The document discusses the ulimit command, which is used to set resource limits for users on Linux systems. It lists the different options for ulimit like -n to set the maximum number of open files, -u for the maximum processes per user, and -t for the maximum CPU time. It then explains that exceeding these limits through things like a fork bomb, which is a denial of service attack where a process continuously replicates itself, can slow down or crash a system through resource starvation. Examples of fork bombs in different programming languages like Bash, Python, and Brainfuck are also provided.

1. ulimit
(built-in)

2. $ ulimit -a <<< etc/security/limits.conf
...display and set resources limits for users.

3. $ ulimit -a ulimit -n
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 17887
max locked memory (kbytes, -l) 16384
max memory size (kbytes, -m) unlimited
open files (-n) 1024 ----------------> DDos attack
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited fork bomb
max user processes (-u) 17887 -----------------> Bad code
virtual memory (kbytes, -v) unlimited recursive
file locks (-x) unlimited

4. Syntax
ulimit [-abcdefHilmnpqrsStTuvx] [limit]
Key
-S Set a soft limit for the given resource.
-H Set a hard limit for the given resource.
-a All current limits are reported.
-l The maximum size that can be locked into memory.
-m The maximum resident set size.
-n The maximum number of open file descriptors.
-p The pipe buffer size.
-q The maximum number of bytes in POSIX message queues.
-r The maximum real-time scheduling priority.
-s The maximum stack size.
-t The maximum amount of cpu time in seconds.
-T The maximum number of threads.
-u The maximum number of processes available to a single user.
-v The maximum amount of virtual memory available to the process.
-x The maximum number of file locks.

5. Fork Bomb
In computing, a fork bomb (also called rabbit virus or
wabbit) is a denial-of-service attack wherein a process
continually replicates itself to deplete available system
resources, slowing down or crashing the system due to
resource starvation.

6. Fork Bomb examples
jAVA
PERL
RUBY
.
.
.
BASH
:(){ :|:& };:
PYTHON
import os
while True:
os.fork()
Brainfuck
+[>+]