Troubleshooting · Lesson 25 of 26 · 8 minutes

DNS and service triage

Separate local name-resolution evidence from web-service health.

Open the lab Course map

Mastery 0/3Run itExplain itApply it

learner@clairos:~ $

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

Your objectives

Hints are always available. Wrong turns never cost XP.

1
Read resolver configuration
Run cat /etc/resolv.conf.
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 /etc/resolv.conf
```
Remember: Reading a source file should precede changing it; avoid printing real secrets.
2
Check the local mapping
Run grep clairos /etc/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 /etc/hosts
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
3
Check the web service
Run systemctl status nginx.
Hint & worked example
Inspect or change a simulated systemd service. Status, active state, and unit configuration reveal different evidence.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
systemctl status nginx
```
Remember: A service can be active while an application endpoint is unhealthy; verify both layers.
4
Check the web listener
Run ss -tulpn.
Hint & worked example
Inspect simulated listening sockets and connection information.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
ss -tulpn
```
Remember: A listening socket is not proof of correct application responses or external reachability.

Understand the pattern

Triaging a name-resolution complaint without assuming a service outage 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.
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.
systemctl: Inspect or change a simulated systemd service. Status, active state, and unit configuration reveal different evidence.
A service can be active while an application endpoint is unhealthy; verify both layers.
ss: Inspect simulated listening sockets and connection information.
A listening socket is not proof of correct application responses or external reachability.

Worked example and expected result

$ cat /etc/resolv.conf

nameserver 192.0.2.53
# fake resolver for training only

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

$ grep clairos /etc/hosts

10.20.30.40 clairos-learn-lab

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

$ systemctl status nginx

nginx.service - simulated nginx
   Loaded: loaded
   Active: active
   Notes: serving the simulated static learn site

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

$ ss -tulpn

Netid State  Local Address:Port  Process
udp   UNCONN 0.0.0.0:68          lab-dhcp
tcp   LISTEN 0.0.0.0:22          sshd
tcp   LISTEN 0.0.0.0:80          nginx
tcp   LISTEN 0.0.0.0:443         nginx

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

Apply it elsewhere

Before adapting the grep workflow from “DNS and service triage” 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: Permission regression triage Try an incident drill

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