Consideration: Name resolution

Conditional forwarding

Any name at the same, lower, or higher level than local zones.

Stub zone

Any name at the same, lower, or higher level than local zones.

Delegation

Only to subdomains of local zones.

Secondary zone

Any name at the same, lower, or higher level than local zones.

Consideration: Query resolution type

Conditional forwarding

Server sends a recursive query to the forwarder.

Stub zone

Server resolves query or passes referral to client for iterative resolution, depending on the query.

Delegation

Server resolves query or passes referral to client for iterative resolution, depending on the query.

Secondary zone

Server resolves the query from the local zone data.

Consideration: Firewall

Conditional forwarding

Firewall-friendly.

Stub zone

Can be affected by firewalls blocking clients.

Delegation

Can be affected by firewalls blocking clients.

Secondary zone

Can be affected by firewalls blocking clients.

Consideration: Replication

Conditional forwarding

Configured per-server.

Stub zone

Automatically replicated if zone is Active Directory integrated, otherwise configured per server.

Delegation

Always replicated to other NS of the parent zone.

Secondary zone

Automatically replicated from the master DNS server by zone transfer.

Consideration: Flexibility

Conditional forwarding

Has to be re-configured when a name server NS is added to or removed from the target zone.

Stub zone

Automatically updates the list of NS for the zone. However, the stub zone has to be reconfigured if the server from which the zone is transferred is removed from the zone.

Delegation

Has to be re-configured when a NS is added to or removed from the target zone.

Secondary zone

Has to be reconfigured if the server from which the zone is transferred is removed from the zone.

Consideration: Fault tolerance

Conditional forwarding

Can be fault-tolerant if multiple forwarders are configured.

Stub zone

Can be fault-tolerant if the stub zone is configured on multiple servers.

Delegation

Fault-tolerant.

Secondary zone

Can be fault-tolerant if multiple servers host the secondary zone.

Consideration: Bandwidth

Conditional forwarding

DNS queries for computers in the zone for which conditional forwarding is configured.
Best used where the amount of forwarded queries would be less than the amount of traffic generated by a zone transfer.

Stub zone

Periodic transfer of NS and glue A records and DNS queries for computers in the stub zone.
Best used where the number of forwarded queries would be less than the amount of traffic generated by a zone transfer.

Delegation

DNS queries for all computers in the zone for which the delegation is configured.
Best used when secondary DNS servers will be used for the parent zone.

Secondary zone

Periodic zone transfer of zone data.
No DNS queries are forwarded for this zone.
Best used when the amount of traffic that would be generated by the zone transfer is less than the amount of traffic that would be generated by forwarded queries.

Consideration: Firewall settings

Conditional forwarding

Firewall must have User Datagram Protocol (UDP) Port 53 open for the two DNS servers on either side of the firewall.

Stub zone

Firewall must have Transmission Control Protocol (TCP) and UDP Port 53 open.

Delegation

Firewall must have UDP Port 53 open.

Secondary zone

Firewall must have TCP Port 53 open.