[Cake] de-natting & host fairness

[Kevin Darbyshire-Bryant]

On 28/09/16 00:08, Jonathan Morton wrote:
>
>> On 26 Sep, 2016, at 06:20, Kevin Darbyshire-Bryant
>> <kevin at darbyshire-bryant.me.uk> wrote:
>>
>> Another github user 'tegularius' presented some beautifully crafted
>> code that did the lookups in a much neater way.  Originally it too
>> had an 'ingress' lookup problem.  This was worked on and I hacked
>> some conditional 'denat' options into cake & tc.
>>
>> For your 'delight' a denat cake
>> https://github.com/kdarbyshirebryant/sch_cake/tree/natoptions along
>> with a matching tc
>> https://github.com/kdarbyshirebryant/tc-adv/tree/denat
>
> As I’m now at the stage of trying to merge this, I’m going to make
> some executive design decisions:
>
> - De-NAT IPv4 packets only.  I think it’s safe to assume that IPv6
> NAT will be rare, and in any case will typically preserve host
> distinctions.  This eliminates switch blocks in favour of simple if
> blocks.

Agree completely.  The IPv6 stuff was inherited/for completeness but 
anyone doing many to one host masquerading with IPv6 really needs a slap!

>
> - Don’t bother with the distinction between src-NAT and dst-NAT
> lookups.  The full lookup has to be done anyway and then masked off,
> the use-case for the limited functionality is nebulous, and all we’re
> doing is adding a lot of nasty conditional branches to the fast
> path.

I winced at every condition as it was being put in believe me!  It is 
horrible and I think now is a left over from when I was trying to 
understand how/why things weren't being translated as expected.  I still 
don't completely trust it, but that's what testing is for :-)

>
> This in turn reduces the configuration interface for the feature to a
> flag, which I’ll call “nat”.

Agreed.

Does this need to be another variable/parameter or could it be the next 
bit along in the flow type?