Add Scrub function

This function will make the message fit for the client's buffer, or
set the TC bit.

middleware/etcd/handler.go

@@ -72,7 +72,7 @@ func (e Etcd) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 	}
 
 	m = dedup(m)
-
+	m, _ = state.Scrub(m)
 	state.W.WriteMsg(m)
 	return 0, nil
 }

middleware/proxy/reverseproxy.go

@@ -3,6 +3,7 @@ package proxy
 
 import (
 	"github.com/miekg/coredns/middleware"
+
 	"github.com/miekg/dns"
 )
 
@@ -18,7 +19,7 @@ func (p ReverseProxy) ServeDNS(w dns.ResponseWriter, r *dns.Msg, extra []dns.RR)
 	)
 	state := middleware.State{W: w, Req: r}
 
-	// tls+tcp ?
+	// We forward the original request, no need to fiddle with EDNS0 opt sizes.
 	if state.Proto() == "tcp" {
 		reply, err = middleware.Exchange(p.Client.TCP, r, p.Host)
 	} else {

middleware/state.go

@@ -102,9 +102,9 @@ func (s State) Size() int {
 	return dns.MinMsgSize
 }
 
-// SizeAndDo returns a ready made OPT record that the reflects the intent
-// from the state. This can be added to upstream requests that will then
-// hopefully return a message that is understandable by the original client.
+// SizeAndDo returns a ready made OPT record that the reflects the intent from
+// state. This can be added to upstream requests that will then hopefully
+// return a message that is fits the buffer in the client.
 func (s State) SizeAndDo() *dns.OPT {
 	size := s.Size()
 	Do := s.Do()
@@ -119,6 +119,40 @@ func (s State) SizeAndDo() *dns.OPT {
 	return o
 }
 
+// Result is the result of Fit.
+type Result int
+
+const (
+	// ScrubIgnored is returned when Scrub did nothing to the message.
+	ScrubIgnored Result = iota
+	// ScrubDone is returned when the reply has been scrubbed.
+	ScrubDone
+)
+
+// Scrub scrubs the reply message so that it will fit the client's buffer. If even after dropping
+// the additional section, it still does not fit the TC bit will be set on the message. Note,
+// the TC bit will be set regardless of protocol, even TCP message will get the bit, the client
+// should then retry with pigeons.
+// TODO(referral).
+func (s State) Scrub(reply *dns.Msg) (*dns.Msg, Result) {
+	size := s.Size()
+	l := reply.Len()
+	if size >= l {
+		return reply, ScrubIgnored
+	}
+	// If not delegation, drop additional section.
+	// TODO(miek): check for delegation
+	reply.Extra = nil
+	l = reply.Len()
+	if size >= l {
+		return reply, ScrubDone
+	}
+	// Still?!! does not fit.
+	reply.Truncated = true
+	return reply, ScrubDone
+
+}
+
 // Type returns the type of the question as a string.
 func (s State) Type() string { return dns.Type(s.Req.Question[0].Qtype).String() }