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Ask HN: How did the internet discover my subdomain?

287 points govideo | 5 comments | 06 Mar 25 22:34 UTC | HN request time: 2.96s | 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. alberth ◴[07 Mar 25 03:29 UTC] No.43287177[source]▶
This site will find any subdomain, for any domain, so long as it previously had a certificate (ssl/tls)

https://crt.sh/

replies(4): >>43287554 #>>43287998 #>>43288382 #>>43289283 #
2. averageRoyalty ◴[07 Mar 25 05:23 UTC] No.43287554[source]▶
This is incorrect (or at least only technically correct). This is only true for subdomains with public, trusted CA signed certificates since certificate transparency has existed and only for subdomains with a specific, non wildcard certificate.
3. govideo ◴[07 Mar 25 07:38 UTC] No.43287998[source]▶
Thanks for mentioning. I checked it out, and am learning lots of new stuff (ie, realize how much I do not know).
4. nvarsj ◴[07 Mar 25 08:46 UTC] No.43288382[source]▶
Doesn’t find any of my semi secret subdomains.
5. socrateslee ◴[07 Mar 25 11:24 UTC] No.43289283[source]▶
https://crt.sh can find your subdomain only when it doesn't have a wildcard certificate(*.somedomain.com)