Security administration · Lesson 32 of 35 · 8 minutes

Known-hosts hygiene

Inspect stored host identities alongside a key fingerprint.

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Your objectives

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1
Read known hosts
Run cat ~/.ssh/known_hosts.
Hint & worked example
Read one or more files to standard output without editing their contents.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
cat ~/.ssh/known_hosts
```
Remember: Reading a source file should precede changing it; avoid printing real secrets.
2
Read a key fingerprint
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.
3
Inspect known-hosts metadata
Run stat ~/.ssh/known_hosts.
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/known_hosts
```
Remember: Metadata and file contents answer different questions.

Understand the pattern

Checking SSH host and key identity evidence helps operators explain what they know before they change anything.

cat: Read one or more files to standard output without editing their contents.
Reading a source file should precede changing it; avoid printing real secrets.
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.
stat: Inspect a file or directory metadata record, including ownership, permissions, and size.
Metadata and file contents answer different questions.

Worked example and expected result

$ cat ~/.ssh/known_hosts

clairos-learn-lab ssh-ed25519 AAAAFAKEHOSTKEY
10.20.30.40 ssh-ed25519 AAAAFAKEHOSTIP

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.

$ stat ~/.ssh/known_hosts

Path: /home/learner/.ssh/known_hosts
Type: file
Mode: -rw-------
Owner: learner
Group: learners
Size: 84 bytes

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 “Known-hosts hygiene”, which interpretation is safe?Do not confuse public keys, private keys, and host fingerprints.A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.

Apply it elsewhere

Before adapting the ssh-keygen workflow from “Known-hosts hygiene” to another server, what should you do?Paste the training paths unchanged into production.Skip output inspection whenever the command is familiar.Confirm the current directory, target identity, permissions, and the installed command behavior first.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Exposed service inventory Try an incident drill

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