[Bug network/14308] getaddrinfo DNS referral response returns host not found when A and AAAA questions are sent and one response is a referral

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https://sourceware.org/bugzilla/show_bug.cgi?id=14308

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=16b293a7a6f65d8ff348a603d19e8fd4372fa3a9

commit 16b293a7a6f65d8ff348a603d19e8fd4372fa3a9
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Wed Apr 30 11:48:43 2014 +0530

    Do not fail if one of the two responses to AF_UNSPEC fails (BZ #14308)

    [Fixes BZ #14308, #12994, #13651]

    AF_UNSPEC results in sending two queries in parallel, one for the A
    record and the other for the AAAA record.  If one of these is a
    referral, then the query fails, which is wrong.  It should return at
    least the one successful response.

    The fix has two parts.  The first part makes the referral fall back to
    the SERVFAIL path, which results in using the successful response.
    There is a bug in that path however, due to which the second part is
    necessary.  The bug here is that if the first response is a failure
    and the second succeeds, __libc_res_nsearch does not detect that and
    assumes a failure.  The case where the first response is a success and
    the second fails, works correctly.

    This condition is produced by buggy routers, so here's a crude
    interposable library that can simulate such a condition.  The library
    overrides the recvfrom syscall and modifies the header of the packet
    received to reproduce this scenario.  It has two key variables:
    mod_packet and first_error.

    The mod_packet variable when set to 0, results in odd packets being
    modified to be a referral.  When set to 1, even packets are modified
    to be a referral.

    The first_error causes the first response to be a failure so that a
    domain-appended search is performed to test the second part of the
    __libc_nsearch fix.

    The driver for this fix is a simple getaddrinfo program that does an
    AF_UNSPEC query.  I have omitted this since it should be easy to
    implement.

    I have tested this on x86_64.

    The interceptor library source:

    /* Override recvfrom and modify the header of the first DNS response to
make it
       a referral and reproduce bz #845218.  We have to resort to this ugly
hack
       because we cannot make bind return the buggy response of a referral for
the
       AAAA record and an authoritative response for the A record.  */
     #define _GNU_SOURCE
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>
     #include <stdio.h>
     #include <stdbool.h>
     #include <endian.h>
     #include <dlfcn.h>
     #include <stdlib.h>

    /* Lifted from resolv/arpa/nameser_compat.h.  */
    typedef struct {
        unsigned        id :16;         /*%< query identification number */
     #if BYTE_ORDER == BIG_ENDIAN
        /* fields in third byte */
        unsigned        qr: 1;          /*%< response flag */
        unsigned        opcode: 4;      /*%< purpose of message */
        unsigned        aa: 1;          /*%< authoritive answer */
        unsigned        tc: 1;          /*%< truncated message */
        unsigned        rd: 1;          /*%< recursion desired */
        /* fields
         * in
         * fourth
         * byte
         * */
        unsigned        ra: 1;          /*%< recursion available */
        unsigned        unused :1;      /*%< unused bits (MBZ as of 4.9.3a3) */
        unsigned        ad: 1;          /*%< authentic data from named */
        unsigned        cd: 1;          /*%< checking disabled by resolver */
        unsigned        rcode :4;       /*%< response code */
     #endif
     #if BYTE_ORDER == LITTLE_ENDIAN || BYTE_ORDER == PDP_ENDIAN
        /* fields
         * in
         * third
         * byte
         * */
        unsigned        rd :1;          /*%< recursion desired */
        unsigned        tc :1;          /*%< truncated message */
        unsigned        aa :1;          /*%< authoritive answer */
        unsigned        opcode :4;      /*%< purpose of message */
        unsigned        qr :1;          /*%< response flag */
        /* fields
         * in
         * fourth
         * byte
         * */
        unsigned        rcode :4;       /*%< response code */
        unsigned        cd: 1;          /*%< checking disabled by resolver */
        unsigned        ad: 1;          /*%< authentic data from named */
        unsigned        unused :1;      /*%< unused bits (MBZ as of 4.9.3a3) */
        unsigned        ra :1;          /*%< recursion available */
     #endif
        /* remaining
         * bytes
         * */
        unsigned        qdcount :16;    /*%< number of question entries */
        unsigned        ancount :16;    /*%< number of answer entries */
        unsigned        nscount :16;    /*%< number of authority entries */
        unsigned        arcount :16;    /*%< number of resource entries */
    } HEADER;

    static int done = 0;

    /* Packets to modify.  0 for the odd packets and 1 for even packets.  */
    static const int mod_packet = 0;

    /* Set to true if the first request should result in an error, resulting in
a
       search query.  */
    static bool first_error = true;

    static ssize_t (*real_recvfrom) (int sockfd, void *buf, size_t len, int
flags,
                  struct sockaddr *src_addr, socklen_t *addrlen);

    void
    __attribute__ ((constructor))
    init (void)
    {
      real_recvfrom = dlsym (RTLD_NEXT, "recvfrom");

      if (real_recvfrom == NULL)
        {
          printf ("Failed to get reference to recvfrom: %s\n", dlerror ());
          printf ("Cannot simulate test\n");
          abort ();
        }
    }

    /* Modify the second packet that we receive to set the header in a manner
as to
       reproduce BZ #845218.  */
    static void
    mod_buf (HEADER *h, int port)
    {
      if (done % 2 == mod_packet || (first_error && done == 1))
        {
          printf ("(Modifying header)");

          if (first_error && done == 1)
        h->rcode = 3;
          else
        h->rcode = 0;    /* NOERROR == 0.  */
          h->ancount = 0;
          h->aa = 0;
          h->ra = 0;
          h->arcount = 0;
        }
      done++;
    }

    ssize_t
    recvfrom (int sockfd, void *buf, size_t len, int flags,
          struct sockaddr *src_addr, socklen_t *addrlen)
    {
      ssize_t ret = real_recvfrom (sockfd, buf, len, flags, src_addr, addrlen);
      int port = htons (((struct sockaddr_in *) src_addr)->sin_port);
      struct in_addr addr = ((struct sockaddr_in *) src_addr)->sin_addr;
      const char *host = inet_ntoa (addr);
      printf ("\n*** From %s:%d: ", host, port);

      mod_buf (buf, port);

      printf ("returned %zd\n", ret);
      return ret;
    }

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog          |   11 +++++++++++
 NEWS               |   14 +++++++-------
 resolv/res_query.c |    7 +++++--
 resolv/res_send.c  |    2 +-
 4 files changed, 24 insertions(+), 10 deletions(-)

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