plugin/tsig: new plugin TSIG (#4957)

* expose tsig secrets via dnsserver.Config
* add tsig plugin

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/transfer/transfer.go

@@ -18,6 +18,7 @@ var log = clog.NewWithPlugin("transfer")
 type Transfer struct {
 	Transferers []Transferer // List of plugins that implement Transferer
 	xfrs        []*xfr
+	tsigSecret  map[string]string
 	Next        plugin.Handler
 }
 
@@ -110,6 +111,9 @@ func (t *Transfer) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 	// Send response to client
 	ch := make(chan *dns.Envelope)
 	tr := new(dns.Transfer)
+	if r.IsTsig() != nil {
+		tr.TsigSecret = t.tsigSecret
+	}
 	errCh := make(chan error)
 	go func() {
 		if err := tr.Out(w, r, ch); err != nil {