From c64a43a734f52a6dd4eb01e9bc72d4b49ae3c776 Mon Sep 17 00:00:00 2001 From: Brad Fitzpatrick Date: Fri, 7 Aug 2020 12:06:32 -0700 Subject: [PATCH] wgengine/router: set MTU on Windows to min(configured,possible) Fixes tailscale/corp#542 Signed-off-by: Brad Fitzpatrick --- wgengine/router/ifconfig_windows.go | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/wgengine/router/ifconfig_windows.go b/wgengine/router/ifconfig_windows.go index a9b99b89b..78388ccc0 100644 --- a/wgengine/router/ifconfig_windows.go +++ b/wgengine/router/ifconfig_windows.go @@ -121,6 +121,13 @@ func monitorDefaultRoutes(device *device.Device, autoMTU bool, tun *tun.NativeTu return err } iface.NlMtu = mtu - 80 + // If the TUN device was created with a smaller MTU, + // though, such as 1280, we don't want to go bigger than + // configured. (See the comment on minimalMTU in the + // wgengine package.) + if min, err := tun.MTU(); err == nil && min < int(iface.NlMtu) { + iface.NlMtu = uint32(min) + } if iface.NlMtu < 576 { iface.NlMtu = 576 }