Kubernetes/Networking

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Pod networking

Networking in Kubernetes is using Linux Network namespace. Each Pod has IP address assosiated with it. It recives this IP from Virtual Ethernet interface pair.

Pod to pod communication on the same node. The pod cidr range was decided during cluster creation kubeadm init --pod-network=10.100.0.0/16 and notified CNI plugin (eg. Flannel, Calico) to use this IP range. 

node-1
 ---pod1---                                        ---pod2---
|10.100.2.9|                                      |10.100.2.7|
|      eth0|----vethc3428d55      vethe10ac769----|eth0      |
 ----------                 \    /                 ----------
                    bridge 10.100.2.1/24
                              |
                     eth0(node-1)172.31.11.11
---------|=====CNI overlay====|---------------------network--------------
 eth0(node-2)172.31.22.22
         |                                         ---pod3---
         |                                        |10.100.1.5|
          \------bridge-----------vetha1bbccdd----|eth0      |
                                                   ----------


Find out node that 'nginx' pod is running on 

kubectl -n default get pods -owide
NAME                     READY   STATUS    RESTARTS   AGE   IP           NODE                NOMINATED NODE   READINESS GATES
nginx-7cdbd8cdc9-89zcf   1/1     Running   1          8d    10.100.2.9   worker-2.acme.com   <none>           <none>


Ssh to worker-2.acme.com 

user@worker-2:~$ ifconfig 
cni0      Link encap:Ethernet  HWaddr 86:5d:c7:93:72:d2  
          inet addr:10.100.2.1  Bcast:0.0.0.0  Mask:255.255.255.0
          inet6 addr: fe80::845d:c7ff:fe93:72d2/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:8951  Metric:1
          RX packets:29 errors:0 dropped:0 overruns:0 frame:0
          TX packets:64 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:1900 (1.9 KB)  TX bytes:7165 (7.1 KB)

docker0   Link encap:Ethernet  HWaddr 02:42:c7:1c:b8:23  
          inet addr:172.17.0.1  Bcast:172.17.255.255  Mask:255.255.0.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth0      Link encap:Ethernet  HWaddr 02:20:2e:90:a8:66  
          inet addr:172.31.122.65  Bcast:172.31.127.255  Mask:255.255.240.0
          inet6 addr: 2a05:d018:85:e101:2177:162b:63d9:3600/128 Scope:Global
          inet6 addr: fe80::20:2eff:fe90:a866/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:9001  Metric:1
          RX packets:20293 errors:0 dropped:0 overruns:0 frame:0
          TX packets:15985 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:15733611 (15.7 MB)  TX bytes:2550893 (2.5 MB)

flannel.1 Link encap:Ethernet  HWaddr b2:3a:21:03:ec:13  
          inet addr:10.100.2.0  Bcast:0.0.0.0  Mask:255.255.255.255
          UP BROADCAST RUNNING MULTICAST  MTU:8951  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:20 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:4741 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4741 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:543426 (543.4 KB)  TX bytes:543426 (543.4 KB)

vethc3428d55 Link encap:Ethernet  HWaddr 52:1e:12:8c:0d:34        #<- 6th interface
          inet6 addr: fe80::501e:12ff:fe8c:d34/64 Scope:Link      #its a pipe to the running pod
          UP BROADCAST RUNNING MULTICAST  MTU:8951  Metric:1
          RX packets:28 errors:0 dropped:0 overruns:0 frame:0
          TX packets:94 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:2264 (2.2 KB)  TX bytes:10623 (10.6 KB)

vethe10ac769 Link encap:Ethernet  HWaddr 56:b5:d4:48:33:cc  
          inet6 addr: fe80::54b5:d4ff:fe48:33cc/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:8951  Metric:1
          RX packets:1 errors:0 dropped:0 overruns:0 frame:0
          TX packets:76 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:42 (42.0 B)  TX bytes:9190 (9.1 KB)


See containers running on this node. The "/pause" container which pertains to nGinx container for the purpose of holding on to the pods network namespace. 

sudo docker ps | grep nginx
CONTAINER ID IMAGE                  COMMAND    CREATED STATUS PORTS NAMES
1fc1244ca7d5 nginx                  "nginx -g" 44 minu Up 44        k8s_nginx_nginx-7cdbd8cdc9-89zcf_default_9c89e271-a07c-11e9-80e8-02f78428aaf6_2
367062cd2852 k8s.gcr.io/pause:3.1   "/pause"   44 minu Up 44          k8s_POD_nginx-7cdbd8cdc9-89zcf_default_9c89e271-a07c-11e9-80e8-02f78428aaf6_10

#See docker IP (has not worked)
docker inspect --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' $INSTANCE_ID

#See the container PID
sudo docker inspect --format '{{ .State.Pid }}' 1fc1244ca7d5
5738
$ sudo nsenter -t 5738 -n ip addr #display the container networking
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
3: eth0@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8951 qdisc noqueue state UP group default 
    link/ether 9e:3a:d3:66:46:bb brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.100.2.9/24 scope global eth0
       valid_lft forever preferred_lft forever

#eth0@if6: :- eth0 on a pod is linked to node's 6th interface

Service networking

Services allow our pods to move around, get deleted, and replicate, all without having to manually keep track of their IP addresses in the cluster. This is accomplished by creating one gateway to distribute packets evenly across all pods.


YAML for nginx NodePort service 

apiVersion: v1
kind: Service
metadata:
  name: nginx-nodeport
spec:
  type: NodePort
  ports:
  - nodePort: 30080 #port on each node on which this service is exposed when type=NodePort or LoadBalancer, by default auto-allocate 
    port: 80        #port that will be exposed by this service
    protocol: TCP
    targetPort: 80  #port to access on the pods targeted by the service, by default same as 'port' above
                    #it's a port exposed by containers
  selector:
    app: nginx    #service will be applied to each pod with this label


Service and endpoint can be seem below 

kubectl get service nginx -owide
NAME    TYPE       CLUSTER-IP       EXTERNAL-IP   PORT(S)        AGE   SELECTOR
nginx   NodePort   10.110.225.169   <none>        80:30080/TCP   9d    run=nginx

kubectl get endpoints -owide
NAME         ENDPOINTS             AGE
kubernetes   172.31.115.255:6443   11d
nginx        10.100.2.11:80        9d


iptables associated with the service above can be seen below 

sudo iptables-save | grep KUBE | grep nginx
-A KUBE-NODEPORTS -p tcp -m comment --comment "default/nginx:" -m tcp --dport 30080 -j KUBE-MARK-MASQ
-A KUBE-NODEPORTS -p tcp -m comment --comment "default/nginx:" -m tcp --dport 30080 -j KUBE-SVC-4N57TFCL4MD7ZTDA
-A KUBE-SERVICES ! -s 10.100.0.0/16 -d 10.110.225.169/32 -p tcp -m comment --comment "default/nginx: cluster IP" -m tcp --dport 80 -j KUBE-MARK-MASQ
#anything from -source (pod cidr 10.100.0.0/16, this will come from nginx service endpoint 10.100.2.11:80) 
#-destined to the service (nginx NodePort ip 10.110.225.169) redirect with a random pod associated with this service
-A KUBE-SERVICES -d 10.110.225.169/32 -p tcp -m comment --comment "default/nginx: cluster IP" -m tcp --dport 80 -j KUBE-SVC-4N57TFCL4MD7ZTDA
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