Fuso is a Rust-based intranet penetration and port forwarding tool that enables exposing local services behind NAT to the public internet via a relay server. It implements a client-server architecture with support for TCP forwarding, SOCKS5 proxy (including UDP association), KCP protocol for reliable UDP transport, LZ4 compression, and RSA+AES encryption. The system is built on a custom async runtime abstraction (supporting tokio and smol), with a modular design using provider and builder patterns. The server (fus) manages client connections and public listeners, while the client (fuc) registers port mappings and forwards traffic. Additional features include bridge mode for cascading proxies, heartbeat health checks, observer hooks for external notifications, and a configuration API. The codebase demonstrates strong Rust async patterns, protocol implementation skills, and a clean separation between core abstractions and runtime specifics.