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I see sites such as that allow people to input an IP or domain and it returns other domains hosted on that IP. How do you find this information?

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It sounds like you're answering your own question. You go to and it'll let you know what other domains share you IP – Nixphoe Aug 4 '11 at 16:58
Yeah because I'm sure that that website is where all people go to find this information. – Rob Aug 4 '11 at 17:03
1 is fairly accurate, though it is far from complete. It misses on quite a few obscure subdomains and smaller websites. I believe that it just runs off of the cache of a few large DNS resolvers. – Hyppy Aug 4 '11 at 17:30
up vote 5 down vote accepted

I do not believe there is a 'programmatic' way to find this out. You would basically have to have a huge list of domain names (a lot easier than you might think) and then resolve them all and put that in a table. If you think about it, the only way that these websites would exist would be 1) a DNS A record in some domain, 2) a virtualhost directive in apache that tells servers what content to serve when asked for a particular domain

Neither one of these methods can easily be found out by external entities.

It's very easy to get lists of domains from server logs, client logs (especially those people that might have spyware), etc.

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What you say makes sense, but I'm going to hold off and hope someone does somehow have a programmatic way of doing it. I can only imagine how large the database would be to store so many domains in it – Rob Aug 4 '11 at 17:22
Nope. It's this way by design. The domain name resolves to an IP address, and sometimes an IP's PTR can resolve to a usable domain name. PTR is the only authoritative answer that can be provided. Your alternative is to create a huge database of every domain name including subdomains ( is different than, etc). Oh, and keep it updated, since some TTLs expire fairly quickly. – Hyppy Aug 4 '11 at 17:27
Alright, I was hoping there was something more simple, such as gethostbyaddr :( Thanks, though! – Rob Aug 4 '11 at 17:34
+1 Thorough run down – Nixphoe Aug 4 '11 at 17:41

All the top level domain registrars have a database of domain and IP addresses.

Tools like the one linked here purchase these databases and use them.

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