Check and reset the default route.

start.py

# -*- coding:utf-8 -*-
import commands
import types
import os
import time
import re

#commands.getstatusoutput('wpa_supplicant -D wext -i wlan0 -B -c /etc/wpa_supplicant/wpa_supplicant.conf')
#commands.getstatusoutput('dhclient wlan0')
pattern = re.compile(r'\d+.\d+.\d+.\d+')
os.system('wvdial &')
while 1:
	(status,output)=commands.getstatusoutput("ifconfig ppp0 | grep 'inet addr' | awk '{ print $2}' | awk -F: '{print $2}'")
	match = pattern.match(output)
	if match:
		ip =output
		#print ip
		(status,output)=commands.getstatusoutput('ping -c 3 www.baidu.com')
		#print status
		if status !=0:
			os.system('route delete default')
			#commands.getstatusoutput('route add default gw 192.168.12.1')
			commands='route add default gw %s' % (ip)
			#print commands
			os.system(commands)
			break
		if status==0:
			print 'ok'
			break
	time.sleep(5)