Make some documentation a bit more accurate.

[netwatch.git] / README

NetWatch!

	This is NetWatch, a system for remote system-management-mode-based
control of a machine without support from or awareness by the OS. It works by
taking over a second network card to provide a standard VNC server, such that
a machine elsewhere on the network can see the text or graphics console of the
machine and inject keystrokes as needed.

	System management mode, introduced with the 386SL, essentially allows
system driver code to run outside of OS control, caused by a special interrupt
pin on the CPU. This was originally intended for applications such as laptop
fan control; it is also the mechansim by which USB legacy keyboard emulation
occurs. When a system management interrupt occurs, the northbridge remaps
portions of memory to expose previously-hidden code, and asserts an SMI# signal,
causing the CPU to save all its state into system management RAM and vector to
a magic entry point.

	This is somewhat slow, and so there is a moderate performance impact
caused by running NetWatch, more significant when a VNC session is open.
Because NetWatch is invisible to the OS, its CPU usage is difficult to monitor;
we do so by comparing the MD5 throughput of the system with NetWatch
running versus without. The only way that the OS could detect this performance
drain is by spinning tightly and watching for a sudden jump in the CPU's time
stamp counters.

	Although it would be possible to start up NetWatch after an OS kernel
has already loaded, it is easier and more useful to load it from GRUB before
the OS boots, such that even the bootloader itself can be controlled over the
network. We do this by providing a stub loader (grubload/) which can be invoked
from GRUB, and takes care of loading the main NetWatch ELF image. Once this is
done and NetWatch is up and running, the loader returns to real mode and
reinvokes GRUB via the BIOS.

	Our current development platform, the Intel ICH2, does not allow SMM
traps on arbitrary PCI accesses. This makes stealing the network card from the
OS somewhat difficult, since there is nothing SMM code can do to cleanly block
access. NetWatch simply chooses its desired network card, and then repeatedly
clobbers the PCI base address registers. Although Linux resets the BARs to sane
values when it probes the PCI bus, by the time it attempts to actually load
the network driver, the card will no longer be accessible; fortunately, the 
driver quickly gives up, and Linux no longer attempts to access the card.

	The northbridge can be configured to invoke a system management
interrupt every 64 milliseconds, and so the bulk of NetWatch's work is done
from this interrupt: checking the network card for incoming packets, invoking
lwIP, and sending any response packets necessary. SMM entry also occurs when
when the OS reads from the keyboard I/O ports, to inject scan codes as needed.

	Much of NetWatch is very hardware-dependent, and although we've tried
to maintain clean interface separation to allow for easy porting, the current
implementation requires:

  - Intel ICH2 system chipset
  - 3C509 Ethernet card to be used by NetWatch, plus another card of
    any type for the OS
  - BIOS which does not set the D_LCK bit. Any system old enough to be
    based on the ICH2 is very likely to have a suitable BIOS.

	Current open issues are listed in the TODO file. See GUIDE for an
overview of which source files do what.