The limit feature in iptables specifies the maximum average number of matches to allow per second. You can specify time intervals in the format /second, /minute, /hour, or /day, or you can use abbreviations so that 3/second is the same as 3/s.In this example, ICMP echo requests are restricted to no more than one per second. When tuned correctly, this feature allows you to filter unusually high volumes of traffic that characterize denial of service (DOS) attacks and Internet worms.

iptables -A INPUT -p icmp --icmp-type echo-request -m limit --limit 1/s -i eth0 -j ACCEPT
iptables -A INPUT -p tcp --syn -m limit --limit 5/s -i eth0 -j ACCEPT

* Boot from debian CD
* Change to console 2 with alt + F2 keystrokes
* Make a new dir to mount the boot partition, for example /disk
* mount the root partition on /disk.
* Do a chroot to the root partition: issuing chroot /disk
* Run grub command to enter grub shell
* Type in the root disk for grub, for example root (hd0,1). This is /dev/hda2 on my system
* Type the following command to install grub on /dev/hda: setup (hd0).
* Last step, type quit, exit from chroot, unmount all disks and reboot