Quantifying the Impact of Blocklisting in the Age of Address Reuse

Blocklists, consisting of known malicious IP addresses, can be used as a simple method to block malicious traffic. However, blocklists can potentially lead to unjust blocking of legitimate users due to IP address reuse, where more users could be blocked than intended. IP addresses can be reused either at the same time (Network Address Translation) or over time (dynamic addressing). We propose two new techniques to identify reused addresses. We built a crawler using the BitTorrent Distributed Hash Table to detect NATed addresses and use the RIPE Atlas measurement logs to detect dynamically allocated address spaces. We then analyze 151 publicly available IPv4 blocklists to show the implications of reused addresses and find that 53-60% of blocklists contain reused addresses having about 30.6K-45.1K listings of reused addresses. We also find that reused addresses can potentially affect as many as 78 legitimate users for as many as 44 days.