dns(3tcl) Domain Name Service dns(3tcl)
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NAME
dns - Tcl Domain Name Service Client
SYNOPSIS
package require Tcl 8.2
package require dns ?1.4.1?
::dns::resolve query ?options?
::dns::configure ?options?
::dns::name token
::dns::address token
::dns::cname token
::dns::result token
::dns::status token
::dns::error token
::dns::reset token
::dns::wait token
::dns::cleanup token
::dns::nameservers
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DESCRIPTION
The dns package provides a Tcl only Domain Name Service client. You
should refer to (1) and (2) for information about the DNS protocol or
read resolver(3) to find out how the C library resolves domain names.
The intention of this package is to insulate Tcl scripts from problems
with using the system library resolver for slow name servers. It may
or may not be of practical use. Internet name resolution is a complex
business and DNS is only one part of the resolver. You may find you are
supposed to be using hosts files, NIS or WINS to name a few other sys-
tems. This package is not a substitute for the C library resolver - it
does however implement name resolution over DNS. The package also ex-
tends the package uri to support DNS URIs (4) of the form
dns:what.host.com or dns://my.nameserver/what.host.com. The dns::re-
solve command can handle DNS URIs or simple domain names as a query.
Note: The package defaults to using DNS over TCP connections. If you
wish to use UDP you will need to have the tcludp package installed and
have a version that correctly handles binary data (> 1.0.4). This is
available at http://tcludp.sourceforge.net/. If the udp package is
present then UDP will be used by default.
Note: The package supports DNS over TLS (RFC 7858) for enhanced privacy
of DNS queries. Using this feature requires the TLS package.
COMMANDS
::dns::resolve query ?options?
Resolve a domain name using the DNS protocol. query is the do-
main name to be lookup up. This should be either a fully quali-
fied domain name or a DNS URI.
-nameserver hostname or -server hostname
Specify an alternative name server for this request.
-protocol tcp|udp
Specify the network protocol to use for this request. Can
be one of tcp or udp.
-port portnum
Specify an alternative port.
-search domainlist
-timeout milliseconds
Override the default timeout.
-type TYPE
Specify the type of DNS record you are interested in.
Valid values are A, NS, MD, MF, CNAME, SOA, MB, MG, MR,
NULL, WKS, PTR, HINFO, MINFO, MX, TXT, SPF, SRV, AAAA,
AXFR, MAILB, MAILA and *. See RFC1035 for details about
the return values. See http://spf.pobox.com/ about SPF.
See (3) about AAAA records and RFC2782 for details of SRV
records.
-class CLASS
Specify the class of domain name. This is usually IN but
may be one of IN for internet domain names, CS, CH, HS or
* for any class.
-recurse boolean
Set to false if you do not want the name server to recur-
sively act upon your request. Normally set to true.
-command procname
Set a procedure to be called upon request completion. The
procedure will be passed the token as its only argument.
-usetls boolean
Set the true to use DNS over TLS. This will force the use
of TCP and change the default port to 853. Certificate
validation is required so a source of trusted certificate
authority certificates must be provided using -cafile or
-cadir.
-cafile filepath
Specify a file containing a collection of trusted cer-
tificate authority certficates. See the update-ca-cer-
tificates command manual page for details or the -CAfile
option help from openssl.
-cadir dirpath
Specify a directory containing trusted certificate au-
thority certificates. This must be provided if -cafile is
not specified for certificate validation to work when
-usetls is enabled. See the openssl documentation for the
required structure of this directory.
::dns::configure ?options?
The ::dns::configure command is used to setup the dns package.
The server to query, the protocol and domain search path are all
set via this command. If no arguments are provided then a list
of all the current settings is returned. If only one argument
then it must the the name of an option and the value for that
option is returned.
-nameserver hostname
Set the default name server to be used by all queries.
The default is localhost.
-protocol tcp|udp
Set the default network protocol to be used. Default is
tcp.
-port portnum
Set the default port to use on the name server. The de-
fault is 53.
-search domainlist
Set the domain search list. This is currently not used.
-timeout milliseconds
Set the default timeout value for DNS lookups. Default is
30 seconds.
-loglevel level
Set the log level used for emitting diagnostic messages
from this package. The default is warn. See the log pack-
age for details of the available levels.
-cafile filepath
Set the default file path to be used for the -cafile op-
tion to dns::resolve.
-cadir dirpath
Set the default directory path to be used for the -cadir
option to dns::resolve.
::dns::name token
Returns a list of all domain names returned as an answer to your
query.
::dns::address token
Returns a list of the address records that match your query.
::dns::cname token
Returns a list of canonical names (usually just one) matching
your query.
::dns::result token
Returns a list of all the decoded answer records provided for
your query. This permits you to extract the result for more un-
usual query types.
::dns::status token
Returns the status flag. For a successfully completed query this
will be ok. May be error or timeout or eof. See also ::dns::er-
ror
::dns::error token
Returns the error message provided for requests whose status is
error. If there is no error message then an empty string is re-
turned.
::dns::reset token
Reset or cancel a DNS query.
::dns::wait token
Wait for a DNS query to complete and return the status upon com-
pletion.
::dns::cleanup token
Remove all state variables associated with the request.
::dns::nameservers
Attempts to return a list of the nameservers currently config-
ured for the users system. On a unix machine this parses the
/etc/resolv.conf file for nameservers (if it exists) and on Win-
dows systems we examine certain parts of the registry. If no
nameserver can be found then the loopback address (127.0.0.1) is
used as a default.
EXAMPLES
% set tok [dns::resolve www.tcl.tk]
::dns::1
% dns::status $tok
ok
% dns::address $tok
199.175.6.239
% dns::name $tok
www.tcl.tk
% dns::cleanup $tok
Using DNS URIs as queries:
% set tok [dns::resolve "dns:tcl.tk;type=MX"]
% set tok [dns::resolve "dns://l.root-servers.net/www.tcl.tk"]
Reverse address lookup:
% set tok [dns::resolve 127.0.0.1]
::dns::1
% dns::name $tok
localhost
% dns::cleanup $tok
Using DNS over TLS (RFC 7858):
% set tok [dns::resolve www.tcl.tk -nameserver dns-tls.bitwiseshift.net -usetls 1 -cafile /etc/ssl/certs/ca-certificates.crt]
::dns::12
% dns::wait $tok
ok
% dns::address $tok
104.25.119.118 104.25.120.118
REFERENCES
[1] Mockapetris, P., "Domain Names - Concepts and Facilities", RFC
1034, November 1987. (http://www.ietf.org/rfc/rfc1034.txt)
[2] Mockapetris, P., "Domain Names - Implementation and Specifica-
tion", RFC 1035, November 1087.
(http://www.ietf.org/rfc/rfc1035.txt)
[3] Thompson, S. and Huitema, C., "DNS Extensions to support IP ver-
sion 6", RFC 1886, December 1995.
(http://www.ietf.org/rfc/rfc1886.txt)
[4] Josefsson, S., "Domain Name System Uniform Resource Identi-
fiers", Internet-Draft, October 2003, (http://www.ietf.org/in-
ternet-drafts/draft-josefsson-dns-url-09.txt)
[5] Gulbrandsen, A., Vixie, P. and Esibov, L., "A DNS RR for speci-
fying the location of services (DNS SRV)", RFC 2782, February
2000, (http://www.ietf.org/rfc/rfc2782.txt)
[6] Ohta, M. "Incremental Zone Transfer in DNS", RFC 1995, August
1996, (http://www.ietf.org/rfc/rfc1995.txt)
[7] Hu, Z., etc al. "Specification for DNS over Transport Layer Se-
curity (TLS)", RFC 7858, May 2016,
(http://www.ietf.org/rfc/rfc7858.txt)
AUTHORS
Pat Thoyts
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain
bugs and other problems. Please report such in the category dns of the
Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please also
report any ideas for enhancements you may have for either package
and/or documentation.
When proposing code changes, please provide unified diffs, i.e the out-
put of diff -u.
Note further that attachments are strongly preferred over inlined
patches. Attachments can be made by going to the Edit form of the
ticket immediately after its creation, and then using the left-most
button in the secondary navigation bar.
SEE ALSO
resolver(5)
KEYWORDS
DNS, domain name service, resolver, rfc 1034, rfc 1035, rfc 1886, rfc
7858
CATEGORY
Networking
COPYRIGHT
Copyright (c) 2002, Pat Thoyts
tcllib 1.4.1 dns(3tcl)