Re: IPv4 specific issue with USB tethering between my Debian laptop and my phone

[Tim Woodall <debianuser@woodall.me.uk>]

On Fri, 17 Dec 2021, Vincent Lefevre wrote:

> Dan Ritter wrote:
> > ip link set USBNET0 mtu 1450, or something like that. I don't
> > know NetworkManager's syntax for that; random googling suggests
> > that they at least had a historical problem with not being able
> > to set MTU on anything other than a pure ethernet or wifi link,
> > but perhaps they've solved that?
>
> "ip link" works on its side. I can see with wireshark that something
> like "ip link set usb0 mtu 1300" is honored, but this doesn't solve
> the problem (I've tried various values from 1000 to 1450).
>
> Note that I could also see with wireshark that the iptables rules
> with --clamp-mss-to-pmtu did not change the packet lengths.

Does this happen on any connection or just some?

You said you could reproduce using ssh? Does that mean you can run
wireshark on both ends and see which packet is getting lost?

Did you try setting both mtu on the interface to something smallish plus
--clamp-mss-to-pmtu?


I'm pretty sure all ipv4 over mobile (which I think is what you are
testing) depends on NAT. So my other guess of a dodgy NAT device isn't
impossible. IIRC your ping tests suggested that ipv4 is being tunnelled
in ipv6