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Re: Cable Modem [really responsible engineering]

  • From: Wojtek Zlobicki
  • Date: Tue Jun 26 21:46:27 2001


----- Original Message -----
From: "Chris Adams" <cmadams@hiwaay.net>
To: <nanog@merit.edu>
Sent: Tuesday, June 26, 2001 9:20 PM
Subject: Re: Cable Modem [really responsible engineering]


>
> Once upon a time, Miquel van Smoorenburg <miquels@cistron-office.nl> said:
> > When the BRAS requests config info when the circuit goes up (using
> > radius) or when it acts as a DHCP relay, it includes the VPI/VCI
> > of the ATM channel in the request. That means that you can assign
> > IP addresses based on the physical connection rather than the MAC
> > address, and this is what we do [well, will do soon anyway ;)]
>
> Okay, but how do you keep the end user from putting a different IP in
> their computer?  We use PPPoA for our "residential" DSL, but someone
> that works here lives outside our service area (small local telcos are
> all over this area), and just got DSL from his local telco/ISP, which
> uses 1483 bridging.  He has multiple computers, so he just picked
> another address, pinged it to see it wasn't in use at the moment, used
> it, and it worked just fine.

Access lists are one way to go :)  You dont get out unless we say so :)

>
> Also, how do you prevent the user from trying to forge someone else's
> IP address or even MAC address in outgoing packets?  Without protecting
> against forged packets, I don't see how to provide accountability when
> someone attacks.

How would anyone find out anothers MAC.  As long as you seperate each
customer into their own bridge group, there is no way for them to find
anothers MAC.  As for forging IP's not much you can do about that.  MAC
address access list.. do they exists ?

>
> DHCP or RADIUS (how did I know you used RADIUS :-) ) is fine for
> assigning things, but how do you _enforce_ those assignments?  I know
> how with PPPoA, but not with a bridged network (the same thing applies
> with cable modems).
>
> --
> Chris Adams <cmadams@hiwaay.net>
> Systems and Network Administrator - HiWAAY Internet Services
> I don't speak for anybody but myself - that's enough trouble.





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