Common error diagnosis in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Nginx startup error output

You run sudo systemctl start nginx but it fails. Which output indicates a missing configuration file error?

A Job for nginx.service failed because the control process exited with error code. See "systemctl status nginx.service" and "journalctl -xe" for details.

B nginx: [alert] could not open error log file: open() "/var/log/nginx/error.log" failed (13: Permission denied)

C nginx: [warn] conflicting server name "example.com" on 0.0.0.0:80, ignored

D nginx: [emerg] open() "/etc/nginx/nginx.conf" failed (2: No such file or directory)

💡 Hint

Look for messages about missing files or 'No such file or directory'.

Option D shows the exact error where nginx cannot find the main configuration file, causing startup failure.

❓ Troubleshoot

intermediate

Diagnosing 502 Bad Gateway error

Your nginx server returns a 502 Bad Gateway error. Which cause is most likely based on this error?

A The SSL certificate has expired.

B The nginx configuration file has a syntax error.

C The backend server (e.g., PHP-FPM or upstream) is down or unreachable.

D The client IP is blocked by firewall rules.

💡 Hint

502 means nginx cannot get a valid response from the backend server.

502 Bad Gateway means nginx tried to connect to an upstream server but failed, often because the backend is down.

❓ Configuration

advanced

Identifying syntax error in nginx config

Which nginx configuration snippet will cause a syntax error when testing with nginx -t?

A server { listen 80 server_name example.com; location / { root /var/www/html; } }

B server { listen 80; server_name example.com; location / { root /var/www/html; index index.html; } }

C server { listen 80; server_name example.com; location / { root /var/www/html; } }

D server { listen 80; server_name example.com; location / { proxy_pass http://localhost:3000; } }

💡 Hint

Check for missing semicolons in the config lines.

Option A is missing a semicolon after 'listen 80', causing a syntax error.

✅ Best Practice

advanced

Proper error log level for debugging

To diagnose intermittent nginx errors in production, which error_log level is best to temporarily set?

A error_log /var/log/nginx/error.log info;

B error_log /var/log/nginx/error.log debug;

C error_log /var/log/nginx/error.log warn;

D error_log /var/log/nginx/error.log crit;

💡 Hint

More detailed logs help find subtle issues but can be verbose.

The 'debug' level logs detailed information useful for troubleshooting complex problems.

🔀 Workflow

expert

Correct order to reload nginx after config change

What is the correct sequence of commands to safely apply a new nginx configuration?

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 2,3,1,4

💡 Hint

You must test config before reloading nginx.

First edit config (1), then test syntax (2), reload nginx (3), and finally check status (4).