docker-compose problem

gistfile1.txt

given

.
|-- docker-compose.yml
|-- node1
|   `-- Dockerfile
|-- node2
|   `-- Dockerfile


and Dockerfile as

FROM centos:centos6.6

docker-compose.yml

version: '2'
services: 
   dn1: 
    build: ./node1/.
    hostname: dn1.foo.com
    command: tail -f /dev/null
   dn2: 
    build: ./node2/.
    hostname: dn2.foo.com
    command: tail -f /dev/null
    
then

docker-compose up -d
docker exec -it test_dn1 bash
$ hostname
 dn1.foo.com
$ ping dn2.foo.com
 PING dn2.foo.com (23.x.x.122) 
$ ping dn1
 64 bytes from test_dn2_1.test_default (172.21.0.3) ....
 
 how to make dn1 resolve dn2 via its hostname dn2.foo.com ?