Securing Your DNS Queries with Curl and DoH: A Comprehensive Guide

DNS-over-HTTPS (DoH) is a protocol that encrypts your DNS queries, enhancing your online privacy and security. Instead of sending your DNS requests in plain text (which can be intercepted and analyzed), DoH sends them over HTTPS, the same secure protocol used for browsing websites. This makes it much more difficult for third parties to monitor your online activity.

curl is a powerful command-line tool that can be used to interact with various network protocols, including HTTPS. This guide will show you how to use curl to query DNS servers using DoH, demonstrating the benefits and providing various examples.

Understanding DNS-over-HTTPS (DoH)

Traditionally, DNS queries are sent over UDP or TCP port 53. This leaves them vulnerable to eavesdropping and manipulation. DoH, on the other hand, leverages the HTTPS protocol (port 443), ensuring that your queries and responses are encrypted and protected from prying eyes. This provides several key advantages:

• Increased Privacy: Prevents ISPs and other network observers from seeing your DNS queries.
• Improved Security: Protects against DNS spoofing and other attacks.
• Better Performance: In some cases, DoH can be faster than traditional DNS because of the inherent benefits of HTTPS.

Using Curl with DoH

To use curl with DoH, you need the URL of a DoH-enabled DNS resolver. Many providers offer this service, including Google Public DNS, Cloudflare, and Quad9. The URL generally follows this pattern:

The query itself is sent as a POST request with the DNS query encoded in the request body. curl handles this encoding automatically.

Example using Google Public DNS

Let's query for the IP address of `google.com` using Google Public DNS with curl:

curl -X POST -H "Content-Type: application/dns-message" --data-binary "@query.bin" https://dns.google/resolve

In this command:

• -X POST specifies a POST request.
• -H "Content-Type: application/dns-message" sets the content type to indicate a DNS message.
• --data-binary "@query.bin" sends the DNS query from a file named `query.bin`. This file needs to be created separately containing the raw DNS query in binary format (more details on this below).
• https://dns.google/resolve is the Google Public DNS DoH endpoint.

Creating the DNS Query File (query.bin)

Creating the `query.bin` file requires specialized tools which are outside the scope of a simple curl example. Dedicated DNS tools or libraries are usually necessary for creating this properly formatted query. However, there are online tools available that can generate the binary query for a given domain name. You would then save the output to the `query.bin` file.

Alternative: Using a JSON-based DoH Endpoint

Several providers offer DoH endpoints that accept JSON requests, making it easier to use curl. Let's consider Cloudflare's DoH endpoint:

curl -s -H "Content-Type: application/json" -d '{"name":"google.com","type":"A"}' 'https://cloudflare-dns.com/dns-query' | jq .Answer

This command uses a JSON payload to specify the domain name (`google.com`) and the record type (`A` for IPv4 address). jq is a command-line JSON processor used to format the output for better readability. You need to have `jq` installed on your system to run this command.

Troubleshooting and Considerations

If you encounter issues, ensure that you have the correct DoH endpoint URL and that your curl command is correctly formatted. Check your network settings to make sure that no firewalls or proxies are blocking HTTPS traffic on port 443. Also, consider installing `jq` if you're using JSON-based DoH endpoints.

Remember to choose a reputable DoH provider to maintain your privacy and security.

This comprehensive guide provides a solid foundation for using curl with DoH. Experiment with different DoH providers and explore the advanced options available with curl to fine-tune your DNS queries.