diff --git a/checker/checker.go b/checker/checker.go
index 6ca8968..eea975c 100644
--- a/checker/checker.go
+++ b/checker/checker.go
@@ -642,6 +642,10 @@ func studentChecker(std *adlin.Student, also_check_matrix bool, offline bool) {
 					if errreg := std.RegisterChallengeError(CheckMap[tunnel_version][DNSDelegation], fmt.Errorf("%s: empty response from the server%s", std.MyDelegatedDomain(), dnsAt)); errreg != nil {
 						log.Printf("Unable to register challenge error for %s: %s\n", std.Login, errreg)
 					}
+				} else if tunnel_version == 3 && dnsIP == stdIP {
+					if errreg := std.RegisterChallengeError(CheckMap[tunnel_version][DNSDelegation], fmt.Errorf("%s: you shouldn't use DNAT on IPv6 (NAT on IPv6 is bad); instead, point your GLUE record to nsauth directly", std.MyDelegatedDomain())); errreg != nil {
+						log.Printf("Unable to register challenge error for %s: %s\n", std.Login, errreg)
+					}
 				} else {
 					if verbose {
 						log.Printf("%s just unlocked DNS challenge\n", std.Login)