Requesting A and AAAA records in single DNS query

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I’m not aware of any nameservers that support multiple questions in a single query.

There’s potential for ambiguity in such a query, since there are per-packet flags (such as AA) which could apply to only one of the questions. If you ask two questions and the server is authoritative for only one of the domains, should the server set the flag or not? I suspect issues such as these have deterred implementors.

There have been a number of proposals to solve the problem you’re talking about (such as this proposal to introduce a QTYPE that combines A and AAAA, and Paul Vixie’s repeated attempts to introduce an EDNS form of multiple questions), but at present programs supporting both IPv4 and 6 tend to perform two separate queries, either AAAA followed (after a timeout) by A, or both simultaneously.

I suppose there’s also the “all” QTYPE, but it can return a lot more data than you need.

Edit: from query.c in the BIND source:

   dns_message_currentname(message, DNS_SECTION_QUESTION,
         &client->query.qname);
   client->query.origqname = client->query.qname;
   result = dns_message_nextname(message, DNS_SECTION_QUESTION);
   if (result != ISC_R_NOMORE) {
     if (result == ISC_R_SUCCESS) {
       /*
        * There's more than one QNAME in the question
        * section.
        */
       query_error(client, DNS_R_FORMERR, __LINE__);
     } else
       query_error(client, result, __LINE__);
     return;
   }

Edit: also, from resolver.c in the BIND source:

    /*
     * XXXRTH  Currently we support only one question.
     */
    if (message->counts[DNS_SECTION_QUESTION] != 1) {
            log_formerr(fctx, "too many questions");
            return (DNS_R_FORMERR);
    }

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