DNS Monitor Using NS Lookup

Problem this snippet solves:

This monitor uses nslookup to verify an expected response from load balanced DNS servers. It accepts 2 command-line arguments: the name to be looked up, and a string found in the expected response.

NOTE: Use external monitors only when a built-in monitor won't do the trick. This example is intended to demonstrate the basic use of nslookup in an external monitor to test UDP-based name services.

How to use this snippet:


  1. Create a new file containing the code below on the LTM filesystem. Recommended location is /config/eav. Permissions on the file must be 700 or better, giving root rwx access to the file. 2. Create a monitor profile of type "External" with the following values:

    • External Program: . . the name of the script file created in step 1
    • Arguments:
    • * "" ""
    • 3. Adjust the interval and timeout as appropriate for your application

Code :

# (c) Copyright 1996-2007 F5 Networks, Inc.
# This software is confidential and may contain trade secrets that are the
# property of F5 Networks, Inc.  No part of the software may be disclosed
# to other parties without the express written consent of F5 Networks, Inc.
# It is against the law to copy the software.  No part of the software may
# be reproduced, transmitted, or distributed in any form or by any means,
# electronic or mechanical, including photocopying, recording, or information
# storage and retrieval systems, for any purpose without the express written
# permission of F5 Networks, Inc.  Our services are only available for legal
# users of the program, for instance in the event that we extend our services
# by offering the updating of files via the Internet.
# these arguments supplied automatically for all external monitors:
# $1 = IP (nnn.nnn.nnn.nnn notation or hostname)
# $2 = port (decimal, host byte order) -- not used in this monitor, assumes default port 53
# $3 = name to be looked up
# $4 = string in expected response

node_ip=`echo $1 | sed 's/::ffff://'`

pidfile="/var/run/`basename $0`.$node_ip..$2.pid"
if [ -f $pidfile ]
   kill -9 `cat $pidfile` > /dev/null 2>&1
echo "$$" > $pidfile

nslookup ${3} $node_ip 2>/dev/null | grep -qi "${4}"

if [ $status -eq 0 ]
    # Need to complete any cleanup activity before sending anything to STDOUT as the script is stopped then.
    rm -f $pidfile
    echo "UP"
   rm -f $pidfile
Published Mar 12, 2015
Version 1.0

Was this article helpful?

No CommentsBe the first to comment