Split-Horizon DNS: One Name, Two Answers, and the Server Decides by Who's Asking

Originally published at https://blog.pathvector.dev/protocol-lab-dns-views-42-split-horizon/ — part of the free Protocol Lab series.

This post is part of Protocol Lab, a free, hands-on series for learning networking protocols by building and breaking them in a container lab. All the lab material — topologies, configs, and scripts — lives in the repo: github.com/pathvector-studio/protocol-lab.

In Lab #41, round-robin DNS rotated the order of one name's records. Split-horizon DNS pulls a more radical trick: it changes the answer itself based on who is asking. An authoritative server keeps several views of the same zone and picks one by the client's source address.

Reading guide: rfc-notes/split-horizon-dns.md

Prerequisite: DNS Lab 05: Resolving a Name Through the Hierarchy