AuroraMiddleware/qt5base-lts
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

QDnsLookup/Unix large replies: use EDNS0 [2/3]

Browse Source 
The current code is inefficient when dealing with replies exceeding the
PACKETSZ default size. This commit adds an EDNS0 footer to the DNS query
informing the DNS server how big our buffer is. We choose a larger
buffer than the old PACKETSZ so more will fit before a Virtual Circuit
(TCP socket) is required.

The choice is based on IPv6 requirements for the minimum MTU. Any
network incapable of transmitting frames with that big a payload must
support fragmenting and reassembly at the Layer 2 level, below IP.
Ethernet MTU is usually 1500, so we leave a bit on the table, but this
avoids having to discover our Path MTU to the DNS server.

This is incomplete: DNS queries above the 1232-byte limit still perform
the same query four times.

Change-Id: I3e3bfef633af4130a03afffd175e72f7fbc9265d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
This commit is contained in:
Thiago Macieira 2023-05-12 09:42:00 -07:00
parent b673fa09c3
commit df07d98243
1 changed files with 31 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

35
src/network/kernel/qdnslookup_unix.cpp
View File

@ -19,18 +19,37 @@ QT_REQUIRE_CONFIG(res_ninit);
#if __has_include(<arpa/nameser_compat.h>)
# include <arpa/nameser_compat.h>
#endif
#include <errno.h>
#include <resolv.h>
#include <array>
#ifndef T_OPT
// the older arpa/nameser_compat.h wasn't updated between 1999 and 2016 in glibc
# define T_OPT ns_t_opt
#endif
QT_BEGIN_NAMESPACE
using namespace Qt::StringLiterals;
static constexpr qsizetype ReplyBufferSize = PACKETSZ;
// minimum IPv6 MTU (1280) minus the IPv6 (40) and UDP headers (8)
static constexpr qsizetype ReplyBufferSize = 1280 - 40 - 8;
// maximum length of a DNS query with a 255-character domain (rounded up to 16)
static constexpr qsizetype QueryBufferSize = HFIXEDSZ + QFIXEDSZ + MAXCDNAME + 1;
// https://www.rfc-editor.org/rfc/rfc6891
static constexpr unsigned char Edns0Record[] = {
0x00, // root label
T_OPT >> 8, T_OPT & 0xff, // type OPT
ReplyBufferSize >> 8, ReplyBufferSize & 0xff, // payload size
NOERROR, // extended rcode
0, // version
0x00, 0x00, // flags
0x00, 0x00, // option length
};
// maximum length of a EDNS0 query with a 255-character domain (rounded up to 16)
static constexpr qsizetype QueryBufferSize =
HFIXEDSZ + QFIXEDSZ + MAXCDNAME + 1 + sizeof(Edns0Record);
using QueryBuffer = std::array<unsigned char, (QueryBufferSize + 15) / 16 * 16>;
#if QT_CONFIG(res_setservers)
@ -112,7 +131,15 @@ prepareQueryBuffer(res_state state, QueryBuffer &buffer, const char *label, ns_r
int queryLength = res_nmkquery(state, QUERY, label, C_IN, type, nullptr, 0, nullptr,
buffer.data(), buffer.size());
Q_ASSERT(queryLength < int(buffer.size()));
return queryLength;
if (Q_UNLIKELY(queryLength < 0))
return queryLength;
// Append EDNS0 record and set the number of additional RRs to 1
Q_ASSERT(queryLength + sizeof(Edns0Record) < buffer.size());
std::copy_n(std::begin(Edns0Record), sizeof(Edns0Record), buffer.begin() + queryLength);
reinterpret_cast<HEADER *>(buffer.data())->arcount = qToBigEndian<quint16>(1);
return queryLength + sizeof(Edns0Record);
}
void QDnsLookupRunnable::query(QDnsLookupReply *reply)