Replace Dropbear to OpenSSH + SFTP
The vanilla OpenWrt out of the box has a small Dropbear SSH server. But it lacks of some other features. You can install another OpenSSH server which is bigger but has more features and default on desktop systems like Ubuntu.
Many routers with OpenWrt as a stock firmware use it out of the box. So for this routers you really don't need anything to do and just start using it.
Installation
To avoid port conflict we need to first move the existing Dropbear from default SSH port, then install OpenSSH, connect to it and only then remove the Dropbear.
Just to be safe if the public key authorization will be broken you'll need to use a password to connect.
So ensure that the root user has a password by using the passwd command.
Set a Dropbear's port to some unused (e.g. 2222) and restart it
uci set dropbear.@dropbear[0].Port=2222 uci commit dropbear /etc/init.d/dropbear restart
Reconnect to the SSH using the new port: ssh root@192.168.1.1 -p 2222
Install OpenSSH server
opkg update
opkg install openssh-server
Allow root access:
sed -i 's/^#PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config
On connection the Dropbear checks that your public key is allowed in a /etc/dropbear/authorized_keys file.
But the OpenSSH checks /root/.ssh/authorized_keys.
So it's recommended to copy the file:
mkdir /root/.ssh/ cp /etc/dropbear/authorized_keys /root/.ssh/
Enable the OpenSSH daemon and restart:
/etc/init.d/sshd enable /etc/init.d/sshd restart
The OpenSSH now use the standard 22 port. Reconnect to SSH over the 22 port ssh root@192.168.1.1
During installation the OpenSSH will generate a new host keys so you'll get a warning that host key was changed.
Once connected now you can disable the Dropbear:
/etc/init.d/dropbear disable /etc/init.d/dropbear stop
Recommended step: Install openssh-sftp-server package to support the SFTP protocol
opkg update
opkg install openssh-sftp-server
If needed, configure the OpenSSH server in /etc/ssh/sshd_config and restart it /etc/init.d/sshd restart