TCP reset attack
Encyclopedia
TCP reset attack, also known as "forged TCP resets", "spoofed TCP reset packets" or "TCP reset attacks". These terms refer to a method of tampering with Internet
communications. Sometimes, the tampering is malicious, other times, it is beneficial.
) or UDP (User Datagram Protocol
). TCP/IP is the protocol set used for email and web browsing. Each protocol has a block of information, called a header, included near the front of each packet. Headers contain information about which computer sent the packet, which computer should receive it, the packet size, etc.
TCP (as opposed to other protocols such as UDP) is used with IP when a virtual connection is required between two computers. TCP software on the two machines which will communicate (for example a workstation with a browser and a web server) by exchanging a stream of packets. Using a TCP connection gives the computers an easy way to exchange data items too big for a single packet, such as video clips, email attachments, or music files. Although some web pages are small enough for a single packet, they are sent over TCP connections for convenience.
When used as designed this can be a useful tool. One common application is the scenario where a computer (we'll call it computer A) crashes while a TCP connection is in progress. The computer on the other end (computer B) will continue to send TCP packets since it does not know computer A has crashed. When computer A reboots, it will then receive packets from the old pre-crash connection. Computer A has no context for these packets and no way of knowing what to do with them, so it might send a TCP reset to the sender of the packets - computer B. This reset lets computer B know that the connection is no longer working. The user on computer B can now try another connection or take other action.
used TCP resets to disrupt suspicious connections as early as 2003.
began using forged TCP resets to cripple peer-to-peer and certain groupware applications on their customers computers . This started a controversy, which was followed by the creation of the Network Neutrality Squad (NNSquad) by Lauren Weinstein
, Vint Cerf
, David Farber, Craig Newmark
and other well-known founders of and champions of openness on the Internet. In 2008 the NNSquad released the NNSquad Network Measurement Agent, a Windows software program written by John Bartas, which could detect Comcast's forged TCP resets and distinguish them from real endpoint-generated resets. Ironically the technology to detect the resets was developed from the earlier Open-source "Buster" software which used forged resets to block malware
and ads in web pages.
In January 2008 the FCC announced it would investigate Comcast's use of forged resets, and on August 21, 2008 it ordered Comcast to terminate the practice.
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
communications. Sometimes, the tampering is malicious, other times, it is beneficial.
Technical Background
The Internet is, in essence, a system for individual computers to exchange electronic messages, or packets of data. This system includes hardware to carry the messages, such as copper and fiber optics cables; and a formalized system for formatting the messages, called "protocols". The basic protocol used on the Internet is the IP protocol, which is usually coupled with additional protocols such as TCP (Transmission Control ProtocolTransmission 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...
) or UDP (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...
). TCP/IP is the protocol set used for email and web browsing. Each protocol has a block of information, called a header, included near the front of each packet. Headers contain information about which computer sent the packet, which computer should receive it, the packet size, etc.
TCP (as opposed to other protocols such as UDP) is used with IP when a virtual connection is required between two computers. TCP software on the two machines which will communicate (for example a workstation with a browser and a web server) by exchanging a stream of packets. Using a TCP connection gives the computers an easy way to exchange data items too big for a single packet, such as video clips, email attachments, or music files. Although some web pages are small enough for a single packet, they are sent over TCP connections for convenience.
TCP resets
The stream of packets in a TCP connection each contains a TCP header. Each of these headers contains a bit known as the "reset" (RST) flag. In most packets this bit is set to 0 and has no effect, however if this bit is set to 1 it indicates to the receiving computer that the computer should immediately stop using the TCP connection - It should not send any more packets using the connections identifying numbers (called ports), and discard any further packets it receives with headers indicating they belong to that connection. A TCP reset basically kills a TCP connection instantly.When used as designed this can be a useful tool. One common application is the scenario where a computer (we'll call it computer A) crashes while a TCP connection is in progress. The computer on the other end (computer B) will continue to send TCP packets since it does not know computer A has crashed. When computer A reboots, it will then receive packets from the old pre-crash connection. Computer A has no context for these packets and no way of knowing what to do with them, so it might send a TCP reset to the sender of the packets - computer B. This reset lets computer B know that the connection is no longer working. The user on computer B can now try another connection or take other action.
Forging TCP resets
In the scenario above the TCP reset bit was sent by a computer which was one of the connection endpoints. It's possible for a 3rd computer to monitor the TCP packets on the connection, and then send a "forged" packet containing a TCP reset to one or both endpoints. The headers in the forged packet must indicate, falsely, that it came from an endpoint, not the forger. This information includes the endpoint IP addresses and port numbers. Every field in the IP and TCP headers must be set to a convincing forged value for the fake reset to trick the endpoint into closing the TCP connection. Properly formatted forged TCP resets can be a very effective way to disrupt any TCP connection the forger can monitor.Are forgeries good or bad?
One obvious application of forged TCP reset is to maliciously disrupt TCP connections without the consent of the two parties which own the endpoints - more on this below. However, network security systems using forged TCP resets have been designed as well. A prototype "Buster" software package was demonstrated in 1995 that would send forged resets to any TCP connection which used port numbers in a short list. Linux volunteers proposed doing something similar with Linux firewalls in 2000 , and the open source SnortSnort (software)
Snort is a free and open source network intrusion prevention system and network intrusion detection system , created by Martin Roesch in 1998...
used TCP resets to disrupt suspicious connections as early as 2003.
Comcast Controversy
By late 2007 ComcastComcast
Comcast Corporation is the largest cable operator, home Internet service provider, and fourth largest home telephone service provider in the United States, providing cable television, broadband Internet, and telephone service to both residential and commercial customers in 39 states and the...
began using forged TCP resets to cripple peer-to-peer and certain groupware applications on their customers computers . This started a controversy, which was followed by the creation of the Network Neutrality Squad (NNSquad) by Lauren Weinstein
Lauren Weinstein
Lauren Weinstein may refer to:* Lauren Weinstein * Lauren Weinstein...
, Vint Cerf
Vint Cerf
Vinton Gray "Vint" Cerf is an American computer scientist, who is recognized as one of "the fathers of the Internet", sharing this title with American computer scientist Bob Kahn...
, David Farber, Craig Newmark
Craig Newmark
Craig Alexander Newmark is an Internet entrepreneur best known for being the founder of the San Francisco-based international website Craigslist.-Biography:...
and other well-known founders of and champions of openness on the Internet. In 2008 the NNSquad released the NNSquad Network Measurement Agent, a Windows software program written by John Bartas, which could detect Comcast's forged TCP resets and distinguish them from real endpoint-generated resets. Ironically the technology to detect the resets was developed from the earlier Open-source "Buster" software which used forged resets to block malware
Malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...
and ads in web pages.
In January 2008 the FCC announced it would investigate Comcast's use of forged resets, and on August 21, 2008 it ordered Comcast to terminate the practice.