Add field keywords to rewrite middleware (#497)

* Require Field for rewrite rules

* review feedback changes

* fix ut

* fix typo, add warning message

middleware/rewrite/class.go

@@ -0,0 +1,31 @@
+// Package rewrite is middleware for rewriting requests internally to something different.
+package rewrite
+
+import (
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+// ClassRule is a class rewrite rule.
+type ClassRule struct {
+	fromClass, toClass uint16
+}
+
+// Initializer
+func (rule ClassRule) New(args ...string) Rule {
+	from, to := args[0], strings.Join(args[1:], " ")
+	return &ClassRule{dns.StringToClass[from], dns.StringToClass[to]}
+
+}
+
+// Rewrite rewrites the the current request.
+func (rule ClassRule) Rewrite(r *dns.Msg) Result {
+	if rule.fromClass > 0 && rule.toClass > 0 {
+		if r.Question[0].Qclass == rule.fromClass {
+			r.Question[0].Qclass = rule.toClass
+			return RewriteDone
+		}
+	}
+	return RewriteIgnored
+}