keep-state and in-kernel NAT exposes local ip on external interface

[bycn82]

for me.  i am totally dont understand all these.   waiting dor example as
well

On Tuesday, July 28, 2015, Ian Smith <smithi at nimnet.asn.au> wrote:

> Way back on Wed, 1 Jul 2015 22:02:53 +0300, Lev Serebryakov wrote:
>  > On 30.06.2015 22:20, Georgios Amanakis via freebsd-ipfw wrote:
>  >
>  >   It is good example for my changes :) All this "skipto / keep-state"
>  > magic is not understandable.
>
> Indeed.  So all we're waiting for, Lev, is some simple usage examples of
> how things should be done with your new stateful operators, especially
> when combining stateful rules with NAT.  Please see what you can do ..
>
> I know it's clear to you, but I for one cannot get my head around these
> without a couple of examples, roughly suitable for inclusion in ipfw(8)
> EXAMPLES section.  Rough illustrations would do fine at first.
>
> In the problems shown lately, including the one below (harder to read
> with the quoting wrapped like that!) it seems the problem of keepalives
> being issued using the internal address would not occur if keep-state
> was only applied _after_ NAT, only on the already-translated source
> address, but like you and apparently many others, I find these rulesets
> very difficult to follow in terms of different possible flows.
>
> cheers, Ian
>
>  > > On FreeBSD 10.1p13 with two interfaces em0(internet) and em1(lan) I
>  > > can fish (tcpdump)packets on em0 which have escaped the in-kernel
>  > > NAT and have as source address an IP on the LAN.
>  > >
>  > > This should not happen and I can confirm that with pf this is not
>  > > the case. I have the following ipfw rules:
>  > >
>  > > nat:  ipfw nat 123 config ip xxx.xxx.xxx.xxx same_ports reset
>  > >
>  > > 00100 reass ip from any to any in 00200 allow ip from any to any
>  > > via lo0 00300 allow ip from any to any via em1 00400 nat 123 ip
>  > > from any to any in recv em0 00500 check-state 00600 skipto 24000 ip
>  > > from any to me dst-port 80,443,22,500,4500,1194,993,8112 in recv
>  > > em0 keep-state 00700 skipto 24000 ip from any to any out xmit em0
>  > > keep-state 00800 deny log ip from any to any 24000 nat 123 ip from
>  > > any to any out xmit em0 24100 allow ip from any to any
>  > >
>  > > Contrary to many online tutorials, including the example of the
>  > > handbook regarding NAT (
>  > > https://www.freebsd.org/doc/handbook/firewalls-ipfw.html), when
>  > > one places the NAT rules with the opposite order (i.e. outbound
>  > > rule first and then the inbound rule) the problem disappears.
>  > >
>  > > i.e. ... 00400 nat 123 ip from any to any out xmit em0 ... 24000
>  > > nat 123 ip from any to any in recv em0 ...
>  > >
>  > > Why is this happening? Any objections to reversing the order of the
>  > > NAT rules?
>
>  > - --
>  > // Lev Serebryakov
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