Embedded Linux 3G pppd dial-up Internet problem

I took over an embedded ARM9 development board with Ethernet port, USB WiFi module, and 3G module, that is, all three methods can access the Internet. There are two methods commonly used, Ethernet and 3G, and the Ethernet Internet access method is more Stable, but the 3G dial-up method sometimes fails to succeed even all day long. I have read some blogs and suggested the following methods based on their experience:

1. Close eth0

ifconfig eth0 down

2. Delete the default route before dialing

route del default

3. Delete the default route and set ppp0 as the default route

route del default
route add default dev ppp0

The above methods have been tested and verified. The most reliable method is the first one, but the best formula is to write an infinite loop regular check in the script. Once it is detected that the Internet cannot be accessed, first shut down the pppd process, and then restart the 3G module. Finally start the pppd process again.

The following is a reference scheme

while true
do
    # whatever eth or wlan or ppp 
    ping -s 1 -c 1 $server > /dev/null
	if [ "$?" != "0" ]
	then
	  #echo "OFF TIME is `date +%Y-%m-%d,%H:%M:%S`" >> /path/offinfo
	  killall pppd
      # function for restart 3g module
	  restart_3gmodule
      sleep 15s
      # function for start pppd,shut eth0 down here
	  pppd_call
	fi
	sleep 300s
done  

According to the current tracking phenomenon, the reason may still lie in the routing (kernel version linux-3.10.x, ppp2.4.7), if you have a clear brother, please let me know in the comment area, I would be very grateful!

Provide a script solution for dynamic network automatic switching, which includes some operations to delete the default route, as follows

#!/bin/sh

#check the wired and wireless network reachable
wired=eth0
#wired=wlan0
wireless=ppp0
needRetryTimes=10
needRestartUdhcpc=0
flag=0

while true; do
Gateway=`route|grep 'default'|grep $wired|awk '{print $2}'`
if [ "$Gateway" != "*" ] && [ -n "$Gateway" ]; then
	ping -s1 -w1 $Gateway > /dev/null
else
	sleep 2s
	continue
fi

if [ "$?" != "0" ]; then
	let flag++
	
	if [ $flag -lt $needRetryTimes ]; then
		echo "we will retry once.." > /dev/null
		continue
	fi
	flag=0

	#if wired if default,delete it
	if [ -n "`route|grep 'default'|grep $wired`" ];then
		echo "route del default $wired" > /dev/null
		route del default $wired
	fi
	
	if [ $needRestartUdhcpc -eq 1 ]; then
		echo "$wired not reachable,so only restart udhcpc once" > /dev/null
		kill -9 `ps|grep 'udhcpc'|grep -v grep|awk '{print $1}'`
		sleep 1s
		/sbin/udhcpc -i $wired -F ZigbeeLinuxGateway -b > /dev/null &
		sleep 2s
		needRestartUdhcpc=0
	fi
	
	#if wireless is not default,add it
	if [ -z "`route|grep 'default'|grep $wireless`" ];then
		echo "route add default $wireless" > /dev/null
		route add default $wireless
	fi
	
	echo "wait 2s and retry" > /dev/null
	sleep 2s
else
	sleep 2s
	needRestartUdhcpc=1
fi
done

Attached are some reference articles:

Linux transplantation of ppp and routing settings of ppp0_tietao's column-CSDN blog_linux ppp0

Linux pppd-GPRS and Ethernet are online at the same time, solving the dial-up default gateway_chenliang0224's column-CSDN blog