plugin/rewrite: fix domain length validation (#2995)

* unit test

* fix domain length validation

* code optimization

* remove unit test

2 files changed, 3 insertions, 19 deletions

diff --git a/plugin/rewrite/name.go b/plugin/rewrite/name.go
index 86a5f1ecd..e1c2a1114 100644
--- a/plugin/rewrite/name.go
+++ b/plugin/rewrite/name.go
@@ -9,8 +9,6 @@ import (
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
-
-	"github.com/miekg/dns"
 )
 
 type exactNameRule struct {
@@ -264,19 +262,6 @@ func (rule *substringNameRule) GetResponseRule() ResponseRule { return ResponseR
 // GetResponseRule return a rule to rewrite the response with.
 func (rule *regexNameRule) GetResponseRule() ResponseRule { return rule.ResponseRule }
 
-// validName returns true if s is valid domain name and shorter than 256 characters.
-func validName(s string) bool {
-	_, ok := dns.IsDomainName(s)
-	if !ok {
-		return false
-	}
-	if len(dns.Name(s).String()) > 255 {
-		return false
-	}
-
-	return true
-}
-
 // hasClosingDot return true if s has a closing dot at the end.
 func hasClosingDot(s string) bool {
 	if strings.HasSuffix(s, ".") {
diff --git a/plugin/rewrite/rewrite.go b/plugin/rewrite/rewrite.go
index 906523326..e72e975b2 100644
--- a/plugin/rewrite/rewrite.go
+++ b/plugin/rewrite/rewrite.go
@@ -44,11 +44,10 @@ func (rw Rewrite) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 	for _, rule := range rw.Rules {
 		switch result := rule.Rewrite(ctx, state); result {
 		case RewriteDone:
-			if !validName(state.Req.Question[0].Name) {
-				x := state.Req.Question[0].Name
-				log.Errorf("Invalid name after rewrite: %s", x)
+			if _, ok := dns.IsDomainName(state.Req.Question[0].Name); !ok {
+				err := fmt.Errorf("invalid name after rewrite: %s", state.Req.Question[0].Name)
 				state.Req.Question[0] = wr.originalQuestion
-				return dns.RcodeServerFailure, fmt.Errorf("invalid name after rewrite: %s", x)
+				return dns.RcodeServerFailure, err
 			}
 			respRule := rule.GetResponseRule()
 			if respRule.Active {