How to avoid that in case the connection to the VPN server drops, the connection start routing via your ISP network .
Script assuming the vpn server is running on UDP port 1194
Start the killswitch, /usr/local/sbin/vpn-killswitch-start
#!/bin/bash
#accept packets to/from dhcp server
iptables -t filter -A OUTPUT -p udp --dport 67 -j ACCEPT