Kubernetes Host Aliases

Kubernetes Host Aliases

We can add entries to a Pod’s /etc/hosts file to provide Pod-level override of hostname resolution when DNS and other options are not applicable. HostAliases field in PodSpec achieves this purpose.

Manual editing of /etc/hosts is not suggested because the file is managed by the kubelet and can be overwritten on during Pod creation/restart.

Before adding any host aliases, let us check the contents of /etc/hosts file of a container.

Create a busybox container and run the command to display the contents of the hosts file.

pradeep@learnk8s$ kubectl run busybox --image=busybox --command -- cat /etc/hosts
pod/busybox created

Verify Pod is created or not

pradeep@learnk8s$ kubectl get pods
NAME      READY   STATUS              RESTARTS   AGE
busybox   0/1     ContainerCreating   0          5s

It is still in ContainerCreating state, verify again

pradeep@learnk8s$ kubectl get pods
NAME      READY   STATUS      RESTARTS   AGE
busybox   0/1     Completed   0          7s

Now that we see Completed state, let us check the logs of this container.

pradeep@learnk8s$ kubectl logs busybox
# Kubernetes-managed hosts file.
127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
fe00::0	ip6-mcastprefix
fe00::1	ip6-allnodes
fe00::2	ip6-allrouters
10.244.151.18	busybox

As expected, in the logs, we do see the expected output, the Kubernetes-managed hosts file.

Apart from the default entries for the localhost, we do see one entry at the end, with an IP, 10.244.151.18 busybox which is nothing but it’s own IP address.

We can verify this with the -o wide option as well.

pradeep@learnk8s$ kubectl get pods -o wide
NAME      READY   STATUS             RESTARTS      AGE   IP              NODE           NOMINATED NODE   READINESS GATES
busybox   0/1     CrashLoopBackOff   1 (13s ago)   23s   10.244.151.18   minikube-m03   <none>           <none>
pradeep@learnk8s$ 

Now let us add some custom entries to this hosts file with the hostalias. We can make use of the kubectl explain pod.spec command to get all the supportedd spec values, once we have the list, just explain the one we are interested in currently (hostAliases).

pradeep@learnk8s$ kubectl explain pod.spec.hostAliases 
KIND:     Pod
VERSION:  v1

RESOURCE: hostAliases <[]Object>

DESCRIPTION:
     HostAliases is an optional list of hosts and IPs that will be injected into
     the pod's hosts file if specified. This is only valid for non-hostNetwork
     pods.

     HostAlias holds the mapping between IP and hostnames that will be injected
     as an entry in the pod's hosts file.

FIELDS:
   hostnames	<[]string>
     Hostnames for the above IP address.

   ip	<string>
     IP address of the host file entry.

pradeep@learnk8s$ 

We can see it takes two fields, hostnames and ip.

Let us run the same command as earlier but with the --dry-run=client option along with -o yaml to get the YAML manifest, to which we can add the hostAliases.

pradeep@learnk8s$ kubectl run busybox --image=busybox --dry-run=client -o yaml --command -- cat /etc/hosts
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: busybox
  name: busybox
spec:
  containers:
  - command:
    - cat
    - /etc/hosts
    image: busybox
    name: busybox
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}
pradeep@learnk8s$

Let us save this output to a file named pod-with-hostalias.yaml

pradeep@learnk8s$ kubectl run busybox --image=busybox --dry-run=client -o yaml --command -- cat /etc/hosts > pod-with-hostalias.yaml

Edit this file to add hostAliases. For example one for kubernetes.io and another for pradeepgadde.com with some random IPs.

pradeep@learnk8s$ vi pod-with-hostalias.yaml 
pradeep@learnk8s$ cat pod-with-hostalias.yaml 
apiVersion: v1
kind: Pod
metadata:
  labels:
    run: busybox
  name: busybox
spec:
  hostAliases:
  - ip: "127.0.0.1"
    hostnames:
    - "pradeepgadde.com"
  - ip: "192.168.1.1"
    hostnames:
    - "kubernetes.io"
  containers:
  - command:
    - cat
    - /etc/hosts
    image: busybox
    name: busybox
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}

Create a Pod from this manifest,

pradeep@learnk8s$ kubectl create -f pod-with-hostalias.yaml 
pod/busybox created

Verify that the Pod is created

pradeep@learnk8s$ kubectl get pods
NAME      READY   STATUS      RESTARTS   AGE
busybox   0/1     Completed   0          6s

As we see Completed state, let us check the logs.

pradeep@learnk8s$ kubectl logs busybox
# Kubernetes-managed hosts file.
127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
fe00::0	ip6-mcastprefix
fe00::1	ip6-allnodes
fe00::2	ip6-allrouters
172.17.0.3	busybox

# Entries added by HostAliases.
127.0.0.1	pradeepgadde.com
192.168.1.1	kubernetes.io
pradeep@learnk8s$ 

We can see that, now, there are two sections , apart from the previous entries, we do see a new section called Entries added by HostAliases.

Also, the entry for its own IP is still there, like earlier.

pradeep@learnk8s$ kubectl get pods -o wide
NAME      READY   STATUS             RESTARTS       AGE    IP           NODE       NOMINATED NODE   READINESS GATES
busybox   0/1     CrashLoopBackOff   5 (105s ago)   5m3s   172.17.0.3   minikube   <none>           <none>

This confirms that hostAliases in the Pod specification is working.