Wednesday, May 19, 2010

Capturing packets from an Interface in a MIB

I had to use pcap to capture packets arriving at a particular interface. For that the snmp daemon needs to run as root. You can configure the options for starting up the snmpd program in /etc/default/snmpd. What's not cool, though, is trying to start it up and shut it down. It seems that nanosleep on Linux doesn't work, or rather it only works with a resolution of 1 second. No matter how many nanoseconds you specify nanosleep doesn't sleep until you fill in the tv_sec field. This is, however, more annoying than a show-stopper. I can now start packet capture via pcap_loop and kill it with pcap_breakloop. Stopping takes a few seconds but mostly works. So now we have a MIB that actually detects DDoS attacks!!!

Thursday, May 13, 2010

Average CPU usage for a terminating process

Someone asked me if we can easily compute the average CPU usage of a particular program over its lifetime when the program is scheduled to terminate at some point in the future. At that time it should print out the result. Here's what I came up with:

# get average CPU for a process
pid=`pgrep "$1"`
while [ -n "$pid" ]; do
temp=`ps -eo pid,pcpu | grep $pid | awk '{print $2}'`
total=`echo "scale=2;$total+$temp" | bc`
sleep 1
pid=`pgrep "$1"`
average=`echo "scale=2;$total/$times" | bc`
echo "Average cpu usage for $1 is: $average"

To run it give it an argument, which should be the name of the process or part or all of the command that launched it, and background the process, e.g.:

./ "java myprogram" &

So, when "java myprogram" terminates, this script tells me its average CPU usage.