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.