# DNS – Basics

How domain names are resolved to IP addresses, and how local DNS differs from public DNS.

For generic networking concepts, see:

- [Network / Basics](./Basics.md)

For my concrete home server DNS setup, see:

- [Home-Server / dnsmasq implementation](../Home-Server/Implementations/DNS/dnsmasq.md)

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## Scope

- What DNS does and why it exists.
- How a query is resolved step by step.
- Common record types.
- Public DNS vs internal / split DNS.
- TTL.

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## 1. What DNS Does

DNS (Domain Name System) translates human-readable names like `google.com` into IP addresses like `142.250.74.46`. Without DNS, every user would need to memorise the IP of every service they use.

DNS is a globally distributed, hierarchical database. No single server knows all names — responsibility is delegated across thousands of authoritative name servers worldwide.

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## 2. How a Query Is Resolved

When your browser wants to reach `www.example.com`:

1. **Local cache** — the OS checks if it already has a recent cached answer.
2. **Stub resolver** — the OS sends the query to its configured DNS server (usually the router or a public resolver).
3. **Recursive resolver** — if not cached, this server performs the full lookup:
   - Asks a **root server**: *"Who handles `.com`?"*
   - Asks the **.com TLD server**: *"Who handles `example.com`?"*
   - Asks the **authoritative name server** for `example.com`: *"What is `www.example.com`?"*
4. The answer (an IP address) travels back and is cached at each level.
5. The OS delivers the IP to the browser, which opens a TCP connection.

This whole process typically takes a few milliseconds.

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## 3. Common Record Types

| Type    | Purpose                                               | Example                                    |
|---------|-------------------------------------------------------|--------------------------------------------|
| `A`     | Maps a name to an IPv4 address                        | `example.com → 93.184.216.34`              |
| `AAAA`  | Maps a name to an IPv6 address                        | `example.com → 2606:2800:…`               |
| `CNAME` | Alias: maps one name to another name                  | `www.example.com → example.com`            |
| `MX`    | Mail server for a domain                              | `@ → mail.example.com`                     |
| `TXT`   | Arbitrary text; used for SPF, DKIM, domain ownership  | `"v=spf1 include:…"`                      |
| `PTR`   | Reverse lookup: IP → name                             | `34.216.184.93.in-addr.arpa → example.com` |
| `NS`    | Authoritative name server(s) for a zone               | `example.com → ns1.example.com`            |

In practice, `A`, `CNAME`, and `TXT` are the records encountered most often.

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## 4. Public DNS vs Internal DNS

### 4.1 Public DNS

Public resolvers answer queries for any registered domain on the internet. Common examples:

- `8.8.8.8` / `8.8.4.4` — Google DNS
- `1.1.1.1` / `1.0.0.1` — Cloudflare DNS
- `9.9.9.9` — Quad9

Your OS or router is configured to use one of these by default.

### 4.2 Internal / Local DNS

For names that only exist inside a private network (like `.lan` domains on a home server), you need an **internal DNS server**. It answers queries for private names and forwards everything else to a public resolver.

This is called **split DNS**: different names are resolved by different servers depending on which network you are on.

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## 5. TTL (Time To Live)

Every DNS record has a **TTL** — a duration in seconds telling resolvers how long they may cache the answer.

- Short TTL (60–300 s): changes propagate quickly, but more queries are made.
- Long TTL (3600–86400 s): reduces query load, but changes are slow to propagate everywhere.

When changing a record (for example updating an IP), it is common to lower the TTL beforehand to speed up propagation.