The current code is inefficient when dealing with replies exceeding the
PACKETSZ default size and especially those that require the use of a
Virtual Circuit (TCP socket). When the TC (Truncated) bit is set in the
DNS reply header, res_nquery() automatically switches to VC so it is
able to return the full size of the reply to us. This means we make the
same request four times:
1) over UDP, getting ~512 bytes of data
2) over TCP, getting the full reply but returning ~512 to us
3) over UDP again, getting (maybe) 1400 bytes of data
4) over TCP again, getting all the data
This commit splits res_nquery() into its two component functions,
res_nmkquery() and res_nsend(). This is incomplete: the four queries
above still happen.
Change-Id: I3e3bfef633af4130a03afffd175e728d96d6a604
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
b673fa09c3