Use an interface for udp conns (#901)

udp/conn.go

@@ -1,6 +1,7 @@
 package udp
 
 import (
+	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/firewall"
 	"github.com/slackhq/nebula/header"
 )
@@ -18,3 +19,29 @@ type EncReader func(
 	q int,
 	localCache firewall.ConntrackCache,
 )
+
+type Conn interface {
+	Rebind() error
+	LocalAddr() (*Addr, error)
+	ListenOut(r EncReader, lhf LightHouseHandlerFunc, cache *firewall.ConntrackCacheTicker, q int)
+	WriteTo(b []byte, addr *Addr) error
+	ReloadConfig(c *config.C)
+}
+
+type NoopConn struct{}
+
+func (NoopConn) Rebind() error {
+	return nil
+}
+func (NoopConn) LocalAddr() (*Addr, error) {
+	return nil, nil
+}
+func (NoopConn) ListenOut(_ EncReader, _ LightHouseHandlerFunc, _ *firewall.ConntrackCacheTicker, _ int) {
+	return
+}
+func (NoopConn) WriteTo(_ []byte, _ *Addr) error {
+	return nil
+}
+func (NoopConn) ReloadConfig(_ *config.C) {
+	return
+}