Linux/Centos server prevents udp packet sending from udp-flood attacks

Some websites are maliciously placed with UDP packet sending tools to attack others, leading to heavy traffic loss. Generally, only the DNS server uses udp protocol, while others can disable UDP packet forwarding.
Therefore, a script is written to only allow Outbound UDP packets from the target DNS server. All other UDP packets are rejected. This method can only prevent malicious UDP packets from being sent, the server itself makes security settings to prevent malicious Trojans.

#/Bin/bash
# Createdby http://www.bkjia.com
# Drop udp Flood
List = 'grep nameserver/etc/resolv. conf | awk '{print $ NF }''
For I in $ list
Do
Iptables-a output-p udp-d $ I -- dport 53-j ACCEPT
Done
Iptables-a output-p udp-j DROP
Service iptables save