Add field keywords to rewrite middleware (#497)

* Require Field for rewrite rules

* review feedback changes

* fix ut

* fix typo, add warning message

middleware/rewrite/rewrite.go

@@ -2,10 +2,7 @@
 package rewrite
 
 import (
-	"strings"
-
 	"github.com/miekg/coredns/middleware"
-
 	"github.com/miekg/dns"
 	"golang.org/x/net/context"
 )
@@ -59,64 +56,6 @@ func (rw Rewrite) Name() string { return "rewrite" }
 type Rule interface {
 	// Rewrite rewrites the internal location of the current request.
 	Rewrite(*dns.Msg) Result
-}
-
-// SimpleRule is a simple rewrite rule. If the From and To look like a type
-// the type of the request is rewritten, otherwise the name is.
-// Note: TSIG signed requests will be invalid.
-type SimpleRule struct {
-	From, To           string
-	fromType, toType   uint16
-	fromClass, toClass uint16
-}
-
-// NewSimpleRule creates a new Simple Rule
-func NewSimpleRule(from, to string) SimpleRule {
-	tpf := dns.StringToType[from]
-	tpt := dns.StringToType[to]
-
-	// ANY is both a type and class, ANY class rewritting is way more less frequent
-	// so we default to ANY as a type.
-	clf := dns.StringToClass[from]
-	clt := dns.StringToClass[to]
-	if from == "ANY" {
-		clf = 0
-		clt = 0
-	}
-
-	// It's only a type/class if uppercase is used.
-	if from != strings.ToUpper(from) {
-		tpf = 0
-		clf = 0
-		from = middleware.Name(from).Normalize()
-	}
-	if to != strings.ToUpper(to) {
-		tpt = 0
-		clt = 0
-		to = middleware.Name(to).Normalize()
-	}
-	return SimpleRule{From: from, To: to, fromType: tpf, toType: tpt, fromClass: clf, toClass: clt}
-}
-
-// Rewrite rewrites the the current request.
-func (s SimpleRule) Rewrite(r *dns.Msg) Result {
-	if s.fromType > 0 && s.toType > 0 {
-		if r.Question[0].Qtype == s.fromType {
-			r.Question[0].Qtype = s.toType
-			return RewriteDone
-		}
-	}
-
-	if s.fromClass > 0 && s.toClass > 0 {
-		if r.Question[0].Qclass == s.fromClass {
-			r.Question[0].Qclass = s.toClass
-			return RewriteDone
-		}
-	}
-
-	if s.From == r.Question[0].Name {
-		r.Question[0].Name = s.To
-		return RewriteDone
-	}
-	return RewriteIgnored
+	// New returns a new rule.
+	New(...string) Rule
 }