L2TP VPN has a fixed (wrong) MTU of 1280

[Rob]

When I setup a L2TP/IPsec VPN and transfer traffic over it from a Linux VM on a Chromebook, it shows an MTU of 1280.  E.g. tracepath says "pmtu 1280".

However, the router to which the connection is made has MTU/MRU 1400 and it establishes the connection at this MTU.

This leads to weird connectivity problems.  Please fix this so the L2TP VPN has the correct MTU instead of some arbitrary "safe value".

[Mike Frysinger]

please file bugs like this using our bug tracker:

https://issuetracker.google.com/

we use the Chrome OS Public Tracker component.

-mike

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[Brian Kennelly]

I have the same problem, but, FWIW, I also see PMTU=1280 connecting to the same VPN from Windows.  In both Crostini and in WSL, this causes connection problems for one of my applications (x3270).  My solution in both cases is to run a shell script in the VM:

#! /bin/sh
ip ro repl 10.3.4.0/23 via $(awk '/nameserver/ {print $2}' /etc/resolv.conf) mtu 1280

It should also be possible to set this up through the LInux startup scripts, but I haven't bothered.

On Tue, Aug 24, 2021 at 10:49 AM 'Rob' via Chromium OS Discussion <chromium-...@chromium.org> wrote:

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Brian Kennelly