I am writing an article about securing servers within a LAN, and the importance of installing a firewall. I found myself wondering though what it is that a firewall actually blocks. I get that they obviously block connections to ports, but if there is nothing listening on that port, what is the risk?
Take for instance a LAN based web server with services (SSH & HTTP) listening on ports 22 and 80. What additional protection is the firewall offering by blocking connections to all other ports if there is nothing listening on those ports?
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