What Is DNS? – Definition and How It Works
Learn what the Domain Name System is, the record types that matter, how TTLs govern propagation, and why DNS decides more than name resolution.
The Domain Name System maps names to addresses and other information. It is the first step of nearly every network operation, and a surprising share of "the site is down" turns out to be a DNS problem.
1. The Record Types That Matter
| Record | Holds |
|---|---|
| A | An IPv4 address |
| AAAA | An IPv6 address |
| CNAME | An alias to another name |
| MX | Mail servers for the domain |
| TXT | Arbitrary text — verification, policy records |
| NS | The authoritative nameservers |
| CAA | Which authorities may issue certificates |
For anyone fetching pages, A and AAAA records decide where the request goes, and NS records decide which servers are authoritative for that answer.
2. The Hierarchy
Names resolve from the right: root servers know the top-level domains, those know each domain's nameservers, and those hold the actual records.
A resolver walks that chain and caches what it learns, which is why the first lookup for a name is slower than the ones that follow.
3. TTL Governs Everything About Change
Each record carries a time to live, telling resolvers how long they may reuse it. That value — not any global mechanism — determines how long a change takes to become visible.
The practical consequence is that TTLs must be lowered before a planned change, not after. Lowering them afterwards does nothing for caches already holding the old value.
4. Why "Propagation" Is a Misnomer
Nothing propagates. Authoritative records change instantly; caches elsewhere expire on their own schedules.
A change that appears live in one place and stale in another is not incomplete propagation — it is two resolvers with different cache states, and the difference resolves as TTLs expire.
5. DNS Decides More Than Addresses
- Content delivery routes users to nearby edges by returning different addresses per region.
- Failover switches traffic by changing records.
- Load distribution returns several addresses in rotation.
- Email authentication lives in TXT records.
- Domain verification for many services works the same way.
This is why an apparently trivial record change can affect delivery, certificates, and email at once.
6. When It Fails
| Symptom | Meaning |
|---|---|
NXDOMAIN |
The name authoritatively does not exist |
SERVFAIL |
The resolver failed — often a broken delegation or signature |
| Timeout | The resolver did not answer |
| Wrong address | Stale cache, or a hosts-file entry |
Querying two public resolvers and the authoritative servers separates a local problem from a real one in seconds.
7. For Collection at Scale
A burst of concurrent lookups can overwhelm a small resolver, producing failures that look like the target refusing traffic. Reusing connections reduces lookups, and respecting TTLs rather than caching indefinitely keeps results correct when a target moves.
8. Summary
DNS resolves names through a hierarchy of authoritative servers, cached along the way according to each record's TTL.
That TTL determines how long a change takes to be seen, "propagation" is really cache expiry, and DNS decides routing, failover, and verification as well as addresses — which is why its failures rarely look like DNS failures.