DNS (Domain Name System)

核心 · Key Idea

In one line: DNS translates www.example.com (memorable for humans) into 93.184.216.34 (usable by machines). Every domain access starts with a DNS query.

What it is#

DNS is a globally distributed database organised hierarchically:

. (root)
└── com (top-level domain, TLD)
    └── example.com (second-level)
        ├── www.example.com  → A record → 93.184.216.34
        └── mail.example.com → A record → 93.184.216.50

When your machine queries, it walks up the tree until it finds the answer.

Analogy#

打个比方 · Analogy

DNS is layered address books:

You ask the front desk "how do I reach the head of marketing, Mr. Li?"
They ask HQ, HQ asks the regional office…
Eventually you get a phone number — then cache it for next time.

Key concepts#

A recordA Record

Domain → IPv4 address. Most common.

AAAA recordAAAA Record

Domain → IPv6 address.

CNAMECanonical Name

Domain → another domain (alias). Common with CDNs.

MXMail Exchange

Mail-receiving server for the domain.

TXTTXT Record

Arbitrary text, often for domain verification / SPF / DKIM.

TTLTime To Live

Cache duration in seconds — determines how fast a change propagates.

Recursive queryRecursive

You ask once; the resolver does the full walk for you.

How it works#

In practice most queries hit the local resolver cache, not the full path.

Practical notes#

dig www.example.com is the standard debug tool. On Windows: nslookup.
dig +trace walks the full path from root.
DNS changes don't apply instantly. Wait for TTL. Lower TTL (e.g. 60 s) hours before changing.
Public DNS: 1.1.1.1 (Cloudflare), 8.8.8.8 (Google), 223.5.5.5 (Alibaba).
DoH / DoT: DNS over HTTPS / TLS, encrypts queries; avoids ISP hijacking and snooping.
DNS poisoning: some networks return wrong IPs — DoH or a different upstream resolver bypasses it.

Easy confusions#

Domain

`example.com`
What DNS resolves.

URL

`https://example.com/path?x=1`
Full resource locator.