DHCP lease without computername

Question

_{Friday, August 24, 2012 1:53 PM}

Hi everyone!

I have a small problem I'd like to solve.
Lately a number of unknown devices has been filling up our DHCP scope, with leases without the computername, but with the MAC Address and the IP, of course.

Does anyone have an idea what kind of devices this could be?

I tried "ping -a IP" to several IP's

It gives me in some cases replies with no computername, in other words, some of the devices are still alive, at least when I pinged the ip.

The scope looks like this.

All replies (3)

_{Friday, August 24, 2012 2:36 PM ✅Answered}

Is this a simple dhcp server in a workgroup or is it a domain member dhcp? Do you have netbios enabled? You should be able to have name resolution within the local subnet through netbios.

Try to identify one of those hosts by mac addresses and decide wether they are legitimate clients or not.

_{Monday, August 27, 2012 8:06 AM ✅Answered}

Hi,

Thank you for the post.

The DHCP lease without hostname means these devices are not windows OS, you could verify it from ping reply TTL value.
Windows: 128
Linux: 64
Cisco: 255
Solaris: 255
https://supportforums.cisco.com/thread/139132

To find the device name, you could find the their name by MAC address in switches MAC tables.
To security your DHCP service, you could set 802.1x on switches ports or set up DHCP with MAC address filtering.
http://technet.microsoft.com/en-us/library/dd759190.aspx
http://blogs.technet.com/b/teamdhcp/archive/2007/10/03/dhcp-server-callout-dll-for-mac-address-based-filtering.aspx

If there are more inquiries on this issue, please feel free to let us know.

Regards

Rick Tan

TechNet Community Support

_{Friday, August 24, 2012 4:05 PM}

The server is part of a domain, and part of a 24 bits subnet, hosting DHCP for a lot! of other subnets.

The NETBIOS setting is set to Default (Use NETBIOS from the DHCP server)

The subnet with these "empty" DHCP leases are not part of the same subnet as the subnet of the DHCP

Freddy