NFS on Debian Stretch

Server installation

# apt install nfs-kernel-server
# cd /usr/share
# mkdir nfs-shared
# chmod 757 nfs-shared
# echo "/usr/share/nfs-shared/ <IP_client>(rw,root_squash)" >> /etc/exports
# service nfs-kernel-server reload

Client configuration (Proxmox)

Go to Datacenter → Storage→ Add → NFS

ID: nfsproxmox
Server: <IP_NFS_server>
Export: /usr/share/nfs-shared
Content: what you want
Nodes: what you want
Enable : check
Max Backups: 1
Add

You can now use the shared NFS filesystem.

If you want to allow other clients to use nfs sharing, add /usr/share/nfs-shared/ <IP_other_client>(rw,root_squash) in /etc/exports.