Adding entries to Pod /etc/hosts with HostAliases

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

Start an Nginx Pod which is assigned a Pod IP:

pod/nginx created

Examine a 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:

# 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
10.200.0.4    nginx

In addition to the default boilerplate, you can add additional entries to the hosts file. For example: to resolve foo.local, to 127.0.0.1 and foo.remote, bar.remote to 10.1.2.3, you can configure HostAliases for a Pod under .spec.hostAliases:

service/networking/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"
    - "foo.remote"
    - "bar.remote"
  containers:
  - name: cat-hosts
    image: busybox
    command:
    - cat
    args:
    - "/etc/hosts"

You can start a Pod with that configuration by running:

kubectl apply -f https://k8s.io/examples/service/networking/hostaliases-pod.yaml

Examine a Pod’s details to see its IPv4 address and its status:

kubectl get pod --output=wide

NAME                           READY     STATUS      RESTARTS   AGE       IP              NODE
hostaliases-pod                0/1       Completed   0          6s        10.200.0.5      worker0

with the additional entries specified at the bottom.

The kubelet the file for each container of the Pod to prevent Docker from modifying the file after the containers have already been started.

Caution:

Avoid making manual changes to the hosts file inside a container.