How do you troubleshoot a 502 Bad Gateway error in Nginx?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Read the error log

   Check /var/log/nginx/error.log for the exact reason: connection refused, upstream timed out, or no live upstreams.

2. Step 2

   Verify the backend is running

   Confirm the app or FastCGI/PHP-FPM/uWSGI process is up and listening on the expected port or socket.

3. Step 3

   Check proxy_pass and upstream

   Make sure proxy_pass or the upstream block points to the correct host, port, or socket path.

4. Step 4

   Test connectivity

   Use curl to the backend directly and check firewall, SELinux, and socket permissions between Nginx and the app.

5. Step 5

   Tune timeouts and buffers

   Raise proxy_read_timeout for slow apps and increase proxy_buffer_size / fastcgi_buffers if headers are too large.

Code Example

# 1. Read the most recent errors
tail -n 50 /var/log/nginx/error.log

# 2. Is the backend process up and listening?
sudo ss -ltnp | grep 8080

# 3. Hit the backend directly, bypassing Nginx
curl -v http://127.0.0.1:8080/

# 4. Example fix: raise the read timeout for a slow app
# location / {
#     proxy_pass http://127.0.0.1:8080;
#     proxy_read_timeout 120s;
#     proxy_buffer_size 16k;
#     proxy_buffers 4 16k;
# }

Follow-up Questions

• What is the difference between a 502 and a 504 Gateway Timeout?
• How do you increase proxy timeouts in Nginx?
• What does 'upstream sent too big header' mean and how do you fix it?
• How does an upstream block with multiple servers affect 502s?
• How would you check if SELinux is blocking Nginx from connecting?