The Jargon File (version 4.4.7, 29 Dec 2003):

Venus flytrap
 n.

    [after the insect-eating plant] See firewall machine.


The Free On-line Dictionary of Computing (30 December 2018):

firewall machine
flytrap
Venus flytrap

    A dedicated gateway server with special
   security precautions on it, used to service external connections
   (typically from the public Internet).  The firewall machine
   protects servers and networks hidden behind it from crackers.

   The typical firewall is an inexpensive microprocessor-based
   Unix machine with no critical data, with public network ports on
   it, but just one carefully watched connection back to the rest of
   the cluster.  The special precautions may include threat
   monitoring, call-back, and even a complete iron box keyable to
   particular incoming IDs or activity patterns.

   The type of network and security environment of a firewall machine
   is often called a De-Militarised Zone (DMZ).  It may contain
   other servers such as e-mail servers or proxy gateways -
   machines that need to be publicly accessible but also need some
   access to internal systems.

   Also known as a (Venus) flytrap after the insect-eating plant.

   (2014-07-15)