listener.rs

use async_std::io;
use async_std::net::{TcpListener, TcpStream};
use async_std::prelude::*;
use async_std::task;
use futures::{AsyncReadExt, future};

async fn in_to_out(incoming: TcpStream, outgoing: TcpStream) {
    let (incoming_reader, incoming_writer) = &mut (&incoming, &incoming);
    let (outgoing_reader, outgoing_writer) = &mut (&outgoing, &outgoing);

    let a = incoming_reader.copy_into(outgoing_writer);
    let b = outgoing_reader.copy_into(incoming_writer);

    future::select(a, b).await;
}

async fn setup_in_to_out(from: &str, to: &str) -> io::Result<()> {
    let listener = TcpListener::bind(from).await?;

    let mut incoming = listener.incoming();

    while let Some(incoming) = incoming.next().await {
        let incoming = incoming?;
        let outgoing = TcpStream::connect(to).await?;
        task::spawn(async {
            in_to_out(incoming, outgoing).await;
        });
    }
    Ok(())
}

fn main() {
    task::block_on(async {
        futures::join!(
            setup_in_to_out("127.0.0.1:8080", "127.0.0.1:3333"),
            setup_in_to_out("127.0.0.1:8081", "127.0.0.1:3333")
        )
    });
}

tcpproxy.rs

use async_std::io;
use async_std::net::{TcpListener, TcpStream};
use async_std::prelude::*;
use async_std::task;
use futures::{AsyncReadExt, future};

async fn handle(incoming: TcpStream, outgoing: TcpStream) {
    let (incoming_reader, incoming_writer) = &mut (&incoming, &incoming);
    let (outgoing_reader, outgoing_writer) = &mut (&outgoing, &outgoing);

    let a = incoming_reader.copy_into(outgoing_writer);
    let b = outgoing_reader.copy_into(incoming_writer);

    future::select(a, b).await;
}

fn main() -> io::Result<()> {
    task::block_on(async {
        let listener = TcpListener::bind("127.0.0.1:8080").await?;

        let mut incoming = listener.incoming();

        while let Some(incoming) = incoming.next().await {
            let incoming = incoming?;
            let outgoing = TcpStream::connect("127.0.0.1:3333").await?;
            task::spawn(async {
                handle(incoming, outgoing).await;
            });
        }
        Ok(())
    })
}