How to get nfsd to serve NFSv4 on High Sierra? I'm hoping to serve an NFS mount from my High Sierra (10.13.4) server to a linux client. The linux client can mount using NFSv3 protocol, but I use ACLs on the served files and directories, so I think we need NFSv4.

I'd like to use version 4 of the protocol. Is this possible? I see the nfsd(8) man page does not mention version 4. I found a manpage for FreeBSD exports(5) that uses the flag V4: at the start of a line in /etc/exports to declare the protocl version. 

I tried this with my nfsd and it failed to parse correctly, using "nfsd checkexports".

If I check rpc:

$ sudo rpcinfo -p
program vers proto   port  
...
100003     2   udp   2049  nfs
100003     3   udp   2049  nfs
100003     2   tcp   2049  nfs
100003     3   tcp   2049  nfs


This makes me think my nfsd supports version 2 and 3 of the protocol. perhaps I can upgrade this server?
The version of nfsd on the Mac does support v4. For ACL support you definitely need to connect using v4.

Your failure to get a v4 connection is probably on the client side. Most NFS clients default to trying v3 first then falling back to v2 if unsuccessful.

On your client side you should specify v4 using the -o vers=4 option to your mount command. Do a man mount_nfs on the client to check that it supports that syntax rather than the old deprecated -o nfsv4 syntax. You could also use -4 but that is also deprecated and highly discouraged.

To get ACL support in your client you may have to add the -o acl option as most clients default to no ACL support.

A close reading of man NFS and man mount_nfs on the client is highly recommended.