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_test.go

@@ -0,0 +1,37 @@
+package middleware
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestEdns0Version(t *testing.T) {
+	m := ednsMsg()
+	m.Extra[0].(*dns.OPT).SetVersion(2)
+
+	_, err := Edns0Version(m)
+	if err == nil {
+		t.Errorf("expected wrong version, but got OK")
+	}
+}
+
+func TestEdns0VersionNoEdns(t *testing.T) {
+	m := ednsMsg()
+	m.Extra = nil
+
+	_, err := Edns0Version(m)
+	if err != nil {
+		t.Errorf("expected no error, but got one: %s", err)
+	}
+}
+
+func ednsMsg() *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	m.Extra = append(m.Extra, o)
+	return m
+}