@fedilab There is another way:
If the subdomain uses HTTPS and a certificate from Let's Encrypt (or another lage CA) then that fact will be logged in the Certificate Authority Log which is public and there are some tools online to search it for all certificates on one domain (and it's subdomains)
But actually this is quite the security feature as this means that if a CA tries to issue a cert for your domain, you have a chance to notice this in the log once all CAs support it and all browsers require it. I think Chrome does already, Firefox doesn't https://wiki.mozilla.org/PKI:CT
@lukas Wow thanks. Could be what I am looking for.
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