Running a background process with Python

Question

I am trying to send GET requests but before starting the requests, I'd like to capture the traffic. Capturing traffic can be done with the command:

dumpcap -i eth0 -f "udp port 53" -w dns.cap

in the background. While I capture packets, I need to make some request by sending some URLs. For now, with the code below, it seems my capturing code does not work, I can not even see a dns.cap file in my folder.

What's the problem?

import requests
import os
import subprocess
import urllib
print "start capturing packets...\n"
#os.system("dumpcap -i eth0 -f \"udp port 53\" -w dns.cap"
os.spawnl(os.P_NOWAIT,'dumpcap -i eth0 -f \"udp port 53\" -w dns.cap')
print urllib.urlopen('http://www.google.com').read()
#resp = requests.get('http://httpbin.org')
#resp=requests.get('http://httpbin.org')
print "ok"

Do you run the script as root? Try to specify full path to 'dns.cap' file. — Pavel Strakhov
– Pavel Strakhov, Commented Dec 23, 2012 at 13:38

Pavel Strakhov · Accepted Answer · 2012-12-23 13:44:53Z

1

os.spanwl is considered old and should be replaced with subprocess.Popen. Replace os.spanwnl call with this:

subprocess.Popen(['/usr/bin/dumpcap', '-i', 'eth0', '-f', 'udp port 53', 
                  '-w', '/tmp/dns.cap'])

It's better to add some pause (sleep) after starting the dumpcap to ensure that capturing is established when you make requests.

answered Dec 23, 2012 at 13:44

Pavel Strakhov

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2 Comments

berkay Over a year ago

i tested, it works thanks, what about following code is there any difference than your post. subprocess.Popen(['/usr/bin/dumpcap', '-i eth0', '-f', 'udp port 53', '-w', '/tmp/dns.cap']), In addition my other code while processing packets really long is there easy way to adapt subprocess.Popen while using long system commands with arguments?

Pavel Strakhov Over a year ago

'-i eth0' will not work, you need to specify each argument in separate array item. There is no problem with long system commands with arguments. You can write '-i', 'eth0' in 2 array items, and you can write '--long-command=eth0' in a single array item.

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