Since I work a lot with clustered VMs, I’ve ended up spending a lot of time trying to figure out how DNS lookups work. I applied ‘fixes’ to my problems from StackOverflow without really understanding why they work (or don’t work) for some time.
Eventually I got fed up with this and decided to figure out how it all hangs together. I couldn’t find a complete guide for this anywhere online, and talking to colleagues they didn’t know of any (or really what happens in detail)
So I’m writing the guide myself.
The first thing to grasp is that there is no single method of getting a DNS lookup done on Linux. It’s not a core system call with a clean interface.
There is, however, a standard C library call called which many programs use:
getaddrinfo. But not all applications use this!
Let’s just take two simple standard programs:
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