Finding addresses in server log that don't exist

Recently, we've been finding "visitors" in our server log that don't appear to exist.. At least, we're unable to resolve them to IPs.
Here're a few examples:

5e0e75f0.bb.sky.com
latvia.local
asadic.com

These "visitors" appear to be up to no good, so we want to block them via .htaccess. But none of the usual DNS tools are able to resolve these to IPs. This is a typical response:

--- 10/25/19 20:31:09 Eastern Daylight Time
--- resolving host "asadic.com", please wait...
--- resolve error: host not found

Anyone seen anything like this before?
 
Never seen it in my Apache web server log. Maybe if you stop resolving addresses in your web server log you'll get the actual IP addresses instead.
 
The majority of visitors appear as standard IPs, so I know we're not resolving by default. And I don't think we've ever added or enabled anything to resolve IPs in the server log. But, just to be sure, can you tell me where the control is that turns off resolving in Apache?
 
UPDATE: Problem solved!

hostnamelookups was set to ON in httpd.conf (not sure who, when or why?). Switched to OFF and everything's pink unicorns, waxed lips and comic books again.
Many thanks for the lead, trev!
 
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