DNS functions (example program)

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DNS functions reference (part 1)
DNS functions reference (part 2)
DNS functions (example program)

DNS functions (example program)

Sorry about this little program. It's not very good, but I made it in a rush. It will find all the MX records for a domain, count them and resolve them, then show what it found:

 [eax@valholl ~] ./mx eax.uni.cc
 ASPMX5.GOOGLEMAIL.COM - 10
 ALT1.ASPMX.L.GOOGLE.COM - 5
 ALT2.ASPMX.L.GOOGLE.COM - 5
 ASPMX.L.GOOGLE.COM - 1
 ASPMX2.GOOGLEMAIL.COM - 10
 ASPMX3.GOOGLEMAIL.COM - 10
 ASPMX4.GOOGLEMAIL.COM - 10
 7
 66.249.83.27
 64.233.171.27
 209.85.163.27
 74.125.47.27
 209.85.135.27
 64.233.167.27
 66.249.93.27

Well, here's the source:

 // $Id: mx.c,v 0.1 2008/04/26 13:51:15 eax/eaxes Exp $ -*- C -*-
 // Julian B. <eax@eax.uni.cc>
 // gcc -Wall -s -O3 -Os -fexpensive-optimizations mx.c -o mx /usr/lib/libresolv.a
 #include <netinet/in.h>
 #include <arpa/nameser.h>
 #include <arpa/inet.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <resolv.h>
 #include <netdb.h>
 #include <err.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 
 #define MAX_MX 10
 
 struct _MX_SERVER
 {
   int priority;
   char MX[MAXDNAME+1];
 } MX_SERVER[MAX_MX];
 
 int MX_NUM = 0;
 
 int findMX(char *domain);
 
 int main(int argc, char *argv[])
 {
   int x;
   struct hostent *host;
   
   if (argc != 2)
     errx(0, "\rusage: %s [domain]", argv[0]);

   if (res_init() == -1)
     errx(0, "Couldn't initialize the DNS resolver");
   
   if (findMX(argv[1]) == 1) 
     exit(EXIT_FAILURE);
   else {
     printf("%i\n", MX_NUM);
     for(x = 0; x < MX_NUM; x++) {
       if ((host = gethostbyname(MX_SERVER[x].MX)) == NULL) {
 	herror("gethostbyname");
        errx(1, "Error resolving hostname!");
       }
       printf("%s\n", inet_ntoa(*((struct in_addr *)host->h_addr)));
     }
   }
   return 0;
 }
 
 int findMX(char *domain)
 {
   unsigned char query_buffer[512];
   int query_response, x;
   ns_msg query_parse_msg;
   ns_rr query_parse_rr;
   
   if ((query_response = res_query(domain, C_IN, T_MX, query_buffer, sizeof(query_buffer))) == -1)
     {
       if (h_errno == HOST_NOT_FOUND) 
         warnx("Please choose an existing domain!");
       else 
         herror("res_query");
       return 1;
     }
     if (ns_initparse(query_buffer, query_response, &query_parse_msg) == -1)
       {
         warnx("Wasn't able to start the DNS query parser.");
         return 1;
       }
     for (x = 0; x < ns_msg_count(query_parse_msg, ns_s_an); x++)
       {
         if (ns_parserr(&query_parse_msg, ns_s_an, x, &query_parse_rr))
         {
 	  warnx("Failed to use ns_parserr.");
 	  return 1;
         }
         if (ns_rr_type(query_parse_rr) == ns_t_mx)
 	  {
             MX_SERVER[x].priority = ns_get16((u_char *)ns_rr_rdata(query_parse_rr));
             if (ns_name_uncompress(ns_msg_base(query_parse_msg), ns_msg_end(query_parse_msg), (u_char  *)ns_rr_rdata(query_parse_rr)+2, MX_SERVER[x].MX, sizeof(MX_SERVER[x].MX)) < 0)
 	      {
                 warnx("We can't uncompress the DNS headers!");
                 return 1;
 	      }
             printf("%s - %i\n", MX_SERVER[x].MX, MX_SERVER[x].priority);
             MX_NUM++;
 	  }
     }
     return 0;
 }