On Fri, 3 Apr 2015, Hans Petter Selasky wrote:
Will you mind if I rephrase that paragraph in the "inet.4" manual page from:
"This closes a minor information leak which allows remote observers to
determine the rate of packet generation on the machine by watching the
counter."
Into:
"This prevents high-speed information exchange between internal and external
observers using packet frequency modulation. An outside observer can ping
the outside facing port at a fixed rate watching the counter. An inside
observer can ping the inside facing port watching the same counter. Even
though packets don't flow between the two ports, data can be exchanged by
watching changes in the packet rate. It is believed that data can be
exchanged in Kb/s range this way. Setting this sysctl also prevents remote
and internal observers to determine the rate of packet generation on the
machine by watching the counter."
Yes, I think this is overly alarmist, and it suggests that other covert
channels might not exist to be exploited if the knob is set -- which isn't
true. We don't promise that there are no covert channels in FreeBSD, and we
would be foolish if we did promise that.
Robert
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