What Is a Firewall Rule?
a Firewall Rule — a single instruction in a firewall's policy that permits or denies specific traffic based on criteria like source, destination, port and protocol.
How it works
Firewalls evaluate traffic against an ordered list of rules, top to bottom, acting on the first match. Each rule specifies conditions (e.g. "allow TCP from 10.0.0.0/24 to any on port 443") and an action (permit/deny). Good policy is specific and ends with a default deny — anything not explicitly allowed is blocked.
Why it matters
Firewall rules are how security policy becomes enforceable reality — and reading, writing and troubleshooting them is daily SOC and network-security work. The order-matters, first-match, default-deny logic mirrors ACLs and is a core security concept.
Frequently asked questions
What is a firewall rule?
A single instruction that permits or denies specific traffic based on criteria like source, destination, port and protocol.
How does a firewall process its rules?
Top to bottom, acting on the first rule that matches the traffic — so rule order matters, and a default deny typically sits at the end.
What is a default-deny rule?
A final catch-all that blocks any traffic not explicitly permitted by earlier rules — the secure baseline for firewall policy.
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