Andy Regan Ramblings of an Irish Sysadmin

Fix Mac OS X DNS Failure on UPC Router

About a week ago, I upgraded my MacBook Pro to OS X 10.6.4. Since then, I've had trouble with the internet connection *only* from my house and *only* on my MBP. Sometimes the connection would work fine, but most of the time Firefox/Safari would only load the first couple of pages and then new pages would start to time out.

Since I could ping/view a site by going to the server's IP address directly, the issue appeared to be with the MBP resolving DNS. DNS is how your computer translates a human readable URL such as andyregan.net to a machine readable IP address.

Since the problem was *only* with my UPC connection (router is Cisco EPC2425), I thought changing DNS provider to Google or OpenDNS might do the trick. Alas, the new DNS servers would work for the first few pages and then pages started timing out again.

After poking around the (extremely limited) options on the UPC router, I noticed that there were a ton of firewall blocks for the 192.168.1.0/24 network over the last week. Devices connecting to the router, such as my MBP, are assigned IP addresses within this range.

On closer inspection, I discovered that the router's firewall was blocking the MBP from the DNS servers for "LAN-side UDP Flood". I turned off the router's firewall and restarted it. Et voila! Bob's your uncle.

My guess is that the firewall blocks must be due to some change in DNS behavior since the last OS X update. Posting here to save some poor soul the heartache. It's important to note that turning off your router's firewall is at your own risk.