In this post we will show how to share a network between two QEMU guests using a bridge.
There are many possible uses of this kind of a setup. One of them is for integration testing of for example, target and initiator code using one guest for the initiator and another for the target.
This post creates a bridge on the host, which the guests both share.
Create Bridge for Shared Network
We will first create the bridge and give it a “local” address, since for now we are not planning on exporting this network off the host. You will also notice we add an IP address for the host on this network 192.168.0.1
sudo ip link add br0 type bridge sudo ip addr add 192.168.0.1/24 dev br0 sudo ip link set br0 up
Now we can check that the bridge exists and is ready (state is UP).
$ ip addr 6: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000 link/ether fe:06:bb:4c:37:a1 brd ff:ff:ff:ff:ff:ff inet 192.168.0.1/24 scope global br0 valid_lft forever preferred_lft forever inet6 fe80::8c39:6fff:fe23:ca06/64 scope link valid_lft forever preferred_lft forever
Next we will tell QEMU about this bridge by adding it to the QEMU bridge configuration file.
$ echo 'allow br0' | sudo tee -a /etc/qemu/bridge.conf
Starting the guests
When we bring up the QEMU guests, we will provide the -netdev option to specify a bridge that our guests will use for their network. Below is an example of these network options.
-netdev bridge,id=hn1 -device virtio-net,netdev=hn1,mac=e6:c8:ff:09:76:99
Here are the full set of options to bring up our aarch64 guests.
Note that we specify a different MAC address for each guest.
guest A: $ sudo qemu-system-aarch64 -nographic -machine virt,gic-version=max -m 8G -cpu max \ -drive file=./ubuntu20-a.img,if=none,id=drive0,cache=writeback \ -device virtio-blk,drive=drive0,bootindex=0 \ -drive file=./flash0-a.img,format=raw,if=pflash \ -drive file=./flash1-a.img,format=raw,if=pflash \ -smp 4 -accel kvm -netdev bridge,id=hn1 \ -device virtio-net,netdev=hn1,mac=e6:c8:ff:09:76:99 guest B: $ sudo qemu-system-aarch64 -nographic -machine virt,gic-version=max -m 8G -cpu max \ -drive file=./ubuntu20-b.img,if=none,id=drive0,cache=writeback \ -device virtio-blk,drive=drive0,bootindex=0 \ -drive file=./flash0-b.img,format=raw,if=pflash \ -drive file=./flash1-b.img,format=raw,if=pflash \ -smp 4 -accel kvm -netdev bridge,id=hn1 \ -device virtio-net,netdev=hn1,mac=e6:c8:ff:09:76:9c
Once the guests are up, you can configure the IP addresses quickly for both guests via the below commands.
Note that we chose IP addresses of 192.168.0.8 for guest a and .16 for guest b.
$ ip addr 2: enp0s3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether e6:c8:ff:09:76:99 brd ff:ff:ff:ff:ff:ff $ sudo ip addr add 192.168.0.8/24 dev enp0s3 $ sudo ip link set enp0s3 up
Testing the Shared Network
To test that the guest’s network is up, check ip addr again. It should show “state UP”.
$ ip addr 2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 link/ether e6:c8:ff:09:76:99 brd ff:ff:ff:ff:ff:ff inet 192.168.0.8/24 scope global enp0s3 valid_lft forever preferred_lft forever inet6 fe80::e4c8:ffff:fe09:7699/64 scope link valid_lft forever preferred_lft forever
Then we can try pinging the host.
$ ping 192.168.0.1 PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data. 64 bytes from 192.168.0.1: icmp_seq=1 ttl=64 time=0.193 ms
It works ! Now let’s try pinging the other guest.
$ ping 192.168.0.16 PING 192.168.0.16 (192.168.0.16) 56(84) bytes of data. 64 bytes from 192.168.0.16: icmp_seq=1 ttl=64 time=0.358 ms
Also worked ! The guests can now see each other and are sharing the same network.
Access to Network Beyond Host
Suppose that we wanted to get access to the external network also. This can be added by simply adding a device to the bridge. In our case the device is: enahisic2i0, and we add it to bridge br0
sudo ip link set enahisic2i0 master br0
After that, we just add public IP addresses on our bridge. You might need to remove it from the device before adding it to the bridge.
sudo ip addr del 18.104.22.168/24 dev enahisic2i0 sudo ip addr add 22.214.171.124/24 dev br0
Finally, inside the guests, give them public IP addreses as well:
sudo ip addr add 126.96.36.199/24 dev enp0s3
Note that to access beyond your local subnet, you might need to add a default route:
sudo ip route add default via 188.8.131.52 dev enops