SNAT rule on LAN - unexpected result

SNAT rule on LAN - unexpected result

[Al Grant]

Hi All,

Doing a little experimenting with iptables and more specifically SNAT.

I have two computers, a Ubunutu box and a Win7 box with a  switch between them.

On the Ubuntu box (192.168.15.200) I have added:

iptables -t nat -A POSTROUTING -o eth4 -j SNAT --to-source 192.168.15.201


Now I run wireshark on the Win7 machine and various bits of broadcast
traffic which was coming from .200 now shows as from .201 - great!

So I tried a ping from the Win7 machine and would expect a reply to
show in wireshark, but that the src address be changed to .201 - but
nope I get a reply from .200. How is this so?

Conversely a ping from the Ubuntu machine to the win7 machine shows
the packets with a src of .201 in wireshark - as expected.

Cheers in advance,

-Al


--
"Beat it punk!"
- Clint Eastwood

Re: SNAT rule on LAN - unexpected result

[Vigneswaran R]

On 07/21/2013 02:21 PM, Al Grant wrote:
> Hi All,
>
> Doing a little experimenting with iptables and more specifically SNAT.
>
> I have two computers, a Ubunutu box and a Win7 box with a  switch between them.
>
> On the Ubuntu box (192.168.15.200) I have added:
>
> iptables -t nat -A POSTROUTING -o eth4 -j SNAT --to-source 192.168.15.201
>
>
> Now I run wireshark on the Win7 machine and various bits of broadcast
> traffic which was coming from .200 now shows as from .201 - great!
>
> So I tried a ping from the Win7 machine and would expect a reply to
> show in wireshark, but that the src address be changed to .201 - but
> nope I get a reply from .200. How is this so?
> Conversely a ping from the Ubuntu machine to the win7 machine shows
> the packets with a src of .201 in wireshark - as expected.

I think, this is due to the fact that 'nat' table will be consulted only 
for new connections. Unlike ping request, the ping *reply* may not be 
considered as part of a new connection.


Regards,
Vignesh