ipfw fwd to closed port

[Slawa Olhovchenkov]

On Thu, Jun 09, 2016 at 09:08:33AM -0400, Kristof Provost wrote:

> 
> 
> On 9 Jun 2016, at 9:06, Slawa Olhovchenkov wrote:
> 
> > On Thu, Jun 09, 2016 at 03:00:17PM +0200, Kristof Provost wrote:
> >
> >> On 2016-06-09 02:02:40 (+0300), Slawa Olhovchenkov <slw at zxy.spb.ru> wrote:
> >>> Forwarding by ipfw to closed local port generating RST packet with
> >>> incorrect checksun. Is this know ussuse? Need open PR?
> >>
> >> Where did you capture the packet? If you've captured the packet on the
> >> machine that generated it tcpdump may indeed claim that the checksum is
> >> wrong, because it's computed by the hardware (so after tcpdump captured
> >> it).
> >
> > On the tun0 (destination of RST packet routed to tun0).
> > tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> metric 0 mtu 1500
> >         options=80000<LINKSTATE>
> >         inet 192.168.4.1 --> 192.168.4.1 netmask 0xffffff00
> >         inet6 fe80::240:63ff:fedc:ac9e%tun0 prefixlen 64 scopeid 0x9
> >         nd6 options=21<PERFORMNUD,AUTO\_LINKLOCAL>
> >         Opened by PID 1345
> >
> > tun0 don't computed checksum.
> 
> I’m not sure I understand what you’re trying to say.
> 
> In any case: either capture the packet outside the machine, or confirm
> that the checksum is wrong by watching the relevant netstat counters.

I am have machine with tun0 (see above) and ipfw rules:

04010  23880  2132855 fwd 127.0.0.1,3129 tcp from 192.168.0.0/16 to not me dst-port 80,3128,8080,8100-8105 recv tun0

# netstat -rn
192.168.4.0/24     192.168.4.1        UGS        tun0
192.168.4.1        link#9             UH         tun0

tun0 handled by coova-chilli.

Initator from network 192.168.4.0/24 (ex: 192.168.4.4) send packet to outside, 8.8.8.8 for example.
fwd on tun0 forwarded tin 127.0.0.1,3129. No listener on 127.0.0.1:3129, RST generated from 8.8.8.8:80
to 192.168.4.4:2345. This packet routed to tun0 an received by chilli.

Checksums must be correct at this point, on tun0 interface for correct handling in chilli.