benweir.com

A few days ago I mistyped a URL that I entered in the Firefox address bar. Usually this would result in the usual "Server not found" error message from Firefox. But instead I got an ad-chocked web page served up by Comcast saying they couldn't find the url. More about this at Ars Technica and plenty of other sites via Google

At the top of Comcast's page is a link to opt-out of this "service". I visited the opt-out page and entered my name, contact info, and cable modem MAC address and submitted the info. This was on August 24. I got an email from them that day and clicked the link therein to confirm my request.

Today (eight days later) I got another email saying that the MAC address I had entered wasn't recognized and that my request was therefore unsuccessful. It turns out I had actually put the router's WAN interface MAC address, not that of the cable modem. My error. So I revisited the opt-out page to try again. It seems they had changed it in the past week and now it requires you to log in with your Comcast username in order to do anything.

The Comcast account at my house is in someone else's name. They're not technical at all. To try and get their username and password or try to get them to complete the opt-out process would be way too much trouble. So I initiated a chat with a Comcast support technician and ultimately got them to opt me out.

At the end of our chat they had me run "ipconfig /flushdns" which I pretended to do on my Linux box. At that point I suspected that the way the opt-out worked was to simply send my cable modem a different DNS server via DHCP. Before all of this, my DNS server was 68.87.76.182. Afterward it changed to 68.87.76.178. Sure enough, they're different. Let's take a look at what hostnames those IP addresses equate to:

[bweir@surge ~]$ host 68.87.76.182
182.76.87.68.in-addr.arpa domain name pointer cns.sanjose.ca.sanfran.comcast.net.
[bweir@surge ~]$ host 68.87.76.178
178.76.87.68.in-addr.arpa domain name pointer nrcns.sanjose.ca.sanfran.comcast.net.

A-ha! So the "domain helper" server is called "cns" and the regular undefiled name server is called "nrcns". This should make it really easy for any Comcast user to opt-out without having to go through Comcast customer service or using some third party DNS server. Simply find out what DNS server IP address Comcast is assigning you, reverse resolve it to a hostname, change "cns" to "nrcns", resolve that hostname back to an IP address, and then manually specify that new IP as your DNS server.

So to go from the "domain helper" name server to using a good one, the steps would look something like this:

[bweir@surge ~]$ cat /etc/resolv.conf 
search hsd1.ca.comcast.net
nameserver 68.87.76.182
[bweir@surge ~]$ host 68.87.76.182
182.76.87.68.in-addr.arpa domain name pointer cns.sanjose.ca.sanfran.comcast.net.
[bweir@surge ~]$ host nrcns.sanjose.ca.sanfran.comcast.net
nrcns.sanjose.ca.sanfran.comcast.net has address 68.87.76.178

Edit: I also found Comcast's DNS servers page as well as a discussion on the topic at dslreports.com