Merge commit from fork

DoH, DoQ, and DNS-over-gRPC unpack messages without the acceptance
checks used by UDP and TCP. An unauthenticated request with a large
QDCOUNT can therefore force excessive allocations while names are
decoded and exhaust server memory.

Enforce the same request policy across all server transports.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/pkg/dnsutil/message.go

@@ -0,0 +1,24 @@
+package dnsutil
+
+import (
+	"encoding/binary"
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+var errRequestRejected = errors.New("dns request rejected")
+
+// UnpackRequest unpacks a request after applying the default miekg/dns request policy.
+func UnpackRequest(msg []byte) (*dns.Msg, error) {
+	var header dns.Header
+	if _, err := binary.Decode(msg, binary.BigEndian, &header); err != nil {
+		return nil, dns.ErrBuf
+	}
+	if dns.DefaultMsgAcceptFunc(header) != dns.MsgAccept {
+		return nil, errRequestRejected
+	}
+
+	request := new(dns.Msg)
+	return request, request.Unpack(msg)
+}

plugin/pkg/dnsutil/message_test.go

@@ -0,0 +1,29 @@
+package dnsutil
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestUnpackRequest(t *testing.T) {
+	request := new(dns.Msg)
+	request.SetQuestion("example.org.", dns.TypeA)
+
+	wire, err := request.Pack()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := UnpackRequest(wire); err != nil {
+		t.Fatalf("UnpackRequest() rejected a valid request: %v", err)
+	}
+
+	request.Question = append(request.Question, request.Question[0])
+	wire, err = request.Pack()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := UnpackRequest(wire); err == nil {
+		t.Fatal("UnpackRequest() accepted multiple questions")
+	}
+}