Support outgoing zone transfers

These can be enabled by adding "transfer out" to the Corefile. Without
it no AXFR is allowed.

For now only AXFR and no IXFR. No TSIG and no ACLs.

middleware/file/zone.go

@@ -1,20 +1,29 @@
 package file
 
 import (
+	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/file/tree"
 
 	"github.com/miekg/dns"
 )
 
+type Transfer struct {
+	Out bool
+	In  bool
+	// more later
+}
+
 type Zone struct {
 	SOA  *dns.SOA
-	SIG  []*dns.RRSIG
+	SIG  []dns.RR
 	name string
 	*tree.Tree
+	Masters  []string
+	Transfer *Transfer
 }
 
 func NewZone(name string) *Zone {
-	return &Zone{name: dns.Fqdn(name), Tree: &tree.Tree{}}
+	return &Zone{name: dns.Fqdn(name), Tree: &tree.Tree{}, Transfer: &Transfer{}}
 }
 
 func (z *Zone) Insert(r dns.RR) {
@@ -24,3 +33,28 @@ func (z *Zone) Insert(r dns.RR) {
 func (z *Zone) Delete(r dns.RR) {
 	z.Tree.Delete(r)
 }
+
+// It the transfer request allowed.
+func (z *Zone) TransferAllowed(state middleware.State) bool {
+	if z.Transfer == nil {
+		return false
+	}
+	return z.Transfer.Out
+}
+
+// All returns all records from the zone, the first record will be the SOA record,
+// otionally followed by all RRSIG(SOA)s.
+func (z *Zone) All() []dns.RR {
+	records := []dns.RR{}
+	allNodes := z.Tree.All()
+	for _, a := range allNodes {
+		records = append(records, a.All()...)
+	}
+
+	if len(z.SIG) > 0 {
+		records = append(z.SIG, records...)
+	}
+	return append([]dns.RR{z.SOA}, records...)
+}
+
+// Apex function?