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Setting nameservers after res_init fails doing res_query

Posted on 2016-09-01
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Last Modified: 2016-09-07
I have an Apache module which needs to find the "real" IP address of its host.  This became an issue after my own server went behind a firewall; the previous use of gethostbyname then returned 192.168.0.x rather than the "real" address.

I found some code that looked likely and wrote a short test program, below.

Unfortunately the results indicate that it is using the default nameservers from resolv.conf, not the nameservers loaded into _res.nsaddr_list after the call to res_init.  This was called out as an issue in the original code, but the original solution (a "dummy" call to res_query) does not work on my system.

Can someone suggest where the problem lies?


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <resolv.h>
#include <netdb.h>

#define N 4096

/* --------------------------------- */
/* CODE BEGINS                       */
/* --------------------------------- */

int main (int argc, char *argv[]) {
    u_char nsbuf[N];
    char dispbuf[N];
    ns_msg msg;
    ns_rr rr;
    int i, l;

    if (argc < 2) {
        printf ("Usage: %s <domain>\n", argv[0]);
        exit (1);
    }

/* Initialize to use the Google nameservers */
     res_init();
     _res.nscount = 2;
     _res.nsaddr_list[0].sin_family = AF_INET;
     _res.nsaddr_list[0].sin_addr.s_addr = inet_addr ("8.8.8.8");
     _res.nsaddr_list[0].sin_port = htons(53);
     _res.nsaddr_list[1].sin_family = AF_INET;
     _res.nsaddr_list[1].sin_addr.s_addr = inet_addr ("8.8.4.4");
     _res.nsaddr_list[1].sin_port = htons(53);

    printf("Domain : %s\n", argv[1]);

 /* Print the A record return or the error code */
    printf("A records : \n");
    l = res_query(argv[1], ns_c_any, ns_t_a, nsbuf, sizeof(nsbuf));
    if (l < 0) perror(argv[1]);
    ns_initparse(nsbuf, l, &msg);
    l = ns_msg_count(msg, ns_s_an);
    for (i = 0; i < l; i++)
    {
      ns_parserr(&msg, ns_s_an, i, &rr);
      ns_sprintrr(&msg, &rr, NULL, NULL, dispbuf, sizeof(dispbuf));
      printf("\t%s \n", dispbuf);
    }


    // NS RECORD
    printf("NS records : \n");
    l = res_query(argv[1], ns_c_any, ns_t_ns, nsbuf, sizeof(nsbuf));
    if (l < 0)
    {
      perror(argv[1]);
    }
    ns_initparse(nsbuf, l, &msg);
    l = ns_msg_count(msg, ns_s_an);
    for (i = 0; i < l; i++)
    {
      ns_parserr(&msg, ns_s_an, 0, &rr);
      ns_sprintrr(&msg, &rr, NULL, NULL, dispbuf, sizeof(dispbuf));
      printf("\t%s \n", dispbuf);
    }
    // ---------
    return 0;
}

Open in new window


sysmgr@www:~$ ./a.out www.*******.com
Domain : www.*******.com
A records :
        www.*******.com.           6H IN A         192.168.0.209
NS records :
        www.*******.com.           6H IN NS        localhost.

Open in new window

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Question by:Dr. Klahn
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2 Comments
 
LVL 33

Accepted Solution

by:
Dr. Klahn earned 0 total points
ID: 41788573
Update:  There is no solution to this problem other than correction of the Debian resolver library.  The bug will therefore also be present in (at least) Ubuntu.

The bug was introduced 10 years ago into debian and was first reported in 2015.

Given that the bug is 10 years old, it took ten years to notice it, the original report is now a year old and the bug priority has been reduced to "normal", it seems unlikely that there will be a solution in the near future.
0
 
LVL 33

Author Closing Comment

by:Dr. Klahn
ID: 41788918
Manual close.  No solution found.
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