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Discard Protocol
The Discard Protocol is a simple and obsolete network service that accepts connections and discards any data received without any response. Originally designed for testing, measuring, or debugging, it silently consumes incoming traffic on both TCP and UDP port 9, confirming network reachability without providing feedback. Due to its minimal function and lack of authentication, it's mostly disabled on modern systems for security reasons..
The Discard Protocol, defined in RFC 863, is a straightforward service designed primarily for diagnostics and simple testing of network communications. It listens on TCP or UDP port 9 and accepts any transmitted data, which it discards immediately without any response or data returned. This simplicity allows developers and network engineers to test reachability, data transmission, and connection establishment without concern for protocol processing overhead.
Typical operation involves a client sending arbitrary data to the Discard service, which consumes it silently regardless of size or content. The absence of any reply is by design, enabling detection of issues such as packet loss, transmission delays, or connection failures purely through the connection state and local reception. Because the protocol is stateless in its UDP form and connection-oriented in its TCP incarnation, it supports flexible testing scenarios.
Historically included on UNIX and other operating systems as part of the 'inetd' superserver, the Discard Protocol serves no application function beyond development and diagnostics. It has been deprecated in most environments due to both redundancy — better tools and commands exist — and security considerations.