Practical UNIX and Internet Security, 3rd Edition

Understanding NFS

Using NFS, clients can mount partitions of a server as if they were physically connected to the client. In addition to allowing remote access to files over the network, NFS allows many (relatively) low-cost computer systems to share the same high-capacity disk drive at the same time. NFS clients and servers have been written for many different operating systems.

NFS is nearly transparent. In practice, a workstation user simply logs into the workstation and begins working, accessing it as if the files were locally stored. In many environments, workstations are set up to mount the disks on the server automatically at boot time or when files on the disk are first referenced. NFS also has a network-mounting program that can be configured to mount the NFS disk automatically when an attempt is made to access files stored on remote disks.

There are several basic security problems with NFS:

• NFS is built on top of Sun’s RPC (Remote Procedure Call), and in most cases uses RPC for user authentication. Unless a secure form of RPC is used, NFS can be easily spoofed.

• Even when Secure RPC is used, information sent by NFS over the network is not encrypted, and is thus subject to monitoring and eavesdropping. The data can be intercepted and replaced (thereby corrupting or Trojaning files being imported via NFS).

• NFS uses the standard Unix filesystem for access control, opening the networked filesystem to many of the same problems as a local filesystem.

One of the key design features ...