andy@rbdc.UUCP (Andy Pitts) (01/25/88)
There have been many articles addressing problems with Hayes compatible
modems and getty wars, but so far no one has noted the most insidious
bug of Hayes type modems used for dial in. The bug goes like this:
Someone using a Hayes modem call into your system, logs in and decides
to place his modem in command mode. He types "+++" which his modem sends
at your system (before going into command mode), your system echo the
"+++" back at your modem putting it in command mode. Now both modems
are in command mode. If the modem at the computer is set to echo characters,
whatever is running (sh or getty) will start talking to itself. Worse
when the user goes back on line he finds the system "dead" as there is no way
to get the modem back on line. All that can be done is hang up and hope
the modem will drop DCD and will reset to a sane mode when the computer drops
DTR.
The fix is to configure the modem so it won't go into command mode.
Some modems have a switch for this. For others setting register S7 to
something like 255 will do it.
The upshot of this is, if you are using a Hayes compatible for a dial in
modem, it Must be configured so it cannot go into command mode while
connected.
Andy Pitts andy@rbdc.UUCP {bakerst, gladys}!rbdc!andy
A line hit is another form of character assassination.