Syn Flood

Flood is a denial of service attack to which every TCP/IP implementation is vulnerable (to some degree). Each half-open TCP connection made to a machine causes the ‘tcpd’ server to add a record to the data structure that stores information describing all pending connections. This data structure is of finite size, and it can be made to overflow by intentionally creating too many partially-open connections.

The half-open connections data structure on the victim server system will eventually fill and the system will be unable to accept any new incoming connections until the table is emptied out. Normally there is a timeout associated with a pending connection, so the half-open connections will eventually expire and the victim server system will recover. However, the attacking system can simply continue sending IP-spoofed packets requesting new connections faster than the victim system can expire the pending connections.

In some cases, the system may exhaust memory, crash, or be rendered otherwise inoperative [13]. Simulation Details The neptune exploit code used in the simulation was compiled from C code originally posted to the bugtraq archive. The neptune program allows the user to specify a victim host, the source address to use in the spoofed packets, the number of packets to send, and the ports to hit on the victim machine (including an ‘infinity’ option that would attack all ports).

The neptune exploit was effective against all three of the victim machines used in the simulation. Every TCP/IP implementation is vulnerable to this attack to a varying degree depending on the size of the data structure used to store incoming connections and the timeout value associated with half-open connections. As a point of reference, sending twenty SYN packets to a port on a Solaris 2. 6 system will cause that port to drop incoming requests for approximately ten minutes.

During the simulation, a neptune attack which sent 20 SYN packets to every port from 1 to 1024 of the Solaris server once every ten minutes was able block incoming connections to any of these ports for more than an hour. Attack Signature A neptune attack can be distinguished from normal network traffic by looking for a number of simultaneous SYN packets destined for a particular machine that are coming from an unreachable host. A host-based intrusion detection system can monitor the size of the tcpd connection data structure and alert a user if this data structure nears its size limit.

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