Nginx returns 502 Bad Gateway when proxying to upstream

Problem

Nginx returns 502 Bad Gateway when reverse proxying to an upstream service. The upstream service is running and accessible directly, but fails through Nginx.

Symptoms

• 502 Bad Gateway in browser
• upstream connect error in nginx error log
• connect() failed (111: Connection refused) in logs

Stack

Solution

Common causes: 1) Upstream is listening on 127.0.0.1 but nginx is in a container — use the container network IP or host.docker.internal. 2) SELinux is blocking the connection — run setsebool httpd_can_network_connect on. 3) Wrong port in proxy_pass.

Code

# Check if SELinux is the issue:
getsebool httpd_can_network_connect
# If off, enable it:
sudo setsebool -P httpd_can_network_connect on

# Correct proxy_pass config:
location /api/ {
    proxy_pass http://127.0.0.1:8080/;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}

Caveats

The trailing slash in proxy_pass matters — it strips the /api/ prefix. Without it, the full path is forwarded.