[reSIProcate] Followup on c-ares support

[Brad Spencer]

First, thanks again for the help so far.  With a little more digging
and some help from others, including the c-ares mailing list, I've
managed to get further along in using c-ares with resiprocate.

I have identified a set of "obvious" features that resiprocate depends
on simply by looking at the API differences between contrib/ares and
c-ares.  I have not searched for embedded behavioural changes that may
have been made to contrib/ares without API changes.  However,
especially if c-ares vs contrib/ares were to remain a choice at
configuration time, I think this is okay, at least as a starting point
because c-ares "works".

The attached patch will allow resiprocate to be configured against the
trunk version of c-ares (1.5.3 + patches).  Of course, it still allows
resiprocate to be configured to use contrib/ares as well.  With this
patch against resiprocate-1.4.1, the following features are missing
only when using c-ares:

 1. TryServersOfNextNetworkUponRcode3

   I've read the mailing list history on item 7, and I'm still
   digesting that.  It seems to be only supported on Windows in the
   current contrib/ares source, though.

 2. AfterSocketCreationFuncPtr

   I haven't been able to find much information about the purpose of
   the AfterSocketCreationFuncPtr feature, except I found some note
   about it being related to QoS.

   Is there a reason only UDP sockets from ares are identified via
   this function?  If I can supply the reasons this is important for
   QoS, and we can figure out what to do about TCP, there's probably a
   good argument to be made for adding an analogue of this to c-ares.
 
 3. DNS servers with IPv6 addresses themselves

   contrib/ares can have IPv4 and IPv6 addresses of DNS _servers_
   configured (via the additionalNameservers argument and via built-in
   config parsing), while c-ares can only contact IPv4 addressed DNS
   servers at the moment.  Note that this has nothing to do with AAAA
   record lookups, which work fine when using c-ares.
    
   This seems like a good candidate for a new c-ares feature, and I
   will ask about it on the c-ares list and report back.

-- 
----------------------------------------------------------
Brad Spencer - spencer at starscale.com - www.starscale.com
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