DNS

The Domain Name System — the distributed directory that turns a name a person can remember (uxlab.tools) into the address a network can route to, and the records beside it that say where a domain's mail goes, who may send it, and what a name is an alias for

Specified by the IETF (RFC 1034 and 1035, 1987) and still the first thing to break when a site is 'down'. The records a web team touches: A / AAAA (name to address), CNAME (name to another name — how next. points at Railway), MX (where mail arrives), TXT (proof of ownership, SPF and DMARC for mail), and the TTL on each, which is how long the answer is cached and therefore how long a mistake takes to undo. This site's names live at Cloudflare, which is also its email routing and its edge; a domain redirect is a DNS record plus a rule, which is what #500's two redirects are.