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Networks not employing destination-side source address validation (DSAV) expose themselves to a class of pernicious attacks which could be easily prevented by filtering inbound traffic purporting to originate from within the network. In this work, we survey the pervasiveness of networks vulnerable to infiltration using spoofed addresses internal to the network. We issue recursive Domain Name System (DNS) queries to a large set of known DNS servers worldwide, using various spoofed-source addresses. We classify roughly half of the 62,000 networks (autonomous systems) we tested as vulnerable to infiltration due to lack of DSAV. As an illustration of the dangers these networks expose themselves to, we demonstrate the ability to fingerprint the operating systems of internal DNS servers. Additionally, we identify nearly 4,000 DNS server instances vulnerable to cache poisoning attacks due to insufficient---and often …