Python raw socket receive issue

Question

I am using the following python script for raw socket packet transfer. Packet transfer is fine, but I am not able to print the incoming packet from the other end.

from socket import socket, AF_PACKET, SOCK_RAW
s = socket(AF_PACKET, SOCK_RAW)
s.bind(("eth0", 0))
src_addr = "\x54\xbe\xf7\x40\xf5\x82"
dst_addr = "\xff\xff\xff\xff\xff\xff"
payload = ("[("*30)+"Hello"+("]"*30)
checksum = "\x1a\x2b\x3c\x4d"
data = payload+checksum
s.send(dst_addr+src_addr+data)
#for receive function 
response=s.recv(4096)
print response
s.close()

• BrandonM: Does the receiving end give any errors? What is the output of the print statement?
• Menglong Li: the server that receiving msgs should be always active, but seems that you closed the socket.
• Balaji subramanian: No I am get any print from "print response" Brandon. Its keep on waiting recv fn. Even after it receives from client.
• Balaji subramanian: I tried without closing socket also but it is not capturing the incoming packets

Answer 1

There is a third argument to the socket function: protocol. If not given, it's defaulting to 0. For AF_PACKET / SOCK_RAW, the protocol argument specifies what kind of packets you're interested in receiving. The values are documented in the packet(7) man page: http://man7.org/linux/man-pages/man7/packet.7.html

I don't think the values are actually defined anywhere in the core python2 modules. Some of them can be found in scapy (http://www.secdev.org/projects/scapy/), or you can just hunt up the linux header file where they are defined (/usr/include/linux/if_ether.h).

So, to fix this, change your code to:

from socket import socket, AF_PACKET, SOCK_RAW, htons
ETH_P_ALL = 3
ETH_P_IP = 0x800 # Alternatively using this will receive the next IP packet
s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))
...

Other comments on your code:

As written, the packet you're sending is unlikely to be comprehensible by anyone. You've got a dst and src MAC address, but then you're not providing an EtherType. Instead the first "[(" will be seen as the EtherType. That probably won't make sense to any receiver of the packet so it will just be discarded.

Also, you should understand that with a raw socket, you're going to receive the next packet of the type you've specified in the protocol. That isn't necessarily (and in fact probably won't be) a response to the packet you just sent.

• repzero: I checked the linux man pages link, no protocol declarations there...Hunting them down in the if_ether.h file did the job...gr8!

Answer 2

Thanks everyone now I am able receive the packet with the following the script. But still I am facing issue with printing multiple response packets(While doing in loop).

from socket import socket, AF_PACKET, SOCK_RAW, htons
from struct import *
import select
import time
ETH_P_ALL = 3
ETH_P_IP = 0x800 
s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))
s.bind(("eth0", 0))
src_addr = "\x54\xbe\xf7\x40\xf7\x82"
dst_addr = "\xff\xff\xff\xff\xff\xff" 
l = "\x00\x21"
ethertype = "\x08\x01"
a ="\x00\x10\x00\x10\x00\x10"
b = "\x00\x11\x00\x11\x00\x11"
payload = ethertype + l + a + b
for i in range(5):
 time.sleep(5) 
 s.send(dst_addr+src_addr+ payload)
 message=s.recv(4096)
 print message

• Meric Ozcan: Could you please give better variable names, such as what is a and what is b