Disable NFSv4 (server) on Debian, allow NFSv3

Turns out modifying the RPCMOUNTDOPTS variable as described in /etc/default/nfs-kernel-server does not work and there's a bug report for that: #738063

This variable is used in the rpc.mountd call:

# systemctl status nfs-kernel-server
● nfs-kernel-server.service - LSB: Kernel NFS server support
   Loaded: loaded (/etc/init.d/nfs-kernel-server)
   Active: active (running) since Sun 2016-06-12 19:46:01 CEST; 6s ago
  Process: 15110 ExecStop=/etc/init.d/nfs-kernel-server stop (code=exited, status=0/SUCCESS)
  Process: 15119 ExecStart=/etc/init.d/nfs-kernel-server start (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/nfs-kernel-server.service
           └─15167 /usr/sbin/rpc.mountd --manage-gids --port 2048 --no-nfs-version 4

However, clients are still able to mount using -o vers=4.

Instead, this option must be passed to rpc.nfsd. Looking at the init script /etc/init.d/nfs-kernel-server, it seems like the RPCNFSDCOUNT variable is the only variable that's passed to rpc.nfsd. It's not intended for that purpose, but it works and it seems to be the only option short of editing the init script.

Solution:

In /etc/default/nfs-kernel-server, add the --no-nfs-version 4 option to RPCNFSDCOUNT instead of RPCMOUNTDOPTS:

# Number of servers to start up
#RPCNFSDCOUNT=8
RPCNFSDCOUNT="8 --no-nfs-version 4"

Restart the NFS service:

# systemctl restart nfs-kernel-server

Test it:

# mount -t nfs -o vers=4 SERVER:/data/public /mnt
mount.nfs: Protocol not supported

Version 3 still works:

# mount -t nfs -o vers=3 SERVER:/data/public /mnt