xref: /bionic/libc/dns/resolv/res_cache.c

/*
 * Copyright (C) 2008 The Android Open Source Project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include "resolv_cache.h"

#include <resolv.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include "pthread.h"

#include <errno.h>
#include <arpa/nameser.h>
#include <sys/system_properties.h>
#include <net/if.h>
#include <netdb.h>
#include <linux/if.h>

#include <arpa/inet.h>
#include "resolv_private.h"
#include "resolv_netid.h"
#include "res_private.h"

#include "private/libc_logging.h"

/* This code implements a small and *simple* DNS resolver cache.
 *
 * It is only used to cache DNS answers for a time defined by the smallest TTL
 * among the answer records in order to reduce DNS traffic. It is not supposed
 * to be a full DNS cache, since we plan to implement that in the future in a
 * dedicated process running on the system.
 *
 * Note that its design is kept simple very intentionally, i.e.:
 *
 *  - it takes raw DNS query packet data as input, and returns raw DNS
 *    answer packet data as output
 *
 *    (this means that two similar queries that encode the DNS name
 *     differently will be treated distinctly).
 *
 *    the smallest TTL value among the answer records are used as the time
 *    to keep an answer in the cache.
 *
 *    this is bad, but we absolutely want to avoid parsing the answer packets
 *    (and should be solved by the later full DNS cache process).
 *
 *  - the implementation is just a (query-data) => (answer-data) hash table
 *    with a trivial least-recently-used expiration policy.
 *
 * Doing this keeps the code simple and avoids to deal with a lot of things
 * that a full DNS cache is expected to do.
 *
 * The API is also very simple:
 *
 *   - the client calls _resolv_cache_get() to obtain a handle to the cache.
 *     this will initialize the cache on first usage. the result can be NULL
 *     if the cache is disabled.
 *
 *   - the client calls _resolv_cache_lookup() before performing a query
 *
 *     if the function returns RESOLV_CACHE_FOUND, a copy of the answer data
 *     has been copied into the client-provided answer buffer.
 *
 *     if the function returns RESOLV_CACHE_NOTFOUND, the client should perform
 *     a request normally, *then* call _resolv_cache_add() to add the received
 *     answer to the cache.
 *
 *     if the function returns RESOLV_CACHE_UNSUPPORTED, the client should
 *     perform a request normally, and *not* call _resolv_cache_add()
 *
 *     note that RESOLV_CACHE_UNSUPPORTED is also returned if the answer buffer
 *     is too short to accomodate the cached result.
 */

/* the name of an environment variable that will be checked the first time
 * this code is called if its value is "0", then the resolver cache is
 * disabled.
 */
#define  CONFIG_ENV  "BIONIC_DNSCACHE"

/* default number of entries kept in the cache. This value has been
 * determined by browsing through various sites and counting the number
 * of corresponding requests. Keep in mind that our framework is currently
 * performing two requests per name lookup (one for IPv4, the other for IPv6)
 *
 *    www.google.com      4
 *    www.ysearch.com     6
 *    www.amazon.com      8
 *    www.nytimes.com     22
 *    www.espn.com        28
 *    www.msn.com         28
 *    www.lemonde.fr      35
 *
 * (determined in 2009-2-17 from Paris, France, results may vary depending
 *  on location)
 *
 * most high-level websites use lots of media/ad servers with different names
 * but these are generally reused when browsing through the site.
 *
 * As such, a value of 64 should be relatively comfortable at the moment.
 *
 * ******************************************
 * * NOTE - this has changed.
 * * 1) we've added IPv6 support so each dns query results in 2 responses
 * * 2) we've made this a system-wide cache, so the cost is less (it's not
 * *    duplicated in each process) and the need is greater (more processes
 * *    making different requests).
 * * Upping by 2x for IPv6
 * * Upping by another 5x for the centralized nature
 * *****************************************
 */
#define  CONFIG_MAX_ENTRIES    64 * 2 * 5
/* name of the system property that can be used to set the cache size */

/****************************************************************************/
/****************************************************************************/
/*****                                                                  *****/
/*****                                                                  *****/
/*****                                                                  *****/
/****************************************************************************/
/****************************************************************************/

/* set to 1 to debug cache operations */
#define  DEBUG       0

/* set to 1 to debug query data */
#define  DEBUG_DATA  0

#if DEBUG
#define __DEBUG__
#else
#define __DEBUG__ __attribute__((unused))
#endif

#undef XLOG

#define XLOG(...) ({ \
    if (DEBUG) { \
        __libc_format_log(ANDROID_LOG_DEBUG,"libc",__VA_ARGS__); \
    } else { \
        ((void)0); \
    } \
})

/** BOUNDED BUFFER FORMATTING
 **/

/* technical note:
 *
 *   the following debugging routines are used to append data to a bounded
 *   buffer they take two parameters that are:
 *
 *   - p : a pointer to the current cursor position in the buffer
 *         this value is initially set to the buffer's address.
 *
 *   - end : the address of the buffer's limit, i.e. of the first byte
 *           after the buffer. this address should never be touched.
 *
 *           IMPORTANT: it is assumed that end > buffer_address, i.e.
 *                      that the buffer is at least one byte.
 *
 *   the _bprint_() functions return the new value of 'p' after the data
 *   has been appended, and also ensure the following:
 *
 *   - the returned value will never be strictly greater than 'end'
 *
 *   - a return value equal to 'end' means that truncation occured
 *     (in which case, end[-1] will be set to 0)
 *
 *   - after returning from a _bprint_() function, the content of the buffer
 *     is always 0-terminated, even in the event of truncation.
 *
 *  these conventions allow you to call _bprint_ functions multiple times and
 *  only check for truncation at the end of the sequence, as in:
 *
 *     char  buff[1000], *p = buff, *end = p + sizeof(buff);
 *
 *     p = _bprint_c(p, end, '"');
 *     p = _bprint_s(p, end, my_string);
 *     p = _bprint_c(p, end, '"');
 *
 *     if (p >= end) {
 *        // buffer was too small
 *     }
 *
 *     printf( "%s", buff );
 */

/* add a char to a bounded buffer */
char*
_bprint_c( char*  p, char*  end, int  c )
{
    if (p < end) {
        if (p+1 == end)
            *p++ = 0;
        else {
            *p++ = (char) c;
            *p   = 0;
        }
    }
    return p;
}

/* add a sequence of bytes to a bounded buffer */
char*
_bprint_b( char*  p, char*  end, const char*  buf, int  len )
{
    int  avail = end - p;

    if (avail <= 0 || len <= 0)
        return p;

    if (avail > len)
        avail = len;

    memcpy( p, buf, avail );
    p += avail;

    if (p < end)
        p[0] = 0;
    else
        end[-1] = 0;

    return p;
}

/* add a string to a bounded buffer */
char*
_bprint_s( char*  p, char*  end, const char*  str )
{
    return _bprint_b(p, end, str, strlen(str));
}

/* add a formatted string to a bounded buffer */
char* _bprint( char*  p, char*  end, const char*  format, ... ) __DEBUG__;
char* _bprint( char*  p, char*  end, const char*  format, ... )
{
    int      avail, n;
    va_list  args;

    avail = end - p;

    if (avail <= 0)
        return p;

    va_start(args, format);
    n = vsnprintf( p, avail, format, args);
    va_end(args);

    /* certain C libraries return -1 in case of truncation */
    if (n < 0 || n > avail)
        n = avail;

    p += n;
    /* certain C libraries do not zero-terminate in case of truncation */
    if (p == end)
        p[-1] = 0;

    return p;
}

/* add a hex value to a bounded buffer, up to 8 digits */
char*
_bprint_hex( char*  p, char*  end, unsigned  value, int  numDigits )
{
    char   text[sizeof(unsigned)*2];
    int    nn = 0;

    while (numDigits-- > 0) {
        text[nn++] = "0123456789abcdef"[(value >> (numDigits*4)) & 15];
    }
    return _bprint_b(p, end, text, nn);
}

/* add the hexadecimal dump of some memory area to a bounded buffer */
char*
_bprint_hexdump( char*  p, char*  end, const uint8_t*  data, int  datalen )
{
    int   lineSize = 16;

    while (datalen > 0) {
        int  avail = datalen;
        int  nn;

        if (avail > lineSize)
            avail = lineSize;

        for (nn = 0; nn < avail; nn++) {
            if (nn > 0)
                p = _bprint_c(p, end, ' ');
            p = _bprint_hex(p, end, data[nn], 2);
        }
        for ( ; nn < lineSize; nn++ ) {
            p = _bprint_s(p, end, "   ");
        }
        p = _bprint_s(p, end, "  ");

        for (nn = 0; nn < avail; nn++) {
            int  c = data[nn];

            if (c < 32 || c > 127)
                c = '.';

            p = _bprint_c(p, end, c);
        }
        p = _bprint_c(p, end, '\n');

        data    += avail;
        datalen -= avail;
    }
    return p;
}

/* dump the content of a query of packet to the log */
void XLOG_BYTES( const void*  base, int  len ) __DEBUG__;
void XLOG_BYTES( const void*  base, int  len )
{
    if (DEBUG_DATA) {
        char  buff[1024];
        char*  p = buff, *end = p + sizeof(buff);

        p = _bprint_hexdump(p, end, base, len);
        XLOG("%s",buff);
    }
} __DEBUG__

static time_t
_time_now( void )
{
    struct timeval  tv;

    gettimeofday( &tv, NULL );
    return tv.tv_sec;
}

/* reminder: the general format of a DNS packet is the following:
 *
 *    HEADER  (12 bytes)
 *    QUESTION  (variable)
 *    ANSWER (variable)
 *    AUTHORITY (variable)
 *    ADDITIONNAL (variable)
 *
 * the HEADER is made of:
 *
 *   ID     : 16 : 16-bit unique query identification field
 *
 *   QR     :  1 : set to 0 for queries, and 1 for responses
 *   Opcode :  4 : set to 0 for queries
 *   AA     :  1 : set to 0 for queries
 *   TC     :  1 : truncation flag, will be set to 0 in queries
 *   RD     :  1 : recursion desired
 *
 *   RA     :  1 : recursion available (0 in queries)
 *   Z      :  3 : three reserved zero bits
 *   RCODE  :  4 : response code (always 0=NOERROR in queries)
 *
 *   QDCount: 16 : question count
 *   ANCount: 16 : Answer count (0 in queries)
 *   NSCount: 16: Authority Record count (0 in queries)
 *   ARCount: 16: Additionnal Record count (0 in queries)
 *
 * the QUESTION is made of QDCount Question Record (QRs)
 * the ANSWER is made of ANCount RRs
 * the AUTHORITY is made of NSCount RRs
 * the ADDITIONNAL is made of ARCount RRs
 *
 * Each Question Record (QR) is made of:
 *
 *   QNAME   : variable : Query DNS NAME
 *   TYPE    : 16       : type of query (A=1, PTR=12, MX=15, AAAA=28, ALL=255)
 *   CLASS   : 16       : class of query (IN=1)
 *
 * Each Resource Record (RR) is made of:
 *
 *   NAME    : variable : DNS NAME
 *   TYPE    : 16       : type of query (A=1, PTR=12, MX=15, AAAA=28, ALL=255)
 *   CLASS   : 16       : class of query (IN=1)
 *   TTL     : 32       : seconds to cache this RR (0=none)
 *   RDLENGTH: 16       : size of RDDATA in bytes
 *   RDDATA  : variable : RR data (depends on TYPE)
 *
 * Each QNAME contains a domain name encoded as a sequence of 'labels'
 * terminated by a zero. Each label has the following format:
 *
 *    LEN  : 8     : lenght of label (MUST be < 64)
 *    NAME : 8*LEN : label length (must exclude dots)
 *
 * A value of 0 in the encoding is interpreted as the 'root' domain and
 * terminates the encoding. So 'www.android.com' will be encoded as:
 *
 *   <3>www<7>android<3>com<0>
 *
 * Where <n> represents the byte with value 'n'
 *
 * Each NAME reflects the QNAME of the question, but has a slightly more
 * complex encoding in order to provide message compression. This is achieved
 * by using a 2-byte pointer, with format:
 *
 *    TYPE   : 2  : 0b11 to indicate a pointer, 0b01 and 0b10 are reserved
 *    OFFSET : 14 : offset to another part of the DNS packet
 *
 * The offset is relative to the start of the DNS packet and must point
 * A pointer terminates the encoding.
 *
 * The NAME can be encoded in one of the following formats:
 *
 *   - a sequence of simple labels terminated by 0 (like QNAMEs)
 *   - a single pointer
 *   - a sequence of simple labels terminated by a pointer
 *
 * A pointer shall always point to either a pointer of a sequence of
 * labels (which can themselves be terminated by either a 0 or a pointer)
 *
 * The expanded length of a given domain name should not exceed 255 bytes.
 *
 * NOTE: we don't parse the answer packets, so don't need to deal with NAME
 *       records, only QNAMEs.
 */

#define  DNS_HEADER_SIZE  12

#define  DNS_TYPE_A   "\00\01"   /* big-endian decimal 1 */
#define  DNS_TYPE_PTR "\00\014"  /* big-endian decimal 12 */
#define  DNS_TYPE_MX  "\00\017"  /* big-endian decimal 15 */
#define  DNS_TYPE_AAAA "\00\034" /* big-endian decimal 28 */
#define  DNS_TYPE_ALL "\00\0377" /* big-endian decimal 255 */

#define  DNS_CLASS_IN "\00\01"   /* big-endian decimal 1 */

typedef struct {
    const uint8_t*  base;
    const uint8_t*  end;
    const uint8_t*  cursor;
} DnsPacket;

static void
_dnsPacket_init( DnsPacket*  packet, const uint8_t*  buff, int  bufflen )
{
    packet->base   = buff;
    packet->end    = buff + bufflen;
    packet->cursor = buff;
}

static void
_dnsPacket_rewind( DnsPacket*  packet )
{
    packet->cursor = packet->base;
}

static void
_dnsPacket_skip( DnsPacket*  packet, int  count )
{
    const uint8_t*  p = packet->cursor + count;

    if (p > packet->end)
        p = packet->end;

    packet->cursor = p;
}

static int
_dnsPacket_readInt16( DnsPacket*  packet )
{
    const uint8_t*  p = packet->cursor;

    if (p+2 > packet->end)
        return -1;

    packet->cursor = p+2;
    return (p[0]<< 8) | p[1];
}

/** QUERY CHECKING
 **/

/* check bytes in a dns packet. returns 1 on success, 0 on failure.
 * the cursor is only advanced in the case of success
 */
static int
_dnsPacket_checkBytes( DnsPacket*  packet, int  numBytes, const void*  bytes )
{
    const uint8_t*  p = packet->cursor;

    if (p + numBytes > packet->end)
        return 0;

    if (memcmp(p, bytes, numBytes) != 0)
        return 0;

    packet->cursor = p + numBytes;
    return 1;
}

/* parse and skip a given QNAME stored in a query packet,
 * from the current cursor position. returns 1 on success,
 * or 0 for malformed data.
 */
static int
_dnsPacket_checkQName( DnsPacket*  packet )
{
    const uint8_t*  p   = packet->cursor;
    const uint8_t*  end = packet->end;

    for (;;) {
        int  c;

        if (p >= end)
            break;

        c = *p++;

        if (c == 0) {
            packet->cursor = p;
            return 1;
        }

        /* we don't expect label compression in QNAMEs */
        if (c >= 64)
            break;

        p += c;
        /* we rely on the bound check at the start
         * of the loop here */
    }
    /* malformed data */
    XLOG("malformed QNAME");
    return 0;
}

/* parse and skip a given QR stored in a packet.
 * returns 1 on success, and 0 on failure
 */
static int
_dnsPacket_checkQR( DnsPacket*  packet )
{
    if (!_dnsPacket_checkQName(packet))
        return 0;

    /* TYPE must be one of the things we support */
    if (!_dnsPacket_checkBytes(packet, 2, DNS_TYPE_A) &&
        !_dnsPacket_checkBytes(packet, 2, DNS_TYPE_PTR) &&
        !_dnsPacket_checkBytes(packet, 2, DNS_TYPE_MX) &&
        !_dnsPacket_checkBytes(packet, 2, DNS_TYPE_AAAA) &&
        !_dnsPacket_checkBytes(packet, 2, DNS_TYPE_ALL))
    {
        XLOG("unsupported TYPE");
        return 0;
    }
    /* CLASS must be IN */
    if (!_dnsPacket_checkBytes(packet, 2, DNS_CLASS_IN)) {
        XLOG("unsupported CLASS");
        return 0;
    }

    return 1;
}

/* check the header of a DNS Query packet, return 1 if it is one
 * type of query we can cache, or 0 otherwise
 */
static int
_dnsPacket_checkQuery( DnsPacket*  packet )
{
    const uint8_t*  p = packet->base;
    int             qdCount, anCount, dnCount, arCount;

    if (p + DNS_HEADER_SIZE > packet->end) {
        XLOG("query packet too small");
        return 0;
    }

    /* QR must be set to 0, opcode must be 0 and AA must be 0 */
    /* RA, Z, and RCODE must be 0 */
    if ((p[2] & 0xFC) != 0 || p[3] != 0) {
        XLOG("query packet flags unsupported");
        return 0;
    }

    /* Note that we ignore the TC and RD bits here for the
     * following reasons:
     *
     * - there is no point for a query packet sent to a server
     *   to have the TC bit set, but the implementation might
     *   set the bit in the query buffer for its own needs
     *   between a _resolv_cache_lookup and a
     *   _resolv_cache_add. We should not freak out if this
     *   is the case.
     *
     * - we consider that the result from a RD=0 or a RD=1
     *   query might be different, hence that the RD bit
     *   should be used to differentiate cached result.
     *
     *   this implies that RD is checked when hashing or
     *   comparing query packets, but not TC
     */

    /* ANCOUNT, DNCOUNT and ARCOUNT must be 0 */
    qdCount = (p[4] << 8) | p[5];
    anCount = (p[6] << 8) | p[7];
    dnCount = (p[8] << 8) | p[9];
    arCount = (p[10]<< 8) | p[11];

    if (anCount != 0 || dnCount != 0 || arCount != 0) {
        XLOG("query packet contains non-query records");
        return 0;
    }

    if (qdCount == 0) {
        XLOG("query packet doesn't contain query record");
        return 0;
    }

    /* Check QDCOUNT QRs */
    packet->cursor = p + DNS_HEADER_SIZE;

    for (;qdCount > 0; qdCount--)
        if (!_dnsPacket_checkQR(packet))
            return 0;

    return 1;
}

/** QUERY DEBUGGING
 **/
#if DEBUG
static char*
_dnsPacket_bprintQName(DnsPacket*  packet, char*  bp, char*  bend)
{
    const uint8_t*  p   = packet->cursor;
    const uint8_t*  end = packet->end;
    int             first = 1;

    for (;;) {
        int  c;

        if (p >= end)
            break;

        c = *p++;

        if (c == 0) {
            packet->cursor = p;
            return bp;
        }

        /* we don't expect label compression in QNAMEs */
        if (c >= 64)
            break;

        if (first)
            first = 0;
        else
            bp = _bprint_c(bp, bend, '.');

        bp = _bprint_b(bp, bend, (const char*)p, c);

        p += c;
        /* we rely on the bound check at the start
         * of the loop here */
    }
    /* malformed data */
    bp = _bprint_s(bp, bend, "<MALFORMED>");
    return bp;
}

static char*
_dnsPacket_bprintQR(DnsPacket*  packet, char*  p, char*  end)
{
#define  QQ(x)   { DNS_TYPE_##x, #x }
    static const struct {
        const char*  typeBytes;
        const char*  typeString;
    } qTypes[] =
    {
        QQ(A), QQ(PTR), QQ(MX), QQ(AAAA), QQ(ALL),
        { NULL, NULL }
    };
    int          nn;
    const char*  typeString = NULL;

    /* dump QNAME */
    p = _dnsPacket_bprintQName(packet, p, end);

    /* dump TYPE */
    p = _bprint_s(p, end, " (");

    for (nn = 0; qTypes[nn].typeBytes != NULL; nn++) {
        if (_dnsPacket_checkBytes(packet, 2, qTypes[nn].typeBytes)) {
            typeString = qTypes[nn].typeString;
            break;
        }
    }

    if (typeString != NULL)
        p = _bprint_s(p, end, typeString);
    else {
        int  typeCode = _dnsPacket_readInt16(packet);
        p = _bprint(p, end, "UNKNOWN-%d", typeCode);
    }

    p = _bprint_c(p, end, ')');

    /* skip CLASS */
    _dnsPacket_skip(packet, 2);
    return p;
}

/* this function assumes the packet has already been checked */
static char*
_dnsPacket_bprintQuery( DnsPacket*  packet, char*  p, char*  end )
{
    int   qdCount;

    if (packet->base[2] & 0x1) {
        p = _bprint_s(p, end, "RECURSIVE ");
    }

    _dnsPacket_skip(packet, 4);
    qdCount = _dnsPacket_readInt16(packet);
    _dnsPacket_skip(packet, 6);

    for ( ; qdCount > 0; qdCount-- ) {
        p = _dnsPacket_bprintQR(packet, p, end);
    }
    return p;
}
#endif


/** QUERY HASHING SUPPORT
 **
 ** THE FOLLOWING CODE ASSUMES THAT THE INPUT PACKET HAS ALREADY
 ** BEEN SUCCESFULLY CHECKED.
 **/

/* use 32-bit FNV hash function */
#define  FNV_MULT   16777619U
#define  FNV_BASIS  2166136261U

static unsigned
_dnsPacket_hashBytes( DnsPacket*  packet, int  numBytes, unsigned  hash )
{
    const uint8_t*  p   = packet->cursor;
    const uint8_t*  end = packet->end;

    while (numBytes > 0 && p < end) {
        hash = hash*FNV_MULT ^ *p++;
    }
    packet->cursor = p;
    return hash;
}


static unsigned
_dnsPacket_hashQName( DnsPacket*  packet, unsigned  hash )
{
    const uint8_t*  p   = packet->cursor;
    const uint8_t*  end = packet->end;

    for (;;) {
        int  c;

        if (p >= end) {  /* should not happen */
            XLOG("%s: INTERNAL_ERROR: read-overflow !!\n", __FUNCTION__);
            break;
        }

        c = *p++;

        if (c == 0)
            break;

        if (c >= 64) {
            XLOG("%s: INTERNAL_ERROR: malformed domain !!\n", __FUNCTION__);
            break;
        }
        if (p + c >= end) {
            XLOG("%s: INTERNAL_ERROR: simple label read-overflow !!\n",
                    __FUNCTION__);
            break;
        }
        while (c > 0) {
            hash = hash*FNV_MULT ^ *p++;
            c   -= 1;
        }
    }
    packet->cursor = p;
    return hash;
}

static unsigned
_dnsPacket_hashQR( DnsPacket*  packet, unsigned  hash )
{
    hash = _dnsPacket_hashQName(packet, hash);
    hash = _dnsPacket_hashBytes(packet, 4, hash); /* TYPE and CLASS */
    return hash;
}

static unsigned
_dnsPacket_hashQuery( DnsPacket*  packet )
{
    unsigned  hash = FNV_BASIS;
    int       count;
    _dnsPacket_rewind(packet);

    /* we ignore the TC bit for reasons explained in
     * _dnsPacket_checkQuery().
     *
     * however we hash the RD bit to differentiate
     * between answers for recursive and non-recursive
     * queries.
     */
    hash = hash*FNV_MULT ^ (packet->base[2] & 1);

    /* assume: other flags are 0 */
    _dnsPacket_skip(packet, 4);

    /* read QDCOUNT */
    count = _dnsPacket_readInt16(packet);

    /* assume: ANcount, NScount, ARcount are 0 */
    _dnsPacket_skip(packet, 6);

    /* hash QDCOUNT QRs */
    for ( ; count > 0; count-- )
        hash = _dnsPacket_hashQR(packet, hash);

    return hash;
}


/** QUERY COMPARISON
 **
 ** THE FOLLOWING CODE ASSUMES THAT THE INPUT PACKETS HAVE ALREADY
 ** BEEN SUCCESFULLY CHECKED.
 **/

static int
_dnsPacket_isEqualDomainName( DnsPacket*  pack1, DnsPacket*  pack2 )
{
    const uint8_t*  p1   = pack1->cursor;
    const uint8_t*  end1 = pack1->end;
    const uint8_t*  p2   = pack2->cursor;
    const uint8_t*  end2 = pack2->end;

    for (;;) {
        int  c1, c2;

        if (p1 >= end1 || p2 >= end2) {
            XLOG("%s: INTERNAL_ERROR: read-overflow !!\n", __FUNCTION__);
            break;
        }
        c1 = *p1++;
        c2 = *p2++;
        if (c1 != c2)
            break;

        if (c1 == 0) {
            pack1->cursor = p1;
            pack2->cursor = p2;
            return 1;
        }
        if (c1 >= 64) {
            XLOG("%s: INTERNAL_ERROR: malformed domain !!\n", __FUNCTION__);
            break;
        }
        if ((p1+c1 > end1) || (p2+c1 > end2)) {
            XLOG("%s: INTERNAL_ERROR: simple label read-overflow !!\n",
                    __FUNCTION__);
            break;
        }
        if (memcmp(p1, p2, c1) != 0)
            break;
        p1 += c1;
        p2 += c1;
        /* we rely on the bound checks at the start of the loop */
    }
    /* not the same, or one is malformed */
    XLOG("different DN");
    return 0;
}

static int
_dnsPacket_isEqualBytes( DnsPacket*  pack1, DnsPacket*  pack2, int  numBytes )
{
    const uint8_t*  p1 = pack1->cursor;
    const uint8_t*  p2 = pack2->cursor;

    if ( p1 + numBytes > pack1->end || p2 + numBytes > pack2->end )
        return 0;

    if ( memcmp(p1, p2, numBytes) != 0 )
        return 0;

    pack1->cursor += numBytes;
    pack2->cursor += numBytes;
    return 1;
}

static int
_dnsPacket_isEqualQR( DnsPacket*  pack1, DnsPacket*  pack2 )
{
    /* compare domain name encoding + TYPE + CLASS */
    if ( !_dnsPacket_isEqualDomainName(pack1, pack2) ||
         !_dnsPacket_isEqualBytes(pack1, pack2, 2+2) )
        return 0;

    return 1;
}

static int
_dnsPacket_isEqualQuery( DnsPacket*  pack1, DnsPacket*  pack2 )
{
    int  count1, count2;

    /* compare the headers, ignore most fields */
    _dnsPacket_rewind(pack1);
    _dnsPacket_rewind(pack2);

    /* compare RD, ignore TC, see comment in _dnsPacket_checkQuery */
    if ((pack1->base[2] & 1) != (pack2->base[2] & 1)) {
        XLOG("different RD");
        return 0;
    }

    /* assume: other flags are all 0 */
    _dnsPacket_skip(pack1, 4);
    _dnsPacket_skip(pack2, 4);

    /* compare QDCOUNT */
    count1 = _dnsPacket_readInt16(pack1);
    count2 = _dnsPacket_readInt16(pack2);
    if (count1 != count2 || count1 < 0) {
        XLOG("different QDCOUNT");
        return 0;
    }

    /* assume: ANcount, NScount and ARcount are all 0 */
    _dnsPacket_skip(pack1, 6);
    _dnsPacket_skip(pack2, 6);

    /* compare the QDCOUNT QRs */
    for ( ; count1 > 0; count1-- ) {
        if (!_dnsPacket_isEqualQR(pack1, pack2)) {
            XLOG("different QR");
            return 0;
        }
    }
    return 1;
}

/****************************************************************************/
/****************************************************************************/
/*****                                                                  *****/
/*****                                                                  *****/
/*****                                                                  *****/
/****************************************************************************/
/****************************************************************************/

/* cache entry. for simplicity, 'hash' and 'hlink' are inlined in this
 * structure though they are conceptually part of the hash table.
 *
 * similarly, mru_next and mru_prev are part of the global MRU list
 */
typedef struct Entry {
    unsigned int     hash;   /* hash value */
    struct Entry*    hlink;  /* next in collision chain */
    struct Entry*    mru_prev;
    struct Entry*    mru_next;

    const uint8_t*   query;
    int              querylen;
    const uint8_t*   answer;
    int              answerlen;
    time_t           expires;   /* time_t when the entry isn't valid any more */
    int              id;        /* for debugging purpose */
} Entry;

/**
 * Find the TTL for a negative DNS result.  This is defined as the minimum
 * of the SOA records TTL and the MINIMUM-TTL field (RFC-2308).
 *
 * Return 0 if not found.
 */
static u_long
answer_getNegativeTTL(ns_msg handle) {
    int n, nscount;
    u_long result = 0;
    ns_rr rr;

    nscount = ns_msg_count(handle, ns_s_ns);
    for (n = 0; n < nscount; n++) {
        if ((ns_parserr(&handle, ns_s_ns, n, &rr) == 0) && (ns_rr_type(rr) == ns_t_soa)) {
            const u_char *rdata = ns_rr_rdata(rr); // find the data
            const u_char *edata = rdata + ns_rr_rdlen(rr); // add the len to find the end
            int len;
            u_long ttl, rec_result = ns_rr_ttl(rr);

            // find the MINIMUM-TTL field from the blob of binary data for this record
            // skip the server name
            len = dn_skipname(rdata, edata);
            if (len == -1) continue; // error skipping
            rdata += len;

            // skip the admin name
            len = dn_skipname(rdata, edata);
            if (len == -1) continue; // error skipping
            rdata += len;

            if (edata - rdata != 5*NS_INT32SZ) continue;
            // skip: serial number + refresh interval + retry interval + expiry
            rdata += NS_INT32SZ * 4;
            // finally read the MINIMUM TTL
            ttl = ns_get32(rdata);
            if (ttl < rec_result) {
                rec_result = ttl;
            }
            // Now that the record is read successfully, apply the new min TTL
            if (n == 0 || rec_result < result) {
                result = rec_result;
            }
        }
    }
    return result;
}

/**
 * Parse the answer records and find the appropriate
 * smallest TTL among the records.  This might be from
 * the answer records if found or from the SOA record
 * if it's a negative result.
 *
 * The returned TTL is the number of seconds to
 * keep the answer in the cache.
 *
 * In case of parse error zero (0) is returned which
 * indicates that the answer shall not be cached.
 */
static u_long
answer_getTTL(const void* answer, int answerlen)
{
    ns_msg handle;
    int ancount, n;
    u_long result, ttl;
    ns_rr rr;

    result = 0;
    if (ns_initparse(answer, answerlen, &handle) >= 0) {
        // get number of answer records
        ancount = ns_msg_count(handle, ns_s_an);

        if (ancount == 0) {
            // a response with no answers?  Cache this negative result.
            result = answer_getNegativeTTL(handle);
        } else {
            for (n = 0; n < ancount; n++) {
                if (ns_parserr(&handle, ns_s_an, n, &rr) == 0) {
                    ttl = ns_rr_ttl(rr);
                    if (n == 0 || ttl < result) {
                        result = ttl;
                    }
                } else {
                    XLOG("ns_parserr failed ancount no = %d. errno = %s\n", n, strerror(errno));
                }
            }
        }
    } else {
        XLOG("ns_parserr failed. %s\n", strerror(errno));
    }

    XLOG("TTL = %lu\n", result);

    return result;
}

static void
entry_free( Entry*  e )
{
    /* everything is allocated in a single memory block */
    if (e) {
        free(e);
    }
}

static __inline__ void
entry_mru_remove( Entry*  e )
{
    e->mru_prev->mru_next = e->mru_next;
    e->mru_next->mru_prev = e->mru_prev;
}

static __inline__ void
entry_mru_add( Entry*  e, Entry*  list )
{
    Entry*  first = list->mru_next;

    e->mru_next = first;
    e->mru_prev = list;

    list->mru_next  = e;
    first->mru_prev = e;
}

/* compute the hash of a given entry, this is a hash of most
 * data in the query (key) */
static unsigned
entry_hash( const Entry*  e )
{
    DnsPacket  pack[1];

    _dnsPacket_init(pack, e->query, e->querylen);
    return _dnsPacket_hashQuery(pack);
}

/* initialize an Entry as a search key, this also checks the input query packet
 * returns 1 on success, or 0 in case of unsupported/malformed data */
static int
entry_init_key( Entry*  e, const void*  query, int  querylen )
{
    DnsPacket  pack[1];

    memset(e, 0, sizeof(*e));

    e->query    = query;
    e->querylen = querylen;
    e->hash     = entry_hash(e);

    _dnsPacket_init(pack, query, querylen);

    return _dnsPacket_checkQuery(pack);
}

/* allocate a new entry as a cache node */
static Entry*
entry_alloc( const Entry*  init, const void*  answer, int  answerlen )
{
    Entry*  e;
    int     size;

    size = sizeof(*e) + init->querylen + answerlen;
    e    = calloc(size, 1);
    if (e == NULL)
        return e;

    e->hash     = init->hash;
    e->query    = (const uint8_t*)(e+1);
    e->querylen = init->querylen;

    memcpy( (char*)e->query, init->query, e->querylen );

    e->answer    = e->query + e->querylen;
    e->answerlen = answerlen;

    memcpy( (char*)e->answer, answer, e->answerlen );

    return e;
}

static int
entry_equals( const Entry*  e1, const Entry*  e2 )
{
    DnsPacket  pack1[1], pack2[1];

    if (e1->querylen != e2->querylen) {
        return 0;
    }
    _dnsPacket_init(pack1, e1->query, e1->querylen);
    _dnsPacket_init(pack2, e2->query, e2->querylen);

    return _dnsPacket_isEqualQuery(pack1, pack2);
}

/****************************************************************************/
/****************************************************************************/
/*****                                                                  *****/
/*****                                                                  *****/
/*****                                                                  *****/
/****************************************************************************/
/****************************************************************************/

/* We use a simple hash table with external collision lists
 * for simplicity, the hash-table fields 'hash' and 'hlink' are
 * inlined in the Entry structure.
 */

/* Maximum time for a thread to wait for an pending request */
#define PENDING_REQUEST_TIMEOUT 20;

typedef struct pending_req_info {
    unsigned int                hash;
    pthread_cond_t              cond;
    struct pending_req_info*    next;
} PendingReqInfo;

typedef struct resolv_cache {
    int              max_entries;
    int              num_entries;
    Entry            mru_list;
    int              last_id;
    Entry*           entries;
    PendingReqInfo   pending_requests;
} Cache;

struct resolv_cache_info {
    unsigned                    netid;
    Cache*                      cache;
    struct resolv_cache_info*   next;
    int                         nscount;
    char*                       nameservers[MAXNS];
    struct addrinfo*            nsaddrinfo[MAXNS];
    int                         revision_id; // # times the nameservers have been replaced
    struct __res_params         params;
    struct __res_stats          nsstats[MAXNS];
    char                        defdname[MAXDNSRCHPATH];
    int                         dnsrch_offset[MAXDNSRCH+1];  // offsets into defdname
};

#define  HTABLE_VALID(x)  ((x) != NULL && (x) != HTABLE_DELETED)

static pthread_once_t        _res_cache_once = PTHREAD_ONCE_INIT;
static void _res_cache_init(void);

// lock protecting everything in the _resolve_cache_info structs (next ptr, etc)
static pthread_mutex_t _res_cache_list_lock;

/* gets cache associated with a network, or NULL if none exists */
static struct resolv_cache* _find_named_cache_locked(unsigned netid);

static void
_cache_flush_pending_requests_locked( struct resolv_cache* cache )
{
    struct pending_req_info *ri, *tmp;
    if (cache) {
        ri = cache->pending_requests.next;

        while (ri) {
            tmp = ri;
            ri = ri->next;
            pthread_cond_broadcast(&tmp->cond);

            pthread_cond_destroy(&tmp->cond);
            free(tmp);
        }

        cache->pending_requests.next = NULL;
    }
}

/* Return 0 if no pending request is found matching the key.
 * If a matching request is found the calling thread will wait until
 * the matching request completes, then update *cache and return 1. */
static int
_cache_check_pending_request_locked( struct resolv_cache** cache, Entry* key, unsigned netid )
{
    struct pending_req_info *ri, *prev;
    int exist = 0;

    if (*cache && key) {
        ri = (*cache)->pending_requests.next;
        prev = &(*cache)->pending_requests;
        while (ri) {
            if (ri->hash == key->hash) {
                exist = 1;
                break;
            }
            prev = ri;
            ri = ri->next;
        }

        if (!exist) {
            ri = calloc(1, sizeof(struct pending_req_info));
            if (ri) {
                ri->hash = key->hash;
                pthread_cond_init(&ri->cond, NULL);
                prev->next = ri;
            }
        } else {
            struct timespec ts = {0,0};
            XLOG("Waiting for previous request");
            ts.tv_sec = _time_now() + PENDING_REQUEST_TIMEOUT;
            pthread_cond_timedwait(&ri->cond, &_res_cache_list_lock, &ts);
            /* Must update *cache as it could have been deleted. */
            *cache = _find_named_cache_locked(netid);
        }
    }

    return exist;
}

/* notify any waiting thread that waiting on a request
 * matching the key has been added to the cache */
static void
_cache_notify_waiting_tid_locked( struct resolv_cache* cache, Entry* key )
{
    struct pending_req_info *ri, *prev;

    if (cache && key) {
        ri = cache->pending_requests.next;
        prev = &cache->pending_requests;
        while (ri) {
            if (ri->hash == key->hash) {
                pthread_cond_broadcast(&ri->cond);
                break;
            }
            prev = ri;
            ri = ri->next;
        }

        // remove item from list and destroy
        if (ri) {
            prev->next = ri->next;
            pthread_cond_destroy(&ri->cond);
            free(ri);
        }
    }
}

/* notify the cache that the query failed */
void
_resolv_cache_query_failed( unsigned    netid,
                   const void* query,
                   int         querylen)
{
    Entry    key[1];
    Cache*   cache;

    if (!entry_init_key(key, query, querylen))
        return;

    pthread_mutex_lock(&_res_cache_list_lock);

    cache = _find_named_cache_locked(netid);

    if (cache) {
        _cache_notify_waiting_tid_locked(cache, key);
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
}

static struct resolv_cache_info* _find_cache_info_locked(unsigned netid);

static void
_cache_flush_locked( Cache*  cache )
{
    int     nn;

    for (nn = 0; nn < cache->max_entries; nn++)
    {
        Entry**  pnode = (Entry**) &cache->entries[nn];

        while (*pnode != NULL) {
            Entry*  node = *pnode;
            *pnode = node->hlink;
            entry_free(node);
        }
    }

    // flush pending request
    _cache_flush_pending_requests_locked(cache);

    cache->mru_list.mru_next = cache->mru_list.mru_prev = &cache->mru_list;
    cache->num_entries       = 0;
    cache->last_id           = 0;

    XLOG("*************************\n"
         "*** DNS CACHE FLUSHED ***\n"
         "*************************");
}

static int
_res_cache_get_max_entries( void )
{
    int cache_size = CONFIG_MAX_ENTRIES;

    const char* cache_mode = getenv("ANDROID_DNS_MODE");
    if (cache_mode == NULL || strcmp(cache_mode, "local") != 0) {
        // Don't use the cache in local mode. This is used by the proxy itself.
        cache_size = 0;
    }

    XLOG("cache size: %d", cache_size);
    return cache_size;
}

static struct resolv_cache*
_resolv_cache_create( void )
{
    struct resolv_cache*  cache;

    cache = calloc(sizeof(*cache), 1);
    if (cache) {
        cache->max_entries = _res_cache_get_max_entries();
        cache->entries = calloc(sizeof(*cache->entries), cache->max_entries);
        if (cache->entries) {
            cache->mru_list.mru_prev = cache->mru_list.mru_next = &cache->mru_list;
            XLOG("%s: cache created\n", __FUNCTION__);
        } else {
            free(cache);
            cache = NULL;
        }
    }
    return cache;
}


#if DEBUG
static void
_dump_query( const uint8_t*  query, int  querylen )
{
    char       temp[256], *p=temp, *end=p+sizeof(temp);
    DnsPacket  pack[1];

    _dnsPacket_init(pack, query, querylen);
    p = _dnsPacket_bprintQuery(pack, p, end);
    XLOG("QUERY: %s", temp);
}

static void
_cache_dump_mru( Cache*  cache )
{
    char    temp[512], *p=temp, *end=p+sizeof(temp);
    Entry*  e;

    p = _bprint(temp, end, "MRU LIST (%2d): ", cache->num_entries);
    for (e = cache->mru_list.mru_next; e != &cache->mru_list; e = e->mru_next)
        p = _bprint(p, end, " %d", e->id);

    XLOG("%s", temp);
}

static void
_dump_answer(const void* answer, int answerlen)
{
    res_state statep;
    FILE* fp;
    char* buf;
    int fileLen;

    fp = fopen("/data/reslog.txt", "w+e");
    if (fp != NULL) {
        statep = __res_get_state();

        res_pquery(statep, answer, answerlen, fp);

        //Get file length
        fseek(fp, 0, SEEK_END);
        fileLen=ftell(fp);
        fseek(fp, 0, SEEK_SET);
        buf = (char *)malloc(fileLen+1);
        if (buf != NULL) {
            //Read file contents into buffer
            fread(buf, fileLen, 1, fp);
            XLOG("%s\n", buf);
            free(buf);
        }
        fclose(fp);
        remove("/data/reslog.txt");
    }
    else {
        errno = 0; // else debug is introducing error signals
        XLOG("%s: can't open file\n", __FUNCTION__);
    }
}
#endif

#if DEBUG
#  define  XLOG_QUERY(q,len)   _dump_query((q), (len))
#  define  XLOG_ANSWER(a, len) _dump_answer((a), (len))
#else
#  define  XLOG_QUERY(q,len)   ((void)0)
#  define  XLOG_ANSWER(a,len)  ((void)0)
#endif

/* This function tries to find a key within the hash table
 * In case of success, it will return a *pointer* to the hashed key.
 * In case of failure, it will return a *pointer* to NULL
 *
 * So, the caller must check '*result' to check for success/failure.
 *
 * The main idea is that the result can later be used directly in
 * calls to _resolv_cache_add or _resolv_cache_remove as the 'lookup'
 * parameter. This makes the code simpler and avoids re-searching
 * for the key position in the htable.
 *
 * The result of a lookup_p is only valid until you alter the hash
 * table.
 */
static Entry**
_cache_lookup_p( Cache*   cache,
                 Entry*   key )
{
    int      index = key->hash % cache->max_entries;
    Entry**  pnode = (Entry**) &cache->entries[ index ];

    while (*pnode != NULL) {
        Entry*  node = *pnode;

        if (node == NULL)
            break;

        if (node->hash == key->hash && entry_equals(node, key))
            break;

        pnode = &node->hlink;
    }
    return pnode;
}

/* Add a new entry to the hash table. 'lookup' must be the
 * result of an immediate previous failed _lookup_p() call
 * (i.e. with *lookup == NULL), and 'e' is the pointer to the
 * newly created entry
 */
static void
_cache_add_p( Cache*   cache,
              Entry**  lookup,
              Entry*   e )
{
    *lookup = e;
    e->id = ++cache->last_id;
    entry_mru_add(e, &cache->mru_list);
    cache->num_entries += 1;

    XLOG("%s: entry %d added (count=%d)", __FUNCTION__,
         e->id, cache->num_entries);
}

/* Remove an existing entry from the hash table,
 * 'lookup' must be the result of an immediate previous
 * and succesful _lookup_p() call.
 */
static void
_cache_remove_p( Cache*   cache,
                 Entry**  lookup )
{
    Entry*  e  = *lookup;

    XLOG("%s: entry %d removed (count=%d)", __FUNCTION__,
         e->id, cache->num_entries-1);

    entry_mru_remove(e);
    *lookup = e->hlink;
    entry_free(e);
    cache->num_entries -= 1;
}

/* Remove the oldest entry from the hash table.
 */
static void
_cache_remove_oldest( Cache*  cache )
{
    Entry*   oldest = cache->mru_list.mru_prev;
    Entry**  lookup = _cache_lookup_p(cache, oldest);

    if (*lookup == NULL) { /* should not happen */
        XLOG("%s: OLDEST NOT IN HTABLE ?", __FUNCTION__);
        return;
    }
    if (DEBUG) {
        XLOG("Cache full - removing oldest");
        XLOG_QUERY(oldest->query, oldest->querylen);
    }
    _cache_remove_p(cache, lookup);
}

/* Remove all expired entries from the hash table.
 */
static void _cache_remove_expired(Cache* cache) {
    Entry* e;
    time_t now = _time_now();

    for (e = cache->mru_list.mru_next; e != &cache->mru_list;) {
        // Entry is old, remove
        if (now >= e->expires) {
            Entry** lookup = _cache_lookup_p(cache, e);
            if (*lookup == NULL) { /* should not happen */
                XLOG("%s: ENTRY NOT IN HTABLE ?", __FUNCTION__);
                return;
            }
            e = e->mru_next;
            _cache_remove_p(cache, lookup);
        } else {
            e = e->mru_next;
        }
    }
}

ResolvCacheStatus
_resolv_cache_lookup( unsigned              netid,
                      const void*           query,
                      int                   querylen,
                      void*                 answer,
                      int                   answersize,
                      int                  *answerlen )
{
    Entry      key[1];
    Entry**    lookup;
    Entry*     e;
    time_t     now;
    Cache*     cache;

    ResolvCacheStatus  result = RESOLV_CACHE_NOTFOUND;

    XLOG("%s: lookup", __FUNCTION__);
    XLOG_QUERY(query, querylen);

    /* we don't cache malformed queries */
    if (!entry_init_key(key, query, querylen)) {
        XLOG("%s: unsupported query", __FUNCTION__);
        return RESOLV_CACHE_UNSUPPORTED;
    }
    /* lookup cache */
    pthread_once(&_res_cache_once, _res_cache_init);
    pthread_mutex_lock(&_res_cache_list_lock);

    cache = _find_named_cache_locked(netid);
    if (cache == NULL) {
        result = RESOLV_CACHE_UNSUPPORTED;
        goto Exit;
    }

    /* see the description of _lookup_p to understand this.
     * the function always return a non-NULL pointer.
     */
    lookup = _cache_lookup_p(cache, key);
    e      = *lookup;

    if (e == NULL) {
        XLOG( "NOT IN CACHE");
        // calling thread will wait if an outstanding request is found
        // that matching this query
        if (!_cache_check_pending_request_locked(&cache, key, netid) || cache == NULL) {
            goto Exit;
        } else {
            lookup = _cache_lookup_p(cache, key);
            e = *lookup;
            if (e == NULL) {
                goto Exit;
            }
        }
    }

    now = _time_now();

    /* remove stale entries here */
    if (now >= e->expires) {
        XLOG( " NOT IN CACHE (STALE ENTRY %p DISCARDED)", *lookup );
        XLOG_QUERY(e->query, e->querylen);
        _cache_remove_p(cache, lookup);
        goto Exit;
    }

    *answerlen = e->answerlen;
    if (e->answerlen > answersize) {
        /* NOTE: we return UNSUPPORTED if the answer buffer is too short */
        result = RESOLV_CACHE_UNSUPPORTED;
        XLOG(" ANSWER TOO LONG");
        goto Exit;
    }

    memcpy( answer, e->answer, e->answerlen );

    /* bump up this entry to the top of the MRU list */
    if (e != cache->mru_list.mru_next) {
        entry_mru_remove( e );
        entry_mru_add( e, &cache->mru_list );
    }

    XLOG( "FOUND IN CACHE entry=%p", e );
    result = RESOLV_CACHE_FOUND;

Exit:
    pthread_mutex_unlock(&_res_cache_list_lock);
    return result;
}


void
_resolv_cache_add( unsigned              netid,
                   const void*           query,
                   int                   querylen,
                   const void*           answer,
                   int                   answerlen )
{
    Entry    key[1];
    Entry*   e;
    Entry**  lookup;
    u_long   ttl;
    Cache*   cache = NULL;

    /* don't assume that the query has already been cached
     */
    if (!entry_init_key( key, query, querylen )) {
        XLOG( "%s: passed invalid query ?", __FUNCTION__);
        return;
    }

    pthread_mutex_lock(&_res_cache_list_lock);

    cache = _find_named_cache_locked(netid);
    if (cache == NULL) {
        goto Exit;
    }

    XLOG( "%s: query:", __FUNCTION__ );
    XLOG_QUERY(query,querylen);
    XLOG_ANSWER(answer, answerlen);
#if DEBUG_DATA
    XLOG( "answer:");
    XLOG_BYTES(answer,answerlen);
#endif

    lookup = _cache_lookup_p(cache, key);
    e      = *lookup;

    if (e != NULL) { /* should not happen */
        XLOG("%s: ALREADY IN CACHE (%p) ? IGNORING ADD",
             __FUNCTION__, e);
        goto Exit;
    }

    if (cache->num_entries >= cache->max_entries) {
        _cache_remove_expired(cache);
        if (cache->num_entries >= cache->max_entries) {
            _cache_remove_oldest(cache);
        }
        /* need to lookup again */
        lookup = _cache_lookup_p(cache, key);
        e      = *lookup;
        if (e != NULL) {
            XLOG("%s: ALREADY IN CACHE (%p) ? IGNORING ADD",
                __FUNCTION__, e);
            goto Exit;
        }
    }

    ttl = answer_getTTL(answer, answerlen);
    if (ttl > 0) {
        e = entry_alloc(key, answer, answerlen);
        if (e != NULL) {
            e->expires = ttl + _time_now();
            _cache_add_p(cache, lookup, e);
        }
    }
#if DEBUG
    _cache_dump_mru(cache);
#endif
Exit:
    if (cache != NULL) {
      _cache_notify_waiting_tid_locked(cache, key);
    }
    pthread_mutex_unlock(&_res_cache_list_lock);
}

/****************************************************************************/
/****************************************************************************/
/*****                                                                  *****/
/*****                                                                  *****/
/*****                                                                  *****/
/****************************************************************************/
/****************************************************************************/

// Head of the list of caches.  Protected by _res_cache_list_lock.
static struct resolv_cache_info _res_cache_list;

/* insert resolv_cache_info into the list of resolv_cache_infos */
static void _insert_cache_info_locked(struct resolv_cache_info* cache_info);
/* creates a resolv_cache_info */
static struct resolv_cache_info* _create_cache_info( void );
/* gets a resolv_cache_info associated with a network, or NULL if not found */
static struct resolv_cache_info* _find_cache_info_locked(unsigned netid);
/* look up the named cache, and creates one if needed */
static struct resolv_cache* _get_res_cache_for_net_locked(unsigned netid);
/* empty the named cache */
static void _flush_cache_for_net_locked(unsigned netid);
/* empty the nameservers set for the named cache */
static void _free_nameservers_locked(struct resolv_cache_info* cache_info);
/* return 1 if the provided list of name servers differs from the list of name servers
 * currently attached to the provided cache_info */
static int _resolv_is_nameservers_equal_locked(struct resolv_cache_info* cache_info,
        const char** servers, int numservers);
/* clears the stats samples contained withing the given cache_info */
static void _res_cache_clear_stats_locked(struct resolv_cache_info* cache_info);

static void
_res_cache_init(void)
{
    const char*  env = getenv(CONFIG_ENV);

    if (env && atoi(env) == 0) {
        /* the cache is disabled */
        return;
    }

    memset(&_res_cache_list, 0, sizeof(_res_cache_list));
    pthread_mutex_init(&_res_cache_list_lock, NULL);
}

static struct resolv_cache*
_get_res_cache_for_net_locked(unsigned netid)
{
    struct resolv_cache* cache = _find_named_cache_locked(netid);
    if (!cache) {
        struct resolv_cache_info* cache_info = _create_cache_info();
        if (cache_info) {
            cache = _resolv_cache_create();
            if (cache) {
                cache_info->cache = cache;
                cache_info->netid = netid;
                _insert_cache_info_locked(cache_info);
            } else {
                free(cache_info);
            }
        }
    }
    return cache;
}

void
_resolv_flush_cache_for_net(unsigned netid)
{
    pthread_once(&_res_cache_once, _res_cache_init);
    pthread_mutex_lock(&_res_cache_list_lock);

    _flush_cache_for_net_locked(netid);

    pthread_mutex_unlock(&_res_cache_list_lock);
}

static void
_flush_cache_for_net_locked(unsigned netid)
{
    struct resolv_cache* cache = _find_named_cache_locked(netid);
    if (cache) {
        _cache_flush_locked(cache);
    }

    // Also clear the NS statistics.
    struct resolv_cache_info* cache_info = _find_cache_info_locked(netid);
    _res_cache_clear_stats_locked(cache_info);
}

void _resolv_delete_cache_for_net(unsigned netid)
{
    pthread_once(&_res_cache_once, _res_cache_init);
    pthread_mutex_lock(&_res_cache_list_lock);

    struct resolv_cache_info* prev_cache_info = &_res_cache_list;

    while (prev_cache_info->next) {
        struct resolv_cache_info* cache_info = prev_cache_info->next;

        if (cache_info->netid == netid) {
            prev_cache_info->next = cache_info->next;
            _cache_flush_locked(cache_info->cache);
            free(cache_info->cache->entries);
            free(cache_info->cache);
            _free_nameservers_locked(cache_info);
            free(cache_info);
            break;
        }

        prev_cache_info = prev_cache_info->next;
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
}

static struct resolv_cache_info*
_create_cache_info(void)
{
    struct resolv_cache_info* cache_info;

    cache_info = calloc(sizeof(*cache_info), 1);
    return cache_info;
}

static void
_insert_cache_info_locked(struct resolv_cache_info* cache_info)
{
    struct resolv_cache_info* last;

    for (last = &_res_cache_list; last->next; last = last->next);

    last->next = cache_info;

}

static struct resolv_cache*
_find_named_cache_locked(unsigned netid) {

    struct resolv_cache_info* info = _find_cache_info_locked(netid);

    if (info != NULL) return info->cache;

    return NULL;
}

static struct resolv_cache_info*
_find_cache_info_locked(unsigned netid)
{
    struct resolv_cache_info* cache_info = _res_cache_list.next;

    while (cache_info) {
        if (cache_info->netid == netid) {
            break;
        }

        cache_info = cache_info->next;
    }
    return cache_info;
}

void
_resolv_set_default_params(struct __res_params* params) {
    params->sample_validity = NSSAMPLE_VALIDITY;
    params->success_threshold = SUCCESS_THRESHOLD;
    params->min_samples = 0;
    params->max_samples = 0;
}

int
_resolv_set_nameservers_for_net(unsigned netid, const char** servers, unsigned numservers,
        const char *domains, const struct __res_params* params)
{
    char sbuf[NI_MAXSERV];
    register char *cp;
    int *offset;
    struct addrinfo* nsaddrinfo[MAXNS];

    if (numservers > MAXNS) {
        XLOG("%s: numservers=%u, MAXNS=%u", __FUNCTION__, numservers, MAXNS);
        return E2BIG;
    }

    // Parse the addresses before actually locking or changing any state, in case there is an error.
    // As a side effect this also reduces the time the lock is kept.
    struct addrinfo hints = {
        .ai_family = AF_UNSPEC,
        .ai_socktype = SOCK_DGRAM,
        .ai_flags = AI_NUMERICHOST
    };
    snprintf(sbuf, sizeof(sbuf), "%u", NAMESERVER_PORT);
    for (unsigned i = 0; i < numservers; i++) {
        // The addrinfo structures allocated here are freed in _free_nameservers_locked().
        int rt = getaddrinfo(servers[i], sbuf, &hints, &nsaddrinfo[i]);
        if (rt != 0) {
            for (unsigned j = 0 ; j < i ; j++) {
                freeaddrinfo(nsaddrinfo[j]);
                nsaddrinfo[j] = NULL;
            }
            XLOG("%s: getaddrinfo(%s)=%s", __FUNCTION__, servers[i], gai_strerror(rt));
            return EINVAL;
        }
    }

    pthread_once(&_res_cache_once, _res_cache_init);
    pthread_mutex_lock(&_res_cache_list_lock);

    // creates the cache if not created
    _get_res_cache_for_net_locked(netid);

    struct resolv_cache_info* cache_info = _find_cache_info_locked(netid);

    if (cache_info != NULL) {
        uint8_t old_max_samples = cache_info->params.max_samples;
        if (params != NULL) {
            cache_info->params = *params;
        } else {
            _resolv_set_default_params(&cache_info->params);
        }

        if (!_resolv_is_nameservers_equal_locked(cache_info, servers, numservers)) {
            // free current before adding new
            _free_nameservers_locked(cache_info);
            unsigned i;
            for (i = 0; i < numservers; i++) {
                cache_info->nsaddrinfo[i] = nsaddrinfo[i];
                cache_info->nameservers[i] = strdup(servers[i]);
                XLOG("%s: netid = %u, addr = %s\n", __FUNCTION__, netid, servers[i]);
            }
            cache_info->nscount = numservers;

            // Flush the cache and reset the stats.
            _flush_cache_for_net_locked(netid);

            // increment the revision id to ensure that sample state is not written back if the
            // servers change; in theory it would suffice to do so only if the servers or
            // max_samples actually change, in practice the overhead of checking is higher than the
            // cost, and overflows are unlikely
            ++cache_info->revision_id;
        } else if (cache_info->params.max_samples != old_max_samples) {
            // If the maximum number of samples changes, the overhead of keeping the most recent
            // samples around is not considered worth the effort, so they are cleared instead. All
            // other parameters do not affect shared state: Changing these parameters does not
            // invalidate the samples, as they only affect aggregation and the conditions under
            // which servers are considered usable.
            _res_cache_clear_stats_locked(cache_info);
            ++cache_info->revision_id;
        }

        // Always update the search paths, since determining whether they actually changed is
        // complex due to the zero-padding, and probably not worth the effort. Cache-flushing
        // however is not // necessary, since the stored cache entries do contain the domain, not
        // just the host name.
        // code moved from res_init.c, load_domain_search_list
        strlcpy(cache_info->defdname, domains, sizeof(cache_info->defdname));
        if ((cp = strchr(cache_info->defdname, '\n')) != NULL)
            *cp = '\0';

        cp = cache_info->defdname;
        offset = cache_info->dnsrch_offset;
        while (offset < cache_info->dnsrch_offset + MAXDNSRCH) {
            while (*cp == ' ' || *cp == '\t') /* skip leading white space */
                cp++;
            if (*cp == '\0') /* stop if nothing more to do */
                break;
            *offset++ = cp - cache_info->defdname; /* record this search domain */
            while (*cp) { /* zero-terminate it */
                if (*cp == ' '|| *cp == '\t') {
                    *cp++ = '\0';
                    break;
                }
                cp++;
            }
        }
        *offset = -1; /* cache_info->dnsrch_offset has MAXDNSRCH+1 items */
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
    return 0;
}

static int
_resolv_is_nameservers_equal_locked(struct resolv_cache_info* cache_info,
        const char** servers, int numservers)
{
    if (cache_info->nscount != numservers) {
        return 0;
    }

    // Compare each name server against current name servers.
    // TODO: this is incorrect if the list of current or previous nameservers
    // contains duplicates. This does not really matter because the framework
    // filters out duplicates, but we should probably fix it. It's also
    // insensitive to the order of the nameservers; we should probably fix that
    // too.
    for (int i = 0; i < numservers; i++) {
        for (int j = 0 ; ; j++) {
            if (j >= numservers) {
                return 0;
            }
            if (strcmp(cache_info->nameservers[i], servers[j]) == 0) {
                break;
            }
        }
    }

    return 1;
}

static void
_free_nameservers_locked(struct resolv_cache_info* cache_info)
{
    int i;
    for (i = 0; i < cache_info->nscount; i++) {
        free(cache_info->nameservers[i]);
        cache_info->nameservers[i] = NULL;
        if (cache_info->nsaddrinfo[i] != NULL) {
            freeaddrinfo(cache_info->nsaddrinfo[i]);
            cache_info->nsaddrinfo[i] = NULL;
        }
        cache_info->nsstats[i].sample_count =
            cache_info->nsstats[i].sample_next = 0;
    }
    cache_info->nscount = 0;
    _res_cache_clear_stats_locked(cache_info);
    ++cache_info->revision_id;
}

void
_resolv_populate_res_for_net(res_state statp)
{
    if (statp == NULL) {
        return;
    }

    pthread_once(&_res_cache_once, _res_cache_init);
    pthread_mutex_lock(&_res_cache_list_lock);

    struct resolv_cache_info* info = _find_cache_info_locked(statp->netid);
    if (info != NULL) {
        int nserv;
        struct addrinfo* ai;
        XLOG("%s: %u\n", __FUNCTION__, statp->netid);
        for (nserv = 0; nserv < MAXNS; nserv++) {
            ai = info->nsaddrinfo[nserv];
            if (ai == NULL) {
                break;
            }

            if ((size_t) ai->ai_addrlen <= sizeof(statp->_u._ext.ext->nsaddrs[0])) {
                if (statp->_u._ext.ext != NULL) {
                    memcpy(&statp->_u._ext.ext->nsaddrs[nserv], ai->ai_addr, ai->ai_addrlen);
                    statp->nsaddr_list[nserv].sin_family = AF_UNSPEC;
                } else {
                    if ((size_t) ai->ai_addrlen
                            <= sizeof(statp->nsaddr_list[0])) {
                        memcpy(&statp->nsaddr_list[nserv], ai->ai_addr,
                                ai->ai_addrlen);
                    } else {
                        statp->nsaddr_list[nserv].sin_family = AF_UNSPEC;
                    }
                }
            } else {
                XLOG("%s: found too long addrlen", __FUNCTION__);
            }
        }
        statp->nscount = nserv;
        // now do search domains.  Note that we cache the offsets as this code runs alot
        // but the setting/offset-computer only runs when set/changed
        // WARNING: Don't use str*cpy() here, this string contains zeroes.
        memcpy(statp->defdname, info->defdname, sizeof(statp->defdname));
        register char **pp = statp->dnsrch;
        register int *p = info->dnsrch_offset;
        while (pp < statp->dnsrch + MAXDNSRCH && *p != -1) {
            *pp++ = &statp->defdname[0] + *p++;
        }
    }
    pthread_mutex_unlock(&_res_cache_list_lock);
}

/* Resolver reachability statistics. */

static void
_res_cache_add_stats_sample_locked(struct __res_stats* stats, const struct __res_sample* sample,
        int max_samples) {
    // Note: This function expects max_samples > 0, otherwise a (harmless) modification of the
    // allocated but supposedly unused memory for samples[0] will happen
    XLOG("%s: adding sample to stats, next = %d, count = %d", __FUNCTION__,
            stats->sample_next, stats->sample_count);
    stats->samples[stats->sample_next] = *sample;
    if (stats->sample_count < max_samples) {
        ++stats->sample_count;
    }
    if (++stats->sample_next >= max_samples) {
        stats->sample_next = 0;
    }
}

static void
_res_cache_clear_stats_locked(struct resolv_cache_info* cache_info) {
    if (cache_info) {
        for (int i = 0 ; i < MAXNS ; ++i) {
            cache_info->nsstats->sample_count = cache_info->nsstats->sample_next = 0;
        }
    }
}

int
android_net_res_stats_get_info_for_net(unsigned netid, int* nscount,
        struct sockaddr_storage servers[MAXNS], int* dcount, char domains[MAXDNSRCH][MAXDNSRCHPATH],
        struct __res_params* params, struct __res_stats stats[MAXNS]) {
    int revision_id = -1;
    pthread_mutex_lock(&_res_cache_list_lock);

    struct resolv_cache_info* info = _find_cache_info_locked(netid);
    if (info) {
        if (info->nscount > MAXNS) {
            pthread_mutex_unlock(&_res_cache_list_lock);
            XLOG("%s: nscount %d > MAXNS %d", __FUNCTION__, info->nscount, MAXNS);
            errno = EFAULT;
            return -1;
        }
        int i;
        for (i = 0; i < info->nscount; i++) {
            // Verify that the following assumptions are held, failure indicates corruption:
            //  - getaddrinfo() may never return a sockaddr > sockaddr_storage
            //  - all addresses are valid
            //  - there is only one address per addrinfo thanks to numeric resolution
            int addrlen = info->nsaddrinfo[i]->ai_addrlen;
            if (addrlen < (int) sizeof(struct sockaddr) ||
                    addrlen > (int) sizeof(servers[0])) {
                pthread_mutex_unlock(&_res_cache_list_lock);
                XLOG("%s: nsaddrinfo[%d].ai_addrlen == %d", __FUNCTION__, i, addrlen);
                errno = EMSGSIZE;
                return -1;
            }
            if (info->nsaddrinfo[i]->ai_addr == NULL) {
                pthread_mutex_unlock(&_res_cache_list_lock);
                XLOG("%s: nsaddrinfo[%d].ai_addr == NULL", __FUNCTION__, i);
                errno = ENOENT;
                return -1;
            }
            if (info->nsaddrinfo[i]->ai_next != NULL) {
                pthread_mutex_unlock(&_res_cache_list_lock);
                XLOG("%s: nsaddrinfo[%d].ai_next != NULL", __FUNCTION__, i);
                errno = ENOTUNIQ;
                return -1;
            }
        }
        *nscount = info->nscount;
        for (i = 0; i < info->nscount; i++) {
            memcpy(&servers[i], info->nsaddrinfo[i]->ai_addr, info->nsaddrinfo[i]->ai_addrlen);
            stats[i] = info->nsstats[i];
        }
        for (i = 0; i < MAXDNSRCH; i++) {
            const char* cur_domain = info->defdname + info->dnsrch_offset[i];
            // dnsrch_offset[i] can either be -1 or point to an empty string to indicate the end
            // of the search offsets. Checking for < 0 is not strictly necessary, but safer.
            // TODO: Pass in a search domain array instead of a string to
            // _resolv_set_nameservers_for_net() and make this double check unnecessary.
            if (info->dnsrch_offset[i] < 0 ||
                    ((size_t)info->dnsrch_offset[i]) >= sizeof(info->defdname) || !cur_domain[0]) {
                break;
            }
            strlcpy(domains[i], cur_domain, MAXDNSRCHPATH);
        }
        *dcount = i;
        *params = info->params;
        revision_id = info->revision_id;
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
    return revision_id;
}

int
_resolv_cache_get_resolver_stats( unsigned netid, struct __res_params* params,
        struct __res_stats stats[MAXNS]) {
    int revision_id = -1;
    pthread_mutex_lock(&_res_cache_list_lock);

    struct resolv_cache_info* info = _find_cache_info_locked(netid);
    if (info) {
        memcpy(stats, info->nsstats, sizeof(info->nsstats));
        *params = info->params;
        revision_id = info->revision_id;
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
    return revision_id;
}

void
_resolv_cache_add_resolver_stats_sample( unsigned netid, int revision_id, int ns,
       const struct __res_sample* sample, int max_samples) {
    if (max_samples <= 0) return;

    pthread_mutex_lock(&_res_cache_list_lock);

    struct resolv_cache_info* info = _find_cache_info_locked(netid);

    if (info && info->revision_id == revision_id) {
        _res_cache_add_stats_sample_locked(&info->nsstats[ns], sample, max_samples);
    }

    pthread_mutex_unlock(&_res_cache_list_lock);
}