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bionic/libc/netbsd/resolv/res_cache.c
The Android Open Source Project 1dc9e472e1 auto import from //depot/cupcake/@135843
2009-03-03 19:28:35 -08:00

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/*
* 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 <stdlib.h>
#include <string.h>
#include <time.h>
#include "pthread.h"
/* This code implements a small and *simple* DNS resolver cache.
*
* It is only used to cache DNS answers for a maximum of CONFIG_SECONDS seconds
* 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 TTLs of answer RRs are ignored. our DNS resolver library does not use
* them anyway, but it means that records with a TTL smaller than
* CONFIG_SECONDS will be kept in the cache anyway.
*
* 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.
*
* - when network settings change, the cache must be flushed since the list
* of DNS servers probably changed. this is done by calling
* _resolv_cache_reset()
*
* the parameter to this function must be an ever-increasing generation
* number corresponding to the current network settings state.
*
* This is done because several threads could detect the same network
* settings change (but at different times) and will all end up calling the
* same function. Comparing with the last used generation number ensures
* that the cache is only flushed once per network change.
*/
/* 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"
/* entries older than CONFIG_SECONDS seconds are always discarded.
*/
#define CONFIG_SECONDS (60*10) /* 10 minutes */
/* maximum 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 valud of 64 should be relatively conformtable at the moment.
*/
#define CONFIG_MAX_ENTRIES 64
/****************************************************************************/
/****************************************************************************/
/***** *****/
/***** *****/
/***** *****/
/****************************************************************************/
/****************************************************************************/
/* set to 1 to debug cache operations */
#define DEBUG 0
/* set to 1 to debug query data */
#define DEBUG_DATA 0
#if DEBUG
# include <logd.h>
# define XLOG(...) \
__libc_android_log_print(ANDROID_LOG_DEBUG,"libc",__VA_ARGS__)
#include <stdio.h>
#include <stdarg.h>
/** 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 */
static 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 */
static 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 */
static 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 */
static 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 = snprintf( 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 */
static 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 */
static 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 */
static void
XLOG_BYTES( const void* base, int len )
{
char buff[1024];
char* p = buff, *end = p + sizeof(buff);
p = _bprint_hexdump(p, end, base, len);
XLOG("%s",buff);
}
#else /* !DEBUG */
# define XLOG(...) ((void)0)
# define XLOG_BYTES(a,b) ((void)0)
#endif
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 )
{
int len;
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 )
{
int len;
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 when; /* time_t when entry was added to table */
int id; /* for debugging purpose */
} Entry;
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 );
e->when = _time_now();
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.
*/
#define MAX_HASH_ENTRIES (2*CONFIG_MAX_ENTRIES)
typedef struct resolv_cache {
int num_entries;
Entry mru_list;
pthread_mutex_t lock;
unsigned generation;
int last_id;
Entry* entries[ MAX_HASH_ENTRIES ];
} Cache;
#define HTABLE_VALID(x) ((x) != NULL && (x) != HTABLE_DELETED)
static void
_cache_flush_locked( Cache* cache )
{
int nn;
time_t now = _time_now();
for (nn = 0; nn < MAX_HASH_ENTRIES; nn++)
{
Entry** pnode = &cache->entries[nn];
while (*pnode != NULL) {
Entry* node = *pnode;
*pnode = node->hlink;
entry_free(node);
}
}
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"
"*************************");
}
struct resolv_cache*
_resolv_cache_create( void )
{
struct resolv_cache* cache;
cache = calloc(sizeof(*cache), 1);
if (cache) {
cache->generation = ~0U;
pthread_mutex_init( &cache->lock, NULL );
cache->mru_list.mru_prev = cache->mru_list.mru_next = &cache->mru_list;
XLOG("%s: cache created\n", __FUNCTION__);
}
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);
}
#endif
#if DEBUG
# define XLOG_QUERY(q,len) _dump_query((q), (len))
#else
# define XLOG_QUERY(q,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 % MAX_HASH_ENTRIES;
Entry** pnode = &cache->entries[ key->hash % MAX_HASH_ENTRIES ];
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;
}
_cache_remove_p(cache, lookup);
}
ResolvCacheStatus
_resolv_cache_lookup( struct resolv_cache* cache,
const void* query,
int querylen,
void* answer,
int answersize,
int *answerlen )
{
DnsPacket pack[1];
Entry key[1];
int index;
Entry** lookup;
Entry* e;
time_t now;
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_mutex_lock( &cache->lock );
/* 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");
goto Exit;
}
now = _time_now();
/* remove stale entries here */
if ( (unsigned)(now - e->when) >= CONFIG_SECONDS ) {
XLOG( " NOT IN CACHE (STALE ENTRY %p DISCARDED)", *lookup );
_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( &cache->lock );
return result;
}
void
_resolv_cache_add( struct resolv_cache* cache,
const void* query,
int querylen,
const void* answer,
int answerlen )
{
Entry key[1];
Entry* e;
Entry** lookup;
/* 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( &cache->lock );
XLOG( "%s: query:", __FUNCTION__ );
XLOG_QUERY(query,querylen);
#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 >= CONFIG_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;
}
}
e = entry_alloc( key, answer, answerlen );
if (e != NULL) {
_cache_add_p(cache, lookup, e);
}
#if DEBUG
_cache_dump_mru(cache);
#endif
Exit:
pthread_mutex_unlock( &cache->lock );
}
/****************************************************************************/
/****************************************************************************/
/***** *****/
/***** *****/
/***** *****/
/****************************************************************************/
/****************************************************************************/
static struct resolv_cache* _res_cache;
static pthread_once_t _res_cache_once;
static void
_res_cache_init( void )
{
const char* env = getenv(CONFIG_ENV);
if (env && atoi(env) == 0) {
/* the cache is disabled */
return;
}
_res_cache = _resolv_cache_create();
}
struct resolv_cache*
__get_res_cache( void )
{
pthread_once( &_res_cache_once, _res_cache_init );
return _res_cache;
}
void
_resolv_cache_reset( unsigned generation )
{
XLOG("%s: generation=%d", __FUNCTION__, generation);
if (_res_cache == NULL)
return;
pthread_mutex_lock( &_res_cache->lock );
if (_res_cache->generation != generation) {
_cache_flush_locked(_res_cache);
_res_cache->generation = generation;
}
pthread_mutex_unlock( &_res_cache->lock );
}
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