Weird Varnish 503 error - what does this mean?

Question

My online whiteboard application has been working previously, but for whatever reason, it no longer works. Now it shows a 503 Varnish cache error as seen here: http://grab.by/eFHG

Do you happen to know where I should start to look to try to resolve this issue?

Thanks! Donny

Answer 1

This error means that Varnish had no response (even not an HTTP error) from the backend within the timeout limit.

You can troubleshoot that in many ways :

• On the backend : do you see requests from Varnish in your webserver log ?

• On Varnish server : Run varnishlog and check the request process. You should have events in this order : RxRequest > TxRequest > RxResponse > TxResponse. Your problem is between TxRequest (request sent to backend) and RxResponse (response received from backend).

• On your Varnish server try connecting on the backend using telnet (telnet ). Does it connect ? If it does, try sending a request (e.g. "GET / "). Do you receive a response ?

Probable causes could be : firewall/selinux blocking between varnish & backend, bad varnish or backend web server config (are backend address & port sync ?), webserver stopped, ...

Answer 2

You could always check your /etc/varnish/default.vcl (CentOS).

backend default {
  .host = "127.0.0.1";
  .port = "80";
}

Make sure the .host value is your server IP Address, and change the port to 8080, and adjust your port setting in /etc/httpd/conf/httpd.conf and make sure Apache listen to 8080.

EDIT
It could also mean web server (Apache) have wrong/default settings.

• Check the virtual host ports, the values might be 80, they should be 8080.
• Check Listen directive and ServerName, they must have 8080 port.
• Make sure you do sudo systemctl enable varnish.
• And for the final touch, do sudo reboot (cache programs are bugging me in the development state).