Raspberry Pi MQTT broker

In my last post I started meddling with the ESP8266 module as a low cost (and smaller) alternative to an Arduino or Raspberry Pi for creating home sensors.

I was toying with the idea of sending measurements to a database using a RESTful API on the existing Postgres database I use to host measurements on Amazon Web Services.

After a bit of reading, I’ve realised that there is a better solution: MQTT, a lightweight protocol designed for machine to machine (M2M) communication for sensors, and similar low powered and simple devices. MQTT uses an event driven publish/subscribe paradigm: sensors publish to a broker, which can then pass on information to subscribers. I’m not 100% sure yet how I will achieve persistence (and store sensor readings for later analysis), HiveMQ has a plugin that will do this at the broker, without the database needing to be a subscriber, which is a nice option.

MQTT looks like the way forward for this kind of project, so I’ll worry about the persistence problem later.

Minibian: a lightweight Raspberry Pi distro

I’ve got a spare Raspberry Pi 2 which I am using for this project. To save space, and avoid raspbian bloat, I’ve used minibian for a very lightweight machine.

Creating a user

You’ll also realise pretty quickly that the usual Raspberry Pi user setup is not there in Minibian. There is just a root user, so I first created a new user with admin privileges:

adduser <newuser>
 
sudo usermod -a -G sudo <newuser> 

You may then also want to get access to the sudo command. So you will need to log into the superuser account, and install it.

# Log in as root (assuming you are logged in as newuser)
 
su
 
# Install sudo
 
apt-get install sudo

Upgrading

The last release of Minibian was 2015-11-12, so a good thing to do is update the few packages that do come installed to the latest versions:

sudo apt-get update -y
sudo apt-get upgrade -y 

Wi-Fi drivers

Minibian is so lightweight it doesn’t ship with Wi-Fi drivers as standard. If you want to use Wi-Fi instead of a wired connection, you will need to install these manually. An explanation is given here, but in brief:

# Install necessary packages
 
apt-get install firmware-ralink 
apt-get install firmware-realtek
 
# Test with a network scan
 
apt-get install wireless-tools iwlist wlan0 scan
 
# Install wpa_supplicant for auto-connect
 
apt-get install wpasupplicant

I usually just copy the /etc/wpa_supplicant/wpa_supplicant.conf file from another one of my pis, to duplicate the Wi-Fi settings. My wpa_supplicant.conf looks something like this:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
 
network={
         ssid="SSID"
         psk="password"
         proto=RSN
         key_mgmt=WPA-PSK
         pairwise=CCMP
         auth_alg=OPEN
        }
 

Using SSH keys

Finally I want to avoid entering a password every time I log on, so I copy the public SSH key from my laptop to the authorized_keys file on the minibian pi.

If you want to use git or similar, you will need to generate an SSH key for the pi, so you may as well run ssh-keygen now, and generate a key. For internal network use, I tend to leave the passwords blank, but if you are particularly security conscious, you can enter something.

This command will create the ~/.ssh/ folder. The next step is to send over your public key from your client machine. I do this without copying pasting with the following command run from the client machine:

cat ~/.ssh/id_rsa.pub | ssh user@192.168.1.100 'cat >>
~/.ssh/authorized_keys' 

You’ll be prompted to enter your password for this command, but after that, you should not have to enter a password again, the next time you SSH in.

Creating a disk image

After all this work, it’s a good idea to create a backup disk image. Thankfully a Minibian image can be nice and small, unlike the standard raspbian, so you are not left with several GB sized files all over the place.

First you need to unmount the drive with umount. It will be mounted in two places on /media/user/, so make sure to umount both.

Then you can run:

sudo dd if=/dev/sdc of=2016-02-15-minibian.img

to create a backup image and compress it. This will produce an 8GB file (depending on the size of your SD card, and with a lot of blank space). We can check this with fdisk -l /tmp/2016-02-15-minibian.img which gives:

Disk /tmp/2016-02-15-minibian.img: 7744 MB, 7744782336 bytes
4 heads, 16 sectors/track, 236352 cylinders, total 15126528 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0008929a
 
                   Device Boot      Start         End      Blocks   Id  System
/tmp/2016-02-15-minibian1              16      125055       62520    b  W95 FAT32
/tmp/2016-02-15-minibian2          125056    14997503     7436224   83  Linux
 

We can then take the block size (1 * 512) and the end of the second partition (14997503), and truncate the end of the image with:

 truncate --size=$[(14997503+1)*512] /tmp/2016-02-15-minibian

and then compress the final image with:

bzip2 /tmp/2016-02-15-minibian.img

Next steps

Next time I’ll install the MQTT broker…

Sources

Create a disk image
Why use MQTT
WiFi on Minibian
Latest version of Minibian
Granting sudo priveleges
Mosquitto MQTT broker
HiveMQ MQTT broker
Good blog on MQTT
Creating bzip2 files
Shrinking disk images

Matt Upson