filtering rule

A network access-control directive configured on a boundary device — such as a firewall or router — that specifies a set of condition attributes (protocol, source address, destination address, port, etc.) together with a decision (accept or deny) to be applied to traffic matching those conditions. It operationalizes an organization's information-flow control policy by restricting traffic between designated sources and destinations, enforced in boundary protection devices through rule sets that may provide packet-filtering capability based on header information or message-filtering capability based on content. Rules are evaluated sequentially against each incoming packet — the first matching rule's action is applied, and if no rule matches the packet is typically rejected — making rule ordering a security-critical property of the overall policy. Standards bodies such as NIST treat filtering rules for routers or firewalls as security-relevant information warranting protection in their own right, placing them alongside cryptographic key material and access control lists.

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