ZyXEL Communications zywall 50 User Manual
ZyWALL 50 Internet Security Gateway
Filter Configuration
15-17
15.4 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 the IP packets.
Generic and TCP/IP filter rules are discussed in more detail in the next section. 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
ZyWALL 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 when the ZyWALL is receiving and
sending the packets; i.e. the interface. The interface can be an Ethernet port or any other hardware port. The
following diagram illustrates this.
Generic Filter rules act on the raw data from/to LAN and WAN. Protocol Filter rules act on the IP packets.
Generic and TCP/IP filter rules are discussed in more detail in the next section. 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
ZyWALL 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 when the ZyWALL is receiving and
sending the packets; i.e. the interface. The interface can be an Ethernet port or any other hardware port. The
following diagram illustrates this.
Figure 15-14 Protocol and Device Filter Sets
15.5 Firewall
Firewall configuration is discussed in the firewall chapters of this manual. Further comparisons are also
made there between filtering, NAT and the firewall.
made there between filtering, NAT and the firewall.