DNS-over-HTTPS (DoH) enhances privacy by encrypting DNS queries, making them harder to intercept. While beneficial for individual privacy, it can pose challenges for network administrators seeking to control and monitor internet traffic. This comprehensive guide details multiple methods to effectively block DoH within your OPNsense firewall, allowing you to maintain network security and visibility.
Blocking DoH isn't as simple as blocking a port or IP address. Because DoH uses standard HTTPS (port 443), blocking it directly would disrupt legitimate HTTPS traffic. Effective DoH blocking requires identifying and filtering DNS queries disguised within HTTPS traffic. This usually involves inspecting the content of HTTPS packets, which is computationally intensive and can impact performance.
The most effective and efficient method is to use a DNS forwarder that natively supports DoH blocking. Many modern DNS servers offer this functionality. Configure OPNsense to use this external DNS server as its primary DNS resolver. This approach offloads the computationally expensive task of DoH detection to the forwarder.
Advantages: Efficient, less resource intensive on OPNsense.
Disadvantages: Requires a compatible external DNS server and relies on the third-party server's accuracy in DoH identification.
OPNsense's integrated Suricata Intrusion Detection/Prevention System (IDS/IPS) can be configured to detect and block DoH traffic. This involves creating custom rules that identify DoH requests based on their content (e.g., specific DNS queries within HTTPS requests). This requires a deep understanding of Suricata rules and regular updates to stay ahead of evolving DoH techniques.
Advantages: Highly customizable, can block specific DoH providers.
Disadvantages: Complex to configure, requires significant expertise in Suricata rule writing and maintenance, potential for false positives, and potential performance impact.
While not a direct DoH blocker, DNSSEC validation can enhance security by verifying the authenticity of DNS responses. This can indirectly mitigate some risks associated with DoH by making it more difficult for attackers to inject malicious responses.
Advantages: Improves DNS security overall.
Disadvantages: Doesn't directly block DoH, requires compatible DNS servers and clients.
Some suggest modifying the OPNsense application-level gateway (ALG) settings for HTTPS. However, this method is generally unreliable and not recommended as a primary DoH blocking solution. Altering ALGs incorrectly can severely disrupt legitimate HTTPS connections.
Advantages: (None significant)
Disadvantages: Highly unreliable, prone to breaking legitimate HTTPS traffic, not a recommended method.
The best approach depends on your technical expertise and network requirements. For most users, utilizing a DNS forwarder with built-in DoH blocking is the most effective and manageable solution. For advanced users who require granular control and are comfortable with Suricata, using custom rules might be an option but requires careful planning and testing.
Blocking DoH effectively requires a multifaceted approach. Understanding the limitations and implications of each method is crucial. Prioritize network security and stability by carefully evaluating your options and testing thoroughly before deploying any changes to your OPNsense configuration.