For every page that cannot be crawled successfully, a network error code is stored. This is only possible, if there are no network errors during crawling.
Source vs. Target Network Error
A network error can occur on both ends of a link, the source page/domain or the target page/domain or even both.
Network Error codes
The following network error codes are possible
|-1||Socket Timeout Exception|
|-4||Too Many Redirects|
|-5||Max Request Duration (timeout fetching the page)|
|-6||Unknown Host Exception (could not resolve IP address for host)|
|-21||SSL Exception with message “Could not generate DH keypair”|
|-99||Other network errors|