Generate a bind9 rndc key
dnssec-keygen -a hmac-md5 -b 256 -n HOST mybrandnewkey
The result will be something like "K%2Ftmp%2Fmybrandnewkey.+157+42885", so you do
cat K%2Ftmp%2Fmybrandnewkey.+157+42885.private
Put the key in /etc/rndc.key like this:
key "rndc-key" { algorithm hmac-md5; secret "Hr1U28EsMhp8r0TEEjAmksoldReEYuiWwQFH0/gkk6I="; };