I wrote some posts about how kubernetes is interacting with docker at CRI level, but what about networking ? Docker has developed it's own solution called CNM to maintain network, to interact with it just use commands under docker network. How it looks like in k8s multi node environment:

# docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
ca3e40762c13        bridge              bridge              local
ae38ae125258        host                host                local
d22e6da563ab        none                null                local

what's stand under network id d22e6da563ab ?

# docker network inspect d22e6da563ab
        "Containers": {
            "03a8117bf896bbd93340382fc71093686f25def0739431aa7c6a4ca8bb0a6102": {

it seems that most of the containers sit here. So why it's using driver null it's because k8s didn't decide to use docker CNM in favour of CNI. Null driver only provides loopback device. In this blog post I won't describe CNI, but I will try to achieve more or less the same by connecting selected container into bridged network using system tools. Create linux network bridge and bind ip address:

# brctl addbr brtest
# ip link set brtest up
# ip addr add dev brtest

create sample container with none network:

# docker run -d --name cni --net=none busybox:latest sleep 86400
# docker exec cni sh -c "ip a"
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet scope host lo
       valid_lft forever preferred_lft forever

docker create linux network namespace with new container, to get list of network namespaces, just provide correct path for iproute2 commands:

# ln -s /var/run/docker/netns/ /var/run/netns
# ip netns list

now we knew which namespace is used, using that name we can create pair of interfaces veth one for host and one for container:

# ip link add v11 type veth peer name v12 netns 5c35ec636755
# ip a show dev v11
5: v11@if2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 0a:fe:9d:1d:6a:2f brd ff:ff:ff:ff:ff:ff link-netnsid 0
# ip netns exec 5c35ec636755 ip a show dev v12
2: v12@if5: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether be:aa:c5:68:b6:3c brd ff:ff:ff:ff:ff:ff link-netnsid 0
# ip link set v11 up

add ip address for interface in container:

# ip netns exec 5c35ec636755 ip link set v12 up
# ip netns exec 5c35ec636755 ip addr add dev v12

now connect host endpoint of veth pair into bridge brtest:

# brctl addif brtest v11

make some tests:

# docker exec cni sh -c "ping -c 3"
PING ( 56 data bytes
64 bytes from seq=0 ttl=64 time=0.061 ms
64 bytes from seq=1 ttl=64 time=0.107 ms
64 bytes from seq=2 ttl=64 time=0.113 ms

--- ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.061/0.093/0.113 ms

communication is established between container and host. It's pretty close to what do CNI bridge plugin, I will take a closer look in next blog post.