You cannot hide anything on the internet anymore, the full IPv4 range is scanned regularly by multiple entities. If you open a port on a public IP it will get found.
If it's a obscure non-standard port it might take longer, but if it's on any of the standard ports it will get probed very quickly and included tools like shodan.io
The reason why I'm repeating this, is that not everyone knows this. People still (albeit less) put up elastic and mongodb instances with no authentication on public IP's.
The second thing which isn't well known is the Certificate Transparency logs. This is the reason why you can't (without a wildcard cert) hide any HTTPS service. When you ask Let's Encrypt (or any CA actually) to generate veryobscure.domain.tld they will send that to the Certificate Transparency logs. You can find every certificate which was minted for a domain on a tool like https://crt.sh
There are many tools like subdomain.center, https://hackertarget.com/find-dns-host-records/ comes to mind. The most impressive one I've seen, which found more much more than expected, is Detectify (which is a paid service, no affiliation), they seem to combine the passive data collection (like subdomain.center) with active brute to find even more subdomains.
But you can probably get 95% there by using CT and a brute-force tool like https://github.com/aboul3la/Sublist3r
If it's a obscure non-standard port it might take longer, but if it's on any of the standard ports it will get probed very quickly and included tools like shodan.io
The reason why I'm repeating this, is that not everyone knows this. People still (albeit less) put up elastic and mongodb instances with no authentication on public IP's.
The second thing which isn't well known is the Certificate Transparency logs. This is the reason why you can't (without a wildcard cert) hide any HTTPS service. When you ask Let's Encrypt (or any CA actually) to generate veryobscure.domain.tld they will send that to the Certificate Transparency logs. You can find every certificate which was minted for a domain on a tool like https://crt.sh
There are many tools like subdomain.center, https://hackertarget.com/find-dns-host-records/ comes to mind. The most impressive one I've seen, which found more much more than expected, is Detectify (which is a paid service, no affiliation), they seem to combine the passive data collection (like subdomain.center) with active brute to find even more subdomains.
But you can probably get 95% there by using CT and a brute-force tool like https://github.com/aboul3la/Sublist3r