Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.44
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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1.44 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.43 2005/03/01 10:09:52 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
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.
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 ,
196: .Dq arcfour ,
197: .Dq blowfish-cbc ,
198: and
199: .Dq cast128-cbc .
1.1 stevesk 200: The default is
201: .Bd -literal
202: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
203: aes192-cbc,aes256-cbc''
204: .Ed
205: .It Cm ClearAllForwardings
206: Specifies that all local, remote and dynamic port forwardings
207: specified in the configuration files or on the command line be
1.7 jmc 208: cleared.
209: This option is primarily useful when used from the
1.1 stevesk 210: .Nm ssh
211: command line to clear port forwardings set in
212: configuration files, and is automatically set by
213: .Xr scp 1
214: and
215: .Xr sftp 1 .
216: The argument must be
217: .Dq yes
218: or
219: .Dq no .
220: The default is
221: .Dq no .
222: .It Cm Compression
223: Specifies whether to use compression.
224: The argument must be
225: .Dq yes
226: or
227: .Dq no .
228: The default is
229: .Dq no .
230: .It Cm CompressionLevel
231: Specifies the compression level to use if compression is enabled.
232: The argument must be an integer from 1 (fast) to 9 (slow, best).
233: The default level is 6, which is good for most applications.
234: The meaning of the values is the same as in
235: .Xr gzip 1 .
236: Note that this option applies to protocol version 1 only.
237: .It Cm ConnectionAttempts
238: Specifies the number of tries (one per second) to make before exiting.
239: The argument must be an integer.
240: This may be useful in scripts if the connection sometimes fails.
241: The default is 1.
1.9 djm 242: .It Cm ConnectTimeout
243: Specifies the timeout (in seconds) used when connecting to the ssh
1.11 jmc 244: server, instead of using the default system TCP timeout.
245: This value is used only when the target is down or really unreachable,
246: not when it refuses the connection.
1.36 djm 247: .It Cm ControlMaster
248: Enables the sharing of multiple sessions over a single network connection.
249: When set to
250: .Dq yes
251: .Nm ssh
252: will listen for connections on a control socket specified using the
253: .Cm ControlPath
254: argument.
255: Additional sessions can connect to this socket using the same
256: .Cm ControlPath
257: with
258: .Cm ControlMaster
259: set to
260: .Dq no
1.38 jmc 261: (the default).
1.36 djm 262: These sessions will reuse the master instance's network connection rather
263: than initiating new ones.
1.37 djm 264: Setting this to
265: .Dq ask
266: will cause
267: .Nm ssh
268: to listen for control connections, but require confirmation using the
269: .Ev SSH_ASKPASS
270: program before they are accepted (see
271: .Xr ssh-add 1
1.38 jmc 272: for details).
1.36 djm 273: .It Cm ControlPath
1.38 jmc 274: Specify the path to the control socket used for connection sharing.
1.36 djm 275: See
276: .Cm ControlMaster
277: above.
1.38 jmc 278: .It Cm DynamicForward
279: Specifies that a TCP/IP port on the local machine be forwarded
280: over the secure channel, and the application
281: protocol is then used to determine where to connect to from the
282: remote machine.
283: The argument must be a port number.
284: Currently the SOCKS4 and SOCKS5 protocols are supported, and
285: .Nm ssh
286: will act as a SOCKS server.
287: Multiple forwardings may be specified, and
288: additional forwardings can be given on the command line.
289: Only the superuser can forward privileged ports.
1.14 markus 290: .It Cm EnableSSHKeysign
291: Setting this option to
292: .Dq yes
293: in the global client configuration file
294: .Pa /etc/ssh/ssh_config
295: enables the use of the helper program
296: .Xr ssh-keysign 8
297: during
298: .Cm HostbasedAuthentication .
299: The argument must be
300: .Dq yes
301: or
302: .Dq no .
303: The default is
304: .Dq no .
1.23 jmc 305: This option should be placed in the non-hostspecific section.
1.14 markus 306: See
307: .Xr ssh-keysign 8
308: for more information.
1.1 stevesk 309: .It Cm EscapeChar
310: Sets the escape character (default:
311: .Ql ~ ) .
312: The escape character can also
313: be set on the command line.
314: The argument should be a single character,
315: .Ql ^
316: followed by a letter, or
317: .Dq none
318: to disable the escape
319: character entirely (making the connection transparent for binary
320: data).
321: .It Cm ForwardAgent
322: Specifies whether the connection to the authentication agent (if any)
323: will be forwarded to the remote machine.
324: The argument must be
325: .Dq yes
326: or
327: .Dq no .
328: The default is
329: .Dq no .
1.3 stevesk 330: .Pp
1.7 jmc 331: Agent forwarding should be enabled with caution.
332: Users with the ability to bypass file permissions on the remote host
333: (for the agent's Unix-domain socket)
334: can access the local agent through the forwarded connection.
335: An attacker cannot obtain key material from the agent,
1.3 stevesk 336: however they can perform operations on the keys that enable them to
337: authenticate using the identities loaded into the agent.
1.1 stevesk 338: .It Cm ForwardX11
339: Specifies whether X11 connections will be automatically redirected
340: over the secure channel and
341: .Ev DISPLAY
342: set.
343: The argument must be
344: .Dq yes
345: or
346: .Dq no .
347: The default is
348: .Dq no .
1.3 stevesk 349: .Pp
1.7 jmc 350: X11 forwarding should be enabled with caution.
351: Users with the ability to bypass file permissions on the remote host
1.22 markus 352: (for the user's X11 authorization database)
1.7 jmc 353: can access the local X11 display through the forwarded connection.
1.22 markus 354: An attacker may then be able to perform activities such as keystroke monitoring
355: if the
356: .Cm ForwardX11Trusted
357: option is also enabled.
358: .It Cm ForwardX11Trusted
1.34 jmc 359: If this option is set to
1.22 markus 360: .Dq yes
361: then remote X11 clients will have full access to the original X11 display.
1.42 djm 362: .Pp
1.22 markus 363: If this option is set to
364: .Dq no
365: then remote X11 clients will be considered untrusted and prevented
366: from stealing or tampering with data belonging to trusted X11
367: clients.
1.42 djm 368: Furthermore, the
369: .Xr xauth 1
370: token used for the session will be set to expire after 20 minutes.
371: Remote clients will be refused access after this time.
1.22 markus 372: .Pp
373: The default is
374: .Dq no .
375: .Pp
376: See the X11 SECURITY extension specification for full details on
377: the restrictions imposed on untrusted clients.
1.1 stevesk 378: .It Cm GatewayPorts
379: Specifies whether remote hosts are allowed to connect to local
380: forwarded ports.
381: By default,
382: .Nm ssh
1.7 jmc 383: binds local port forwardings to the loopback address.
384: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 385: .Cm GatewayPorts
386: can be used to specify that
387: .Nm ssh
388: should bind local port forwardings to the wildcard address,
389: thus allowing remote hosts to connect to forwarded ports.
390: The argument must be
391: .Dq yes
392: or
393: .Dq no .
394: The default is
395: .Dq no .
396: .It Cm GlobalKnownHostsFile
397: Specifies a file to use for the global
398: host key database instead of
399: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 400: .It Cm GSSAPIAuthentication
1.27 markus 401: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 402: The default is
1.21 markus 403: .Dq no .
1.18 markus 404: Note that this option applies to protocol version 2 only.
405: .It Cm GSSAPIDelegateCredentials
406: Forward (delegate) credentials to the server.
407: The default is
408: .Dq no .
409: Note that this option applies to protocol version 2 only.
1.44 ! djm 410: .It Cm HashKnownHosts
! 411: Indicates that
! 412: .Nm ssh
! 413: should hash host names and addresses when they are added to
! 414: .Pa $HOME/.ssh/known_hosts .
! 415: These hashed names may be used normally by
! 416: .Nm ssh
! 417: and
! 418: .Nm sshd ,
! 419: but they do not reveal identifying information should the file's contents
! 420: be disclosed.
! 421: The default is
! 422: .Dq no .
! 423: Note that hashing of names and addresses will not be retrospectively applied
! 424: to existing known hosts files.
1.1 stevesk 425: .It Cm HostbasedAuthentication
426: Specifies whether to try rhosts based authentication with public key
427: authentication.
428: The argument must be
429: .Dq yes
430: or
431: .Dq no .
432: The default is
433: .Dq no .
434: This option applies to protocol version 2 only and
435: is similar to
436: .Cm RhostsRSAAuthentication .
437: .It Cm HostKeyAlgorithms
438: Specifies the protocol version 2 host key algorithms
439: that the client wants to use in order of preference.
440: The default for this option is:
441: .Dq ssh-rsa,ssh-dss .
442: .It Cm HostKeyAlias
443: Specifies an alias that should be used instead of the
444: real host name when looking up or saving the host key
445: in the host key database files.
446: This option is useful for tunneling ssh connections
447: or for multiple servers running on a single host.
448: .It Cm HostName
449: Specifies the real host name to log into.
450: This can be used to specify nicknames or abbreviations for hosts.
451: Default is the name given on the command line.
452: Numeric IP addresses are also permitted (both on the command line and in
453: .Cm HostName
454: specifications).
455: .It Cm IdentityFile
456: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 457: is read.
458: The default is
1.1 stevesk 459: .Pa $HOME/.ssh/identity
460: for protocol version 1, and
461: .Pa $HOME/.ssh/id_rsa
462: and
463: .Pa $HOME/.ssh/id_dsa
464: for protocol version 2.
465: Additionally, any identities represented by the authentication agent
466: will be used for authentication.
467: The file name may use the tilde
468: syntax to refer to a user's home directory.
469: It is possible to have
470: multiple identity files specified in configuration files; all these
471: identities will be tried in sequence.
1.29 markus 472: .It Cm IdentitiesOnly
473: Specifies that
474: .Nm ssh
475: should only use the authentication identity files configured in the
1.31 jmc 476: .Nm
1.29 markus 477: files,
478: even if the
479: .Nm ssh-agent
480: offers more identities.
481: The argument to this keyword must be
482: .Dq yes
483: or
484: .Dq no .
485: This option is intented for situations where
486: .Nm ssh-agent
487: offers many different identities.
488: The default is
489: .Dq no .
1.39 djm 490: .It Cm KbdInteractiveDevices
491: Specifies the list of methods to use in keyboard-interactive authentication.
492: Multiple method names must be comma-separated.
493: The default is to use the server specified list.
1.1 stevesk 494: .It Cm LocalForward
495: Specifies that a TCP/IP port on the local machine be forwarded over
496: the secure channel to the specified host and port from the remote machine.
497: The first argument must be a port number, and the second must be
1.43 djm 498: .Xo
499: .Sm off
500: .Oo Ar bind_address : Oc
501: .Ar host:port
502: .Sm on
503: .Xc .
504: IPv6 addresses can be specified by enclosing addresses in square brackets or
505: by using an alternative syntax:
506: .Xo
507: .Sm off
508: .Oo Ar bind_address / Oc
509: .Ar host/port
510: .Sm on
511: .Xc .
512: Multiple forwardings may be specified, and additional forwardings can be
513: given on the command line.
1.1 stevesk 514: Only the superuser can forward privileged ports.
1.43 djm 515: By default, the local port is bound in accordance with the
516: .Cm GatewayPorts
517: setting.
518: However, an explicit
519: .Ar bind_address
520: may be used to bind the connection to a specific address.
521: The
522: .Ar bind_address
523: of
524: .Dq localhost
525: indicates that the listening port be bound for local use only, while an
526: empty address or
527: .Dq *
528: indicates that the port should be available from all interfaces.
1.1 stevesk 529: .It Cm LogLevel
530: Gives the verbosity level that is used when logging messages from
531: .Nm ssh .
532: The possible values are:
533: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 534: The default is INFO.
535: DEBUG and DEBUG1 are equivalent.
536: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 537: .It Cm MACs
538: Specifies the MAC (message authentication code) algorithms
539: in order of preference.
540: The MAC algorithm is used in protocol version 2
541: for data integrity protection.
542: Multiple algorithms must be comma-separated.
543: The default is
544: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
545: .It Cm NoHostAuthenticationForLocalhost
546: This option can be used if the home directory is shared across machines.
547: In this case localhost will refer to a different machine on each of
548: the machines and the user will get many warnings about changed host keys.
549: However, this option disables host authentication for localhost.
550: The argument to this keyword must be
551: .Dq yes
552: or
553: .Dq no .
554: The default is to check the host key for localhost.
555: .It Cm NumberOfPasswordPrompts
556: Specifies the number of password prompts before giving up.
557: The argument to this keyword must be an integer.
558: Default is 3.
559: .It Cm PasswordAuthentication
560: Specifies whether to use password authentication.
561: The argument to this keyword must be
562: .Dq yes
563: or
564: .Dq no .
565: The default is
566: .Dq yes .
567: .It Cm Port
568: Specifies the port number to connect on the remote host.
569: Default is 22.
570: .It Cm PreferredAuthentications
571: Specifies the order in which the client should try protocol 2
1.11 jmc 572: authentication methods.
573: This allows a client to prefer one method (e.g.
1.1 stevesk 574: .Cm keyboard-interactive )
575: over another method (e.g.
576: .Cm password )
577: The default for this option is:
578: .Dq hostbased,publickey,keyboard-interactive,password .
579: .It Cm Protocol
580: Specifies the protocol versions
581: .Nm ssh
582: should support in order of preference.
583: The possible values are
584: .Dq 1
585: and
586: .Dq 2 .
587: Multiple versions must be comma-separated.
588: The default is
589: .Dq 2,1 .
590: This means that
591: .Nm ssh
592: tries version 2 and falls back to version 1
593: if version 2 is not available.
594: .It Cm ProxyCommand
595: Specifies the command to use to connect to the server.
596: The command
597: string extends to the end of the line, and is executed with
598: .Pa /bin/sh .
599: In the command string,
600: .Ql %h
601: will be substituted by the host name to
602: connect and
603: .Ql %p
604: by the port.
605: The command can be basically anything,
606: and should read from its standard input and write to its standard output.
607: It should eventually connect an
608: .Xr sshd 8
609: server running on some machine, or execute
610: .Ic sshd -i
611: somewhere.
612: Host key management will be done using the
613: HostName of the host being connected (defaulting to the name typed by
614: the user).
1.7 jmc 615: Setting the command to
616: .Dq none
1.6 markus 617: disables this option entirely.
1.1 stevesk 618: Note that
619: .Cm CheckHostIP
620: is not available for connects with a proxy command.
621: .Pp
622: .It Cm PubkeyAuthentication
623: Specifies whether to try public key authentication.
624: The argument to this keyword must be
625: .Dq yes
626: or
627: .Dq no .
628: The default is
629: .Dq yes .
630: This option applies to protocol version 2 only.
631: .It Cm RemoteForward
632: Specifies that a TCP/IP port on the remote machine be forwarded over
633: the secure channel to the specified host and port from the local machine.
634: The first argument must be a port number, and the second must be
1.43 djm 635: .Xo
636: .Sm off
637: .Oo Ar bind_address : Oc
638: .Ar host:port
639: .Sm on
640: .Xc .
641: IPv6 addresses can be specified by enclosing any addresses in square brackets
642: or by using the alternative syntax:
643: .Xo
644: .Sm off
645: .Oo Ar bind_address / Oc
646: .Ar host/port
647: .Sm on
648: .Xc .
1.1 stevesk 649: Multiple forwardings may be specified, and additional
650: forwardings can be given on the command line.
651: Only the superuser can forward privileged ports.
1.43 djm 652: .Pp
653: If the
654: .Ar bind_address
655: is not specified, the default is to only bind to loopback addresses.
656: If the
657: .Ar bind_address
658: is
659: .Ql *
660: or an empty string, then the forwarding is requested to listen on all
661: interfaces.
662: Specifying a remote
663: .Ar bind_address
664: will only succeed if the server's
665: .Cm GatewayPorts
666: option is enabled (see
667: .Xr sshd_config 5 ).
1.1 stevesk 668: .It Cm RhostsRSAAuthentication
669: Specifies whether to try rhosts based authentication with RSA host
670: authentication.
671: The argument must be
672: .Dq yes
673: or
674: .Dq no .
675: The default is
676: .Dq no .
677: This option applies to protocol version 1 only and requires
678: .Nm ssh
679: to be setuid root.
680: .It Cm RSAAuthentication
681: Specifies whether to try RSA authentication.
682: The argument to this keyword must be
683: .Dq yes
684: or
685: .Dq no .
686: RSA authentication will only be
687: attempted if the identity file exists, or an authentication agent is
688: running.
689: The default is
690: .Dq yes .
691: Note that this option applies to protocol version 1 only.
1.32 djm 692: .It Cm SendEnv
693: Specifies what variables from the local
694: .Xr environ 7
695: should be sent to the server.
696: Note that environment passing is only supported for protocol 2, the
1.33 djm 697: server must also support it, and the server must be configured to
698: accept these environment variables.
1.32 djm 699: Refer to
700: .Cm AcceptEnv
701: in
702: .Xr sshd_config 5
703: for how to configure the server.
704: Variables are specified by name, which may contain the wildcard characters
705: .Ql \&*
706: and
707: .Ql \&? .
1.33 djm 708: Multiple environment variables may be separated by whitespace or spread
1.32 djm 709: across multiple
710: .Cm SendEnv
711: directives.
712: The default is not to send any environment variables.
1.28 markus 713: .It Cm ServerAliveInterval
714: Sets a timeout interval in seconds after which if no data has been received
715: from the server,
716: .Nm ssh
717: will send a message through the encrypted
718: channel to request a response from the server.
719: The default
720: is 0, indicating that these messages will not be sent to the server.
721: This option applies to protocol version 2 only.
722: .It Cm ServerAliveCountMax
723: Sets the number of server alive messages (see above) which may be
724: sent without
725: .Nm ssh
726: receiving any messages back from the server.
727: If this threshold is reached while server alive messages are being sent,
728: .Nm ssh
729: will disconnect from the server, terminating the session.
730: It is important to note that the use of server alive messages is very
731: different from
732: .Cm TCPKeepAlive
733: (below).
734: The server alive messages are sent through the encrypted channel
735: and therefore will not be spoofable.
736: The TCP keepalive option enabled by
737: .Cm TCPKeepAlive
738: is spoofable.
739: The server alive mechanism is valuable when the client or
740: server depend on knowing when a connection has become inactive.
741: .Pp
742: The default value is 3.
743: If, for example,
744: .Cm ServerAliveInterval
745: (above) is set to 15, and
746: .Cm ServerAliveCountMax
747: is left at the default, if the server becomes unresponsive ssh
748: will disconnect after approximately 45 seconds.
1.1 stevesk 749: .It Cm SmartcardDevice
1.11 jmc 750: Specifies which smartcard device to use.
751: The argument to this keyword is the device
1.1 stevesk 752: .Nm ssh
753: should use to communicate with a smartcard used for storing the user's
1.11 jmc 754: private RSA key.
755: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 756: .It Cm StrictHostKeyChecking
757: If this flag is set to
758: .Dq yes ,
759: .Nm ssh
760: will never automatically add host keys to the
761: .Pa $HOME/.ssh/known_hosts
762: file, and refuses to connect to hosts whose host key has changed.
763: This provides maximum protection against trojan horse attacks,
764: however, can be annoying when the
765: .Pa /etc/ssh/ssh_known_hosts
766: file is poorly maintained, or connections to new hosts are
767: frequently made.
768: This option forces the user to manually
769: add all new hosts.
770: If this flag is set to
771: .Dq no ,
772: .Nm ssh
773: will automatically add new host keys to the
774: user known hosts files.
775: If this flag is set to
776: .Dq ask ,
777: new host keys
778: will be added to the user known host files only after the user
779: has confirmed that is what they really want to do, and
780: .Nm ssh
781: will refuse to connect to hosts whose host key has changed.
782: The host keys of
783: known hosts will be verified automatically in all cases.
784: The argument must be
785: .Dq yes ,
786: .Dq no
787: or
788: .Dq ask .
789: The default is
790: .Dq ask .
1.26 markus 791: .It Cm TCPKeepAlive
792: Specifies whether the system should send TCP keepalive messages to the
793: other side.
794: If they are sent, death of the connection or crash of one
795: of the machines will be properly noticed.
796: However, this means that
797: connections will die if the route is down temporarily, and some people
798: find it annoying.
799: .Pp
800: The default is
801: .Dq yes
802: (to send TCP keepalive messages), and the client will notice
803: if the network goes down or the remote host dies.
804: This is important in scripts, and many users want it too.
805: .Pp
806: To disable TCP keepalive messages, the value should be set to
807: .Dq no .
1.1 stevesk 808: .It Cm UsePrivilegedPort
809: Specifies whether to use a privileged port for outgoing connections.
810: The argument must be
811: .Dq yes
812: or
813: .Dq no .
814: The default is
815: .Dq no .
1.4 stevesk 816: If set to
817: .Dq yes
818: .Nm ssh
819: must be setuid root.
1.1 stevesk 820: Note that this option must be set to
821: .Dq yes
1.17 markus 822: for
1.1 stevesk 823: .Cm RhostsRSAAuthentication
1.17 markus 824: with older servers.
1.1 stevesk 825: .It Cm User
826: Specifies the user to log in as.
827: This can be useful when a different user name is used on different machines.
828: This saves the trouble of
829: having to remember to give the user name on the command line.
830: .It Cm UserKnownHostsFile
831: Specifies a file to use for the user
832: host key database instead of
833: .Pa $HOME/.ssh/known_hosts .
1.8 jakob 834: .It Cm VerifyHostKeyDNS
835: Specifies whether to verify the remote key using DNS and SSHFP resource
836: records.
1.24 jakob 837: If this option is set to
838: .Dq yes ,
1.25 jmc 839: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 840: from DNS.
841: Insecure fingerprints will be handled as if this option was set to
842: .Dq ask .
843: If this option is set to
844: .Dq ask ,
845: information on fingerprint match will be displayed, but the user will still
846: need to confirm new host keys according to the
847: .Cm StrictHostKeyChecking
848: option.
849: The argument must be
850: .Dq yes ,
851: .Dq no
1.25 jmc 852: or
853: .Dq ask .
1.8 jakob 854: The default is
855: .Dq no .
1.12 jakob 856: Note that this option applies to protocol version 2 only.
1.1 stevesk 857: .It Cm XAuthLocation
1.5 stevesk 858: Specifies the full pathname of the
1.1 stevesk 859: .Xr xauth 1
860: program.
861: The default is
862: .Pa /usr/X11R6/bin/xauth .
863: .El
864: .Sh FILES
865: .Bl -tag -width Ds
866: .It Pa $HOME/.ssh/config
867: This is the per-user configuration file.
868: The format of this file is described above.
869: This file is used by the
870: .Nm ssh
871: client.
1.30 djm 872: Because of the potential for abuse, this file must have strict permissions:
873: read/write for the user, and not accessible by others.
1.1 stevesk 874: .It Pa /etc/ssh/ssh_config
875: Systemwide configuration file.
876: This file provides defaults for those
877: values that are not specified in the user's configuration file, and
878: for those users who do not have a configuration file.
879: This file must be world-readable.
880: .El
1.13 jmc 881: .Sh SEE ALSO
882: .Xr ssh 1
1.1 stevesk 883: .Sh AUTHORS
884: OpenSSH is a derivative of the original and free
885: ssh 1.2.12 release by Tatu Ylonen.
886: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
887: Theo de Raadt and Dug Song
888: removed many bugs, re-added newer features and
889: created OpenSSH.
890: Markus Friedl contributed the support for SSH
891: protocol versions 1.5 and 2.0.