Merge pull request 'docs: Real DNS + empty upstream Quick Start' (#43) from docs/quick-start-real-dns into main

docs: Real DNS + empty upstream Quick Start

Fixes #32

README.md

@@ -10,8 +10,8 @@ SteamCache2 is a blazing fast download cache for Steam, designed to reduce bandw
 - Reduces bandwidth usage
 - Easy to set up and configure aside from dns stuff to trick Steam into using it
 - Supports multiple clients
-- **NEW:** YAML configuration system with automatic config generation
-- **NEW:** Simple Makefile for development workflow
+- YAML configuration with automatic config generation on first run
+- Makefile for development and validation workflows
 - Cross-platform builds (Linux, macOS, Windows)
 
 ## Quick Start
@@ -34,7 +34,10 @@ SteamCache2 is a blazing fast download cache for Steam, designed to reduce bandw
 
    The application will automatically create a `config.yaml` file with default settings and exit, allowing you to customize it.
 
-3. **Edit the configuration** (`config.yaml`):
+3. **Edit the configuration** (`config.yaml`) for a real Steam front-door:
+
+   Leave `upstream` empty. With an empty upstream, steamcache2 fetches from the request `Host` (the same pattern SteamPrefill / real Steam clients use when DNS points them at your cache). Do **not** paste a fake host like `https://steam.cdn.com` — that is not a Steam CDN and will not put you in front of Steam.
+
    ```yaml
    listen_address: :80
    cache:
@@ -45,10 +48,19 @@ SteamCache2 is a blazing fast download cache for Steam, designed to reduce bandw
            size: 10GB
            path: ./disk
            gc_algorithm: hybrid
-   upstream: "https://steam.cdn.com"  # Set your upstream server
+   # Empty upstream = use the client Host as the origin (Steam CDN names only).
+   upstream: ""
    ```
 
-4. **Run the application again:**
+4. **Point Steam (or SteamPrefill) at this cache** before you expect hits:
+
+   - **LAN DNS:** resolve `lancache.steamcontent.com` (and other Steam content names your clients use) to this server's LAN IP.
+   - **Single Windows PC:** add a hosts override — see [Windows Hosts File Override](#windows-hosts-file-override) (`<cache-ip> lancache.steamcontent.com`).
+   - Restart Steam (or your prefill tool) after DNS/hosts changes.
+
+   Empty-upstream direct fetch only allows Steam CDN host suffixes (`steamcontent.com`, `steampowered.com`, `steamstatic.com`). Literal IPs and unrelated hosts are rejected.
+
+5. **Run the application again:**
    ```bash
    make run  # or ./steamcache2
    ```
@@ -238,8 +250,9 @@ cache:
     gc_algorithm: hybrid
 
 # Upstream server configuration
-# The upstream server to proxy requests to
-upstream: "https://steam.cdn.com"
+# Leave empty to fetch from the request Host (Steam CDN names only).
+# Set only when chaining caches (table RAM cache -> room disk cache).
+upstream: ""
 ```
 
 #### Startup Validation