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.
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.

Practice

1. What does a 502 Bad Gateway error usually indicate in nginx?

easy

A. nginx cannot get a valid response from the backend server

B. nginx configuration file has a syntax error

C. nginx server is overloaded with too many requests

D. nginx is running out of disk space

Solution

1. Step 1: Understand the meaning of 502 Bad Gateway

   A 502 error means the gateway (nginx) got an invalid response from the backend server it tries to connect to.

2. Step 2: Identify the cause related to backend communication

   This error is not about nginx config syntax or server load, but about backend response failure.

3. Final Answer:

   nginx cannot get a valid response from the backend server -> Option A

4. Quick Check:

   502 Bad Gateway = backend response failure [OK]

Hint: 502 means backend server response problem [OK]

Common Mistakes:

• Confusing 502 with nginx config syntax errors
• Thinking 502 means nginx is overloaded
• Assuming 502 is due to disk space issues

2. Which of the following nginx configuration directives is essential to correctly proxy requests to a backend server to avoid 502 errors?

easy

A. listen 8080;

B. root /var/www/html;

C. server_name localhost;

D. proxy_pass http://backend_server;

Solution

1. Step 1: Identify the directive that defines backend server address

   The proxy_pass directive tells nginx where to send requests, crucial for proxying.

2. Step 2: Understand other directives' roles

   listen sets port, server_name sets hostname, root sets static file location; none define backend proxy.

3. Final Answer:

   proxy_pass http://backend_server; -> Option D

4. Quick Check:

   proxy_pass sets backend URL [OK]

Hint: proxy_pass sets backend URL to avoid 502 [OK]

Common Mistakes:

• Using listen instead of proxy_pass for backend
• Confusing server_name with backend address
• Setting root instead of proxy_pass for proxying

3. Given this nginx error log snippet:
connect() failed (111: Connection refused) while connecting to upstream
What is the most likely cause of the 502 error?

medium

A. nginx has insufficient permissions to read files

B. nginx configuration syntax error

C. Backend server is not running or not accepting connections

D. DNS resolution failure for backend server

Solution

1. Step 1: Interpret the error message

   "Connection refused" means nginx tried to connect but backend server refused, likely because it is down or not listening.

2. Step 2: Rule out other causes

   Syntax errors cause config reload failures, permissions affect file access, DNS failure shows different errors.

3. Final Answer:

   Backend server is not running or not accepting connections -> Option C

4. Quick Check:

   Connection refused = backend down [OK]

Hint: Connection refused means backend server is down [OK]

Common Mistakes:

• Assuming syntax error from connection refused
• Blaming nginx file permissions
• Thinking DNS failure causes connection refused

4. You see a 502 error in nginx logs caused by upstream timed out. Which fix is most appropriate?

medium

A. Clear browser cache and cookies

B. Increase proxy_read_timeout in nginx config

C. Change server_name directive

D. Restart nginx service immediately

Solution

1. Step 1: Understand the timeout error

   "upstream timed out" means nginx waited too long for backend response.

2. Step 2: Adjust nginx timeout settings

   Increasing proxy_read_timeout lets nginx wait longer before giving 502 error.

3. Final Answer:

   Increase proxy_read_timeout in nginx config -> Option B

4. Quick Check:

   Timeout error fixed by increasing proxy_read_timeout [OK]

Hint: Increase proxy_read_timeout to fix upstream timeout [OK]

Common Mistakes:

• Restarting nginx without config change
• Changing server_name unrelated to timeout
• Clearing browser cache won't fix backend timeout

5. You fixed a backend server crash causing 502 errors. What is the correct next step to apply nginx changes and confirm the fix?

hard

A. Reload or restart nginx and check error logs for 502 errors

B. Clear browser cache and refresh the page

C. Edit nginx config to change server_name

D. Delete nginx access logs to free space

Solution

1. Step 1: Apply nginx configuration and backend fixes

   After fixing backend, reload or restart nginx to apply any config changes and reset connections.

2. Step 2: Verify fix by checking logs

   Check nginx error logs to confirm 502 errors no longer appear, ensuring the problem is solved.

3. Final Answer:

   Reload or restart nginx and check error logs for 502 errors -> Option A

4. Quick Check:

   Reload nginx and check logs to confirm fix [OK]

Hint: Reload nginx and check logs after backend fix [OK]

Common Mistakes:

• Only clearing browser cache without server reload
• Changing unrelated config like server_name
• Deleting logs instead of reading them