Q: What are the functions of a firewall?
Every process on a computer that communicates with another computer (file sharing, printing, web server, etc.) communicates through a logical port. These ports are numbered 1–65,535. For example, when you open a browser page to Quora, your browser goes to the server named www within the Quora domain, and knocks on port 80 (websites typically communicate through port 80), when answered by the quora server software, it will send what will become the webpage within your browser.
To communicate, the receiving computer (in this case, www at quora) must meet the following requirements:
- The designated port must be “open” (available to accept incoming packets)
- There must be an application that is listening on that port (in the case of websites, this is often Apache)
- The communication coming into the port must be in a language that is understood by the application.
A firewall can have several functions:
- Close down a port completely
- Open a port to only specific IP addresses
- Block (blacklist) specific IP addresses
- Keep ports closed, except when “invited” in by the user. This is called Stateful Packet Inspection. If the user “invites” another computer, that computer can come in through a port specified by the user. More specifically, the user launches an application, the application reaches out to a server, the server responds and is allowed past the firewall.
- Open ports only during specific time frames
- Many firewalls include VPN services as well, allowing encrypted communication with designated users from outside of the local area network