HTTP Response Codes and Possible Failure Issues

HTTP response codes from 1xx to 5xx and their possible failure issues:

Out-going request:

• Own server --> ISP proxies --> Customer proxy (Nginx) -->  Customer server

Inward request:

• Customer server --> ISP proxies --> Own proxy (Nginx) --> Own server

1xx:

• Exactly a response from server
• No failures, informational response

2xx:

• Exactly a response from server
• Successful response from server

3xx:

• Response from server or proxies
• HTTP redirect
• Possible issues:
• Bad redirect by server, for example, infinite redirection
• Bad redirect by proxies, for example, inifinite redirection

4xx:

• Exactly a response from server
• Error due to client request data
• Possible issues:
• Bad headers
• Bad parameters in URL
• Bad data in POST body

5xx:

• Response from server or proxies
• Error due to server side
• Possible issues:
• Server code error, exception, crash, etc.
• Proxy (Nginx) fails to proxy_pass to a service

Timeout:

• Target IP, domain, subdomain don't exist
• Target machine having firewall that dropouts connections
• No internet connection
• Server doesn't respond

Reference:
https://en.wikipedia.org/wiki/List_of_HTTP_status_codes