Script to perform DNS lookup and email if it fails

116Fanatic · 02-24-2014, 10:17 AM

I have been searching for a script that will be hosted on an external network that performs a DNS lookup (put it in a cron job) and if it fails it will send an email alert. I plan to use a google account for smtp. Does anyone have a script that would do this?

Thank you!

TB0ne · 02-24-2014, 10:44 AM

Quote:

Originally Posted by 116Fanatic

I have been searching for a script that will be hosted on an external network that performs a DNS lookup (put it in a cron job) and if it fails it will send an email alert. I plan to use a google account for smtp. Does anyone have a script that would do this?

Thank you!

Well, if you can't find something on Google that does what you want, post what you have written/tried on your own, and tell us where you're stuck. Otherwise, there are many easily-found scripting tutorials (like the one in my posting signature for starters), that will get you going. But we're not going to write scripts for you.

You can probably find scripts that run ping tests against a list of IP addresses....modify it to run a dig against a domain list.

TenTenths · 02-24-2014, 12:11 PM

Pay www.nodeping.com $8 a month

szboardstretcher · 02-24-2014, 12:24 PM

do something and when it FAILS do something else. Sounds perfect for an OR operation.

Code:

nslookup somewhere.com || #send mail to someone saying it didnt work!

116Fanatic · 02-25-2014, 10:36 AM

I wasn't expecting someone to write a script for me, I was just hoping someone already had something in place. Here is what I used in case it helps anyone in the future:

The first script creates a temporary file with the result of the lookup. I grep for the address. It calls the second script to check the temp file to see if it is empty. It then deletes the temp file:
#!/bin/bash
dig @yourdnsserver test.com | grep 1.1.1.1 > dnsfile.tmp
dnsfilecompare.sh dnsfile.tmp
rm dnsfile.tmp

Script 2 sends an email if the file is empty and only echos if the file has contents:

#!/bin/bash
_file="$1"
[ $# -eq 0 ] && { echo "Usage: $0 filename"; exit 1; }
[ ! -f "$_file" ] && { echo "Error: $0 file note found."; exit 2; }

if [ -s "$_file" ]
then
echo "All is well"
else
echo "DNS lookup of test.com failed" | mail -s "DNS lookup of test.com failed" user@test.com
fi

szboardstretcher · 02-25-2014, 10:45 AM

thats cool. you can do this same procedure with just this oneliner though:

Code:

nslookup oaiwhefoih.com || echo "DNS lookup of test.com failed" | mail -s "DNS lookup of test.com failed" user@test.com

schneidz · 02-25-2014, 10:56 AM

Quote:

Originally Posted by szboardstretcher

do something and when it FAILS do something else. Sounds perfect for an OR operation.

Code:

nslookup somewhere.com || #send mail to someone saying it didnt work!

good idea but nslookup still returns 0 even if the site cant be found (its not that the program failed -- its reporting correctly that the site cant be found).

maybe ping -c 1 ... would be better here ?

this is what i would do:

Code:

server=does_not_exist.com
if [ -n "`nslookup $server | grep can\'t\ find`" ]
then 
 echo "DNS lookup of $server failed" | mail -s "DNS lookup of $server failed" user@test.com
fi

szboardstretcher · 02-25-2014, 11:00 AM

Quote:

Originally Posted by schneidz

good idea but nslookup still returns 0 even if the site cant be found (its not that the program failed -- its reporting correctly that the site cant be found).

No it doesn't.

Code:

# nslookup asdfioejf.com 

** server can't find asdfioejf.com: NXDOMAIN

# echo $?
1

schneidz · 02-25-2014, 11:12 AM

Quote:

Originally Posted by szboardstretcher

No it doesn't.

Code:

# nslookup asdfioejf.com 

** server can't find asdfioejf.com: NXDOMAIN

# echo $?
1

thats so weird:

Code:

[schneidz@hyper sd-bak-04.04.2013]$ nslookup asdfioejf.com
Server:		192.168.1.1
Address:	192.168.1.1#53

** server can't find asdfioejf.com: NXDOMAIN

[schneidz@hyper sd-bak-04.04.2013]$ echo $?
0

szboardstretcher · 02-25-2014, 11:18 AM

Well then. Seems we are using different versions that treat exit codes differently?

In that case, dig it:

Code:

dig asdfefefefaf.com | grep NOERROR 1>/dev/null || echo "DNS lookup of test.com failed" | mail -s "DNS lookup of test.com failed" user@localhost

That work on yours?

schneidz · 02-25-2014, 11:28 AM

this is what i get:

Code:

[schneidz@hyper sd-bak-04.04.2013]$ dig asdfefefefaf.com | grep NOERROR 1>/dev/null || echo "DNS lookup of test.com failed" 
DNS lookup of test.com failed
[schneidz@hyper sd-bak-04.04.2013]$ dig asdfioejf.com

; <<>> DiG 9.8.0-P4-RedHat-9.8.0-7.P4.fc15 <<>> asdfioejf.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 3073
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;asdfioejf.com.			IN	A

;; AUTHORITY SECTION:
com.			627	IN	SOA	a.gtld-servers.net. nstld.verisign-grs.com. 1393345357 1800 900 604800 86400

;; Query time: 14 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Tue Feb 25 11:27:28 2014
;; MSG SIZE  rcvd: 104

[schneidz@hyper sd-bak-04.04.2013]$ echo $?
0

this old nettop is running fc-15 (prolly has something to do with it ?).

szboardstretcher · 02-25-2014, 11:31 AM

That worked as expected. If dig works (NOERROR) then it will do nothing. If dig does not work (NOERROR not found) it mails a message.

Wow.. yeah, the nslookup difference is definitely related to fedora15. You should really consider updating to something newer.