add allocation backend that performs no data forwarding

For environments that support a host having a whole subnet, it is
useful to let Rudder just do the address allocation and use native
networking for data flows.

backend/alloc/alloc.go

@@ -0,0 +1,53 @@
+package alloc
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/coreos/rudder/backend"
+	"github.com/coreos/rudder/pkg/ip"
+	"github.com/coreos/rudder/pkg/task"
+	"github.com/coreos/rudder/subnet"
+)
+
+
+type AllocBackend struct {
+	sm   *subnet.SubnetManager
+	stop chan bool
+}
+
+func New(sm *subnet.SubnetManager) backend.Backend {
+	return &AllocBackend{
+		sm: sm,
+		stop: make(chan bool),
+	}
+}
+
+func (m *AllocBackend) Init(extIface *net.Interface, extIP net.IP, ipMasq bool) (ip.IP4Net, int, error) {
+	attrs := subnet.BaseAttrs{
+		PublicIP: ip.FromIP(extIP),
+	}
+
+	sn, err := m.sm.AcquireLease(ip.FromIP(extIP), &attrs, m.stop)
+	if err != nil {
+		if err == task.ErrCanceled {
+			return ip.IP4Net{}, 0, err
+		} else {
+			return ip.IP4Net{}, 0, fmt.Errorf("Failed to acquire lease: %v", err)
+		}
+	}
+
+	return sn, extIface.MTU, nil
+}
+
+func (m *AllocBackend) Run() {
+	m.sm.LeaseRenewer(m.stop)
+}
+
+func (m *AllocBackend) Stop() {
+	close(m.stop)
+}
+
+func (m *AllocBackend) Name() string {
+	return "allocation"
+}

main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/coreos/rudder/pkg/ip"
 	"github.com/coreos/rudder/pkg/task"
 	"github.com/coreos/rudder/subnet"
+	"github.com/coreos/rudder/backend/alloc"
 	"github.com/coreos/rudder/backend/udp"
 )
 
@@ -128,6 +129,8 @@ func newBackend() (backend.Backend, error) {
 	switch strings.ToLower(bt.Type) {
 	case "udp":
 		return udp.New(sm, config.Backend), nil
+	case "alloc":
+		return alloc.New(sm), nil
 	default:
 		return nil, fmt.Errorf("'%v': unknown backend type", bt.Type)
 	}