Introduction

While on the flight to Derbycon 2019 I wanted to have a way to broadcast the conference schedule to WiFi so I didn't have to pull the program out. I didn't finish the build in time, but it may still be useful for others.

https://github.com/vgrsec/wifi-messages

Overview

This project is designed to provide messages over wifi. It has two operating modes.

• Default: A single file will be loaded and broadcast. This file is located in /boot/wifi-messages/input/_default.txt
• Date-based: ssid-rotator.py grabs current date and time and broadcasts any lines that are for this hour, and the next hour. It loads the schedule from /boot/wifi-messages/input/%date%.txt

To perform the rotation, this project contains a systemd timer, a shell script to broadcast ssid messages, and a ssid rotation python script.

The input file is located in /boot so that one can remove the SD Card from the raspberry pi and load new messages without having to muck with the device.

Setup

Required Equipment

• Raspberry Pi
• Alfa AWUS036NH
• USB Battery
• MicroUSB Cable
• Network Cable
• Network Adapter that supports crossover detection
• Adafruit PiRTC – PCF8523 Real Time Clock (RTC)
• CR1220 coin cell battery for RTC

Setup Raspberry Pi

• Install the RTC
• Download Raspbian Lite
• Image SD Card
• Enable SSH

The first time you setup your wifi-messages pi make sure to be connected to the internet. Otherwise the RTC may not be programmed with the correct time and the clock rotation will fail.

• Connect network cable from RPi to computer
• SSH: ssh pi@raspberrypi.local
  Password: raspberry
• Clone the wifi-messages repo and run the setup script:
  sudo -i
cd /usr/local/bin/
apt-get install git -y
git clone https://github.com/vgrsec/wifi-messages.git
cd /usr/local/bin/wifi-messages
./setup.sh

Setup WiFi Messages

Modify /boot/wifi-messages/input/_default.txt for static messages.
OR
Create files for the days you wish to broadcast messages in /boot/wifi-messages/input/YYYYMMDD.txt

The easiest way to modify the input files is to remove the sd card from the raspberry pi and put it into another device. This way you can use a text editor to edit the file without having to ssh into the pi.

Files

ssid-rotator.py

ssid-rotator is looking for times in half hour increments. I am not a developer, and so the logic is only looking for the top of the hour and the half hour. It is also not capable of rotating over at midnight.

wifi-messages.sh

1. NTP time sync
2. Set Timezone
3. airmon-ng is used to put the wifi card into broadcast mode.
4. mdk3 broadcasts the ssid's from /usr/local/bin/wifi-messages/output/ssid.txt

input/_test.txt

This file shows the format that ssid-rotator.py expects. The first four characters for the time is what the rotator looks at.

To perform a function test of your wifi-rotator copy input/_test.txt to input/%YYYYMMDD%.txt and watch it go.

Resources

• https://timemachines.io/blog/2015/6/3/connect-directly-to-raspberry-pi-from-mac-over-ethernet
• https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi?view=all