Sunday, July 12, 2015

QEMU/KVM Bridged Network with TAP interfaces

In my previous post, Rapid Linux Kernel Dev/Test with QEMU, KVM and Dracut, I described how build and boot a Linux kernel quickly, making use of port forwarding between hypervisor and guest VM for virtual network traffic.

This post describes how to plumb the Linux VM directly into a hypervisor network, through the use of a bridge.

Start by creating a bridge on the hypervisor system:
> sudo /sbin/brctl addbr br0

Clear the IP address on the network interface that you'll be bridging (e.g. eth0).
Note: This will disable network traffic on eth0!
> sudo ip addr flush dev eth0
Add the interface to the bridge:
> sudo /sbin/brctl addif br0 eth0


Next up, create a TAP interface:
> sudo /sbin/tunctl -u $(whoami)
Set 'tap0' persistent and owned by uid 1001
The -u parameter ensures that the current user will be able to connect to the TAP interface.

Add the TAP interface to the bridge:
> sudo /sbin/brctl addif br0 tap0

Make sure everything is up:
> sudo ip link set dev br0 up
> sudo ip link set dev tap0 up

The TAP interface is now ready for use. Assuming that a DHCP server is available on the bridged network, the VM can now obtain an IP address during boot via:
> qemu-kvm -kernel arch/x86/boot/bzImage \
           -initrd initramfs \
           -device e1000,netdev=network0,mac=52:55:00:d1:55:01 \
           -netdev tap,id=network0,ifname=tap0,script=no,downscript=no \
           -append "ip=dhcp rd.shell=1 console=ttyS0" -nographic

The MAC address is explicitly specified, so care should be taken to ensure its uniqueness.

The DHCP server response details are printed alongside network interface configuration. E.g.
[    3.792570] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[    3.796085] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[    3.812083] Sending DHCP requests ., OK
[    4.824174] IP-Config: Got DHCP answer from 10.155.0.42, my address is 10.155.0.1
[    4.825119] IP-Config: Complete:
[    4.825476]      device=eth0, hwaddr=52:55:00:d1:55:01, ipaddr=10.155.0.1, mask=255.255.0.0, gw=10.155.0.254
[    4.826546]      host=rocksolid-sles, domain=suse.de, nis-domain=suse.de
...

Didn't get an IP address? There are a few things to check:
  • Confirm that the kernel is built with boot-time DHCP client (CONFIG_IP_PNP_DHCP=y) and E1000 network driver (CONFIG_E1000=y) support.
  • Check the -device and -netdev arguments specify a valid e1000 TAP interface.
  • Ensure that ip=dhcp is provided as a kernel boot parameter, and that the DHCP server is up and running.
Happy hacking!

5 comments:

  1. I was looking for solid, simple examples for how to bridge a QEMU guest to my host without using SLIRP. This is the most distro agnostic and to the point example I've found so far, thank you for putting this together!

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  2. Glad to hear it was helpful. Thanks for the feedback Kenneth!

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  3. Thank you for simple instructions you put together. It has been great help to us.

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  4. Great tutorial, will be using this to create VM to VM taps for SDN services.

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  5. Thank you very .. it was very helpful for me

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