The parameter yarn.resourcemanager.webapp.address in yarn-default.xml is in "host:port" format,which is noted in the cluster set up guide (http://hadoop.apache.org/common/docs/r2.0.0-alpha/hadoop-yarn/hadoop-yarn-site/ClusterSetup.html).
When i read though the code,i find "host" format is also supported. In "host" format,the port will be random.
So we may add more documentation in yarn-default.xml for easy understood.
I will submit a patch if it's helpful.