Add scripts to automatically update the LAN DNS records.

./sbin/update-dns.sh may be called at regular intervals (one hour
or so) via crontab.

It will automatically detect:
- its local host name (*.lan.dougal.aaltronav.eu); and
- which IP has internet access, if any.

Armed with that information and with the dynamic DNS API password
stored in DYNDNS_PASSWD in ~/.dougalrc, it will update the relevant
DNS record.

For this to work, the first `lan.dougal` hostname in the Nginx
configuration must be the one that is set up for dynamic update.
Other `lan.dougal` hostnames should be CNAME records pointing to
the first one.

sbin/get-ip.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# Find the interface used for the default route (e.g., to reach 8.8.8.8)
+INTERFACE=$(ip route get 8.8.8.8 | awk '{for(i=1;i<=NF;i++) if ($i=="dev") {print $(i+1); exit}}')
+
+if [ -z "$INTERFACE" ] || [ "$INTERFACE" = "lo" ]; then
+    echo "No valid default route found." >/dev/stderr
+    exit 1
+fi
+
+# Extract the primary IPv4 address from the interface (excluding loopback)
+IP=$(ip -4 addr show dev "$INTERFACE" scope global | grep -oP '(?<=inet\s)\d{1,3}(\.\d{1,3}){3}(?=/)' | head -n1)
+
+if [ -z "$IP" ]; then
+    echo "No global IPv4 address found on $INTERFACE." >/dev/stderr
+    exit 1
+fi
+
+echo "$IP"