/** \page design Design

The following image shows the various modules of libdns and their
functionality.

\image html libdnsoverview.png

\section central_structures Central structures

  - \ref ldns_pkt A DNS Packet, which can contain a query, answers, and additional information.
  - \ref ldns_rr A Resource Record, which holds a bit of information about a specific domain name.
  - \ref ldns_rdf An RDATA field, every Resource Record contains one or more RDATA fields, depending on the type of RR. These are the 'basic types' of DNS data.
  - \ref ldns_rr_list A list of resource records
  - \ref ldns_zone A representation of a DNS Zone.

The actual structure definitions are named \c ldns_struct_<name> which are all typedeffed to \c ldns_<name>


A DNS Resource record looks like this:

<pre>
nlnetlabs.nl.   600     IN      MX             10    open.nlnetlabs.nl.
 \              \       \       \              \                     /
  owner          ttl     class   type           \        rdf[]      /
  (rdf)     (uint32_t) (rr_class) (rr_type)
                                                 10          := rdf[0]
                                                 open.nlnetlabs.nl. := rdf[1]
</pre>

* Owner:
The owner name is put in an \c ldns_rdf structure, which is a normal rdata field but always
has the type LDNS_RDF_TYPE_DNAME.

An \ref ldns_rdf structure has 3 members; the size, the type of rdata and a void *
pointer to the data. The data is always in uncompressed wireformat.

The RSH (rdata) is put in an array of rdf's (in this case 2).

The entire resource record is put in a RR structure, which has
the fields described above (under the RR):
  - owner	(nlnetlabs.nl.)
  - ttl		(600)
  - class	(LDNS_RR_CLASS_IN: 'IN')
  - type         (LDNS_RR_TYPE_MX: 'MX')
  - rd_count	(2)
  - rdata_fields[rd_count]
                - rdf[0]  (10)
                - rdf[1]  (open.nlnetlabs.nl.)

* RR list structure:
An RR list structure is simply a structure with a counter
and an array of RR structures. Different kinds of RR's can
be grouped together this way.

* RRset structure:
An RRset structure is an RR list structure, but its accessor
function checks if the \c ldns_rr structures in there are: 
 - from the same type
 - have the same TTL
 - have the same ownername

This is the RFC definition of an RRset.

* pkt structure:
A pkt structure consists out of a header structure where 
packet specific flags are kept, TC, RD, IP from the server which
sent the packet, etc.
Further more it is divided in 4 sections: question, authority, answer
and additional.

All four sections have the type RRlist that simply hold a list of RR's


\section wire_module Wire module and central structures Interface

As the WIRE module takes care of the compression/decompression
it needs a buffer which holds all the binary DNS data.
All functions will operate on such a buffer to extract specific
information which is then stored in RR structures.


\section resolver_module Resolver module and central structures Interface

The resolver module always returns a pkt structure. Either with
the answer or a SERVFAIL pkt.

The exact function-call parameters have not yet been 
decided on.

Also the resolver module will need to access some of the
to_wire and from_wire function to creates ldn_pkt's from
the data it receives (arrow not drawn).


\section str_module str module and central structures Interface

Convert to and from strings. This module could be used
to read in a zone file (list of RRs) and convert the text strings to
the format used by ldns. Or the other way around.


\section net_module Net module and resolver module interface

The resolver module will get a packet and will mold it so that
it can be sent off to a nameserver. 
It might need to interface with the wire module (arrow not drawn).

\section Net module and OS/libc interface
OS/network calls will be used here. The Net module is the only part of 
the library where the underlying OS matters. 

\section Client program ldns interface
Any client program will have access to
- Wire module
- Central structures
- Resolver module
- str module (arrow not drawn in the above figure)

\section dnssec_module DNSSEC module
The DNSSEC types are handled in the RR module, but the crypto
routines are contained in this module. This module will depend
on OpenSSL for the crypto routines. 

*/