Feature: add fallback filters (#105)

dns/filters.go

@@ -0,0 +1,26 @@
+package dns
+
+import "net"
+
+type fallbackFilter interface {
+	Match(net.IP) bool
+}
+
+type geoipFilter struct{}
+
+func (gf *geoipFilter) Match(ip net.IP) bool {
+	if mmdb == nil {
+		return false
+	}
+
+	record, _ := mmdb.Country(ip)
+	return record.Country.IsoCode == "CN" || record.Country.IsoCode == ""
+}
+
+type ipnetFilter struct {
+	ipnet *net.IPNet
+}
+
+func (inf *ipnetFilter) Match(ip net.IP) bool {
+	return inf.ipnet.Contains(ip)
+}