How to detect and stop port scan from VPS node?

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http://www.linuxquestions.org – Hello, I'm running a HyperVM VPS node on CentOS 5.8, I got reported that some of VPS doing port scan to another network, also one of the reported IPs isn't assigned to any VPS and isn't pingable! Could you please guide me how to check the issue and prevent any hosted VPS from doing so? The report shows that all my IPs using port 80 and scanning the other network on port 1234 Thanks a lot, John (HowTos)