Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.43
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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1.43 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.42 2005/02/28 00:54:10 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.
1.1 stevesk 410: .It Cm HostbasedAuthentication
411: Specifies whether to try rhosts based authentication with public key
412: authentication.
413: The argument must be
414: .Dq yes
415: or
416: .Dq no .
417: The default is
418: .Dq no .
419: This option applies to protocol version 2 only and
420: is similar to
421: .Cm RhostsRSAAuthentication .
422: .It Cm HostKeyAlgorithms
423: Specifies the protocol version 2 host key algorithms
424: that the client wants to use in order of preference.
425: The default for this option is:
426: .Dq ssh-rsa,ssh-dss .
427: .It Cm HostKeyAlias
428: Specifies an alias that should be used instead of the
429: real host name when looking up or saving the host key
430: in the host key database files.
431: This option is useful for tunneling ssh connections
432: or for multiple servers running on a single host.
433: .It Cm HostName
434: Specifies the real host name to log into.
435: This can be used to specify nicknames or abbreviations for hosts.
436: Default is the name given on the command line.
437: Numeric IP addresses are also permitted (both on the command line and in
438: .Cm HostName
439: specifications).
440: .It Cm IdentityFile
441: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 442: is read.
443: The default is
1.1 stevesk 444: .Pa $HOME/.ssh/identity
445: for protocol version 1, and
446: .Pa $HOME/.ssh/id_rsa
447: and
448: .Pa $HOME/.ssh/id_dsa
449: for protocol version 2.
450: Additionally, any identities represented by the authentication agent
451: will be used for authentication.
452: The file name may use the tilde
453: syntax to refer to a user's home directory.
454: It is possible to have
455: multiple identity files specified in configuration files; all these
456: identities will be tried in sequence.
1.29 markus 457: .It Cm IdentitiesOnly
458: Specifies that
459: .Nm ssh
460: should only use the authentication identity files configured in the
1.31 jmc 461: .Nm
1.29 markus 462: files,
463: even if the
464: .Nm ssh-agent
465: offers more identities.
466: The argument to this keyword must be
467: .Dq yes
468: or
469: .Dq no .
470: This option is intented for situations where
471: .Nm ssh-agent
472: offers many different identities.
473: The default is
474: .Dq no .
1.39 djm 475: .It Cm KbdInteractiveDevices
476: Specifies the list of methods to use in keyboard-interactive authentication.
477: Multiple method names must be comma-separated.
478: The default is to use the server specified list.
1.1 stevesk 479: .It Cm LocalForward
480: Specifies that a TCP/IP port on the local machine be forwarded over
481: the secure channel to the specified host and port from the remote machine.
482: The first argument must be a port number, and the second must be
1.43 ! djm 483: .Xo
! 484: .Sm off
! 485: .Oo Ar bind_address : Oc
! 486: .Ar host:port
! 487: .Sm on
! 488: .Xc .
! 489: IPv6 addresses can be specified by enclosing addresses in square brackets or
! 490: by using an alternative syntax:
! 491: .Xo
! 492: .Sm off
! 493: .Oo Ar bind_address / Oc
! 494: .Ar host/port
! 495: .Sm on
! 496: .Xc .
! 497: Multiple forwardings may be specified, and additional forwardings can be
! 498: given on the command line.
1.1 stevesk 499: Only the superuser can forward privileged ports.
1.43 ! djm 500: By default, the local port is bound in accordance with the
! 501: .Cm GatewayPorts
! 502: setting.
! 503: However, an explicit
! 504: .Ar bind_address
! 505: may be used to bind the connection to a specific address.
! 506: The
! 507: .Ar bind_address
! 508: of
! 509: .Dq localhost
! 510: indicates that the listening port be bound for local use only, while an
! 511: empty address or
! 512: .Dq *
! 513: indicates that the port should be available from all interfaces.
1.1 stevesk 514: .It Cm LogLevel
515: Gives the verbosity level that is used when logging messages from
516: .Nm ssh .
517: The possible values are:
518: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 519: The default is INFO.
520: DEBUG and DEBUG1 are equivalent.
521: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 522: .It Cm MACs
523: Specifies the MAC (message authentication code) algorithms
524: in order of preference.
525: The MAC algorithm is used in protocol version 2
526: for data integrity protection.
527: Multiple algorithms must be comma-separated.
528: The default is
529: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
530: .It Cm NoHostAuthenticationForLocalhost
531: This option can be used if the home directory is shared across machines.
532: In this case localhost will refer to a different machine on each of
533: the machines and the user will get many warnings about changed host keys.
534: However, this option disables host authentication for localhost.
535: The argument to this keyword must be
536: .Dq yes
537: or
538: .Dq no .
539: The default is to check the host key for localhost.
540: .It Cm NumberOfPasswordPrompts
541: Specifies the number of password prompts before giving up.
542: The argument to this keyword must be an integer.
543: Default is 3.
544: .It Cm PasswordAuthentication
545: Specifies whether to use password authentication.
546: The argument to this keyword must be
547: .Dq yes
548: or
549: .Dq no .
550: The default is
551: .Dq yes .
552: .It Cm Port
553: Specifies the port number to connect on the remote host.
554: Default is 22.
555: .It Cm PreferredAuthentications
556: Specifies the order in which the client should try protocol 2
1.11 jmc 557: authentication methods.
558: This allows a client to prefer one method (e.g.
1.1 stevesk 559: .Cm keyboard-interactive )
560: over another method (e.g.
561: .Cm password )
562: The default for this option is:
563: .Dq hostbased,publickey,keyboard-interactive,password .
564: .It Cm Protocol
565: Specifies the protocol versions
566: .Nm ssh
567: should support in order of preference.
568: The possible values are
569: .Dq 1
570: and
571: .Dq 2 .
572: Multiple versions must be comma-separated.
573: The default is
574: .Dq 2,1 .
575: This means that
576: .Nm ssh
577: tries version 2 and falls back to version 1
578: if version 2 is not available.
579: .It Cm ProxyCommand
580: Specifies the command to use to connect to the server.
581: The command
582: string extends to the end of the line, and is executed with
583: .Pa /bin/sh .
584: In the command string,
585: .Ql %h
586: will be substituted by the host name to
587: connect and
588: .Ql %p
589: by the port.
590: The command can be basically anything,
591: and should read from its standard input and write to its standard output.
592: It should eventually connect an
593: .Xr sshd 8
594: server running on some machine, or execute
595: .Ic sshd -i
596: somewhere.
597: Host key management will be done using the
598: HostName of the host being connected (defaulting to the name typed by
599: the user).
1.7 jmc 600: Setting the command to
601: .Dq none
1.6 markus 602: disables this option entirely.
1.1 stevesk 603: Note that
604: .Cm CheckHostIP
605: is not available for connects with a proxy command.
606: .Pp
607: .It Cm PubkeyAuthentication
608: Specifies whether to try public key authentication.
609: The argument to this keyword must be
610: .Dq yes
611: or
612: .Dq no .
613: The default is
614: .Dq yes .
615: This option applies to protocol version 2 only.
616: .It Cm RemoteForward
617: Specifies that a TCP/IP port on the remote machine be forwarded over
618: the secure channel to the specified host and port from the local machine.
619: The first argument must be a port number, and the second must be
1.43 ! djm 620: .Xo
! 621: .Sm off
! 622: .Oo Ar bind_address : Oc
! 623: .Ar host:port
! 624: .Sm on
! 625: .Xc .
! 626: IPv6 addresses can be specified by enclosing any addresses in square brackets
! 627: or by using the alternative syntax:
! 628: .Xo
! 629: .Sm off
! 630: .Oo Ar bind_address / Oc
! 631: .Ar host/port
! 632: .Sm on
! 633: .Xc .
1.1 stevesk 634: Multiple forwardings may be specified, and additional
635: forwardings can be given on the command line.
636: Only the superuser can forward privileged ports.
1.43 ! djm 637: .Pp
! 638: If the
! 639: .Ar bind_address
! 640: is not specified, the default is to only bind to loopback addresses.
! 641: If the
! 642: .Ar bind_address
! 643: is
! 644: .Ql *
! 645: or an empty string, then the forwarding is requested to listen on all
! 646: interfaces.
! 647: Specifying a remote
! 648: .Ar bind_address
! 649: will only succeed if the server's
! 650: .Cm GatewayPorts
! 651: option is enabled (see
! 652: .Xr sshd_config 5 ).
1.1 stevesk 653: .It Cm RhostsRSAAuthentication
654: Specifies whether to try rhosts based authentication with RSA host
655: authentication.
656: The argument must be
657: .Dq yes
658: or
659: .Dq no .
660: The default is
661: .Dq no .
662: This option applies to protocol version 1 only and requires
663: .Nm ssh
664: to be setuid root.
665: .It Cm RSAAuthentication
666: Specifies whether to try RSA authentication.
667: The argument to this keyword must be
668: .Dq yes
669: or
670: .Dq no .
671: RSA authentication will only be
672: attempted if the identity file exists, or an authentication agent is
673: running.
674: The default is
675: .Dq yes .
676: Note that this option applies to protocol version 1 only.
1.32 djm 677: .It Cm SendEnv
678: Specifies what variables from the local
679: .Xr environ 7
680: should be sent to the server.
681: Note that environment passing is only supported for protocol 2, the
1.33 djm 682: server must also support it, and the server must be configured to
683: accept these environment variables.
1.32 djm 684: Refer to
685: .Cm AcceptEnv
686: in
687: .Xr sshd_config 5
688: for how to configure the server.
689: Variables are specified by name, which may contain the wildcard characters
690: .Ql \&*
691: and
692: .Ql \&? .
1.33 djm 693: Multiple environment variables may be separated by whitespace or spread
1.32 djm 694: across multiple
695: .Cm SendEnv
696: directives.
697: The default is not to send any environment variables.
1.28 markus 698: .It Cm ServerAliveInterval
699: Sets a timeout interval in seconds after which if no data has been received
700: from the server,
701: .Nm ssh
702: will send a message through the encrypted
703: channel to request a response from the server.
704: The default
705: is 0, indicating that these messages will not be sent to the server.
706: This option applies to protocol version 2 only.
707: .It Cm ServerAliveCountMax
708: Sets the number of server alive messages (see above) which may be
709: sent without
710: .Nm ssh
711: receiving any messages back from the server.
712: If this threshold is reached while server alive messages are being sent,
713: .Nm ssh
714: will disconnect from the server, terminating the session.
715: It is important to note that the use of server alive messages is very
716: different from
717: .Cm TCPKeepAlive
718: (below).
719: The server alive messages are sent through the encrypted channel
720: and therefore will not be spoofable.
721: The TCP keepalive option enabled by
722: .Cm TCPKeepAlive
723: is spoofable.
724: The server alive mechanism is valuable when the client or
725: server depend on knowing when a connection has become inactive.
726: .Pp
727: The default value is 3.
728: If, for example,
729: .Cm ServerAliveInterval
730: (above) is set to 15, and
731: .Cm ServerAliveCountMax
732: is left at the default, if the server becomes unresponsive ssh
733: will disconnect after approximately 45 seconds.
1.1 stevesk 734: .It Cm SmartcardDevice
1.11 jmc 735: Specifies which smartcard device to use.
736: The argument to this keyword is the device
1.1 stevesk 737: .Nm ssh
738: should use to communicate with a smartcard used for storing the user's
1.11 jmc 739: private RSA key.
740: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 741: .It Cm StrictHostKeyChecking
742: If this flag is set to
743: .Dq yes ,
744: .Nm ssh
745: will never automatically add host keys to the
746: .Pa $HOME/.ssh/known_hosts
747: file, and refuses to connect to hosts whose host key has changed.
748: This provides maximum protection against trojan horse attacks,
749: however, can be annoying when the
750: .Pa /etc/ssh/ssh_known_hosts
751: file is poorly maintained, or connections to new hosts are
752: frequently made.
753: This option forces the user to manually
754: add all new hosts.
755: If this flag is set to
756: .Dq no ,
757: .Nm ssh
758: will automatically add new host keys to the
759: user known hosts files.
760: If this flag is set to
761: .Dq ask ,
762: new host keys
763: will be added to the user known host files only after the user
764: has confirmed that is what they really want to do, and
765: .Nm ssh
766: will refuse to connect to hosts whose host key has changed.
767: The host keys of
768: known hosts will be verified automatically in all cases.
769: The argument must be
770: .Dq yes ,
771: .Dq no
772: or
773: .Dq ask .
774: The default is
775: .Dq ask .
1.26 markus 776: .It Cm TCPKeepAlive
777: Specifies whether the system should send TCP keepalive messages to the
778: other side.
779: If they are sent, death of the connection or crash of one
780: of the machines will be properly noticed.
781: However, this means that
782: connections will die if the route is down temporarily, and some people
783: find it annoying.
784: .Pp
785: The default is
786: .Dq yes
787: (to send TCP keepalive messages), and the client will notice
788: if the network goes down or the remote host dies.
789: This is important in scripts, and many users want it too.
790: .Pp
791: To disable TCP keepalive messages, the value should be set to
792: .Dq no .
1.1 stevesk 793: .It Cm UsePrivilegedPort
794: Specifies whether to use a privileged port for outgoing connections.
795: The argument must be
796: .Dq yes
797: or
798: .Dq no .
799: The default is
800: .Dq no .
1.4 stevesk 801: If set to
802: .Dq yes
803: .Nm ssh
804: must be setuid root.
1.1 stevesk 805: Note that this option must be set to
806: .Dq yes
1.17 markus 807: for
1.1 stevesk 808: .Cm RhostsRSAAuthentication
1.17 markus 809: with older servers.
1.1 stevesk 810: .It Cm User
811: Specifies the user to log in as.
812: This can be useful when a different user name is used on different machines.
813: This saves the trouble of
814: having to remember to give the user name on the command line.
815: .It Cm UserKnownHostsFile
816: Specifies a file to use for the user
817: host key database instead of
818: .Pa $HOME/.ssh/known_hosts .
1.8 jakob 819: .It Cm VerifyHostKeyDNS
820: Specifies whether to verify the remote key using DNS and SSHFP resource
821: records.
1.24 jakob 822: If this option is set to
823: .Dq yes ,
1.25 jmc 824: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 825: from DNS.
826: Insecure fingerprints will be handled as if this option was set to
827: .Dq ask .
828: If this option is set to
829: .Dq ask ,
830: information on fingerprint match will be displayed, but the user will still
831: need to confirm new host keys according to the
832: .Cm StrictHostKeyChecking
833: option.
834: The argument must be
835: .Dq yes ,
836: .Dq no
1.25 jmc 837: or
838: .Dq ask .
1.8 jakob 839: The default is
840: .Dq no .
1.12 jakob 841: Note that this option applies to protocol version 2 only.
1.1 stevesk 842: .It Cm XAuthLocation
1.5 stevesk 843: Specifies the full pathname of the
1.1 stevesk 844: .Xr xauth 1
845: program.
846: The default is
847: .Pa /usr/X11R6/bin/xauth .
848: .El
849: .Sh FILES
850: .Bl -tag -width Ds
851: .It Pa $HOME/.ssh/config
852: This is the per-user configuration file.
853: The format of this file is described above.
854: This file is used by the
855: .Nm ssh
856: client.
1.30 djm 857: Because of the potential for abuse, this file must have strict permissions:
858: read/write for the user, and not accessible by others.
1.1 stevesk 859: .It Pa /etc/ssh/ssh_config
860: Systemwide configuration file.
861: This file provides defaults for those
862: values that are not specified in the user's configuration file, and
863: for those users who do not have a configuration file.
864: This file must be world-readable.
865: .El
1.13 jmc 866: .Sh SEE ALSO
867: .Xr ssh 1
1.1 stevesk 868: .Sh AUTHORS
869: OpenSSH is a derivative of the original and free
870: ssh 1.2.12 release by Tatu Ylonen.
871: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
872: Theo de Raadt and Dug Song
873: removed many bugs, re-added newer features and
874: created OpenSSH.
875: Markus Friedl contributed the support for SSH
876: protocol versions 1.5 and 2.0.
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