Worked on the coturn/coturn project to restore backward compatibility for legacy STUN clients, focusing on stability and protocol compliance. Addressed a regression introduced in version 4.7.0 by correcting logic errors in the C codebase, specifically within ns_turn_server.c and dtls_listener.c, to ensure proper handling of old STUN packets when backward compatibility is enabled. Applied targeted changes to both TCP and UDP processing paths, removing stray negations and adding missing compatibility branches. This work improved interoperability with legacy clients, reduced support overhead, and reinforced the reliability of STUN protocol implementation using C programming and network programming expertise.