discovery.zen.ping.multicast.enabled: false
discovery.zen.ping.unicast.hosts: ["xy2.compute.amazonaws.com"]
# and on other node use:
# discovery.zen.ping.unicast.hosts: ["xy1.compute.amazonaws.com"]

# to use ec2 api (you need the security keys!) for discovery you need to install the aws plugin
# read
# http://www.elasticsearch.org/tutorials/2011/08/22/elasticsearch-on-ec2.html
# http://www.elasticsearch.org/guide/reference/modules/discovery/ec2.html