Detecting DHCP servers..

[dawks]

A tech that works on our VoIP system claims there is a rogue DHCP server somewhere on our network causing issues with our phones. I've checked all the possible places they might be, and ran a tool called DHCPloc.exe from MSFT, and all it finds is our proper DHCP server. Nothing else. Is there another Windows based tool I can use to look for rogue DHCP servers?

 

[ebaycj]

turn off your proper dhcp for a minute, then run it.

 

[spyordie007]

What I would do:
1. Get the IP of the rogue DHCP server from your IP tech (any device/client that picks up an address of it should know).
2. Reverse that IP to a MAC.
3. Find out what port that MAC is connected to and shut it off.

As for using the DHCPloc.exe tool; verify you are running it on the correct VLAN (presumably it's sitting on the Voice VLAN if it's causing issues w/ the phones). Running it on your data VLAN wont do any good 😉

 

[Nothinman]

A tech that works on our VoIP system claims there is a rogue DHCP server somewhere on our network causing issues with our phones.

He told you it's a DHCP server but didn't give you the IP of the server?

 

[dawks]

Originally posted by: Nothinman

A tech that works on our VoIP system claims there is a rogue DHCP server somewhere on our network causing issues with our phones.

He told you it's a DHCP server but didn't give you the IP of the server?

When our VoIP phones boot, they need to get an IP address from a DHCP server. Some of them take along time, and often have an error "Option 128 missing". He believes this is because there is some random DHCP server out there. I think its just a configuration problem or a bug with Win2K server. I'm just looking for a tool to detect a rogue DHCP server. I'm sure thats NOT the problem, but he won't listen till I can prove it.

 

[spyordie007]

Cisco phones?

Look at the network configuration and you'll see the IP of the DHCP server they got an address from.

 

[Nothinman]

Yea, any device worth it's salt should tell you the IP of the DHCP server that gave it the lease.

 

[Zugzwang152]

i like the idea of unplugging the real DHCP server... but I'm no networking guy 😛

 

[Nothinman]

i like the idea of unplugging the real DHCP server... but I'm no networking guy

But you have to find it first and that's what we were talking about.