I have a high-availability cluster (Heartbeat) connected via serial line and two ethernet NICs. I'd like to set up a monitoring script capable of recognizing disconnected serial line (basically the same question was answered at SO, however I am not satisfied with such a general answer).
I cannot simply open the serial device and read the data myself, since the serial line is opened by Heartbeat.
So I started to look for some indirect clues. The only difference I have found so far is in the contents of
/proc/tty/driver/serial. This is how it looks like when it's connected:
# cat /proc/tty/driver/serial serinfo:1.0 driver revision: 0: uart:16550A port:000003F8 irq:4 tx:2722759 rx:2718165 brk:1 RTS|CTS|DTR|DSR|CD
And when disconnected:
# cat /proc/tty/driver/serial serinfo:1.0 driver revision: 0: uart:16550A port:000003F8 irq:4 tx:2725233 rx:2720703 brk:1 RTS|DTR
I'm not confident enough to decide that the signals listed at the end of the line have the very meaning of connected/disconnected cable as I have not found any documentation on the contents of the /proc/tty/driver/serial. I can only assume that the presence of the signal means the given signal is on "right now" (or was in recent past? or?). The Serial HOWTO says that additional signals present when the cable is connected (CTS flow control signal, DSR "I'm ready to communicate", CD "Modem connected to another") are all in the "input" direction. So there has to be somebody alive at the other end.
Assuming that meaning of signals is as described in the Serial HOWTO, I can base my decision on the presence of, say CD signal. However I am not really sure.
So the question is: is my method "right", or do I have any better options I am not aware of?
EDIT: I did some additional observations and had a talk with my colleague. Turns out the presence or absence of signals at the end of the line is quite good indicator of the serial port activity, on both ends. However, it's not an indicator of physical presence of a cable. Whenever there was a program writing to serial port outgoing signals were present (RTS|DTR). When the other side was writing incoming signals were present (CTS|DSR|CD). When none of the sides communicates there are no signals at all (that does not necessarily mean there is no cable present). Don't forget that the exact signals depend on the wiring of the cable (I have "null modem with partial handshaking").