How to use the ` sr ` function in the ` scapy ` module to sniff the response packets for a custom protocol?

I made a custom packet layer by the module "Scapy". I want to use the sr function in the same module to send a UDP packet carrying this custom layer as its payload to a remote device and sniff responses from the device. Below is my code to implement for this purpose:

from scapy.all import *

PORTNUMBER = 8888

class MyLayer(Packet):

  name = "MyLayer"

  fields_desc = [BitField("id", 1, 16),
                 FieldLenField("len", None, length_of = "data"),
                 StrLenField("data", "", length_from = lambda pkt: pkt.len)]

  def answers(self, other):
    print(">>> Running answers of MyLayer")
    if other.__class__ == self.__class__:
      return 1
    return 0


bind_layers(UDP, MyLayer, dport = PORTNUMBER)

if __name__ == "__main__":

  src = "192.168.1.200"
  dst = "192.168.1.255"

  ipHeader = IP(src = src, dst = dst)
  udpHeader = UDP(sport = PORTNUMBER, dport = PORTNUMBER)
  myLayer = MyLayer(data = "abcd1234")
  reqPacket = ipHeader/udpHeader/myLayer

  results, _ = sr(reqPacket, 
                   timeout = 2,
                   filter = f"udp dst port {PORTNUMBER}", 
                   iface = "NIC for Test")

As the code above illustrates, I create the custom protocol layer called MyLayer which is the custom protocol I need. I hope sr will use the answers function in the MyLayer class to check if a sniffed packet should be regarded as a response for the packet it sent. And to confirm that the answers function of MyLayer is called by sr, I print the message ">>> Running answers of MyLayer" in the beginning of the answers function.

However, this code does not work. The response packets from the remote device have all been disregarded by the sr function. And the message ">>> Running answers of MyLayer" has never been printed. I think the reason for why the code above did not work is that the received packet failed to pass check of sr function by the answer function of the IP layer.

Can anyone tell me if there is any way to make the sr function able to regard the received packets as legitimate answers for the packet it sent? Thank you in advance.

Solution

Thanks to the reply of MonteChrist0, I found that the root cause of my problem is related to the broadcast IP address of the request packet. So I add the line conf.checkIPaddr = 0 to my code. Moreover, the multi parameter of the sr function has to be assigned the value True or the function will only return the request packet as the answer. Below is the code which works for me:

from scapy.all import *

PORTNUMBER = 8888
conf.checkIPaddr = 0

class MyLayer(Packet):

  name = "MyLayer"

  fields_desc = [BitField("id", 1, 16),
                 FieldLenField("len", None, length_of = "data"),
                 StrLenField("data", "", length_from = lambda pkt: pkt.len)]

if __name__ == "__main__":

  src = "192.168.1.200"
  dst = "192.168.1.255"

  ipHeader = IP(src = src, dst = dst)
  udpHeader = UDP(sport = PORTNUMBER, dport = PORTNUMBER)
  myLayer = MyLayer(data = "abcd1234")
  reqPacket = ipHeader/udpHeader/myLayer

  results, _ = sr(reqPacket, 
                  timeout = 2,
                  filter = f"udp dst port {PORTNUMBER}", 
                  multi = True,
                  iface = "NIC for Test")

I remove the lines for the answers function and bind_layers in the code above to prove that adding the line conf.checkIPaddr = 0 alone is enough to make sc to recognize the response packets for the broadcast packets sent by sr as answers in my case.

But as MonteChrist0 pointed out earlier, if we want to implement the answers function of the MyLayer class and want it to be called by sr, the line for the bind_layers will be required. So a more complete version of the code will be as follows:

from scapy.all import *

PORTNUMBER = 8888
conf.checkIPaddr = 0

class MyLayer(Packet):

  name = "MyLayer"

  fields_desc = [BitField("id", 1, 16),
                 FieldLenField("len", None, length_of = "data"),
                 StrLenField("data", "", length_from = lambda pkt: pkt.len)]

  def answers(self, other):
    if other.__class__ == self.__class__:
      return 1
    return 0


bind_layers(UDP, MyLayer, dport = PORTNUMBER)

if __name__ == "__main__":

  nic = "192.168.1.200"
  dst = "192.168.1.255"

  ipHeader = IP(src = nic, dst = dst)
  udpHeader = UDP(sport = PORTNUMBER, dport = PORTNUMBER)
  myLayer = MyLayer(data = "abcd1234")
  reqPacket = ipHeader/udpHeader/myLayer

  results, _ = sr(reqPacket, 
                  timeout = 2,
                  filter = f"dst host {nic} and udp dst port {PORTNUMBER}", 
                  multi = True,
                  iface = "NIC for Test")

( I think it should be better to filter the request packet by the filter parameter of sr so that the request packet will not be regarded as an answer packet by sr. That is why I modify the condition following the filter parameter in the code above as well. )