Raspberry Pi into a 3G / 4G Hotspot

This guide allows you to convert a Raspberry Pi into a 3G or 4G Hotspot, using a 3G or 4G Modem and a WiFi card.

Prerequisites:

1. You must have a 3G or 4G USB Modem (in this example, I am using a ZTE MF112 3G Modem from Three)
2. You must have a wireless adaptor (either build in to the Pi or a compatible external one)
3. You must have imaged the Pi with the latest Raspian build. The version I used is downloadable here.

Preparing the Raspberry Pi:

Firstly, you need update the Raspberry Pi image.

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Then, you need to download and install some packages, as follows:

This installs usb-modeswitch (we’ll see this later) which allows you to configure your 3G Modem to be used as a 3G Modem, ppp which is the Point-to-Point Protocol daemon to establish internet links over dialup modems, dnsmasq which provides a DHCP server for the wireless LAN, and hostapd which allows you to create the wireless hotspot.

You’ll also need to download UTMS keeper, as follows:

Configuring your 3G Modem:

A lot of 3G / 4G Modems come with file systems which contain installers for Windows and OSX. The Raspberry Pi often detects these and registers the modem as a storage device, instead of its true purpose. This is why usb-modeswitch is used, to switch the mode of the USB Device.

To do this, we need to find all the USB devices on the Pi.

This will churn out something like this:

My device (ZTE WCDMA Technologies…) is showing up here. To confirm that your device is acting as a modem as not as removable storage, you need to edit the modeswitch config file. For this, you need the DefaultVendor and DefaultProduct parameters, which usually can be found by going a quick Google search for ‘<<Dongle Model Number>> usb_modeswitch’.

At the bottom of this file, add the following lines:

Where <<VendorID>> and <<ProductID>> are the information you found online. Sometimes, they are revelaed in the lsusb command, but you must ensure that they are correct (i.e. actually a modem). In my lsusb, the VendorID is 19d2 and the ProductID is 0031 but in my modeswitch config file, I enter 0103 for my ProductID.

Connecting to the cellular network:

We installed UMTSkeeper earlier, who’s job it is to automatically connect (and re-connect should the connection drop out) the 3G connection. This uses sakis3g to do that. It’s prudent to check the 3G connection before we script the connection automatically with UMTSkeeper .

Firstly, we need to sakis3g and utmskeeper executable.

Then, we can run sakis3g with the interactive flag to test our 3G connection:

This will guide you through the process of connecting to the internet using your 3G Modem.

1. Choose ‘Connect with 3G’
2. In the ‘Select Modem Type’, choose ‘1. USB device’
3. Under ‘Please select USB modem’, select your USB Modem (in my case, it’s ZTE WCDMA Technologies MSM).
4. Under ‘Please select an APN’, either choose your carrier’s APN or select ‘Custom APN’ (in my case, it’s offered ‘Mobile Device (three.co.uk)’ which I know is the correct APN’
5. You should receive a notification confirming the device is connected. In my case, it says ‘MF112 connected to 3’.
6. Under ‘Please select an action’, click ‘Exit’ and you will return to the command line.

To make sure the device is actually connected:

Now we’ve proven the connection, we need to automate it. Let’s disconnect from 3G to not waste that data! Hop back in to the sakis3g interactive session and select ‘Disconnect’.

In doing this, we’ve established some settings we’ll need to use with UTMSkeeper. We’ve established that:

• The APN is three.co.uk
• The APN does not require a username or password
  
• The USB Modem is running correctly

We can run that connection in one command:

Here are the components of that command:

• APN – the APNof your carrier
• APN_USER – the APN user of your carrier. If you didn’t need one, enter 0
• APN_PASS – the APN password of your carrier. If you didn’t need one, enter 0
• USBINTERFACE – the ID of your USB Modem, found above when we did lsusb.
• USBMODEM – also the ID of your USB Modem

So running that command connects the device up to the internet again. We can confirm this by performing ifconfig again. Now we’ve confirmed the parameters we need for UMTSkeeper. To disconnect, just run sakis3g with disconnect.

The UMTSkeeper command is similar, as follows:

Ensure that the options you set up using the sakis3g executable are placed in the correct bits in the UMTSkeeper executable..

Clearly, we want this to start when the pi boots, so we need to put it into /etc/rc.local which will run that command on start.

I added the command straight above the commands to print the IP Address of the Pi when it boots.

Creating a WiFi Hotspot:

As we haven’t configured anything, we need to stop dnsmasq and hostapd:

We then need to configure a static IP address on to the Wireless interface. Hop in to:

At the bottom (but above any other interface configuration options), enter the following information:

The name of the interface can be found by running the ifconfig command. Change <<IP_Address>> with your intended IP Address.

To configure the DHCP Server, we need to edit options in the DHCP config file:

Then, enter the following information:

Change <<Subnet_Start>> and <<Subnet_Stop>> with your intended IP Addresses.

Now we’re ready to configure the access point:

Enter the following information:

Replace <<Your_Network_Name>> and <<Your_Network_Passcode>> with your own values.

We need to tell the system where to find the configuration file:

Find the line with #DAEMON_CONF and replace it with the following:

Finally, reboot the Pi.

NATting the Connection:

Firstly, we need to enable IP Forwarding:

Remove the # before the line that reads #net.ipv4.ip_forward=1 and save the file.

Then, run the following commands:

To have the IP Tables load each time the Pi starts:

And add the following line above the UMTSkeeper command we added earlier.

Finally, reboot the Pi.

Congratulations! When the Pi boots back up, it should be a fully functioning 3G WiFi hotspot.

Firewalling

Since your Pi will be out on the open internet, it’s probably a good idea to have some sort of a firewall going! We’ll use UFW.

If you’re SSHing in to your pie, you’ll want to add:

You’ll also need to tell UFW to allow IP Forwarding, as follows:

Change ‘DEFAULT_FORWARD_POLICY’ from ‘DROP’ to ‘ACCEPT’. Then, run the final command.