This is a bidirectional router that shows how to implement I/O multiplexing in Go. The function is similar to netcat.

simplerouter.go

/*
 * This is a bidirectional router that shows how to implement
 * I/O multiplexing in Go. The function is similar to netcat.
 * Test this program in terminal:
 * 
 *   $ go run simplerouter.go
 *   GET / HTTP/1.0<CR>
 *   <CR>
 *   HTTP/1.0 302 Found
 *   ...
 *
 */

package main

import (
    "io"
    "os"
    "log"
    "net"
    "strings"
)

// Bidirectional routing.
//   ar ---------> bw
//   aw <--------- br
func router(ar, aw, br, bw chan string) {
    for {
        select {
        case str, open := <-ar:
            if !open {
                close(bw)
                return
            }
            bw <- str  // may block
        case str, open := <-br:
            if !open {
                close(aw)
                return
            }
            aw <- str  // may block
        }
    }
}

// Read data from an io.Reader, then send it to a chan.
func io2chan(src io.Reader, dst chan string) {
    buf := make([]byte, 8192)
    for {
        n, err := src.Read(buf)
        if err != nil {
            if err != io.EOF {
                log.Println("io2chan error:", err)
            }
            close(dst)
            return
        }
        dst <- string(buf[:n])
    }
}

// Read data from a chan, then send it to an io.Writer.
func chan2io(src chan string, dst io.Writer) {
    for {
        select {
            case str, open := <-src:
                if !open {
                    return
                }
                _, err := dst.Write([]byte(str))
                if err != nil {
                    log.Println("chan2io error:", err)
                    return
                }
        }
    }
}

func chan2io2(src chan string, dst io.Writer) {
    for {
        select {
            case str, open := <-src:
                if !open {
                    return
                }
                str2 := strings.Replace(str, "\n", "\r\n", -1)
                _, err := dst.Write([]byte(str2))
                if err != nil {
                    log.Println("chan2io2 error:", err)
                    return
                }
        }
    }
}

func main() {
    conn, err := net.Dial("tcp", "www.google.com:80")
    if err != nil {
        log.Println("failed to connect:", err)
        os.Exit(1)
    }

    si := make(chan string)
    so := make(chan string)
    ni := make(chan string)
    no := make(chan string)

    //      /--- stdin  --> si ----> no -> sock.w ---\
    //  tty-|                                        |-sock <-> server
    //      \--- stdout <-- so <---- ni <- sock.r ---/
    go io2chan(os.Stdin, si)
    go chan2io(so, os.Stdout)
    go io2chan(conn, ni)
    go chan2io2(no, conn)

    router(si, so, ni, no)
}