Osmo-ggsn MTU issue with Kernel GTP

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Keith A. Milner maillist at superlative.org
Tue Apr 13 15:02:05 UTC 2021


On Tuesday, 13 April 2021 13:14:48 BST Harald Welte wrote:
> Hi Keith,
> 
> On Tue, Apr 13, 2021 at 11:10:18AM +0100, Keith A. Milner wrote:
> > However, if I change to using Kernel GTP, by changing the gtpu-mode to
> > "kernel-gtp", traffic will not route (and, yes, I have done modprobe gtp).
> > After some exploration, it appears the tun device is being created with
> > an MTU of
> > zero which then, cannot pass traffic:
> I think this one is fixed in
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/dr
> ivers/net?id=e21eb3a065a2d90ee3bb06cc2e77acad403ec7cd but "of course" Debian
> 10 is using a too old kernel to contain that fix.

Ah, missed that. Thanks Harald.

> 
> You could ask Debian if they'd back-port it to the stable kernel?

It's not that big of an issue for me, as this is just for some testing. I was 
mainly concerned I had missed some configuration somewhere. As I can use the 
ipup-script this will work for me.

> 
> We would certainly also be happy to accept a patch mentioning this in the
> user manual, or a patch that makes osmo-ggsn check the MTU at startup time
> and print some big error message into the log.

I will have a look at that, probably at the former rather than the latter.

Thanks once again.

Keith







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