Background: 

 

This guide aims to provide a basic understanding of how DHCP works. And what you can do to troubleshoot if you experience a DHCP issue.

 

This Article Applies to:

 

All routers and switches with DHCP function.

 

Application Scenario:

 

 

Interaction:

 

Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to dynamically assign IP addresses to clients on an Internet Protocol (IP) network, simplifying the job for network administrators.

 

This process can be broken down into four main steps: Discover, Offer, Request, and Acknowledge.

 

1. Discover

 

When a client joins a new network, it doesn't know its IP address or any other network configuration details. So, it broadcasts a DHCP Discover message on the network, looking for available DHCP servers.

 

2. Offer

Upon receiving the client's Discover message, a DHCP server selects an IP address from its pool and sends an Offer message back to the client, which includes the IP address. If there are multiple DHCP servers on the network, the client might receive several Offer messages.

 

3. Request

The client chooses one of the offers and responds to the corresponding DHCP server with a Request message, asking to use the IP address provided. This request is also broadcasted, informing all other DHCP servers that the client has accepted an offer.

 

4. Acknowledge

Finally, the DHCP server sends an Acknowledge message to the client, confirming the assigned IP address and other configuration information, such as subnet mask, default gateway, and DNS server addresses. With this acknowledgment, the client can start using the assigned IP address.

This process not only automates the assignment of IP addresses but also greatly simplifies network configuration and management. Clients can request updates or changes to their IP addresses at any time without needing manual network settings configuration by network administrators. This is why DHCP is so important and widespread in modern networks.

 

Appendix:

 

Other status or messages you might see in DHCP interaction:

 

1. ACK (Acknowledgement)

The Acknowledgement (ACK) message is sent by the DHCP server to the client, confirming that an IP address and other configuration information have been successfully assigned or updated. This message marks the end of a successful DHCP interaction.

 

2. NAK (Negative Acknowledgement)

The Negative Acknowledgement (NAK) message is also sent by the DHCP server but is used to reject a client's request for an IP address. This typically occurs when the requested IP address is no longer available (for example, if it has been assigned to another client), or the client's lease has expired and cannot be renewed.

 

3. DECLINE

A client sends a DECLINE message to the server indicating that it has detected the offered IP address is already in use on the network. After receiving a DECLINE message, the DHCP server will mark that address as conflicting and will attempt to allocate another address.

 

4. RELEASE

When a client no longer needs the IP address assigned to it, it sends a RELEASE message to the DHCP server to relinquish the address. This typically occurs when a client is shutting down normally or leaving the network.

 

5. INFORM

Clients use an INFORM message to request local network configuration parameters without requesting an IP address. This is used by clients that already have an IP address (for example, through static configuration) but need other network information, such as the addresses of DNS servers.

 

 

Wireshark:

 

1. What does a basic interaction look like?

 

 

2. DHCP (Dynamic Host Configuration Protocol) uses the UDP protocol. The client-side uses port 68, while the server side uses port 67.

Server:

 

 

Client:

 

 

3. How DHCP Option reflect on the DHCP offer.

 

 

Note:

1. Since it is UDP, and with a specific port, you can block it by ACL. 

2. You should replug your Ethernet cable when you assign a new IP to it. This equals a reboot to your device.

3. The DHCP renew and release commands have different purposes:

• DHCP Renew: This command is used to request a renewal of the IP address lease from the DHCP server. When you run "ipconfig /renew" command in Windows Command Prompt, it sends a broadcast message to all available DHCP servers on the network, asking for an extension or renewal of the current lease. If successful, it will update the IP configuration with any changes provided by the DHCP server.
• DHCP Release: The "ipconfig /release" command is used to release or relinquish your current IP address lease back to the DHCP server. It informs the DHCP server that you no longer need that particular IP address and allows other devices on the network to use it if necessary.

 

Update Log:

 

Mar 11th, 2024:

Release of this KB.

 

Recommended Threads:

 

Troubleshooting Online Detection and Link Backup (Failover) Don't Take Effect

Troubleshooting Custom DDNS Is Not Working

 

Feedback:

 

• If this was helpful, welcome to give us Kudos by clicking the upward triangle below.
• If there is anything unclear in this solution post, please feel free to comment below.
• If you encounter such an issue, please follow the troubleshooting above to check your settings. Besides, ensure your Omada Controller and Gateway are running with the latest firmware.
• If the issue still exists after you try the suggestion above, please feel free to comment below or contact our support team with a detailed description of your issue and the steps you have tried.

 

Thank you in advance for your valuable feedback!

 

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