SystemRescueCd 1.4.0 introduces two new options that allow you to boot from NFS v3 or NBD. Basically, if the computer on which you work had no CDRom drive, or if you just want to boot SystemRescueCd from the network for any other reason, you can install network services somewhere else on your network (dhcp server, tftp server, http/nfs/nbd server) and you will be able to boot SystemRescueCd from the network.
Old SystemRescueCd versions were already able to boot from the network. But the third stage of the boot process (after dhcp and tftp) was based either on http or tftp. These protocols are used to transfer the compressed root filesystem image (sysrcd.dat) to the memory during the boot process. After this file is copied it’s then running entirely from the memory and you can disconnect the network if you want.
The problem with that is that it may take some time to transfer the whole root filesystem image through the network, and you cannot use it on computers which have less than ~ 400 MB of memory since there would not be enough memory to store the root filesystem image and for the programs.
With NFS and NBD the root filesystem is mounted through the network. It means the root filesystem is not stored locally, and then data are transmitted through the network only when necessary. It allows computers with only 256 MB of memory to boot SystemRescueCd from the network. If you want to boot SystemRescueCd from either NFS or NBD you still need DHCP and TFTP in the first steps of the boot process. NFS or NBD only replaces the third step of the process which makes the root filesystem accessible after the kernel has started.
Even if NFS is more popular that NBD, the latter one must be easier to configure. NBD only requires a single TCP port to work so it’s less hassle than NFS to administrate. NBD is a service that provides a block device on the client side, and which sends a copy of sysrcd.dat on the server side.