I thought a useful addition to our blog might be a running series of handy scripts.  Just for the sake of things (and because I couldn’t find a better title), I’m calling the series “David’s Handy Scripts”.  I’ll get this show on the road with a quick script I put together to track 2 things that can get you into a mess if they’re not monitored – disk usage and bandwidth usage.

There are easier ways to track bandwidth usage (vnstat comes to mind), but where’s the fun in that?  I’m using iptables to track packets.  You can do a lot with iptables and it’s really quite simple to track bandwidth by port/protocol if you wanted some granularity.

We need to setup a few rules to record our stats.  Execute each of these:

iptables -N output
iptables -A OUTPUT -j output
iptables -N input
iptables -A INPUT -j input

You can pop those 4 commands in /etc/rc.local to make them persistent or integrate them into any existing firewall ruleset you’re currently using.  After those are in place, you’re basically all set.  Just edit the top of the following script to indicate when you want to receive alerts.  Pop that in a cronjob to run daily and you’ll get an alert when those thresholds are exceeded.

#!/bin/bash
#####
# script to monitor badnwidth and disk space usage
# bandwidth - send alert if usage > X
# disk - send alert if used space > Y
#####

MAILTO="address@example.com"
HOSTNAME=`hostname`

#BANDWIDTH THRESHOLD
BANDWIDTHLIMIT=42949672960 #40GB (1 GB = 1073741824 bytes)

#DISK SPACE THRESHOLD
USEDDISKLIMIT=85 #80%

###
# BANDWIDTH CHECK

#reset counter if it is first of month & create log if it does not exist
let DATE=`date +%d`
if [ $DATE -eq 0 ] || [ ! -e /root/bandwidth.log ]; then
echo "0" > /root/bandwidth.log
fi

#calculate usage
let OLDTOTAL=`cat /root/bandwidth.log`
let IN=`iptables -L INPUT -v -x | grep input | awk '{print $2}'`
let OUT=`iptables -L OUTPUT -v -x | grep output | awk '{print $2}'`
let TOTAL=IN+OUT
let NEWTOTAL=OLDTOTAL+$TOTAL

#write updated total out to counter log
echo $NEWTOTAL > /root/bandwidth.log

#check if total exceeds our warning threshold and send alert if it does
if [ $NEWTOTAL -gt $BANDWIDTHLIMIT ]; then
echo $HOSTNAME "$NEWTOTAL bytes used this month" | mail -s "$HOSTNAME bandwidth warning" $MAILTO
fi

#clear counters
iptables -L INPUT -Z -v > /dev/null
iptables -L OUTPUT -Z -v > /dev/null

###
# DISK UTILIZATION CHECK

#grab disk usage
let DISKUSAGE=`df | grep xvda1 | awk '{print $5}' | cut -d'%' -f1`

#check if usage exceeds threshold and send alert if it does
if [ $DISKUSAGE -gt $USEDDISKLIMIT ]; then
echo $HOSTNAME "current disk usage - $DISKUSAGE%" | mail -s "$HOSTNAME disk usage warning" $MAILTO
fi

exit 0