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Adding entries to Pod /etc/hosts with HostAliases |
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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 a 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:
{{< code file="hostaliases-pod.yaml" >}}
This Pod can be started with the following commands:
$ kubectl apply -f hostaliases-pod.yaml
pod "hostaliases-pod" created
$ kubectl get pod -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.