openwrt-packages-dist/net/adblock/files/samples/firewall.user.sample

# redirect/force all dns queries to port 53 of your router
# configuration found in /etc/firewall.user
#
iptables -t nat -I PREROUTING -p tcp --dport 53 -j REDIRECT --to-ports 53
iptables -t nat -I PREROUTING -p udp --dport 53 -j REDIRECT --to-ports 53