do not decode ipv6 fragments after the 1st fragment (#576)

* do not decode ipv6 fragments after the 1st fragment
* add a unit test

dpkt/ip6.py

@@ -91,6 +91,12 @@ def unpack(self, buf):
         if next_ext_hdr is not None:
             self.p = next_ext_hdr
 
+        # do not decode fragments after the first fragment
+        # https://github.com/kbandla/dpkt/issues/575
+        if self.nxt == 44 and ext.frag_off > 0:  # 44 = IP_PROTO_FRAGMENT
+            self.data = buf
+            return
+
         try:
             self.data = self._protosw[next_ext_hdr](buf)
             setattr(self, self.data.__class__.__name__.lower(), self.data)
@@ -557,3 +563,25 @@ class Proto:
     assert 'PROTO' not in IP6._protosw
     IP6.set_proto('PROTO', Proto)
     assert IP6.get_proto('PROTO') == Proto
+
+
+def test_ip6_fragment_no_decode():  # https://github.com/kbandla/dpkt/issues/575
+    from . import udp
+
+    # fragment 0
+    s = (b'\x60\x00'
+         b'\x00\x00\x00\x2c\x11\x3f\x20\x01\x06\x38\x05\x01\x8e\xfe\xcc\x4a'
+         b'\x48\x39\xfa\x79\x04\xdc\x20\x01\x05\x00\x00\x60\x00\x00\x00\x00'
+         b'\x00\x00\x00\x00\x00\x30\xde\xf2\x00\x35\x00\x2c\x61\x50\x4d\x8b'
+         b'\x01\x20\x00\x01\x00\x00\x00\x00\x00\x01\x03\x69\x73\x63\x03\x6f'
+         b'\x72\x67\x00\x00\xff\x00\x01\x00\x00\x29\x10\x00\x00\x00\x80\x00'
+         b'\x00\x00')
+    frag0 = IP6(s)
+    assert type(frag0.data) == udp.UDP
+
+    s = (b'\x60\x00\x00\x00\x01\x34\x2c\x35\x20\x01\x05\x00\x00\x60\x00\x00'
+         b'\x00\x00\x00\x00\x00\x00\x00\x30\x20\x01\x06\x38\x05\x01\x8e\xfe'
+         b'\xcc\x4a\x48\x39\xfa\x79\x04\xdc'
+         b'\x61\x72\x31\xb9\xc1\x0f\xcf\x7c\x61\x62\x63\x64\x65\x66\x67\x68')  # partial data
+    frag2 = IP6(s)
+    assert type(frag2.data) == bytes