Comcast Mangles FTP Traffic To Some Hosts

We recently ran into a problem where a customer became unable to access the FTP site of a partner. They had been accessing this FTP site regularly for years. About two weeks ago, it abruptly stopped working. When connecting via any FTP client in either Active and Passive modes, they were unable to login. The customer is using the Comcast provided router/modem with a static IP.

At first look, it appeared as if the partner was blocking them since we were able to connect from our office and from other locations. During our testing, the customer was able to connect to other FTP sites fine. We did find, however, that they were unable to connect to any FTP site hosted by Network Solutions. Their partner’s FTP site is also on Network Solutions.

Comcast replaced their modem/router and the problem persists. Here’s the kicker: We put a laptop behind their device with the static IP and it worked.

Comcast then switched the customer to a Dynamic IP, and it now works. At this point, I can only conclude that an upstream Comcast router on the static IP network is mangling FTP traffic to some hosts. I’ll update this post when I know more.