nsupdate.info/docs/examples/bind9/named.conf

// configuration snippet for bind 9 nameserver (put it into /etc/bind9/named.conf )

key "nsupdate.info." {
    // must be same algorithm as in the Domain record of the nsupdate.info based service
    algorithm hmac-sha512;
    // the secret is just a shared secret in base64-encoding, you don't need
    // to use a special tool to create it. Some random in base64 encoding should
    // be OK. must be same secret as in the Domain record of the nsupdate.info based service
    secret "YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYQ==";
};

zone nsupdate.info {
        type master;
        // bind9 needs write permissions into that directory and into that file:
        file "/etc/bind/zones/nsupdate.info";
        // everyone who has that key may update this zone:
        allow-update { key "nsupdate.info."; };
};