Attacking our pc router at work

Michal Mertl michal.mertl at
Wed Apr 5 13:19:16 UTC 2006

Mark Jayson Alvarez wrote:
> Hi,

>    I have one question. What if I change my ip and mac address at the
>  same time to that of our pcrouter's ip and mac... Will this going to
>  kick out that router in our network, causing the rest of the entire
>  lan to be out of service?? No one's gonna caught me right?? Arpwatch
>  can only watch if an ip address has moved to another mac address but
>  not when both ip and mac has moved to another ip and mac... Do you
>  know any possible solution to this??  

Your question is off topic for this list.

Use inteligent switches (not hubs) and port security (you can allow only
a specific MAC address behind a switch port). You could also use static 
entries on the switch for some MAC addresses (entry on a switch is a MAC
address + port behind which the address can be found) but that isn't as
safe. An attacker can generate traffic with lots of source MAC
addresses. Every switch has limited memory to store the MAC addresses
and usually when the table is full it starts working as a hub. A
sophisticate attacker may still be able to contaminate end stations - if
he sends a gratuitous ARP reply to a station where he pretends he is the
router (changes the MAC address), he will receive the traffic for the
router and can also then make man-in-the-middle attacks (insert himself
into forwarding chain of the station).

More sophisticated solution is using 802.1x - port-based authentication
- a switch will only start forwarding traffic to you once you
authenticate and you of course shouldn't be able to authenticate as the

On FreeBSD you can disable ARP and/or create static ARP entries and it
will protect you a little but you also need to configure some protection
on the network infrastructure.

It's quite a complex issue to protect against this type of attack and I
am no real guru so please take what I said with a grain of salt.



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