"There's a lot of issues related to Linux on a mainframe," Leahy said. "In terms of Linux's history in that kind of environment, it's relatively short. Just from experience from a reliability and scalability standpoint, Linux has a long way to go to mature yet."

Linux vendor Red Hat, which is based in Raleigh, North Carolina and supports Linux as an enterprise system, says its operating system is popular inside the federal government -- whether top management knows or not.

"I've visited a whole lot of government organizations. Virtually every government agency I've visited has Linux somewhere in the enterprise," said Robert Hibbard, Red Hat's director of professional consulting. "The question is: Does anyone know about it?"

Hibbard said that the grass-roots adoption of Linux at federal agencies often obscures the real popularity: "I suspect at least half of those who say they don't use it have it in their enterprise but don't know about it."

Open-source software is a generic term applying to programs for which the source code is available -- but it may still cost money. Free software is a subset of open-source software, and generally means that the software is available to use at no cost, subject to license agreements like the GNU General Public License. The Linux operating system, sometimes called the GNU Linux operating system is free software.