Over the last few weeks I built my own DNS client. Mostly because I thought dig (the standard DNS client) was kinda clunky. Partly because I wanted to learn more about DNS. So here's how I built it, and how you can build your own too. It's a great weekend project, and I learned a lot from finishing it.
Programming languages generally only manipulate bytes (groups of 8 bits). It can be pretty tricky to manipulate single bits. But sometimes you need to -- for example, a DNS header has some 4-bit numbers, and encodes some boolean flags into single bits. So we really need a way to parse binary data without chunking it up into bytes of 8 bits.
Luckily, Nom can do this! In the last blog post, we learned how to parse text files with Nom. The trick is to start with simple parsers that parse a few characters at a time. Then, using combinators, combine those simple parsers into more complex parsers that can deserialize an entire structured file. We can reuse this approach for parsing binary data too. Let's see how!