How do I improve the security of a Linux server?

List its public services, administrative users and the applications that depend on it. A small internal file server and an internet-facing application have different requirements. The review should focus on the actual system rather than apply an unexplained checklist.

A practical scope can cover update status, administrative accounts, remote access, unnecessary services and relevant configuration. Record the findings, the changes agreed and anything left for a later decision.

SSH changes can lock out a remote administrator. Ubuntu's OpenSSH guidance recommends testing configuration before restarting the service and warns about losing access after an incorrect change.

For a remote server, confirm a working recovery console or another agreed way back in. Then check the intended user's access and the application after each relevant change. This makes the effect of hardening reviewable.

Routine hardening is appropriate when you want to improve a known system. If there are signs of compromise, say so at the outset: the immediate task may be incident triage rather than ordinary configuration changes.

A hardening report should be explicit about what was inspected and what was outside scope. It does not establish that every vulnerability has been found or confer a certification.