DHCP relay binding

Question

As per my knowledge, the Relay agent does the job of forwarding the DHCPDISCOVER and DHCPREQUEST from the Client to the Server and DHCPOFFER and DHCPACK from the Server to the Client. But it not directly forwards it, but makes some changes in the packet by changing the Source IP to itself. Now my question would be,

1)Does the Relay gets BOUND to the Server and then forward the packet to the Client?

2)And once the Client gets the IP, does it get BOUND to the Server as well as the Relay?

Im so confused with the Relay Bindings. Please help me on this.

Answer 1

DHCP relaying is no hack, it's specified in RFC 3046. When forwarding, the agent puts its own IP address in the GIADDR field, so the DHCP server knows that a gateway is required.

The DHCP lease is associated with the client's MAC address. The gateway's MAC is only used for hop-to-hop delivery.

Answer 2

There's an additional field in the DHCP header to indicate the IP address of the relay agent. This will be the address of the interface upon which the router/relay agent receives a DHCPDISCOVER from a client. More importantly, this address informs the DHCP server of which DHCP pool it should offer an address from.

The DHCP address for the client is not bound to the relay agent as well -- the DHCP server is only using its knowledge of the relay to supply the correct address. Take a look at the diagram here as well, depicting the client/relay/server relationship.

Note the MAC address of our "host":

Host#show int gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is iGbE, address is 0c79.14cc.1500 (bia 0c79.14cc.1500)

This is the DHCPDISCOVER being relayed, with the client MAC address and relay agent IP address:

And this is the DHCPOFFER as it returns to the host:

So yes, the relay does make a change to the packet by inserting its address in the relay agent field, but that isn't part of the binding issued by the server. The only binding that exists is between the DHCP server and the client.

The relay does not need to remember any associations as it's simply broadcasting replies (OFFER/ACK) from the DHCP server onto the appropriate connected network, where the clients reside. (Which it can determine via seeing which relay address is specified.)

Also for future reference, when I get stuck on understanding exactly what a protocol is doing, I find it's best to configure it in GNS3 and do a few packet captures. Then reference what you see with what you're reading.