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BCM 4.0 Networking Configuration Guide
Appendix D
Stateful Packet Filters
Overview
BCM supports stateful packet filtering for IP protocols. Stateful packet filters monitor active
sessions and record session information such as IP addresses and port numbers. They maintain
state information for each flow Stateful filters use the state information to determine if a packet is
responding to an earlier request that has been validated by the rule set. If the packet is in response
to a previous request, the packet is treated in the same manner. It will either be blocked or allowed
through. Stateful packet filters protect networks against Internet attacks such as source spoofing,
where an attacker pretends to be a trusted user by using an IP address that is within the accepted
range of IP addresses of an internal network. Business Communications Manager 4.0 stateful
packet filtering validates that addresses coming from outside the network are valid outside
addresses. Stateful packet filters also protect networks from a denial-of-service attack, where an
attacker tries to block valid users from accessing a resource or a server.
Understanding Stateful IP Policy Filter Rules
The concept of stateful rules versus non-stateful rules is rather complex. The following describes
the IP firewall mechanisms used for the processing of inbound and outbound IP Policy filter rules
with the help of flowcharts. In addition, information on stateful session creation and timeout is
provided. Finally, to increase the understanding a set of examples is presented to the reader with
the help of packet sequence charts.
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