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Kubernetes/Networking

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< Kubernetes

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 has been created on every node to listen on port 30080

sudo lsof -i6 | grep 30080
kube-prox 3202  root    9u  IPv6  52872      0t0  TCP *:30080 (LISTEN)


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

# Another example, showing an application 'rediness-app' is having 3 pods behind its service
$ kubectl get endpoints
NAME            ENDPOINTS                                        AGE
kubernetes      35.205.145.199:443                               26d
readiness-app   10.60.0.23:8080,10.60.1.9:8080,10.60.2.24:8080   7m6s


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

Port-forward

You can port forward traffic only to a pod not service. If that be possible you would know where your traffic ends up. This feature port-forward is only used to debug application.


This is a status of deployed applications, note frond-end application, we are trying to access localy

ClipCapIt-190808-125724.PNG


We can port-forward remote kubernetes service port to local machine. Let's assume we have config like this:

ClipCapIt-190808-125049.PNG

Create port-forwarding, your host will start listening on port TCP:8079. If your pod is exposing port below <1024 you should then you need to specify local port to target pod, then use eg: 8089:80. First port is local port, second is the container port.

$ kubectl port-forward front-end 8079
Forwarding from 127.0.0.1:8079 -> 8079
Forwarding from [::1]:8079 -> 8079
Handling connection for 8079

#In a new terminal try to reach to the frond-end
curl http://localhost:8079

Ssh to node

gcloud compute ssh gke-cluster-1-default-pool-5d74cca4-6hwx


$ gcloud compute ssh gke-cluster-1-default-pool-5d74cca4-6hwx
WARNING: The public SSH key file for gcloud does not exist.
WARNING: The private SSH key file for gcloud does not exist.
WARNING: You do not have an SSH key for gcloud.
WARNING: SSH keygen will be executed to generate a key.
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/vagrant/.ssh/google_compute_engine.
Your public key has been saved in /home/vagrant/.ssh/google_compute_engine.pub.
The key fingerprint is:
SHA256:V8S2tzis6NPGfFdKzUak0IX0aRUBin3jW+wCMFqfoYA vagrant@u18cli-2
The key's randomart image is:
+---[RSA 2048]----+
|           ..+o==|
|      .   o.= o.+|
|     E . = =.= =.|
|        + =.* * .|
|       .S..= + B |
|         .  = * =|
|         = . = = |
|        o * . +  |
|       ..o . .   |
+----[SHA256]-----+
Updating project ssh metadata...⠹Updated [https://www.googleapis.com/compute/v1/projects/responsive-sun-246311].            
Updating project ssh metadata...done.                                                                                       
Waiting for SSH key to propagate.
Warning: Permanently added 'compute.7391123279603721012' (ECDSA) to the list of known hosts.
##############################################################################
# WARNING: Any changes on the boot disk of the node must be made via
#          DaemonSet in order to preserve them across node (re)creations.
#          Node will be (re)created during manual-upgrade, auto-upgrade,
#          auto-repair or auto-scaling.
#          See https://cloud.google.com/kubernetes-engine/docs/concepts/node-images#modifications
#          for more information.
##############################################################################
##############################################################################
# WARNING: Any changes on the boot disk of the node must be made via
#          DaemonSet in order to preserve them across node (re)creations.
#          Node will be (re)created during manual-upgrade, auto-upgrade,
#          auto-repair or auto-scaling.
#          See https://cloud.google.com/kubernetes-engine/docs/concepts/node-images#modifications
#          for more information.
##############################################################################
Welcome to Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-1034-gke x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

This system has been minimized by removing packages and content that are
not required on a system that users do not log into.

To restore this content, you can run the 'unminimize' command.

0 packages can be updated.
0 updates are security updates.


Welcome to Kubernetes v1.13.7-gke.8!

You can find documentation for Kubernetes at:
  http://docs.kubernetes.io/

The source for this release can be found at:
  /home/kubernetes/kubernetes-src.tar.gz
Or you can download it at:
  https://storage.googleapis.com/kubernetes-release-gke/release/v1.13.7-gke.8/kubernetes-src.tar.gz

It is based on the Kubernetes source at:
  https://github.com/kubernetes/kubernetes/tree/v1.13.7-gke.8

For Kubernetes copyright and licensing information, see:
  /home/kubernetes/LICENSES

LoadBalancer networking

Loadbalancer is an extension of NodePort type of service. Loadbalancer redirect traffic to all nodes and NodePort. LoadBalancers are not pod aware, as nodes are its backend. The traffic to the right pod is controlled by IPtables. See below. If request from LoadBalancer is sent to Node-3, to reach a pod:80. There is no pod serving port 80 on Node-3. Therefore IPTables will route traffic to another node here: Node-1 or 2 (this is overlay network). Then the reply, will be routed back to Node-3 and out through LoadBalancer. This all means extra hops and latency.

.
                   LoadBalancer
                 IP: 22.111.222.33
              /          |           \
Node-1               Node-2              Node-3
172.10.10.11         172.10.11.22        172.10.12.33
NodePort:33623       NodePort:33623      NodePort:33623     
Service:8080:80      Service:8080:80     Service:8080:80

pod1 10.100.1.1:80   pod4 10.100.2.1:80  pod6 10.100.3.1:81
pod2 10.100.1.2:81   pod5 10.100.2.2:81
pod3 10.100.1.3:82


LoadBalancer YAML spec. NodePort is not specifed as K8s will assign one and will manage it.

apiVersion: v1
kind: Service
metadata:
  name: nginx-loadbalancer
spec:
  type: LoadBalancer
  ports:
  - port: 80
    targetPort: 80
  selector:
    app: nginx


Create a loadbalancer by exposing a deployment <souce lang=bash> kubectl run nginx-loadbalancer --image=nginx kubectl scale deployment/nginx-loadbalancer --replicas=2 kubectl expose deployment nginx-loadbalancer --port 80 --target-port 8080 --type LoadBalancer </source>


Influence traffic flow <souce lang=bash> kubectl describe service nginx-loadbalancer Name: nginx-loadbalancer Namespace: default Labels: run=nginx-loadbalancer Annotations: <none> #annotated -> ExternalTrafficPolicy: Local Selector: run=nginx-loadbalancer Type: LoadBalancer IP: 10.97.188.223 Port: <unset> 80/TCP TargetPort: 8080/TCP NodePort: <unset> 31154/TCP Endpoints: 10.100.1.10:8080 Session Affinity: None External Traffic Policy: Cluster Events: <none>

  1. Add addnotation, so traffic is routed to a pod on a local node if exists.

kubectl annotate service nginx-loadbalancer ExternalTrafficPolicy=Local </source>

Ingress networking

Ingress YAML spec

cat > ingress.yml <<EOF
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: service-ingress
spec:
  rules:
  - host: acme.example.com #must be valid domain
    http:
      paths:
      - backend:
          serviceName: nginx-1
          servicePort: 80
  - host: app.example.com
    http:
      paths:
      - backend:
          serviceName: nginx-2
          servicePort: 80
  - http:                #any traffic not matching HEADER: <hostnames> above
      paths:
      - backend:
          serviceName: httpd-3
          servicePort: 80
EOF


Resource can be managed like any other K8s resources

kubectl apply -f ingress
kubectl edit ingress service-ingress

Summary diagram of AWS Kubernetes networking

ClipCapIt-190716-085801.PNG

DNS

Since 1.13, CoreOS has replaced kube-dns<tt> with <tt>core-dns written in Go. It supports DNS over TLS in short dot. core-dns pods are running as a deployment.

#svc-name     ns   /BaseDomainName\ 
kubernetes.default.svc.cluster.local
10-10-20-1.default.pod.cluster.local
#pod ip       ns   \BaseDomainName/


Core-dns runs as a deployment

kubectl -n kube-system get pod -owide | grep core
coredns-86c58d9df4-7dl5d                                  1/1     Running   59         12d   10.100.0.18      master-1.acme.com   <none>           <none>
coredns-86c58d9df4-rsxct                                  1/1     Running   59         12d   10.100.0.19      master-1.acme.com   <none>           <none>

kubectl -n kube-system get deployments -owide
NAME      READY   UP-TO-DATE   AVAILABLE   AGE   CONTAINERS   IMAGES                     SELECTOR
coredns   2/2     2            2           12d   coredns      k8s.gcr.io/coredns:1.2.6   k8s-app=kube-dns

#Service that perform LoadBalancing. note it's named 'kube-dns' to support backward compatibnility for workloads relaying on kube-dns
ubectl -n kube-system get service -owide
NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)         AGE   SELECTOR
kube-dns   ClusterIP   10.96.0.10   <none>        53/UDP,53/TCP   12d   k8s-app=kube-dns


Interact with DNS, using BusyBox container

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  namespace: default
spec:
  containers:
  - image: busybox:1.28.4
    command:
      - sleep
      - "3600"
    imagePullPolicy: IfNotPresent
    name: busybox
  restartPolicy: Always

#Deploy
kubectl apply -f busybox.yaml

#Verify dns settings
kubectl exec -t busybox -- cat /etc/resolv.conf 
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local mylabserver.com
options ndots:5

#Check DNS operations
kubectl exec -it busybox -- nslookup  <service-name>
kubectl exec -it busybox -- nslookup  <pod-ip-v-4>.default.pod.cluster.local #pod
kubectl exec -it busybox -- nslookup     <svcName>.default.svc.cluster.local #service
kubectl exec -it busybox -- nslookup  kube-dns.kube-system.svc.cluster.local #service


CoreDNS manages following records for Services:

  • A records (not headless) - Services are assigned a DNS A record for a name of the form my-svc.my-namespace.svc.cluster-domain.example. This resolves to the cluster IP of the Service.
  • A records (headless without cluster IP) - Services are also assigned a DNS A record for a name of the form my-svc.my-namespace.svc.cluster-domain.example. Unlike normal Services, this resolves to the set of IPs of the pods selected by the Service
  • SRV records - SRV Records are created for named ports that are part of normal or Headless Services. read more...


Headless services, its a service without a clusterIP, will respond with set of IPs that belong to a POD. These IPs are current IPs that service consider healthy pods.

apiVersion: v1
kind: Service
metadata:
  name: kube-headless
spec:
  clusterIP: None #set to none
  ports:
  - port: 80
    targetPort: 8080
  selector:
    app: kubserve2


Custom dns settings can be set per pod basis. Default is cluster first, pod inherits DNS settings from a node it's running on

apiVersion: v1
kind: Pod
metadata:
  namespace: default
  name: pod-dns-custom-setup
spec:
  containers:
    - name: test
      image: nginx
  dnsPolicy: "None" #do not inherit settings from a node
  dnsConfig:
    nameservers:
      - 8.8.4.4
    searches:
      - ns1.svc.cluster.local
      - my.dns.search.suffix
    options:
      - name: ndots
        value: "3"
      - name: edns0


Troubleshot core-dns

kubectl -n kube-system get pods | grep dns
coredns-86c58d9df4-7dl5d                                  1/1     Running   60         13d
coredns-86c58d9df4-rsxct                                  1/1     Running   60         13d

#See logs
kubectl -n kube-system logs coredns-86c58d9df4-7dl5d -f
.:53
2019-07-18T06:33:08.165Z [INFO] CoreDNS-1.2.6
2019-07-18T06:33:08.165Z [INFO] linux/amd64, go1.11.2, 756749c
CoreDNS-1.2.6
linux/amd64, go1.11.2, 756749c
 [INFO] plugin/reload: Running configuration MD5 = f65c4821c8a9b7b5eb30fa4fbc167769
 [ERROR] plugin/errors: 2 8726527267836830687.8090630885983783330. HINFO: unreachable backend: read udp 10.100.0.20:33021->172.31.0.2:53: i/o timeout

References