feature: plugin/rewrite: rewrite ANSWER SECTION (#1318)

Resolves: #1313

plugin/rewrite/reverter.go

@@ -1,27 +1,61 @@
 package rewrite
 
-import "github.com/miekg/dns"
+import (
+	"github.com/miekg/dns"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// ResponseRule contains a rule to rewrite a response with.
+type ResponseRule struct {
+	Active      bool
+	Pattern     *regexp.Regexp
+	Replacement string
+}
 
 // ResponseReverter reverses the operations done on the question section of a packet.
 // This is need because the client will otherwise disregards the response, i.e.
 // dig will complain with ';; Question section mismatch: got miek.nl/HINFO/IN'
 type ResponseReverter struct {
 	dns.ResponseWriter
-	original dns.Question
+	originalQuestion dns.Question
+	ResponseRewrite  bool
+	ResponseRules    []ResponseRule
 }
 
 // NewResponseReverter returns a pointer to a new ResponseReverter.
 func NewResponseReverter(w dns.ResponseWriter, r *dns.Msg) *ResponseReverter {
 	return &ResponseReverter{
-		ResponseWriter: w,
-		original:       r.Question[0],
+		ResponseWriter:   w,
+		originalQuestion: r.Question[0],
 	}
 }
 
 // WriteMsg records the status code and calls the
 // underlying ResponseWriter's WriteMsg method.
 func (r *ResponseReverter) WriteMsg(res *dns.Msg) error {
-	res.Question[0] = r.original
+	res.Question[0] = r.originalQuestion
+	if r.ResponseRewrite {
+		for _, rr := range res.Answer {
+			name := rr.(*dns.A).Hdr.Name
+			for _, rule := range r.ResponseRules {
+				regexGroups := rule.Pattern.FindStringSubmatch(name)
+				if len(regexGroups) == 0 {
+					continue
+				}
+				s := rule.Replacement
+				for groupIndex, groupValue := range regexGroups {
+					groupIndexStr := "{" + strconv.Itoa(groupIndex) + "}"
+					if strings.Contains(s, groupIndexStr) {
+						s = strings.Replace(s, groupIndexStr, groupValue, -1)
+					}
+				}
+				name = s
+			}
+			rr.(*dns.A).Hdr.Name = name
+		}
+	}
 	return r.ResponseWriter.WriteMsg(res)
 }