On the BIND 9.x server the private key corresponding to the ZSK, stored on name servers accepting dynamic updates, must be group owned by root.

The private ZSK key must be protected from unauthorized access. This strategy is not feasible in situations in which the DNSSEC-aware name server has to support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) has to have both the zone file master copy and the private key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures for the updated RRsets.