echo server in go

echo.go

package main

import (
	"flag"
	"fmt"
	"net"
	"os"
	"strings"
)

var quiet *bool

func main() {
	port := flag.Int("port", 0, "port to run on")
	addr := flag.String("addr", "", "addr to run on")
	quiet = flag.Bool("quiet", false, "Surpress logs")
	flag.Parse()

	var as string
	if !strings.Contains(*addr, ":") {
		as = fmt.Sprintf("%s:%d", *addr, *port)
	} else {
		as = fmt.Sprintf("[%s]:%d", *addr, *port)
	}

	tcpAddr, err := net.ResolveTCPAddr("tcp", as)
	if err != nil {
		fmt.Fprintf(os.Stderr, "[!] resolve failed: %s\n", err)
		os.Exit(-1)
	}

	tcpLn, err := net.ListenTCP("tcp", tcpAddr)
	if err != nil {
		fmt.Fprintf(os.Stderr, "[!] couldn't start listening: %s\n", err)
		os.Exit(-1)
	}

	lnAddr := tcpLn.Addr()
	tcpAddr = lnAddr.(*net.TCPAddr)
	if *port == 0 {
		fmt.Printf("%d\n", tcpAddr.Port)
	}

	udpAddr, err := net.ResolveUDPAddr("udp", tcpAddr.String())
	if err != nil {
		fmt.Fprintf(os.Stderr, "[!] resolve failed: %s\n", err)
		os.Exit(-1)
	}

	udpLn, err := net.ListenUDP("udp", udpAddr)
	if err != nil {
		fmt.Fprintf(os.Stderr, "[!] couldn't start listening: %s\n", err)
		os.Exit(-1)
	}

	go runTCPEcho(tcpLn)
	runUDPEcho(udpLn)
}

func runUDPEcho(ln *net.UDPConn) {
	var (
		pos int
		arr = make([]string, 10, 10)
	)

	var buf [4096]byte
	for {
		n, raddr, err := ln.ReadFromUDP(buf[:])
		if err != nil {
			fmt.Fprintf(os.Stderr, "[-] read failed: %s\n", err)
			continue
		}
		ln.WriteToUDP([]byte(fmt.Sprintf("%d\n", n)), raddr)
		m := fmt.Sprintf("udp://%s\n", raddr.String())
		for _, v := range arr {
			if v == m {
				m = ""
				break
			}
		}
		if m != "" {
			arr[pos] = m
			pos = (pos + 1) % len(arr)
			if *quiet == false {
				fmt.Printf("%s", m)
			}
		}
	}
}

func runTCPEcho(ln *net.TCPListener) {
	for {
		conn, err := ln.Accept()
		if conn == nil {
			fmt.Fprintf(os.Stderr, "[-] accept failed: %s\n", err)
			continue
		}
		if *quiet == false {
			fmt.Printf("tcp://%s\n", conn.RemoteAddr())
		}
		go handleTCP(conn)
	}
}

func handleTCP(conn net.Conn) {
	var buf [4096]byte
	for {
		n, err := conn.Read(buf[:])
		if err != nil {
			break
		}
		_, err = conn.Write(buf[:n])
		if err != nil {
			break
		}
	}
	conn.Close()
}