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New Member

Cisco IOS Interface Blocked by IPv4 Packets


My routers are all affected by the recent IOS vulnerability. I have implement the access-list recommonded by Cisco on my Internet router. I also have a internal router but i have not apply any work around on this router as there are a lot of application/service within my LAN. If i were to implement the access-list, i will need to test all the service. As i will be having a major network migration in a month time, i was thinking to upgrde the IOS than.

How vulnerable is my internal router when i have the internet router put up with the recommanded access-list. How does the specifically crafted IPv4 packets work? Furthermore, my internal router is private ip address. Will the specifically crafted IPv4 packets slip throught my Internet router and reach my internal router since i already have the 1st level block and assuming that there won't be any internal DoS.



Re: Cisco IOS Interface Blocked by IPv4 Packets

Hi Lye,

Have you read the following Cisco Advisory document on IPv4 problem:


Hope this helps --

New Member

Re: Cisco IOS Interface Blocked by IPv4 Packets

The packet must have a destination address of the router. If your router is using non-routable addresses you are only vulnerable from the inside of your network. The access-lists only block seldom used protocols. If you are not sure if they are in use on your network create the access-list on the router and do not apply it to an interface, then after a few days type "show ip access-list" and look for hits on that access-list.