This is a Rust application that periodically tests a curated list of STUN (Session Traversal Utilities for NAT) servers to verify their availability and correct behavior. The system sends STUN Binding Requests over UDP and TCP to each candidate server, compares the returned mapped address against a reference server's result to detect NAT-induced mapping changes, and optionally checks for the OTHER-ADDRESS attribute to identify servers that support NAT behavior testing (RFC 5780). Healthy servers are written to several text files (hostnames, IPv4 addresses, IPv6 addresses) with and without the NAT-testing capability filter. The output lists are shuffled hourly to distribute load across servers. The architecture is a single binary built with Tokio async runtime, modularized into server loading, STUN query logic, output formatting, and a GeoIP caching layer. It is not a library; it is a self-contained service intended to run as a GitHub Actions cron job. The code is clean, idiomatic Rust with proper error handling and logging.