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bt-tether

Description

bt-tether is a small script that enables your Jolla Phone to share its mobile internet connection over PAN Bluetooth Tethering. I could not find any software that did that realiably on up-to-date SailfishOS so I tried my hand at it.

It is quite experimental and a little hacky. It’s only been tested on my Jolla Phone in developer mode. I’m distributing it here without any warranty, however I’d be glad to receive feedback, bug reports and pull requests; I’ll do my best to improve it.

bt-tether can connect several devices at once to whatever is the current connection of the phone (Mobile or WLAN). It uses the pand daemon that should be installed already, along with specially crafted scripts and an adhoc DNS relay.

Licence

This software is distributed under the terms of the MIT Lincence. See the included LICENCE file.

Building RPM package

Run the rpm/prep script with a version (x.y) and a release number as arguments, it should create a .tgz and a .spec file that can be used by rpmbuild. You can use the following command in the mersdk VM of the SailfishOS dev environment:

sb2 -t SailfishOS-3.0.3.9-armv7hl rpmbuild -v -bb SPECS/bt-tether-0.1-3.spec

(you may have to change some version numbers and path)

Notes

  • The pand util doesn’t seem to be packaged with bluez5, so for now this package most likely only works with bluez version 4.
  • There is no GUI, everything should work right away!
  • My tablet, using PAN Bluetooth Tethering to my phone, which is connected to the Mobile network, achieves a bandwidth of 1.5Mbps on average (for download and upload, not at the same time). This is usually what I get when transfering files from my phone using Bluetooth.
  • After some casual testing, it appears that battery consumption seems lower than WLAN Tethering, which is good! In my test, the phone’s battery went down 30% during 24h (with very little direct use) with both Mobile Data and Bluetooth activated, while my tablet was constantly connected to the Bluetooth Tethering and running Telegram.
  • If you had a paired device before you installed bt-tether, you have to re-pair your device for it to see that your phone now has PAN capability.
  • While bt-tether should work right away after being installed, you may have to reboot the phone if it doesn’t.
  • Since the package has to modify a configuration file that it doesn’t own during installation, an update of SailfishOS may break bt-tether. If this happens, try to reinstall the package.

How does it work?

This section is based on my partial understanding of bluez, it may be inaccurate!

The package contains a systemd service configuration that will launch pand at boot. A post-install script will also disable the native network plugin of the bluetooth stack that doesn’t work by modifying /etc/bluetooth/main.conf. (A pre-uninstall script reverts this modification).

Once pand runs, all devices pairing with the phone will see the PAN capability.

When a paired device wants to use the PAN capability, pand will spawn an interface on the phone and launch a up-script. This script will configure the interface with a sensible IP, add a MASQUERADE target with iptables, launch a tiny DHCP server (udhcpd) and the dns-relay utility that I coded for the specific purpose of this package. A down-script reverts everything when the device disconnects.

All this happens for each device requesting to use the PAN capability. Fortunately, udhcpd and dns-relay both have extremely tiny CPU and memory footprints. At some point I’ll try to use only one instance of udhcpd and dns-relay.

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