UNCA Computer Science Linux Playpen

The purpose of this page is to explain how to access the "playpen," that collection of four computers at the back of the Robinson 004 lab. The playpen provides a place for students in upper-level courses to perform system experiments that cannot be performed on "normal" lab computers which must be tightly managed for users in introductory courses.

Although students may be granted privileged system access within the playpen, they still must be very careful. If a student performs an operation that renders a computer incapable of booting, this will certainly have an impact on fellow students.

Direct physical access

There is only one monitor for the four computers. It is assumed that most access to the computers will be over the network. If you need to access them at the console, you'll have to use the KVM switch next to the monitor to select the desired computer. You'll probably be disappointed with the result. When running Linux, these computers do not run the graphical X11 display on the console. So if you want to use the mouse or see pictures, you'll need to connect to the systems remotely.

Network access

The four computers of the playpen are not directly connected to the UNCA Computer Science network. Instead the four are connected to a router that provides network connectivity via NAT, Network Address Translation. This means that the four computers share a single IP address and IP name, playpen.cs.unca.edu, to the outside "world." It also means that the only server applications that can be seen from the outside are those explicitly enabled in the router. One thing that is absolutely verboten is fooling with the network connections to these four computers or the router.

You connect to the computers using ssh, secure shell. Although the computers share a common IP address, each runs its secure shell daemon at a different TCP port to allow connectivity to the individual computers. The names and ports used by each computer is given in the following table.

Computer Port
barnard 22010
broadway 22011
merrimon 22012
wtweaver 22013

Since the domain name of the NAT router is playpen.cs.unca.edu, you can connect to these computers with an ssh command similar to the following:
  ssh -X -p portnum playpen.cs.unca.edu
The -X option enables X11 forwarding and only needed it if you want to start graphical applications on the playpen computer.

Unfortunately there is one potential problem. The four computers have different keys for SSH, and some SSH clients object strongly when they see a second key from the same computer. This can happen with the Linux ssh, because ssh which assumes the four computers are really just one computer named playpen.cs.unca.edu. The easiest solution for this is to only connect to one of the four from the "outside". If you need to go the a second computer, then make the connection on the "inside". There are other solutions, but they aren't much better. By the way, PuTTY, the popular Windows SSH client has no problem with seeing different keys from the same computer.