A Weekend Project: Build Your Own DNS Resolver

Julia Evans has released a free, interactive guide for implementing a DNS resolver from scratch in about 200 lines of Python. The project is designed for a weekend’s worth of work—beta testers report completing it in roughly 2 to 4 hours—and it covers binary protocol parsing, DNS query mechanics, and the real-world behavior behind domain name resolution.

The guide is available at implement-dns.wizardzines.com.

What the Resolver Does

A DNS resolver is the piece of software that translates a domain name into an IP address. The project walks through building one that accepts a domain from the command line and outputs the corresponding IP address, giving you a hands-on view of what happens behind the scenes when you make a DNS query.

$ python3 resolve.py example.com
93.184.216.34

Format and Tooling

The content is delivered as a Jupyter notebook, which allows for mixing runnable code with explanatory text. The author chose this format after struggling to find a structure that kept both code and prose in sync. The notebooks are converted to a website using Jupyter Book, which reruns every notebook before rendering HTML so the displayed outputs are guaranteed to match what the code actually produces.

Readers can also download the notebooks and run them locally via the “download the code” button on the homepage.

Why Python and the Standard Library

Python was chosen over lower-level languages like Go or Rust to lower the barrier for people who are new to networking or systems programming. Binary data handling in Python is straightforward using struct.pack and struct.unpack, so the higher-level language doesn’t get in the way. The guide’s command-line resolver uses only Python’s standard library modules:

  • random for generating DNS query IDs
  • socket for making the UDP connection
  • struct for converting data to and from binary format
  • dataclasses to make record serialization and deserialization more ergonomic
  • io for BytesIO, which provides a reader interface that tracks how much of the packet has been parsed

Exercises and Extensions

The toy resolver intentionally omits features that a production resolver would need. The guide closes with suggested exercises for extending the implementation—such as adding additional record types or fixing protocol edge cases—so readers can push their implementation closer to a real-world resolver.

The author also notes plans for future projects along similar lines, including a potential “Implement TLS in a Weekend” guide, though no timeline is promised.