Charter ARP Leak
jra at baylink.com
Mon Dec 29 17:51:04 UTC 2014
----- Original Message -----
> From: "Brett Frankenberger" <rbf at rbfnet.com>
> On Mon, Dec 29, 2014 at 12:27:04PM -0500, Jay Ashworth wrote:
> > >
> > > Valdis, you are correct. What your seeing is caused by multiple IP
> > > blocks being assigned to the same CMTS interface.
> > Am I incorrect, though, in believing that ARP packets should only be
> > visible
> > within a broadcast domain,
> broadcast domain != subnet
Yeah; I didn't use the right term. That's why my networks are small. :-)
> > and that because of that, they should not be
> > being passed through a cablemodem attached to such a CMTS interface
> > unless
> > they're within the IP network in which that interface lives (which
> > is
> > probably not 0/0)?
> > This sounds like a firmware bug in either the CMTS or the
> > cablemodem.
> int ethernet 0/0
> ip address 10.0.0.1 255.255.0.0
> ip address 18.104.22.168 255.255.0.0 secondary
> ip address 22.214.171.124 255.255.0.0 secondary
> The broadcast domain will have ARP broadcasts for all three subnets.
> Doing it over a CMTS doesn't change that.
Ok. But the interface to which the cablemodem is attached, in the general
single-DHCP-IP case, is a /24, is it not?
The example Valdis posted had 5 or 6 different /24s from all over the v4
address space; that seems exceptionally sloppy routing...
I have seen ARP-traffic-not-for-me come through a cablemodem in the past as
well, but it was *uniformly* for the /24 in which my modem's address lived
Jay R. Ashworth Baylink jra at baylink.com
Designer The Things I Think RFC 2100
Ashworth & Associates http://www.bcp38.info 2000 Land Rover DII
St Petersburg FL USA BCP38: Ask For It By Name! +1 727 647 1274
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