debian-live persistence problem with /etc/network/interfaces

[PAM <pam@mifamofi.net>]

Hi,

I'm trying to create a live usb key, but I'm stuck to a problem I can't 
solve.

I'm using boot=live persistence toram option as kernel options

On a second partition of the live usb I put an ext3 partition with a 
label persistence

whatever the options I put in persistence.conf,
for example:
/ union

or
/etc union
/usr union
/home
/root

or
/etc link
/home
/root

or
/etc
/home
/root

/root seems to work (I can touch files, and delete them accross reboot)
but /etc/network/interfaces is always rewritten!

even if I boot without persistence, mod the file and reboot, the file is 
overwritten (on the persistence partition also!)

What mechanism I didn't understand? Any Idea?

PS: Sorry for my english it's not my natural language.