[PLUG] Ubuntu And Comcast Digital Cable?

Sean Whitney sean.whitney at gmail.com
Sun Feb 11 23:53:58 UTC 2007



Aaron Ten Clay wrote:
> m0gely writes:
>> Keith Lofstrom wrote:
>>> Comcast looks for a specific MAC address for one machine on the
>>> customer end.
>>
>> It's the modem's MAC, not the machine behind it.

This is true and false.  The CMTS is looking for the MAC of the cable
modem for provisioning purposes.  However some CMTS's have feature sets
beyond this.  Cisco UBR's for example have a command called "cable
source verify" which limits the CPE (your computer) MAC to specific
MAC's the CMTS see's and associates with your Cable modem after it comes
online.  This was an attempt by the Cable industry to "limit" how many
computers that a customer could use in a house.

It's similar to the argument that the cable company wants to install
separate set top boxes for every tv in your house and charge you
accordingly.  They wanted to sell you modems for every computer in your
house.  I have seen cable customers with up to 5 cable modems in one
house online at the same time. It has taken a while for networking
concepts to penetrate the television model.  In the past several years
they have wised up about home networking and don't care so much about
what's behind the router.  Cable labs has been working on a project
called "Cablehome" which was an attempt to manage devices behind a NAT
device.  I think it's largely in the process of being abandoned.

One person in the cable industry's favorite saying was "We shouldn't
work so hard pissing off the customer."

In a previous life I wrote a perl script to log into Cisco CMTSs and
reset this value, so call center people could reset it for irate customers.

This also explains why "MAC cloning" is a feature seen on
firewall/routers that you can by for DSL/DOCSIS networks.

>> -- 
>> - m0gely
> 
> Actually, the configuration file Comcast provides customers' modems
> tells the modem to allow only one MAC address to access the modem. This
> means the first MAC address the modem sees is the only one it will talk to.

Cable modem config files don't do much MAC level limitations, there is
some basic port, source/destination address filtering available but not
much more.

> Some modems will offer private addresses (usually in the
> 192.168.100.0/24 range) to any other MAC addresses they see. Some modems
> will offer these addresses to any MAC address they see if it hasn't yet
> received the configuration file from Comcast (at bootup or if it loses
> sync for more than 120 seconds).
This is to allow techs (or users) to troubleshoot connectivity issues by
connecting to the modem default address of 192.168.100.1 which should
work for all modems.

My firewall is set up to route this out my outside interface, which
allows me to access my modem webpage even when it's fully functional.

Sean
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