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Security Fundamentals & Crypto · Foundational

Packet-Filtering Firewall

Stateless firewall that allows or denies individual packets based on header information such as IP, port, and protocol.

Definition

Packet filters examine each packet independently at Layers 3 and 4 without tracking connection state. They are fast but cannot detect attacks spread across packets or verify that responses belong to legitimate sessions. Router ACLs and cloud network ACLs are stateless packet filters.

Example

An AWS network ACL must explicitly allow ephemeral return ports because it is stateless.

Exam tip

Stateless filters need rules for return traffic; stateful firewalls handle it automatically.

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