I am trying to back up the working files on my Macintosh to a NAS server. The server runs FreeNAS software. I mount the server over AFP (Apple Filing Protocol). The server runs Netatalk software which provides AFP.
Some of the files I backed up had a special group membership, xampp, on my Mac. On the AFP volume, their group membership is the default staff. So I tried to correct their group membership manually.
% sudo chown :xampp /Volumes/FreeNAS-vol/a/b/c/doc.mp4
chown: /Volumes/FreeNAS-vol/a/b/c/doc.mp4: Operation not permitted
However, group xampp exists on the FreeNAS server. When I connect to a shell on that server via ssh, I am able to perform this chown with no problem.
Why does the Mac say "Operation not permitted" when I chown a file on FreeNAS mounted via AFP? How can I make this chown operation succeed?
My underlying goal is to be able to back up all files on my Mac to the FreeNAS server via rsync. Rsync is able to copy the file data and some metadata, but it is unable to correct the group membership of the files on the server.
N.B. My FreeNAS version is FreeNAS-11.3-U4.1, which dates from 2020. Current versions are now named TrueNAS Core. I will upgrade my server to the latest version someday. I am skeptical that upgrading will solve this problem, but we shall see.
macOS version is 10.14.6 Mojave.
Update: the file system for FreeNAS is ZFS. Thus the files for which I am trying to change group membership are stored as ZFS, and delivered over AFP via Netatalk, to a client on a macOS shell, where I am using the chown command. (To answer a comment on this question.)
