tcp forwarder that will retry upon connection refused error

forwarder.go

package main

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"log/slog"
	"net"
	"os"
	"syscall"
	"time"
)

func handleConn(downConn net.Conn, remoteAddr string, maxRetries int) {
	defer downConn.Close()

	slog.Info("Handle connection")

	var (
		upConn net.Conn
		err    error
	)

	for i := range maxRetries {
		upConn, err = net.Dial("tcp", remoteAddr)
		if err == nil {
			break
		}
		if !errors.Is(err, syscall.ECONNREFUSED) {
			slog.Error("Fail to dial", "error", err)
			return
		}
		if i == maxRetries-1 {
			slog.Error("Fail to dial after retries", "error", err)
			return
		}
		slog.Info("Retry dial after 3 secs", "error", err)
		time.Sleep(3 * time.Second)
	}

	slog.Info("Connected to upstream")
	defer upConn.Close()
	done := make(chan struct{})
	go func() {
		_, _ = io.Copy(upConn, downConn)
		close(done)
	}()
	_, _ = io.Copy(downConn, upConn)
	<-done
}

func run(localAddr, remoteAddr string, maxRetries int) error {
	slog.Info("Waiting for connections", "localAddr", localAddr, "remoteAddr", remoteAddr)
	lis, err := net.Listen("tcp", localAddr)
	if err != nil {
		return err
	}
	for {
		downConn, err := lis.Accept()
		if err != nil {
			var tempErr interface{ Temporary() bool }
			if errors.As(err, &tempErr) && tempErr.Temporary() {
				continue
			}
			return fmt.Errorf("accept: %w", err)
		}
		go handleConn(downConn, remoteAddr, maxRetries)
	}
}

func main() {
	var (
		localAddr  string
		remoteAddr string
		retries    int
	)
	flag.StringVar(&localAddr, "l", "127.0.0.1:8090", "local listen address")
	flag.StringVar(&remoteAddr, "u", "127.0.0.1:8091", "upstream address")
	flag.IntVar(&retries, "r", 10, "max retries")
	flag.Parse()
	err := run(localAddr, remoteAddr, retries)
	if err != nil {
		slog.Error("Fail to run", "error", err)
		os.Exit(1)
	}
}