Clarifying DNS Resolver Behavior for the Recursion Desired (RD) Flag

This document addresses inconsistencies observed in the handling of the Recursion Desired (RD) flag in DNS queries by various DNS resolver implementations, particularly when the RD flag is cleared (set to 0). Such inconsistencies have been shown to be exploitable, leading to potent Denial of Service (DoS) amplification attacks. This document provides clear guidance and recommendations for DNS resolver (including forwarding and recursive resolvers) behavior when processing queries with different RD flag settings. The goal is to enhance DNS security, mitigate specific amplification attack vectors, and ensure more predictable and robust DNS operations.