<p>I set up 1 master 2 nodes k8s cluster in according to documentation. A pod can ping the other pod on the same node but can't ping the pod on the other node.</p>

<p>To demonstrate the problem I deployed below deployments which has 3 replica. While two of them sits on the same node, the other pod sits on the other node.</p>

<pre>

    $ cat nginx.yaml
    apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: nginx-deployment
      labels:
        app: nginx
    spec:
      replicas: 2
      selector:
        matchLabels:
          app: nginx
      template:
        metadata:
          labels:
            app: nginx
        spec:
          containers:
          - name: nginx
            image: nginx
            ports:
            - containerPort: 80
    ---
    kind: Service
    apiVersion: v1
    metadata:
      name: nginx-svc
    spec:
      selector:
        app: nginx
      ports:
      - protocol: TCP
        port: 80

    $ kubectl get nodes
    NAME                                          STATUS    ROLES     AGE       VERSION
    ip-172-31-21-115.us-west-2.compute.internal   Ready     master    20m       v1.11.2
    ip-172-31-26-62.us-west-2.compute.internal    Ready         19m       v1.11.2
    ip-172-31-29-204.us-west-2.compute.internal   Ready         14m       v1.11.2

    $ kubectl get pods -o wide
    NAME                               READY     STATUS    RESTARTS   AGE       IP           NODE                                          NOMINATED NODE
    nginx-deployment-966857787-22qq7   1/1       Running   0          11m       10.244.2.3   ip-172-31-29-204.us-west-2.compute.internal   
    nginx-deployment-966857787-lv7dd   1/1       Running   0          11m       10.244.1.2   ip-172-31-26-62.us-west-2.compute.internal    
    nginx-deployment-966857787-zkzg6   1/1       Running   0          11m       10.244.2.2   ip-172-31-29-204.us-west-2.compute.internal   

    $ kubectl get svc
    NAME         TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)   AGE
    kubernetes   ClusterIP   10.96.0.1               443/TCP   21m
    nginx-svc    ClusterIP   10.105.205.10           80/TCP    11m

</pre>

<p>Everything looks fine.</p>

<p>Let me show you containers.</p>

<pre>

    # docker exec -it 489b180f512b /bin/bash
    root@nginx-deployment-966857787-zkzg6:/# ifconfig
    eth0: flags=4163  mtu 8951
            inet 10.244.2.2  netmask 255.255.255.0  broadcast 0.0.0.0
            inet6 fe80::cc4d:61ff:fe8a:5aeb  prefixlen 64  scopeid 0x20

    root@nginx-deployment-966857787-zkzg6:/# ping 10.244.2.3
    PING 10.244.2.3 (10.244.2.3) 56(84) bytes of data.
    64 bytes from 10.244.2.3: icmp_seq=1 ttl=64 time=0.066 ms
    64 bytes from 10.244.2.3: icmp_seq=2 ttl=64 time=0.055 ms
    ^C

</pre>

<p>So it pings its neighbor pod on the same node.</p>

<pre>

    root@nginx-deployment-966857787-zkzg6:/# ping 10.244.1.2
    PING 10.244.1.2 (10.244.1.2) 56(84) bytes of data.
    ^C
    --- 10.244.1.2 ping statistics ---
    2 packets transmitted, 0 received, 100% packet loss, time 1059ms

</pre>

<p>And can't ping its replica on the other node.</p>

<p>Here is host interfaces:</p>

<pre>

    # ifconfig
    cni0: flags=4163  mtu 8951
            inet 10.244.2.1  netmask 255.255.255.0  broadcast 0.0.0.0

    docker0: flags=4099  mtu 1500
            inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255

    eth0: flags=4163  mtu 9001
            inet 172.31.29.204  netmask 255.255.240.0  broadcast 172.31.31.255

    flannel.1: flags=4163  mtu 8951
            inet 10.244.2.0  netmask 255.255.255.255  broadcast 0.0.0.0

    lo: flags=73  mtu 65536
            inet 127.0.0.1  netmask 255.0.0.0

    veth09fb984a: flags=4163  mtu 8951
            inet6 fe80::d819:14ff:fe06:174c  prefixlen 64  scopeid 0x20

    veth87b3563e: flags=4163  mtu 8951
            inet6 fe80::d09c:d2ff:fe7b:7dd7  prefixlen 64  scopeid 0x20

    # ifconfig
    cni0: flags=4163  mtu 8951
            inet 10.244.1.1  netmask 255.255.255.0  broadcast 0.0.0.0

    docker0: flags=4099  mtu 1500
            inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255

    eth0: flags=4163  mtu 9001
            inet 172.31.26.62  netmask 255.255.240.0  broadcast 172.31.31.255

    flannel.1: flags=4163  mtu 8951
            inet 10.244.1.0  netmask 255.255.255.255  broadcast 0.0.0.0

    lo: flags=73  mtu 65536
            inet 127.0.0.1  netmask 255.0.0.0

    veth9733e2e6: flags=4163  mtu 8951
            inet6 fe80::8003:46ff:fee2:abc2  prefixlen 64  scopeid 0x20

</pre>

<p>Processes on the nodes:</p>

<pre>

    # ps auxww|grep kube
    root      4059  0.1  2.8  43568 28316 ?        Ssl  00:31   0:01 /usr/local/bin/kube-proxy --config=/var/lib/kube-proxy/config.conf
    root      4260  0.0  3.4 358984 34288 ?        Ssl  00:31   0:00 /opt/bin/flanneld --ip-masq --kube-subnet-mgr
    root      4455  1.1  9.6 760868 97260 ?        Ssl  00:31   0:14 /usr/bin/kubelet --bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf --kubeconfig=/etc/kubernetes/kubelet.conf --config=/var/lib/kubelet/config.yaml --cgroup-driver=systemd --cni-bin-dir=/opt/cni/bin --cni-conf-dir=/etc/cni/net.d --network-plugin=cni

</pre>

<p>Because of this network problem clusterIP is also unreachable:</p>

<p>$ curl 10.105.205.10:80</p>

<p>Any suggestion?</p>

<p>Thanks.</p>

<p>I found the problem.</p>

<p>Flannel uses UDP port 8285 and 8472 which was being blocked by AWS security groups. I had only opened TCP ports. </p>

<p>I enable UDP port 8285 and UDP port 8472 as well as TCP 6443, 10250, 10256.</p>
