After setting up, trying out and debugging my own exports I simply tried out the loopback example mentioned in the documentation by copying and pasting it straight into the terminal window.
In all cases I get the same thing: The mount command "hangs" and does nothing.
Trying out "vers=3" does not make things better (local on the box it says "unsupported protocol version" though NFS3 should be supported as said in the documentation [or does the mount.nfs command only support Version 4 but the server does it all?], from remote it still hangs).
OK, NFS seems to be an issue with newer OpenWRT versions and not a functionality which is focused by development.
I found lots of users complaining that NFS stopped working after upgrade or that after a fresh installation they don't get NFS to work. But in the cases I was able to find (also in this forum which in general is a very big help and has a great community as I can tell) no really solution or discussion on those threads took place.
So in the end I gave up and use sshfs instead.
What a pity! I am running two Ubuntu servers and several clients with NFS for years and don't get it to work on my new OpenWRT device.
Perhaps in the next release
I wanted to actually test a Samba server, but since I saw this problem I decided to use a NFS server.
Fresh compiled "master", loop-device was already selected for testing DM-Crypt so I only added nfs-server and nfs-utils (not sure they are needed).
Works just fine when exporting external usb hard drive or an sd-card (mmc). Untweaked I am getting around 9MB/s so I should look into that. Might be a slow SD-card or the fact that I am using dm-crypt on the external drive.
BUT: file transfer back and forth between OpenWRT as server and my linux box works just fine.
I have no experience with exporting the Overlayfs, but since that didn't change and you had that working before, those settings should be the same as before.
@drbrains :
I didn't compile from source but installed the latest release which was available last week (OpenWrt 21.02.0 r16279-5cc0535800). So if it works in the master branch perhaps the next release will fix something?
@Barney :
Keep in mind that I followed the installation and setup instructions for NFS on a fresh system and even the local mount which is mentioned in the doc as a test did not work.
So I don't think there is an incompatible configuration between server and client
But when anyone has an idea how to fix this issue I would be very thankful
Per now my backup works using sshfs and so I have no actual use case, but having it up and running is no bad idea in an complete Linux environment
I haven't read these instructions, so can't say anything about their correctness.
I've set up many NFS systems (client & server) during the last decades and never had a problem with NFS. It just worked out of the box. But I must admit, that all these systems have been Linux hosts. I have no experience, if one part (either server or client) is a non-Linux host.
So instead of reading OpenWRT's NFS instructions I was guided by my previous Linux NFS experience and I configured the NFS server on the OpenWRT router the same way I did on a Linux host. And it worked.
Maybe, I don't know. But there is definitely something wrong with your NFS configuration (either on the server or the client or both). Otherwise it should work.