How to use {libresolv, libbind}.

dns_packet_parse.c

/*
 * # Advance preparation are required.
 * # ipfw add divert 10000 udp from me to [nameserver_ip] via [IF]
 * # ipfw add divert 10000 udp from [nameserver_ip] to me via [IF]
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/queue.h>

#include <nameser.h>


void hexdump (char *desc, void *addr, int len)
{
    int i;
    unsigned char buff[17];
    unsigned char *pc = addr;

    if (desc != NULL)
        printf ("%s:\n", desc);

    for (i = 0; i < len; i++) {
        if ((i % 16) == 0) {
            if (i != 0) {
                printf ("  %s\n", buff);
            }
            printf ("  %04x ", i);
        }
        printf (" %02x", pc[i]);
        if ((pc[i] < 0x20) || (pc[i] > 0x7e)) {
            buff[i % 16] = '.';
        } else {
            buff[i % 16] = pc[i];
        }
        buff[(i % 16) + 1] = '\0';
    }
    while ((i % 16) != 0) {
        printf ("   ");
        i++;
    }
    printf ("  %s\n", buff);

    return;
}

int main(int argc, char** argv)
{
    int i;
    int sock;
    struct sockaddr_in sin_divert;
    ssize_t size;
    char buf[4096];
    struct sockaddr_in recv_addr;
    socklen_t addr_size = sizeof(recv_addr);

    ns_msg ns_handle;

    sin_divert.sin_family   = AF_INET;
    sin_divert.sin_port     = htons(10000);

    sock = socket(AF_INET, SOCK_RAW, IPPROTO_DIVERT);
    if(sock < 0){
        perror("socket.");
        exit(EXIT_FAILURE);
    }

    if(bind(sock, (struct sockaddr*)&sin_divert, sizeof(sin_divert)) < 0){
        perror("bind.");
        exit(EXIT_FAILURE);
    }

    while(1) {

        memset(&recv_addr, 0, sizeof(struct sockaddr_in));
        memset(&ns_handle, 0, sizeof(ns_handle));

        size = recvfrom(sock, buf, sizeof(buf), 0,(struct sockaddr*)&recv_addr, &addr_size);
        if (size < 0) {
            exit(EXIT_FAILURE);
        }


        /*
         * パケットのパース部分省略！！！
         */
        if(ns_initparse((uint8_t*)buf+28,  size-28, &ns_handle)) {
           exit(EXIT_FAILURE);
        }


        /*
         * -- identifier
         */
        int name_id = ns_msg_id(ns_handle);
        printf("-- identifier -\n");
        printf("%d\n", name_id);
        printf("---------------\n");

        /* 
         * -- flags --
         * 0    1                5    6    7    8           11               15
         * +----+----------------+----+----+----+-----------+----------------+
         * | QR | Operation Code | AA | TC | RA |   Zero    |    Recode      |
         * +----+----------------+----+----+----+-----------+----------------+
         * 
         * Question/Response          : ns_f_qr
         * Operation code             : ns_f_opcode
         * Authoritative Answer       : ns_f_aa
         * Truncation occurred        : ns_f_tc
         * Recursion Desired          : ns_f_rd
         * Recursion Available        : ns_f_ra
         * MBZ                        : ns_f_z
         * Authentic Data (DNSSEC)    : ns_f_ad
         * Checking Disabled (DNSSEC) : ns_f_cd
         * Response code              : ns_f_rcode
         */
        printf("-- flag -------\n");
        printf("ns_f_qr    : %d\n", ns_msg_getflag(ns_handle, ns_f_qr));
        printf("ns_f_opcode: %d\n", ns_msg_getflag(ns_handle, ns_f_opcode));
        printf("ns_f_aa    : %d\n", ns_msg_getflag(ns_handle, ns_f_aa));
        printf("ns_f_tc    : %d\n", ns_msg_getflag(ns_handle, ns_f_tc));
        printf("ns_f_rd    : %d\n", ns_msg_getflag(ns_handle, ns_f_rd));
        printf("ns_f_ra    : %d\n", ns_msg_getflag(ns_handle, ns_f_ra));
        printf("ns_f_z     : %d\n", ns_msg_getflag(ns_handle, ns_f_z));
        printf("ns_f_ad    : %d\n", ns_msg_getflag(ns_handle, ns_f_ad));
        printf("ns_f_cd    : %d\n", ns_msg_getflag(ns_handle, ns_f_cd));
        printf("ns_f_rcode : %d\n", ns_msg_getflag(ns_handle, ns_f_rcode));
        printf("---------------\n");

        /*
        Question     ns_s_qd 
        Answer       ns_s_an 
        Name servers ns_s_ns 
        Add records  ns_s_ar 
        */
        int name_count_qd = ns_msg_count(ns_handle, ns_s_qd);
        int name_count_an = ns_msg_count(ns_handle, ns_s_an);
        int name_count_ns = ns_msg_count(ns_handle, ns_s_ns);
        int name_count_ar = ns_msg_count(ns_handle, ns_s_ar);
        /*
        printf("-- count ------\n");
        printf("ns_s_qd    : %d\n", name_count_qd);
        printf("ns_s_an    : %d\n", name_count_an);
        printf("ns_s_ns    : %d\n", name_count_ns);
        printf("ns_s_ar    : %d\n", name_count_ar);
        printf("---------------\n");
        */

        ns_rr rr;
        char pbuf[0xFFFF];

        // query
        printf("Questionrecord\n");
        for (i = 0; i < name_count_qd; i++) {
            memset(&rr, 0, sizeof(rr));
            if (ns_parserr(&ns_handle, ns_s_qd, i, &rr) == 0) {
                printf("\t-- %d --\n", i+1);
                ns_sprintrr(&ns_handle, &rr, NULL, NULL, pbuf, sizeof(pbuf));
                printf("\t%s\n", pbuf);
                printf("\tfqdn :%s\n", ns_rr_name(rr));
                printf("\ttype :%d\n", ns_rr_type(rr));
                printf("\tclass:%d\n", ns_rr_class(rr));
                printf("\tttl  :%d\n", ns_rr_ttl(rr));
                //printf("\trdlen:%d\n", ns_rr_rdlen(rr));
                //hexdump("qd", (uint8_t*)ns_rr_rdata(rr), ns_rr_rdlen(rr));
            }
        }

        printf("AnswerRecord\n");
        for (i = 0; i < name_count_an; i++) {
            memset(&rr, 0, sizeof(rr));
            if (ns_parserr(&ns_handle, ns_s_an, i, &rr) == 0) {
                printf("\t-- %d --\n", i+1);
                ns_sprintrr(&ns_handle, &rr, NULL, NULL, pbuf, sizeof(pbuf));
                printf("\t%s\n", pbuf);
                printf("\tfqdn :%s\n", ns_rr_name(rr));
                printf("\ttype :%d\n", ns_rr_type(rr));
                printf("\tclass:%d\n", ns_rr_class(rr));
                printf("\tttl  :%d\n", ns_rr_ttl(rr));
                //printf("\trdlen:%d\n", ns_rr_rdlen(rr));
                //hexdump("an", (uint8_t*)ns_rr_rdata(rr), ns_rr_rdlen(rr));
            }
        }

        printf("NameServerRecord\n");
        for (i = 0; i < name_count_ns; i++) {
            memset(&rr, 0, sizeof(rr));
            if (ns_parserr(&ns_handle, ns_s_ns, i, &rr) == 0) {
                printf("\t-- %d --\n", i+1);
                ns_sprintrr(&ns_handle, &rr, NULL, NULL, pbuf, sizeof(pbuf));
                printf("\t%s\n", pbuf);
                printf("\tfqdn :%s\n", ns_rr_name(rr));
                printf("\ttype :%d\n", ns_rr_type(rr));
                printf("\tclass:%d\n", ns_rr_class(rr));
                printf("\tttl  :%d\n", ns_rr_ttl(rr));
                //printf("\trdata:%s\n", ns_rr_rdata(rr));
                //hexdump("ns", (uint8_t*)ns_rr_rdata(rr), ns_rr_rdlen(rr));
            }
        }

        printf("AdditionalRecord\n");
        for (i = 0; i < name_count_ar; i++) {
            memset(&rr, 0, sizeof(rr));
            if (ns_parserr(&ns_handle, ns_s_ar, i, &rr) == 0) {
                printf("\t-- %d --\n", i+1);
                ns_sprintrr (&ns_handle, &rr, NULL, NULL, pbuf, sizeof(pbuf));
                printf("\t%s\n", pbuf);
                printf("\tfqdn :%s\n", ns_rr_name(rr));
                printf("\ttype :%d\n", ns_rr_type(rr));
                printf("\tclass:%d\n", ns_rr_class(rr));
                printf("\tttl  :%d\n", ns_rr_ttl(rr));
                //printf("\trdlen:%d\n", ns_rr_rdlen(rr));
                //hexdump("ar", (uint8_t*)ns_rr_rdata(rr), ns_rr_rdlen(rr));
            }
        }

        size = sendto(sock, buf, size, 0, (struct sockaddr*)&recv_addr, addr_size);

    }
    return 0;
}