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/xfr.go

@@ -0,0 +1,61 @@
+package file
+
+import (
+	"fmt"
+
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+type (
+	Xfr struct {
+		*Zone
+	}
+)
+
+// Serve an AXFR (or maybe later an IXFR) as well.
+func (x Xfr) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := middleware.State{W: w, Req: r}
+	if !x.TransferAllowed(state) {
+		return dns.RcodeServerFailure, nil
+	}
+	if state.QType() != dns.TypeAXFR {
+		return 0, fmt.Errorf("file: xfr called with non xfr type: %d", state.QType())
+	}
+	if state.Proto() == "udp" {
+		return 0, fmt.Errorf("file: xfr called with udp")
+	}
+
+	records := x.All()
+	if len(records) == 0 {
+		return dns.RcodeServerFailure, nil
+	}
+
+	ch := make(chan *dns.Envelope)
+	defer close(ch)
+	tr := new(dns.Transfer)
+	go tr.Out(w, r, ch)
+
+	j, l := 0, 0
+	records = append(records, records[0])
+	for i, r := range records {
+		l += dns.Len(r)
+		if l > transferLength {
+			ch <- &dns.Envelope{RR: records[j:i]}
+			l = 0
+			j = i
+		}
+	}
+	if j < len(records) {
+		ch <- &dns.Envelope{RR: records[j:]}
+	}
+
+	w.Hijack()
+	// w.Close() // Client closes connection
+	return dns.RcodeSuccess, nil
+}
+
+//const transferLength = 10e3 // Start a new envelop after message reaches this size.
+const transferLength = 100 // Start a new envelop after message reaches this size.