0) Introduction
Recently it appeared to me, that receiving SMS-based one time passwords (OTPs) on your mobile phone is quite a ugly situation actually. One should rather use a authenticator app or offload the SMS receiving from the mobile phone entirely.
So that’s what I did. Hardcore separation of concerns. I got a cellular modem with accompanying SIM card, connected it to my usual do-it-all-housekeeping-server and set up a dedicated container for the cellular modem in there. So now the OTP SMS receiving is actually decoupled from my personal mobile phone and instead placed on its own dedicated service on my internal network. Quite nice.
Quick note before starting: You can replace every occurrence of incus with lxc in the following commands if you’d like. I will be using Incus as per personal preference however.
Alright, thats enough introduction; let’s setup a SMS send-and-receive capable LXC/Incus container!
1) The hardware
There are two pieces of hardware you need here: An actual cellular modem and a SIM card for it to be able to connect to a cellular network.
I’ve settled on a Teltonika TRM 240 4G modem here, a proper industrial LTE modem which would also be great to use in conjunction with a data plan. We’ll be using SMS only however.
Because we’ll only be neeing SMS capability, I’ve settled on a basic prepaid SIM card with no monthly fees (you pay only for actual usage). Here it is a Vodafone CallYa Classic SIM card, but any SIM is fine really.
2) Setup the LXC/Incus container
2.1) Setup on your host machine
With the hardware ready, we can launch the LXC/Incus container now.
incus launch ubuntu:24.04 cellular --profile default --profile bridged_192.168.20.149Here I am starting up a Ubuntu 24.04-based Incus container named cellular with a addidtional profile called bridged_192.168.20.149. The optional profile is there, so that the Incus container gets its own IP address on my network and behaves like a standalone server basically. You can find details on how to setup a bridged profile in my article about that topic.
After launching your container, plug in your modem to your host machines USB port. The dmesg log should then look something like this:
[ 218.398264] usb 2-1: New USB device found, idVendor=2c7c, idProduct=0121, bcdDevice=3.18
[ 218.398275] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 218.398280] usb 2-1: Product: Android
[ 218.398283] usb 2-1: Manufacturer: Android
[ 218.433359] usbcore: registered new interface driver usbserial_generic
[ 218.433389] usbserial: USB Serial support registered for generic
[ 218.447695] usbcore: registered new interface driver option
[ 218.447837] usbserial: USB Serial support registered for GSM modem (1-port)
[ 218.447920] option 2-1:1.0: GSM modem (1-port) converter detected
[ 218.449442] usb 2-1: GSM modem (1-port) converter now attached to ttyUSB0
[ 218.450025] option 2-1:1.1: GSM modem (1-port) converter detected
[ 218.450356] usb 2-1: GSM modem (1-port) converter now attached to ttyUSB1
[ 218.450545] option 2-1:1.2: GSM modem (1-port) converter detected
[ 218.452706] usb 2-1: GSM modem (1-port) converter now attached to ttyUSB2
[ 218.452786] option 2-1:1.3: GSM modem (1-port) converter detected
[ 218.452913] usb 2-1: GSM modem (1-port) converter now attached to ttyUSB3
[ 218.457417] usbcore: registered new interface driver cdc_wdm
[ 218.462247] qmi_wwan 2-1:1.4: cdc-wdm0: USB WDM device
[ 218.464152] qmi_wwan 2-1:1.4 wwan0: register 'qmi_wwan' at usb-0000:02:1b.0-1, WWAN/QMI device
[ 218.464183] usbcore: registered new interface driver qmi_wwanAs you can see, this exposes the new devices /dev/ttyUSB0, /dev/ttyUSB1, /dev/ttyUSB2, /dev/ttyUSB3 and /dev/cdc-wdm0 on your host. Depending on your modem model, these devices would be similar ones, compared to my Teltonika TRM 240 here.
These devices are used for GNSS like GPS/GLONASS/Galileo, debug or broadband data. We are actually interested in /dev/ttyUSB2 however, as this exposes the AT modem, we’ll be using to send and receive SMS.
Depending on your modem, this device could be a different one. Here you can use a script like the following one, to iterate trough every ttyUSB device and look out for what device actually responds to our AT command with an OK.
for p in /dev/ttyUSB*; do
echo -e "Testing $p"
echo -e "AT\r" > $p
timeout 1 cat $p
doneNow with the device known, proceed to pass it into yout LXC/Incus container:
incus config device add cellular teltonika_trm_240 unix-char source=/dev/ttyUSB2 path=/dev/ttyUSB2Adjust the device name teltonika_trm_240 and the device /dev/ttyUSB2 for your specific setup. Proceed to restart your LXC/Incus container and change your console into it afterwards.
incus restart cellular
incus exec cellular bash2.2) Setup within your container
Now clean up your container after changing into it for the first time. We’ll be deleting the default user, update and upgrade the packages, install some base utilities and remove the SSH server:
deluser --remove-home ubuntu
apt-get -y update && apt-get -y upgrade
apt-get -y install htop nano screen zip unzip net-tools
apt-get -y purge openssh-server
apt-get -y autoremove3) SMS tools: gammu and gammu-smsd
In my testing, gammu and gammu-smsd were easier to use then other utilities like smstools. So we’ll be using gammu/gammu-smsd here.
You can think of gammu as an application, which interfaces with your SIM card to execute different functions like sending SMS or reading SMS saved on the SIM card itself.
gammu-smsd however, is a daemon which runs in the background and manages your SMS all the time, so not only just on execution like base gammu. Here your SMS get offloaded from the SIM card storage upon receiving and saved in a pre-defined folder (more on that in just a moment).
3.1) Setup gammu
You can install gammu from just apt-get:
apt-get install gammuOpen up and edit the settings file for gammu at /etc/gammurc next:
[gammu]
device = /dev/ttyUSB2
name = teltonika_trm_240
connection = atAgain, please adjust the device and name depending on your setup.
After that, you can test out your cellular modem and SIM card for functionality:
gammu --identifyThe result should look similar to this then:
Device : /dev/ttyUSB2
Manufacturer : Quectel
Model : unknown (EC21)
Firmware : <SOME-PRIVATE-INFO>
IMEI : <SOME-PRIVATE-INFO>
SIM IMSI : <SOME-PRIVATE-INFO>Or send out your first SMS:
echo "Hello! All your base are belong to us." | gammu --sendsms TEXT 015230000001Or read (and then delete) your received SMS like so:
gammu --getallsms
gammu deleteallsms 13.2) Setup gammu-smsd
This is nice and all, however you now have to completely manually manage your SMS. A bit of more automation would be nice indeed. Proceed to install gammu-smsd for that:
apt-get install gammu-smsd usb-modeswitchOpen up and edit the settings file for gammu-smsd at /etc/gammu-smsdrc next:
[gammu]
device = /dev/ttyUSB2
name = teltonika_trm_240
connection = at
[smsd]
pin = 1196
service = files
logfile = syslog
debuglevel = 0
# Paths where messages are stored
inboxpath = /var/spool/gammu/inbox/
outboxpath = /var/spool/gammu/outbox/
sentsmspath = /var/spool/gammu/sent/
errorsmspath = /var/spool/gammu/error/Notice how the gammu setting are also gammu-smsd. In fact, gammu-smsd blocks the modem, while the service is running, so keep that in mind.
As for the different folders now defined in the config file:
inboxpath= This is the directory, wheregammu-smsdstores incoming SMS messages received from the modemoutboxpath= This is the directory, where you place outgoing SMS message files thatgammu-smsdwill read and send through the modemsentsmspath= This is the directory, where successfully sent SMS message files are moved after being deliverederrorsmspath= This is the directory, where SMS message files are moved if sending fails due to an error
Now you can restart the gammu-smsd daemon and test it out by sening out a SMS:
service gammu-smsd restart
echo "Hello! All your base are belong to us." | gammu-smsd-inject TEXT 015230000001Alright! Now gammu-smsd is also ready for usage now. You can see and read your OTP SMS in your inbox directory /var/spool/gammu/inbox, like we defined in the config file.
4) Bonus: Get your SIM card balance
Basically, we are finished now, but special functions, like retrieving the SIM cards balance, the procedure differes a tiny bit from sending/receiving plain SMS. For simplicity I’ll stick with the SIM card balance example here.
4.1) Method 1: Use gammu-smsd
is is the easy way. Just use the gammu-smsd-inject command and gammu-smsd does the rest for you:
gammu-smsd-inject USSD "*100#"After you issed this command, see in your inbox directory /var/spool/gammu/inbox for a response.
4.2) Method 2: Use interactively via gammu
You can also use plain gammu if your prefer it this way. You have to stop the gammu-smsd service first however, as it blocks access to the modem.
systemctl stop gammu-smsdNow you can issue the USSD query for SIM card balance:
gammu getussd "*100#"For my German Vodafone SIM the result looks like this:
Press Ctrl+C to break…
USSD received
Status : Action needed
Service reply : "Aktuelles Guthaben: 4,20 EUR
Wähl bitte aus:
1 Aufladen
2 Guthaben & Verbrauch
3 Tarife & Optionen
4 Spracheinstellungen
5 Vorteilsangebot"Don’t forget to start up the gammu-smsd service after your manual query:
systemctl start gammu-smsd5) Finished
Alright! That’s how you can send and receive SMS within LXC/Incus containers with your own cellular modem.
The next step would be to have a WebGUI (inside your internal network), to interface with your directories under /var/spool/gammu, so you can manage your SMS from your webbrowser, without having to have console access to your container. Stay tuned.
