Saturday, July 24, 2010

Troubleshooting ATT DSL

There's been a lot of news lately about hacks of home/SOHO routers like Linksys and D-Link. Even though my router-and-firmware combo is on the 'ok' list (D-Link DIR-628 v2.22) I decided to upgrade it to the latest v2.24 firmware. Unfortunately this set off a series of mishaps that took me hours to resolve.

The instructions from D-Link warned that the upgrade would erase the configuration but that you could make a backup beforehand, then restore it afterwards. The upgrade itself went fine as did the restore of the configuration. But then my Internet connection would freeze after only a couple of minutes. I tried power-cycling both the router and the AT&T modem (a Westell modem/router) but the problem only got worse. Finally, I could no longer get the router to login anymore and so I called AT&T Tech Support.

They wanted to 'upsell' me to their 'Advanced Support Plus' department, i.e., paid support! I resisted this since I assumed the problem was on their end. The regular technician was very polite and tried resetting my password for me, but to no avail. Finally I acquiesced to paying for the advanced support, but then it was too late -- they'd closed for the night. At this point, the technician asked, "Did you say that the 'Internet' LED on your modem was lit?" Yes, it was. That was the turning point! I'd been assuming it was an 'activity' indicator when in fact it was a logged-in indicator, i.e., the modem was doing the PPPoE login instead of my router.

When I first received the service I had assumed that the built-in router was a NAT device. So I'd disabled the auto-login and switched the modem to 'Bridge' mode -- just act as a pure modem-to-ethernet device. At some point in the evening's testing the modem had done a full reset? Probably, AT&T had triggered this when they saw the link trying/failing to connect. Once we realized what had happened we were easily able to switch the modem back to bridge mode and I was back online.

The next day, unfortunately, the connection had failed again. I decided there must be some change in the way my D-Link router was performing the PPPoE login that was incompatible with AT&T's DSLAM. Or, maybe the modem just really really wanted to be the router :-)

At this point I decided to test my other assumption, that the built-in router was performing NAT. It wasn't! I switched the Westell device back to it's original router/PPPoE configuration and programmed it with my new/updated login. I also had to reconfigure my router to use a different IP range. But then I was successfully able to open a Bittorrent app and get full two-way communication, so it was clear I wasn't double-NAT'd.

In the long run this goes to show the value of relying on a vendor's default configuration as much as possible. I should have tried to use my router with AT&T's default modem configuration instead of assuming I had to work around it. If I had I never would have had any of these problems.

UPDATE 8/15/10 - I take it back: I am double-NAT'd and I am not able to get 2-way Bittorrent connections...