DHCP – Relay Agent for Windows Server 2003
Instead of deploying a DHCP server on every subnet, discover how to install and configure a DHCP relay agent. By far the hardest part of mastering the DHCP relay agent is installing it. Once you have found the relay agent, configuring it to listen for DHCPDiscover packets, is the proverbial piece of cake.
DHCP Relay Agent – Concept
DHCPDiscover packets, like all broadcasts, cannot pass across routers. In fact that was a lie, if you have a modern Router which is RFC 1542 compliant, then you can forward the DHCPDiscover packets to a DHCP server in a different subnet. In this instance, the Router acts as a Relay Agent.
Relay Agent – Installation
It is rare for Microsoft to remove functionality, but while NT 4.0 Workstations could act as DHCP Relay agents, XP and W2K Pro cannot. So you need to install the relay agent on a Windows Server 2003.
What is not obvious is where you find the relay agent, the answer is in Routing and Remote Access. When you think about it, the relay agent is a type of router, hence the RRAS location to install and configure the DHCP Relay agent makes sense.
As I say once you find and install the Relay Agent, configuring is easy, all you need to do is tell the router or DHCP relay agent the IP address of the real DHCP servers. Just right click the DHCP Relay Agent, and then select properties from the shortcut menu.
Trap: you forget to add an interface. See that ‘ISP’ interface in the screen shot is Enabled.
Relay Agent – In action
Let us turn this tutorial to see how the Relay Agent works. What happens is the Relay agent intercepts DHCPDiscover packets from clients and then unicasts to the DHCP on their behalf. The secret of successful relaying is to create the appropriate scope on the DHCP server. The first time it worked I thought that it was a miracle that the client got the correct IP address. On reflection, I realized that the Relay Agent adds the Source IP address when it contacts DHCP. So now I understand how the server knows, from its list of scopes, which subnet to offer an IP address.
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