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Dropbear is a lightweight SSH server and client implementation, often used on embedded systems and routers. If you use the dropbear SSH client (dbclient) and want to use a private key, it needs to be in the dropbear format and cannot have a passphrase. Dropbear provides a conversion utility to convert openssh style keys to dropbear style keys, dropbearconvert, but on Ubuntu it's not in your $PATH. This means you have to provide the full path to execute it, which is cumbersome. There is a bug in Ubuntu that has been reported in 2012, but in Ubuntu 18.04 (2019) it's still not fixed.

This is the full path to dropbearconvert:

/usr/lib/dropbear/dropbearconvert

This is an example to convert an OpenSSH style private key to a dropbear style private key:

usr/lib/dropbear/dropbearconvert openssh dropbear ~/.ssh/id_rsa ~/.ssh/id_rsa_dropbear

Your dropear style private key will be located in ~/.ssh/id_rsa ~/.ssh/id_rsa_dropbear.

Do note that your private key cannot be protected with a passphrase. Dropbear does not support this.

Tags: bug , dropbear , openssh , snippets , ssh , ubuntu