Kubernetes Deployment From Scratch – Networking (Part 2)

12 February 2018 · 15 min read

Kubernetes Deployment From Scratch – Networking (Part 2)

In our last blog post on kubernetes from scratch, we created a pseudo cluster to show how Kubernetes works inside. Today we are going to add a second node and make sure the cluster utilizes it.

Creating second node

What about having more than one node? What if we would like to schedule pods on 2 nodes? It is as simple as running kubelet on another node and making sure it connects to our API Server.

First of all, we assume that we have the first node from our previous blog post, with 10.135.53.41 internal IP, 46.101.177.76 external IP, running API Server, etcd and nginx deployment.

Now let’s create a second one, as before, replace --ssh-keys with your SSH key. After that, copy kubelet config file from the master node.

$ doctl compute droplet create k8s-worker --region fra1 --size 2gb --image ubuntu-18-04-x64 --enable-private-networking --ssh-keys 79:29:54:77:13:2f:9c:b8:06:3e:8b:fe:8d:c0:d7:ba
ID          Name          Public IPv4      Private IPv4    Public IPv6    Memory    VCPUs    Disk    Region    Image                 Status    Tags
63460608    k8s-worker    46.101.98.124                                   2048      2        40      fra1      Ubuntu 18.04.3 x64    new
$ scp -3 root@46.101.177.76:/var/lib/kubelet/config.yaml root@46.101.98.124:~/
$ ssh root@46.101.98.124

In the next step, you have to install essential prerequisites - Docker and Kubernetes node binaries. Install Docker following instructions from the previous part, and then install Kubernetes with:

root@k8s-worker:~$ wget -q --show-progress https://dl.k8s.io/v1.17.3/kubernetes-node-linux-amd64.tar.gz
root@k8s-worker:~$ tar xzf kubernetes-node-linux-amd64.tar.gz
root@k8s-worker:~$ mv kubernetes/node/bin/* /usr/local/bin/
root@k8s-worker:~$ rm -rf kubernetes*

In order to connect the kubelet with the API server, you have to set the kubeconfig.

root@k8s-worker:~$ mkdir -p /etc/kubernetes /var/lib/kubelet
root@k8s-worker:~$ mv config.yaml /var/lib/kubelet
root@k8s-worker:~$ export MASTER_IP=10.135.53.41
root@k8s-worker:~$ kubectl config set-cluster kubernetes \
  --server=http://${MASTER_IP}:8080 \
  --kubeconfig=kubelet.conf
root@k8s-worker:~$ kubectl config set-context default \
  --cluster=kubernetes \
  --user=system:node:k8s-worker \
  --kubeconfig=kubelet.conf
root@k8s-worker:~$ kubectl config use-context default --kubeconfig=kubelet.conf
root@k8s-worker:~$ mv kubelet.conf /etc/kubernetes

Finally, you can run kubelet:

root@k8s-worker:~$ kubelet \
  --config=/var/lib/kubelet/config.yaml \
  --kubeconfig=/etc/kubernetes/kubelet.conf \
  &> /tmp/kubelet.log &

Now on k8s-master, we can check if the node has been recognized:

root@k8s-master:~$ kubectl get nodes
NAME         STATUS   ROLES    AGE   VERSION
k8s-master   Ready    <none>   81m   v1.17.3
k8s-worker   Ready    <none>   61m   v1.17.3

This is how our nodes look like now:

Next, let’s scale up our nginx deployment:

root@k8s-master:~$ kubectl scale deploy nginx --replicas=6
root@k8s-master:~$ kubectl get pods -o=wide
NAME                   READY     STATUS    RESTARTS   AGE       IP           NODE
nginx                  1/1       Running   0          1h        172.17.0.3   k8s-master
nginx-31893996-3dnx7   1/1       Running   0          1h        172.17.0.5   k8s-master
nginx-31893996-5d1ts   1/1       Running   0          1h        172.17.0.6   k8s-master
nginx-31893996-5xnhc   1/1       Running   0          17s       172.17.0.2   k8s-worker
nginx-31893996-9k93w   1/1       Running   0          1h        172.17.0.4   k8s-master
nginx-31893996-lfrzl   1/1       Running   0          17s       172.17.0.4   k8s-worker
nginx-31893996-q99cp   1/1       Running   0          17s       172.17.0.3   k8s-worker
nginx2                 1/1       Running   0          1h        172.17.0.2   k8s-master

We can see that they are scheduled on both nodes.

Wait, can you see that pods have duplicated IP addresses? This is because we don’t have a way to manage IP address for pods among all nodes, this also means that there is no communication between pods located on different nodes.
To fix that we have to introduce another cluster component – network fabric.

Kubernetes networking using Flannel

Kubernetes makes specific assumptions about networking in the cluster:

pods can communicate with each other by using unique pod’s IP address
nodes can communicate with pods using unique pod’s IP address
the IP that a container sees itself as is the same IP that others see it as

Kubernetes assumes that each pod and service in a cluster has a unique IP address and can communicate with other pods using their IP addresses. To achieve that we need a way to assign subnet of IP address for each node and ask Docker to use it when spawning containers, then we have to establish a non-NAT communication between these IP address. There is a lot of ways to do that, here we are going to focus on Flannel.

Flannel is one of the easiest ways to achieve these assumptions. Basically, Flannel runs as an agent on each node and is responsible for allocating a subnet for that node out of configured address space. That subnet is used by docker to obtain IP addresses for pods. Each subnet together with node’s IP address is stored in etcd and is readable by all agents. This allows flannel to obtain node location for given pod’s IP and forward traffic to that node.

This is how our networking will look like, it also shows how flannel works in the big picture.

Applying what we’ve just learned, now we can run flannel on our nodes. The tricky part is to configure docker to use flannel. This is what we are going to do:

create CNI configuration file and send it to both nodes with scp
insert initial flannel configuration to etcd using etcdctl
run flannel on both nodes pointing it to our etcd
rerun kubelet on both nodes with --network-plugin=cni option. That will use previously created config to allocate new pods to the subnet created by flannel

10-flannel.conflist:

{
  "name": "cbr0",
  "cniVersion": "0.3.1",
  "plugins": [
    {
      "type": "flannel",
      "delegate": {
        "hairpinMode": true,
        "isDefaultGateway": true
      }
    },
    {
      "type": "portmap",
      "capabilities": {
        "portMappings": true
      }
    }
  ]
}

$ scp 10-flannel.conflist root@46.101.177.76:~/
$ scp 10-flannel.conflist root@46.101.98.124:~/

root@k8s-master:~$ mkdir -p /opt/cni/bin
root@k8s-master:~$ curl -L "https://github.com/containernetworking/plugins/releases/download/0.8.2/cni-plugins-linux-amd64-0.8.2.tgz" | tar -C /opt/cni/bin -xz
root@k8s-master:~$ mkdir -p /etc/cni/net.d
root@k8s-master:~$ mv 10-flannel.conflist /etc/cni/net.d
root@k8s-master:~$ export MASTER_IP=10.135.53.41
root@k8s-master:~$ wget -q --show-progress https://github.com/coreos/flannel/releases/download/v0.11.0/flannel-v0.11.0-linux-amd64.tar.gz
flannel-v0.11.0-linux-amd64.tar.gz.1                      100%[==================================================================================================================================>]   4.51M  1.15MB/s    in 3.9s
root@k8s-master:~$ tar xzf flannel-v0.11.0-linux-amd64.tar.gz
root@k8s-master:~$ mv flanneld /usr/local/bin/
root@k8s-master:~$ ETCDCTL_API=2 etcdctl set /coreos.com/network/config '{"Network": "10.0.0.0/8", "SubnetLen": 20, "SubnetMin": "10.10.0.0","SubnetMax": "10.99.0.0","Backend": {"Type": "vxlan","VNI": 100,"Port": 8472}}'
root@k8s-master:~$ flanneld -iface=$MASTER_IP &> /tmp/flanneld.log &
root@k8s-master:~$ pkill kubelet
root@k8s-master:~$ kubelet \
  --config=/var/lib/kubelet/config.yaml \
  --kubeconfig=/etc/kubernetes/kubelet.conf \
  --network-plugin=cni \
  &> /tmp/kubelet.log &
root@k8s-master:~$ systemctl restart docker

Similarly on the worker node, except the etcdctl part:

root@k8s-worker:~$ mkdir -p /opt/cni/bin
root@k8s-worker:~$ curl -L "https://github.com/containernetworking/plugins/releases/download/0.8.2/cni-plugins-linux-amd64-0.8.2.tgz" | tar -C /opt/cni/bin -xz
root@k8s-worker:~$ mkdir -p /etc/cni/net.d
root@k8s-worker:~$ mv 10-flannel.conflist /etc/cni/net.d
root@k8s-worker:~$ export MASTER_IP=10.135.53.41
root@k8s-worker:~$ export NODE_IP=10.135.53.42
root@k8s-worker:~$ wget -q --show-progress https://github.com/coreos/flannel/releases/download/v0.11.0/flannel-v0.11.0-linux-amd64.tar.gz
flannel-v0.11.0-linux-amd64.tar.gz.1                      100%[==================================================================================================================================>]   4.51M  1.15MB/s    in 3.9s
root@k8s-worker:~$ tar xzf flannel-v0.11.0-linux-amd64.tar.gz
root@k8s-worker:~$ mv flanneld /usr/local/bin/
root@k8s-worker:~$ flanneld -iface=$NODE_IP -etcd-endpoints http://$MASTER_IP:2379 &> /tmp/flanneld.log &
root@k8s-worker:~$ pkill kubelet
root@k8s-worker:~$ kubelet \
  --config=/var/lib/kubelet/config.yaml \
  --kubeconfig=/etc/kubernetes/kubelet.conf \
  --network-plugin=cni \
  &> /tmp/kubelet.log &
root@k8s-worker:~$ systemctl restart docker

and now we can check pods’ IP addresses and try to ping them on different nodes:

root@k8s-master:~$ kubectl get pods -owide
NAME                     READY   STATUS    RESTARTS   AGE   IP            NODE      
nginx-86c57db685-6brvp   1/1     Running   0          26h   10.11.112.7   k8s-master
nginx-86c57db685-lphqr   1/1     Running   0          26h   10.15.176.4   k8s-worker
nginx-86c57db685-qc687   1/1     Running   0          26h   10.11.112.8   k8s-master
nginx-86c57db685-s6gx7   1/1     Running   0          26h   10.15.176.3   k8s-worker
nginx-86c57db685-zqtxx   1/1     Running   0          26h   10.15.176.2   k8s-worker

root@k8s-master:~$ kubectl run --generator=run-pod/v1 -it curl --image=ulamlabs/curlping --command -- bash
root@curl:/$ ping 10.11.112.7 -c 1 && ping 10.15.176.4 -c 1
PING 10.11.112.7 (10.11.112.7): 56 data bytes
64 bytes from 10.11.112.7: icmp_seq=0 ttl=64 time=0.051 ms
--- 10.11.112.7 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.051/0.051/0.051/0.000 ms
PING 10.15.176.4 (10.15.176.4): 56 data bytes
64 bytes from 10.15.176.4: icmp_seq=0 ttl=62 time=1.999 ms
--- 10.15.176.4 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 1.999/1.999/1.999/0.000 ms

Sweet, we have a pod to pod communication and this is how our nodes look like now:

Kubernetes deployment - networking - Ulam Labs

Load balancing between nodes

So now we have both nodes fully capable of running pods, what about receiving traffic?
Currently, we accept traffic only on the first node, it will be forwarded to pods on the second node (by flannel) but this is not high availability solution – the first node is single a point of failure.
To solve that we should install Kube Proxy on all worker nodes, after doing that we can add an icing on our cake – DigitalOcean Load Balancer and balance ingress between nodes.

root@k8s-worker:~$ kube-proxy --master=http://$MASTER_IP:8080 &> /tmp/proxy.log &
$ doctl compute load-balancer create --name lb --region fra1 --forwarding-rules entry_protocol:http,entry_port:80,target_protocol:http,target_port:30073 --health-check protocol:http,port:30073,path:/,check_interval_seconds:10,response_timeout_seconds:5,healthy_threshold:5,unhealthy_threshold:3
$ doctl compute droplet list "k8s*"
ID          Name          Public IPv4      Private IPv4    Public IPv6    Memory    VCPUs    Disk    Region    Image                 Status    Tags
63370004    k8s-master    46.101.177.76    10.135.53.41                   2048      2        40      fra1      Ubuntu 18.04.3 x64    active
63460608    k8s-worker    46.101.98.124    10.135.40.58                   2048      2        40      fra1      Ubuntu 18.04.3 x64    active
$ doctl compute load-balancer add-droplets 58f02699-5717-43e6-bbfe-51ef4cc0a227 --droplet-ids 63370004,63460608
$ doctl compute load-balancer get 58f02699-5717-43e6-bbfe-51ef4cc0a227
ID                                      IP               Name    Status    Created At              Algorithm      Region    Tag    Droplet IDs          SSL      Sticky Sessions                                Health Check                                                                                                                      Forwarding Rules
58f02699-5717-43e6-bbfe-51ef4cc0a227    67.207.79.225    lb      active    2017-09-27T09:40:56Z    round_robin    fra1             63370004,63460608    false    type:none,cookie_name:,cookie_ttl_seconds:0    protocol:http,port:30073,path:/,check_interval_seconds:10,response_timeout_seconds:5,healthy_threshold:5,unhealthy_threshold:3    entry_protocol:http,entry_port:80,target_protocol:http,target_port:30073,certificate_id:,tls_passthrough:false
$ curl http://67.207.79.225
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>

Everything works, we could even assume that we have production ready cluster 😀 but of course, we are far from that.

This is how our cluster looks now:

Setting up Kubernetes component - Ingress

Load balancer is a great tool to expose our cluster to the public, but still it's not the best what you can get. As your cluster will grow, possibly you'll end up in situation where you'll need more and more load balancers. In production environment, usually you will want to replicate your control plane node for high availability, so you will have to set up a load balancer balancing traffic between control plane nodes. Usually you will deploy a few or lot of applications inside your cluster. That will lead you to setting up a load balancer of each application. The problem is, every load balancer costs money. Single load balancer does not cost a fortune, but as your kubernetes cluster grows, cost will become significant. You can solve this problem with another Kubernetes component - Ingress. Its task is to define routing rules to the services inside the cluster. For example, you can configure which service will be targeted depending on Host HTTP header.

First you will need an ingress controller which will be deployed as a NodePort service and previously created load balancer will lead to it instead of nginx service. There is a variety of ingress controllers available, we will use NGINX controller here.

Let's start with creating nginx-ingress.yaml manifest file, don't forget to replace server IP with internal IP of your master node in nginx-ingress-kubeconfig:

apiVersion: v1
kind: Namespace
metadata:
  name: nginx-ingress
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-ingress-kubeconfig
  namespace: nginx-ingress
data:
  ingress-controller.kubeconfig: |
    apiVersion: v1
    kind: Config
    clusters:
    - cluster:
        server: http://10.135.53.41:8080  # replace with your master node internal IP
      name: kubernetes
    contexts:
    - context:
        cluster: kubernetes
        user: nginx-ingress-controller
      name: default
    current-context: default
    preferences: {}
    users: null
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-ingress
  namespace: nginx-ingress
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx-ingress
  template:
    metadata:
      labels:
        app: nginx-ingress
    spec:
      containers:
      - image: quay.io/kubernetes-ingress-controller/nginx-ingress-controller:0.30.0
        args:
        - /nginx-ingress-controller
        - --kubeconfig=/etc/kubernetes/ingress-controller.kubeconfig
        name: nginx-ingress
        ports:
        - name: http
          containerPort: 80
        securityContext:
          allowPrivilegeEscalation: true
          runAsUser: 101
          capabilities:
            drop:
            - ALL
            add:
            - NET_BIND_SERVICE
        volumeMounts:
         - mountPath: /etc/kubernetes
           name: nginx-ingress-kubeconfig
        env:
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: POD_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
      automountServiceAccountToken: false
      volumes:
      - name: nginx-ingress-kubeconfig
        configMap:
          name: nginx-ingress-kubeconfig
---
apiVersion: v1
kind: Service
metadata:
  name: nginx-ingress
  namespace: nginx-ingress
spec:
  type: NodePort
  ports:
  - port: 80
    targetPort: 80
    nodePort: 30073
    protocol: TCP
    name: http
  selector:
    app: nginx-ingress

Next, rerun the nginx service with ClusterIP type and run the ingress controller

root@k8s-master:~$ kubectl delete svc nginx
root@k8s-master:~$ kubectl expose deploy nginx --port=80
root@k8s-master:~$ kubectl apply -f nginx-ingress.yaml

Now we can create a simple Ingress resource which will forward all requests to the nginx service. Create ingress.yaml manifest:

apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: simple-ingress
spec:
  rules:
  - http:
      paths:
      - path: /
        backend:
          serviceName: nginx
          servicePort: 80

root@k8s-master:~$ kubectl apply -f ingress.yaml
root@k8s-master:~$ curl http://67.207.79.225
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>

Everything work as before, we can access the nginx service by the load balancer. Benefits from using Ingress rise when there are, let's say, 10 applications with different domains on your cluster. Instead of setting up a load balancer for each application, you'll need only one for the ingress controller, and ingress will do the rest later on.

Running Flannel as a DaemonSet

When you were setting up the second node, probably you have noticed that there is a lot of repetitive work - running and configuring kube-proxy and flannel. As the cluster grows, likely there will be more services that will be needed on each node. For this purpose there are another Kubernetes resource - DaemonSet. DaemonSet does exactly that - it runs a pod on each node in the kubernetes cluster.

At first, on both nodes kill flanneld process with pkill flanneld, next, on the master node create flannel manifest file, replace IP in --etcd-endpoints option with your master node internal IP:

apiVersion: v1
kind: ConfigMap
metadata:
  name: flannel-cfg
  namespace: kube-system
  labels:
    app: flannel
data:
  cni-conf.json: |
    {
      "name": "cbr0",
      "cniVersion": "0.3.1",
      "plugins": [
        {
          "type": "flannel",
          "delegate": {
            "hairpinMode": true,
            "isDefaultGateway": true
          }
        },
        {
          "type": "portmap",
          "capabilities": {
            "portMappings": true
          }
        }
      ]
    }
---
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: kube-flannel
  namespace: kube-system
  labels:
    app: flannel
spec:
  selector:
    matchLabels:
      app: flannel
  template:
    metadata:
      labels:
        app: flannel
    spec:
      hostNetwork: true
      tolerations:
        - operator: Exists
          effect: NoSchedule
      initContainers:
        - name: install-cni
          image: quay.io/coreos/flannel:v0.11.0-amd64
          command:
            - cp
          args:
            - -f
            - /etc/kube-flannel/cni-conf.json
            - /etc/cni/net.d/10-flannel.conflist
          volumeMounts:
            - name: cni
              mountPath: /etc/cni/net.d
            - name: flannel-cfg
              mountPath: /etc/kube-flannel
      containers:
        - name: kube-flannel
          image: quay.io/coreos/flannel:v0.11.0-amd64
          securityContext:
            capabilities:
              add: ["NET_ADMIN"]
          command:
            - /opt/bin/flanneld
          args:
            - --iface=$(NODE_IP)
            - --etcd-endpoints=http://10.135.53.41:2379  # replace with your master node internal IP
          env:
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  fieldPath: metadata.name
            - name: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: NODE_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.hostIP
          volumeMounts:
            - name: run
              mountPath: /run/flannel
            - name: flannel-cfg
              mountPath: /etc/kube-flannel
      volumes:
        - name: run
          hostPath:
            path: /run/flannel
        - name: cni
          hostPath:
            path: /etc/cni/net.d
        - name: flannel-cfg
          configMap:
            name: flannel-cfg

root@k8s-master:~$ kubectl apply -f flannel.yaml
root@k8s-master:~$ kubectl get pods -n kube-system
NAME                 READY   STATUS    RESTARTS   AGE
kube-flannel-mjk7w   1/1     Running   1          4m12s
kube-flannel-smk6t   1/1     Running   1          3m58s
root@k8s-master:~$ systemctl restart docker

root@k8s-worker:~$ systemctl restart docker

$ curl http://67.207.79.225
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>

As you see, everything works as before, and now, if you add a node to the cluster, flannel will be set up automatically. You can now try setting up kube-proxy as DaemonSet yourself.

Wrapping up on Kubernetes deployment from scratch

In this blog post, we have learned how nodes in the kubernetes cluster communicate together and how pods are exposed to the outer world through services. Next we have learnt about two extremely useful Kubernetes resources - Ingresses and DaemonSets. There are more aspects of Kubernetes that we need to cover before we can say that our cluster is production ready. We are going to cover them in future blog posts, so stay tuned!

If you’d like to know more about our services for your business, don’t hesitate to get in touch.

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Konrad RotkiewiczSeasoned Lead Full Stack Python Developer and System Engineer

Konrad has 6 years of strong background in Embedded & Networking programming, as well as experience leading multiple teams for UK and US customers, which has laid the foundations for him to become a natural Technology leader. A man obsessed with continuous improvements, Konrad is never satisfied with his results.