I recently bought a Kobo Mini ebook reader because I wanted to play around with the e-ink display. To do that I obviously needed a shell on the device, and here I'll describe how I got that.
There are basically two ways to get the (already built-in) telnet- and ftp-server to start. One way is to open up the Kobo Mini, take out it's internal MicroSD card, mount that on a Linux computer, and modify some scripts. The other way it to build a fake update package.
I choose the way of opening the device and accessing the root filesystem directly.
To actually use the telnet-server we need pseudo-shells, and for that we need to create an init-script that mounts /dev/pts on the device.
So I created /etc/init.d/rcS2 (like other people on the internet did) and filled it with the following little code snippet:
#!/bin/sh
# /etc/init.d/rcS2
mkdir -p /dev/pts
mount -t devpts devpts /dev/pts
The file should be executable, so chmod a+x etc/init.d/rcS2 and we are good to go.
Next we need to actually get the telnet server to start, for that we modify /etc/inetd.conf and add the following lines:
21 stream tcp nowait root /bin/busybox ftpd -w -S /
23 stream tcp nowait root /bin/busybox telnetd -i
To get our init-script and inetd to run we also need to append some lines to /etc/inittab:
::respawn:/usr/sbin/inetd -f /etc/inetd.conf
If you want to go the route with the fake update package you'll have to put all these new and modified files into a gzip compressed tar-file and put it as .kobo/KoboRoot.tgz on the device.
Keep in mind that you will need the original content of /etc/inittab, so you may need to extract that from an official firmware update or get it from somewhere else on the internet.
After putting the MicroSD back into my Kobo Mini i started it up and ran the Webbrowser to start the wifi connection.
After all that telnet was finally reachable, and I could login with user root without a password (probably a bad idea to do this on a public wifi network...).
On the shell you can run killall nickel to get rid of the original eBook-software (until next reboot), which also disables the automatic disconnect of the wifi connection (which will obviously drain your battery).
