4 Firewalling

Figure 4: Processing of a packet in the IP layer.

For this project, you should implement a simple firewall based on packet filtering. In packet filtering, packets are dropped when they fulfill certain criteria. Figure 4 shows the processing of packets in the IP layer. A packet originates either from the link layer or it is generated by a local process. As can be seen from the figure, filtering can take place at three places: packets destined to the local host go through input filtering, packets received from another host and destined to a remote machine go through forward filtering, and locally generated packets go through output filtering.