If the answer to an upstream query is a CNAME which points to an
A/AAAA record which also exists in /etc/hosts and friends, then
caching is suppressed, to avoid inconsistent answers. This is
now modified to allow caching when the upstream and local A/AAAA
records have the same value.

This should solve problems people are seeing when a file changes
twice within a second and thus is missed for polling.

Make sure dst_addr is assigned the correct address in receive_query when OPTNOWILD is
enabled so the assigned mark can be correctly retrieved and set in forward_query when
conntrack is enabled.
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

While testing https://github.com/sbyx/odhcp6c client I have noticed it
permanently crashes after startup.

The reason was it (odhcp6c) doesn't expect empty IA options in ADVERTISE
message without any suboptions.

Despite this validation bug of odhcp6c, dnsmasq should not generate
ADVERTISE messages with IA if there's nothing to advert per  RFC 3315
17.2.2:

   If the server will not assign any addresses to any IAs in a

   subsequent Request from the client, the server MUST send an Advertise

   message to the client that includes only a Status Code option with

   code NoAddrsAvail and a status message for the user, a Server

   Identifier option with the server's DUID, and a Client Identifier

   option with the client's DUID.

Meanwhile it's need to add status code for every IA in REPLY message per
RFC3315 18.2.1:

   If the server cannot assign any addresses to an IA in the message
   from the client, the server MUST include the IA in the Reply message
   with no addresses in the IA and a Status Code option in the IA
   containing status code NoAddrsAvail.

So, I've changed the logic to skip IA completely from ADVERTISE messages and
to add NoAddrsAvail subcode into IA of REPLY messages.

As for overhead, yes, I believe it's ok to return NoAddrsAvail twice in IA
and in global section for compatibility with all old and new clients.

Simon's fault.

SO_MARK has to be done before issuing the connect() call on the
TCP socket.