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

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

Hi,

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

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.

It should be possible to add a new netlink attribute

NFTA_NAT_REG_PROTO_SHIFT

which allows for -2^16 .. +2^16 to express the (positive/negative)
delta offset.

Parser would need to be taught to deal with negative and positive
offset, we probably need a new special type for named maps too
(port-shift).

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.