nbd-bind

Description

nbd-bind is a convenient wrapper for using QEMU's qemu-nbd to access disk images.

qemu-nbd, among other things, allows you to bind a disk image to a block device, with a name of the form /dev/nbd[0-9]+. However, there are two problems with this: only root can run qemu-nbd or access the resulting block devices, and you can't know the name of the block device in advance (/dev/nbd0 may already be in use, and unavailable for binding a disk image to). nbd-bind solves these two problems by:

• Running as suid root, and checking the calling user has read/write permissions on the disk image and target
• Binding to any available nbd device, and symlinking from the target name to the block devices
• Giving the calling user ownership of the nbd device while their disk image is attached

Examples

Binding a disk image called disk-image.vdi, which has two partitions on it, to block:

$ nbd-bind disk-image.vdi block
$ ls
block  block-p1  block-p2  disk-image.vdi
$ nbd-unbind block
$ ls
disk-image.vdi

nbd-bind can be used in conjunction with an appropriate entry in fstab to have a Makefile which installs to a disk image without running with sudo.

fstab entry:

/home/<username>/<path>/block-p1 /home/<username>/<path>/mnt auto user,noauto 0 0

install target in /home/<username>/<path>/Makefile:

install: all
        nbd-bind disk-image.vdi block
        mount mnt
        <commands to install to mnt>
        umount mnt
        nbd-unbind block

Installation

If your copy of QEMU isn't located in /usr/bin, or you want to install nbd-bind somewhere other than /usr/local/bin, then edit the QEMUNBD and PREFIX variables in the Makefile. Then run:

make; sudo make install