Original Patch From Brad Spencer:
https://c-ares.haxx.se/mail/c-ares-archive-2016-04/0000.shtml
My modifications include:
* Dynamically find GetBestRoute2 since it is a Windows Vista+ symbol, and will fall back to prior behavior when not available.
* Prefer get_DNS_AdaptersAddresses as the modifications should alleviate the concerns which caused us to prefer get_DNS_NetworkParams
* Update AppVeyor to use MinGW-w64 instead of the legacy MinGW
* Fix compile error in test suite for Windows.
Original message from patch below:
From: Brad Spencer <bspencer@blackberry.com>
Date: Fri, 29 Apr 2016 14:26:23 -0300
On Windows, the c-ares DNS resolver tries first to get a full list of
DNS server addresses by enumerating the system's IPv4/v6 interfaces and
then getting the per-interface DNS server lists from those interfaces
and joining them together. The OS, at least in the way the c-ares
prefers to query them (which also may be the only or best way in some
environments), does not provide a unified list of DNS servers ordered
according to "current network conditions". Currently, c-ares will then
try to use them in whatever order the nested enumeration produces, which
may result in DNS requests being sent to servers on one interface
(hosting the current default route, for example) that are only intended
to be used via another interface (intended to be used when the first
interface is not available, for example). This, in turn, can lead to
spurious failures and timeouts simply because of the server address
order that resulted because of the enumeration process.
This patch makes the (safe?) assumption that there is no other better
rule to chose which interface's DNS server list should be prioritized.
After all, a DNS lookup isn't something "per network"; applications
don't look up "these DNS names on this interface and those DNS names on
that interface". There is a single resource pool of DNS servers and the
application should presume that any server will give it the "right"
answer. However, even if all DNS servers are assumed to give equally
useful responses, it is reasonable to expect that some DNS servers will
not accept requests on all interfaces. This patch avoids the problem by
sorting the DNS server addresses using the Windows IPv4/v6 routing tables.
For example, a request to DNS server C on interface 2 that is actually
sent over interface 1 (which may happen to have the default route) may
be rejected by or not delivered to DNS server C. So, better to use DNS
servers A and B associated with interface 1, at least as a first try.
By using the metric of the route to the DNS server itself as a proxy for
priority of the DNS server in the list, this patch is able to adapt
dynamically to changes in the interface list, the DNS server lists per
interface, which interfaces are active, the routing table, and so on,
while always picking a good "best" DNS server first.
In cases where any DNS server on any interface will do, this patch still
seems useful because it will prioritize a lower-metric route's (and thus
interface's) servers.
The added api requires both some typedefs not previously imported
into the test build + the test code did not fully deal with
socket differences on windows.
* Added test case macro to automatically run tests twice, once "normal",
once with virtual IO.
* Changed most "live" query tests to run in dual mode to verify
at least simple socket IO via virtual functions
* Added test case for settings/duping socket functions & callback data
Uses virtual socket IO functions when set on a channel.
Note that no socket options are set, nor is any binding
done by the library in this case, since the client defining
these is probably more suited to deal with this.
Changed the way usage requirements (include dirs, compile defs, dependent libraries) are specified, to match the recommended standard practice for modern CMake. This involves using target-specific functions (target_include_directories, target_compile_definitions, etc.), along with the PUBLIC, PRIVATE or INTERFACE modifiers.
Updated chain-building support to imitate new-style Find modules (import libs), instead of old-style Find modules (cache variables).
Built tarballs are not including all of the files needed
to build the test suite because they are missing from the
<target>_SOURCES variable in Makefile.am.
Michael Osei
Brad House
Sergii Pylypenko
Calle Wilund
Calle Wilund
David Drysdale
Stephen Sorley
Gregor Jasny
Chris Araman
Daniel Stenberg
Aaron Bieber