Rogue DHCP Server

Unanswered Question
Jan 9th, 2008

The other day the network had an instance where someone plugged in a netgear router into a port on vlan X. This obviously started answering DHCP discoveries from clients with its offers.

My question is this...If the client attempted to get an address on a vlan where the helper address was pointing to the correct DHCP server on vlan Y, why did this rogue server cause a DOS on vlan X?

The lease time did not expire on the server, and the client typically remembers the address it used last

The DHCP servers are on vlan Y, is it because the rogue server was plugged into vlan x ( walong with the clients)which has closer proximity to the clients, obviously???

Thanks in advance!!!

I have this problem too.
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johnd2310 Thu, 01/10/2008 - 03:31

Hi,

Dhcp request is a broadcast and a dhcp client will accept the first offer it gets. Because the rouge dhcp server was on the same subnet as the client and it responded before the correct server, its addresses were accepted over the addresses from the correct server. When a client renews an address, it will sent a unicast request to the server since it now knows the address of the server and this is why your clients kept using the same rouge addresses.

To protect your network from this kind of thing, consider looking at features like dhcp snooping.

http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_22_ea2/configuration/guide/swdhcp82.html#wp1078853

thanks

John

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