On another note, if you allowed them to get root, you should consider your device to be compromised and wipe it completely.

hmm, that's really weird. Are you running 6.0.1 stock or CM13? I'm currently using CM13, and it worked without netId when I last tried it (in January). Looking at the source code for 6.0.0, the second argument (the one just after setnetdns) is passed to stringToNetId, but I'm not familiar enough with the Android source code and C++ to tell what it does precisely.

Ah, that's too bad. I guess they changed the code some time after Lollipop so you don't need to know the netId anymore, only the devname. Not completely sure, but I think dnsdomain is the search domain. For example if you have it set to "lan" and you type "my-home-pc", the system will try to resolve "my-home-pc.lan".

@vishalbiswas Yeah, and how are you going to restore it with dd if the bootloader isn't working?

You could try the service CLI tool which can call system service methods.

You shouldn't need USB debugging for fastboot, as Android isn't even involved in the fastboot mode.