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Troubleshooting Cloudflare 5XX errors – Cloudflare Help Center

​​ Error analytics

Error Analytics per domain are available within the support portal for your account.  Error Analytics allows insight into overall errors by HTTP error code and provides the URLs, responses, origin server IP addresses, and Cloudflare data centers needed to diagnose and resolve the issue.  Error Analytics are based on a 1% traffic sample.

To view Error Analytics:

  • Navigate to the Cloudflare support portal.  Refer to  instructions about filing a support ticket for information on how to reach the support portal.
  • Scroll down to the Error Analytics section.
  • Click Visit Error Analytics.
  • Enter the domain to investigate.
  • A graph of Errors over time is displayed.
  • Click on a status code in the table beneath the graph to expand traffic error details.

​​ Error 500: internal server error

Error 500 generally indicates an issue with your origin web server.  Error establishing database connection is a common HTTP 500 error message generated by your origin web server.  Contact your hosting provider to resolve.

Resolution

Provide details to your hosting provider to assist troubleshooting the issue.

However, if the 500 error contains “cloudflare” or “cloudflare-nginx” in the HTML response body, provide  Cloudflare support with the following information:

  1. Your domain name
  2. The time and timezone of the 500 error occurrence
  3. The output of www.example.com/cdn-cgi/trace from the browser where the 500 error was observed (replace www.example.com with your actual domain and host name)

​​ Error 502 bad gateway or error 504 gateway timeout

An HTTP 502 or 504 error occurs when Cloudflare is unable to establish contact with your origin web server.

There are two possible causes:

​​ 502/504 from your origin web server

Cloudflare returns an Cloudflare-branded HTTP 502 or 504 error when your origin web server responds with a standard HTTP 502 bad gateway or 504 gateway timeout error:

Example of a Cloudflare-branded error 502.

Resolution

Contact your hosting provider to troubleshoot these common causes at your origin web server:

  • Ensure the origin server responds to requests for the hostname and domain within the visitor’s URL that generated the 502 or 504 error.
  • Investigate excessive server loads, crashes, or network failures.
  • Identify applications or services that timed out or were blocked.

​​ 502/504 from Cloudflare

A 502 or 504 error originating from Cloudflare appears as follows:

Example of an unbranded error 502.

If the error does not mention “cloudflare,” contact your hosting provider for assistance on 502/504 errors from your origin.

Resolution

To avoid delays processing your inquiry, provide these required details to  Cloudflare Support:

  1. Time and timezone the issue occurred.
  2. URL that resulted in the HTTP 502 or 504 response (for example:  https://www.example.com/images/icons/image1.png)
  3. Output from browsing to  www.example.com/cdn-cgi/trace (replace  www.example.com with the domain and host name that caused the HTTP 502 or 504 error)

​​ Error 503: service temporarily unavailable

HTTP error 503 occurs when your origin web server is overloaded. There are two possible causes discernible by error message:

  • Error doesn’t contain “cloudflare” or “cloudflare-nginx” in the HTML response body.

Resolution: Contact your hosting provider to verify if they rate limit requests to your origin web server.

  • Error contains “cloudflare” or “cloudflare-nginx” in the HTML response body.

Resolution: A connectivity issue occured in a Cloudflare data center. Provide  Cloudflare support with the following information:

  1. Your domain name
  2. The time and timezone of the 503 error occurrence
  3. The output of  www.example.com/cdn-cgi/trace from the browser where the 503 error was observed (replace  www.example.com with your actual domain and host name)

​​ Error 520: web server returns an unknown error

Error 520 occurs when the origin server returns an empty, unknown, or unexpected response to Cloudflare.

Resolution

Contact your hosting provider or site administrator and request a review of your origin web server error logs for crashes and to check for these common causes:

  • Origin web server application crashes
  • Cloudflare IPs not allowed at your origin
  • Headers exceeding 16 KB (typically due to too many cookies)
  • An empty response from the origin web server that lacks an HTTP status code or response body
  • Missing response headers or origin web server not returning  proper HTTP error responses.
    • upstream prematurely closed connection while reading response header from upstream is a common error we may notice in our logs. This indicates the origin web server was having issues which caused Cloudflare to generate 520 errors.

If 520 errors continue after contacting your hosting provider or site administrator, provide the following information to  Cloudflare Support:

​​ Error 521: web server is down

Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain  Cloudflare IP addresses.

The two most common causes of 521 errors are:

  • Offlined origin web server application
  • Blocked Cloudflare requests

Resolution

Contact your site administrator or hosting provider to eliminate these common causes:

​​ Error 522: connection timed out

Error 522 occurs when Cloudflare times out contacting the origin web server. Two different timeouts cause HTTP error 522 depending on when they occur between Cloudflare and the origin web server:

  1. Before a connection is established, the origin web server does not return a SYN+ACK to Cloudflare within 15 seconds of Cloudflare sending a SYN.
  2. After a connection is established, the origin web server doesn’t acknowledge (ACK) Cloudflare’s resource request within 90 seconds.

Resolution

Contact your hosting provider to check the following common causes at your origin web server:

  • (Most common cause)  Cloudflare IP addresses are rate limited or blocked in .htaccess, iptables, or firewalls. Confirm your hosting provider allows Cloudflare IP addresses.
  • An overloaded or offline origin web server drops incoming requests.
  • Keepalives are disabled at the origin web server.
  • The origin IP address in your Cloudflare DNS app does not match the IP address currently provisioned to your origin web server by your hosting provider.
  • Packets were dropped at your origin web server.

If you are using Cloudflare Pages, verify that you have a custom domain set up and that your CNAME record is pointed to your custom Pages domain. Instructions on how to set up a custom Pages domain can be found here.

If none of the above leads to a resolution, request the following information from your hosting provider or site administrator before  contacting Cloudflare support:

​​ Error 523: origin is unreachable

Error 523 occurs when Cloudflare cannot contact your origin web server. This typically occurs when a network device between Cloudflare and the origin web server doesn’t have a route to the origin’s IP address.

Resolution Contact your hosting provider to exclude the following common causes at your origin web server:

  • Confirm the correct origin IP address is listed for A or AAAA records within your Cloudflare DNS app.
  • Troubleshoot Internet routing issues between your origin and Cloudflare, or with the origin itself.

If none of the above leads to a resolution, request the following information from your hosting provider or site administrator:

​​ Error 524: a timeout occurred

Error 524 indicates that Cloudflare successfully connected to the origin web server, but the origin did not provide an HTTP response before the default 100 second connection timed out. This can happen if the origin server is simply taking too long because it has too much work to do - e.g. a large data query, or because the server is struggling for resources and cannot return any data in time.

Resolution

Here are the options we’d suggest to work around this issue:

  • Implement status polling of large HTTP processes to avoid hitting this error.
  • Contact your hosting provider to exclude the following common causes at your origin web server:
    • A long-running process on the origin web server.
    • An overloaded origin web server.
  • Enterprise customers can increase the 524 timeout up to 6000 seconds using the proxy_read_timeout API endpoint.
  • If you regularly run HTTP requests that take over 100 seconds to complete (for example large data exports), move those processes behind a subdomain not proxied (grey clouded) in the Cloudflare DNS app.
  • If error 524 occurs for a domain using Cloudflare Railgun, ensure the lan.timeout is set higher than the default of 30 seconds and restart the railgun service.

​​ Error 525: SSL handshake failed

525 errors indicate that the SSL handshake between Cloudflare and the origin web server failed. Error 525 occurs when these two conditions are true:

  1. The  SSL handshake fails between Cloudflare and the origin web server, and
  2. Full or Full (Strict) SSL is set in the Overview tab of your Cloudflare SSL/TLS app.

Resolution

Contact your hosting provider to exclude the following common causes at your origin web server:

Additional checks

  • Check if you have a certificate installed on your origin server. You can check this article for more details on how to run some tests. In case you don’t have any certificate, you can create and install our free Cloudflare origin CA certificate. Using Origin CA certificates allows you to encrypt traffic between Cloudflare and your origin web server.
  • Review the cipher suites your server is using to ensure they match what is supported by Cloudflare.
  • Check your server’s error logs from the timestamps you see 525s to ensure there are errors that could be causing the connection to be reset during the SSL handshake.

​​ Error 526: invalid SSL certificate

Error 526 occurs when these two conditions are true:

  1. Cloudflare cannot validate the SSL certificate at your origin web server, and
  2. Full SSL (Strict) SSL is set in the Overview tab of your Cloudflare SSL/TLS app.

Resolution

Request your server administrator or hosting provider to review the origin web server’s SSL certificates and verify that:

Screen showing an SSL certificate with no errors.

If the origin server uses a self-signed certificate, configure the domain to use Full SSL instead of Full SSL (Strict). Refer to recommended SSL settings for your origin.

​​ 527 Error: Railgun Listener to origin error

A 527 error indicates an interrupted connection between Cloudflare and your origin’s  Railgun server (rg-listener). Common causes include:

  • Firewall interference
  • Network incidents or packet loss between the Railgun server and Cloudflare

Common causes of 527 errors include:

If contacting Cloudflare support, provide the following information from the Railgun Listener:

  • The full content of the railgun.conf file
  • The full content of the railgun-nat.conf file
  • Railgun log files that detail the observed errors

​​ Connection timeouts

The following Railgun log errors indicate a connection failure between the Railgun Listener and your origin web server:

connection failed 0.0.0.0:443/example.com: dial tcp 0.0.0.0:443: i/o timeout
no response from origin (timeout) 0.0.0.0:80/example.com

Resolution

Contact your hosting provider for assistance to test for connectivity issues between your origin web server and your Railgun Listener. For example, a netcat command tests connectivity when run from the Railgun Listener to the origin web server’s SERVERIP and PORT (80 for HTTP or 443 for HTTPS):

nc -vz SERVERIP PORT

​​ LAN timeout exceeded

The following Railgun Listener log error is generated if the origin web server does not send an HTTP response to the Railgun Listener within the 30 second default timeout:

connection failed 0.0.0.0:443/example.com: dial tcp 0.0.0.0:443: i/o timeout

The time is adjusted by the lan.timeout parameter of the railgun.conf file.

Resolution

Either increase the lan.timeout limit in railgun.conf, or review the web server configuration. Contact your hosting provider to confirm if the origin web server is overloaded.

​​ Connection refusals

The following errors appear in the Railgun logs when requests from the Railgun Listener are refused:

Error getting page: dial tcp 0.0.0.0:80:connection refused

Resolution

Allow the IP of your Railgun Listener at your origin web server’s firewall.

The following errors appear in the Railgun logs if TLS connections fail:

connection failed 0.0.0.0:443/example.com: remote error: handshake failure
connection failed 0.0.0.0:443/example.com: dial tcp 0.0.0.0:443:connection refused
connection failed 127.0.0.1:443/www.example.com: x509: certificate is valid for
example.com, not www.example.com

Resolution

If TLS/SSL errors occur, check the following on the origin web server and ensure that:

  • Port 443 is open
  • An SSL certificate is presented by the origin web server
  • the SAN or Common Name of the origin web server’s SSL certificate contains the requested or target hostname
  • SSL is set to Full or Full (Strict) in the Overview tab of the Cloudflare SSL/TLS app

​​ Error 530

HTTP error 530 is returned with an accompanying 1XXX error displayed. Search for the specific  1XXX error within the Cloudflare Help Center for troubleshooting information.