netfilter is a very important part of the Linux kernel in terms of security, packet mangling, and manipulation. The front end for netfilter is iptables, which “tells” the kernel what the user wants to do with the IP packets arriving into, passing through, or leaving the Linux box.
The most used features of netfilter are packet filtering and network address translation, but there are a lot of other things that we can do with netfilter, such as packet mangling Layer 7 filtering.