Ip Rule Evaluation - D-Link DFL-1660 User Manual

3.6.2. IP Rule Evaluation

This description of traffic flow is an extremely simplified version of the full flow description found
in Section 1.3, "NetDefendOS State Engine Packet Flow".
For example, before the route lookup is done, NetDefendOS first checks that traffic from the source
network should, in fact, be arriving on the interface where it was received. This is done by
NetDefendOS performing a reverse route lookup which means that the routing tables are searched
for a route that indicates the network should be found on that interface.
This second route should logically exist if a connection is bi-directional and it must have a pair of
routes associated with it, one for each direction.
3.6.2. IP Rule Evaluation
When a new connection, such as a TCP/IP connection, is being established through the NetDefend
Firewall, the list of IP rules are evaluated from top to bottom until a rule that matches the parameters
of the new connection is found. The first matching rule's Action is then performed.
If the action allows it then the establishment of the new connection will go ahead. A new entry or
state representing the new connection will then be added to the NetDefendOS internal state table
which allows monitoring of opened and active connections passing through the NetDefend Firewall.
If the action is Drop or Reject then the new connection is refused.
Stateful Inspection
After initial rule evaluation of the opening connection, subsequent packets belonging to that
connection will not need to be evaluated individually against the rule set. Instead, a highly efficient
algorithm searches the state table for each packet to determine if it belongs to an established
connection.
Figure 3.3. Simplified NetDefendOS Traffic Flow
Tip: Rules in the wrong order sometimes cause problems
It is important to remember the principle that NetDefendOS searches the IP rules from
top to bottom, looking for the first matching rule.
If an IP rule seems to be ignored, check that some other rule above it is not being
triggered first.
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Chapter 3. Fundamentals