dpkg/dselect/methods/disk/desc.nfs

Installation across the network, from an NFS server which not
currently mounted.

The server should have contain the Packages.gz file from each
distribution area being installed (usually stable, contrib and
non-free) as well as the corresponding binary/*/*.deb files.

The easiest way for them to get this is to make a (partial) copy of
the distribution site's directory hierarchy, if possible.