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I want to be able to check whether a particular port is open from outside world to a particular machine. But i wish to do this test from the same machine. For example i want to check if port 80 is open to a host called host.mydomain.com and perform this check from host.mydomain.com.
The problem here is that the machine will only have certain incoming and outgoing ports open. And i want to run a perl script on that machine to determine if the firewall rules created are working fine. Outgoing can be checked easily but how do i check incoming ports ?
I was thinking to spoof the src port to the required incoming port and then send a packet out to one of the allowed outgoing addresses. Then create a socket that is already listening on the incoming port. When the host replies it will come to my spoofed port where i would have one sock instance waiting to accept.
If thats possible ! Because the reply will not be the initial sync packet ?
AFAIK, you'll need some form of proxy to do this correctly.
It's easy to verify 1) an allow rule in a firewall ruleset; 2) a listening TCP socket. But the problem is a scan from a host to itself is more or less meaningless. (I don't think spoofing IPs is going to be a good test.)
I recommend either setting up a simple (external) scan host, or making use of a service like GRC's Shields Up.