In computing, a fork bomb (also called rabbit virus) is a denial-of-service (DoS) attack wherein a process continually replicates itself to deplete available system resources, slowing down or crashing the system due to resource starvation.
In computing, a fork bomb (also called rabbit virus) is a denial-of-service (DoS) attack wherein a process continually replicates itself to deplete available system resources, slowing down or crashing the system due to resource starvation.
Around 1978, an early variant of a fork bomb called wabbit was reported to run on a System/360. It may have descended from a similar attack called RABBITS reported from 1969 on a Burroughs 5500 at the University of Washington. [1]
Fork bombs operate both by consuming CPU time in the process of forking, and by saturating the operating system's process table. [2] [3] A basic implementation of a fork bomb is an infinite loop that repeatedly launches new copies of itself.
In Unix-like operating systems, fork bombs are generally written to use the fork system call. [3] As forked processes are also copies of the first program, once they resume execution from the next address at the frame pointer, they continue forking endlessly within their own copy of the same infinite loop. this has the effect of causing an exponential growth in processes. As modern Unix systems generally use a copy-on-write resource management technique when forking new processes, [4] a fork bomb generally will not saturate such a system's memory.
Microsoft Windows operating systems do not have an equivalent functionality to the Unix fork system call; [5] a fork bomb on such an operating system must therefore create a new process instead of forking from an existing one, such as with batch echo %0^|%0 > $_.cmd & $_. In this batch script, %0|%0 is written to $_.cmd, which is then executed by & $_. [6]
A classic example of a fork bomb is one written in Unix shell :(){ :|:& };:, possibly dating back to 1999, [7] which can be more easily understood as
fork(){fork|fork&} fork In it, a function is defined (fork()) as calling itself (fork), then piping (|) its result into itself, all in a background job (&).
The code using a colon : as the function name is not valid in a shell as defined by POSIX, which only permits alphanumeric characters and underscores in function names. [8] However, its usage is allowed in GNU Bash as an extension. [9]
As a fork bomb's mode of operation is entirely encapsulated by creating new processes, one way of preventing a fork bomb from severely affecting the entire system is to limit the maximum number of processes that a single user may own. On Linux, this can be achieved by using the ulimit utility; for example, the command ulimit -u 30 would limit the affected user to a maximum of thirty owned processes. [10] On PAM-enabled systems, this limit can also be set in /etc/security/limits.conf, [11] and on *BSD, the system administrator can put limits in /etc/login.conf. [12] Modern Linux systems also allow finer-grained fork bomb prevention through cgroups and process number (PID) controllers. [13]
Bash, short for Bourne-Again SHell, is a shell program and command language supported by the Free Software Foundation and first developed for the GNU Project by Brian Fox. Designed as a 100% free software alternative for the Bourne shell, it was initially released in 1989. Its moniker is a play on words, referencing both its predecessor, the Bourne shell, and the concept of rebirth.
Cygwin is a free and open-source Unix-like environment and command-line interface (CLI) for Microsoft Windows. The project also provides a software repository containing open-source packages. Cygwin allows source code for Unix-like operating systems to be compiled and run on Windows. Cygwin provides native integration of Windows-based applications.
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be command languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text. A script which sets up the environment, runs the program, and does any necessary cleanup or logging, is called a wrapper.
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.
The Bourne shell (sh) is a shell command-line interpreter for computer operating systems. It first appeared on Version 7 Unix, as its default shell. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link or hard link to a compatible shell—even when other shells are used by most users.
Almquist shell is a lightweight Unix shell originally written by Kenneth Almquist in the late 1980s. Initially a clone of the System V.4 variant of the Bourne shell, it replaced the original Bourne shell in the BSD versions of Unix released in the early 1990s.
In Unix-like and some other operating systems, the pwd command writes the full pathname of the current working directory to the standard output.
In computing, particularly in the context of the Unix operating system and its workalikes, fork is an operation whereby a process creates a copy of itself. It is an interface which is required for compliance with the POSIX and Single UNIX Specification standards. It is usually implemented as a C standard library wrapper to the fork, clone, or other system calls of the kernel. Fork is the primary method of process creation on Unix-like operating systems.
In computer programming, glob patterns specify sets of filenames with wildcard characters. For example, the Unix Bash shell command mv *.txttextfiles/ moves all files with names ending in .txt from the current directory to the directory textfiles. Here, * is a wildcard and *.txt is a glob pattern. The wildcard * stands for "any string of any length including empty, but excluding the path separator characters ".
In computing, time is a command in Unix and Unix-like operating systems. It is used to determine the duration of execution of a particular command.
bc, for basic calculator, is "an arbitrary-precision calculator language" with syntax similar to the C programming language. bc is typically used as either a mathematical scripting language or as an interactive mathematical shell.
In computing, echo is a command that outputs the strings that are passed to it as arguments. It is a command available in various operating system shells and typically used in shell scripts and batch files to output status text to the screen or a computer file, or as a source part of a pipeline.
In computing, kill is a command that is used in several popular operating systems to send signals to running processes.
A command shell is a command-line interface to interact with and manipulate a computer's operating system.
test is a command-line utility found in Unix, Plan 9, and Unix-like operating systems that evaluates conditional expressions. test was turned into a shell builtin command in 1981 with UNIX System III and at the same time made available under the alternate name [.
In computing, a shell builtin is a command or a function, called from a shell, that is executed directly in the shell itself, instead of an external executable program which the shell would load and execute.
In computing, a shebang is the character sequence #!, consisting of the characters number sign and exclamation mark, at the beginning of a script. It is also called sharp-exclamation, sha-bang, hashbang, pound-bang, or hash-pling.
In Unix and Unix-like operating systems, type is a command that describes how its arguments would be interpreted if used as command names.
In Unix and Unix-like operating systems, printf is a shell builtin that formats and outputs text like the same-named C function.
OpenRC is a dependency-based init system for Unix-like computer operating systems. It was created by Roy Marples, a NetBSD developer who was also active in the Gentoo project. It became more broadly adopted as an init system outside of Gentoo following the decision by some Linux distributions not to adopt systemd.
bash$ :(){ :|:&};:}Name: In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set. The first character of a name is not a digit.
When the shell is in POSIX mode (see Bash POSIX Mode), fname must be a valid shell name and may not be the same as one of the special builtins (see Special Builtins). In default mode, a function name can be any unquoted shell word that does not contain '$'.
