[CentOS] OT : iptables/arptables question

Fabian Arrotin fabian.arrotin at arrfab.net
Tue Jan 27 19:14:41 UTC 2009

Filipe Brandenburger wrote:
> Hi Fabian:
> On Tue, Jan 27, 2009 at 08:16, Fabian Arrotin <fabian.arrotin at arrfab.net> wrote:
>> Question is : how can i "mangle" output packets to appear coming from
>> public ip and not from 192.168.X.X ?
> Found this that might help you (google for: linux default outgoing ip):
> """
>> On a machine with multiple interfaces, is it possible to set the default
>> outgoing IP address to something other than the address for the interface
>> on the outgoing route?
> Yes.
> ip route add via src
>                                                           ^^^
> The src parameter tells the routing code to use this address when sending
> packets. The address only needs to be on the system. IE:
> ip addr add dev dummy0
> And send the packets out of eth0.
> """
> From: http://lkml.indiana.edu/hypermail/linux/kernel/0112.1/0359.html
> Just make sure you keep a separate route for your ISP's side of the
> private network (maybe the one created when your interface goes up
> will do), otherwise your routing protocol might fail.
> HTH,
> Filipe

Hi Filipe,

thanks for the link, i completely missed that point from the 'ip route' 
On the other hand, 10 minutes after i had sent my mail (and 3 coffee 
later to be precise) i saw also a picture from wikipedia 
explaining how packets went through the different ip tables and it was 
directly clear : even packets leaving the local box (and being processed 
in the OUTPUT filter) are still processed in the nat table (postrouting 
filter) so a simple SNAT rule did the job perfectly too ;-) In fact it's 
the first time that i have to modify packets leaving a linux gateway and 
i thought that only packets being forwarded (and so traversing the 
FORWARD filter) could also being modified in the nat table ...
I've also had a look in the sysconfig.txt file to see how your solution 
could be applied but it's still not very clear how that can be done. But 
using GATEWAYDEV=eth3 (eth3 having my public-ip/32 while eth3:1 having 
my 192.168.X.X/24 ip) in the /etc/sysconfig/network and declaring a 
GATEWAY=192.168.X.X (isp router ip) in the ifcfg-eth3:1 does also the 
job. But a `route -n` is strange though : "         U     0      0        0 eth3" like for the old ppp 

So multiple ways to solve the initial question ...
Fabian Arrotin
  idea=`grep -i clue /dev/brain` ; test -z "$idea" && echo "sorry, init 
6 in progress" || sh ./answer.sh

Fabian Arrotin
  idea=`grep -i clue /dev/brain` ; test -z "$idea" && echo "sorry, init 
6 in progress" || sh ./answer.sh

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