Blocking DNS-over-HTTPS (DoH) on pfSense: A Comprehensive Guide

DNS-over-HTTPS (DoH) offers enhanced privacy by encrypting DNS queries, but it can also present challenges for network administrators seeking to control and monitor DNS traffic. This guide provides a detailed explanation of how to effectively block DoH on your pfSense firewall, covering various methods and their implications.

Understanding the Need to Block DoH

While DoH improves privacy for individual users, it can hinder network security and management in several ways:

• Reduced Visibility: Encrypted DoH traffic is opaque to traditional network monitoring tools, making it difficult to track DNS queries and identify potential threats.
• Circumvention of DNS Filtering and Security Controls: DoH can bypass parental controls, content filtering, and other security measures implemented at the DNS level.
• Difficulty in Troubleshooting: Troubleshooting DNS-related issues becomes more challenging when DoH is used, as network administrators lack visibility into the encrypted DNS queries.
• Compliance Concerns: In some regulated environments, the lack of visibility into DNS traffic may violate compliance requirements.

Methods for Blocking DoH on pfSense

pfSense offers several methods to block DoH, each with its strengths and weaknesses:

1. Blocking Specific DoH Servers

This method involves explicitly blocking the IP addresses of known DoH servers used by popular browsers and applications. This is a relatively simple approach but requires constant updating as new DoH providers emerge. You can achieve this using pfSense's firewall rules. You'll need to identify the IP addresses of the DoH servers you want to block and create rules to drop or reject traffic to those addresses on port 443 (HTTPS).

Warning: This method is not foolproof as new DoH providers continually appear, and some might use dynamic IP addresses.

2. Using a DNS Forwarder with DoH Blocking Capabilities

Several DNS forwarders are designed to block DoH traffic. These forwarders act as an intermediary between your pfSense firewall and the clients. By configuring your pfSense to forward DNS requests to a DoH-blocking forwarder, you can effectively prevent clients from using DoH. Examples include some enterprise-grade DNS solutions.

This approach offers better control and is less prone to being bypassed than blocking individual servers.

3. Deep Packet Inspection (DPI)

More sophisticated firewalls with DPI capabilities can inspect the content of encrypted traffic (though this might require additional licenses or hardware) and identify DoH traffic based on its characteristics. This allows for more accurate blocking, even if the DoH server's IP address changes.

Warning: DPI can be resource-intensive and may impact overall network performance. It's crucial to properly configure DPI to avoid unintended consequences.

4. Application-Level Gateway (ALG)

While not directly designed for blocking DoH, correctly configuring the pfSense ALG settings for HTTPS might help in identifying and potentially influencing DoH traffic. However, it's not a reliable method for complete DoH blocking and is unlikely to be effective against newer DoH implementations.

Implementing DoH Blocking on pfSense (Example: Blocking Specific Servers)

Let's outline the process of blocking specific DoH servers using pfSense's firewall rules. This is a simplified example, and you'll need to adapt it to your specific network configuration.

1. Identify DoH server IP addresses: Research and find the IP addresses of the DoH servers you wish to block (e.g., Cloudflare's DoH server).
2. Navigate to Firewall > Rules: In your pfSense web interface, go to the Firewall > Rules section.
3. Create a new rule: Add a new rule at the top of the LAN or WAN ruleset (depending on where you want to block the traffic). Set the action to "block" or "reject".
4. Configure the rule: Specify the following:
   • Interface: LAN or WAN
   • Protocol: TCP
   • Destination port: 443
   • Destination: Enter the IP addresses of the DoH servers you identified.
5. Save the rule: Save your changes.

Conclusion

Blocking DoH on pfSense requires a thoughtful approach. The best method depends on your specific security requirements, network resources, and level of technical expertise. While completely blocking DoH may be challenging due to its constantly evolving nature, combining multiple techniques can significantly reduce its usage and improve your network's visibility and security.

Remember to regularly review and update your blocking rules to account for new DoH servers and evolving techniques.