Denial Isn't Just a River in Egypt

June 1997 / BYTE Software Lab Report / Firewall Software for NT and Unix / Denial Isn't Just a River in Egypt

Pete Loshin
Denial-of-service attacks are a serious threat, and any good firewall should be able to stop at least some of them. Two have gotten a lot of att ention because they take advantage of the openness of TCP/IP protocols.
Ping of Death: IP datagrams larger than 65,535 bytes are "illegal," but some TCP/IP implementations incorrectly attempt to process them. Because large datagrams are almost always fragmente d and hosts don't start reconstructing datagrams until receiving the last fragment, when illegal datagrams are accepted, some TCP/IP stacks will crash the system that is attempting to process them. This attack uses Ping (a semi-acronym for Packet Internet Groper) because all TCP/IP hosts support it and it's easy to use; the attack itself requires only one command line.
SYN flooding: When hosts use Transmission Control Protocol (TCP of TCP/IP) for virtual circuit service, they use a three-way handshake protocol to negotiate the link. Every time a host is asked to open a TCP link, it responds with the second part of the handshake and waits for acknowledgment from the requesting host to open the circuit. Attackers generate a flood of TCP SYN (for "synchronize") requests to a server, but they never answer the server's responses. The server must allocate resources to handle these phony requests, in some cases tying up all the server's available resources.

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Matthew Wilson
My approach to software engineering is far more pragmatic than it is theoretical--and no language better exemplifies this than C++.

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