Echo — the server sends back every byte you send it, one of the original inetd debugging services.
Echo is defined in RFC 862: over TCP the server returns the stream unchanged until the client closes, and over UDP it returns each datagram to its sender. It exists as a reachability and round-trip test from the era before ping and traceroute were universal, and it shipped as an inetd or xinetd built-in on classic UNIX. Modern distributions ship it disabled. Its UDP form is a documented reflection and amplification vector, because a spoofed source address gets an equal-sized reply sent to the victim.
Do not expose
The UDP form is a spoofable reflector documented in CISA's amplification-attack alert, and the service has no legitimate internet-facing use.
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