Update prod documentation

docs/SETUP_PROD.md

@@ -1,5 +1,7 @@
 # Configure project for production
 
+Note: This guide assumes that you use the default hostname, `central.internal` as hostname for your central system.
+
 ## Create production build
 
 ### Central
@@ -44,6 +46,70 @@ The OTA update is then located in `build/main.bin`
 * A server running a recent Linux (Debian / Ubuntu preferred) with `central` as hostname
 * DHCP configured on the network
 
+## Configure DNS server
+
+If you need to setup a DNS server / proxy to point `central.internal` to the central server IP, you can follow this guide.
+
+### Retrieve DNS server binary
+Use [DNSProxy](https://gitlab.com/pierre42100/dnsproxy) as DNS server. Get and compile the sources:
+
+```bash
+git clone https://gitlab.com/pierre42100/dnsproxy
+cd dnsproxy
+cargo build --release
+scp target/release/dns_proxy USER@CENTRAL_IP:/home/USER
+```
+
+Then, on the target server, install the binary to its final destination:
+
+```bash
+sudo mv dns_proxy /usr/local/bin/
+```
+
+### Configure DNS server
+Configure the server as a service `/etc/systemd/system/dns.service`:
+
+```conf
+[Unit]
+Description=DNS server
+After=syslog.target
+After=network.target
+
+[Service]
+RestartSec=2s
+Type=simple
+User=root
+Group=root
+WorkingDirectory=/tmp
+ExecStart=/usr/local/bin/dns_proxy -l "CENTRAL_IP:53" -c "central.internal. A CENTRAL_IP"
+Restart=always
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Enable and start the new service:
+
+```bash
+sudo systemctl enable dns
+sudo systemctl start dns
+```
+
+Check that it works correctly:
+
+```bash
+dig central.internal. @CENTRAL_IP
+```
+
+You should get an entry like this if it works:
+
+```
+;; ANSWER SECTION:
+central.internal.	0	IN	A	CENTRAL_IP
+```
+
+Then, in your DHCP service, define the central as the DNS server.
+
 ## Configure server
 
 ### Create a user dedicated to the central
@@ -82,7 +148,7 @@ COOKIE_SECURE=true
 LISTEN_ADDRESS=0.0.0.0:443
 ADMIN_USERNAME=admin
 ADMIN_PASSWORD=FIXME
-HOSTNAME=central.local
+HOSTNAME=central.internal
 STORAGE=/home/central/storage
 FRONIUS_ORIG=http://10.0.0.10
 ```