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Ask HN: How did the internet discover my subdomain?

287 points govideo | 1 comments | 06 Mar 25 22:34 UTC | HN request time: 0.218s | source

I have a domain that is not live. As expected, loading the domain returns: Error 1016.

However...I have a subdomain with a not obvious name, like: userfileupload.sampledomain.com

This subdomain IS LIVE but has NOT been publicized/posted anywhere. It's a custom URL for authenticated users to upload media with presigned url to my Cloudflare r2 bucket.

I am using CloudFlare for my DNS.

How did the internet find my subdomain? Some sample user agents are: "Expanse, a Palo Alto Networks company, searches across the global IPv4 space multiple times per day to identify customers' presences on the Internet. If you would like to be excluded from our scans, please send IP addresses/domains to: scaninfo@paloaltonetworks.com", "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_7; en-us) AppleWebKit/534.20.8 (KHTML, like Gecko) Version/5.1 Safari/534.20.8", "Mozilla/5.0 (Linux; Android 9; Redmi Note 5 Pro) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.89 Mobile Safari/537.36",

The bots are GET requests which are failing, as designed, but I'm wondering how the bots even knew the subdomain existed?!

1. webpagealert ◴[10 Mar 25 05:31 UTC] No.43317274[source]▶
DNS Leaks or Public Records DNS Propagation: When you create a subdomain (e.g., blog.yoursite.com), your DNS provider (e.g., Cloudflare, GoDaddy) updates global DNS servers. These records are public and visible to anyone who queries the DNS (e.g., via dig blog.yoursite.com). WHOIS Data: If your domain registration details are public (not privacy-protected), your subdomain’s ownership info may be exposed.