A Ruby Toy DNS

What is this, exactly?

This is an implementation of Julia Evans' Implement DNS in a Weekend.

It does more or less what you'd think.

Ask it about a domain, it gives you the IP address, and copies it to your clipboard.

How do I run it?

Easy (assuming you have Ruby installed)!

Clone the repo, and run ruby runner.rb. Then follow the cues!

Tell me about the directory structure

The code is organized around Julia's pedagogical strategy. She built her DNS server from the ground up, starting with a query. Then she showed how to build the response, and brought it all together in the resolver.

This means that some methods may be redundant across these directories. The bulk of the functional code lives in the resolver directory.

Any interesting revelations along the way?

The thorniest thing for me was figuring out DNS name compression.

When you query a DNS server for a domain name, the response comes back to you as a (mostly) hexadecimal bytestream.

It looks something like this:

\x81\x80\x00\x01\x00\x01\x00\x00\x00\x00\x03www\x07example\x03com\x00\x00\x01\x00\x01\xc0\x0c\x00\x01\x00\x01\x00\x00R\x9b\x00\x04]\xb8\xd8"

In the course of reading the bytestream, one of the more important pieces of information is, of course, the domain name. In this case, www.example.com.

But the DNS response may need to reference the domain name multiple times. And we don't want to have www and example and com to show up duplicatively in our string. So what do we do?

DNS Compression, of course! (Here's a link to the relevant spec).

The compression algo goes like this.

1. Read the string until we encounter the hexadecimal figure for a binary number, 11000000. That's c0 in hex.
2. That number is a signal -- "we have a compressed item here". The byte following our 11000000 number contains the index for where our domain name shows up in the string. In this case, the following hex number is 0c, or 12.
3. That refers us back to the domain name, which we can use in our code.

Here's a little diagram:

Far from the only thing I learned over the course of this project. If you're at all interested in this kind of thing, you should give Julia's project a try!