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

firewall machine
 n.

    A dedicated gateway machine with special security precautions on it, used
    to service outside network connections and dial-in lines. The idea is to
    protect a cluster of more loosely administered machines hidden behind it
    from crackers. The typical firewall is an inexpensive micro-based Unix
    box kept clean of critical data, with a bunch of modems and 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,
    callback, and even a complete iron box keyable to particular incoming IDs
    or activity patterns. Syn. flytrap, Venus flytrap. See also wild side
    .

    [When first coined in the mid-1980s this term was pure jargon. Now (1999)
    it is techspeak, and has been retained only as an example of uptake ?ESR]


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)