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>

infra/conf/fakedns.go

@@ -0,0 +1,65 @@
+package conf
+
+import (
+	"github.com/golang/protobuf/proto"
+	"github.com/xtls/xray-core/app/dns/fakedns"
+)
+
+type FakeDNSConfig struct {
+	IPPool  string `json:"ipPool"`
+	LruSize int64  `json:"poolSize"`
+}
+
+func (f FakeDNSConfig) Build() (proto.Message, error) {
+	return &fakedns.FakeDnsPool{
+		IpPool:  f.IPPool,
+		LruSize: f.LruSize,
+	}, nil
+}
+
+type FakeDNSPostProcessingStage struct{}
+
+func (FakeDNSPostProcessingStage) Process(conf *Config) error {
+	var fakeDNSInUse bool
+
+	if conf.DNSConfig != nil {
+		for _, v := range conf.DNSConfig.Servers {
+			if v.Address.Family().IsDomain() {
+				if v.Address.Domain() == "fakedns" {
+					fakeDNSInUse = true
+				}
+			}
+		}
+	}
+
+	if fakeDNSInUse {
+		if conf.FakeDNS == nil {
+			// Add a Fake DNS Config if there is none
+			conf.FakeDNS = &FakeDNSConfig{
+				IPPool:  "240.0.0.0/8",
+				LruSize: 65535,
+			}
+		}
+		found := false
+		// Check if there is a Outbound with necessary sniffer on
+		var inbounds []InboundDetourConfig
+
+		if len(conf.InboundConfigs) > 0 {
+			inbounds = append(inbounds, conf.InboundConfigs...)
+		}
+		for _, v := range inbounds {
+			if v.SniffingConfig != nil && v.SniffingConfig.Enabled && v.SniffingConfig.DestOverride != nil {
+				for _, dov := range *v.SniffingConfig.DestOverride {
+					if dov == "fakedns" {
+						found = true
+					}
+				}
+			}
+		}
+		if !found {
+			newError("Defined Fake DNS but haven't enabled fake dns sniffing at any inbound.").AtWarning().WriteToLog()
+		}
+	}
+
+	return nil
+}