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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>4. How PDNS translates DNS queries into backend queries</title><link rel="stylesheet" href="docbook.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="PowerDNS manual" /><link rel="up" href="pdns-internals.html" title="Appendix B. PDNS internals" /><link rel="prev" href="modules.html" title="3. Modules & Backends" /><link rel="next" href="adding-rr-types.html" title="5. Adding new DNS record types" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4. How PDNS translates DNS queries into backend queries</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="modules.html">Prev</a> </td><th width="60%" align="center">Appendix B. PDNS internals</th><td width="20%" align="right"> <a accesskey="n" href="adding-rr-types.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="4. How PDNS translates DNS queries into backend queries"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="dns-to-query"></a>4. How PDNS translates DNS queries into backend queries</h2></div></div></div><p>
A DNS query is not a straightforward lookup. Many DNS queries need to check the backend for additional data, for example to
determine of an unfound record should lead to an NXDOMAIN ('we know about this domain, but that record does not exist') or an
unauthoritative response.
</p><p>
Simplified, without CNAME processing and wildcards, the algorithm is like this:
</p><p>
When a query for a <span class="command"><strong>qname</strong></span>/<span class="command"><strong>qtype</strong></span> tuple comes in, it is requested directly from the backend.
If present, PDNS adds the contents of the reply to the list of records to return. A question tuple may generate multiple answer
records.
</p><p>
Each of these records is now investigated to see if it needs 'additional processing'. This holds for example for MX records which may
point to hosts for which the PDNS backends also contain data. This involves further lookups for A or AAAA records.
</p><p>
After all additional processing has been performed, PDNS sieves out all double records which may well have appeared. The resulting set of
records is added to the answer packet, and sent out.
</p><p>
A zone transfer works by looking up the <span class="command"><strong>domain_id</strong></span> of the SOA record of the name and then listing all records of that
<span class="command"><strong>domain_id</strong></span>. This is why all records in a domain need to have the same domain_id.
</p><p>
When a query comes in for a specific qname, PDNS starts looking for SOA records of all subdomains of the qname, so
no.such.powerdns.com turns into a SOA query for no.such.powerdns.com, such.powerdns.com, powerdns.com, com, ''. When a SOA is found,
that zone is consulted for relevant NS instructions which lead to a referral. If nothing is found within the zone, an authoritative
NXDOMAIN is sent out.
</p><p>
If no SOA was found, an unauthoritative no-error is returned.
</p><p>
PDNS breaks strict RFC compatibility by not always checking for the presence of a SOA record first. This is unlikely to lead to
problems though.
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