utils: fall back to the writer linktype for unbound layers

scapy/utils.py

@@ -2211,9 +2211,12 @@ def write_packet(self,
         ifname = getattr(packet, "sniffed_on", None)
         direction = getattr(packet, "direction", None)
         if not isinstance(packet, bytes):
-            linktype: int = conf.l2types.layer2num[
-                packet.__class__
-            ]
+            # The class may not be bound to any linktype (e.g. conf.raw_layer),
+            # in which case fall back to the one write_header() settled on.
+            linktype: int = conf.l2types.layer2num.get(
+                packet.__class__,
+                self.linktype,
+            )
         else:
             linktype = self.linktype
         if ifname is not None:

test/regression.uts

@@ -2779,6 +2779,21 @@ with RawPcapWriter(fd) as w:
     w.write(b"test")
     assert w.linktype == 1
 
+= Check PcapWriter with a layer that is not bound to any linktype
+~ pcap
+
+from io import BytesIO
+
+f = BytesIO()
+w = PcapWriter(f, linktype=DLT_RAW, sync=True)
+w.write(Raw(b"test"))
+assert w.linktype == DLT_RAW
+
+f.seek(0) or None
+r = RawPcapReader(f)
+assert r.linktype == DLT_RAW
+assert [x for (x, y) in r] == [b"test"]
+
 = Check tcpdump()
 ~ tcpdump
 from io import BytesIO