Demonstrates how to send a DNS query and parse response in C using libresolv.

libresolv_query.c

/*
 * Copyright (c) 2013 by Wil Tan <[email protected]>
 *
 * Based on dump_dns.c from the dnscap <https://www.dns-oarc.net/tools/dnscap>
 * originally written by Paul Vixie.
 *
 * Copyright (c) 2007 by Internet Systems Consortium, Inc. ("ISC")
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>


extern const char *_res_opcodes[];

#define MY_GET16(s, cp) do { \
        register const u_char *t_cp = (const u_char *)(cp); \
        (s) = ((u_int16_t)t_cp[0] << 8) \
            | ((u_int16_t)t_cp[1]) \
            ; \
        (cp) += NS_INT16SZ; \
} while (0)

#define MY_GET32(l, cp) do { \
        register const u_char *t_cp = (const u_char *)(cp); \
        (l) = ((u_int32_t)t_cp[0] << 24) \
            | ((u_int32_t)t_cp[1] << 16) \
            | ((u_int32_t)t_cp[2] << 8) \
            | ((u_int32_t)t_cp[3]) \
            ; \
        (cp) += NS_INT32SZ; \
} while (0)

static void
dump_dns_rr(ns_msg *msg, ns_rr *rr, ns_sect sect, FILE *trace) {
	char buf[NS_MAXDNAME];
	u_int class, type;
	const u_char *rd;
	u_int32_t soa[5];
	u_int16_t mx;
	int n;

	class = ns_rr_class(*rr);
	type = ns_rr_type(*rr);
	fprintf(trace, "%s,%s,%s",
		ns_rr_name(*rr),
		p_class(class),
		p_type(type));
	if (sect == ns_s_qd)
		return;
	fprintf(trace, ",%lu", (u_long)ns_rr_ttl(*rr));
	rd = ns_rr_rdata(*rr);
	switch (type) {
	case ns_t_soa:
		n = ns_name_uncompress(ns_msg_base(*msg), ns_msg_end(*msg),
				       rd, buf, sizeof buf);
		if (n < 0)
			goto error;
		putc(',', trace);
		fputs(buf, trace);
		rd += n;
		n = ns_name_uncompress(ns_msg_base(*msg), ns_msg_end(*msg),
				       rd, buf, sizeof buf);
		if (n < 0)
			goto error;
		putc(',', trace);
		fputs(buf, trace);
		rd += n;
		if (ns_msg_end(*msg) - rd < 5*NS_INT32SZ)
			goto error;
		for (n = 0; n < 5; n++)
			MY_GET32(soa[n], rd);
		sprintf(buf, "%u,%u,%u,%u,%u",
			soa[0], soa[1], soa[2], soa[3], soa[4]);
		break;
	case ns_t_a:
		inet_ntop(AF_INET, rd, buf, sizeof buf);
		break;
	case ns_t_aaaa:
		inet_ntop(AF_INET6, rd, buf, sizeof buf);
		break;
	case ns_t_mx:
		MY_GET16(mx, rd);
		fprintf(trace, ",%u", mx);
		/* FALLTHROUGH */
	case ns_t_ns:
	case ns_t_ptr:
	case ns_t_cname:
		n = ns_name_uncompress(ns_msg_base(*msg), ns_msg_end(*msg),
				       rd, buf, sizeof buf);
		if (n < 0)
			goto error;
		break;
	case ns_t_txt:
		snprintf(buf, (size_t)rd[0]+1, "%s", rd+1);
		break;
	default:
 error:
		sprintf(buf, "[%u]", ns_rr_rdlen(*rr));
	}
	if (buf[0] != '\0') {
		putc(',', trace);
		fputs(buf, trace);
	}
}


static void
dump_dns_sect(ns_msg *msg, ns_sect sect, FILE *trace, const char *endline) {
        int rrnum, rrmax;
        const char *sep;
        ns_rr rr;

        rrmax = ns_msg_count(*msg, sect);
        if (rrmax == 0) {
                fputs(" 0", trace);
                return;
        }
        fprintf(trace, " %s%d", endline, rrmax);
        sep = "";
        for (rrnum = 0; rrnum < rrmax; rrnum++) {
                if (ns_parserr(msg, sect, rrnum, &rr)) {
                        fputs(strerror(errno), trace);
                        return;
                }
                fprintf(trace, " %s", sep);
                dump_dns_rr(msg, &rr, sect, trace);
                sep = endline;
        }
}

void
dump_dns(const u_char *payload, size_t paylen,
	  FILE *trace, const char *endline)
{
	u_int opcode, rcode, id;
	const char *sep;
	ns_msg msg;

	fprintf(trace, " %sdns ", endline);
	if (ns_initparse(payload, paylen, &msg) < 0) {
		fputs(strerror(errno), trace);
		return;
	}
	opcode = ns_msg_getflag(msg, ns_f_opcode);
	rcode = ns_msg_getflag(msg, ns_f_rcode);
	id = ns_msg_id(msg);
	fprintf(trace, "%s,%s,%u", _res_opcodes[opcode], p_rcode(rcode), id);
	sep = ",";
#define FLAG(t,f) if (ns_msg_getflag(msg, f)) { \
			fprintf(trace, "%s%s", sep, t); \
			sep = "|"; \
		  }
	FLAG("qr", ns_f_qr);
	FLAG("aa", ns_f_aa);
	FLAG("tc", ns_f_tc);
	FLAG("rd", ns_f_rd);
	FLAG("ra", ns_f_ra);
	FLAG("z", ns_f_z);
	FLAG("ad", ns_f_ad);
	FLAG("cd", ns_f_cd);
#undef FLAG

	dump_dns_sect(&msg, ns_s_an, trace, endline);
}


int main() {
	u_char answer[1024] = "";
	res_init();
	int rv = res_query("google.com", ns_c_in, ns_t_txt, answer, sizeof(answer));
	printf("rv=%d\n", rv);
	dump_dns(answer, rv, stdout, "\n");
	printf("\n");

}

Is it possible to specify the DNS server to be used for the query?

Is it possible to specify the DNS server to be used for the query?

Yes. All you need is to modify the global variable "_res" initialized by "res_init" function. In this structure is an array of used nameservers called "nsaddr_list" with IP addresses of nameservers from the "/etc/resolv.conf" file. You can easily replace them.