iptables: built-in INPUT chain in nat table?

Posted by ughmandaem on Server Fault See other posts from Server Fault or by ughmandaem
Published on 2011-03-10T07:26:58Z Indexed on 2011/03/10 8:11 UTC
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iptables

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nat

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input

I have a Gentoo Linux system running linux 2.6.38-rc8. I also have a machine running Ubuntu with linux 2.6.35-27. I also have a virtual machine running Debian Unstable with linux 2.6.37-2.

On the Gentoo and Debian systems I have an INPUT chain built into my nat table in addition to PREROUTING, OUTPUT, and POSTROUTING. On Ubuntu, I only have PREROUTING, OUTPUT, and POSTROUTING.

I am able to use this INPUT chain to use SNAT to modify the source of a packet that is destined to the local machine (imagine simulating an incoming spoofed IP to a local application or just to test a virtual host configuration). This is possible with 2 firewall rules on Gentoo and Debian but seemingly not so on Ubuntu. I looked around for documentation on changes to the SNAT target and the INPUT chain of the nat table and I couldn't find anything.

Does anyone know if this is a configuration issue or is it something that was just added in more recent versions of linux?

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