"Inveniam viam aut faciam" : I will either
find a way, or I shall make one
- FORK BOMB
First off lets get the basics
sorted first, the threat of a fork bomb can be classified as a
Denial-of-Service attack, as it will totally exhaust resources on the
server being victimized. But, it can only be made possible if the
server has not been configured fully - most especially the limits
placed upon the users of the system. I know that each possible
classification has its advocates, but no matter what you think the
threat is real.
What Is It? The threat is simply that any user
able to execute commands on a system can bring the system to a grinding
halt by using the resources. And you did not read wrong, anyone can do
it. It does not require any major technical knowledge, in fact here it
is here; as a simple shell script..
Do not test this on any server you
cannot reboot or which is being used for production purposes. What the
above script does is just create multiple instances of itself and just
carrys on doing it until the CPU just grinds to a halt, and if the CPU
does not give out, then the memory or some other resource will. And you
can get nasty as well..
echo "oh darn it" >> fork.$RANDOM &
This will not only create multiple
instances of the process, but each process will create a file with a
short message inside. Thus adding a new threat to the disk space of the
targeted system. In fact you could add other attacks in as well.
What To Do
About It? Well, as I started off saying, this
threat is made possible if the server has not been fully configured.
You see, what you can do is use the ulimit command to limit the number of possible
processes a user can have. Add the following -purely a suggested limit-
to the startup files (/etc/profile)
of the users.
ulimit -u 200
Or you can do the following to
apply the limit to all users upon system startup
echo "* hard nproc 200" >>
As far as threat of disk usage
goes, the only thing you can do is to have the system files and the
user directories -including the /tmp folder- seperate. And it is actually as
simple as that, so there is really no reason to not have it set on your
Final Words Have fun trying it, lets face it,
there is nothing like actually typing a couple of commands and bringing
a server to a grinding halt. Just bear in mind that I warned you about
testing it on production machines and defintely on machines you do not
have responsibility for. As always, learn, have fun and make your