A bug that survived 15 years of upgrades. One of my servers lost DNS after every reboot. ping 8.8.8.8 worked; ping https://google.com didn't resolve. I'd been "fixing" it by hand-editing /etc/resolv.conf, which, of course, was reset on the next boot. 🙈 Turned out to be a config from 2010. I finally debugged it properly with Claude. https://blog.pecar.me/the-15-year-old-iptables-rule-that-broke-my-dns