Prestige 791R G.SHDSL Router

12.4 Filter Types and NAT

There are two classes of filter rules, Generic Filter Device rules and Protocol Filter (TCP/IP) rules. Generic Filter rules act on the raw data from/to LAN and WAN. Protocol Filter rules act on IP packets.

When NAT (Network Address Translation) is enabled, the inside IP address and port number are replaced on a connection-by-connection basis, which makes it impossible to know the exact address and port on the wire. Therefore, the Prestige applies the protocol filters to the “native” IP address and port number before NAT for outgoing packets and after NAT for incoming packets. On the other hand, the generic (or device) filters are applied to the raw packets that appear on the wire. They are applied at the point where the Prestige is receiving and sending the packets; for instance, the interface. The interface can be an Ethernet, or any other hardware port. The following figure illustrates this.

Figure 12-14 Protocol and Device Filter Sets

12.5 Example Filter

Let’s look at an example to block outside users from Telnetting into the Prestige.

12-16

Filter Configuration