UDP Port 67 Explained

UDP Port 67 is a key component in modern network infrastructures, allowing vital automatic assignment of IP addresses and configurations to clients.

By using a simple broadcast/response protocol, the Bootstrap Protocol (BOOTP) and its successor DHCP dynamically assign addresses without needing human intervention. When a new device boots up on a network, UDP messages to and from port 67 occur behind the scenes to ensure it obtains the required networking parameters.

For administrators, this automation means easier scaling to large networks and less manual upkeep when adding new clients. For users, it enables a seamless “plug-and-play” experience.

The ubiquitous adoption of DHCP has cemented the importance of its foundational protocol and UDP port 67 in enabling dynamic network configuration.

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What is UDP Port 67?

UDP port 67 is used for Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP). Both BOOTP and DHCP allow network devices to obtain IP addresses and other parameters automatically.

Some key points about UDP port 67:

  • It is used by BOOTP and DHCP clients to communicate with BOOTP and DHCP servers
  • DHCP has largely replaced BOOTP, but the port usage remains the same
  • It allows devices to automatically obtain IP addresses and network configurations on boot-up
  • Communication occurs between clients and servers using UDP over port 67

Why is UDP Port 67 Important?

Automatic IP Addressing

UDP port 67 is central to automatic IP addressing services like DHCP. When a client device boots up, it broadcasts requests using UDP on port 67.

A DHCP server listens on port 67 and can assign the client an IP address, subnet mask, default gateway, DNS servers, and other network parameters. This automation makes life easier for network administrators.

Network Configuration

In addition to automatic IP addressing, DHCP provides other network configuration parameters to clients via communications on UDP 67. This includes the default domain name, NetBIOS name servers, NetBIOS scope, and more. Central management of these core network settings is achieved thanks to DHCP and UDP 67.

Ease of Administration

UDP port 67 allows network administrators to set up “plug and play” environments where devices can be added to the network without manual configuration. As long as a DHCP server listens on UDP 67, new hosts can boot up and automatically obtain configurations. This ease of administration is a key benefit of DHCP and UDP 67 in modern networks.

How Does Communication Over UDP Port 67 Work?

Broadcast Requests

When a DHCP client computer or device boots up, it broadcasts DHCP discover messages over UDP on port 67. This allows DHCP clients to locate DHCP servers available for service requests. The message contains unique client identifiers to allow DHCP servers to offer configurations.

Address Offers

DHCP servers receive the DHCP discover broadcasts on UDP 67. In response, the servers send a DHCP offer message back to the client. This offer contains an IP address along with other network configuration parameters. As the offer is sent directly to the requesting client IP, UDP port 68 is used for replies.

Address Requests

When the client receives offers from one or more DHCP servers, it chooses one and broadcasts a DHCP request message on UDP port 67. This formally requests the use of the offered IP address and network configuration. Other DHCP servers will see these requests and withdraw any other offers.

Configuration Acknowledgement

Finally, the DHCP server whose offer was accepted will send a DHCP acknowledgment message to the client using UDP port 68. This confirms the offered configuration can be used by the client. The client device can then continue the boot process using the auto-configured IP address and network parameters.

What Happens If UDP Port 67 is Blocked?

If a network has a DHCP server but UDP traffic on port 67 is blocked at any point, clients will be unable to complete DHCP requests. This means they will fail to obtain IP addresses automatically. Some problems seen if UDP 67 is blocked include:

Limited Connectivity

Clients may have limited or no network connectivity without assigned IP addresses. Although auto-assigned private addresses or link-local addresses may work for some connectivity, full network access will usually be impaired.

Manual Configuration Required

Without a working DHCP server, clients will need to be manually configured with static IP addresses. For large networks, this administrative overhead can be substantial.

Network Communication Issues

Many clients will be unable to communicate reliably beyond their local network segment. Routed connections rely on correct IP configuration, which DHCP would normally provide.

Increase Security Risks

Clients may use random IP addresses or accept old DHCP configurations from memory. This can make IP address management challenging and increase vulnerabilities.

Should UDP Port 67 be Blocked By Firewalls?

Generally No

For most networks, UDP port 67 should not be blocked at the firewall. As DHCP is critical for easy administration and network communication, port blocking would cause more problems than it solves.

When to Block?

However, there are some specific situations when blocking makes sense:

  • Networks with no DHCP server or DHCP functionality
  • Isolated network segments where no DHCP is required
  • Environments where static IP assignment is enforced as a security policy

In these cases, UDP traffic on port 67 serves no valid purpose. The port can be blocked at firewalls to reduce the network attack surface.

How Can UDP Port 67 be Secured?

Despite the importance of DHCP, the protocol has some inherent vulnerabilities on UDP ports 67 and 68. Some good practices to secure DHCP include:

Server Hardening

Apply security best practices to DHCP servers like patch management, access controls, and logging. As DHCP servers directly influence network configurations, extra protection here is wise.

IP Address Filters

Configure DHCP servers to only respond to discover messages from specific address ranges, network segments, or client MAC addresses. This limits the ability of rogue devices to obtain network access.

DHCP Snooping

On network switches, enable DHCP snooping. This adds a layer of security by tracking DHCP messaging and filtering requests on other ports.

Separate Helpers

Use DHCP relay agents and helpers to forward requests rather than allowing wider Layer 2 broadcast traffic between clients and servers. This reduces the exposure of DHCP to network attacks.

These measures can help restrict the open nature of DHCP and UDP ports 67 and 68. Security groups should consider protections based on their specific network architecture and risks.

UDP Port 67 and DHCP Future Usage

DHCP and the underlying UDP ports 67 and 68 continue to be widely used in modern networks. The automatic assignment of networking parameters remains essential as networks grow larger and more complex. As new protocols like IPv6 become more prevalent, updated versions of DHCP are seeing expanded usage as well.

For these reasons, it is unlikely UDP port 67 will decline in usage moving forward. DHCP helps reduce administrative overheads and complexity. The ongoing need for these efficiencies will see DHCP, BOOTP, and UDP port 67 remain core network technologies for the foreseeable future.


UDP port 67 has become essential to the self-configuring nature of modern networks. By enabling automatic delivery of dynamic IP addresses and other parameters, it eliminates much of the overhead needed to scale up networks. The set-and-forget automation has pushed manual configurations to the sidelines as DHCP rules the roost.

Critical networking protocols often persist over decades, which should ensure the need for UDP 67 will remain well into the future. Whether for existing IPv4 networks or new IPv6 implementations, DHCP and its device discovery process will continue to rely on the simple but important UDP 67 port.

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