Simple Python UDP Server: trouble receiving packets from clients other than localhost

Question

So, the very simple code that I'm trying to use is here: http://wiki.python.org/moin/UdpCommunication

(also here): Sending:

import socket
UDP_IP = "127.0.0.1"
UDP_PORT = 5005
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.sendto(MESSAGE, (UDP_IP, UDP_PORT))

Receiving:

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

The code works fine when I run both applications on my computer. I put the sending code on my laptop with:

UDP_IP="IP address for my computer"

Everything else is the same. Yet, nothing happens. What am I doing incorrectly? I used wireshark and determined that the packet is being sent and received; however, the python program isn't receiving the packet. I'm very confused.

Any help is much appreciated. Thanks in advance.

Answer 1

eventually figured out my issue and it was pretty complex and highly localized,

I had a very similar problem happen to me. I realize that you have already solved this problem, however I thought it would be good to share how I solved the issue for me.

The issue I found was with my firewall settings. I found that the packets were being blocked by Windows Firewall.

I too had used Wireshark which showed that packets were being sent and received. It is important to note that Wireshark grabs packets at a much lower level than a Python application.

By running my code locally with a listener on one port and a client on another port on the same PC, the firewall wasn't blocking the packets. When I moved to interfacing with an external machine, the firewall rules came into play blocking the incoming packets.

Changing the firewall policy fixed this issue. There are numerous ways and inherent security risks to changing the firewall to make this work so I will leave that part up to the IT professionals. :-)

Answer 2

Try binding to all local interfaces on the receiving side:

sock.bind(("", UDP_PORT)) # could also use "0.0.0.0"

Note that the behavior of operating systems is not entirely logical (nor consistent) in terms of binding when receiving UDP packets, especially for multicast traffic. This is the behavior you get:

Linux: Binding to a specific IP will filter the incoming UDP packets and only the ones targeted at this specific IP will get through the filter. This means for example that multicast UDP packets received by an interface with IP 192.168.1.100 will not be received when binding to IP 192.168.1.100. On Linux the normal bind does not bind to an interface. Use setsockopt(SO_BINDTODEVICE) for this. Binding to 0.0.0.0 (or "" on Python) will always receive all UDP packets received by the machine on all interfaces, regardless of the destination IP, so this is usually the most useful option on Linux.

Windows: Binding to a specific IP will bind to the interface belonging to this IP, pretty much like setsockopt(SO_BINDTODEVICE) does on Linux. Incoming UDP packets are not filtered by this IP, so multicast traffic can be received even when binding to a concrete IP. (This is probably the first time the Windows behavior seems more consistent to me than the Linux behavior.)

Python does not abstract these OS specific differences away for sockets (as it does in other areas). As long as you have no explicit reason not to do so I suggest to always bind to 0.0.0.0.