ITPEC FE Morning April 2023 Question 44
A packet filtering firewall is installed at the connection point of an internal network and the Internet. Which of the following is the appropriate combination of filtering rules that enables PCs on the internal network to access a web server on the Internet (port number 80)?
Explanation
Request from port 1024 or higher to 80, reply from 80 to 1024 or higher — a client talks from a temporary port to the server's well-known port, and the reply reverses the pair.
- •Request, PC → web server: the PC's OS picks an ephemeral source port (1024 or higher), and the destination port is 80, where the web server listens
- •Reply, web server → PC: the server answers from port 80 to the PC's ephemeral port (1024 or higher)
A packet filter needs both rules, otherwise the request leaves and the reply is dropped. Only option (c) has this pair.
Answer (c)
Why not others:
- •(a) — the reply row is right, but the request row has the ports reversed (source 80, destination 1024 or higher), so the request to the server's port 80 is blocked
- •(b) — both rows have source and destination ports swapped: the PC would send from port 80 and the server would reply to port 80
- •(d) — the request row is right, but the reply row goes from 1024 or higher to port 80, whereas the server replies from port 80 to the PC's ephemeral port
Key rule: In a client–server exchange the well-known port sits on the server's side in both directions: the destination in the request, the source in the reply.
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