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I have a problem but not sure where it originates...

I've set up a very small business network and they've got an old Belkin cable router (F5D7231-4) that connects to the building LAN. The building LAN is provided by the landlord and other businesses within the building use the network also. We've basically created a small LAN inside a bigger LAN. We have no access to the building LAN configuration. It's either use it or don't use it.

The router is configured to obtain its public IP by DHCP.

The LAN side of the router has the DHCP server disabled (as we have a Windows server with DHCP server configured - 192.168.100.x range).

The router is also used to provide wifi to the office so company laptops can access the server resources.

Everything works fine sometimes but we're experiencing a problem where the Windows Server DHCP service keeps stopping because another DHCP server is on the network. The network being picked up by devices is the building network (10.10.50.x range) because we see the Windows network profile show "propertylink.local" rather than our "ISA.local".

My question is: Is it possible that a SoHo cable router (the Belkin that they've provided) could be letting through DHCP requests via the WAN port - and thus allowing client computers to obtain incorrect IP addresses?

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By all means it is possible. It is called a dhcrelay, and it does exactly what its name states: it relays DHCP and BOOTP requests from a subnet where a DHCP server does not exist to another subnet where such server does exist.

I am a bit surprised, though, that you router's GUI, old though it may be, does not have an explicit entry to enable/disable the dhcrelay agent. Are you sure you looked carefully?

At any rate, a partial solution might be to make sure communications on ports UDP 67-68 are blocked on the router, for instance by routing ports 67 and 68 to a non-existent IP address.

The above is however a hack, because it does not address the problem (why do you have the dhcrelay enabled?), and because some parts of the DHCP communication cannot be blocked because DHCP servers often use raw packets, which do not use ports. It may however do for some finite time to restore communications on your own LAN, as you search for ways to disable the dhcrelay agent.

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  • I did wonder if it was DHCP relay... but I would have thought that the WAN port wouldn't allow this... The router is pretty old... an F5D7231-4 and doesn't have very many features - certainly not anything regarding DHCp relay... oh well, looks like they'll be investing in a better router. Thanks MariusMatutiae.
    – Kinnectus
    Feb 15, 2016 at 11:29

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