What does "mapping=identity" mean in /etc/exports?

[Greg Scott]

Hello -

I'm migrating an old NFS server to newer hardware.  /etc/exports in the old
environment looks like this:

/shares/IMSHCS01       
*(rw,insecure,sync,no_root_squash,all_squash,mapping=identity,anonuid=0,anongid=0

So the old system is exporting the directory, /shares/IMSHCS01 to anyone and everyone who
wants to connect, with a bunch of switches essentially making it wide open.

Wonderful.  

But on the new system, when I set up  /etc/exports the same way, I get this error: 

[root@nfs2013 etc]# exportfs -av
exportfs: /etc/exports:1: unknown keyword "mapping=identity"

[root@nfs2013 etc]#

Getting rid of "mapping=identity", my shared directory exports just fine.  

I can find no mention of "mapping=identity" in any man pages, but several google
references show this option in pasted-in copies of /etc/exports files.  My hunch is, this
is an obsolete option and no longer supported - but what did it do in case my new server
needs to behave the same way the old server behaved?

Thanks

- Greg Scott