EDNS: return error on wrong version. (#95)

Split up the previous changes a bit. This PR only returns the expected
error when the received packet has the wrong EDNS version.

EDNS0 handling in the middleware needs a nicer abstraction, like
ReflectEdns() or something.

middleware/edns.go

@@ -0,0 +1,34 @@
+package middleware
+
+import (
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+// Edns0Version checks the EDNS version in the request. If error
+// is nil everything is OK and we can invoke the middleware. If non-nil, the
+// returned Msg is valid to be returned to the client (and should). For some
+// reason this response should not contain a question RR in the question section.
+func Edns0Version(req *dns.Msg) (*dns.Msg, error) {
+	opt := req.IsEdns0()
+	if opt == nil {
+		return nil, nil
+	}
+	if opt.Version() == 0 {
+		return nil, nil
+	}
+	m := new(dns.Msg)
+	m.SetReply(req)
+	// zero out question section, wtf.
+	m.Question = nil
+
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	o.SetVersion(0)
+	o.SetExtendedRcode(dns.RcodeBadVers)
+	m.Extra = []dns.RR{o}
+
+	return m, errors.New("EDNS0 BADVERS")
+}