Raspberry Pi LTE hotspot on Raspbian Lite

Recently, I was looking for a solution to share my LTE USB modem connection to the rPi's LAN port. I've searched for hours for a solution with no luck. Others just recommended me to buy an LTE router. Thanks, no. Then a fantastic guy on Stackexchange helped me a lot. And the solution was born. In my case I am going to connect rPi LAN port with my Unify USG secondary port to use rPi as a LTE failover connection.

We have 3 connections on the rPi:

1. WiFi (wlan0) - main internet connection on rPi. I am going to use this connection to connect to rPi (SSH).
2. LTE USB modem (wwan0) - LTE internet connection
3. LAN (eth0) - we are going to share wwan0's connection with this interface.

I am not going to show you here how to setup your devices. Please do it before (WiFi, LTE). This is not a step by step guide. I'll show you the relevant parts of my config files.

First, install dnsmasq package:

sudo apt-get install dnsmasq

/etc/network/interfaces:

#please use different subnet as your wlan0
#eth0:
auto eth0  
iface eth0 inet static  
	address 192.168.2.1
	netmask 255.255.255.0

#wlan0:
allow-hotplug wlan0
iface wlan0 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
    metric 0  

#wwan0:
allow-hotplug wwan0
iface wwan0 inet dhcp
    metric 1000
    pre-up /bin/sleep 10
    pre-up /bin/echo -e "AT^NDISDUP=1,1,\"internet\"\r" > /dev/ttyUSB0
    post-up /etc/wwan2lan.sh
    post-down /bin/echo -ne 'AT^NDISDUP=1,0\r\n' > /dev/ttyUSB0

From wwan0 config you only need metric 1000, post-up /etc/wwan2lan.sh lines. Other commands are for setting up internet connection on my Huawei E3372.

Turn on NAT IPV4 forwarding. Add at the end of/etc/sysctl.conf:

net.ipv4.ip_forward=1

Setup internet connection sharing in Iptables:

sudo iptables -t nat -A POSTROUTING -o wwan0 -j MASQUERADE

Configure it to load on reboot by first saving it to a file:

sudo sh -c "iptables-save > /etc/iptables.ipv4.nat"

Then create a 'hook' file with a line to restore the ip tables :

sudo nano /lib/dhcpcd/dhcpcd-hooks/70-ipv4-nat

Add:

iptables-restore < /etc/iptables.ipv4.nat

The following script makes the real deal, so it's important:

/etc/wwan2lan.sh

#!/bin/bash

sleep 10

# define interfaces

LOCALIF="wlan0"
WWANIF="wwan0"
LANIF="eth0"

#get gateway IP address

LOCALGATEWAY=$(ip route show 0.0.0.0/0 dev $LOCALIF | cut -d\  -f3)
WWANGATEWAY=$(ip route show 0.0.0.0/0 dev $WWANIF | cut -d\  -f3)
 
#local traffic gets one private default route:

ip rule add iif lo priority 48000 table 2
ip route add default via $LOCALGATEWAY dev $LOCALIF table 2

#eth0 traffic gets another private default route:

ip rule add iif $LANIF priority 48010 table 3
ip route add default via $WWANGATEWAY dev $WWANIF table 3

#delete shared gateways

ip route delete default dev $LOCALIF
ip route delete default dev $WWANIF

#enable wwan0 traffic from localhost if we specify the adapter (example: curl --interface wwan0 google.com)
INETADDR=$(/sbin/ifconfig $WWANIF | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}')
ip rule add from $INETADDR/32 iif lo priority 47010 table 3

Setup Dnsmasq, if you used different subnet for eth0, you must change it here as well:

/etc/dnsmasq.conf

interface=eth0      # Use interface eth0  
listen-address=192.168.2.1 # Explicitly specify the address to listen on  
#bind-interfaces      # Bind to the interface to make sure we aren't sending things elsewhere  
#server=8.8.8.8       # Forward DNS requests to Google DNS  
domain-needed        # Don't forward short names  
#bogus-priv           # Never forward addresses in the non-routed address spaces.  
dhcp-range=192.168.2.1,192.168.2.50,12h # Assign IP addresses between 192.168.2.1 and 293.168.2.50 with a 12 hour lease time  
log-queries

That's all. You are successfully shared LTE (wwan0) connection to your LAN (eth0) port. You can also add check for monitoring internet connection and if it is needed, restart it:

#!/bin/bash
# ping checker tool

TMP_FILE="/tmp/ping_checker_tool.tmp"
if [ -r $TMP_FILE ]; then
    FAILS=`cat $TMP_FILE`
else
    FAILS=0
fi
SERVER="google.com"
INTERFACE="wwan0"
#IPADDRESS=$(ifconfig wwan0 | grep 'inet addr:'| grep -v '127.0.0.1' | cut -d: -f2 | awk '{ print $1}')

curl --interface $INTERFACE --connect-timeout 15 $SERVER >/dev/null 2>&1
if [ $? -ne 0 ] ; then #if ping exits nonzero...
    FAILS=$[FAILS + 1]
else
    FAILS=0
fi

if [ $FAILS -gt 2 ]; then
    FAILS=0
	echo "wwan0 failed, going to restart interface..." >> /var/log/watchdog/watchdog.log
	/sbin/ifdown wwan0
	sleep 20
	/sbin/ifup wwan0
	sleep 20
	IPADDRESS=$(/sbin/ifconfig wwan0 | grep 'inet addr:'| grep -v '127.0.0.1' | cut -d: -f2 | awk '{ print $1}')
	echo "Time: $(date) - Server $SERVER is offline! IP is: $IPADDRESS" >> /var/log/watchdog/watchdog.log
fi
echo $FAILS > $TMP_FILE

Language corrections are welcomed.

p.s: I LOVE coffee! Buy me a coffee at: