If the mount fails, check with your system administrator to ensure that you are authorized to access the AIX or UNIX file system listed in the exports control file. The NFS client also provides UNIX authentication for security and provides the UNIX client's UID, GID, and a list of the GIDs from the UNIX client's groups to the remote NFS server for authorization checking. When the remote NFS server is the NFS server, the mvslogin command can be used to provide additional security checking through RACF authentication. The privilege level required to enter mount and unmount commands is superuser. When requesting service from the NFS server, if the z/OS system operator issues the freeze=on operand of the modify command, all new tries to mount a z/OS UNIX file system fail until the z/OS system operator issues the freeze=off operand. If the UNIX system operator issues the freeze=onhfs operand of the modify command, z/OS conventional MVS data sets can still be mounted, but all new tries to mount UNIX files fail until the system operator issues the freeze=offhfs operand.