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serat

Member

Registered: 2020-01-19

Posts: 30

qemu upgrade breaks bridge-networking

qemu 9.1.2-1 works, but upgrading to 9.2.0-1 breaks the networking.

I had set up to have a bridge (created by systemd) called qlan-bridge. I also have a dnsmasq listening on this interface and handing out IP addresses. I have also set up the forwarding net.ipv4.*.forwarding = 1

With qemu 9.1.2-1 everything works, and the vm receives the IP address from the dnsmasq dhcp server. With 9.2.0-1, the dnsmasq does receive the dhcp request and responds with the IP address (as I see from the dnsmasq logs), but this response does not seem to reach the vm since it never gets the IP address.

The qemu command used to start the vm is given below. Any ideas why networking breaks with the newer qemu version?

/usr/bin/qemu-system-x86_64  -nographic -device intel-hda -device hda-duplex \
-drive file=/usr/share/edk2-ovmf/x64/OVMF_CODE.4m.fd,if=pflash,format=raw,unit=0,readonly=on -drive file=/home/qemu-user/.config/qemu.d/debian-uefi-vars.fd,if=pflash,format=raw,unit=1 \
-vga std -spice port=10421,addr=127.0.0.1,disable-ticketing \
-nic bridge,br=qlan-bridge,model=virtio,mac=52:54:00:21:2E:EE -k en-us \
-enable-kvm -object iothread,id=io1 -device virtio-blk-pci,drive=disk0,iothread=io1 \
-drive if=none,id=disk0,cache=unsafe,format=raw,aio=threads,file=/dev/vg-nvme/qemu-debian \
-cpu host,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time -smp cores=2,threads=1,sockets=1 \
-fsdev local,security_model=passthrough,id=fsdev0,path=/home/qemu-user/Documents/qemu-share \
-device virtio-9p-pci,id=fs0,fsdev=fsdev0,mount_tag=hostshare -device virtio-balloon -m 2G

osr

Member

Registered: 2025-01-02

Posts: 2

Re: qemu upgrade breaks bridge-networking

I have the same issue with several home assistant instances. Downgrading qemu solves it.

serat

Member

Registered: 2020-01-19

Posts: 30

Re: qemu upgrade breaks bridge-networking

The problem is not resolved with the update (qemu  9.2.0-2).

I have narrowed it down to some problem with forwarding the dhcp response. Specifically, if I have a static IP address in the qemu guest, then the guest can access the network and forwarding etc set up from the host works. But if I have the guest use dhcp to configure its IP address, then the dhcp request reaches the dhcp server running on the host, and the dhcp server responds to it, but for some reason it gets dropped somewhere along the line and never seems to reach the guest.

Any ideas why this might be the case? Or is there any ideas on how I can track the dhcp response packet and see why it is being dropped/misrouted/whatever?

On the host, my systemd-network config files for the qlan-bridge are as follows.

qlan-bridge.network

[Match]
Name=qlan-bridge

[Network]
Address=172.21.1.1/24
IPv4Forwarding=yes
IPMasquerade=yes

qlan-bridge.netdev

[NetDev]
Name=qlan-bridge
Kind=bridge

Update:
I had dnsmasq running the dhcp server on the qlan-bridge, which was not working. But when I changed the qlan-bridge.network to run systemd's DHCP server, the guest can now obtain the IP address. So the problem seems to be some setting in the qlan-bridge which prevents the dhcp response from reaching the guest.

Is there some specific setting to ensure that the dhcp response, which I am guessing is some kind broadcast, passes through the interface? Or is this qemu problem I am seeing because of some systemd change that happened previously (because I am using systemd-networkd to set up the bridge)?

Last edited by serat (Yesterday 03:21:13)