DNS¶

DNS = phonebook: hostname → IP.

On Linux, name resolution isn't only DNS. /etc/nsswitch.conf sets the order — usually files (/etc/hosts) then dns. That's why getent and dig can disagree.
Resolver (recursive: 8.8.8.8, systemd-resolved) answers by asking around. Authoritative server actually owns the record.
TTL = how long an answer is cached before it's re-fetched.
Most modern Debian/Ubuntu run systemd-resolved: a stub resolver at 127.0.0.53, which is what /etc/resolv.conf usually points at.

Lookup tools¶

Command	What it does
`nslookup <hostname>`	simple lookup (uses configured resolver)
`dig <hostname>`	detailed lookup, full DNS answer
`dig +short <hostname>`	just the IP, no noise
`dig <hostname> MX`	query a specific record type (A/AAAA/MX/TXT/NS)
`dig @8.8.8.8 <hostname>`	ask a specific server (bypass your resolver)
`dig -x <ip>`	reverse lookup (IP → name)
`dig +trace <hostname>`	follow delegation from root → authoritative
`host <hostname>`	quick one-line lookup
`getent hosts <hostname>`	resolves via nsswitch (files + dns, like apps do)
`resolvectl query <host>`	query through systemd-resolved

Command	What it does
`resolvectl status`	which DNS servers per interface
`resolvectl flush-caches`	clear the resolver cache
`resolvectl statistics`	cache hits/misses

File	Purpose
`/etc/resolv.conf`	which resolver(s) to use (often → 127.0.0.53)
`/etc/hosts`	static hostname → IP overrides
`/etc/nsswitch.conf`	resolution order (`files dns`)
`/etc/systemd/resolved.conf`	systemd-resolved settings

Symptom / need	Check
`getent` vs `dig` mismatch	it's `/etc/hosts` or nsswitch, not DNS itself
Stale record after a change	`resolvectl flush-caches` (or check TTL)
Which server am I even using?	`resolvectl status` or `cat /etc/resolv.conf`
Works with `@8.8.8.8`, not default	your configured resolver is the problem