Multiple VMs Might Break Network Connections

[iwinux]

OS / App Versions

• macOS 13.5 (x86-64)
• QEMU 7.2.1
• socket_vmnet 1.1.2 built from source

Problem

The network of currently started QEMU VM stops working (symptom: ping 1.1.1.1 times out) when another VM (with unique MAC address) boots up but DOES NOT initiate any DHCP request.

However, after the second VM finishes its DHCP request, the network on first VM resumes to normal.

Steps to Reproduce

1. start socket_vmnet

sudo env DEBUG=true /usr/local/bin/socket_vmnet \
	--socket-group=admin \
	--vmnet-mode=shared \
	--vmnet-gateway=10.233.2.1 \
	--vmnet-dhcp-end=10.233.2.250 \
	--vmnet-mask=255.255.255.0 \
	/run/vmnet.sock

2. start VM `vmnet-test-01`

# Alpine Live ISOs available at https://www.alpinelinux.org/downloads/
# The "Virtual" ones are smallest

/usr/local/bin/socket_vmnet_client \
    /run/vmnet.sock \
    qemu-system-x86_64 \
        -name "vmnet-test-01" \
        -machine type=q35,accel=hvf \
        -cpu host \
        -smp cpus=1,sockets=1,cores=1,threads=1 \
        -m 1024 \
        -cdrom "$HOME/Downloads/alpine-virt-3.18.2-x86_64.iso" \
        -netdev socket,id=net0,fd=3 \
        -device "virtio-net-pci,netdev=net0,mac=de:ad:be:ef:00:01" \
        -object rng-random,filename=/dev/urandom,id=rng0 \
        -device virtio-rng-pci,rng=rng0 \
        -device virtio-gpu-pci \
        -nographic

3. acquire IP via DHCP for vmnet-test-01

# login with user "root"

cat > /etc/network/interfaces <<-END
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
END

rc-service networking start

# it pings successfully
ping -c 5 -w 1 1.1.1.1

4. start VM `vmnet-test-02`

/usr/local/bin/socket_vmnet_client \
    /run/vmnet.sock \
    qemu-system-x86_64 \
        -name "vmnet-test-02" \
        -machine type=q35,accel=hvf \
        -cpu host \
        -smp cpus=1,sockets=1,cores=1,threads=1 \
        -m 1024 \
        -cdrom "$HOME/Downloads/alpine-virt-3.18.2-x86_64.iso" \
        -netdev socket,id=net0,fd=3 \
        -device "virtio-net-pci,netdev=net0,mac=de:ad:be:ef:00:02" \
        -object rng-random,filename=/dev/urandom,id=rng0 \
        -device virtio-rng-pci,rng=rng0 \
        -device virtio-gpu-pci \
        -nographic

5. after 1-2 minutes, network on vmnet-test-01 stops working

# timeout
ping -c 5 -w 1 1.1.1.1

# DHCP cannot renew lease
rc-service networking restart

6. repeat step 3 for vmnet-test-02

# login with user "root"

cat > /etc/network/interfaces <<-END
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
END

rc-service networking start

# it pings successfully
ping -c 5 -w 1 1.1.1.1

7. surprisingly, now networks on both VMs resume to normal

Debug Log

https://gist.github.com/iwinux/305a9bea6b77c5c44494bb1883354f72