Planetlab Ping-of-Death (POD)
-----------------------------
The Planetlab POD is a Linux kernel patch that creates the feature
of resetting and rebooting a computer when a particularily
formatted ICMP "ping" commmand is received.

A PlanetLab node's kernel gets a host, mask and hash value through
sysctl's that are performed on the node.  The node then looks
for an ICMP packet from a host matching the host/mask and, if the
packet payload is the hash value, the node forces an immediate
reboot.

From a controlling host's side, one invokes commands on the
remote node to set the host, mask and hash values and remembering
the hash value.  When the rebooting of a remote node is required,
the remembered hash value is looked up and sent to the node
in a specially constructed ICMP packet.

This POD system uses the same packet definition as EMULAB and uses
the some code from EMULAB to generate and transmit the packet.
No reason to invent something new.

The remote execution (ssh commands) are all done as "idsl1".

Contolling host tools:
pl-podset:  set the host and hash on a remote node
        Call: pl-podset hostIP maskIP targetName
        Uses ssh to call 'pl-podcntl' on the remote node to
        set the host and hash in that node.  Also saves the
        information into the file './HashFile'.
pl-podzap: send ICMP packet to node to resetit
        Call: pl-podzap targetName
        Fetches parameters from the file './HashFile' and
        calls 'pl-poddoit' on the 'hostIP' machine to send the 
        reset ICMP packet to the specified target node.
        Note that this relies on the planetlab node that
        was specified in the 'pl-podset' command to actually
        do the packet transmission.
pl-poddoit: send the ICMP packet (called by 'pl-podzap')
        Routine that constructs an ICMP packet of the proper
        format and send it to the node.

Planetlab node scripts:
pl-podcntl: set the host and hash into the kernel
        Call: pl-podcntl hostIP maskIP hashValue
        Sets the specified values into the kernel and enables
        the POD filtering operation.

EXAMPLE
-------
Setting a node to be able to reset (assuming running on 123.123.12.23):
    pl-podset 123.123.12.23 255.255.255.255 planetlab-1.stanford.edu

To later reset the node (command performed on 123.123.12.23):
    pl-podzap planetlab-1.stanford.edu