9.5. Securing NFS
NFS is well suited for sharing entire file systems with a large number of known hosts in a transparent manner. However, with ease of use comes a variety of potential security problems.
The following points should be considered when exporting NFS file systems on a server or mounting them on a client. Doing so minimizes NFS security risks and better protects data on the server.
For a concise listing of steps administrators can take to secure NFS servers, refer the the chapter titled Server Security in the Security Guide.
9.5.1. Host Access
Depending on which version of NFS you plan to implement, depends on your existing network environment, and your security concerns. The following sections explain the differences between implementing security measures with NFSv2, NFSv3, and NFSv4. If at all possible, use of NFSv4 is recommended over other versions of NFS.
9.5.1.1. Using NFSv2 or NFSv3
NFS controls who can mount an exported file system based on the host making the mount request, not the user that actually uses the file system. Hosts must be given explicit rights to mount the exported file system. Access control is not possible for users, other than through file and directory permissions. In other words, once a file system is exported via NFS, any user on any remote host connected to the NFS server can access the shared data. To limit the potential risks, administrators often allow read-only access or squash user permissions to a common user and group ID. Unfortunately, these solutions prevent the NFS share from being used in the way it was originally intended.
Additionally, if an attacker gains control of the DNS server used by the system exporting the NFS file system, the system associated with a particular hostname or fully qualified domain name can be pointed to an unauthorized machine. At this point, the unauthorized machine is the system permitted to mount the NFS share, since no username or password information is exchanged to provide additional security for the NFS mount.
Wildcards should be used sparingly when exporting directories via NFS as it is possible for the scope of the wildcard to encompass more systems than intended.
It is also possible to restrict access to the
portmap service via TCP wrappers. Access to ports used by portmap, rpc.mountd, and rpc.nfsd can also be limited by creating firewall rules with iptables.
For more information on securing NFS and
portmap, refer to the chapter titled Server Security in the Security Guide. Additional information about firewalls can be found in Chapter 18, iptables.
9.5.1.2. Using NFSv4
The release of NFSv4 brought a revolution to authentication and security to NFS exports. NFSv4 mandates the implementation of the RPCSEC_GSS kernel module, the Kerberos version 5 GSS-API mechanism, SPKM-3, and LIPKEY. With NFSv4, the mandatory security mechanisms are oriented towards authenticating individual users, and not client machines as used in NFSv2 and NFSv3.
Note
It is assumed that a Kerberos ticket-granting server (KDC) is installed and configured correctly, prior to configuring an NFSv4 server.
NFSv4 includes ACL support based on the Microsoft Windows NT model, not the POSIX model, because of its features and because it is widely deployed. NFSv2 and NFSv3 do not have support for native ACL attributes.
Another important security feature of NFSv4 is its removal of the
rpc.mountd daemon. The rpc.mountd daemon presented possible security holes because of the way it dealt with filehandlers.
For more information on the RPCSEC_GSS framework, including how
rpc.svcgssd and rpc.gssd interoperate, refer to http://www.citi.umich.edu/projects/nfsv4/gssd/.
