[TriLUG] dhclient and iptables

[Greg Brown]

I've got a script that does that.  It monitors eth0 by default.  Just 
copy the ip address that you are currently using to a file named 
"currentIP" and change /home/ghome/scripts to wherever you want to 
store the script currentIP and the script and you're good to go.  
Here's the script:

#!/bin/bash

###############################################################
#
# testIP.sh
# version 1.0
# Greg Brown
#
# watches the external interface on my cable modem attached
# box (eth0) and compaires the IP address to what is stored
# in the flat file /home/ghome/scripts/currentIP.  If a change
# has been detected it sends an e-mail to me at work and
# at home so I can change my dynamic hostname resolution.
#
# June 12, 2003
#
# version 1.1
#
# Added a test to make sure that the results caputred in
# $IPADDRESS actually contained something.  If the results
# in $IPADDRESS are null we now exit the script without
# processesing further.
#
# June 13, 2003
#
###############################################################

# get the current ip address on the system
IPADDRESS=`/sbin/ifconfig eth0 | grep inet | tr ':' ' ' | awk '{ print 
$3 }'`

# test line
# echo "current ip $IPADDRESS"

# now use test to see if $IPADDRESS has a value stored in it
/usr/bin/test $IPADDRESS

# now check the results of $? which is were the results of the above
# test are stored.  If the result is 1 then it is null, if the result
# is 0 then it is NOT null
if /usr/bin/test $? = 1
then

	# debug line	
	# echo "we don't have a IP address stored in $IPADDRESS"

	# we don't have an ip address so exit the script
	# we can assume the interface is down and since there is no
	# point in sending an e-mail - we can't without the inteface
	# being up - well, just exit.
	exit

fi

# debug line
# echo "we made it past the first test"

# now get the last known IP address
LASTIP=`/bin/cat /home/ghome/scripts/currentIP`

# echo "lastip $LASTIP"

# now test to see if the IP addresses match
if /usr/bin/test "$IPADDRESS" = "$LASTIP"
then

	# echo "we have the same ip address"
	# echo "last: $LASTIP current: $IPADDRESS"
	# do nothing, we have matching IP addresses
	# just exit
	exit	

else

	# echo "we do not have matching IPs"
	# echo "last: $LASTIP current: $IPADDRESS"
	
	# we do NOT have matching IP addresses so
	# we have to overwrite the currentIP file
	echo $IPADDRESS >/home/ghome/scripts/currentIP

	# get the hostname for the e-mail
	HOSTN=`/bin/hostname`

	# and get the time as well
	DANDT=`/bin/date`

	# and we have to notify Greg what has happened
	# create a file with the info we need to send
	# the email
	echo "$DANDT" >/tmp/$$
	echo "$HOSTN has changed IP addresses" >>/tmp/$$
	echo "IP was $LASTIP" >>/tmp/$$
	echo "IP is now $IPADDRESS" >>/tmp/$$

	# now mail the file
	/bin/mail -s "$HOSTN has changed IP Addresses" 
gregbrown at mindspring.com </tmp/$$

	# now remove /tmp/$$
	/bin/rm /tmp/$$

	# and we're done
	exit

fi