Emil Lerch

Husband, Father, Technologist, Cloud Architect

Rasperry Pi Headless Bootstrap

Recently I picked up a new Rasberry Pi Zero W and was excited but also lamenting the fact that I’d have to dig out a keyboard and mouse. Being lazy, and being willing to work really hard to remain lazy, I was determined to find a way around this.

I grabbed a MicroSD card and put Raspian Jessie Lite on it. I then extended the root partition which I found easier to do before first boot since a) it was a virgin distro install and b) since I wasn’t doing it from the running system there were no reboots involved - I could simply eject the card and plug it back in to refresh the block device listing.

From there, I found a pretty good run-through of adding the ability to connect via bluetooth here: https://hacks.mozilla.org/2017/02/headless-raspberry-pi-configuration-over-bluetooth/ The article is pretty good and though the script details are a little odd I went with most of it. One big difference is that I did not allow automatic login as I want to allow bluetooth serial connections permanently (so I can add public hotspot networks later if I’m travelling). That was an easy change - just remove the “-a pi” off the end of the ExecStart=/usr/bin/rfcomm line, and it removes the need for the “Security” section near the bottom, since, sure, you can find/pair/connect, but all you’ll get is a login prompt. I followed the instructions in the article, so I won’t repeat them here, but this is the script I used (with a generic device name) without the “-a pi” on the getty line.

#!/bin/bash -e

# Display name in BT discovery list
echo PRETTY_HOSTNAME=MY-DEVICE-NAME

# Edit /lib/systemd/system/bluetooth.service to enable BT services
sudo sed -i: 's|^Exec.*toothd$| \
ExecStart=/usr/lib/bluetooth/bluetoothd -C \
ExecStartPost=/usr/bin/sdptool add SP \
ExecStartPost=/bin/hciconfig hci0 piscan \
|g' /lib/systemd/system/bluetooth.service

# create /etc/systemd/system/rfcomm.serfvice to enable BT serial from systemctl
sudo cat <<EOF | sudo tee /etc/systemd/system/rfcomm.service > /dev/null
[Unit]
Description=RFCOMM service
After=bluetooth.service
Requires=bluetooth.service

[Service]
ExecStart=/usr/bin/rfcomm watch hci0 1 getty rfcomm0 115200 vt100

[Install]
WantedBy=multi-user.target
EOF

#enable the new rfcomm service
sudo systemctl enable rfcomm

#start the rfcomm service
sudo systemctl restart rfcomm

With the script in place to allow bluetooth I needed one more thing. The terminal emulators for Android over bluetooth serial connections kind of suck, so for that reason I wanted to be able to get in and get out of the bluetooth side of things as quickly as I could, basically bootstrapping to wifi.

I therefore created an additional script new-wifi that looked like this:

#!/bin/bash
sudo iwlist wlan0 scan |grep -F SSID
read -p 'SSID:' ssid
read -s -p 'Password:' password
sudo sh -c "wpa_passphrase $ssid $password | sed '/#psk/d' \
  >> /etc/wpa_supplicant/wpa_supplicant.conf"
sudo wpa_cli reconfigure

This handles networks that are broadcasting SSIDs just fine and doesn’t expose passwords in wpa_supplicant.conf. Make sure to chmod 700 on the script. For non-broadcast SSID networks, I’ll likely need to modify the script to add the SSID as documented here: https://www.raspberrypi.org/documentation/configuration/wireless/wireless-cli.md

General Connection overview and Mac specifics

Next up, we want to connect to this thing. On Linux and Mac I found the easiest way was to use screen. I didn’t try without specifying the baud rate so I don’t know if that will work, but the general steps are:

  • Pair with the Pi
  • Find the device
  • (sometimes) rfcomm bind to the device
  • screen 115200

The baud rate should match the script above, so if you change it, just make sure it matches. On mac, the device just “shows up” after pairing, so you’ll look for it in /dev/cu.MY-DEVICE-NAME-SerialPort. You’ll also see a /dev/tty.MY-DEVICE-NAME-SerialPort, but you’ll want to use the cu version. There’s a great description of the differences here: http://stackoverflow.com/questions/8632586/macos-whats-the-difference-between-dev-tty-and-dev-cu

Linux

On Linux, something like this might also show up automagically. I like to use arch linux when I can since nothing is automatic and therefore I learn much more about the internals, so after pairing and making sure my system could work with bluetooth, I was able to create a serial device using rfcomm. A great overview of this process is in this gist: https://gist.github.com/0/c73e2557d875446b9603 Note that the device becomes /dev/rfcomm0, but a simple screen /dev/rfcomm0 115200 is all you need to connect. For me, I got ‘screen is terminating’ when I tried this. I then realized my user didn’t have permissions to read/write to /dev/rfcomm0. So, I used ‘sudo screen…’ until I added myself to the uucp group. The uucp group has rw permissions on the file, and you can add yourself with: usermod -a -G uucp myusername

Android

On Android I needed an appy-app to connect to Bluetooth. As I mentioned earlier they mostly suck as they’re usually optimized for non-terminal super-raw usage. The two that I found the best was BlueTerm2 (a fork of BlueTerm that uses volume buttons to send control characters instead of the jog wheel that hasn’t existed in forever), and SENA BTerm Bluetooth Terminal. I’m not linking to BlueTerm2 here because I don’t want to send folks over there. It’s mostly superior to SENA with one glaring issue - it doesn’t seem to be able to send upper case characters. The source code is available so if someone fixes it I’ll use BlueTerm2 in a heartbeat, but for now I’ll stick with SENA.

Windows

For Windows, putty should be your friend as shown here: http://www.hobbyist.co.nz/?q=bluetooth-module-device I haven’t tried this though as I don’t have a use case for it and my Windows machine doesn’t have a bluetooth adapter.

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