Many sites connected to the Internet make use of IPv4 addresses that are not globally unique. Examples are the addresses designated in RFC 1918 for private use within individual sites. Hosts should never normally send DNS reverse-mapping queries for those addresses on the public Internet. However, such queries are frequently observed. Authoritative servers are deployed to provide authoritative answers to such queries as part of a loosely coordinated effort known as the AS112 project. Since queries sent to AS112 servers are usually not intentional, the replies received back from those servers are typically unexpected. Unexpected inbound traffic can trigger alarms on intrusion detection systems and firewalls, and operators of such systems often mistakenly believe that they are being attacked. This document provides background information and technical advice to those firewall operators. This document is not an Internet Standards Track specification; it is published for informational purposes.