Security administration · Lesson 21 of 35 · 8 minutes

SSH hardening deep review

Review SSH config, key mode, and host identity.

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Your objectives

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1
Check root login
Run sudo grep PermitRootLogin /etc/ssh/sshd_config.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
sudo grep PermitRootLogin /etc/ssh/sshd_config
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
2
Inspect key mode
Run stat ~/.ssh/authorized_keys.
Hint & worked example
Inspect a file or directory metadata record, including ownership, permissions, and size.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
stat ~/.ssh/authorized_keys
```
Remember: Metadata and file contents answer different questions.
3
Fingerprint key
Run ssh-keygen -lf ~/.ssh/authorized_keys.
Hint & worked example
Review or create a simulated SSH-key artifact. It is not a usable cryptographic credential.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
ssh-keygen -lf ~/.ssh/authorized_keys
```
Remember: Do not confuse public keys, private keys, and host fingerprints.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

grep: Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
No matches is exit status 1, which is different from a malformed invocation.
stat: Inspect a file or directory metadata record, including ownership, permissions, and size.
Metadata and file contents answer different questions.
ssh-keygen: Review or create a simulated SSH-key artifact. It is not a usable cryptographic credential.
Do not confuse public keys, private keys, and host fingerprints.

Worked example and expected result

$ sudo grep PermitRootLogin /etc/ssh/sshd_config

PermitRootLogin no

Exit status: 0. The command completed successfully in the training fixture.

$ stat ~/.ssh/authorized_keys

Path: /home/learner/.ssh/authorized_keys
Type: file
Mode: -rw-------
Owner: learner
Group: learners
Size: 40 bytes

Exit status: 0. The command completed successfully in the training fixture.

$ ssh-keygen -lf ~/.ssh/authorized_keys

256 SHA256:36a69dc636a69dc636a69dc636a69dc6 /home/learner/.ssh/authorized_keys (ED25519)

Exit status: 0. The command completed successfully in the training fixture.

Try a variation

Repeat this workflow in the sandbox with a different scratch filename. Keep the system fixtures unchanged and compare the observed output with your expectation before moving on.

Explain it

In “SSH hardening deep review”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.Metadata and file contents answer different questions.

Apply it elsewhere

Before adapting the stat workflow from “SSH hardening deep review” to another server, what should you do?Skip output inspection whenever the command is familiar.Confirm the current directory, target identity, permissions, and the installed command behavior first.Paste the training paths unchanged into production.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Firewall listener reconcile Try an incident drill

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