demasi@paisano.UUCP (Michael C. De Masi) (01/28/88)
Hello Again,
First of all, I'd like to thank everybody for their kind
responses to my original posting. If you remember, the
question involved the possibility of using the regular
I/O ports from a standard 3b (or, I suppose, any reasonable
Unix* machine) to monitor the console ports of other similar
machines in situations where a one console/machine ratio was
impossible due to space considerations. Additional requirements
were that the monitoring machine should keep a log of any
output from the monitored machine's console, and that the
solution should not involve any special hardware or radical
changes to either machine's software or configuration.
The responses I received were many, and about evenly split
between ideas for performing the task and requests for any
solution that came in. Many of the solutions received
were quite feasible, but either involved hardware I did not
have or proprietary software I did not intend to buy. Some
of the solutions were extremely complex, and others either
did not really hit the mark, performance wise.
The best solution, of course, was very simple. In the '/etc/inittab'
file of the monitoring machine, install the line:
XX:2:respawn:sh -c '(/bin/stty 9600 evenp;/bin/cat) </dev/ttyXX >>LOGFILE 2>/dev/null'
where 'XX' is replaced by the port number to which you will
attach the wire from the monitored machine's console and
'LOGFILE' is replaced by the full path name of a file, writable
by 'root' on the monitoring machine, which you choose to hold
the output from the monitored console port.
The trick here is to open the port simultaneously for both the
'getty' which sets the port for the proper options and the 'cat'
which redirects the input from the port to the log file. The
generic 'open' issued by shell will wait until it can return
a useful file descriptor (Ie, until an operational console port
from a running machine gets plugged into the appropriate I/O
port on the monitoring machine) before executing the 'getty'
and 'cat'. 'cat' will run, redirecting the output from the
monitored console to the logfile until it reaches an EOF
condition (the connection gets broken, the monitored machine
goes down), and the process respawns. The 'sh -c' format of
the line is necessary to keep 'init' from going into a rapid
respawn error if the connection goes down. Simple, no?
In fact, it is this feature which allows the further generation
of the 'inittab' entry to be written:
61:2:respawn:sh -c '(/bin/stty 9600 evenp;/bin/echo "\n\nRESUME: \c";/bin/date;/bin/cat;/bin/echo "\n\nBREAK: \c";/bin/date) </dev/tty61 >>/usr4/console/log 2>/dev/null'
which in addition to keeping a log of the monitored console's
output, also adds a nice trail of informative records and time
stamps to the collected data every time the connection gets
broken or the monitored machine goes down. This should also
give you an idea what the real 'inittab' entry should look like.
The only modification on the monitored machine that might be
necessary would be to adjust '/etc/gettydefs' to loop around
9600 Baud at the console entry, to keep the two machines from
going out of synch.
Of course, this setup is only good for readonly monitoring,
as attaching to the monitored console in interactive mode
through the monitoring machine is presently not possible.
Since our consoles are all fed through a patch panel and
hard disconnects/reconnects are easy, this is not a big
problem. If anybody has any great ideas about how one
might get around this, given some variation on the above
stated theme, please post. (I tried doing a 'cu' directly
to the I/O port, but since the 'cat' is still running, the
I/O gets garbled. Kill the 'cat', it immediately respawns.)
Special thanks go to Tony Hansen, AT&T-IS Labs and
Don Ritchie, AT&T Bell Labs (Gee, those guys at AT&T are smart! :-)
who gave me the above ideas.
Until we meet again,
Michael C. De Masi - AT&T Communications (For whom I work and not speak)
2340 Dulles Corner Blvd. Herndon, Virginia 22071 Phone: 703-834-8123
UUCP: decuac!grebyn!paisano!demasi
"All things considered, I'd rather be in Philadelphia" - W C Fields
*Unix is a registered trademark of AT&T - Bell Laboratories