denoland-deno/docs/examples/tcp_echo.md

TCP echo server

This is an example of a simple server which accepts connections on port 8080, and returns to the client anything it sends.

const listener = Deno.listen({ port: 8080 });
console.log("listening on 0.0.0.0:8080");
for await (const conn of listener) {
  Deno.copy(conn, conn);
}

When this program is started, it throws PermissionDenied error.

$ deno run https://deno.land/std/examples/echo_server.ts
error: Uncaught PermissionDenied: network access to "0.0.0.0:8080", run again with the --allow-net flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    ...

For security reasons, Deno does not allow programs to access the network without explicit permission. To allow accessing the network, use a command-line flag:

$ deno run --allow-net https://deno.land/std/examples/echo_server.ts

To test it, try sending data to it with netcat:

$ nc localhost 8080
hello world
hello world

Like the cat.ts example, the copy() function here also does not make unnecessary memory copies. It receives a packet from the kernel and sends back, without further complexity.