How to forward a port from guest to host in qemu (kvm)

Minimal working setup

I can access both the host and external network with this QEMU command:

qemu-system-x86_64 \
  -append 'root=/dev/vda console=ttyS0' \
  -device rtl8139,netdev=net0 \
  -drive file=./out/buildroot/build/default/x86_64/images/rootfs.ext2.qcow2,format=qcow2,if=virtio,snapshot \
  -enable-kvm \
  -kernel ./out/linux/default/x86_64/arch/x86/boot/bzImage \
  -machine pc \
  -netdev user,id=net0 \
  -nographic \
  -serial mon:stdio \
;

Then in the host, setup a file server on port 8000:

python -m SimpleHTTPServer 8000

nc -kl 0.0.0.0 8000 also works, but on one of my host systems it only worked with 0.0.0.0 but not localhost.

Now in the guest:

# Done by init in most non-minimal systems.
ifup -a
ip route

This gives:

default via 10.0.2.2 dev eth0
10.0.2.0/24 dev eth0 scope link  src 10.0.2.15

which tells us that the port we need is 10.0.2.2:

cd /tmp
wget 10.0.2.2:8000
cat index.html

Tested with the following images:

• minimal QEMU + Buildroot setup
• Ubuntu amd64 cloud image: https://askubuntu.com/questions/281763/is-there-any-prebuilt-qemu-ubuntu-image32bit-online/1081171#1081171

Tested on an Ubuntu 18.10 host, QEMU 2.12.