Beginning in SLE 15 SP6, NFS Server no longer supports UDP by default

After upgrading to SLES 15 SP6 or SP7, the kernel-based NFS Server no longer has support for UDP enabled by default.

SUSE recommends avoiding NFS over UDP (see the "Additional Information" section for more info).  The default for NFS support for UDP recently changed to "disabled", but it can still be enabled, if necessary.

Edit /etc/sysconfig/nfs

Find:
NFSD_OPTIONS=""
 
and alter it to either of these (which are equivalent to each other):
 
NFSD_OPTIONS="--udp"
or
NFSD_OPTIONS="-u"
 
Then do "systemctl restart nfsserver"

The default behavior was changed.  This may have involved kernel changes, and/or it may involve changed in the source code for the nfs-utils package, from which the "nfs-kernel-server" package is built.  The output of "man 8 rpc.nfsd" has also been changed to reflect the new default:

       -u  or  --udp
              Instruct the kernel nfs server to open and listen on a UDP socket.

       -U  or  --no-udp
              Instruct the kernel nfs server not to open and listen on a UDP socket. This is the default.

NFS over UDP is not considered fully reliable.  This goes beyond the fact that UDP is considered an unreliable protocol, in contrast to TCP, which is specifically designed to be reliable, with verified delivery.  When using NFS over UDP, there are risks of silent data corruption when using modern network speeds (even 1 1 Gbps).

For more details, see "man 5 nfs" and look for the section titled, "Using NFS over UDP on high-speed links".