While trying to diagnose a network access issue (I wanted to be able to access my PowerMac G4 when not at home and it wasn't working), I used the Network Utility to scan the open ports on one of the Macs on my network. In doing so I discovered what I assume is an odd problem.
Each of my Macs is running 10.4.1. Each Mac has a manually configured IP address in Network preferences. Each Mac has OS X's built-in firewall enabled. Each Mac has Personal File Sharing, iChat Bonjour, iTunes Music Sharing, iPhoto Bonjour Sharing, and port 8100 open in the Sharing preferences.
I use a graphite AirPort base station.
All works well under this configuration. Each Mac can go on-line, access the Internet, exchange e-mail, and can exchange/share files over the network.
Anyway, using the Network Utility on one of my PowerBooks, I did a Port Scan on my PowerMac G4. Under the above settings, Port Scan ran for about 20 minutes before stopping. When it did stop, it found no ports open.
Next, I did a Port Scan for a specific range of ports. I made sure the service that used the port range was active. I then scanned for those open ports on my PowerMac G4. Again, after about 20 minutes, the port scan found no open ports.
At this point, I disabled the built-in firewall on my PowerMac G4. This time when I ran Port Scan, it discovered all kinds of open ports on the PowerMac.
Is this behavior odd? Shouldn't enabling sharing services in the built-in firewall allow the ports in question to be seen as open during a port scan?
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