NATPMP

NAT-PMP, or NAT Port Mapping Protocol, is a lightweight network protocol that allows devices on a private LAN to automatically request and manage port mappings on a NAT gateway. By creating an inbound path, NAT-PMP enables applications that require direct external access, such as certain peer-to-peer clients or servers behind a NAT, to function without manual router configuration.

The protocol operates over UDP, typically using port 5351 on the NAT device. A client sends a mapping request that specifies the desired internal IP address, protocol (TCP or UDP), and the external port to be opened. The gateway responds with the assigned external port and a lease duration, after which the mapping may need to be refreshed. NAT-PMP is designed to be simple to implement and requires minimal state on the gateway.

NAT-PMP was developed by Apple and is defined in RFC 6886. It predates and differs from the Port Control Protocol (PCP), which provides a more flexible and extensible framework and supports broader features, including IPv6. Because of its simplicity, NAT-PMP has enjoyed support on various consumer routers and operating systems, though many networks now favor PCP or UPnP IGD, depending on vendor and security considerations.

Security and interoperability considerations are important: NAT-PMP offers little built-in authentication and relies on trusted networks. Mappings persist according to their lease settings and can expose internal services to the external network. Administrators and users should enable NAT-PMP only where necessary and consider disabling it in untrusted environments.

See also: UPnP IGD, PCP, NAT.