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dns(n) 1.5.0 tcllib "Domain Name Service"


dns - Tcl Domain Name Service Client

Table Of Contents



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 systems. This package is not a substitute for the C library resolver - it does however implement name resolution over DNS. The package also extends the package uri to support DNS URIs (4) of the form dns:what.host.com or dns://my.nameserver/what.host.com. The dns::resolve 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.


::dns::resolve query ?options?

Resolve a domain name using the DNS protocol. query is the domain name to be lookup up. This should be either a fully qualified 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 recursively 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 certificate authority certficates. See the update-ca-certificates command manual page for details or the -CAfile option help from openssl.

-cadir dirpath

Specify a directory containing trusted certificate authority 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 default 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 package for details of the available levels.

-cafile filepath

Set the default file path to be used for the -cafile option 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 unusual 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::error

::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 returned.

::dns::reset token

Reset or cancel a DNS query.

::dns::wait token

Wait for a DNS query to complete and return the status upon completion.

::dns::cleanup token

Remove all state variables associated with the request.


Attempts to return a list of the nameservers currently configured for the users system. On a unix machine this parses the /etc/resolv.conf file for nameservers (if it exists) and on Windows systems we examine certain parts of the registry. If no nameserver can be found then the loopback address ( is used as a default.


% set tok [dns::resolve www.tcl.tk]
% dns::status $tok
% dns::address $tok
% dns::name $tok
% 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]
% dns::name $tok
% 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::wait $tok
% dns::address $tok


  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 Specification", RFC 1035, November 1087. (http://www.ietf.org/rfc/rfc1035.txt)

  3. Thompson, S. and Huitema, C., "DNS Extensions to support IP version 6", RFC 1886, December 1995. (http://www.ietf.org/rfc/rfc1886.txt)

  4. Josefsson, S., "Domain Name System Uniform Resource Identifiers", Internet-Draft, October 2003, (http://www.ietf.org/internet-drafts/draft-josefsson-dns-url-09.txt)

  5. Gulbrandsen, A., Vixie, P. and Esibov, L., "A DNS RR for specifying 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 Security (TLS)", RFC 7858, May 2016, (http://www.ietf.org/rfc/rfc7858.txt)


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. 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 output 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



DNS, domain name service, resolver, rfc 1034, rfc 1035, rfc 1886, rfc 7858