Add middleware/dnssec (#133)

This adds an online dnssec middleware. The middleware will sign
responses on the fly. Negative responses are signed with NSEC black
lies.

middleware/state.go

@@ -15,9 +15,13 @@ type State struct {
 	Req *dns.Msg
 	W   dns.ResponseWriter
 
-	// Cache size after first call to Size or Do
+	// Cache size after first call to Size or Do.
 	size int
 	do   int // 0: not, 1: true: 2: false
+	// TODO(miek): opt record itself as well.
+
+	// Cache name as (lowercase) well
+	name string
 }
 
 // Now returns the current timestamp in the specified format.
@@ -26,12 +30,6 @@ func (s *State) Now(format string) string { return time.Now().Format(format) }
 // NowDate returns the current date/time that can be used in other time functions.
 func (s *State) NowDate() time.Time { return time.Now() }
 
-// Header gets the heaser of the request in State.
-func (s *State) Header() *dns.RR_Header {
-	// TODO(miek)
-	return nil
-}
-
 // IP gets the (remote) IP address of the client making the request.
 func (s *State) IP() string {
 	ip, _, err := net.SplitHostPort(s.W.RemoteAddr().String())
@@ -191,7 +189,13 @@ func (s *State) QType() uint16 { return s.Req.Question[0].Qtype }
 
 // Name returns the name of the question in the request. Note
 // this name will always have a closing dot and will be lower cased.
-func (s *State) Name() string { return strings.ToLower(dns.Name(s.Req.Question[0].Name).String()) }
+func (s *State) Name() string {
+	if s.name != "" {
+		return s.name
+	}
+	s.name = strings.ToLower(dns.Name(s.Req.Question[0].Name).String())
+	return s.name
+}
 
 // QName returns the name of the question in the request.
 func (s *State) QName() string { return dns.Name(s.Req.Question[0].Name).String() }
@@ -210,6 +214,11 @@ func (s *State) ErrorMessage(rcode int) *dns.Msg {
 	return m
 }
 
+// Clear clears all caching from State s.
+func (s *State) Clear() {
+	s.name = ""
+}
+
 const (
 	doTrue  = 1
 	doFalse = 2