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It will be hard to find and block all downloading apps. If your concern is high data transfer, it is much easier to setup quotas for each user. I think. This will automatically prevent large downloads from being saved.
Wireshark lets you look at packets that pass through an ethernet port. You can see what the source and destination ports are, protocol, IPs etc. Set it up (it should be in one of the RH repos) and start the various applications you are concerned about on a computer attached to the server. You'll soon see ewhat ports the applications are requesting.
I fact, you could just install wireshark on a client and watch there. There are Windows and Linux versions.
The destination ports would be useless, as they will be the same ports anything else uses. You won't be able to differentiate between program Foo and program Bar by using destination ports. You could use source ports, but who's to say the programs limit themselves to a specific range which doesn't overlap with what your browser (for example) uses? This kind of stuff is simply not something iptables is a good choice for half the time.
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