8.8. Securing NFS

NFS is suitable for transparent sharing of entire file systems with a large number of known hosts. However, with ease-of-use comes a variety of potential security problems. To minimize NFS security risks and protect data on the server, consider the following sections when exporting NFS file systems on a server or mounting them on a client.

8.8.1. NFS Security with AUTH_SYS and Export Controls

Traditionally, NFS has given two options in order to control access to exported files.
First, the server restricts which hosts are allowed to mount which filesystems either by IP address or by host name.
Second, the server enforces file system permissions for users on NFS clients in the same way it does local users. Traditionally it does this using AUTH_SYS (also called AUTH_UNIX) which relies on the client to state the UID and GID's of the user. Be aware that this means a malicious or misconfigured client can easily get this wrong and allow a user access to files that it should not.
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 through 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 rpcbind[1] service with TCP wrappers. Creating rules with iptables can also limit access to ports used by rpcbind, rpc.mountd, and rpc.nfsd.
For more information on securing NFS and rpcbind, refer to man iptables.

8.8.2. NFS Security with AUTH_GSS

NFSv4 revolutionized NFS security by mandating the implementation of RPCSEC_GSS and the Kerberos version 5 GSS-API mechanism. However, RPCSEC_GSS and the Kerberos mechanism are also available for all versions of NFS. In FIPS mode, only FIPS-approved algorithms can be used.
Unlike AUTH_SYS, with the RPCSEC_GSS Kerberos mechanism, the server does not depend on the client to correctly represent which user is accessing the file. Instead, cryptography is used to authenticate users to the server, which prevents a malicious client from impersonating a user without having that user's Kerberos credentials. Using the RPCSEC_GSS Kerberos mechanism is the most straightforward way to secure mounts because after configuring Kerberos, no additional setup is needed.

Configuring Kerberos

Before configuring an NFSv4 Kerberos-aware server, you need to install and configure a Kerberos Key Distribution Centre (KDC). Kerberos is a network authentication system that allows clients and servers to authenticate to each other by using symmetric encryption and a trusted third party, the KDC. Red Hat recommends using Identity Management (IdM) for setting up Kerberos.

Procedure 8.2. Configuring an NFS Server and Client for IdM to Use RPCSEC_GSS

    • Create the nfs/hostname.domain@REALM principal on the NFS server side.
    • Create the host/hostname.domain@REALM principal on both the server and the client side.
    • Add the corresponding keys to keytabs for the client and server.
  1. On the server side, use the sec= option to enable the wanted security flavors. To enable all security flavors as well as non-cryptographic mounts:
    /export *(sec=sys:krb5:krb5i:krb5p)
    
    Valid security flavors to use with the sec= option are:
    • sys: no cryptographic protection, the default
    • krb5: authentication only
    • krb5i: integrity protection
    • krb5p: privacy protection
  2. On the client side, add sec=krb5 (or sec=krb5i, or sec=krb5p, depending on the setup) to the mount options:
    # mount -o sec=krb5 server:/export /mnt
    
    For information on how to configure a NFS client, see the Setting up a Kerberos-aware NFS Client section in the Red Hat Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide.
Although Red Hat recommends using IdM, Active Directory (AD) Kerberos servers are also supported. For details, see the following Red Hat Knowledgebase article: How to set up NFS using Kerberos authentication on RHEL 7 using SSSD and Active Directory.
For more information, see the exports(5) and nfs(5) manual pages, and Section 8.5, “Common NFS Mount Options”.
For further information on the RPCSEC_GSS framework, including how gssproxy and rpc.gssd inter-operate, see the GSSD flow description.

8.8.2.1. NFS Security with NFSv4

NFSv4 includes ACL support based on the Microsoft Windows NT model, not the POSIX model, because of the Microsoft Windows NT model's features and wide deployment.
Another important security feature of NFSv4 is the removal of the use of the MOUNT protocol for mounting file systems. The MOUNT protocol presented a security risk because of the way the protocol processed file handles.

8.8.3. File Permissions

Once the NFS file system is mounted read/write by a remote host, the only protection each shared file has is its permissions. If two users that share the same user ID value mount the same NFS file system, they can modify each others' files. Additionally, anyone logged in as root on the client system can use the su - command to access any files with the NFS share.
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. Red Hat recommends that this feature is kept enabled.
By default, NFS uses root squashing when exporting a file system. This sets the user ID of anyone accessing the NFS share as the root user on their local machine to nobody. Root squashing is controlled by the default option root_squash; for more information about this option, refer to Section 8.7.1, “The /etc/exports Configuration File”. If possible, never disable root squashing.
When exporting an NFS share as read-only, consider using the all_squash option. This option makes every user accessing the exported file system take the user ID of the nfsnobody user.