502 Bad Gateway troubleshooting in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a 502 Bad Gateway error indicate in nginx?

It means nginx received an invalid response from the upstream server it tried to contact.

beginner

Name two common causes of a 502 Bad Gateway error in nginx.

1. The upstream server is down or unreachable.<br>2. The upstream server is overloaded or crashing.

intermediate

Which nginx configuration directive specifies the upstream server address?

The proxy_pass directive inside a location block tells nginx where to send requests.

beginner

How can you check if the upstream server is reachable from the nginx server?

Use commands like curl or telnet to test connection to the upstream server's IP and port.

intermediate

What nginx log files help diagnose 502 Bad Gateway errors?

The error.log file usually contains detailed error messages about upstream failures.

What does a 502 Bad Gateway error mean in nginx?

A The client sent a bad request

B Nginx server is overloaded

C Nginx cannot reach the upstream server or got a bad response

D DNS resolution failed

A 502 error means nginx tried to contact the upstream server but got an invalid or no response.

Which nginx directive sets the upstream server address?

A proxy_pass

B server_name

C listen

D root

The proxy_pass directive tells nginx where to forward requests.

If you see a 502 error, what is a good first step to troubleshoot?

A Restart the client browser

B Check if the upstream server is running and reachable

C Change DNS settings

D Clear nginx cache

The upstream server might be down or unreachable, causing the 502 error.

Where do you find nginx error messages related to 502 errors?

A /var/log/nginx/error.log

B /usr/share/nginx/html/index.html

C /etc/nginx/nginx.conf

D /var/log/nginx/access.log

The error.log file contains detailed error messages.

Which command can you use to test connectivity to the upstream server?

A chmod

B ls

C cat

D curl

curl can test if the upstream server responds on the expected port.

Explain the main reasons why nginx might return a 502 Bad Gateway error and how you would check for them.

Think about the connection between nginx and the server it forwards requests to.

Describe how to use nginx configuration to specify the upstream server and how misconfiguration can cause 502 errors.

Focus on the proxy_pass setting inside location blocks.