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Detailed explanations of Kubernetes system concepts and abstractions.

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Adding entries to Pod /etc/hosts with HostAliases

Adding entries to a Pod’s /etc/hosts file provides Pod-level override of hostname resolution when DNS and other options are not applicable. In 1.7, users can add these custom entries with the HostAliases field in PodSpec.

Modification not using HostAliases is not suggested because the file is managed by Kubelet and can be overwritten on during Pod creation/restart.

Default Hosts File Content

Lets start an Nginx Pod which is assigned an Pod IP:

$ kubectl run nginx --image nginx --generator=run-pod/v1
pod "nginx" created

$ kubectl get pods --output=wide
NAME     READY     STATUS    RESTARTS   AGE    IP           NODE
nginx    1/1       Running   0          13s    10.200.0.4   worker0

The hosts file content would look like this:

$ kubectl exec nginx -- cat /etc/hosts
# 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
10.200.0.4	nginx

by default, the hosts file only includes ipv4 and ipv6 boilerplates like localhost and its own hostname.

Adding Additional Entries with HostAliases

In addition to the default boilerplate, we can add additional entries to the hosts file to resolve foo.local, bar.local to 127.0.0.1 and foo.remote, bar.remote to 10.1.2.3, we can by adding HostAliases to the Pod under .spec.hostAliases:

hostaliases-pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: hostaliases-pod
spec:
  restartPolicy: Never
  hostAliases:
  - ip: "127.0.0.1"
    hostnames:
    - "foo.local"
    - "bar.local"
  - ip: "10.1.2.3"
    hostnames:
    - "foo.remote"
    - "bar.remote"
  containers:
  - name: cat-hosts
    image: busybox
    command:
    - cat
    args:
    - "/etc/hosts"

This Pod can be started with the following commands:

$ kubectl apply -f hostaliases-pod.yaml
pod "hostaliases-pod" created

$ kubectl get pod -a -o=wide
NAME                           READY     STATUS      RESTARTS   AGE       IP              NODE
hostaliases-pod                0/1       Completed   0          6s        10.244.135.10   node3

The hosts file content would look like this:

$ kubectl logs hostaliases-pod
# 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
10.244.135.10	hostaliases-pod
127.0.0.1	foo.local
127.0.0.1	bar.local
10.1.2.3	foo.remote
10.1.2.3	bar.remote

With the additional entries specified at the bottom.

Limitations

HostAlias is only supported in 1.7+.

HostAlias support in 1.7 is limited to non-hostNetwork Pods because kubelet only manages the hosts file for non-hostNetwork Pods.

In 1.8, HostAlias is supported for all Pods regardless of network configuration.

Why Does Kubelet Manage the Hosts File?

Kubelet manages the hosts file for each container of the Pod to prevent Docker from modifying the file after the containers have already been started.

Because of the managed-nature of the file, any user-written content will be overwritten whenever the hosts file is remounted by Kubelet in the event of a container restart or a Pod reschedule. Thus, it is not suggested to modify the contents of the file.

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