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asterisk-func_dns: Asterisk 1.4 DNS() dialplan function

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I'm doing another small code release. This one is asterisk-func_dns, a dialplan function DNS() to do an explicit DNS query without requiring you to launch an external program. It's an alpha release currently intended for Asterisk 1.4 and only supporting IPv4 / DNS A record types.

I'm using this to implement DNS-based load-balancing for outgoing calls across a series of proxies and internet connections.

In my dialplan, I request the IP addresses of my proxy servers ahead of any attempt to Dial(). This module returns the list of IP addresses published in the record, separated by commas. This allows me to sequentially fork across the proxy servers, and since I don't need to rely on Dial()'s forking support, I can add additional processing in between attempts. Since I obtain the proxy set by looking up a single DNS name, my Asterisk dialer configurations do not have to change if I add more proxy servers to my network, also meaning that those Asterisk dialers will not waste time trying to contact outbound proxy servers that have gone offline for maintenance or due to a failure. Each Asterisk dialer will try every call amongst all the working proxy servers, up to one attempt each, in a random order.

This code could benefit from some obvious todos: forward port to modern Asterisk versions, implementation of the ability to grab other record types like SRV or AAAA, etc. I may address these eventually but at the moment this is "good enough for me". I release this code (under the same Asterisk licensing terms: GPLv2) with the hopes that someone finds it useful.

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