NBD is a client and server allowing one machine to serve a block device over a TCP connection to another machine (currently, a Linux client). Think iSCSI, without all the complications.

The NBD server included in the nbd-2.7.1 package does not compile on non-Linux hosts, even though it doesn't need to do anything fundamentally unportable. I needed to access a block device on an OpenBSD machine, so I wrote a replacement NBD server.