RD, RA and CD really only make sense when the opcode is QUERY.  The
change in behaviour was noted in CHANGES.

4534.  [bug]           Only set RD, RA and CD in QUERY responses. [RT #43879]

In theory one could extend UPDATE to recurse where the UPDATE message
is forwarded by a recursive server to the servers for the zone in
which case RD and RA would have meaning.  The same would apply to
any other future opcode.

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka@isc.org