This rule > -A INPUT -i eth0 -p udp -j udpincoming_packets
makes these two worthless > -A udpincoming_packets -i eth1 -p udp -m udp --sport 67:68 -j ACCEPT > -A udpincoming_packets -i eth1 -p udp -m udp --dport 67:68 -j ACCEPT There's no way a packet can simultaneously match -i eth0 and -i eth1. So no bootp.dhcp traffic is actually allowed by this. Delete the sport rule and put the dport rule in the main INPUT chain, no need to consider broadcast and unicast separately.
