why does nfsv4 client open an extra port ? how to close it if it is actually useless ?
i'm working with an ubuntu ( xenial ) bunch of servers and need to understand why mounting an nfs v4 share opens a random port on the client side. the port has no associated process and seems to be directly open by the nfs kernel module. the port is closed if i unmount and a different one is opened if i remount the share. no traffic ever hits that port neither when mounting nor afterwards ( possibly because the share is read-only ).
nmap reports the port ( the number ????? changes from time to time using an apparently random high range port ) as :
PORT STATE SERVICE VERSION
?????/tcp open fmproduct 1-4 (RPC #1073741824)
as far as i remember, nfsv4 does not need a port mapper to work so i don't really get the point of whatever RPC service is open on the client side. is that correct ?
if the above is correct, anybody knows how to instruct ubuntu not to open that port ?
( please don't tell me to use the firewall or hosts.deny : i do not want the port to be open in the first place )