This is just a short post logging something I newly learned. It turned out that when launching VMs with QEMU, if the official cloud server images (available at this link) by Ubuntu is used, the networking will be missing by default (seemed to be related to some cloud-init issues).

To fix it, I referred to this page, the simple takeaway here is that adding a home-made seed.img file can fix this issue.

For example, the original command I used to launch a Ubuntu VM using a x86 cloud server disk image was:

qemu-system-x86_64 \
  -machine q35,accel=kvm \
  -cpu host \
  -smp 4 -m 4000 \
  -nographic \
  -kernel linux-6.11.7/arch/x86/boot/bzImage \
  --append "console=ttyS0 root=/dev/vda1" \
  -netdev user,id=hostnet0,hostfwd=tcp::2222-:22 \
  -device virtio-net-pci,netdev=hostnet0,mac=52:54:00:12:34:56 \
  -drive if=none,file=noble-server-cloudimg-amd64.img,id=hd0 \
  -device virtio-blk-pci,drive=hd0

However, to get networking, I need to first install cloud-image-utils with

sudo apt install cloud-image-utils

Then create a file named user-data:

#cloud-config
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true

And a file named meta-data:

instance-id: iid-001

Then create the seed.img:

cloud-localds seed.img user-data meta-data

Add this seed.img into the QEMU command, and network is configured correctly.

qemu-system-x86_64 \
  -machine q35,accel=kvm \
  -cpu host \
  -smp 4 -m 4000 \
  -nographic \
  -kernel linux-6.11.7/arch/x86/boot/bzImage \
  --append "console=ttyS0 root=/dev/vda1" \
  -netdev user,id=hostnet0,hostfwd=tcp::2222-:22 \
  -device virtio-net-pci,netdev=hostnet0,mac=52:54:00:12:34:56 \
  -drive if=none,file=noble-server-cloudimg-amd64.img,id=hd0 \
  -drive if=virtio,format=raw,file=seed.img \
  -device virtio-blk-pci,drive=hd0

Meanwhile, when I tested cloud images for ARM64, it turned out that this extra step is not necessary, the cloud-init feature came out of the box.

If you happen to catch what mistake I made here in the QEMU commands, please let me know!