iptables drop negative DNS responses

Solution 1:

Offset for DNS flags in ip packet is 28bytes and you need to check last 2 bits in 2byte field u32 rule for that is "28&0x000F=0x03"

My test rule is: iptables -A INPUT -m u32 -p udp --sport 53 --u32 "28&0x000F=0x03" -j LOG

BTW I like the idea to use this firewall rule for split horizon DNS, going to test it for some of my networks.

Solution 2:

This drops BIND9 "rejected" replies to the current spoofed UDP queries!

iptables -F OUTPUT 
#iptables -A OUTPUT -p udp --sport 53 -j LOG --log-prefix="OUT-UPD-RAW : "
#iptables -A OUTPUT -m u32 -p udp --sport 53 --u32 "28&0xFFFF=0x8105" -j LOG --log-prefix="STOPPED-DNS-REJECTED-REPLY "
iptables -A OUTPUT -m u32 -p udp --sport 53 --u32 "28&0xFFFF=0x8105" -j DROP
iptables -A OUTPUT -p udp --sport 53 -j LOG --log-prefix="OUT-UPD-SENT: "