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Old 03-16-2009, 12:32 AM   #1
Registered: Apr 2004
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scan network for DHCP server requests in real-time

I would like to scan my entire network (in real-time) to see which IP is making DHCP request to which DHCP server. The idea is to see if there is any rogue DHCP server on the network.

I have a server which can see all my vlan (it's a sniffer box) and from there I can do: tcpdump -i eth0 -n port 67 and port 68. But this command doesn't show which DHCP server it's accessing. I'm getting:

<date> IP <IP_ADDRESS> > : BOOTP/DHCP, Request from <MAC_ADDRESS>, length 300 - doesn't tell me the DHCP server name - just a broadcast packet.

Thanks for any help.
Old 03-17-2009, 12:16 PM   #2
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This is maybe over my head, but I think you can use nmap to discover dhcp servers. Try something like:
root@vaio:~# nmap -sU -p 67-68

Starting Nmap 4.53 ( ) at 2009-03-17 16:59 GMT
Interesting ports on (
67/udp open|filtered dhcps
68/udp closed        dhcpc
MAC Address: 00:18:4D:B9:DA:44 (Netgear)

Interesting ports on (
67/udp closed dhcps
68/udp closed dhcpc
MAC Address: 00:15:AF:6A:1D:F3 (AzureWave Technologies)

Interesting ports on (
67/udp closed dhcps
68/udp closed dhcpc

Nmap done: 255 IP addresses (3 hosts up) scanned in 6.944 seconds



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