Fix Nginx 502 Bad Gateway — Upstream Connection Failed

Direct answer

Nginx returns 502 Bad Gateway when it cannot get a valid response from the upstream server. This means the backend process is either down, unreachable, or timing out. Check the upstream process status and Nginx error logs.

Structured breakdown

Cause

A 502 Bad Gateway from Nginx means the upstream server (your app) is not responding. Check if the upstream process is running, verify the proxy_pass address, and inspect Nginx error logs.

Fix

  • Check if the upstream application is running: systemctl status <service> or ps aux
  • Verify proxy_pass in Nginx config points to the correct host and port
  • Check Nginx error logs: tail -f /var/log/nginx/error.log

Outcome

Upstream responses are received correctly and 502 errors stop.

Common causes

  • Upstream application process crashed or is not running
  • Incorrect proxy_pass address or port in Nginx config
  • Upstream server is overloaded and dropping connections
  • PHP-FPM, Node.js, or application process pool exhausted
  • Firewall or security group blocking Nginx-to-upstream traffic

Fix steps

  1. 1

    Check if the upstream application is running: systemctl status <service> or ps aux

  2. 2

    Verify proxy_pass in Nginx config points to the correct host and port

  3. 3

    Check Nginx error logs: tail -f /var/log/nginx/error.log

  4. 4

    Increase upstream timeout values: proxy_connect_timeout, proxy_read_timeout

  5. 5

    Restart the upstream service and monitor for stability

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Frequently asked questions

Related technical context

These examples show the commands, logs, and configuration patterns most often used to verify this issue.

Command examples

  • tail -f /var/log/nginx/error.log
  • systemctl status <upstream-service>
  • curl -I http://127.0.0.1:3000/health

Log snippet

upstream prematurely closed connection while reading response header from upstream
connect() failed (111: Connection refused) while connecting to upstream

Config snippet

upstream backend {
    server 127.0.0.1:3000;
}
server {
    location / {
        proxy_pass http://backend;
        proxy_connect_timeout 60s;
        proxy_read_timeout 60s;
    }
}