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643 posts

Ultimate Geek


Topic # 11890 16-Feb-2007 12:06

it won't be news to most of you techies that ARP data from 172.16.0.0/12 leaks out of the Motorola cable modems on Telstra's network. I've always been happy to ignore this but a friend of mine told me an interesting story last night.

He recently replaced a dying linksys router with a 3com Office Connect one, very nice router. A couple of days later Telstra shut down his modem and refused to start his service again until he changed his local area network's IP addresses and subnet. He was using the RFC1918-defined private network address space of 172.16.0.0/12 also. So it would seem that the 3com router was smarter than the Linksys and seeing ARP data on it's external/WAN interface decided to proxy ARP between networks.

Spurious ARP data being allowed to enter private networks is a slight concern but the fact that his ARP data was allowed to enter and cause problems in the TCL network is a major concern. People could be poisioning Telstra's cache's and potentially sniffing other people's Internet connections, switched ethernet networks still are vulnerable and I'd like to know what Telstra's excuse for this huge vulnerability is.




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  Reply # 61010 16-Feb-2007 12:45
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This issue has been debated many times over the years and never seems to reach a resolution. I was one of a handful of people who got to trial the cable modem network around 10yrs ago now and it was something that the guys were talking about (not that I actually understood what they were really talking about back then!) because the cable modem is really just a ethernet bridge and you can snoop on all ARP traffic on the network.

Is this issue in particular only affecting 172.16.0.0/12?




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  Reply # 61011 16-Feb-2007 12:55

yes AFAIK it's the 172.16.0.0/12 traffic eminating from the modem causing the problem. the 3com isn't doing anything particularly wrong by proxying ARP it sees on both interfaces.

But surely they could VLAN the customer traffic seperately to the network/management traffic?




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  Reply # 61015 16-Feb-2007 13:49
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The amount of ARP traffic is fairly large as well.  I switched to a WRT54G router running DD-WRT to get some SNMP monitoring and while I use most of my 20 Gb cap, the amount of extra traffic seen by my router is well into 10 gb.




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  Reply # 61048 16-Feb-2007 20:17
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barf:  the 3com isn't doing anything particularly wrong by proxying ARP it sees on both interfaces.


What, sounds like a gaping security hole if that default behaviour.

I agree VLAN'ing off of management traffic is a sensible idea, I assume there is some technical reason for this not being implemented however as it seems blindingly obvious :)

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  Reply # 61085 17-Feb-2007 14:46
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Fraktul:
barf: the 3com isn't doing anything particularly wrong by proxying ARP it sees on both interfaces.


What, sounds like a gaping security hole if that default behaviour.


The router has a selectable option for 'Proxy ARP' that is not on by default, and I have never had it on.

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