D-Link DFL-2500 User Guide - Page 129
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110 Chapter 14. IP Rules connection is found. The action of the rule is then carried out. If the action is Allow, the connection will be established and a state representing the connection is added to the firewall's internal state table. If the action is Drop, the new connection will be refused. First matching principle - If there are several matching rules, the first matching one decides what will happen to the connection. (Except for SAT rules, shown in Example.) Consecutive packets belonging to an existing connection will not need to be evaluated again. Instead, a highly optimized state-lookup algorithm will search the internal state table for an existing state representing the connection. This methodology is applied not only on TCP connections, but on UDP and ICMP traffic as well. Thus, the size of the firewall ruleset does not affect the throughput of the firewall. A rule is expressed in a definite form, consisting of two logical parts: the fields and the action. The subsections below explain the parameters of a rule that are available in D-Link firewalls. 14.1.1 Fields Fields are some pre-defined and reusable network objects, such as Addresses and Services, which are used by every rule for matching purpose. The following fields in the rule list are used by the firewall to check a packet in the traffic flow. All these filtering fields have to match the contents of a packet for any rule to trigger. ◦ Service: the protocol type that the packet must match. (Services are defined as logical objects before configuring the rules, see 8.2 Services ) ◦ Source Interface: one or a group of interfaces where the packet is received on the firewall. ◦ Source Network: the network that the source IP address of the packet matches. ◦ Destination Interface: one or a group of interfaces where the packet is aiming at. ◦ Destination Network: the network that the destination IP address of the packet matches. D-Link Firewalls User's Guide
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