Python UDP socket does not receive message on loopback address

Question

I'm facing an issue to send and receive message using UDP socket with Python.

The problem is when I try to send and receive a message using this code i don't receive anything: Receiver.py:

import socket

UDP_IP = "127.0.0.1"
UDP_PORT = 55681
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((UDP_IP, UDP_PORT))
while True:
     data, addr = sock.recvfrom(1024) # buffer size is 1024 bytes
print "received message:", data

Sender.py:

import socket

UDP_IP = "192.168.1.161"
UDP_PORT = 55681
MESSAGE = "Hello, World!"

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

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

But if I change the code of the Receiver by this :

import socket

UDP_IP = "192.168.1.161"
UDP_PORT = 55681

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

It works perfectly. Also it is the same case if I put 127.0.0.1 as addresses in both programs.Why ?

I work on linux with python 2.7 and this is the result of ifconfig:

lo        Link encap:Local Loopback  
      inet addr:127.0.0.1  Mask:255.0.0.0
      inet6 addr: ::1/128 Scope:Host
      UP LOOPBACK RUNNING  MTU:16436  Metric:1
      RX packets:12252 errors:0 dropped:0 overruns:0 frame:0
      TX packets:12252 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:1528301 (1.5 MB)  TX bytes:1528301 (1.5 MB)

wlan0     Link encap:Ethernet  HWaddr 00:21:6a:5b:b4:dc  
      inet addr:192.168.1.161  Bcast:192.168.1.255  Mask:255.255.255.0
      inet6 addr: XXXX::XXX:XXXX:XXXX:XXXX/64 Scope:Link
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:532561 errors:0 dropped:0 overruns:0 frame:0
      TX packets:318747 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000 
      RX bytes:629969205 (629.9 MB)  TX bytes:32669218 (32.6 MB)

Thanks by advance for your answers.

Answer 1

When you execute this line in your receiver:

sock.bind((UDP_IP, UDP_PORT))

you're binding only to the address specified by UDP_IP. Thus, your socket will only receive packets sent to that address. This is why it works when you match your sender's and receiver's addresses.

If you want to receive packets on any address, bind like this instead:

sock.bind(('', UDP_PORT))

As documented, specifying '' makes bind bind to all available addresses.