UDP checksum computation fixed

scapy/layers/inet.py

@@ -557,7 +557,10 @@ class UDP(Packet):
                                      inet_aton(self.underlayer.dst),
                                      self.underlayer.proto,
                                      ln)
-                ck=checksum(psdhdr+p)
+                ck = checksum(psdhdr+p)
+                # According to RFC768 if the result checksum is 0, it should be set to 0xFFFF
+                if ck == 0:
+                    ck = 0xFFFF
                 p = p[:6]+struct.pack("!H", ck)+p[8:]
             elif isinstance(self.underlayer, scapy.layers.inet6.IPv6) or isinstance(self.underlayer, scapy.layers.inet6._IPv6ExtHdr):
                 ck = scapy.layers.inet6.in6_chksum(socket.IPPROTO_UDP, self.underlayer, p)