When sending DHCP requests, a client can indicate if it wants an answer in uni or broadcast, by setting the “broadcast flag”. How can I force my WIN client to set this flag to unicast?
This question is far beyond what I personally understand, so I’m again grateful that my friend Jim Boyce was willing to step in and help answer it. I know enough to know that DHCP is the Dynamic Host Configuration Protocol and that here’s what the informative DHCP Handbook web site says about the broadcast flag:
“The “broadcast flag”: DHCP includes a way in which client implementations unable to receive a packet with a specific IP address can ask the server or relay agent to use the broadcast IP address in the replies (a “flag” set by the client in the requests). The definition of DHCP states that implementations “should” honor this flag, but it doesn’t say they “must”. Some Microsoft TCP/IP implementations used this flag, which meant in practical terms, relay agents and servers had to implement it. A number of BOOTP-relay-agent implementations (e.g. in routers) handled DHCP just fine except for the need for this feature, thus they announced new versions stated to handle DHCP.”
Anyway, here’s Jim’s answer:
This isn’t so much a client issue as it is a server issue. In order for your Windows clients to receive DHCP responses by unicast rather than broadcast, you need to configure the DHCP server accordingly to allow clients to request a unicast response. To do so, you must modify the registry on the DHCP server (assuming a Windows-based DHCP server).
1. On the server, open the Registry Editor and navigate to the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer\Parameters.
2. If the IgnoreBroadcastFlag value does not exist in this key, create it as a DWORD value.
3. Set the value of IgnoreBroadcastFlag to to 1 to cause the server to ignore the client broadcast flag and always respond with multicast. Set the value of IgnoreBroadcastFlag to 0 to allow the clients to request unicast.
4. Close the Registry Editor and restart the DHCP server.
Jim and I both hope that’ll help you get things going properly on your network.