Residential proxy detection is one of the most challenging problems in IP-based threat intelligence. VPNs run on known datacenter IP ranges; you can blocklist them. Tor exit nodes publish their addresses. Datacenter proxies sit on hosting ASNs that reputation databases flag in hours. Residential proxies do none of that. They route traffic through real consumer ISP connections (Comcast, Vodafone, Jio, Deutsche Telekom), making every request look like it comes from someone's living room. The FBI published a PSA in March 2026 warning that residential proxies have become a standard tool for credential stuffing, account takeovers, and purchase fraud. Google's Threat Intelligence team disrupted what they called the largest residential proxy network a month earlier. The threat is not emerging. It's operational. This tutorial walks through detecting residential proxy traffic using an IP security API, with production code in cURL, Python, and Node.js.…