Excellent. Of course I assumed that, but I felt that it was worth saying anyhow for-the-record, because I actually have encountered applications that got blacklisted by DNS servers.
My sincere understanding remains that Apache by-default has precisely the information that is in the HTTP header, and the IP-address from whence the request came and to which the reply should be directed, and ... that is all. As others have noted, you can have Apache do it, and as you have noted, you can do it yourself. But in normal practice this does not occur. Nearly all requests that any web-server expects to receive will not be associated with IPs that have any associated domain-name at all.
Perhaps, in your apparent-edge case, that Apache directive might be just what the doctor ordered. Perhaps that’s exactly the use-case that it was built for. (And if you have local DNS servers doing cached lookups for you, performance might well be perfectly acceptable.) Having said that, I would be quick to point out that I have no personal experience with it ... I have never yet had the need.
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