CODEWITHSUNDEEP
pythonnetwork-programmingpcapy
Newbie Developer
Newbie Developer

Reputation: 19

Byte slicing using brackets for unpack

I'm trying to slice a variable containing a number of bytes, so I can use unpack with a buffer of 14 bytes. But apparently it's not working. What am I doing wrong here?

import pcapy
from struct import *
import sys

devs = pcapy.findalldevs()

cap = pcapy.open_live(devs[4], 65535, 0, 1)

while 1:
    (header, payload) = cap.next()
    print(sys.getsizeof(payload))
    l2hdr = payload[:14]
    print(sys.getsizeof(l2hdr))
    l2data = unpack("!6s6sH", l2hdr)
    srcmac = "%.2x:%.2x:%.2x:%.2x:%.2x:%.2x" % (l2hdr[0]), (l2hdr[1]), (l2hdr[2]), (l2hdr[3]), (l2hdr[4]), (l2hdr[5])
    dstmac = "%.2x:%.2x:%.2x:%.2x:%.2x:%.2x" % (l2hdr[6]), (l2hdr[7]), (l2hdr[8]), (l2hdr[9]), (l2hdr[10]), (l2hdr[11])
    print("Source MAC: ", srcmac, " Destination MAC: ", dstmac)

and here's the result

Traceback (most recent call last):
  File "C:/Users/admin/Downloads/test1.py", line 14, in <module>
    l2data = unpack("!6s6sH", l2hdr)
17
struct.error: unpack requires a buffer of 14 bytes
17

Process finished with exit code 1

Upvotes: 1

Views: 242

Answers (1)

Michael Robinson
Michael Robinson

Reputation: 956

I suspect your payload is empty. The 17 bytes is too small.

If you check the len of the payload and/or header, it should be greater than 0.

Use that as a test to see if you have any data. 

while 1:
    (header, payload) = cap.next()
    while len(payload) == 0:
        (header, payload) = cap.next()

Though that's now showing an error in the rest of your code. 

Traceback (most recent call last):
  File "go.py", line 18, in <module>
    srcmac = "%.2x:%.2x:%.2x:%.2x:%.2x:%.2x" % (l2hdr[0]), (l2hdr[1]), (l2hdr[2]), (l2hdr[3]), (l2hdr[4]), (l2hdr[5])
TypeError: not enough arguments for format string

I rewrote your original to this with fstrings and it's working well

import pcapy
from struct import unpack

devs = pcapy.findalldevs()

cap = pcapy.open_live(devs[4], 65535, 0, 1)

while 1:
    (header, payload) = cap.next()
    while len(payload) == 0:
        (header, payload) = cap.next()
    l2hdr = payload[:14]
    l2data = unpack("!6s6sH", l2hdr)
    srcmac = f"{l2hdr[0]:2x}:{l2hdr[1]:2x}:{l2hdr[2]:2x}:{l2hdr[3]:2x}:{l2hdr[4]:2x}:{l2hdr[5]:2x}"
    dstmac = f"{l2hdr[6]:2x}:{l2hdr[7]:2x}:{l2hdr[8]:2x}:{l2hdr[9]:2x}:{l2hdr[10]:2x}:{l2hdr[11]:2x}"
    print("Source MAC: ", srcmac, " Destination MAC: ", dstmac)

Upvotes: 0

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