[krbdev.mit.edu #8688] URI lookups don't work on Windows

[Greg Hudson via RT]

The wshelper res_search() (invoked from dnsglue.c) calls 
DnsQuery_A(). (The _A here refers to ANSI encoding, not DNS A 
records).  This Windows function helpfully parses the response packet 
and yields a linked list of records, each decoded according to the 
query type.  wshelper undoes this work, reassembling a fake DNS 
response packet from the query domain and resource records, so that 
we can parse it again in dnsglue.c and dnssrv.c.

DnsQuery_A() has no support for URI records.  Therefore, 
res_quer.c:build_rr() has no code to piece together a URI record 
encoding from a PDNS_RECORD.  Although it is experimentally possible 
to make a URI query and find the record encoding (un-decoded) in the 
PDNS_RECORD function where the union rests, retrieving the record 
this way seems unlikely to keep working if Windows ever implements 
the URI type.

I am hoping to use the DNS_QUERY_RETURN_MESSAGE flag to get the 
original DNS response packet so we don't have to reassemble it; 
however, this flag is not documented well enough to use and I can 
find no examples of code using it.  I have asked Microsoft dochelp 
for clarification and will report back with their answer.