Implement a DNS zone

Full implementation, DNS (and in the future DNSSEC). Returns answer in a
hopefully standards compliant way.
Testing with my miek.nl zone are included as well.
This should correctly handle nodata, nxdomain and cnames.

middleware/file/lookup.go

@@ -0,0 +1,75 @@
+package file
+
+import "github.com/miekg/dns"
+
+// Result is the result of a Lookup
+type Result int
+
+const (
+	Success Result = iota
+	NameError
+	NoData // aint no offical NoData return code.
+)
+
+// Lookup looks up qname and qtype in the zone, when do is true DNSSEC are included as well.
+// Two sets of records are returned, one for the answer and one for the additional section.
+func (z *Zone) Lookup(qname string, qtype uint16, do bool) ([]dns.RR, []dns.RR, Result) {
+	// TODO(miek): implement DNSSEC
+	var rr dns.RR
+	mk, known := dns.TypeToRR[qtype]
+	if !known {
+		return nil, nil, NameError
+		// Uhm...?
+		// rr = new(RFC3597)
+	} else {
+		rr = mk()
+	}
+	if qtype == dns.TypeSOA {
+		return z.lookupSOA(do)
+	}
+
+	rr.Header().Name = qname
+	elem := z.Tree.Get(rr)
+	if elem == nil {
+		return []dns.RR{z.SOA}, nil, NameError
+	}
+	rrs := elem.Types(dns.TypeCNAME)
+	if len(rrs) > 0 { // should only ever be 1 actually; TODO(miek) check for this?
+		// lookup target from the cname
+		rr.Header().Name = rrs[0].(*dns.CNAME).Target
+		elem := z.Tree.Get(rr)
+		if elem == nil {
+			return rrs, nil, Success
+		}
+		return rrs, elem.All(), Success
+	}
+
+	rrs = elem.Types(qtype)
+	if len(rrs) == 0 {
+		return []dns.RR{z.SOA}, nil, NoData
+	}
+	// Need to check sub-type on RRSIG records to only include the correctly
+	// typed ones.
+	return rrs, nil, Success
+}
+
+func (z *Zone) lookupSOA(do bool) ([]dns.RR, []dns.RR, Result) {
+	return []dns.RR{z.SOA}, nil, Success
+}
+
+// signatureForSubType range through the signature and return the correct
+// ones for the subtype.
+func (z *Zone) signatureForSubType(rrs []dns.RR, subtype uint16, do bool) []dns.RR {
+	if !do {
+		return nil
+	}
+	sigs := []dns.RR{}
+	for _, sig := range rrs {
+		if s, ok := sig.(*dns.RRSIG); ok {
+			if s.TypeCovered == subtype {
+				sigs = append(sigs, s)
+			}
+		}
+	}
+	return sigs
+}