Hi there Infoconalpha1, and welcome!
Have spent my career working with various types of "high technology", most of which involves various government Agencies. Can verify that just
about every type of OS is used, in some way, on all classifications of networks.
The DOD undertook the development of the common desktop environment or CDE, which produces a standard look and feel, no matter the OS. The CDE
provides the “window dressing” for, at least, Sun Solaris, HP-UX, and Microsoft Windows based network systems; very much like Gnome or the KDE but
those function in the *UX-only world. Although developed for internal use, that, and ADA, discussed below, are now in wide use.
Even with the existence of such a common-denominator system, just about every OS on the market is in use but, honestly, there are not that many LINUX
implementations. Some, assuredly, but most systems utilize the "Normal" versions of MS Windows and the big name *UXes. As in the "outside
world", implementations of the various LINUX varieties are rising and integrate nicely since they are, after all, *Xes. As an aside, the other
categories of Open Source Software (OSS) tend to be even less prevalent, especially on the classified systems, but again, the implementations are
rising when the software fits the requirements.
Another type of OS used primarily on heavy iron is based upon the ADA language and is; well, quite esoteric, that may be what d1k had in mind(?). ADA
was primarily developed by Cii Honeywell/Bull from 1977-1980 with numerous updates since. Note: ADA had many 2 digit years... Y2K fun!
Most often, the rather outrageous price of the "Trusted" versions makes implementation nearly infeasible, especially when considering the ROI of
such systems versus the "Normal" versions. In addition, there are a number of software titles that will not run, or have difficulties, on the
“Trusted” versions and require costly revisions that are normally not necessary.
In general, would state that the "Normal" versions can be made just as secure, if not more so, due to the existing knowledge of what is needed to
secure the systems. Often discover systems where the engineers and administrators have disabled much of what makes the “Trusted” versions
different, or more secure, just to get them to function as desired or needed- HA!
The guidance from the various Websites listed below, and mentioned by d1k, will harden a system quite well. Remember that Physical, Operational, and
Personnel Security figure significantly into the level of security obtainable, along with some trade secrets not listed in the guides.
Some good information sites are:
Definition, History, and Links re: ADA from U of Michigan
Computer Security Resource Center by the NIST Computer Security Division
NSA - System and Network Attack Center
Center for Internet Security (CIS)
Large collection of checklists, regulations, and the like, by the Information Assurance Support
Environment of DISA
If prodded at gunpoint to say what server OSes are the most prevalent within the US Government overall, would have to say:
Various versions of Microsoft Windows
LINUX and other commercial and proprietary OSes
*The last three entries change position often, but that is a best guess as of today.
Are you perchance looking to work with, or sell to, the government in some fashion or just curious?
By the way, let us all keep XphilesPhan's final sentence in mind!
Since purposefully omitting some OSes, looking forward to seeing which OS d1k
has in mind.
Hope this helps to answer your query, and serves to further the discussion,