wgengine/monitor: fix crash on Linux on type 21 messages

Fixes #532

wgengine/monitor/monitor.go

@@ -20,6 +20,13 @@ type message interface {
 	ignore() bool
 }
 
+// unspecifiedMessage is a minimal message implementation that should not
+// be ignored. In general, OS-specific implementations should use better
+// types and avoid this if they can.
+type unspecifiedMessage struct{}
+
+func (unspecifiedMessage) ignore() bool { return false }
+
 // osMon is the interface that each operating system-specific
 // implementation of the link monitor must implement.
 type osMon interface {

wgengine/monitor/monitor_freebsd.go

@@ -48,9 +48,3 @@ func (c *devdConn) Receive() (message, error) {
 		return unspecifiedMessage{}, nil
 	}
 }
-
-// unspecifiedMessage is a minimal message implementation that should not
-// be ignored. TODO: make specific messages like monitor_linux.go.
-type unspecifiedMessage struct{}
-
-func (unspecifiedMessage) ignore() bool { return false }

wgengine/monitor/monitor_linux.go

@@ -92,8 +92,9 @@ func (c *nlConn) Receive() (message, error) {
 			Gateway: netaddrIP(rmsg.Attributes.Gateway),
 		}, nil
 	default:
+		// TODO(bradfitz): parse type 21 too (https://github.com/tailscale/tailscale/issues/532)
 		c.logf("netlink msg %+v, %q", msg.Header, msg.Data)
-		return nil, nil
+		return unspecifiedMessage{}, nil
 	}
 }