NAT Traversal, peer discovery and the stark realisation that you’re an idiot

In my last post I unveiled groundstation, a supremely pre-beta cut of a tool I’m building to automagically sync objects in several git repos with any and all nearby peers. Up until tonight, I had been testing with two laptops, connected to the same wireless network (more or less the usecase I envisage).

This evening, I had only my laptop with me: but “Not to worry,” I thought, “I’ll just light up my dev VM!”. At work we use vagrant to light up ondemand VM’s, bootstrap them with babushka and get on with it. We use some trickery in the vagrant-dns gem to make the VM addressible from the host, with vagrant taking care of NAT for us.

Which is where things got interesting. groundstation uses UDP broadcast to find it’s peers, which WILL penetrate most NAT configurations, but with the caveat that the source address will be rewritten- in this instance rewriting it to my external IP address, causing my daemon to attempt to connect to it’s “peer” and sync it’s objects- with itself.

About richo

I enjoy exploring interesting concepts in weird languages. I also like hacking on all of the things.
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