DoD is massive and there's a ton of different deployment requirements all over the place. On the systems I've worked on, they've ranged from old RHEL 4.x versio

DoD is massive and there's a ton of different deployment requirements all over the place. On the systems I've worked on, they've ranged from old RHEL 4.x versions up to CentOS 7 now and also SunOS and Solaris and everything in between. There's been some movement away from COTS software in favor of just paying a contractor to do the support for you when you already pay them tens of thousands per week for development and support in the first place, but this is also tough to generalize. I can only say that DoD has been increasingly budget strapped in most R&D areas and has been clamping down hard on costs (in probably the wrong places as usual, but there's structural problems that keep DoD and federal sector in general from doing the most cost-effective purchasing).