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Hey. I know this isn't exactly linux-related, but I'd like to ask something about a possible hack attempt. Recently I checked my apache2 httpd logs and found multiple entries of this strange IP address scanning my webserver for vulnerabilities. While I can't say they found any holes as I regularly update my software, I was curious to see who it was. I did a reverse DNS lookup on the IP in question, but the lookup failed. I've tried multiple reverse-DNS-lookup websites and they were all unable to resolve the IP. I know the IP is valid since it appears in the logs, but why can it not be resolved to a hostname?
There is no requirement that IP addresses be tied to names. This is done for convenience. Computers would work just fine using only IP addresses but we humans would have a hard time remember IPs for everything we want to use so we assign names.
We use DNS to find the names when we don't know them but it requires that someone registered the name and published its association with the IP address. Sometimes you don't want people knowing what your systems are for legitimate purposes. Sometimes you don't want them knowing because you're a hacker and are doing something bad.
There are sites that will approximate the IP address' geographic location. If you see it is coming out of some exotic foreign land (e.g. Russia) you probably just want to blacklist the address so it doesn't do any queries.