Yarn web UI is not running

I'm working with Hadoop 2.9 with a very minimal configuration. In particular, I didn't set any ports, everything is running with default ports.

When I start Hadoop with the command start-all.sh, the Yarn ResourceManager and NodeManager are running, according to the result of command ps -ef | grep yarn.

Yet the Yarn web UI doesn't seem to be running as netstat -l returns no port 8088 listening. I haven't figured out how it is supposed to start. Is there a configuration parameter that will fire it ?