IPv4 NAT has limited the spread of IoT botnets considerably by default-denying bots' incoming connection requests to in-home devices unless the owner has explicitly allowed them. As the Internet transitions to majority IPv6, however, residential connections no longer require the use of NAT. This paper therefore asks: has the transition from IPv4 to IPv6 ultimately made residential networks more vulnerable to attack, thereby empowering the next generation of IPv6-based IoT botnets? To answer this question, we introduce a large-scale IPv6 scanning methodology that, unlike those that rely on computationally intensive algorithms, can be run on low-resource devices common in IoT botnets. We use this methodology to perform the largest-scale measurement of IPv6 residential networks to date, and compare which devices are publicly accessible to comparable IPv4 networks. We received responses from 93,832,220 distinct IPv6 addresses, 14,901,892 of which are inside of residential networks (i.e., not the external-facing gateway IP). These residential network internal addresses span 4,412 ASes across 144 countries. These responses come from protocols commonly exploited by IoT botnets (including telnet and FTP), as well as protocols typically associated with end-user devices (including iPhone-Sync and IPP). Compared with IPv4, we reach 11x more HP printers over IPv6 than Shodan finds across the entire IPv4 Internet, and thousands of iPhones and smart lights whose services IPv4 NAT made unreachable by default. Collectively, our results show that NAT has indeed acted as the de facto firewall of the Internet, and the IPv4-to-IPv6 transition of residential networks is opening up new devices to attack. Finally, we discuss potential mitigations to prevent in-home network reachability as IPv6 adoption continues to grow.

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