It's Always DNS

Definitions

It's Always DNS is a running joke in the IT and DevOps community expressing the observation that a surprisingly large proportion of mysterious network failures, connectivity issues, and service outages ultimately trace back to a DNS (Domain Name System) problem. The humor comes from its persistent truth: after hours of debugging network configurations, firewall rules, application code, and infrastructure, the root cause is often a misconfigured DNS record, a stale DNS cache, a propagation delay after a record change, an expired domain, or a failing nameserver. The phrase has become a troubleshooting heuristic: when something network-related breaks in a confusing way, check DNS first. The joke has spawned merchandise, conference talks, and the website isitdns.com. Its enduring popularity reflects the fact that DNS is a foundational layer of the internet that is easy to misconfigure and whose failures produce symptoms that mimic many other types of problems. The DevOps wisdom behind the joke is genuinely useful: many hours of debugging can be saved by ruling out DNS issues early.
In plain English: An internet truth: when something weird is happening with your network or website, the problem is almost always DNS (the internet's address book).