Re: [PATCH iptabes-nft] iptables-nft: allow removal of empty builtin chains

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Sun, Aug 15, 2021 at 03:27:33PM +0200, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > On Sat, Aug 14, 2021 at 10:53:14PM +0200, Florian Westphal wrote:
> > > Indeed.  Since this removes the base chain, it implicitly reverts
> > > a DROP policy too.
> > 
> > User still has to iptables -F on that given chain before deleting,
> > right?
> 
> Yes, -X fails if the chain has rules.
> 
> > If NLM_F_NONREC is used, the EBUSY is reported when trying to delete
> > a chain with rules.
> 
> Yes.

But we really do not need NLM_F_NONREC for this new feature, right? I
mean, a quick shortcut to remove the basechain and its content should
be fine.

> > My assumption is that the user will perform:
> > 
> > iptables-nft -F -t filter
> > iptables-nft -D -t filter
> 
> Yes, assuminy you meant -X instead of -D.

Oh well, embarrasing, yes.

> This behaves just like before, it deletes all rules (-F) and all user-defined
> chains (-X).
>
> > I mean, by when the user has an empty basechain with default policy to
> > DROP, if they remove the chain, then they are really meaning to remove
> > the chain and this default policy to DROP.
> 
> ATM iptables -X $BUILTIN will always fail.
> In -legecy there is no kernel API to allow for its removal,
> for -nft there is an extra check that throws an error.
> 
> > Or am I missing anything else?
> 
> No, I don't think so.  I would prefer if
> iptables-nft -F -t filter
> iptables-nft -X -t filter
> 
> ... would result in an empty "filter" table.

Your concern is that this would change the default behaviour?

> I could also add a patch that requests removal
> of the table as well for the -X case; but unlike base chain the
> presence of the table alone has no impact on dataplane.

Then, probably add a new command for this?

iptables-nft -K INPUT -t filter => to remove the INPUT/filter basechain.

Then:

iptables-nft -N INPUT -t filter

to bring it back (if it was removed).