fix(edns): modify SourceNetmask for IPv6 (#260)

Google public dns required ipv6 edns `SourceNetmask` to be no more than 56.

Author: tuzi3040

core/common/edns.go

@@ -33,10 +33,10 @@ func SetEDNSClientSubnet(m *dns.Msg, ip string, isNoCookie bool) {
 		nes.Address = net.ParseIP(ip)
 		if nes.Address.To4() != nil {
 			nes.Family = 1         // 1 for IPv4 source address, 2 for IPv6
-			nes.SourceNetmask = 24 // 24 for IPV4, 64 for IPv6
+			nes.SourceNetmask = 24 // 24 for IPV4, 56 for IPv6
 		} else {
 			nes.Family = 2          // 1 for IPv4 source address, 2 for IPv6
-			nes.SourceNetmask = 64 // 24 for IPV4, 64 for IPv6
+			nes.SourceNetmask = 56 // 24 for IPV4, 56 for IPv6
 		}
 		nes.SourceScope = 0
 		if es != nil && es.Address.IsUnspecified() {