Added fwmark code

src/conn_linux.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"golang.org/x/sys/unix"
+	"net"
+)
+
+func setMark(conn *net.UDPConn, value int) error {
+	if conn == nil || value == 0 {
+		return nil
+	}
+
+	file, err := conn.File()
+	if err != nil {
+		return err
+	}
+
+	return unix.SetsockoptInt(
+		int(file.Fd()),
+		unix.SOL_SOCKET,
+		unix.SO_MARK,
+		value,
+	)
+}