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11.2. Backing up the system

Remember that if you are sure the system has been compromised you cannot trust the installed software or any information that it gives back to you. Applications might have been trojanized, kernel modules might be installed, etc.
The best thing to do is a complete file system backup copy (using dd) after booting from a safe medium. Debian GNU/Linux CD-ROMs can be handy for this since they provide a shell in console 2 when the installation is started (jump to it using Alt+2 and pressing Enter). From this shell, backup the information to another host if possible (maybe a network file server through NFS/FTP). Then any analysis of the compromise or re-installation can be performed while the affected system is offline.
If you are sure that the only compromise is a Trojan kernel module, you can try to run the kernel image from the Debian CD-ROM in rescue mode. Make sure to startup in single user mode, so no other Trojan processes run after the kernel.