Add a tutorial on CNI

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+title = "Using CNI"
+weight = 1
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+
+This tutorial will show you how to set up networking for a gVisor sandbox using
+the [Container Networking Interface (CNI)](https://github.com/containernetworking/cni).
+
+## Install CNI Plugins
+
+First you will need to install the CNI plugins. CNI plugins are used to set up
+a network namespace that `runsc` can use with the sandbox.
+
+Start by creating the directories for CNI plugin binaries:
+
+```
+sudo mkdir -p /opt/cni/bin
+```
+
+Download the CNI plugins:
+
+```
+wget https://github.com/containernetworking/plugins/releases/download/v0.8.3/cni-plugins-linux-amd64-v0.8.3.tgz
+```
+
+Next, unpack the plugins into the CNI binary directory:
+
+```
+sudo tar -xvf cni-plugins-linux-amd64-v0.8.3.tgz -C /opt/cni/bin/
+```
+
+## Configure CNI Plugins
+
+This section will show you how to configure CNI plugins. This tutorial will use
+the "bridge" and "loopback" plugins which will create the necessary bridge and
+loopback devices in our network namespace. However, you should be able to use
+any CNI compatible plugin to set up networking for gVisor sandboxes.
+
+The bridge plugin configuration specifies the IP address subnet range for IP
+addresses that will be assigned to sandboxes as well as the network routing
+configuration. This tutorial will assign IP addresses from the `10.22.0.0/16`
+range and allow all outbound traffic, however you can modify this configuration
+to suit your use case.
+
+Create the bridge and loopback plugin configurations:
+
+```
+sudo mkdir -p /etc/cni/net.d
+
+sudo sh -c 'cat > /etc/cni/net.d/10-bridge.conf << EOF
+{
+  "cniVersion": "0.4.0",
+  "name": "mynet",
+  "type": "bridge",
+  "bridge": "cni0",
+  "isGateway": true,
+  "ipMasq": true,
+  "ipam": {
+    "type": "host-local",
+    "subnet": "10.22.0.0/16",
+    "routes": [
+      { "dst": "0.0.0.0/0" }
+    ]
+  }
+}
+EOF'
+
+sudo sh -c 'cat > /etc/cni/net.d/99-loopback.conf << EOF
+{
+  "cniVersion": "0.4.0",
+  "name": "lo",
+  "type": "loopback"
+}
+EOF'
+```
+
+## Create a Network Namespace
+
+For each gVisor sandbox you will create a network namespace and configure it
+using CNI. First, create a random network namespace name and then create
+the namespace.
+
+The network namespace path will then be `/var/run/netns/${CNI_CONTAINERID}`.
+
+```
+export CNI_PATH=/opt/cni/bin
+export CNI_CONTAINERID=$(printf '%x%x%x%x' $RANDOM $RANDOM $RANDOM $RANDOM)
+export CNI_COMMAND=ADD
+export CNI_NETNS=/var/run/netns/${CNI_CONTAINERID}
+
+sudo ip netns add ${CNI_CONTAINERID}
+```
+
+Next, run the bridge and loopback plugins to apply the configuration that was
+created earlier to the namespace. Each plugin outputs some JSON indicating the
+results of executing hte plugin. For example, The bridge plugin's response
+includes the IP address assigned to the ethernet device created in the network
+namespace. Take note of the IP address for use later.
+
+```
+export CNI_IFNAME="eth0"
+sudo -E /opt/cni/bin/bridge < /etc/cni/net.d/10-bridge.conf
+export CNI_IFNAME="lo"
+sudo -E /opt/cni/bin/loopback < /etc/cni/net.d/99-loopback.conf
+```
+
+Get the IP address assigned to our sandbox:
+
+```
+POD_IP=$(sudo ip netns exec ${CNI_CONTAINERID} ip -4 addr show eth0 | grep -oP '(?<=inet\s)\d+(\.\d+){3}')
+```
+
+## Create the OCI Bundle
+
+Now that our network namespace is created and configured, we can create the OCI
+bundle for our container. As part of the bundle's `config.json` we will specify
+that the container use the network namespace that we created.
+
+The container will run a simple python webserver that we will be able to
+connect to via the IP address assigned to it via the bridge CNI plugin.
+
+Create the bundle and root filesystem directories:
+
+```
+sudo mkdir -p bundle
+cd bundle
+sudo mkdir rootfs
+sudo docker export $(docker create python) | sudo tar --same-owner -pxf - -C rootfs
+sudo mkdir -p rootfs/var/www/html
+sudo sh -c 'echo "Hello World!" > rootfs/var/www/html/index.html'
+```
+
+Next create the `config.json` specifying the network namespace.
+```   
+sudo /usr/local/bin/runsc spec
+sudo sed -i 's;"sh";"python", "-m", "http.server";' config.json
+sudo sed -i "s;\"cwd\": \"/\";\"cwd\": \"/var/www/html\";" config.json
+sudo sed -i "s;\"type\": \"network\";\"type\": \"network\",\n\t\t\t\t\"path\": \"/var/run/netns/${CNI_CONTAINERID}\";" config.json
+```
+
+## Run the Container
+
+Now we can run and connect to the webserver. Run the container in gVisor. Use
+the same ID used for the network namespace to be consistent:
+
+```
+sudo runsc run -detach ${CNI_CONTAINERID}
+```
+
+Connect to the server via the sandbox's IP address:
+
+```
+curl http://${POD_IP}:8000/
+```
+
+You should see the server returning `Hello World!`.
+
+## Cleanup
+
+After you are finished running the container, you can clean up the network
+namespace .
+
+```
+sudo runsc kill ${CNI_CONTAINERID}
+sudo runsc delete ${CNI_CONTAINERID}
+
+export CNI_COMMAND=DEL
+
+export CNI_IFNAME="lo"
+sudo -E /opt/cni/bin/loopback < /etc/cni/net.d/99-loopback.conf
+export CNI_IFNAME="eth0"
+sudo -E /opt/cni/bin/bridge < /etc/cni/net.d/10-bridge.conf
+
+sudo ip netns delete ${CNI_CONTAINERID}
+```