I am trying to create a raw socket in Python that listens for UDP packets only:
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_UDP)
s.bind(('0.0.0.0', 1337))
while True:
print s.recvfrom(65535)
This needs to be run as root, and creates a raw socket on port 1337, which listens for UDP packets and prints them whenever they are received; no problems there.
Now let's make a little client to test if this works:
import socket
c = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
c.connect(('127.0.0.1', 1337))
c.send('message 1')
c.send('message 2')
c.send('message 3')
c.send('message 4')
c.send('message 5')
c.send('message 6')
Consistently, only the first, third, and fifth message (message 1
, message 3
and message 5
) will get through and be printed in the server output. The second, fourth, and sixth messages do not show up on the server output, and instead the client gets an exception:
>>> c.send('message 2')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
socket.error: [Errno 111] Connection refused
Running this in Wireshark shows that it is getting an ICMP reply for "Destination unreachable". I have been able to reproduce this on 3 distinct machines (all running Linux though). Am I missing something? Is this expected behaviour for UDP to consistently drop packets, since protocols using it are supposed to be tolerant of packet loss? Even so, why would packets be dropped when sent on the local interface?
Binding the server to 127.0.0.1
instead of 0.0.0.0
has the same result.