I have a minimal, uncustomized Ubuntu 16.04 xenial Nginx server environment with only these incoming data TCP ports unfiltered:
22, 80, 443, 9000
/etc/csf/csf.conf I found out that the following incoming data UPD ports are unfiltered:
20, 21, 53
Should I filter these 3 so no one could try to access my system through them?