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I had to tackle this problem as well for a small project and found that I could just send a GET request to https://www.cloudflare.com/cdn-cgi/trace and parse the response with a simple regex: /loc=(\w*)$/gm

The response time averages 10ms and is free, granted I'm not sure if there is a ToS or anything attached to this endpoint, I've only found non-conclusive discussion here https://community.cloudflare.com/t/what-are-the-terms-of-use...

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I mean, you can do that but it's a hideous hack and we might change that output at any time and you could easily hit DDoS mitigation or other security features if you do that. That endpoint is used for debugging and is in no way an API.

I'd advise you to just create a Cloudflare Worker if you want do this. You'll get more reliability etc. and Workers have a very generous free tier.

It is worth mentioning that /cdn-cgi/trace doesn't only work under www.cloudflare.com. It works for any https?://DOMAIN_WITH_CLOUDFLARE/cdn-cgi/trace . It is an easy to determine whether a website is using CloudFlare or not too.

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