CNI for LXC

It’s now possible to use CNI (container networking interface) with lxc. Here is an example. This requires some recent upstream patches, so for simplicity let’s use the lxc packages for zesty in ppa:serge-hallyn/atom. Setup a zesty host with that ppa, i.e.

sudo add-apt-repository ppa:serge-hallyn/atom
sudo add-apt-repository ppa:projectatomic/ppa
sudo apt update
sudo apt -y install lxc1 skopeo skopeo-containers jq

(To run the oci template below, you’ll also need to install git://github.com/openSUSE/umoci. Alternatively, you can use any standard container, the oci template is not strictly needed, just a nice point to make)

Next setup CNI configuration, i.e.

cat >> EOF | sudo tee /etc/lxc/simplebridge.cni
{
  "cniVersion": "0.3.1",
  "name": "simplenet",
  "type": "bridge",
  "bridge": "cnibr0",
  "isDefaultGateway": true,
  "forceAddress": false,
  "ipMasq": true,
  "hairpinMode": true,
  "ipam": {
    "type": "host-local",
    "subnet": "10.10.0.0/16"
  }
}
EOF

The way lxc will use CNI is to call out to it using a start-host hook, that is, a program (hook) which is called in the host namespaces right before the container starts. We create the hook using:

cat >> EOF | sudo tee /usr/share/lxc/hooks/cni
#!/bin/sh

CNIPATH=/usr/share/cni

CNI_COMMAND=ADD CNI_CONTAINERID=${LXC_NAME} CNI_NETNS=/proc/${LXC_PID}/ns/net CNI_IFNAME=eth0 CNI_PATH=${CNIPATH} ${CNIPATH}/bridge < /etc/lxc/simplebridge.cni
EOF

This tells the ‘bridge’ CNI program our container name and the network namespace in which the container is running, and sends it the contents of the configuration file which we wrote above.

Now create a container,

sudo lxc-create -t oci -n a1 -- -u docker://alpine

We need to edit the container configuration file, telling it to use our new hook,

sudo sed -i '/^lxc.net/d' /var/lib/lxc/a1/config
cat >> EOF | sudo tee -a /var/lib/lxc/a1/config
lxc.net.0.type = empty
lxc.hook.start-host = /usr/share/lxc/hooks/cni
EOF

Now we’re ready! Just start the container with

lxc-execute -n a1

and you’ll get a shell in the alpine container with networking configured.

Disclaimer

The opinions expressed in this blog are my own views and not those of Cisco.