NFS question

Don Holmgren djholm at
Fri Jul 13 08:45:59 PDT 2001

In the (old) user space nfs server, there was the following switch available to
nfsd (from the man page for nfs-server-2.2):

       -r or --re-export
              Allow remotely mounted file-systems to be exported.
              This can be used to turn a machine  into  a  multi-
              plier  for NFS or Novell servers. Caution should be
              used when re-exporting loopback NFS mounts  because
              re-entering the mount point will result in deadlock
              between the NFS client and the NFS server.

              I should be noted that (on Linux) nfsd looks at the
              major  device number of the file system to find out
              whether it is a remote volume; if the major  number
              is not 0, it assumes the file system is local. How-
              ever, not only remote file systems use major number
              0, also procfs does. If you choose to re-export NFS
              file systems, beware that this potentially includes
              /proc  if  you  have the file system root exported.

I've used this successfully when a mirror I operated ran out of space and I
needed to quickly borrow additional space w/out adding a disk (i.e., a 3rd
system, analogous to X, supplied extra space via NFS which the mirror, HEAD,
exported to the world).  But, this was a read-only export.  I'd be leary of
doing this for a less restricted export.

Don Holmgren

On Fri, 13 Jul 2001, Robert Sand wrote:

> Christoph Wasshuber wrote:
> > 
> > I have a fundamental NFS question. Assume
> > that we have three computers, X, HEAD, NODE1.
> > 
> >         X ----- HEAD ----- NODE1
> > 
> >       /vol      /mnt/v1    /mnt/v2
> > 
> > X     is some computer on a larger network
> > HEAD  is also member of this larger network and
> >       knows about X and NODE1
> > NODE1 is one of my beowulf nodes and does not
> >       know anything about the larger network.
> Not a chance.  Your reasoning sounds good but here is were the problem
> starts.  X is sharing the vol via nfs to any number of systems and HEAD
> is one of those systems.  HEAD is mounting the nfs filesystem on /mnt/v1
> therefore /mnt/v1 is an nfs filesystem.  The nfs server on head can not
> share a fiel system that does not belong to it and it shouldn't because
> of the implicit security and permission problems involved in it.  The
> nfs server on HEAD can only share filesystems that are local to it, be
> they HDD's, floppy's, CDROM's, or ZIP drives but never an NFS mounted
> filesystem.
> -- 
> Robert Sand.
> mailto:rsand at
> University of Minnesota, Duluth
> Information Technology Systems and Services
> 144 MWAH
> 218-726-6122        fax 218-726-7674
> "Walk behind me I may not lead, Walk in front of me I may not follow,
>  Walk beside me and we walk together"  UTE Tribal proverb.
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