Update inet.py

Fix UDP packet unable to calculate validity when carrying AH extension header

Author: Shu-xueyuan

scapy/layers/inet.py

@@ -755,6 +755,12 @@ def post_build(self, p, pay):
             elif conf.ipv6_enabled and isinstance(self.underlayer, scapy.layers.inet6.IPv6) or isinstance(self.underlayer, scapy.layers.inet6._IPv6ExtHdr):  # noqa: E501
                 ck = scapy.layers.inet6.in6_chksum(socket.IPPROTO_TCP, self.underlayer, p)  # noqa: E501
                 p = p[:16] + struct.pack("!H", ck) + p[18:]
+            elif self.underlayer.name == "AH" and isinstance(self.underlayer.underlayer, IP):
+                ck = in4_chksum(socket.IPPROTO_TCP, self.underlayer.underlayer, p)
+                p = p[:16] + struct.pack("!H", ck) + p[18:]
+            elif self.underlayer.name == "AH" and (conf.ipv6_enabled and isinstance(self.underlayer.underlayer, scapy.layers.inet6.IPv6) or isinstance(self.underlayer.underlayer, scapy.layers.inet6._IPv6ExtHdr)):  # noqa: E501
+                ck = scapy.layers.inet6.in6_chksum(socket.IPPROTO_TCP, self.underlayer.underlayer, p)  # noqa: E501
+                p = p[:16] + struct.pack("!H", ck) + p[18:]
             else:
                 log_runtime.info(
                     "No IP underlayer to compute checksum. Leaving null."
@@ -833,6 +839,18 @@ def post_build(self, p, pay):
                 if ck == 0:
                     ck = 0xFFFF
                 p = p[:6] + struct.pack("!H", ck) + p[8:]
+            elif self.underlayer.name == "AH" and isinstance(self.underlayer.underlayer, IP):
+                ck = in4_chksum(socket.IPPROTO_UDP, self.underlayer.underlayer, p)
+                # According to RFC768 if the result checksum is 0, it should be set to 0xFFFF  # noqa: E501
+                if ck == 0:
+                    ck = 0xFFFF
+                p = p[:6] + struct.pack("!H", ck) + p[8:]
+            elif self.underlayer.name == "AH" and (isinstance(self.underlayer.underlayer, scapy.layers.inet6.IPv6) or isinstance(self.underlayer.underlayer, scapy.layers.inet6._IPv6ExtHdr)):  # noqa: E501
+                ck = scapy.layers.inet6.in6_chksum(socket.IPPROTO_UDP, self.underlayer.underlayer, p)  # noqa: E501
+                # According to RFC2460 if the result checksum is 0, it should be set to 0xFFFF  # noqa: E501
+                if ck == 0:
+                    ck = 0xFFFF
+                p = p[:6] + struct.pack("!H", ck) + p[8:]
             else:
                 log_runtime.info(
                     "No IP underlayer to compute checksum. Leaving null."