Re: [Linux-decnet-user] Linux DECNet & Termnal servers

[Patrick C. <pa...@ty...>]

On Tue, Jul 08, 2003 at 03:05:31PM +0100, Rich Buckley wrote:
> Patrick wrote:
> > If you really want to intercept packets and do nefarious things with them
> then
> > you could write a PF_PACKET program to advertise a LAT service from the
> Linux
> > box that forwards the information (after suitable NATing) to the terminal
> > server.
> 
> This is what I want to do. I would like the ability to "understand" what the
> VAX
> and VT terminal are saying to each other and periodically inject information
> into
> the data stream. The old fashioned way of doing this would be to feed the
> terminal
> connection through two serial ports and sniff. This is however not very
> scalable
> when dealing with a large number of terminals.

The PF_PACKET way is the "neatest" but probably the hardest to code because you
would need to understand quite a lot of the LAT protocol in the program.

What might be OK is for you to write a small program that simply behaves as a
LAT service on Linux and forwards the information either into llogin or a
/dev/lat port to VMS. That way you only have to deal with the raw data and not
the protocol. Plus you get a copy of the filter per user, making it easier to 
keep a track of any context you may need.

latcp -A -a myservice -C /usr/local/bin/myfilter -u someuser

is the LAT command to create custom service, all your myfilter program has
to do then is to fork llogin to the VMS box and watch the packets go to-and-fro,
changing them as necessary.

I recommend you use the -u switch for this command otherwise your filter will
run as root and that's asking for trouble :-)

patrick