About the project
I recently got back into reading and, being a tech enthusiast, that more or less instantly translated into "I need an e-reader". I have always been curious about e-ink and wanted to see how much the technology had matured over the last few years. Looking around, the current e-reader space turned out to be quite divided and dominated by the Amazon Kindle, which locks you into its own ecosystem more than I am comfortable with, so I went looking for a more open alternative. That search quickly led me to KOReader, an open source document reader that runs on top of the existing firmware of many e-readers. It is easy to install on a lot of devices but particularly painless on Kobo hardware. Since I also like to read magazines I went with the Kobo Clara Colour.
Managing a growing e-book library can quickly get messy and that is where Calibre comes in. It is an open source library manager that handles conversion, organisation and metadata enrichment. KOReader ships with a Calibre companion plugin that talks directly to a running Calibre instance over the network, which is a nice way to push new books to the device without plugging it in.
That last part only really works when the e-reader and the Calibre machine are on the same network. The obvious alternative is to port forward Calibre to the internet, but exposing it directly is not something I want to do. I run a WireGuard server at home anyway, so the natural fix was to just bring the e-reader onto that VPN whenever I needed to sync.
There is an existing Tailscale plugin for KOReader that ships with an install script and works out of the box, which is great if you are already on Tailscale. I am not, and I did not want to add another overlay network just for this, so I went looking for a WireGuard equivalent. That turned out not to exist. Digging a bit further made it clear why: there are no reputable pre-built WireGuard binaries for the Kobo and the kernel on these devices is far too old to include the in-kernel WireGuard module. The userspace implementation wireguard-go sidesteps the kernel problem but still has to be cross-compiled for the device's architecture (armv7l on the Clara Colour), together with the wg tool from wireguard-tools.
I cross-compiled both binaries using Docker, with a Debian armv7 container under emulation for wg and a regular Go toolchain targeting armv7 for wireguard-go. Once they were copied onto the device the binaries themselves worked fine, so the plugin itself is really just a thin Lua wrapper around them. It adds a WireGuard entry under KOReader's Network menu with connect, disconnect, toggle and status actions, picks up any .conf file dropped into its configs/ folder, and on the back of that handles the usual bring-up and tear-down steps so a single tap is enough to get the tunnel cleanly up or down.
The result is a small plugin, but it closes the last gap in my reading setup: the e-reader can now reach my Calibre library from anywhere without exposing it to the open internet.