Monitoring an ADSL link using a simple script

The script obtains the default gateway from the tun0 interface and attempts to ping it. If the ping fails it will ping a secondary IP address which you hard code in the script. If both these tests fail a certain number of times (2 by default) it restarts the ppp daemon. It will keep bouncing ppp until the ping test works again.

The script keeps a count of how many times the ping test failed so when the ping test works it can send you an email to let you know what happened and how many times the test failed. By default this email goes to root. Based on how often you're running the script from cron you'll be able to tell the length of the outage from this email.

The following script should be installed as pingmonitor.sh in the /usr/local/etc/ directory.

#!/bin/sh

#
# This script tests for network connectivity and restarts ppp if it is found
# to be down.
#

# --- User-modifiable variables ---

# Use this IP address if the primary address cannot be determined
secondaryaddr="139.134.2.2"

# Number of failed pings required to signify link failure
failedtrigger=2

# File to keep track of total number of failed pings
failedcountfile="/usr/local/etc/pingmonitor.missedping.count"

# Email address to send reports to
emailaccount="root"

# Set to the appropriate label in /etc/ppp/ppp.conf
isp="bigpond"

# These options will be given to /usr/sbin/ppp
ppp_opts="-quiet -ddial -nat"

# --- End of user-modifiable variables ---

# Load in system configuration.
if [ -f /etc/defaults/rc.conf ]; then
        . /etc/defaults/rc.conf
        source_rc_confs
elif [ -f /etc/rc.conf ]; then
        . /etc/rc.conf
fi

# only continue if the ppp link should be up
if [ ! $ppp_enable ]; then
        # PPP is not configured in the rc files.
        exit 0
elif [ "$ppp_enable" = "NO" ] || [ "$ppp_enable" = "no" ]; then
        # PPP is not wanted
        exit 0
fi

# Set umask
umask 137

# Determine the default gateway for the ADSL link
primaryaddr=`ifconfig tun0 | grep 'inet ' | grep -v 255.255.255.255 | tail -1 | cut -f 2 -d '>' | cut -f 2 -d ' '`

if [ "$primaryaddr" = "" ]; then
        primaryaddr="0.0.0.0"
        secondaryaddr="0.0.0.0"
fi


# Check if we've had any previous failures
if [ -f $failedcountfile ]; then
        pingfailed=`head -n 1 $failedcountfile`
else
        pingfailed=0
fi


# Run the ping for the primary address - our default gateway.
/sbin/ping -c 5 -t 5 -q -m 2 $primaryaddr > /dev/null 2> /dev/null


# If the ping failed. Check to see if the gateway is filtered.
if [ $? -ne 0 ]; then
        # Try to pull a TTL EXCEEDED message from the gateway.
        if [ `/sbin/ping -c 1 -m 0 -n -t 1 $primaryaddr 2> /dev/null | grep -i "time to live exceeded" | grep $primaryaddr | wc -l` -eq 1 ]; then
                ping_error=0
        else
                ping_error=1
        fi
else
        # No filtering. The default gateway responded to the initial ping.
        ping_error=0
fi


#
# Ping returns a non-zero error condition if ALL the ECHO_REQUEST packets did
# not return a ECHO_REPLY. If we received just one answer then ping returns
# a zero error condition which is perfect for our tests.
#


# Check the ping status and try pinging the secondary address if it failed
if [ $ping_error -ne 0 ]; then
        /sbin/ping -c 5 -t 5 -q -m 7 $secondaryaddr > /dev/null 2> /dev/null

        if [ $? -eq 0 ]; then
                ping_error=0
        else
                ping_error=1
        fi
fi


# Test the error condition
if [ $ping_error -ne 0 ]; then
        # Update and record the failure count
        pingfailed=$(($pingfailed + 1))
        echo $pingfailed > $failedcountfile

        # Test if we've hit our failure trigger
        if [ $pingfailed -ge $failedtrigger ]; then
                # time to restart ppp so kill it first
                /usr/bin/killall ppp > /dev/null 2> /dev/null

                # wait for it to die
                sleep 5

                # really ensure ppp is dead - we can't risk two running
                /usr/bin/killall -9 ppp > /dev/null 2> /dev/null

                # wait again
                sleep 5

                # start up ppp again
                /usr/sbin/ppp $ppp_opts $isp > /dev/null 2> /dev/null

                # our work here is done
        fi
else
        # the ping worked so check if we've just recovered from a failure
        if [ $pingfailed -ge $failedtrigger ]; then

                # we have just recovered so let the admin know
                echo "PING test failed $pingfailed times" | /usr/bin/mail -s "PPP restart on `/bin/hostname -s` at `date '+%H:%M %d/%m/%y'`" $emailaccount > /dev/null 2> /dev/null
        fi


        # all's well now so remove the failure count
        rm -f $failedcountfile > /dev/null 2> /dev/null
fi


# that's it

Use the following steps to modify the script for use:

The script should be run periodically using cron. Add the following entry to /etc/crontab:

# PING Monitor - check the ADSL connection every two minutes
0-59/2  *       *       *       *       root     /usr/local/etc/pingmonitor.sh

Restart cron by running: kill -HUP cron.

Test the script. Try pulling the phone line from the back of the modem or such for enough time to trigger the script.