NameNode UI only listens on HOSTNAME:9870

The UI of the NameNode is only accessible via http://HOSTNAME:9870/ but not localhost or 127.0.0.1.

Port-forwarding with kubectl does not work and proxies, like the oauth-proxy, must specify the hostname.

NameNode logs:

INFO  hdfs.DFSUtil (DFSUtil.java:httpServerTemplateForNNAndJN(1716)) - Starting Web-server for hdfs at: http://simple-hdfs-namenode-default-0.simple-hdfs-namenode-default.default.svc.cluster.local:9870
INFO  http.HttpServer2 (HttpServer2.java:bindListener(1355)) - Jetty bound to port 9870

Trying to access the UI on 127.0.0.1:

$ kubectl exec -ti simple-hdfs-namenode-default-0 -c namenode -- curl -L -I http://127.0.0.1:9870/
curl: (7) Failed to connect to 127.0.0.1 port 9870: Connection refused

On the other hand, the UI of the DataNode listens on 0.0.0.0:

INFO  web.DatanodeHttpServer (DatanodeHttpServer.java:start(318)) - Listening HTTP traffic on /0.0.0.0:9864

and can be accessed via 127.0.0.1:

$ kubectl exec -ti simple-hdfs-datanode-default-0 -c datanode -- curl -L -I http://127.0.0.1:9864/
HTTP/1.1 302 Found

Start with the NameNode HTTP setup around DFSUtil.httpServerTemplateForNNAndJN and HttpServer2.bindListener, then compare it with the DataNode behavior reported from DatanodeHttpServer.start. Reproduce the requests to 127.0.0.1:9870 and the service hostname, and consider the issue done when the NameNode UI accepts localhost access and kubectl port-forwarding without requiring the pod hostname.