Programs that use the resolver library but query IPv6 resolvers directly require access to the IPv6 extensions in resolv.h.  However, the definition for struct __res_state_ext is missing from the standard system header files.

Fix: 

Include a definition for struct __res_state_ext in resolv.h

struct __res_state_ext {
        union res_sockaddr_union nsaddrs[MAXNS];
        struct sort_list {
                int     af;
                union {
                        struct in_addr  ina;
                        struct in6_addr in6a;
                } addr, mask;
        } sort_list[MAXRESOLVSORT];
        char nsuffix[64];
        char nsuffix2[64];
};
How-To-Repeat: The following will fail to compile:

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

int main(void) {
  struct __res_state myres;
  struct __res_state_ext *myres6;
  struct sockaddr_in6 *mysockaddr6;

  res_ninit(&myres);
  myres6 = myres._u._ext.ext;
  mysockaddr6 = &(myres6->nsaddrs[0].sin6); 
}