Nortel BSR222, Configuration, Page: 165 / 451

Chapter 10 Firewalls 165
Nortel Business Secure Router 222 Configuration — Basics
3 The packet is inspected by a firewall rule to determine and record information
about the state of the packet's connection. This information is recorded in a
new state table entry created for the new connection. If there is not a firewall
rule for this packet and it is not an attack, the
Action for packets that don’t
match firewall rules field determines the action for this packet.
4
Based on the obtained state information, a firewall rule creates a temporary
access list entry that is inserted at the beginning of the WAN interface's
inbound extended access list. This temporary access list entry is designed to
permit inbound packets of the same connection as the outbound packet just
inspected.
5 The outbound packet is forwarded out through the interface.
6
Later, an inbound packet reaches the interface. This packet is part of the
connection previously established with the outbound packet. The inbound
packet is evaluated against the inbound access list, and is permitted because of
the temporary access list entry previously created.
7 The packet is inspected by a firewall rule, and the connection's state table
entry is updated as necessary. You can modify the inbound extended access
list temporary entries based on the updated state information, in order to
permit only packets that are valid for the current state of the connection.
8
Any additional inbound or outbound packets that belong to the connection are
inspected to update the state table entry and to modify the temporary inbound
access list entries as required, and are forwarded through the interface.
9 When the connection terminates or times out, the connection's state table
entry is deleted and the connection's temporary inbound access list entries are
deleted.
Stateful inspection and the Business Secure Router
Additional rules can be defined to extend or override the default rules. For
example, a rule can be created that will:
• Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the
LAN to the Internet.
• Allow certain types of traffic from the Internet to specific hosts on the LAN.
• Allow access to a Web server to everyone but competitors.