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From: campbell@maynard.UUCP (Larry Campbell)
Newsgroups: net.unix
Subject: Re: unix system enhancements
Message-ID: <359@maynard.UUCP>
Date: Sat, 4-Oct-86 16:18:37 EDT
Article-I.D.: maynard.359
Posted: Sat Oct  4 16:18:37 1986
Date-Received: Mon, 6-Oct-86 18:49:30 EDT
References: <4119@brl-smoke.ARPA> <702@hropus.UUCP> <1986Oct1.180413.29885@sq.uucp>
Reply-To: campbell@maynard.UUCP (Larry Campbell)
Organization: The Boston Software Works Inc., Maynard, MA
Lines: 40
Summary: Let's learn from TOPS-2O before it's gone forever

In article <702@hropus.UUCP> jrw@hropus.UUCP (Jim Webb) writes:
>> 	3.  And lastly modify the tty driver to support "advising."  This would
>> 	allow root to watch the characters sent to a terminal and have the
>> 	characters that he types inserted into that port's input character
>> 	stream.
>
>	A hackers' delight, a simple rm -rf will remove this "advising" :-)

Of course such a capability would be restricted to the root.  One of
TOPS-20s useful and, as far as I know, unique features is that of
terminal "linking".  The output queue for a terminal device may be
"linked" to the output queues of up to four other terminal devices.
When a link exists, all characters entered into the queue are also
entered into the output queue of the other terminal(s).

Whether a terminal can be linked to depends on a bit that the user
controls;  the default is to give permission.  The commands ACCEPT LINKS
and REFUSE LINKS control this bit.  You would typically say REFUSE LINKS
if you're typing out something confidential, or just don't want to be
bothered by requests for help, etc.  Of course, a privileged user can
link regardless of the state of the permission bit.

Most links are two-way, although a popular program called SPY allows
a privileged user to spy silently on someone else's terminal session
(very useful for catching system crackers in the act!).

That's the output side.  On the input side, the STI% (Simulate Terminal
Input) system call allows a suitably privileged process to insert
characters in the input stream of a terminal.  This is used to
implement the ADVISE command, which links two terminals, and copies
all characters typed on the advisor's terminal to the advisee's
terminal input stream.  Of course, this requires privileges.  It is
very useful in demonstrating things to beginners.  Demonstrating video
editors and such requires that the two users be on similar terminals,
but since most TOPS-20 users (and even most UNIX users?) are on VT100
compatible terminals, this isn't a problem in practice.
-- 
Larry Campbell                             The Boston Software Works, Inc.
ARPA: campbell%maynard.uucp@harvard.ARPA   120 Fulton Street, Boston MA 02109
UUCP: {alliant,wjh12}!maynard!campbell     (617) 367-6846