Hello, I am the Network and Systems Administrator in an organization of just under 500 users. We have a number of Windows Servers, and that is certainly my area of expertise.

We also have a very small handful of Debian servers.

We are about to terminate the sysadmin of these Debian systems.

Short of powering down the systems, I would like to know how I can ensure that the previous admin does not have control of these systems in the future, at least until we hire a replacement linux sysadmin.

I have physical/virtual-console access to each of the systems, so I can reboot them in various user-modes. I just don't know what to do.

Please assume that I do not currently have root access to all of these systems (an oversight on my part that I now recognize.)

I have some experience in Linux, and use it on my desktop on a daily basis, but I must admit that I am a competent user of linux, not a systems admin.

I have no fear of the command line however....

Is there a list of steps that one should take to "secure" a system from somebody else?

Again, I assure you that this is legit, I am re-taking control of my employer's systems, at the request of my employer.

I hope to not have to shut the systems down permanently and still be reasonably certain that they are secure.

Thanks for your time.