How does a firewall decide what gets in?
Network firewalls started as ordinary routers told to be picky, and a 1983 hacker film may have given them their name.
▶ Start the storyA firewall checks each packet against a list of rules and lets it through or throws it away. In computing, a firewall is a network security system that monitors and controls incoming and outgoing traffic based on configurable rules. It typically stands as a barrier between a , like your home, and an untrusted one, like the internet.
The rules look at things you already know from earlier in this path. A packet must match certain attributes defined in a rule to be allowed through. The first kind of firewall, the packet filter, kept an , with the default action set to . Packets could be filtered by source and destination address, by protocol, or by source and destination ports. Because most traffic used TCP or UDP with well-known ports, firewalls could tell web browsing from email or file transfers just by the port number.
Step 1: Packet arrives
From the untrusted network
Step 2: Check the rules
Addresses, protocol, ports
Step 3: Match: let it through
It reaches the
Step 4: No match:
The default action
Firewalls grew smarter. Second-generation firewalls also remembered which ports two addresses were using, so they could examine a whole conversation rather than single packets. Later ones, working at the , understand protocols such as DNS and HTTP, so they can spot an unwanted program hiding on a non-standard port, or an allowed protocol being abused.
The name itself is a borrowing. A firewall was originally a wall built to confine a fire within a line of adjacent buildings. Before it was used in real-life computing, the word appeared in the 1983 hacking film WarGames, which possibly inspired its later use.
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Recap
Firewalls moved from checking one packet's addresses and ports to remembering conversations and understanding applications.
💡 A trick to remember it · A firewall is a bouncer with a list: first he checks names and doors, then he remembers who's inside, then he listens to what they say.
Surprising fact · Early firewalls grew out of ordinary 1980s routers that were set to filter packets.
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No source, no claim. Every fact in this lesson (17 claims) cites at least one of these.