I recently had the case, where I lost connexion of my NFS client connexion because NFS server crashed. The problem I had is simple, some of clients couldn’t recover their connexion because the old one was still shown as already connected.
And when I tried to remount NFS clients connexions, I got:
mount.nfs: mount(2): Protocol not supported
mount.nfs: trying text-based options 'udp,sec=sys,rsize=8192,wsize=8192,intr,hard,addr=10.0.0.1'
mount.nfs: prog 100003, trying vers=3, prot=17
mount.nfs: trying 10.0.0.1 prog 100003 vers 3 prot UDP port 2049
mount.nfs: prog 100005, trying vers=3, prot=17
mount.nfs: trying 10.0.0.1 prog 100005 vers 3 prot UDP port 54874
The solution to recover was to unexport connexions from the server:
exportfs -u <ip>:/mnt/nfs/dev/xmlcache</ip>
If you want to know a little bit more about it, you can check my wiki page on NFS. I think this kind of issue may be less present in the version 4 of the NFS protocol, but I didn’t try to reproduce it with the 4 version. Any feedback will be appreciated 😉
Hope this will help