Add Fake DNS support (#309)

Co-authored-by: Xiaokang Wang <xiaokangwang@outlook.com>
Co-authored-by: loyalsoldier <10487845+Loyalsoldier@users.noreply.github.com>
Co-authored-by: kslr <kslrwang@gmail.com>

app/dispatcher/fakednssniffer.go

@@ -0,0 +1,51 @@
+// +build !confonly
+
+package dispatcher
+
+import (
+	"context"
+
+	"github.com/xtls/xray-core/core"
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/common/net"
+	"github.com/xtls/xray-core/common/session"
+	"github.com/xtls/xray-core/features/dns"
+)
+
+// newFakeDNSSniffer Create a Fake DNS metadata sniffer
+func newFakeDNSSniffer(ctx context.Context) (protocolSnifferWithMetadata, error) {
+	var fakeDNSEngine dns.FakeDNSEngine
+	err := core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) {
+		fakeDNSEngine = fdns
+	})
+	if err != nil {
+		return protocolSnifferWithMetadata{}, err
+	}
+	if fakeDNSEngine == nil {
+		errNotInit := newError("FakeDNSEngine is not initialized, but such a sniffer is used").AtError()
+		return protocolSnifferWithMetadata{}, errNotInit
+	}
+	return protocolSnifferWithMetadata{protocolSniffer: func(ctx context.Context, bytes []byte) (SniffResult, error) {
+		Target := session.OutboundFromContext(ctx).Target
+		if Target.Network == net.Network_TCP || Target.Network == net.Network_UDP {
+			domainFromFakeDNS := fakeDNSEngine.GetDomainFromFakeDNS(Target.Address)
+			if domainFromFakeDNS != "" {
+				newError("fake dns got domain: ", domainFromFakeDNS, " for ip: ", Target.Address.String()).WriteToLog(session.ExportIDToError(ctx))
+				return &fakeDNSSniffResult{domainName: domainFromFakeDNS}, nil
+			}
+		}
+		return nil, common.ErrNoClue
+	}, metadataSniffer: true}, nil
+}
+
+type fakeDNSSniffResult struct {
+	domainName string
+}
+
+func (fakeDNSSniffResult) Protocol() string {
+	return "fakedns"
+}
+
+func (f fakeDNSSniffResult) Domain() string {
+	return f.domainName
+}