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

  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;
}

	[BZ #14308]
	* resolv/res_query.c (__libc_res_nsearch): Return if at least
	one response is valid.
	* resolv/res_send.c (send_dg): Check for validity of other
	response if the current response is a referral.

---
 resolv/res_query.c | 7 +++++--
 resolv/res_send.c  | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

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

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