programming/ReverseDNS written at 22:36:27; Add Comment
How to properly look up hostnames from IP addresses
Looking up reverse DNS to determine the hostname of an IP address that you are talking to is one of those almost simple things that software keeps getting wrong. So here is how you do it, in two variants.
First, if you use DNS and have an (almost) empty
The first two steps are the 'reverse lookup'; the third is the 'forward lookup'.
Adding a dot to the end of the claimed hostname forces a rooted DNS lookup without any use of your DNS search list of default domains. Without this, it's possible that a local host will block a successful check on an outside one because the local host has a short name that is the same as the full name of the outside host.
If you have an
Depending on what you are using the hostname for, you may want to map it to a consistent (ASCII) case or preserve it as the reverse lookup gave it to you. (If you are presenting it to the user and you care, you may then want to worry about IDNA issues.)
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