FedFsNfsReferrals
From Linux NFS
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Project: fedfs-utils
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Introduction
FedFS relies on existing support for referrals in standard network file system clients like the NFS or CIFS client built into Linux.
A referral is a file server response that tells a client to look elsewhere for the shared file system it wants. During a referral event, the file server provides a list of locations a client can try. Each location consists of an export path paired with a server hostname or IP address. A client can use any one of these pairs to mount the file system.
NFSv4 referral support is enabled by default (when client NFSv4 support is enabled) in all recent Linux kernel versions. When a referral is encountered, the Linux NFS client automatically retrieves a location list from the file server and attempts to mount each location in the list until one succeeds.
Note that when a file server returns a location containing a DNS hostname, the client must resolve that to an IP address before it can mount a new server.
Legacy referral support
With kernels before 2.6.36, an upcall is needed to perform DNS resolution. The kernel executes a script in /sbin/nfs_cache_getent to resolve a hostname.
If your Linux distributor doesn't provide this script, you can find it in the Linux kernel source in either Documentation/filesystems/nfs.txt or Documentation/filesystems/nfs/nfs.txt. Simply copy the script into /sbin/nfs_cache_getent and make the script executable. After the client is rebooted (or the NFS module is reloaded), NFSv4 referral locations containing hostnames should be handled correctly.
New-style referral support
With kernel 2.6.36 and following, the kernel NFS client can continue to use the upcall mechanism described above if the CONFIG_NFS_USE_LEGACY_DNS build option is set to Y. However, an in-kernel DNS resolver is used if this build option is set to N.
Typically your Linux distributor provides the build option settings used to compile their kernel in /boot/config-foo (where foo is the kernel version). You can look in this file to see how the CONFIG_NFS_USE_LEGACY_DNS build option is set in your distribution.
