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Learn and Troubleshoot 502 and 504 Errors

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Christopher Hotze
Updated:

Overview

500-class response codes indicate a server failure occurred while the CDN processed a request.

When integrated with the StackPath CDN, it's possible you may encounter 502 or 504 errors. These errors are commonly caused by either incorrect settings, origin server firewalls, origin server issues or connection timeouts.

You can use this document to learn how to prevent and troubleshoot 502 and 504 errors that your users may see. 

Reviewing Examples

Error type Description
502 - Bad Gateway

You use the edge server (CDN) as a proxy, and then the WAF receives an invalid response from the origin server, also known as the upstream server.

  • For example, imagine the request as a phone call. You answer the phone; however, the caller was not who you were expecting. 
504 - Gateway Timeout

You use the edge server (WAF) as a proxy, and then the WAF does not receive a timely response from the origin server. 

  • For example, imagine the request as a phone call; however, the phone never rings. Instead, the caller hears, "Please try your call again later." 

Troubleshooting 502 Errors

Review common causes for 502 errors and how to troubleshoot them:

Cause Troubleshoot
The CDN does not connect to the correct server.

Ensure that the IP address set in your origin is correct. 

If you use a domain as your origin, then ensure that the domain resolves to the correct IP address. 

A firewall (or other security device) blocks the request.

  • In other words, some thing that is not your origin is blocking the request from the CDN.

Verify that your firewall does not drop requests from the CDN.

Verify that DDoS protection settings have not been triggered by origin pull requests from the CDN.

Disable CMS security plug-ins, and then refresh your site. 

  • Some CMS security plug-ins may cause complications; as a result, disable one plug-in at a time, and then refresh your site.

Verify that you have allowlisted StackPath IP blocks on your server's firewall. 

The domain is not resolving properly

Verify that your DNS records are properly configured and propagated.

Origin pull encounters a redirect loop

cURL the origin URL to see where the redirect is going. 

  • Many times, the origin tries to redirect to itself or to the CDN, which can cause a redirect loop. 

 

 

Troubleshooting 504 Errors

By default, the CDN will maintain a connection with an origin before the connection times out for 20 seconds. 

  • This time limit is configured to avoid negatively affecting the network and overall performance of the CDN.

Review the following ways to troubleshoot a 504 error:

  • Verify that the Host Header is set correctly in the StackPath Control Portal.
  • Verify that the HTACCESS file is correct on the server, especially if WordPress has been recently reinstalled or restored.
  • Verify that you have allowlisted StackPath IP blocks on your server's firewall. 
  • Verify that the server is not having problems at the WordPress, the Apache/NGINX/WebServer, or at the Operating System level, such as:
    • Permissions issues in the operating system
    • Database corruption
    • Server hardware/infrastructure issues
    • Issues with server's resources (RAM, Disk i/o, CPU)
    • Server issues that prevent WordPress from properly accessing its database or other resources
    • If you are unsure how or what to check or you do not have access to your server's operating system, then contact your hosting provider.

Viewing 5xx Errors in the Browser

A 502 or a 504 error in the browser indicates that the web server is unable to respond to the CDN's initial request to your server for that HTML, which will return a 5xx error. 

Review the following table to view examples of different message types you may see in a browser: 

502 Error Examples 504 Error Examples
  • “502 Bad Gateway”
  • “HTTP Error 502 – Bad Gateway”
  • “502 Service Temporarily Overloaded”
  • “Error 502”
  • “502 Proxy Error”
  • “HTTP 502”
  • “502 Bad Gateway NGINX”
  • “504 Gateway Timeout”
  • “HTTP 504 Gateway Timeout”
  • “HTTP 504 Error”
  • “HTTP 504”
  • “504 Gateway Time-Out”
  • “Nginx 504 Gateway Timeout”
  • “Gateway Timeout (504)”
  • “504 Gateway Timeout NGINX”
Each error has multiple examples with different wording, which can vary from site to site; however, all examples indicate the same overall message.

Viewing 5xx Errors in the Terminal

To view these errors via terminal, use a cURL command to send an HTTP request to your site via your Edge Address. 

cURL against the Edge Address to make sure that the CDN is proxying the connection to the origin. 

Review the following example:

curl -I myEDGEADDRESS.stackpathcdn.com
HTTP/1.1 504 Gateway Timeout
date: Thu, 13 Dec 2018 14:53:45 GMT
accept-ranges: bytes
cache-control: max-age=10
x-hw: 1651863782.cds085.da2.hn,1651863782.cds239.da2.sc,1651863782.cdn2-redis01-dfw1.stackpath.systems.-.wx,1651863782.cds239.da2.p
access-control-allow-origin: *
Connection: keep-alive
Content-Length: 0

In the above example, note that: 

  • A response of 504 Gateway Timeout is present
  • The X-HW header ends in p, which indicates that this request was proxied to the origin.
    • The X-HW is inserted by StackPath and indicates where an asset is served from, what PoP the request is going to, and if the response came from CDN cache or from a proxied connection to the origin. 

When the CDN proxies a request, the CDN server passes the request to the origin server and then reports to the browser what was received. In this case, the server gave a 504. 

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