Force SYN packets check
Make sure NEW incoming tcp connections are SYN packets; otherwise we need to drop them:iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP
Force Fragments packets check
Packets with incoming fragments drop them. This attack result into Linux server panic such data loss.iptables -A INPUT -f -j DROP
XMAS packets
Incoming malformed XMAS packets drop them:iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP
Drop all NULL packets
Incoming malformed NULL packets:iptables -A INPIT -p tcp --tcp-flags ALL NONE -j DROP
Block Spoofing and bad addresses
Using iptables you can filter to drop suspicious source address. Network server should not accept packets claiming from the Internet that claim to originate from inside your network. Spoofing can be classified as:a) IP spoofing – Disable the source address of authentication, for example rhosts based authentication. Filter RPC based services such as portmap and NFS,
b) DNS spoofing
Please see Iptables: How to avoid Spoofing and bad addresses attack tip for more information.
Also use NAT for your internal network. This makes difficult for attacker to spoof IP address from outside.
Filter incoming ICMP, PING traffic
It includes the ping of death attack and ICMP floods. You should block all ICMP and PING traffic for outside except for your own internal network (so that you can ping to see status of your own server) . See Linux : Iptables Allow or block ICMP ping request article.Once system is secured, test your firewall with nmap or hping2 command:
# nmap -v -f FIREWALL-IP
# nmap -v -sX FIREWALL-IP
# nmap -v -sN FIREWALL-IP
# hping2 -X FIREWALL-IP
