Posts Tagged ‘resolver’

DNS Monitoring: Check and Alert if DNS nameserver resolver of Linux machine is not properly resolving shell script. Monitor if /etc/resolv.conf DNS runs Okay

Thursday, March 14th, 2024

linux-monitor-check-dns-is-resolving-fine

If you happen to have issues occasionally with DNS resolvers and you want to keep up an eye on it and alert if DNS is not properly resolving Domains, because sometimes you seem to have issues due to network disconnects, disturbances (modifications), whatever and you want to have another mean to see whether a DNS was reachable or unreachable for a time, here is a little bash shell script that does the "trick".

Script work mechacnism is pretty straight forward as you can see we check what are the configured nameservers if they properly resolve and if they're properly resolving we write to log everything is okay, otherwise we write to the log DNS is not properly resolvable and send an ALERT email to preconfigured Email address.

Below is the check_dns_resolver.sh script:

 

#!/bin/bash
# Simple script to Monitor DNS set resolvers hosts for availability and trigger alarm  via preset email if any of the nameservers on the host cannot resolve
# Use a configured RESOLVE_HOST to try to resolve it via available configured nameservers in /etc/resolv.conf
# if machines are not reachable send notification email to a preconfigured email
# script returns OK 1 if working correctly or 0 if there is issue with resolving $RESOLVE_HOST on $SELF_HOSTNAME and mail on $ALERT_EMAIL
# output of script is to be kept inside DNS_status.log

ALERT_EMAIL='your.email.address@email-fqdn.com';
log=/var/log/dns_status.log;
TIMEOUT=3; DNS=($(grep -R nameserver /etc/resolv.conf | cut -d ' ' -f2));  

SELF_HOSTNAME=$(hostname –fqdn);
RESOLVE_HOST=$(hostname –fqdn);

for i in ${DNS[@]}; do dns_status=$(timeout $TIMEOUT nslookup $RESOLVE_HOST  $i); 

if [[ “$?” == ‘0’ ]]; then echo "$(date "+%y.%m.%d %T") $RESOLVE_HOST $i on host $SELF_HOST OK 1" | tee -a $log; 
else 
echo "$(date "+%y.%m.%d %T")$RESOLVE_HOST $i on host $SELF_HOST NOT_OK 0" | tee -a $log; 

echo "$(date "+%y.%m.%d %T") $RESOLVE_HOST $i DNS on host $SELF_HOST resolve ERROR" | mail -s "$RESOLVE_HOST /etc/resolv.conf $i DNS on host $SELF_HOST resolve ERROR";

fi

 done

Download check_dns_resolver.sh here set the script to run via a cron job every lets say 5 minutes, for example you can set a cronjob like this:
 

# crontab -u root -e
*/5 * * * *  check_dns_resolver.sh 2>&1 >/dev/null

 

Then Voila, check the log /var/log/dns_status.log if you happen to run inside a service downtime and check its output with the rest of infrastructure componets, network switch equipment, other connected services etc, that should keep you in-line to proof during eventual RCA (Root Cause Analysis) if complete high availability system gets down to proof your managed Linux servers was not the reason for the occuring service unavailability.

A simplified variant of the check_dns_resolver.sh can be easily integrated to do Monitoring with Zabbix userparameter script and DNS Check Template containing few Triggers, Items and Action if I have time some time in the future perhaps, I'll blog a short article on how to configure such DNS zabbix monitoring, the script zabbix variant of the DNS monitor script is like this:

[root@linux-server bin]# cat check_dns_resolver.sh 
#!/bin/bash
TIMEOUT=3; DNS=($(grep -R nameserver /etc/resolv.conf | cut -d ' ' -f2));  for i in ${DNS[@]}; do dns_status=$(timeout $TIMEOUT nslookup $(hostname –fqdn) $i); if [[ “$?” == ‘0’ ]]; then echo "$i OK 1"; else echo "$i NOT OK 0"; fi; done

[root@linux-server bin]#


Hope this article, will help someone to improve his Unix server Infrastucture monitoring.

Enjoy and Cheers !

Universal way to configure a static IP address on ethernet lan (eth0) interface in Linux

Friday, April 29th, 2011

One of the most precious commands I ever learned to use in Linux is ifconfig and route .

They have saved my life in configuring the static IP based internet of numerous Desktop Linux computers & notebooks.

Though the usage is very much known by most of the people who are into Linux, I believe it’s likely that the newer people who entered the world of Linux or some Unix system administrators are still lacking the knowledge on how to manually configure their eth0 lan card, thus I thought it might be handy for someone to share it, I know that for most unix users & admins especially the advanced ones this post might be funny, so if you’re an advanced administrator just skip the post and don’t laught at it 😉

Now the universal commands (works on each and every Linux host) to configure manually static IP internet connection on Linux are:

linux:~# /sbin/ifconfig eth0 192.168.0.3 netmask 255.255.255.0
linux:~# /sbin/route add default gw 192.168.0.1
linux:~# echo 'nameserver 192.168.0.1' >> /etc/resolv.conf

I’ve used this simple commands on thousands ot Linux hosts and it’s still handy 🙂

In above example 192.168.0.3 is the static IP address provided by the ISP, netmask is the netmask and the second /sbin/route add default gw would set the default gateway to the example ip 192.168.0.1

The third final line would add up a resolver nameserver the Linux host would use.

Cheers 😉