Blocking DNS over HTTPS (DoH): A Comprehensive Guide for Network Administrators and Home Users

DNS over HTTPS (DoH) offers improved privacy and security for DNS lookups by encrypting them over HTTPS. However, this encryption also presents challenges for network administrators who need to monitor and control internet traffic, implement security policies, and prevent malicious activity. This guide explores the reasons behind blocking DoH, the methods available to do so, and the potential implications of such actions.

Why Block DNS over HTTPS?

• Network Monitoring and Security: DoH encrypts DNS queries, making it difficult for network administrators to monitor traffic for malicious activity, parental control enforcement, or compliance with security policies. This lack of visibility can hinder the detection and prevention of threats like malware infections and phishing attacks.
• Content Filtering and Parental Controls: With DoH, standard methods of content filtering that rely on inspecting DNS queries become ineffective. This makes it challenging for parents to enforce safe internet browsing practices for their children or for organizations to block inappropriate websites.
• Data Loss Prevention (DLP): Organizations using DLP solutions may find their effectiveness compromised by DoH, as the encrypted DNS traffic prevents the inspection of domain names being accessed, potentially leading to sensitive data leakage.
• Legal Compliance: In some jurisdictions, network administrators are required to monitor and log internet traffic for compliance with legal requirements. DoH can make it difficult to fulfill these obligations.
• Corporate Security Policies: Companies may have security policies that prohibit employees from using specific DNS resolvers or accessing certain websites. DoH can circumvent these policies if not properly managed.

Methods for Blocking DNS over HTTPS

Several methods exist to block or mitigate the impact of DoH. The most effective approach often depends on the network environment and available resources.

1. Router-Level Blocking:

Many modern routers offer built-in DoH blocking capabilities. This method is generally the most effective and easiest to implement, as it blocks DoH traffic at the network perimeter. Check your router's documentation for specific instructions on how to enable DoH blocking. Look for settings related to DNS filtering or security.

2. Firewall Rules:

Network firewalls can be configured to block traffic to known DoH resolvers (e.g., Cloudflare's 1.1.1.1, Google's 8.8.8.8). This requires identifying the specific IP addresses or domain names used by these services and creating firewall rules to block outbound connections to them. This approach is less effective, as new DoH resolvers can emerge.

3. DNS Filtering Services:

Several commercial DNS filtering services offer DoH blocking capabilities as part of their broader security suite. These services often provide more sophisticated features like content filtering, threat intelligence, and reporting.

4. DPI (Deep Packet Inspection):

Advanced firewalls or security appliances with DPI capabilities can inspect the encrypted DoH traffic to identify DNS queries even when encrypted. However, this method is often resource-intensive and may not be feasible for smaller networks.

5. Client-Side Solutions (Less Effective):

While less effective than network-level blocking, some client-side solutions exist. These usually involve configuring the operating system or applications to use a specific DNS resolver that doesn't support DoH. However, this approach can be easily bypassed.

Implications of Blocking DoH

Blocking DoH can have unintended consequences. While it enhances network monitoring and control, it can also compromise user privacy and security by forcing users to rely on less secure DNS protocols. This may lead to DNS spoofing or other attacks if the alternative DNS infrastructure is not properly secured. It's essential to carefully consider these implications and balance the need for network control with user security and privacy.

Conclusion

The decision to block DoH is a complex one that requires careful consideration of the benefits and drawbacks. Network administrators should carefully evaluate their specific security needs and choose the most appropriate method for their environment. Remember to prioritize the security and privacy of your users while maintaining control over network traffic.