Revert diagnostic ServeDNS log line

plugin.go

@@ -75,7 +75,6 @@ func (p *RFC2136) Name() string { return "rfc2136" }
 //	Anything else  → pass through to Next (the auto plugin handles
 //	                 queries against the zone files we maintain).
 func (p *RFC2136) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	log.Infof("ServeDNS: opcode=%d (UPDATE=5) qcount=%d", r.Opcode, len(r.Question))
 	if r.Opcode == dns.OpcodeUpdate {
 		if err := p.checkTSIG(w, r); err != nil {
 			log.Warningf("UPDATE rejected: %v", err)

setup.go

@@ -11,6 +11,7 @@ import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/miekg/dns"
 )
 
 // log is the package logger, scoped so messages are prefixed `[rfc2136]`.
@@ -18,6 +19,59 @@ var log = clog.NewWithPlugin("rfc2136")
 
 func init() {
 	plugin.Register("rfc2136", setup)
+
+	// miekg/dns's default MsgAcceptFunc rejects UPDATE opcode messages
+	// with NOTIMP before they ever reach the plugin chain (see the
+	// comment in miekg/dns/acceptfunc.go: "Don't allow dynamic updates,
+	// because then the sections can contain a whole bunch of RRs").
+	//
+	// CoreDNS constructs its dns.Server instances without setting a
+	// per-server MsgAcceptFunc, so the package-level default is the
+	// one that runs. We override it here at plugin init() time -- well
+	// before any dns.Server starts listening -- to permit UPDATE
+	// through. The plugin itself does proper validation in the
+	// UPDATE handler, so opening this gate doesn't lower security.
+	dns.DefaultMsgAcceptFunc = msgAcceptFunc
+}
+
+// msgAcceptFunc mirrors miekg/dns's defaultMsgAcceptFunc but additionally
+// allows OpcodeUpdate. For UPDATE messages, the conservative Ancount/
+// Nscount limits in the default function don't apply -- per RFC 2136
+// those sections (Prerequisite / Update) can carry many RRs.
+func msgAcceptFunc(dh dns.Header) dns.MsgAcceptAction {
+	// Responses are silently ignored regardless of opcode (default behaviour).
+	if isResponse := dh.Bits&0x8000 != 0; isResponse {
+		return dns.MsgIgnore
+	}
+
+	opcode := int(dh.Bits>>11) & 0xF
+	switch opcode {
+	case dns.OpcodeQuery, dns.OpcodeNotify, dns.OpcodeUpdate:
+		// allowed
+	default:
+		return dns.MsgRejectNotImplemented
+	}
+
+	if dh.Qdcount != 1 {
+		return dns.MsgReject
+	}
+
+	// UPDATE messages legitimately carry multiple RRs in the
+	// Prerequisite (Ancount) and Update (Nscount) sections -- skip the
+	// "exactly 1" check that the default function applies for queries.
+	if opcode != dns.OpcodeUpdate {
+		if dh.Ancount > 1 {
+			return dns.MsgReject
+		}
+		if dh.Nscount > 1 {
+			return dns.MsgReject
+		}
+	}
+
+	if dh.Arcount > 2 {
+		return dns.MsgReject
+	}
+	return dns.MsgAccept
 }
 
 // setup is invoked by the CoreDNS plugin registry once per Corefile