nsd users - Jul 2011 - NSD returns SERVFAIL instead of REFUSED

I was wondering why NSD returns SERVFAIL for recursive queries, normally it
is REFUSED ?

NSD:
;; ->>HEADER<<- opcode: QUERY, status: *SERVFAIL*, id: 39603
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

BIND:
;; ->>HEADER<<- opcode: QUERY, status: *REFUSED*, id: 4023
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

Is it configurable ? At least I could not find any options.

Thanks,
Arun
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On Sun, Jul 17, 2011 at 10:34:56AM +0300,
 Arun N S <ns.arunns at gmail.com> wrote 
 a message of 55 lines which said:
> I was wondering why NSD returns SERVFAIL for recursive queries,
> normally it is REFUSED ?
AFAIK, no RFC explicitely says what a name server should return when
it does not accept out-of-zone queries.