Question regarding packet src/dst rewriting

Question regarding packet src/dst rewriting

[Russell Handorf]

Hello all,

I have a linux router over here with 4 ethernet nics running on 2.6.4 
kernel. The respective IP's are as follows:
eth0 : peering point to ISP
eth1: public routable IP block (/23)
eth2 : 172.16.0.1
eth3 : 192.168.0.1

The weird behavior I am having is that whenever a packet from the 
Internet comes in interface eth0, and gets forwarded to a server on 
eth1, the server which resides on the eth1 side of the network sees the 
source IP as coming from the router and not the real host on the 
Internet. Vice versa applies as well (a node behind eth1 sends a packet 
out to, say, google.com. Google.com sees the data as coming from the 
router, and not the node which is behind eth1).

I dont understand why the packet headers are being re-written. I also 
dont understand if this is a specific problem with iptables, or the 
linux kernel itself.

eth2 and eth3 are masquerading, but eth1 is not.

Any help would be greatly appreciated.

Thanks again,

Russ

Re: Question regarding packet src/dst rewriting

[Samuel Jean]

On Fri, August 20, 2004 12:32 pm, Russell Handorf said:
> The weird behavior I am having is that whenever a packet from the
> Internet comes in interface eth0, and gets forwarded to a server on
> eth1, the server which resides on the eth1 side of the network sees the
> source IP as coming from the router and not the real host on the
> Internet. Vice versa applies as well (a node behind eth1 sends a packet
> out to, say, google.com. Google.com sees the data as coming from the
> router, and not the node which is behind eth1).
>

My assumption is that you are using a MASQUERADE rule regardless of the
outgoing device.

> I dont understand why the packet headers are being re-written. I also
> dont understand if this is a specific problem with iptables, or the
> linux kernel itself.
>
> eth2 and eth3 are masquerading, but eth1 is not.
>

Okay, so you are using 2 MASQUERADE rules with specific outgoing device ?
Please post your ruleset. PREROUTING and POSTROUTING chains should be enough.