Fixed a TypeError that triggered when reading a pcap file containing a NTP packet. (#1186)

* Fixed an error (TypeError) that triggered when reading a pcap file that
contained ntp packets. The type of variable "cls" was string not a class
type (invalid argument for method: issubclass()). By importing the type
'TCP' from 'scapy.layers.inet' and by converting cls to a classtype by
reading the key from globals, the TypeError is mitigated.

* Added test to provide insight into the cls variable inconsistencies.
Fixed a small mistake

* https://github.com/secdev/scapy/pull/1186/files/d0a2544ae13ba353d25e86be2cfc13cceda36a10

* Moved tests.

* fix backward incompatibility of scapy.utils.RawPcapReader.read_packet() for scapy's v2.4 againts v2.3.2 & v2.3.3
named tuple instead of tuple as result of RawPcap[Ng]Reader.read_block_???()/read_packet():

* Fixed an error (TypeError) that triggered when calling sessions() on a PacketList that
contained ntp packets. The type of variable "cls" was a string not a class
type (invalid argument for method: issubclass()).

* Added test to provide insight into the cls variable inconsistencies.
Fixed a small mistake

* https://github.com/secdev/scapy/pull/1186/files/d0a2544ae13ba353d25e86be2cfc13cceda36a10

* Forgot to delete ntp.uts during rebase...

Author: Neurocinetics

scapy/layers/ntp.py

@@ -214,7 +214,7 @@ def haslayer(self, cls):
         if cls == "NTP":
             if isinstance(self, NTP):
                 return True
-        elif issubclass(cls, NTP):
+        elif not isinstance(cls, str) and issubclass(cls, NTP):
             if isinstance(self, cls):
                 return True
         return super(NTP, self).haslayer(cls)

test/regression.uts

@@ -7053,7 +7053,6 @@ assert(not NTPControl in p)
 assert(NTPPrivate in p)
 assert(NTP in p)
 
-
 = NTP - Layers (2)
 p = NTPHeader()
 assert(type(p[NTP]) == NTPHeader)
@@ -7062,6 +7061,17 @@ assert(type(p[NTP]) == NTPControl)
 p = NTPPrivate()
 assert(type(p[NTP]) == NTPPrivate)
 
+= NTP - sessions (1)
+p = IP()/TCP()/NTP()
+l = PacketList(p)
+s = l.sessions()  # Crashed on commit: e42ecdc54556c4852ca06b1a6da6c1ccbf3f522e
+assert len(s) == 1
+
+= NTP - sessions (2)
+p = IP()/UDP()/NTP()
+l = PacketList(p)
+s = l.sessions()  # Crashed on commit: e42ecdc54556c4852ca06b1a6da6c1ccbf3f522e
+assert len(s) == 1
 
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