What is a Worm?
A computer WORM is a self-contained program (or set of programs), that
is able to spread functional copies of itself or its segments to other
computer systems (usually via network connections).
Note that unlike viruses, worms do not need to attach themselves to a
host program. There are two types of worms--host computer worms and
Host computer worms are entirely contained in the computer they run on
and use network connections only to copy themselves to other computers.
Host computer worms where the original terminates itself after launching
a copy on another host (so there is only one copy of the worm running
somewhere on the network at any given moment), are sometimes called
Network worms consist of multiple parts (called "segments"), each
running on different machines (and possibly performing different
actions) and using the network for several communication purposes.
Propagating a segment from one machine to another is only one of those
purposes. Network worms that have one main segment which coordinates
the work of the other segments are sometimes called "octopuses."
The infamous Internet Worm (perhaps covered best in "The Internet Worm
Program: An Analysis," Eugene H. Spafford, Purdue Technical Report CSD-
TR-823) was a host computer worm, while the Xerox PARC worms were
network worms (a good starting point for these is "The Worm Programs--
Early Experience with a Distributed Computation," Communications of the
ACM, 25, no.3, March 1982, pp. 172-180).
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