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New Member

How do you determine the DHCP server on a subnet

I'm trying to find the DHCP server on a subnet several time zones away (the local admins are in bed). There is no IP helper address configured on the ethernet interface. I am now doing a port scan on that subnet. Is there a more effiicient way to do this? "debug ip udp" does not seem to help because I only see the broadcasts from the clients. Any ideas? If this was a tcp port, I would be doing "telnet IP_address Port" in succession. Is there a similar trick for udp ports?

2 REPLIES
New Member

Re: How do you determine the DHCP server on a subnet

Two things come to mind. If there is no helper address on the interface and it works, the DHCP server must be local. I'm guessing that it doesn't work because you only see the broadcasts from the clients.

If it doesn't work, my best guess would be that it is the same DHCP server used for other subnets. Can you find out what that is?

I know its not much help, but its all I have!!

Cisco Employee

Re: How do you determine the DHCP server on a subnet

Depending on type of switch, you may be able to do a VACL and match against ports 67 and 68 to find the server.

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