What is 520 Error "Unknown Error": Meaning and How to Fix Issue

What is 520 Error

Free Complete Site Audit

Access a full website audit with over 300 technical insights.

Something went wrong. Please, try again later.
Trusted by
Sitechecker trusted company

Free Website SEO Checker & Audit Tool

  • Scan the site for 300+ technical issues
  • Monitor your site health 24/7
  • Track website rankings in any geo

What is the 520 HTTP Status Code?

The 520 status code, labeled as “Unknown Error,” is an HTTP response status code utilized mainly by reverse proxies. It indicates that the proxy server, while attempting to forward a request to the origin server, received an unexpected response that it couldn’t understand or interpret according to the standard HTTP protocol. This lack of clarity or coherence in the response leads to the general designation as an “Unknown Error,” since the specific cause of the error is not readily apparent.

Identifying and resolving the underlying issue may require a detailed investigation into the server configurations, network conditions, or application behavior.

Impact 520 Status Code on SEO

Search engines like Google use web crawlers (or spiders) to scan and index the content of websites. These crawlers expect to receive standard HTTP status codes to understand how to deal with a particular page.

Inability to Crawl If a crawler encounters a 520 status code on a webpage, it recognizes this as a server-side error. Since the error does not provide specific information about what went wrong, the crawler may not be able to access the page's content at all.
This means the page might not get indexed.
Repeated Failures If the 520 errors are widespread across a site or if they persist over time, search engines might reduce the crawling frequency for that site.
This can lead to delayed indexing of new or updated content, which in turn impacts the site's ability to rank well in search results.
User Experience SEO is not just about pleasing search engines; it's also about providing a good experience for human users. A site that frequently returns 520 errors may frustrate visitors, leading to increased bounce rates.
Search engines might detect this pattern and interpret it as a sign of poor user experience, which can further diminish search ranking.
Loss of Link Equity If other sites link to a page on your website that's returning a 520 error, the “link juice” or equity from those links may be wasted, as search engines can't pass that equity onto a page they can't access.
This can reduce the overall authority of your site.

The 520 status code, while seemingly a technical server issue, can, directly and indirectly, affect a site’s SEO. Ensuring that such errors are minimized and quickly resolved is key to maintaining optimal search visibility and user experience.

520 Status Code Common Reasons and How to Fix Them

Here are some common reasons for encountering a 520 status code and ways to troubleshoot and fix them:

Misconfigured Server or Proxy

Sometimes, the server or reverse proxy may be misconfigured, leading to ambiguous responses and causing the 520 status code.

Review the server and proxy settings and ensure that they comply with standard HTTP protocols. Check logs for any specific errors and rectify them accordingly. Tools like Nginx or Apache logs can help identify misconfigurations.

Security Software or Firewall Conflicts

Security systems and firewalls might unintentionally block or alter the responses between the origin server and the reverse proxy, causing the 520 error.

Examine firewall rules and security settings, making adjustments if necessary to ensure proper communication between the servers. Whitelisting the IP addresses of known proxies can prevent inadvertent blocking.

Network Connectivity Issues

Issues in the network connection between the origin server and the reverse proxy can lead to the 520 status code.

Investigate the network route between the proxy and origin server, checking for timeouts or other connectivity problems. Tools like traceroute or network monitoring software can be valuable in identifying network bottlenecks or failures.

Server Overload

When the origin server is overwhelmed with requests, it might send back a response that the proxy doesn't understand, resulting in a 520 error.

Monitor server load regularly and scale resources as needed to handle peak traffic. Implementing load balancing or upgrading server hardware might be necessary. Tools like New Relic or other server monitoring solutions can provide insights into server performance.

SSL/TLS Certificate Errors

Invalid or expired SSL/TLS certificates on the origin server may lead to conflicts with the reverse proxy, causing a 520 status code.

Regularly check the validity of SSL/TLS certificates, renewing them before expiration. Make sure to configure them correctly. Tools like SSL Labs' SSL Server Test can help in diagnosing certificate-related issues.
Troubleshooting 520 status codes involves identifying the underlying issue through careful examination of server configurations, network conditions, security settings, and more. Employing proper monitoring tools and conducting regular audits can prevent these errors from arising, ensuring a smooth user experience and maintaining SEO integrity. Many of the aforementioned issues can be complex, so seeking professional help from a network or server administrator may be necessary in some cases.

HTTP Status Code Checker Tool for Identifying HTTP 520 Errors

http status code checker

The Sitechecker HTTP Status Codes tool is designed to quickly identify HTTP 520 Errors. By entering the URL, users can detect 520 status codes, enabling prompt attention to potential issues that may affect user experience and SEO.
The tool also analyzes the possible causes behind the 520 Errors, examining server response and other relevant details. This information can guide the troubleshooting process, making it easier to understand and resolve the problem.

Utilizing a user-friendly interface, the Sitechecker tool is a valuable tool for webmasters and SEO professionals. It aids in maintaining accessible and error-free web pages, thus contributing to an optimized online presence.


The 520 status code, known as “Unknown Error,” occurs mainly in the context of reverse proxies and signifies an unexpected response that the proxy can’t interpret. This error can have several detrimental effects on SEO, including the inability of search engines to crawl and index content, delayed indexing, loss of link equity, and diminished user experience. Various underlying issues such as misconfigured servers, firewall conflicts, network problems, server overload, and certificate errors can cause the 520 error, and specific methods are needed to troubleshoot and fix them.

Tools like the Sitechecker HTTP Status Codes can play a vital role in quickly identifying and analyzing these errors, aiding webmasters and SEO professionals in maintaining an optimized online presence.

An unknown error may require a detailed investigation into server configurations, network conditions, or application behavior. Check logs, server settings, and consider utilizing tools like Sitechecker to identify and resolve the issue.
Error code 520 in Java doesn't specifically relate to Java itself. Instead, it's a standard HTTP status code indicating an "Unknown Error," usually involving a reverse proxy receiving an unexpected response from the origin server.
Error code 520 on Firefox represents an HTTP error where the browser received an unexpected response from the server. This might be due to server misconfigurations, network issues, or other problems between the server and the proxy. Refreshing the page or checking the server's status may resolve the issue.
Fast Links

You may also like

View More Posts
Understanding the HTML Onclick Attribute
SEO Basics
Understanding the HTML Onclick Attribute
Ivan Palii
Oct 16, 2023
What is 414 Error: Meaning and How to Fix Issue
SEO Basics
What is 414 Error: Meaning and How to Fix Issue
Ivan Palii
Sep 27, 2023
Average Session Duration: Calculation, Tips, and Best Practice
SEO Basics
Average Session Duration: Calculation, Tips, and Best Practice
Ivan Palii
Sep 29, 2023