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Firewall Rule

A configured statement that tells a firewall whether to allow or block specific network traffic, based on attributes like source and destination address, port, and protocol. Rules are how a firewall's policy is actually expressed.

A firewall enforces policy through an ordered set of rules, each matching traffic by criteria (source/destination IP, port, protocol, direction) and specifying allow or deny. Together, ideally built on a default-deny base, they define exactly what may cross a boundary. Understanding firewall rules is fundamental to segmentation, egress control, and reading why traffic was permitted or blocked during investigation.

Introduced in: Networking Foundations

Examples

  • A rule allowing inbound traffic to port 443 on a web server.
  • A default-deny base with explicit allow rules for required flows.
  • Blocking egress to an unapproved destination with a deny rule.

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