Sniffer with RAW SOCKETS

Hi all. I'm trying to make a simple icmp sniffer by using SOCK_RAW.
The code below works but ONLY if I first use the sendto() function.
Does anybody knows why?
Regards

from socket import *
import select
def recv():
while 1:
if s in select.select([s],[],[],99)[0]:
reply = s.recvfrom(2000)[0]
print reply
s = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)
s.setsockopt(IPPROTO_IP, IP_HDRINCL, 1)
s.sendto('test', ('127.0.0.1', 0)) # without this it doesn't work.
recv()


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