I have been bashing my head against the fact that on the iPhone you seemingly cannot access /etc/resolv.conf, or use functions from the Mac such as SCDynamicStore stuff. I have seen many people asking this question, a few related to the same package I was trying to use – ares.

Eventually I managed to find a fix which uses the libresolv to find the DNS servers. This seems to work just fine and I have shown the patch to ares_init.c below for reference.

#if TARGET_OS_IPHONE
#include <resolv.h>
#endif

...

#if TARGET_OS_IPHONE
// XXX: On the iPhone we need to get the DNS servers using resolv.h magic
if ((_res.options & RES_INIT) == 0) res_init();
channel->nservers = _res.nscount;
channel->servers = malloc(channel->nservers * sizeof(struct server_state));
memset(channel->servers, '\0', channel->nservers * sizeof(struct server_state));

int i;
for (i = 0; i < channel->nservers; i++)
{
    memcpy(&channel->servers[i].addr, &_res.nsaddr_list[i].sin_addr, sizeof(struct in_addr));
}
#endif

[As per most of my posts these days, this is just a brain dump. I'll try to pad it out as soon as possible]