Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.38.2.2
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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1.38.2.2! brad 37: .\" $OpenBSD: ssh_config.5,v 1.49 2005/03/16 11:10:38 jmc 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.
1.38.2.1 brad 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
1.38.2.1 brad 123: (use IPv4 only) or
1.10 djm 124: .Dq inet6
1.38.2.1 brad 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.
1.38.2.1 brad 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.
1.38.2.1 brad 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.
1.38.2.1 brad 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, but these may be manually hashed using
425: .Xr ssh-keygen 1 .
1.1 stevesk 426: .It Cm HostbasedAuthentication
427: Specifies whether to try rhosts based authentication with public key
428: authentication.
429: The argument must be
430: .Dq yes
431: or
432: .Dq no .
433: The default is
434: .Dq no .
435: This option applies to protocol version 2 only and
436: is similar to
437: .Cm RhostsRSAAuthentication .
438: .It Cm HostKeyAlgorithms
439: Specifies the protocol version 2 host key algorithms
440: that the client wants to use in order of preference.
441: The default for this option is:
442: .Dq ssh-rsa,ssh-dss .
443: .It Cm HostKeyAlias
444: Specifies an alias that should be used instead of the
445: real host name when looking up or saving the host key
446: in the host key database files.
447: This option is useful for tunneling ssh connections
448: or for multiple servers running on a single host.
449: .It Cm HostName
450: Specifies the real host name to log into.
451: This can be used to specify nicknames or abbreviations for hosts.
452: Default is the name given on the command line.
453: Numeric IP addresses are also permitted (both on the command line and in
454: .Cm HostName
455: specifications).
456: .It Cm IdentityFile
457: Specifies a file from which the user's RSA or DSA authentication identity
1.11 jmc 458: is read.
459: The default is
1.1 stevesk 460: .Pa $HOME/.ssh/identity
461: for protocol version 1, and
462: .Pa $HOME/.ssh/id_rsa
463: and
464: .Pa $HOME/.ssh/id_dsa
465: for protocol version 2.
466: Additionally, any identities represented by the authentication agent
467: will be used for authentication.
468: The file name may use the tilde
469: syntax to refer to a user's home directory.
470: It is possible to have
471: multiple identity files specified in configuration files; all these
472: identities will be tried in sequence.
1.29 markus 473: .It Cm IdentitiesOnly
474: Specifies that
475: .Nm ssh
476: should only use the authentication identity files configured in the
1.31 jmc 477: .Nm
1.29 markus 478: files,
479: even if the
480: .Nm ssh-agent
481: offers more identities.
482: The argument to this keyword must be
483: .Dq yes
484: or
485: .Dq no .
486: This option is intented for situations where
487: .Nm ssh-agent
488: offers many different identities.
489: The default is
490: .Dq no .
1.38.2.1 brad 491: .It Cm KbdInteractiveDevices
492: Specifies the list of methods to use in keyboard-interactive authentication.
493: Multiple method names must be comma-separated.
494: The default is to use the server specified list.
1.1 stevesk 495: .It Cm LocalForward
496: Specifies that a TCP/IP port on the local machine be forwarded over
497: the secure channel to the specified host and port from the remote machine.
1.38.2.2! brad 498: The first argument must be
1.38.2.1 brad 499: .Sm off
1.38.2.2! brad 500: .Oo Ar bind_address : Oc Ar port
1.38.2.1 brad 501: .Sm on
1.38.2.2! brad 502: and the second argument must be
! 503: .Ar host : Ns Ar hostport .
1.38.2.1 brad 504: IPv6 addresses can be specified by enclosing addresses in square brackets or
505: by using an alternative syntax:
1.38.2.2! brad 506: .Oo Ar bind_address Ns / Oc Ns Ar port
! 507: and
! 508: .Ar host Ns / Ns Ar hostport .
1.38.2.1 brad 509: Multiple forwardings may be specified, and additional forwardings can be
510: given on the command line.
1.1 stevesk 511: Only the superuser can forward privileged ports.
1.38.2.1 brad 512: By default, the local port is bound in accordance with the
513: .Cm GatewayPorts
514: setting.
515: However, an explicit
516: .Ar bind_address
517: may be used to bind the connection to a specific address.
518: The
519: .Ar bind_address
520: of
521: .Dq localhost
522: indicates that the listening port be bound for local use only, while an
523: empty address or
524: .Sq *
525: indicates that the port should be available from all interfaces.
1.1 stevesk 526: .It Cm LogLevel
527: Gives the verbosity level that is used when logging messages from
528: .Nm ssh .
529: The possible values are:
530: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.7 jmc 531: The default is INFO.
532: DEBUG and DEBUG1 are equivalent.
533: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 534: .It Cm MACs
535: Specifies the MAC (message authentication code) algorithms
536: in order of preference.
537: The MAC algorithm is used in protocol version 2
538: for data integrity protection.
539: Multiple algorithms must be comma-separated.
540: The default is
541: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
542: .It Cm NoHostAuthenticationForLocalhost
543: This option can be used if the home directory is shared across machines.
544: In this case localhost will refer to a different machine on each of
545: the machines and the user will get many warnings about changed host keys.
546: However, this option disables host authentication for localhost.
547: The argument to this keyword must be
548: .Dq yes
549: or
550: .Dq no .
551: The default is to check the host key for localhost.
552: .It Cm NumberOfPasswordPrompts
553: Specifies the number of password prompts before giving up.
554: The argument to this keyword must be an integer.
555: Default is 3.
556: .It Cm PasswordAuthentication
557: Specifies whether to use password authentication.
558: The argument to this keyword must be
559: .Dq yes
560: or
561: .Dq no .
562: The default is
563: .Dq yes .
564: .It Cm Port
565: Specifies the port number to connect on the remote host.
566: Default is 22.
567: .It Cm PreferredAuthentications
568: Specifies the order in which the client should try protocol 2
1.11 jmc 569: authentication methods.
1.38.2.2! brad 570: This allows a client to prefer one method (e.g.\&
1.1 stevesk 571: .Cm keyboard-interactive )
1.38.2.2! brad 572: over another method (e.g.\&
1.1 stevesk 573: .Cm password )
574: The default for this option is:
575: .Dq hostbased,publickey,keyboard-interactive,password .
576: .It Cm Protocol
577: Specifies the protocol versions
578: .Nm ssh
579: should support in order of preference.
580: The possible values are
581: .Dq 1
582: and
583: .Dq 2 .
584: Multiple versions must be comma-separated.
585: The default is
586: .Dq 2,1 .
587: This means that
588: .Nm ssh
589: tries version 2 and falls back to version 1
590: if version 2 is not available.
591: .It Cm ProxyCommand
592: Specifies the command to use to connect to the server.
593: The command
594: string extends to the end of the line, and is executed with
595: .Pa /bin/sh .
596: In the command string,
597: .Ql %h
598: will be substituted by the host name to
599: connect and
600: .Ql %p
601: by the port.
602: The command can be basically anything,
603: and should read from its standard input and write to its standard output.
604: It should eventually connect an
605: .Xr sshd 8
606: server running on some machine, or execute
607: .Ic sshd -i
608: somewhere.
609: Host key management will be done using the
610: HostName of the host being connected (defaulting to the name typed by
611: the user).
1.7 jmc 612: Setting the command to
613: .Dq none
1.6 markus 614: disables this option entirely.
1.1 stevesk 615: Note that
616: .Cm CheckHostIP
617: is not available for connects with a proxy command.
618: .Pp
619: .It Cm PubkeyAuthentication
620: Specifies whether to try public key authentication.
621: The argument to this keyword must be
622: .Dq yes
623: or
624: .Dq no .
625: The default is
626: .Dq yes .
627: This option applies to protocol version 2 only.
628: .It Cm RemoteForward
629: Specifies that a TCP/IP port on the remote machine be forwarded over
630: the secure channel to the specified host and port from the local machine.
1.38.2.2! brad 631: The first argument must be
1.38.2.1 brad 632: .Sm off
1.38.2.2! brad 633: .Oo Ar bind_address : Oc Ar port
1.38.2.1 brad 634: .Sm on
1.38.2.2! brad 635: and the second argument must be
! 636: .Ar host : Ns Ar hostport .
! 637: IPv6 addresses can be specified by enclosing addresses in square brackets
! 638: or by using an alternative syntax:
! 639: .Oo Ar bind_address Ns / Oc Ns Ar port
! 640: and
! 641: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 642: Multiple forwardings may be specified, and additional
643: forwardings can be given on the command line.
644: Only the superuser can forward privileged ports.
1.38.2.1 brad 645: .Pp
646: If the
647: .Ar bind_address
648: is not specified, the default is to only bind to loopback addresses.
649: If the
650: .Ar bind_address
651: is
652: .Ql *
653: or an empty string, then the forwarding is requested to listen on all
654: interfaces.
655: Specifying a remote
656: .Ar bind_address
657: will only succeed if the server's
658: .Cm GatewayPorts
659: option is enabled (see
660: .Xr sshd_config 5 ) .
1.1 stevesk 661: .It Cm RhostsRSAAuthentication
662: Specifies whether to try rhosts based authentication with RSA host
663: authentication.
664: The argument must be
665: .Dq yes
666: or
667: .Dq no .
668: The default is
669: .Dq no .
670: This option applies to protocol version 1 only and requires
671: .Nm ssh
672: to be setuid root.
673: .It Cm RSAAuthentication
674: Specifies whether to try RSA authentication.
675: The argument to this keyword must be
676: .Dq yes
677: or
678: .Dq no .
679: RSA authentication will only be
680: attempted if the identity file exists, or an authentication agent is
681: running.
682: The default is
683: .Dq yes .
684: Note that this option applies to protocol version 1 only.
1.32 djm 685: .It Cm SendEnv
686: Specifies what variables from the local
687: .Xr environ 7
688: should be sent to the server.
689: Note that environment passing is only supported for protocol 2, the
1.33 djm 690: server must also support it, and the server must be configured to
691: accept these environment variables.
1.32 djm 692: Refer to
693: .Cm AcceptEnv
694: in
695: .Xr sshd_config 5
696: for how to configure the server.
697: Variables are specified by name, which may contain the wildcard characters
698: .Ql \&*
699: and
700: .Ql \&? .
1.33 djm 701: Multiple environment variables may be separated by whitespace or spread
1.32 djm 702: across multiple
703: .Cm SendEnv
704: directives.
705: The default is not to send any environment variables.
1.28 markus 706: .It Cm ServerAliveInterval
707: Sets a timeout interval in seconds after which if no data has been received
708: from the server,
709: .Nm ssh
710: will send a message through the encrypted
711: channel to request a response from the server.
712: The default
713: is 0, indicating that these messages will not be sent to the server.
714: This option applies to protocol version 2 only.
715: .It Cm ServerAliveCountMax
716: Sets the number of server alive messages (see above) which may be
717: sent without
718: .Nm ssh
719: receiving any messages back from the server.
720: If this threshold is reached while server alive messages are being sent,
721: .Nm ssh
722: will disconnect from the server, terminating the session.
723: It is important to note that the use of server alive messages is very
724: different from
725: .Cm TCPKeepAlive
726: (below).
727: The server alive messages are sent through the encrypted channel
728: and therefore will not be spoofable.
729: The TCP keepalive option enabled by
730: .Cm TCPKeepAlive
731: is spoofable.
732: The server alive mechanism is valuable when the client or
733: server depend on knowing when a connection has become inactive.
734: .Pp
735: The default value is 3.
736: If, for example,
737: .Cm ServerAliveInterval
738: (above) is set to 15, and
739: .Cm ServerAliveCountMax
740: is left at the default, if the server becomes unresponsive ssh
741: will disconnect after approximately 45 seconds.
1.1 stevesk 742: .It Cm SmartcardDevice
1.11 jmc 743: Specifies which smartcard device to use.
744: The argument to this keyword is the device
1.1 stevesk 745: .Nm ssh
746: should use to communicate with a smartcard used for storing the user's
1.11 jmc 747: private RSA key.
748: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 749: .It Cm StrictHostKeyChecking
750: If this flag is set to
751: .Dq yes ,
752: .Nm ssh
753: will never automatically add host keys to the
754: .Pa $HOME/.ssh/known_hosts
755: file, and refuses to connect to hosts whose host key has changed.
756: This provides maximum protection against trojan horse attacks,
757: however, can be annoying when the
758: .Pa /etc/ssh/ssh_known_hosts
759: file is poorly maintained, or connections to new hosts are
760: frequently made.
761: This option forces the user to manually
762: add all new hosts.
763: If this flag is set to
764: .Dq no ,
765: .Nm ssh
766: will automatically add new host keys to the
767: user known hosts files.
768: If this flag is set to
769: .Dq ask ,
770: new host keys
771: will be added to the user known host files only after the user
772: has confirmed that is what they really want to do, and
773: .Nm ssh
774: will refuse to connect to hosts whose host key has changed.
775: The host keys of
776: known hosts will be verified automatically in all cases.
777: The argument must be
778: .Dq yes ,
779: .Dq no
780: or
781: .Dq ask .
782: The default is
783: .Dq ask .
1.26 markus 784: .It Cm TCPKeepAlive
785: Specifies whether the system should send TCP keepalive messages to the
786: other side.
787: If they are sent, death of the connection or crash of one
788: of the machines will be properly noticed.
789: However, this means that
790: connections will die if the route is down temporarily, and some people
791: find it annoying.
792: .Pp
793: The default is
794: .Dq yes
795: (to send TCP keepalive messages), and the client will notice
796: if the network goes down or the remote host dies.
797: This is important in scripts, and many users want it too.
798: .Pp
799: To disable TCP keepalive messages, the value should be set to
800: .Dq no .
1.1 stevesk 801: .It Cm UsePrivilegedPort
802: Specifies whether to use a privileged port for outgoing connections.
803: The argument must be
804: .Dq yes
805: or
806: .Dq no .
807: The default is
808: .Dq no .
1.4 stevesk 809: If set to
810: .Dq yes
811: .Nm ssh
812: must be setuid root.
1.1 stevesk 813: Note that this option must be set to
814: .Dq yes
1.17 markus 815: for
1.1 stevesk 816: .Cm RhostsRSAAuthentication
1.17 markus 817: with older servers.
1.1 stevesk 818: .It Cm User
819: Specifies the user to log in as.
820: This can be useful when a different user name is used on different machines.
821: This saves the trouble of
822: having to remember to give the user name on the command line.
823: .It Cm UserKnownHostsFile
824: Specifies a file to use for the user
825: host key database instead of
826: .Pa $HOME/.ssh/known_hosts .
1.8 jakob 827: .It Cm VerifyHostKeyDNS
828: Specifies whether to verify the remote key using DNS and SSHFP resource
829: records.
1.24 jakob 830: If this option is set to
831: .Dq yes ,
1.25 jmc 832: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 833: from DNS.
834: Insecure fingerprints will be handled as if this option was set to
835: .Dq ask .
836: If this option is set to
837: .Dq ask ,
838: information on fingerprint match will be displayed, but the user will still
839: need to confirm new host keys according to the
840: .Cm StrictHostKeyChecking
841: option.
842: The argument must be
843: .Dq yes ,
844: .Dq no
1.25 jmc 845: or
846: .Dq ask .
1.8 jakob 847: The default is
848: .Dq no .
1.12 jakob 849: Note that this option applies to protocol version 2 only.
1.1 stevesk 850: .It Cm XAuthLocation
1.5 stevesk 851: Specifies the full pathname of the
1.1 stevesk 852: .Xr xauth 1
853: program.
854: The default is
855: .Pa /usr/X11R6/bin/xauth .
856: .El
857: .Sh FILES
858: .Bl -tag -width Ds
859: .It Pa $HOME/.ssh/config
860: This is the per-user configuration file.
861: The format of this file is described above.
862: This file is used by the
863: .Nm ssh
864: client.
1.30 djm 865: Because of the potential for abuse, this file must have strict permissions:
866: read/write for the user, and not accessible by others.
1.1 stevesk 867: .It Pa /etc/ssh/ssh_config
868: Systemwide configuration file.
869: This file provides defaults for those
870: values that are not specified in the user's configuration file, and
871: for those users who do not have a configuration file.
872: This file must be world-readable.
873: .El
1.13 jmc 874: .Sh SEE ALSO
875: .Xr ssh 1
1.1 stevesk 876: .Sh AUTHORS
877: OpenSSH is a derivative of the original and free
878: ssh 1.2.12 release by Tatu Ylonen.
879: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
880: Theo de Raadt and Dug Song
881: removed many bugs, re-added newer features and
882: created OpenSSH.
883: Markus Friedl contributed the support for SSH
884: protocol versions 1.5 and 2.0.