Host Reachable Through cURL but Cannot Be Resolved by nslookup

Recently I was doing some weird shit with docker and docker compose that required a weird DNS setup. While debugging a problem with DNS resolution in nginx I came across a strange issue: I could reach a host by curl and ping but wasn't able to resolve it using nslookup and dig.

A quick Google search wasn't really helpful so I decided to run strace on both tools and see if there's an obvious difference in how they handle name resolution. After looking at the trace I finally spotted the difference: curl was opening /etc/hosts and dig wasn't!

Turns out extra_hosts is adding hostnames to the /etc/hosts file which isn't checked by the DNS tools or nginx resolver.