nbd-server binary package in Ubuntu Precise amd64

 Network Block Device (NBD) is a client/server protocol that
 emulates a block device (such as a hard disk, a floppy, or a CD-ROM)
 over the network, thus giving the system the ability to swap over the
 network, or to use raw network disk space for other purposes.
 .
 However, writing to one Network Block Device from different clients
 simultaneously is not recommended, and would probably result in data
 loss. If you want multiple clients to share a remote resource, use a
 network file system such as NFS or Coda.
 .
 This package provides the server binary for NBD.

Source package