You are looking for Components, Admin Tools, Web Application Firewall, Configure WAF, Exceptions from Blocking, Whitelisted domains. The downside is that you need to do a reverse DNS lookup for every request. Moreover, using a dynamic DNS service it's not guaranteed that you'll get that subdomain as the reverse DNS lookup of your IP.
The other approach would require us to let you define a list of fixed, whitelisted hostnames. However, that would require making N DNS requests for every request made to your site, where N is the number of hostnames you have. With just 3 hostnames you'd get ~1 second page load time increase. Most hosts deal with that by caching the DNS queries for an arbitrary amount of time -typically around 1 hours- which would render that feature useless. Since such a feature would either kill your site's performance or not work due to no fault of ours I decided to not implement it.
What you could do for your use case is write a CRON script which performs a DNS query against a live DNS server (e.g. 8.8.8.8) and replace that result in #__admintools_adminiplist (the Administrator IP Whitelist feature's database), provided that you have turned on that feature in Admin Tools. However, this is very specific to your use case so we won't be making this into an Admin Tools feature.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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