Surfguard resolves a hostname to the public IP addresses it points at and refuses anything that would reach an internal network: private, loopback, link-local and carrier-grade NAT space, plus the IPv6 transition ranges a naive guard misses (IPv4-mapped, SIIT, NAT64, 6to4, Teredo). It resolves and classifies only; the caller owns the fetch and pins the connection to a returned address so DNS rebinding cannot swap in a blocked one. Standard library only, no runtime dependencies.