How does a Client Verify if the DNS server is Alive or down

[Wah Peng]

Hello,

Network issue should be detected by other ways.
But DNS health check can be done by sending a normal DNS query to the 
server and try get a valid response.

This is the script what I use to check the health of my DNS server. :D

#!/usr/bin/perl
use strict;
use Net::DNS;
use POSIX 'strftime';
use MIME::Lite;
use MIME::Words qw(encode_mimewords);

my $debug = 0;
my $test_rr = 'alive.example.net';
my $test_val = '8.8.8.8';
my @nameservers = qw(1.1.1.1 2.2.2.2);

for my $ns (@nameservers) {
     test_query($ns);
}

sub test_query {
     my $ns = shift;
     my $res = Net::DNS::Resolver->new(nameservers => [$ns]);
     $res->tcp_timeout(10);
     $res->udp_timeout(10);
     my $answer = $res->query($test_rr);

     if (defined $answer) {
         my @rr = $answer->answer;
         if ($rr[0]->address ne $test_val) {
            sendmail( "DNS not matched: $ns" );
            print "$ns wrong\n";
         }
         if ($debug) {
            print "$ns expected value for $test_rr: $test_val\n";
            print "$ns got value for $test_rr: ", $rr[0]->address,"\n";
         }
     } else {
         sendmail( "Can't query to: $ns" );
         print "$ns wrong\n";
     }
}


sub sendmail {
     # your email function to send the alerts
}

Regards.


On 2015/10/20 星期二 14:26, Harshith Mulky wrote:
> How can a Client verify if the DNS Server is Running(named service is
> Running) or Down?
> Does it periodically send any messages to the server. What Kind of
> messages are required by the client to be sent towards server to
> determine if the DNS IP is reachable or not?