/************************************
编译：
gcc -g -o dns dns.c -lresolv

测试：
./dns gmail.com

结果：
pref    domain name
20      alt2.gmail-smtp-in.l.google.com
30      alt3.gmail-smtp-in.l.google.com
40      alt4.gmail-smtp-in.l.google.com
5       gmail-smtp-in.l.google.com
10      alt1.gmail-smtp-in.l.google.com
*************************************/

//-----------------------------------
// dns.h
#ifndef     DNS_H
#define     DNS_H
 
#define     DNS_MSG_END -2
 
#define     dns_mx_query(str)       dns_resolve((str), T_MX)
#define     dns_mx_expand()         dns_findmx(T_MX)
 
#define     foreach_mxrr(p, dn)     while(dns_mx_expand() != DNS_MSG_END \
                                        && (!dns_get_mxrr(&p, dn, MAXDNAME)))
 
void dns_init(void);
int dns_get_mxrr(unsigned short *, unsigned char *, unsigned int);
int dns_resolve(char *, int);
int dns_findmx(int);
 
#endif
//-----------------------------------

//dns.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <sys types.h="">
#include <netinet in.h="">
#include <arpa nameser.h="">
#include <resolv.h>
#include <errno.h>
#include "dns.h"
 
extern int res_query();
extern int res_search();
extern int errno;
extern int h_errno;
 
HEADER  *hdr;
unsigned char *buf;
static int response_len;
static unsigned char *response_end;
static unsigned char *response_pos;
static int num_answers;
static char name[MAXDNAME];
unsigned short pref;
 
static unsigned short getshort(unsigned char *c)
{
    unsigned short u;
    u = c[0];
    return (u << 8) + c[1];
}
 
int dns_resolve(char *domain, int type)
{
    int     n;
    int     i;
    errno = 0;
    if (domain == NULL)
        return -1;
 
    response_len = res_search(domain, C_IN, type, buf, PACKETSZ);
 
    if (response_len <= 0)
        return -1;
 
    if (response_len >= PACKETSZ)
        response_len = PACKETSZ;
 
    response_end = buf + response_len;
    response_pos = buf + sizeof(HEADER);
    n = ntohs(hdr->qdcount);
    while (n-- > 0) {
        i = dn_expand(buf, response_end, response_pos, name, MAXDNAME);
        response_pos += i;
        i = response_end - response_pos;
        if (i < QFIXEDSZ)   return -1;
        response_pos += QFIXEDSZ;
    }
 
    num_answers = ntohs(hdr->ancount);
    return num_answers;
}
 
int dns_findmx(int want_type)
{
    unsigned short rrtype;
    unsigned short rrdlen;
    int             i;
 
    if (num_answers <= 0) return DNS_MSG_END;
    num_answers--;
 
    if (response_pos == response_end) return -1;
    i = dn_expand(buf, response_end, response_pos, name, MAXDNAME);
    if (i < 0) return -1;
    response_pos += i;
 
    i = response_end - response_pos;
    if (i < 10) return -1;
 
    rrtype = getshort(response_pos);
    rrdlen = getshort(response_pos + 8);
    response_pos += 10;
    if (rrtype == want_type) {
        if (rrdlen < 3) return -1;
        pref = (response_pos[0] << 8) + response_pos[1];
        memset(name, 0, MAXDNAME);
        if (dn_expand(buf, response_end, response_pos + 2, name, MAXDNAME) < 0)
            return -1;
        response_pos += rrdlen;
        return strlen(name);
    }
 
    response_pos += rrdlen;
    return 0;
}
 
void dns_init()
{
    res_init();
    memset(name, 0, MAXDNAME);
}
 
int dns_get_mxrr(unsigned short *p, unsigned char *dn, unsigned int len)
{
    *p = pref;
    strncpy(dn, name, len);
    if (len < (strlen(name) + 1))
        return -1;
    return 0;
}
 
int main(int argc, char **argv)
{
    char dname[MAXDNAME];
    int     i;
    unsigned short p;
 
    buf = (unsigned char *)calloc(1, PACKETSZ);
    if (buf == NULL) {
        fprintf(stderr, "calloc for response.buf failed");
        exit(-1);
    }
    hdr = (void *)buf;
 
    dns_init();
    if (argc != 2) {
        fprintf(stderr, "bad argument\n");
        exit(-1);
    }
 
    i = dns_mx_query(argv[1]);
    if (i < 0) {
        fprintf(stderr, "err\n");
        return 0;
    }
    printf("pref\tdomain name\n");
    foreach_mxrr(p, dname) {
        printf("%d\t%s\n", p, dname);
    }
 
    if (buf) {
        free(buf);
        buf = NULL;
    }
 
    return 0;
}