c语言编程获得mx记录

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/* dns.c
 *
 * char *getmxbyname(char *domain) - gets the DNS MX records for host/domain char *domain
 *   it returns a colon delimited list of valid MXs.
 */

#include <msmtpd.h>
#include <stdio.h>
#include <ctype.h>
#include <errno.h>
#include <signal.h>
#include <setjmp.h>

#include <sys/types.h>          /* not always automatically included */
#include <sys/param.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <netdb.h>
#undef NOERROR                  /* in <sys/streams.h> on solaris 2.x */
#include <arpa/nameser.h>
#include <resolv.h>       

 
/*
 * Copyright (c) 1983 Eric P. Allman
 * Copyright (c) 1988 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted provided
 * that: (1) source distributions retain this entire copyright notice and
 * comment, and (2) distributions including binaries display the following
 * acknowledgement:  ``This product includes software developed by the
 * University of California, Berkeley and its contributors'' in the
 * documentation or other materials provided with the distribution and in
 * all advertising materials mentioning features or use of this software.
 * Neither the name of the University nor the names of its contributors may
 * be used to endorse or promote products derived from this software without
 * specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

/*silly hackery */

#if defined(BIND_493)
typedef u_char  qbuf_t;
#else
typedef char    qbuf_t;
#endif               

#if defined(BIND_493)
typedef char    nbuf_t;
#else
typedef u_char  nbuf_t;
#endif
          

 

/**/
/*
** GETMXBYNAME -- Fetch mx hosts for a domain
** ------------------------------------------
**
** Returns:
**  Number of mx hosts found.
**
** Outputs:
**  The  contains the mx names.
*/

char *getmxbyname(char *domain)  /*find mxs for this domain*/
{
 //int verbose = 0;
 //int debug = 0;

 /* #define HFIXEDSZ  sizeof(HEADER)    actually 12 */
 #define MAXPACKET  8192   /* max size of packet */
 //#define MAXMXHOSTS  20   /* max num of mx records we want to see */
 enum { MAXMXHOSTS = 20 };
 //char _arr[MAXBUF][MAXBUF];
 enum { MAXMXBUFSIZ = (MAXMXHOSTS * (MAXBUF+1)) };

typedef union {
 HEADER hdr;
 u_char buf[MAXPACKET];
} querybuf;

 static char hostbuf[MAXMXBUFSIZ];

 char *MxHosts[MAXMXHOSTS];
 querybuf answer;  /* answer buffer from nameserver */
 HEADER *hp;   /* answer buffer header */
 int ancount, qdcount;  /* answer count and query count */
 u_char *msg, *eom, *cp;  /* answer buffer positions */
 int type, class, dlen;  /* record type, class and length */
 u_short pref;   /* mx preference value */
 u_short prefer[MAXMXHOSTS]; /* saved preferences of mx records */
 char *bp;   /* hostbuf pointer */
 int nmx;   /* number of mx hosts found */
 register int i;
 register int j;
 register int n;

 char *str;   /* final answer string buffer. */
 str = xmalloc(MAXBUF);

/*
 * Query the nameserver to retrieve mx records for the given domain.
 */
 errno = 0;   /* reset before querying nameserver */
 h_errno = 0;

 n = res_search(domain, C_IN, T_MX, (u_char *)&answer, sizeof(answer));
 if (n < 0)
 {
  if (_res.options & RES_DEBUG)
   debug("sres_search failed/n");
  return(0);
 }

 errno = 0;   /* reset after we got an answer */

 if (n < HFIXEDSZ)
 {
  h_errno = NO_RECOVERY;
  return(0);
 }

 /* avoid problems after truncation in tcp packets */
 if (n > sizeof(answer))
  n = sizeof(answer);

/*
 * Valid answer received. Skip the query record.
 */
 hp = (HEADER *)&answer;
 qdcount = ntohs((u_short)hp->qdcount);
 ancount = ntohs((u_short)hp->ancount);

 msg = (u_char *)&answer;
 eom = (u_char *)&answer + n;
 cp  = (u_char *)&answer + HFIXEDSZ;

 while (qdcount-- > 0 && cp < eom)
 {
  n = dn_skipname(cp, eom);
  if (n < 0)
   return(0);
  cp += n;
  cp += QFIXEDSZ;
 }

/*
 * Loop through the answer buffer and extract mx records.
 */
 nmx = 0;
 bp = hostbuf;

 while (ancount-- > 0 && cp < eom && nmx < MAXMXHOSTS)
 {
  //if (verbose >= 4 || debug)
  // (void) p_rr((qbuf_t *)cp, (qbuf_t *)msg, stdout);

  n = dn_expand(msg, eom, cp, (nbuf_t *)bp, MAXBUF);
  if (n < 0)
   break;
  cp += n;

  type = _getshort(cp);
   cp += INT16SZ;

  class = _getshort(cp);
   cp += INT16SZ;

  /* ttl = _getlong(cp); */
   cp += INT32SZ;

  dlen = _getshort(cp);
  cp += INT16SZ;

  if (type != T_MX || class != C_IN)
  {
   cp += dlen;
   continue;
  }

  pref = _getshort(cp);
  cp += INT16SZ;

  n = dn_expand(msg, eom, cp, (nbuf_t *)bp, MAXBUF);
  if (n < 0)
   break;
  cp += n;

  prefer[nmx] = pref;
  MxHosts[nmx] = bp;
  nmx++;

  //n = strlength(bp) + 1;
  n = strlen(bp) + 1;
  bp += n;
 }

/*
 * Sort all records by preference.
 */
 for (i = 0; i < nmx; i++)
 {
  for (j = i + 1; j < nmx; j++)
  {
   if (prefer[i] > prefer[j])
   {
    register u_short tmppref;
    register char *tmphost;

    tmppref = prefer[i];
    prefer[i] = prefer[j];
    prefer[j] = tmppref;

    tmphost = MxHosts[i];
    MxHosts[i] = MxHosts[j];
    MxHosts[j] = tmphost;
   }
  }
 }

 for (i = 0; i< nmx; i++){
                strcat(str, MxHosts[i]);
                strcat(str, ":");
 }
      
 return(str);

}