Recently, awareness has been raised about a number of "blind" attacks that can be performed against the Transmission Control Protocol (TCP) and similar protocols. The consequences of these attacks range from throughput-reduction to broken connections or data corruption. These attacks rely on the attackers ability to guess or know the four- tuple (Source Address, Destination Address, Source port, Destination Port) that identifies the transport protocol instance to be attacked. This document describes a simple and efficient method for random selection of the client port number, such that the possibility of an attacker guessing the exact value is reduced. While this is not a replacement for cryptographic methods, the described port number randomization algorithms provide improved security/obfuscation with very little effort and without any key management overhead. Both text and pdf formats of this paper are included.
