How to fix a 502 Bad Gateway error on your server

Last changed 2026-11-02. CortexHosting is not open yet, so you can’t order a server today.

A 502 means your web server, such as nginx or Caddy, is running but can't get a good answer from the app behind it. The usual cause is that the app has stopped. Start by checking the app.

These steps are for a server you can log in to. If you only visit the site, wait a few minutes and try again, or tell the people who run it.

Find the problem in four steps

1. Is your app running?

Check the service by its name. Use your own app's name in place of myapp:

sudo systemctl status myapp

If it says inactive or failed, start it and read why it stopped:

sudo systemctl restart myapp
sudo journalctl -u myapp -n 50

If your app runs in Docker, use these:

sudo docker ps -a
sudo docker logs --tail 50 myapp

2. Does your app answer on the server itself?

Ask it directly, without going through the web server. Use your app's own port in place of 3000:

curl -I http://127.0.0.1:3000

If you get an answer, your app is fine, so go to step 3. If you get Connection refused or Couldn't connect to server, your app is not listening on that port. This command shows every port in use:

sudo ss -ltnp

3. Is the web server sending traffic to the right place?

The address in your web server's settings must match the port from step 2.

Web server	Setting to look for	Where it usually is
nginx	`proxy_pass` or `fastcgi_pass`	`/etc/nginx/`
Caddy	`reverse_proxy`	`/etc/caddy/Caddyfile`

After a change, test nginx's settings with sudo nginx -t, then run sudo systemctl reload nginx. For Caddy, run sudo systemctl reload caddy.

4. What does the error log say?

sudo tail -n 50 /var/log/nginx/error.log

On Caddy, use sudo journalctl -u caddy -n 50.

What the log means

What the nginx log says	What it means	What to do
`connect() failed (111: Connection refused)`	Nothing is listening at that address	Start your app, or fix the port (steps 1 to 3)
`failed (13: Permission denied)`	The system is blocking nginx	See "On Rocky Linux, AlmaLinux or RHEL" below
`upstream prematurely closed connection`	Your app crashed or quit while it was answering	Read your app's own log (step 1)
`upstream sent too big header`	Your app sent more headers than nginx allows	Make `proxy_buffer_size` bigger
`No such file or directory` with `.sock`	The socket file is missing, often because PHP has stopped	Restart PHP (see below)

If your site runs on PHP, such as WordPress

The PHP service may have stopped. Its name depends on your system:

System	Command
Ubuntu or Debian	`sudo systemctl restart php8.3-fpm`, with your PHP version in the name
Rocky Linux, AlmaLinux or RHEL	`sudo systemctl restart php-fpm`

On Rocky Linux, AlmaLinux or RHEL

A system called SELinux stops nginx from connecting to other programs unless you allow it. If your log says Permission denied but your app is running, run this:

sudo setsebool -P httpd_can_network_connect 1

If your app keeps stopping

Check if the server has run out of memory or disk:

free -h
df -h
sudo journalctl -k | grep -i "out of memory"

A full disk or too little memory can stop an app again and again. Free some space, make your app use less, or move to a bigger plan.

Doing this on CortexHosting

Open your server and go to Services to see what is running. Open Docker if your app runs there.
Go to Console to run the commands above in your browser, or connect with SSH.
If your server runs Caddy, go to Domains and press Change… beside your domain. Check that Where it points shows your app's port.

On a Managed server, no account has sudo, so the commands above that use it won't work. Use the dashboard for that work.