For the record, I honestly believe an investigation into these topics by the OP could result in a real solution to their actual problem. It certainly seems more productive than identifying edge cases of what was probably a poorly phrased question.
The only person who can know whether the domain is "used" for an actual web site, is the owner of the domain.
The reason why, is because you have to define what "used" means.
Only the owner of the domain can set up a public DNS entry for that domain. But he can also set up a private DNS entry for that domain, or a public DNS entry with a private IP address.
For example, lassell.lan.informatimago.com is a public DNS entry, with a private IP address. Can you know whether there's an actual website on this domain?
If a domain shows a generic landing page like this one http://www.domainlandingpage.com/ then you know it is not being used but people are just keeping it so they can sell it to someone who actually wants to use it. I guess the question would then be: how can you detect if a domain is just being parked.
You could try checking if the nameservers assigned to the site are operated by the domain's registrar and if so if they are used primarily for that registrar's parked domains.
It might be being used for email rather than website hosting. I have a few websites with only MX records, and one domain registered which only has a bunch of SRV records in DNS.
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[ 3.9 ms ] story [ 50.0 ms ] threadThe reason why, is because you have to define what "used" means.
Only the owner of the domain can set up a public DNS entry for that domain. But he can also set up a private DNS entry for that domain, or a public DNS entry with a private IP address.
For example, lassell.lan.informatimago.com is a public DNS entry, with a private IP address. Can you know whether there's an actual website on this domain?
Then of course, you would have to know what an actual web site is! Is http://hubble.informatimago.com:8117 an actual website?
What if I activated the web server only for 2 seconds after receiving a specific packet on a specific port?
It can't. Are you really suggesting releasing other people's domains "with software"?