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/dns/nameserver_fakedns.go

@@ -0,0 +1,43 @@
+// +build !confonly
+
+package dns
+
+import (
+	"context"
+
+	"github.com/xtls/xray-core/core"
+	"github.com/xtls/xray-core/common/net"
+	"github.com/xtls/xray-core/features/dns"
+)
+
+type FakeDNSServer struct {
+	fakeDNSEngine dns.FakeDNSEngine
+}
+
+func NewFakeDNSServer() *FakeDNSServer {
+	return &FakeDNSServer{}
+}
+
+func (FakeDNSServer) Name() string {
+	return "FakeDNS"
+}
+
+func (f *FakeDNSServer) QueryIP(ctx context.Context, domain string, _ dns.IPOption) ([]net.IP, error) {
+	if f.fakeDNSEngine == nil {
+		if err := core.RequireFeatures(ctx, func(fd dns.FakeDNSEngine) {
+			f.fakeDNSEngine = fd
+		}); err != nil {
+			return nil, newError("Unable to locate a fake DNS Engine").Base(err).AtError()
+		}
+	}
+	ips := f.fakeDNSEngine.GetFakeIPForDomain(domain)
+
+	netIP := toNetIP(ips)
+	if netIP == nil {
+		return nil, newError("Unable to convert IP to net ip").AtError()
+	}
+
+	newError(f.Name(), " got answer: ", domain, " -> ", ips).AtInfo().WriteToLog()
+
+	return netIP, nil
+}