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    <p><code>MX</code> records were used because there was a need for SMTP traffic to <code>user@domain</code> to be routed differently to other traffic for that domain, and <code>SRV</code> records hadn't been invented yet.</p> <p>The modern convention that you can type <code>http://example.com/</code> in your browser without a <code>www</code> prefix and still get to the required website is actually a bit odd. To explain in more detail, consider how a zone would normally be setup to achieve this prefix-less access:</p> <pre><code>$ORIGIN example.com @ IN A 192.168.1.1 IN MX mail.example.com www IN A 192.168.1.1 mail IN A 192.168.1.2 </code></pre> <p>So, any traffic addressed to <code>example.com</code> goes to that IP address, regardless of the protocol in use (unless it's e-mail which will use the MX record).</p> <p>In practise it would be preferable for <em>all</em> applications to make use of <code>SRV</code> records, and then we could do away with the application specific prefixes all together, and use A records for their real purpose - specifically mapping real <em>hostnames</em> to IP addresses.</p> <p>If SRV records were used in this way that zone file would look instead like:</p> <pre><code>$ORIGIN example.com _http._tcp IN SRV 0 0 80 www.example.com _smtp._tcp IN SRV 0 0 25 mail.example.com www IN A 192.168.1.1 mail IN A 192.168.1.2 </code></pre> <p>This assumption that the primary <code>A</code> record at a domain is actually for HTTP service is also part of the reason why Verisign's <a href="http://en.wikipedia.org/wiki/Site_Finder" rel="noreferrer">SiteFinder</a> "service" caused as many problems as it did when it was (briefly) introduced in 2003. By intercepting all DNS <code>A</code> record lookups for unknown domains and returning one of their own addresses, Verisign broke all sorts of protocols that assumed that they could fail-over to other address database mechanisms if the DNS lookup failed.</p>
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    1. COInteresting side-effect of SRV records for http -- there's a natural failover to an error page (or stale static server) when your primary web server is completely unavailable. We need more of this.
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    2. COtrue, although in that case you'd want the 'www' entry to fail-over correctly too, which in this example (with an explicit A record) it can't.
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