Issues with sending lines over netcat to Python

Question

I am piping a program's stdout to netcat: nc -u localhost 50000. Listening on UDP 50000 is a Python program that does something like this:

  lstsock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
  lstsock.setblocking(0)

  while True:
        print '1'
        try:
            tmp = lstsock.recv(SOCK_BUFSZ)
        except socket.error, e:
            if e.args[0] == errno.EWOULDBLOCK:
                sleep(.5)
                continue
            else:
                print("Socket error: {}".format(e))
                return
        print tmp

I'll always get a few lines, but then the program hangs on print '1'. When I run the line-generating program, the output is a line to stdin about every second. What's going on here?

Edit, in case it's somehow related: The program producing lines is in docker (run with --net="host", and the server (accepting lines) is on the host running docker. Docker is sending it over 127.0.0.1.

Another edit: It seems to stop accepting input when SOCK_BUFSZ bytes were received. It's not recycling the buffer?

Update: this seems to be an issue with Docker. It works on localhost, but not from the container. I have connectivity (I can ping the server, and the first burst of data gets through).

May I ask you to show us your netcat script?

user3657941
– user3657941

2016-09-22 05:01:47 +00:00
Commented Sep 22, 2016 at 5:01 — user3657941
– user3657941, Commented Sep 22, 2016 at 5:01
@DavidCullen - ./command | nc -u localhost 50000

Ken - Enough about Monica
– Ken - Enough about Monica

2016-09-22 09:09:15 +00:00
Commented Sep 22, 2016 at 9:09 — Ken - Enough about Monica
– Ken - Enough about Monica, Commented Sep 22, 2016 at 9:09

Community · Accepted Answer · 2017-05-23 12:32:45Z

1

This Python script worked for me:

import socket

lstsock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
lstsock.bind(('127.0.0.1', 50000))

SOCK_BUFSZ = 4096
counter = 1
while True:
    print "counter = %s" % counter
    counter += 1
    data, addr = lstsock.recvfrom(SOCK_BUFSZ)
    print data

I used this bash script to send lines:

while true ;
do
    echo "Running..."
    echo -n "hello" | nc -w 0 -u "127.0.0.1" 50000
    sleep 1
done

I didn't see the point of setting the socket to be non-blocking when time was wasted in a sleep.

After a comment from horse_hair, I put put the server in a thread:

import socket
import threading
import time

SOCK_BUFSZ = 4096

def udp_server(quit_flag):
    udp_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    udp_sock.bind(('127.0.0.1', 50000))
    counter = 1
    while not quit_flag.is_set():
        print "counter = %s" % counter
        counter += 1
        data, addr = udp_sock.recvfrom(SOCK_BUFSZ)
        print data

def main():
    quit_flag = threading.Event()
    udp_thread = threading.Thread(target=udp_server, args=(quit_flag,))
    udp_thread.daemon = True
    udp_thread.start()
    try:
        while True:
            time.sleep(1.0)
    except KeyboardInterrupt:
        print "Exiting due to keyboard interrupt"
    quit_flag.set()
    udp_thread.join()

if __name__ == '__main__':
    main()

Everything still works.

edited May 23, 2017 at 12:32

CommunityBot

11 silver badge

answered Sep 22, 2016 at 3:52

user3657941

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2 Comments

Ken - Enough about Monica Over a year ago

I get the first 'hello' then nothing for a long time, then a 'hello' again, repeat. Also, I get 'invalid wait time' when using the -w 0 option with netcat. The only difference is that my netcat is apparently different than yours, and I'm running through docker. Oh, and my receive loop is in a thread. Simply looping and printing 'hello' in the thread (without receiving from the network) works, so doesn't seem like there's delay in the run loop.

user3657941 Over a year ago

What version of netcat are you using? The nc on both OS X and Linux allows me to use -w 0. What is the output of ls -hl $(which nc)?

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