DNS Rebind Attack Message

[jdegolyer]

We are hosting a web service and we have configured our DNS server to point to multiple IP addresses to forward requests to that are being set to a specific URL over the web. One of these IP addresses within our DNS server is the IP address of our firewalls WAN port. When I give the OPNSense firewall a floating rule to allow ALL to ALL over any interface (obviously not secure), it gave the DNS rebinding attack message. Could anyone please explain as to why this may happen?