If by "your machine" you mean the gateway in my diagram, yes it is effectively "directly" connected to the internet. x.mydom.com is a static IP address hosted by Comcast: Physically a cable modem.
I don't know what you mean by "hairpin configuration." I suppose a split-dns configuration would be a solution but this just feels like I have somehow mis-configured IPTables at the gateway for the internal traffic.
To answer pschaff, the error is that I get no response from the server. As I said
http://x.pvt works but
http://x.mydom.com gets lost in the bit bucket somewhere.
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