Feature: add UDP TPROXY support on Linux (#562)

proxy/redir/utils_linux.go

@@ -0,0 +1,96 @@
+// +build linux
+
+package redir
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"syscall"
+)
+
+// dialUDP acts like net.DialUDP for transparent proxy.
+// It binds to a non-local address(`lAddr`).
+func dialUDP(network string, lAddr *net.UDPAddr, rAddr *net.UDPAddr) (*net.UDPConn, error) {
+	rSockAddr, err := udpAddrToSockAddr(rAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	lSockAddr, err := udpAddrToSockAddr(lAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	fd, err := syscall.Socket(udpAddrFamily(network, lAddr, rAddr), syscall.SOCK_DGRAM, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	if err = syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
+		syscall.Close(fd)
+		return nil, err
+	}
+
+	if err = syscall.SetsockoptInt(fd, syscall.SOL_IP, syscall.IP_TRANSPARENT, 1); err != nil {
+		syscall.Close(fd)
+		return nil, err
+	}
+
+	if err = syscall.Bind(fd, lSockAddr); err != nil {
+		syscall.Close(fd)
+		return nil, err
+	}
+
+	if err = syscall.Connect(fd, rSockAddr); err != nil {
+		syscall.Close(fd)
+		return nil, err
+	}
+
+	fdFile := os.NewFile(uintptr(fd), fmt.Sprintf("net-udp-dial-%s", rAddr.String()))
+	defer fdFile.Close()
+
+	c, err := net.FileConn(fdFile)
+	if err != nil {
+		syscall.Close(fd)
+		return nil, err
+	}
+
+	return c.(*net.UDPConn), nil
+}
+
+func udpAddrToSockAddr(addr *net.UDPAddr) (syscall.Sockaddr, error) {
+	switch {
+	case addr.IP.To4() != nil:
+		ip := [4]byte{}
+		copy(ip[:], addr.IP.To4())
+
+		return &syscall.SockaddrInet4{Addr: ip, Port: addr.Port}, nil
+
+	default:
+		ip := [16]byte{}
+		copy(ip[:], addr.IP.To16())
+
+		zoneID, err := strconv.ParseUint(addr.Zone, 10, 32)
+		if err != nil {
+			zoneID = 0
+		}
+
+		return &syscall.SockaddrInet6{Addr: ip, Port: addr.Port, ZoneId: uint32(zoneID)}, nil
+	}
+}
+
+func udpAddrFamily(net string, lAddr, rAddr *net.UDPAddr) int {
+	switch net[len(net)-1] {
+	case '4':
+		return syscall.AF_INET
+	case '6':
+		return syscall.AF_INET6
+	}
+
+	if (lAddr == nil || lAddr.IP.To4() != nil) && (rAddr == nil || lAddr.IP.To4() != nil) {
+		return syscall.AF_INET
+	}
+	return syscall.AF_INET6
+}