Effectively Blocking DNS-over-HTTPS (DoH) with Pi-hole: A Comprehensive Guide

DNS-over-HTTPS (DoH) enhances privacy by encrypting DNS queries, making them harder to intercept. However, this encryption can also hinder the effectiveness of DNS-based ad blockers like Pi-hole. This comprehensive guide explores how to mitigate this challenge and ensure your Pi-hole continues to function optimally even when clients use DoH.

Understanding the Challenge: DoH and Pi-hole

Pi-hole works by intercepting DNS queries from your devices. With DoH, these queries are encrypted and sent directly to a DoH resolver (like Cloudflare's 1.1.1.1 or Google's 8.8.8.8), bypassing your Pi-hole entirely. This means ads and trackers aren't blocked, negating the primary function of your Pi-hole setup.

Strategies to Block or Mitigate DoH

Several strategies can be employed to address this issue, ranging from simple configuration changes to more involved network modifications. The optimal approach depends on your network setup and technical expertise.

1. Modifying Client DNS Settings (Least Effective)

This approach involves manually configuring devices to use your Pi-hole's IP address as their primary DNS server. While straightforward, it's often unreliable as many applications and operating systems automatically use DoH regardless of the manually configured DNS server. This method is generally ineffective for completely blocking DoH.

2. Using Pi-hole's Upstream DNS Resolver (Partial Effectiveness)

Configure your Pi-hole to use a DNS resolver that doesn't support DoH, or at least doesn't aggressively promote it. This will prevent your Pi-hole from initiating DoH queries. However, devices that stubbornly use a different upstream DoH resolver will still bypass your Pi-hole.

To do this: In your Pi-hole admin panel, navigate to the settings section and adjust your upstream DNS servers. Consider using a resolver known to have robust DoH blocking features.

3. Network-Level Blocking (Most Effective)

This is the most comprehensive solution. It involves blocking DoH traffic at your router or firewall level. This prevents devices from even attempting to connect to DoH resolvers. This approach requires deeper technical understanding and access to your router's configuration.

• Router-level configuration: Many modern routers allow you to block specific DNS ports (usually TCP/UDP port 443, the port used by DoH). Consult your router's manual for instructions on setting up firewall rules or DNS filtering.
• Firewall rules (iptables): If you're comfortable with command-line interfaces, you can use iptables (on Linux systems) to create custom firewall rules that block traffic to known DoH servers. This requires careful configuration and a good understanding of network security.

4. Using a DNSSEC Validator

Implementing DNSSEC validation on your Pi-hole adds another layer of security and integrity check. While not directly blocking DoH, it helps ensure the DNS responses your Pi-hole receives are authentic and haven't been tampered with. This enhances the overall security of your network even if DoH is in use.

Choosing the Right Approach

The best solution depends on your technical skills and the level of control you have over your network. If you're comfortable with command-line tools and router configuration, network-level blocking is the most effective method. If not, modifying your Pi-hole's upstream DNS servers and educating users about disabling DoH on their devices might be a more feasible approach.

Advanced Considerations

Consider these points for a more robust solution:

• Regular updates: Keep your Pi-hole, router firmware, and other network components up-to-date to benefit from security patches and improved DoH blocking capabilities.
• Monitoring: Regularly monitor your network traffic to identify any attempts to bypass your Pi-hole and adjust your blocking strategies as needed.
• User education: Inform users about the benefits of using your Pi-hole and how DoH can potentially compromise its effectiveness. Encourage them to disable DoH on their devices whenever possible.

By implementing these strategies, you can effectively mitigate the challenges posed by DoH and maintain the functionality of your Pi-hole, ensuring a safer and more controlled network environment.