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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 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 the 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:
  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"

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
10.200.0.4	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

As of 1.7, Pods with hostNetwork enabled will not be able to use this feature. This is because kubelet only manages the hosts file for non-hostNetwork Pods. There are ongoing discussions to change this.

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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