Security administration · Lesson 23 of 35 · 8 minutes

Known host key context

Review SSH host identity without connecting.

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learner@clairos:~ $

↑ history · Tab complete · Ctrl+C cancel · Esc, Tab to leaveExit 0

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
Find host key
Run grep clairos ~/.ssh/known_hosts.
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.
```
grep clairos ~/.ssh/known_hosts
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
3
Resolve SSH target
Run ssh -G lab.
Hint & worked example
Inspect a fixed SSH training scenario; no real remote connection is made.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
ssh -G lab
```
Remember: Verify host identity and choose credentials deliberately on real systems.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

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.
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.
ssh: Inspect a fixed SSH training scenario; no real remote connection is made.
Verify host identity and choose credentials deliberately on real systems.

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.

$ grep clairos ~/.ssh/known_hosts

clairos-learn-lab ssh-ed25519 AAAAFAKEHOSTKEY

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

$ ssh -G lab

host lab
user learner
hostname clairos-learn-lab
identityfile ~/.ssh/training_key
pubkeyauthentication yes
passwordauthentication no

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 host key context”, which interpretation is safe?A successful first check guarantees that every dependent service is healthy.No matches is exit status 1, which is different from a malformed invocation.A command appearing in history proves it completed successfully.

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Sudo boundary review Try an incident drill

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