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

proxy/redir/udp_linux.go

@@ -0,0 +1,69 @@
+// +build linux
+
+package redir
+
+import (
+	"encoding/binary"
+	"errors"
+	"net"
+	"syscall"
+)
+
+const (
+	IPV6_TRANSPARENT     = 0x4b
+	IPV6_RECVORIGDSTADDR = 0x4a
+)
+
+func setsockopt(c *net.UDPConn, addr string) error {
+	isIPv6 := true
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return err
+	}
+	ip := net.ParseIP(host)
+	if ip != nil && ip.To4() != nil {
+		isIPv6 = false
+	}
+
+	rc, err := c.SyscallConn()
+	if err != nil {
+		return err
+	}
+
+	rc.Control(func(fd uintptr) {
+		err = syscall.SetsockoptInt(int(fd), syscall.SOL_IP, syscall.IP_TRANSPARENT, 1)
+		if err == nil && isIPv6 {
+			err = syscall.SetsockoptInt(int(fd), syscall.SOL_IPV6, IPV6_TRANSPARENT, 1)
+		}
+
+		if err == nil {
+			err = syscall.SetsockoptInt(int(fd), syscall.SOL_IP, syscall.IP_RECVORIGDSTADDR, 1)
+		}
+		if err == nil && isIPv6 {
+			err = syscall.SetsockoptInt(int(fd), syscall.SOL_IPV6, IPV6_RECVORIGDSTADDR, 1)
+		}
+	})
+
+	return err
+}
+
+func getOrigDst(oob []byte, oobn int) (*net.UDPAddr, error) {
+	msgs, err := syscall.ParseSocketControlMessage(oob[:oobn])
+	if err != nil {
+		return nil, err
+	}
+
+	for _, msg := range msgs {
+		if msg.Header.Level == syscall.SOL_IP && msg.Header.Type == syscall.IP_RECVORIGDSTADDR {
+			ip := net.IP(msg.Data[4:8])
+			port := binary.BigEndian.Uint16(msg.Data[2:4])
+			return &net.UDPAddr{IP: ip, Port: int(port)}, nil
+		} else if msg.Header.Level == syscall.SOL_IPV6 && msg.Header.Type == IPV6_RECVORIGDSTADDR {
+			ip := net.IP(msg.Data[8:24])
+			port := binary.BigEndian.Uint16(msg.Data[2:4])
+			return &net.UDPAddr{IP: ip, Port: int(port)}, nil
+		}
+	}
+
+	return nil, errors.New("cannot find origDst")
+}