The "host=" setting in /etc/nfs.conf isn't fit for the function it needs
to perform.  Of note it doesn't allow binding to multiple addresses.  It
also doesn't allow restricting to binding IPv4-only nor IPv6-only.  In
fact I was unable to get it to restrict IPv6 binding at all.

Multiple addresses is a common configuration.  Disabling IPv4 or IPv6
are also common.

It also appears stopping/starting `/etc/init.d/nfs-kernel-server` appears
to leave old address bindings in place.  If the "host=" setting is added,
then the daemon is restarted it will remain bound to the wildcard
address.

NFS is a service which should be filtered at firewalls, but it should
still be possible to restrict binding as a backup security measure.  I'm
tempted to rate this as pretty high importance.