Setting up a new firewall

(Traditional format)

Wed Oct 6 22:17:25 2004 Setting up a new firewall
Posted by Tony Lawrence
Search Keys: network|security

I was configuring a new firewall/router today that was replacing an existing system attached to a DSL line. As the new hardware would have a different MAC address, I reset the DSL modem so that it would lose its arp cache. To test, I tried pinging the DSL gateway address. Unfortunately, no response.

I double checked my work, didn't see anything wrong. Checked it again, power cycled this hardware and the DSL device. Still couldn't ping the gateway. Put it back on the old router, power cycled the DSL, and that could ping the gateway. I then removed all firewalls (very briefly!) and configured a machine to sit right on the DSL - it could ping the gateway IP address too, so it wasn't that the DSL wouldn't give up its MAC cache.

Hmmm. Hated to do it, but time to call the manufacturer. Reset everything once more, and walked through every detail with their tech rep. He couldn't see anything wrong either.

While he was muttering to himself, I idly typed "ping yahoo.com" and to my complete astonishment, got a response. "Hold on a sec", I said, and explained to the other tech what had just happened. I tried the gateway address again, still no response.

It took a few minutes more to figure this out, but what was happening was that the gateway ping was responding from a *different ip address*. The other firewall didn't think that was a problem, but this new system didn't feel that was acceptable, and had blocked the response - as it should. I got caught by it because my first instinct is to try the gateway - if the gateway IP address doesn't respond, normally there's no point in trying anything else. In this case, though, nothing was actually wrong, and if I had tried any other site earlier I would have known that.

Something to remember.

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