ZyXEL Communications G.SHDSL.bis 4-port Security Gateway P-793H - Manual (Page 311)

P-793H User’s Guide
Chapter 30 Filter Configuration 311
2 Enter 1 or 2 to open Menu 11.x - Remote Node Profile.
3 Go to the Edit Filter Sets field, press [SPACE BAR] to select Yes and press [ENTER].
4 This brings you to menu 11.1.4. Apply a filter set (our example filter set 3) as shown in
Figure 152 on page 276 .
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; i.e., the interface. The interface can be an Ethernet port or
any other hardware port. The following diagram illustrates this.
Figure 192 Protocol and Device Filter Sets
30.5 Firewall Versus Filters
Firewall configuration is discussed in Chapter 9 on page 131 . Further comparisons are also
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.
Note: If you do not activate the firewall, it is advisable to apply filters.