Problem

Validation Authority log files sometimes reference Curl Error Code numbers and it is not clear what they mean. For example, there might be entries like the following:

E: [01/Jan/2017:00:00:00.000 -0000 TID:00000000] Failed to fetch data from http://example.com:80/example.crl location.Curl Error Code:6.Reason:Unknown Error.

Resolution

These refer to the open source program, cURL and the error numbers can be looked up in the cURL documentation. For example, the Curl Error Code:6 shown in the sample above is listed in the documentation as:

CURLE_COULDNT_RESOLVE_HOST (6)

Couldn't resolve host. The given remote host was not resolved.

That one means that DNS was unable to resolve the host (example.com) for the URL displayed in the error message.