[Nbd] [RFC] NBD protocol improvement

[Laurent Vivier <Laurent.Vivier@...154...>]

Hi,

I use the NBD protocol to connect a virtual machine to a disk image.

I'd like to allow the client to specify the file to use.

Here is what I propose, all your comments are welcome:

* NBD server side:

  - put all the disk images in a directory, for instance:

    # ls /ISO
    ubuntu-6.06-desktop-amd64.iso
    ubuntu-7.10-desktop-amd64.iso
    ubuntu-8.04-desktop-i386.iso

  - run the NBD server with a new parameter to specify the directory
    instead of the filename

    # nbd-server 1024 --read-only --directory /ISO

* NBD client side

  nbd-client isoserver 1024 /dev/nbd0 ubuntu-8.04-desktop-i386.iso

* protocol changes:

- when used with "--directory", nbd-server sends a disk size of 0 in the
negotiate operation.
- when nbd-client receive a disk size of zero, it sends a filename
(first bytes is the size of the filename, following bytes are the
filename).
- nbd-server receives the filename, opens the file (to avoid security
issues, we use "basename(filename)"), and re-sends a negotiate packet
with the good size.
- next is as ususal.

Regards,
Laurent

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