Add network & containerisation

Doc/Network/DNS.md

@@ -1,18 +1,93 @@
 # DNS – Basics
 
-This page is for generic DNS concepts I reuse across projects.
+How domain names are resolved to IP addresses, and how local DNS differs from public DNS.
 
-## Scope
+For generic networking concepts, see:
 
-- What DNS does (names → IPs).
-- Difference between public DNS and internal DNS.
-- Common record types (A, AAAA, CNAME, etc.).
-
-## Notes / TODO
-
-- Add examples with public domains.
-- Explain how internal `.lan` domains fit into this picture.
+- [Network / Basics](./Basics.md)
 
 For my concrete home server DNS setup, see:
 
 - [Home-Server / dnsmasq implementation](../Home-Server/Implementations/DNS/dnsmasq.md)
+
+---
+
+## 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.
+
+---
+
+## 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.
+
+---
+
+## 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.
+
+---
+
+## 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.
+
+---
+
+## 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.
+
+---
+
+## 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.