Re: qemu-nbd acting as a server or not ?

[Connor Kuehl]

On Mon Apr 25, 2022 at 5:09 AM CDT, Bob Power wrote:
> Hi,
> The man page for qemu-nbd reads:  qemu−nbd − QEMU Disk Network Block Device 
> Server
> I'm accessing a qcow2 image on a host using: qemu-nbd -c /dev/nbd0  test.qcow2
>
> It appears that when used this way qemu-nbd does not operate as a server ...
>
> While the fs on the image is mounted and in use :
>
> ]# nmap -p 10809 localhost
> 10809/tcp closed nbd
>
> ]# systemctl list-units --type=service | grep nbd
> ]#
> So is it the case that when used in this manner it does not and is not 
> intended to act as a server ?

The manual also says this about the -c switch you're using:

        Expose the guest-visible contents of a qcow2 file via a block device
        /dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for partitions
        found within), then disconnect the device when done. Access to bind
        qemu-nbd to a /dev/nbd device generally requires root privileges, and
        may also require the execution of modprobe nbd to enable the kernel NBD
        client module.  CAUTION: Do not use this method to mount filesystems
        from an untrusted guest image - a malicious guest may have prepared the
        image to attempt to trigger kernel bugs in partition probing or file
        system mounting.

        qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
        qemu-nbd -d /dev/nbd0

Are you able to interact with the image with /dev/nbd0*?

If so, I wonder if what's happening here is that your host kernel is the
client and the qemu-nbd server process is just exiting when its client
hangs up. That's qemu-nbd's default behavior, unless you also supply the
-p or --persistent switch.

I'm just guessing here. I haven't used qemu-nbd with the -c switch
before.

--
Connor