Refactor: TPROXY inbound UDP write back

https://t.me/projectXray/119670

虽然不一定是最终的版本，但值得记录，感谢协助测试的各位朋友，特别感谢 @yichya @huyz

proxy/dokodemo/fakeudp_linux.go

@@ -0,0 +1,85 @@
+// +build linux
+
+package dokodemo
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"syscall"
+)
+
+func FakeUDP(addr *net.UDPAddr, mark int) (net.PacketConn, error) {
+
+	if addr == nil {
+		addr = &net.UDPAddr{
+			IP:   []byte{0, 0, 0, 0},
+			Port: 0,
+		}
+	}
+
+	localSocketAddress, af, err := udpAddrToSocketAddr(addr)
+	if err != nil {
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("build local socket address: %s", err)}
+	}
+
+	fileDescriptor, err := syscall.Socket(af, syscall.SOCK_DGRAM, 0)
+	if err != nil {
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("socket open: %s", err)}
+	}
+
+	if mark != 0 {
+		if err = syscall.SetsockoptInt(fileDescriptor, syscall.SOL_SOCKET, syscall.SO_MARK, mark); err != nil {
+			syscall.Close(fileDescriptor)
+			return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("set socket option: SO_MARK: %s", err)}
+		}
+	}
+
+	if err = syscall.SetsockoptInt(fileDescriptor, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
+		syscall.Close(fileDescriptor)
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("set socket option: SO_REUSEADDR: %s", err)}
+	}
+
+	if err = syscall.SetsockoptInt(fileDescriptor, syscall.SOL_IP, syscall.IP_TRANSPARENT, 1); err != nil {
+		syscall.Close(fileDescriptor)
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("set socket option: IP_TRANSPARENT: %s", err)}
+	}
+
+	if err = syscall.Bind(fileDescriptor, localSocketAddress); err != nil {
+		syscall.Close(fileDescriptor)
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("socket bind: %s", err)}
+	}
+
+	fdFile := os.NewFile(uintptr(fileDescriptor), fmt.Sprintf("net-udp-fake-%s", addr.String()))
+	defer fdFile.Close()
+
+	packetConn, err := net.FilePacketConn(fdFile)
+	if err != nil {
+		syscall.Close(fileDescriptor)
+		return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("convert file descriptor to connection: %s", err)}
+	}
+
+	return packetConn, nil
+}
+
+func udpAddrToSocketAddr(addr *net.UDPAddr) (syscall.Sockaddr, int, error) {
+	switch {
+	case addr.IP.To4() != nil:
+		ip := [4]byte{}
+		copy(ip[:], addr.IP.To4())
+
+		return &syscall.SockaddrInet4{Addr: ip, Port: addr.Port}, syscall.AF_INET, nil
+
+	default:
+		ip := [16]byte{}
+		copy(ip[:], addr.IP.To16())
+
+		zoneID, err := strconv.ParseUint(addr.Zone, 10, 32)
+		if err != nil {
+			return nil, 0, err
+		}
+
+		return &syscall.SockaddrInet6{Addr: ip, Port: addr.Port, ZoneId: uint32(zoneID)}, syscall.AF_INET6, nil
+	}
+}