Mount a filesystem using sshfs using the Dropbear server on Yocto firmware

Question

I built a firmware for an embedded board using Yocto. ssh server Dropbear seems to be up and running and working properly. I can login with the root user without a password. Nevertheless, I cannot mount the filesystem from an Ubuntu desktop using sshfs. On the desktop I'm getting:

sudo sshfs -o allow_other root@10.42.0.68:/ /mountpoint
remote host has disconnected

in Poky instead I can see in /var/log/messages:

May  7 00:25:37 raspberrypi3 authpriv.info dropbear[537]: Child connection from 10.42.0.1:48010
May  7 00:25:38 raspberrypi3 authpriv.notice dropbear[537]: Auth succeeded with blank password for 'root' from 10.42.0.1:48010
May  7 00:25:38 raspberrypi3 authpriv.info dropbear[537]: Exit (root): Disconnect received

Is it possible to increase the verbosity somehow? I tried to add "verbose = 1" in /etc/default/dropbear but this is probably wrong as the server does not even start anymore. Maybe sshfs is not supported at all by dropbear?

Answer 1

As for you trying to do SSHFS with Dropbear: the question is that SSHFS needs SFTP, while Dropbear only supports SCP.

So there is not much a point in debugging why it is happening.

From the dd-wrt wiki: https://www.dd-wrt.com/wiki/index.php/Sshfs

Since Dropbear (the default ssh server) apparently does not support sshfs, you will need to install and run Openssh instead.

So indeed, SSHFS is not supported by Dropbear as you suspected.

P.S. For the benefit of other readers, Dropbear is a lightweight replacement for OpenSSH used widely in embedded systems/routers/ iOTs.

Answer 2

SSHFS uses SFTP, and Dropbear doesn't provide SFTP. So when you try to use it, the Dropbear server sees a request for a subsystem that it doesn't understand and drops the connection.

You can provide Dropbear with an SFTP server, such as the one from OpenSSH. (Or you can replace Dropbear by OpenSSH altogether, but you don't have to.) You'll need to find an sftp-server binary that runs on your system, i.e. one compiled for the architecture and for the standard library on your system. On Yocto, the executable from the openssh-sftp-server package should work with Dropbear, but I haven't checked whether the package management works out.