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HTTP Error Codes Troubleshooting Guide

HTTP error codes occurring on web servers provide critical information for understanding why a request failed.

In this guide, you will learn the most common 4xx and 5xx HTTP error codes, their causes and applicable solution methods on Apache and Nginx servers.

What Will You Learn in This Guide?​

The following topics will be covered in this document:

  • Meanings of 4xx and 5xx HTTP error codes
  • Common error scenarios on Apache and Nginx servers
  • Fast problem detection via log files
  • Practical solution methods for each error code

Overview of HTTP Error Codes​

Each HTTP request is responded to by the server with a three-digit status code.

Code GroupMeaning
1xxInformation
2xxSuccessful
3xxRedirect
4xxClient Error
5xxServer Error

This guide focuses specifically on client errors (4xx) and server errors (5xx).

General Troubleshooting Tips​

Before resolving HTTP errors, it is recommended that you perform the following basic checks:

  • Clear browser cache after server configuration changes
  • Examine files access.log and error.log
  • Determine whether the error is caused by the web server or the application

4xx – Client Errors​

400 Bad Requests​

The request was sent by the server with an unintelligible error.

Possible Causes​

  • Corrupted browser cookies
  • Incorrect URL or HTTP request
  • Incorrectly used curl or API calls

Solution​

  • Clear browser cache and cookies
  • Verify request format

401 Unauthorized​

The server has received the request but authentication is required.

Possible Causes​

  • Incorrect username or password
  • Expired session
  • Incorrect HTTP authentication configuration

Solution​

  • Verify user information
  • Check .htpasswd or HTTP Auth configuration

403 Forbidden​

The request is valid, but the server does not grant access.

Possible Causes​

  • Incorrect file permissions
  • Incorrect .htaccess rules
  • There is no index file in the directory

Sample Solution​

sudo chmod o=r /var/www/siteadi/index.html

This command makes the file readable.

404 Not Found

The server is running, but the requested resource could not be found.

Possible Causes​

Incorrect URL File has been deleted or moved Incorrect DocumentRoot configuration

Solution​

Check file path Verify VirtualHost configuration

5xx – Server Errors​

500 Internal Server Error

The server cannot process the request.

Possible Causes​

Incorrect .htaccess file Missing PHP modules Incorrect file permissions

Solution​

Examine file error.log Undo recent configuration changes 502 Bad Gateway

The proxy server could not receive a valid response from the backend (backend) server.

Possible Causes​

Backend application not working Incorrect proxy configuration Firewall is blocking traffic

Solution​

Verify that the backend service is running Check reverse proxy settings

503 Service Unavailable

The server is temporarily unavailable.

Possible Causes​

Insufficient CPU or RAM Server maintenance mode Worker or connection limits reached

Solution​

Check server resource usage Increase Apache or Nginx worker limits

504 Gateway Timeout

The proxy server did not receive a timely response from the backend server.

Possible Causes​

Backend application is running very slowly and Timeout settings are low

Solution​

Increase backend performance Increase timeout values

Frequently Asked Questions (FAQ)​

Do HTTP error codes affect SEO?

Yes. In particular, 404 and 5xx errors can negatively affect search engine rankings.

Which log file should I check?

First, error.log and then access.log files should be examined.

What is the difference between 401 and 403? 401 → Authentication required 403 → Access completely denied

What is the difference between error codes 502 and 504?

502 Bad Gateway → Backend returns invalid response 504 Gateway Timeout → Backend does not respond within the specified time

Result​

In this guide, you learned the meanings and solutions to the most common HTTP error codes.

Thanks to this information:

You can detect errors faster on Apache and Nginx-based servers, easily solve problems with log analysis, and ensure uninterrupted operation of your web applications.

You can use high-performance servers on GenixNode to make your server infrastructure more reliable.