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fix: CoAP Ping response uses RST instead of ACK per RFC 7252 §4.2 #655

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fix: CoAP Ping response uses RST instead of ACK per RFC 7252 §4.2 #655

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d1e0008
fix: CoAP Ping response uses RST instead of ACK per RFC 7252 §4.2
sandrolain Apr 1, 2026
1fb1d0a
fix: simplify SetReadDeadline call in TestConnPingResponseIsReset
sandrolain Apr 20, 2026
35aadb0
Merge branch 'master' into fix/coap-ping-rst
sandrolain Apr 20, 2026
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4 changes: 3 additions & 1 deletion udp/client/conn.go
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Original file line number Diff line number Diff line change
Expand Up @@ -647,7 +647,9 @@ func (cc *Conn) sendPong(w *responsewriter.ResponseWriter[*Conn], r *pool.Messag
cc.errors(fmt.Errorf("cannot send pong response: %w", err))
}
if r.Type() == message.Confirmable {
w.Message().SetType(message.Acknowledgement)
// RFC 7252 §4.2: An Empty Confirmable message (CoAP Ping) is rejected
// with a matching Reset message.
w.Message().SetType(message.Reset)
w.Message().SetMessageID(r.MessageID())
} else {
if w.Message().Type() != message.Reset {
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54 changes: 54 additions & 0 deletions udp/client/conn_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ import (
"errors"
"io"
"log"
"net"
"sync"
"testing"
"time"
Expand Down Expand Up @@ -833,6 +834,59 @@ func TestConnPing(t *testing.T) {
require.NoError(t, err)
}

func TestConnPingResponseIsReset(t *testing.T) {
l, err := coapNet.NewListenUDP("udp", "")
require.NoError(t, err)
defer func() {
errC := l.Close()
require.NoError(t, errC)
}()
var wg sync.WaitGroup
defer wg.Wait()

s := udp.NewServer()
defer s.Stop()

wg.Add(1)
go func() {
defer wg.Done()
errS := s.Serve(l)
assert.NoError(t, errS)
}()

// Send a raw CoAP Ping (Empty CON) and verify the response is RST per RFC 7252 §4.2
conn, errD := net.Dial("udp", l.LocalAddr().String())
require.NoError(t, errD)
defer func() {
errC := conn.Close()
require.NoError(t, errC)
}()

// Build empty CON message: Ver=1, T=CON(0), TKL=0, Code=0.00, MID=1234
ping := []byte{
0x40, // Ver=1, T=CON, TKL=0
0x00, // Code=0.00 (Empty)
0x04, 0xd2, // MID=1234
}
_, errW := conn.Write(ping)
require.NoError(t, errW)

err = conn.(*net.UDPConn).SetReadDeadline(time.Now().Add(time.Second * 2))
require.NoError(t, err)
buf := make([]byte, 1024)
n, errR := conn.Read(buf)
require.NoError(t, errR)
require.GreaterOrEqual(t, n, 4)

// Parse response: verify type is RST (3) and MID matches
respType := message.Type((buf[0] >> 4) & 0x3)
respCode := codes.Code(buf[1])
respMID := uint16(buf[2])<<8 | uint16(buf[3])
require.Equal(t, message.Reset, respType, "CoAP Ping response must be RST per RFC 7252 §4.2")
require.Equal(t, codes.Empty, respCode, "CoAP Ping response code must be 0.00 (Empty)")
require.Equal(t, uint16(0x04d2), respMID, "CoAP Ping response MID must match request MID")
}

func TestConnRequestMonitorCloseConnection(t *testing.T) {
l, err := coapNet.NewListenUDP("udp", "")
require.NoError(t, err)
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