Ask HN: How did the internet discover my subdomain?

The options how to find it are basically limitless. Best source is probably Certificate Transparency project as others suggested. But it does not end there, some other things that we do are things like internet crawl, domain bruteforcing on wildcard dns, dangling vhosts identification, default certs on servers (connect to IP on 443 and get default cert) and many others.

Security by obscurity does not work. You can not rely on "people won't find it". Once it's online, everyone can find it. No matter how you hide it.

This is one of those false voyeur OS internet tennets designed to get people to publish their stuff.

Obscurity is a fine strategy, if you don't post your source that's good. If you post your source, that's a risk.

The fact that you can't rely on that security measure is just a basic security tennet that applies to everything: don't rely on a single security measure, use redundant barriers.

Truth is we don't know how the subdomain got leaked. Subdomains can be passwords and a well crafted subdomain should not leak, if it leaks there is a reason.

The saying is "security by obscurity is not security" which is absolutely true.

If your security relies on the attacker not finding it or not knowing how it works, it's not actually secure.

Obscurity has its own value of course, I strongly recommend running any service that's likely to be scanned for regularly on non-standard ports wherever practical simply to reduce the number of connection logs you need to sort through. Obscurity works for what it actually offers. That has nothing to do with security though, and unfortunately it's hard in cases where a human is likely to want to type in your service address because most user-facing services have little to no support for SRV records.

Two of the few services that do have widespread SRV support are SIP VoIP and Minecraft, and coincidentally the former is my day job while I've also run a personal Minecraft server for over a decade. I can say that the couple of systems I still have running public-facing SIP on port 5060 get scanned tens of thousands of times per hour while the ones running on non-standard ports get maybe one or two activations of fail2ban a month. Likewise my Minecraft server has never seen a single probe from anyone other than an actual player.