roslaunch remote pc

edit

There are two PCs named tzh-pdl and alienware and they can ssh to each other successfully.

tzh@tzh-pdl:~$ ssh allen@alienware 
Welcome to Ubuntu 16.04 LTS (GNU/Linux 4.4.0-21-generic x86_64)
...
Last login: Mon Sep  5 22:13:30 2016 from 10.0.5.47
allen@alienware:~$

There is my launch file.

<launch>
   <machine name="wrsb" address="alienware" env-loader="/opt/ros/kinetic/env.sh" user="allen"/>
   <node machine="wrsb" name="listener" pkg="rospy_tutorials" type="listener.py"/>
   <node machine="wrsb" name="talker" pkg="rospy_tutorials" type="talker.py"/>
</launch>

I want to launch the node in alienware pc through roslaunch the launch file in tzh-pdl pc.But I got the problem.

tzh@tzh-pdl:~$ roslaunch catkin_ws/test.launch 
... logging to /home/tzh/.ros/log/d00dda00-7373-11e6-b5ad-7a51613db0d9/roslaunch-tzh-pdl-8639.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://tzh-pdl:45748/
remote[alienware-0] starting roslaunch
remote[alienware-0]: creating ssh connection to alienware:22, user[allen]
remote[alienware-0]: failed to launch on wrsb:

Unable to establish ssh connection to [allen@alienware:22]: Server u'alienware' not found in known_hosts

[alienware-0] killing on exit
unable to start remote roslaunch child: alienware-0
The traceback for the exception was written to the log file
tzh@tzh-pdl:~$

How can I fix it? Thanks a lot.