ARP poisoning troubleshooting

Posted on 2014-11-20
Last Modified: 2014-12-04
It looks like my network got attacked by ARP poisoning. I see spoofing MAC addresses. When I shut down one, another one po up somewhere else. I am not sure how to deal with this. Any thoughts will be greatly appreciated. Thanks
Question by:leblanc
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LVL 29

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 125 total points
ID: 40455448
Defence depends on options that are available on switch.
LVL 33

Expert Comment

by:Dave Howe
ID: 40456864
many switches allow you to shut down a port if more than a certain number of mac addresses are seen from it - that is the usual method (if you have it). Cisco call that "port security" and obviously other vendors call it something else :)

As Predrag says, we would need further info to give a more detailed answer.

Author Comment

ID: 40458013
Sorry, My network is all Juniper ex2200 and ex4200. So if I understand correctly, you can shut down some of the ports now to stop this issue? Thx
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LVL 33

Accepted Solution

Dave Howe earned 375 total points
ID: 40458122
Its a little more subtle than that.

As I read this page by setting a mac limit of (say) 5 you can say that any port on the switch will only accept the first five mac addresses it sees, and any mac addresses beyond those will be dropped (silently) and the action logged.

this means that you can both mitigate the extra macs and identify which port the spoofer is connected to, in order to track them down.

you can also hard-lock certain macs to certain ports if you wish - so that the port will only accept that one mac or list of macs - but that is considerable administrative overhead unless the attached nics are pretty static.

my reading is that, in config mode, at the ethernet-switching-options>>secure-access-port level, the command

set interface all mac-limit 5

would cause the mac limit to go to 5 and drop as the action (default)

you might first want to clear already learned macs with

clear ethernet-switching-table

to prevent any valid users being accidentally locked out by this move; you can also clear down individual ports with the same command (and a final argument of the port should you want to just release one port) should a sixth device (laptop, say) be added to a port legitimately in future.

Author Comment

ID: 40458941
When you set interface all mac-limit 5, will it set for all access ports? I have a stack of 4 48-port switches.
LVL 33

Assisted Solution

by:Dave Howe
Dave Howe earned 375 total points
ID: 40459391
the <all> designates the port on the switch (as in all of them) - you can substitute the actual port if you want to do this at a finer grained level (see the link I posted)
it will need doing per switch, and you should also set the switches to send their logs via syslog to a central machine so you can monitor them for the drop alerts.

Author Comment

ID: 40460830
That is a very good idea. I will try that. Now what if my access port is connected to an AP. There are a lot of hosts going through an AP. Correct? So how will I deal with that? Thanks
LVL 33

Assisted Solution

by:Dave Howe
Dave Howe earned 375 total points
ID: 40460864
normally you wouldn't do that with a trunk port, and a AP counts as a trunk (so, exclude the links between switches too)

if its an AP though, normally it will have its own mac table and so forth, and you will have to deal with that separately.

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