Now that you have a continuously running, replicated application you can expose it on a network. Before discussing the Kubernetes approach to networking, it is worthwhile to contrast it with the "normal" way networking works with Docker.
By default, Docker uses host-private networking, so containers can talk to other containers only if they are on the same machine. In order for Docker containers to communicate across nodes, they must be allocated ports on the machine's own IP address, which are then forwarded or proxied to the containers. This obviously means that containers must either coordinate which ports they use very carefully or else be allocated ports dynamically.
Coordinating ports across multiple developers is very difficult to do at scale and exposes users to cluster-level issues outside of their control. Kubernetes assumes that pods can communicate with other pods, regardless of which host they land on. We give every pod its own cluster-private-IP address so you do not need to explicitly create links between pods or mapping container ports to host ports. This means that containers within a Pod can all reach each other's ports on localhost, and all pods in a cluster can see each other without NAT. The rest of this document will elaborate on how you can run reliable services on such a networking model.
This guide uses a simple nginx server to demonstrate proof of concept. The same principles are embodied in a more complete [Jenkins CI application](http://blog.kubernetes.io/2015/07/strong-simple-ssl-for-kubernetes.html).
We did this in a previous example, but lets do it once again and focus on the networking perspective. Create an nginx pod, and note that it has a container port specification:
You should be able to ssh into any node in your cluster and curl both IPs. Note that the containers are *not* using port 80 on the node, nor are there any special NAT rules to route traffic to the pod. This means you can run multiple nginx pods on the same node all using the same containerPort and access them from any other pod or node in your cluster using IP. Like Docker, ports can still be published to the host node's interface(s), but the need for this is radically diminished because of the networking model.
So we have pods running nginx in a flat, cluster wide, address space. In theory, you could talk to these pods directly, but what happens when a node dies? The pods die with it, and the Deployment will create new ones, with different IPs. This is the problem a Service solves.
A Kubernetes Service is an abstraction which defines a logical set of Pods running somewhere in your cluster, that all provide the same functionality. When created, each Service is assigned a unique IP address (also called clusterIP). This address is tied to the lifespan of the Service, and will not change while the Service is alive. Pods can be configured to talk to the Service, and know that communication to the Service will be automatically load-balanced out to some pod that is a member of the Service.
This specification will create a Service which targets TCP port 80 on any Pod with the `app=nginx` label, and expose it on an abstracted Service port (`targetPort`: is the port the container accepts traffic on, `port`: is the abstracted Service port, which can be any port other pods use to access the Service). View [service API object](/docs/api-reference/v1/definitions/#_v1_service) to see the list of supported fields in service definition.
As mentioned previously, a Service is backed by a group of pods. These pods are exposed through `endpoints`. The Service's selector will be evaluated continuously and the results will be POSTed to an Endpoints object also named `my-nginx`. When a pod dies, it is automatically removed from the endpoints, and new pods matching the Service's selector will automatically get added to the endpoints. Check the endpoints, and note that the IPs are the same as the pods created in the first step:
You should now be able to curl the nginx Service on `<CLUSTER-IP>:<PORT>` from any node in your cluster. Note that the Service IP is completely virtual, it never hits the wire, if you're curious about how this works you can read more about the [service proxy](/docs/user-guide/services/#virtual-ips-and-service-proxies).
Kubernetes supports 2 primary modes of finding a Service - environment variables and DNS. The former works out of the box while the latter requires the [kube-dns cluster addon](http://releases.k8s.io/{{page.githubbranch}}/cluster/addons/dns/README.md).
When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. This introduces an ordering problem. To see why, inspect the environment of your running nginx pods (your pod name will be different):
Note there's no mention of your Service. This is because you created the replicas before the Service. Another disadvantage of doing this is that the scheduler might put both pods on the same machine, which will take your entire Service down if it dies. We can do this the right way by killing the 2 pods and waiting for the Deployment to recreate them. This time around the Service exists *before* the replicas. This will given you scheduler level Service spreading of your pods (provided all your nodes have equal capacity), as well as the right environment variables:
Kubernetes offers a DNS cluster addon Service that uses skydns to automatically assign dns names to other Services. You can check if it's running on your cluster:
```shell
$ kubectl get services kube-dns --namespace=kube-system
If it isn't running, you can [enable it](http://releases.k8s.io/{{page.githubbranch}}/cluster/addons/dns/README.md#how-do-i-configure-it). The rest of this section will assume you have a Service with a long lived IP (my-nginx), and a dns server that has assigned a name to that IP (the kube-dns cluster addon), so you can talk to the Service from any pod in your cluster using standard methods (e.g. gethostbyname). Let's run another curl application to test this:
Till now we have only accessed the nginx server from within the cluster. Before exposing the Service to the internet, you want to make sure the communication channel is secure. For this, you will need:
* Self signed certificates for https (unless you already have an identity certificate)
* An nginx server configured to use the certificates
You can acquire all these from the [nginx https example](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/examples/https-nginx/), in short:
```shell
$ make keys secret KEY=/tmp/nginx.key CERT=/tmp/nginx.crt SECRET=/tmp/secret.json
- It contains both rc and service specification in the same file
- The [nginx server](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/examples/https-nginx/default.conf) serves http traffic on port 80 and https traffic on 443, and nginx Service exposes both ports.
- Each container has access to the keys through a volume mounted at /etc/nginx/ssl. This is setup *before* the nginx server is started.
Note how we supplied the `-k` parameter to curl in the last step, this is because we don't know anything about the pods running nginx at certificate generation time,
so we have to tell curl to ignore the CName mismatch. By creating a Service we linked the CName used in the certificate with the actual DNS name used by pods during Service lookup.
Lets test this from a pod (the same secret is being reused for simplicity, the pod only needs nginx.crt to access the Service):
For some parts of your applications you may want to expose a Service onto an external IP address. Kubernetes supports two ways of doing this: NodePorts and LoadBalancers. The Service created in the last section already used `NodePort`, so your nginx https replica is ready to serve traffic on the internet if your node has a public IP.
