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Filters and Network Address Translation
156
BLACK BOX
®
Advanced Console Server
Filters and Network Address Translation
The Filter feature is available for firmware version 2.1.0 and above; the Network Address
Translation (NAT) feature is available for firmware version 2.1.1 and above.
Description
IP filtering consists of blocking or not the passage of IP packets, based on rules which
describe the characteristics of the packet, such as the contents of the IP header, the input/
output interface, or the protocol. This feature is used mainly in firewall applications, which
filter the packets which could crack the network system or generate unnecessary traffic in
the network.
Network Address Translation (NAT) allows the IP packets to be translated from local network
to global network, and vice-versa. This feature is particularly useful when there is demand for
more IP addresses in the local network than available as global IP addresses. In the BLACK
BOX
®
Advanced Console Server, this feature will be used mainly for clustering (one “Master”
Console server works as the interface between the global network and the “slave” Console
servers).
The BLACK BOX
®
Advanced Console Server uses the Linux utility
iptables
to set up, main-
tain and inspect both the filter and the NAT tables of IP packet rules in the Linux kernel.
Besides filtering or translating packets, the iptables utility is able to count the packets which
match a rule, and to create logs for specific rules.
Structure of the iptables
The iptables are structured in three levels: table, chain, and rule. A table can contain several
chains, and each chain can contain several rules.
Table
The table indicates how the iptables will work. There are currently three independent tables
supported by the iptables, but only two will be used:
Chain
Each table contains a number of built-in chains and may also contain user-defined chains. The
built-in chains will be called according to the type of packet. User-defined chains will be