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Ask HN: How did the internet discover my subdomain?

287 points govideo | 2 comments | 06 Mar 25 22:34 UTC | HN request time: 0.001s | 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. xg15 ◴[07 Mar 25 10:56 UTC] No.43289141[source]▶
TIL (from this thread) : You can abuse TLS handshakes to effectively reverse-DNS an IP address without ever talking to a DNS server! Is this built into dig yet? :)

(Alright, some IP addresses, not all of them)

I also wonder if this is a potential footgun for eSNI deployments: If you add eSNI support to a server, you must remember to also make regular SNI mandatory - otherwise, an eavesdropper can just ask your server nicely for the domain that the eSNI encryption was trying to hide from it.

replies(1): >>43290795 #
2. yatralalala ◴[07 Mar 25 15:14 UTC] No.43290795[source]▶
Lifehack - it's especially awesome in cases where server operator is using self-signed certs / private cert authorities. Because you will not find these in public cert logs.