libcurl versions greatet than 7.21.7 including the current version 7.27.0 no longer function with c-ares versions between 1.5.0 and 1.6.0.
The problem was introduced in the following commit:
Author: Vsevolod Novikov <email@example.com>
Date: Sat Jan 29 20:12:10 2011 +0100
In file lib/asyn-ares.c:
Curl uses the macro ARES_VERSION to decide the value of macro HAVE_CARES_CALLBACK_TIMEOUTS, which affects whether to use a new style or old style prototype for "query_completed_cb". This macro is defines in <ares_version.h>. Curl does not include this header directly, it only includes <ares.h>.
In newer versions of c-ares, <ares.h> includes <ares_version.h> itself, so there is no problem.
In older versions of c-ares, <ares.h> does not include <ares_version.h>, so the macro ARES_VERSION stays undefined. If the actual version of c-ares is 1.5.x or 1.6.0, this means that the wrong prototype is used for query_completed_cb. The compiler issues a warning but it is easy to miss that. The end result is that no host lookups work. (Crashes are also possible.)
Proposed solutions (one or the other):
- Re-add #include <ares_version.h> in asyn-ares.h after #include <ares.h>
- Drop support for versions of c-ares less than 1.6.1 and update the file lib/README.ares and m4/curl-confopts.m4 to reflect this.