Filter Types And Nat; Firewall Versus Filters; Applying A Filter; Figure 197 Protocol And Device Filter Sets - ZyXEL Communications G.SHDSL.bis 4-port Security Gateway P-793H User Manual

Chapter 30 Filter Configuration
5 Press [ENTER] to confirm after you enter the set numbers and to leave menu 11.1.4.

30.4 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 ZyXEL Device 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 ZyXEL Device is
receiving and sending the packets; in other words the interface. The interface can be an
Ethernet port or any other hardware port. The following diagram illustrates this.

Figure 197 Protocol and Device Filter Sets

30.5 Firewall Versus Filters

Firewall configuration is discussed in
made between filtering, NAT and the firewall.

30.6 Applying a Filter

This section shows you where to apply the filter(s) after you design it (them). The ZyXEL
Device already has filters to prevent NetBIOS traffic from triggering calls, and block
incoming telnet, FTP and HTTP connections.
If you do not activate the firewall, it is advisable to apply filters.
306
Chapter 9 on page 129. Further comparisons are also
P-793H User's Guide