Network Address Translator (NAT) is the well-known, transitional method to mitigate the problem of IPv4 address depletion in today's Internet. However, the assignment, translation, and export of address/port in a NAT at run time affect application functions. Accordingly, application servers behind the NAT cannot accept requests directly from public networks. Sensitive applications cannot hold their end-to-end security mechanisms. Applications lose connections after the NAT reboots or changes the binding address/port. However, current proposals for NAT traversal hardly solve the problems. Against the problems, we propose Smart Tunnel Union for NAT Traversal (STUNT) in the paper. STUNT permits applications behind the NAT to be actively contacted by Internet clients, keeps end-to-end security mechanisms, and avoids the risk of exporting binding information of the NAT to connection endpoints. Meanwhile, it permits applications to traverse the NAT and keeps the NAT intact.