fixes

internal/mtprotoedge/exchange_compat.go

@@ -38,6 +38,7 @@ func (s *Server) runServerExchange(ctx context.Context, conn transport.Conn) (ex
 		timeout:   exchange.DefaultTimeout,
 		key:       s.key,
 		dc:        s.dc,
+		strictDC:  s.strictDC,
 		log:       s.log.Named("exchange"),
 		rng:       compatServerRNG{rand: s.rand},
 		commitKey: s.commitExchangeAuthKey,
@@ -65,6 +66,7 @@ type serverExchangeCompat struct {
 	timeout   time.Duration
 	key       exchange.PrivateKey
 	dc        int
+	strictDC  bool
 	log       *zap.Logger
 	rng       compatServerRNG
 	commitKey func(context.Context, exchange.ServerExchangeResult, int) error
@@ -345,10 +347,23 @@ func (s serverExchangeCompat) validatePQInnerDataDC(d mt.PQInnerDataClass) error
 	switch innerDataDC := d.(type) {
 	case *mt.PQInnerDataDC:
 		if innerDataDC.DC != s.dc {
+			if !s.strictDC {
+				// Lenient by default (Options.StrictDC doc has the full
+				// rationale): telesrv is a single physical backend, and the
+				// OwpenGram client forks deliberately alias dc_id 1..5 to this
+				// one server, so a client-chosen dc_id that isn't our
+				// configured DC is expected, not an error. dc_id plays no
+				// role in key derivation, so accepting it doesn't weaken the
+				// exchange.
+				s.log.Debug("Accepted permanent auth key DC mismatch (lenient mode)",
+					zap.Int("server_dc", s.dc),
+					zap.Int("client_dc", innerDataDC.DC))
+				return nil
+			}
 			return wrongDCError(s.dc, innerDataDC.DC)
 		}
 	case *mt.PQInnerDataTempDC:
-		if !sameDCByAbs(innerDataDC.DC, s.dc) {
+		if !sameDCByAbs(innerDataDC.DC, s.dc) && s.strictDC {
 			return wrongDCError(s.dc, innerDataDC.DC)
 		}
 		if innerDataDC.DC < 0 {

internal/mtprotoedge/exchange_test.go

@@ -366,8 +366,8 @@ func TestKeyExchangeAcceptsAndroidMediaTempNegativeDC(t *testing.T) {
 	}
 }
 
-func TestKeyExchangeRejectsWrongNegativeTempDC(t *testing.T) {
-	ex := serverExchangeCompat{dc: 2, log: zaptest.NewLogger(t)}
+func TestKeyExchangeRejectsWrongNegativeTempDCWhenStrict(t *testing.T) {
+	ex := serverExchangeCompat{dc: 2, strictDC: true, log: zaptest.NewLogger(t)}
 	err := ex.validatePQInnerDataDC(&mt.PQInnerDataTempDC{DC: -3})
 	var exErr *exchange.ServerExchangeError
 	if !errors.As(err, &exErr) {
@@ -378,6 +378,38 @@ func TestKeyExchangeRejectsWrongNegativeTempDC(t *testing.T) {
 	}
 }
 
+// TestKeyExchangeAcceptsMismatchedDCByDefault asserts that, in the default
+// lenient mode, neither permanent nor temp key exchange requires dc_id to
+// equal the server's configured DC. telesrv is always a single physical
+// backend; the OwpenGram client forks deliberately alias dc_id 1..5 to it
+// (see Options.StrictDC doc), so a mismatched client-chosen dc_id must not be
+// rejected — doing so previously broke every account whose client picked a
+// starting dc_id other than the server's.
+func TestKeyExchangeAcceptsMismatchedDCByDefault(t *testing.T) {
+	ex := serverExchangeCompat{dc: 2, log: zaptest.NewLogger(t)}
+	if err := ex.validatePQInnerDataDC(&mt.PQInnerDataDC{DC: 3}); err != nil {
+		t.Fatalf("permanent DC mismatch: err = %v, want nil (lenient by default)", err)
+	}
+	if err := ex.validatePQInnerDataDC(&mt.PQInnerDataTempDC{DC: -3}); err != nil {
+		t.Fatalf("temp DC mismatch: err = %v, want nil (lenient by default)", err)
+	}
+}
+
+// TestKeyExchangeRejectsMismatchedPermanentDCWhenStrict asserts that
+// strictDC=true still enforces exact DC-ID equality for permanent-key
+// exchange (kept for a hypothetical future real multi-DC deployment).
+func TestKeyExchangeRejectsMismatchedPermanentDCWhenStrict(t *testing.T) {
+	ex := serverExchangeCompat{dc: 2, strictDC: true, log: zaptest.NewLogger(t)}
+	err := ex.validatePQInnerDataDC(&mt.PQInnerDataDC{DC: 3})
+	var exErr *exchange.ServerExchangeError
+	if !errors.As(err, &exErr) {
+		t.Fatalf("err = %T %v, want ServerExchangeError", err, err)
+	}
+	if exErr.Code != codec.CodeWrongDC {
+		t.Fatalf("error code = %d, want %d", exErr.Code, codec.CodeWrongDC)
+	}
+}
+
 func TestDecodeCompatPQInnerDataTemp(t *testing.T) {
 	want := mt.PQInnerData{
 		Pq:          []byte{0x0f},

internal/mtprotoedge/server.go

@@ -301,6 +301,22 @@ type Options struct {
 
 	// DC 是本 server 的 DC ID。默认 2。
 	DC int
+	// StrictDC turns on exact DC-ID validation for the permanent-key exchange
+	// (default off = lenient). telesrv is always a single physical backend —
+	// there is no real multi-DC federation behind it — but the OwpenGram
+	// client forks intentionally run in "single-server backend" mode, where
+	// dc_id 1..5 all alias to this one server (see owpengram_servers.cpp /
+	// ApplyServerToDcOptions in the desktop client) so that any old data
+	// referencing a specific dc_id still resolves correctly. When tdesktop
+	// adds a new local account it picks its own starting dc_id (its usual
+	// multi-DC load-spreading behavior, unrelated to which physical server
+	// it's actually talking to) — that choice is not guaranteed to equal our
+	// configured DC. Strict validation would reject those accounts with
+	// "-444 wrong dc_id" even though they are connecting to the right (and
+	// only) server; dc_id is a client-side routing label here, not part of
+	// key derivation, so accepting the mismatch does not weaken the exchange.
+	// The switch exists for a hypothetical future real multi-DC deployment.
+	StrictDC bool
 	// RSAKey 是 server RSA 私钥，用于密钥交换。nil 时无法完成握手。
 	RSAKey *rsa.PrivateKey
 	// AuthKeys 持久化 auth key。默认内存实现。
@@ -480,6 +496,7 @@ type Server struct {
 	outboundScratchPool      *outboundScratchPool
 
 	dc        int
+	strictDC  bool
 	key       exchange.PrivateKey
 	authKeys  store.AuthKeyStore
 	conns     *SessionManager
@@ -531,6 +548,7 @@ func New(opts Options) *Server {
 		outboundControlBudget:    newOutboundTrackedBudget(defaultOutboundControlMaxBytes),
 		outboundScratchPool:      newOutboundScratchPool(opts.OutboundWriteGlobalMaxBytes),
 		dc:                       opts.DC,
+		strictDC:                 opts.StrictDC,
 		key:                      exchange.PrivateKey{RSA: opts.RSAKey},
 		authKeys:                 opts.AuthKeys,
 		conns:                    conns,