Fix crash caused due to unknown or missing network or transport layers

Parser would crash when confronted with no network or transport layer
information. This issue is fixed by checking if these layers are `nil`
before attempting to reference them.

parser/parse.go

@@ -112,9 +112,36 @@ PACKETLOOP:
 			continue
 		}
 
+		// Parse network layer information
+		networkLayer := packet.NetworkLayer()
+		if networkLayer == nil {
+			log.Error("Unknown/missing network layer for packet")
+			stats.PacketErrors += 1
+			continue
+		}
+		switch networkLayer.LayerType() {
+		case layers.LayerTypeIPv4:
+			ip4 = networkLayer.(*layers.IPv4)
+			schema.SourceAddress = ip4.SrcIP.String()
+			schema.DestinationAddress = ip4.DstIP.String()
+			schema.Ipv4 = true
+			stats.PacketIPv4 += 1
+		case layers.LayerTypeIPv6:
+			ip6 = networkLayer.(*layers.IPv6)
+			schema.SourceAddress = ip6.SrcIP.String()
+			schema.DestinationAddress = ip6.DstIP.String()
+			schema.Ipv4 = false
+			stats.PacketIPv6 += 1
+		}
+
 		// Parse DNS and transport layer information
 		msg = nil
 		transportLayer := packet.TransportLayer()
+		if transportLayer == nil {
+			log.Error("Unknown/missing transport layer for packet")
+			stats.PacketErrors += 1
+			continue
+		}
 		switch transportLayer.LayerType() {
 		case layers.LayerTypeTCP:
 			tcp = transportLayer.(*layers.TCP)
@@ -161,23 +188,6 @@ PACKETLOOP:
 			continue PACKETLOOP
 		}
 
-		// Parse network layer information
-		networkLayer := packet.NetworkLayer()
-		switch networkLayer.LayerType() {
-		case layers.LayerTypeIPv4:
-			ip4 = networkLayer.(*layers.IPv4)
-			schema.SourceAddress = ip4.SrcIP.String()
-			schema.DestinationAddress = ip4.DstIP.String()
-			schema.Ipv4 = true
-			stats.PacketIPv4 += 1
-		case layers.LayerTypeIPv6:
-			ip6 = networkLayer.(*layers.IPv6)
-			schema.SourceAddress = ip6.SrcIP.String()
-			schema.DestinationAddress = ip6.DstIP.String()
-			schema.Ipv4 = false
-			stats.PacketIPv6 += 1
-		}
-
 		// Ignore questions unless flag set
 		if !msg.Response && !DoParseQuestions && !DoParseQuestionsEcs {
 			continue PACKETLOOP