The Free On-line Dictionary of Computing (19 January 2023):

firewall machine
flytrap
Venus flytrap

    A network firewall implemented as a
   separate piece of hardware or a virtual machine.

   (2020-06-25)


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]