Scapy packet sent cannot be received

Question

I'm trying to send UDP Packets with scapy with the following command:

>> send(IP(dst="127.0.0.1",src="111.111.111.111")/UDP(dport=5005)/"Hello")
.
Sent 1 packets.

And from tcpdump I can see:

22:02:58.384730 IP 111.111.111.111.domain > localhost.5005: [|domain]

I'm trying to receive this packet with the following code:

import socket

UDP_IP = "127.0.0.1"
UDP_PORT = 5005

sock = socket.socket(socket.AF_INET, # Internet
                     socket.SOCK_DGRAM) # UDP
sock.bind((UDP_IP, UDP_PORT))

while True:
    data, addr = sock.recvfrom(1024) # buffer size is 1024 bytes
    print "received message:", data

But cannot receive the message.

I have tested the network by sending udp packets normally with the following code and the packet can be received:

import socket
import time

UDP_IP = "127.0.0.1"
UDP_PORT = 5005

print "UDP target IP:", UDP_IP
print "UDP target port:", UDP_PORT

sock = socket.socket(socket.AF_INET, # Internet
                     socket.SOCK_DGRAM) # UDP
num = 0
while True:
  sock.sendto(str(num), (UDP_IP, UDP_PORT))
  print "Message sent: " + str(num)
  num += 1
  time.sleep(1)

Any help will be greatly appreciated.

----------------Updates-----------------------

A packet sent by Scapy that can not be received:

13:22:52.984862 IP (tos 0x0, ttl 64, id 1, offset 0, flags [DF], proto UDP (17), length 33)
    127.0.0.1.5555 > 127.0.0.1.12345: [udp sum ok] UDP, length 5
    0x0000:  4500 0021 0001 4000 4011 3cc9 7f00 0001  E..!..@.@.<.....
    0x0010:  7f00 0001 15b3 3039 000d 9813 4865 6c6c  ......09....Hell
    0x0020:  6f     

                              o

While a packet sent by normal python script that can be received:

13:20:02.374481 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 33)
    127.0.0.1.53143 > 127.0.0.1.12345: [bad udp cksum 0xfe20 -> 0xde2e!] UDP, length 5
    0x0000:  4500 0021 0000 4000 4011 3cca 7f00 0001  E..!..@.@.<.....
    0x0010:  7f00 0001 cf97 3039 000d fe20 4865 6c6c  ......09....Hell
    0x0020:  6f

Solution

Looks like you are using Scapy to send the UDP traffic to your localhost interface. In the send() function, specify the appropriate outbound interface to send the traffic out.

Example:

send((IP(dst="127.0.0.1",src="111.111.111.111")/UDP(dport=5005)/"Hello"),iface="lo0")

On my computer, the lo0 is my local loopback interface. To see or set the default interface for scapy, check out the bottom half of this post: http://thepacketgeek.com/scapy-p-02-installing-python-and-scapy/

Comments

[ethanjyx]

Thanks Mat! I found ifconfig and found eth0, eth1 and lo. I tried every of them but it is still not working...

[Mat Wood]

Since you can sniff the Scapy packet with tcpdump, I don't think it's a Scapy issue. I found a similar thread with this issue: stackoverflow.com/questions/17688061/…

[Ethan Heilman]

I have the same issue. I created a non-scapy program and can receive packets. Comparing the packets that work and the ones that don't in wireshark shows the only difference is the scapy packets are using a layer 2 broadcast address. If I specify a MAC addresses in the packet scapy overwrites the MAC addresses I specify with a broadcast address.