netdev - Re: [PATCH V6] netfilter: netns nf_conntrack: per-netns net.netfilter.nf_conntrack

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Message-ID: <aC97x6CHFleb54i0@strlen.de>
Date: Thu, 22 May 2025 21:32:23 +0200
From: Florian Westphal <fw@...len.de>
To: Pablo Neira Ayuso <pablo@...filter.org>
Cc: lvxiafei <xiafei_xupt@....com>, coreteam@...filter.org,
	davem@...emloft.net, edumazet@...gle.com, horms@...nel.org,
	kadlec@...filter.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
	lvxiafei@...setime.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH V6] netfilter: netns nf_conntrack: per-netns
 net.netfilter.nf_conntrack_max sysctl

Pablo Neira Ayuso <pablo@...filter.org> wrote:
> > -        to nf_conntrack_buckets by default.
> > -        Note that connection tracking entries are added to the table twice -- once
> > -        for the original direction and once for the reply direction (i.e., with
> > -        the reversed address). This means that with default settings a maxed-out
> > -        table will have a average hash chain length of 2, not 1.
> > +    - 0 - disabled (unlimited)
> 
> unlimited is too much, and the number of buckets is also global, how
> does this work?

Its an historic wart going back to ip_conntrack - it was never the
default but you could disable any and all limits even in the original
version.

Wether its time to disallow 0 is a different topic and not related to this patch.

I would argue: "yes", disallow 0 -- users can still set INT_MAX if they
 want and that should provide enough rope to strangle yourself.

> > +    The limit of other netns cannot be greater than init_net netns.
> > +    +----------------+-------------+----------------+
> > +    | init_net netns | other netns | limit behavior |
> > +    +----------------+-------------+----------------+
> > +    | 0              | 0           | unlimited      |
> > +    +----------------+-------------+----------------+
> > +    | 0              | not 0       | other          |
> > +    +----------------+-------------+----------------+
> > +    | not 0          | 0           | init_net       |
> > +    +----------------+-------------+----------------+
> > +    | not 0          | not 0       | min            |
> > +    +----------------+-------------+----------------+

I think this is fine, it doesn't really change things from init_net
point of view.