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I ran an nmap scan against myself from a separate network. The results came back as this:
Not shown: 1675 filtered ports
PORT STATE SERVICE
80/tcp open http
1720/tcp open H.323/Q.931
3128/tcp open squid-http
6000/tcp closed X11
6001/tcp closed X11:1
6002/tcp closed X11:2
6003/tcp closed X11:3
6004/tcp closed X11:4
6005/tcp closed X11:5
6006/tcp closed X11:6
6007/tcp closed X11:7
6008/tcp closed X11:8
6009/tcp closed X11:9
6017/tcp closed xmail-ctrl
6050/tcp closed arcserve
Nmap finished: 1 IP address (1 host up) scanned in 25.000 seconds
I have a firewall up (ipcop) I would assume that it would block most of this from showing up to the outside world. Even before this, most of this showed up when I ran this. I thought perhaps it was the router or modem doing this? Router is out of the equation now. Any ideas?
What about these others, arcserv, xmail-ctrl, squid-http (from ipcop? I haven't set it up yet), H.323/Q.931 (what is this?)
Distribution: OpenBSD 4.6, OS X 10.6.2, CentOS 4 & 5
It could be that your ISP is filtering some ports to cut down on the traffic they receive and prevent their customer machines from being exploited. The best way to test a firewall is to plug a machine directly into it's external NIC with a crossover cable, or put a machine on the same switch as the external NIC, and scan from there. That way you can rule out ISP.