.. _Extended5DNS:

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.. avmetadata::
   :author: Michael S. Kirkpatrick
   :requires:
   :satisfies: 
   :topic: 

Extended Example: DNS Client
============================

This Extended Example is a minimal client for performing a DNS query for IPv4
addresses. Given a domain name (such as ``example.com``), the client sets up
the DNS question in ``setup_dns_request()`` (lines 200 – 234). The ``packed``
attribute of the ``dns_record_a_t`` (lines 31 – 39) ensures that the compiler
does not add any unnecessary padding that would make the question improperly
structured. The ``build_domain_qname()`` function (lines 95 – 134) replaces the
dots in the domain name with an integer to denote the length of the next field.
Lines 65 – 82 perform the actual query, sending the request to OpenDNS using a
UDP socket. The client is only designed to support DNS ``A`` type records for
IPv4. I.e., this client does not support ``CNAME``, ``NS``, or ``MX`` records.
The ``print_dns_response()`` function (lines 153 – 198) will stop if any other
record types are encountered.

.. codeinclude:: Internet/ExtEx-5.c
   :linenos: true