How to get QEMU to use a named Docker interface?

Question

Question

Is it possible to get QEMU to use a named Docker interface and network on the same host machine?

Motivating exmaple:

Let's say I have a DHCP server in a Docker container with static IP running as follows:

pxe_1    | Interface: eth0
pxe_1    | IP: 172.16.100.11
pxe_1    | Subnet: 172.16.100
pxe_1    | Starting PXE server...
...

The Docker (compose) DHCP server is in its own network on the same host with a named interface pxe0:

networks:
  pxenet:
    driver: bridge
    ipam:
      driver: default
      config:
        - subnet: 172.16.100.0/24
          gateway: 172.16.100.1
    driver_opts:
      com.docker.network.bridge.name: pxe0

Say I want to run a bootloader in QEMU and have the DHCP broadcasts reach the above Docker container. This is my command:

qemu-system-aarch64 \
-serial stdio \
-machine virt,gic-version=3 \
-cpu cortex-a53 \
-m 256M \
-smp 4 \
-bios u-boot.bin

The bootloader startup then looks like this:

U-Boot 2021.01-rc1-g896cc5aa (Nov 06 2020 - 23:33:35 -0800)
...    
BOOTP broadcast 1
DHCP client bound to address 10.0.2.15 (2 ms)
Using virtio-net#32 device
TFTP from server 10.0.2.2; our IP address is 10.0.2.15
...

The packets from QEMU VM are not reaching the Docker container on the same host.

What network/nic/net QEMU settings are needed for the QEMU VM to be on the same custom Docker network (not using a bridge on the host)?

I'm thinking along the lines of something like this,

-netdev tap,id=pxe0,ifname=pxe0,script=no,downscript=no \
-device e1000,netdev=pxe0,mac=52:55:00:d1:55:01

but fails with

qemu-system-aarch64: could not configure /dev/net/tun (pxe0): Invalid argument

even though ip a reveals

477: pxe0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:8f:81:41:e3 brd ff:ff:ff:ff:ff:ff
    inet 172.16.100.1/24 brd 172.16.100.255 scope global pxe0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:8fff:fe81:41e3/64 scope link 
       valid_lft forever preferred_lft forever

score 1 · Accepted Answer · answered Nov 08 '20 at 16:22

What network/nic/net QEMU settings are needed for the QEMU VM to be on the same custom Docker network (not using a bridge on the host)?

After a lot of malarkey with reading in nuanced detail about how QEMU networking works, and Docker networking, QEMU can only use a tun/tap interface to bypass the internal VM gateway and internal VM DHCP service. Therefore, the only solution I can find is to:

Add a tap called 'pxe-tap'

# sudo modprobe tap
sudo ip tuntap add mode tap pxe-tap

Attach the tap to the named Docker interface pxe0 (which is a bridge - important) and raise it.

sudo ip link set pxe-tap master pxe0
sudo ip link set dev pxe-tap up

Communication from QEMU will then be on the named Docker network.

How to get QEMU to use a named Docker interface?

Question

Motivating exmaple:

1 Answers1