Missing 10G interface on Intel 82599/X520: enabling allow_unsupported_sfp persistently on CentOS and Ubuntu

A Linux server with an Intel 82599ES 10-Gigabit SFI/SFP+ controller showed no 10G interface and dmesg contained failed to load because an unsupported SFP+ or QSFP module type was detected after an untested SFP+ module was inserted. The article explains that the ixgbe driver only enables modules it has been validated with and shows how to enable others persistently on CentOS 7 and Ubuntu 16/17/18.

• Hardware: Intel 82599ES / X520
• Transceivers: untested third-party SFP+
• Form factor: SFP+

Symptoms / messages:

failed to load because an unsupported SFP+ or QSFP module type was detected
interface missing