Attempt to decode SCTPChunkData data when possible

scapy/contrib/diameter.py

@@ -4828,7 +4828,5 @@ def DiamAns(cmd, **fields):
 
 bind_layers(TCP, DiamG, dport=3868)
 bind_layers(TCP, DiamG, sport=3868)
-bind_layers(SCTPChunkData, DiamG, dport=3868)
-bind_layers(SCTPChunkData, DiamG, sport=3868)
 bind_layers(SCTPChunkData, DiamG, proto_id=46)
 bind_layers(SCTPChunkData, DiamG, proto_id=47)

scapy/layers/sctp.py

@@ -24,6 +24,7 @@
     IntEnumField,
     IntField,
     MultipleTypeField,
+    PacketLenField,
     PacketListField,
     PadField,
     ShortEnumField,
@@ -624,6 +625,26 @@ class SCTPChunkParamAdaptationLayer(_SCTPChunkParam, Packet):
 }
 
 
+class _SCTPChunkDataField(PacketLenField):
+    """PacketLenField that dispatches using bind_layers bindings."""
+
+    def m2i(self, pkt, m):
+        # Only dissect complete messages
+        if pkt.beginning != 1 or pkt.ending != 1:
+            return m
+        # Check bind_layers bindings
+        for fval, cls in pkt.payload_guess:
+            if all(
+                hasattr(pkt, k) and v == pkt.getfieldval(k)
+                for k, v in fval.items()
+            ):
+                try:
+                    return cls(m)
+                except Exception:
+                    pass
+        return m
+
+
 class SCTPChunkData(_SCTPChunkGuessPayload, Packet):
     # TODO : add a padding function in post build if this layer is used to generate SCTP chunk data  # noqa: E501
     fields_desc = [ByteEnumField("type", 0, sctpchunktypes),
@@ -637,7 +658,8 @@ class SCTPChunkData(_SCTPChunkGuessPayload, Packet):
                    XShortField("stream_id", None),
                    XShortField("stream_seq", None),
                    IntEnumField("proto_id", None, SCTP_PAYLOAD_PROTOCOL_INDENTIFIERS),  # noqa: E501
-                   PadField(StrLenField("data", None, length_from=lambda pkt: pkt.len - 16),  # noqa: E501
+                   PadField(_SCTPChunkDataField("data", None, conf.raw_layer,
+                                                length_from=lambda pkt: pkt.len - 16),  # noqa: E501
                             4, padwith=b"\x00"),
                    ]
 

test/contrib/diameter.uts

@@ -253,3 +253,33 @@ r3b = DiamReq ('Multimedia-Auth', drHbHId=0x5478, drEtEId=0x1234,
 
 raw(r3b) == raw(r3)
 
+
+#######################################################################
++ Diameter over SCTP
+#######################################################################
+
+= Diameter decoded from SCTPChunkData via proto_id binding
+
+from scapy.layers.sctp import SCTP, SCTPChunkData
+
+diam_pkt = DiamAns('Capabilities-Exchange', drHbHId=0x1234, drEtEId=0x5678,
+                    avpList=[AVP('Origin-Host', val='host.example.com'),
+                             AVP('Origin-Realm', val='example.com')])
+
+pkt = SCTP(raw(SCTP() / SCTPChunkData(proto_id=46, beginning=1, ending=1, data=raw(diam_pkt))))
+chunk = pkt[SCTPChunkData]
+assert isinstance(chunk.data, DiamG)
+assert chunk.proto_id == 46
+assert chunk.data.drHbHId == 0x1234
+assert chunk.data.avpList[0].avpCode == 264
+
+= SCTPChunkData with unknown proto_id keeps raw bytes
+
+pkt = SCTP(raw(SCTP() / SCTPChunkData(proto_id=0, data=b"test")))
+assert pkt[SCTPChunkData].data == b"test"
+
+= SCTPChunkData fragment is not decoded
+
+pkt = SCTP(raw(SCTP() / SCTPChunkData(proto_id=46, beginning=1, ending=0, data=raw(diam_pkt))))
+assert not isinstance(pkt[SCTPChunkData].data, DiamG)
+