Subject: Re: [PATCH nf] netfilter: nf_conntrack: Add allow_clash to generic protocol handler

From: Florian Westphal <[email-masked]>
Date: Mon, 22 Dec 2025 12:22:43 +0100

Pablo Neira Ayuso <[email-masked]> wrote:
> On Sat, Dec 20, 2025 at 09:05:59PM +0100, Florian Westphal wrote:
> > Yuto Hamaguchi <[email-masked]> wrote:
> > > The upstream commit, 71d8c47fc653711c41bc3282e5b0e605b3727956
> > >  ("netfilter: conntrack: introduce clash resolution on insertion race"),
> > > sets allow_clash=true in the UDP/UDPLITE protocol handler
> > > but does not set it in the generic protocol handler.
> > > 
> > > As a result, packets composed of connectionless protocols at each layer,
> > > such as UDP over IP-in-IP, still drop packets due to conflicts during conntrack insertion.
> > > 
> > > To resolve this, this patch sets allow_clash in the nf_conntrack_l4proto_generic.
> > 
> > Makes sense to me, thanks.
> 
> Would it be possible to follow a more conservative approach?
> ie. restrict this to protocols where we know clashes are possible
> such as IPIP.

I don't think there is a need to be more conservative here.

If we have to fallback to the generic tracker that means we don't have a
L4 aware tracker for the protocol, so we can always merge entries instead
of dropping them.

Whats the concern?