"incorrect mount option was specified" when mounting krb5p nfs4 partition on Ubuntu

I have a NFS4 share running with krb5p. I have no problems accessing it from CentOS clients, all that is required is: yum install krb5-workstation setup krb5 (edit krb5.conf, setup keytab) systemctl enable nfs-secure.service && systemctl start nfs-secure.service systemctl enable nfs-client.target && systemctl start nfs-client.target mkdir /mnt/x Add the following to fstab: server.example.com:/srv/share/subdir /mnt/x nfs4 defaults,sec=krb5p,noexec,nosuid,_netdev,auto 0 0 This works great on CentOS, I've setup a dozen client hosts so far that way. However on Ubuntu, I get : mount.nfs4: an incorrect mount option was specified I think the Ubuntu error is to do with nfs-secure.service however there seems to be no equivalent on Ubuntu that gets installed with NFS client ? (I am using Ubuntu 16.04.5 LTS). ***UPDATE:*** I have tried: systemctl enable rpc-gssd.service && systemctl start rpc-gssd.service That launches OK: # systemctl status rpc-gssd.service ● rpc-gssd.service - RPC security service for NFS client and server Loaded: loaded (/lib/systemd/system/rpc-gssd.service; static; vendor preset: enabled) Active: active (running) since Thu 2018-10-04 16:49:40 BST; 6min ago Process: 51689 ExecStart=/usr/sbin/rpc.gssd $GSSDARGS (code=exited, status=0/SUCCESS) Main PID: 51691 (rpc.gssd) Tasks: 1 Memory: 516.0K CPU: 13ms CGroup: /system.slice/rpc-gssd.service └─51691 /usr/sbin/rpc.gssd But Ubuntu just hangs when trying to mount ? # mount -v -t nfs4 -o defaults,sec=krb5p,noexec,nosuid,_netdev,auto server.example.com:/srv/dir/example /mnt/example mount.nfs4: timeout set for Thu Oct 4 16:54:40 2018 mount.nfs4: trying text-based options 'sec=krb5p,addr=10.10.10.10,clientaddr=10.10.10.9' # NOTHING ELSE HAPPENS.....