The Character Generator Protocol (CHARGEN) is a service of the Internet Protocol Suite

Internet protocol suite

The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...

 defined in RFC 864 in 1983 by Jon Postel

Jon Postel

Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards...

. It is intended for testing, debugging, and measurement purposes.

A host may connect to a server that supports the Character Generator Protocol on either Transmission Control Protocol

Transmission Control Protocol

The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

 (TCP) or User Datagram Protocol

User Datagram Protocol

The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...

 (UDP) port number 19. Upon opening a TCP connection, the server starts sending arbitrary characters to the connecting host and continues until the host closes the connection. In the UDP implementation of the protocol, the server sends a UDP datagram containing a random number (between 0 and 512) of characters every time it receives a datagram from the connecting host. Any data received by the server is discarded.

Inetd implementation

On most UNIX-like

Unix-like

A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 operating systems a CHARGEN server is built into the inetd

Inetd

inetd is a super-server daemon on many Unix systems that manages Internet services. First appearing in 4.3BSD , it is generally located at /usr/sbin/inetd.-Function:...

 (or xinetd) daemon

Daemon (computer software)

In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...

. The CHARGEN service is usually not enabled by default. It may be enabled by adding the following lines to the file /etc/inetd.conf and telling inetd to reload its configuration:
chargen stream tcp nowait root internal
chargen dgram udp wait root internal

Applications

The CHARGEN service may be used as a source of a byte-stream for debugging TCP network code for proper bounds checking and buffer management. It may also be a source of generic payload for bandwidth measurement and/or QoS fine-tuning. Although consideration must be given if hardware compression is active, as the output from the CHARGEN service is easily and efficiently compressed. This compression can cause bandwidth tests to report the size of the data after decompression, instead of the actual amount of data which passed the wire.

Sample session

A typical CHARGEN service session looks like this: The user connects to the host using a telnet

TELNET

Telnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility using a virtual terminal connection...

 client.
The user receives a stream of byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s. Although the specific format of the output is not prescribed by RFC 864, the recommended pattern (and a de-facto standard) is shifted lines of 72 ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

 characters repeating.

$ telnet localhost chargen

Trying 127.0.0.1...

Connected to localhost.

Escape character is '^]'.

!"#$%&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefgh

"#$%&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghi

$%&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghij


$%&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijk

%&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijkl

&'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklm

'*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmn

*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmno

)*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnop

+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopq


+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqr

,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrs

-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrst

./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstu

/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuv

^]

telnet> quit

Connection closed.

This continues until the TCP connection is closed as shown in the trace by ending the telnet session.

Abuse

The service was used maliciously to crash MS DNS servers running Microsoft Windows NT 4.0 by piping the arbitrary characters straight into the DNS server listening port (telnet ntbox 19 | telnet ntbox 53). However, the attack was presumably a symptom of improper buffer management on the part of Microsoft's DNS service and not directly related to the CHARGEN service.

See also

• Barber pole
• List of well-known ports (computing)
• Echo Protocol
  ECHO protocol
  The Echo Protocol is a service in the Internet Protocol Suite defined in RFC 862. It was originally proposed for testing and measurement of round-trip times in IP networks....
  
  
• Discard Protocol
• Daytime Protocol
• Time Protocol
  TIME protocol
  The Time Protocol is a network protocol in the Internet Protocol Suite defined in 1983 in RFC 868. Its purpose is to provide a site-independent, machine readable date and time....