Using Curl with DNS over HTTPS (DoH): A Comprehensive Guide

DNS over HTTPS (DoH) enhances your DNS queries by encrypting them over HTTPS, improving privacy and security. This guide demonstrates how to leverage the power of Curl, a versatile command-line tool, to interact with DoH resolvers.

Understanding DNS over HTTPS

Traditionally, DNS queries are sent over UDP or TCP, leaving them vulnerable to eavesdropping and manipulation. DoH encapsulates these queries within HTTPS requests, providing several advantages:

• Privacy: Your DNS queries are encrypted, preventing your ISP or other network observers from seeing which websites you're accessing.
• Security: Protects against DNS spoofing and cache poisoning attacks.
• Performance: Can potentially improve performance in certain scenarios, though this is not always guaranteed.

Using Curl with DoH

Curl's flexibility allows you to easily interact with DoH resolvers. The key is using the appropriate URL structure and HTTP headers. The general format is:

curl -X POST -H "Content-Type: application/dns-message" -d "" 

Let's break this down:

• -X POST: Specifies a POST request, which is the standard method for DoH.
• -H "Content-Type: application/dns-message": Sets the content type to indicate that the request body contains a DNS message.
• -d "": This is where you provide your DNS query. This typically requires encoding the query in the DNS message format (using tools like dig or creating a raw DNS message). We'll provide simpler examples below.
• : The URL of the DoH resolver you want to use. Examples include:

• Cloudflare: https://cloudflare-dns.com/dns-query
• Google Public DNS: https://dns.google/dns-query
• Quad9: https://dns.quad9.net/dns-query

Simple Examples

While sending raw DNS messages is powerful, for simpler queries, many DoH providers offer alternative methods. Here are examples using query parameters:

Example 1: Resolving google.com with Cloudflare using query parameters (easier, but not all providers support it):

curl "https://cloudflare-dns.com/dns-query?name=google.com&type=A"

This uses the query parameters name (the domain) and type (the record type, A for IPv4 addresses). The response will be in JSON format.

Example 2: Resolving example.com using a dedicated DoH endpoint that accepts JSON (check the provider's documentation):

curl -X POST -H "Content-Type: application/json" -d '{"name":"example.com","type":"A"}' ""

This uses JSON for the request body. Again, check your provider's documentation to see if they accept this type of request.

Advanced Usage

For more complex scenarios, you might need to:

• Handle different response formats: DoH resolvers may return responses in different formats (JSON, binary DNS messages). Curl's output options (like -s for silent, -w for writing data) and tools like jq (for JSON processing) can be helpful.
• Use DNS message encoding: For full control, construct raw DNS messages using tools specific to your operating system (or using a programming language).
• Implement error handling: Check the HTTP status code to ensure successful queries.

Conclusion

Curl, combined with the right DoH resolver and the proper command-line options, provides a flexible and powerful way to utilize DNS over HTTPS. Remember to consult your chosen DoH resolver's documentation for specific instructions and supported query methods.