Securing Your Network with Opnsense and DNS over HTTPS (DoH)

DNS over HTTPS (DoH) is a method of encrypting DNS queries, enhancing privacy and security by preventing eavesdropping and manipulation of your DNS traffic. This guide explains how to configure DoH on your Opnsense firewall for improved network protection.

Why Use DNS over HTTPS?

Traditional DNS queries are sent in plain text, making them vulnerable to various attacks:

• DNS Spoofing/Cache Poisoning: Attackers can redirect your traffic to malicious websites by manipulating DNS responses.
• DNS Snooping: Your ISP or other network observers can see which websites you visit.
• Man-in-the-Middle (MITM) Attacks: Attackers can intercept and modify DNS queries to redirect your traffic.

DoH mitigates these risks by encrypting DNS queries and responses using HTTPS, the same protocol used for secure web browsing. This ensures confidentiality and integrity of your DNS traffic.

Configuring DoH on Opnsense

Opnsense offers several ways to implement DoH. The most common approach involves using a DNS resolver that supports DoH, such as Cloudflare's 1.1.1.1 or Google Public DNS. You can configure this at the network level or on a per-interface basis. We'll outline the network-wide configuration here, which is generally preferred for simplicity and consistency.

Step 1: Accessing the Opnsense Web Interface

Log into your Opnsense web interface using your administrative credentials. The address will typically be the IP address of your Opnsense box.

Step 2: Navigating to DNS Resolver Settings

Go to Services > DNS Resolver.

Step 3: Enabling and Configuring DoH

In the DNS Resolver settings, you'll find options to enable DoH. Enable the feature and specify a DoH server. For example, for Cloudflare's 1.1.1.1, use the following URL: https://cloudflare-dns.com/dns-query. You may also choose alternative providers like Google Public DNS (https://dns.google/resolve). Consider your preference for privacy and security when selecting a provider.

Step 4: Advanced Settings (Optional)

You may wish to explore the advanced settings, including specifying a specific port if necessary (although the default is typically sufficient), and possibly setting up a fallback DNS server in case the primary DoH server is unavailable.

Step 5: Testing your configuration

After making changes, save the configuration. You can test your DoH configuration using tools such as dig +trace example.com @ from your local network and observing that the query uses HTTPS.

Step 6: Client Configuration (if necessary)

While the Opnsense configuration handles DoH for traffic passing through it, some clients may require additional configuration to send DNS traffic through the Opnsense box. For instance, you might need to specify Opnsense's IP address as the DNS server in your client's network settings. Consult your client's documentation for specific instructions.

Troubleshooting

If you encounter issues, check the following:

• Verify that you have correctly entered the DoH server URL.
• Ensure that your Opnsense firewall allows traffic to the specified DoH server's port (typically port 443).
• Check the Opnsense logs for any errors related to the DNS resolver.
• Restart the DNS resolver service after making changes.

Conclusion

Implementing DoH on your Opnsense firewall provides a significant boost to your network's privacy and security. By encrypting DNS traffic, you protect yourself against various attacks and ensure that your internet activity remains confidential. While this guide focused on network-wide configuration, remember to adapt these steps based on your specific network needs and client requirements.