The main reason the NFS protocol is not recommended for use across a firewall or a security domain is that.

A.
it is UDP based. As a result, its state is difficult to track.

B.
This protocol uses a range of ports, and firewalls have difficulty opening the proper entry points to allow traffic.

C.
File permissions are easily modified in the requests, and the security of the protocol is not stringent.

D.
Industry technicians do not understand NFS well, but is actually appropriate to run across various security domains.