Security feature when connecting to ethernet cable?

I have a Mac OS X 10.5.8. that had never given me any trouble. I recently encountered problems connecting to the Internet via Ethernet cable, in that I could not connect when I booted the Mac up with the Ethernet cable connected. (Still can't.) What I can do is boot the Mac up, then connect the Ethernet cable, then it gives me a window asking whether I will deny/allow the application "configd" to accept incoming network connections. If I allow it, it requires my admin login, then requires permission to allow a DNS connect, and only then can I finally get to the Internet. I had been using this Mac for nearly two years regularly connecting to the Internet over Ethernet with the cable plugged in (in addition to using AirPort in some locations) and had never encountered this or any other problem. This showed up after I had logged on to my employer's new secure wifi system. Any words of wisdom on a) what happened in my system to cause this and b) whether I can undo it? (I have no intention of logging into the secure wifi again, it that is the cause of the problem.)

Thanks!
Tamara

This can happen if there's some corruption in the firewall settings. This thread details the process to reset to factory defaults. That should do the trick.