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handle FIN flag in order of the sequence numbers #2002

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3ab1a3a
handle FIN flag in order of the sequence numbers
matthiasklein Oct 23, 2025
68d12d5
ExamplesTest: X509Toolkit: test_pcap_extract_to_file: adjust TLS mess…
matthiasklein Oct 27, 2025
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4 changes: 3 additions & 1 deletion Packet++/header/TcpReassembly.h
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Original file line number Diff line number Diff line change
Expand Up @@ -404,9 +404,11 @@ namespace pcpp
uint32_t sequence;
size_t dataLength;
uint8_t* data;
uint32_t flowKey;
bool isFin;
std::chrono::time_point<std::chrono::high_resolution_clock> timestamp;

TcpFragment() : sequence(0), dataLength(0), data(nullptr)
TcpFragment() : sequence(0), dataLength(0), data(nullptr), flowKey(0), isFin(false)
{}
~TcpFragment()
{
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47 changes: 34 additions & 13 deletions Packet++/src/TcpReassembly.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -268,10 +268,10 @@ namespace pcpp
return Ignore_PacketOfClosedFlow;
}

// handle FIN/RST packets that don't contain additional TCP data
if (isFinOrRst && tcpPayloadSize == 0)
// handle RST packets that don't contain additional TCP data
if (isRst && tcpPayloadSize == 0)
{
PCPP_LOG_DEBUG("Got FIN or RST packet without data on side " << sideIndex);
PCPP_LOG_DEBUG("Got RST packet without data on side " << sideIndex);

handleFinOrRst(tcpReassemblyData, sideIndex, flowKey, isRst);
return FIN_RSTWithNoData;
Expand Down Expand Up @@ -441,27 +441,36 @@ namespace pcpp
{
PCPP_LOG_DEBUG("Payload length is 0, doing nothing");

// handle case where this packet is FIN or RST
if (isFinOrRst)
// handle case where this packet is RST
if (isRst)
{
handleFinOrRst(tcpReassemblyData, sideIndex, flowKey, isRst);
status = FIN_RSTWithNoData;
return status;
}
else

if(!isFin)
{
status = Ignore_PacketWithNoData;
}

return status;
return status;
}
}

// create a new TcpFragment, copy the TCP data to it and add this packet to the the out-of-order packet list
TcpFragment* newTcpFrag = new TcpFragment();
newTcpFrag->data = new uint8_t[tcpPayloadSize];

if(tcpPayloadSize)
{
newTcpFrag->data = new uint8_t[tcpPayloadSize];
memcpy(newTcpFrag->data, tcpLayer->getLayerPayload(), tcpPayloadSize);
}

newTcpFrag->dataLength = tcpPayloadSize;
newTcpFrag->sequence = sequence;
newTcpFrag->timestamp = currTime;
memcpy(newTcpFrag->data, tcpLayer->getLayerPayload(), tcpPayloadSize);
newTcpFrag->isFin = isFin;
newTcpFrag->flowKey = flowKey;

tcpReassemblyData->twoSides[sideIndex].tcpFragmentList.pushBack(newTcpFrag);

PCPP_LOG_DEBUG("Found out-of-order packet and added a new TCP fragment with size "
Expand All @@ -476,8 +485,8 @@ namespace pcpp
checkOutOfOrderFragments(tcpReassemblyData, sideIndex, false);
}

// handle case where this packet is FIN or RST
if (isFinOrRst)
// handle case where this packet is RST
if (isRst)
{
handleFinOrRst(tcpReassemblyData, sideIndex, flowKey, isRst);
}
Expand Down Expand Up @@ -580,6 +589,12 @@ namespace pcpp

foundSomething = true;

if(curTcpFrag->isFin)
{
PCPP_LOG_DEBUG("handle saved FIN flag on sequence match");
handleFinOrRst(tcpReassemblyData, sideIndex, curTcpFrag->flowKey, false);
}

continue;
}

Expand Down Expand Up @@ -616,6 +631,12 @@ namespace pcpp
}

foundSomething = true;

if(curTcpFrag->isFin)
{
PCPP_LOG_DEBUG("handle saved FIN flag on lower sequence");
handleFinOrRst(tcpReassemblyData, sideIndex, curTcpFrag->flowKey, false);
}
}
else
{
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4 changes: 2 additions & 2 deletions Tests/ExamplesTest/expected_output/x509_pcap_extract.txt
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
Packet count: 7451
TLS messages: 7229
TLS messages: 7216
TLS Flows: 334
TLS handshake messages: 1086
TLS handshake messages: 1085
Certificates parsed: 31
Certificates failed parsing: 0
Incomplete Certificates: 104
103 changes: 0 additions & 103 deletions Tests/Pcap++Test/PcapExamples/one_http_stream_fin2_output2.txt
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This file was deleted.

4 changes: 2 additions & 2 deletions Tests/Pcap++Test/Tests/TcpReassemblyTests.cpp
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Expand Up @@ -721,13 +721,13 @@ PTF_TEST_CASE(TestTcpReassemblyWithFIN_RST)
tcpReassemblyTest(packetStream, tcpReassemblyResults, true, false);

PTF_ASSERT_EQUAL(stats.size(), 1);
PTF_ASSERT_EQUAL(stats.begin()->second.numOfDataPackets, 5);
PTF_ASSERT_EQUAL(stats.begin()->second.numOfDataPackets, 6);
PTF_ASSERT_EQUAL(stats.begin()->second.numOfMessagesFromSide[0], 1);
PTF_ASSERT_EQUAL(stats.begin()->second.numOfMessagesFromSide[1], 1);
PTF_ASSERT_TRUE(stats.begin()->second.connectionsStarted);
PTF_ASSERT_TRUE(stats.begin()->second.connectionsEnded);
PTF_ASSERT_FALSE(stats.begin()->second.connectionsEndedManually);
expectedReassemblyData = readFileIntoString(std::string("PcapExamples/one_http_stream_fin2_output2.txt"));
expectedReassemblyData = readFileIntoString(std::string("PcapExamples/one_http_stream_fin2_output.txt"));
PTF_ASSERT_EQUAL(expectedReassemblyData, stats.begin()->second.reassembledData);
} // TestTcpReassemblyWithFIN_RST

Expand Down