modbus packet not being parsed correctly in 2.6.0

[jpinedaa]

Brief description

packets that were being correctly parsed as modbus in 2.5.0 are now not being recognized as Read Holding Registers Response in 2.6.0

Scapy version

2.6.0

Python version

3.10

Operating system

Ubuntu 22.04.4 LTS

Additional environment information

No response

How to reproduce

import sys
from scapy.contrib.modbus import *
from scapy.utils import rdpcap

pcap_file = sys.argv[1]
print(f"Reading pcap file: {pcap_file}")
packets = rdpcap(filename=pcap_file, count=1000)
print(packets[32].show2(dump=True))

with provided pcap file

Actual result

Reading pcap file: error.pcap
###[ cooked linux v2 ]###
proto = IPv4
reserved = 0
ifindex = 4
lladdrtype= 0x1
pkttype = unicast-to-another-host
lladdrlen = 6
src = b',\xcfg,\xe2?'
###[ IP ]###
version = 4
ihl = 5
tos = 0x0
len = 307
id = 18082
flags = DF
frag = 0
ttl = 64
proto = tcp
chksum = 0xd998
src = 10.1.2.168
dst = 10.1.2.225
\options \
###[ TCP ]###
sport = 502
dport = 58136
seq = 499266911
ack = 1928828226
dataofs = 8
reserved = 0
flags = PA
window = 249
chksum = 0x7860
urgptr = 0
options = [('NOP', None), ('NOP', None), ('Timestamp', (3747823981, 2872918422))]
###[ ModbusADU ]###
transId = 0x0
protoId = 0x0
len = 249
unitId = 0x1
###[ Raw ]###
load =
b'\x03\xf6\x02\xbd\x00\x99\x00\x02\x00\x01\x00\x03\x00\x01\x00\x00\x00\x00\xff\xff\xff\xff\x03\xd0\x03\xd1\x00\x00\x03\xe7
x00\x0c\x139\x0b\x19\x00\x00\xeaL\x00\x00\x00\x00\x00\x00\x00\x96\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
x00\x00\t\x00\x00\x00\x00\x00\x00\x00\x00\xff\xf6\x02&\x02\x8a\x01\xf4\x01\x90\x01\xa4\x01E\x01F\x00\x00\x03\xe7\x00\x04
\x13;\x0b\x1b\x00\x00\x00\x00\x00\x00\xa9[\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00|\x00\x00\x00\x00
x00\x00\x12\xf8\x01E\x01F\x00\x00\x03\xe7\x00\x04\x137\x0b\x19\x00\x00\x00\x00\x00\x00\xa9\x14\x00\x00\x00\x00\x00\x00
x00\x00\x00\x00\x00\x00\x00\x00\x00\x84\x00\x00\x00\x00\x00\x00\x13\x0c\x01E\x01F\x00\x00\x03\xe7\x00\x04\x139\x0b\x17
\x00\x00\x00\x00\x00\x00\xa9R\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x7f\x00\x00\x00\x00\x00\x00\x
13\x04\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfd\x00\x00\xff\xfd\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff'

Expected result

Reading pcap file: error.pcap
###[ cooked linux v2 ]###
proto = IPv4
reserved = 0
ifindex = 4
lladdrtype= 0x1
pkttype = unicast-to-another-host
lladdrlen = 6
src = ',\xcfg,\xe2?'
###[ IP ]###
version = 4
ihl = 5
tos = 0x0
len = 307
id = 18082
flags = DF
frag = 0
ttl = 64
proto = tcp
chksum = 0xd998
src = 10.1.2.168
dst = 10.1.2.225
\options \
###[ TCP ]###
sport = 502
dport = 58136
seq = 499266911
ack = 1928828226
dataofs = 8
reserved = 0
flags = PA
window = 249
chksum = 0x7860
urgptr = 0
options = [('NOP', None), ('NOP', None), ('Timestamp', (3747823981, 2872918422))]
###[ ModbusADU ]###
transId = 0x0
protoId = 0x0
len = 249
unitId = 0x1
###[ Read Holding Registers Response ]###
funcCode = 0x3
byteCount = 246
registerVal= [701, 153, 2, 1, 3, 1, 0, 0, 65535, 65535, 976, 977, 0, 999, 12, 4921, 2841, 0, 59980, 0, 0, 0, 150, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0,
0, 0, 65526, 550, 650, 500, 400, 420, 325, 326, 0, 999, 4, 4923, 2843, 0, 0, 0, 43355, 0, 0, 0, 0, 0, 0, 0, 124, 0, 0, 0, 4856, 325, 326, 0,
999, 4, 4919, 2841, 0, 0, 0, 43284, 0, 0, 0, 0, 0, 0, 0, 132, 0, 0, 0, 4876, 325, 326, 0, 999, 4, 4921, 2839, 0, 0, 0, 43346, 0, 0, 0, 0, 0, 0, 0,
127, 0, 0, 0, 4868, 65535, 65535, 65535, 65535, 65535, 65533, 0, 65533, 0, 0, 0, 0, 65535]

Related resources

error.zip