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

[Phil Sutter <phil@nwl.cc>]

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.

Initially, a map came to mind. Something like:

| dnat to : tcp dport map { 1000-2000 : 5000-6000 }

To my surprise, nft accepts the syntax (listing is broken, though). But
IIUC, it means "return 5000-6000 for any port in [1000;2000]" and dnat
does round-robin? At least it's not what one would expect. Maybe one
could control the lookup behaviour somehow via a flag?

Cheers, Phil