DORA Process in DHCP - Explained in detail

DORA is a process used by DHCP (Dynamic Host Configuration Protocol). DHCP provides an automatic IP address to Hosts that want to connect to a network. In this article, we will discuss the DORA process in detail.

What is the DORA Process in DHCP?

DORA is a process that DHCP uses to provide an IP address to hosts or client machines. The DORA process has four messages.

Discover
Offer
Request
Acknowledgment

Now, take a look at the below diagram. This diagram will show you how the client and server exchange these messages.

Now, we will look at exactly what happens when a DHCP client requests an IP address from DHCP Server. Some messages are exchanged between the DHCP Server and Client. These messages are explained below.

Keypoint: All DHCP Messages are broadcast at Network Layer (i.e Layer 3).

DHCP Discover Message – The Dora First Message

Discover Message is the first message of the DORA process. In this message, the DHCP client wants to discover a DHCP Server and sends a DHCP Discover message. Different fields of Discover message are :

Source IP: 0.0.0.0

Destination IP: 255.255.255.255

Source MAC: DHCP Client Machine MAC Address

Destination MAC: FF:FF:FF:FF:FF:FF

So, we can say that the DHCP Discover message is always broadcast at the network and data link layer.

DHCP Offer Message – The Dora Second Message

As soon as the DHCP Server receives the Client Discover message, the DHCP server replies to the DHCP client as an offer message.

Source IP: DHCP Server IP Address

Destination IP: 255.255.255.255

Source MAC: DHCP Server Machine MAC Address

Destination MAC: DHCP client MAC Address

Now, as you can see, still the Destination IP address in the DHCP Offer Message header has a broadcast IP address. This is because the client didn’t get an IP address from DHCP Server. But, this time, this message is broadcast on Network Layer only.

DHCP Request Message – The Dora Third Message

The DHCP client machine receives the DHCP Offer message and replies with a DHCP request message. This message tells the DHCP server that I’m fine with this IP address. Please allocate this IP address to me. The DHCP Request Message header has the following main fields:

Source IP: 0.0.0.0

Destination IP: 255.255.255.255

Source MAC: DHCP Client Machine MAC Address

Destination MAC: DHCP Server MAC Address

Now, as you can see, still Client Source IP address is 0.0.0.0. This is because the client still didn’t get an IP address from DHCP Server. Also, this message is broadcast only at the network layer of the OSI Model.

DHCP Acknowledge Message – The Dora Fourth Message

DHCP acknowledge message is the last message of the DORA process. It is sent by the DHCP Server to the DHCP Client. This message is a reply to the DHCP Request message. This message header has the following main fields:

Source IP: DHCP Server IP Address

Destination IP: 255.255.255.255

Source MAC: DHCP Server Machine MAC Address

Destination MAC: DHCP client MAC Address

After this message, the DHCP client will get an IP address. This message has broadcast at the network layer but unicast on the Data Link layer.

Understanding DORA Using Video Demonstration

Continue Reading

References

Summary

DORA is a sequence of messages of the DHCP process. The DHCP Server and DHCP Client exchange some messages and after that, DHCP provides an IP address to the DHCP client.