400 The Plain Http Request Was Sent To Https Port

This error message is a common one when troubleshooting Nginx errors in Linux or Unix-based systems. When an HTTP request is sent to HTTPS port (443), Nginx returns a 400 status code, indicating that the request was unsuccessful due to a problem with the client's certificate or connection.

The following 400 status codes were encountered by Nginx:

• 401 Unauthorized
• 403 Forbidden
• 404 Not Found
• 405 Method Not Allowed
• 406 Not Acceptable
• 407 Proxy Authentication Required
• 408 Request Timeout
• 409 Conflict
• 410 Gone
• 411 Length Required
• 412 Precondition Failed
• 413 Payload Too Large
• 414 URI Too Long
• 415 Unsupported Media Type
• 416 Range Not Satisfiable
• 417 Expectation Failed
• 418 I'm a teapot
• 421 Bad Gateway
• 422 Unprocessable Entity
• 423 Payload Too Large
• 424 Protocol Evolution Required
• 425 Too Early
• 426 Upgrade Required
• 428 Precondition Required
• 429 Too Many Requests
• 431 Request Header Fields Too Large
• 451 Refused to Close

In some cases, the error message might be more detailed, such as:

Nginx received a TLS certificate error or a connection refused error.

To troubleshoot Nginx errors in your Linux or Unix-based system environment, check the following:

• Verify that the client's certificate is up-to-date and valid.
• Check the connection settings for HTTPS port (443).
• Run a `telnet` test to verify that the connection is established.
• Review the Nginx configuration files for any errors or warnings.

Learn more about troubleshooting Nginx errors in Linux or Unix-based systems on IceClog.