You are trying to mount and connect to an image using qemu-nbd:

qemu-nbd -c /dev/nbd0 rtt.qcow2

qemu-nbd: Failed to set NBD socket
qemu-nbd: Disconnect client, due to: Failed to read request: Unexpected end-of-file before all bytes were read

Another related error:

qemu-nbd: Failed to unlink socket /var/lock/qemu-nbd-nbd0: Operation not permitted
Error with nbd, exiting

The above can happen if you don't have permission to access the nbd socket.

qemu-nbd is not that smart, this is caused by /dev/nbd0 being connected

This can happen due to a failed mount or perhaps you just forgot to disconnect the previous time.

We'll solve it by disconnecting the nbd0 device with the -d flag (for disconnect).

qemu-nbd -d /dev/nbd0
/dev/nbd0 disconnected

Now can see that it works without error
 

qemu-nbd -c /dev/nbd0 1234.img
 

fdisk -l /dev/nbd0



Disk /dev/nbd0: 15 GiB, 16106127360 bytes, 31457280 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Simple Way To Test if /dev/nbd0 is in use

Also remember by default you probably have up to /dev/nbd15.

You can use a tool like fdisk to check if the nbd is in use:

fdisk -l /dev/nbd0
fdisk: cannot open /dev/nbd0: Inappropriate ioctl for device

If you get an error like "fdisk: cannot open /dev/nbd0: Inappropriate ioctl for device", it is NOT in use.

If it's in use you should be able to see the disk info:

fdisk -l /dev/nbd0
Disk /dev/nbd0: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


qemu, nbd, socket, disconnect, bytes, readqemu,

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