From 530b0a5ff2ad68cc0421f10dd93568945cc671c9 Mon Sep 17 00:00:00 2001
From: Ville Vesilehto <ville@vesilehto.fi>
Date: Thu, 16 Jul 2026 03:54:46 +0300
Subject: [PATCH] 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>
---
 core/dnsserver/server_grpc.go      |  4 ++--
 core/dnsserver/server_quic.go      |  4 ++--
 plugin/pkg/dnsutil/message.go      | 24 ++++++++++++++++++++++++
 plugin/pkg/dnsutil/message_test.go | 29 +++++++++++++++++++++++++++++
 plugin/pkg/doh/doh.go              | 13 +++++++++----
 5 files changed, 66 insertions(+), 8 deletions(-)
 create mode 100644 plugin/pkg/dnsutil/message.go
 create mode 100644 plugin/pkg/dnsutil/message_test.go

diff --git a/core/dnsserver/server_grpc.go b/core/dnsserver/server_grpc.go
index 42f54eaff..972a3b2c0 100644
--- a/core/dnsserver/server_grpc.go
+++ b/core/dnsserver/server_grpc.go
@@ -9,6 +9,7 @@ import (
 
 	"github.com/coredns/caddy"
 	"github.com/coredns/coredns/pb"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	"github.com/coredns/coredns/plugin/pkg/reuseport"
 	"github.com/coredns/coredns/plugin/pkg/transport"
 
@@ -177,8 +178,7 @@ func (s *ServergRPC) Query(ctx context.Context, in *pb.DnsPacket) (*pb.DnsPacket
 	if len(in.GetMsg()) > dns.MaxMsgSize {
 		return nil, fmt.Errorf("dns message exceeds size limit: %d", len(in.GetMsg()))
 	}
-	msg := new(dns.Msg)
-	err := msg.Unpack(in.GetMsg())
+	msg, err := dnsutil.UnpackRequest(in.GetMsg())
 	if err != nil {
 		return nil, err
 	}
diff --git a/core/dnsserver/server_quic.go b/core/dnsserver/server_quic.go
index aff71e626..9c89334b9 100644
--- a/core/dnsserver/server_quic.go
+++ b/core/dnsserver/server_quic.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/coredns/coredns/plugin/metrics/vars"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 	cproxyproto "github.com/coredns/coredns/plugin/pkg/proxyproto"
 	"github.com/coredns/coredns/plugin/pkg/reuseport"
@@ -211,8 +212,7 @@ func (s *ServerQUIC) serveQUICStream(stream *quic.Stream, conn *quic.Conn) {
 		return
 	}
 
-	req := &dns.Msg{}
-	err = req.Unpack(buf)
+	req, err := dnsutil.UnpackRequest(buf)
 	if err != nil {
 		clog.Debugf("unpacking quic packet: %s", err)
 		s.closeQUICConn(conn, DoQCodeProtocolError)
diff --git a/plugin/pkg/dnsutil/message.go b/plugin/pkg/dnsutil/message.go
new file mode 100644
index 000000000..c2da9dba5
--- /dev/null
+++ b/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)
+}
diff --git a/plugin/pkg/dnsutil/message_test.go b/plugin/pkg/dnsutil/message_test.go
new file mode 100644
index 000000000..a913a5165
--- /dev/null
+++ b/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")
+	}
+}
diff --git a/plugin/pkg/doh/doh.go b/plugin/pkg/doh/doh.go
index 45d25a0f3..a5c46202d 100644
--- a/plugin/pkg/doh/doh.go
+++ b/plugin/pkg/doh/doh.go
@@ -9,6 +9,8 @@ import (
 	"net/http"
 	"strings"
 
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+
 	"github.com/miekg/dns"
 )
 
@@ -106,7 +108,12 @@ func RequestToMsgWire(req *http.Request) (*dns.Msg, []byte, error) {
 // requestToMsgPost extracts the dns message from the request body.
 func requestToMsgPost(req *http.Request) (*dns.Msg, []byte, error) {
 	defer req.Body.Close()
-	return toMsgWire(req.Body)
+	buf, err := io.ReadAll(http.MaxBytesReader(nil, req.Body, maxDNSQuerySize))
+	if err != nil {
+		return nil, nil, err
+	}
+	m, err := dnsutil.UnpackRequest(buf)
+	return m, buf, err
 }
 
 const maxDNSQuerySize = 65536
@@ -149,9 +156,7 @@ func base64ToMsgWire(b64 string) (*dns.Msg, []byte, error) {
 		return nil, nil, err
 	}
 
-	m := new(dns.Msg)
-	err = m.Unpack(buf)
-
+	m, err := dnsutil.UnpackRequest(buf)
 	return m, buf, err
 }