For easy allow or disallowing of dynamic IP-adresses you can add them as a hostname in a .htaccess file.

As I have read from: .htaccess allow from hostname?

it does a reverse lookup on the connecting ip address, seeing if the response matches the allowed name.

(Well, actually Apache is doing a double lookup, first a reverse lookup and then a forward lookup on the result of the reverse.)

This is the reason we are currently not using dynamic-ip hostnames in the .htaccess: this "sounds" quite heavy: 2 extra lookups for every request.

• Is this indeed quite heavy, and would a reasonably busy server that is rather looking for less then more load get away with this :)? (e.g.: how does this 'load' compare to the rest? If a request is 1000 times more expensive then the lookups it might be negligible. otoh, it could be that final straw :) )
• Are there other solutions? I can write a script that does a lookup of the hostname and put it in .htaccess files ofcourse, but this feels a bit like a hack.