The Distributed Denial-of-Service (DDos) attack could be transported by various Internet procotols, such as ICMP, TCP, UDP, etc.

A simple measure to mitigate the attack on TCP could be to set an overall cap on the inbound and outbound TCP connections respectively, and drop any more once a cap is reached, by using the iptables utility,

iptables -A INPUT -p tcp -m connlimit --connlimit-above 10000 --connlimit-mask 0 -j DROP
iptables -A OUTPUT -p tcp -m connlimit --connlimit-above 10000 --connlimit-mask 0 -j DROP

To check the rules just set up,

iptables -L

to obtain result

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       tcp  --  anywhere             anywhere             #conn src/0 > 10000

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       tcp  --  anywhere             anywhere             #conn src/0 > 10000

It turns out after rebooting the rules will be reset, so set them up every time at reboot.