Connect with explicit options

The first test names the operator user, server address, nondefault port if any, and intended identity file explicitly. This is a small part of a safe operator path from a local machine to a disposable Ubuntu server, including file transfer, tunnels, automation, and recovery, but the boundary it creates affects everything that follows.

SSH combines command-line options, per-host configuration, wildcard configuration, and system defaults using precedence rules. The useful target is not encyclopedic coverage. Make one deliberate choice, observe its consequences, and know which requirement would make you revise it.

Watch for one shortcut in particular: trying to change several client and server settings at once until a connection succeeds. It makes later failures harder to locate. Instead, start with an explicit command, use effective-configuration and verbose output, and change one diagnosed boundary at a time. Prefer an implementation you can explain from the outside before optimizing it.

Keep the wider goal in view: Connect to the intended host and know exactly which identity, network path, and trust decision made it possible. A short observation with concrete evidence is more useful than a confident sentence with no reproduction path.

Connect with one intentionally wrong user, port, and identity in separate attempts and identify each failure layer. Hand the result to someone else and see whether the behavior is clear without an oral explanation.