Now we know how to filter on ipaddresses and interfaces

Time to learn the fun stuff... stateful filtering.

Stateful filtering is a concept where we know filter on the state of connections.
ip_conntrack
this is the kernelmodule that implements the connectiontracking.

We can filter on diffrent states of a connection.
NEW
packet that's about to create a new connection
ESTABLISHED
packets that's part of an already existing connection
RELATED
packets that's related to another connection, ie the dataconnections in a ftp session
...