MTU of the Wireguard interface when WAN uses a PPPoE connection

[KuddelSK]

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Describe the bug

I connect to my ISP using a PPPoE connection. When i configure wireguard and look at the wg0 interface using ifconfig i see a MTU of 1420 (1500 - 80 for the Wireguard header). This is wrong in case of a PPPoE connection as PPPoE adds 8 Byte on its own. Therefore the MTU should be 1412 in this case.

This is not critical as one can manually adjust the tunnel MTU.

I don't know if its relevant to the bug but i have to use a VLAN which provides the ethernet layer for the PPPoE connection.

To Reproduce

Steps to reproduce the behavior:

1. Use a PPPoE Wan connection
2. Install the Wireguard service
3. Configure Wireguard (leave MTU empty for automatic calculation)
4. Execute "ifconfig wg0" in a router shell

Expected behavior

MTU of 1412 for wg0 in case of a PPPoE connection.

Describe alternatives you considered

One can set the tunnel MTU manually.

Environment

OpnSense 21.7.6
APU 4D4 (GX-412TC CPU; 4 Nics i211AT )

[nzkiwi68]

I'm not sure that statement of 80 bytes for the Wireguard header is correct. From what I am reading, the Wireguard header is only 60 bytes and thus 1420 automatic MTU is fine for PPPoE and non PPPoE connections for IPv4.

Only IPv6 over PPPoE would require an MTU of 1412.

My reference is here:
https://lists.zx2c4.com/pipermail/wireguard/2017-December/002201.html

[KuddelSK]

Sorry. I forgot to mention this information inside the issue description.

I configured WAN to use IPv4 and IPv6. I connect to Wireguard using the public IPv6. I guess this is quite common as a non RFC1918 IPv4 is rare these days for home users.

So i guess my suggestion is that an automatic MTU of 1412 should be used if IPv6 is configured for a WAN interface.

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