Hi again, :-)
Since the values of /proc/net/dev on 32-bit Linux systems are 32-bit, the values easily overflow and rollover... about 40 seconds for a 1 GB interface.
The attached patch monitors the delta of the /proc/net/dev entries and adds 2**32 if it has rolled over and sums the difference to the "now" value. 64-bit values should continue to work.
As long as a daemon delay of 30 is used a 1 GB interfaces can be monitored.
If there was a way to call "ethtool -S eth0" instead then it could be used to obtain all of the Linux net statistics values, but given this is in libmonit that probably isn't an option.
While this may not be an ideal solution, is there a better fix ?