root# cat /var/lib/nfs/rmtab
From the rpc.mountd(8) man page:
The rmtab File
The rpc.mountd daemon registers every successful MNT request by adding
an entry to the /var/lib/nfs/rmtab file. When receivng a UMNT request
from an NFS client, rpc.mountd simply removes the matching entry from
/var/lib/nfs/rmtab, as long as the access control list for that export
allows that sender to access the export.
Clients can discover the list of file systems an NFS server is cur-
rently exporting, or the list of other clients that have mounted its
exports, by using the showmount(8) command. showmount(8) uses other
procedures in the NFS MOUNT protocol to report information about the
server’s exported file systems.
Note, however, that there is little to guarantee that the contents of
/var/lib/nfs/rmtab are accurate. A client may continue accessing an
export even after invoking UMNT. If the client reboots without sending
a UMNT request, stale entries remain for that client in
/var/lib/nfs/rmtab.