From 4b75a279696f7bab74d85832db65762d03922d87 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Tue, 7 Jul 2020 10:44:54 -0700
Subject: [PATCH] wgengine/monitor: fix crash on Linux on type 21 messages

Fixes #532
---
 wgengine/monitor/monitor.go         | 7 +++++++
 wgengine/monitor/monitor_freebsd.go | 6 ------
 wgengine/monitor/monitor_linux.go   | 3 ++-
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/wgengine/monitor/monitor.go b/wgengine/monitor/monitor.go
index 38cdbaa54..2fc503596 100644
--- a/wgengine/monitor/monitor.go
+++ b/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 {
diff --git a/wgengine/monitor/monitor_freebsd.go b/wgengine/monitor/monitor_freebsd.go
index cb77ac095..832772f09 100644
--- a/wgengine/monitor/monitor_freebsd.go
+++ b/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 }
diff --git a/wgengine/monitor/monitor_linux.go b/wgengine/monitor/monitor_linux.go
index d67cac381..7368bdc88 100644
--- a/wgengine/monitor/monitor_linux.go
+++ b/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
 	}
 }