HTTP response status codes indicates the status of the Web Page , Server or API . Client error and Server error are the common faced errors. Error are mainly analyzed during browsing a web page, API Testing and Load Testing

1. HTTP Response Codes

2. Five Classes of Responses
• 100-199 considered for Informational Response.
• 200-299 considered for Successful Response.
• 300-399 considered for Redirects.
• 400-499 considered for Client errors.
• 500-599 considered for Server errors.

3. 400 Bad Request
• 400 Bad Request response status code indicates that the server
cannot or will not process the request due to something that is
perceived to be a client error.
• Forms of Client Error can be invalid request, invalid request message
framing, or deceptive request routing.
• By deleting cookies and by resetting the browser this error can be
resolved in some cases.
• Sometimes, HTTP header is too long. In principle, headers don't have
a size limit and also target server may have to set a limit.

4. 403 Forbidden Response
• 403 Forbidden Response means that user does not have permission to
access page or requested resource.
• This error can occur intentionally or misconfigured permission.
• Intentionally means server understood the request, but is refusing to
authorize it.
• It can be with the Request for URL paths that correspond to file system
directories when directory listings have been disabled in the server
• Sometimes certain administrators configure extension like Mod proxy
extension to Apache to block such requests and in this case it also return
403 Forbidden.

5. 404 Server Not Found
• 404 that shows Response Message “ Server not found” is a client-side error.
• Appears due to broken URL or the page may have been removed from the
website.
• Suppose resource of the page has been deployed from one to another
server so it creates New URL .
When redirecting to Old URL will show us 404 Error.
• While Using Internet Explorer and showing Website cannot found their can
be possibility of 400 (Bad Request) or 404.
• Sometimes by retrying or refreshing the web page can resolve this problem.

6. 500 Internal Server Error
• 500 Internal Server Error is also a "server-side" error, In this type of error
with website's server not PC or Internet connection.
• Sometimes by retrying or refreshing the web page can resolve this
problem. It is a general error on the web site's servers and not your
computer.
• This type of issue can be resolved by Clearing browser history or cache.
There can be possibility that the page can be cached version.

7. 502 Bad Gateway
• Indication for that the server, when act as a gateway or proxy,
received an invalid response from the upstream server.
• Fixing should be done gateway server or proxies.
• Mainly it happens due to three points below
Unable to Resolve Domain Name.
Problem with Origin Server.
Blocked request by Firewall.
• Different Ways to show 502 error responses are Service Temporarily
Unloaded, Proxy Error or Gateway error.

8. 503 Service Unavailable
• Indication that server is not able to handle the request.
• Overloading or Server under Maintenance are the most probable
reason for this response code error.
• Error can be handled by taking care of caching-related headers that
are sent along with this response, as these temporary condition
responses should usually not be cached.
• Troubleshooting can be done using
Reboot the Server or by Checking Logs.
Check for Unexpected Maintenance.

9. 504 Gateway Timeout
• This type of error happens when server attempts to load a web page
but does get a response in time from another server.
• Most of the time error is with website itself when web server is
overwhelmed with requests .
• Website under maintenance causes this type of error.
• Troubleshooting can be done using
By Checking for any DNS changes.
Shift through logs.
Fixing of faulty firewall configurations.
Comb through your website’s code to find bugs.