Re: [PATCH nftables 8/8] test: py: add tests for shifted nat port-ranges

[Florian Westphal <fw@strlen.de>]

Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Fri, Mar 24, 2023 at 11:59:04PM +0100, Florian Westphal wrote:
> > Jeremy Sowden <jeremy@azazel.net> wrote:
> > > +ip daddr 10.0.0.1 tcp dport 55900-55910 dnat ip to 192.168.127.1:5900-5910/55900;ok
> > > +ip6 daddr 10::1 tcp dport 55900-55910 dnat ip6 to [::c0:a8:7f:1]:5900-5910/55900;ok
> > 
> > This syntax is horrible (yes, I know, xtables fault).
> > 
> > Do you think this series could be changed to grab the offset register from the
> > left edge of the range rather than requiring the user to specify it a
> > second time?  Something like:
> > 
> > ip daddr 10.0.0.1 tcp dport 55900-55910 dnat ip to 192.168.127.1:5900-5910
> > 
> > I'm open to other suggestions of course.
> 
> To allow to mix this with maps, I think the best approach is to add a
> new flag (port-shift) and then allow the user to specify the
> port-shift 'delta'.
> 
> ip daddr 10.0.0.1 tcp dport 55900-55910 dnat ip to ip saddr map { \
>         192.168.127.0-129.168.127.128 : 1.2.3.4 . -55000 } port-shift

Sorry, I don't see the usecase for different deltas.
But even if we assume that, kernel already takes the dnat target port
number from a register.

> where -55000 means, subtract -55000 to the tcp dport in the packet, it
> is an incremental update.
> 
> This requires a kernel patch to add the new port-shift flag.

... so I don't see why we need a new port-shift flag at all.
I think best approach is to provide the actual new dport in a register,
like we already do right now.

So we need an 'add' operation in kernel to compute

portreg = sreg_with_port + sreg_with_offset
> 
> Florian, this is based on your idea to support 'add' command, which is
> still needed for other usecases. I think nat is special in the sense
> that the goal is to feed the registers that instruct the NAT engine
> what kind of mangling is needed.

See above.  I don't think we should go with the existing NAT flag,
its very much a hack to overcome iptables design limitations.