rsyncd matches host names in the "hosts allow" line of rsyncd.conf with reverse lookups only. This fails when you don't have control over reverse DNS, which is common these days.
To save me from having to type IP addresses instead of host names into my config file, please make it do forward lookups instead (or, in addition). Call gethostbyname on the items in "hosts allow" and compare those IPs against the incoming IP.
[Are insults really necessary?]
From the rsync 3.1.0 manual:
Controls whether the daemon performs a forward lookup on any hostname specified in an hosts allow/deny setting. By default this is enabled, allowing the use of an explicit hostname that would not be returned by reverse DNS of the connecting IP.