[techtalk] MAC addr from Cable Modem

moebius at ip-solutions.net moebius at ip-solutions.net
Thu Jan 20 10:31:25 EST 2000

Hey All,
   I tried tcpdump with the appropriate flags and was 
able to see my gateway. It was responding to arp's from 
my IP. So, i thought that perhaps my modem was indeed my 
gateway and the @home people were just incorrect. I then 
portscanned the gateway (nmap) and found telnet open 
(23). I telnetted over and got a Unix like login prompt. 
I guess that this couldn't be my modem, I doubt that it 
has an OS to run login, so I'm guessing that the gateway 
is indeed on the cable company's side. I never read the 
full man page on tcpdump. There is a lot of options.

> >The idea with running tcpdump on the local section of 
the ISP's
> >network is that you can see the arp requests, which 
will be (roughly)
> >in the form of "who-is foo tell bar" with foo being 
an IP address and
> >bar being the MAC address of the requesting party.  

> Wouldn't this method show the MAC address of the 
ethernet card connected to
> the
> modem, not the modem itself?
> Your machine will ARP your gateway for outgoing 
packets not destined for
> the
> local network instead of the modem. And people who 
send packets to you are
> going to ARP on your IP, probably not the modem's IP, 
if it even has one...
> My tcpdump allows a -e switch which shows the source 
and destination MAC
> address for each packet, not just ARPs. In my case, I 
get the MAC address
> of my
> gateway instead of the modem though.
> Andy
> ************
> techtalk at linuxchix.org   http://www.linuxchix.org

Harry Hoffman
Product Systems Specialist
Restaurants Unlimited Inc.
(206) 634-3082 x. 270

techtalk at linuxchix.org   http://www.linuxchix.org

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