Cisco ASA's appear to be linux under the hood based on watching versions of ASA804-3/12/19/23/31 boot on the console
They are Linux, and run two copies of IOS simultaneously in a VM each.
Kind of like how VMWare ESX is Linux - technically it is, but you don't really treat it as such.
Not to nit-pick, but VMware ESX uses RedHat Enterprise Linux for it's service console on versions previous to ESXi. The purpose of the service console is to provide support for booting the ESX Hypervisor which itself IS NOT Linux. It does, however, implement a Linux Driver compatability layer so that un-modified Linux drivers can be used w/ the Vmware ESX Hypervisor. The stated goal of this layer is to allow existing third party drivers to be rapidly added to the ESX Hypervisor w/out a lengthy porting process or a requirement for a company to maintain a completely separate driver source code tree for Vmware ESX. Here is a link to some info on Wikipedia: http://en.wikipedia.org/wiki/VMware_ESX_Server Specifically; "VMware states that the ESX Server product runs on "bare metal".[3] In contrast to other VMware products, it does not run atop a third-party operating system[4], but instead includes its own kernel. Up through the current ESX version 3.5, a Linux kernel is started first[5] and is used to load a variety of specialized virtualization components, including VMware's 'vmkernel' component. This previously-booted Linux kernel then becomes the first running virtual machine and is called the service console. Thus, at normal run-time, the vmkernel is running on the bare computer and the Linux-based service console runs as the first virtual machine (and cannot be terminated or shutdown without shutting down the entire system)." It is a common misconception that the ESX Hypervisor is Linux based, but that is an urban legend.