help how make iptables for bloc dos attacker

[thxyou]

hi and sorry for my bad english
pls help me how make iptables in linux ubuntu
i find this

Code:

iptables -A INPUT -p udp --dport 27015 -m length --length 28 -j DROP

27015 this my srcds port
is all what i need to stop dos attacker or what

but if the attacker atack me with random port ex: 20000 or 81 or 12345 ...
he can lag my server ?

what i need to do pls help me iam new in Linux