The SERVFAIL RCODE can be generated for many reasons which may not be related
to lack of feature support. For example, the Stubby resolver generates
SERVFAIL when a request times out. Such transient failures can cause
unnecessary downgrades to both the transaction and the server's feature level.
The consequences of this are especially severe if the server is in DNSSEC
strict mode. In this case repeated downgrades eventually cause the server to
stop resolving entirely with the error "incompatible-server".
To avoid unnecessary downgrades the request should be retried once with the
current level before the transaction's feature level is downgraded.
Steven Siloti