Comcast modem fun

Well, the Comcast guy showed up today and dropped off the modem self-installation kit. About two or three hours later and it still wasn’t working quite right.

Here is my setup. The modem is plugged into eth0 on a Linux box, and eth1 runs to a switch. Traffic routed out eth0 is masqueraded (also known as NATed). If the modem and Linux box are turned on then everything works fine. But if I bring eth0 down and back up, then some odd behavior begins. All traffic that originates on the Linux box behaves normally — I can use elinks to browse the web and irssi to chat on IRC without any problems. But any traffic that is masqueraded, meaning that it comes from another computer on my network, does not behave normally. The connection establishes and works for a split second and then is silent.

I’d suspect a routing problem on the Linux box, but tcpdump there confirms everything is working as it should. However, ifconfig reports RX errors on eth0. This makes no sense — traffic originating from the LAN side of the router box triggers receive errors on eth0. Unless I reboot the router and the modem. I have not observed this behavior with any other ISP or uplink switch.

Anyone have any theories?

6 Replies to “Comcast modem fun”

  1. First you would need to connect directly to the modem. It needs to pull info from our network. Once you can connect directly, then you may be able to introduce more hardware into the picture.

    1. I am unsure how much more directly I could be connected to the modem. Every box on my network works fine when plugged directly into the modem (if I reboot the modem first) but if any traffic is routed through any of those boxes, I start getting RX errors from the modem. Since this same configuration has worked with several other ISPs, I am reluctant to conclude that there is a hardware problem behind the modem; the most likely place seems to be on the modem itself or upstream.

      If you have any diagnostic direction I would appreciate it. I am running Debian Lenny on the box doing the routing. If you would like to continue this via email let me know and I will send you a quick message.

  2. The solution I went with for the moment is “buy a wireless router.” It seems to be happier than my server when attached to the modem. I’m still not sure exactly what the problem is, but my best guess is that it is some bizarre incompatibility between my network cards (both the same model) and the cable modem. Nothing else makes a whole lot of sense given the information I have.

    1. It’s a TRENDnet TEW-652BRP, one of the cheapest 802.11n routers I’ve seen. The only issue I’ve had with it so far is I can’t access the configuration wirelessly from my laptop. Wireshark says that the FIN packet that comes in to end the HTTP session has a bad checksum… repeatedly. So who knows what that’s about.

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