Firmware-mod-kit Extract ilegal image after build.

[fazzianeric]

Greetings, i've been spending time on this a while now, i'm trying to create something like an stock firmware for an router for my ISP use. And for this I used many tools to extract and recompile de .bin file, I succeded on extracting, altering the files I needed but when i recompile inevitably the checksumd5 alters and the router doesn't accept the firmware, everything I did till here was with regular tools like Binwalk, unsquashfs and mksquashfs and I decided to give it a try with the mod-kit, but then I saw it is more linke a script that uses the very same tools i've been using so far, the main issue here is that when the image is the checksum canges and I know it is very hard to avoid this, but the router checks this information and I don't know the parameter that it uses to reconize an legal firmware from an "ilegal". So I guess the question is, is there a way to do this without changing the checksum of it?