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

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

On Wed, 15 Dec 2021, Vincent Lefevre wrote:

> On 2021-12-15 06:09:12 +0000, Tim Woodall wrote:
> > 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.
>
> Indeed, this works up to
>  ping -4 -s 1472 joooj.vinc17.net
>  ping -6 -s 1452 joooj.vinc17.net
>
> > Yes, probably - --clamp-mss-to-ptmu in iptables might be what is needed.
>
> Thanks, but taking the rules from
>
>  https://tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.cookbook.mtu-mss.html
>
> I've tried
>
>  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
>
> and
>
>  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1024
I think you possibly want -A OUTPUT rules too. And also ip6tables rules.

When I look at where I've configured this in the past I've also done it
on the mangle chain:

iptables -t mangle -A FORWARD -p tcp -o $NET_IF --tcp-flags SYN,RST SYN
-j TCPMSS --clamp-mss-to-pmtu

ip6tables -t mangle -A FORWARD -p tcp -o $NETv6_IF --tcp-flags SYN,RST
SYN -j TCPMSS --clamp-mss-to-pmtu

etc. (you may or may not want to bother writing explicit rules for each
interface)

If you're going via a firewall then you (usually) only need -A FORWARD
But if you're trying to configure it on your laptop itself you probably
need -A OUTPUT (and possibly -A INPUT).

> and none of them have any effect. Or did I need something else?
>
> Any other suggestion?