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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: 0.49s | 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. ciaovietnam ◴[07 Mar 25 03:08 UTC] No.43287109[source]▶
There is a chance that your subdomain is the first/default virtual host in your web server setup (or the subdomain's access log is the default log file) so any requests to the server's IP address get logged to this virtual host. That means they didn't access your subdomain, they accessed via your server IP address but got logged in your subdomain's access log.
replies(1): >>43287735 #
2. BrandoElFollito ◴[07 Mar 25 06:21 UTC] No.43287735[source]▶
And this is the correct answer, thank you.

Transparency logs are fine except if you have a wildcard cert (or no https, obviously).

IP scans are just this: scans for live ports. If you do not provide a host header in your call you get whatever the default response was set up. This can be a default site, a 404 or anything else.