python arp header unpacking with struct module

Question

I'm going to make my own python sniffer but i have problem with unpacking arp protocol header.

here is my code:

def Sniffer():
    try:
        # AF_PACKET, That's basically packet level.
        # 0X0003, That's every packet. (We can find it here: /usr/include/linux/if_ether.h) 
        SK = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(0x0003))
    except socket.error as MSG:
        print "Socket creation error:\n", MSG

    try:
        while True:
            Receive = SK.recvfrom(65565)
            Packet = Receive[0]
            Ethernet(Packet)
    except socket.error as MSG:
        print "Receive error:\n", MSG



# Ethernet Decapsulation (We need EtherType field value)
def Ethernet(Packet):
    ETHERNET_LENGTH = 14
    ETHERNET_HEADER = Packet[:ETHERNET_LENGTH]
    ETHERNET_HEADER_UNPACK = struct.unpack("!6s6sH", ETHERNET_HEADER)

    EtherType = ETHERNET_HEADER_UNPACK[2]
    print EtherType

    if EtherType == 2054:
        ARP(ETHERNET_LENGTH, Packet)
    if EtherType == 2048:
        IPV4(Packet)



# ARP Decapsulation (We need OPCODE field value)
def ARP(ETHERNET_LENGTH, Packet):
    ARP_LENGTH = 42
    ARP_HEADER = Packet[ETHERNET_LENGTH:ARP_LENGTH]
    ARP_HEADER_UNPACK = struct.unpack("!2s2s1s1s2s6s4s6s4s", ARP_HEADER)

    OPCODE = ARP_HEADER_UNPACK[4]

    if OPCODE == 1:
        print "ARP Request (Some one scann your network)"

That's my error:

Traceback (most recent call last):
  File "HoneySniffer.py", line 130, in <module>
    Sniffer()
  File "HoneySniffer.py", line 22, in Sniffer
    Ethernet(Packet)
  File "HoneySniffer.py", line 38, in Ethernet
    ARP(ETHERNET_LENGTH, Packet)
  File "HoneySniffer.py", line 48, in ARP
    ARP_HEADER_UNPACK = struct.unpack("!2s2s1s1s2s6s4s6s4s", ARP_HEADER)
struct.error: unpack requires a string argument of length 28

why this is happening?
How can i to fix it?

I find it here: Python arp sniffing raw socket no reply packets

score 0 · Answer 1 · answered Apr 03 '19 at 17:33

I have the exact same problem and i solved it using your reference link Python arp sniffing raw socket no reply packets and little research of my own.

conn=socket.socket(socket.AF_PACKET,socket.SOCK_RAW,socket.ntohs(0x0003))

def arp_header(packet):

(a ,b ,c ,d ,e ,f ,g ,h ,i ) = struct.unpack('2s2s1s1s2s6s4s6s4s',packet[14:42])

hw_type=(binascii.hexlify(a)).decode('utf-8')
proto_type=(binascii.hexlify(b)).decode('utf-8')
hw_size=(binascii.hexlify(c)).decode('utf-8')
proto_size=(binascii.hexlify(d)).decode('utf-8')
opcode=(binascii.hexlify(e)).decode('utf-8')

return (hw_type,proto_type,hw_size,proto_size,opcode,socket.inet_ntoa(g),socket.inet_ntoa(i))

use struct.unpack('2s2s1s1s2s6s4s6s4s',packet[14:42]) not struct.unpack('!2s2s1s1s2s6s4s6s4s',packet[14:42]) this solved my struct.error: unpack requires a string argument of length 28 error

i have also used individual variables,due to my project requirements,

Now ,for debugging i used print(packet[14:42]) -which gave me byte hex literals(like b'\x00\x01\x08\x00\x06\x04\x00\x01\xe4\x11[I&\xbe\n\x00..... etc)

so i have to decode in utf-8 after using hexlify,since hexlify is again returning byte objects to me.

Python version i used:3.6.5

Date:03/April/2019

Result:- Arp Packet: - H/W Type: 0001, Protocol Type: 0800, H/W Size: 06 ,Protocol Size: 04 - Opcode: 0001, Source IP: 10.0.15.141, Destination IP: 10.0.10.2

Verdict:This method worked for me,i hope it will work for you also :)

python arp header unpacking with struct module

1 Answers1