MTU on wireguard-apple for IPv6

["Rémi Lapeyre" <remi.lapeyre@lenstra.fr>]

Hi, we have users that are reporting many issues when using the
WireGuard client on MacOS laptops with IPv6 networks. It seems that
most of those issues disappear when we set the MTU to 1280 in the
configuration.

Since they are using laptops, some are frequently on badly behaved
networks. Reading the source code, it seems that this situation is
handled differently on iOS vs MacOS in generateNetworkSettings():


        let mtu = tunnelConfiguration.interface.mtu ?? 0

        /* 0 means automatic MTU. In theory, we should just do
         * `networkSettings.tunnelOverheadBytes = 80` but in
         * practice there are too many broken networks out there.
         * Instead set it to 1280. Boohoo. Maybe someday we'll
         * add a nob, maybe, or iOS will do probing for us.
         */
        if mtu == 0 {
            #if os(iOS)
            networkSettings.mtu = NSNumber(value: 1280)
            #elseif os(macOS)
            networkSettings.tunnelOverheadBytes = 80
            #else
            #error("Unimplemented")
            #endif
        } else {
            networkSettings.mtu = NSNumber(value: mtu)
        }


Does `networkSettings.tunnelOverheadBytes = 80` means the MTU will
used be the one of the interface minus 80 bits of overhead for the
WireGuard protocol?

Some of our users where tethering over their phone connection so it
seems that 1280 is appropriate in that case, but I’m confused as to
why connections not going through the WireGuard tunnel where not
impacted in that case.

Does it really make sense to treat laptops and iOS devices differently
are sometime connected to random networks? Should this be changed to
`networkSettings.mtu = NSNumber(value: 1280)` in all cases?

Rémi
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