middleware/secondary: multiple fixes (#745)

Fix transferring the zone from a master and the matching of notifies
to source and dst IP addresses.

Add `upstream` keyword as well, because it is needed for the same
reasons as in the *file* middlware.
Add some dire warning about upstream in the readme of both middlewares.

Out of band testing, hidden by net build tag was added. Integration
testing still needs to be setup.

middleware/file/notify.go

@@ -3,8 +3,8 @@ package file
 import (
 	"fmt"
 	"log"
+	"net"
 
-	"github.com/coredns/coredns/middleware"
 	"github.com/coredns/coredns/middleware/pkg/rcode"
 	"github.com/coredns/coredns/request"
 
@@ -21,8 +21,13 @@ func (z *Zone) isNotify(state request.Request) bool {
 	if len(z.TransferFrom) == 0 {
 		return false
 	}
-	remote := middleware.Addr(state.IP()).Normalize()
-	for _, from := range z.TransferFrom {
+	// If remote IP matches we accept.
+	remote := state.IP()
+	for _, f := range z.TransferFrom {
+		from, _, err := net.SplitHostPort(f)
+		if err != nil {
+			continue
+		}
 		if from == remote {
 			return true
 		}