“A network appliance is a device designed to perform a specific
function, such as load balancers, firewalls, bandwidth optimizers,
and Web accelerators. The appliance should perform its job in an
easier, more efficient, and more economical manner than the task
could otherwise be performed.”
“Developers examining general-purpose OSs often feel constrained
by the lack of facilities the OS provides for building a “headless”
device such as a network appliance….”
“… a general-purpose OS does not typically allow economical
access to the source code or kernel, and overloads the developer
with unnecessary features.”
“… vendors spend a lot of resources on engineering, and the
appliance they build is not closely tied to the OS. Therefore, it
does not make sense for them to spend a lot of money on the
OS.”
“While UNIX is technically a general-purpose OS, many
versions offer open source code, so developers of network
appliances can pick what they need and tune it to suit the
particular appliance they are building.”