wordpress 503 Backend fetch failed

Any 500 error like this is referring to an issue related to the web server itself. Specifically, it's stating that the Varnish service is unable to make an HTTP response from the web server.

Check the logs

While it might provide exact answers, your best bet is to start by review your web server's logs. The fix will depend on your set up and issues you find while digging more into your Linode's logs. The specific logs for your web server should be located in /var/log/.

Firewall

The way Varnish works is by communicating between its service and the web server itself over public ports. Because of this workflow, a firewall setting can interrupt its ability to communicate. If this is the case, you should see something in the logs pointing to connection errors.

We have a couple of guides that should get you pointed in the right direction if it's a firewall issue:

• How to Configure a Firewall with UFW
• Introduction to FirewallD on CentOS
• Control Network Traffic with iptables

I don't know what your set up is like, but we have a guide that explains and walks through setting up Varnish with NGINX over SSL & HTTP on a Debian 8 Linode. That guide should provide more information on Varnish.

Plugins

Since you're using Wordpress, Varnish may have been added to your set up by a plugin that you've added. One way to troubleshoot this would be by disabling and reenabling plugins, testing the connection as you do so.

Usual Suspects - PHP and MySQL

Two usual suspects for 500 errors can be PHP and MySQL. Some commands you can run to check into these services are:

ps aux | grep -E "php|cgi|sql"
sudo netstat -plntu | grep -E "php|cgi|sql"

The Community post I linked above about checking logs will also help point you in the direction for these services.

If you encounter any log output that you're not sure of, feel free to share it here and we can try to assist in deciphering with you.