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Discovered: February 16, 2000
Updated: February 13, 2007 11:33:34 AM
Also Known As: Tribal Flood Network 2000
Type: Trojan Horse
Systems Affected: Linux, UNIX, Windows

TFN and TFN2K are not viruses, but attack tools that can be used to perform a distributed DoS attack. TFN and TFN2K can perform various attacks such as UDP flood attacks (similar to Trinoo), ICMP flood attack (similar to Smurf), and TCP SYN flood attacks.

Antivirus Protection Dates

  • Initial Rapid Release version December 18, 2000
  • Latest Rapid Release version August 08, 2016 revision 023
  • Initial Daily Certified version December 18, 2000
  • Latest Daily Certified version August 09, 2016 revision 001

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Discovered: February 16, 2000
Updated: February 13, 2007 11:33:34 AM
Also Known As: Tribal Flood Network 2000
Type: Trojan Horse
Systems Affected: Linux, UNIX, Windows

When a TCP connection is made from system to system, it exchanges the following sequence of messages before making the connection:
1. The client sends SYN message to a server.
2. The server sends a SYN-ACK message back to client.
3. The client sends an ACK message to confirm and the connection is opened.

Once the connection is made, service-specific data can be exchanged between client and server. During these initial communications, each SYN/ACK request is tracked in a table since multiple machines may want to connect to the server. If too many half connections are made, it may cause an overflow in the table and the server will not be able to accept new connections.

TCP SYN flood attacks are performed by sending a SYN with a spoofed IP, for which the server will wait for a SYN-ACK that will never arrive. These half connections will eventually time out. However, if you can pump enough half connections before the server is waiting for the time-out, it will cause the server to not accept new connections, resulting in a denial of service.

It may be possible to detect these attacks by looking for SYN_RECEIVED state using tools such as NetStat. You may also want to install a filter on the router to check if external packets actually have external IPs and also check to see if internal IPs have internal IP addresses.

TFN2K is a newer version of TFN that supports Windows NT and UNIX platforms. It also has some new features that make detection more difficult, including sending decoy information to avoid being traced.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.