5. Stateful and stateless packet filtering

A packet filter is a multi-port system placed between two networks and responsible for filtering IP packets using filtering rules. The filtering rules implemented in packet filters are commonly referred to as ACLs, for Access Control List.

Filtering is based on the information usually found in the header of IP and TCP packets [7] :

• protocol numbers ;

• source and/or destination IP addresses ;

• source and/or destination port numbers (included in the TCP or UDP (User Datagram Protocol ) message);

• TCP connection flags ;

• other options.

This filtering is implemented in the network layer, i.e. in the kernel of an operating system (§