Understanding Bash fork() Bomb :(){ :|:& };: code

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Can you explain the following bash code or bash fork() bomb code?
:(){ :|:& };:

The fork bomb is a form of denial-of-service (DoS) attack against a Linux or Unix-based system. It makes use of the fork operation. The :(){ :|:& };: is nothing but a bash function. This function get executed recursively. It is often used by sysadmin to test user process limitations on server. Linux process limits can be configured via /etc/security/limits.conf and PAM to avoid bash fork() bomb. Once a successful fork bomb has been activated in a system it may not be possible to resume normal operation without rebooting the system as the only solution to a fork bomb is to destroy all instances of it.

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WARNING! These examples may crash your computer if executed.

Understanding :(){ :|:& };: fork() bomb code

:() – Defined the function called :. This function accepts no arguments. The syntax for bash function is as follows:

foo(){
 arg1=$1
 arg2=$2
 echo 'Bar..'
 #do_something on $arg argument
}

fork() bomb is defined as follows:

:(){
 :|:&
};:

:|: – Next it will call itself using programming technique called recursion and pipes the output to another call of the function ‘:’. The worst part is function get called two times to bomb your system.

& – Puts the function call in the background so child cannot die at all and start eating system resources.

; – Terminate the function definition.

: – Call (run) the function aka set the fork() bomb.
Here is more human readable code:

bomb() { 
 bomb | bomb &
}; bomb

Properly configured Linux / UNIX box should not go down when fork() bomb sets off. See the comment # 5 below for more fork bomb examples created in Perl, Windows XP (batch) and C.

Related: How to: Prevent a fork bomb by limiting user process under Linux.

Preventing fork bomb on Linux

Type the following ulimit command to find out the current maximum processes you can run on Linux:
ulimit -u
OR
ulimit -a
Preventing Fork Bomb on Linux
The number 128038 indicates that you can run 128038 processes. To protect your Linux system from a fork bomb, you need to lower that number. To limit your session to 5000 processes, use the following command
ulimit -S -u 5000

WARNING! Please don’t set ulimit numbers too low. This will prevent you from working on your system.

Now run fork bomb again:
:(){ :|:& };:
And you will see messages as follows:

bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable
bash: fork: Resource temporarily unavailable

You just avoided fork bomb on Linux. Run the following pgrep command to see the current threads limit:
pgrep -wcu $USER
Sample outputs:

5002

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Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

Start the discussion at www.nixcraft.com

Historical Comment Archive

32 comment

  1. i tested this on a fresh install of CentOS and it totaly crashed :P

    How about a nice “howto” for protecting against this attacks?

  4. Yes How to must be started, for the benefits of the innocent users, as the viruses are spread by entities whose brains are configured differently.

  5. Perl exmaple:

    perl -e "fork while fork" &

    Python example:

    import os
  while(1):
      os.fork()

    Windows XP / Vista bat file example:

    :bomb
start %0
goto bomb

    UNIX style for Windows:

    %0|%0

    C program example:

    #include 
 int main() {   while(1)      fork();  }

    Plz note that the fork bomb is a form of denial of service, so don’t run on production or unauthorized system.

  6. An other one is that when you have set a quota for your mailboxes and crontab is generating mail over and over eventually the quota will exceed, after a while the mailq will fill up with 1000’s of mail and the system will crash.

  7. Hi,

    i run it on my testing server and it start consuming my server process.
    so one thing is clear from this form bomb that it starting issuing new processes rather than threads.

    how we prevent our sites from such fork attacks

    Thanks in advance

    1. Yes. And when I don’t tie my shoe laces, I can fall…

      And, and, … And, … when I eat too much, I get stomach ache

  11. Sleek code :)

    I tried on FC4. Took only 1 minute as root.

    In WinXP, I waited till 7 minutes. Displayed some errors and all, but was responding. Tried MS Word, Task Manager, nothing could be executed. The machine was useless untill reset.

  14. A fork bomb process which can no longer fork exits. The following short Z Shell code will typically get rid of the above fork bomb in about a minute:

    while (sleep 100 &!) do; done

    It keeps trying and will eventually start a new do-nothing process; Each new do-nothing process reduces the number of rampant “fork bomb” processes by one, until eventually all of them are eradicated, at which point the do-nothing processes can exit.

  15. Use ulimit -u 30 (where 30 it is a max number of process ) to protect yourself from this attack(or config your limits.conf)

  17. This did 100% of nothing on my AuroraUX^ machine.
    Load went up for about min to min and a half, then the kernel killed the process for me.
    Silly Linux..

    1. On my Ubuntu 10.04 LTS the fork bomb launched about 2500 processes and then my kernel killed it. Total time ~< 5 min.

  18. How would i send a forkbomb/ebomb to an ip without requiring passwords like the ones controled by keys when u stimotaunisusley press keys it opens up stuff on the targets ip.

  21. WARNING! These examples may crash your computer if executed.

    may?
    They WILL crash your comp.
    Win7 – batch WILL kill (also input lag)

  24. :(){ : | : & }; :
    • :() is a function name with the open and close parenthesis representing no parameters. The body of this function is within the braces {}.
    • : | : is a recursive call since : is the name of the function. The function gets called twice and is piping input from one call to the other.
    • & backgrounds the previous function call so that it will not die.
    • ; finishes the function declaration.
    • : Calls the function just defined.

  25. I did the %0|%0 command in Windows XP. Type into Notepad and saved as FORKBOMB.BAT then double-clicked. Fortunately, I already had task manager running, so I right-clicked explorer and hit “end process tree” which left me with a blank desktop. I then used Alt+Ctrl+Del to reopen Task Manager and from there, typed explorer.exe in the RUN dialog to get Windows back up without rebooting.

    If you add an additional command, for example:

    PING [insert url of hated website]

    you can spam out DOS attacks as well while you watch your computer crash. Seriously people, don’t do that…

  26. It’s almost a right-of-passage to get tricked into running this command when you get your first system. Part of unwinding why your system just crashed actually teaches you quite a bit about unix pipes.

    For that reason I’m partly sad that in at least Ubuntu 12.04, the default ulimit setting prevents the bomb from taking down the system.

  27. There is a minimal “cure” for this.

    Classic REISUB Linux magic key combo always works for rebooting computer, however, using Alt+SysRq+K will stop all processes on current tty. In the case it doesn’t work, probably you’re on X server and need to use Alt+SysRq+RK.

    1. Ooops! Totally crashed. fork: Cannot allocate memory. Then Call Trace. Then, not responding to anything, just call trace. Awesome! Is this second to vmsplice?

      More power nixCraft!

  29. Why can you create a function named : at all?

    :(){ echo colonfunc; }
    :
    colonfunc

    This works for in interactive shell, but not if you put this into a script.
    cat < colon.sh
    #!/bin/sh
    :(){ echo colonfunc; }
    :
    NNNN
    chmod +x colon.sh
    ./colon.sh
    ./colon.sh: line 2: `:': not a valid identifier

    This is what I would expect.
    I’ve noticed some but not all versions of bash also allow you to use:
    * @ , ? . + – =
    and other punctuation characters as shell function names.

    % @(){ echo colonfunc; }
    bash: syntax error near unexpected token `}’

    This behavior I would expect. However, try this on my OSX Macbook:
    % @(){ echo colonfunc; }
    % @
    colonfunc

    This is strange, it worked.

    What is the difference? Bash versions.
    @ works with:
    GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)
    @ does not work with:
    GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu)

    Even worse, you can call some functions with punctuation names, but you cannot call them as functions. You can list shell functions with “set | less”. Now search /colonfunc in less for all of the shell functions:

    % () 
{ 
    echo colonfunc
}
, () 
{ 
    echo colonfunc
}
. () 
{ 
    echo colonfunc
}
= () 
{ 
    echo colonfunc
}

    % is a function, but you cannot call it.

    % %
    bash: fg: %: no such job

    But this works just fine:
    % =
    colonfunc

    This behavior varies between versions of bash. The fact that shell does allow some punctuation characters as function names seems like a bug. The fact not all versions of bash allows the same set of punctuation characters as function names seems to verify this theory.

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