NFS/Kerberos

Kerberos is one of the few security mechanisms available for NFS. It provides strong per-user authentication, strong data encryption, and (with NFSv4) removes the requirement for matching UIDs/GIDs between client and server.

NFSv4 is recommended for Kerberos. Although it is possible to enable Kerberos in NFSv3, it will not provide 100% coverage as the auxiliary NFSv3 protocols (e.g. the "mount" protocol) remain insecure, and the lack of idmapping in NFSv3 severely reduces the usability of Kerberos as well.

Prerequisites

Kerberos requires a KDC (Key Distribution Center) service to be running somewhere. The krb5-kdc.service that comes with MIT Kerberos (krb5) is a good option for small setups that do not warrant the complexity of Active Directory or FreeIPA. The second option is Heimdal, found on some BSDs and as heimdal^AUR on Arch.

NFSv4 idmapping becomes much more important to have with Kerberos. Both the server and the clients should have the same idmapping domain configured:

/etc/idmapd.conf

[General]
Domain = example.com

Server configuration

Each NFS server needs a Kerberos principal for nfs/server.fqdn to be created on the KDC, and its keys added to the server's /etc/krb5.keytab.

Note:

It is recommended to also have the standard host/ principal there as well, for other purposes.

fileserv# kadmin -p frob/admin
Password for frob/admin@EXAMPLE.COM: *********
kadmin:  addprinc -nokey nfs/fs.example.com
kadmin:  ktadd nfs/fs.example.com

gssproxy.service must be enabled and started on the server.

Tip: Earlier systems used rpc.svcgssd, but it is no longer present in modern nfs-utils and has been fully superseded by gssproxy.

gssproxy comes pre-configured with NFS server support (the /etc/gssproxy/24-nfs-server.conf file is part of the standard package) and does not need any tweaking, other than placing the NFS service keytab in the standard location.

Your /etc/exports should offer the Kerberos authentication flavors in the sec= option:

/home    *(rw,sec=krb5p)
/usr     10.147.0.0/16(rw,sec=krb5p:krb5i:krb5:sys) *(rw,sec=krb5p)

Note: Do not use the obsolete gss/krb5p(...) syntax. It still works, but you should not be using it anymore.

The available flavors are:

• krb5p provides 'privacy' (Kerberos-based encryption). It is sufficiently secure to be used over Internet, but might provide poor throughput over a LAN – consider using krb5 inside RPC-with-TLS instead.
• krb5i provides 'integrity' (Kerberos-based MAC) but not encryption. It might be useful for serving static data as it still protects against packet tampering.
• krb5 provides only authentication, without data integrity or encryption. It is a good choice if you have RPC-over-TLS enabled via xprtsec= or if Kerberos is being run over an otherwise "secure" LAN (e.g. over a WireGuard tunnel), but certainly not in the clear over public networks.
• sys is the traditional UID-based (non-Kerberos) NFS security mode.

Client configuration

In addition to users, each NFS client should have a machine Kerberos principal in /etc/krb5.keytab, which will be used in situations where user Kerberos tickets are not yet available – in particular, it will be needed to actually mount the filesystem at boot time before any users have logged in yet (or if mounting is done via autofs). More generally, all operations done "as root" will be authenticated as the machine principal.

Tip: The machine identity is also mandatory for some internal NFS operations if NFSv4.0 or NFSv4.1 are used, although no longer mandatory in NFSv4.2.

Unlike in the server case, the client machine does not need an nfs/ principal specifically – it is enough to have the generic host/the.fqdn principal. (See the rpc.gssd manual page for what it looks for.)

The client must have rpc-gssd.service active (i.e. the rpc.gssd daemon).

Options used when mounting the filesystems are very similar to the options used in /etc/exports; you can specify one or more flavors using the sec= option. Although the client will automatically use the strongest mode offered, it is nevertheless recommended to explicitly require e.g. sec=krb5p to prevent downgrade attacks.

Once the filesystem has been mounted, root may already access it (using the machine's Kerberos credentials), but every non-root user needs their own Kerberos tickets to be present. This means either having the user manually run kinit for themselves, or setting up pam-krb5 to acquire tickets during login (which only works for password-based logins), or using gssproxy or k5start(1) (from kstart^AUR) to acquire tickets from a keytab file.

Tip: You will likely need to set up k5start to renew tickets as they expire a few hours after being acquired.