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

What is DORA Process in DHCP?

DORA is a process which is used by DHCP in order to provide an IP address to hosts or client machine. 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 take a look at exactly what happens when DHCP client requests an IP address from DHCP Server.  There are some messages which 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 hence 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

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

DHCP Offer Message – The Dora Second Message

As soon as DHCP Server receives Client Discover message, the DHCP server reply to 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 at Network Layer only.

DHCP Request Message – The Dora Third Message

Now, the DHCP client machine receives the DHCP Offer message and replies with a DHCP request message. This message basically 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 has 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 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

• How to configure Palo Alto Networks Firewall as a DHCP Server
• What is the difference between TCP/IP and the OSI Model

References

• RFC – 2131

Summary

DORA is a sequence of messages of the DHCP process. The DHCP Server and DHCP Client exchanges some message and after that DHCP provide an IP address to DHCP client.

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