feat(core): expose TLS ConnectionState (SNI) for DoQ (#8129)

DoQWriter previously stored only the QUIC stream, so plugins reading
TLS state via dns.ConnectionStater (e.g. for SNI-based routing or
auditing) could not see anything for DoQ connections, even
though the underlying QUIC connection carries a full tls.ConnectionState.

This change adds a *quic.Conn reference to DoQWriter and wires it in serveQUICStream.

It implements dns.ConnectionStater on *DoQWriter, returning the TLS
state from the underlying QUIC connection (mirrors the DoT behavior
that miekg/dns already provides for *tls.Conn)

Forwards ConnectionState through request.ScrubWriter, which wraps
every response writer before the plugin chain runs; the embedded
dns.ResponseWriter interface does not promote ConnectionState (it
belongs to a separate interface), so without this plugins would
still see nil for both DoQ and DoT

Signed-off-by: Nicholas Amorim <nicholas@santos.ee>

core/dnsserver/quic.go

@@ -1,6 +1,7 @@
 package dnsserver
 
 import (
+	"crypto/tls"
 	"encoding/binary"
 	"errors"
 	"net"
@@ -13,6 +14,7 @@ type DoQWriter struct {
 	localAddr  net.Addr
 	remoteAddr net.Addr
 	stream     *quic.Stream
+	conn       *quic.Conn
 	Msg        *dns.Msg
 	tsigStatus error
 }
@@ -68,3 +70,15 @@ func (w *DoQWriter) Hijack()                {}
 func (w *DoQWriter) LocalAddr() net.Addr    { return w.localAddr }
 func (w *DoQWriter) RemoteAddr() net.Addr   { return w.remoteAddr }
 func (w *DoQWriter) Network() string        { return "" }
+
+// ConnectionState implements the dns.ConnectionStater interface, exposing the
+// TLS state of the underlying QUIC connection (e.g. for plugins that need to
+// read the SNI ServerName). Mirrors the DoT behavior already provided by the
+// miekg/dns response writer.
+func (w *DoQWriter) ConnectionState() *tls.ConnectionState {
+	if w.conn == nil {
+		return nil
+	}
+	state := w.conn.ConnectionState().TLS
+	return &state
+}

core/dnsserver/quic_test.go

@@ -48,3 +48,11 @@ func TestDoQWriter_ResponseWriterMethods(t *testing.T) {
 		t.Errorf("RemoteAddr() = %v, want %v", addr, remoteAddr)
 	}
 }
+
+func TestDoQWriter_ConnectionStateNilConn(t *testing.T) {
+	writer := &DoQWriter{}
+
+	if state := writer.ConnectionState(); state != nil {
+		t.Errorf("ConnectionState() = %v, want nil when conn is unset", state)
+	}
+}

core/dnsserver/server_quic.go

@@ -221,6 +221,7 @@ func (s *ServerQUIC) serveQUICStream(stream *quic.Stream, conn *quic.Conn) {
 		localAddr:  conn.LocalAddr(),
 		remoteAddr: conn.RemoteAddr(),
 		stream:     stream,
+		conn:       conn,
 		Msg:        req,
 	}
 

core/dnsserver/server_quic_test.go

@@ -683,3 +683,125 @@ func TestServerQUIC_ServeQUIC_TSIGValidSigLeavesTsigStatusNil(t *testing.T) {
 		t.Fatal("ServeDNS() was not called")
 	}
 }
+
+// connectionStateCapturePlugin records the *tls.ConnectionState observed via
+// the dns.ConnectionStater interface implemented by the DoQ response writer.
+type connectionStateCapturePlugin struct {
+	t      *testing.T
+	called chan struct{}
+	state  chan *tls.ConnectionState
+}
+
+func (p connectionStateCapturePlugin) Name() string { return "connection-state-capture" }
+
+func (p connectionStateCapturePlugin) ServeDNS(_ context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	p.t.Helper()
+
+	if cs, ok := w.(dns.ConnectionStater); ok {
+		p.state <- cs.ConnectionState()
+	} else {
+		p.state <- nil
+	}
+	if p.called != nil {
+		p.called <- struct{}{}
+	}
+
+	m := new(dns.Msg)
+	m.SetReply(r)
+	if err := w.WriteMsg(m); err != nil {
+		p.t.Fatalf("WriteMsg() failed: %v", err)
+	}
+	return dns.RcodeSuccess, nil
+}
+
+func TestServerQUIC_ServeQUIC_ConnectionStateExposesSNI(t *testing.T) {
+	const sni = "doq.example.com"
+
+	called := make(chan struct{}, 1)
+	stateCh := make(chan *tls.ConnectionState, 1)
+
+	config := testConfig("quic", connectionStateCapturePlugin{
+		t:      t,
+		called: called,
+		state:  stateCh,
+	})
+	config.TLSConfig = mustMakeQUICServerTLSConfig(t)
+
+	server, err := NewServerQUIC(transport.QUIC+"://127.0.0.1:0", []*Config{config})
+	if err != nil {
+		t.Fatalf("NewServerQUIC() failed: %v", err)
+	}
+
+	pc, err := server.ListenPacket()
+	if err != nil {
+		t.Fatalf("ListenPacket() failed: %v", err)
+	}
+	defer pc.Close()
+
+	serveErrCh := make(chan error, 1)
+	go func() {
+		serveErrCh <- server.ServeQUIC()
+	}()
+
+	defer func() {
+		_ = server.Stop()
+		select {
+		case <-serveErrCh:
+		case <-time.After(2 * time.Second):
+		}
+	}()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	clientTLS := mustMakeQUICClientTLSConfig()
+	clientTLS.ServerName = sni
+
+	conn, err := quic.DialAddr(ctx, pc.LocalAddr().String(), clientTLS, &quic.Config{})
+	if err != nil {
+		t.Fatalf("quic.DialAddr() failed: %v", err)
+	}
+	defer conn.CloseWithError(DoQCodeNoError, "")
+
+	stream, err := conn.OpenStreamSync(ctx)
+	if err != nil {
+		t.Fatalf("OpenStreamSync() failed: %v", err)
+	}
+
+	q := new(dns.Msg)
+	q.SetQuestion("example.com.", dns.TypeA)
+	q.Id = 0
+	wire, err := q.Pack()
+	if err != nil {
+		t.Fatalf("dns.Msg.Pack() failed: %v", err)
+	}
+
+	if _, err := stream.Write(AddPrefix(wire)); err != nil {
+		t.Fatalf("stream.Write() failed: %v", err)
+	}
+	if err := stream.Close(); err != nil {
+		t.Fatalf("stream.Close() failed: %v", err)
+	}
+
+	if _, err := readDOQMessage(stream); err != nil {
+		t.Fatalf("readDOQMessage() failed: %v", err)
+	}
+
+	select {
+	case <-called:
+	case <-time.After(5 * time.Second):
+		t.Fatal("ServeDNS() was not called")
+	}
+
+	select {
+	case state := <-stateCh:
+		if state == nil {
+			t.Fatal("ConnectionState() = nil, want non-nil TLS state for DoQ request")
+		}
+		if state.ServerName != sni {
+			t.Errorf("ConnectionState().ServerName = %q, want %q", state.ServerName, sni)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("did not receive connection state from plugin")
+	}
+}