#!/usr/bin/env bash
#
# Usage:
#	1) Save as: set_dhcp_requested_ip.sh
#	2) Make executable:
#		chmod +x set_dhcp_requested_ip.sh
#	3) Run as root, passing the desired IP:
#		sudo ./set_dhcp_requested_ip.sh 192.168.1.50
#
# What it does:
#	- detects the primary network interface
#	- writes a Netplan config with DHCP enabled
#	- adds a systemd-networkd DHCPv4 RequestAddress=<IP> override
#	- applies the config
#
# Important:
#	- the router/DHCP server may ignore the requested IP
#	- for guaranteed assignment, make a DHCP reservation on the router too

set -euo pipefail

requested_ip="${1:-}"

if [[ -z "$requested_ip" ]]; then
	echo "Error: IP address argument is required."
	echo "Example: sudo $0 192.168.1.50"
	exit 1
fi

if [[ "${EUID}" -ne 0 ]]; then
	echo "Run this script as root: sudo $0 $requested_ip"
	exit 1
fi

if ! [[ "$requested_ip" =~ ^([0-9]{1,3}\.){3}[0-9]{1,3}$ ]]; then
	echo "Error: invalid IPv4 format: $requested_ip"
	exit 1
fi

for octet in ${requested_ip//./ }; do
	if (( octet < 0 || octet > 255 )); then
		echo "Error: invalid IPv4 octet in $requested_ip"
		exit 1
	fi
done

iface="$(ip route | awk '/^default/ {print $5; exit}')"

if [[ -z "$iface" ]]; then
	echo "Error: could not detect the primary network interface."
	exit 1
fi

netplan_file="/etc/netplan/99-dhcp-requested-ip.yaml"
networkd_dropin_dir="/etc/systemd/network/10-${iface}.network.d"
networkd_dropin_file="${networkd_dropin_dir}/request-address.conf"

mkdir -p "$networkd_dropin_dir"

cat > "$netplan_file" <<EOF
network:
	version: 2
	renderer: networkd
	ethernets:
		${iface}:
			dhcp4: true
EOF

cat > "$networkd_dropin_file" <<EOF
[DHCPv4]
RequestAddress=${requested_ip}
EOF

chmod 600 "$netplan_file" "$networkd_dropin_file"

echo "Generated Netplan config:"
cat "$netplan_file"
echo

echo "Generated systemd-networkd DHCP override:"
cat "$networkd_dropin_file"
echo

netplan generate
netplan apply

systemctl restart systemd-networkd

echo
echo "Done."
echo "Interface: $iface"
echo "Requested DHCP IPv4: $requested_ip"
echo
echo "Current addresses on $iface:"
ip -4 addr show dev "$iface"
echo
echo "Lease info:"
networkctl status "$iface" || true