feat: add tcpfwd to pipe connections

cmd/tcpfwd/main.go

@@ -0,0 +1,86 @@
+package main
+
+import (
+	"flag"
+	"io"
+	"log"
+	"net"
+	"os"
+	"strings"
+)
+
+func main() {
+	var listenPort string
+	var target string
+	flag.StringVar(&listenPort, "port", "", "Local port to listen on (same as target by default)")
+	flag.StringVar(&target, "target", "", "Target host:port (required)")
+	flag.Parse()
+
+	if target == "" {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	if len(listenPort) == 0 {
+		i := strings.LastIndex(target, ":")
+		listenPort = target[i+1:]
+	}
+	listenAddr := ":" + listenPort
+	log.Printf("TCP bridge %s → %s", listenAddr, target)
+
+	// Note: allow unprivileged users to use this like so:
+	// echo 'net.ipv4.ip_unprivileged_port_start=1' | sudo tee /etc/sysctl.d/01-deprivilege-ports.conf
+   // sudo sysctl -p /etc/sysctl.d/01-deprivilege-ports.conf
+	listener, err := net.Listen("tcp", listenAddr)
+
+	if err != nil {
+		log.Fatalf("Failed to bind %s: %v", listenAddr, err)
+	}
+	log.Printf("TCP bridge listening on %s → %s", listenAddr, target)
+
+	for {
+		client, err := listener.Accept()
+		if err != nil {
+			log.Printf("Accept error: %v", err)
+			continue
+		}
+		go handleConn(client, target)
+	}
+}
+
+func handleConn(client net.Conn, target string) {
+	defer client.Close()
+
+	remote, err := net.Dial("tcp", target)
+	if err != nil {
+		log.Printf("Failed to connect to %s: %v", target, err)
+		return
+	}
+	defer remote.Close()
+
+	log.Printf("New connection %s ↔ %s", client.RemoteAddr(), remote.RemoteAddr())
+
+	// Bidirectional copy with error handling
+	go func() { _ = copyAndClose(remote, client) }()
+	func() { _ = copyAndClose(client, remote) }()
+}
+
+// copyAndClose copies until EOF or error, then closes dst
+func copyAndClose(dst, src net.Conn) error {
+	_, err := io.Copy(dst, src)
+	dst.Close()
+	if err != nil && !isClosedConn(err) {
+		log.Printf("Copy error (%s → %s): %v",
+			src.RemoteAddr(), dst.RemoteAddr(), err)
+	}
+	return err
+}
+
+// isClosedConn detects common closed-connection errors
+func isClosedConn(err error) bool {
+	if err == nil {
+		return false
+	}
+	opErr, ok := err.(*net.OpError)
+	return ok && opErr.Err.Error() == "use of closed network connection"
+}