Annoying DNS Issues in Mozilla

I've finally figured out why visits to some Web sites take forever. I've maintained for years that "if something works, but takes a long time, blame DNS." Sure enough, a combination of Mozilla's behavior and uncooperative DNS servers are conspiring against Web users.

Here's how Mozilla resolves a host name when the remote DNS server cooperates. First Mozilla causes a DNS query for an AAAA record. This is an IPv6 record. The name server (here a forwarding name server) replies that it doesn't know an AAAA record for xlonhcld.xlontech.net. Mozilla promptly asks for the A record, which is returned in the last packet. So far so good. 

18:24:04.604363 192.168.2.5.49203 > 172.27.20.1.53:  56494+ AAAA? xlonhcld.xlontech.net. (39)

18:24:04.611197 172.27.20.1.53 > 192.168.2.5.49203:  56494 0/1/0 (104)

18:24:04.611474 192.168.2.5.49204 > 172.27.20.1.53:  56495+ A? xlonhcld.xlontech.net. (39)

18:24:04.619886 172.27.20.1.53 > 192.168.2.5.49204:  56495 18/7/0 A[|domain]

This is what happens with sites that don't answer AAAA queries properly. Mozilla asks for the AAAA record four times 

18:20:11.292864 192.168.2.5.49188 > 172.27.20.1.53:  4329+ AAAA? dclkcorp.rpts.net. (35)

18:20:16.304730 192.168.2.5.49189 > 172.27.20.1.53:  4329+ AAAA? dclkcorp.rpts.net. (35)

18:20:26.319837 192.168.2.5.49190 > 172.27.20.1.53:  4329+ AAAA? dclkcorp.rpts.net. (35)

18:20:46.334627 192.168.2.5.49191 > 172.27.20.1.53:  4329+ AAAA? dclkcorp.rpts.net. (35)

After 75 seconds it gives up and asks for the A record. The name server promptly responds and the page loads. 

18:21:26.345147 192.168.2.5.49192 > 172.27.20.1.53:  4330+ A? dclkcorp.rpts.net. (35)

18:21:26.378344 172.27.20.1.53 > 192.168.2.5.49192:  4330 2/6/6[|domain]

Here are the replies for the AAAA record requests. I haven't figured out the purpose of the ICMP port unreachable messages. 

18:21:59.533098 172.27.20.1.53 > 192.168.2.5.49190:  4329 ServFail 0/0/0 (35)

18:21:59.533170 192.168.2.5 > 172.27.20.1: icmp: 192.168.2.5 udp port 49190 unreachable

18:22:00.197621 172.27.20.1.53 > 192.168.2.5.49189:  4329 ServFail 0/0/0 (35)

18:22:00.197688 192.168.2.5 > 172.27.20.1: icmp: 192.168.2.5 udp port 49189 unreachable

18:22:11.193955 172.27.20.1.53 > 192.168.2.5.49188:  4329 ServFail 0/0/0 (35)

18:22:11.194029 192.168.2.5 > 172.27.20.1: icmp: 192.168.2.5 udp port 49188 unreachable

18:22:44.194926 172.27.20.1.53 > 192.168.2.5.49191:  4329 ServFail 0/0/0 (35)

18:22:44.195000 192.168.2.5 > 172.27.20.1: icmp: 192.168.2.5 udp port 49191 unreachable

The Mozilla team has had this bug report open for a long time. Unfortunately, the remote name servers are to blame, as shown by this Internet draft. The issue was also discussed on freebsd-current.

For sites that don't answer AAA requests properly, like doubleclick, entries like these in /etc/hosts might work: 

::0                     ad.doubleclick.com

::0                     ad.doubleclick.net

::1 is also an option, which is localhost.

The Mozilla bug report offered this code to check IPv6 resolutions using gethostbyname2: 

#include <stdio.h>

#include <stdlib.h>

#include <sys/socket.h>

#include <netdb.h>

#include <arpa/inet.h>

#include <errno.h>

#include <string.h>



int main(int argc, char *argv[])

{

    struct hostent *hent;

    char addrstr[64];

    int i;



    if (argc != 2) {

        fprintf(stderr, "Usage: %s \n", argv[0]);

        exit(1);

    }



    hent = gethostbyname2(argv[1], AF_INET6);

    if (hent == NULL) {

        fprintf(stderr, "gethostbyname2 failed: %d\n", h_errno);

        exit(1);

    }

    printf("h_name = %s\n", hent->h_name);

    if (hent->h_aliases) {

        for (i = 0; hent->h_aliases[i]; i++) {

            printf("h_aliases[%d] = %s\n", i, hent->h_aliases[i]);

        }

    }

    if (hent->h_addrtype == AF_INET) {

        printf("h_addrtype = AF_INET\n");

    } else if (hent->h_addrtype == AF_INET6) {

        printf("h_addrtype = AF_INET6\n");

    } else {

        printf("h_addrtype = %d\n", hent->h_addrtype);

    }

    printf("h_length = %d\n", hent->h_length);

    if (hent->h_addr_list) {

        for (i = 0; hent->h_addr_list[i]; i++) {

            printf("h_addr_list[%d] = %s\n", i,

                inet_ntop(hent->h_addrtype, hent->h_addr_list[i],

                addrstr, sizeof(addrstr)));

        }

    }

    return 0;

}

It compiles fine on FreeBSD 5.2 REL and shows how different sites respond. Here's a site that responds with an IPv6 address: 

orr# ./gethost2 www.netbsd.org

h_name = www.netbsd.org

h_addrtype = AF_INET6

h_length = 16

h_addr_list[0] = 2001:4f8:4:7:290:27ff:feab:19a7

Here's a site with no IPv6 address: 

orr# ./gethost2 www.bejtlich.net

gethostbyname2 failed: 4

Here's what happens when I query for a site with an entry in /etc/hosts: 

orr# ./gethost2 ad.doubleclick.net

h_name = ad.doubleclick.net

h_addrtype = AF_INET6

h_length = 16

h_addr_list[0] = ::

This is how a site with a broken respond behaves. The program times out after waiting for a minute and 15 seconds. 

orr# time ./gethost2 www.apcc.com

gethostbyname2 failed: 2

0.000u 0.004s 1:15.04 0.0%      0+0k 0+0io 0pf+0w

Watching the Mozilla bug report, a fix appears to be in the works.

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