arcfour ,
199: .Dq blowfish-cbc ,
200: and
201: .Dq cast128-cbc .
1.1 stevesk 202: The default is
203: .Bd -literal
1.54 djm 204: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
205: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
206: aes192-ctr,aes256-ctr''
1.1 stevesk 207: .Ed
208: .It Cm ClearAllForwardings
209: Specifies that all local, remote and dynamic port forwardings
210: specified in the configuration files or on the command line be
1.7 jmc 211: cleared.
212: This option is primarily useful when used from the
1.1 stevesk 213: .Nm ssh
214: command line to clear port forwardings set in
215: configuration files, and is automatically set by
216: .Xr scp 1
217: and
218: .Xr sftp 1 .
219: The argument must be
220: .Dq yes
221: or
222: .Dq no .
223: The default is
224: .Dq no .
225: .It Cm Compression
226: Specifies whether to use compression.
227: The argument must be
228: .Dq yes
229: or
230: .Dq no .
231: The default is
232: .Dq no .
233: .It Cm CompressionLevel
234: Specifies the compression level to use if compression is enabled.
235: The argument must be an integer from 1 (fast) to 9 (slow, best).
236: The default level is 6, which is good for most applications.
237: The meaning of the values is the same as in
238: .Xr gzip 1 .
239: Note that this option applies to protocol version 1 only.
240: .It Cm ConnectionAttempts
241: Specifies the number of tries (one per second) to make before exiting.
242: The argument must be an integer.
243: This may be useful in scripts if the connection sometimes fails.
244: The default is 1.
1.9 djm 245: .It Cm ConnectTimeout
246: Specifies the timeout (in seconds) used when connecting to the ssh
1.11 jmc 247: server, instead of using the default system TCP timeout.
248: This value is used only when the target is down or really unreachable,
249: not when it refuses the connection.
1.36 djm 250: .It Cm ControlMaster
251: Enables the sharing of multiple sessions over a single network connection.
252: When set to
253: .Dq yes
254: .Nm ssh
255: will listen for connections on a control socket specified using the
256: .Cm ControlPath
257: argument.
258: Additional sessions can connect to this socket using the same
259: .Cm ControlPath
260: with
261: .Cm ControlMaster
262: set to
263: .Dq no
1.38 jmc 264: (the default).
1.36 djm 265: These sessions will reuse the master instance's network connection rather
266: than initiating new ones.
1.37 djm 267: Setting this to
268: .Dq ask
269: will cause
270: .Nm ssh
271: to listen for control connections, but require confirmation using the
272: .Ev SSH_ASKPASS
273: program before they are accepted (see
274: .Xr ssh-add 1
1.38 jmc 275: for details).
1.51 jakob 276: If the
277: .Cm ControlPath
278: can not be opened,
279: .Nm ssh
280: will continue without connecting to a master instance.
1.56 djm 281: .Pp
282: Two additional options allow for opportunistic multiplexing: try to use a
283: master connection but fall back to creating a new one if one does not already
284: exist.
285: These options are:
286: .Dq auto
287: and
288: .Dq autoask .
289: The latter requires confirmation like the
290: .Dq ask
291: option.
1.36 djm 292: .It Cm ControlPath
1.55 djm 293: Specify the path to the control socket used for connection sharing as described
294: in the
1.36 djm 295: .Cm ControlMaster
1.57 ! djm 296: section above or the string
! 297: .Dq none
! 298: to disable connection sharing.
1.55 djm 299: In the path,
300: .Ql %h
301: will be substituted by the target host name,
302: .Ql %p
303: the port and
304: .Ql %r
305: by the remote login username.
1.56 djm 306: It is recommended that any
307: .Cm ControlPath
308: used for opportunistic connection sharing include
309: all three of these escape sequences.
310: This ensures that shared connections are uniquely identified.
1.38 jmc 311: .It Cm DynamicForward
312: Specifies that a TCP/IP port on the local machine be forwarded
313: over the secure channel, and the application
314: protocol is then used to determine where to connect to from the
315: remote machine.
316: The argument must be a port number.
317: Currently the SOCKS4 and SOCKS5 protocols are supported, and
318: .Nm ssh
319: will act as a SOCKS server.
320: Multiple forwardings may be specified, and
321: additional forwardings can be given on the command line.
322: Only the superuser can forward privileged ports.
1.14 markus 323: .It Cm EnableSSHKeysign
324: Setting this option to
325: .Dq yes
326: in the global client configuration file
327: .Pa /etc/ssh/ssh_config
328: enables the use of the helper program
329: .Xr ssh-keysign 8
330: during
331: .Cm HostbasedAuthentication .
332: The argument must be
333: .Dq yes
334: or
335: .Dq no .
336: The default is
337: .Dq no .
1.23 jmc 338: This option should be placed in the non-hostspecific section.
1.14 markus 339: See
340: .Xr ssh-keysign 8
341: for more information.
1.1 stevesk 342: .It Cm EscapeChar
343: Sets the escape character (default:
344: .Ql ~ ) .
345: The escape character can also
346: be set on the command line.
347: The argument should be a single character,
348: .Ql ^
349: followed by a letter, or
350: .Dq none
351: to disable the escape
352: character entirely (making the connection transparent for binary
353: data).
354: .It Cm ForwardAgent
355: Specifies whether the connection to the authentication agent (if any)
356: will be forwarded to the remote machine.
357: The argument must be
358: .Dq yes
359: or
360: .Dq no .
361: The default is
362: .Dq no .
1.3 stevesk 363: .Pp
1.7 jmc 364: Agent forwarding should be enabled with caution.
365: Users with the ability to bypass file permissions on the remote host
366: (for the agent's Unix-domain socket)
367: can access the local agent through the forwarded connection.
368: An attacker cannot obtain key material from the agent,
1.3 stevesk 369: however they can perform operations on the keys that enable them to
370: authenticate using the identities loaded into the agent.
1.1 stevesk 371: .It Cm ForwardX11
372: Specifies whether X11 connections will be automatically redirected
373: over the secure channel and
374: .Ev DISPLAY
375: set.
376: The argument must be
377: .Dq yes
378: or
379: .Dq no .
380: The default is
381: .Dq no .
1.3 stevesk 382: .Pp
1.7 jmc 383: X11 forwarding should be enabled with caution.
384: Users with the ability to bypass file permissions on the remote host
1.22 markus 385: (for the user's X11 authorization database)
1.7 jmc 386: can access the local X11 display through the forwarded connection.
1.22 markus 387: An attacker may then be able to perform activities such as keystroke monitoring
388: if the
389: .Cm ForwardX11Trusted
390: option is also enabled.
391: .It Cm ForwardX11Trusted
1.34 jmc 392: If this option is set to
1.22 markus 393: .Dq yes
394: then remote X11 clients will have full access to the original X11 display.
1.42 djm 395: .Pp
1.22 markus 396: If this option is set to
397: .Dq no
398: then remote X11 clients will be considered untrusted and prevented
399: from stealing or tampering with data belonging to trusted X11
400: clients.
1.42 djm 401: Furthermore, the
402: .Xr xauth 1
403: token used for the session will be set to expire after 20 minutes.
404: Remote clients will be refused access after this time.
1.22 markus 405: .Pp
406: The default is
407: .Dq no .
408: .Pp
409: See the X11 SECURITY extension specification for full details on
410: the restrictions imposed on untrusted clients.
1.1 stevesk 411: .It Cm GatewayPorts
412: Specifies whether remote hosts are allowed to connect to local
413: forwarded ports.
414: By default,
415: .Nm ssh
1.7 jmc 416: binds local port forwardings to the loopback address.
417: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 418: .Cm GatewayPorts
419: can be used to specify that
420: .Nm ssh
421: should bind local port forwardings to the wildcard address,
422: thus allowing remote hosts to connect to forwarded ports.
423: The argument must be
424: .Dq yes
425: or
426: .Dq no .
427: The default is
428: .Dq no .
429: .It Cm GlobalKnownHostsFile
430: Specifies a file to use for the global
431: host key database instead of
432: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 433: .It Cm GSSAPIAuthentication
1.27 markus 434: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 435: The default is
1.21 markus 436: .Dq no .
1.18 markus 437: Note that this option applies to protocol version 2 only.
438: .It Cm GSSAPIDelegateCredentials
439: Forward (delegate) credentials to the server.
440: The default is
441: .Dq no .
442: Note that this option applies to protocol version 2 only.
1.44 djm 443: .It Cm HashKnownHosts
444: Indicates that
445: .Nm ssh
446: should hash host names and addresses when they are added to
1.50 djm 447: .Pa ~/.ssh/known_hosts .
1.44 djm 448: These hashed names may be used normally by
449: .Nm ssh
450: and
451: .Nm sshd ,
452: but they do not reveal identifying information should the file's contents
453: be disclosed.
454: The default is
455: .Dq no .
1.46 jmc 456: Note that hashing of names and addresses will not be retrospectively applied
1.45 djm 457: to existing known hosts files, but these may be manually hashed using
458: .Xr ssh-keygen 1 .
1.1 stevesk 459: .It Cm HostbasedAuthentication
460: Specifies whether to try rhosts based authentication with public key
461: authentication.
462: The argument must be
463: .Dq yes
464: or
465: .Dq no .
466: The default is
467: .Dq no .
468: This option applies to protocol version 2 only and
469: is similar to
470: .Cm RhostsRSAAuthentication .
471: .It Cm HostKeyAlgorithms
472: Specifies the protocol version 2 host key algorithms
473: that the client wants to use in order of preference.
474: The default for this option is:
475: .Dq ssh-rsa,ssh-dss .
476: .It Cm HostKeyAlias
477: Specifies an alias that should be used instead of the
478: real host name when looking up or saving the host key
479: in the host key database files.
480: This option is useful for tunneling ssh connections
481: or for multiple servers running on a single host.
482: .It Cm HostName
483: Specifies the real host name to log into.
484: This can be used to specify nicknames or abbreviations for hosts.
485: Default is the name given on the command line.
486: Numeric IP addresses are also permitted (both on the command line and in
487: .Cm HostName
488: specifications).
489: .It Cm IdentityFile
490: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 491: is read.
492: The default is
1.50 djm 493: .Pa ~/.ssh/identity
1.1 stevesk 494: for protocol version 1, and
1.50 djm 495: .Pa ~/.ssh/id_rsa
1.1 stevesk 496: and
1.50 djm 497: .Pa ~/.ssh/id_dsa
1.1 stevesk 498: for protocol version 2.
499: Additionally, any identities represented by the authentication agent
500: will be used for authentication.
501: The file name may use the tilde
502: syntax to refer to a user's home directory.
503: It is possible to have
504: multiple identity files specified in configuration files; all these
505: identities will be tried in sequence.
1.29 markus 506: .It Cm IdentitiesOnly
507: Specifies that
508: .Nm ssh
509: should only use the authentication identity files configured in the
1.31 jmc 510: .Nm
1.29 markus 511: files,
512: even if the
513: .Nm ssh-agent
514: offers more identities.
515: The argument to this keyword must be
516: .Dq yes
517: or
518: .Dq no .
519: This option is intented for situations where
520: .Nm ssh-agent
521: offers many different identities.
522: The default is
523: .Dq no .
1.39 djm 524: .It Cm KbdInteractiveDevices
525: Specifies the list of methods to use in keyboard-interactive authentication.
526: Multiple method names must be comma-separated.
527: The default is to use the server specified list.
1.1 stevesk 528: .It Cm LocalForward
529: Specifies that a TCP/IP port on the local machine be forwarded over
530: the secure channel to the specified host and port from the remote machine.
1.49 jmc 531: The first argument must be
1.43 djm 532: .Sm off
1.49 jmc 533: .Oo Ar bind_address : Oc Ar port
1.43 djm 534: .Sm on
1.49 jmc 535: and the second argument must be
536: .Ar host : Ns Ar hostport .
1.46 jmc 537: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 538: by using an alternative syntax:
1.49 jmc 539: .Oo Ar bind_address Ns / Oc Ns Ar port
540: and
541: .Ar host Ns / Ns Ar hostport .
1.46 jmc 542: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 543: given on the command line.
1.1 stevesk 544: Only the superuser can forward privileged ports.
1.43 djm 545: By default, the local port is bound in accordance with the
546: .Cm GatewayPorts
547: setting.
548: However, an explicit
549: .Ar bind_address
550: may be used to bind the connection to a specific address.
551: The
552: .Ar bind_address
553: of
554: .Dq localhost
1.46 jmc 555: indicates that the listening port be bound for local use only, while an
556: empty address or
557: .Sq *
1.43 djm 558: indicates that the port should be available from all interfaces.
1.1 stevesk 559: .It Cm LogLevel
560: Gives the verbosity level that is used when logging messages from
561: .Nm ssh .
562: The possible values are:
563: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 564: The default is INFO.
565: DEBUG and DEBUG1 are equivalent.
566: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 567: .It Cm MACs
568: Specifies the MAC (message authentication code) algorithms
569: in order of preference.
570: The MAC algorithm is used in protocol version 2
571: for data integrity protection.
572: Multiple algorithms must be comma-separated.
573: The default is
574: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
575: .It Cm NoHostAuthenticationForLocalhost
576: This option can be used if the home directory is shared across machines.
577: In this case localhost will refer to a different machine on each of
578: the machines and the user will get many warnings about changed host keys.
579: However, this option disables host authentication for localhost.
580: The argument to this keyword must be
581: .Dq yes
582: or
583: .Dq no .
584: The default is to check the host key for localhost.
585: .It Cm NumberOfPasswordPrompts
586: Specifies the number of password prompts before giving up.
587: The argument to this keyword must be an integer.
588: Default is 3.
589: .It Cm PasswordAuthentication
590: Specifies whether to use password authentication.
591: The argument to this keyword must be
592: .Dq yes
593: or
594: .Dq no .
595: The default is
596: .Dq yes .
597: .It Cm Port
598: Specifies the port number to connect on the remote host.
599: Default is 22.
600: .It Cm PreferredAuthentications
601: Specifies the order in which the client should try protocol 2
1.11 jmc 602: authentication methods.
1.48 jmc 603: This allows a client to prefer one method (e.g.\&
1.1 stevesk 604: .Cm keyboard-interactive )
1.48 jmc 605: over another method (e.g.\&
1.1 stevesk 606: .Cm password )
607: The default for this option is:
608: .Dq hostbased,publickey,keyboard-interactive,password .
609: .It Cm Protocol
610: Specifies the protocol versions
611: .Nm ssh
612: should support in order of preference.
613: The possible values are
614: .Dq 1
615: and
616: .Dq 2 .
617: Multiple versions must be comma-separated.
618: The default is
619: .Dq 2,1 .
620: This means that
621: .Nm ssh
622: tries version 2 and falls back to version 1
623: if version 2 is not available.
624: .It Cm ProxyCommand
625: Specifies the command to use to connect to the server.
626: The command
627: string extends to the end of the line, and is executed with
628: .Pa /bin/sh .
629: In the command string,
630: .Ql %h
631: will be substituted by the host name to
632: connect and
633: .Ql %p
634: by the port.
635: The command can be basically anything,
636: and should read from its standard input and write to its standard output.
637: It should eventually connect an
638: .Xr sshd 8
639: server running on some machine, or execute
640: .Ic sshd -i
641: somewhere.
642: Host key management will be done using the
643: HostName of the host being connected (defaulting to the name typed by
644: the user).
1.7 jmc 645: Setting the command to
646: .Dq none
1.6 markus 647: disables this option entirely.
1.1 stevesk 648: Note that
649: .Cm CheckHostIP
650: is not available for connects with a proxy command.
1.52 djm 651: .Pp
652: This directive is useful in conjunction with
653: .Xr nc 1
654: and its proxy support.
1.53 jmc 655: For example, the following directive would connect via an HTTP proxy at
1.52 djm 656: 192.0.2.0:
657: .Bd -literal -offset 3n
658: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
659: .Ed
1.1 stevesk 660: .It Cm PubkeyAuthentication
661: Specifies whether to try public key authentication.
662: The argument to this keyword must be
663: .Dq yes
664: or
665: .Dq no .
666: The default is
667: .Dq yes .
668: This option applies to protocol version 2 only.
669: .It Cm RemoteForward
670: Specifies that a TCP/IP port on the remote machine be forwarded over
671: the secure channel to the specified host and port from the local machine.
1.49 jmc 672: The first argument must be
1.43 djm 673: .Sm off
1.49 jmc 674: .Oo Ar bind_address : Oc Ar port
1.43 djm 675: .Sm on
1.49 jmc 676: and the second argument must be
677: .Ar host : Ns Ar hostport .
678: IPv6 addresses can be specified by enclosing addresses in square brackets
679: or by using an alternative syntax:
680: .Oo Ar bind_address Ns / Oc Ns Ar port
681: and
682: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 683: Multiple forwardings may be specified, and additional
684: forwardings can be given on the command line.
685: Only the superuser can forward privileged ports.
1.43 djm 686: .Pp
687: If the
688: .Ar bind_address
689: is not specified, the default is to only bind to loopback addresses.
690: If the
691: .Ar bind_address
692: is
693: .Ql *
694: or an empty string, then the forwarding is requested to listen on all
695: interfaces.
696: Specifying a remote
697: .Ar bind_address
1.46 jmc 698: will only succeed if the server's
699: .Cm GatewayPorts
1.43 djm 700: option is enabled (see
1.46 jmc 701: .Xr sshd_config 5 ) .
1.1 stevesk 702: .It Cm RhostsRSAAuthentication
703: Specifies whether to try rhosts based authentication with RSA host
704: authentication.
705: The argument must be
706: .Dq yes
707: or
708: .Dq no .
709: The default is
710: .Dq no .
711: This option applies to protocol version 1 only and requires
712: .Nm ssh
713: to be setuid root.
714: .It Cm RSAAuthentication
715: Specifies whether to try RSA authentication.
716: The argument to this keyword must be
717: .Dq yes
718: or
719: .Dq no .
720: RSA authentication will only be
721: attempted if the identity file exists, or an authentication agent is
722: running.
723: The default is
724: .Dq yes .
725: Note that this option applies to protocol version 1 only.
1.32 djm 726: .It Cm SendEnv
727: Specifies what variables from the local
728: .Xr environ 7
729: should be sent to the server.
730: Note that environment passing is only supported for protocol 2, the
1.33 djm 731: server must also support it, and the server must be configured to
732: accept these environment variables.
1.32 djm 733: Refer to
734: .Cm AcceptEnv
735: in
736: .Xr sshd_config 5
737: for how to configure the server.
738: Variables are specified by name, which may contain the wildcard characters
739: .Ql \&*
740: and
741: .Ql \&? .
1.33 djm 742: Multiple environment variables may be separated by whitespace or spread
1.32 djm 743: across multiple
744: .Cm SendEnv
745: directives.
746: The default is not to send any environment variables.
1.28 markus 747: .It Cm ServerAliveInterval
748: Sets a timeout interval in seconds after which if no data has been received
749: from the server,
750: .Nm ssh
751: will send a message through the encrypted
752: channel to request a response from the server.
753: The default
754: is 0, indicating that these messages will not be sent to the server.
755: This option applies to protocol version 2 only.
756: .It Cm ServerAliveCountMax
757: Sets the number of server alive messages (see above) which may be
758: sent without
759: .Nm ssh
760: receiving any messages back from the server.
761: If this threshold is reached while server alive messages are being sent,
762: .Nm ssh
763: will disconnect from the server, terminating the session.
764: It is important to note that the use of server alive messages is very
765: different from
766: .Cm TCPKeepAlive
767: (below).
768: The server alive messages are sent through the encrypted channel
769: and therefore will not be spoofable.
770: The TCP keepalive option enabled by
771: .Cm TCPKeepAlive
772: is spoofable.
773: The server alive mechanism is valuable when the client or
774: server depend on knowing when a connection has become inactive.
775: .Pp
776: The default value is 3.
777: If, for example,
778: .Cm ServerAliveInterval
779: (above) is set to 15, and
780: .Cm ServerAliveCountMax
781: is left at the default, if the server becomes unresponsive ssh
782: will disconnect after approximately 45 seconds.
1.1 stevesk 783: .It Cm SmartcardDevice
1.11 jmc 784: Specifies which smartcard device to use.
785: The argument to this keyword is the device
1.1 stevesk 786: .Nm ssh
787: should use to communicate with a smartcard used for storing the user's
1.11 jmc 788: private RSA key.
789: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 790: .It Cm StrictHostKeyChecking
791: If this flag is set to
792: .Dq yes ,
793: .Nm ssh
794: will never automatically add host keys to the
1.50 djm 795: .Pa ~/.ssh/known_hosts
1.1 stevesk 796: file, and refuses to connect to hosts whose host key has changed.
797: This provides maximum protection against trojan horse attacks,
798: however, can be annoying when the
799: .Pa /etc/ssh/ssh_known_hosts
800: file is poorly maintained, or connections to new hosts are
801: frequently made.
802: This option forces the user to manually
803: add all new hosts.
804: If this flag is set to
805: .Dq no ,
806: .Nm ssh
807: will automatically add new host keys to the
808: user known hosts files.
809: If this flag is set to
810: .Dq ask ,
811: new host keys
812: will be added to the user known host files only after the user
813: has confirmed that is what they really want to do, and
814: .Nm ssh
815: will refuse to connect to hosts whose host key has changed.
816: The host keys of
817: known hosts will be verified automatically in all cases.
818: The argument must be
819: .Dq yes ,
820: .Dq no
821: or
822: .Dq ask .
823: The default is
824: .Dq ask .
1.26 markus 825: .It Cm TCPKeepAlive
826: Specifies whether the system should send TCP keepalive messages to the
827: other side.
828: If they are sent, death of the connection or crash of one
829: of the machines will be properly noticed.
830: However, this means that
831: connections will die if the route is down temporarily, and some people
832: find it annoying.
833: .Pp
834: The default is
835: .Dq yes
836: (to send TCP keepalive messages), and the client will notice
837: if the network goes down or the remote host dies.
838: This is important in scripts, and many users want it too.
839: .Pp
840: To disable TCP keepalive messages, the value should be set to
841: .Dq no .
1.1 stevesk 842: .It Cm UsePrivilegedPort
843: Specifies whether to use a privileged port for outgoing connections.
844: The argument must be
845: .Dq yes
846: or
847: .Dq no .
848: The default is
849: .Dq no .
1.4 stevesk 850: If set to
851: .Dq yes
852: .Nm ssh
853: must be setuid root.
1.1 stevesk 854: Note that this option must be set to
855: .Dq yes
1.17 markus 856: for
1.1 stevesk 857: .Cm RhostsRSAAuthentication
1.17 markus 858: with older servers.
1.1 stevesk 859: .It Cm User
860: Specifies the user to log in as.
861: This can be useful when a different user name is used on different machines.
862: This saves the trouble of
863: having to remember to give the user name on the command line.
864: .It Cm UserKnownHostsFile
865: Specifies a file to use for the user
866: host key database instead of
1.50 djm 867: .Pa ~/.ssh/known_hosts .
1.8 jakob 868: .It Cm VerifyHostKeyDNS
869: Specifies whether to verify the remote key using DNS and SSHFP resource
870: records.
1.24 jakob 871: If this option is set to
872: .Dq yes ,
1.25 jmc 873: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 874: from DNS.
875: Insecure fingerprints will be handled as if this option was set to
876: .Dq ask .
877: If this option is set to
878: .Dq ask ,
879: information on fingerprint match will be displayed, but the user will still
880: need to confirm new host keys according to the
881: .Cm StrictHostKeyChecking
882: option.
883: The argument must be
884: .Dq yes ,
885: .Dq no
1.25 jmc 886: or
887: .Dq ask .
1.8 jakob 888: The default is
889: .Dq no .
1.12 jakob 890: Note that this option applies to protocol version 2 only.
1.1 stevesk 891: .It Cm XAuthLocation
1.5 stevesk 892: Specifies the full pathname of the
1.1 stevesk 893: .Xr xauth 1
894: program.
895: The default is
896: .Pa /usr/X11R6/bin/xauth .
897: .El
898: .Sh FILES
899: .Bl -tag -width Ds
1.50 djm 900: .It Pa ~/.ssh/config
1.1 stevesk 901: This is the per-user configuration file.
902: The format of this file is described above.
903: This file is used by the
904: .Nm ssh
905: client.
1.30 djm 906: Because of the potential for abuse, this file must have strict permissions:
907: read/write for the user, and not accessible by others.
1.1 stevesk 908: .It Pa /etc/ssh/ssh_config
909: Systemwide configuration file.
910: This file provides defaults for those
911: values that are not specified in the user's configuration file, and
912: for those users who do not have a configuration file.
913: This file must be world-readable.
914: .El
1.13 jmc 915: .Sh SEE ALSO
916: .Xr ssh 1
1.1 stevesk 917: .Sh AUTHORS
918: OpenSSH is a derivative of the original and free
919: ssh 1.2.12 release by Tatu Ylonen.
920: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
921: Theo de Raadt and Dug Song
922: removed many bugs, re-added newer features and
923: created OpenSSH.
924: Markus Friedl contributed the support for SSH
925: protocol versions 1.5 and 2.0.