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

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

On Tue, 14 Dec 2021, Dan Ritter wrote:

> Vincent Lefevre wrote:
> > Hi,
> >
> > When I use USB tethering between my Debian/unstable laptop and my
> > Samsung Galaxy Note10+ phone (Android 11), everything is OK with
> > IPv6 connections (e.g. "wget -6"), but IPv4 connections freeze
> > (e.g. "wget -4" or "ssh -4"). I can notice this in particular when
> > testing with the same remote server. For instance, with "wget -4",
> > a few dozens of KB are downloaded, then the connection is completely
> > frozen.
> >
> > I don't have any issue with the mobile hotspot (thus wifi between the
> > laptop and the phone) or with a Cosmo under Android 9 (instead of the
> > Samsung Galaxy Note10+), but in both cases, this is much slower, and
> > I sometimes get wifi disconnections.
> >
> > On my Debian laptop, I'm using NetworkManager (nmcli).
>
> Have you investigated the MTU? This sounds like a mismatch
> between what your laptop and phone are using.
>
> Diagnosis (besides looking at connection details on both sides):
> ping with increasingly large packet sizes. If it works at small
> IPv4 packet sizes and then stops, that's the problem.

Yes, probably - --clamp-mss-to-ptmu in iptables might be what is needed.

The other thing I thought of was dodgy natting somewhere. Especially if
there's a firewall somewhere that drops 'middle of stream' packets where
no SYN has been seen first.