NFSREF(8) System Manager's Manual NFSREF(8)

nfsref - manage NFS referrals

nfsref [-?d] [-t type] add pathname server export [ server export ... ]

nfsref [-?d] [-t type] remove pathname

nfsref [-?d] [-t type] lookup pathname

NFS version 4 introduces the concept of file system referrals to NFS. A file system referral is like a symbolic link on a file server to another file system share, possibly on another file server. On an NFS client, a referral behaves like an automounted directory. The client, under the server's direction, mounts a new NFS export automatically when an application first accesses that directory.

Referrals are typically used to construct a single file name space across multiple file servers. Because file servers control the shape of the name space, no client configuration is required, and all clients see the same referral information.

The Linux NFS server supports NFS version 4 referrals. Administrators can specify the refer= export option in /etc/exports to configure a list of exports from which the client can choose. See exports(5) for details.

The nfsref(8) command is a simple way to get started managing junction metadata. Other administrative commands provide richer access to junction information.

Valid nfsref(8) subcommands are:

Adds junction information to the directory named by pathname. The named directory must already exist, and must not already contain junction information. Regular directory contents are obscured to NFS clients by this operation.
A list of one or more file server and export path pairs is also specified on the command line. When creating an NFS basic junction, this list is stored in an extended attribute of the directory.
If junction creation is successful, the nfsref(8) command flushes the kernel's export cache to remove previously cached junction information.
Removes junction information from the directory named by pathname. The named directory must exist, and must contain junction information. Regular directory contents are made visible to NFS clients again by this operation.
If junction deletion is successful, the nfsref(8) command flushes the kernel's export cache to remove previously cached junction information.
Displays junction information stored in the directory named by pathname. The named directory must exist, and must contain junction information.
When looking up an NFS basic junction, the junction information in the directory is listed on stdout.

Enables debugging messages during operation.
Specifies the junction type for the operation. Valid values for junction-type are nfs-basic or nfs-fedfs.
For the add subcommand, the default value if this option is not specified is nfs-basic. For the remove and lookup subcommands, the --type option is not required. The nfsref(8) command operates on whatever junction contents are available.

Suppose you have two file servers, top.example.net and home.example.net. You want all your clients to mount top.example.net:/ and then see the files under home.example.net:/ automatically in top:/home.

On top.example.net, you might issue this command as root:

# mkdir /home
# nfsref --type=nfs-basic add /home home.example.net /
Created junction /home.

/etc/exports
NFS server export table

exports(5)

RFC 5661 for a description of NFS version 4 referrals

Chuck Lever <chuck.lever@oracle.com>

9 Jan 2018