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. You can add these custom entries with the HostAliases field in PodSpec.
Modification not using HostAliases is not suggested because the file is managed by the kubelet and can be overwritten on during Pod creation/restart.
Default hosts file content
Start an Nginx Pod which is assigned a Pod IP:
kubectl run nginx --image nginx
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:
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, you can add additional entries to the
For example: to resolve
10.1.2.3, you can configure HostAliases for a Pod under
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"
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
hosts file content looks 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 10.200.0.5 hostaliases-pod # Entries added by HostAliases. 127.0.0.1 foo.local bar.local 10.1.2.3 foo.remote bar.remote
with the additional entries specified at the bottom.
Why does the kubelet manage the hosts file?
The kubelet manages the
hosts file for each container of the Pod to prevent the container runtime from
modifying the file after the containers have already been started.
Historically, Kubernetes always used Docker Engine as its container runtime, and Docker Engine would
then modify the
/etc/hosts file after each container had started.
Current Kubernetes can use a variety of container runtimes; even so, the kubelet manages the hosts file within each container so that the outcome is as intended regardless of which container runtime you use.
Avoid making manual changes to the hosts file inside a container.
If you make manual changes to the hosts file, those changes are lost when the container exits.